UNIVERSITI TEKNOLOGI MARA
FACULTY OF HEALTH SCIENCES
ENVIRONMENTAL TOXICOLOGY
INSTITUT PERKEMBANGAN PENDIDIKAN (InED)
UNIVERSITI TEKNOLOGI MARA (UiTM)
40450 SHAH ALAM
STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
ENVIRONMENTAL TOXICOLOGY (ENV 410)
for the
Bachelor in Environmental Safety and Health
(Honours) Program (e- pjj)
Faculty of Health Sciences
Universiti Teknologi Mara (UiTM)
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
Course Description:
The course covers how organisms are exposed to and respond to chemicals exposures.
Uptake, metabolism and elimination of chemicals will be discussed. The spectrum of
toxic effects will be covered from acute to chronic toxicity. Emphasis will be on humans.
Environmental fate of chemicals will be presented. Methods for assessing potential
environmental exposures will be covered coupled with toxicological principles to predict
likely outcomes of environmental exposures to chemicals.
Course Outcomes:
Upon successful completion of this course the student should be able to:
1.
Identify basic terminologies and toxicological concepts.
2.
Describe the basic types of toxins, their sources, their sinks, and the
transformations within organisms and ecosystems.
3.
Interpret dose-response relationship Discuss the role of man in production and
distribution of toxic substances.
4.
Explain toxicant absorption, distribution, biotransformation and elimination in the
human body.
5.
Discuss the significance of hazard assessment.
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
Course Description:
The course covers how organisms are exposed to and respond to chemicals exposures.
Uptake, metabolism and elimination of chemicals will be discussed. The spectrum of
toxic effects will be covered from acute to chronic toxicity. Emphasis will be on humans.
Environmental fate of chemicals will be presented. Methods for assessing potential
environmental exposures will be covered coupled with toxicological principles to predict
likely outcomes of environmental exposures to chemicals.
Course Outcomes:
Upon successful completion of this course the student should be able to:
1.
2.
3.
4.
5.
Identify basic terminologies and toxicological concepts.
Describe the basic types of toxins, their sources, their sinks, and the
transformations within organisms and ecosystems.
Interpret dose-response relationship Discuss the role of man in production and
distribution of toxic substances.
Explain toxicant absorption, distribution, biotransformation and elimination in the
human body.
Discuss the significance of hazard assessment.
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
CONTENTS
1.
2.
3.
4.
INTRODUCTION TO ENVIRONMENTAL TOXICOLOGY
1.1
What is environmental Toxicology?
1.2
Terminology
1.3
Subdisciplines of Toxicology
1.4
Ecological Concepts
1.5
Relevance of Environmental Toxicology to Human Species
1.6
Structural Levels of Organization
TOXICOLOGICAL CONCEPTS
2.1
Toxicity
2.2
Toxicokinetics and Toxicodynamics
2.3
Classification of Toxicants
2.4
Determination of Toxicity
2.5
Determining the doses to Test
DOSE-RESPONSE RELATIONSHIPS
3.1
Dose and Response Relationships
3.2
Dose Response Graphs
3.3
How Individuals May Respond in a Population
3.4
Dose Referenced to time
3.5
Referencing dose to environmental media
ABSORPTION MECHANISMS OF TOXICANTS
4.1
Interactions of Toxicants with Cells
4.2
Cell Membrane Structure
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
4.3
Process of Cellular Uptake of Toxicants: passive diffusion, filtration,
special transport and endocytosis
4.4
Routes of absorption
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5.
6.
7.
4.5
Distribution of toxicants
4.6
Factors Affecting Distribution of Toxicants to Tissues
4.7
Storage of Toxicants
METABOLISM OF TOXICANTS
5.1
Why metabolize?
5.2
Phase 1 (oxidations, reductions and hydrolysis in the FAD and
cytochrome P-450 enzymes systems)
5.3
Phase 2 reactions (glycoside, sulphate, and glutathione
conjugations and others)
5.3
Elimination Mechanisms
CHEMICAL FATE
6.1
Mobility and Persistence Indicators
6.2
Bioavailabilty, Bioaccumulation, Biomagnifications
6.3
DDT
6.4
Malathion
ENVIRONMENTAL TOXICANTS
7.1
Introduction to Environmental Toxicants
7.2
Pesticides
7.3
Plastics
7.4
Metals
7.5
Organic solvents
7.6
Other Environmental Toxicants
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
8.
RISK ASSESSMENT
8.1
Introduction to Risk
8.2
Risk Assessment
8.3
Risk Management
Teaching Methodology:
Lectures and tutorials
Assessment:
Continuous Assessment
40%
Final Exam
60%
Total
100%
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
SYLLABUS CONTENTS
CHAPTER 1 : INTRODUCTION TO ENVIRONMENTAL TOXICOLOGY
1.1
What is Toxicology?
It is the study of the adverse effects of chemicals in biological systems. A
biological system can be as complex as an entire organism or can be a less
complicated in vitro cell culture. As a public health professionals we direct most of
our attention to human health effects : however, we must also recognized the
adverse effects of chemicals as we share the planet with plants and other
animals.
Toxic chemicals
Terms commonly used to refer to toxic chemicals are as follows :



Toxicant : Any chemical that can potentially produce harm. May affect
specific tissues or organs ( target tissues, target organs), benzene may
affect the blood and blood-forming tissues. May also be relatively nonspecific, affecting the entire body. Sodium cyanide..a systemic toxicant
which has the ability to interfere with all body cell utilization of oxygen.
Toxicant may be heavy metal, pesticide, an organic solvent or even a
toxin.
Toxin : Those chemicals that are produced by living organisms, Aflatoxin
B produced by Aspergillus flavus can cause liver necrosis and cancer,
Tetrodotoxin produced by puffer fish affects nervous system.
Poison : Any substance when ingested, inhaled or absorbed or when
applied to, injected into or developed within a body in relatively small
amounts may, by its chemical action, cause death or injury. Some
examples are synthetic chemicals or toxin. “All substances are poisons;
there is none which is not a poison.
The right dose differentiates a poison from a remedy.”
Paracelsus (1493-1541)

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Xenobiotics : any synthetic chemical that has no beneficial effect on the
body.
STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
Examples of xenobiotics
Toxicant and Source
Deltamethrin (insecticide)
Ethylene glycol monomethylether
(solvent)
Paraquat
Soman gas
Example of Tissue/System Affected
Nervous system
Testis
Lung
Nervous system
Toxicology Effects:
Home environment, Current events, Workplace, School, Government
Decisions, Global and local environment
Toxicology and Government Agencies
1. Food and Drug Administration (FDA)
Function:
- promotes and protects the public health by helping safe and effective
products reach the market.
- monitors products for continued safety after their usage
2.The Agency for Toxic Substances and Disease Registry (ATSDR)
Function:
-Established in 1983 to provide health-based information for use of cleanup of
chemical waste disposal sites mandated by CERCLA
-Concerned with the health effects that may occur from exposure to toxic
chemicals.
The Agency publishes
� Toxicological Profiles – which provide information on specific chemicals
and possible health effects
� Case Studies in Environmental Medicine – which provide information to
health care providers about the toxic effects of chemicals
� Public Health Statements – which contain information on toxic chemical
exposures
� ToxFAQ’s – which are fact sheets summarizing hazardous substances
3. EPA – Environmental Protection Agency
Function:
-Enforce federal laws designed to protect human health and the environment
-Oversee cleanup of hazardous waste sites
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-Regulate specific chemicals
4. OSHA - Occupational Safety and Health Administration
Function:
-Develop rules and regulations that activate the requirements of
environmental laws
5. CDC - Centers for Disease Control and Prevention
Mission:
To promote health and quality of life by preventing and controlling disease,
injury, and disability.
6. The American Conference of Governmental Industrial Hygienists (ACGIH)
Produces:
Listing of Threshold Limit, Values (TLV) and Biological Exposure
Indices (BEI) for several hundred chemicals. The lists are updated every
year.
7. NIOSH
Function:
-Investigates potentially hazardous work conditions
-Evaluates chemical hazards in the workplace
-Conducts research on chemicals
-Provides information to OSHA for use in setting standards
8. Material Safety Data Sheets (MSDS)
9. HazDat (Hazardous Substances and Health Effects Database)
Define environmental toxicology?
Environmental Toxicology is concerned with the study of chemicals that
contaminate food, water, soil, or the atmosphere. It also deals with toxic
substances that enter bodies of waters such as lakes, streams, rivers, and
oceans. This sub-discipline addresses the question of how various plants,
animals, and humans are affected by exposure to toxic substances.
1.2
Terminology
Toxicology, Atmosphere, Biosphere, Hydrosphere, Lithosphere, Ecosystem,
Hazardous waste, Infinite dilution, Environmental Toxicology, Forensic toxicology,
Mechanistic toxicology, Industrial toxicology, Clinical toxicology, Descriptive
toxicology, Molecules, Macromolecules, Organelles, Cells, Organs, Organ system
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1.3
2009
Subdisciplines of Toxicology
-
Environmental Toxicology
Studies chemicals that are contaminants of food, water, soil, or the air. Deals with
toxic substances that enter the waterways, such as lakes, streams, rivers and
oceans.
-
Occupational (Industrial) Toxicology
Concerns with health effects exposure to chemicals in the workplace. This field
grew out of a need to protect workers from toxic substances and to make their
work environment safe. Occupational diseases caused by industrial chemicals
account for an estimated 50,000 to 70,000 deaths, and 350,000 new cases of
illness each year in the United States.
-
Regulatory Toxicology
Gathers and evaluates existing toxicological information to establish
concentration-based standards of “safe” exposure. The standard is the level of a
chemical that a person can be exposed to without any harmful health effects.
-
Food Toxicology
Involves in delivering a safe and edible supply of foods to the consumer.
Toxicologists must determine the acceptable daily intake level for those
substances.
- Clinical Toxicology
Involves in the diagnosis and treatment of poisoned patients
- Descriptive Toxicology:
Involves in devising risk assessment tests for potentially toxic substances.
Descriptive Toxicology is concerned with gathering toxicological information from
animal experimentation. These types of experiments are used to establish how
much of a chemical would cause illness or death. (EPA), (OSHA), (FDA), use
information from these studies to set regulatory exposure limits.
- Forensic Toxicology
Helps to establish cause and effect relationships between exposure to a drug or
chemical and the toxic or lethal effects that result ( the medical-legal interface)
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- Mechanistic Toxicology
Study of molecular mechanisms involved in the adverse effect makes
observations on how toxic substances cause their effects. The effects of exposure
can depend on a number of factors, including the size of the molecule, the
specific tissue type or cellular components affected, whether the substance is
easily dissolved in water or fatty tissues, all of which are important when trying to
determine the way a toxic substance causes harm, and whether effects seen in
animals can be expected in humans.
- Analytical Toxicology
Identifies the toxicant through analysis of body fluids, stomach content,
excrement,skin, or suspected containers.
Toxicity testing
Essentially the main objectives in toxicity testing are to :
1.5
i.
assess a substance toxic potential
ii.
aid in the prediction of a potential hazard
iii.
produce data for use in risk-benefit assessments
iv.
provide information on the mechanisms of toxication
Ecological Concepts
Ecotoxicology
Ecotoxicology is "the study of the harmful effects of chemicals upon ecosystems"
(Walker et al, 1996). In toxicology the organisms sets the limit of the investigation
whereas Ecotoxicology aspires to assess the impact of chemicals not only on
individuals but also on populations and whole ecosystems. It was after World War
II that increasing concern about the impact of toxic discharges on the environment
led toxicology to expand from the study of impacts on man to that of impacts on
the environment. This became known as Environmental Toxicology.
The major tools of Environmental Toxicology were: detection of toxic residues in
the environment or in individual organisms and testing for the toxicity of chemicals
on animals other than man. It was however, a very big jump from a fish in a jar or
a rat in a cage to a complex, multivariate environment and ecotoxicology
developed from the need to measure and predict the impact of pollutants on
populations, communities and whole ecosystems rather than on individuals.
There are THREE main objectives in ecotoxicology:


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To obtain data for risk assessment and environmental management.
To meet the legal requirements for the development and release of new
chemicals into the environment.
STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
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2009
To develop empirical or theoretical principles to improve knowledge of the
behaviour and effects of chemicals in living systems.
(Forbes & Forbes 1994)
In order to achieve these objectives, the main areas of study are:


distribution of pollutants in the environment, their entry, movement,
storage and transformation within the environment.
effects of pollutants on living organisms.
At an individual level, toxicants may disrupt the biochemical, molecular and
physiological structure and function which will in turn have consequences for the
structure and function of communities and ecosystems.
At the population level it may be possible to detect changes in the numbers of
individuals, in gene frequency (as in resistance of insects to insecticides) or
changes in ecosystem function (e.g. soil nitrification) which are attributable to
pollution.
It may be possible to use biomarkers to establish that a natural population has
been exposed to pollution and these can provide a valuable guide to whether or
not a natural population is at risk or in need of further investigation.
For the purposes of the Regulation and Registration of chemicals the toxicity of
individual chemicals is principally investigated via toxicity testing, the main tool of
which is the Standard Toxicity Test (STT) which usually tests the Dose or
concentration of a particular chemical that is toxic to under controlled, laboratory
conditions. Toxicity tests are mainly carried out using individual animals although
there has been a move towards the use of more complex systems known as
Mesocosms. In some situations, particularly in the case of pesticides, it may be
possible to carry out Field trials to assess toxicity.
Toxicity data are used to make assessments of the HAZARD and the RISK posed
by a particular chemical.
A more complete definition of Ecotoxicology comes from Forbes & Forbes 1994
“the field of study which integrates the ecological and toxicological effects of
chemical pollutants on populations, communities and ecosystems with the
fate (Transport, transformation and breakdown) of such pollutants in the
environment".
Pollutant or Contaminant, Xenobiotics or Environmetal chemicals
Depending on the source, environmental chemical may be used to describe simply
a chemical that occurs in the environment (Walker et al 1996) or substances which
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
enter the environment as a result of human activity or occur in higher
concentrations than they would in nature (Römbke & Moltmann 1995).
The terms contaminant and pollutant can be described separately but are often in
effect synonomous. Both are used to describe chemicals that are found at levels
judged to be above those that would normally be expected. Pollutants have the
potential to cause harm, whereas contaminants are not harmful. This is however,
not an easy distinction to make.
Whether or not a contaminant is a pollutant may depend on its level in the
environment and the organism or system being considered, thus one particular
substance may be a contaminant relative to one species but pollutant relative to
another. Finally, in practice it is often difficult to demonstrate that harm is not being
caused so that in effect pollutant and contaminant become synonomous. (Walker et
al 1996).
Harm or Damage?
There is a fundamental difference is viewpoint between these two definitions, one
defines harm as an effect regardless of any compensation that the population might
make, the other defines damage as occurring only if there is an effect subsequent
to any compensation.
Harm:
Damage
biochemical or physical changes which adversely affect individual
organisms' birth, growth or mortality rates. Such changes would
necessarily produce population declines were it not that other processes
may compensate. (Walker et al 1996).
"the interaction between a substance and a biological system. The
substance's potential to cause damage is weighed against the protective
potential inherent in the biological system (e.g. excretion or metabolic
reactions, adaptation or regeneration)" (Römbke & Moltmann 1995).
Endpoints, Dose and Concentartion
There are many different ways in which toxicity can be measured but they are
nearly all assessed relative to a particular outcome or end point.
Examples of toxicity endpoints are:
Acute SystemicToxicity, Carcinogenicity, Dermal Ecotoxicity Penetration, Eye
Irritation/Corrosion, Genotoxicity, Neurotoxicity, Pharmacokinetics & Metabolism,
Repeated Dose/Organ Toxicity, Reproductive & Developmental Toxicity, Skin
Irritation/Corrosion, Skin Sensitization
Initially, most Toxicity Tests measured the number of organisms killed by a
particular DOSE or CONCENTRATION of the chemical being tested.
With terrestrial animals the DOSE of chemical (taken orally, applied to the skin or
injected) administered is usually recorded. DOSE is usually used where the dietary
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
dose of a test chemical can be accurately determined. For aquatic organisms or
where the test chemical is dosed on the surrounding medium, the tests usually
measure the CONCENTRATION of chemical in the surrounding water/medium.
The following measures, known as a group as EDs or ECs (Effective Doses or
Effective Concentrations) are frequently used to describe data from toxicity tests:
LD50
LC50
ED50/
EC50
Median lethal dose, that is the dose that kills 50% of the population
Median lethal concentration.
Median effect dose/concentration that is the dose that produced a
defined effect to 50% of the population.
NOED/
NOEC
No Observed Effect Dose (or Concentration)
No Observed Effect Level. Sometimes this more general term is
NOEL
used to describe either of the above. It can be defined as the
highest level (that is dose or concentration) of the test chemical that
does not cause a statistically significant difference from the control.
Lowest Observed Effect Dose (or Concentration)
There has been a move away from the use of lethal end points in
LOED/
toxicity testing towards the measurement of EFFECTS rather than
LOE
death. Examples of EFFECTS which can be used include changes
in: reproduction (eg. number of eggs laid or young hatched); growth
(e.g. biomass or body length) and biochemical or physiological
effects (e.g. enzyme synthesis or respiration).
Hazard and arisk
Toxicity data is used to make assessments of the HAZARD and the RISK posed
by a particular chemical. Where:
HAZARD : the potential to cause harm
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RISK : the probability that harm will be caused.
Defining HAZARD involves answering two questions, 'how much damage are we
prepared to tolerate' and 'how much proof is enough'. The first is a question for
society, alleviating/avoiding/repairing damage involves costs, how much are we
prepared to pay? The second is largely a scientific problem of providing sufficient
evidence that damage is due to pollution. HAZARD is not necessarily directly
related to toxicity, it is a product of exposure and toxicity, a compound with
moderate toxicity but very high exposure may cause more damage that a very
toxic chemical with very low exposure.
RISK if usually defined using the predicted environmental concentration (PEC)
and the predicted environmental no effect concentration (PNEC). Information on
the movement and behaviour of pollutants in the environment are used to
calculate the PEC whereas data from Toxicity Testing must be extrapolated to
calculate the PNEC. The making of these calculations is not a precise art, apart
from doubts about the extrapolation of Toxicity data from the lab to the field it can
be very difficult to estimate the degree of exposure, particularly for mobile species
such as birds and mammals.
Biomarkers
A biomarker can be defined as a "biological response to a chemical or chemicals
that gives a measure of exposure and sometimes, also, of toxic effect" (Walker et
al 1996), they can be divided into biomarkers of exposure and of toxic effect.
Examples of biomarkers range from the inhibition of AChE (acetylcholinesterase)
in the nervous system of animals to the thinning of eggshells in birds.
Biomarkers can help to bridge the gap between the laboratory and the field by
giving direct evidence of whether or not a particular animal, plant or ecosystem is
being affected by pollution. They will often provide more reliable evidence of
exposure than measurements of the pollutants themselves in the environment,
the latter are often short-lived and difficult to detect, whereas their effects
(detectable via biomarkers) may be much longer-term.
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Scale and Accuracy
The difficulty in extrapolating from simple, highly artificial, single-species toxicity
tests to complex, multi-variate ecosystems has led to attempts to develop more
complex systems which can be used in toxicity tests. Such systems are usually
termed microcosms, mesocosms or macrocosms that is small, medium or large
multispecies systems. It must be possible to control conditions in these systems
to such an extent that they can provide meaningful, reproducable (that is, the
system could be accurately copied elsewhere), replicable (that is, two replicates
of the same experiment would produce the same results) data in toxicity tests.
MIXTURES OF CHEMICALS, ADDITION OR MULTIPLICATION
In natural systems, organisms are often exposed to more than one pollutant at the
same time. Regulatory authorities usually assume - unless there is evidence to
the contrary - that the toxicity of combinations of chemicals is roughly additive,
and in many cases this is quite correct. However, in some cases, toxicity is more
than additive that is there is POTENTIATION of toxicity. One particular type of
potentiation called SYNERGISM occurs where only one of the chemicals present
would cause a toxic effect on its own, when a particular second chemical is
present it is not toxic in its own right but acts as a SYNERGIST to greatly increase
the toxicity of the first chemical.
1.6
Relevance of Environmental Toxicology to Human Species
Summarize the relevance of environmental toxicology to the human species.
1.7
Structural Levels of Organization
Chemical level
Cellular level
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Tissue level
Organ level
Organ system level
Organism level
REVIEW QUESTION
Homework on Silent Spring
1. Examine the ideas presented in Silent Spring and the country’s reaction to those
ideas.
2. Examine the impact of pesticide use on wildlife and the environment.
3. Recognize Silent Spring as a forerunner of the environmental movement.
PRACTICE PROBLEMS / ASSIGNMENTS
List the Malaysian agencies involved in handling toxicological issues.
CHAPTER 2 : TOXICOLOGICAL CONCEPTS
2.1
TERMS
Terms
Description
Harmful
or
Adverse effects
Harmful or adverse effects are those that are damaging to either
the survival or normal function of the individual.
Toxicity
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The word “toxicity” describes the degree to which a substance is
poisonous or can cause injury. Before toxicity can develop, a
substance must come into contact with a body surface such as
skin, eye or mucosa of the alimentary or respiratory tract.
The toxicity depends on a variety of factors: dose, duration and
STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
route of exposure, shape and structure of the chemical itself, and
individual human factors.
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Toxic
2009
This term relates to poisonous or deadly effects on the body by
inhalation (breathing), ingestion (eating), or absorption, or by
direct contact with a chemical.
“Toxicant” refers to toxic substances that are produced by or are
a byproduct of human-made activities.
Toxicant
Toxin
Poison
Xenobiotics
Toxic Symptom
Toxic Effects
Selective
Toxicity
21
A toxicant is any chemical that can injure or kill humans,
animals, or plants; a poison. The term “toxicant” is used when
talking about toxic substances that are produced by or are a byproduct of human-made activities. For example, dioxin (2,3-7,8
tetrachlorodibenzop-dioxin {TCDD}), produced as a by-product of
certain chlorinated chemicals, is a toxicant. On the other hand,
arsenic, a toxic metal, may occur as a natural contaminant of
groundwater or may contaminate groundwater as a by-product of
industrial activities. If the second case is true, such toxic
substances are referred to as toxicants, rather than toxins.
The term “toxin” usually is used when talking about toxic
substances produced naturally. A toxin is any poisonous
substance of microbial (bacteria or other tiny plants or animals),
vegetable, or synthetic chemical origin that reacts with specific
cellular components to kill cells, alter growth or development, or
kill the organism. Those chemicals that are produced by living
organisms, Aflatoxin B produced by Aspergillus flavus can cause
liver necrosis and cancer, Tetrodotoxin produced by puffer fish
affects nervous system.
Any substance when ingested, inhaled or absorbed or when
applied to, injected into or developed within a body in relatively
small amounts may, by its chemical action, cause death or injury.
Some examples are synthetic chemicals or toxin. “All substances
are poisons; there is none which is not a poison.
any synthetic chemical that has no beneficial effect on the body
Deltamethrin (insecticide), Ethylene glycol monomethylether
(solvent),Paraquat, Soman gas
This term includes any feeling or sign indicating the presence of
a poison in the system.
This term refers to the health effects that occur due to exposure
to a toxic substance; also known as a poisonous effect on the
body.
“Selective toxicity” means that a chemical will produce injury to
one kind of living matter without harming another form of life,
even though the two may exist close together.
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Toxicity Development
Before toxicity can develop, a substance must come into contact with a body
surface such as skin, eye or mucosa of the digestive or respiratory tract. The
dose of the chemical, or the amount one comes into contact with, is important
when discussing how “toxic” a substance can be.
.
2.2
Toxicokinetics and Toxicodynamics
Toxicokinetics is the study of the five time dependent processes related to
toxicant as they interact with living organism. These process are :
I.
II.
III.
IV.
Absorption
Distribution
Storage
Biotransformation
V.
Elimination
: How toxicant enter the organism
: How toxicant travel within the organism
: How some tissues preferentially harbor a toxicant
: How toxicant are altered (of detoxified) by
chemical in the organism
: How toxicant are removed from the organism
Toxidcodynamics examines the mechanisms by which toxicant produce
unique cellular effects within the organism. As expected, if toxicant exert their
influence at the level of the cell, the mechanisms will involve cellular
component.Included in the mechanisms of toxic action are alterations to the
cell plasma membrane, organelles, nucleus, cytoplasm, enzyme system,
biosynhetic pathway, development or production. Reversible or irreversible
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2009
cellular injury occurs will depend on the duration of exposure as well as the
specific toxicokinetics properties of the toxicant.
Chemical disposition and toxicokinetic
Chemical Disposition and Toxicokinetics studies are necessary to determine
how a chemical can be absorbed, distributed to the tissues and organs of the
body, stored, metabolised and eliminated.The disposition and biological
reactivity of a toxicant ( its toxicodynamics) are the detrmining factors in the
severityof toxicity. There are specific aspects of disposition that are of primary
importance :
-
The duration and concentration of substance at the site of entry
The rate of absorption
The total amount of toxicant absorbed
The distribution within the body and the presence at spesific sites
The efficiency of biotransformation
The toxicity of metabolites
The storage of toxicant and its metabolites in the body
The rate and sites of elimination
Models of disposition
The movement of toxicants throughout the body, over time, has been
described by the use of models of disposition. Models of disposition integrate
the processes of distribution, metabolism and elimination.
Several different theoreticalmodels have been described :
-
One- compartment model
Two-compartment model
Multicompartment model
Physiologically based model
Assignment :
Explain briefly the above models.
2.3
Classification of Toxicants
Many classification schemes for toxic agents have been proposed.
Dioscorides classified substances using the general characteristics of
whether they were toxic or therapeutic. Additionally, the source of the toxicant
has long been recognized as means of classification.An early scheme by
orfila classified substances as being of animal, vegetable or mineral origin.
No single classification system can be expected to adequately distinguish all
known toxicants. As more data related to toxicant becomes available there
will undoubtedly be more characteristics that can be used for classification
purposes.It is important to pay careful attention in the selection and
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2009
presentation of a classification system or conbination of systems so they will
be informative and appropriate for the intended audience.
Some of the ways commonly used to classify toxicants
2.4
Classification
Categories
Physical state
Gas, liquid, solid, dust
Use
Pesticides, solvent, food additives
Chemical structure
Aromatic amines, aliphatics, glycols
General action
Air pollutant, chronic poisons, industrial toxins
Effects
Carcinogens, mutagens, teratogens
Target organ
Neurotoxins, hepatotoxins, nephrotoxins
Mechanism of action
Stimulants, inhibitors, blockers
Poisoning potential
Slightly toxic, moderately toxic, super toxic
Labeling requirement
Oxidizer, acid explosive
General or use class
Plastics, organics chemicals, heavy metal
Determination of Toxicity
Toxicity is the state of being poisonous, it is imperative that cause and effect
relationship between substances be established. Toxicity is determined when,
on the administration of a substances, an observable and well defined end
effect is identified. Paracelsus recognised the value of cause and effect
relationship and the specificity with which different doses of chemical agents
produced well defined toxic or therapeutic effects. Toxicity involves for steps :
Toxicity Steps
-Test organism :
Plant or animal can be used. Algae, bacteria, mice, rats rabbit or nonhumans
primates are often selected as the test organism
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2009
In vivo (in life) studies use the whole organism fot toxicity testing. Humans, for
moral and ethical reasons which are culturally defined are normally not
chosen as the test organism
In vitro (in glass or test tube) studies do not use the whole organism but
instead make use of cultured cells or tissue cultures, providing an attractive
alternative in terms of cost and ethics to in vivo toxicity testing.
Response (end effect)
The response need to be easily observed and quantifiable. Of the many
possible responses, some that are commonly used include;
i.
ii.
iii.
iv.
v.
vi.
Changes in the total number of cells in a bacterial colony
The presence or absence of biochemical product produced by cultured
cell
Changes in cell morphology
Numbers of tumor produced
Alteration in sleep patterns
Changes in growth and development of an organism
Duration of the test
The duration may range from a few second to years, depending on the type of
testing being performed. Eye irritant test may only take a few second,
whereas reproductive studies may take a years, particulalrly when multiple
generation are examined.
Doses
In vivo studies the dose is expressed as the weight in miligrams (mg) of the
substances being tested per kilogram (kg) of body weight of the experiment
organism.
For in vitro toxicity testing, the weight in miligram (mg) of the substances
being tested per milimeter (mL) of medium containing the cells expresses the
dose, written as mg/ml.
2.5
Determining the doses to Test
The ability to acurately determine which doses are responsible for producing
a specific end result is critical. If the selected doses all produced the
predetermined response, then questions relating to the minimum dose
required to produced the selected response cannot be answered.
This is of particular concern when toxicity testing is done to determine the
minimum dose required to produce a response. For this reason, the series of
25
STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
doses selected usually will be in logarithm dose sequence instead of a linear
dose sequence.
Logarithm doses have an advantage over linear doses in their ability to
maximize the range of doses being tested, while minimizing the possibility of
overlooking a small dose that may represent the response threshold or
minimum dose at which the end effect will first be observed. Estimates of the
range of doses to test for a suspected toxicant are often made from previous
experience, including toxicity test result from similar chemical substances, or
a range finding subchronic study.
REVIEW QUESTION
1. Which of these groups is usually designated as one of the most sensitive subpopulations for exposures to toxic substances?
a.
b.
c.
d.
Adult women
Infants
Adult men
Adolescents
2. You have worked at a chemical facility for 10 years. The facility does not require
protective equipment, and you have developed a number of serious health affects in
the last 7 years. You are possibly experiencing what type of exposure?
a. Chronic
b. Acute
3. You are worried about contamination of vegetables grown in contaminated soils.
What type of toxicologist would you contact?
a.
b.
c.
Descriptive
Environmental
Regulatory d. Food
4. You are concerned about risks associated with growing vegetables in soil with high
lead and arsenic concentrations. You are speaking of what type of substance?
a.
b.
Toxin
Toxicant
5. The larger the amount of exposure and the greater the dose, the greater the
observed response, or effect.
a.
b.
True
False
6. What type of toxicologist takes samples of your blood, urine and hair for testing?
26
STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
a.
b.
c.
d.
2009
Descriptive
Analytical
Mechanistic
Forensic
7. Toxic agents can be classified in terms of their physical state, their effects, and their
source.
a.
b.
True
False
8. Which agency deals with the health effects that may occur from environmental
exposure to toxic chemicals?
a.
b.
c.
d.
The Environmental Protection Agency
The Centers for Disease Control and Prevention
The Agency for Toxic Substances and Disease Registry
The Nuclear Regulatory Commission
11. The no observed adverse effect level (NOAEL) is the same as the no effect level
(NEL).
a.
True b. False
12. The term toxicant is used when talking about toxic substances that are produced
by or are a by-product of human-made activities.
a. True b. False
PRACTICE PROBLEMS
Section A
In small groups, complete the toxicology cross-word puzzle, using the knowledge gained
through this module.
PUZZLE QUESTIONS
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
Across
1 toxic substances produced naturally
4 contains information on hazardous substances found at NPL and non-NPL sites
6 the dose or exposure level below which the harmful or adverse effects of a substance
are not seen in a population
7 the differences between two or more persons in the level of their response to exposure
8 implements regulations that control and abate air emissions from stationary and
mobile sources
11 the act of coming into contact with a hazardous substance
12 long-term exposure
Down
2 no observed adverse effect level
3 short-term exposure, usually less than 24 hours
5 any chemical that can injure or kill humans, animals, or plants: a poison
6 toxicology data network
9 examines the health effects from exposure to contaminants at NPL and non-NPL sites
10 poisonous or deadly effects on the body by inhalation, ingestion, absorption, or
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
contact with a chemical
11 deals with the environmental impacts of exposure to hazardous substance
Section B
1. There are many potentially toxic materials in our normal diet, however the dose is
below a "threshold" for effect. Why do you think we can consume low levels of noted
poisons like arsenic, lead, pesticides and mycotoxins without acute effects?
2. You may have heard the expression "dilution is the solution to pollution". Think back
to yesterday...did you drink the recommended 2 quarts (64 ounces) of water?
2.4
Determination of Toxicity
Toxicity is the state of being poisonous, it is imperative that cause and effect
relationship between substances be established. Toxicity is determined when, on
the administration of a substances, an observable and well defined end effect is
identified. Paracelsus recognised the value of cause and effect relationship and
the specificity with which different doses of chemical agents produced well
29
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2009
defined toxic or therapeutic effects. Essentially the main objectives in toxicity
testing are to :
v.
assess a substance toxic potential
vi.
aid in the prediction of a potential hazard
vii.
produce data for use in risk-benefit assessments
viii.
provide information on the mechanisms of toxication
Determination of toxicity involves FOUR steps :
Toxicity
Descriptions
Steps
Plant or animal can be used. Algae, bacteria, mice, rats rabbit or nonhumans
primates are often selected as the test organism
In vivo (in life) studies use the whole organism fot toxicity testing. Humans,
Test
for moral and ethical reasons which are culturally defined are normally not
organism
chosen as the test organism. In vitro ( in glass or test tube) studies do not
use the whole organism but instead make use of cultured cells or tissue
cultures, providing an attractive alternative in terms of cost and ethics to in
vivo toxicity testing.
The response need to be easily observed and quantifiable. O f the many
possible responses , some that are commonly used include ;
Response
i.
Changes in the total number of cells in a bacterial colony
ii.
The presence or absence of biochemical product produced by
(end
effect)
cultured cell
iii.
Changes in cell morphology
iv.
Numbers of tumor produced
v.
Alteration in sleep patterns
vi.
Changes in growth and development of an organism
The duration may range from a few second to years, depending on the type
Duration
of the test
of testing being performed. Eye irritant test may only take a few second,
whereas reproductive studies may take a years, particulalrly when multiple
generation are examined.
Doses
30
In vivo studies the dose is expressed as the weight in miligrams (mg) of the
STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
substances being tested per kilogram (kg) of body weight of the experiment
organism. For in vitro toxicity testing, the weight in miligram (mg) of the
substances being tested per milimeter (mL) of medium containing the cells
expresses the dose, written as mg/ml.
2.2
Determining the doses to Test
The ability to acurately determine which doses are responsible for producing a
specific end result is critical. If the selected doses all produced the predetermined
response, then questions relating to the minimum dose required to produced the
selected response cannot be answered.
This is of particular concern when toxicity testing is done to determine the
minimum dose required to produce a response. For this reason, the series of
doses selected usually will be in logarithm dose sequence instead of a linear
dose sequence.
Logarithm doses have an advantage over linear doses in their ability to maximize
the range of doses being tested, while minimizing the possibility of overlooking a
small dose that may represent the response threshold or minimum dose at which
the end effect will first be observed. Estimates of the range of doses to test for a
suspected toxicant are often made from previous experience, including toxicity
test result from similar chemical substances, or a range finding subchronic study.
REVIEW QUESTION
1. Which of these groups is usually designated as one of the most sensitive subpopulations for exposures to toxic substances?
a.
Adult women
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
b.
c.
d.
2009
Infants
Adult men
Adolescents
2. You have worked at a chemical facility for 10 years. The facility does not require
protective equipment, and you have developed a number of serious health affects in
the last 7 years. You are possibly experiencing what type of exposure?
a. Chronic
b. Acute
3. You are worried about contamination of vegetables grown in contaminated soils. What
type of toxicologist would you contact?
a.
b.
c.
Descriptive
Environmental
Regulatory d. Food
4. You are concerned about risks associated with growing vegetables in soil with high
lead and arsenic concentrations. You are speaking of what type of substance?
a.
b.
Toxin
Toxicant
5. The larger the amount of exposure and the greater the dose, the greater the observed
response, or effect.
a.
b.
True
False
6. What type of toxicologist takes samples of your blood, urine and hair for testing?
a.
b.
c.
d.
Descriptive
Analytical
Mechanistic
Forensic
7. Toxic agents can be classified in terms of their physical state, their effects, and their
source.
a.
b.
True
False
8. Which agency deals with the health effects that may occur from environmental
exposure to toxic chemicals?
a.
b.
The Environmental Protection Agency
The Centers for Disease Control and Prevention
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
c.
d.
2009
The Agency for Toxic Substances and Disease Registry
The Nuclear Regulatory Commission
11. The no observed adverse effect level (NOAEL) is the same as the no effect level
(NEL).
a.
True b. False
12. The term toxicant is used when talking about toxic substances that are produced by
or are a by-product of human-made activities.
a. True b. False
CHAPTER 3 : DOSE-RESPONSE RELATIONSHIPS
"The right dose differentiates a poison and a remedy." -Paracelsus
The most fundamental of all principles of toxicology is that of the relationship
between the amount of a toxicant that is received by the organism (the dose) and
the effect(s) that results from the dose (the response). The basis of establishing
this relationship, for most chemicals, comes primarily from experimental data
using laboratory animals, in vitro studies, and to a much lesser extent, information
from humans.
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3.1
2009
The dose-relationship

Can establish the lowest dose where an objectively measurable effect first
occurs (threshold level)
Establishes a quantitative relationship between the dose and the response
Provides the basis for establishing a causal relationship between the dose
and the response
Provides information to assess the relative toxicity of a chemical when
compared with others tested under similar conditions of exposure.



The relationship between the dose and response was recognized by Paracelsus in
the 16th century as fundamental to understanding how the same chemical can
produce benefit or injury to a person. His statement ‘solely the dose determines
that a thing is not a poison’ was the recognition that any substance could be a
poison and that the right dose differentiates whether that substance will act as a
poison or as a remedy.
He hypothesized that:


There must be a dose below which no response can occur or can be
measured
There must be a maximum response in which any further increase in the
dose will not result in any increase in the effect
Dose can be defined in several ways:




The administered or applied dose is the amount presented to an
absorption barrier and available for absorption. This is the dose reffered to
in toxicity testing unless otherwise specified.
The absorbed dose is the amount crossing a specific absorption barrier (
e.g. the exchange boundaries of the skin, lung, and digestive tract)
through uptake processes
The internal dose is a more general term denoting the amount absorp
without respect to specific absorption barriers or exchange boundaries
The delivered dose is the amount of the chemical available for interaction
with any particular organ or cell.
Relating Dose to Response
For a dose-response relationship to be scientifically valid in toxicology, a number
of conditions must be satisfied:




34
A method is needed to objectively measure an adverse response
The adverse response occurs after the dose is administered
The dose response is due solely to the dose
The type of adverse response(s) measured is the same or similar for each
individual or in vitro system that is used for testing
STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY

2009
The intensity for the adverse response to the dose
The dose and the magnitude of a toxic response may break down when we talk
about allergic reactions. Why?
In individuals who have been sensitized to some chemical, it is not the chemical
that causes the allergic response but rather the mediators of inflammation that are
produced secondary to the chemical that act an allergic trigger.
3.2
Dose Response Graphs
Dose-response curves can be used to plot the results of many kinds of
experiments. The X-axis plots concentration of a drug or hormone. The Y-axis
plots response, which could be almost anything. For example, the response might
be enzyme activity, accumulation of an intracellular second messenger,
membrane potential, secretion of a hormone, heart rate or contraction of a
muscle.
The term "dose-response curve" is also used more loosely to describe in vitro
experiments where you apply known concentrations of drugs. The term
"concentration-response curve" is a more precise label for the results of these
experiments. The term "dose-response curve" is occasionally used even more
loosely to refer to experiments where you vary levels of some other variable, such
as temperature or voltage.
An agonist is a drug that causes a response. If you administer various
concentrations of an agonist, the dose-response curve will go uphill as you go
from left (low concentration) to right (high concentration). A full agonist is a drug
that appears able to produce the full tissue response. A partial agonist is a drug
that provokes a response, but the maximum response is less than the maximum
response to a full agonist. An antagonist is a drug that does not provoke a
response itself, but blocks agonist-mediated responses. If you vary the
concentration of antagonist (in the presence of a fixed concentration of agonist),
the dose-response curve will run downhill.
The shape of dose-response curves
Many steps can occur between the binding of the agonist to a receptor and the
production of the response. So depending on which drug you use and which
response you measure, dose-response curves can have almost any shape.
However, in very many systems dose-response curves follow a standard shape
shown below (generally sigmoidal)
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
.
Dose-response experiments typically use 10-20 doses of agonist, approximately
equally spaced on a logarithmic scale. For example doses might be 1, 3, 10, 30,
100, 300, 1000, 3000, and 10000 nM. When converted to logarithms, these
values are equally spaced: 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0.
Note: The logarithm of 3 is actually 0.4771, not 0.50. The antilog of 0.5 is 3.1623.
So to make the doses truly equally spaced on a log scale, the concentrations
ought to be 1.0, 3.1623, 10.0, 31.623 etc.
Theoretically, the threshold dose represents a dose that is large enough to show
an inability of normal repair mechanisms to keep pace with the level of chemical
insult. This of course would need to be referenced to the toxic end-point that we
are measuring.
Two terms found in the toxicological literature that represent empirically
determined dose levels are:


LOAEL (Lowest observed adverse effect level) – represents the lowest
dose at which an observed toxic or adverse effect occurred)
NOEAL (Highest dose at which there was no measurable toxic or adverse
response)
The EC50
A standard dose-response curve is defined by four parameters: the baseline
response (Bottom), the maximum response (Top), the slope, and the drug
concentration that provokes a response halfway between baseline and maximum
(EC50).
It is easy to misunderstand the definition of EC50. It is defined quite simply as the
concentration of agonist that provokes a response half way between the baseline
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
(Bottom) and maximum response (Top). It is impossible to define the EC50 until
you first define the baseline and maximum response.
Depending on how you have normalized your data, this may not be the same as
the concentration that provokes a response of Y=50. For example, in the example
below, the data are normalized to percent of maximum response, without
subtracting a baseline. The baseline is about 20%, and the maximum is 100%, so
the EC50 is the concentration of agonist that evokes a response of about 60%
(half way between 20% and 100%).
Don't over interpret the EC50. It is simply the concentration of agonist required to
provoke a response halfway between the baseline and maximum responses. It is
usually not the same as the Kd for the binding of agonist to its receptor.
The steepness of a dose-response curve
Many dose-response curves follow exactly the shape of a receptor binding curve.
As shown below, 81 times more agonist is needed to achieve 90% response than
a 10% response.
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2009
Some dose-response curves however, are steeper or shallower than the standard
curve. The steepness is quantified by the Hill slope, also called a slope factor. A
dose-response curve with a standard slope has a Hill slope of 1.0. A steeper
curve has a higher slope factor, and a shallower curve has a lower slope factor. If
you use a single concentration of agonist and varying concentrations of
antagonist, the curve goes downhill and the slope factor is negative
.
How individuals may respond in a population
In a population of individuals, one can observe biological variation in response to
the same dose of chemical. Consider a hypothetical group of 100 individuals
exposed to exactly the same manner to the chemical in the air that typically
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2009
produces respiratory irritation ( e.g. sulfur dioxide), futher assumptions are the
concentration of that chemical and the internal dose receives by each individual
are the same. We then see a bell-shaped distribution curve that theoritically never
reaches 0 or 100%.
The majority of responses to the toxicant are similar, some are hyporesponsive
and some are hyperresponsive. The responses have a typically Gaussian
distribution and can be expressed as plus one or two standard deviations of that
mean. One standard deviation includes 68% of the individuals, and two standard
deviations include 95% of the indivuduals.
Class exercise : Draw graphs to represent the response of the group.
The dose must be referenced to time
A dose can be a single event or multiple events over a specified period of time,
for example an adult male may consume, a total of 500mg/ 3x a day/ for 10 If this
consecutive days over the course of a month ( total dose , 15 000 mg or 15 g) of
amoxicillin, if properly taken, should have therapeutic value, if he were to take the
total dose on day 1, then there would be little therapeutic value and indeed there
may be edverse effects, including injury to the liver and kidneys. If this individual
weighed 70 kg, approximately 21.4 mg/kg of body weight, as compared to all
taken as a single dose, 214mg/kg or a dose of 10 x greater than the daily
therapeutic level. If we were to reference the doses over an 8 hr period instead of
a 24 hr period, then if correctly taken the therapeutic dose becomes 7.14mg/kg
compared with 214 mg/kg if all the antibiotic is taken in a single dose. This latter
dose is approximately 30 x greater than the therapeutic dose. Clearly, the way
taht time is referenced with respect to dose can greatly impact our assessment
about toxicity.
Dose standardization Based on Body Weight
For both efficacy and toxicity studies, doses are standardised based on body
weight. For example, if a single daily dose of 0.1 mg/kg of a medication has been
shown to be efficacous in men, a 120kg male should receive a daily dose of 12
mg, on the contrary a 70 kg male would require only 7mg. If these men were
drinking at a pub and each consumed the same amount of toxicant ethanol, over
the same period of time they would have different blood alcohol concentration
based on weight difference alone, assuming their blood alcohol levels were
similarly determined
Referencing dose to environmental media
A dose may be represented as an exposure dose or the concentration a chemical
present in a medium and would be expressed as a gram unit of a chemical per
cubic metre of air, liter of water, kilogram of soil or koligram of
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2009
edibldes(vegetable, fruit, fish). For inhaled chemicals- exposure concentration in
ppm or mg/cibic metre of air, nevertheless this is not equivalent to the actual dose
because it does not tell us how much of the chemical has entered the body. So
the actual absorbed dose depends on factors such as exposure time, body wt.,
pulmonary retention, walking, sitting or excercising.
It is possible to calculate, the dose of inhaled aerosol using equations that
incorporate these factors. As an example, the dose of an inhaled aerosol may be
described by the following formula :
D = α x ( VT x f x C) t / M
D = Dose (mg/kg)
α = the fraction of aerosol deposited in the lungs (% retained)
VT= tidal volume (ml of air moved with each minute)
f= ventilation frequency (breaths per minute)
C= exposyre concentration (mg/metre cube)
T= exposure duration (minutes)
M= body weight (kg)
The total dose of a any chemical may be the result of combined individual doses,
from different media, over a specified period of time.
Hypothetical case :
Consider a family outing for a day of camping, hiking, and fishing for mum, dad,
and their 3-year old.The campsite has well water for drinking, bass in the lake for
the angler of the family, and lots of dirt for junior to play. At the end of the day,
dad has brought back enough fish for dinner. Unknown to the family, some recent
environmental sampling show concentration s of benzene in the different media :
5µg/l of well water, 3 mg/kg of soil, and 2 mg/kg in the flesh of the lake bass.
Based on the above discussion, the total dose of benzene for each member of
the family can be determined for that day. See the table below :
Benzene Consumption and Oral Daily Dose
Dad
Mom
Child
B.wt
kg
80
60
25
Amount Of media
consumed
water Fish soil
l
mg
mg
3
500 50
2
300 10
1
200 200
Amount of Benzene
consumed
water Fish soil
µg
mg
mg
15
1
0.00015
10
0.6
0.00003
5
0.4
0.0006
Consumed
µg
1015.15
610.03
405.6
TD
µg/kg/dy
12.7
10.2
16.2
The next morning , Junior is complains of stomach pain, and dad has learned that the
campsite has just been closed down due to benzene contamination. The family now is
extremely concerned taht Junior may have become ill from benzene and wants to know
how toxic benzene is and how it got to the campsite.
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2009
How would you react to this if you were the state toxicologist ? What information
do you need to have ? Animal lethality data? Human lethality data ?
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2009
CHAPTER 4 : ABSORPTION MECHANISMS OF TOXICANTS
Toxicant entry into the body
To produce a systemic effect, a toxicant has to defeat barriers to absorption and
enter into the internal compartments of the body ; otherwise all effects are
confined to the site of exposure, i.e. all toxicity will be local ( irritation to skin or
respiratory tract)
4.1
Interactions of Toxicants with Cells
In general, toxicants exert their effects when they interact with cells. This cellular
interaction may occur ;
i.
ii.
iii.
iv.
42
on the surface of the cell
within the cell
in the underlying tissues
extracellular (interstitial) space.
STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
Although there are four different types of tissues in the human body (epithelial,
connective, muscle and nerve), it is a collection of epithelial cells that forms the
epithelium that covers the surface of the body and forms the lining of the lumen of
the respiratory and digestive systems.
Chemical characterictics of both the toxicant and cell membrane determine
whether any interaction occurs on the surface of the cell or whether the barrier
will be effective in keeping the toxicant out of the organism. Under normal
condition , the contact between adjacent epithelial cells will not permit the
passage of substances. This is due the presence of occluding cell junctions,
which are formed by intramembranous proteins arranged to ‘stitch’ the membrane
of adjacent cell together.
4.2
Cell Membrane Structure
A knowledge of the structure of epithelial cells, particularly the characteristic of
the cell membrane, or plasma membrane is important when considering why
some toxicant move through the barrier with relative ease while other toxicant find
entrance into the body difficult or impossible.
The cell membrane is composed of phospholipid molecules. As the term
phospolipid implies, there are two components to the molecule : phosphate and
lipids. The phosphate head is a region that is hydrophilic. This means that this
portion of the molecules prefers associating with water (hydrowater ; philic,
atracttion for of love of). In contrast, the lipid tai lis a hydrophobic region which is
repelled by water. Additionally, this region is said to be lipophilic, or attractive to
lipid soluble substances.
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4.2
2009
Processess of Cellular Uptake of Toxicants:
Toxicants move across cell membranes by either simple diffusion or specialized
transport. Specialized transport mechanisms included active transport, special
transport (facilitated or carrier-mediated diffusion and active transport), and
endocytosis.
The primary mechanism that moves toxicants into and out of cell membranes is
by simple diffusion. Several primary factors determine net diffusion and the
diffusion rate are:




Size of the molecule
Molecular charge and degree of ionization
Water solubility
Concentration gradient
Simple Diffusion : In this process the molecules relies on its concentration
gradient to enter the cell. This process is passive, as opposed to active since no
cellular energy is used to “power: the toxicant across the cell membrane.
Diffusion is the movement of a substance from a region of high concentration to
low concentration and depending on the direction of the concentration gradient;
substances will continue to move into or out of a cell until equilibrium is reached.
In the absence of a concentration gradient, no net movement of substances will
occur.
Facilitated Diffusion :
In this process molecules become bound to specific carrier proteins found on the
outer surface of the cell membrane. The molecules is then passed or passively
transported by the membrane protein into a cell. The energy for transport derived
from the potential energy stored in the concentration gradient not from the cell
energy input.
Facilitated diffusion is a well known mechanism in the transport of nutrient such
as glucose across the cell membrane.
Active Transport :
As a means to enter the cell, involves the consumption of cellularly produced
energy such as adenosine triphosphate (ATP). Active transport enables the cell
to transport molecules against, or up their concentration gradient.
Although not a major route of toxicant entry into the cell, active transport play a
vital role in the elimination of toxicant or their metabolic intermediates from a cell.
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Endocytosis and exocytosis :
During endocytosis the cell membrane will flow around and engulf the
macromolecules that are in close proximity to the cell. Once engulfed, the
particulate will invaginate or turn inward to form a visicle.
The visice will detach from the adjacent cell membrane and become part of the
cytoplasm. Phagocytosis (cellular eating) and pinocytosis (cellular drinking) are
two types of endocytosis. Phagocytosis performed by special white blood cells is
responsible for removing particulates from the small sacs (alveoli) in the lung
4.4
Routes of absorption
Absorption is the process by which toxicant cross the epithelial cell barrier.
Depending on the nature of the toxicant, dose, duration and type of exposure, a
toxicant may limit its contact to the outer surface of the epithelial cell barrier or
cross the cell membrane, enter the cell and possibly move completely through the
cell and into the underlying lymphatic or cardiovascular divisions of the circulatory
system.
There are three primary routes of absorption : (1) percutaneous ( integumentary
system) or through the skin, (2) the respiratory system and (3) digestive system.
Under accidental circumstances where there is an unnatural interruption to the
integrity of the barrier, absorption can also take place in the exposed tissue found
beneath the epithelium such as subcutaneous fat and muscle.
Each route of absorption has it own special type of epithelial cells that unite to
form specific tissues. These unique cell and tissue characteristic present the
potential toxicant with a different set of structural and functional features that
must be overcome to gain entrance into the body. There are also route specific
structures, such as hair follicles in the skin, that actually facilitate the absorption
of some toxicant. An awareness of anatomical and physiological characteristics
associated with each route of absorption is important as a first step in
understanding how toxicants enter the body.
Other exposure routes are intravenous and intraarterial routes provide direct
entry of chemicals into the blood vascular system, injection of chemical into the
skin and muscle. All these route of exposure are termed as parenteral route.
4.5
Chemical Disposition and toxicokinetics
Chemical disposition and toxikokinetics studies are necessary to determine how a
chemical can be absorbed, distributed to the tissues and organs of the body,
stored ( bioaccumulation), metabolized, and eliminated. The disposition and
biological reactivity of a toxicant ( its toxicodynamics) are the determining factors
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in the severity of toxicity. There are specific aspects of disposition that are of
primary importance:
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The duration and concentration of the substance at the site of entry
The rate of absorption
The total amount of toxicant absorbed
The distribution within the body and presence of specific sites
The efficiency of biotransformation
The toxicity of metabolites
The storage of toxicants and its metabolites within the body
The rates and sites of elimination
Distribution occurs when a toxicant is absorbed and subsequently enters the
lymph or blood supply for transport to other regions of the body. The lymphatic
system is a part of the circulatory system and drains excess fluid from the tissues.
Included in the lymphatic system are lymph capillaries, lymph nodes, aggregation
of lymphoid tissue (tonsil, spleen and thymus) and circulatory lymphocytes.
The precise location where the toxicant enters the bloodstream is important
because once a toxicant gains entrance into the organism, the other
toxicokinetics processes such as storage, biotransformation and elimination will
affect the concentration of the toxicant in the lymph or blood. When these
toxicokinetics processes occurs soon after the entrance of the toxicants into the
blood supply, or immediately “downstream” from the point of entry, the blood
levels of te toxicant may be diminished or eliminated thus reducing or eliminating
toxicity.
4.6
Factors Affecting Distribution of Toxicants to Tissues
The distribution of toxicants to tissues is dependent on several factors;
i.
physical and chemical properties of the toxicant
ii.
concentration of the toxicant in the blood and in the tissue
iii.
volume of blood flowing through a specific tissue
iv.
tissue specificity or preference of the toxicant
v.
Presence of special “barrier” to slow down toxicant entrance
4.7
Routes of absorption
Absorption is the process by which toxicant cross the epithelial cell barrier.
Depending on the nature of the toxicant, dose, duration and type of exposure, a
toxicant may limit its contact to the outer surface of the epithelial cell barrier or
cross the cell membrane, enter the cell and possibly move completely through the
cell and into the underlying lymphatic or cardiovascular divisions of the circulatory
system.
There are three primary routes of absorption : (1) percutaneous ( integumentary
system) or through the skin, (2) the respiratory system and (3) digestive system.
Under accidental circumstances where there is an unnatural interruption to the
integrity of the barrier, absorption can also take place in the exposed tissue found
beneath the epithelium such as subcutaneous fat and muscle.
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Each route of absorption has it own special type of epithelial cells that unite to
form specific tissues. These unique cell and tissue characteristic present the
potential toxicant with a different set of structural and functional features that
must be overcome to gain entrance into the body. There are also route specific
structures, such as hair follicles in the skin, that actually facilitate the absorption
of some toxicant. An awareness of anatomical and physiological characteristics
associated with each route of absorption is important as a first step in
understanding how toxicants enter the body.
Other exposure routes are intravenous and intraarterial routes provide direct
entry of chemicals into the blood vascular system, injection of chemical into the
skin and muscle. All these route of exposure are termed as parenteral route.
4.8
Storage of Toxicants
Once distribution occurs, toxicants can undergo other toxicokinetic processes
such as storage, biotransformation and elimination. Storage results when
toxicants accumulate in specific tissues or become bound to circulating plasma
protein. Both mechanisms reduce the concentration of the free toxicant in the
blood plasma.
Class exercise:
Where are the common storage locations for various toxicants in the human
body?
REVIEW QUESTIONS
1. Define xenobiotics ?
2. Why are lipophilic xenobiotics generally more harmful and pervasive then
hydrophilic xenobiotic in the environment ?
3. Even though DDT has been out of use in the US for about 2 decades, most if
not all of us, have a residual amount of DDT and metabolites still in our adipose
tissues (even kids). What are your thoughts about this?
4. Describe the main routes of exposure of lead. Why are children most
susceptible to lead poisoning ? How lead poisoning be treated ?
5.
MULTIPLE CHOICE QUESTIONS
Toxicity of environmental tobacco smoke
1. Which of the following alveolar cell types clean particles deposited in the lungs?
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A. macrophages
B. epithelium type I
C. epithelium type II
D. fibroblasts
E. capillaries
2. One of the primary functions of the alveoli is to create a large surface area in
the lungs. Why is a large surface area so important?
A. for energy storage
B. to remove toxins from the blood
C. to store oxygen for future use
D. for gas exchange
E. for the Krebs cycle
3. Which of the following causes the most deaths in the US?
A. AIDS
B. motor vehicles
C. homicide
D. smoking
E. alcohol
4. Which statement do you agree with?
A. Environmental tobacco smoke (ETS), also known as secondhand smoke, has LESS
toxic compounds than directly inhaled tobacco smoke.
B. Environmental tobacco smoke (ETS), also known as secondhand smoke, has MORE
toxic compounds than directly inhaled tobacco smoke.
5. What is PM10?
A. the number of packs per day that cause lung cancer in 10% of the population
B. particles which are small enough to be deposited in the lungs
C. a measure of the amount of pollen in a certain volume of air
D. a measure of the severity of an asthma attack
6. Asthma is caused by decreased airflow in and out of the lungs due to:
A. small abnormalities in airways
B. reversible bronchial spasms
C. destruction of alveolar walls
D. allergic reaction in lung tissues
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CHAPTER 5 : METABOLISM OF TOXICANTS
5.1
Why Metabolize?
Many endogenous and xenobiotic compounds are lipophilic.Lipophilic compounds
easily cross lipid bilayers and can be transported by lipoproteins. Metabolism of
endogenous compounds and xenobiotics allows organisms to convert lipophilic
compounds to more water soluble forms which facilitate excretion. Otherwise,
these compounds will accumulate in fat.
Biotransformation
Biotransformation is the process whereby a substance is changed from one
chemical to another (transformed) by a chemical reaction within the body.
Metabolism or metabolic transformations are terms frequently used for the
biotransformation process. However, metabolism is sometimes not specific for the
transformation process but may include other phases of toxicokinetics.
Biotransformation is vital to survival in that it transforms absorbed nutrients (food,
oxygen, etc.) into substances required for normal body functions. For some
pharmaceuticals, it is a metabolite that is therapeutic and not the absorbed drug.
For example, phenoxybenzamine (Dibenzyline®), a drug given to relieve
hypertension, is biotransformed into a metabolite, which is the active agent.
Biotransformation also serves as an important defense mechanism in those toxic
xenobiotics and body wastes are converted into less harmful substances and
substances that can be excreted from the body.
If you recall, toxicants that are lipophilic ('lipid-loving', dissolve easily in lipids),
non-polar, and of low molecular weight are readily absorbed through the cell
membranes of the skin, gastrointestinal (GI) tract, and lung. These same
chemical and physical properties control the distribution of a chemical throughout
the body and its penetration into tissue cells. Lipophilic toxicants are hard for the
body to eliminate and can accumulate to hazardous levels. However, most
lipophilic toxicants can be transformed into hydrophilic ('water-loving', dissolve
easily in water) metabolites that are less likely to pass through membranes of
critical cells. Hydrophilic chemicals are easier for the body to eliminate than
lipophilic substances. Biotransformation is thus a key body defense mechanism.
Fortunately, the human body has a well-developed capacity to biotransform most
xenobiotics as well as body wastes. An example of a body waste that must be
eliminated is hemoglobin, the oxygen-carrying iron-protein complex in red blood
cells. Hemoglobin is released during the normal destruction of red blood cells.
Under normal conditions hemoglobin is initially biotransformed to bilirubin, one of
a number of hemoglobin metabolites. Bilirubin is toxic to the brain of newborns
and, if present in high concentrations, may cause irreversible brain injury.
Biotransformation of the lipophilic bilirubin molecule in the liver results in the
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production of water-soluble (hydrophilic) metabolites excreted into bile and
eliminated via the feces.
The biotransformation process is not perfect. When biotransformation results in
metabolites of lower toxicity, the process is known as detoxification. In many
cases, however, the metabolites are more toxic than the parent substance. This is
known as bioactivation. Occasionally, biotransformation can produce an
unusually reactive metabolite that may interact with cellular macromolecules (e.g.,
DNA). This can lead to very serious health effects, for example, cancer or birth
defects. An example is the biotransformation of vinyl chloride to vinyl chloride
epoxide, which covalently binds to DNA and RNA, a step leading to cancer of the
liver.
5.2
Chemical Reactions
Chemical reactions are continually taking place in the body. They are a normal
aspect of life, participating in the building up of new tissue, tearing down of old
tissue, conversion of food to energy, disposal of waste materials, and elimination
of toxic xenobiotics. Within the body is a magnificent assembly of chemical
reactions, which is well-orchestrated and called upon as needed. Most of these
chemical reactions occur at significant rates only because specific proteins,
known as enzymes, are present to catalyze them, that is, accelerate the reaction.
A catalyst is a substance that can accelerate a chemical reaction of another
substance without itself undergoing a permanent chemical change.
Enzymes are the catalysts for nearly all biochemical reactions in the body.
Without these enzymes, essential biotransformation reactions would take place
slowly or not at all, causing major health problems. An example is the inability of
persons that have phenylketonuria (PKU) to use the artificial sweetener,
aspartame (in Equal®). Aspartame is basically phenylalanine, a natural
constituent of most protein-containing foods.
Some persons are born with a genetic condition in which the enzyme that can
biotransform phenylalanine to tyrosine (another amino acid), is defective. As the
result, phenylalanine can build up in the body and cause severe mental
retardation. Babies are routinely checked at birth for PKU. If they have PKU, they
must be given a special diet to restrict the intake of phenylalanine in infancy and
childhood.
These enzymatic reactions are not always simple biochemical reactions. Some
enzymes require the presence of cofactors or co-enzymes in addition to the
substrate (the substance to be catalyzed) before their catalytic activity can be
exerted. These co-factors exist as a normal component in most cells and are
frequently involved in common reactions to convert nutrients into energy (vitamins
are an example of co-factors). It is the drug or chemical transforming enzymes
that hold the key to xenobiotic transformation. The relationship of substrate,
enzyme, co-enzyme, and transformed product is illustrated below:
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Most biotransforming enzymes are high molecular weight proteins, composed of
chains of amino acids linked together by peptide bonds. A wide variety of
biotransforming enzymes exist. Most enzymes will catalyze the reaction of only a
few substrates, meaning that they have high "specificity". Specificity is a function
of the enzyme's structure and its catalytic sites. While an enzyme may encounter
many different chemicals, only those chemicals (substrates) that fit within the
enzymes convoluted structure and spatial arrangement will be locked on and
affected.
This is sometimes referred to as the "lock and key" relationship. As shown in
Figure 1, when a substrate fits into the enzyme's structure, an enzyme-substrate
complex can be formed. This allows the enzyme to react with the substrate with
the result that two different products are formed. If the substrate does not fit into
the enzyme, no complex will be formed and thus no reaction can occur.
Fig 1 lock and key" relationship
The array of enzymes range from those that have absolute specificity to those
that have broad and overlapping specificity. In general, there are three main types
of specificity:
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For example, formaldehyde dehydrogenase has absolute specificity since it
catalyzes only the reaction for formaldehyde. Acetylcholinesterase has absolute
specificity for biotransforming the neurotransmitting chemical, acetylcholine.
Alcohol dehydrogenase has group specificity since it can biotransform several
different alcohols, including methanol and ethanol. N-oxidation can catalyze a
reaction of a nitrogen bond, replacing the nitrogen with oxygen.
The names assigned to enzymes may seem confusing at first. However, except
for some of the originally studied enzymes (such as pepsin and trypsin), a
convention has been adopted to name enzymes. Enzyme names end in "ase"
and usually combine the substrate acted on and the type of reaction catalyzed.
For example, alcohol dehydrogenase is an enzyme that biotransforms alcohols by
the removal of a hydrogen. The result is a completely different chemical, an
aldehyde or ketone.
The biotransformation of ethyl alcohol to acetaldehyde is depicted below:
ADH = alcohol dehydrogenase, a specific catalyzing enzyme NAD = nicotinamide
adenine dinucleotide, a common cellular reducing agent
By now you know that the transformation of a specific xenobiotic can be either
beneficial or harmful—perhaps both depending on the dose and circumstances. A
good example is the biotransformation of acetaminophen (Tylenol®), a commonly
used drug to reduce pain and fever. When the prescribed doses are taken, the
desired therapeutic response is observed with little or no toxicity. However, when
excessive doses of acetaminophen are taken, hepatotoxicity can occur. This is
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because acetaminophen normally undergoes rapid biotransformation with the
metabolites quickly eliminated in the urine and feces.
At high doses, the normal level of enzymes may be depleted and the
acetaminophen is available to undergo reaction by an additional biosynthetic
pathway, which produces a reactive metabolite that is toxic to the liver. For this
reason, a user of Tylenol® is warned not to take the prescribed dose more
frequently than every 4-6 hours and not to consume more than four doses within
a 24-hour period. Biotransforming enzymes, like most other biochemicals, are
available in a normal amount and in some situations can be "used up" at a rate
that exceeds the bodies ability to replenish them. This illustrates the frequently
used phrase, the "Dose Makes the Poison."
Biotransformation reactions are categorized not only by the nature of their
reactions, e.g., oxidation, but also by the normal sequence with which they tend to
react with a xenobiotic (substances foreign to living organisms). They are
usually classified as Phase I and Phase II reactions. Phase I reactions are
generally reactions which modify the chemical by adding a functional structure.
This allows the substance to "fit" into the Phase II enzyme so that it can become
conjugated (joined together) with another substance. Phase II reactions consist of
those enzymatic reactions that conjugate the modified xenobiotic with another
substance. The conjugated products are larger molecules than the substrate and
generally polar in nature (water-soluble). Thus, they can be readily excreted from
the body. Conjugated compounds also have poor ability to cross cell membranes.
In some cases, the xenobiotic already has a functional group that can be
conjugated and the xenobiotic can be biotransformed by a Phase II reaction
without going through a Phase I reaction. A good example is phenol that can be
directly conjugated into a metabolite that can then be excreted. The
biotransformation of benzene requires both Phase I and Phase II reactions. As
illustrated below, benzene is biotransformed initially to phenol by a Phase I
reaction (oxidation). Phenol has the functional hydroxyl group that is then
conjugated by a Phase II reaction (sulphation) to phenyl sulfate.
The major transformation reactions for xenobiotics are listed below:
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5.3
2009
Phase I Reactions
Phase I biotransformation reactions are simple reactions as compared to Phase II
reactions. In Phase I reactions, a small polar group (containing both positive and
negative charges) is either exposed on the toxicant or added to the toxicant. The
three main Phase I reactions are oxidation, reduction, and hydrolysis.
Oxidation is a chemical reaction in which a substrate loses electrons. There are a
number of reactions that can achieve the removal of electrons from the substrate.
Addition of oxygen was the first of these reactions discovered and thus the
reaction was named oxidation. However, many of the oxidizing reactions do not
involve oxygen. The simplest type of oxidation reaction is dehydrogenation, that is
the removal of hydrogen from the molecule. Another example of oxidation is
electron transfer that consists simply of the transfer of an electron from the
substrate.
Examples of these types of oxidizing reactions are illustrated below:
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The specific oxidizing reactions and oxidizing enzymes are numerous and several
textbooks are devoted to this subject. Most of the reactions are self-evident from the
name of the reaction or enzyme involved. Listed are several of these oxidizing
reactions.
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alcohol dehydrogenation
aldehyde dehydrogenation
alkyl/acyclic hydroxylation
aromatic hydroxylation
deamination
desulfuration
N-dealkylation
N-hydroxylation
N-oxidation
O-dealkylation
sulphoxidation
Reduction is a chemical reaction in which the substrate gains electrons. Reductions
are most likely to occur with xenobiotics in which oxygen content is low. Reductions
can occur across nitrogen-nitrogen double bonds (azo reduction) or on nitro groups
(NO2). Frequently, the resulting amino compounds are oxidized forming toxic
metabolites. Some chemicals such as carbon tetrachloride can be reduced to free
radicals, which are quite reactive with biological tissues. Thus, reduction reactions
frequently result in activation of a xenobiotic rather than detoxification. An example of
a reduction reaction in which the nitro group is reduced is illustrated below:
There are fewer specific reduction reactions than oxidizing reactions. The nature of
these reactions is also self-evident from their name. Listed are several of the
reducing reactions.
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2009
azo reduction
dehalogenation
disulfide reduction
nitro reduction
N-oxide reduction
sulfoxide reduction
Hydrolysis is a chemical reaction in which the addition of water splits the toxicant into
two fragments or smaller molecules. The hydroxyl group (OH-) is incorporated into
one fragment and the hydrogen atom is incorporated into the other. Larger chemicals
such as esters, amines, hydrazines, and carbamates are generally biotransformed by
hydrolysis. The example of the biotransformation of procaine (local anesthetic) which
is hydrolyzed to two smaller chemicals is illustrated below:
Toxicants that have undergone Phase I biotransformation are converted to
metabolites that are sufficiently ionized, or hydrophilic, to be either eliminated
from the body without further biotransformation or converted to an intermediate
metabolite that is ready for Phase II biotransformation. The intermediates from
Phase I transformations may be pharmacologically more effective and in many
cases more toxic than the parent xenobiotic.
5.4
Phase II Reactions
A xenobiotic that has undergone a Phase I reaction is now a new intermediate
metabolite that contains a reactive chemical group, e.g., hydroxyl (-OH), amino (NH2), and carboxyl (-COOH). Many of these intermediate metabolites do not
possess sufficient hydrophilicity to permit elimination from the body. These
metabolites must undergo additional biotransformation as a Phase II reaction.
Phase II reactions are conjugation reactions, that is, a molecule normally present
in the body is added to the reactive site of the Phase I metabolite. The result is a
conjugated metabolite that is more water-soluble than the original xenobiotic or
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Phase I metabolite. Usually the Phase II metabolite is quite hydrophilic and can
be readily eliminated from the body.
The primary Phase II reactions are:

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
glucuronide conjugation - most important reaction
sulfate conjugation - important reaction
acetylation
amino acid conjugation
glutathione conjugation
methylation
Glucuronide conjugation is one of the most important and common Phase II
reactions. One of the most popular molecules added directly to the toxicant or its
phase I metabolite is glucuronic acid, a molecule derived from glucose, a
common carbohydrate (sugar) that is the primary source of energy for cells. The
sites of glucuronidation reactions are substrates having an oxygen, nitrogen, or
sulfur bond. This includes a wide array of xenobiotics as well as endogenous
substances, such as bilirubin, steroid hormones and thyroid hormones.
Glucuronidation is a high-capacity pathway for xenobiotic conjugation.
Glucuronide conjugation usually decreases toxicity, although there are some
notable exceptions, for example, the production of carcinogenic substances. The
glucuronide conjugates are generally quite hydrophilic and are excreted by the
kidney or bile, depending on the size of the conjugate. The glucuronide
conjugation of aniline is illustrated below:
Sulfate conjugation is another important Phase II reaction that occurs with many
xenobiotics. In general, sulfation decreases the toxicity of xenobiotics. Unlike
glucuronic acid conjugates that are often eliminated in the bile, the highly polar
sulfate conjugates are readily secreted in the urine. In general, sulfation is a lowcapacity pathway for xenobiotic conjugation. Often glucuronidation or sulfation
can conjugate the same xenobiotics.
5.5
Biotransformation Sites
Biotransforming enzymes are widely distributed throughout the body. However,
the liver is the primary biotransforming organ due to its large size and high
concentration of biotransforming enzymes. The kidneys and lungs are next with
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10-30% of the liver's capacity. A low capacity exists in the skin, intestines, testes,
and placenta. Since the liver is the primary site for biotransformation, it is also
potentially quite vulnerable to the toxic action of a xenobiotic that is activated to a
more toxic compound.
Within the liver cell, the primary subcellular components that contain the
transforming enzymes are the microsomes (small vesicles) of the endoplasmic
reticulum and the soluble fraction of the cytoplasm (cytosol). The mitochondria,
nuclei, and lysosomes contain a small level of transforming activity.
Microsomal enzymes are associated with most Phase I reactions. Glucuronidation
enzymes, however, are contained in microsomes. Cytosolic enzymes are nonmembrane-bound and occur free within the cytoplasm. They are generally
associated with Phase II reactions, although some oxidation and reduction
enzymes are contained in the cytosol. The most important enzyme system
involved in Phase I reactions it the cytochrome P-450 enzyme system.
This system is frequently referred to as the "mixed function oxidase (MFO) "
system. It is found in microsomes and is responsible for oxidation reactions of a
wide array of chemicals.
The fact that the liver biotransforms most xenobiotics and that it receives blood
directly from the gastrointestinal tract renders it particularly susceptible to damage
by ingested toxicants. Blood leaving the gastrointestinal tract does not directly
flow into the general circulatory system. Instead, it flows into the liver first via the
portal vein. This is known as the "first pass" phenomena. Blood leaving the liver is
eventually distributed to all other areas of the body; however, much of the
absorbed xenobiotic has undergone detoxication or bioactivation. Thus, the liver
may have removed most of the potentially toxic chemical. On the other hand,
some toxic metabolites are in high concentration in the liver.
5.6
Modifiers of Biotransformation
The relative effectiveness of biotransformation depends on several factors,
including species, age, gender, genetic variability, nutrition, disease, exposure to
other chemicals that can inhibit or induce enzymes, and dose levels. Differences
in species capability to biotransform specific chemicals are well known. Such
differences are normally the basis for selective toxicity, used to develop chemicals
effective as pesticides but relatively safe in humans. For example, malathion in
mammals is biotransformed by hydrolysis to relatively safe metabolites, but in
insects, it is oxidized to malaoxon, which is lethal to insects.
Safety testing of pharmaceuticals, environmental and occupational substances is
conducted with laboratory animals. Often, differences between animal and human
biotransformation are not known at the time of initial laboratory testing since
information is lacking in humans. Humans have a higher capacity for glutamine
conjugation than laboratory rodents. Otherwise, the types of enzymes and
biotransforming reactions are basically comparable. For this reason,
determination of biotransformation of drugs and other chemicals using laboratory
animals is an accepted procedure in safety testing.
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Age may affect the efficiency of biotransformation. In general, human fetuses and
neonates (newborns) have limited abilities for xenobiotic biotransformations. This
is due to inherent deficiencies in many, but not all, of the enzymes responsible for
catalyzing Phase I and Phase II biotransformations. While the capacity for
biotransformation fluctuates with age in adolescents, by early adulthood the
enzyme activities have essentially stabilized. Biotransformation capability is also
decreased in the aged. Gender may influence the efficiency of biotransformation
for specific xenobiotics. This is usually limited to hormone-related differences in
the oxidizing cytochrome P-450 enzymes.
Genetic variability in biotransforming capability accounts for most of the large
variation among humans. The Phase II acetylation reaction in particular is
influenced by genetic differences in humans. Some persons are rapid and some
are slow acetylators. The most serious drug-related toxicity occurs in the slow
acetylators, often referred to as "slow metabolizers". With slow acetylators,
acetylation is so slow that blood or tissue levels of certain drugs (or Phase I
metabolites) exceeds their toxic threshold.
Examples of drugs that build up to toxic levels in slow metabolizers that have
specific genetic-related defects in biotransforming enzymes are listed below:
Poor nutrition can have a detrimental effect on biotransforming ability. This is
related to inadequate levels of protein, vitamins, and essential metals. These
deficiencies can decrease the ability to synthesize biotransforming enzymes.
Many diseases can impair an individual's capacity to biotransform xenobiotics. A
good example, is hepatitis (a liver disease), which is well known to reduce hepatic
biotransformation to less than half normal capacity.
Enzyme inhibition and enzyme induction can be caused by prior or simultaneous
exposure to xenobiotics. In some situations exposure to a substance will inhibit
the biotransformation capacity for another chemical due to inhibition of specific
enzymes. A major mechanism for the inhibition is competition between the two
substances for the available oxidizing or conjugating enzymes, that is the
presence of one substance uses up the enzyme that is needed to metabolize the
second substance.
Enzyme induction is a situation where prior exposure to certain environmental
chemicals and drugs results in an enhanced capability for biotransforming a
xenobiotic. The prior exposures stimulate the body to increase the production of
some enzymes. This increased level of enzyme activity results in increased
biotransformation of a chemical subsequently absorbed. Examples of enzyme
inducers are alcohol, isoniazid, polycyclic halogenated aromatic hydrocarbons
(e.g., dioxin), phenobarbital, and cigarette smoke. The most commonly induced
enzyme reactions involve the cytochrome P-450 enzymes.
Dose level can affect the nature of the biotransformation. In certain situations, the
biotransformation may be quite different at high doses versus that seen at low
dose levels. This contributes to the existence of a dose threshold for toxicity. The
mechanism that causes this dose-related difference in biotransformation usually
can be explained by the existence of different biotransformation pathways. At low
doses, a xenobiotic may follow a biotransformation pathway that detoxifies the
substance. However, if the amount of xenobiotic exceeds the specific enzyme
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capacity, the biotransformation pathway is "saturated". In that case, it is possible
that the level of parent toxin builds up. In other cases, the xenobiotic may enter a
different biotransformation pathway that may result in the production of a toxic
metabolite.
An example of a dose-related difference in biotransformation occurs with
acetaminophen (Tylenol®). At normal doses, approximately 96% of
acetaminophen is biotransformed to non-toxic metabolites by sulfate and
glucuronide conjugation. At the normal dose, about 4% of the acetaminophen is
oxidized to a toxic metabolite; however, that toxic metabolite is conjugated with
glutathione and excreted. With 7-10 times the recommended therapeutic level,
the sulphate and glucuronide conjugation pathways become saturated and more
of the toxic metabolite is formed. In addition, the glutathione in the liver may also
be depleted so that the toxic metabolite is not detoxified and eliminated. It can
react with liver proteins and cause fatal liver damage.
5.7
Elimination
The two conversion routes are conversion of substance into metabolites
and excretion of the unchanged substance and the metabolites from the
organism. (Metabolites: other substances than the initial one, having their own
chemical and toxicological properties and their own toxicological profiles).
Metabolism: Metabolism or biotransformation is generally catalyzed by
enzymes. It can take place in all organs and tissues. For the transformation of
most xenobiotics, however, the liver plays the quantitatively most important role.
Liver - The liver is the body’s factory. It metabolizes food, filters toxins and
converts ingredients into substances that are needed in all parts of the body. Your
liver is one of the largest and most important organs in your body. The liver, when
healthy, will store vitamins, sugars, fats and other nutrients from the food that you
eat. The liver builds chemicals that your body needs to stay healthy and break
down harmful substances, like alcohol and other toxic (poisonous) chemicals. It
also removes waste products from your blood and makes sure that your body has
just the right amount of other chemicals that it needs.
Lungs – Volatile substances and gases can be eliminated via the lung by
exhalation.
Lymphatic system - The lymphatic is the body’s filter system which supports
immune function. A healthy lymphatic system filters out bacteria and other foreign
particles.
Blood - The blood is a liquid organ which transfers and transports substances
throughout the body. It is what delivers the needed nutrition to those areas that
are in need. Red clover and chlorella are wonderful natural cleansers of the blood
system. A regular exercise routine stimulates the blood system and assists the
body in eliminating waste.
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Skin – Perspiration is also an elimination route. The final product of normal
protein metabolism, urea and a number of drugs has been shown to be eliminated
by this route. Arsenic and thallium can be readily detected in hair because they
accumulate in the skin and its appendages.
Colon – The intestinal tract includes the small and large intestines. Substances
which are excreted with the feces could have entered the intestinal tract from the
liver via the bile or have excreted directly through the intestinal membrane. To be
excreted with the bile, substances must not only be of certain minimum size (MW
> than 300) but must also contain a polar group (fulfilled after the substance has
undergone Phase II conjugation reactions.
Kidney – Substances below a certain molecular size are excreted via the healthy
kidney; in man these are mostly substances with molecular weights below 300.
The kidneys produce urine which is the waste in the body. Often kidney problems
are the result of dehydration. A natural supplement that cleanses and provides
needed nutrition for the kidneys is corn silk.
Mammary glands – The elimination of xenobiotics with contaminated breast milk
can result in an internal exposure of the body. Examples are: the highly lipophilic
polychlorinated dibenzodioxins and polychlorinated dibenzofurans ( PCDD/PCDF)
are eliminated with the milk.
Depending on its polarity a compound follows different pathways of elimination.
Although a single elimination pathway can dominate, all pathways can work
silmultaneously. Source : Schober (2000)
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CHAPTER 6 : CHEMICAL FATE
6.1
Mobility and Persistence Indicators
Mobility refers to a biological or chemical contaminant’s ability to move within soil
or ground water over time. A contaminant may move under the influence of
gravity as with light or dense non-aqueous phase liquids or under the influence of
ground water flow as with dissolved constituents. As the contaminant moves
through porous medium, conditions within the medium tend to resist the mobility
of the contaminant. For example, molecules (e.g., cations and pesticides) tend to
adsorb onto particles of the porous medium in proportion to their concentration in
ground water. Adsorption also depends on the physical and chemical
characteristics of the medium such as carbon content and pH. Dissolved
contaminants also exhibit the tendency to diffuse within the solute, although
diffusivity is a minor mechanism of mobility in the case of rapid ground water flow
such as with injection wells.
Persistence is the ability of a biological or chemical contaminant to remain
unchanged in composition, chemical state, and physical state over time.
Persistence depends on chemical structure, conditions in the aquifer conducive to
degradation such as microbial population, nutrients available to support microbial
growth, and the type and quantity of ions in the background ground water. In
some cases, the degradation of one constituent occurs along with a chemical
change in another as when electrons are accepted by manganese ions in the
breakdown of petroleum hydrocarbon. In this case, the relatively immobile
manganese3+ becomes mobile manganese2+.
Persistence can be expressed in terms of half-lives, or the time it takes for a
chemical to change one-half of its mass to a different form or compound. The
half-life is an empirical, global measurement that encompasses all of the operable
degradation mechanisms. Accordingly, the half-life of a chemical is expressed as
a range of values for a particular medium.
6.2
Bioavailability, Bioaccumulation, Biomagnification
Bioavailability of the chemical refers to that portion present within the medium that
is potentially available for direct uptake by organism. If a chemical is in high
concentration in the sediment of the lake and a fish spends most of its time
residing in the water column, then the bioavailability of this chemical to the fish is
low.
Bioaccumulation/bioconcentration refers to how pollutants enter a food chain
while biomagnification refers to the tendency of pollutants to concentrate as they
move from one trophic level to the next.
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Bioavailability
Bioaccumulation
Biomagnification
2009
Bioavailability of the chemical refers to that portion
present within the medium that is potentially available for
direct uptake by organism.
increase in concentration of a pollutant from the
environment to the first organism in a food chain
increase in concentration of a pollutant from one link in a
food chain to another
We are concerned about these phenomena because together they mean that
even small concentrations of chemicals in the environment can find their way into
organisms in high enough dosages to cause problems. In order for
biomagnification to occur, the pollutant must be:




long-lived
mobile
soluble in fats
biologically active
If a pollutant is short-lived, it will be broken down before it can become
dangerous. If it is not mobile, it will stay in one place and is unlikely to be taken
up by organisms. If the pollutant is soluble in water it will be excreted by the
organism. Pollutants that dissolve in fats, however, may be retained for a long
time. It is traditional to measure the amount of pollutants in fatty tissues of
organisms such as fish. In mammals, we often test the milk produced by females,
since the milk has a lot of fat in it and because the very young are often more
susceptible to damage from toxins (poisons). If a pollutant is not active
biologically, it may biomagnify, but we really don't worry about it much, since it
probably won't cause any problems.
6.3
DDT (bioavailability, bioaccumulation, biomagnifications)
DDT
-
-
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lipophilic and environmentally persistent chemical
can be stored and accumulated/concentrated in fatty tissue
bioconcentration can result in levels of a thousand orders of magnitude
greater than what might be observed as a background level in the water
Bioconcentration factor (BCF): it is a concentration of a pollutant in the fish
divided by its concentration in the water. Calculated from the ratio of the
toxicant concentration in the whole animal ( or a particular tissue)at its steady
state, to its concentration in the environment.
If the BCF is > than 1, it indicates the accumulation is higher in the fish than in
the medium in which it resides.
Chemicals such as DDT and methylmercury are examples of chemicals with
biological half-lives long enough in the body for quantitative assessments.
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2009
DDT concentration in the fish has resulted from direct uptake from the water as
well as uptake through dietary accumulation. Ecologists refer this as
biomagnifications to desiganate the accumulation of a pollutant from trophic
transfer.
DDT had been traced to be present in mothers’ milk?
 Compare the above to ozone, is quantitative assessment possible?
Consider its half life, reactivity, and high injury-producing potential
6.4
Malathion
Malathion is an organophosphate insecticide commonly used to control mosquitos
and other flying insects, especially during outbreaks of vector-borne diseases, to
protect public health. Malathion degrades rapidly in the environment via
hydrolysis, biodegradation, photochemical degradation, and photolysis. Malathion
is toxic to aquatic organisms, but has a relatively low toxicity for birds and
mammals. Malathion can be absorbed after inhalation, oral, or dermal exposure,
but is readily excreted in the urine, and does not accumulate in organs or tissues.
The dermal absorption by humans has been reported to be about 10%. The major
metabolites of malathion are mono- and di-carboxylic acid derivatives, and
malaoxon is a minor metabolite. The principal toxicological effect of malathion is
cholinesterase inhibition, due primarily to malaoxon and to phosphorus thionate
impurities. Early signs and symptoms of malathion poisoning in humans include
pinpoint pupils, headache, nausea, dizziness, muscle weakness, drowsiness, and
anxiety. Moderate poisoning can result in chest tightness, difficulty breathing,
bradycardia, tachycardia, tremor/ataxia, blurred vision, and confusion. Severe,
life-threatening signs include coma, seizures, respiratory arrest, and paralysis.
Malathion may also be irritating to the skin and eyes.
Levels of malathion used for wide-area treatment to protect the public from
mosquito-carrying diseases are not likely to result in harmful effects in individuals
who are not directly exposed during spraying. The carcinogenicity data for
malathion is insufficient to assess human carcinogenic potential. Using effects
levels in laboratory animals exposed to malathion, EPA has proposed risk
assessment values (an acute oral RfD of 0.5 mg/kg; a chronic oral RfD of 0.02
mg/kg/day; an inhalation risk assessment value of 0.03 mg/kg/day for short,
intermediate, and long term inhalation exposure; and a dermal risk assessment
value of 0.5 mg/kg/day for short term and intermediate dermal exposure). Using
these values, EPA concluded that adult and toddler risk estimates did not exceed
the levels of EPA's concern for residential bystander exposure for malathionwide-area treatment to protect the public from mosquito-carrying diseases.
ATSDR finds this assessment to be reasonable. Nevertheless, it is recommended
that appropriate precautions be taken such as to avoid being outside when
spraying occurs and to minimize exposure during and after the spraying.
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REVIEW QUESTIONS
What properties determine pesticide persistence?
Once a pesticide has be introduced into the environment, its chemical and
physical properties determine its fate: where it goes and how long it lasts. Each
pesticide has its own unique set of properties. Pesticides that break down quickly
do not offer much opportunity for exposure. How quickly a pesticide breaks down
depends on the pesticide’s chemistry, as well as environmental factors, such as
temperature, rainfall, soil pH. Pesticides are designed to last long enough to do
their job -- control pests, then break down to non-toxic substances. However,
pesticide persistence is highly variable, from a few hours to days, months or
years. Most pesticides used today last from a few days to a few months.
What properties determine pesticide mobility?
A pesticide’s mobility depends on its water solubility, solubility in fat, adsorption to
soil, and its tendency to become a vapor. A pesticide that is adsorbed to or taken
up into a plant is less likely to become a vapor, be washed off onto the soil, or be
transferred to the skin if the plant is touched. Pesticides that strongly adsorb to
soil are not very mobile in water that infiltrates toward groundwater, or water that
runs off into surface water, such as a pond, lake or stream. Pesticides strongly
adsorbed to soil may still enter surface water if there is soil erosion. Pesticides
strongly adsorbed to soil do not volatilize easily.
PRACTICE PROBLEMS
What is the difference between biomagnification and bioaccumulation?
Both describe the increase in concentration of a particular element or compound in
biological organisms.
The difference is:
- bioconcentration and bioaccumulation occur within a single organism
- biomagnification occurs across trophic (food chain) levels
What are Endocrine Disrupting Chemicals (EDCs) and why are they dangerous?
Endocrine disrupting chemicals (EDCs) are chemicals that interfere with natural hormone
systems, which control the development of an individual from conception to birth, and the
later functioning of that individual.
Some man-made EDCs are persistent in the environment and bioaccumulate - that is,
they accumulate in the fatty tissue of organisms and increase in concentration as they
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move up through the food chain. They are now ubiquitous contaminants found
throughout the world.
Scientific investigations have provided steadily growing evidence linking synthetic
endocrine-disrupting compounds to impaired health in wildlife and humans. For example,
EDCs can alter sexual development, impair reproduction and undermine the immune and
thyroid systems. They have been linked to decreased intelligence, changes in behavior,
reproductive problems, species declines, reduced resistance to disease and, at high
exposure, birth defects.
Due to their persistence and mobility, they can accumulate in and harm species far from
their original source. For example:
Whales in the world's oceans carry PCBs and other contaminants at concentrations
which cause development defects in humans.
Marine gastropods (whelks and periwinkle) suffer sex determination defects due to
tributyltin leaching from antifouling paints on ships' hulls. Bivalves such as mussels suffer
from malformations and growth anomalies. The effects on female dog whelk are striking,
as they become masculinised and grow penises.
Bald eagles in contaminated industrialized regions continue to have difficulty
reproducing.
Albatrosses nesting on remote Midway Island in the Northern Pacific are carrying levels
of PCBs, DDT, dioxins, and furans that have been shown to be hazardous to bird species
in the industrialized Great Lakes region of the United States and Canada.
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CHAPTER 7: ENVIRONMENTAL TOXICANTS
7.1
Introduction to Environmental Toxicants
Environmental toxicants are agents in our surroundings that are harmful to
human health. Some substances, such as air and water pollutants, are
recognized for their toxicity. No one would knowingly breath air polluted with
sulfuric acid or drink water containing pesticides. However, other toxic agents
equally harmful to human health (food additives and contaminants,
bacteriotoxins, fungitoxins, phytotoxins, household product and industrial
chemicals) often go unrecognized for their serious toxicity.
7.2
Pesticides
The EPA defines pesticide as any substance or mixture of substances intended to
prevent, destroy, repel, or mitigate any pest. Pesticides may also be described as
any physical, chemical, or biological agent that will kill an undesirable plant or
animal pest List pesticides they are familiar with either through personal use or in
relation to hazardous chemicals in their community.
They are classified as to the organism they destroy, as the terms fungicides
(fungi), herbicides (plant), insecticides (insect) and rodenticides (rodent imply.)
Insecticides : Most insecticides are neurotoxicants that disrupt the transmission
of a nerve impulse either as it passes along the axon or the synapse. Insects
exposed to neurotoxicants respond with twitching, weakness and paralysis which
leads to death. Similar symptoms are also seen in human
Organophosphates : Parathion, and malathion
Carbamates : aldicarb, carbaryl
Organochlorines : Dichlorodiphenylltrichoroethane (DDT)
Herbicides : Herbicides act to eliminate unwanted plants (e.g weeds) by
interfering with hormonal system that regulate growth or by promoting water loss.
Although effective in eliminating plants, most herbicides are weakly toxic to
humans, most likely due to inherent differences in plant and animal cell structure
(cell membrane) and function (biochemical pathway).
Bipyridyls : diquat, paraquat
Chlorophenoxy compounds: 2,4 Dichlorophenoxyacetic (2,4-D)
Dinitrophenol : 2,4-dinitrophenol (DNP)
Fungicides : Fungi, the intended victims of fungicides, are themselves
responsible for producing some of the deadliest toxins (fungitoxins).
Examples Hexachlorobenzene, Organomercurials, Phthalimides,
Dithiocarbamates.
Rodenticides :Numerous vertebrate organisms (such as bats, coyotes, rabbits,
skunks and wolves) have at one time or another been considered pests.
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Rodenticides ( agent lethal to rodent s)are of particular importance to human
health since rodent often serve as vectors for the transmission of disease.
Examples of rodenticides include coagulants, inhibitor of cellular respiration,
vasoconstrictors and diabetogenics.
7.3
Plastics
Polymers that can be shaped by pressure or heat to the form of a cavity or mold
are termed plastics. Two forms of plastics are generally recognized :
thermoplastic and thermosetting plastics. Thermoplastics, which comprise over
80% of all plastics, can be remelted and remolded (e.g polyethyelene,
polypropylene, polyvinylchloride, polystyrene) and as such are of interest to
numerous recycling efforts. Plastics that once molded cannot be remelted and
remolded are termed thermosetting plastics.
Concern about plastic as environmental toxicants is twofold. Firstly many plastics
are nondegradable. They resist biological degradation (nonbiodegradable) and
degradation from ultraviolet radiation (nonphotedegradable). The posibility is that
plastics could persists in landfills or as roadside pollutant for hundred of year.
Secondly attempts to incinerate some plastics ( e.g PVC), to reduce their
contribution to landfills, result in the production of toxic chemicals. Once such
toxic chemicals, dioxane are demonstrated carcinogens, most probably involving
an epigenetic mechanism.
7.4
Heavy Metals
Metals differ from other toxic substances in that they are neither created nor
destroyed by humans. Their use by humans plays an important role in
determining their potential for health effects. Their effect on health could occur
through at least two mechanisms: first, by increasing the presence of heavy
metals in air, water, soil, and food, and second, by changing the structure of the
chemical. For example, chromium III can be converted to or from chromium VI,
the more toxic form of the metal.
Virtually all metals can produce toxicity when ingested in sufficient quantities, but
there are several which are especially important because either they are so
pervasive, or produce toxicity at such low concentrations. When speaking of
heavy metals we generally mean, lead, mercury, iron, copper, manganese,
cadmium, arsenic, nickel, aluminum, silver, and beryllium.
In general heavy metals produce their toxicity by forming complexes or "ligands"
with organic compounds. These modified biological molecules lose their ability to
function properly, and result in malfunction or death of the affected cells. The
most common groups involved in ligand formation are oxygen, sulfur, and
nitrogen. When metals bind to these groups they may inactive important enzyme
systems, or affect protein structure.
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7.5
2009
Organic solvents and Vapors
Nearly everyone is exposed to solvents. Occupational exposures can range from
the use of “white-out” by administrative personnel, to the use of chemicals by
technicians in a nail salon. When a solvent evaporates, the vapors may also pose
a threat to the exposed population.
7.6
Other Environmental Toxicants
Radiation and Radioactive Materials
Radiation is the release and propagation of energy in space or through a material
medium in the form of waves, the transfer of heat or light by waves of energy, or
the stream of particles from a nuclear reactor
An example for discussion purposes would be the dropping of the atom ic bom b
during World War II, or the Chernobyl Accident in Russia.
Dioxin/Furans
Dioxin, (or TCDD) was originally discovered as a contaminant in the herbicide
Agent Orange. Dioxin is also a by-product of chlorine processing in paper
producing industries.
Plant Toxins
Different portions of a plant may contain different concentrations of chemicals.
Some chemicals made by plants can be lethal. For example, taxon, used in
chemotherapy to kill cancer cells, is produced by a species of the yew plant.
Animal Toxins
These toxins can result from venomous or poisonous animal releases. Venomous
animals are usually defined as those that are capable of producing a poison in a
highly developed gland or group of cells, and can deliver that toxin through biting
or stinging. Poisonous animals are generally regarded as those whose tissues,
either in part or in their whole, are toxic.
Examples of venomous animals, such as snakes, spiders, etc., and poisonous
anim als, such as puffer fish, or oysters, which m ay be toxic to some individuals
when contam inated with Vibrio vulnificus.
Subcategories of Toxic Substance Classifications
All of these substances may also be further classified according to their:
 Effect on target organs (liver, kidney, hematopoietic system),
 Use (pesticide, solvent, food additive),
 Source of the agent (animal and plant toxins),
 Effects (cancer mutation, liver injury),
 Physical state (gas, dust, liquid),
 Labeling requirements (explosive, flammable, oxidizer),
 Chemistry (aromatic amine, halogenated hydrocarbon), or
 Poisoning potential (extremely toxic, very toxic, slightly toxic)
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General Classifications of Interest to Communities
 Air pollutants
 Occupation-related
 Acute and chronic poisons
All chemicals (or any chemical) may be poisonous at a given dose and through a
particular route. For example, breathing too much pure oxygen, drinking
excessive amounts of water or eating too much salt can cause poisoning or death
REVIEW QUESTIONS
Is our food safe?
What are the advantages of pesticide use?
What are the disadvantages of pesticide use?
Give an example of how a broad-spectrum, persistent insecticide can distrupt the
entire ecosystem.
5. Describe alternative ways to control insect populations other than use of synthetic
insecticides
6. Relate how toxicant accumulation in the food chain led to the minamata disasters.
Why is the conversion to metthylmercury the key step in mercury toxicity ?
1.
2.
3.
4.
PRACTICE PROBLEMS
1. Which of the following is NOT a normal function/role of metal in the body?
A. iron in the heme of hemoglobin
B. calcium in bone
C. cobalt in Vitamin B12
D. phosphorus in ATP
E. arsenic in ATP
2. What is one primary function of the tubules in the kidney?
A. filtering water and solutes out of the blood
B. reabsorption of water and solutes
C. producing bile
D. transporting filtered blood to the lungs
3. Which of the following is NOT characteristic of metals?
A. Metals are often charged ions.
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B. Metals can be destroyed or degraded in the body.
C. Metals easily bond to other molecules.
D. Metals can have various oxidation states.
4. Who is LEAST likely to be exposed to toxic metals?
A. a technician working in a computer component board assembly line
B. a person who drinks water from a ground water well
C. a person smoking a cigarette
D. a person working on a new home computer
E. a painter renovating an 70 year old home
5. One of the ways that lead makes people sick is by interfering with the protein that
helps make hemoglobin. The result is an anemic like condition where your blood can't
carry enough oxygen to keep you healthy. This is an example of what mechanism of
metal toxicity?
A.
B.
C.
D.
Enzyme inhibition
carcinogenicity
Disruption of cellular organelles
Corrosion
6. Why is the kidney often a target for toxic chemicals such as metals?
A. because toxicants enter the body through the stomach and intestines and are then
transported to the kidney
B. because the kidney has a very large surface area in direct contact with the blood
C. because there is very high blood flow to the kidney and it can concentrate substances
D. because the kidney cannot regenerate damaged cells
7. Amino acids are filtered out of the blood in the glomerulus of the kidney, but amino
acids are not a waste product. The body needs the amino acids, so they are
reabsorbed from the filtrate back into the tubule cells of the kidney. After the amino
acids are concentrated in the tubule cells they must be transported back into blood.
What type of transport is used to move the amino acids from the filtrate to the inside of
the kidney cells?
A. passive diffusion
B. protein channels
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C. active transport
8. Itai-itai byo is a disease found among Japanese women. Scientists believe that it is
caused by eating rice grown in soil containing _____________.
A. lead
B. mercury
C. cadmium
D. chromium
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CHAPTER 8 : RISK ASSESSMENT
8.1
Introduction to Risk
Risk is defined as the possibility of loss injury. The term environmental risk to health is
the probability of an adverse effect on human health resulting from exposure to a
particular environmental agent or combination of agents (toxicants).
Risk may be expressed as a probability (e.g., P= 0.00001) or incidence (e.g., 1 in
100,000) of a particular response for a given exposure. Risk (probability or incidence) is
based on statistical estimates from sample population studied during toxicity testing and
on other observation, such as those from epidemiological studies.
8.2
Risk Assessment
Assessment of environmental risk to human health involves a sequence of interrelated
steps ( Omenn and Faustman, 1997), beginning with the identification of the causative
agent (toxicant) or exposure situation, and culminating in an evaluation of the number of
persons who are ultimately affected and the severity of their effects.
Four steps characterize the process:
i)
toxicant identification
ii) toxicant evaluation
iii) exposure evaluation
iv) risk estimation
Steps
Description
Toxicant identification
A review of existing literature, which may include toxicity tests and
epidemiology, may be used to identify a toxicant. In the absence
of the relevant literature, descriptive toxicity testing must be done.
Dose response conclusions of descriptive toxicology are sufficient
for toxicant identification. It is not necessary to understand the
mode of action of a toxicant (mechanistic toxicology)
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Toxicant Evaluation
Exposure Evaluation
Risk Estimation
8.3
2009
Toxicity test are carried out in nonhuman species, the applicability
of those test to human must be evaluated. An awareness of
interspecific (between species) and intraspecific (within species)
variability in toxicity testing is necessary when evaluating
nonhuman test.
Careful attention to variability in age, sex, diet circadian rhythms,
hormonal status and biotransformation capacity is required.
With good experimental design, exposure to a suspected toxicant
is precisely regulated during toxicity testing. Descriptive
toxicologist predetermine the exposure parameter including the ;
i. organism exposed
ii. route of entry
iii. dose
iv. frequency (how often)
v. duration of dosing
Exposure assessment needs to be pursued with the same
diligence as toxicant identification and evaluation. The
assessment should examine exposures currently experienced as
well as those anticipated under different conditions
Exposure for noncarcinogenic toxicants is expressed as maximum
daily dose (MDD)(mg/kg/day), whereas exposure to carcinogens
is stated
as lifetime average
daily dose (LADD)
(mg/kg/day/lifetime)
Risk is the probability of an undesirable biological response
resulting from exposure to a toxicant. Estimating risk requires an
integration of the toxicity conclusions (toxicant identification and
dose response evaluation) and exposure assessment (MDD or
LADD)Risk is approximate by equation
( Risk = Toxicity x Exposure)
Since dose response relationship is not linear, exposure is more
accurately expressed as a function (f) as seen in the following
equation:
R = T x f (E)
Risk Management
Risk is often presented as a declarative statement, devoid of interpretation. To be of value
of humanity, risk once characterized must be implemented into regulatory policies that
benefit society. The intent of risk management is to examine risk assessment data and
where needed, develop regulatory options that address public health and social and
economic concerns. Federal agencies charged with overseeing risk management often
must overcome the influence of negative public perceptions and legislative mandates to
arrive responsible decisions.
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8.4
2009
Summary
Research
Risk assessment
Laboratory and field
observations
Information on
extrapolation methods
Exposure assessment
Emissions
characterization
Risk assessment relies on evaluation techniques
75
Development of
regulatory options
Toxicity
assessment: Hazard
identification and
dose response
assessment
Research needs identified from
risk assessment process
Field measuments,
characterization of
populations
Risk management
Risk
characte
rization
Evaluation of public
health, economic, social,
political consequences of
regulatory options
Agency decisions and
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REVIEW QUESTIONS
1. Which of the following is NOT a step in the Risk Assessment Process?
a. Hazard identification
b. Hazard evaluation or dose-response assessment
c. Exposure dose
d. Risk characterization
2. Epidemiology is the study of causative factors associated with the occurrence and number of
cases of disease and illness in a specific population.
a. True b. False
3. Exposure tells the toxicologist what dose causes a “response” usually illness or death, in the
test animal.
a. True b. False
4. What activities should be conducted during the hazard identification step of the risk
assessment?
a. Identifying the substance name
b. Describing the physical/chemical properties of the toxic substances
c. Identifying the sources of toxicity information
d. Identifying the exposure pathway
e. All of the above
5. Prospective epidemiological studies gather information from the past.
a. True b. False
6. The exposure assessment step in the risk assessment process identifies all EXCEPT
which of the following?
a. Frequency of exposure
b. Type of chemical exposure
c. Length of time of exposure
d. Route of exposure
e. The amount of exposure
7. Susceptible populations that may be more at risk for illness than others includes the
following EXCEPT:
a. Young children
b. Older adults
c. Teenagers
d. Women of Childbearing Age
IV. Activity Lab
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Divide the participants into small groups. Have them perform a mock risk assessment on an
environmental issue or contaminant of concern to them. To keep the process simple, inform the
participants that they are to perform this activity in the manner described below.
The four steps in the Risk Assessment process are:
1) Hazard identification
2) Dose response evaluation
3) Exposure assessment
4) Risk characterization
The Risk Assessment asks five basic questions, which the participants must answer:
1) What is the hazard?
2) Are the people really exposed to the hazard?
3) If so, how long and how long will it take to determine the amount of exposure?
4) Is there evidence to prove that exposure occurred or is occurring?
5) Given the information collected, is there a risk of adverse health effects from an
exposure?
PRACTICE PROBLEMS
Use the information below to assess a toxicant of choice in the environment.
Types of Information Collected and Considered When Performing the Risk Assessment
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A. Hazard Identification
Collection of Data
a. Name of Substance
b. Physical/Chemical Properties
c. Source of Information
d. Exposure to Toxic Substances
- Route of exposure
STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
COURSEWORK AND FINAL EXAMINATION
MODULE LEADER
78
:
Rodziah Bt. Ismail
2009
STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
Room:
:
2009
Block A, Fakulti Sains Kesihatan, Jalan Othman
Petaling Jaya.
Contact No
:
0192812011
e-mail address
:
rodzia02@salam.uitm.edu.my,
rodzia_hi@yahoo.com
Module leader is responsible for the individual timetabling and content of the module. You will be
providing with the necessary background information for the module.
A.
Group of 4 -(15%)
Any topics on environmental issues affecting human health such as:












Chemical plant explosion In Bhopal, India on December, 1986
Nuclear reactor meltdown at Chenobyl, April 1986
Thalidomide babies
Itai-itai disease
Environmental Tobacco Smoke
Asthma and urban air pollution
Blue Baby Syndrome
DDT
Food additives
HINI issues
Love canal
Malamine incident
Other topics accidental or non-accidental release of toxicants to the environment
affecting human health is acceptable.
Length of paper

8-12 pages double spaced, excluding the reference page.
References
B.

You must cite references

At least 4 real references

At least 2 non-web references
You are requested to write an essay of 1000 to 1,300 words on anyone of Environmental
toxicology subdisciplines mentioned in section 1.4.
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
Assessment
Assessment
Submission Date
Contribution towards final
Individual Write-Up
10 %
Class Test – (15%)
C.
Your class test will be held in two sessions and consist of TWO Parts. Part 1 class test
will be held somewhere in August after before your Semester break. Part 2 will be held in
September. You have to answer mainly long and short question based on the module
content. Part 1 and 2 will be worth 7.5% each.
Final Examination Assessment – (60%)
D.
Your understanding of the material contained in this module will be assessed in a 3 hour
examination. You have to answer mainly long and short question and some essay-type
questions based on the module content. There are previous year exam papers for this
module and you are encouraged to go to the Library to get them.
Please note that in the time available, seminars can only present a summary
framework of the topics in this module. Typically, you should spend an hour or two
of private study for every two hour of formal seminar contact. If you are given
references referring to material covered in seminar, make sure you read them; it is
unlikely that you will attain more than a 2.0 CGPA in the exam if you just regurgitate
lecture notes. Do not just photocopy material “to read later” (i.e the night before the
exam), you will gain most enjoyment and satisfaction from this module if you
consistently set aside time for private study of material contained in lectures as the
module progresses.
RESPONSIBILITIES AND MY EXPECTATION
You are expected to spend a minimum of 42 hours working on this module. The time includes the
scheduled lectures, private study, group work, report writing etc, and thus you should put in at
least 3 hours of private study time each week. You are encouraged to attend all scheduled
seminars given to you and attendance will be taken.
PLAGARISM
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
It is important to remember that it is a university-wide assignment offence to copy material
directly from any reference source (books, website or journals) without correct citation.
Failure to do this may result the student obtaining zero for that particular assignment.
WHAT’S NEXT
During your study in this Program, time management is important. While your workload will be
heavier than the final year, it becomes increasingly important to produce high quality
assignments. The attendance to lectures, practical, etc is absolutely paramount in order to:
1.
Provide you with the necessary material and training which will be expected from you by
future employers.
2.
Obviously pass your examinations.
3.
Provide additional material which may be used in written assignments.
One essential point to remember is that as you have enrolled as Environmental Health and
Safety students, honors level, this means that studying outside the seminars, etc is crucial
in order to pass. Reading relevant material in addition to the one listed in handouts or
lectures is important if you wish to obtain a good honors degree.
References
1. Environmental Toxicology. W.H.C. Basetti and M.H. Yu. CRC Press (2004)
2. Environmental Toxicology: Biological and Health Effects of Pollutants. Yu Ming-Ho. CRC
Press (2004)
3. Occupational. Environmental and Industrial Toxicology. M.I. Greenberg, R.J. Hamilton, J.
Richards and S.D. Phillips. C.V. Mosbt (2003)
4. Principles of Ecotoxicology (2 nd ed). Walker C.H., S.P. Hopkin, R.M., Sibly and D.B. Peakall
(2002)
5. Environmental Toxicology. S.F. Zakrzewski. Oxford University Press (2002)
6. Principles of Toxicology (2nd ed). P.L. Williams, R.C James and S.M. Roberts. John Wiley and
Sons, Ltd (2000)
7. Toxicology and Risk Assessment: A Comprehensive Introduction. Helmut Greim and Robert
Snyder . John Wiley and Sons, Ltd (2008)
8. http://www.atsdr.cdc.gov/training/toxmanual/modules/2/lecturenotes.html
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
CONTINOUS ASSESSMENT (SAMPLE QUESTIONS)
1. Which statement is the most correct?
A. Chemicals manufactured by humans are more dangerous to human health than naturally
occurring chemicals.
B. Both natural and human-made chemicals are potentially toxic to humans.
C. Naturally occurring chemicals are more poisonous to humans than synthetic chemicals.
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
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2. One of the items below is a hazardous substance. Four are sources of a hazardous
substances. Which one is a hazardous substance?
A.
B.
C.
D.
E.
clogged furnace
cigarette
a dog
paint applied before 1978
dust mite parts
3. Which of the following is NOT a possible route of entry for a hazard?
A.
B.
C.
D.
ingestion
absorption
exposure
inhalation
4. When DDT, a pesticide, enters the human body, it is ______________________
A.
B.
C.
D.
water soluble and is easily excreted in urine.
stored in the bones.
not toxic, but is processed by enzymes and becomes a different compound which is toxic.
fat soluble and can be stored in fat tissue.
5. Who took the largest dosage of aspirin?
A.
B.
C.
D.
an adult woman who weighs 125 lb and took 300 mg of aspirin
a teenage boy who weighs 135 lb and took 600 mg of aspirin
a baby who weighs 20 lb and took 100 mg of aspirin
a chihuahua who weighs 5 lb and took 50 mg of aspirin
6. Which will NOT help you determine the dose of a hazardous gas received by a person?
A.
B.
C.
D.
E.
F.
their respiration rate
their length of exposure to the gas
the source of the gas
their frequency of exposure to the gas
the concentration of the gas
the gas's chemical and biological properties
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
7. Most hazardous substances exhibit a "dose-response relationship." What does this mean?
A. The harm caused by the hazard increases as the amount of hazard entering the body (dose)
increases.
B. It does not matter how big a dose you receive, you will always have same amount of
harm/sickness.
C. Exposure to the hazard always results in harm.
D. Fifty percent of the people will die when exposed to 0.1 mg/kg.
8. A family home has a clogged furnace that is producing carbon monoxide, a hazardous gas.
Which family member is likely to be harmed the most?
A.
B.
C.
D.
E.
Atan, the son who is in 1st grade
Ani, who is going to be in preschool next year
Kalsom, the nanny who cares for the toddler every weekday morning
Fatima, the mother who works at home.
Omar, the father who works at the University
9. All of the people listed below live in the same house. Who is most likely to experience toxic
effects from the second-hand smoke?
A.
B.
C.
D.
E.
the grandmother, who is very fit
the mother, who smokes
the father, who smokes
the teenage daughter, who has asthma
the son, who is in 5th grade
10. There are several ways to control or reduce your exposure to a hazard. Opening a window in
a room full of people who are smoking is an example of controlling your exposure to
environmental tobacco smoke by __________________.
A.
B.
C.
D.
treating the symptoms of the hazard
diluting the hazard
distancing yourself from the hazard
removing the hazard
11. Which environmental health scientist would determine ways to prevent and reduce exposure
to second hand smoke?
A.
B.
C.
D.
E.
a toxicologist
an epidemiologist
an industrial hygienist
an occupational and environmental medicine physician
a pharmacologist
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
FINAL EXAM (SAMPLE 1)
PART A
(60 marks)
Answer ALL questions.
1. a) Explain the basis for studying environmental toxicology.
b) Contrast between acute and chronic injuries.
(4 marks)
(6 marks)
2. Define the following terms:
a) endocrine disrupters.
(5 marks)
b) teratogenesis.
(5 marks)
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
3. a) Draw a generalized dose response curve.
b) State the factors that influence the severity of a dose.
2009
(5 marks)
(5 marks)
4. a) State the significance of biotransformation in the body’s response to environmental
chemicals.
(5 marks)
b) Explain how cytochrome P450s may be related to cancer.
5. a) Define exposure.
b) Classify environmental carcinogens.
6. a) Summarize the effect of UV radiation on DNA.
b) Explain the meaning of imposex.
PART B
(5 marks)
(4 marks)
(6 marks)
(5 marks)
(5 marks)
( 60 marks)
Answer FIVE (5) questions only.
QUESTION 1
Describe different portals of entry of a toxicant.
(12 marks)
QUESTION 2
Identify the factors that influence the severity of a toxin once it has been absorbed into
the human body.
(12 marks)
QUESTION 3
Explain renal elimination of toxicants from the human body.
(12 marks)
QUESTION 4
Contrast the toxicity of organochlorine pesticides and pyrethroids.
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
(12 marks)
QUESTION 5
Describe the biotic degradation process of a toxicant.
(12 marks)
QUESTION 6
Outline Phase I and Phase II reactions as detoxication mechanisms.
(12 marks)
FINAL EXAM (SAMPLE 2)
PART A
(60 marks)
Answer ALL questions.
QUESTION 1
Define the following terms:
a) Necrosis.
(5 marks)
b) Apoptosis.
(5 marks)
QUESTION 2
a) Describe the factors that affect bioaccumulation in organisms.
(5 marks)
b)
What are the common storage sites of toxicants in the human body?
(5 marks)
QUESTION 3
a) Explain the processes involved in Phase II reactions.
(5 marks)
b) What is meant by reactive metabolites?
(3 marks)
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
2009
c) List FOUR sites where biotransformation occurs.
(2 marks)
QUESTION 4
a) Discuss toxicity preventive measures.
(5 marks)
b) Explain what the following
diethylstilbestrol and accutane.
compounds
have
in
common:
thalidomide,
(5 marks)
QUESTION 5
a) Explain renal elimination of toxicants from the body.
(5 marks)
b) Describe the characteristics of these toxicants.
(5 marks)
QUESTION 6
a) Summarize the common toxic mechanisms of metals in the human body.
(5 marks)
b) Describe the condition of delayed neurotoxicity.
(5 marks)
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STUDY GUIDE for ENVIRONMENTAL TOXICOLOGY
PART B
2009
(60 marks)
Answer FIVE questions only.
QUESTION 1
Toxicokinetics is the five time-dependent processes related to toxicants as they interact
with living organisms. Explain briefly these five processes.
(12 marks)
QUESTION 2
Describe the anatomical structures associated with the route of absorption in either ONE
of the following: percutaneous, respiratory system, or digestive system.
(12 marks)
QUESTION 3
Describe the abiotic degradation process of a toxicant.
(12 marks)
QUESTION 4
When a toxicant is released, transport processes will determine its spatial and temporal
distribution. Describe the physical transport of a toxicant.
(12 marks)
QUESTION 5
Outline a testing protocol for a multigenerational reproductive study.
(12 marks)
QUESTION 6
Discuss the approaches that can be used when we estimate a risk.
(12 marks)
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2009