CLINICAL BIOCHEMISTRY
INTRODUCTION
Clinical biochemistry is also known as clinical chemistry,
chemical pathology, medical biochemistry or pure blood
chemistry.
Clinical biochemistry is the area of pathology that is
generally concerned with analysis of body fluids.
Biochemical tests including:
 Tests for various components of blood and urine.
 Measurement of the activities of different enzymes.
 Spectrometric assay.
 Electrophoresis.
 Immunoassay.
 Endocrinology.
Most current laboratories are now highly automated and
use assays that are closely monitored and quality
controlled.
Serum is the yellow watery part of blood that is left after
blood has been allowed to clot and all blood cells have
been removed.
This is most easily done by centrifugation which packs the
denser blood cells and platelets to the bottom of the
centrifuge tube, leaving the liquid serum fraction resting
above the packed cells.
Plasma is essentially the same as serum, but is obtained
by
centrifuging
the
blood
without clotting. Plasma
therefore contains all of the clotting factors, including
fibrinogen.
ENDOCRINOLOGY
Endocrinology is a branch of medicine dealing with
disorder
of
the
endocrine
system
and
its
specific
secretions (hormones).
Endocrinology is concerned with the study of the:
 Biosynthesis of hormones
 Storage of hormones
 Chemistry of hormones
 Physiological function of hormones
 Cells of the endocrine glands
 Tissues that secrete hormones
The endocrine system consists of several glands, in
different parts of the body that secrete hormones directly
into the blood rather than into a duct system.
2
Hormones have many different functions and modes of
action.
One hormone may have several effects on different target
organs.
One target organ may be affected by more than one
hormone.
Chemical that classified as a hormone should be:
 Produced by an organ.
 Released in small amounts into the blood.
 Transported by the blood to a distant organ to exert
its specific function.
A hormone is a chemical released by one or more cells that
affects cells in other parts of the organism. Only a small
amount of hormone is required to alter cell metabolism.
Three mechanisms of chemical signaling of hormone can
be distinguished:
1. Autocrine signaling
Chemical signal acts on the same cell. The cell signals
itself through a chemical that it synthesizes and then
responds to.
This effect can occur within the cytoplasm of the cell or by
an interacting with receptors on the surface of the same
cell.
3
2. Paracrine signaling
Chemical signals that diffuse into the area and interact
with receptors on nearby cells. Paracrine effect is a
chemical communication between cells within a tissue or
organ, e.g. the release of neurotransmitters at synapses in
the nervous system.
3. Endocrine signaling
The chemicals are secreted into the blood and carried to
the cells they act upon.
A neuroendocrine signal is a "classical" hormone that is
released into the blood by a neurosecretory neuron.
Hormones act by binding to specific receptors in the target
organ.
Hormone
is
essentially
a
chemical
messenger
that
transports a signal from one cell to another.
Endocrine hormone molecules are secreted (released)
directly into the bloodstream.
Exocrine hormone molecules are secreted directly into a
duct, and from the duct they either flow into the
bloodstream or they flow from cell to cell.
Endocrinology as a profession (job)
Although every organ system (including the brain, lungs,
heart, intestine, skin, and the kidney)
secretes and
4
responds
to
hormones,
the
clinical
specialty
of
endocrinology focuses primarily on the endocrine organs
(the organs whose primary function is hormone secretion).
These endocrine organs include the:
 pituitary gland
 thyroid gland
 adrenal gland
 ovaries
 testes
 pancreas
An endocrinologist is a doctor who specializes in treating
disorders of the endocrine system, such as diabetes,
hyperthyroidism, and many others.
The medical specialty of endocrinology involves the
diagnostic evaluation of a wide variety of symptoms and
variations and the long-term management of disorders of
deficiency or excess of one or more hormones.
The diagnosis and treatment of endocrine diseases are
guided by laboratory tests to a greater extent than for
most specialties.
Many
diseases
are
investigated
through
excitation/stimulation or inhibition/suppression Testing.
5
This might involve injection with a stimulating agent to
test the function of an endocrine organ. Blood is then
sampled to assess the changes of the relevant hormones
or metabolites.
Most endocrine disorders are chronic diseases that need
life-long care.
Some of the most common endocrine diseases include:
 diabetes mellitus
 hypothyroidism
 The metabolic syndrome.
6
Hormone
A hormone is a chemical released by one or more cells that
affects cells in other parts of the organism. Only a small
amount of hormone is required to alter cell metabolism.
Hormone
is
essentially
a
chemical
messenger
that
transports a signal from one cell to another.
All multi-cellular organisms produce hormones; plant
hormones are also called phyto hormones.
Hormones in animals are often transported in the blood.
Cells respond to a hormone when they express a specific
receptor for that hormone.
The hormone binds to the receptor protein, resulting in the
activation of a signal transduction mechanism that
ultimately leads to cell type-specific responses.
Endocrine hormone molecules are secreted (released)
directly into the bloodstream.
Exocrine
hormone
(or
ecto-hormones)
are
secreted
directly into a duct, and from the duct they either flow into
the bloodstream or they flow from cell to cell by diffusion
in a process known as paracrine signalling.
7
Chemical classes of hormones
There are three different classes of hormone based on
their chemical composition:
1. Amines
Amines,
such
as
nor-epinephrine,
epinephrine,
and
dopamine, are derived from single amino acids, in this
case tyrosine.
Thyroid hormones such as 3,5,3’-tri-iodothyronine (T3)
and 3,5,3’,5’-tetra-iodothyronine (thyroxine, T4) make up
a subset of this class because they derive from the
combination
of
two
iodinated
tyrosine
amino
acid
residues.
8
2. Peptide and protein
Peptide hormones and protein hormones consist of three
(in the case of thyrotropin-releasing hormone) to more
than 200 (in the case of follicle-stimulating hormone)
amino acid residues and can have molecular weights as
large as 30,000.
All hormones secreted by the pituitary gland are peptide
hormones, as are:
 Leptin from adipocytes
 Ghrelin from the stomach
 insulin from the pancreas
3. Steroid
Steroid
hormones
are
converted
from
their
parent
compound, cholesterol.
Vitamin D3 (steroid hormone)
9
Mammalian steroid hormones can be grouped into five
groups by the receptors to which they bind:
1. Gluco corticoids
2. Mineralo corticoids
3. Androgens
4. Estrogens
5. Progestagens
Hormones as a signal
Hormonal
signaling
across
this
hierarchy
(chain
of
commands) involves the following:
1. Biosynthesis of a particular hormone in a particular
tissue
2. Storage and secretion of the hormone
3. Transport of the hormone to the target cell(s)
4. Recognition of the hormone by an associated cell
membrane or intracellular receptor protein.
5. Relay (send) and amplification of the received
hormonal signal via a signal transduction process:
This then leads to a cellular response. The reaction of
the target cells may then be recognized by the
original hormone-producing cells, leading to a downregulation in hormone production. This is an example
of a homeostatic negative feedback loop.
6. Degradation of the hormone.
10
As can be inferred from the hierarchical diagram, hormone
biosynthetic cells are typically of a specialized cell type,
residing within a particular endocrine gland, such as
thyroid gland, ovaries, and testes.
Hormones exit their cell of origin via exocytosis or another
means of membrane transport. The hierarchical model is
an over simplification of the hormonal signaling process.
Cellular recipients of a particular hormonal signal may be
one of several cell types that reside within a number of
different tissues, as is the case for insulin, which triggers a
diverse (varied) range of systemic physiological affects.
Different tissue types may also respond differently to the
same hormonal signal. Because of this, hormonal signaling
is elaborate (complicated) and hard to dissect (divided).
11
Interactions with receptors
Most hormones initiate a cellular response by initially
combining with either a specific intracellular or cell
membrane associated receptor protein.
A cell may have several different receptors that recognize
the
same
transduction
hormone
and
pathways,
activate
or
different
alternatively
signal
different
hormones and their receptors may invoke (refer to) the
same biochemical pathway.
For many hormones, including most protein hormones, the
receptor is membrane associated and embedded in the
plasma membrane at the surface of the cell.
The interaction of hormone and receptor typically triggers
a cascade (flow) of secondary effects within the cytoplasm
of the cell, often involving phosphorylation or dephosphorylation of various other cytoplasmic proteins,
changes
in
ion
channel
permeability,
or
increased
concentrations of intracellular molecules that may act as
secondary messengers (e.g. cyclic AMP).
Some protein hormones also interact with intracellular
receptors located in the cytoplasm or nucleus by an
intracrine mechanism.
12
For hormones such as steroid or thyroid hormones, their
receptors are located intracellularly within the cytoplasm
of their target cell.
In order to bind their receptors these hormones must:
 Cross the cell membrane.
 Combined with receptor.
 The combined hormone-receptor complex moves
across the nuclear membrane into the nucleus of the
cell.
 Hormone binds to specific DNA sequences.
 Hormone is effectively amplifying or suppressing the
action of certain genes.
 Hormones affecting protein synthesis.
However, it has been shown that not all steroid receptors
are located intracellularly, some are plasma membrane
associated.
An important consideration, dictating the level at which
cellular signal transduction pathways are activated in
response
to
a
hormonal
signal
is
the
effective
concentration of hormone-receptor complexes that are
formed.
13
Hormone-receptor complex concentrations are effectively
determined by three factors:
1. The number of hormone molecules available for
complex formation
2. The number of receptor molecules available for
complex formation and
3. The binding affinity between hormone and receptor.
The number of hormone molecules available for complex
formation is usually the key factor in determining the level
at which signal transduction pathways are activated.
The
number
of
hormone
molecules
available
being
determined by the concentration of circulating hormone,
which is in turn influenced by the level and rate at which
they are secreted by biosynthetic cells.
The number of receptors at the cell surface of the
receiving cell can also be varied as can the affinity
between the hormone and its receptor.
14
Physiology of hormones
Most cells are capable of producing one or more
molecules, which act as signaling molecules to other cells,
altering their growth, function, or metabolism.
The classical hormones produced by cells in the endocrine
glands are cellular products, specialized to serve as
regulators at the overall organism level. They may also
exert their effects solely (exclusively) within the tissue in
which they are produced and originally released.
We explain before the negative feedback mechanism as
the following: Relayed and amplified hormone leads to a
cellular response. The reaction of the target cells may then
be recognized by the original hormone-producing cells,
leading to a down-regulation in hormone production.
The rate of hormone biosynthesis and secretion is often
regulated by a homeostatic negative feedback control
mechanism. Such mechanism depends on factors which
influence the metabolism and excretion of hormones.
Thus, higher hormone concentration alone cannot trigger
the negative feedback mechanism. Negative feedback
must be triggered by overproduction of an "effect" of the
hormone.
15
Hormone secretion can be stimulated or inhibited by:

Other
hormones
(stimulating-
or
releasing-
hormones)

Plasma concentrations of ions or nutrients, as well as
binding globulins

Neurons and mental activity

Environmental changes, e.g., of light or temperature
One special group of hormones is the TROPIC HORMONES
that stimulate the hormone production of other endocrine
glands. For example, thyroid-stimulating hormone (TSH)
causes growth and increased activity of another endocrine
gland, the thyroid, which increases output of thyroid
hormones.
A recently-identified class of hormones is that of the
HUNGER HORMONES ‫ هرمونات الجوع‬such as ghrelin, orexin
and PYY 3-36 - and SATIETY HORMONES
‫هرمونات الشبع‬
such as leptin, obestatin, nesfatin-1.
In order to release active hormones quickly into the
circulation, hormone biosynthetic cells may produce and
store biologically inactive hormones in the form of pre- or
pro-hormones. These can then be quickly converted into
their active hormone form in response to a particular
stimulus.
16
Effects of hormone
Hormones have the following effects on the body:

stimulation or inhibition of growth

mood swings ‫تغيير الحالة المزاجية‬

induction or suppression of apoptosis (programmed
cell death)

activation or inhibition of the immune system

regulation of metabolism

preparation of the body for fighting, sex, fleeing
‫تجنب‬, mating, and other activity

preparation of the body for a new phase of life, such
as puberty ‫البلوغ‬
, parenting ‫ األبوة واألمومة‬, and
menopause ‫سن اليأس‬

control of the reproductive cycle

hunger cravings ‫الرغبة الملحة‬
A hormone may also regulate the production and release
of other hormones.
Hormone signals control the internal environment of the
body through homeostasis.
17
Pharmacology
Many
hormones
and
their
analogues
are
used
as
methods
of
medication.
The most commonly prescribed hormones are:
 Estrogens
and
progestagens
(as
hormonal contraception ‫)منع الحمل‬.
 Thyroxine (as levothyroxine, for hypothyroidism).
 Steroids (for autoimmune diseases and several
respiratory disorders.
 Insulin is used by many diabetics.
 Local preparations for use in otolaryngology ‫طب األنف‬
‫والحنجرة‬
‫واألدن‬
often
contain
pharmacologic
equivalents of adrenaline.
 Steroid and vitamin D creams are used extensively in
dermatological practice.
A "pharmacologic dose" of a hormone is a medical usage
referring to an amount of a hormone far greater than
naturally occurs in a healthy body.
The effects of pharmacologic doses of hormones may be
different from responses to naturally-occurring amounts
and may be therapeutically useful. An example is the
ability of pharmacologic doses of gluco-corticoid to
suppress inflammation.
18
ENDOCRINE DISEASES
A disease due to disorder of the endocrine system is often
called a "HORMONE IMBALANCE", but is technically known
as an ENDOCRINOPATHY or ENDOCRINOSIS.
Endocrine disease
Classification and external resources
Major endocrine glands. (Male left, female on the right.)
1. Pineal gland
2. Pituitary gland
3. Thyroid gland
4. Thymus
5. Adrenal gland
6. Pancreas
7. Ovary
8. Testes
19
Among the hundreds of endocrine diseases are:

Adrenal disorders:

Addison's disease (hypofunctioning of the
adrenal
cortex).
It
leads
to
adrenal
crisis
(disaster) with cardiovascular collapse.


Adrenal hyperplasia.

Mineralo corticoid deficiency.
Glucose homeostasis disorders:
o
Diabetes mellitus, is a condition in which the body either
does not produce enough, or does not properly respond
to, insulin, a hormone produced in the pancreas. Insulin
enables cells to absorb glucose in order to turn it into
energy. In diabetes, the body either fails to properly
respond to its own insulin, does not make enough
insulin, or both. This causes glucose to accumulate in
the blood, often leading to various complications.
o
Hypoglycemia

Idiopathic hypoglycemia, is a medical condition in
which the glucose level in the blood (blood
glucose ) is abnormally low.

Insulinoma, An insulinoma is a tumour of the
pancreas that is derived from beta cell s and
secretes insulin .

Metabolic bone disease:
o
Osteoporosis, is a disease of bone that leads to an
increased risk of fracture .
o
Osteitis deformans (Paget's disease of bone),
is a
general term for inflammation of bone
20
o
Rickets and osteomalacia, Osteomalacia term for the
softening of the bones due to defective bone ...
Osteomalacia in children is known as rickets

Pituitary gland disorders:
o
Diabetes insipidus, such as diabetes mellitus
o
Hypopituitarism
o
Pituitary tumors

Pituitary adenomas, Pituitary adenomas are
tumor s that occur in the pituitary gland , and
account for about 10% of intracranial neoplasms

Prolactinoma (or Hyperprolactinemia), A
prolactinoma is a benign tumor (adenoma ) of the
pituitary gland that produces a hormone called
prolactin.

Acromegaly, gigantism, An excess of secretion of
growth
hormone
after
puberty
can
lead
to
acromegaly


Parathyroid gland disorders:
o
hyper parathyroidism
o
Hypo parathyroidism
Sex hormone disorders:
o
Disorders
of
sex
development
or
intersex
disorders

Gonadal dysgenesis: (the gonads are ovaries or
testes), Gonadal dysgenesis is a condition of
21
unusual and asymmetrical gonadal development
leading to an unassigned sex differentiation.
It is a type of female hypogonadism in which no
functional ovaries are present to induce puberty
o
Hypogonadism, Low male or female hormones

Gonadotropin deficiency, Ovarian failure or
Testicular failure.
o

Ovarian failure

Testicular failure
Disorders of Gender

Gender identity disorder, is the formal diagnosis
used by psychologists and physicians to describe persons
who experience significant gender dysphoria (discontent
with the biological sex they were born with).
o
Disorders of Puberty

Delayed puberty, described as delayed puberty
when a boy or girl has passed the usual age.

Precocious puberty, is an unusually early onset of
puberty, the process of sexual maturation triggered by
the brain or exogenous chemicals, which usually begins in
late childhood and results in reproductive maturity and
completion of growth. Early puberty may be a variation of
normal development, or may be a result of a disease or
abnormal hormone exposure.
o

Menstrual function or fertility disorders
Thyroid disorders:
o
Goiter, also called a bronchocele, is a swelling in the thyroid
gland, which can lead to a swelling of the neck or larynx (voice
box). Goitre usually occurs when the thyroid gland is not
22
functioning properly. Worldwide, the most common cause for
goiter is iodine deficiency
o
Hyperthyroidism
o
Hypothyroidism, is the disease state in humans and in
animals caused by insufficient production of thyroid hormone by
the thyroid gland.

o
Thyroiditis, is the inflammation of the thyroid gland.
o
Thyroid cancer
Tumours of the endocrine glands
Hierarchical nature of hormonal control
Hormonal regulation of some physiological activities
involves a hierarchy (chain of command) of cell types
acting on each other either to stimulate or to modulate the
release and action of a particular hormone.
The secretion of hormones from successive levels of
endocrine
cells
is
stimulated
by
chemical
signals
originating from cells higher up the hierarchical system.
The master coordinator of hormonal activity in mammals is
the hypothalamus, which acts on input that it receives
from the central nervous system.
Other hormone secretion occurs in response to local
conditions, such as the rate of secretion of parathyroid
23
hormone
by
the
parathyroid
cells
in
response
to
fluctuations of ionized calcium levels in extracellular fluid.
As can be inferred from the hierarchical diagram, hormone
biosynthetic cells are typically of a specialized cell type,
residing within a particular endocrine gland (e.g., the
thyroid gland, the ovaries, or the testes).
Hormones may exit their cell of origin via exocytosis or
another means of membrane transport. However, the
hierarchical model is an over simplification of the
hormonal signaling process. Cellular recipients of a
particular hormonal signal may be one of several cell types
that reside within a number of different tissues, as is the
case for insulin, which triggers a diverse range of systemic
physiological effects.
Different tissue types may also respond differently to the
same hormonal signal. Because of this, hormonal signaling
is elaborate and hard to dissect.
24
The Hormones of the Human:
Proteins, peptides, and modified amino acids
These hydrophilic (and mostly large) hormone molecules
bind to receptors on the surface of "target" cells; that is,
cells able to respond to the presence of the hormone.
These receptors are transmembrane proteins. Binding of
the hormone to its receptor initiates a sequence of
intracellular signals that may

alter the behavior of the cell (such as by opening or
closing membrane channels) or

stimulate (or repress) gene expression in the nucleus
by turning on (or off) the promoters and enhancers of
the genes.
This is the sequence of events:

The hormone binds to a site on the extracellular
portion of the receptor.
o
The receptors are transmembrane proteins that
pass through the plasma membrane 7 times,
with their N-terminal exposed at the exterior of
the cell and their C-terminal projecting into the
cytoplasm.

Binding of the hormone to the receptor
o
activates a G protein associated with the
cytoplasmic C-terminal
25
o
This initiates the production of a "second
messenger". The most common of these are

cyclic AMP, (cAMP) which is produced by
adenylyl cyclase from ATP,
o
The second messenger, in turn, initiates a series
of intracellular events (shown here as short
arrows) such as

phosphorylation and activation of enzymes;

release of Ca2+ into the cytosol from stores
within the endoplasmic reticulum.
o
In the case of cAMP, these enzymatic changes
activate the transcription factor CREB.
26
o
The cell begins to produce the appropriate gene
products in response to the hormonal signal it
had received at its surface.
Steroid Hormones
Steroid hormones, being hydrophobic molecules, diffuse
freely into all cells. However, their "target" cells contain
cytoplasmic
and/or
nuclear
proteins
that
serve
as
receptors of the hormone.
27
The hormone binds to the receptor and the complex binds
to hormone response elements - stretches of DNA within
the promoters of genes responsive to the hormone.
The hormone/receptor complex acts as a transcription
factor turning target genes "on" (or "off").
Hormone Regulation
The levels of hormones circulating in the blood are tightly
controlled by three homeostatic mechanisms:
1. When one hormone stimulates the production of a
second, the second suppresses the production of the
first.
Example: The follicle
‫ جرررا‬stimulating hormone
(FSH) stimulates the release of estrogens from the
ovarian follicle. A high level of estrogen, in turn,
suppresses the further production of FSH.
2. Antagonistic pairs of hormones.
Example: Insulin causes the level of blood sugar
(glucose) to drop when it has risen. Glucagon causes
it to rise when it has fallen.
3. Hormone secretion is increased (or decreased) by the
same
substance
whose
level
is
decreased
(or
increased) by the hormone.
Example:
suppresses
a rising
the
level of
production
Ca2+ in the
of
the
blood
parathyroid
hormone (PTH). A low level of Ca2+ stimulates it.
28
Hormone Transport
Although a few hormones circulate simply dissolved in the
blood, most are carried in the blood bound to plasma
proteins. For example, all the steroid hormones, being
highly hydrophobic, are transported bound to plasma
proteins.
29
Hormones of the Reproductive System
FEMALES
The ovaries of sexually-mature females secrete:

a mixture of estrogens of which 17β-estradiol is the
most abundant (and most potent).

progesterone.
ESTROGENS
Estrogens are steroids. They
 are primarily responsible for the conversion of girls
into sexually-mature women.
 development of breasts
 further development of the uterus and vagina
 broadening of the pelvis
 growth of hair
 increase in adipose (fat) tissue
 participate in the monthly preparation of the body for
a possible pregnancy
 participate in pregnancy if it occurs
30
Estrogens also have non-reproductive effects.

They antagonize the effects of the parathyroid
hormone, minimizing the loss of calcium from bones
and thus helping to keep bones strong.

They promote blood clotting.
PROGESTERONE
Progesterone is also a steroid. It has many effects in the
body. Here we shall focus on the role of progesterone in
the menstrual cycle and pregnancy
Regulation of Estrogen and Progesterone
The synthesis and secretion of estrogens is stimulated by
follicle-stimulating hormone (FSH), which is, in turn,
controlled by the hypothalamic gonadotropin releasing
hormone (GnRH).
31
Hypothalamus → GnRH → Pituitary → FSH → Follicle → Estrogens
High levels of estrogens suppress the release of GnRH
(bar) providing a negative-feedback control of hormone
levels.
Progesterone production is stimulated by luteinizing
hormone (LH), which is also stimulated by GnRH.
Hypothalamus → GnRH → Pituitary → LH → Corpus luteum → Progesterone
32
Elevated levels of progesterone control themselves by the
same negative feedback loop used by estrogen (and
testosterone).
The Menstrual Cycle
About every 28 days, some blood and other products of
the disintegration of the inner lining of the uterus (the
endometrium
‫ ) بطانررة الررر‬are discharged from the
33
uterus, a process called menstruation. During this time a
new follicle begins to develop in one of the ovaries. After
menstruation ceases, the follicle continues to develop,
secreting an increasing amount of estrogen as it does so.

The rising level of estrogen causes the endometrium
to become thicker and more richly supplied with
blood vessels and glands.

A rising level of LH causes the developing egg within
the follicle to complete the first meiotic division
(meiosis I), forming a secondary oocyte.

After about two weeks, there is a sudden surge ‫اندفاع‬
in the production of LH.

This surge in LH triggers ovulation: the release of the
secondary oocyte ‫ البويضة‬into the fallopian tube.

Under the continued influence of LH, the now-empty
follicle develops into a corpus luteum (hence the
name luteinizing hormone for LH).

Stimulated by LH, the corpus luteum secretes
progesterone which
o
continues the preparation of the endometrium
for a possible pregnancy

o
inhibits the contraction of the uterus
o
inhibits the development of a new follicle
If fertilization does not occur (which is usually the
case),
34
o
the rising level of progesterone inhibits the
release of GnRH which, in turn,
o

inhibits further production of progesterone.
As the progesterone level drops,
o
the corpus luteum begins to degenerate;
o
the endometrium begins to break down, its cells
committing programmed cell death (apoptosis);
o
the inhibition of uterine contraction is lifted, and
o
the bleeding and cramps of menstruation begin.
Pregnancy
Fertilization of the egg takes place within the fallopian
tube. As the fertilized egg passes down the tube, it
undergoes its first mitotic divisions. By the end of the
week, the developing embryo has become a hollow ball of
cells called a blastocyst. At this time, the blastocyst
reaches the uterus and embeds itself in the endometrium,
a
process
called
implantation.
With
implantation,
pregnancy is established.
The blastocyst has two parts:

the inner cell mass, which will become the baby, and

the trophoblast, which will
o
develop into the placenta and umbilical cord
o
and
begin
to
secrete
human
chorionic
gonadotropin (HCG).
35
HCG is a glycoprotein. It is a dimer of

the same alpha subunit (of 89 amino acids) used by
TSH, FSH, and LH) and

a unique beta subunit (of 148 amino acids).
HCG behaves much like FSH and LH with one crucial
exception: it is NOT inhibited by a rising level of
progesterone. Thus HCG prevents the deterioration of the
corpus luteum at the end of the fourth week and enables
pregnancy to continue beyond the end of the normal
menstrual cycle.
Because only the implanted trophoblast makes HCG, its
early appearance in the urine of pregnant women provides
the basis for the most widely used test for pregnancy
(which
can
provide
a
positive
signal
even
before
menstruation would have otherwise begun).
As pregnancy continues, the placenta becomes a major
source of progesterone, and its presence is essential to
maintain pregnancy. Mothers at risk of giving birth too
soon can be given a synthetic progestin to help them
retain the fetus until it is full-term.
Birth
Toward the end of pregnancy,
36

The placenta releases large amounts of CRH which
stimulates the pituitary glands of both mother and
her fetus to secrete

ACTH, which acts on their adrenal glands causing
them
to
release
the
estrogen
precursor
dehydroepiandrosterone sulfate (DHEAS).

This is converted into estrogen by the placenta.

The rising level of estrogen causes the smooth
muscle cells of the uterus to
o
synthesize connexins and form gap junctions.
Gap junctions connect the cells electrically so
that they contract together as labor begins.
o

express receptors for oxytocin.
Oxytocin is secreted by the posterior lobe of the
pituitary as well as by the uterus.

Prostaglandins are synthesized in the placenta and
uterus.

The normal inhibition of uterine contraction by
progesterone is turned off by several mechanisms
while
37

both oxytocin and prostaglandins cause the uterus to
contract and labor begins.
Three or four days after the baby is born, the breasts begin
to secrete milk.

Milk synthesis is stimulated by the pituitary hormone
prolactin (PRL), and

its release from the breast is stimulated by oxytocin.

Milk contains an inhibitory peptide. If the breasts are
not fully emptied, the peptide accumulates and
inhibits milk production. This autocrine action thus
matches supply with demand.
Other Hormones
Relaxin
As the time of birth approaches in some animals (e.g.,
pigs, rats) , this polypeptide has been found to:
o
relax the pubic ligaments
o
soften and enlarge the opening to the cervix.
Relaxin is found in pregnant humans but at higher levels
early in pregnancy than close to the time of birth. Relaxin
promotes angiogenesis, and in humans it probably plays a
more important role in the development of the interface
between the uterus and the placenta that it does in the
birth process.
38
Activins, Inhibins, Follistatin.
These proteins are synthesized within the follicle. Activins
and inhibins bind to follistatin. Activins increase the action
of FSH; inhibins, as their name suggests, inhibit it. How
important they are in humans remains to be seen.
However the important role that activin and follistatin play
in the embryonic development of vertebrates justifies
mentioning them here.
Oral contraceptives: the "pill"
The feedback inhibition of GnRH secretion by estrogens
and progesterone provides the basis for the most widelyused
form
formulations
of
of
contraception.
synthetic
Dozens
estrogens
of
or
different
progestins
(progesterone relatives) — or both — are available. Their
inhibition of GnRH prevents the mid-cycle surge of LH and
ovulation. Hence there is no egg to be fertilized.
Usually the preparation is taken for about three weeks and
then stopped long enough for normal menstruation to
occur.
The main side-effects of the pill stem from an increased
tendency for blood clots to form (estrogen enhances
clotting of the blood).
RU-486
39
RU-486 (also known as mifepristone) is a synthetic steroid
related to progesterone. Unlike the synthetic progestins
used in oral contraceptives that mimic the actions of
progesterone, RU-486 is a progesterone antagonist; that
is, it blocks the action of progesterone. It does this by
binding more tightly to the progesterone receptor than
progesterone itself but without the normal biological
effects:
 The RU-486/receptor complex is not active as a
transcription factor.
 Thus genes that are turned on by progesterone are
turned off by RU-486.
 The proteins needed to establish and maintain
pregnancy are no longer synthesized.
 The endometrium breaks down.
 The embryo detaches from it and can no longer make
chorionic gonadotropin (HCG).
 Consequently
the
corpus
luteum
ceases
its
production of progesterone.
 The inhibition on uterine contraction is lifted.
 Soon the embryo and the breakdown products of the
endometrium are expelled.
These properties of RU-486 have caused it to be used to
induce abortion of an unwanted fetus. In practice, the
physician assists the process by giving a synthetic
40
prostaglandin (e.g., misoprostol [Cytotec®]) 36–48 hours
after giving the dose of RU-486. Use of RU-486 is
generally limited to the first seven weeks of pregnancy.
RU-486 has been used for many years in some countries.
However, the controversies surrounding abortion in the
United States kept it from being authorized for use here
until September 2000.
MENOPAUSE
The menstrual cycle continues for many years. But
eventually, usually between 42 and 52 years of age, the
follicles become less responsive to FSH and LH. They begin
to secrete less estrogen. Ovulation and menstruation
become irregular and finally cease. This cessation is called
menopause.
With levels of estrogen now running one-tenth or less of
what they had been, the hypothalamus is released from
their inhibitory influence (bar). As a result it now
stimulates
the
pituitary
to
increased
activity.
The
concentrations of FSH and LH in the blood rise to ten or
more times their former values. These elevated levels may
cause a variety of unpleasant physical and emotional
symptoms.
41
Hormone replacement therapy (HRT)
Many menopausal women elect to take a combination of
estrogen and progesterone after they cease to make their
own. The benefits are:

reduction
in
the
unpleasant
symptoms
of
the
menopause

a reduction in the loss of calcium from bones and
thus a reduction in osteoporosis and the fractures
that accompany it.
It was also believed that HRT reduced the risk of
cardiovascular disease. However, a recent study of 16,000
menopausal women was stopped 3 years early when it
was found that, in fact, HRT increased (albeit only slightly)
not decreased the incidence of cardiovascular disease.
Environmental estrogens
Some substances that find their way into the environment,
such as

DDE, a breakdown product of the once widely-used
insecticide DDT,

DDT itself (still used in some countries (e.g., Mexico),
and

PCBs, chemicals once used in a wide variety of
industrial applications
42
can bind to the estrogen (and androgen) receptors and
mimic (weakly) the effects of the hormone. This has
created anxiety that they may be responsible for harmful
effects such as cancer and low sperm counts.
However, there is as yet little evidence to support these
worries.
 No epidemiological relationship has been found
between the incidence of breast cancer and the levels
of these compounds in the body.
 As for laboratory studies that found a synergistic
effect of two of these substances on receptor binding
(findings that created the great alarm), these have
not been replicated in other laboratories, and the
authors of the original report have since withdrawn it
as invalid.
MALES
The
principal
testosterone.
androgen
This
steroid
(male
is
sex
hormone)
manufactured
by
is
the
interstitial (Leydig) cells of the testes. Secretion of
testosterone
responsible
increases
for
the
sharply
at
development
puberty
of
the
and
is
so-called
secondary sexual characteristics (e.g., beard) of men.
Testosterone is also essential for the production of sperm.
43
Production of testosterone is controlled by the release of
luteinizing hormone (LH) from the anterior lobe of the
pituitary gland, which is in turn controlled by the release
of GnRH from the hypothalamus. LH is also called
interstitial cell stimulating hormone (ICSH).
Hypothalamus → GnRH → Pituitary → LH → Testes → Testosterone
The level of testosterone is under negative-feedback
control: a rising level of testosterone suppresses the
release of GnRH from the hypothalamus. This is exactly
parallel to the control of estrogen secretion in females.
Males need estrogen, too!
44
Important human hormones
Spelling is not uniform for many hormones. Current North
American
and
international
usage
is
estrogen,
gonadotropin, while British usage retains the Greek
diphthong in oestrogen and the unvoiced aspirant h in
gonadotrophin.
Name
Abbreviation
Melatonin (N-acetyl-
antioxidant and causes
5-methoxytryptamine)
Serotonin
drowsiness
5-HT
(a thyroid hormone)
Controls mood, appetite, and
sleep
less active form of thyroid
Thyroxine
(or tetraiodothyronine)
Effect
hormone: increase the basal
T4
metabolic rate & sensitivity to
catecholamines,
affect protein synthesis
potent form of thyroid hormone:
Triiodothyronine
(a thyroid hormone)
T3
increase the basal metabolic rate
& sensitivity to catecholamines,
affect protein synthesis
Fight-or-flight response:
Boosts the supply of oxygen and
Epinephrine (or
adrenaline)
glucose to the brain and muscles
EPI
(by increasing heart rate and
stroke volume, vasodilation,
increasing catalysis of glycogen
in liver, breakdown of lipids in
45
fat cells. dilate the pupils
Suppress non-emergency bodily
processes (e.g. digestion)
Suppress immune system
Fight-or-flight response:
Boosts the supply of oxygen and
glucose to the brain and muscles
Norepinephrine
(or noradrenaline)
(by increasing heart rate and
NRE
stroke volume, vasoconstriction
and increased blood pressure,
breakdown of lipids in fat cells.
Increase skeletal muscle
readiness.
Dopamine
(or prolactin inhibiting
hormone
Increase heart rate and blood
DPM, PIH or pressure
DA
TRH from anterior pituitary
Antimullerian hormone
(or mullerian inhibiting
AMH
factor or hormone)
Adiponectin
angiotensin
ACTH
vasopressin)
(glucocorticoids and androgens)
in adrenocortical cells
vasoconstriction
AGT
release of aldosterone from
adrenal cortex dipsogen.
Antidiuretic hormone
(or vasopressin, arginine
TRH from anterior pituitary
synthesis of corticosteroids
corticotropin)
Angiotensinogen and
Inhibit release of prolactin and
Acrp30
Adrenocorticotropic
hormone (or
Inhibit release of prolactin and
retention of water in kidneys
ADH
moderate vasoconstriction
Release ACTH in anterior
46
pituitary
Atrial-natriuretic
peptide (or atriopeptin)
Calcitonin
ANP
CT
Construct bone, reduce blood
Ca2+
Release of digestive enzymes
Cholecystokinin
CCK
from pancreas
Release of bile from gallbladder
hunger suppressant
Corticotropin-releasing
hormone
Erythropoietin
CRH
EPO
Release ACTH from anterior
pituitary
Stimulate erythrocyte production
In female: stimulates
maturation of Graafian follicles
Follicle-stimulating
hormone
in ovary.
FSH
In male: spermatogenesis,
enhances production of
androgen-binding protein by the
Sertoli cells of the testes
Gastrin
GRP
Secretion of gastric acid by
parietal cells
Stimulate appetite,
Ghrelin
secretion of growth hormone
from anterior pituitary gland
glycogenolysis and
Glucagon
GCG
gluconeogenesis in liver
increases blood glucose level
Gonadotropin-releasing
hormone
GnRH
Release of FSH and LH from
anterior pituitary.
47
Growth hormonereleasing hormone
GHRH
Release GH from anterior
pituitary
promote maintenance of corpus
Human chorionic
gonadotropin
luteum during beginning of
hCG
pregnancy
Inhibit immune response,
towards the human embryo.
increase production of insulin
Human placental
lactogen
HPL
and IGF-1
increase insulin resistance and
carbohydrate intolerance
stimulates growth and cell
Growth hormone
GH or hGH
reproduction
Release Insulin-like growth
factor 1 from liver
Inhibin
Intake of glucose, glycogenesis
and glycolysis in liver and muscle
Insulin
INS
from blood
intake of lipids and synthesis of
triglycerides in adipocytes Other
anabolic effects
Insulin-like growth
factor (or somatomedin)
Leptin
insulin-like effects
IGF
regulate cell growth and
development
LEP
decrease of appetite and
increase of metabolism.
In female: ovulation
Luteinizing hormone
LH
In male: stimulates Leydig cell
production of testosterone
48
Melanocyte stimulating
hormone
MSH or αMSH
melanogenesis by melanocytes
in skin and hair
release breast milk. Contraction
of cervix and vagina Involved in
Oxytocin
OXT
orgasm, trust between people.
and circadian homeostasis (body
temperature, activity level,
wakefulness).
increase blood Ca2+: *indirectly
stimulate osteoclasts

Ca2+ reabsorption in
kidney

Parathyroid hormone
PTH
activate vitamin D
(Slightly) decrease blood
phosphate:

(decreased reuptake in
kidney but increased
uptake from bones

activate vitamin D)
milk production in mammary
Prolactin
PRL
glands
sexual gratification after sexual
acts
Relaxin
RLN
Unclear in humans
Secretion of bicarbonate from
liver, pancreas and duodenal
Secretin
SCT
Brunner's glands
Enhances effects of
cholecystokinin Stops production
of gastric juice
49
Inhibit release of GH and TRH
from anterior pituitary
Suppress release of gastrin,
cholecystokinin (CCK), secretin,
motilin, vasoactive intestinal
peptide (VIP), gastric inhibitory
polypeptide (GIP),
enteroglucagon in
gastrointestinal system
Somatostatin
SRIF
Lowers rate of gastric emptying
Reduces smooth muscle
contractions and blood flow
within the intestine
Inhibit release of insulin from
beta cells.
Inhibit release of glucagon from
beta cells . Suppress the
exocrine secretory action of
pancreas.
Thrombopoietin
TPO
Thyroid-stimulating
hormone (or
TSH
thyrotropin)
Thyrotropin-releasing
hormone
produce platelets.
secrete thyroxine (T4) and
triiodothyronine (T3)
Release thyroid-stimulating
TRH
hormone (primarily)
Stimulate prolactin release
Stimulation of gluconeogenesis
Inhibition of glucose uptake in
Cortisol
muscle and adipose tissue
Mobilization of amino acids from
extrahepatic tissues Stimulation
50
of fat breakdown in adipose
tissue anti-inflammatory and
immunosuppressive
Increase blood volume by
reabsorption of sodium in
Aldosterone
kidneys (primarily)
Potassium and H+ secretion in
kidney.
Anabolic: growth of muscle mass
and strength, increased bone
density, growth and strength,
Testosterone
Virilizing: maturation of sex
organs, formation of scrotum,
deepening of voice, growth of
beard and axillary hair.
Dehydroepiandrosterone
DHEA
Androstenedione
Dihydrotestosterone
Virilization, anabolic
Substrate for estrogen
DHT
Females:
Structural:

promote formation of
female secondary sex
characteristics
Estradiol
E2

accelerate height growth

accelerate metabolism
(burn fat)

reduce muscle mass

stimulate endometrial
growth
51

increase uterine growth

maintenance of blood
vessels and skin

reduce bone resorption,
increase bone formation
Protein synthesis:

increase hepatic
production of binding
proteins
Coagulation:

increase circulating level of
factors 2, 7, 9, 10,
antithrombin III,
plasminogen

increase platelet
adhesiveness
Increase HDL, triglyceride,
height growth Decrease LDL, fat
depositition Fluid balance:

salt (sodium) and water
retention

increase growth hormone

increase cortisol, SHBG
Gastrointestinal tract:

reduce bowel motility

increase cholesterol in bile
Melanin:

increase pheomelanin,
reduce eumelanin
Cancer: support hormonesensitive breast cancers [11]
52
Suppression of production in the
body of estrogen is a treatment
for these cancers.
Lung function:

promote lung function by
supporting alveoli[12].
Males: Prevent apoptosis of germ
cells[13]
Estrone
Estriol
Support pregnancy:
Convert endometrium to
secretory stage Make cervical
mucus permeable to sperm.
Inhibit immune response, e.g.
towards the human embryo.
Decrease uterine smooth muscle
contractility[14] Inhibit lactation
Inhibit onset of labor. Support
fetal production of adrenal
Progesterone
mineralo- and glucosteroids.
Other: Raise epidermal growth
factor-1 levels Increase core
temperature during ovulation
Reduce spasm and relax smooth
muscle (widen bronchi and
regulate mucus)
Antiinflammatory Reduce gallbladder activity Normalize blood
clotting and vascular tone, zinc
and copper levels, cell oxygen
53
levels, and use of fat stores for
energy. Assist in thyroid function
and bone growth by osteoblasts
Relsilience in bone, teeth, gums,
joint, tendon, ligament and skin
Healing by regulating collagen
Nerve function and healing by
regulating myelin Prevent
endometrial cancer by regulating
effects of estrogen.
Active form of vitamin D3
Calcitriol
Increase absorption of calcium
(1,25-dihydroxy
and phosphate from
vitamin D3)
gastrointestinal tract and
kidneys inhibit release of PTH
Calcidiol
Inactive form of Vitamin D3
(25-hydroxyvitamin D3)
Prostaglandins
PG
Leukotrienes
LT
Prostacyclin
PGI2
Thromboxane
TXA2
Prolactin releasing
hormone
PRH
Release prolactin from anterior
pituitary
lipolysis and steroidogenesis,
Lipotropin
PRH
stimulates melanocytes to
produce melanin
(To a minor degree than ANP)
Brain natriuretic
peptide
BNP
reduce blood pressure by:
reducing systemic vascular
resistance, reducing blood water,
54
sodium and fats
Neuropeptide Y
Histamine
Endothelin
Pancreatic polypeptide
NPY
increased food intake and
decreased physical activity
stimulate gastric acid secretion
Smooth muscle contraction of
stomach [17]
Unknown
Activates the renin-angiotensin
Renin
system by producing angiotensin
I of angiotensinogen
Enkephalin
Regulate pain
55
STEROID HORMONE METABOLISM
Steroid hormones can be grouped into five groups by the
receptors to which they bind:
1. Glucocorticoids
2. Mineralocorticoids
3. Androgens
4. Estrogens
5. Progestagens
Steroids are lipophilic, low-molecular weight compounds
derived from cholesterol that play a number of important
physiological roles.
The steroid hormones are synthesized mainly by endocrine
glands such as:
 The gonads (testes and ovary)
 The adrenals
 The fetoplacental unit , during gestation
Steroid hormones act both on peripheral target tissues
and the central nervous system (CNS).
An important function of the steroid hormones is to
coordinate physiological and behavioural responses for
specific biological purposes, e.g. reproduction.
56
Thus, gonadal steroids influence the sexual differentiation
of the genitalia and of the brain, determine secondary
sexual characteristics during development and sexual
maturation, contribute to the maintenance of their
functional state in adulthood and control or modulate
sexual behaviour.
It has been recently discovered that, in addition to the
endocrine glands, the CNS is also able to form a number of
biologically active steroids directly from cholesterol (the
so-called "NEUROSTEROIDS").
These neurosteroids, however, are more likely to have
"autocrine" or "paracrine" functions rather than true
endocrine effects.
Despite their relatively simple chemical structure, steroids
occur in a wide variety of biologically active forms. This
variety is not only due to the large range of compounds
secreted by steroid-synthesizing tissues, but also to the
fact that circulating steroids are extensively metabolized
peripherally, notably in the liver, and in their target
tissues, where conversion to an active form is sometimes
required before they can elicit their biological responses.
57
Steroid metabolism is therefore important not only for the
production of these hormones, but also for the regulation
of their cellular and physiological actions.
Structure, nomenclature and classification
The parent compound from which all steroids are derived is
cholesterol.
58
As shown before cholesterol is made up of three hexagonal carbon
rings (A,B,C) and a pentagonal carbon ring (D) to which a sidechain (carbons 20-27) is attached (at position 17 of the polycyclic
hydrocarbon). Two angular methyl groups are also found at
position 18 and 19.
Removal of part of the side-chain gives rise to C21-compounds of
the pregnane series (progestins and corticosteroids).
59
Total removal produces C19-steroids of the androstane series
(including the androgens), whereas loss of the 19-methyl group
(usually after conversion of the A-ring to a phenolic structure,
hence the term "aromatization") yields the estrane series, to which
estrogens belong.
60
Individual compounds are characterised by the presence or
absence of specific functional groups (mainly hydroxy, keto(oxo)
and aldehyde functions for the naturally occurring steroids) at
certain positions of the carbon skeleton (particularly at positions
3,5,11,17,18,20 and 21).
Given that at most positions, the functional groups can be oriented
either in equatorial or axial position, this type of structure gives
rise to a great number of possible stereoisomers (i.e. molecules
having the same chemical formula, but a different threedimensional conformation).
Stereoisomerism is very important for biological activity (i.e. for
steroid-protein interactions).
Steroid hormone biosynthesis:
 The
adrenals
produce
both
androgens
and
corticosteroids (mineralo- and glucocorticoids).
 The ovaries can secrete estrogens and progestins.
 The testis mainly androgens.
However, the biochemical pathways involved are strikingly
similar in all tissues, the difference in secretory capacity
being mostly due to the presence or absence of specific
enzymes. It is therefore possible to give a general outline
61
of the major biosynthetic pathways which is applicable to
all steroid-secreting glands.
A general outline of the major biosynthetic pathways
62
Cholesterol can be synthetized in all steroid-producing tissues from
acetate, but the main production sites are the liver, the skin and
the intestinal mucosa.
Examples of some routes of steroid metabolism:
63
64
Enzymes involved in steroid biosynthesis:
 Desmolases
(or
lyases):
these
enzymes
catalyse
reactions which result in the removal of parts of the
original cholesterol side-chain. This involves sequential
hydroxylation of adjacent C (e.g., of C-20 and C-22 for P450scc) and requires a cytochrome P-450, molecular
oxygen (O2) and nicotinamide dinucleotide phosphate,
reduced form (NADPH) as a cofactor.
 Hydroxylases: these enzymes are membrane-bound and
are present either in the mitochondrial or in the
microsomal fraction of the cell. They also require a
cytochrome P-450, molecular oxygen and NADPH, as for
lyases.
 Hydroxysteroid dehydrogenases (oxido-reductases):
these enzymes catalyze reversible reactions and
depend either on NADP(H) or NAD(H). They are found
both in the cell cytosol and in the microsomal
fraction.
 Aromatase: conversion of the A-ring to a phenolic
structure (i.e. with a phenolic HO-group at C-3), a
process known as " aromatization ", involves a
complex sequence of hydroxylation reactions and
loss of the angular C-19 methyl group (10).
Correlation between structure and function: the role of
metabolism
The biological activity of a steroid molecule depends on its
ability to interact with a specific binding site on the
65
corresponding receptor. In most cases, biological activity
can be directly correlated with binding affinity. The affinity
(usually characterized by the binding constant KD, which
is the molar concentration required to saturate half of the
available binding sites) of a steroid for its specific receptor
is dependent upon the presence or absence of particular
functional groups and
the overall three-dimensional
structure of the molecule.
66
CLINICAL BIOCHEMISTRY, summary
Serum is the yellow watery part of blood that is left after blood has been allowed
to clot and all blood cells have been removed. This is most easily done by
centrifugation which packs the denser blood cells and platelets to the bottom of
the centrifuge tube, leaving the liquid serum fraction resting above the packed
cells.
Plasma is essentially the same as serum, but is obtained by centrifuging the blood
without clotting. Plasma therefore contains all of the clotting factors, including
fibrinogen.
Large array of laboratory tests can be sub-categorized into sub-specialties of:

General or routine chemistry

Endocrinology - the study of hormones

Immunology - the study of the immune system and antibodies

Pharmacology or Toxicology - the study of drugs
Endocrinology is a branch of medicine dealing with:

Disorder of the endocrine system and its secretions (hormones).

The integration of developmental events such as:
1. proliferation

The
coordination
2. growth
of
metabolism,
3. differentiation
respiration,
excretion,
movement,
reproduction, and sensory perception.
Endocrinology is concerned with the study of the:

Biosynthesis of hormones

Storage of hormones

Chemistry of hormones

Physiological function of hormones

Cells of the endocrine glands

Tissues that secrete hormones
The endocrine system consists of several glands, in different parts of the body
that secrete hormones directly into the blood rather than into a duct system.
67
Hormones have many different functions and modes of action; one hormone may
have several effects on different target organs, and, conversely, one target organ
may be affected by more than one hormone.
Any chemical to be classified as hormone, must be:

Produced by an organ.

Released in small amounts into the blood.

Transported by the blood to a distant organ to exert its specific function.
A hormone is a chemical released by one or more cells that affects cells in other
parts of the organism. Only a small amount of hormone is required to alter cell
metabolism.
Three mechanisms of chemical signaling of hormone can be distinguished:
1. Autocrine signaling
Chemical signal acts on the same cell. The cell signals itself through a chemical
that it synthesizes and then responds to.
This effect can occur within the cytoplasm of the cell or by an interacting with
receptors on the surface of the same cell.
2. Paracrine signaling
Chemical signals that diffuse into the area and interact with receptors on nearby
cells. Paracrine effect is a chemical communication between cells within a tissue
or organ, e.g. the release of neurotransmitters at synapses in the nervous system.
3. Endocrine signaling
The chemicals are secreted into the blood and carried to the cells they act upon.
A neuroendocrine signal is a "classical" hormone that is released into the blood by
a neurosecretory neuron.
Hormone is essentially a chemical messenger that transports a signal from one
cell to another.
Endocrine
hormone
molecules
are
secreted
(released)
directly
into
the
bloodstream.
Exocrine hormone molecules are secreted directly into a duct, and from the duct
they either flow into the bloodstream or they flow from cell to cell.
Endocrinology as a profession (job)
Although every organ system (including the brain, lungs, heart, intestine, skin,
and the kidney)
secretes and responds to hormones, the clinical specialty of
68
endocrinology focuses primarily on the endocrine organs (the organs whose
primary function is hormone secretion).
These endocrine organs include the pituitary gland, thyroid gland, adrenal gland,
ovaries, testes and pancreas.
An endocrinologist is a doctor who specializes in treating disorders of the
endocrine system, such as diabetes, hyperthyroidism, and many others.
The diagnosis and treatment of endocrine diseases are guided by laboratory tests
to a greater extent than for most specialties.
Many
diseases
are
investigated
through
excitation/stimulation
or
inhibition/suppression Testing. This might involve injection with a stimulating
agent to test the function of an endocrine organ. Blood is then sampled to assess
the changes of the relevant hormones or metabolites.
Most endocrine disorders are chronic diseases that need life-long care.
Some of the most common endocrine diseases include:

diabetes mellitus

hypothyroidism

The metabolic syndrome.
There are 3 different classes of hormone based on their chemical composition:
1. Amine hormones
Amine hormones, such as nor-epinephrine, epinephrine, and dopamine, are
derived from single amino acid (tyrosine).
Thyroid hormones such as 3,5,3’-tri-iodothyronine (T3) and 3,5,3’,5’-tetraiodothyronine (thyroxine, T4) make up a subset of this class because they derive
from the combination of two iodinated tyrosine amino acid residues.
2. Peptide and protein hormones
Peptide and protein hormones consist of 3 to ›200 amino acid residues and have
molecular weights as large as 30,000.
69
All hormones secreted by the pituitary gland are peptide hormones. Insuline,
liptin, ghrelin are examples for this group.
3. Steroid hormones
Steroid hormones are converted from their parent compound, cholesterol. Vitamin
D3, estrogen, progesterone are examples for this group.
Hormonal signaling caused across hierarchy (chain of commands) involves the
following:
7. Biosynthesis of a particular hormone in a particular tissue
8. Storage and secretion of the hormone
9. Transport of the hormone to the target cell(s)
10. Recognition of the hormone by an
associated cell membrane
or
intracellular receptor protein.
11. Relay (send) and amplification of the received hormonal signal via a signal
transduction process that leads to a cellular response.
12. Degradation of the hormone.
The master coordinator of hormonal activity in mammals is the hypothalamus,
which acts on input that it receives from the central nervous system.
Hormones act by binding to specific receptors in the target organ. The receptor
has at least two basic constituents:

Recognition site , to which the hormone binds

Effector site , which precipitates the modification of cellular function.
Transduction mechanism of hormone:
The hormone binding induces allosteric
modification that, in turn, produces the appropriate response.
The hormone binds to the receptor protein, resulting in the activation of a signal
transduction mechanism that ultimately leads to cell type-specific responses.
A cell may have several different receptors that recognize the same hormone and
activate different signal transduction pathways.
In order to bind their receptors these hormones must:

Cross the cell membrane.

Combined with receptor.
70

The combined hormone-receptor complex moves across the nuclear
membrane into the nucleus of the cell.

Hormone binds to specific DNA sequences.

Hormone is effectively amplifying or suppressing the action of certain
genes.

Hormones affecting protein synthesis.
Hormone-receptor complex concentrations are effectively determined by three
factors:
4. The number of hormone molecules available for complex formation
5. The number of receptor molecules available for complex formation and
6. The binding affinity between hormone and receptor.
Hormone secretion can be stimulated or inhibited by:

Other hormones (stimulating- or releasing-hormones)

Plasma concentrations of ions or nutrients, as well as binding globulins

Neurons and mental activity

Environmental changes, e.g., of light or temperature
TROPIC HORMONES are special group of hormones that stimulate the hormone
production of other endocrine glands. For example, thyroid-stimulating hormone
(TSH) causes growth and increased activity of another endocrine gland (thyroid
gland), which increases output of thyroid hormones.
In order to release active hormones quickly into the circulation, hormone
biosynthetic cells may produce and store biologically inactive hormones in the
form of pre- or pro-hormones. These can then be quickly converted into their
active hormone form in response to a particular stimulus (HUNGER HORMONES
and SATIETY HORMONES).
71
The levels of hormones circulating in the blood are tightly controlled by three
homeostatic mechanisms:
4. When one hormone stimulates the production of a second, the second
suppresses the production of the first, e.g. FSH hormone stimulates the
release of estrogens from the ovaries. A high level of estrogen, in turn,
suppresses the further production of FSH.
5. Antagonistic pairs of hormones.
Insulin drops the level of blood sugar when it has risen. Glucagon rise the
blood sugar level when it has fallen.
6. Hormone secretion is increased (or decreased) by the same substance
whose level is decreased (or increased) by the hormone, e.g., a rising level
of Ca2+ in the blood suppresses the production of the parathyroid hormone
(PTH), while the low level of Ca2+ stimulates the production of PTH.
Hormones have the following effects on the body:

stimulation or inhibition of growth

mood swings

induction or suppression of apoptosis (programmed cell death)

activation or inhibition of the immune system

regulation of metabolism

preparation of the body for a new phase of life, such as puberty
,
parenting , and menopause

control of the reproductive cycle

regulate the production and release of other hormones.
The most commonly prescribed hormones are:

Estrogens and progestagens as methods of hormonal contraception.

Thyroxine for hypothyroidism.

Steroids for autoimmune diseases and several respiratory disorders.

Insulin is used by many diabetics.

Adrenaline for use in otolaryngology

Steroid and vitamin D creams are used extensively in dermatological
practice.
A "pharmacologic dose" of a hormone is a medical usage referring to an amount of
a hormone far greater than naturally occurs in a healthy body. The effects of
pharmacologic doses of hormones may be different from responses to naturally-
72
occurring amounts and may be therapeutically useful. An example is the ability of
pharmacologic doses of gluco-corticoid to suppress inflammation.
A disease due to a disorder of the endocrine system is often called a "HORMONE
IMBALANCE",
but
is
technically
known
as
an
ENDOCRINOPATHY
or
ENDOCRINOSIS.
Among the hundreds of endocrine diseases are:

Adrenal hyperplasia

Diabetes mellitus

Hypoglycemia

Insulinoma

Metabolic bone disease (osteoporosis, rickets and osteomalacia)

Pituitary tumors

Hyper or hypoparathyroidism

Disorders of sex development or intersex disorders

Ovarian failure

Testicular failure

Delayed puberty

Menstrual function or fertility disorders (amenorrhea)

Goiter, swelling in the thyroid gland due to iodine deficiency.

Hyper or hypothyroidism

Thyroiditis, is the inflammation of the thyroid gland.

Thyroid cancer

Tumours of the endocrine glands (Incidentaloma)
The ovaries of sexually-mature females secrete:

Estrogens

Progesterone.
Estrogens are primarily responsible for the conversion of girls into sexuallymature women, participate in the monthly preparation of the body for a possible
pregnancy and participate in pregnancy if it occurs
Estrogens also have non-reproductive effects such as:

They antagonize the effects of the parathyroid hormone, minimizing the
loss of calcium from bones and thus helping to keep bones strong.
73

They promote blood clotting.

Estrogen is needed for both sexes for normal bone development.
Progesterone has many effects in the body (menstrual cycle and pregnancy).
Many menopausal women elect to take a combination of estrogen and
progesterone as a hormone replacement therapy (HRT) for:

reduction in the unpleasant symptoms of the menopause

a reduction in the loss of calcium from bones and thus a reduction in
osteoporosis and the fractures that accompany it.
Some chemicals that find their way into the environment, such as DDT, DDE and
PCBs called ENVIRONMENTAL ESTROGENS because they can bind to the estrogen
receptors and mimic (weakly) the effects of the hormone. This may be responsible
for harmful effects such as cancer and low sperm counts
Secretion of testosterone increases sharply at puberty and is responsible for the
development
of
the
so-called
secondary
sexual
characteristics
of
men.
Testosterone is also essential for the production of sperm.
Steroid hormones can be grouped into five groups by the receptors to which they
bind:
6. Glucocorticoids
7. Mineralocorticoids
8. Androgens
9. Estrogens
10. Progestagens
Steroids are lipophilic, low-molecular weight compounds derived from cholesterol
that play a number of important physiological roles.
The steroid hormones are synthesized mainly by endocrine glands such as:

The gonads (testis and ovary)

The adrenals

The fetoplacental unit , during gestation
74
It has been recently discovered that, in addition to the endocrine glands, the CNS
is also able to form a number of biologically
active steroids directly from
cholesterol (the so-called "NEUROSTEROIDS").
These neurosteroids, however, are more likely to have "autocrine" or "paracrine"
functions rather than true endocrine effects.
Steroid metabolism is therefore important not only for the production of these
hormones, but also for the regulation of their cellular and physiological actions.
The parent compound from which all steroids are derived is cholesterol.
As shown before cholesterol is made up of three hexagonal carbon rings (A,B,C)
and a pentagonal carbon ring (D) to which a side-chain (carbons 20-27) is
attached (at position 17 of the polycyclic hydrocarbon). Two angular methyl
groups are also found at position 18 and 19.
Removal of part of the side-chain gives rise to C21-compounds of the pregnane
series (progestins and corticosteroids).
Total removal produces C19-steroids of the androstane series (including the
androgens), whereas loss of the 19-methyl group (usually after conversion of the
A-ring to a phenolic structure, hence the term "aromatization") yields the estrane
series, to which estrogens belong.
75
Steroid hormone biosynthesis:

The adrenals produce both androgens and corticosteroids (mineralo- and
glucocorticoids).

The ovaries can secrete estrogens and progestins.

The testis mainly androgens.
76
A general outline of the major biosynthetic pathways
Examples of some routes of steroid metabolism:
Cholesterol can be synthetized in all steroid-producing tissues from acetate, but
the main production sites are the liver, the skin and the intestinal mucosa.
77
78
Enzymes involved in steroid biosynthesis:

Desmolases (or lyases): these enzymes catalyse reactions which result in
the removal of parts of the original cholesterol side-chain. This requires a
cytochrome P-450, molecular oxygen (O2) and nicotinamide dinucleotide
phosphate, reduced form (NADPH) as a cofactor.

Hydroxylases: these enzymes are membrane-bound and are present either
in the mitochondrial or in the microsomal fraction of the cell. They also
require a cytochrome P-450, molecular oxygen and NADPH, as for lyases.

Hydroxysteroid
dehydrogenases
(oxido-reductases):
these
enzymes
catalyze reversible reactions and depend either on NADP(H) or NAD(H).
They are found both in the cell cytosol and in the microsomal fraction.

Aromatase: conversion of the A-ring to a phenolic structure (i.e. with a
phenolic HO-group at C-3), a process known as " aromatization ", involves
a complex sequence of hydroxylation reactions and loss of the angular C19 methyl group (10).
Questions:
I. Put circle around the most suitable answer
79
1. Serum is
(a) obtained by centrifuging the blood without clotting
(b) obtained by centrifuging the blood after clotting
(c) containing all blood cells and platelets
(d) containing fibrinogen
2. If the chemical signal acts on the same cell and then responds to, the
mechanism called
(a) autocrine signaling
(b) paracrine signaling
(c) endocrine signaling
3. If the Chemical signals diffuse into the area and interact with receptors on
nearby cells, the mechanism called
(a) autocrine signaling
(b) paracrine signaling
(c) endocrine signaling
4. If the chemicals are secreted into the blood and carried to the cells they act
upon, the mechanism called
(a) autocrine signaling
(b) paracrine signaling
(c) endocrine signaling
5. Hormone secretion can be stimulated or inhibited by:
(a) other hormones
(b) plasma concentrations of ions or nutrients
(c) environmental changes
(d) all answers are correct
II. Complete the following statements:
1. The endocrine system consists of several glands secrete hormones directly into
the blood rather than into a duct system.
80
2. Any chemical to be classified as hormone must be produced by an organ,
released in small amounts into the blood and transported to its specific function.
3. Endocrine hormones are secreted directly into the bloodstream while Exocrine
hormones are secreted directly into a duct and then either flow into the
bloodstream or from cell to cell.
4. Endocrine organs include the pituitary gland, thyroid gland, adrenal gland,
ovaries, testes and pancreas.
5. Amine hormones such as nor-epinephrine, epinephrine, and dopamine, are
derived from the amino acid tyrosine.
6. Steroid hormones can be grouped into five groups by the receptors to which
they bind:
1. Glucocorticoids
2. Mineralocorticoids
3. Androgens
4. Estrogens
5. Progestagens
III.
Draw the structure and name of the missing compounds in the
following metabolic pathway.
81
82
STEROID INACTIVATION AND CATABOLISM
Inactivation refers to the metabolic conversion of a
biologically active compound into an inactive one.
Inactivation can occur at various stages of hormone
action.
Peripheral inactivation (e.g. by liver enzymes) is required
to ensure steady-state levels of plasma hormones as
steroids are more or less continuously secreted into the
bloodstream.
Hormone inactivation can also occur in target tissues,
after the hormone has triggered the relevant biological
effects in order to ensure termination of hormone action.
The main site of peripheral steroid inactivation and
catabolism is the liver, but some catabolic activity also
occurs in the kidneys.
Inactive hormones are mainly eliminated as urinary
(mostly conjugated) metabolites. Usually, steroids are
eliminated once they have been inactivated (i.e., they are
not " recycled "). This elimination (e.g. as a urinary
excretion products) requires conversion to hydrophilic
83
compounds in order to ensure their solubility in biological
fluids at rather high concentrations.
Depending on the structure of the starting steroid, the
following reactions may be involved:

Reduction of a double bond at C-4 and reduction of
an oxo (keto) group at C-3 to a secondary alcoholic
group.

Reduction of an oxo group at C-20 to a secondary
alcoholic group.

Oxidation of a 17ß-hydroxyl group.

Further hydroxylations at various positions of the
steroid nucleus (e.g. 7-hydroxylation of 5a-reduced
androgens).
84

Conjugation
(sulphate
and/or
glucuronide
derivatives).
Formation of steroid conjugates
Conjugation (formation of hydrophilic molecules) is an
important step in steroid catabolism. Most excretory
products are in conjugated form.
Two major pathways are used:
(a) Formation of glucuronides:
This reaction requires uridine diphosphoglucuronic acid
(UDPGA) and a glucuronyl transferase. Glucuronic acid is
attached to a HO-group on the steroid molecule:
1. Activation of conjugating agent: Uronic acid +
UTP
to
uridine
diphosphoglucuronic
acid
(UDPGA).
Uronic acid + UTP
UDPGA
2. Active conjugating agent (UDPGA) + Substrate
(steroid)
glucuronyl transferase
Steroid-OH + UDPGA
Steroid glucuronide
(b) Formation of sulphates:
This conversion is catalyzed by sulpho kinases, which
occur in the cytosol of liver, testicular, adrenal and fetal
85
tissues. The substrates are steroids with an HO-group and
phosphoadenosine-5’-phosphosulphate (PAPS).
This is a three-step reaction which requires Mg++ ions:
Activation of congugatig agent (SO4) to PAPS
ATP sulphurylase
(1) SO4-- + ATP
Adenosine-5’-phosphosulphate
(APS) + pyrophosphate (P-Pi)
ATP kinase
(2) APS + ATP
Phosphoadenosine-5’-
phosphosulphate (PAPS)
Substrate + PAPS
ATP sulphokinase
(3) Steroid-OH + PAPS
Steroid-O-SO3-
+ 3’,5’-phosphoadenosine (PAP) + H+
86
Amine hormone metabolism
Epinephrine (Adrenaline)
Epinephrine (Adrenaline)
Epinephrine is a hormone and neurotransmitter. When
produced in the body it increases heart rate, dilates blood
vessels.
Epinephrine plays a central role in the short-term stress
reaction. It is released from the adrenal glands when
danger threatens or in an emergency, hence an Adrenaline
rush. Such triggers may be threatening, exciting, or
environmental stressor conditions such as high noise
levels, or bright light and high ambient temperature.
87
Mechanism of action
β-adrenergic receptors
Epinephrine's actions are mediated through adrenergic
receptors. Epinephrine is a non-selective agonist of all
adrenergic receptors. It activates α1, α2, β1, and β2
receptors to different extents.
Specific functions include:

It binds to α1 receptors of liver cells, which activate
inositol-phospholipid signaling pathway, signaling
the
phosphorylation
of
glycogen
synthase
and
phosphorylase kinase leading to the latter activating
another
enzyme—glycogen
phosphorylase—which
catalyzes breakdown of glycogen (glycogenolysis) so
as to release glucose to the bloodstream.
88
Simultaneously
protein
phosphatase-1
(PP1)
is
inactivated, as in the active state PP1 would reverse all
the previous phosphorylations.

Epinephrine also activates β-adrenergic receptors of
the liver and muscle cells, thereby activating the
adenylate cyclase signaling pathway, which will in
turn increase glycogenolysis.
β2 receptors are found primarily in skeletal muscle blood
vessels where they trigger vasodilation. However, αadrenergic receptors are found in most smooth muscles
and epinephrine triggers vasoconstriction in those vessels.
Norepinephrine Synthesis and Release
89
Norepinephrine (NE) is the primary neurotransmitter for
postganglionic
sympathetic
adrenergic
nerves.
It
is
synthesized inside the nerve axon, stored within vesicles,
then released by the nerve when an action potential
travels down the nerve. Below are the details for release
and synthesis of NE:
1. The amino acid tyrosine is transported into the
sympathetic nerve axon.
2. Tyrosine (Tyr) is converted to DOPA by tyrosine
hydroxylase (rate-limiting step for NE synthesis).
3. DOPA is converted to dopamine (DA) by DOPA
decarboxylase.
4. Dopamine is transported into vesicles then converted
to norepinephrine (NE) by dopamine β-hydroxylase
(DBH); transport into the vesicle can by blocked by
the drug reserpine.
5. An
action
potential
traveling
down
the
axon
depolarizes the membrane and causes calcium to
enter the axon.
6. Increased intracellular calcium causes the vesicles to
migrate to the axonal membrane and fuse with the
membrane, which permits the NE to diffuse out of the
vesicle into the extracellular (junctional) space. DBH,
and
depending
on
the
nerve
other
secondary
neurotransmitters (e.g., ATP), is released along with
the NE.
90
7. The NE binds to the postjunctional receptor and
stimulates the effector organ response.
Epinephrine Synthesis and Release
Epinephrine is synthesized from norepinephrine within the
adrenal medulla, which are small glands associated with
the kidneys. Preganglionic fibers sympathetic adrenergic
nerves synapse within the adrenals. Activation of these
fibers releases acetylcholine, which binds to postjunctional
nicotinic receptors in the tissue. This leads to stimulation
of NE synthesis within adenomedullary cells, but unlike
sympathetic neurons, there is an additional enzyme
(phenylethanolamine-N-methyltransferase) that adds a
methyl group to the NE molecule to form epinephrine. The
epinephrine is released into the blood perfusing the glands
and carried throughout the body.
Norepinephrine and Epinephrine Removal and Metabolism
The binding of NE to its receptor depends on the
concentration of NE in the vicinity of the receptor. If the
nerve stops releasing NE, then the NE concentration in the
junctional cleft will decrease and NE will leave the
receptor. There are several mechanisms by which the
norepinephrine
is
removed
from
the
intercellular
(junctional) space and therefore from the postjunctional
receptor:
91
1. Most (~90%) of the NE is transported back into the
nerve terminal by a neuronal reuptake transport
system. This transporter is blocked by cocaine;
therefore,
cocaine
concentrations
by
increases
blocking
its
junctional
NE
reuptake
and
subsequent metabolism. (This is a major mechanism
by which cocaine stimulates cardiac function and
raises blood pressure.)
2. Some of the junctional NE diffuses into capillaries
and is carried out of the tissue by the circulation.
Therefore, high levels of sympathetic activation in the
body increase the plasma concentration of NE and its
metabolites.
3. Some of the junctional NE is metabolized within the
extracellular space before reaching the capillaries.
4. A small amount of NE (~5%) is taken up by the
postjunctional
tissue
(termed
"extraneuronal
uptake") and metabolized.
92
From this point:
93
NE (and epinephrine) is metabolized by catechol-Omethytransferase (COMT) and monoamine oxidase (MAO).
The final product of these pathways is vanillylmandelic
acid (VMA). This final product, along with its precursors
normetanephrine and metanephrine, is measured in urine
and plasma in the diagnosis of pheochromocytoma, which
can cause severe hypertension and cardiac arrhythmias.
94
Acetylcholine Synthesis and Metabolism
Acetyl-CoA is synthesized from pyruvate by mitochondria
within cholinergic nerves. This acetyl-CoA combines with
choline that is transported into the nerve axon to form
acetylcholine (ACh). The enzyme responsible for this is
choline acetyltransferase. The newly formed ACh is then
transported into vesicles for storage and subsequent
release similar to what occurs for NE. After ACh is
released, it is rapidly degraded within the synapse by
acetylcholineesterase, to form acetate and choline.
Medical Uses:
Epinephrine is used as a drug to treat cardiac arrest ‫توقف‬
‫ القلب‬and other cardiac dysrhythmias
Due to its vasocontriction effects, epinephrine is the drug
of choice for treating anaphylaxis ‫فرط الحساسية‬. It is also
useful in treating sepsis ‫تعفن الدم‬
. Allergy patients
undergoing immunotherapy may receive an epinephrine
rinse before the allergen extract is administered, thus
reducing the immune response to the administered
allergen. It is also used as a bronchodilator for asthma.
Epinephrine can also be found in some brands of nasal
spray. Its use in this form is to open air passages.
95
96
Epinephrine is synthesized from norepinephrine in a
synthetic pathway shared by all catecholamines, including
L-dopa, dopamine, and epinephrine.
Epinephrine is synthesized via methylation of the primary
distal amine of norepinephrine by phenylethanolamine
N-methyltransferase (PNMT) in the cytosol of adrenergic
neurons and cells of the adrenal medulla (so-called
chromaffin cells).
cells
of
adrenal
PNMT is only found in the cytosol of
medullary
cells.
PNMT
uses
S-
adenosylmethionine (SAMe) as a cofactor to donate the
methyl group to norepinephrine, creating epinephrine.
For norepinephrine to be acted upon by PNMT in the
cytosol, it must first be shipped out of granules of the
97
chromaffin cells. This may occur via the catecholamine-H+
exchanger
VMAT1.
VMAT1
is
also
responsible
for
transporting newly synthesized epinephrine from the
cytosol back into chromaffin granules in preparation for
release.
Regulation
Epinephrine synthesis is solely under the control of the
Central Nervous System (CNS). Several levels of regulation
dominate epinephrine synthesis.
Adrenocorticotropic hormone (ACTH) and the sympathetic
nervous system stimulate the synthesis of epinephrine
precursors by enhancing the activity of enzymes involved
in catecholamine synthesis. The specific enzymes are
tyrosine hydroxylase in the synthesis of dopa and enzyme
dopamine-β-hydroxylase
in
the
release
of
synthesis
of
epinephrine
(and
norepinephrine.
Calcium
triggers
the
norepinephrine) into the bloodstream.
98
Dopamine
3,4-dihydroxyphenethylamine
Dopamine is a neurotransmitter occurring in a wide
variety
of
animals,
including
both
vertebrates
and
invertebrates.
In
the
brain,
this
phenethylamine
functions
as
a
neurotransmitter, activating the five types of dopamine
receptors — D1, D2, D3, D4, and D5, and their variants.
Biosynthesis of dopamine
99
As a member of the catecholamine family, dopamine is a
precursor to norepinephrine (noradrenaline) and then
epinephrine (adrenaline) in the biosynthetic pathways for
these neurotransmitters.
Dopamine is biosynthesized in the body (mainly by
nervous tissue and the medulla of the adrenal glands) first
by the hydroxylation of the amino acid L-tyrosine to LDOPA via the enzyme tyrosine 3-monooxygenase, also
known
as
tyrosine
hydroxylase,
and
then
by
the
decarboxylation of L-DOPA by aromatic L-amino acid
decarboxylase (which is often referred to as dopa
decarboxylase). In some neurons, dopamine is further
processed
into
norepinephrine
by
dopamine
beta-
hydroxylase.
100
In neurons, dopamine is packaged after synthesis into
vesicles, which are then released into the synapse in
response to a presynaptic action potential.
Therapeutic use:
Dopamine can be supplied as a medication that acts on the
sympathetic nervous system, producing effects such as
increased
heart rate and
blood
pressure.
However,
because dopamine cannot cross the blood-brain barrier,
dopamine given as a drug does not directly affect the
central nervous system. To increase the amount of
dopamine in the brains of patients with diseases such as
Parkinson's disease and dopa-responsive dystonia, LDOPA, which is the precursor of dopamine, can be given
because it can cross the blood-brain barrier.
101
Peptide hormone metabolism
Insulin metabolism
Insulin is a peptide hormone composed of 51 amino acids
and has a molecular weight of 5808 Da. It is produced in
the islets of Langerhans in the pancreas. The name comes
from the Latin insula for "island".
Insulin has extensive effects on metabolism and other
body functions. Insulin causes cells in the liver, muscle,
and fat tissue to take up glucose from the blood, storing it
as glycogen in the liver and muscle, and stopping use of
fat as an energy source.
When insulin is absent (or low), glucose is not taken up by
body cells, and the body begins to use fat as an energy
source, for example, by transfer of lipids from adipose
tissue to the liver for mobilization as an energy source.
When control of insulin levels fails, diabetes mellitus will
result. Insulin is used medically to treat some forms of
diabetes mellitus.
Patients with Type 1 diabetes mellitus depend on external
insulin (most commonly injected subcutaneously) for their
survival because the hormone is no longer produced
internally.
102
Patients with Type 2 diabetes mellitus
are insulin
resistant, and because of such resistance, may suffer from
a relative insulin deficiency. Some patients with Type 2
diabetes may eventually require insulin when other
medications
fail
to
control
blood
glucose
levels
adequately.
Protein structure of insulin:
Within vertebrates, the similarity of insulins is extremely
close. Bovine insulin differs from human in only three
amino acid residues, and pig insulin in one. Even insulin
from some species of fish is similar enough to human to be
clinically effective in humans.
Insulin is produced and stored in the body as a hexamer (a
unit of six insulin molecules), while the active form is the
monomer. The hexamer is an inactive form with long-term
stability which serves as a way to keep the highly reactive
insulin protected, yet readily available.
The hexamer-monomer conversion is one of the central
aspects of insulin formulations for injection. The hexamer
is far more stable than the monomer, which is desirable
for practical reasons, however the monomer is a much
faster reacting drug because diffusion rate is inversely
related to particle size. A fast reacting drug means that
insulin injections do not have to precede mealtimes by
103
hours, which in turn gives diabetics more flexibility in their
daily schedule.
Synthesis, physiological effects, and degradation of
insulin:
Synthesis:
Insulin is produced in the pancreas and released when any
of the several stimuli is detected. The stimuli include
ingested protein and glucose in the blood produced from
digested food. Carbohydrate produces glucose, although
not all types of carbohydrate produce glucose and thereby
increase blood glucose levels. In target cells, they initiate
a signal transduction, which has the effect of increasing
glucose uptake and storage. Finally, insulin is degraded,
terminating the response.
In mammals, insulin is synthesized in the pancreas within
the beta cells (β-cells) of the islets of Langerhans. One
million to three million islets of Langerhans (pancreatic
islets) form the endocrine part of the pancreas, which is
primarily an exocrine gland. The endocrine portion only
accounts for 2% of the total mass of the pancreas. Within
the islets of Langerhans, beta cells constitute 60–80% of
all the cells.
104
In beta cells, insulin is synthesized from the proinsulin
precursor molecule by the action of proteolytic enzymes,
known as prohormone convertases (PC1 and PC2), as well
as
the
exoprotease
carboxypeptidase
E.
These
modifications of proinsulin remove the center portion of
the molecule (ie, C-peptide), from the C- and N- terminal
ends of proinsulin. The remaining polypeptides (51 amino
acids in total), the B- and A- chains, are bound together by
disulfide bonds/disulphide bonds.
It has been shown that insulin and its related proteins, are
also produced inside the brain and that reduced levels of
these proteins are linked to Alzheimer's disease.
Release:
Beta cells in the islets of Langerhans release insulin in two
phases. The first phase insulin release is rapidly triggered
in response to increased blood glucose levels. The second
phase is a sustained, slow release of newly formed vesicles
that are triggered independently of sugar.
105
The description of first phase release is as follows:

Glucose enters the beta cells through the glucose
transporter GLUT2

Glucose goes into glycolysis and the respiratory cycle
where
multiple
high-energy
ATP
molecules
are
produced by oxidation

Dependent on ATP levels, and hence blood glucose
levels, the ATP-controlled potassium channels (K+)
close and the cell membrane depolarizes

On
depolarization,
voltage
controlled
calcium
channels (Ca2+) open and calcium flows into the cells
106

An increased calcium level causes activation of
phospholipase
C,
which
cleaves
the
membrane
phospholipid phosphatidyl inositol 4,5-bisphosphate
into inositol 1,4,5-triphosphate and diacylglycerol.

Inositol 1,4,5-triphosphate (IP3) binds to receptor
proteins in the membrane of endoplasmic reticulum
(ER). This allows the release of Ca2+ from the ER via
IP3 gated channels, and further raises the cell
concentration of calcium.

Significantly increased amounts of calcium in the
cells causes release of previously synthesized insulin,
which has been stored in secretory vesicles
This is the main mechanism for release of insulin. In
addition some insulin release takes place generally with
food intake, not just glucose or carbohydrate intake, and
the beta cells are also somewhat influenced by the
autonomic nervous system. The signaling mechanisms
controlling these linkages are not fully understood.
Other substances known to stimulate insulin release
include amino acids from ingested proteins, acetylcholine.
Three amino acids (alanine, glycine and arginine) act
similarly to glucose by altering the beta cell's membrane
potential. Acetylcholine triggers insulin release through
107
phospholipase
C,
while
the
last
acts
through
the
mechanism of adenylate cyclase.
When the glucose level comes down to the usual
physiologic value, insulin release from the beta cells slows
or stops. If blood glucose levels drop lower than this,
especially
to
dangerously
low
levels,
release
of
hyperglycemic hormones (most prominently glucagon
from Islet of Langerhans' alpha cells) forces release of
glucose into the blood from cellular stores, primarily liver
cell stores of glycogen. By increasing blood glucose, the
hyperglycemic
hormones
prevent
or
correct
life-
threatening hypoglycemia.
Release of insulin is strongly inhibited by the stress
hormone norepinephrine (noradrenaline), which leads to
increased blood glucose levels during stress.
108
Physiological effects
Effect of insulin on glucose uptake and metabolism
Insulin binds to its receptor (1) which in turn starts many
protein
activation
translocation
of
cascades
Glut-4
(2).
transporter
These
to
the
include:
plasma
membrane and influx of glucose (3), glycogen synthesis
(4), glycolysis (5) and fatty acid synthesis (6).
The actions of insulin on the global human metabolism
level include:

Control of cellular intake of certain substances, most
prominently glucose in muscle and adipose tissue.
109

Increase of DNA replication and protein synthesis via
control of amino acid uptake.

Modification of the activity of numerous enzymes.
Degradation
Once an insulin molecule has docked onto the receptor and
effected its action, it may be released back into the
extracellular environment, or it may be degraded by the
cell.
Degradation normally involves endocytosis of the insulinreceptor complex followed by the action of insulin
degrading enzyme. Most insulin molecules are degraded
by liver cells. It has been estimated that an insulin
molecule produced endogenously by the pancreatic beta
cells is degraded within approximately one hour after its
initial release into circulation (insulin half-life ~ 4-6
minutes).
Diseases and syndromes
There are several conditions in which insulin disturbance
is pathologic:

Diabetes mellitus – general term referring to all
states characterized by hyperglycemia.

Insulinoma - a tumor of pancreatic beta cells
producing excess of insulin or reactive hypoglycemia.
110

Metabolic
syndrome – leading to
hypertension,
obesity ‫ السمنة‬, Type 2 diabetes, and cardiovascular
disease (CVD) develop.

Polycystic ovary syndrome – a complex syndrome in
women in the reproductive years.
111
Leptin hormone
Leptin is a 16,000 Da protein hormone with important
effects
in
regulating
body
weight,
metabolism
and
reproductive function. It plays also a key role in regulating
energy intake and energy expenditure. It is one of the
most important adipose derived hormones.
Leptin is expressed predominantly by adipocytes, which
fits with the idea that body weight is sensed as the total
mass of fat in the body.
Smaller amounts of leptin are also secreted by cells in the
epithelium of the stomach and in the placenta.
Leptin receptors are highly expressed in areas of the
hypothalamus known to be important in regulating body
weight, as well as in T lymphocytes and vascular
endothelial cells.
Physiologic Effects of Leptin:
In addition to being a biomarker for body fat, serum leptin
levels also reflect individual energy balance.
Several studies have shown that fasting or following a
very low calorie diet lowers leptin levels. It might be that
on short term leptin is an indicator of energy balance. This
system is more sensitive to starvation than to overfeeding.
112
Leptin levels do not rise extensively after overfeeding. It
might be that the dynamics of leptin due to an acute
change in energy balance are related to appetite and
eventually to food intake.
Leptin appears to result from a combination of at least two
fundamental effects:

Decreased hunger and food consumption, mediated
at least in part by inhibition of neuropeptide Y
synthesis. Neuropeptide Y is a very potent stimulator
of feeding behavior.

Increased energy expenditure ‫انفاق‬, measured as
increased
oxygen
consumption,
higher
body
temperature and loss of adipose tissue mass.
Reproductive Function
It has long been known that starvation adversely affect
reproductive function. For example, very low body fat in
human females is often associated with cessation (stop) of
menstrual cycles, and similar effects are seen in starving
or nutritionally-deprived animals. Also, the onset of
puberty is known to correlate with body condition as well
as age.
Leptin concentrations are low in people and animals with
low body fat, and leptin appears to be a significant
regulator of reproductive function. These effects are
113
probably due in part to the ability of leptin to enhance
secretion of gonadotropin-releasing hormone, and thus
luteinizing and follicle-stimulating hormones from the
anterior pituitary.
Synthesis
In addition to white adipose tissue—the major source of
leptin—it can also be produced by brown adipose tissue,
placenta, ovaries, skeletal muscle, stomach, mammary
epithelial cells, bone marrow, pituitary and liver.
Control of Leptin Synthesis and Secretion
The amount of leptin expressed by adipocytes is well
correlated with the lipid content of the cells. Once
synthesized, leptin is secreted through a constitutive
pathway and not stored in the cell.
114
BLOOD BIOCHEMISTRY
Blood carries hormones and disease-fighting substances.
Blood picks up oxygen from the lungs and nutrients from
the gastrointestinal tract and carries them to cells
throughout the body for metabolism. It picks up carbon
dioxide and other wastes from those cells and transports
them to the lungs and excretory organs.
Blood consists of plasma, red and white cells (erythrocytes
and leukocytes), and platelets (thrombocytes).
Blood disorders include:
 Polycythemia (abnormal increase in the number of
circulating red blood cells).
 Anemia.
 Leukemia
 Hemophilia ‫نزيف وراثي‬
Technically, blood is a transport liquid pumped by the
heart to all parts of the body, after which it is returned to
the heart to repeat the process.
Blood is both a tissue and a fluid. It is a tissue because it is
a collection of similar specialized cells that serve particular
functions. These cells are suspended in a liquid matrix
(plasma), which makes the blood as fluid.
115
If blood flow ceases, death will occur within minutes
because of the effects of an unfavorable environment on
highly susceptible cells.
The constancy of the composition of the blood is made
possible by the circulation, which conveys blood through
the organs that regulate the concentrations of its
components.
In the lungs, blood acquires oxygen and releases carbon
dioxide transported from the tissues.
Nutrient
substances
derived
from
food
reach
the
bloodstream after absorption by the gastrointestinal tract.
Glands of the endocrine system release their secretions
into the blood, which transports these hormones to the
tissues in which they exert their effects.
Many substances are recycled through the blood; for
example, iron released during the destruction of old red
cells is conveyed by the plasma to sites of new red cell
production where it is reused.
Each of the numerous components of the blood is kept
within appropriate concentration limits by an effective
regulatory mechanism.
116
In
many
instances,
feedback
control
systems
are
operative; thus, a declining level of blood sugar (glucose)
leads to accelerated release of glucose into the blood so
that a potentially hazardous depletion of glucose does not
occur.
The red pigment hemoglobin, which contains iron, is found
in
all
vertebrates
including
humans,
hemoglobin
is
contained exclusively within the red cells (erythrocytes).
The pigment concentrations required would cause the
blood to be so viscous as to impede circulation.
Hemoglobin
It is the iron-containing oxygen-transport metalloprotein
in the red blood cells of vertebrates.
In mammals, the protein makes up about 97% of the red
blood cell's dry content, and around 35% of the total
content (including water).
Hemoglobin transports oxygen from the lungs to the rest
of the body (i.e. the tissues) where it releases the oxygen
for cell use. It also has a variety of other roles of gas
transport and effect-modulation which vary from species
to species, and are quite diverse in some invertebrates.
For example, hemoglobin transports CO2 back from the
tissues to the lungs.
117
Hemoglobin has an oxygen binding capacity of between
1.36 and 1.37 ml O2 per gram of hemoglobin, which
increases the total blood oxygen capacity seventyfold.
Synthesis
Hemoglobin (Hb) is synthesized in a complex series of
steps. The heme part is synthesized in a series of steps in
the mitochondria and the cytosol of immature red blood
cells, while the globin protein parts are synthesized by
ribosomes in the cytosol. Production of Hb continues in the
cell
throughout
its
early
development
from
the
proerythroblast to the reticulocyte in the bone marrow.
At this point, the nucleus is lost in mammalian red blood
cells. Even after the loss of the nucleus in mammals,
residual ribosomal RNA allows further synthesis of Hb until
the reticulocyte loses its RNA soon after entering the
vasculature (this hemoglobin-synthetic RNA in fact gives
the reticulocyte its reticulated appearance and name).
Structure
118
Heme group
Hemoglobin exhibits characteristics of both the tertiary
and quaternary structures of proteins. Most of the amino
acids in hemoglobin form alpha helices, connected by
short non-helical segments. Hydrogen bonds stabilize the
helical sections inside this protein, causing attractions
within the molecule, folding each polypeptide chain into a
specific shape.
Hemoglobin's quaternary structure comes from its four
subunits in roughly a tetrahedral arrangement.
In most humans, the hemoglobin molecule is an assembly
of
four
globular
protein
subunits.
Each
subunit
is
composed of a protein chain tightly associated with a nonprotein heme group. Each protein chain arranges into a set
119
of alpha-helix structural segments connected together in a
globin
fold
arrangement,
so
called
because
this
arrangement is the same folding motif used in other
heme/globin proteins such as myoglobin. This folding
pattern contains a pocket which strongly binds the heme
group.
A heme group consists of an iron (Fe) ion (charged atom)
held in a heterocyclic ring, known as a porphyrin. A
porphyrin ring consists of four pyrrole molecules cyclically
linked together with the iron ion bound in the centre.
That set
The iron ion, which is the site of oxygen binding (i.e.
transporting the oxygen and CO2 in the blood), coordinates
with the four nitrogens in the center of the ring, which all
lie in one plane. The iron is bound strongly to the globular
protein via the imidazole ring of the F8 histidine residue
below the porphyrin ring. A sixth position can reversibly
bind oxygen by a coordinate covalent bond, completing
the octahedral group of six ligands. Oxygen binds in an
"end-on bent" geometry where one oxygen atom binds Fe
and the other protrudes at an angle. When oxygen is not
bound, a very weakly bonded water molecule fills the site,
forming a distorted octahedron.
120
The iron ion may either be in the Fe2+ or Fe3+ state, but
ferrihemoglobin
(methemoglobin)
(Fe3+)
cannot
bind
oxygen. In binding, oxygen temporarily oxidizes (Fe2+) to
(Fe3+), so iron must exist in the +2 oxidation state to bind
oxygen.
The
enzyme
methemoglobin
reductase
reactivates
hemoglobin found in the inactive (Fe3+) state by reducing
the iron center.
Generally speaking, hemoglobin can be saturated with
oxygen molecules (oxyhemoglobin), or desaturated with
oxygen molecules (deoxyhemoglobin).
Oxyhemoglobin is formed during respiration when oxygen
binds to the heme component of the protein hemoglobin in
red blood cells. This process occurs in the pulmonary
capillaries adjacent to the alveoli of the lungs. The oxygen
then travels through the blood stream to be dropped off at
cells where it is utilized in aerobic glycolysis and in the
production
of
ATP
by
the
process
of
oxidative
phosphorylation. It does not, however, help to counteract
a decrease in blood pH. Ventilation, or breathing, may
reverse this condition by removal of carbon dioxide, thus
causing a shift up in pH.
121
Deoxyhemoglobin is the form of hemoglobin without the
bound oxygen. The absorption spectra of oxyhemoglobin
and deoxyhemoglobin differ. The oxyhemoglobin has
significantly lower absorption of the 660 nm wavelength
than deoxyhemoglobin, while at 940 nm its absorption is
slightly higher. This accounts for hemoglobin's red colour
and deoxyhemoglobin's blue colour. This difference is used
for measurement of the amount of oxygen in patient's
blood by an instrument called pulse oximeter.
Degradation in vertebrate animals
When red cells reach the end of their life due to aging or
defects, they are broken down, the hemoglobin molecule
is broken up and the iron gets recycled. When the
porphyrin ring is broken up, the fragments are normally
secreted in the bile by the liver.
This process also produces one molecule of carbon
monoxide for every molecule of heme degraded. This is
one of the few natural sources of carbon monoxide
production in the human body, and is responsible for the
normal blood levels of carbon monoxide even in people
breathing pure air.
The other major final product of heme degradation is
bilirubin. Increased levels of this chemical are detected in
the blood if red cells are being destroyed more rapidly
122
than usual. Improperly degraded hemoglobin protein or
hemoglobin that has been released from the blood cells
too rapidly can clog small blood vessels, especially the
delicate blood filtering vessels of the kidneys, causing
kidney damage.
123
Role of blood in disease
In sickle cell hemoglobin (HbS),
GLUTAMIC ACID is
mutated to VALINE.
This change allows the deoxygenated form of the
hemoglobin to stick to each other.
Hemoglobin deficiency can be caused by either one of the
followings:
1. decreased amount of hemoglobin molecules as in
anemia.
2. decreased ability of each molecule to bind oxygen at
the same partial pressure of oxygen.
Hemoglobinopathies (genetic defects resulting in abnormal
structure of the hemoglobin molecule) may cause both of above.
In any case, hemoglobin deficiency decreases blood
oxygen-carrying capacity.
Hemoglobin deficiency is generally strictly distinguished
from hypoxemia (defined as decreased partial pressure of
oxygen in blood).
124
Both of Hemoglobin deficiency and hypoxemia are causes
hypoxia (insufficient oxygen supply to tissues).
Other common causes of low hemoglobin include:
 loss of blood
 nutritional deficiency
 bone marrow problems
 chemotherapy
 kidney failure
 abnormal hemoglobin (such as that of sickle cell
disease)
High hemoglobin levels may be caused by:
 exposure to high altitudes
 smoking
 dehydration
 tumors
The ability of each hemoglobin molecule to carry oxygen is
normally modified by altered blood pH or CO2, causing an
altered oxygen-hemoglobin dissociation curve. However, it
can also be pathologically altered in e.g. carbon monoxide
poisoning.
Decrease of hemoglobin, with or without an absolute
decrease of red blood cells, leads to symptoms of anemia.
125
Anemia
has
many
different
causes,
although
iron
deficiency and its resultant iron deficiency anemia are the
most common causes in the Western world.
As absence of iron decreases heme synthesis, red blood
cells in iron deficiency anemia are HYPOCHROMIC (lacking
the red hemoglobin pigment) and MICROCYTIC (smaller
than normal).
In hemolysis (accelerated breakdown of red blood cells),
associated jaundice ‫ شعور بالضيق‬is caused by the
hemoglobin metabolite bilirubin,
and the circulating
hemoglobin can cause renal failure.
Some mutations in the globin chain are associated with
the hemoglobinopathies, such as sickle-cell disease and
thalassemia. Other mutations are referred to merely as
hemoglobin variants.
There is a group of genetic disorders, known as the
PORPHYRIAS that are characterized by errors in metabolic
pathways of heme synthesis.
GLYCOSYLATED HEMOGLOBIN is the form of hemoglobin
to which glucose is bound. Glucose stays attached to the
hemoglobin for the life of the red blood cells, which is
approximately 120 days. The levels of glycosylated
126
hemoglobin are tested to monitor the long-term control of
the chronic disease of type 2 diabetes mellitus (T2DM).
Poor control of T2DM results in high levels of glycosylated
hemoglobin in the red blood cells.
The normal reference range is approximately 4 %–5.9 %.
Though difficult to obtain, values less than 7 % are
recommended for people with T2DM.
Levels greater than 9 % are associated with poor control
of the glycosylated hemoglobin and levels greater than
12 % are associated with very poor control.
Diabetics who keep their glycosylated hemoglobin levels
close to 7 % have a much better chance of avoiding the
complications that can sometimes accompany diabetes
(than those whose levels are 8 % or higher).
127
URINE BIOCHEMISTRY
Urine is a liquid waste product of the body secreted by the
kidneys by a process of filtration from blood called
urination.
Cellular metabolism generates many waste compounds,
many rich in nitrogen, that require elimination from the
bloodstream.
This waste is eventually expelled from the body in a
process known as MICTURITION, the primary method for
excreting water-soluble chemicals from the body. These
chemicals can be detected and analyzed by urinalysis.
Renal physiology
Soluble wastes are excreted through urination process.
The
kidneys
extract
the
soluble
wastes
from
the
bloodstream, as well as excess water, sugars, and a
variety of other compounds.
The composition of urine is adjusted in the process of
REABSORPTION whereby certain solutes, such as glucose,
are reabsorbed back into the blood stream via carrier
128
molecules.
The
remaining
fluid
contains
high
concentrations of urea and other substances, including
toxins.
Urine is produced by a process of
1. Filtration
2. Reabsorption
3. Tubular secretion
COMPOSITION
Urine is an aqueous solution of approximately
 95% water
 5% metabolic wastes such as
1. urea
2. dissolved inorganic salts
3. organic
compounds,
including
proteins,
hormones, and a wide range of metabolites.
Fluid and materials being filtered by the kidneys, destined
(intended) to become urine, come from the blood or
interstitial fluid.
Except in cases of kidney or urinary tract infection, urine is
nearly odorless. Subsequent to elimination from the body,
urine can acquire strong odors due to bacterial action.
129
Urea structure
Ammonia is produced by breakdown of urea. Some
diseases alter the quantity and consistency of the urine,
such as sugar as a consequence of diabetes.
Characteristics
The typical color of urine is caused by the pigment
UROCHROME as well as the degradation products of
BILIRUBIN and UROBILIN. It can range from clear to a
dark amber, depending mostly upon the level of hydration
of the body, among other factors.
Unusual color
 Colorless- indicates over-hydration, which is usually
considered
much
healthier
than
dehydration.
Yellowing/light orange may be caused by removal of
excess B vitamins from the bloodstream.
 Certain medications such as rifampin and pyridium
can cause orange urine.
 Bloody urine is termed HEMATURIA, potentially a sign
of a bladder infection.
130
 Consumption of beets can cause urine to have a
pinkish tint (color); the condition is harmless and
temporary.
 Dark orange to brown urine can be a symptom of
jaundice, rhabdomyolysis, or Gilbert's syndrome.
 Black
or
dark-colored
urine
is
referred
to
as
melanuria and may be caused by a melanoma.
 Reddish or brown urine may be caused by porphyria.
Again, the consumption of beets can cause the urine
to have a harmless, temporary pink or reddish tint.
 Greenish color is usually a consequence of consuming
asparagus.
 Fluorescent yellow / greenish urine may be caused by
dietary supplemental vitamins, especially the B
vitamins.
 Dark yellow urine is usually indicative of dehydration.
Odor
The smell of urine can be affected by the consumption of
food. Eating asparagus is known to produce a strong odour
in human urine. This is due to the body's breakdown of
asparagusic acid. Other foods that contribute to odor
include curry, alcohol, coffee, turkey and onion.
Turbidity
131
Turbid urine may be a symptom of a bacterial infection,
but can also be due to crystallization of salts such as
calcium phosphate.
pH
The pH of urine is close to neutral (7) but can normally
vary between 6.5 and 7.4
In persons with HYPER URICOS URIA, acidic urine can
contribute to the formation of stones of uric acid in the
kidneys or bladder.
Volume
The amount of urine produced depends on numerous
factors
including
state
of
hydration,
activities,
environmental factors, size, and health. In adult humans
the average production is about 1 - 2 L/day.
Producing too much or too little urine needs medical
attention:
Polyuria is a condition of excessive production of urine (>
2.5 L/day), in contrast to oliguria where < 400 mL are
produced per day, or anuria with a production of < 100 mL
per day.
132
Chemical synapse
Chemical synapses are specialized junctions through which
neurons signal to each other and to non-neuronal cells such as
those in muscles or glands. Chemical synapses allow neurons to
form circuits within the central nervous system. They are crucial to
the biological computations that underlie perception and thought.
They allow the nervous system to connect to and control other
systems of the body.
At a chemical synapse, one neuron releases a neurotransmitter
into a small space (the synapse) that is adjacent to another
neuron. Neurotransmitters must then be cleared out of the
synapse efficiently so that the synapse can be ready to function
again as soon as possible.
The adult human brain is estimated to contain from 1014 to 5 ×
1014 (100-500 trillion) synapses. Every cubic millimeter of cerebral
cortex contains roughly a billion of them.
The word "synapse" comes from "synaptein", which Sir Charles
Scott Sherrington and colleagues coined from the Greek "syn-"
("together") and "haptein" ("to clasp"). Chemical synapses are not
the only type of biological synapse: electrical and immunological
synapses also exist. Without a qualifier, however, "synapse"
commonly means chemical synapse.
133
Illustration of the major elements in chemical synaptic transmission. An electrochemical wave
called an action potential travels along the axon of a neuron. When the wave reaches a
synapse, it provokes release of a puff of neurotransmitter molecules, which bind to chemical
receptor molecules located in the membrane of another neuron, on the opposite side of the
synapse.
134
Structure
Synapses are functional connections between neurons, or
between neurons and other types of cells. A typical neuron
gives rise to several thousand synapses, although there
are some types that make far fewer. Most synapses
connect axons to dendrites, but there are also other types
of connections, including axon-to-cell-body, axon-to-axon,
and dendrite-to-dendrite.
Synapses are generally too small to be recognizable using
a light microscope except as points where the membranes
of two cells appear to touch, but their cellular elements
can be visualized clearly using an electron microscope.
Chemical synapses pass information directionally from a
pre-synaptic cell to a post-synaptic cell and are therefore
asymmetric in structure and function. The pre-synaptic
terminal, or synaptic bouton, is a specialized area within
the
axon
of
the
pre-synaptic
cell
that
contains
neurotransmitters enclosed in small membrane-bound
spheres called synaptic vesicles. Synaptic vesicles are
docked at the pre-synaptic plasma membrane at regions
called active zones (AZ)
Immediately opposite is a region of the postsynaptic cell
containing
neurotransmitter
receptors;
for
synapses
between two neurons the postsynaptic region may be
found on the dendrites or cell body. Immediately behind
135
the postsynaptic membrane is an elaborate complex of
interlinked
proteins called
the
post-synaptic density
(PSD).
Proteins in the PSD are involved in anchoring and
trafficking neurotransmitter receptors and modulating the
activity of these receptors. The receptors and PSDs are
often found in specialized protrusions from the main
dendritic shaft called dendritic spines.
Between the pre- and postsynaptic cells is a gap about
20 nm wide called the synaptic cleft. The small volume of
the cleft allows neurotransmitter concentration to be
raised and lowered rapidly. The membranes of the two
adjacent cells are held together by cell adhesion proteins.
Signaling in chemical synapses
Here is a summary of the sequence of events that take
place in synaptic transmission from a pre-synaptic neuron
to a post-synaptic cell. Each step is explained in more
detail below. Note that with the exception of the final
step, the entire process may run only a few tenths of a
millisecond, in the fastest synapses.
1. The process begins with a wave of electrochemical
excitation called an action potential traveling along
the membrane of the pre-synaptic cell, until it
reaches the synapse.
136
2. The electrical depolarization of the membrane at the
synapse causes channels to open that are permeable
to calcium ions.
3. Calcium
ions
membrane,
flow
rapidly
through
increasing
the
presynaptic
the
calcium
concentration in the interior.
137
4. The high calcium concentration activates a set of
calcium-sensitive proteins attached to vesicles that
contain a neurotransmitter chemical.
5. These
proteins
change
shape,
causing
the
membranes of some "docked" vesicles to fuse with
the membrane of the presynaptic cell, thereby
opening
the
vesicles
and
dumping
their
neurotransmitter contents into the synaptic cleft, the
narrow space between the membranes of the preand post-synaptic cells.
6. The neurotransmitter diffuses within the cleft. Some
of it escapes, but some of it binds to chemical
receptor molecules located on the membrane of the
postsynaptic cell.
7. The binding of neurotransmitter causes the receptor
molecule to be activated in some way. Several types
of activation are possible, as described in more detail
below. In any case, this is the key step by which the
synaptic
process
affects
the
behavior
of
the
postsynaptic cell.
8. Due to thermal shaking, neurotransmitter molecules
eventually break loose from the receptors and drift
away.
9. The neurotransmitter is either reabsorbed by the
presynaptic cell, and then repackaged for future
release, or else it is broken down metabolically.
138
Neurotransmitter release
The release of a neurotransmitter is triggered by the
arrival of a nerve impulse (or action potential) and occurs
through an unusually rapid process of cellular secretion,
also known as exocytosis: Within the presynaptic nerve
terminal,
vesicles
containing
neurotransmitter
sit
"docked" and ready at the synaptic membrane. The
arriving action potential produces an influx of calcium ions
through
voltage-dependent,
calcium-selective
ion
channels at the down stroke of the action potential (tail
current). Calcium ions then trigger a biochemical cascade
which results in vesicles fusing with the pre-synaptic
membrane and releasing their contents to the synaptic
cleft within 180µsec of calcium entry. Vesicle fusion is
driven by the action of a set of proteins in the pre-synaptic
terminal known as SNAREs.
As calcium ions enter into the pre-synaptic neuron, they
bind with the proteins found within the membranes of the
synaptic vesicles that allow the vesicles to "dock."
Triggered by the binding of the calcium ions, the synaptic
vesicle proteins begin to move apart, resulting in the
creation of a fusion pore. The presence of the pore allows
for the release of neurotransmitter into the synapse.
The membrane added by this fusion is later retrieved by
endocytosis and recycled for the formation of fresh
neurotransmitter-filled vesicles.
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Receptor binding
Receptors on the opposite side of the synaptic gap bind
neurotransmitter molecules and respond by opening
nearby ion channels in the postsynaptic cell membrane,
causing ions to rush in or out and changing the local
transmembrane potential of the cell. The resulting change
in voltage is called a postsynaptic potential.
In general, the result is excitatory, in the case of
depolarizing
currents,
or
inhibitory in the case of
hyperpolarizing currents. Whether a synapse is excitatory
or inhibitory depends on what type(s) of ion channel
conduct the postsynaptic current display(s), which in turn
is a function of the type of receptors and neurotransmitter
employed at the synapse.
Termination
After a neurotransmitter molecule binds to a receptor
molecule, it does not stay bound forever: sooner or later it
is shaken loose by random temperature-related jiggling.
Once the neurotransmitter breaks loose, it can either drift
away, or bind again to another receptor molecule. The
pool
of
neurotransmitter
molecules
undergoing
this
binding-loosening cycle steadily diminishes, however.
Neurotransmitter molecules are typically removed in one
of two ways, depending on the type of synapse: either
they are taken up by the presynaptic cell (and then
140
processed for re-release during a later action potential), or
else they are broken down by special enzymes. The time
course of these "clearing" processes varies greatly for
different types of synapses, ranging from a few tenths of a
millisecond for the fastest, to several seconds for the
slowest.
Desensitization
Desensitization of the postsynaptic receptors is a decrease
in response to the same neurotransmitter stimulus. It
means that the strength of a synapse may in effect
diminish as a train of action potentials arrive in rapid
succession--a phenomenon that gives rise to the so-called
frequency dependence of synapses. The nervous system
exploits this property for computational purposes, and can
tune its synapses through such means as phosphorylation
of the proteins involved.
Effects of drugs
One of the most important features of chemical synapses
is that they are the site of action for the majority of
psychoactive drugs. Synapses are affected by drugs such
as curare, strychnine, cocaine, morphine, alcohol, LSD, and
countless others. These drugs have different effects on
synaptic function, and often are restricted to synapses
that use a specific neurotransmitter. For example, curare
is a poison which stops acetylcholine from depolarising the
141
post-synaptic membrane, causing paralysis. Strychnine
blocks the inhibitory effects of the neurotransmitter
glycine, which causes the body to pick up and react to
weaker and previously ignored stimuli, resulting in
uncontrollable muscle contractions. Morphine acts on
synapses that use endorphin neurotransmitters, and
alcohol
increases
the
inhibitory
effects
of
the
neurotransmitter GABA. LSD interferes with synapses that
use the neurotransmitter serotonin to cause hallucination.
Cocaine blocks reuptake of dopamine and therefore
increases its effects.
Neurotransmitters
They are packaged into synaptic vesicles that cluster
beneath the membrane on the presynaptic side of a
synapse, and are released into the synaptic cleft, where
they
bind
to
receptors
in
the
membrane
on
the
postsynaptic side of the synapse.
Release of neurotransmitters usually follows arrival of an
action potential at the synapse, but may follow graded
electrical potentials. Low level "baseline" release also
occurs without electrical stimulation.
Identifying neurotransmitters
Chemicals can be classified as neurotransmitters if they
meet the following conditions:
142

There are precursors and/or synthesis enzymes
located in the presynaptic side of the synapse.

The chemical is present in the presynaptic element.

It is available in sufficient quantity in the presynaptic
neuron to affect the postsynaptic neuron.

There are postsynaptic receptors and the chemical is
able to bind to them.

A biochemical mechanism for inactivation is present.
Types of neurotransmitters
There
are
many
neurotransmitters.
different
Dividing
ways
them
into
to
amino
classify
acids,
peptides, and monoamines is sufficient for some purposes.
Major neurotransmitters
1. Amino
acids:
glutamate,
aspartate,
serine,
-aminobutyric acid (GABA), glycine.
2. Monoamines:
dopamine
(DA),
norepinephrine
(noradrenaline; NE, NA), epinephrine (adrenaline),
serotonin (SE, 5-HT), melatonin
3. Others: acetylcholine (ACh), adenosine, anandamide,
histamine, nitric oxide, etc.
143
Not all neurotransmitters are equally important. By far the
most prevalent transmitter is glutamate, which is used at
well over 90% of the synapses in the human brain. The
next most prevalent is GABA, which is used at more than
90% of the synapses that don't use glutamate.
Note, however, that even though other transmitters are
used in far fewer synapses, they may be very important
functionally: the great majority of psychoactive drugs
exert their effects by altering the actions of some
neurotransmitter system, and the great majority of these
act through transmitters other than glutamate or GABA.
Addictive drugs such as cocaine, amphetamine, and
heroin, for example, exert their effects primarily on the
dopamine system.
Excitatory and inhibitory
Some neurotransmitters are commonly described as
"excitatory" or "inhibitory". The only direct effect of a
neurotransmitter is to activate one or more types of
receptors.
The effect on the postsynaptic cell depends, therefore,
entirely on the properties of those receptors. It happens
that for some neurotransmitters (for example, glutamate),
the most important receptors all have excitatory effects:
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that is, they increase the probability that the target cell
will fire an action potential.
For other neurotransmitters (such as GABA), the most
important receptors all have inhibitory effects. There are,
however, other neurotransmitters, such as acetylcholine,
for which both excitatory and inhibitory receptors exist;
and there are some types of receptors that activate
complex metabolic pathways in the postsynaptic cell to
produce effects that cannot appropriately be called either
excitatory or inhibitory. Thus, it is an oversimplification to
call
a
neurotransmitter
nevertheless
it
is
so
excitatory
convenient
or
to
inhibitory—
call
glutamate
excitatory and GABA inhibitory that this usage is seen very
frequently.
Actions
As
explained
above,
the
only
direct
action
of
a
neurotransmitter is to activate a receptor. Therefore, the
effects of a neurotransmitter system depend on the
connections of the neurons that use the transmitter, and
the
chemical
properties
of
the
receptors
that
the
transmitter binds to.
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Degradation and elimination
Neurotransmitter must be broken down once it reaches
the post-synaptic cell to prevent further excitatory or
inhibitory signal transduction. For example, acetylcholine,
(ACh) (an excitatory neurotransmitter), is broken down by
acetylcholinesterase (AChE).
Choline is taken up and recycled by the pre-synaptic
neuron to synthesize more ACh. Other neurotransmitters
such as dopamine are able to diffuse away from their
targeted synaptic junctions and are eliminated from the
body via the kidneys, or destroyed in the liver. Each
neurotransmitter has very specific degradation pathways
at regulatory points, which may be the target of the body's
own regulatory system or recreational drugs.
Acetylcholine
Acetylcholine (ACh) is a simple molecule synthesized from
choline and acetyl-CoA through the action of choline
acetyltransferase.
Neurons that synthesize and release ACh are termed
cholinergic neurons. When an action potential reaches the
terminal button of a presynaptic neuron a voltage-gated
calcium channel is opened.
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The influx of calcium ions, Ca2+, stimulates the exocytosis
of presynaptic vesicles containing ACh, which is thereby
released into the synaptic cleft. Once released, ACh must
be removed rapidly in order to allow repolarization to take
place; this step, hydrolysis, is carried out by the enzyme,
acetylcholinesterase.
The acetylcholinesterase found
at nerve endings is
anchored to the plasma membrane through a glycolipid.
Two main classes of ACh receptors have been identified on
the basis of their responsiveness to the toadstool alkaloid,
muscarine, and to nicotine, respectively: the muscarinic
receptors and the nicotinic receptors.
Both receptor classes are abundant in the human brain.
Nicotinic receptors are further divided into those found at
neuromuscular junctions and those found at neuronal
synapses. The activation of ACh receptors by the binding
of ACh leads to an influx of Na+ into the cell and an efflux
of K+, resulting in a depolarization of the postsynaptic
neuron and the initiation of a new action potential.
Cholinergic Agonists and Antagonists
Numerous compounds have been identified that act as
either agonists or antagonists of cholinergic neurons. The
principal action of cholinergic agonists is the excitation or
147
inhibition of autonomic effector cells that are innervated
by postganglionic parasympathetic neurons and as such
are refered to as parasympathomimetic agents. The
cholinergic agonists include choline esters (such as ACh
itself) as well as protein- or alkaloid-based compounds.
Several naturally occurring compounds have been shown
to affect cholinergic nerons, either positively or negatively.
The
responses
enhanced
by
of
cholinergic
administration
neurons
of
can
also
cholinesterase
be
(ChE)
inhibitors. ChE inhibitors have been used as components
of
nerve
gases
but
also
have
significant
medical
application in the treatment of disorders such as glaucoma
and myasthenia gravis as well as in terminating the effects
of neuromuscular blocking agents such as atropine.
Catecholamines
The
principal
catecholamines
are
norepinephrine,
epinephrine and dopamine. These compounds are formed
from phenylalanine and tyrosine.
Tyrosine is produced in the liver from phenylalanine
through the action of phenylalanine hydroxylase. The
tyrosine is then transported to catecholamine-secreting
neurons where a series of reactions convert it to
dopamine, to norepinephrine and finally to epinephrine
(see Specialized Products of Amino Acids).
148
Catecholamines
exhibit
peripheral
nervous
system
excitatory and inhibitory effects as well as actions in the
CNS such as respiratory stimulation and an increase in
psychomotor activity. The excitatory effects are exerted
upon smooth muscle cells of the vessels that supply blood
to the skin and mucous membranes.
Cardiac function is also subject to excitatory effects, which
lead to an increase in heart rate and in the force of
contraction. Inhibitory effects, by contrast, are exerted
upon smooth muscle cells in the wall of the gut, the
bronchial tree of the lungs, and the vessels that supply
blood to skeletal muscle.
In
addition
to
their
effects
as
neurotransmitters,
norepinephrine and epinephrine can influence the rate of
metabolism. This influence works both by modulating
endocrine function such as insulin secretion and by
increasing the rate of glycogenolysis and fatty acid
mobilization.
The catecholamines bind to two different classes of
-
-adrenergic receptors. The
catecholamines therefore are also known as adrenergic
neurotransmitters;
neurons
that
secrete
them
are
adrenergic neurons.
149
Norepinephrine-secreting neurons are noradrenergic. The
adrenergic receptors are classical serpentine receptors
that couple to intracellular G-proteins. Some of the
norepinephrine released from presynaptic noradrenergic
neurons recycled in the presynaptic neuron by a reuptake
mechanism.
Catecholamine Catabolism
Epinephrine
and
norepinephrine
are
catabolized
to
inactive compounds through the sequential actions of
catecholamine-O-methyltransferase
(COMT)
and
monoamine oxidase (MAO).
Compounds that inhibit the action of MAO have been
shown to have beneficial effects in the treatment of
clinical depression, even when tricyclic antidepressants
are ineffective.
The
utility
of
MAO
inhibitors
was
discovered
serendipitously when patients treated for tuberculosis
with isoniazid showed signs of an improvement in mood;
isoniazid was subsequently found to work by inhibiting
MAO.
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Serotonin
Serotonin (5-hydroxytryptamine, 5HT) is formed by the
hydroxylation and decarboxylation of tryptophan (see
Specialized Products of Amino Acids). The greatest
concentration
of
5HT
(90%)
is
found
in
the
enterochromaffin cells of the gastrointestinal tract. Most
of the remainder of the body's 5HT is found in platelets
and the CNS.
151
The effects of 5HT are felt most prominently in the
cardiovascular system, with additional effects in the
respiratory system and the intestines. Vasoconstriction is
a classic response to the administration of 5HT.
Neurons that secrete 5HT are termed serotonergic.
Following the release of 5HT, a portion is taken back up by
the presynaptic serotonergic neuron in a manner similar to
that of the reuptake of norepinephrine.
GABA
Several amino acids have distinct excitatory or inhibitory
effects
upon
the
nervous
system.
The
amino
acid
-aminobutyrate, also called 4-aminobutyrate,
(GABA)
is
a
well-known
inhibitor
of
presynaptic
transmission in the CNS, and also in the retina. The
formation of GABA occurs by the decarboxylation of
glutamate catalyzed by glutamate decarboxylase (GAD).
GAD is present in many nerve endings of the brain as well
-cells of the pancreas. Neurons that secrete
GABA are termed GABAergic.
GABA exerts its effects by binding to two distinct
receptors, GABA-A and GABA-B. The GABA-A receptors
form a Cl- channel. The binding of GABA to GABA-A
receptors increases the Cl- conductance of presynaptic
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neurons. The anxiolytic drugs of the benzodiazepine family
exert their soothing effects by potentiating the responses
of GABA-A receptors to GABA binding. The GABA-B
receptors are coupled to an intracellular G-protein and act
by increasing conductance of an associated K+ channel.
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