Biochemistry of Digestion, Absorption and
Detoxification
27/9-9/10/2011
1.
2.
3.
4.
5.
6.
Lecture 1: Introduction to the Biochemistry of Digestion, absorption and detoxification.
Lecture 2: Digestion and Digestive secretion from mouth and stomach
Lecture 3: Digestive secretion from pancreas and liver
Lecture 4: Detoxification in the liver
Lecture 5: Secretion and absorption in the small intestine
Lecture 6: Secretion and absorption in the large intestine
Aim and objective of the above six lectures is to understand:
The biochemistry and mechanism of digestion of food
1. The absorption of basic nutrients
2. The detoxification mechanism
References:
1. "Biochemistry" by Lubert Stryer
(textbook)
2. "Textbook of Biochemistry with Clinical Correlations" by T.M.Devlin
(additional reading)
3. "Lippincott's Illustrated Reviews in Biochemistry" by P.C.Champe, R.A.Harvey and
D.R.Ferrier
(additional reading)
4. "Harper's Biochemistry" by R.K.Murray, D.K.Granner, P.A. Mayes and V.W.Rodwell.
(additional reading)
Prof. Dr.H.D.El-Yassin1
2011
LECTURE 1
Tuesday 27/9/2011
Introduction to the Biochemistry of Digestion,
Absorption and Detoxification
Introduction
Digestion is the chemical breakdown of large food molecules into smaller
molecules that can be used by cells. The breakdown occurs when certain
specific enzymes are mixed with the food.
Enzymes involved in Digestion
polysaccharides
maltose
proteins
peptides
fats
fatty acids and glycerol
glucose
amino acids
Prof. Dr.H.D.El-Yassin2
2011
LECTURE 1
Tuesday 27/9/2011
Review of Food Chemistry
The diet of any animal contains hundreds if not thousands of different
molecules, but the bulk of the ingested nutrients are in the form of huge
macromolecules that cannot be absorbed into blood without first being
reduced to much simpler and smaller forms. The most important enzymatic
reaction in digestion of foodstuffs is hydrolysis - the breaking of a chemical
bond by the addition of a water molecule.
Proteins
Proteins are polymers of amino acids linked together by peptide bonds. Chain
length varies tremendously and many dietary proteins have been modified
after translation by addition of carbohydrate (glycoproteins) or lipid
(lipoprotein) moieties. Very short proteins, typically 3 to 10 amino acids in
length, are called peptides.
Lipids
Fatty acids are present in only small amounts in animal and plant tissues, but
are the building blocks of many important complex lipids. True fatty acids
possess a long hydrocarbon chain terminating in a carboxyl group. Nearly all
fatty acids have an even number of carbons and have chains between 14 and
22 carbons in length. The principle differences among the many fatty acids
are the length of the chain (usually 16 or 18 carbons) and the positions of
unsaturated or double bonds.
The most abundant storage form of fat in animals and plants, and hence the
most important dietary lipid, is triglyceride. A molecule of triglyceride is
composed of a molecule of glycerol in which each of the three carbons is
linked through an ester bond to a fatty acid. Triglycerides cannot be efficiently
absorbed, and are enzymatically digested by pancreatic lipase into a 2monoglyceride and two free fatty acids, all of which can be absorbed. Other
lipases hydrolyse a triglyceride into glycerol and three fatty acids.
Prof. Dr.H.D.El-Yassin3
2011
LECTURE 1
Tuesday 27/9/2011
Carbohydrates
1. Monosaccharides or simple sugars are either hexoses (6-carbon) like
glucose, galactose and fructose, or pentoses (5-carbon) like ribose.
These are the breakdown products of more complex carbohydrates and
can be efficiently absorbed across the wall of the digestive tube and
transported into blood.
2. Disaccharides are simply two monosaccharides linked together by a
glycosidic bond. The disaccharides most important in nutrition and
digestion are:




lactose or "milk sugar": glucose + galactose
sucrose or "table sugar": glucose + fructose
maltose: glucose + glucose
Oligosaccharides are relatively short chains of monosaccharides which
typically are intermediates in the breakdown of polysaccharides to
monosaccharides.
3. Polysaccharides :

Starch is a major plant storage form of glucose. It occurs in two forms:
alpha-amylose, in which the glucoses are linked together in straight
chains, and amylopectin, in which the glucose chains are highly
branched. Except for the branch points of amylopectin, the glucose
monomers in starch are linked via alpha(1->4) glycosidic bonds, which,
in the digestive tract of mammals, are hydrolyzed by amylases.

Cellulose is the other major plant carbohydrate. It is the major
constituent of plant cell walls, and more than half of the organic carbon
on earth is found in cellulose. Cellulose is composed on unbranched,
linear chains of D-glucose molecules, linked to one another by beta(1>4) glycosidic bonds, which no vertebrate has the capacity to
enzymatically digest.

Glycogen is the third large polymer of glucose and is the major animal
storage carbohydrate. Like starch, the glucose molecules in glycogen
are linked together by alpha(1->4) glycosidic bonds.
Prof. Dr.H.D.El-Yassin4
2011
LECTURE 1
Tuesday 27/9/2011
The process of digestion produces glucose, amino acids, glycerol, and fatty
acids (see above). The energy in glucose is used to produce ATP via the
reactions of glycolysis, cellular respiration, and the electron transport system
(see diagram below). The body uses amino acids to construct proteins.
Excess amino acids can be used to synthesize pyruvate, acetyl CoA, and
alpha ketogluterate, which enters the Krebs cycle. Glycerol and fatty acids can
be converted to pyruvate and Acetyl CoA and then enter cellular respiration.
Mouth
Chewing breaks food into smaller particles so that chemical digestion can
occur faster.
 Enzymes: Salivary amylase breaks starch (a polysaccharide) down to
maltose (a disaccharide).
 Bicarbonate ions in saliva act as buffers, maintaining a pH between 6.5
and 7.5.
 Mucins (mucous) lubricate and help hold chewed food together in a
clump called a bolus.
Prof. Dr.H.D.El-Yassin5
2011
LECTURE 1
Tuesday 27/9/2011
Stomach
The stomach stores up to 2 liters of food. Gastric glands within the stomach
produce secretions called gastric juice.
The muscular walls of the stomach contract vigorously to mix food with gastric
juice, producing a mixture called chyme.
Gastric juice
 Pepsinogen is converted to pepsin, which digests proteins. Pepsinogen
production is stimulated by the presence of gastrin in the blood.
 HCl
Hydrochloric acid (HCl) converts pepsinogen to pepsin which breaks down
proteins to peptides. HCl maintains a pH in the stomach of approximately 2.0.
It also dissolves food and kills microorganisms.
Mucous protects the stomach from HCl and pepsin.
Secretion of Gastric Juice: Gastrin is a hormone that stimulates the
stomach to secrete gastric juice.
Duodenum
The duodenum is the first part of the small intestine.
Chyme enters in tiny spurts. At this point, proteins and carbohydrates are only
partially digested and lipid digestion has not begun.
Pancreas
The pancreas acts as an exocrine gland by producing pancreatic juice which
empties into the small intestine via a duct.
The pancreas also acts as an endocrine gland to produce insulin.
 Pancreatic Juice
Pancreatic juice contains sodium bicarbonate which neutralizes the acidic
material from the stomach.
 Pancreatic amylase digests starch to maltose.
 Trypsin and Chymotrypsin digest proteins to peptides. Like pepsin
(produced in the stomach), they are specific for certain amino acids, not
all of them. They therefore produce peptides.
 Lipase digests fats to glycerol and fatty acids.
Prof. Dr.H.D.El-Yassin6
2011
LECTURE 1
Tuesday 27/9/2011
Liver
The liver produces bile which is stored in gallbladder and sent to the
duodenum through a duct.
Bile emulsifies fats (separates it into small droplets) so they can mix with
water and be acted upon by enzymes.
Other Functions of the Liver
 The liver detoxifies blood from intestines that it receives via the hepatic
portal vein.
 The liver stores glucose as glycogen (animal starch) and breaks down
glycogen to release glucose as needed. This storage-release process
maintains a constant glucose concentration in the blood (0.1%). If
glycogen and glucose run short, proteins can be converted to glucose.
 It produces blood proteins.
 It destroys old red blood cells and converts hemoglobin from these cells
to bilirubin and biliverdin which are components of bile.
 Ammonia produced by the digestion of proteins is converted to a less
toxic compound (urea) by the liver.
Hormones Involved in Digestion
1. Gastrin: The presence of food in the stomach stimulates specific
receptors which in turn stimulates endocrine cells in the stomach to
secrete the hormone gastrin into the circulatory system. Gastrin
stimulates the stomach to secrete gastric juice.
2. Secretin: Secretin is produced by cells of the duodenum.
It’s production is
stimulated by acid
chyme
from stomach.
It stimulates the
pancreas
to produce sodium
bicarbonate, which neutralizes the acidic chyme. It also stimulates the liver to
secrete bile.
3. CCK (cholecystokinin): CCK production is stimulated by the presence
of food in the duodenum. It stimulates the gallbladder to release bile and
the pancreas to produce pancreatic enzymes.
4. GIP (Gastric Inhibitory Peptide):Food in the duodenum stimulates
certain endocrine cells to produce GIP.
It has the opposite effects of gastrin; it inhibits gastric glands in the stomach
and it inhibits the mixing and churning movement of stomach muscles. This
slows the rate of stomach emptying when the duodenum contains food.
Prof. Dr.H.D.El-Yassin7
2011
LECTURE 1
Tuesday 27/9/2011
Small Intestine
The small intestine is approximately 3 m long. Like the stomach, it contains
numerous ridges and furrows. In addition, there are numerous projections
called villi that function to increase the surface area of the intestine. Individual
villus cells have microvilli which greatly increase absorptive surface area.
The total absorptive surface area is equivalent to 500 or 600 square meters.
Each villus contains blood vessels and a lacteal (lymph vessel).
Peptidases and maltase are embedded within the plasma membrane of the
microvilli.
Peptidases complete the digestion of peptides to amino acids.
Maltase completes the digestion of disaccharides.
Absorption:
The Large Intestine:
It functions in three processes:
 Recovery of water and electrolytes from ingesta: By the time ingesta
reaches the terminal ileum, roughly 90% of its water has been
absorbed, but considerable water and electrolytes like sodium and
chloride remain and must be recovered by absorption in the large gut.
 Formation and storage of feces: As ingesta is moved through the large
intestine, it is dehydrated, mixed with bacteria and mucus, and formed
into feces.
 Microbial fermentation: The large intestine of all species teems with
microbial life. Those microbes produce enzymes capable of digesting
many of molecules that to vertebrates are indigestible, cellulose being a
premier example. Absorption: water, sodium ions and chloride ions
 Secretion: bicarbonate ions and mucus
Prof. Dr.H.D.El-Yassin8
2011
LECTURE 1
Tuesday 27/9/2011
Summary of Digestive Enzymes
The digestive enzymes in the table below are summarized according to
type of food that they digest.
FOOD TYPE
ENZYME
SOURCE
PRODUCTS
CARBOHYDRATES
Salivary amylase
Pancreatic amylase
Maltase
Salivary glands
Pancreas
Small intestine
Maltose
Maltose
Glucose
PROTEINS
Pepsin
Trypsin
Peptidases
Stomach mucosa
Pancreas
Intestinal mucosa
Peptides
Peptides
Amino acids
FATS
Lipase
Pancreas
Fatty acids
and glycerol
The table below shows digestive enzymes grouped by source of the
enzyme.
SOURCE
ENZYME
FOOD
PRODUCT
MOUTH (salivary
glands)
Salivary amylase
Polysaccharides
Maltose
STOMACH
Pepsin
Proteins
Peptides
PANCREAS
Pancreatic
amylase
Trypsin
Lipase
Polysaccharides
Proteins
Fats
Maltose
Peptides
Fatty acids
and
glycerol
SMALL INTESTINE
Maltase
Peptidases
Maltose
Peptides
Glucose
Amino
acids
Prof. Dr.H.D.El-Yassin9
2011
LECTURE 2
Thursday 29/9/2011
Digestion and Digestive Secretion from
Mouth and Stomach
The Mouth
Complex food substances taken by animals must be broken down
into simple, soluble and diffusible substances before they can be
absorbed into the body. In the mouth, salivary glands secrete αamylase, which digests starch into small segments of multiple
sugars and into the individual soluble sugars.
Salivary glands also secrete lysozyme, which kills bacteria but is not
classified as a digestive enzyme.
The Stomach
Foodstuffs entering the stomach have been, crushed and reduced in
size by mastication, with saliva. The stomach provides four basic
functions that assist in the early stages of digestion and prepare the
ingesta for further processing in the small intestine:
1. It serves as a short-term storage reservoir, allowing a rather
large meal to be consumed quickly and dealt with over an extended
period.
2. It is in the stomach that substantial chemical and enzymatic
digestion is initiated, particularly of proteins.
3. Vigorous contractions of gastric smooth muscle mix and grind
foodstuffs with gastric secretions, resulting in liquefaction of
food, a prerequisite for delivery of the ingesta to the small intestine.
4. As food is liquefied in the stomach, it is slowly released into
the small intestine for further processing.
If the lining of the stomach is examined with a hand lens, one can
see that it is covered with numerous small holes. These are the
openings of gastric pits which extend into the mucosa as straight and
branched tubules, forming gastric glands.
Prof. Dr.H.D.El-Yassin
10
2011
LECTURE 2
Thursday 29/9/2011
Four major types of secretory epithelial cells cover the surface of the
stomach and extend down into gastric pits and glands:
1. Mucous cells: secrete an alkaline mucus that protects the
2. epithelium against shear stress and acid
3. Parietal cells: secrete hydrochloric acid.
4. Chief cells: secrete pepsin, a proteolytic enzyme
5. G cells: secrete the hormone gastrin
Prof. Dr.H.D.El-Yassin
11
2011
LECTURE 2
Thursday 29/9/2011
Gastric secretions
1. Mucosal Protection
Mucus layer on gastric surface forms a mucosal barrier to damage
against several forms of potential injury to the gastric mucosa.
1. A gel 0.2mm thick; 80% CHO; 20% protein
2. Secreted by neck cells, surface epithelium
3. Can be cleaved by pepsin, so continual production is required
4. Release is stimulated by acetylcholine from nerve endings
5. Also rich in bicarbonate
a. HCO3- content creates a "micro-environment" around surface
cells to prevent acid damage
b. HCO3- secretion is inhibited by adrenergic input (prominent in
stress)
2. Acid Secretion
Hydrochloric acid is secreted from parietal cells into the lumen where
it establishes an extremely acidic environment. This acid is important
for activation of pepsinogen and inactivation of ingested
microorganisms such as bacteria.
2.1. Function of Gastric acid
1. To kill micro-organisms: (but H. pylori survives by making ammonia
(basic) from urea using urease).
2. to provide the optimal pH for pepsin action
3. to activate pepsinogens (cleaved to form pepsin)
4. Facilitating absorption of iron by converting colloidal iron into ionic
form.
5. stimulating duodenum to liberate secretin
6. breaks down connective tissue in food
Prof. Dr.H.D.El-Yassin
12
2011
LECTURE 2
Thursday 29/9/2011
2.2.Mechanism of gastric acid secretion
The hydrogen ion concentration in parietal cell secretions is roughly
3 million fold higher than in blood, HC1 at a concentration of roughly
160 mM (equivalent to a pH of 0.8). And chloride is secreted against
both a concentration and electric gradient. Thus, the ability of the
parietal cell to secrete acid is dependent on active transport.
Acid secretion mechanisms in the parietal cell
The key player in acid secretion is a H+/K+ ATPase or "proton pump"
located in the cannalicular membrane. This ATPase is magnesiumdependent.
Prof. Dr.H.D.El-Yassin
13
2011
LECTURE 2
Thursday 29/9/2011
The H+/K+ ATPase
The parietal cells in the stomach use this pump to secrete gastric juice.
These cells transport protons (H+) from a concentration of about 4 x 10-8
M within the cell to a concentration of about 0.15 M in the gastric juice
(giving it a pH close to 1). Small wonder that parietal cells are stuffed
with mitochondria and uses huge amounts of energy as they carry out
this three-million fold concentration of protons.
The current model for explaining acid secretion is as follows:
• Hydrogen ions are generated within the parietal cell from dissociation
of water. The hydroxyl ions formed in this process rapidly combine with
carbon dioxide to form bicarbonate ion, a reaction cataylzed by carbonic
anhydrase.
• Bicarbonate is transported out of the basolateral membrane in
exchange for chloride. The outflow of bicarbonate into blood results in a
slight elevation of blood pH known as the "alkaline tide". This process
serves to maintain intracellular pH in the parietal cell.
• Chloride and potassium ions are transported into the lumen of the
cannaliculus by conductance channels, and such is necessary for
secretion of acid.
• Hydrogen ion is pumped out of the cell, into the lumen, in exchange
for potassium through the action of the proton pump; potassium is thus
effectively recycled.
Accumulation of osmotically-active hydrogen ion in the cannaliculus
generates an osmotic gradient across the membrane that results in
outward diffusion of water - the resulting gastric juice is 155 mM
HC1 and 15 mM KC1 with a small amount of NaCl.
Prof. Dr.H.D.El-Yassin
14
2011
LECTURE 2
Thursday 29/9/2011
2.3. Control of gastric acid secretion
Parietal cells bear receptors for three stimulators of acid secretion,
reflecting a neural, paracrine and endocrine control:
 ACETYLCHOLINE
o
o
o
o

released from cholinergic nerve fibres
binds to (M3) receptor on cell surface
opens Ca++ channels in apical surface
promotes release of Ca++ from intracellular stores
GASTRIN
o binds to CCK-B receptor on cell surface
o releases intracellular Ca++

HISTAMINE
o released from mast cells
o binds to parietal cell surface receptor
o activates adenyl cyclase (increases cyclic AMP, an
intracellular messenger)
Prof. Dr.H.D.El-Yassin
15
2011
LECTURE 2
Thursday 29/9/2011
Histamine's effect on the parietal cell is to activate adenylate cyclase, leading
to elevation of intracellular cyclic AMP concentrations and activation of protein
kinase A (PKA). One effect of PKA activation is phosphorylation of
cytoskeletal proteins involved in transport of the H+/K+ ATPase from
cytoplasm to plasma membrane. Binding of acetylcholine and gastrin both
result in elevation of intracellular calcium concentrations.
INHIBITORY CONTROL
• acid at less than pH 2 is a direct inhibitor of acid release
• acid in duodenum releases secretin which inhibits gastric secretion
• fatty acids, peptides stimulate release of GIF (gastric inhibitory
polypeptide) and CCK (cholecystokinin)
Several additional mediators have been shown to result in gastric acid
secretion when infused into animals and people, including e.g. calcium.
Calcium simulates gastrin release. lt is unclear whether these molecules
have a significant physiologic role in parietal cell function.
Alkaline tide during gastric secretion: Owing to secreation of a lage
amount of H+ as HCl, there is surplus of OH- in the parietal cell
which is taken up not only by the CO2 to form HCO3- but also by
other buffer systems of parietal cell initially and later by those of
plasma.
HPO 42
H 2 PO4
HCO3
H 2 CO3
Lactate
Lactic acid
All tend to increase on the side of the base i.e.:HPO4-2, HCO3and lactate, with the result that the pH of plasma is raised and an
alkaline urine is excreted for some hours following intake of food
and gastric secretion. This is known as the alkaline tide.
Prof. Dr.H.D.El-Yassin
16
2011
LECTURE 2
Thursday 29/9/2011
3. Proteases:
Pepsinogen, an inactive zymogen, is secreted into gastric juice from
both mucous cells and chief cells. Once secreted, pepsinogen is
activated by stomach acid into the active protease pepsin, which is
largely responsible for the stomach's ability to initiate digestion of
proteins, in young animals; chief cells also secrete chymosin (rennin),
a protease that coagulates milk protein allowing it to be retained more
than briefly in the stomach.
Pepsinogens and Pepsins
Pepsinogens are secreted in a form such that the activation
peptide assumes a compact structure that occludes the active site.
On exposure to an acidic (pH < 4) environment such as occurs in the
lumen of the stomach, the activation peptide unfolds, allowing the
active site to clip it off, yielding mature, catalytically active pepsin.
Optimal activity of pepsins is at pH of 1.8 to 3.5, depending on the
isoform, They are reversibly inactivated at about pH 5 and irreversibly
inactivated at pH 7 to 8.
The mature, active enzymes are roughly 325 amino acids with a
mass of approximately 35 kDa.
Pepsin initiates protein digestion by splitting certain amino acid linkages
in proteins (Cleaves preferentially C-terminal. It does not cleave at V, A or
G. Other residues may be cleaved, with very variable rates) to yield
peptide fragments.
17
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 2
Thursday 29/9/2011
Because pepsin can digest protein, it must be stored and secreted in an
inactive form so that it does not digest the cells in which it is formed.
In general, secretion of pepsinogens is coupled to secretion of acid
from the parietal cell. In vitro studies have demonstrated that
secretion is effectively stimulated by agents that stimulate either of two
conditions:
1.
Elevated intracellular levels of cyclic AMP: examples include
secretin, vasoactive intestinal peptide and epinephrine.
2.
Elevated intracellular calcium: the principal mediators
investigated include acetylcholine and peptides of the
gastrin/cholecystokinin family
Pepsin was discovered by Theodor Schwann in 1836. It was the first
animal enzyme to be discovered.
Chymosin (Rennin) and the Coagulation of Milk
Chymosin, known also as rennin, is a proteolytic enzyme synthesized
by chief cells in the stomach. Its role in digestion is to coagulate milk
in the stomach, a process of considerable importance in the very
young animal. If milk were not coagulated, it would rapidly flow
through the stomach and miss the opportunity for initial digestion of its
proteins.
Chymosin efficiently converts liquid milk to a semisolid like cottage
cheese, allowing it to be retained for longer periods in the stomach.
Chymosin secretion is maximal during the first few days after birth,
and declines thereafter, replaced in effect by secretion of pepsin as
the major gastric protease.
18
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 2
Thursday 29/9/2011
Chymosin is secreted as an inactive proenzyme called prochymosin
that, like pepsin, is activated on exposure to acid. Chymosin is also
similar to pepsin in being most active in acidic environments, which
makes sense considering its mission.
Aside from its physiologic role, chymosin is also a very important
industrial enzyme because it is widely used in cheese making.
4. Hormones
The principle hormone secreted from the gastric epithelium is gastrin,
a peptide that is important in control of acid secretion and gastric
motility. Gastrin is secreted by G-cells and released into the blood
where it travels to the parietal cells to stimulate acid secretion, and to
Enterochromaffin-Like (ECL) Cells to stimulate histamine secretion.
The net result of gastrin secretion is increased acid production
through two mechanisms:
1. Direct stimulation of the parietal cells,
2. Tropic action on parietal cells increasing their number.
N.B. in gastrinoma (Zollinger-Ellison syndrome) increased production of
gastrin causes hypersecretion of acid which is not subject to normal
inhibitory mechanisms.
A number of other enzymes are secreted by gastric epithelial cells,
including a lipase and gelatinase. One secretory product of
considerable importance in man is intrinsic factor, a glycoprotein
secreted by parietal cells that is necessary for intestinal absorption of
vitamin B12.
19
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 2
Thursday 29/9/2011
Intrinsic Factor
Intrinsic factor is a glycoprotein secreted by parietal (humans) of the
gastric mucosa. In humans, it has an important role in the absorption of
vitamin B12 (cobalamin) in the intestine, and failure to produce or
utilize intrinsic factor results in the condition pernicious anemia.—
as a result of an autoimmune attack against parietal cells —
Dietary vitamin B12 is released from ingested proteins in the
stomach through the action of pepsin and acid. It is rapidly bound
by one of two vitamin B12-binding proteins that are present in gastric
juice; at acid pH, these binding proteins have a greater affinity for
the vitamin than does intrinsic factor. In the small intestine
pancreatic proteases digest the binding proteins, releasing vitamin
B12 which then becomes bound to intrinsic factor. Finally, there are
receptors for intrinsic factor on the ileal mucosa which bind the
complex, allowing vitamin B12 to be absorbed into portal blood.
In all mammals, vitamin B12is necessary for maturation of erythrocytes,
and a deficiency of this vitamin leads to development of anemia.
Since efficient absorption of vitamin B12in humans depends on
intrinsic factor, diseases which decrease the secretion of intrinsic
factor (e.g. atrophic gastritis), interfere with cleavage of the binding
proteins (e.g. pancreatic exocrine insufficiency) or decrease binding
and absorption of the intrinsic factor-vitamin B12 complex (e.g. ileal
disease or resection) can result in this type of anemia.
Absorption in the Stomach
The stomach absorbs very few substances, although small amounts
of certain lipid-soluble compounds can be taken up, including
aspirin, other non-steroidal anti-inflammatory drugs, and ethanol.
Notably, these substances are also well-recognized causes of
gastric irritation and their use (especially overuse) is commonly
associated with development of gastritis and gastric ulcers.
20
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 3
Sunday 2/10/2011
Digestive Secretion from Pancreas and Liver
The Pancreas
The pancreas plays a vital role in accomplishing the followings:
• Acid must be quickly and efficiently neutralized to prevent damage to the
duodenal mucosa
• Macromolecular nutrients - proteins, fats and starch - must be broken down
much further before their constituents can be absorbed through the mucosa into
blood
Insufficient exocrine secretion by the pancreas leads to starvation, even if the body is
consuming adequate quantities of high quality food.
In addition to its role as an exocrine organ, the pancreas is also an endocrine organ.
The major hormones it secretes - insulin and glucagon - play a vital role in
carbohydrate and lipid metabolism.
Exocrine Secretions of the Pancreas
21
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 3
Sunday 2/10/2011
Pancreatic juice is composed of two secretory products critical to proper
digestion: digestive enzymes and bicarbonate. The enzymes are synthesized and
secreted from the exocrine acinar cells, whereas bicarbonate is secreted from the
epithelial cells lining small pancreatic ducts.
1. Digestive Enzymes:
a. Proteases
Digestion of proteins is initiated by pepsin in the stomach, but the bulk of protein
digestion is due to the pancreatic proteases. Several proteases are synthesized in
the pancreas and secreted into the lumen of the small intestine. The two
major
pancreatic
proteases
are
trypsin
and
chymotrypsin
both
are
endopeptidases, which are synthesized and packaged into secretory vesicles as
the inactive proenzymes trypsinogen and chymotrypsinogen.
 Trypsin: Cleaves peptide bonds on the C-terminal side of arginines and lysines.
 Chymotrypsin: Cuts on the C-terminal side of tyrosine, phenylalanine, and
tryptophan residues (the same bonds as pepsin, whose action ceases when
the NaHCOs raises the pH of the intestinal contents).
22
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 3
Sunday 2/10/2011
Once trypsinogen and chymotrypsinogen are released into the lumen of the small
intestine, they must be converted into their active forms in order to digest proteins,
Trypsinogen is activated by the enzyme enterokinase, which is embedded in the
intestinal mucosa.
Once trypsin is formed, it activates chymotrypsinogen, as well as additional molecules
of trypsinogen. The net result is a rather explosive appearance of active protease
once the pancreatic secretions reach the small intestine.
Trypsin and chymotrypsin digest proteins into peptides and peptides into smaller
peptides, but they cannot digest proteins and peptides to single amino acids. Some
of the other proteases from the pancreas, for instance carboxypeptidase
(exopeptidase) (This enzyme removes, one by one, the amino acids at the C-terminal of peptides).
But the final digestion of peptides into amino acids is largely the effect of peptidases in small intestinal
epithelial cells.
23
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 3
Sunday 2/10/2011
b. Pancreatic Lipase
The major form of dietary fat is triglyceride, or neutral lipid. A triglyceride molecule
cannot be directly absorbed across the intestinal mucosa. It must first be digested
into a 2-monoglyceride and two free fatty acids. The enzyme that performs
this hydrolysis is pancreatic lipase.
Sufficient quantities of bile salts must also be present in the lumen of the intestine
in order for lipase to efficiently digest dietary triglyceride and for the resulting
fatty acids and monoglyceride to be absorbed. This means that normal
digestion and absorption of dietary fat is critically dependent on secretions from
both the pancreas and liver.
Pancreatic lipase has recently been in the limelight as a target for management
of obesity. The drug orlistat (Xenical) is a pancreatic lipase inhibitor that
interferes with digestion of triglyceride and thereby reduces absorption of
dietary fat. Clinical trials support the contention that inhibiting lipase can lead to
significant reductions in body weight in some patients.
c. Amylase
The major dietary carbohydrate for many species is starch, a storage form of glucose in
plants. Amylase is the enzyme that hydrolyses starch to maltose (a glucose-glucose
disaccharide), as well as the trisaccharide maltotriose and small branchpoints fragments
called dextrins.
d. Other Pancreatic Enzymes
In addition to the proteases, lipase and amylase, the pancreas produces a host of
other digestive enzymes, including nucleases, gelatinase and elastase.
 Nucleases. These hydrolyze ingested nucleic acids (RNA and DNA) into their
component nucleotides.
 Elastase: Cuts peptide bonds next to small, uncharged side chains such as
those of alanine and serine.
24
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 3
Sunday 2/10/2011
2. Bicarbonate and Water
Epithelial cells in pancreatic ducts are the source of the bicarbonate and water
secreted by the pancreas. The mechanism of bicarbonate secretion is essentially the
same as for acid secretion parietal cells and is dependent on the enzyme
carbonic anhydrase. In pancreatic duct cells, the bicarbonate is secreted into the
lumen of the duct and hence into pancreatic juice.
Control of Pancreatic Exocrine Secretion
Secretion from the exocrine pancreas is regulated by both neural and endocrine controls.
During interdigestive periods, very little secretion takes place, but as food enters the
stomach and, a little later, chyme flows into the small intestine, pancreatic secretion is
strongly stimulated.
25
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 3
Sunday 2/10/2011
The most important stimuli for pancreatic secretion come from three hormones secreted
by the enteric endocrine system:
• Cholecystokinin: This hormone is synthesized and secreted by enteric endocrine
cells located in the duodenum. Its secretion is strongly stimulated by the presence of
partially digested proteins and fats in the small intestine. As chyme floods into the
small intestine, cholecystokinin is released into blood and binds to receptors on
pancreatic acinar cells, ordering them to secrete large quantities of digestive
enzymes. It also stimulates the gallbladder to release bile and the pancreas to
produce pancreatic enzymes.
• Secretin: This hormone is secreted in response to acid in the duodenum. The
predominant effect of secretin on the pancreas is to stimulate duct cells to
secrete water and bicarbonate. As soon as this occurs, the enzymes secreted by
the acinar cells are flushed out of the pancreas, through the pancreatic duct
into the duodenum. It also stimulates the liver to secrete bile.
•
Gastrin: This hormone, which is very similar to cholecystokinin, is secreted in
large amounts by the stomach in response to gastric distention and irritation, in
addition to stimulating acid secretion by the parietal cell; gastrin stimulates
pancreatic acinar cells to secrete digestive enzymes.
26
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 3
Sunday 2/10/2011
The Liver
The liver is the largest gland in the body and performs an astonishingly large
number of tasks that impact all body systems. One consequence of this complexity is
that hepatic disease has widespread effects on virtually all other organ systems.
The three fundamental roles of the liver are:
1. Vascular functions: including formation of lymph and hepatic phagocytic system.
2. Metabolic achievements in control of synthesis and utilization of
carbohydrates, lipids and proteins.
3. Secretory and excretory functions, particularly with respect to the synthesis
of secretion of bile.
The latter is the only one of the three that directly affects digestion - the liver,
through its biliary tract, secretes bile acids into the small intestine where they
assume a critical role in the digestion and absorption of dietary lipids.
27
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 3
Sunday 2/10/2011
Secretion of Bile and the Role of Bile Acids in Digestion
Bile is a complex fluid containing water, electrolytes and a battery of organic
molecules including bile acids, cholesterol, phospholipids and bilirubin that flows
through the biliary tract into the small intestine. There are two fundamentally
important functions of bile in all species:
 Bile contains bile acids, which are critical for digestion and absorption
of fats and fat-soluble vitamins in the small intestine.
 Many waste products are eliminated from the body by secretion into bile
and elimination in feces.
The secretion of bile can be considered to occur in two stages:
• Initially, hepatocytes secrete bile into canaliculi, from which it flows into bile
ducts. This hepatic bile contains large quantities of bile acids, cholesterol and
other organic molecules.
• As bile flows through the bile ducts it is modified by addition of a watery,
bicarbonate-rich secretion from ductal epithelial cells.
In humans: the gall bladder stores and concentrates bile during the fasting state.
Typically, bile is concentrated five-fold in the gall bladder by absorption of water and
small electrolytes - virtually all of the organic molecules are retained.
Secretion into bile is a major route for eliminating cholesterol. Free cholesterol is
virtually insoluble in aqueous solutions, but in bile, it is made soluble by bile acids
and lipids like lethicin. Gallstones (Cholelithiasis) most of which are composed
predominantly of cholesterol, result from processes that allow cholesterol to
precipitate from solution in bile.
28
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 3
Sunday 2/10/2011
Role of Bile Acids in Fat Digestion and Absorption
Bile salts are formed in the hepatocytes by a series of enzymatic steps that
convert cholesterol to cholic or chenodeoxycholic acids. The rate limiting step is
hydroxylation at the 7-alpha position. These reactions include the activity of 8
enzymes belonging to either monooxygenase or dehydrogenase enzyme
classes.
There are four major bile acids found in the body:
1. Cholic acid
2. Deoxycholic acid
3. Decholin
4. Chenodiol (Chenix), is used to dissolve gallstones in patients who cannot tolerate
surgery. Chenodiol is a natural bile acid that blocks production of cholesterol .
This action leads to gradual dissolution of cholesterol gallstones.
29
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 3
Sunday 2/10/2011
Synthesis of bile acids is one of the predominant mechanisms for the excretion of
excess cholesterol. However, the excretion of cholesterol in the form of bile acids is
insufficient to compensate for an excess dietary intake of cholesterol.
These acids are then conjugated with glycine or taurine and secreted as Na+ (or K+)
salts. Conjugation causes a decrease in their pKa values, making them more water
soluble.
The most abundant bile acids in human bile are chenodeoxycholic acid (45%) and
cholic acid (31%). These are referred to as the primary bile acids. Within the
intestines the primary bile acids are acted upon by bacteria and converted to the
secondary bile acids, identified as deoxycholate (from cholate) and lithocholate
(from chenodeoxycholate). Both primary and secondary bile acids are reabsorbed by
the intestines and delivered back to the liver via the portal circulation. Within the
liver the carboxyl group of primary and secondary bile acids is conjugated via an
amide bond to either glycine or taurine before their being resecreted into the bile
canaliculi.
These conjugation reactions yield glycoconjugates and tauroconjugates, respectively.
The bile canaliculi join with the bile ductless, which then form the bile ducts. Bile acids
are carried from the liver through these ducts to the gallbladder, where they are stored
for future use. The ultimate fate of bile acids is secretion into the intestine, where they
aid in the emulsification of dietary lipids. In the gut the glycine and taurine residues are
removed and the bile acids are either excreted (only a small percentage) or
reabsorbed by the gut and returned to the liver. This process of secretion from the liver
to the gallbladder, to the intestines and finally reabsorbtion is termed the enterohepatic
circulation.
30
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 3
Sunday 2/10/2011
Enterohepatic Recirculation
After the bile acids has been released into the small intestine via the bile duct to play an
integral role in the absorption of dietary lipids and lipid soluble vitamins. More than
90% of the bile salts are actively reabsorbed (by a sodium-dependent cotransport process) from the ileum into the hepatic-portal circulation from where they are
cleared and resecreted by the liver to once again be stored in the gall bladder. This
secretion/reabsorption cycle is called the Enterohepatic Circulation.
Systemic circulation: supplies nourishment to all of the tissue located throughout your
body, with the exception of the heart and lungs because they have their own systems.
Systemic circulation is a major part of the overall circulatory system.
Portal circulation: Blood from the gut and spleen flow to and through the liver before
returning to the right side of the heart. This is called the portal circulation and the large vein
through which blood is brought to the liver is called the portal vein.
The net effect of this enterohepatic recirculation is that each bile salt molecule is
reused about 20 times, often two or three times during a single digestive phase.
31
Prof. Dr. Hedef Dhafir El-Yassin 2011
LECTURE 3
Sunday 2/10/2011
Note: liver disease can dramatically alter this pattern of recirculation - for instance, sick
hepatocytes have decreased ability to extract bile acids from portal blood and damage to the
canalicular system can result in escape of bile acids into the systemic circulation. Assay of
systemic levels of bile acids is used clinically as a sensitive indicator of hepatic disease.
Bile acids are facial amphipathic, that is, they contain both hydrophobic
(lipid soluble) and polar (hydrophilic) faces. The cholesterol-derived portion of
a bile acid has one face that is hydrophobic (that with methyl groups) and one
that is hydrophilic (that with the hydroxyl groups); the amino acid conjugate is
polar and hydrophilic.
Their amphipathic nature enables bile acids to carry out two important
functions:
1. Emulsification of lipid aggregates: Bile acids have detergent action
on particles of dietary fat, which causes fat globules to break down
or be emulsified into minute, microscopic droplets. Emulsification is not
digestion per se, but is of importance because it greatly increases the
surface area of fat, making it available for digestion by lipases, which
cannot access the inside of lipid droplets.
2. Solubilization and transport of lipids in an aqueous environment: Bile
acids are lipid carriers and are able to solubilize many lipids by
forming micelles - aggregates of lipids such as fatty acids, cholesterol
and monoglycerides - that remain suspended in water. Bile acids are
also critical for transport and absorption of the fat-soluble vitamins.
Solubility properties of bile acids in aqueous solutions. Abbreviation: CMC, critical
micellar concentration
32
Prof.Dr H.D.El-Yassin
2011
LECTURE 3
Sunday 2/10/2011
Clinical Significance of Bile Acid Synthesis
Bile acids perform four physiologically significant functions:
1. Their synthesis and subsequent excretion in the feces represent the only
significant mechanism for the elimination of excess cholesterol.
2. Bile acids and phospholipids solubilize cholesterol in the bile, thereby
preventing the precipitation of cholesterol in the gallbladder.
3. They facilitate the digestion of dietary triacylglycerols by acting as
emulsifying agents that render fats accessible to pancreatic lipases.
4. They facilitate the intestinal absorption of fat-soluble vitamins.
Role of Bile Acids in Cholesterol Homeostasis
Hepatic synthesis of bile acids accounts for the majority of cholesterol
breakdown in the body. In humans, roughly 500 mg of cholesterol are converted
to bile acids and eliminated in bile every day. This route for elimination of
excess cholesterol is probably important in all animals, but particularly in
situations of massive cholesterol ingestion.
Interestingly, it has recently been demonstrated that bile acids participate in
cholesterol metabolism by functioning as hormones that alter the transcription of
the rate-limiting enzyme in cholesterol biosynthesis.
Pattern and Control of Bile Secretion
The flow of bile is lowest during fasting, and a majority of that is diverted into the
gallbladder for concentration. When chyme from an ingested meal enters the
small intestine, acid and partially digested fats and proteins stimulate secretion
of cholecystokinin and secretin. These enteric hormones have important effects
on pancreatic exocrine secretion. They are both also important for secretion and
flow of bile:
33
Prof.Dr H.D.El-Yassin
2011
LECTURE 3
Sunday 2/10/2011
1. Cholecystokinin: The name of this hormone describes its effect on the
biliary system - cholecysto = gallbladder and kinin = movement. The
most potent stimulus for release of cholecystokinin is the presence of fat in
the duodenum. Once released, it stimulates contractions of the gallbladder
and common bile duct, resulting in delivery of bile into the gut.
2. Secretin: This hormone is secreted in response to acid in the duodenum.
Its effect on the biliary system is very similar to what was seen in the
pancreas - it simulates biliary duct cells to secrete bicarbonate and water,
which expands the volume of bile and increases its flow out into the intestine.
34
Prof.Dr H.D.El-Yassin
2011
LECTURE 4
Tuesday 4/10/2011
Detoxification in the Liver
The liver is one of the most important organs in the body when it comes to
detoxifying or getting rid of foreign substances or toxins, especially from the
gut.
The liver detoxifies harmful substances by a complex series of chemical
reactions. The role of these various enzyme activities in the liver is to
convert fat soluble toxins into water soluble substances that can be
excreted in the urine or the bile depending on the particular characteristics
of the end product. Many of the toxic chemicals that enter the body are fatsoluble, which means they dissolve only in fatty or oily solutions and not in
water. This makes them difficult for the body to excrete. Fat soluble
chemicals have a high affinity for fat tissues and cell membranes, which are
composed of fatty acids and proteins. In these fatty tissues of the body,
toxins may be stored for years, being released during times of exercise,
stress or fasting.
The liver plays several roles in detoxification: it filters the blood to remove
large toxins, synthesizes and secretes bile full of cholesterol and other fatsoluble toxins, and enzymatically disassembles unwanted chemicals.
LECTURE 4
Tuesday 4/10/2011
35
Prof.Dr H.D.El-Yassin
2011
LECTURE 4
Tuesday 4/10/2011
This enzymatic process usually occurs in two steps referred to as:


phase I and
phase II.
Phase I either directly neutralizes a toxin, or modifies the toxic chemical to
form activated intermediates which are then neutralized by one of more of
the several phase II enzyme systems.
The level of exposure to environmental carcinogens varies widely, as does
the efficiency of the detoxification enzymes, particularly phase II. High
levels of exposure to carcinogens coupled with slow detoxification enzymes
significantly increases susceptibility to cancer.
36
Prof.Dr H.D.El-Yassin
2011
LECTURE 4
Tuesday 4/10/2011
Phase I Detoxification
This pathway converts a toxic chemical into a less harmful chemical. This is
achieved by various chemical reactions (such as oxidation, reduction and
hydrolysis), and during this process free radicals are produced which, if
excessive, can damage the liver cells. Antioxidants reduce the damage
caused by these free radicals. If antioxidants are lacking and toxin exposure
is high, toxic chemicals become far more dangerous. Some may be
converted from relatively harmless substances into potentially carcinogenic
substances.
The effects of exposure to toxins varies from individual to individual. Some
people are highly sensitive to different endogenous and exogenous toxins.
Others, because their bodies are more resilient and their livers can detoxify
more efficiently, aren't as sensitive.
CYTOCHROME P450 MONOOXYGENASE SYSTEM
Monooxygenase (mixed function oxidases) incorporate one atom from
molecular oxygen into a substrate (creating a hydroxyl group), with the
other atom being reduced to water. In the cytochrome P450
monooxygenase system NADPH provides the reducing equivalents
required by the series of reactions. This system performs different
functions in two separate locations in cells.The overall reaction catalyzed
by a cytochrome P450 enzyme is:
R-H + O2 + NADPH + H+
R-OH + H2O + NADP+
where R may be a steroid, drug or other chemical.
1. Mitochondrial System: the function of the mitochondrial
cytochrome P450 monooxygenase system is to participate in the
hydroxylation of steroids, a process that makes theses hydrophobic
compounds more water soluble. For example, in the steroid
hormone producing tissues, such as placenta, ovaries, testes and
adrenal cortex, it is used to hydroxylate intermediates in the
conversion of cholesterol to steroid hormones. The liver uses this
system in bile acid synthesis, and kidney uses it to hydroxylat
vitamin 25-hydroxycholecalciferol (vitamin D) to its biologically
active 1, 25-hydroxylated form.
37
Prof.Dr H.D.El-Yassin
2011
LECTURE 4
Tuesday 4/10/2011
2. Microsomal System: An extremely important function the function
of the microsomal cytochrome P450 monooxygenase system
found associated with the membranes of the endoplasmic
reticulum (particularly in the liver) is the detoxification of foreign
compounds (xenobiotics). Theses include numerous drugs such as
varied pollutants as petroleum products, carcinogens and
pesticides. The cytochrome P450 monooxygenase system can be
used to hydroxylate theses toxins again using NADPH as the
source of reducing equivalents. The purpose of these modifications
is:
a. it may itself activate or deactivate a drug,
b. or make a toxic compound more soluble, thus facilitating its
excretion in the urine or feces. Frequently, however, the new
hydroxyl group will serve as a site for conjugation with polar
compound, such as glucuronic acid, which will significantly
increase the compound's solubility.
Excessive amounts of toxic chemicals such as pesticides can disrupt the P450 enzyme system by causing hyper activity or what is called 'induction' of
this pathway. This will result in high levels of damaging free radicals being
produced. Substances that may cause hyperactivity of the P- 450 enzymes:
Caffeine, Alcohol, Dioxin, Saturated fats, Organophosphorus pesticides,
Paint fumes, Sulfonamides, Exhaust fumes, Barbiturates.
Transforming a toxin to a more chemically reactive form makes it more
easily metabolized by the phase II enzymes.
If the phase II detoxification systems are not working adequately, these
intermediates can cause substantial damage, including the initiation of
carcinogenic processes. Each enzyme works best in detoxifying certain
types of chemicals, but with considerable overlap in activity among the
enzymes.
The activity of the various cytochrome P450 enzymes varies significantly
from one individual to another, based on genetics, the individual's level of
exposure to chemical toxins, and his or her nutritional status. Since the
activity of cytochrome P450 varies so much, so does an individual's risk for
various diseases. This variability of cytochrome P450 enzymes is seen in
the variability of people's ability to detoxify the carcinogens found in
cigarette smoke and helps to explain why some people can smoke with only
modest damage to their lungs, while others develop lung cancer after only a
few decades of smoking.
LECTURE 4
Tuesday 4/10/2011
38
Prof.Dr H.D.El-Yassin
2011
LECTURE 4
Tuesday 4/10/2011
A significant side-effect of phase I detoxification is the production of free
radicals as the toxins are transformed--for each molecule of toxin
metabolized by phase I, one molecule of free radical is generated. Without
adequate free radical defenses, every time the liver neutralizes a toxin
exposure, it is damaged by the free radicals produced.
The most important antioxidant for neutralizing the free radicals produced in
phase I is glutathione. In the process of neutralizing free radicals, however,
glutathione (GSH) is oxidized to glutathione disulfide (GSSG). Glutathione
is required for one of the key phase II detoxification processes. When high
levels of toxin exposure produce so many free radicals from phase I
detoxification that the glutathione is depleted, the phase II processes
dependent upon glutathione stop, producing oxidative stress or liver
damage. The toxins transformed into activated intermediates by phase I are
substantially more reactive than the phase I toxins were. Unless quickly
removed from the body by phase II detoxification mechanisms, they can
cause widespread problems, especially carcinogenesis. Therefore, the rate
at which phase I produces activated intermediates must be balanced by the
rate at which phase II finishes their processing. People with a very active
phase I detoxification system coupled with slow or inactive phase II
enzymes are termed pathological detoxifiers. These people suffer unusually
severe toxic reactions to environmental poisons.
An efficient liver detoxification system is vital to health and in order to
support this process it is essential that many key nutrients are included in
the diet. Vitamins and minerals – particularly the B vitamins – play a major
role, acting as cofactors for many enzyme systems including those of liver
detoxification. Depletion of vitamin C may also impair the detoxification
process; vitamin C also prevents free radical formation. Vitamin E and
selenium are cofactors for glutathione peroxidase activity as well as being
powerful antioxidants. Other nutrients which play vital roles in the Phase II
pathway include amino acids glycine, cysteine, glutamine, methionine,
taurine, glutamic acid and aspartic acid. Grapefruit juice, which contains
naringenin, slows down Phase I enzyme activity.
As with all enzymes, the cytochrome P450s require several nutrients to
function, such as copper, magnesium, zinc and vitamin C.
39
Prof.Dr H.D.El-Yassin
2011
LECTURE 4
Tuesday 4/10/2011
Phase II Detoxification
This is called the conjugation pathway, whereby the liver cells add
another substance (eg. cysteine, glycine or a sulphur molecule) to a toxic
chemical or drug. This makes the toxin or drug water-soluble, so it can then
be excreted from the body via watery fluids such as bile or urine. Individual
xenobiotics and metabolites usually follow one or two distinct pathways.
There are essentially six phase II detoxification pathways:
1.
2.
3.
4.
5.
6.
Glutathione conjugation
Amino acid conjugation
Methylation
Sulfation
Acetylation
Glucuronidation
1. Glutathione conjugation
A primary phase II detoxification route is
conjugation with glutathione(glutamylcysteinylglycine), (a tripeptide
composed of three amino acids--cysteine,
glutamic acid, and glycine).
Glutathione conjugation produces watersoluble mercaptates which are excreted via
the kidneys.
The elimination of fat-soluble compounds,
especially heavy metals like mercury and lead, is dependent upon adequate
levels of glutathione, which in turn is dependent upon adequate levels of
methionine and cysteine. When increased levels of toxic compounds are
present, more methionine is utilized for cysteine and glutathione synthesis.
Methionine and cysteine have a protective effect on glutathione and prevent
depletion during toxic overload. This, in turn, protects the liver from the
damaging effects of toxic compounds and promotes their elimination.
If the availability of methionine is reduced, not only will the capability of
the liver to detoxify be impaired, but there will also be less glutathione
available to complex with foreign substances.
Studies have demonstrated that a deficiency of methionine can, in itself,
cause liver cancer without the presence of a carcinogen, and also that
the deficiency of methionine can permit a heavy metal to cause toxic
effects.
LECTURE 4
Tuesday 4/10/2011
40
Prof.Dr H.D.El-Yassin
2011
LECTURE 4
Tuesday 4/10/2011
Glutathione is also an important antioxidant. This combination of
detoxification and free radical protection, results in glutathione being one of
the most important anticarcinogens and antioxidants in our cells, which
means that a deficiency is cause of serious liver dysfunction and damage.
Exposure to high levels of toxins depletes glutathione faster than it can be
produced or absorbed from the diet. This results in increased susceptibility
to toxin-induced diseases, such as cancer, especially if phase I
detoxification system is highly active.
A deficiency can be induced either by diseases that increase the need for
glutathione, deficiencies of the nutrients needed for synthesis, or diseases
that inhibit its formation. Glutathione is available through two routes: diet
and synthesis. Dietary glutathione (found in fresh fruits and vegetables,
cooked fish, and meat) is absorbed well by the intestines and does not
appear to be affected by the digestive processes. Dietary glutathione in
foods appears to be efficiently absorbed into the blood.
2. Amino acid conjugation
Several amino acids (glyucine, taurine, glutamine, arginine, and ornithine)
are used to combine with and neutralize toxins. Of these, glycine is the
most commonly utilized in phase II amino acid detoxification.
Patients suffering from hepatitis, alcoholic liver disorders, carcinomas,
chronic arthritis, hypothyroidism, toxemia of pregnancy, and excessive
chemical exposure are commonly found to have a poorly functioning amino
acid conjugation system.
Even in normal adults, a wide variation exists in the activity of the glycine
conjugation pathway. This is due not only to genetic variation, but also to
the availability of glycine in the liver. Glycine, and the other amino acids
used for conjugation, become deficient on a low-protein diet and when
chronic exposure to toxins results in depletion.
3. Methylation
Methylation involves conjugating methyl groups to toxins.
Most of the methyl groups used for detoxification comes from Sadenosylmethionine (SAM). SAM is synthesized from the amino acid
methionine, a process which requires the nutrients choline, the active form
of B12 --methyl cobalamin, and the active form of folic acid --5methyltetrahydrofolate. Methionine is a major source of numerous sulfurcontaining compounds, including the amino acids cysteine and taurine.
41
Prof.Dr H.D.El-Yassin
2011
LECTURE 4
Tuesday 4/10/2011
4. Sulfation
Sulfation is the conjugation of toxins with sulfur-containing compounds. The
sulfation system is important for detoxifying several drugs, food additives,
and, especially, toxins from intestinal bacteria and the environment. In
addition to environmental toxins, sulfation is also used to detoxify some
normal body chemicals and is the main pathway for the elimination of
steroid and thyroid hormones. Since sulfation is also the primary route for
the elimination of neurotransmitters, dysfunction in this system may
contribute to the development of some nervous system disorders.
Many factors influence the activity of sulfate conjugation. For example, a
diet low in methionine and cysteine has been shown to reduce sulfation.
5. Acetylation
Conjugation of toxins with acetyl-CoA is the primary method by which the
body eliminates sulfa drugs. This system appears to be especially sensitive
to genetic variation, with those having a poor acetylation system being far
more susceptible to sulfa drugs and other antibiotics. While not much is
known about how to directly improve the activity of this system, it is known
that acetylation is dependent on thiamine, pantothenic acid, and vitamin C.
6. Glucuronidation
Glucuronidation, the combining of glucuronic acid with toxins, in Phase II
can be reversed by Beta glucuronidase enzymes produced by pathological
bacteria and cause toxins to be reabsorbed increasing toxicity. Many of the
commonly prescribed drugs are detoxified through this pathway. It also
helps to detoxify aspirin, menthol, vanillin (synthetic vanilla), food additives
such as benzoates, and some hormones.
Sulfoxidation
Sulfoxidation is the process by which the sulfur-containing molecules in
drugs and foods are metabolized. It is also the process by which the body
eliminates the sulfite food additives used to preserve many foods and
drugs. Normally, the enzyme sulfite oxidase (molybdenum
dependentenzyme) metabolizes sulfites to safer sulfates, which are then
excreted in the urine. Those with a poorly functioning sulfoxidation system,
however, have an increased ratio of sulfite to sulfate in their urine. Those
with a poorly functioning sulfoxidation detoxification pathway are more
sensitive to sulfur-containing drugs and foods containing sulfur or sulfite
additives.
Lecture 5
Thursday 6/10/2011
42
Prof.Dr H.D.El-Yassin
2011
Lecture 5
Thursday 6/10/2011
SECRETION AND ABSORPTION IN
THE SMALL INTESTINE
The small intestine is the portal for absorption of virtually all nutrients into blood.
Accomplishing this transport requires breaking down large supramolecular aggregates into
small molecules that can be transported across the epithelium.
By the time ingesta reaches the small intestine, foodstuffs have been mechanically broken
down and reduced to a liquid by mastication and grinding in the stomach. Once within the
small intestine, these macromolecular aggregates are exposed to pancreatic enzymes and bile,
which enables digestion to molecules capable or almost capable of being absorbed. The final
stages of digestion occur on the surface of the small intestinal epithelium.
The net effect of passage through the small intestine is absorption of most of the water and
electrolytes (sodium, chloride, potassium) and essentially all dietary organic molecules
(including glucose, amino acids and fatty acids). Through these activities, the small intestine
not only provides nutrients to the body, but plays a critical role in water and acid-base
balance.
Secretion in the Small Intestine
Large quantities of water are secreted into the lumen of the small intestine during the
digestive process. Almost all of this water is also reabsorbed in the small intestine.
Regardless of whether it is being secreted or absorbed, water flows across the
mucosa in response to osmotic gradients. In the case of secretion, two distinct
processes establish an osmotic gradient that pulls water into the lumen of the
intestine:
1. Increases in luminal osmotic pressure resulting from influx and
digestion of foodstuffs: The chyme that floods into the intestine from the
stomach typically is not hyperosmotic, but as its macromolecular
components are digested, osmolarlity of that solution increases
dramatically.
Starch, for example, is a huge molecule that contributes only a small amount to
osmotic pressure, but as it is digested, thousands of molecules of maltose are
generated, each of which is as osmotically active as the original starch molecule.
43
Prof.Dr H.D.El-Yassin
2011
Lecture 5
Thursday 6/10/2011
Thus, as digestion proceeds lumenal osmolarity increases dramatically and water is
pulled into the lumen. Then, as the osmotically active molecules (maltose, glucose,
amino acids) are absorbed, osmolarity of the intestinal contents decreases and water
can be absorbed.
2. Crypt cells actively secrete electrolytes, leading to water secretion: The
apical or lumenal membrane of crypt epithelial cells contain an ion
channel of immense medical significance - a cyclic AMP-dependent
chloride channel known also as the cystic fibrosis transmembrane
conductance regulator or CFTR. Mutations in the gene for this ion
channel result in the disease cystic fibrosis. This channel is responsible
for secretion of water by the following steps:
1. Chloride ions enter the crypt epithelial cell by cotransport with
sodium and potassium; sodium is pumped back out via sodium
pumps, and potassium is exported via a number of channels.
2. Activation of adenylyl cyclase by a number of so-called
secretagogues leads to generation of cyclic AMP.
3. Elevated intracellular concentrations of cAMP in crypt cells activate
the CFTR, resulting in secretion of chloride ions into the lumen.
4. Accumulation of negatively-charged chloride anions in the crypt
creates an electric potential that attracts sodium, pulling it into the
lumen, apparently across tight junctions - the net result is secretion
of NaCl.
5. Secretion of NaCl into the crypt creates an osmotic gradient across
the tight junction and water is drawn into the lumen.
44
Prof.Dr H.D.El-Yassin
2011
Lecture 5
Thursday 6/10/2011
CLINICAL CORRELATION
Cystic fibrosis
Abnormal activation of the cAMP-dependent chloride channel (CFTR) in crypt cells has
resulted in the deaths of millions upon millions of people. Several types of bacteria produce
toxins that strongly, often permanently, activate the adenylate cyclase in crypt enterocytes.
This leads to elevated levels of cAMP, causing the chloride channels to essentially become
stuck in the "open" position". The result is massive secretion of water that is manifest as
severe diarrhea. Cholera toxin, produced by cholera bacteria, is the best known example of
this phenomenon, but several other bacteria produce toxins that act similarly.
Absorption in the Small Intestine: General Mechanisms
Virtually all nutrients from the diet are absorbed into blood across the mucosa of the
small intestine.To remain viable, all cells are required to maintain a low intracellular
concentration of sodium. In polarized epithelial cells like enterocytes, low intracellular
sodium is maintained by a large number of Na+/K+ ATPases - so-called sodium
pumps - embedded in the basolateral membrane. These pumps export 3 sodium ions
from the cell in exchange for 2 potassium ions, thus establishing a gradient of both
charge and sodium concentration across the basolateral membrane.
Aside from the electrochemical gradient of sodium, several other concepts are
required to understand absorption in the small intestine. Also, dietary sources of
protein, carbohydrate and fat must all undergo the final stages of chemical digestion
just prior to absorption of, for example, amino acids, glucose and fatty acids.




Water and electrolytes
Carbohydrates, after digestion to monosaccharides
Proteins, after digestion to small peptides and amino acids
Neutral fat, after digestion to monoglyceride and free fatty acids
45
Prof.Dr H.D.El-Yassin
2011
Lecture 5
Thursday 6/10/2011
1) Absorption of Water and Electrolytes
The small intestine must absorb massive quantities of water. A normal person akes in roughly
1 to 2 liters of dietary fluid every day. On top of that, another 6 to 7 liters of fluid is received
by the small intestine daily as secretions from salivary glands, stomach, pancreas, liver and
the small intestine itself.
By the time the ingesta enters the large intestine, approximately 80% of this fluid has been
absorbed. Net movement of water across cell membranes always occurs by osmosis, and the
fundamental concept needed to understand absorption in the small gut is that absorption of
water is absolutely dependent on absorption of solutes, particularly sodium:




Sodium is absorbed into the cell by several mechanisms, but chief among
them is by co-transport with glucose and amino acids - this means that
efficient sodium absorption is dependent on absorption of these organic
solutes.
Absorbed sodium is rapidly exported from the cell via sodium pumps - when a
lot of sodium is entering the cell, a lot of sodium is pumped out of the cell,
which establishes a high osmolarity in the small intercellular spaces between
adjacent enterocytes.
Water diffuses in response to the osmotic gradient established by sodium - in
this case into the intercellular space. It seems that the bulk of the water
absorption is transcellular, but some also diffuses through the tight junctions.
Water, as well as sodium, then diffuses into capillary blood within the villus.
Water is thus absorbed into the intercellular space by diffusion down an osmotic gradient.
However, looking at the process as a whole, transport of water from lumen to blood is often
against an osmotic gradient - this is important because it means that the intestine can absorb
water into blood even when the osmolarity in the lumen is higher than osmolarity of blood.
2) Absorption of Monosaccharides
Monosaccharides, are only rarely found in normal diets. Rather, they are derived by
enzymatic digestion of more complex carbohydrates within the digestive tube.
Particularly important dietary carbohydrates include starch and disaccharides such as lactose
and sucrose. None of these molecules can be absorbed for the simple reason that they cannot
cross cell membranes unaided and, unlike the situation for monosaccharides, there are no
transporters to carry them across.
Brush Border Hydrolases Generate Monosaccharides
Polysaccharides and disaccharides must be digested to monosaccharides prior to absorption
and the key players in these processes are the brush border hydrolases, which include maltase,
lactase and sucrase. Dietary lactose and sucrose are "ready" for digestion by their respective
brush border enzymes. Starch, as discussed previously, is first digested to maltose by amylase
in pancreatic secretions and, saliva.
46
Prof.Dr H.D.El-Yassin
2011
Lecture 5
Thursday 6/10/2011
Dietary lactose and sucrose, and maltose derived from digestion of starch, diffuse in
the small intestinal lumen and come in contact with the surface of absorptive
epithelial cells covering the villi where they engage with brush border hydrolases:
 maltase cleaves maltose into two molecules of glucose
 lactase cleaves lactose into a glucose and a galactose
 sucrase cleaves sucrose into a glucose and a fructose
Glucose and galactose are taken into the enterocyte by cotransport with sodium
using the same transporter. Fructose enters the cell from the intestinal lumen via
facilitated diffusion through another transporter.
Absorption of Glucose and other Monosaccharides:
Transport across the Intestinal Epithelium
Absorption of glucose entails transport from the intestinal lumen, across the
epithelium and into blood. The transporter that carries glucose and galactose into the
enterocyte is the sodium-dependent hexose transporter, known more formally as
SGLUT-1. As the name indicates, this molecule transports both glucose and sodium
ion into the cell and in fact, will not transport either alone.
The essence of transport by the sodium-dependent hexose transporter involves a
series of conformational changes induced by binding and release of sodium and
glucose, and can be summarized as follows:
1. the transporter is initially oriented facing into the lumen - at this point it is capable of
binding sodium, but not glucose
2. sodium binds, inducing a conformational change that opens the glucose-binding
pocket
3. glucose binds and the transporter reorients in the membrane such that the pockets
holding sodium and glucose are moved inside the cell
4. sodium dissociates into the cytoplasm, causing glucose binding to destabilize
5. glucose dissociates into the cytoplasm and the unloaded transporter reorients back to
its original, outward-facing position
Fructose is not co-transported with sodium. Rather it enters the enterocyte by
another hexose transporter (GLUT5).
Once inside the enterocyte, glucose and sodium must be exported from the cell into
blood. Sodium is rapidly shuttled out in exchange for potassium by the battery of
sodium pumps on the basolateral membrane, this process maintains the
electrochemical gradient across the epithelium. The massive transport of sodium out
of the cell establishes the osmotic gradient responsible for absorption of water.
Digestion and absorption of carbohydrates
47
Prof.Dr H.D.El-Yassin
2011
Lecture 5
Thursday 6/10/2011
CLINICAL CORRELATION
Disaccharidase Deficiency
Intestinal disaccharidase deficiencies are encountered relatively frequently in humans.
Deficiency can be present in one enzyme or several enzymes for a variety of reasons (genetic
defect, physiological decline with age, or the result of "injuries" to the mucosa). Of the
disaccharidases, lactase is the most common enzyme with an absolute or relative deficiency,
which is experienced as milk intolerance. The consequences of an inability to hydrolyze
lactose in the upper small intestine are inability to absorb lactose and bacterial fermentation of
ingested lactose in the lower small intestine. Bacterial fermentation results in the production
of gas (distension of gut and flatulence) and osmotically active solutes that draw water into
the intestinal lumen (diarrhea). The lactose in yogurt has already been partially hydrolyzed
during the fermentation process of making yogurt. Thus individuals with lactase deficiency
can often tolerate yogurt better than unfermented dairy products. The enzyme lactase is
commercially available to pretreat milk so that the lactose is hydrolyzed.
3) Absorption of Amino Acids and Peptides
Dietary proteins are, with very few exceptions, not absorbed. Rather, they must be
digested into amino acids or di- and tripeptides first, through the action of gastric and
pancreatic proteases. The brush border of the small intestine is equipped with a
family of peptidases. Like lactase and maltase, these peptidases are integral
membrane proteins rather than soluble enzymes. They function to further the
hydrolysis of lumenal peptides, converting them to free amino acids and very small
peptides. These endproducts of digestion, formed on the surface of the enterocyte,
are ready for absorption.
a) Absorption of Amino Acids
The mechanism by which amino acids are absorbed is conceptually identical to that of
monosaccharides. The lumenal plasma membrane of the absorptive cell bears at least four
sodium-dependent amino acid transporters - one each for acidic, basic, neutral and amino
acids. These transporters bind amino acids only after binding sodium. The fully loaded
transporter then undergoes a conformational change that dumps sodium and the amino acid
into the cytoplasm, followed by its reorientation back to the original form.
Thus, absorption of amino acids is also absolutely dependent on the electrochemical gradient
of sodium across the epithelium. Further, absorption of amino acids, like that of
monosaccharides, contributes to generating the osmotic gradient that drives water absorption.
The basolateral membrane of the enterocyte contains additional transporters which export
amino acids from the cell into blood. These are not dependent on sodium gradients.
48
Prof.Dr H.D.El-Yassin
2011
Lecture 5
Thursday 6/10/2011
CLINICAL CORRELATION
Neutral Amino Aciduria (Hartnup Disease)
Transport functions, like enzymatic functions, are subject to modification by mutations. An
example of a genetic lesion in epithelial amino acid transport is Hartnup disease, named after
the family in which the disease entity resulting from the defect was first recognized. The
disease is characterized by the inability of renal and intestinal epithelial cells to absorb neutral
amino acids from the lumen. In the kidney, in which plasma amino acids reach the lumen of
the proximal tubule through the ultrafiltrate, the inability to reabsorb amino acids manifests
itself as excretion of amino acids in the urine (amino aciduria). The intestinal defect results in
malabsorption of free amino acids from the diet. Therefore the clinical symptoms of patients
with this disease are mainly those due to essential amino acid and nicotinamide deficiencies.
The pellagra-like features are explained by a deficiency of tryptophan, which serves as
precursor for nicotinamide. Investigations of patients with Hartnup disease revealed the
existence of intestinal transport systems for di- or tripeptides, which are different from the
ones for free amino acids. The genetic lesion does not affect transport of peptides, which
remains as a pathway for absorption of protein digestion products
b) Absorption of Peptides
There is virtually no absorption of peptides longer than four amino acids. However, there is
abundant absorption of di- and tripeptides in the small intestine. These small peptides are
absorbed into the small intestinal epithelial cell by cotransport with H+ ions via a transporter
called PepT1.
Once inside the enterocyte, the vast bulk of absorbed di- and tripeptides are digested into
amino acids by cytoplasmic peptidases and exported from the cell into blood. Only a very
small number of these small peptides enter blood intact.
c) Absorption of Intact Proteins
Absorption of intact proteins occurs only in a few circumstances. Normal" enterocytes do not
have transporters to carry proteins across the plasma membrane and they certainly cannot
permeate tight junctions.
One important exception is that for a very few days after birth, neonates have the ability to
absorb intact proteins. This ability, which is rapidly lost, is of immense importance because it
allows the newborn animal to acquire passive immunity by absorbing immunoglobulins in
colostral milk. The small intestine rapidly loses the capacity to absorb intact proteins.
4) Absorption of Lipids
The bulk of dietary lipid is neutral fat or triglyceride, composed of a glycerol backbone with
each carbon linked to a fatty acid. Additionally, most foodstuffs contain phospholipids, sterols
like cholesterol and many minor lipids, including fat-soluble vitamins. In order for the
triglyceride to be absorbed, two processes must occur:


Large aggregates of dietary triglyceride, which are virtually insoluble in an
aqueous environment, must be broken down physically and held in
suspension - a process called emulsification.
Triglyceride molecules must be enzymatically digested to yield monoglyceride
and fatty acids, both of which can efficiently diffuse into the enterocyte
The key players in these two transformations are bile salts and pancreatic lipase, both of
which are mixed with chyme and act in the lumen of the small intestine.
49
Prof.Dr H.D.El-Yassin
2011
Lecture 5
Thursday 6/10/2011
Digestion and absorption of lipids
Changes in physical state during triacylglycerol digestion.
Abbreviations: TG, triacylglycerol; DG, diacylglycerol; MG, monoacylglycerol; FA, fatty acid
CLINICAL CORRELATION
A--Lipoproteinemia
A-b-lipoproteinemia is an autosomal recessive disorder characterized by the absence of all
lipoproteins containing apo--lipoprotein, that is, chylomicrons, very low density lipoproteins (VLDLs),
and low density lipoproteins (LDLs). Serum cholesterol is extremely low. This defect is associated with
severe malabsorption of triacylglycerol and lipid-soluble vitamins (especially tocopherol and vitamin E)
and accumulation of apo B in enterocytes and hepatocytes. The defect does not appear to involve the
gene for apo B, but rather one of several proteins involved in processing of apo B in liver and intestinal
mucosa, or in assembly and secretion of triacylglycerol-rich lipoproteins, that is, chylomicrons and
VLDLs from these tissues, respectively.
50
Prof.Dr H.D.El-Yassin
2011
Lecture 5
Thursday 6/10/2011
5) Absorption of Minerals and Metals
The vast bulk of mineral absorption occurs in the small intestine. The best-studied
mechanisms of absorption are clearly for calcium and
iron, deficiencies of which are significant health
problems throughout the world.
a) Calcium
The quantity of calcium absorbed in the intestine is
controlled by how much calcium has been in the diet
during recent periods of time. Calcium is absorbed by
two distinct mechanisms:
1. Active, transcellular absorption occurs only in the
duodenum when calcium intake has been low. This
process involves import of calcium into the enterocyte, transport across the cell, and export
into extracellular fluid and blood. The rate limiting step in transcellular calcium absorption is
transport across the epithelial cell, which is greatly enhanced by the carrier protein calbindin,
the synthesis of which is totally dependent on vitamin D.
2. Passive, paracellular absorption occurs in the jejunum and ileum, and, to a much lesser
extent, in the colon when dietary calcium levels have been moderate or high. In this case,
ionized calcium diffuses through tight junctions into the basolateral spaces around
enterocytes, and hence into blood. Such transport depends on having higher concentrations of
free calcium in the intestinal lumen than in blood.
b) Phosphorus
Phosphorus is predominantly absorbed as inorganic phosphate in the upper small intestine.
Phosphate is transported into the epithelial cells by cotransport with sodium, and expression
of this (or these) transporters is enhanced by vitamin D.
c) Iron
Iron homeostasis is regulated at the level of intestinal
absorption, and it is important that adequate but not excessive
quantities of iron be absorbed from the diet. Inadequate
absorption can lead to iron-deficiency disorders such as
anemia. On the other hand, excessive iron is toxic because
mammals do not have a physiologic pathway for its
elimination.
51
Prof.Dr H.D.El-Yassin
2011
Lecture 5
Thursday 6/10/2011
Iron is absorbed by villus enterocytes in the proximal duodenum. Efficient absorption requires
an acidic environment.
Ferric iron (Fe+++) in the duodenal lumen is reduced to its ferrous form through the action of
a brush border ferrireductase. Iron is then co transported with a proton into the enterocyte via
the divalent metal transporter DMT-1. This transporter is not specific for iron, and also
transports many divalent metal ions.
Once inside the enterocyte, iron follows one of two major pathways:

Iron abundance states: iron within the enterocyte is trapped by incorporation
into ferritin and hence, not transported into blood. When the enterocyte dies
and is shed, this iron is lost.

Iron limiting states: iron is exported out of the enterocyte via a transporter
(ferroportin) located in the basolateral membrane. It then binds to the ironcarrier transferrin for transport throughout the body.
d) Copper
There appear to be two processes responsible for copper absorption:
i) a rapid, low capacity system and
ii) a slower, high capacity system, which may be similar to the two processes seen
with calcium absorption.
Many of the molecular details of copper absorption remain to be elucidated. Inactivating
mutations in the gene encoding an intracellular copper ATPase have been shown responsible
for the failure of intestinal copper absorption in Menkes disease.
A number of dietary factors have been shown to influence copper absorption. For example,
excessive dietary intake of either zinc or molybdenum can induce secondary copper
deficiency states.
e) Zinc
Zinc homeostasis is largely regulated by its uptake and loss through the small intestine.
Although a number of zinc transporters and binding proteins have been identified in villus
epithelial cells, a detailed picture of the molecules involved in zinc absorption is not yet in
hand.
A number of nutritional factors have been identified that modulate zinc absorption. Certain
animal proteins in the diet enhance zinc absorption. Phytates from dietary plant material
(including cereal grains, corn, rice) chelate zinc and inhibit its absorption. Subsistence on
phytate-rich diets is thought responsible for a considerable fraction of human zinc
deficiencies.
52
Prof.Dr H.D.El-Yassin
2011
Lecture 6
Sunday 9/10/2011
Secretion and Absorption in the Large
Intestine
The large intestine is the last attraction in digestive tube and the location of the
terminal phases of digestion. It functions in three processes:
 Recovery of water and electrolytes from ingesta: By the time ingesta
reaches the terminal ileum, roughly 90% of its water has been absorbed, but
considerable water and electrolytes like sodium and chloride remain and must
be recovered by absorption in the large gut.
 Formation and storage of feces:
 Microbial fermentation: The large intestine of all species teems with
microbial life. Those microbes produce enzymes capable of digesting many of
molecules that to vertebrates are indigestible, cellulose being a premier
example. The extent and benefit of fermentation also varies greatly among
species.
Absorption, Secretion and Formation of Feces in the Large Intestine
1. Absorption: water, sodium ions and chloride ions
2. Secretion: bicarbonate ions and mucus
Water, as always, is absorbed in response to an osmotic gradient. The mechanism
responsible for generating this osmotic pressure is essentially identical to what was
seen in the small intestine - sodium ions are transported from the lumen across the
epithelium by virtue of the epithelial cells having very active sodium pumps on their
basolateral membranes and a means of absorbing sodium through their lumenal
membranes. The colonic epithelium is actually more efficient at absorbing water than
the small intestine and sodium absorption in the colon is enhanced by the hormone
aldosterone.
Chloride is absorbed by exchange with bicarbonate. The resulting secretion of
bicarbonate ions into the lumen aids in neutralization of the acids generated by
microbial fermentation in the large gut.
53
Prof.Dr H.D.El-Yassin
2011
Lecture 6
Sunday 9/10/2011
Model for electrogenic NaCl absorption in the large intestine
This Na+ flux is electrogenic; that is, it is associated with an electrical current, and it
can be inhibited by the diuretic drug amiloride at micromolar concentrations
Microbial Fermentation
Fermentation is the enzymatic decomposition and utililization of foodstuffs,
particularly carbohydrates, by microbes
The large intestine does not produce its own digestive enzymes, but contains huge
numbers of bacteria which have the enzymes to digest and utilize many substrates.
In all animals, two processes are attributed to the microbial flora of the large
intestine:
1. Digestion of carbohydrates not digested in the small intestine
2. Synthesis of vitamin K and certain B vitamins
Cellulose is common constituent in the diet of many animals, including man, but no
mammalian cell is known to produce a cellulase. Several species of bacteria in the
large bowel synthesize cellulases and digest cellulose. Importantly, the major end
products of microbial digestion of cellulose and other carbohydrates are volatile fatty
acids, lactic acid, methane, hydrogen and carbon dioxide. Fermentation is thus the
major source of intestinal gas. Volatile fatty acids (acetic, proprionic and butyric
acids) generated from fermentation can be absorbed by diffusion in the colon.
Synthesis of vitamin K by colonic bacteria provides a valuable supplement to dietary
sources and makes clinical vitamin K deficiency rare. Similarly, formation of B
vitamins by the microbial flora in the large intestine is useful to many animals. They
are not absorbed in the large intestine, but are present in feces.
54
Prof.Dr H.D.El-Yassin
2011
Lecture 6
Sunday 9/10/2011
Intestinal Gas Production
A considerable amount of gas is present in the gastrointestinal contents of all
animals.
Five gases constitute greater than 99% of the gases passed: N2, O2, CO2, H2 and
methane. None of these gases has an odor, and the characteristic odor of feces is
due to very small quantities of a few other gases, including hydrogen sulfide.
There are three principal sources of the five major intestinal gases:
1. Air swallowing is the major source of gas in the stomach.
2. Intraluminal generation of gases results from two major processes;
First, in the proximal intestine, the interaction of hydrogen and bicarbonate
ions (principally from gastric and pancreatic secretions) leads to generation of
CO2. The amount of gas generated by this pathway is not great, because the
lumenal contents do not contain carbonic anhydrase and the dissociation of
H2CO3 is thus quite slow. Additionally, most of the CO2 produced in this way is
absorbed into blood.
The second and much more productive source of gas is fermentation by
colonic bacteria. Microbes appear to be the sole source of all of the hydrogen
and methane produced in the intestine. A variety of fruits and vegetables
contain polysaccharides that are not digested in the small intestine and lead to
voluminous gas production by microbes. Indeed, the primary medical
treatment for excessive gas production is dietary manipulation to eliminate
foodstuffs that the individual cannot digest and absorb.
The Gastrointestinal Barrier
The gastrointestinal mucosa forms a barrier between the body and a
lumenal environment which not only contains nutrients, but is loaded with
potentially hostile microorganisms and toxins. The challenge is to allow
efficient transport of nutrients across the epithelium while rigorously
excluding passage of harmful molecules and organisms into the animal.
The exclusionary properties of the gastric and intestinal mucosa are
referred to as the "gastrointestinal barrier".
The gastrointestinal barrier is often discussed as having two
components:
1. The intrinsic barrier is composed of the epithelial cells lining the
digestive tube and the tight junctions that tie them together.
2. The extrinsic barrier consists of secretions and other influences
that are not physically part of the epithelium, but which affect the
epithelial cells and maintain their barrier function.
a. Mucus and Bicarbonate
55
Prof.Dr H.D.El-Yassin
2011
Lecture 6
Sunday 9/10/2011
The entire gastrointestinal epithelium is coated with mucus, which serves
an important role in mitigating shear stresses on the epithelium and
contributes to barrier function in several ways. The abundant
carbohydrates on mucin molecules bind to bacteria, which aids in
preventing epithelial colonization and, by causing aggregation,
accelerates clearance. Diffusion of hydrophilic molecules is considerably
lower in mucus than in aqueous solution, which is thought to retard
diffusion of a variety of damaging chemicals, including gastric acid, to the
epithelial surface.
b. Hormones and Cytokines
Normal proliferation of gastric and intestinal epithelial cells, as well as
proliferation in response to such injury as ulceration, is known to be
affected by a large number of endocrine and paracrine factors. Several
of the enteric hormones are known to enhance rates of proliferation.
Different forms of injury to the epithelium can lead to either enhanced or
suppressed rates of cell proliferation.
Prostaglandins, particularly prostaglandin E2 and prostacyclin, have long
been known to have "cytoprotective" effects on the gastrointestinal
epithelium. A common clinical correlate in many mammals is that use of
aspirin and other non-steroidal antiinflammatory drugs (NSAIDs) which
inhibit prostaglandin synthesis is commonly associated with gastric
erosions and ulcers.
c. Antibiotic Peptides and Antibodies
56
Prof.Dr H.D.El-Yassin
2011
Lecture 6
Sunday 9/10/2011
Digestive Disorders
1. Stomach and Intestine
Peptic ulcers
Peptic ulcer is a general term that refers to ulcers occurring in the lower
esophagus, the stomach, or the duodenum (upper part of the small
intestine).
What is the difference between a duodenal ulcer and a gastric
ulcer?
A duodenal ulcer is a break in the lining of the upper part of the small
intestine (the duodenum); a gastric ulcer is a break in the lining of the
stomach.
What causes duodenal and gastric ulcers?
Duodenal ulcers are much more common than gastric ulcers. The
primary cause of duodenal ulcers is increased production of acid by the
stomach.
Gastric ulcers, on the other hand, are thought to be caused by changes
in the stomach lining that make it more susceptible to damage by the
acid normally produced by the stomach.
Factors in the development of peptic ulcers include:
 Helicobacter pylori
Research shows that most ulcers develop as a result of infection with
bacterium called Helicobacter pylori (H. pylori).
 Smoking
Studies show smoking increases the chances of getting an ulcer, slows
the healing process of existing ulcers, and contributes to ulcer
recurrence.
57
Prof.Dr H.D.El-Yassin
2011
Lecture 6
Sunday 9/10/2011
 Caffeine
Caffeine seems to stimulate acid secretion in the stomach, which can
aggravate the pain of an existing ulcer. However, the stimulation of
stomach acid cannot be attributed solely to caffeine.
 Alcohol
Although no proven link has been found between alcohol consumption
and peptic ulcers, ulcers are more common in people who have cirrhosis
of the liver, a disease often linked to heavy alcohol consumption.
 Stress
Mucos HCO3 content creates a "micro-environment" around surface cells
to prevent acid damage, but its secretion is inhibited by adrenergic input
(prominent in stress!)
 Acid and pepsin
It is believed that the stomach's inability to defend itself against the
powerful digestive fluids, hydrochloric acid and pepsin, contributes to
ulcer formation.
 nonsteroidal anti-inflammatory drugs (NSAIDs)
These drugs (such as aspirin, ibuprofen, and naproxen sodium) make
the stomach vulnerable to the harmful effects of acid and pepsin.
2. Bile and the Biliary System
a. Gallstones (Cholelithiasis)
There are two major types of gallstones, which form due to distinctly different
pathogenetic mechanisms.
1. Cholesterol Stones
About 90% of gallstones are of this type. These stones can be almost
pure cholesterol or mixtures of cholesterol and substances such as mucin.
The key event leading to formation and progression of cholesterol stones is
precipitation of cholesterol in bile. There are clearly important genetic
determinants for cholesterol stone formation. There is also an important
gender bias in development of stones - the prevalence in adult females is two
to three times that seen in males and use of contraceptive steroids is a risk
factor for development of gallstones.
2. Pigment Stones
Roughly 10% of human gallstones are pigment stones composed of
large quantities of bile pigments, along with lesser amounts of cholesterol and
calcium salts. The most important risk factor for development of these
stones is chronic hemolysis from almost any cause - bilirubin is a major
constituent of these stones. Additionally, some forms of pigment stones are
associated with bacterial infections. Apparently, some bacteria release
deconjugate bilirubin, leading to precipitation as calcium salts.
58
Prof.Dr H.D.El-Yassin
2011
Lecture 6
Sunday 9/10/2011
b. Jaundice
Jaundice, is yellowing of the skin, sclera (the white of the eyes) and
mucous membranes caused by increased levels of bilirubin in the human
body. Usually the concentration of bilirubin in the blood must exceed 23mg/dL for the coloration to be easily visible. Jaundice comes from the
French word jaune, meaning yellow.
Causes of jaundice
When red blood cells
die, the heme in their
hemoglobin is converted
to bilirubin in the spleen.
The bilirubin is
processed by the liver, enters bile and is eventually excreted through
faeces.
Consequently, there are three different classes of causes for jaundice.
Pre-hepatic or hemolytic causes, where too many red blood cells are
broken down, hepatic causes where the processing of bilirubin in the
liver does not function correctly, and post-hepatic or extrahepatic
causes, where the removal of bile is disturbed.
1. Pre-hepatic
Pre-hepatic (or hemolytic) jaundice is caused by anything which causes
an increased rate of hemolysis (breakdown of red blood cells). Malaria
can cause jaundice. Certain genetic diseases, such as glucose 6phosphate dehydrogenase deficiency can lead to increase red cell lysis
and therefore hemolytic jaundice. Defects in bilirubin metabolism also
present as jaundice.
2. Hepatic
Hepatic causes include acute hepatitis, hepatotoxicity and alcoholic liver
disease. Jaundice commonly seen in the newborn baby is another
example of hepatic jaundice.
59
Prof.Dr H.D.El-Yassin
2011
Lecture 6
Sunday 9/10/2011
 Neonatal jaundice
Neonatal jaundice is usually harmless: this condition is often seen in
infants around the second day after birth, lasting till day 8 in normal
births, or to around day 14 in premature births. Serum bilirubin normally
drops to a low level without any intervention required: the jaundice is
presumably a consequence of metabolic and physiological adjustments
after birth. Infants with neonatal jaundice are typically treated by
exposing them to high levels of colored light to break down the bilirubin.
Lecture 6
Sunday 9/10/2011
This works due to a photo oxidation process occurring on the bilirubin in
the subcutaneous tissues of the neonate. Light energy creates
isomerization of the bilirubin and consequently transformation into
compounds that the new born can excrete via urine and stools.
3. Post-hepatic
Post-hepatic (or obstructive) jaundice, also called cholestasis, is caused
by an interruption to the drainage of bile in the biliary system. The most
common causes are gallstones in the common bile duct and pancreatic
cancer in the head of the pancreas.
The van den Bergh test:
When a mixture of sulphanic acid, hydrochloric acid and sodium nitrite (diazo reagent) is added to serum
containing an excess of biliriubin glucuronide a reddish-violet color results, the maximum color intensity being
reached within 30 seconds (direct reaction) (for hepatic and post hepatic jaundice)
When the same above reagents are mixed with serum containing an excess of billirubin itself or bilirubi-protien
complex no color develops until alcohol is added, then the reddish-violet color appears.(indirect reaction) (for pre
hepatic jaundice)
Note: the addition of alcohol solvent provides the means of solution for the water insoluble bilirubin which is thus
enabled to react with the diazo reagent
c. Cirrhosis
Cirrhosis is characterized anatomically by widespread nodules in the
liver combined with fibrosis. The fibrosis and nodule formation causes
distortion of the normal liver architecture which interferes with blood flow
through the liver. Cirrhosis can also lead to an inability of the liver to
perform its biochemical functions.
Causes of Cirrhosis
 Alcoholic liver disease
 Chronic viral hepatitis B, C and D
 Chronic autoimmune hepatitis
 Inherited metabolic diseases
 Chronic bile duct diseases
 Chronic congestive heart failure infections
 Parasitic infections
 liver inflammation that can be caused by fatty liver
 long term exposure to toxins or drugs
60
Prof.Dr H.D.El-Yassin
2011
Lecture 6
Sunday 9/10/2011
3. Intestine
Diarrhea is an increase in the volume of stool or frequency of defecation.
It is one of the most common clinical signs of gastrointestinal disease,
but also can reflect primary disorders outside of the digestive system
There are numerous causes of diarrhea, but in almost all cases, this
disorder is a manifestation of one of the four basic mechanisms
described below.
1. Osmotic Diarrhea
Absorption of water in the intestines is dependent on adequate
absorption of solutes. If excessive amounts of solutes are retained in the
intestinal lumen, water will not be absorbed and diarrhea will result.
Osmotic diarrhea typically results from one of two situations:
 Ingestion of a poorly absorbed substrate: The offending molecule
is usually a carbohydrate or divalent ion. Common examples include
mannitol or sorbitol, epson salt (MgSO4) and some antacids (MgOH2).
 Malabsorption: Inability to absorb certain carbohydrates is the most
common deficit in this category of diarrhea, but it can result virtually
any type of malabsorption. A common example is lactose intolerance
resulting from a deficiency in the brush border enzyme lactase. In
such cases, a moderate quantity of lactose is consumed (usually as
milk), but the intestinal epithelium is deficient in lactase, and lactose
cannot be effectively hydrolyzed into glucose and galactose for
absorption. The osmotically-active lactose is retained in the intestinal
lumen, where it "holds" water.
 A distinguishing feature of osmotic diarrhea is that it stops after the
patient is fasted or stops consuming the poorly absorbed solute.
2. Secretory Diarrhea
Large volumes of water are normally secreted into the small intestinal
lumen, but a large majority of this water is efficiently absorbed before
reaching the large intestine. Diarrhea occurs when secretion of water into
the intestinal lumen exceeds absorption.
Many millions of people have died of the secretory diarrhea associated
with cholera. The responsible organism, Vibrio cholerae, produces
cholera toxin, which strongly activates adenylyl cyclase, causing a
prolonged increase in intracellular concentration of cyclic AMP within
crypt enterocytes. This change results in prolonged opening of the
chloride channels that are instrumental in secretion of water from the
crypts, allowing uncontrolled secretion of water.
Exposure to toxins from several other types of bacteria (e.g. E. coli heatlabile toxin) induce the same series of steps and massive secretory
61
Prof.Dr H.D.El-Yassin
2011
Lecture 6
Sunday 9/10/2011
diarrhea that is often lethal unless the person or animal is aggressively
treated to maintain hydration. In addition to bacterial toxins, a large
number of other agents can induce secretory diarrhea by turning on the
intestinal secretory machinery, including:
 some laxatives
 hormones secreted by certain types of tumors (e.g. vasoactive
intestinal peptide)
 a broad range of drugs (e.g. some types of asthma medications,
antidepressants, cardiac drugs)
 certain metals, organic toxins, and plant products (e.g. arsenic,
insecticides, mushroom toxins, caffeine)
In most cases, secretory diarrheas will not resolve during a 2-3 day fast.
3. Inflammatory and Infectious Diarrhea
The epithelium of the digestive tube is protected from insult by a number
of mechanisms constituting the gastrointestinal barrier, but like many
barriers, it can be breached and often associated with widespread
destruction of absorptive epithelium. In such cases, absorption of water
occurs very inefficiently and diarrhea results. Examples of pathogens
frequently associated with infectious diarrhea include:
Bacteria: Salmonella, E. coli, Campylobacter
Viruses: rotaviruses
Protozoa:
4. Diarrhea Associated with Deranged Motility
In order for nutrients and water to be efficiently absorbed, the intestinal
contents must be adequately exposed to the mucosal epithelium and
retained long enough to allow absorption. Disorders in motility than
accelerate transit time could decrease absorption, resulting in diarrhea
even if the absorptive process per se was proceeding properly.
62
Prof.Dr H.D.El-Yassin
2011