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Gastro-intestinal Enzymes
Pepsin
• Proteolytic enzyme of the stomach
• At least two immunologically distinct pepsinogens (1 and
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2) are secreted by the stomach
Pepsinogen 1 is secreted by the chief cells in the oxyntic
glandular area
Pepsinogen 2 by cells through out the stomach as well as in
Brunner’s glands in the duodenum
It is an endopeptidase
It preferentially hydrolyses peptide linkage where one of
the amino acids is aromatic (eg, tyrosin) or dicarboxylic
amino acids (eg, glutamate)
Pepsin potentiates rather than initiates ulcer formation
Clinical Significance
• Serum concentration of pepsinogen I reflects the parietal
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cell mass and correlate well with the maximum acidsecreting capacity
Increased pepsinogen I:
– Duodenal ulcer
– Acute and chronic superficial gastritis.
– H. pylori sero-positive patients
• Decreased levels of pepsinogen I:
– Atrophic gastritis
– Gastric carcinoma
• Increased pepsinogen II:
– Acute and chronic superficial gastritis
• Major risk factor for gastric ulcer
Pancreatic Juice
• Alkaline secretion: secreted by the duct cells
– Composition: HCO3-, Na+, K+, Mg2+, Ca2+, Cl– Function: together with the other alkaline secretions (bile and
intestinal juice) neutralize the acid chyme arriving from the
stomach.
Why this is important?
• The pancreatic enzymes require a neutral or slightly alkaline
pH for their activity
• The absorption of fat depends on the formation of micelles, a
process which only takes place at neutral or slightly alkaline pH
• It protects the intestinal mucosa, excess acid in the duodenum
can damage the mucosa and can lead to ulcer formation
Pancreatic Juice
• Pancreatic enzymes: secreted by acinar cells
– In their inactive form;
• Trypsinogen
• Chymotrypsinogen
• Proelastase
• Procarboxypeptidase
• Prophospholipase A2
– In their active form;
• Lipase
• Alpha-amylase
• Ribonuclease
• Deoxyribonuclease
• Cholesterol esterase
Trypsin
• Two types, trypsinogen-1 and -2
• Trypsin is a serine proteinase charaterized by the presence at
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the active site of serine and histidine, both of which participate
in the catalytic process
Trypsins are considered endopeptidases
The enzymatic mechanism is like all other serine proteases: A
catalytic triad serves to make the active site serine nucleophilic.
This is achieved by modifying the electrostatic environment of
the serine residue
The aspartate residue (Asp 189) located in the catalytic pocket
(S1) of trypsins is responsible for attracting and stabilizing
positively-charged lysine and/or arginine, and is thus
responsible for the specificity of the enzyme.
Trypsin
• Trypsin activity is stimulated by calcium and magnesium
ions and to a lesser extent by cobalt and manganese ions .
Cyanide, sulfide, citrate, fluoride, and heavy metals inhibit
activity as do those organic phosphorous compounds that
combine with serine at the active site.
• Clinical Significance:
– deficiency of trypsin might lead to the disorder termed
meconium ileus
– After an attack of acute pancreatitis, serum
immunoreative trypsin rises in parallel with serum
amylase activity.
Chymotrypsin
• Two types, chymotrypsinogen-1 and -2
• Chymotrypsin is synthesized in the acinar
cells of the human pancreas
• Chymotrypsin-1 describes as anionic and
chymotrypsin-2 as cationic because of their
differing electrophoretic mobilities; the
cationic form predominates.
Action and kinetics of chymotrypsin:
• For an enzyme-mediated reaction to take place, the
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reacting molecule or molecules, called substrates, must fit
into a specific section of the enzymes structure called the
active site.
Each active site has:
(1) a shape that fits a specific substrate or substrates
(2) side chains that attract the enzymes particular
substrates
(3) side chains specifically positioned to speed the reaction.
Mechanism of peptide bond cleavage in chymotrypsin (1)
Mechanism of peptide bond cleavage in chymotrypsin (2)
Mechanism of peptide bond cleavage in chymotrypsin (3)
Mechanism of peptide bond cleavage in chymotrypsin (4)
Mechanism of peptide bond cleavage in chymotrypsin (5)
Mechanism of peptide bond cleavage in chymotrypsin (6)
Chymotrypsin
• Clinical Significance:
– Chymotrypsin is more resistant than
trypsin to degradation in the intestine; it
is therefore the enzyme of choice for assay
in feces.
– The major application of chymotrypsin
assay is the investigation of pancreatic
disease.
Acute Pancreatitis
• Acute pancreatitis is a disease in which the pancreatic
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tissue is destroyed by digestive enzymes
The pancreas normally secrets a polypeptide known as
Kazal inhibitor, that inhibits any small amounts of
activated trypsin which may find its way into the ducts, by
forming a complex with it
Enzyme Y is also exhibiting a protective function, which is
activated by traces of active trypsin degrades zymogen
The alkaline pH (8.0-9.5) and low Ca2+ concentration in
pancreatic secretions promote the degradation rather than
the activation of trypsinogen
In acute pancreatitis activated trypsin and other enzymes
are present in the ducts of the pancreas
Elastase
• Elastase, like trypsin and chymotrypsin, is a
serine protease that also hydrolyses amides
and esters
• Attacking bonds next to small amino acids
residues such as glycine, alanine and serine
• Two types;
– Elastase I, which is anionic exists in serum both
free and as a complex with alpha-1 proteinase
inhibitor;
– Elastase II which is cationic mainly in the
bound form with alpha-1 proteinase inhibitor
Elastase
• Clinical Significance:
– Elastase I is increased in acute and relapsing chronic
pancreatitis
– There is some evidence that this enzyme is considerably
more specific for pancreatitis than is amylase.
– Elevation of elastase I have also been observed in
carcinoma of the pancreas, especially carcinoma of the
head of the pancreas.
– Elastase II is likewise increased in acute pacreatitis
• Enteropeptidase:
– Is a serine protease enzyme secreted by intestinal mucusa
– Cleaves after Lysine if the Lys is preceded by four Asp and not followed
by a Pro.
• Carboxypeptidase:
– Carboxypeptidases are usually classified into one of several families
based on their active site mechanism
• Enzymes that use a metal in the active site are called "metallocarboxypeptidases".
• Carboxypeptidases that use active site serine residues are called
"serine carboxypeptidases".
• Carboxypeptidases that use an active site cysteine are called
"cysteine carboxypeptidase" (or "thiol carboxypeptidases").
– By substrate preference:
• In this classification system, carboxypeptidases that have a stronger
preference for those amino acids containing aromatic or branched
hydrocarbon chains are called carboxypeptidase A (A for
aromatic/aliphatic).
• Carboxypeptidases that cleave positively charged amino acids
(arginine, lysine) are called carboxypeptidase B (B for basic).
Induced conformation (carboxypeptidase A)
Brush Border Enzymes of The Intestine
• Aminopeptidase, Leucine aminopeptidase
• Oligopeptidases, degrade small peptides such as
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tetrapeptides
Dipeptidyl aminopeptidases, remove dipeptides from the
N-terminal of proteins
Tripeptidases and Dipeptidases in the endothelial cells
Lipases
• Human lipase is a glycoprotien
• Lipases are defined as enzymes that hydrolyze glycerol
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esters of long-chain fatty acids.
For full catalytic activity and greatest specificity, the
presence of bile salts and a cofactor, called co-lipase is
required.
Both lipase and colipase are synthesized in the pancreatic
acinar cells and secreted by the pancreas in roughly
equimolar quantities.
Lipase measurements are used exclusively to investigate
pancreatic disorders, usually pancreatitis
Brush Border Enzymes of The Intestine
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Isomaltase (alpha 1,4-glycosidase)
Glucoamylase
Maltase
Sucrase
Lactase
Trehalase
Sucrase and isomaltase are synthesized as a single
polypeptide chain inside the cell
Lactase
• Secreted by Brunner's glands of the duodenum and glands of
Lieberkun, works at pH 5.4-6.0 hydrolyze lactose to glucose and
galactose.
• Lactase deficiency:
– Symptoms of flatulence, abdominal discomfort, bloating, or
diarrhea
• Lactase deficiency of two types:
– Congenital Lactase Deficiency
• Symptoms occur as soon as milk is taken
• Absent or low intestinal lactase in the neonate, however, cannot
be taken as proof of congenital deficiency because lactase is
normally the slowest of the oligosaccharidases to reach normal
levels in the newborn’s gut
• An abnormal oral lactose tolerance test obtained a few months
after birth could also be due to congenital glucose-galactose
intolerance
Lactase
• Acquired Lactase Deficiency
– Lactase activity declines as a child ages
– The age at which decline begins is genetically determined in an
Autosomal recessive fashion and differs among ethnic groups
• Secondary Lactase Intolerance
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– Occurs as a result of reduced enzyme activity following diffuse
intestinal damage from;
– Infections (Giardiasis, bacterial overgrowth, or viruses),
– Ulcerative colitis,
– Celiac disease, and
– Tropical sprue
Testing: hydrogen breath test is used to measure the amount of
hydrogen in the breath.
• Ribonuclease (RNase): digestion of dietary nucleic acids
– Ribonuclease A catalyzes cleavage of the phosphodiester bond
between the 5'-ribose of a nucleotide and the phosphate group
attached to the 3'-ribose of an adjacent pyrimidine nucleotide
forming a 2',3'-cyclic phosphate
– Ribonuclease B: is a glycosylated derivative of RNase A.
• Deoxyribonuclease (DNase): digestion of dietary nucleic acid
• Polynucleotidases: nucleic acids into nucleotides
• Nucleosidases: nucleosides to give free nitrogen base plus pentose
phosphate
• Phosphatases: remove phosphate
• Phospholipases: phospholipids to produce glycerol, fatty acids,
phosphoric acid, and bases such as choline
• Cholesterol esterase: cholesterol and fatty acids
• Sucrase-Isomaltase deficiency: flatulence, abdominal
discomfort, bloating, diarrhea
• Colipase deficiency: steatorrhea
• Absorption of undigested polypeptides may cause antigenic
reactions: celiac disease
• Monosaccharide Malabsorption: caused by single mutation
resulted in a defect in the Na+-glucose co-transporter
carrier mechanism (SGLT1)
Disturbances due to malabsorption
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Anemia: Iron, Vitamin B12, Folate
Edema: Products of protein digestion
Tetany: Calcium, Magnesium, Vitamin D
Osteoporosis: Calcium, Product of protein digestion,
Vitamin D
Milk intolerance: Lactose
Bleeding Bruising: Vitamin K
Steatorrhea (fatty stools): Lipids and fat-soluble vitamins
Hartnup disease (defect in intestinal neutral amino acid
carrier): Neutral amino acids