Monitor peptide, also known as pancreatic secretory trypsin inhibitor I (PSTI-I) or pancreatic secretory trypsin inhibitor 61 (PSTI-61), is a

peptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. A ...

that plays an important role in the regulation of the

digestive system The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food into smaller and smaller compone ...

, specifically the release of

cholecystokinin Cholecystokinin (CCK or CCK-PZ; from Greek ''chole'', "bile"; ''cysto'', "sac"; ''kinin'', "move"; hence, ''move the bile-sac (gallbladder)'') is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and ...

(CCK).

Function

One of the primary functions of monitor peptide is to stimulate the release of CCK from the

enteroendocrine cells Enteroendocrine cells are specialized cells of the gastrointestinal tract and pancreas with endocrine function. They produce gastrointestinal hormones or peptides in response to various stimuli and release them into the bloodstream for systemic ef ...

of the small intestine. CCK then acts on the

gallbladder In vertebrates, the gallbladder, also known as the cholecyst, is a small hollow organ where bile is stored and concentrated before it is released into the small intestine. In humans, the pear-shaped gallbladder lies beneath the liver, although ...

to release

bile Bile (from Latin ''bilis''), or gall, is a dark-green-to-yellowish-brown fluid produced by the liver of most vertebrates that aids the digestion of lipids in the small intestine. In humans, bile is produced continuously by the liver (liver bile ...

and on the

pancreas The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an end ...

to release

digestive enzymes Digestive enzymes are a group of enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption into the cells of the body. Digestive enzymes are found in the digestive tracts of anima ...

, which help to further break down the food. This coordinated response helps to ensure efficient digestion and absorption of nutrients. Another function is to act as a

competitive inhibitor Competitive inhibition is interruption of a chemical pathway owing to one chemical substance inhibiting the effect of another by competing with it for binding or bonding. Any metabolic or chemical messenger system can potentially be affected b ...

trypsin Trypsin is an enzyme in the first section of the small intestine that starts the digestion of protein molecules by cutting these long chains of amino acids into smaller pieces. It is a serine protease from the PA clan superfamily, found in the dig ...

, which is a

protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...

that can activate other proteases. It has been shown to prevent premature activation of pancreatic enzymes. Its role as a feedback regulator has been well-described for decades. Monitor peptide binds to intestinal epithelial cells and induces CCK-release, which enhances pancreatic secretion in the presence of nutritional

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

in the

duodenum The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine m ...

. When all nutritional protein is digested, monitor peptide is bound by trypsin and subsequently degraded, resulting in decreasing CCK-release and a reduction of pancreatic secretion.

History

Monitor peptide was first discovered in 1984 by Fushiki ''et al.'' It was purified from rat bile-pancreatic juice and the peptide sequence was elucidated.

Description

Monitor peptide is composed of 61 amino acids with a

molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

of approximately 6500 daltons and is basic ( PI = 9.0), acid stable, and heat resistant. It is only found in the

zymogen In biochemistry, a zymogen (), also called a proenzyme (), is an inactive precursor of an enzyme. A zymogen requires a biochemical change (such as a hydrolysis reaction revealing the active site, or changing the configuration to reveal the activ ...

granules of pancreatic acinar cells. Similar to CCK-releasing peptide (CCK-RP), it is trypsin sensitive and stimulates CCK release. It is possible that it also stimulates the growth of intestinal epithelial cells.

References

{{reflist Peptides Cholecystokinin