Monitor peptide

Monitor peptide, also known as pancreatic secretory trypsin inhibitor I (PSTI-I) or pancreatic secretory trypsin inhibitor 61 (PSTI-61), is a peptide that plays an important role in the regulation of the digestive system, specifically the release of cholecystokinin (CCK).

Function
One of the primary functions of monitor peptide is to stimulate the release of CCK from the enteroendocrine cells of the small intestine. CCK then acts on the gallbladder to release bile and on the pancreas to release digestive enzymes, 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 of trypsin, which is a protease 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 in the duodenum. 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 of approximately 6500 daltons and is basic (PI = 9.0), acid stable, and heat resistant. It is only found in the zymogen 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.