Digestion in the small intestinal lumen

The products of HCI and pepsin digestion enter the duodenum of the small intestine through the pyloric sphincter. In the duodenum, proteins and polypeptides serve as substrate for enzymes secreted from the pancreas and small intestine. Initially, protein and polypeptides entering the duodenum are broken down further in the intestine by pancreatic exopeptidases trypsin, chymotrypsin, elastase, and pancreatic endopeptidases car-boxypeptidase A and B. The pancreas secretes proenzymes into the duodenum, which when activated hydrolyse peptide bonds. The conversion of inactive trypsinogen into active trypsin requires removal of an N-terminal peptide and is catalysed by the brush border enzyme enteropeptidase (formerly known as enterokinase). Enteropeptidase selectively cleaves a hexapeptide (H2N-Val-Asp-Asp-Asp-Asp-Lys) from the amino terminus of trypsinogen resulting in trypsin (Kitamoto et al., 1994). Enteropeptidase activity may be regulated by pancreatic secretion (Kwong et al., 1978) and thus possibly by dietary protein content. Following its conversion from trypsinogen, trypsin activates the other zymogens and to a lesser degree tripsinogen.

Trypsin, chymotrypsin, and elastase catalyse the breakdown of proteins, polypeptides, and peptides into smaller peptides and amino acids in the duodenum. Each pancreatic protease has a unique and complementary action. Trypsin catalyses the breakdown of bonds that involve lysine and/or arginine, whereas linkages involving aromatic amino acid residues are susceptible to chymotrypsin catalysis (Alpers, 1994). Elastases are less specific with regard to the type of peptide bonds, but in general catalyse the breakdown of peptide bonds containing aliphatic residues. The action of trypsin, chymotrypsin, and elastase releases numerous terminal peptide bonds, which in turn are further digested by aminopeptidases, carboxypeptidases, and other specific peptidases present in the lumen or mucosa of the small intestine. Pancreatic carboxypeptidase A and B are exopeptidases that catalyse the hydrolysis of the carboxy-ter-minal bonds in polypeptide chains, removing the amino acids in sequence. Proteolysis of an approximately 100-residue segment from the amino-terminal region results in the activation of procarboxypeptidases (Aviles et al., 1985). Carboxy-terminal aromatic or non-polar amino acids exposed by the action of chymotrypsin and elastase are available to be cleaved by carboxypeptidase A, while car-boxy-terminal basic amino acids exposed by trypsin can be cleaved by carboxypeptidase B. Both carboxypeptidase A and B are inhibited by proline. The products of pancreatic digestion are oligopeptides of up to six amino acid residues (approximately 60%) as well as free amino acids (approximately 40%; Alpers, 1994).

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