Pepsins

Pepsins are the major enzyme secretions of the stomach, digesting protein in the diet to yield peptide fragments. Pepsins are members of the aspartate proteinase family with two aspartate residues at the active site. They are broad specificity endopepti-dases with a preference for peptide bonds between hydrophobic amino acids. Human gastric juice contains two groups of pepsins: pepsin A, which contains six isoenzymes; and pepsin C, which contains two isoenzymes (Table 3). Once secreted into the lumen, the acidic conditions convert pepsinogens to pepsin. Unlike the activation of other proteinases, e.g., trypsinogen to trypsin via a proteolytic cleavage by another enzyme (enterokinase), pepsinogen activation is autocatalytic.

The process is understood in detail for porcine pepsinogen, which when activated produces an enzyme very similar to human pepsin 3b. Secreted pepsinogen has an N-terminal peptide of 44 amino acids blocking the active site. On exposure to pH values below 5, carboxyl groups become protonated abolishing charge-charge interactions. This allows part of the N-terminal protein into the active site where it is cleaved, releasing a 16-amino acid peptide. The enzyme is now partially activated; full activation occurs with cleavage of a further 28 amino acids from the N-terminal by either another partially activated pepsin or a fully activated enzyme.

The majority of proteinase activity in human gastric juice is due to pepsin 3 (70.3 ±2.6%); variable amounts of pepsin 5 are also present (16.9±2.0%). The only other isoenzyme found in significant amounts is pepsin 1. This differs from other pepsins in that it is an ionic complex of a

Table 3 Pepsins of human gastric juice

Pepsin group

Chromosome location

Individual pepsins

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