Pepsin is a gastric enzyme released on cleavage of pepsinogens. Pepsinogens are precursor proteins produced by the gastric glands. Secretion in an inactive form limits self-digestion.

Cleavage is triggered by:

• gastric acidity
• the auto-catalytic activity of pepsin on pepsinogen

Pepsin acts to break bonds within polypeptide chains, particularly those near phenylalanine, tyrosine or leucine residues, to yield smaller peptides. It is an endopeptidase.

Pepsin is optimally active at gastric acidity; within the duodenum, the increase in pH reduces proteolytic activity.