pepsin, enzyme produced in the mucosal lining of the stomach that acts to degrade protein. Pepsin is one of three principal protein-degrading, or proteolytic, enzymes in the digestive system, the other two being chymotrypsin and trypsin. The three enzymes were among the first to be isolated in crystalline form. During the process of digestion, these enzymes, each of which is particularly effective in severing links between particular types of amino acids, collaborate to break down dietary proteins to their components, i.e., peptides and amino acids, which can be readily absorbed by the intestinal lining. In the laboratory studies pepsin is most efficient in cleaving bonds involving the aromatic amino acids, phenylalanine, tryptophan, and tyrosine. Pepsin is synthesized in an inactive form by the stomach lining; hydrochloric acid, also produced by the gastric mucosa, is necessary to convert the inactive enzyme and to maintain the optimum acidity ( p H 1–3) for pepsin function. Pepsin and other proteolytic enzymes are used in the laboratory analysis of various proteins; pepsin is also used in the preparation of cheese and other protein-containing foods.