a proteolytic enzyme of the hydrolase class, found in the gastric juice of mammals, birds, reptiles, and most species of fish. Pepsin breaks down proteins and peptides. It was first recognized in 1836 by T. Schwann and isolated in crystal form in 1930 by J. Northrop.

Pepsin is a globular protein with a molecular weight of approximately 34,500. A pepsin molecule is a polypeptide chain consisting of 340 amino-acid residues, three disulfide bonds (—S—S—), and phosphoric acid. The isoelectric point of pepsin is approximately equal to pH 1.0. Therefore, pepsin is stable in a strongly acid medium and reaches its maximum activity at pH 1–2, the pH of gastric juice. It undergoes denaturation at pH 6.0.

Pepsin is an endopeptidase, that is, an enzyme that splits the central peptide bonds in protein and peptide molecules, except keratins and other scleroproteins, to form simpler peptides and free amino acids. It very rapidly hydrolyzes peptide bonds formed by the aromatic amino acids tyrosine and phenylalanine; however, unlike the proteolytic enzymes trypsin and chymotrypsin, it does not exhibit a strong specificity.

Pepsin is produced by the gastric chief cells in the form of inactive pepsinogen. Pepsinogen is then converted to pepsin by the splitting off of several peptides, including a pepsin inhibitor, from the N-terminal section of pepsinogen. The activation process involves several stages and is catalyzed by the hydrochloric acid in gastric juice and by pepsin itself (autocatalysis).

Pepsin is used in the laboratory investigation of the primary structure of proteins, in cheese-making, and in the treatment of certain gastrointestinal diseases.