A proteolytic enzyme found in the gastric juice of mammals, birds, reptiles, and fish. It is formed from a precursor, pepsinogen, which is found in the stomach mucosa. Pepsinogen is converted to pepsin either by hydrochloric acid, naturally present in the stomach, or by pepsin itself. A pepsin inhibitor, peptide, remains attached to the pepsin molecule at pH values above 5.0, preventing activation of the enzyme. The approximate molecular weight of pepsin is 34,500 and that of pepsinogen is 42,600. Maximum activity occurs at pH 1.8, and broad specificity for peptide bonds is demonstrated.