Digestion of proteins occurs in stomach and intestine. The protein hydrolyzing enzymes are called proteases and peptidases. All these enzymes are secreted in inactive form and are called proenzymes, because if secreted in active form, these would hydrolyse cellular and extracellular proteins of the organism itself. Proenzymes (propepsin, prorennin etc) are activated either by optimal pH or some specific protease only at the sites of their action.