Proteolytic enzymes are secreted as inactive zymogens. This prevents autodigestion of the secretory acini, e.g. trypsinogen. The chief cells of the stomach secrete pepsin. Optimum pH for its activity is 2.0. Pepsin catalyses hydrolysis of the bonds formed by carboxyl groups of Phe, Tyr, Trp and Met. Activation of trypsin is by ‘enterokinase’ present in the intestinal microvilli. Acute pancreatitis is a condition resulting from premature activation of the zymogen trypsinogen inside the pancreas itself. Trypsin catalyses hydrolysis of the bonds formed by carboxyl groups of ARG and LYA. Important proteolytic enzymes of the body are Pepsin, Trypsin, Chymotrypsin, carboxypeptidase A and B, and elastase. Amino acids have five different transport systems for their absorption viz. for neutral amino acids, for basic amino acids, for imino acids and glycine, for acidic amino acids and for beta amino acids. Absorption of neutral amino acids in renal tubules and brain is by the ‘gamma glutamyl cycle’. Transaminases require pyridoxal phosphate (PLP) as cofactor. Reactions that form ammonia in the body are transamination, trans-deamination, oxidative deamination and non-oxidative deamination. First line of defense against ammonia toxicity in the body is its trapping by glutamine, especially in the brain cells. Second line of defense is constituted by the urea cycle. The two nitrogen atoms of urea are derived from ammonia and aspartic acid. Carbamoyl synthase I is the rate-limiting enzyme of the urea cycle. The reaction is irreversible. Deficiency of all the urea cycle enzymes is known. They constitute the set of disorders called ‘urea cycle defects’. Normal plasma urea level is 20–40 mg/dl. The one carbon units are; N5 formyl THFA, N10 formyl THFA, N5N10 Methylene THFA, N5N10 Methenyl THFA and N5 methyl THFA.