The role of individual amino acid in diet was examined in regulation of some amino acid-catabolizing enzymes in the liver of protein-depleted rats. Feeding rats with a mixture containing 18 amino acids in equimolar proportion produced marked increase in activity of all enzymes examined; serine dehydratase (EC 4.2.1.13), glutamic-oxaloacetic transaminase (GOT, EC 2.6.1.1), branched-chain a-ketoacid dehydrogenase (BCADH, EC 2.6.1.42), glutamic-pyruvic transaminase (GPT, EC 2.6.1.2) and histidase (EC 4.3.1.3). The induction of serine dehydratase, GOT and histidase by complete amino acids mixture(CAA) could essentially be attributed to the action of tryptophan. The removal of non-essential amino acid mixture (NEAA) from the CAA in the diet caused enhancement of increase in the activity of serine dehydratase, GOT and GPT whilst it caused no further increase in that of both BCADH and histidase. These results suggest that NEAA in diet may inhibit the induction of some amino acid-catabolizing enzymes by essential...