UNIQUE STEREOSPECIFICITY OF D-AMINO-ACID AMINOTRANSFERASE AND BRANCHED-CHAIN L-AMINO-ACID AMINOTRANSFERASE FOR C-4' HYDROGEN-TRANSFER OF THE COENZYME

D-Amino acid aminotransferase and branched-chain L-amino acid aminotransferase, which show a significant sequence homology,6 are unique in their stereospecific catalysis of pro-R C-4' hydrogen transfer through the coenzyme-substrate Schiff base intermediates in contrast to other various aminotransferases catalyzing the pro-S hydrogen transfer. D-Amino acid aminotransferase abstracted (R)-H-1 from (4'S)-[4'-H-2]pyridoxamine in a half reaction of transamination with an amino acceptor. Branched-chain L-amino acid aminotransferase catalyzed the pro-R specific hydrogen exchange of pyridoxamine 5'-phosphate with the solvent hydrogen. When D-amino acid aminotransferase and branched chain L-amino acid aminotransferase were incubated with (4'R)-[4'-H-3]pyridoxamine 5'-phosphate and (4'S)-[4'-H-3]pyridoxamine 5'-phosphate in a half reaction of transamination with an amino acceptor, and also an overall transamination, H-3 was released into the solvent exclusively from (4'R)-[4'-H-3]pyridoxamine. Thus, these two enzymes are categorized into the same group on the basis of stereospecificity of the hydrogen transfer but different from all other aminotransferases showing the pro-S stereospecificity.