Branched Chain Amino Acid Aminotransferase

The kinetics of transamination reactions catalyzed by pig heart branched chain amino aminotransferase was investigated. The activation of aged preparations by 2-mercaptoethanol, previously noted, was associated with increases in both the maximum velocity and enzyme-substrate affinities. This implies a protein conformational change. The sharp pH optimum observed with standard assay conditions reflects a change in the rate-limiting half-reaction from that of leucine with the phosphopyridoxal form of the enzyme to that of α-ketoglutarate with the phosphopyridoxamine form as the pH is raised. Whereas nonpolar monocarboxylic acids and dicarboxylic acids which are substrate analogues both inhibited strongly at the optimum pH of 8.3, only the monocarboxylic acids were effective at higher pH values. The kinetics of the reaction of l-leucine with α-ketoglutarate could be adequately described by a binary rate equation as would be expected for a transamination reaction. Exchange transamination reactions between homologous amino acid keto acid substrate pairs were also shown and found to proceed at rates comparable to that observed with leucine and α-ketoglutarate.