Chemical modification of bacterial 4-aminobutyrate aminotransferase by phenylglyoxal.

4-Aminobutyrate aminotransferase (EC 2.6.1.19), obtained from Pseudomonas fluorescens, was irreversibly inhibited by phenylglyoxal, a reagent that specifically modifies arginyl residues. The inactivation appeared to be the result of a simple, bimolecular reaction since no evidence of a reversible complex between inhibitor and enzyme emerged. The second-order rate constant was 0.221 +/- 0.077 M-1 sec-1. The concentration of either substrate had no effect on the inhibition, but an increase in the concentration of pyridoxal 5'-phosphate reduced the rate of inactivation by phenylglyoxal. The data are consistent with the modification of amino acid residues at the cofactor binding site on the enzyme.