CHEMICAL INACTIVATION OF BACTERIAL GABA AMINOTRANSFERASE

The effects of three potential irreversible inhibitors of gamma-aminob utyrate aminotransferase from Pseudomonas fluorescens were studied in order to throw more light on the nature of the active site of the enzy me. The thiol group reagent mercuric chloride inactivated the enzyme i n a concentration-dependent manner. Inhibition kinetics were consisten t with a simple bimolecular reaction. The second-order rate constant w as 4.2 x 10(3) +/- 0.61 M-1 sec(-1). In contrast to either of the subs trates, the cofactor pyridoxal 5'-phosphate could protect the enzyme f rom the inhibition, suggesting cysteinyl residues are important for co factor binding at the active site. p-Chloromercuribenzoic acid produce d a similar inactivation of the enzyme. 4-Amino-2-fluorobutanoic acid also inhibited enzymic activity but in this case the inhibition was re versible and competitive with respect to gamma-aminobutyric acid (GABA ). The inhibitor constant (K-i) was 0.83 +/- 0.44 mM. We found no evid ence that this fluorinated analogue of GABA could act as a substrate f or the enzyme.