Different modes of action of inhibitors of bacterial D-amino acid transaminase. A target enzyme for the design of new antibacterial agents.

D-Amino acid transaminase from Bacillus sphaericus shows a deuterium kinetic isotope effect (VH/VD) between 2 and 3 in the transamination of alpha-protio- or alpha-deuterio-D-alanine and alpha-ketoglutarate, suggesting that alpha-proton abstraction is at least partially rate-limiting for this reaction. This transaminase also catalyzes a beta-elimination reaction with substrates such as beta-fluoroalanine with no detectable deuterium isotope effect (VH/BD = 1). These results, taken together with previous work (Soper, T. S., and Manning, J. M. (1978) Biochemistry 17, 3377-3384) suggest that the rate-limiting step in the beta-elimination reaction is solvolysis of an alpha-aminoacrylate-pyridoxal-P Schiff's base intermediate. D-Cycloserine is an active site titrant of D-amino acid transaminase. Inactivation by cycloserine can be completely reversed by dialysis against pyridoxal phosphate at neutral pH. Gabaculine is also an efficient inhibitor of this enzyme and possesses some antibacterial activity. The latter two inhibitors probably act by sequestration of the coenzyme rather than by alkylation of the protein as with the beta-halo derivatives of D-alanine.