The Reaction of β-Chloroglutamic Acid with Glutamate-Aspartate Transaminase.

Porcine glutamate-aspartate transaminase catalyzes a β-elimination reaction with both th threo-and erythro-isomers of β-chloroglutamate; chloride, ammonia, and α-ketoglutarate are formed in equimolar amounts. The latter product was characterized as the 2,4-dinitrophenyl-hydrazone and by catalytic hydrogenation of this derivative to glutamic acid. The β-elimination reaction is catalyzed by highly purified glutamate-aspartate transaminase; the reaction is not catalyzed by the phosphopyridoxamine form of the enzyme, the apoenzyme, or by free pyridoxal 5;-phosphate. There is no detectable transamination between β-chloroglutamate and either oxaloacetate or αketoglutarate. Incubation of either the holoenzyme or the apoenzyme with β-chlorogultamate does not result in any detectable loss of enzyme activity. In the presence of N-ethylmaleimide much less α-ketoglutarate than ammonia is formed; this is consistent with the idea that a reactive carbanion intermediate reacts with N-ethylmaleimide. The novel β-elimination reaction with glutamate-aspartate transaminase demonstrated here supports the idea that in some instances the specificity of an enzymatic reaction can be determined by the structure of the substrate. [ABSTRACT FROM AUTHOR]