The Reaction of β‐Chloroglutamic Acid with Glutamate‐Aspartate Transaminase

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Abstract
Porcine glutamate‐aspartate transaminase catalyzes a β‐elimination reaction with both the 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‐dinitrophenylhydrazone 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 α‐ketoglutarate. Incubation of either the holoenzyme or the apoenzyme with β‐chloroglutamate 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.
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