The Reaction Mechanisms and Substrate-Stereospecificities of L-Alloisocitrate Dehydrogenase and Oxalosuccinate Decarboxylase

Abstract

A sequential reaction was suggested for the conversion of L-alloisocitrate to α-oxoglutarate by an enzyme complex of L-alloisocitrate dehydrogenase and oxalosuccinate decarboxylase from Pseudomonas strain No. 2, during which oxalosuccinate was not released from the enzyme-substrate complex. The stereochemistry of oxalosuccinate formed by L-alloisocitrate dehydrogenase and decarboxylated by oxalosuccinate decarboxylase was opposite to that of the substrate for D-isocitrate dehydrogenase. Incubation of L-alloisocitrate with the dehydrogenase and decarboxylase in deuterium oxide provided [3-²H]-α-oxoglutarate, the configuration of which turned out to be the same as that produced by D-isocitrate dehydrogenase from D-isocitrate. The data suggested that enol form of α-oxoglutarate was involved as an intermediate in decarboxylation of oxalosuccinate by oxalosuccinate decarboxylase. L-Alloisocitrate dehydrogenase was shown to react with pro-S proton of NADH.