Stereospecificity of Citrate Synthetase in Relation to Glutamate Biosynthesis by Extracts of Chloropseudomonas ethylicum

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The synthesis of citric and glutamic acids by extracts of Chloropseudomonas ethylicum was studied with labeled precursors. When acetyl-coenzyme A-1-14C was used as substrate, only 0.1% of the total radioactivity was found in the C-5 position of citric acid; whereas, with oxalacetate-4-14C as substrate, 100% of the total radioactivity was found in C-5. These results demonstrated that the Chloropseudomonas citrate synthetase had an absolute stereospecificity, identical to that of the pig heart synthetase. The distribution of radioactivity in the glutamic acid synthesized from acetyl-coenzyme A-1-14C was 0% in C-1 and 94.0% in C-5; whereas the glutamic acid formed from oxalacetate-4-14C contained 89.6% in C-1 and 0.5% in C-5. This distribution is entirely consistent with the biosynthesis of glutamic acid from citric acid via aconitase, ds-isocitrate, and l-glutamate dehydrogenases. The presence of l-glutamate dehydrogenase in extracts was demonstrated. The stereospecificity of the citrate synthetase and the pattern of glutamate labeling further establish that the aconitase of Chloropseudomonas is completely stereospecific.

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