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A comparative study of the citrate synthases purified from the facultatively photosynthetic bacterium Rhodospirillum rubrum (Gram negative) and the thermophile Bacillus stearothermophilus (Gram positive) was made. The citrate synthase from R. rubrum was activated by KCl (6-fold at 0.1 M KCl) and, less effectively, by NaCl and NH4Cl. Its molecular weight was about 300,000. The enzyme was strongly inhibited by NADH, and this inhibition was counteracted by AMP. The citrate synthase from B. stearothermophilus was little affected by KCl, NaCl and NH4Cl, all of which activated by about 25% at 0.1 M. Its molecular weight was ca 100,000. The enzyme was not affected by NADH or AMP. Both citrate synthases were insensitive to alpah-oxoglutarate concentrations up to 5 mM, and were inhibited by ATP; the B. stearothermophilus enzyme was more strongly inhibited than the R. rubrum enzyme. In both cases the ATP inhibition was strictly competitive towards acetyl-CoA and non-competitive towards oxaloacetate. Both enzymes, in spite of the peculiar physiological properties of their bacterial sources, followed the close correlation between the properties of the citrate synthase and the taxonomical position of the microorganism, proposed by Weitzman and his co-workers.

Citrate synthase, purified 600-fold from Rhodospirillum rubrum, is activated by KCl and inhibited by ATP and NADH; the effect of the latter inhibitor is completely counteracted by AMP and partially counteracted by KCl.