Regulation of the TCA and Glyoxylate Cycles in Brevibacterium flavum: I. Inhibition of lsocitrate Lyase and Isocitrate Dehydrogenase by Organic Acids Related to the TCA and Glyoxylate Cycles*
1. NADP-specific isocitrate dehydrogenase [EC 1.1.1.42] and isocitrate lyase [EC 4.1.3.1] were separated from a sonic extract of Brevi bacterium flavum No. 2247 cells grown on acetate and were partially purified by repeated ammonium-sulfate fractionation. The pH optima and various kinetic constants of the two enzymes were determined. 2. The inhibitory effects on them of intermediates of the TCA and glyoxylate cycles and related amino acids were examined. Isocitrate dehydrogenase was completely inhibited by the presence of both oxal-acetate and glyoxylate at low concentrations. It was also inhibited by oxalacetate, α-ketoglutarate and glyoxylate. Isocitrate lyase was competitively inhibited by oxalate, glyoxylate, malate, oxalacetate, and α-ketoglutarate. Succinate is a ‘mixed-type’ inhibitor for this enzyme. Intermediates of the TCA cycle inhibited isocitrate lyase much more than isocitrate dehydrogenase, and their inhibitory effects were additive, so that simultaneous addition of all the intermediates, even at low concentration, caused strong inhibition. Thus, the TCA cycle intermediates, as a whole, may have a strong control on isocitrate lyase activity in vivo.