| Abstract: | Acinetobacter calcoaceticus contains two forms of NADP+-dependent isocitrate dehydrogenases differing, among others, by their molecular weights and regulatory properties. The regulation of the high-molecular form of isocitrate dehydrogenase and of isocitrate lyase by organic acids, either belonging or related to the citrate and glyoxalate cycle, is investigated. While alpha-ketoglutarate and oxalacetate competitively inhibit the isocitrate dehydrogenase against Ds-isocitrate, glyoxylate and pyruvate were found to increase Vmax and to lower the KM value for Ds-isocitrate and NADP+. Simultaneous addition of oxalacetate and glyoxylate (not, however, addition of the nonenzymatically formed condensation product of both compound) nullified the activation of isocitrate dehydrogenase by glyoxylate, and potentiates the inhibitory effect of oxalacetate. Alpha-ketoglutarate, succinate, and phosphoenolpyruvate inhibit the isocitrate lyase in a noncompetitive fashion against DS-isocitrate; L-malate, oxalacetate and glyoxylate inhibit competitively. The intermediates of the citrate and glyoxylate cycle afford additive inhibition of the isocitrate lyase. The importance of organic acids of the citrate and glyoxylate cycle and of phosphoenolpyruvate for the regulation of the citrate and glyoxylate cycle at the level of isocitrate dehydrogenase and isocitrate lyase is discussed. |
