REGULATING THE ACTIVITY OF NAD-SPECIFIC ISOCITRATE DEHYDROGENASE FROM MAIZE SCUTELLA

NAD-specific isocitrate dehydrogenase was partially purified from maize scutella (Zea mays L.), and some regulatory properties of this preparation were examined. Enzyme activity in vivo is likely to be controlled by the intracellular NAD/NADH ratio as well as by the level of several organic acids related to the tricarboxylic acid cycle. Citrate positively effected the partially purified enzyme, whereas 2-oxoglutarate, cis-, trans-aconitate, and a combination of glyoxylate and oxaloacetate were inhibitory. It is suggested that the enzyme molecule binds at least four molecules of substrate, and that this substrate binding is highly cooperative. Mn2+ and Mg2+ ions stimulated enzymatic activity, although Mn2+ was more efficient than Mg2+. Neither AMP nor ADP had any effect on the enzyme, while ATP was inhibitory.