The activation of ox-brain NAD+-dependent isocitrate dehydrogenase by magnesium ions

Two independent methods were used to assess the dependence of the activity of ox brain NAD+-dependent isocitrate dehydrogenase on the concentration of magnesium ions. The results indicated the complex between magnesium and isocitrate to be the true substrate for the enzyme. Free isocitrate is neither a substrate nor an inhibitor of the enzyme but free magnesium ions inhibit competitively with respect to the magnesium-isocitrate complex. The inhibition of the enzyme by ATP and citrate appears to be largely explicable in terms of their effects on the concentration of the complex between Mg2+ and isocitrate. The dependence of the activation of the enzyme by ADP on the concentration of magnesium ions suggests that free ADP, rather than its complex with Mg2+, is the activator.