Nicotinamide Adenine Dinucleotide-specific Isocitrate Dehydrogenase from a Higher Plant

The stability constants of the magnesium complexes of citrate, isocitrate, and NAD+ were measured and found to be 10.1 x 103 m-1, 1.35 x 103 m-1, and 0.106 x 103 m-1, respectively. These data were used to calculate the concentrations of free isocitrate, free magnesium, and magnesium isocitrate in mixtures of magnesium, NAD+, and isocitrate. The effect of these variables on the NAD+-specific isocitrate dehydrogenase from peas (Pisum sativum L.) was investigated and it was concluded that (a) the enzyme has a substrate site specific for the magnesium isocitrate complex, (b) the enzyme has an activator site specific for free isocitrate, and (c) it is probable that the enzyme has no requirement for free magnesium.