Nicotinamide Adenine Dinucleotide Phosphate-specific Isocitrate Dehydrogenase from a Higher Plant

Nicotinamide adenine dinucleotide phosphate-specific isocitrate dehydrogenase was extracted from etiolated pea (Pisum sativum L.) seedlings and was purified 65-fold. The purified enzyme exhibits one predominant protein band by polyacrylamide gel electrophoresis, which corresponds to the dehydrogenase activity as measured by the nitro blue tetrazolium technique. The reaction is readily reversible, the pH optima for the forward (nicotinamide adenine dinucleotide phosphate reduction) and reverse reactions being 8.4 and 6.0, respectively. The enzyme has different cofactor and inhibitor characteristics in the two directions. Manganese ions can be used as a cofactor for the reaction in each direction but magnesium ions only act as a cofactor in the forward reaction. Zinc ions, and to a lesser extent calcium ions, inhibit the enzyme at low concentrations when magnesium but not manganese is the metal activator. It is suggested that there is a fundamental difference between magnesium and manganese in the activation of the enzyme. The enzyme shows normal kinetics and the Michaelis contant for each substrate was determined. The inhibition by nucleotides, nucleosides, reaction products, and related compounds was studied. The enzyme shows a linear response to the mole fraction of reduced nicotinamide adenine dinucleotide phosphate when total nicotinamide adenine dinucleotide phosphate (nicotinamide adenine dinucleotide phosphate plus reduced nicotinamide adenine dinucleotide phosphate) is kept constant. Isocitrate in the presence of divalent metal ions will protect the enzyme from inactivation by p-chloromercuribenzoate. Protection is also afforded by manganese ions alone but not by magnesium ions alone There is a concerted inhibition of the enzyme by oxalacetate and glyoxylate.