Properties of the nicotinamide adenine dinucleotide phosphate-specific isocitrate dehydrogenase from Blastocladiella emersonii.

The nicotinamide adenine dinucleotide phosphate (NADP)-specific isocitrate dehydrogenase from Blastocladiella emersonii was purified. The enzyme was very unstable. Satisfactory stability was obtained in the presence of 0.2% ovalbumin. The enzyme had a molecular weight of about 100,000. It did not exhibit homotropic cooperativity for any of it substrates and was not affected by the allosteric modifiers citrate and adenosine monophosphate, diphosphate, and tri-phosphate. The substrate saturation studies showed both intercept and slope effects in Lineweaver-Burk plots. The Km values for isocitrate and NADP were found to be 20 and 10 muM, respectively. The product inhibition pattern was compatible with a random sequential reaction mechanism. The enzyme catalyzed the oxidative decarboxylation of isocitrate about six times better than the reductive carboxylation of alpha-ketoglutarate. The enzyme was inhibited by glyoxylate plus oxalacetate. Assays conducted in the presence of low Mg2+ concentrations exhibited a lag. This lag could be abolished by the addition of reduced NADP to the assay mixture.