Eubacterial isocitrate dehydrogenase with dual specificity for NAD and NADP from Rhodomicrobium vannielii

Cell-free extracts of the photosynthetic eubacterium Rhodomicrobium vannielii contained both NADP and NAD-linked isocitrate dehydrogenase activities. Apparent Km values of 12 µM for NADP, 0.75 mM for NAD, 9.3 µM for isocitrate (NADP utilising) and 8.2 µM for isocitrate (NAD utilising) were determined in such extracts. Four lines of evidence indicated that one enzyme was responsible for the two activities; (i) non-additivity of reaction rates in the presence of both NADP and NAD (ii) the presence of one band which stained for activity with both cofactors on non-denaturing polyacrylamide gels (iii) identical heat inactivation kinetics for both activities (iv) co-elution of both activities after ion-exchange and hydrophobic interaction chromatography. This is the first report of a eubacterial isocitrate dehydrogenase with dual cofactor specificity.