The role of the mitochondrial NAD+: isocitrate dehydrogenase in lipid accumulation by the oleaginous yeast Rhodosporidium toruloides CBS 14

Abstract

The mitochondrial NAD+: isocitrate dehydrogenase from Rhodosporidium toruloides CBS 14 had a molecular radius of 191 800 daltons, a pH optimum of 7.8, and an apparent Kms for isocitrate and NAD+ of 109 and 365 μM, respectively. The enzyme was absolutely dependent on AMP for activity and showed sigmoidal activation curves with respect to isocitrate concentration. ATP was shown to be a potent inhibitor even in the presence of saturating AMP and isocitrate concentrations. NADH was the most potent inhibitor, followed by L-glutamate and α-ketoglutarate, and less strongly by citrate and oxaloacetate. The inhibition by L-glutamate and α-ketoglutarate was noncompetitive, with respect to isocitrate, and indicated that the enzyme could be very susceptible to regulation by the products of organic nitrogen metabolism in this yeast. Inhibition by citrate and more so the strong inhibition by NADH illustrated the interplay between the inactivation of NAD+: isocitrate dehydrogenase and the steps leading to the biosynthesis of acetyl-CoA. The key role of this enzyme in lipid biosynthesis in oleaginous yeasts is discussed.