Role of a peroxisomal NADP-specific isocitrate dehydrogenase in the metabolism of the riboflavin overproducer Ashbya gossypii

During growth on soybean oil as carbon source a high activity of NADP-specific isocitrate dehydrogenase (ICDH) of up to 3 U/mg was found in cell-free extracts of the filamentous fungus Ashbya gossypii. Since this activity could mean a loss of carbon for riboflavin formation, the subcellular localization of this enzyme and its role in the metabolism were studied. While the NADP-specific ICDH, localized by density gradient centrifugation in peroxisomes, followed Michaelis–Menten-type kinetics for the substrate isocitrate, the mitochondrial NAD-specific isoenzyme exhibited an allosteric regulation by adenine nucleotides. Localization of enzymes involved in the substrate supply and the conversion of reaction products was investigated to explain the metabolic function of the peroxisomal ICDH. NADPH-oxidizing 2,4-dienoyl-CoA reductase was exclusively found in peroxisomes, while citrate synthase and α-ketoglutarate dehydrogenase complex (KGDH) were found only in mitochondria, and NAD-specific glutamate dehydrogenase was found in the cytosol. The data shown are consistent with the assumption that the NADP-specific ICDH of A. gossypii provides reducing equivalents for the peroxisomal metabolism, probably for the degradation of unsaturated fatty acids.