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-speci fic isocitrate dehydrogenase (ICDH) of up to 3 U/mg was found in cell-free extracts of the filamentous fungus Ashbya gossypii. Since this activity cou ld mean a loss of carbon for riboflavin formation, the subcellular localiza tion of this enzyme and its role in the metabolism were studied. While the NADP-specific ICDH, localized by density gradient centrifugation in peroxis omes, followed Michaelis-Menten-type kinetics for the substrate isocitrate, the mitochondrial NAD-specific isoenzyme exhibited an allosteric regulatio n 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-dienoy l-CoA reductase was exclusively found in peroxisomes, while citrate synthas e and alpha-ketoglutarate dehydrogenase complex (KGDH) were found only in m itochondria, and NAD-specific glutamate dehydrogenase was found in the cyto sol. The data shown are consistent with the assumption that the NADP-specif ic ICDH of A. gossypii provides reducing equivalents for the peroxisomal me tabolism, probably for the degradation of unsaturated fatty acids. (C) 2000 Elsevier Science B.V. All rights reserved.