NADP-specific isocitrate dehydrogenase from the citric acid-accumulating fungus Aspergillus niger

NADP-specific isocitrate dehydrogenase [threo-DS-isocitrate: NADP+ oxidoreductase (decarboxylating), EC 1.1.1.42] was purified 200-300-fold from the citric acid-accumulating fungus Aspergillus niger. The enzyme consists of a single polypeptide chain with a molecular mass of 60 +/- 4 kDa and has a pI of 5.9 +/- 0.2. Only a single enzyme protein was found, although the enzyme appears to occur both in the mitochondrion and in the cytoplasm. Growth on organic acids as carbon sources or on NO3- as nitrogen source led to increased activities, whereas the presence of amino acids led to lower activities. The enzyme exhibits hyperbolic kinetics with respect to its substrates isocitrate and NADP+. Mn2+ and Mg2+ are obligatory for enzyme activity. The enzyme is inhibited by its products alpha-oxoglutarate and NADPH. Among various metabolites, ATP and citrate appear to inhibit the enzyme at a concentration considered to occur intracellularly. In both cases, however, the mechanism is a removal of the metal ion cofactor from the substrates. It is concluded that under physiological conditions, where the Mg2+ content is around 10 mM, the observed inhibition by ATP or citrate is of poor regulatory significance.