Ammonium assimilation by Candida albicans and other yeasts: evidence for activity of glutamate synthase.

Activities and properties of the ammonium assimilation enzymes NADP+-dependent glutamate dehydrogenase (GDH), glutamate synthase (GOGAT) and glutamine synthetase (GS) were determined in batch and continuous cultures of Candida albicans. NADP+-dependent GDH activity showed allosteric kinetics, with an S0.5 for 2-oxoglutarate of 7.5 mM and an apparent Km for ammonium of 5.0 mM. GOGAT activity was affected by the buffer used for extraction and assay, but in phosphate buffer, kinetics were hyperbolic, yielding Km values for glutamine of 750 microM and for 2-oxoglutarate of 65 microM. The enzymes GOGAT and NADP+-dependent GDH were also assayed in batch cultures of Saccharomyces cerevisiae and three other pathogenic Candida spp.: Candida tropicalis, Candida pseudotropicalis and Candida parapsilosis. Evidence is presented that GS/GOGAT is a major pathway for ammonium assimilation in Candida albicans and that this pathway is also significant in other Candida species.