l-Glutamate and l-aspartate are the natural nitrogen sources preferred by the fungus Pleurotus ostreatus. In the vegetative mycelium NAD-dependent glutamate dehydrogenase (NAD-GDH) and glutamine synthetase (GS) were both present, but the highest level of these enzymes was found in the fruiting body. NAD-GDH was derepressed by ammonia and repressed by higher concentrations of l-glutamate. Otherwise, the level of the enzyme in mycelium was independent of the nitrogen source. GS was derepressed by ammonia, l-glutamate, l-aspartate and l-alanine. Repression of GS was observed in the presence of l-glutamine, l-arginine, nitrate, urea and low concentrations of ammonia. An increase in the ratio of glucose to nitrogen resulted in a decrease in the level of GS in the mycelium. The Km values of NAD-GDH for 2-oxoglutarate, ammonia, NADH, l-glutamate and NAD+ were 2·9 mM, 3·3 mM, 11 μM, 0·18 mM and 62 μM, respectively. The Km values of GS for l-glutamate and ammonia were 2·5 and 0·27 mM, respectively. The repression of GS and derepression of NAD-GDH in the presence of low concentrations of ammonia together with glutamate or aspartate could be explained by an inhibition or repression of some amino acid permeases by ammonia. The absence of NADP-dependent glutamate dehydrogenase and glutamate synthase together with the apparent mode of regulation of NAD-GDH and GS synthesis suggest that P. ostreatus assimilates ammonia via GS and NAD-GDH. However, a dissimilatory role of NAD-GDH in the deamination of l-glutamate, due to its very low Km for l-glutamate, is not excluded.
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