The central role of NH4+-assimilation in the microbial metabolization of several inorganic nitrogen sources and the regulation of its key enzymes--glutamate dehydrogenases (GDH--EC 1.4.1.2. and EC 1.4.1.4.), glutamine synthetase (GS--EC 6.3.1.2.) and glutamate synthase (GOGAT--EC 1.4.1.3.)--are presented. In excess of ammonia gramnegative bacteria as well as yeasts assimilate this ion in a NADH + H+ or NADPH + H+ dependent reaction by GDH. Under NH4+-limitation--in natural environments rather the rule than the exception--the ammonia assimilation is ATP dependent and catalyzed via GS and GOGAT. Subsequently the connection between the nitrogen metabolization and the resulting changes in the extracellular pH of growing yeast cultures is discussed. The stoichiometric exchange between NH4+ and H+ led to the assumption that in the physiological pH-range an energy dependent NH4+/H+ transport is the preferred++ mechanism of ammonia uptake for NH4+ excess as well as for NH4+ shortage.
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