Regulation by Ammonium of Glutamate Dehydrogenase (NADP+) from Saccharomyces cerevisiae

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Abstract
The activity of glutamate dehydrogenase (NADP+) (NADP-GDH) of S. cerevisiae is decreased under conditions in which intracellular NH3 concentration increases. A high internal NH3 concentration can be obtained by increasing the ammonium sulfate concentration in the culture medium, and by growing the yeast either in acetate + NH3 media, where the pH of the medium rises during growth, or in heavily buffered glucose + NH3 media at pH 7.5. Under these conditions cellular oxoglutarate concentrations do not vary and changes in NADP-GDH activity appear to provide a constant rate of oxoglutarate utilization. The following results suggest that the decrease in NADP-GDH activity in NH3-accumulating yeast cells is brought about by repression of synthesis: after a shift to high ammonium sulfate concentrations, the number of units of activity/cell decreased as the inverse of cell doubling; and the rate of degradation of labeled NADP-GDH was essentially the same in NH3-accumulating yeast cells and in controls, whereas the synthesis constant was much lower in the NH3-accumulating cells than in the controls.