This study investigates how ammonia concentration regulates glutamate dehydrogenase (NADP+) (NADP-GDH) in the yeast Saccharomyces cerevisiae. The researchers found that increased intracellular ammonia concentration, achieved either by raising external ammonium sulfate levels or by increasing medium pH to promote ammonia diffusion, causes a substantial decrease in NADP-GDH activity. Interestingly, this reduction occurs through repression of enzyme synthesis rather than increased protein degradation, distinguishing it from other regulatory mechanisms during nutrient starvation. Radioactive labeling experiments showed that the synthesis rate constant decreased significantly in ammonia-accumulating cells while degradation rates remained unchanged. The physiological significance appears to be maintaining constant intracellular 2-oxoglutarate concentrations across varying ammonia conditions. The enzyme's activity adjustments help maintain a relatively constant metabolic flux through the amination pathway despite changing substrate availability, suggesting the yeast cell prioritizes preserving adequate 2-oxoglutarate levels for biosynthesis and central metabolism.

Key findings

• NADP-GDH activity decreases when intracellular ammonia concentration increases, regardless of whether ammonia accumulates from high external ammonium sulfate or pH-dependent diffusion
• The activity decrease results from repression of enzyme synthesis (lower synthesis rate constant) rather than increased protein degradation, unlike carbon starvation effects
• Changes in NADP-GDH activity maintain relatively constant intracellular 2-oxoglutarate concentrations and sustained metabolic flux through the amination reaction across varying ammonia conditions
• The enzyme serves as a high-capacity, low-affinity ammonia assimilation system that operates mainly under ammonia excess in S. cerevisiae