Regulation of nicotinamide adenine dinucleotide phosphate levels in yeast

The steady-state levels and redox states of pyridine nucleotide pools have been studied in yeast as a function of external growth conditions. Yeast grown aerobically on 0.8% glucose show two distinct phases of logarithmic growth, a first phase utilizing glucose with ethanol accumulation, and a second phase utilizing ethanol. During growth on glucose, the size of the NADP pool (NADP⁺ + NADPH) is maintained at approximately 12% the size of the NAD pool (NAD⁺ + NADH). Upon exhaustion of glucose, the mechanism(s) that maintain the levels of NADP relative to NAD are altered, resulting in a rapid 2- to 2.5-fold decrease in the size of the NADP pool relative to the size of the NAD pool. The lower levels of NADP are maintained during growth on ethanol. The NAD pool is approximately 50% NADH during both the glucose and ethanol phases of growth, while the NADP pool is approximately 67 and 48% NADPH during the glucose and ethanol phases of growth, respectively. Rapid media transfer experiments show that the decrease in NADP is reversible, that it does not require the net synthesis of pyridine nucleotide or protein, and that changes in the size of the NADP pool relative to the total pyridine nucleotide pool are correlated with changes in the redox state of the NADP pool.