Energy metabolism in Saccharomyces cerevisiae. Effect of progressive starvation on respiration and pyridine nucleotide reduction linked to ethanol oxidation in intact cells

The progressive effects of aerobic starvation on endogenous and ethanol-linked respiration and pyridine nucleotide reduction have been studied in the yeast Saccharomyces cerevisiae. Three distinct phases of pyridine nucleotide reduction were observed when ethanol was added to unstarved yeast: an initial phase of rapid reduction and accelerating respiration (A); a steady-state phase of reduction with maximal respiration (B); a final phase of rapid reduction at anaerobiosis (C).During the first 5 hr of aeration, the steady-state Phase B was replaced by a phase of slow pyridine nucleotide reduction, while Phases A and C were unaffected. During this period, both endogenous pyridine nucleotide reduction and endogenous respiration decreased sharply.Between 5 and 22 hr of aeration, the endogenous level of reduced pyridine nucleotide declined further, while endogenous respiration remained unchanged. Concurrently, the extent of the Phase A reduction doubled.The addition of ethanol to aerobic, unstarved yeast stimulated a rapid pyridine nucleotide reduction, with further reduction occurring at anaerobiosis. Under anaerobic conditions, the addition of ethanol to unstarved yeast caused little further reduction of pyridine nucleotide. Two hours of starvation decreased the extent of the endogenously supported anaerobic reduction and correspondingly increased the ethanol-induced reduction. These results suggest that, in unstarved yeast, reducing equivalents derived from ethanol under aerobic conditions and those derived from endogenous carbohydrate under anaerobic conditions have access to the same pool of pyridine nucleotide. With starvation, this pool becomes accessible to ethanol-derived (or ethanol-mobilized) reducing equivalents under anaerobic conditions.