INTERACTIONS BETWEEN GLUCOSE-METABOLISM AND OXIDATIVE PHOSPHORYLATIONS ON RESPIRATORY-COMPETENT SACCHAROMYCES-CEREVISIAE CELLS

The purpose of this work was to analyze the interactions between oxida tive phosphorylations and glucose metabolism on yeast cells aerobicall y grown on lactate as carbon source and incubated in a resting cell me dium. On such respiratory-competent yeast cells, four different metabo lic steady states have particularly been studied: (a) glucose feeding under anaerobiosis, (b) ethanol supply under aerobiosis, (c) glucose s upply under aerobiosis and (d) glucose plus ethanol under aerobiosis. For each condition, we measured: (a) the cellular ATP/ADP ratio and NA DH content sustained under these conditions, (b) the glucose consumpti on rate (glucose conditions) and the respiratory rate (aerobic conditi ons). Under aerobic conditions, when ethanol is used as substrate, the ATP/ADP ratio and NADH level are very high as compared with glucose f eeding. However, the rate of oxygen consumption is similar under both conditions. The main observation is a large increase in the respirator y rate when both glucose and ethanol are added. This increase correspo nds to an ATP/ADP ratio and a NADH level lower than those observed wit h ethanol but higher than those with glucose. Therefore the response o f the respiratory rate to the ATP/ADP ratio depends on the redox poten tial. We studied the way in which the ATP-consuming activity was incre ased under glucose + ethanol conditions. By NMR experiments, it appear s that neither the futile cycle at the level of the phosphofructo-1-ki nase/fructo-1,6-bisphosphatase couple nor the synthesis of carbohydrat e stores could account for the increase in oxidative phosphorylation. However, it is shown that, in the presence of glucose + ethanol, ATP c onsumption is strongly stimulated. It is hypothesized that this consum ption is essentially due to the combination of the well-known plasma m embrane proton-ATPase activation by glucose and the high phosphate pot ential due to oxidative ethanol metabolism. While it is well documente d that oxidative phosphorylations inhibit the glycolytic flux, i.e. th e Pasteur effect, we clearly show in this work that the glycolytic pat hway limits the ability of mitochondria to maintain a cellular phospha te potential.