The purpose of this work was to analyze the interactions between oxidative phosphorylations and glucose metabolism on yeast cells aerobically grown on lactate as carbon source and incubated in a resting cell medium. On such respiratory-competent yeast cells, four different metabolic steady states have particularly been studied: (a) glucose feeding under anaerobiosis, (b) ethanol supply under aerobiosis, (c) glucose supply under aerobiosis and (d) glucose plus ethanol under aerobiosis. For each condition, we measured: (a) the cellular ATP/ADP ratio and NADH content sustained under these conditions, (b) the glucose consumption rate (glucose conditions) and the respiratory rate (aerobic conditions).
Under aerobic conditions, when ethanol is used as substrate, the ATP/ADP ratio and NADH level are very high as compared with glucose feeding. However, the rate of oxygen consumption is similar under both conditions. The main observation is a large increase in the respiratory rate when both glucose and ethanol are added. This increase corresponds to an ATP/ADP ratio and a NADH level lower than those observed with ethanol but higher than those with glucose. Therefore the response of the respiratory rate to the ATP/ADP ratio depends on the redox potential. We studied the way in which the ATP-consuming activity was increased under glucose + ethanol conditions. By NMR experiments, it appears that neither the futile cycle at the level of the phosphofructo-1-kinase/fructo-1,6-bisphosphatase couple nor the synthesis of carbohydrate stores could account for the increase in oxidative phosphorylation. However, it is shown that, in the presence of glucose + ethanol, ATP consumption is strongly stimulated. It is hypothesized that this consumption is essentially due to the combination of the well-known plasma membrane proton-ATPase activation by glucose and the high phosphate potential due to oxidative ethanol metabolism. While it is well documented that oxidative phosphorylations inhibit the glycolytic flux, i.e. the Pasteur effect, we clearly show in this work that the glycolytic pathway limits the ability of mitochondria to maintain a cellular phosphate potential.