Effect of the pH of the incubation medium on glycolysis and respiration in Saccharomyces cerevisiae

Abstract

An investigation was undertaken in order to explain the effect of pH (4.0 and 7.5) of the incubation medium on the anaerobic glycolysis and respiration in yeast, in which a higher ethanol production and glucose consumption, as well as a higher respiratory rate at the high pH are observed. The measurement of the changes in the concentrations of the metabolites of the glycolytic sequence, NADH, ADP, ATP, and P i that appeared when the pH was suddenly increased, permitted us to detect an increase of ADP and P i and a decrease of the ATP levels, which in conjunction with the changes in the concentrations of the glycolytic intermediates indicate that at the high pH there was an increase in the activity of an ATPase system which is responsible for the acceleration of glycolysis. This same ATPase activity could be shown under similar conditions with ethanol as substrate in respiring yeast, and the increased levels of ADP seem to be responsible also for the increased respiratory rate at the high pH. When the pH was increased from 4.0 to 7.5, this ATPase activity was induced if potassium ions were absent from the incubation medium, but no important changes in the levels of the adenosine phosphates were observed upon the pH increase when these ions were previously included. The data could be explained on the basis of the existence of an energy-requiring proton pump, which might be of the kind described by Mitchell; it would have the role of transporting H + to the outside of the cell. Such an anisotropic system ought to be inhibited by increasing concentrations of H + on the outside, and stimulated at the high pH. At the low pH, K + would allow the function of this system, by exchanging for H +. At the high pH, this ATP-utilizing system could work without K +, and anions would leave the cell accompanying protons; in the presence of K +, the cation could be taken up by yeast in exchange for the same protons, lowering the amount of anions expelled, without a further increase of the ATP utilization. The results presented in this communication, furthermore do not support the idea that the pH of the yeast cell remains constant with variations of the pH of the incubation medium. This inconstancy has important implications for the regulation of both glycolysis and respiration in yeast as a consequence of the changes of the external pH.