Intracellular acidification does not account for inhibition of Saccharomyces cerevisiae growth in the presence of ethanol.

Intracellular acidification has been considered one of a number of mechanisms underlying the inhibition of growth and fermentation by ethanol in yeast. However, most of the studies on the effect of ethanol on yeast intracellular pH (pHi) were carried out by using unadapted cells to which ethanol was added. In this paper we show that the pHi of exponential cells of Saccharomyces cerevisiae IGC 3507 III grown in a medium with glucose and inhibitory concentrations of ethanol only decreased to values below those in unstressed cells (6.9) for concentrations equal to or above 7% (v/v). Only at these supracritical levels (7-10% (v/v)) was pH homeostasis in ethanol-adapted yeast affected. This is consistent with the significant increase of plasma membrane permeability and decrease of plasma membrane H(+)-ATPase in comparison with the corresponding values in unstressed cells. These deleterious effects were only observed with those high concentrations of toxin. These results indicate that intracellular acidification does not account for inhibition of yeast growth in the presence of ethanol. In fact, growth was inhibited by ethanol concentrations (3-6% (v/v)) that did not lead to the decrease of pHi. Furthermore, even for supracritical concentrations, close to the maximal that allowed growth (10% (v/v)), the decrease of pHi was not important reaching, at the most, values of 6.5-6.6.