Synergistic effects of ethanol and temperature on yeast mitochondria

At temperatures lower than 37°C, the ethanol inhibition constant (Ki) for growth or fermentation inrho⁺ cells of theSaccharomyces cerevisiae strain S288C was always higher (1.1M) than inrho^– mutants (0.7M). At 37°C these differences disappeared, and both strains were equally inhibited by ethanol (Ki=0.7m). Mitochondrial activity can be inhibited by high ethanol concentration and temperature. In fact, the stronger inhibition by ethanol of therho⁺ strain at 37°C was due to the fact that, under these conditions, this strain loses the advantage conferred by mitochondrial activity since the induction ofrho^– cells in the population is very high. This does not result in an increase in the frequency ofrho^– mutants because of the poor viability of these mutants in conditions of high temperature and ethanol. In consequence, S288C strain becomes as strongly inhibited by ethanol as therho^– mutant strains. Differences in viability were not related to the fatty acids and ergosterol composition of the strain. In the presence of ethanol, bothrho⁺ andrho^– strains modified their lipids in the same way, but these changes did not improve their ethanol tolerance. They were not due to differences in adaptation to ethanol either, since after successive transfers in ethanol, growth () and fermentation () rates in therho^– mutants were increasingly inhibited with time, whereas in the S288C strain inhibition of and by ethanol remained unaltered. Rather,rho^– mutants are less viable thanrho⁺ cells because of the inability of the former to respire. At 37°C the Ki increased to 0.9M ethanol either when mitochondrial from highly ethanol-tolerant wine yeasts were transferred torho^– mutants of the strain S288C or when the mitochondria of strain S288C were preadapted by growing the strain in glycerol instead of glucose before it was cultivated in ethanol.