Acquisition of Ethanol Tolerance in Saccharomyces cerevisiae: The Key Role of the Mitochondrial Superoxide Dismutase

Saccharomyces cerevisiae aBR10 cells are able to develop resistance to lethal ethanol concentrations (14%, v/v), by preexposure to a sublethal heat shock (37°C) or ethanol stress (8%, v/v). Heat shock and 8% ethanol stress had no effect on the concentrations of glutathione [reduced (GSH) and oxidized (GSSG) forms] and on glutathione reductase and CuZn superoxide dismutase (SOD) activities, suggesting that the development of resistance to lethal ethanol concentrations is independent of these antioxidant defenses. In fact, a S. cerevisiae mutant, deficient in CuZnSOD, had an even higher ethanol tolerance, compared to the wild-type strain, and this mutation did not impair a further acquisition of ethanol tolerance. In contrast to CuZnSOD, the MnSOD activity seems to play a more important role in ethanol resistance. The MnSOD activity of the S. cerevisiae aBR10 cells increased upon exposure to heat shock or 8% ethanol. The higher tolerance to 14% ethanol in CuZnSOD deficient cells was also associated to a higher MnSOD activity, as compared to the aBR10 cells; this activity decreased during both stress pretreatments (while still higher than that observed in the wild-type strain). The results obtained suggest that maximum ethanol tolerance is attained with a MnSOD activity close to 1.0 U/mg protein. On either side of this value, the increased sensitivity of S. cerevisiae cells to 14% ethanol might be due to an inability to prevent either superoxide radical- or hydrogen peroxide-induced damages, respectively. These results are supported by the fact that a MnSOD deficiency renders yeast cells more ethanol sensitive.