Ethanol Dissipates the Proton-motive Force across the Plasma Membrane of Saccharomyces cerevisiae

Summary: Populations of Saccharomyces cerevisiae NCYC 431, harvested after 16 h incubation from self-induced anaerobic cultures, were more tolerant to the inhibitory effect of ethanol on fermentation rate and viability than organisms harvested from 8 h cultures. Ethanol increased the rate of passive influx of protons into de-energized organisms at a rate which was greater with organisms from 8 h compared with 16 h cultures. Rates of passive influx of protons into spheroplasts were significantly greater than into intact organisms, although culture age did not affect rates of ethanol-induced influx of protons into spheroplasts. Ethanol retarded both the initial net rate of proton efflux and the final extent of acidification produced by suspensions of energized organisms, both effects being more pronounced with organisms from 8 h as compared with 16 h cultures. The magnitude of the proton-motive force (Δp) was decreased by ethanol in both energized and de-energized organisms. Although culture age did not affect the extent of ethanol-induced decrease in Δp in de-energized organisms, in energized organisms harvested from 8 h cultures ethanol produced a significantly greater decrease in Δp as compared with organisms from 16 h cultures. If the ability of ethanol to decrease the Δp value is important in its inhibitory effect on growth, it is suggested that some phenomenon other than proton uncoupling is involved.