EFFECTS OF ETHANOL AND OTHER ALKANOLS ON TRANSPORT OF ACETIC-ACID IN SACCHAROMYCES-CEREVISIAE

In glucose-grown cells of Saccharomyces cerevisiae IGC 4072, acetic ac id enters only by simple diffusion of the undissociated acid. In these cells, ethanol and other alkanols enhanced the passive influx of labe lled acetic acid. The influx of the acid followed first-order kinetics with a rate constant that increased exponentially with the alcohol co ncentration, and an exponential enhancement constant for each alkanol was estimated. The intracellular concentration of labelled acetic acid aas also enhanced by alkanols, and the effect increased exponentially with alcohol concentration. Acetic acid is transported across the pla sma membrane of acetic acid-, lactic acid-, and ethanol-groan cells by acetate-proton symports. We found that in these cells ethanol and but anol inhibited the transport of labelled acetic acid in a noncompetiti ve way; the maximum transport velocity decreased with alcohol concentr ation, while the affinity of the system for acetate was not significan tly affected by the alcohol. Semilog plots of V-max versus alcohol con centration yielded straight lines with negative slopes from which esti mates of the inhibition constant for each alkanol could be obtained. T he intracellular concentration of labelled acid was significantly redu ced in the presence of ethanol or butanol, and the effect increased wi th the alcohol concentration. We postulate that the absence of an oper ational carrier for acetate in glucose-grown tells of S. cerevisiae, c ombined with the relatively high permeability of the plasma membrane f or the undissociated acid and the inability of the organism to metabol ize acetic acid, could be one of the reasons why this species exhibits low tolerance to acidic environments containing ethanol.