Effects of weak acids on cation accumulation, delta pH and delta psi in yeast

With glucose as carbon source, as reported by other authors (Conway et al., 1950), yeast cells accumulated large amounts of CO2, carbonic acid, bicarbonate and K+ than when grown with ethanol. The addition of propionic acid to the cells produced an increase in the potassium ion accumulation when cells were incubated with ethanol as substrate, but not when incubated with glucose. In disagreement with Ryan et al. (1971) and Ryan and Ryan (1972), the internal pH of the cells with ethanol was lower than with glucose, eliminating this factor as the limiting factor for potassium accumulation. When the membrane potential difference was estimated, it was found that it was higher with glucose as substrate than with ethanol. In addition, with ethanol the addition of propionic acid produced an increase of the membrane potential, in agreement with the idea of an accumulation of the anions of monoprotic acids, which, being not diffusible, increase the negative membrane potential inside, which can drive the accumulation of larger amounts of monovalent cations. It was also found that the addition of propionic acid to cells incubated with glucose as substrate produced an efflux of CO2 from the cells, so that an exchange of the acid appears to take place.