Processes involved in the creation of buffering capacity and in substrate-induced proton extrusion in the yeast Saccharomyces cerevisiae.

The high pH-maintaining capacity of yeast suspension after glucose-induced acidification, measured as its ability to neutralize added alkali, was found to be due mainly to actively extruded acidity (H+). The buffering action of passively excreted metabolites (CO2, organic acids) and cell surface polyelectrolytes contributed only 15--40% to the overall pH-maintaining capacity which was 10 mmol NaOH/l per pH unit between pH 3 and 4 and 3.5 nmol NaOH/l per pH unit between pH 4 and 7. The buffering capacity of yeast cell-free extract was still higher (up to 4.5-times) than that of glucose-supplied cell suspension; addition of glucose to the extract thus produced considerable titratable acidity but negligible net acidity. The glucose-induced acidification of yeast suspension was stimulated by univalent cations in the sequence K+ greater than Rb+ much greater than Li+ congruent to Cs+ congruent to Na+. The processes participating in the acidification and probably also in the creation of extracellular buffering capacity include excretion of CO2 and organic acids, net extrusion of H+ and K+ (in K+-free media; in K+-containing media this is preceded by an initial rapid K+ uptake), and movements of some anions (phosphate, chlorides). The overall process appears to be electrically silent.