Different sources of acidity in glucose-elicited extracellular acidification in the yeast Saccharomyces cerevisiae

Three wild-type strains of Saccharomyces cerevisiae, viz. K, Y55 and Sigma 1278b, two mutants lacking one or both of the putative K+ transporters, trk 1 Delta and trk1 Delta trk2 Delta and a mutant in the plasma membrane H+-AT Pase, viz. pma1-105, were compared in their extracellular acidification fol lowing addition of glucose and subsequent addition of KCI; in ATPase activi ty in purified plasma membranes; and in respiration on glucose. The glucose -induced acidification was the greater the higher the respiratory quotient, i.e. the higher the anaerobic metabolism. A markedly lower acidification w as found in the ATPase-deficient pma1-105 strain but also in the TRK-defici ent double mutant. The acidification pattern after addition of KCl correspo nds to expectations in the TRK mutants; however, a similarly decreased acid production was found in the ATPase-deficient mutant pma1-105. The highest rate of ATP hydrolysis in vitro was found with the trk1 Delta trk2 Delta mu tant where glucose-, as well as KCl-induced acidification were lowest. Like wise, the pma1-105 mutant with extremely low acidification showed only a mi nutely lower ATP hydrolysis than did its parent Y55 strain. Apparently, sev eral different sources of acidity are involved in the glucose-induced acidi fication (including extrusion of organic acids); in fact, contrary to the g eneral belief, the H+-ATPase may play a minor role in this process in some strains.