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 Σ1278b, two mutants lacking one or both of the putative K+ transporters, trklΔ and trklΔtrk2Δ, and a mutant in the plasma membrane H+‐ATPase, viz. pmal‐105, were compared in their extracellular acidification following addition of glucose and subsequent addition of KCl; in ATPase activity 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 was found in the ATPase‐deficient pmal‐105 strain but also in the TRK‐deficient double mutant. The acidification pattern after addition of KCl corresponds to expectations in the TRK mutants; however, a similarly decreased acid production was found in the ATPase‐deficient mutant pmal‐105. The highest rate of ATP hydrolysis in vitro was found with the trklΔtrk2Δ mutant where glucose‐, as well as KCl‐induced acidification were lowest. Likewise, the pmal‐105 mutant with extremely low acidification showed only a minutely lower ATP hydrolysis than did its parent Y55 strain. Apparently, several different sources of acidity are involved in the glucose‐induced acidification (including extrusion of organic acids); in fact, contrary to the general belief, the H+‐ATPase may play a minor role in this process in some strains.