Pb2+ inhibits competitively flocculation of Saccharomyces cerevisiae

Pb2+ inhibited calcium-induced flocculation in Flo1 (S646-1B) and NewFlo phenotype strains (NCYC 1190, NCYC 1195 and NCYC 1364) of Saccharomyces cerevisiae. Flocculation was restored after washing with water or EDTA, which suggests a reversible binding of Pb2+ to yeast cell walls. Pb2+ probably inhibited flocculation by competing with Ca2+, since Pb2+ inhibition was alleviated by excess of Ca2+. Using a fluorescent avidin-fluorescein isothiocyanate (Avidin-FITC) probe, active cell surface flocculation lectins, in the presence of Ca2+ ions, were visualized. Conversely, Avidin-FITC was not fixed to yeast walls of flocculent cells, in the presence of Pb2+ ions or in the simultaneous presence of 0.05 mM Ca2+ and 0.4 mM Pb2+. These results suggest that Pb2+ ions were not able to induce the correct conformation of the lectin-like component and reinforces the hypothesis that Pb2+ ions compete for the same "calcium site" of flocculation zymolectins.