Effect of Temperature on the Uptake of 35S(-II) by Wine Yeasts

Abstract

The rates of uptake of 35S from S(-II) solutions by wine yeasts, Saccharomyces cerevisiae strains R92 and R104 and Saccharomyces chevalieri strain R93, were measured at pH 3.1 and 7.2 over the temperature range 5 ° C to 80 ° C and at 0.3 (or 0.5) mM and 5.0 mM S(-II) concentrations. Three critical temperatures were observed; the first, at ca 20 ° C is attributed to a phase change of the yeast cell membrane from a crystalline to a liquid crystalline state; the second, at the temperature of maximum activity at 30 ° C to 40 ° C is thought to arise from a switch from a metastable to a thermodynamically more stable state which is less effective in supporting the transport functions; and the third, at tempera tures greater than 50 ° C correlates well with the thermal viability of the yeasts. Variation of the activation energy, Ea, with extracellular S(-II) concen tration was observed and Ea for the uptake of S(-II) from a solution of 5 mM S(-II) at pH 7.2 was higher than at pH 3.1. The values of Ea support the postulate of a simple diffusion of H2S(aq) and carrier mediated transport of HS-(aq) for the transport of S(-II).