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

Department of Chemistry University of Auckland Private Bag Auckland, New Zealand

Ragina M.D. Singh

Department of Chemistry University of Auckland Private Bag Auckland, New Zealand

Peter Walde

Department of Chemistry University of Auckland Private Bag Auckland, New Zealand

D. John Spedding

Department of Chemistry University of Auckland Private Bag Auckland, New Zealand

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 a variety of solution pH values between pH 3.1 and pH 7.8. A pH effect was observed, the rates of uptake being higher at the lower pH values, but this effect was not related entirely to changes in the H₂S or HS- concentration. The transport process of S(-II) appeared to be due to simple diffusion of H₂S(aq) and carrier mediated transport of HS-(aq). The kinetic constants K_m and V _max were calculated for the carrier component of the mechanism at pH 7.2 and the permeability coefficient P was calculated for the diffusion of H₂S(aq) at pH 3.1 and 7.2. By using these parameters, it was possible to calculate a theoretical ini tial rate of uptake over a range of extracellular S(-II) concentrations (0 to 50 mmoll^-1) at pH 3.1 and pH 7.2. The experimentally determined initial rates were found to agree, within the experimental error, with the theoretical values. The initial rate of uptake of S(-II) and the values of K_m for yeast strain R104 (a low sulfide producer) were found to be less than those for both strain R92 (a normal sulfide producer) and for strain R93 (a high sulfide producer).

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