Differential utilisation of sulfur compounds for H₂S liberation by nitrogen-starved wine yeasts

Abstract

Hydrogen sulfide (H2S) liberation by Saccharomyces cerevisiae wine yeast was studied as a function of different inorganic sulfurous precursors using a model winemaking system. In a medium analogous to grape juice, and following nitrogen starvation of the yeast culture, a combination of 5 mM sulfate and 300 μM sulfite was more potent than 5 mM sulfate alone as a substrate for H2S liberation. Sulfate plus sulfite allowed H2S liberation in greater amounts, at higher rates and over a longer period than sulfate alone, by most of the five yeast strains examined. Nevertheless, a sulfate-only medium still supported the liberation of between 11 and 86% of the H2S observed in the sulfate plus sulfite medium. After nitrogen depletion of the medium, sulfate uptake by yeast cells was stable for at least 7 h and even increased in a cycloheximide-sensitive manner by ca 35%. The relatively limited formation of H2S from sulfate is therefore not attributable to losses of sulfate transport by the culture as might occur through elevated permease turnover. Similarly, an inhibition of sulfate transport could not account for the lesser formation of H2S from sulfate because cultures starved of nitrogen in the presence of sulfate continued to accumulate sulfate in excess of the rates of H2S liberation. Instead, sulfate deprivation/supplementation experiments implied that limited utilisation of sulfate was due to an inhibition of sulfate reduction to sulfide. Experiments also highlighted a contribution by intracellular sulfur pools of between 35 and 70% to the total H2S liberated from sulfate by nitrogen starved yeasts. As a component of this pool, glutathione was a precursor for 40% of the H2S liberated from sulfate-containing medium.