Sulfite formation by wine yeasts

Trichosporon jirovecii-mediated synthesis of cadmium sulfide nanoparticles

Cadmium sulphide is one of the most promising materials for solar cells and of great interest due to its useful applications in photonics and electronics, thus the development of bio-mediated synthesis of cadmium sulphide nanoparticles (CdS NPs) is one of the essential areas in nanoparticles. The present study demonstrates for the first time the eco-friendly biosynthesis of CdS NPs using the yeast Trichosporon jirovecii. The biosynthesis of CdS NPs were confirmed by UV-Vis spectrum and characterized by X-ray diffraction assay and electron microscopy. Scanning and transmission electron microscope analyses shows the formation of spherical CdS NPs with a size range of about 6-15 nm with a mean Cd:S molar ratio of 1.0:0.98. T. jirovecii produced hydrogen sulfide on cysteine containing medium confirmed by positive cysteine-desulfhydrase activity and the colony color turned yellow on 0.1 mM cadmium containing medium. T. jirovecii tolerance to cadmium was increased by the UV treatment and three 0.6 mM cadmium tolerant mutants were generated upon the UV radiation treatment. The overall results indicated that T. jirovecii could tolerate cadmium toxicity by its conversion into CdS NPs on cysteine containing medium using cysteine-desulfhydrase as a defense response mechanism.

Inactivation of MET2 in brewer's yeast increases the level of sulfite in beer

Brewer's yeasts sometimes produce inadequate or excessive amounts of sulfite, an antioxidant and flavour stabilizer, so means of controlling the sulfite production are desired. Understanding the physiology and regulation of the sulfur assimilation pathway of Saccharomyces yeasts is the key to change sulfite production. The MET2 gene of Saccharomyces yeasts encodes homoserine O-acetyl transferase, which catalyzes the conversion of homoserine to O-acetyl homoserine which in turn combines with hydrogen sulfide to form homocysteine, the immediate precursor of methionine. We expected that inactivation of MET2 would lead to accumulation of sulfide and derepression of the entire sulfur assimilation pathway and, therefore, possibly also to sulfite accumulation. Brewer's yeasts were constructed in which several of the four MET2 gene copies were inactivated. Sulfite production was increased in strains with one remaining MET2 gene and even more so when no active MET2 was present. In both cases, hydrogen sulfide production was also increased. To the extent that excess sulfide can be removed, this strategy may be applied to control sulfite accumulation by brewer's yeast in beer production.

Determination of sulphite reductase activity and its response to assimilable nitrogen status in a commercial Saccharomyces cerevisiae wine yeast

The liberation of H2S is a common problem afflicting wine fermentation. Sulphite reductase activity of a commercial wine yeast was investigated to define its involvement in this process. The activity studied here differed from those characterized previously from cider and bakers' yeasts by displaying a greater sensitivity to cold, low ionic strength and possibly, proteolytic action. These differences necessitated the development of a new method of quantification. Through this method, the onset of H2S liberation was shown not to be a result of variations in the levels of sulphite reductase activity. Thus, high levels of activity which occurred during the exponential phase of growth were not necessarily accompanied by the liberation of H2S. Similarly, nitrogen-starved cultures which liberated H2S showed no corresponding increase in sulphite reductase activity from prestarvation levels. In fact, rates of H2S liberation from cultures and in enzyme assays agreed closely. A short-term independence of sulphite reductase activity from culture nitrogen status was therefore evident. The only influence of nitrogen was achieved in its absence when enzyme activity decayed with a half-life (4.25 h) which was comparable to that induced by the presence of cycloheximide (5.75 h). A proposed transcriptional control mechanism mediated by methionine derivatives was only partly effective in this strain although an in vitro inhibitory effect of methionine was implicated. These data therefore support the notion that H2S liberation in response to nitrogen starvation stems from a failure of metabolism to sequester H2S which continues to be formed, at least initially, at prestarvation rates.

The effect of sulfite on the yeast Saccharomyces cerevisiae

After a short period of tolerance, living cells of Saccharomyces cerevisiae were irreversibly damaged by low concentrations of sulfite. The length of the period of tolerance and the rate of the damaging effect depended on the concentration on sulfite, pH-value, temperature, the physiological state of the cells, and incubation time. Inhibitors of protein synthesis and mitochondrial ATP synthesis did not alter the deleterious effect of sulfite on living cells. Furthermore, cell damage leading to inhibition of colony formation occured under aerobic as well as under anaerobic conditions. Prior to cell inactivation sulfite induced the formation of respiratory deficient cells. The active agent was shown to be SO2.