The Influence of Assimilable Nitrogen Compounds in Wort on the Ability of Yeast to Reduce Dimethyl Sulphoxide

Dimethyl sulphoxide (DMSO) reductase activity increased at the end of the growth cycle of Saccharomyces cerevisiae NCYC 240 on complex medium. On defined medium, DMSO reductase and thioredoxin reductase activities and thioredoxin were all elevated in cultures limiting in nitrogen, but not in nitrogen-sufficient cultures. In nitrogen limiting cultures it was also shown that the reduction of DMSO (measured as disappearance of [14C] from the medium) was much increased compared with conditions of nitrogen excess. This applied whether ammonium or glutamate was the limiting nitrogen source. When methionine was used as the limiting nitrogen source, DMSO reduction was higher still. Methionine-limited cells accumulated [17C] methionine much more rapidly than did cells from methionine-sufficient cultures, and [14C] methionine sulphoxide could be identified in the cells after 1 min incubation with labelled methionine. It is concluded that DMSO reductase activity is controlled by nitrogen catabolite repression and may be involved in methionine uptake by yeast. The amino nitrogen content of wort will in consequence have an important effect on levels of dimethylsulphide (DMS) produced during fermentation.