FEMS Microbiology Letters 26 (1985) 89-94 89 Published by Elsevier FEM 01974 The involvement of thioredoxin and thioredoxin reductase in the dimethyl sulphoxide reductase system of Saccharomyces cerevisiae (Yeast; methionine( + )-sulphoxide) Richard M. Gibson * and Peter J. Large ** Department of Biochemistry, University of Hull, Hull HU6 7RX, U.K. Received 28 September 1984 Accepted 2 October 1984 1. SUMMARY Dimethyl sulphoxide (DMSO) reductase activ- ity in crude extracts of Saccharomyces cereo&iae NCYC240 was stimulated by addition of thioredoxin, but not by addition of thioredoxin reductase. The activity was partially purified. DEAE-cellulose could be used to separate thioredoxin and its reductase (which bound to the column) from the terminal DMSO-reductase pro- tein (which failed to bind). The highly unstable purified terminal reductase so obtained required both thioredoxin and thioredoxin reductase to reconstitute activity with either dithiothreitol (DTT) or NADPH as electron donor. Partially purified terminal reductase had an M r of about 15 000. 2. INTRODUCTION Dimethyl sulphide (DMS) at extremely low concentrations (< 100 /~g/1) is one of the most * Present address: Carlsberg Laboratories, Department of Chemistry, Gamle Carlsberg Vej 10, DK-2500 Copenhagen-Valby, Denmark. **To whom correspondence should be addressed. important contributors to the flavour of lager beers [1]. It can be formed in beer by a number of different routes [2], one of which is the reduction of DMSO during fermentation [3]. The DMSO is thought to be formed from S-methylmethionine during kilning of malt [4]. An enzyme in crude extracts of yeast that catalyses the NADPH-de- pendent reduction of DMSO to DMS has been described [4,5]. Although this enzyme is relatively low in specific activity, the sensitivity with which the product can be detected means that it can still play an important role in the chemical changes of fermentation. The enzyme was competitively inhibited by methionine (_+)-sulphoxide [4], and evidence was obtained implicating a fraction thought to contain thioredoxin and thioredoxin reductase (EC 1.6.4.5) in the DMSO-reductase sys- tem, although a requirement for both proteins was not directly demonstrated [4,5]. To clarify the role of DMSO reductase in the metabolism of S. cerevisiae, the terminal enzyme of the DMSO reductase system has now been partially purified. Some of its properties and its dependence on thioredoxin and thioredoxin re- ductase are described in this paper. In the paper, the phrases 'DMSO reductase' or 'DMSO-re- ductase activity' are used to denote the DTI" (or NADPH)-dependent reduction of DMSO, and so 0378-1097/85/$03.30 (~ 1985 Federation of European Microbiological Societies