DIMETHYL SULPHOXIDE REDUCTASE OF SACCHAROMYCES SPP

Effect of l-Cysteine and Transition Metal Ions on Dimethyl Sulfide Oxidation

During malt kilning, significant amounts of dimethyl sulfide (DMS) oxidize leading to the formation of dimethyl sulfoxide (DMSO), a precursor of DMS during fermentation. Yet, knowledge regarding reaction mechanisms of DMSO formation during malt production is limited. The role of thiols in sulfide oxidation is unclear as they possess sulfoxide reducing ability as well as pro- and antioxidative properties. This study investigated the effects of the thiol l-cysteine (Cys), molecular oxygen, transition metal ions, and EDTA on DMS oxidation in aqueous model solutions. Highest oxidative DMS consumption was observed when Cys was combined with iron(II) (∼12%) and copper(II) (∼40%). Response surface modeling (RSM) revealed that Cys together with copper(II) had a strictly prooxidative effect and no antioxidative behavior was found. Hydrogen peroxide, as generated via autoxidation of Cys-Cu(I)-Cys complexes, was supposed to be the primary DMS oxidant in this work. Based on redox kinetics, potential reaction mechanisms, and their impact on oxidative processes in thermal food processing, such as malt and beer production, are discussed.

The microbiology of malting and brewing

Brewing beer involves microbial activity at every stage, from raw material production and malting to stability in the package. Most of these activities are desirable, as beer is the result of a traditional food fermentation, but others represent threats to the quality of the final product and must be controlled actively through careful management, the daily task of maltsters and brewers globally. This review collates current knowledge relevant to the biology of brewing yeast, fermentation management, and the microbial ecology of beer and brewing.

The level of MXR1 gene expression in brewing yeast during beer fermentation is a major determinant for the concentration of dimethyl sulfide in beer

DMS (dimethyl sulfide) is an important beer flavor compound which is derived either from the beer wort production process or via the brewing yeast metabolism. We investigated the contribution of yeast MXR1 gene activity to the final beer DMS content. The MXR1-CA gene from Saccharomyces carlsbergensis (synonym of Saccharomyces pastorianus) lager brewing yeast was isolated and sequenced, and found to be 88% identical with Saccharomyces cerevisiae MXR1. Inactive deletion alleles of both genes were substituted for their functional counterparts in S. carlsbergensis. Such yeasts fermented well and did not form DMS from dimethyl sulfoxide. Overexpression in brewing yeast of MXR1 from non-native promoters with various strengths and transcription profiles resulted in an enhanced and correlated DMS production. The promoters of MXR1 and MXR1-CA contain conserved Met31p/Met32p binding sites, and in accordance with this were found to be co-regulated with the genes of the sulfur assimilation pathway. In addition, conserved YRE-like DNA sequences are present in these promoters, indicating that Yap1p may also take part in the control of these genes.

Microbial activity in aquatic environments measured by dimethyl sulfoxide reduction and intercomparison with commonly used methods

A new method to determine microbial (bacterial and fungal) activity in various freshwater habitats is described. Based on microbial reduction of dimethyl sulfoxide (DMSO) to dimethyl sulfide (DMS), our DMSO reduction method allows measurement of the respiratory activity in interstitial water, as well as in the water column. DMSO is added to water samples at a concentration (0.75% [vol/vol] or 106 mM) high enough to compete with other naturally occurring electron acceptors, as determined with oxygen and nitrate, without stimulating or inhibiting microbial activity. Addition of NaN(3), KCN, and formaldehyde, as well as autoclaving, inhibited the production of DMS, which proves that the reduction of DMSO is a biotic process. DMSO reduction is readily detectable via the formation of DMS even at low microbial activities. All water samples showed significant DMSO reduction over several hours. Microbially reduced DMSO is recovered in the form of DMS from water samples by a purge and trap system and is quantified by gas chromatography and detection with a flame photometric detector. The DMSO reduction method was compared with other methods commonly used for assessment of microbial activity. DMSO reduction activity correlated well with bacterial production in predator-free batch cultures. Cell-production-specific DMSO reduction rates did not differ significantly in batch cultures with different nutrient regimes but were different in different growth phases. Overall, a cell-production-specific DMSO reduction rate of 1.26 x 10(-17) +/- 0. 12 x 10(-17) mol of DMS per produced cell (mean +/- standard error; R(2) = 0.78) was calculated. We suggest that the relationship of DMSO reduction rates to thymidine and leucine incorporation is linear (the R(2) values ranged from 0.783 to 0.944), whereas there is an exponential relationship between DMSO reduction rates and glucose uptake, as well as incorporation (the R(2) values ranged from 0.821 to 0.931). Based on our results, we conclude that the DMSO reduction method is a nonradioactive alternative to other methods commonly used to assess microbial activity.

Inactivation of MXR1 abolishes formation of dimethyl sulfide from dimethyl sulfoxide in Saccharomyces cerevisiae

Dimethyl sulfide (DMS) is a sulfur compound of importance for the organoleptic properties of beer, especially some lager beers. Synthesis of DMS during beer production occurs partly during wort production and partly during fermentation. Methionine sulfoxide reductases are the enzymes responsible for reduction of oxidized cellular methionines. These enzymes have been suggested to be able to reduce dimethyl sulfoxide (DMSO) as well, with DMS as the product. A gene for an enzymatic activity leading to methionine sulfoxide reduction in Saccharomyces yeast was recently identified. We confirmed that the Saccharomyces cerevisiae open reading frame YER042w appears to encode a methionine sulfoxide reductase, and propose the name MXR1 for the gene. We found that Mxr1p catalyzes reduction of DMSO to DMS and that an mxr1 disruption mutant cannot reduce DMSO to DMS. Mutant strains appear to have unchanged fitness under several laboratory conditions, and in this paper I hypothesize that disruption of MXR1 in brewing yeasts would neutralize the contribution of the yeast to the DMS content in beer.

Chemostat enrichment and isolation of Hyphomicrobium EG. A dimethyl-sulphide oxidizing methylotroph and reevaluation of Thiobacillus MS1

A stable mixed bacterial culture was obtained by chemostat enrichment using dimethyl-sulphoxide as a carbon and energy source. This culture could not only rapidly oxidize dimethyl-sulphoxide but also dimethyl-sulphide. Enzyme determinations indicated that an important part of it consisted of methylotrophs, which assimilated carbon via the serine pathway. Indeed plate counts revealed the majority of the community to be a Hyphomicrobium species. This organism, designated Hyphomicrobium EG, is an obligate methylotroph which can only grow aerobically on several different C1-compounds. Its performance on dimethyl-sulphoxide was compared with that of the community and of another recently isolated strain, Hyphomicrobium S. The mixed culture, Hyphomicrobium EG and Hyphomicrobium S had a mu max of 0.08, 0.08 and 0.014 h-1 respectively. The KS for dimethyl-sulphoxide was the same for all three cultures (3-6 microM), whereas that for dimethyl-sulphide of Hyphomicrobium EG after growth on dimethyl-sulphoxide was 3-fold higher than that of the other two cultures (48 and 16 microM respectively). After growth on dimethyl-sulphide it improved to 3 microM. Dimethyl-sulphide respiration was maximal at a concentration of 100 microM; higher concentrations were inhibitory. One of the accompanying organisms, a pink methylotroph, was able to derive energy from the oxidation of thiosulphate. Available cultures of Thiobacillus MS1 that were reported to be able to utilize dimethyl-sulphide could no longer metabolize this compound.