S-Methylcysteine sulfoxide in Brassica vegetables and formation of methyl methanethiosulfinate from Brussels sprouts

The naturally occurring sulfur compound S-methylcysteine sulfoxide (SMCSO) was determined for five common Brassica vegetables, broccoflower, broccoli, Brussels sprouts, cabbage, and cauliflower. This natural chemical has a role in the aroma and flavor of these vegetables and, perhaps more importantly, appears to be involved with the inhibition of experimental carcinogenesis. Brussels sprouts contained the highest level of SMCSO. This vegetable was then used to identify the cystine lyase-mediated enzymatic conversion products including methyl methanethiosulfinate (MMTSO). After it was demonstrated that MMTSO, dimethyl trisulfide, and pyruvate can be formed facilely in a simplified enzymatic model system, generation of MMTSO was confirmed in a water extract of macerated Brussels sprouts. Formation of MMTSO was shown to be strongly dependent on pH of the vegetable juice. After 24 h, only trace amounts of MMTSO were observed at the developed acidic pH of Brussels sprouts, while considerable amounts were formed at an adjusted basic pH. This is the first evidence that MMTSO is enzymatically derived from SMCSO under natural conditions.