Metabolization of Free and Peptide-Bound Oxidized Methionine Derivatives by <i>Saccharomyces cerevisiae</i> in a Model System

In the brewing process, methionine is a decisive amino acid for (off-)flavor formation. A significant part of methionine is oxidized to methionine sulfoxide (MetSO) in malt. We hypothesized that MetSO and MetSO₂ are metabolized to volatile compounds during yeast fermentation and examined whether the yeast <i>Saccharomyces cerevisiae</i> is able to catabolize l-MetSO and l-MetSO₂ in free and dipeptide-bound forms. We also investigated the stability of l-methionine sulfoximine and <i>S</i>-methylmethionine. Cell viability in the presence of the test compounds was at least 90%. Both free and peptide-bound test substances were metabolized by <i>Saccharomyces cerevisiae</i>. l-MetSO was degraded most rapidly as the free amino acid, while l-MetSO₂ was degraded most rapidly bound in dipeptides. We observed a different degradation behavior of the (<i>R</i>) and (<i>S</i>) diastereoisomers for l-MetSO and l-methionine sulfoximine. Furthermore, we detected methionol as the only metabolite of MetSO. Methionol sulfoxide was not formed. MetSO₂ was not converted to methionol or methionol sulfone but to the respective α-hydroxy acid. We conclude that the reduction of MetSO to methionine proceeds faster than transamination. The occurrence of MetSO or MetSO₂ in brewing malt will not lead to the formation of hitherto unknown volatile metabolites of the Ehrlich pathway.