Metabolization of Free and Peptide-Bound Oxidized Methionine Derivatives by 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 MetSO are metabolized to volatile compounds during yeast fermentation and examined whether the yeast is able to catabolize l-MetSO and l-MetSO in free and dipeptide-bound forms. We also investigated the stability of l-methionine sulfoximine and -methylmethionine. Cell viability in the presence of the test compounds was at least 90%. Both free and peptide-bound test substances were metabolized by . l-MetSO was degraded most rapidly as the free amino acid, while l-MetSO was degraded most rapidly bound in dipeptides. We observed a different degradation behavior of the () and () diastereoisomers for l-MetSO and l-methionine sulfoximine. Furthermore, we detected methionol as the only metabolite of MetSO. Methionol sulfoxide was not formed. MetSO 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 MetSO in brewing malt will not lead to the formation of hitherto unknown volatile metabolites of the Ehrlich pathway.