Metabolization\nof Free and Peptide-Bound Oxidized\nMethionine Derivatives by <i>Saccharomyces cerevisiae</i> in a Model System

In the brewing process, methionine is a decisive amino\nacid for\n(off-)flavor formation. A significant part of methionine is oxidized\nto methionine sulfoxide (MetSO) in malt. We hypothesized that MetSO\nand MetSO<sub>2</sub> are metabolized to volatile compounds during\nyeast fermentation and examined whether the yeast <i>Saccharomyces\ncerevisiae</i> is able to catabolize l-MetSO and l-MetSO<sub>2</sub> in free and dipeptide-bound forms. We also\ninvestigated the stability of l-methionine sulfoximine and <i>S</i>-methylmethionine. Cell viability in the presence of the\ntest compounds was at least 90%. Both free and peptide-bound test\nsubstances were metabolized by <i>Saccharomyces cerevisiae</i>. l-MetSO was degraded most rapidly as the free amino acid,\nwhile l-MetSO<sub>2</sub> was degraded most rapidly bound\nin dipeptides. We observed a different degradation behavior of the\n(<i>R</i>) and (<i>S</i>) diastereoisomers for l-MetSO and l-methionine sulfoximine. Furthermore,\nwe detected methionol as the only metabolite of MetSO. Methionol sulfoxide\nwas not formed. MetSO<sub>2</sub> was not converted to methionol or\nmethionol sulfone but to the respective α-hydroxy acid. We conclude\nthat the reduction of MetSO to methionine proceeds faster than transamination.\nThe occurrence of MetSO or MetSO<sub>2</sub> in brewing malt will\nnot lead to the formation of hitherto unknown volatile metabolites\nof the Ehrlich pathway.