Methionine sulphoxide reductases revisited: free methionine as a primary target of H₂O₂stress in auxotrophic fission yeast.

Amino acid methionine can suffer reversible oxidation to sulphoxide and further irreversible over-oxidation to methionine sulphone. As part of the cellular antioxidant scavenging activities are the methionine sulphoxide reductases (Msrs), with a reported role in methionine sulphoxide reduction, both free and in proteins. Three families of Msrs have been described, but the fission yeast genome only includes one representative for two of these families: MsrA/Mxr1 and MsrB/Mxr2. We have investigated their role in methionine reduction and H2 O2 sensitivity. We show here that MsrA/Mxr1 is able to reduce free oxidized methionine. Cells lacking each one of the genes are not significantly sensitive to different types of oxidative stresses, neither display altered life span. However, only when deletion of msrA/mxr1 is combined with deletion of met6, which confers methionine auxotrophy, the survival upon H2 O2 stress decreases by 100-fold. In fact, cells lacking only Met6, and which therefore require addition of methionine to the growth media, are extremely sensitive to H2 O2 stress. These and other evidences suggest that oxidation of free methionine is a primary target of peroxide toxicity in cells devoid of methionine biosynthetic capacity, and that an important role of Msrs is to recycle this oxidized free amino acid.