Biochemistry of protein methionine oxidation and reduction. Oxidation of protein methionine residues produces 2 sulfoxide diastereomers, methionine-S-sulfoxide and methionine-R-sulfoxide, which can be stereospecifically reduced back to methionine by 2 classes of mammalian methionine sulfoxide reductases, MSRA and MSRB, respectively. Further oxidation of methionine sulfoxide to methionine sulfone is biologically irreversible.