[Isoform 1]: Catalyzes the reversible oxidation-reduction of methionine sulfoxide in proteins to methionine. Has an important function as a repair enzyme for proteins that have been inactivated by oxidation (PubMed:10452521, PubMed:12039877). Plays a crucial role in repairing oxidized methionine residues in key lens proteins, such as alpha-crystallin (CRYAA/CRYAB) and cytochrome c (CYCS), thereby restoring their function and maintaining lens transparency (By similarity). {ECO:0000250|UniProtKB:Q9D6Y7, ECO:0000269|PubMed:10452521, ECO:0000269|PubMed:12039877}. This is the function of MSRA (methionine sulfoxide reductase A, Ensembl gene identifier ENSG00000175806).