Has an important function as a repair enzyme for proteins that have been inactivated by oxidation. Catalyzes the reversible oxidation-reduction of methionine sulfoxide in proteins to methionine. [thioredoxin]-disulfide + H2O + L-methionyl-[protein] = [thioredoxin]-dithiol + L-methionyl-(S)-S-oxide-[protein] [thioredoxin]-disulfide + H2O + L-methionine = [thioredoxin]-dithiol + L-methionine (S)-S-oxide Only about 25% of mRNAs are initiated at the mitochondrial isoform 1 codon. Ubiquitous. Highest expression in adult kidney and cerebellum, followed by liver, heart ventricles, bone marrow and hippocampus. Mitochondrial. Produced by alternative splicing. Cytoplasmic. Produced by alternative promoter usage. Cytoplasmic and nuclear. Produced by alternative promoter usage. Produced by alternative splicing. Cytoplasmic. Produced by alternative initiation. Belongs to the MsrA Met sulfoxide reductase family.