Has an important function as a repair enzyme for proteins that have been inactivated by oxidation (By similarity). 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 [thioredoxin]-disulfide + H2O + L-methionyl-[protein] = [thioredoxin]-dithiol + L-methionyl-(R)-S-oxide-[protein] Mutant exhibits decreased binding to GalNAcbeta1-4Gal containing receptors that are present on type II lung cells and vascular endothelial cells. In the N-terminal section; belongs to the MsrA Met sulfoxide reductase family. In the C-terminal section; belongs to the MsrB Met sulfoxide reductase family.