This enzyme may serve as a scavenger, allowing the cell to utilize biotin sulfoxide as a biotin source. It reduces a spontaneous oxidation product of biotin, D-biotin D-sulfoxide (BSO or BDS), back to biotin. Also exhibits methionine-(S)-sulfoxide (Met-S-SO) reductase activity, acting specifically on the (S) enantiomer in the free, but not the protein-bound form. It thus plays a role in assimilation of oxidized methionines. [thioredoxin]-disulfide + H2O + L-methionine = [thioredoxin]-dithiol + L-methionine (S)-S-oxide Binds 1 molybdenum-bis(molybdopterin guanine dinucleotide) (Mo-bis-MGD) cofactor per subunit. kcat is 3.2 min(-1) for the reduction of Met-S-SO using benzyl viologen as an artificial electron donor. The N-terminal 39 residues are essential for activity. Cells lacking this gene together with metB1, msrA and msrB are unable to use Met-S-SO for growth but retain the ability to use the enantiomer Met-R-SO, while a metB1/msrA/msrB deletion mutant is able to use both compounds. Requires a small thioredoxin-like protein for activity. Belongs to the prokaryotic molybdopterin-containing oxidoreductase family.