Major contributor to the demethylation of dimethylsulfonioproprionate (DMSP). Demethylates DMSP to methyl-mercaptopropionate (MMPA). (6S)-5,6,7,8-tetrahydrofolate + S,S-dimethyl-beta-propiothetin = (6S)-5-methyl-5,6,7,8-tetrahydrofolate + 3-(methylsulfanyl)propanoate + H(+) Non-competitively inhibited by MMPA, weakly inhibited by substrate analogs dimethylsulfoniobutanoate and dimethylsulfoniopentanoate. It is used as an intracellular osmolyte. Upon expression and purification from E.coli. Optimum pH is 7.0-8.0. Homodimer. DMSP is used as an intracellular osmolyte, predator deterrent and antioxidant. Belongs to the GcvT family. DmdA subfamily.