DMSP: tetrahydrofolate methyltransferase from the marine sulfate-reducing bacterium strain WN

Dimethylsulfoniopropionate (DMSP), an important compatible solute of many m arine algae, can be metabolised by bacteria via cleavage to dimethylsulfide and acrylate or via an initial demethylation. This is the first report on the purification of an enzyme that specifically catalyses the demethylation of DMSP. The enzyme was isolated from the sulfate-reducing bacterium strai n WN, which grows on DMSP and demethylates it to methylthiopropionate. DMSP :tetrahydrofolate (THF) methyltransferase from strain WN was purified 76-fo ld [to a specific activity of 40.5 mu mol min(-1) (mg protein)(-1)]. SDS po lyacrylamide gel electrophoresis showed two bands of approximately 10 and 3 5 kDa; in particular the 35 kDa polypeptide became significantly enriched d uring the purification. Storage of the purified fraction at -20 degrees C u nder nitrogen resulted in a 99% loss of activity in two days. The activity could be partially restored by addition of 200 mu M cyanocobalamin, hydroxo cobalamin or coenzyme B-12-ATP did not have any positive effect on activity . Reduction of the assay mixture by titanium(III)nitrilotriacetic acid slig htly stimulated the activity. Gel filtration chromatography revealed a nati ve molecular mass between 45 and 60 kDa for the DMSP:THF methyltransferase. The enzyme was most active at 35 degrees C and pH 7.8. Glycine betaine, wh ich can be considered an N-containing structural analogue of DMSP, did not serve as a methyl donor for DMSP:THF methyltransferase. Various sulfur-cont aining DMSP-analogues were tested but only methylethylsolfoniopropionate se rved as methyl donor. None of these compounds inhibited methyl transfer fro m DMSP to THF. Strain WN did not grow on any of the sulfur-containing DMSP- analogues. (C) 2000 Elsevier Science B.V. All rights reserved.