Biochemical Evidence for Two Novel Enzymes in the Biosynthesis of 3-Dimethylsulfoniopropionate in Spartina alterniflora1

Abstract

3-Dimethylsulfoniopropionate (DMSP) is an osmoprotectant accumulated by the cordgrass Spartina alterniflora and other salt-tolerant plants. Previous in vivo isotope tracer and metabolic modeling studies demonstrated that S. alterniflora synthesizes DMSP via the route S-methyl-Met 3 3-dimethylsulfoniopropylamine (DMSP-amine) 3 3-dimethylsulfoniopropionaldehyde 3 DMSP and indicated that the first reaction requires a far higher substrate concentration than the second to attain one-half-maximal rate. As neither of these reactions is known from other organisms, two novel enzymes are predicted. Two corresponding activities were identified in S. alterniflora leaf extracts using specific radioassays. The first, S-methyl-Met decarboxylase (SDC), strongly prefers the l-enantiomer of S-methyl-Met, is pyridoxal 5Ј-phosphate-dependent, generates equimolar amounts of CO 2 and DMSP-amine, and has a high apparent K m (approximately 18 mm) for its substrate. The second enzyme, DMSP-amine oxidase (DOX), requires O 2 for activity, shows an apparent K m for DMSP-amine of 1.8 mm, and is not accompanied by DMSP-amine dehydrogenase or transaminase activity. Very little SDC or DOX activity was found in grasses lacking DMSP. These data indicate that SDC and DOX are the predicted novel enzymes of DMSP synthesis.

Plants Synthesize Ethanolamine by Direct Decarboxylation of Serine Using a Pyridoxal Phosphate Enzyme

“…In some cases, a portion of the eluate was freeze-dried, redissolved, and analyzed by TLC on silica gel G plates developed with methanol:acetone: concentrated HCl (90:10:4 v/v/v); radioactive zones were detected by autoradiography. To determine the stoichiometry of the reaction and when using L-or D-[1- 14 C]serine as substrate, the assays were run at 21-22°C and 14 CO 2 was trapped as described (32). Other L-amino acid decarboxylase activities were assayed by using U- 14 as for SDC assays except that reactions were stopped with 1 ml of ice-cold water, appropriate amines replaced Etn as carrier, and amine products other than histamine were isolated using different ion exchange columns (1 ml).…”

Evolution of DMSP (dimethylsulfoniopropionate) biosynthesis pathway: Origin and phylogenetic distribution in polyploid Spartina (Poaceae, Chloridoideae)

“…The two steps of the Spartina DMSP biosynthesis pathway that are specific to DMSP+ species involve a S-methyl-Met decarboxylase (SDC) and a DMSP-amine oxidase (DOX) (Kocsis and Hanson, 2000).…”

“…The first step, catalyzed by the MMT enzyme, corresponds to the conversion of methionine into SMM (Bourgis et al, 1999) and the fourth step, catalyzed by BADH enzyme, converts DMSP-aldehyde into DMSP (Trossat et al, 1997;Vojtěchová et al, 1997). The intermediate specific steps involve (i) the conversion of SMM to DMSP-amine by an Smethyl-Met decarboxylase (SDC) related to an ornithine decarboxylase, and (ii) the conversion of DMSP-amine to DMSP-aldehyde by an oxidase (DOX) related to a diamine oxidase (Kocsis et al, 1998;Kocsis and Hanson, 2000). The protein sequences of these enzymes, as well as their corresponding genes, are unknown.…”

The chemical biology of dimethylsulfoniopropionate

“…20 The likely intermediate, 4-(dimethylsulfonio)-2-oxobutyrate (3), was shown to be instable and has therefore never been observed. 19,21 In both plants a final oxidation step yields DMSP. 19,21 In both plants a final oxidation step yields DMSP.…”

“…Along this pathway DMSP is first demethylated by the methyl transferase DmdA that transfers a methyl group to tetrahydrofolate (FH 4 ) to yield 3-methylmercaptopropionic acid (21). Along this pathway DMSP is first demethylated by the methyl transferase DmdA that transfers a methyl group to tetrahydrofolate (FH 4 ) to yield 3-methylmercaptopropionic acid (21).…”