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-dimethylsulfoniopropylamine (DMSP-amine) --> 3-dimethylsulfoniopropionaldehyde --> 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 5'-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.