Evidence implicating dimethylsulfoniopropionaldehyde as an intermediate in dimethylsulfoniopropionate biosynthesis

3-Dimethylsulfoniopropionate (DMSP) is an osmoprotectant accumulated by certain flowering plants and algae. In Wollastonia biflora (L.) DC. (Compositae) the first intermediate in DMSP biosynthesis has been shown to be S-methylmethionine (SMM) (A.D. Hanson, J. Rivoal, L. Paquet, D.A. Gage [1994] Plant Physiol 105: 103-110). Other possible intermediates were investigated by radiolabeling methods using W. biflora leaf discs. In pulse-chase experiments with [35S]SMM, 3-dimethylsulfoniopropionaldehyde (DMSP-ald) acquired label rapidly and lost it during the chase period. Conversely, 3-dimethylsulfoniopropylamine (DMSP-amine), 3-dimethylsulfoniopropionamide (DMSP-amide), and 4-dimethylsulfonio-2-hydroxybutyrate (DMSHB) labeled slowly and continuously during both pulse and chase. When unlabeled compounds were supplied along with [35S]SMM, DMSP-ald promoted [35S]DMSP-ald accumulation but DMSHB, DMSP-amide, and DMSP-amine had no such trapping effect. These data indicate that DMSP-ald is an intermediate in DMSP biosynthesis and that the other three compounds are not. Consistent with this, [35S]DMSHB was not metabolized to DMSP. Although [14C]DMSP-amine and [14C]DMSP-amide were converted slowly to DMSP, similar or higher conversion rates were found in plants that do not naturally accumulate DMSP, indicating that nonspecific reactions were responsible. These nonaccumulating species did not form [35S]DMSP-ald from [35S]SMM, implying that DMSP-ald is specific to DMSP biosynthesis. W. biflora leaf discs catabolized supplied sulfonium compounds to dimethylsulfide at differing rates, in the order DMSP-ald >> DMSP-amine > SMM > DMSP-amide > DMSHB > DMSP.