Evidence that the pathway of dimethylsulfoniopropionate biosynthesis begins in the cytosol and ends in the chloroplast

In the flowering plant Wollastonia biflora (L.) DC. the first step in 3-dimethylsulfoniopropionate (DMSP) synthesis is conversion of methionine to S-methylmethionine (SMM) and the last is oxidation of 3-dimethylsulfoniopropionaldehyde (DMSP-ald) (F. James, L. Paquet, S.A. Sparace, D.A. Gage, A.D. Hanson [1995] Plant Physiol 108: 1439-1448). DMSP-ald was shown to undergo rapid, spontaneous decomposition to dimethylsulfide and acrolein. However, it was stable enough (half-life greater than or equal to 1 h) in tertiary amine buffers to use as a substrate for enzyme assays. A dehydrogenase catalyzing DMSP-ald oxidation was detected in extracts of W. biflora mesophyll protoplasts. This enzyme had a high affinity for DMSP-ald (K-m = 1.5 mu M), was subject to substrate inhibition, preferred NAD to NADP, and was immunologically related to plant betaine aldehyde dehydrogenases. After fractionation of protoplast lysates, greater than or equal to 90% of DMSP-ald dehydrogenase activity was recovered from the chloroplast stromal fraction, whereas the enzyme that mediates SMM synthesis, S-adenosylmethionine:methionine S-methyltransferase, was found exclusively in the cytosolic fraction. Immunohistochemical analysis confirmed that the S-methyltransferase was cytosolic. Intact W. biflora chloroplasts were able to metabolize supplied [S-35]SMM to [S-35]DMSP. These findings indicate that SMM is made in the cytosol, imported into the chloroplast, and there converted successively to DMSP-ald and DMSP.