In marine algae, DMSP synthesis occurs via a route which is entirely different from that in higher plants (Gage et al, 1997). Methionine is first transaminated to 4-methylthio-2-oxobutyrate (MTOB or KMTB), which is then reduced to 4-methylthio-2-hydroxybutyrate (MTHB). S-Methylation of MTHB then yields 4-dimethylsulfonio-2-hydroxybutyrate (DMSHB) which is subsequently oxidatively decarboxylated to DMSP (Gage et al, 1997). The first three enzymes of the pathway have been identified in Enteromorpha intestinalis (Summers et al, 1998); a methionine:2-oxoglutarate aminotransferase with a high affinity for methionine, an NADPH-dependent MTOB reductase, and an S-adenosylmethionine-dependent MTHB S-methyltransferase. The activities of the enzymes are sufficient to account for flux to DMSP in Enteromorpha and other DMSP-accumulating algae, but were over 20-fold lower in alga lacking DMSP (Summers et al, 1998).

Because DMSP does not contain N, the accumulation of DMSP could offer advantages over accumulation of nitrogenous onium compounds for osmotic adjustment in N-limiting environments; N-deficiency increases DMSP levels in marine algae (Gage et al, 1997). The enzyme DMSP lyase generates the gas dimethyl sulfide (DMS) and the potentially toxic compound, acrylate, from DMSP. Acrylate generated from DMSP may play a role in defense against herbivores (Wolfe et al, 1997).

Gage DA, Rhodes D, Nolte KD, Hicks WA, Leustek T, Cooper AJL, Hanson AD 1997 A new route for synthesis of dimethylsulphoniopropionate in marine algae. Nature 387: 891-894.

Summers PS, Nolte KD, Cooper AJL, Borgeas H, Leustek T, Rhodes D, Hanson AD 1998 Identification and stereospecificity of the first three enzymes of 3-dimethylsulfoniopropionate biosynthesis in a chlorophyte alga. Plant Physiol. 116: 369-378.

Wolfe GV, Steinke M, Kirst GO 1997 Grazing-activated chemical defence in a unicellular marine alga. Nature 387: 894-897.