Identification and Stereospecificity of the First Three Enzymes of 3-Dimethylsulfoniopropionate Biosynthesis in a Chlorophyte Alga¹

Peter S. Summers², Kurt D. Nolte, Arthur J.L. Cooper, Heidi Borgeas, Thomas Leustek, David Rhodes, and Andrew D. Hanson^*

Horticultural Sciences Department, University of Florida, Gainesville, Florida 32611 (P.S.S., K.D.N., A.D.H.); Department of Biochemistry, Burke Medical Research Institute of Cornell University Medical College, White Plains, New York 10605 (A.J.L.C.); Botany Department, University of Hawaii at Manoa, Honolulu, Hawaii 96822 (H.B.); Center for Agricultural Molecular Biology, Rutgers University, New Brunswick, New Jersey 08903 (T.L.); and Department of Horticulture, Purdue University, West Lafayette, Indiana 47907 (D.R.)

Many marine algae produce 3-dimethylsulfoniopropionate (DMSP), a potent osmoprotective compound whose degradation product dimethylsulfide plays a central role in the biogeochemical S cycle. Algae are known to synthesize DMSP via the four-step pathway, l-Met  4-methylthio-2-oxobutyrate  4-methylthio-2-hydroxybutyrate  4-dimethylsulfonio-2-hydroxy-butyrate (DMSHB)  DMSP. Substrate-specific enzymes catalyzing the first three steps in this pathway were detected and partially characterized in cell-free extracts of the chlorophyte alga Enteromorpha intestinalis. The first is a 2-oxoglutarate-dependent aminotransferase, the second an NADPH-linked reductase, and the third an S-adenosylmethionine-dependent methyltransferase. Sensitive radiometric assays were developed for these enzymes, and used to show that their activities are high enough to account for the estimated in vivo flux from Met to DMSP. The activities of these enzymes in other DMSP-rich chlorophyte algae were at least as high as those in E. intestinalis, but were 20-fold lower in algae without DMSP. The reductase and methyltransferase were specific for the d-enantiomer of 4-methylthio-2-hydroxybutyrate in vitro, and both the methyltransferase step and the step(s) converting DMSHB to DMSP were shown to prefer d-enantiomers in vivo. The intermediate DMSHB was shown to act as an osmoprotectant, which indicates that the first three steps of the DMSP synthesis pathway may be sufficient to confer osmotolerance.

Plant Physiol. (1998) 116: 369-378
Copyright Clearance Center:   0032-0889/98/116/0369/10
© 1998 American Society of Plant Physiologists

This article has been cited by other articles:

				S. D. McNeil, M. L. Nuccio, and A. D. Hanson Betaines and Related Osmoprotectants. Targets for Metabolic Engineering of Stress Resistance Plant Physiology, August 1, 1999; 120(4): 945 - 950. [Full Text]

				M. G. Kocsis, K. D. Nolte, D. Rhodes, T.-L. Shen, D. A. Gage, and A. D. Hanson Dimethylsulfoniopropionate Biosynthesis in Spartina alterniflora1 . Evidence That S-Methylmethionine and Dimethylsulfoniopropylamine Are Intermediates Plant Physiology, May 1, 1998; 117(1): 273 - 281. [Abstract] [Full Text]