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Plant Cell 5:809-20, Weger, H, G, & Dasgupta, R' 1993, Regulation of alternative pathway respiration in Chlamydomonas reinhardtii (Chloro- phyceae),/ Phycol. 29:300-8, ' Weger, H, G,, Guy, R, D, & Turpin, D, H, 1990, Cytochrome and alternative pathway respiration in green algae. Mea- surements using inhibitors and "O, discrimination. Plant Physiol. 93:356-60, J. Phycol. 31, 567-574 (1995) ISOLATION, PURIFICATION, AND CHARACTERIZATION OF DMSP LYASE (DIMETHYLPROPIOTHETIN DETHIOMETHYLASE (4.4.1.3)) FROM THE RED ALCA POLYSIPHONIA PANICULATA' Michele K. Nishiguchi^ and Lynda J. Goff Department of Biology, University of California, Santa Cruz, California 95064 ABSTRACT Polysiphonia paniculata Montagne is an intertidal red alga known to produce large amounts ofthe compound dimethylsulfoniopropionate (DMSP). Conversion of this substrate into dimethylsulfide is accomplished in P, pan- iculata by an enzyme called DMSP lya.ie (dimelhylpro- piothetin dethiomethyla.se (4.4.1.3)). DMSP lyase has been purified and characterized from P, paniculata. Enzyjtie actix'ity is found in tiuo different proteins: the larger with a molecular weight of 9.26 x W daltons and the smaller with a molecular weight of 3.65 x 10* daltons. Specific activity of the enzyme is 526 ^mols-min~'-mg~' for the smaller protein a nd 263 nmols • min '' • mg~ 'for the la rger protein. The Michaelis-Menten constant (KJ is 72.8 nM ± 17.15 and the v^^ is 1.62 nmols-min'' ± 0.928for the 92.6-kDa protein. The pi of the larger protein is 5.8 and 5.9 for the smaller protein. Interaction with cysteine protease inhibitors L-iran!i-epo.\y.succinyl-leucylamido (4- guanidino)-butane, dithiobis-(2-nitrobenzoate), or N-ethyhnaleimide inactivated enzyme actix'ity. The pres- ence of either magnesium or calcium with DMSP lyase enhanced activity al concentrations between 20 and 40 but had little effect above these lex'els. Addition ofthe ' Received 7 July 1994, Accepted 21 April 1995, • Present address and author for reprint requests: Department of Biological Sciences, University of Southern California, Uni- versity Park, Los Angeles, California 90089-0371, dix'alent chelalors ethylenebis(oxyethytene'nitrito) tetraa- cetic acid and ethylenediaminetetraacetate decreased ac- tivity of the enzyme, but actii'ity xvas restored whe7i either chelator was removed and magnesium or calcium was added to the enzyme. Key index words: dimethylsulfoniopropionate; dimelh- yimlfide; DMSP lyase; osmolyte; osmoregulation; solute DMSP lyase (dimethylpropiothetin dethiomethy- lase (4.4,1.3)) is an enzyme that converts the im- portant osmotic solute dimethylsulfoniopropionate (DMSP) into dimethylsulfide (DMS), a volatile gas. DMSP is found in some species of macroalgae and phytoplankton (Reed 1983a, Vairavamurthy et al. 1985, Dickson and Kirst 1986, Edwards et al'. 1987) and has been extensively studied for its properties as an osmoregulatory solute (Kadota and Ishida 1968, Dickson etal. 1980, Dickson and Kirst 1986). DMSP production has also been associated with changes in irradiance (Karsten et al. 1990, 1991) and nitrate concentration (Turner et al. 1988, Grone and Kirst 1992). These previous studies indicate that abiotic factors have a .substantial influence on the produc- tion of DMSP, but there is little evidence about the mechanisms that control the accumulation and sub- sequent conversion of DMSP into DMS, The conversion properties of this enzyme have global implications. DMS contributes approximately