Characteristics of DMSP-lyase in <i>Phaeocystis</i> sp. (Prymnesiophyceae)

The marine phytoplankton species Phaeocystis sp, is one of the few microalgae known to be able to convert dimethylsulfoniopropionate (DMSP) enzymatically into dimethyl sulfide (DMS) and acrylic acid. The function of this enzymatic process for the organism is not known. From experiments with crude extracts and whole cells of axenic cultures of Phaeocystis it was concluded that DMSP-lyase is membrane-bound and located extracellularly because: (1) the enzyme activity in extracts and in whole cells varied in a similar manner with pH; (2) between 50 and 80% of the DMSP-lyase activity was associated with the membrane fraction; (3) lyase activity in whole cells was inhibited by the nonpermeable thiol-reagent p-chloromercuribenzenesulfonic acid (pCMBS). The pH optimum was 10.5 or higher, which is in contrast with available data for the enzyme from other organisms. The pH profile, the requirement for reduced thiol groups in extracts and the inhibition by pCMBS suggest the involvement of cysteine residues at the active site. Production of DMSP as well as its cleavage by DMSP-lyase are apparently not involved in the short term regulation of the osmotic potential of cells upon changes in salinity.