O-18-labelling pattern of okadaic acid from (H2O)-O-18 in dinoflagellate Prorocentrum lima elucidated by tandem mass spectrometry

Okadaic acid is a metabolite of the unicellular algae dinoflagellate. Its b iosynthesis has attracted considerable attention since the skeletal structu re was shown to be synthesized via an unprecedented route. However, its rel evant intermediates or enzymes are unknown. In the course of our previous i nvestigations on the oxygen source of okadaic acid by tandem mass spectrome try (CID MS/MS), we determined the level of O-18 incorporation for each oxy gen site from O-18(2) and [O-18(2)]acetate. In the present study, we examin ed (H2O)-O-18-labelling patterns of okadaic acid from dinoflagellates in co mparison with salinomycin from actinomycetes and has provided intriguing in formation regarding biosynthesis. Unexpectedly, oxygen atoms originating fr om acetate were not labelled from (H2O)-O-18; this can not be accounted for by the usual metabolic route where acetyl-CoA is biosynthesized via pyruva te. Similar experiments for salinomycin revealed that all of its oxygen ato ms derived from acetate or propionate were labelled by (H2O)-O-18. Another interesting feature is that two oxygen sires were derived from both O-2 and H2O while the others were labelled only from O-2. These results imply that an oxidation mechanism other than those in actinomycetes polyethers may be involved in the biosynthesis of okadaic acid.