18O-labelling pattern of okadaic acid from H218O in dinoflagellate Prorocentrum lima elucidated by tandem mass spectrometry

Okadaic acid is a metabolite of the unicellular algae dinoflagellate. Its biosynthesis has attracted considerable attention since the skeletal structure was shown to be synthesized via an unprecedented route. However, its relevant intermediates or enzymes are unknown. In the course of our previous investigations on the oxygen source of okadaic acid by tandem mass spectrometry (CID MS/MS), we determined the level of O-18 incorporation for each oxygen site from O-18(2) and [O-18(2)]acetate. In the present study, we examined (H2O)-O-18-labelling patterns of okadaic acid from dinoflagellates in comparison with salinomycin from actinomycetes and has provided intriguing information regarding biosynthesis. Unexpectedly, oxygen atoms originating from 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 pyruvate. Similar experiments for salinomycin revealed that all of its oxygen atoms 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.