COMPARATIVE TOXICITY OF THE DIARRHETIC SHELLFISH POISONS, OKADAIC ACID, OKADAIC ACID DIOL-ESTER AND DINOPHYSISTOXIN-4, TO THE DIATOM THALASSIOSIRA-WEISSFLOGII

Quantitative structure-activity relationships were determined for the diarrhetic shellfish poisoning (DSP) toxins, okadaic acid (OA), OA dio l-ester and dinophysistoxin-4 (DTX-4), using a sensitive bioassay proc edure with the diatom Thalassiosira weissflogii. OA diol-ester was fou nd to be nearly as toxic as OA. This result contradicted the accepted idea that only the free acid toxins, such as DTX-1 and OA, are potent phosphatase inhibitors. Postassay analyses using liquid chromatography -mass spectrometry (LC-MS) of cultures incubated with OA diol-ester sh owed that the ester had partially decomposed to OA, which explained so me but not all of the observed toxicity, The formation of OA during th e bioassay raised the possibility that cells exposed to inactive DSP t oxin esters could metabolically activate them. This was examined in an additional experiment which showed that the hydrolysis of both DTX-4 and OA diol-ester was spontaneous and apparently not mediated by the p resence of T. weissflogii cells, However, cells of T, weissflogii chal lenged with OA diol-ester rapidly metabolized most of the toxin to a m ore water-soluble product. From interpretation of mass spectral data o btained using ion-spray LC-MS, the metabolite was identified as an oxy genated diol-ester of OA, implying that it was the product of a monoox ygenase-detoxification pathway. It is postulated that OA diol-ester, a s a lipid-soluble, uncharged molecule with a propensity to hydrolyse t o OA, may facilitate the transfer of OA across cell walls and membrane s. (C) 1997 Elsevier Science Ltd.