Studies on metabolisms of paralytic shellfish toxins in scallop in association with the reductive transformation by thiols

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Paralytic shellfish poisoning caused by toxic dinoflagellates such as Alexandrium spp. is the most prominent shell poison because of acuteness of the symptoms and high fatality. Once contaminated by paralytic shellfish toxins, it takes a time to depurate toxins in bivalves. A long-time closure of the shellfish market due to this phenomenon poses severe damage to the shellfish industry. However, no effective method to remove paralytic shellfish toxins from bivalves have been developed, because there is little knowledge about the metabolism of the toxins. Gonyautoxins (GTXs), 11-O-sulfate derivatives of STX, are major components of Alexandrium spp. which distribute in northern Pacific area. Toxin profile of the shellfish during a bloom of toxic dinoflagellates reflects that of the causative dinoflagellate. In the toxin deputation period after disappearance of the causative dinoflagellates, however, toxin profile of the shellfish gradually changes, for example, a part of GTXs transforms to STXs, though the biological process of transformation has not been elucidated. Recently, we have found that bacterial extracts transform GTXs to STXs. In this transformation, glutathione (GSH) in the extract is found to be involved. A stable conjugate of GSH and STXs is formed in a process of the reaction. These findings suggest that GSH in the shellfish tissue is involved in the transformation of GTXs to STXs of the shellfish during deputation period, and that GS-STXs conjugates are formed and accumulated in the shellfish. However, the conjugates were not detected in the tonic shellfish, suggesting that something happens on the conjugates. In this study, we demonstrate that STXs moiety of the GS-STXs conjugates are decomposed when glutamic acid residue is released by g-glutamyltranspeptidase, indicating that GSH is involved also in decomposition of GTXs accumulated in the shellfish.