Brevetoxin Depuration in Shellfish via Production of Non-toxic Metabolites: Consequences for Seafood Safety and the Environmental Fate of Biotoxins.

During blooms of the dinoflagellate Karenia brevis, filter-feeders such as oysters and clams bioaccumulate brevetoxins, often to levels that are toxic to humans. In controlled aquarium experiments, we exposed live oysters to bloom levels of toxic K. brevis, followed by 10 weeks of exposure to non-toxic microalgae. Oysters were harvested weekly and analyzed for brevetoxins and brevetoxin metabolites to quantify toxin bioaccumulation and depuration. All of the PbTx-2 concentrated by oysters was immediately converted to a mixture of polar metabolites that were then slowly eliminated from the oysters. However, 90% of measured PbTx-3 was eliminated within two weeks of toxic exposure but without apparent biotransformation. Extracts of oysters containing high levels of PbTx-3 were toxic to mice by intraperitoneal (IP) injection. Extracts of oysters harvested after PbTx-3 had been eliminated were non-toxic despite high concentrations of PbTx-2 metabolites. Oysters collected in Florida during and after a bloom of K. brevis contained polar metabolites of PbTx-2 as well as PbTx-3, but no PbTx-2. Again, PbTx-3 concentration was a good predictor of mouse toxicity. One hundred percent conversion of PbTx-2 to polar metabolites was also accomplished in vitro by spiking oyster or clam homogenate with PbTx-2, followed by a brief incubation at room temperature. These PbTx-2 metabolites did not kill mice, either orally or by intraperitoneal injection, even at concentrations 30 times greater than toxic PbTx-3 levels.