Temperature and base requirements for the alkaline hydrolysis of okadaite's esters

Portuguese bivalves are recurrently contaminated with okadaic acid (OA) and dinophysistoxin-2 (DTX2), found mainly in esterified forms. Throughout the years different conditions have been reported in the literature for releasing the parent toxins through an alkaline hydrolysis step, in order to simplify their detection by HPLC-FLD or LC–MS. In order to clearly understand toxin stability and reaction end-point the binominous temperature/time course and base concentration were studied using naturally contaminated bivalve samples. The results showed a strong temperature dependence of the reaction. At 60 °C and 70 °C the hydrolysis was fast, and 40 min were sufficient for maximal recovery of OA and DTX2, while at 40 °C and 50 °C it was only complete after 100 min and 60 min, respectively. At room temperature the reaction was slow and incomplete even after 2 h. Stability of OA and DTX2 in semi-purified bivalve matrix at 70 °C for 2 h was demonstrated. Concentrations of sodium hydroxide lower than 2.5 M, corresponding to a final incubation concentration of 0.23 M, resulted in incomplete release of parent toxins, demonstrating that high concentrations are needed when taking into account the dilution in the supernatant extract.