The total activity of a mixture of okadaic acid-group compounds can be calculated by those of individual analogues in a phosphoprotein phosphatase 2A assay

Monitoring of okadaic acid (OA)-group toxins in seafood is of paramount importance for the protection of consumer health from diarrheic shellfish poisoning. The property of OA-group compounds to inhibit type 2A serine/threonine phosphoprotein phosphatase (PP2A) has been exploited for the detection of OA in several experimental settings, but the performance of PP2A inhibition assays in the quantification of mixtures of OA-group compounds has not been reported yet. We have used a PP2A inhibition assay to analyze the total effect of mixtures including OA and one of its analogues, okadaol (OOH), by measuring the activity of individual compounds and of toxin mixtures through the inhibition they exert on the PP2A enzyme. We found that both OA and OOH inhibit PP2A under our experimental conditions, with IC50 values of 0.37 ± 0.04 nM and 4.3 ± 0.8 nM, respectively, confirming that OOH is about ten-fold less potent than OA. PP2A assays were also carried out with predefined mixtures of OA and OOH, covering the full dose–response of one compound in the presence of increasing concentrations of the other toxin. The experimental data we obtained were used to analyze their correlation with those that could be calculated by adding the relative effects exerted by individual analogues, and we found that a good correlation exists between the observed and the expected data, when the predicted effect was calculated on the basis of toxicity equivalence factors. Our findings show that an additive model based on the use of toxicity equivalence factors of individual toxins is appropriate for the calculation of the total activity of multi-component mixtures of OA-group compounds in unknown samples.