The behaviour of mixtures of paralytic shellfish toxins in receptor dependent assays

Using two unrelated STX-binding proteins (the rat brain Na channel and a saxiphilin from the xanthid crab Liomera tristis), this study experimentally validated the predicted combined effects of binary and ternary mixtures of paralytic shellfish toxins (PSTs). All toxin dilutions (tritiated saxitoxin ([3H]STX), STX dihydrochloride, neoSTX, dcSTX, and GTX5) were in 0.01 M acetic acid. The L. tristis saxiphilin assay was conducted in high protein binding 96 well filtration plates and the rat brain Na channel binding of [3H]STX was measured using a microtiter plate method.The model predicts the amount of inhibition by toxin mixtures in competitive binding assays from which an IC50 (concentration that inhibits radioligand bindingby 50%) can be calculated. IC50 values and their 95% confidence limits from experimental data were derived using the single site curve equation of Graphpad Prism (Ver 4.0, Graphpad Software, San Diego, CA), with fitting constrained to a slope of 1 and a baseline of 0. To understand how toxin mixtures may act at the Na channel receptor via which PSTs exert their neurotoxicity.To test if the presence of weaker toxins dilutes stronger toxins The strong dominance of a mixture by the most potent toxins has implications for measurement of toxic test samples and for standards that may contain low levels of highly potent bioactive impurities.