The behavior of mixtures of paralytic shellfish toxins in competitive binding assays

Organisms that contain paralytic shellfish toxins (PSTs) may contain many members of this toxin family. PSTs block voltage-gated sodium channels (Na channel) and elicit neurotoxicity. Animals, including humans, may encounter PST mixtures via consumption of tainted seafood, contaminated water, or the microalgae that produce the toxins. PST binding by the Na channel as well as other proteins such as antibodies and saxiphilin have been used to develop biomolecular assays for PSTs. An equation that predicts the combined effects of binary and ternary PST mixtures has been experimentally validated for two unrelated STX-binding proteins, the rat brain Na channel and a saxiphilin from the xanthid crab Liomera tristis. It was found that the most potent toxin or toxins in any mixture profoundly affect the cumulative potency of the mixture, overwhelming weaker toxins with the transition from strong to weak toxicity and changing in a curvilinear manner. Less active PSTs must be several orders of magnitude more concentrated than stronger toxins for the mixture to reflect their potency. This behavior is important in understanding how toxin mixtures may act at the Na channel receptor via which PSTs exert their neurotoxicity and that the presence of weaker toxins does not dilute the effect of stronger toxins in a linear fashion. This strong dominance of a mixture by the most potent toxins also has implications for measurement of toxic test samples and for standards that may contain low levels of highly potent bioactive impurities. This equation has been extended to mixtures of PSTs containing more than three toxins and may be applicable to other natural contaminants and any competitive binding assays used to detect their presence and measure their concentration.