Binding of scorpion toxin to receptor sites associated with sodium channels in frog muscle. Correlation of voltage-dependent binding with activation.

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Abstract
Purified scorpion toxin (Leiurus quinquestriatus) slows inactivation of Na channels in frog muscle at concentrations in the range of 17-170 nM. Mono[125I]iodo scorpion toxin binds to a single class of sites in frog sartorius muscle with a dissociation constant of 14 nM and a binding capacity of 13 femtomol/mg wet weight. Specific binding is inhibited more than 90% by 3 .mu.M sea anemone toxin II and by depolarization with 165 mM K+. Half-maximal inhibition of binding is observed on depolarization to -41 mV. The voltage dependence of scorpion toxin binding is correlated with the voltage dependence of activation of Na channels. Removal of Ca from the bathing medium shifts activation and inhibition of scorpion toxin binding to more negative membrane potentials. Activation of sodium channels may cause a conformational change in the scorpion toxin receptor site resulting in reduced affinity for scorpion toxin.