One-to-one binding of a purified scorpion toxin to Na channels

Abstract

DETAILED elucidation of the molecular organisation which controls ionic flow through the excitable membranes has been prevented by a difficulty in isolating substances to which characteristic features of the ionic channels can be attributed^1–3. In the study of the cholinergic receptor, polypeptide α-toxins from certain snakes have been successfully used for the identification and purification of nicotinic receptors⁴. Thus, agents analogous to snake toxins may be of a decisive value in the molecular approach to ionic channels, and toxins from certain scorpions are among the most promising candidates^5–8. Scorpion toxins seem to act by modifying kinetic properties of Na channels as well as by suppressing the current through K channels^9–11. But, no quantitative analysis was made on an electrophysiological basis to show the precise nature of the toxin-binding. We describe here a study of the effects of a toxin from Leiurus quinquestriatus on Na, Ca and K currents in the tunicate egg membrane, where each ionic current proved to be essentially identical with that in other excitable membranes^12,13. The discrete critical membrane potentials for the activation of these currents in the egg facilitate discrimination of the respective currents only by adjusting potential steps in the voltage-clamp condition. Thus, quantitative aspects of the inactivation kinetics of Na current can be analysed conveniently in this preparation.