Membrane potential-dependent binding of scorpion toxin to the action potential Na+ ionophore. Studies with a toxin derivative prepared by lactoperoxidase-catalyzed iodination.

Scorpion toxin enhances activation of the action potential Na+ ionophore by the alkaloid neurotoxins veratridine, batrachotoxin, and aconitine. The pure toxin can be iodinated in a lactoperoxidase-catalyzed reaction. Both the monoiodoand diiodotoxin derivatives retain physiologic activity. Both monoiodoand diiodoscorpion toxin bind to a single class of saturable binding sites in electrically excitable neuroblastoma cells. The concentration dependence of saturable binding of the toxin and the concentration dependence of activation of the action potential Na+ ionophore by scorpion toxin are identical. Both saturable binding and activation of the Na+ ionophore are inhibited by depolarization of the cells. Variant neuroblastoma clones specifically lacking the action potential Na+ ionophore have no saturable or membrane potential-dependent binding of scorpion toxin. These results indicate that scorpion toxin binds specifically to the action potential Na+ ionophore. Scatchard plots of the binding data are linear, indicating the presence of a single homogeneous class of binding sites. These sites have a K,] of 0.5 to 2 nM for monoiodoscorpion toxin and 1.4 to 4 nM for diiodoscorpion toxin. The binding capacity is 47 + 6 fmol of toxinlmg of cell protein or 25 sites/pm* of cell surface membrane. A single action potential Na+ channel transports 1 x 10” ionslmin and has a conductance of 2.9 psiemens at physiologic ion concentration. Depolarization of the cells increases the K, for scorpion toxin binding lo-fold for each 31 mV of depolarization but does not affect the number of sites observed. Tetrodotoxin and saxitoxin have no effect on scorpion toxin binding. Batrachotoxin enhances scorpion toxin binding. Data are presented supporting the conclusion that the membrane potential dependence of scorpion toxin binding is due to a membrane potential-dependent conformational change in the scorpion toxin binding site that is associated with the process of inactivation of the Na+ ionophore during * The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby markedl~aduertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. $ Present address, Department of Pharmacology, University of Washington, Seattle, Washington 98105. prolonged depolarization. A two-state conformational change mode1 based on this assumption gives a qualitative fit of the data.