Purification of a toxic protein from scorpion venom which activates the action potential Na+ ionophore

Venom of the scorpion Leiurus quinquestriatus acts cooperatively with the alkaloids veratridine, aconitine, and batrachotoxin in activating the action potential Na+ ionophore. A small (Mr = 6700), basic (pI approximately 9.8), toxic polypeptide purified approximately 80-fold from this venom by ion exchange chromatography appears homogeneous by gel electrophoresis and isoelectric focusing and, like whole venom, acts cooperatively with the alkaloids veratridine, aconitine, and batrachotoxin to activate the action potential Na+ ionophore. The action of the scorpion toxin is slowly reversible. Concentration-response curves suggest interaction with a single class of sites with KD - 1.3 to 2.4 nM. The scorpion toxin is a poor activator of the Na+ ionophore when tested alone. However, treatment of cells sequentially with scorpion toxin followed by veratridine activates as well as treatment with both simultaneously suggesting that scorpion toxin binds in the absence of veratridine but does not activate the Na+ ionophore unless veratridine is present. In contrast, scorpion toxin causes 3- to 20-fold decreases in apparent KD for aconitine, veratridine, and batrachotoxin. The effect of the toxin is inhibited competitively by divalent cations and noncompetitively by tetrodotoxin (KI - 4 nM).