Isolation and pharmacological characterization of four novel Na+ channel-blocking toxins from the scorpion...

Four novel Na+ channel-blocking toxins (numbered 6 to 9) were purified from the venom of the scorpion Centruroides noxius Hoffmann by gel filtration and high-performance liquid chromatography and their chemical and pharmacological properties were characterized. Amino acid analysis and SDS-PAGE of the pure toxins showed them to be composed of approximately 65 amino acid residues with a molecular mass of approximately 7,500 Da. The amino acid sequences of the newly isolated toxins displayed substantial similarity to those of previously isolated and characterized C. noxius toxins. Toxin 7, the most active toxin from this venom, selectively blocked the whole-cell inward Na+ current (INa) from guinea pig ventricular myocytes without altering the K+ or Ca2+ currents. Using rat brain synaptosomes, the specific binding parameters of 125I-Toxin 7 were determined: KD = 40 pM and Bmax = 1.8 pmol/mg protein. The binding was independent of membrane potential and was displaced by all Na+ channel-toxins from C. noxius venom, with K0.5s ranging from 60 pM to 60 nM. Tityus gamma-toxin, a toxin representative of beta-scorpion toxins, totally displaced 125I-Toxin 7 binding, but AaH II and Lqq V toxins, representative of alpha-scorpion toxins, had no effect. All four C. noxius toxins inhibited [3H]GABA uptake by synaptosomes with IC50 s similar to KD s. The toxin effect was not synergistic with veratridine. From these results, it was concluded that the newly purified toxins exert their effects by binding to Site 4 of the voltage-sensitive Na+ channel and must, therefore, be classified as beta-scorpion toxins.