Purification of protein toxins from Leiurus quinquestriatus hebraeus that modify Na channels

Two protein toxins (Lqh1 and Lqh2) were purified from crude venom obtained from Middle Eastern scorpions, Leiurus quinquestriatus hebraeus, by using cationic exchange chromatography. Lqh1 and Lqh2 were compared to toxin V (Lqq5) obtained from the venom of the North African scorpion Leiurus quinquestriatus quinquestriatus. Lqh1 and Lqh2 were purified to homogeneity; they had mol. wts of 6390 and 5870, respectively; thus both toxins differ in size from Lqq5 (7462). Electrophysiological experiments also suggested that all three toxins are different. In a dose-dependent manner, Lqh1, Lqh2 and Lqq5 lengthened and attenuated propagated compound action potentials (AP) recorded from frog sciatic nerves using the single sucrose-gap technique. Toxins Lqh1 and Lqh2 were found to be more effective than Lqq5 in both lengthening and blocking APs. Voltage-clamp experiments using the vaseline-gap technique on frog skeletal muscle fibres showed that Lqh1 and Lqh2 attenuated the Na current amplitude and slowed inactivation, while Lqq5 primarily lengthened the Na current duration. Increases in the holding potential increase the current attenuation caused by all three toxins. Evidence from sucrose-gap and voltage-clamp experiments suggests that all three toxins bind to Na channels and block them, besides their well-known ability to slow inactivation kinetics. The increased effectiveness of Lqh1 appears to be produced by a slowed rate of exit of the toxin from its binding site.