Tityus bahiensis toxin IV-5b selectively affects Na channel inactivation in chick dorsal root ganglion neurons

Abstract

A novel toxin was isolated from the venom of the Brazilian scorpion Tityus (T.) bahiensis. The N-terminal amino acid sequence of this toxin was shown to be 80% identical to the corresponding segment of T. serrulatus toxin IV-5. The new toxin was thus named toxin IV-5b. Toxin IV-5b was found to markedly slow inactivation of Na channel in dorsal root ganglion neurons from chick embryo. By contrast, Na channel activation was only negligibly delayed, and deactivation completely unaffected. Similarly unaffected by the toxin were K and Ca currents. The slowing effect of the toxin starts to appear at concentrations of c. 80 nM, and shows a KD of 143 nM. With a toxin concentration of 2.4 microM, the Na channel inactivation time constant was increased c. 3-fold with respect to the control. The slowing of inactivation was voltage dependent, and increased with depolarization.