MODE OF ACTION OF IBERIOTOXIN, A POTENT BLOCKER OF THE LARGE CONDUCTANCE CA-2+-ACTIVATED K+ CHANNEL

Iberiotoxin, a toxin purified from the scorpion Buthus tamulus is a 37 amino acid peptide having 68% homology with charybdotoxin. Charibdotoxin blocks large conductance Ca2+-activated K+ channels at namolar concentrations from the external side only (Miller, C., E. Moczydlowski, R. Latorre, and M. Phillips. 1985. Nature (Lond.). 313:316-318). Like charybdotoxin, iberiotoxin is only able to block the skeletal muscle membrane Ca2+-activated K+ channel incorporated into neutral planar bilayers when applied to the external side. In the presence of iberiotoxin, channel activity is interrupted by quiescent periods that can last for several minutes. From single-channel records it was possible to determine that iberiotoxin binds to Ca2+-activate K+ channel in a bimolecular reaction. When the solution bathing the membrane are 300 mM K+ internal and 300 mM Na+ external the toxin second order association rate constant is 3.3 x 10(6) S-1 M-1 and the first order dissociation rate constant is 3.8 X 10(-3) S-1, yielding an apparent equilibrium dissociation constant of 1.16 nM. This constant is 10-fold lower than that of charibdotoxin, and the values for the rate constants showed above indicate that this is mainly due to the very low dissociation rate constant; mean blocked time approximately 5 min. The fact that tetraethylammonium competitively inhibits the iberiotoxin binding to the channel is a strong suggestion that this toxin binds to the channel external vestibule. Increasing the external K+ concentration makes the association rate constant to decrease with no effect on the dissociation reaction indicating that the surface charges located in the external channel vestibule play an important role in modulating toxin binding.