Specific binding of toxin II from Centruroides suffusus suffusus to the sodium channel in electroplaque membranes

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Abstract
The binding of toxin II from the scorpion C. s. suffusus (CssII) to electroplaque membranes from Electrophorus electricus was studied with the use of a radiolabeled derivative of the toxin ([125I]CssII). Specific binding of the latter to the membranes required the protonation of a group (either in the membrane or in the toxin itself) with an apparent pKa value of 7.5, and also the presence of a certain minimum concentration of ions, though there was no requirement for a specific ion. At 20.degree. C and pH 6 the 2nd-order rate constant for formation of the [125I]CssII-membrane complex was about 5 .times. 106 M-1 s-1, while the 1st-order constant for its dissociation was about 2 .times. 10-3 s-1. Under equilibrium conditions specific binding of [125I]CssII was a simple saturable function of [125I]CssII concentration, characterized by a Kd of 0.4-0.7 nM and a maximum capacity of 0.9-2.4 pmol of toxin/mg of membrane protein. The latter value was the same as the number of membrane sites that could specifically bind a radiolabeled derivative of tetrodotoxin. Unlabeled CssII displaced bound [125I]CssII with an apparent Kd of about 1 nM. None of 19 other neurotoxins or local anesthetics known to interact with Na+ channels in excitable cells affected [125I]CssII binding, but it was completely inhibited by toxin .gamma. from the scorpion Tityus serrulatus serrulatus. The Na+ channel probably possesses a distinct class of binding sites to which these 2 scorpion toxins bind with high affinities. On the other hand, no CssII receptor was detected in crab [Cancer pagurus] axonal membranes, indicating that it is not a characteristic feature of all Na+ channels.