The binding to rat brain synaptosomes of a beta-scorpion toxin, i.e., toxin II of Centruroides suffusus suffusus (Css II), was studied as a function of pH, temperature, and concentration of some monovalent and divalent cations. At 10 degrees C and pH 6.0, the specific binding of 125I-labeled Css II corresponds to a single class of noninteracting high-affinity binding sites (KD = 0.18 nM) with a capacity (4.2 pmol/mg of protein) that is almost identical with that generally accepted for saxitoxin. The equilibrium dissociation constant of beta-scorpion toxin is pH independent, but the maximum binding capacity is reduced with increasing pH. Li+, guanidinium, Ca2+, Mg2+, and Mn2+ modified the apparent KD of the 125I-labeled Css II toxin. The equilibrium dissociation constant varies markedly with the temperature. The van't Hoff plot of the data is curvilinear, corresponding to a standard free-energy change associated with an entropy-driven process. The association rate constant also varies considerably with the temperature whereas the Arrhenius plot is linear between 1 and 30 degrees C. The energy of activation determined from these data is 17.6 kcal/mol. These results support the hypothesis that a cluster of nonpolar amino acid residues present on one face of the molecule is involved in the toxin-receptor interaction.

Neurotoxins purified from scorpion venoms previously had been divided into two classes according to their binding properties in rat brain synaptosomes. However, the pharmacological action of β‐scorpion toxin (β‐ScTx) on this preparation has not yet been described. In this report we show that a β‐ScTx induced an increase in 22Na+ uptake through synaptosomal voltage‐sensitive sodium channels since this stimulation was abolished by tetrodotoxin (TTX). The increase was smaller than with veratridine and no synergy was observed between β‐ScTx and veratridine, as is the case for α‐scorpion toxin (α‐ScTx) and veratridine. The effects of α‐ and β‐ScTx were additive and the concentration‐effect curves for each type of toxin were not modified by the other, suggesting that these two types of toxins act through distinct and noninteracting receptor sites. This was confirmed by the absence of mutual modification of the equilibrium and kinetic binding properties. β‐ScTx was shown to inhibit the uptake and to stimulate the release of [3H]γ‐aminobutyric acid. These effects were blocked by TTX, and no synergy was observed with veratridine. It was concluded that all these effects are mediated by the activation of voltagesensitive sodium channels induced by the binding of β‐ScTx to a receptor site (site 4) distinct from those for other neurotoxins acting on sodium channels.