Dendrotoxin (DTX), an Mr7000 convulsant polypeptide from the venom of Dendroaspis angusticeps, or its facilitatory homologues act through blockade of certain voltage-sensitive K+currents in a variety of neurons. High-affinity acceptors for DTX have been demonstrated in synaptic plasma membranes of rat or chick brain, and a fraction of these avidly bind β-bungarotoxin (β-BuTX), a presy-naptically active protein whose lighter B polypeptide is homologous to this toxin. Extraction of rat synaptic plasma membranes using Triton X-100 in K+-containing buffer yielded binding sites with KDvalues of ~0.5 and 0.7 nM for125I-labeled DTX and β-BuTX, respectively. The content of high-affinity sites obtained forβ-BuTX, including the contribution of a lower affinity component, approximates to the Bmax(~ 1.3 pmol/mg of protein) obtained for the apparent single set of DTX acceptors. On solubilization, the pharmacological specificity of the acceptor for neurotoxic DTX congeners was retained.125I-β-BuTX binding (2.1 nM) was blocked efficaciously by DTX (IC50= 1.6 nM) while the binding of 2.1 nM125I-DTX was inhibited completely by β-BuTX (IC50= 25 nM); the lower potency of the latter could relate to the noncompetitive nature of the mutual competition and to the presence of high- and low-affinity sites for β-BuTX. On gel filtration, or sedimentation analysis in H20/sucrose and2H2O/sucrose gradients, one peak of DTX binding activity was observed, and this was inhibitable by β-BuTX. From the hydrodynamic properties of the acceptor/detergent/lipid complex (s20,w= 13.2 S; Stokes radius = 8.6 nm), a molecular weight of 405 000–465 000 was estimated. The agreement of these values with those reported for β-BuTX binding activity of chick brain, together with the acceptor binding parameters observed for both toxins, and published cross-linking experiments, suggests that they share common acceptors which are likely constituents of some voltage-dependent K+channels.
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