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Binding properties of a neurotoxin from the venom of the green mamba, Dendroaspis viridis.

A toxin, alpha-mambatoxin, was purified from the venom of the green mamba Dendroaspis viridis using the procedures of Shipolini et al. (Shipolini, R. A., Bailey, G. S., Edwardson, J. A., and Banks, E. C. (1973) Eur. J. Biochem. 40, 337-344). The purified toxin blocks agonist-induced activation of acetylcholine receptors on muscle cells but, like alpha-bungarotoxin, it does not affect agonist-induced activation of receptors on a clonal sympathetic nerve cell line (PC12), an endothelial cell line, cultured chick ciliary ganglion neurons, or frog cardiac ganglion neurons. The toxin does block binding of alpha-bungarotoxin to cultures of muscle and nerve, and iodinated alpha-mambatoxin binds to cultures of muscle and nerve. The alpha-mambatoxin binding component on muscle was identified as acetylcholine receptor on the basis of sedimentation, immunoprecipitation, and rate of degradation. The alpha-mambatoxin binding component on PC12 cells, like the alpha-bungarotoxin binding component on these cells, is not recognized by anti-acetylcholine receptor antisera which do recognize acetylcholine receptor on these cells. The number of alpha-mambatoxin binding sites on both nerve and muscle when assayed in situ is twice that of alpha-bungarotoxin binding sites. However, when muscle cells are solubilized in nonionic detergents and then labeled with toxins, the number of alpha-mambatoxin binding sites is reduced and the two toxins bind in equal molar amounts. Finally, unlike alpha-bungarotoxin, which dissociates from complexes formed with nerve, alpha-mambatoxin forms complexes with nerve which dissociate only very slowly if at all.