Muscarinic Toxins from Dendroaspis (Mamba) Venom

Jolkkonen, M., 1996. Muscarinic Toxins from Dendroaspis (Mamba) Venom: Peptides Selective for Subtypes of Muscarinic Acetylcholine Receptors. Acta Univ. Ups., Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 183. 52 pp. Uppsala. ISBN 91-554-3675-7 Snake venoms from various mamba species contain peptides which bind to the muscarinic but not to the nicotinic acetylcholine receptors. Peptides with this function have not been isolated from any other organism. These peptides were called muscarinic toxins, although the toxicity of the purified peptides is very limited. A number of muscarinic toxins were isolated from venom of the Eastern green mamba Dendroaspis angusticeps and the black mamba D. polylepis. The sequences of seven muscarinic toxins were determined. They contain 65 or 66 amino acids and four disulfides, and are homologous to other snake toxins such as fasciculins, a-neurotoxins, k-neurotoxins and cytotoxins. Studies of the binding the muscarinic toxins to cloned human receptors of subtype m1 through m5, expressed in Chinese hamster ovary (CHO) cells, revealed that several of the toxins have a high degree of subtype selectivity, up to 10 000-fold for a single subtype over others. Attempts to synthesise subtype-selective ligands have consistently met with very limited success, and the superior selectivity of the muscarinic toxins renders them valuable tools in receptor studies. This was exemplified by the use of radioiodinated muscarinic toxin 1 (MT1) to autoradiographically investigate the distribution of m1-receptors in brain. The method gave a narrower distribution of binding than the less selective, commonly used ligand pirenzepine. Sequence comparison of muscarinic toxins with different selectivity allowed the identification of one region, residues 31Ð33, crucial for the selective binding to receptor. The affinities of the toxins varies from 200 pM for the preferred subtype to several μM for the less favoured subtypes. Different toxins not only have very different affinities and selectivities, but also different binding rates. Studies of the binding kinetics revealed that the binding includes a fast and readily reversible association step followed by the slow formation of a more stable toxin-receptor complex. In contrast to well-documented results on the related m1toxin from D. angusticeps, no allosteric effects on antagonist binding could be observed with D. angusticeps toxin MT3. Preliminary investigations indicate that the toxins have agonistic effects. This is supported by results of other workers in the field, again with the exception of m1-toxin. Thus there seems to exist qualitative as well as quantitative differencies between individual muscarinic toxins. The structure-function relationship of the toxins is discussed, and it is concluded that the toxins are likely to find use as biochemical probes, and hopefully also in studies of receptor function and in ligand design.