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Journal of Neuroscience, Vol 13, 4293-4300, Copyright © 1993 by Society for Neuroscience
Purification and properties of m1-toxin, a specific antagonist of m1 muscarinic receptors
SI Max, JS Liang and LT Potter
Department of Molecular and Cellular Pharmacology, University of Miami School of Medicine, Florida 33101.
The venom of the Eastern green mamba from Africa, Dendroaspis angusticeps,
was found to block the binding of 3H-quinuclidinyl benzilate to pure m1 and
m4 muscarinic ACh receptors expressed in Chinese hamster ovary cells. The
principal toxin in the venom with anti- m1 muscarinic activity was purified
by gel filtration and reversed- phase HPLC. This toxin has 64 amino acids,
a molecular mass of 7361 Da, and an isoelectric point of 7.04. Its cysteine
residues are homologous with those in curare-mimetic alpha-neurotoxins, and
with those in fasciculin, which inhibits AChE. At low concentrations the
toxin blocked m1 receptors fully and pseudoirreversibly while having no
antagonist activity on m2-m5 receptors; the toxin is therefore named
"m1-toxin." At higher concentrations m1-toxin interacted reversibly with m4
receptors, and half of the toxin dissociated in 20 min at 25 degrees C. The
affinity of m1-toxin is therefore much higher for m1 than for m4 receptors.
By comparison with m1-toxin, pirenzepine has sixfold higher affinity for m1
than for m4 receptors. Autoradiographs of muscarinic receptors in the rat
brain demonstrated that m1-toxin blocked the binding of 2 nM 3H-pirenzepine
only in regions known to bind m1-specific antibodies. Thus, m1-toxin is a
much more selective ligand than pirenzepine for functional and binding
studies of m1 muscarinic receptors.
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