Muscarinic acetylcholine receptor (mAChR) inhibitor from snake venom: Interaction with subtypes of human mAChR

Snake venoms can contain a variety of well-studied neurotoxins, especially nicotinic acetylcholine receptor inhibitor, normally called postsynaptic ne urotoxin. Karlsson first reported muscarinic acetylcholine receptor (mAChR) inhibitor from snake venom. In a previous study in our laboratory, we foun d a mAChR inhibitor from Naja naja sputatrix venom that bound to rat brain synaptosomes. Brain synaptosomes contain all subtypes of mAChRs, and thus t he exact selectivity of the inhibitor could not be determined. mAChR inhibi tor from N. naja sputatrix venom was purified and the binding to all human mAChR subtypes (M1, M2, M3, M4, and M5) was investigated and is reported in this communication. The inhibitor bound to all subtypes of the human mAChR , but showed considerably high selectivity for the R15 subtype. It was also found that the reduction of disulfide bonds in the inhibitor eliminated th e binding to the mAChR. This suggests that a specific tertiary conformation maintained by disulfide bonds is essential for binding to the mAChR. An ol igo peptide, QIHDNCYNE, comparable to a part of the inhibitor molecule, was synthesized and studied for its binding to the mAChR. The synthetic peptid e did not show any binding activity, suggesting this portion of the inhibit or molecule is not involved in mAChR binding. The selective binding of the M5 mAChR subtype to antagonists has not yet been reported. Therefore, the p urified inhibitor reported in this communication may be a useful tool to cl arify the mechanism of muscarinic cholinergic transmission. (C) 1999 Academ ic Press.