A snake venom inhibitor to muscarinic acetylcholine receptor (mAChR): Isolation and interaction with cloned human mAChR

An inhibitor to the muscarinic acetylcholine receptor (mAChR) was purified from the venom of Crotalus atrox (western diamondback rattlesnake). The inh ibitor was found to be a 30-kDa homodimer protein with phospholipase A, act ivity. In order to determine the subtype selectivity of the purified inhibi tor, the inhibitory effect on the binding of two orthosteric antagonists, [ H-3]quinuclidinyl benzilate ([H-3]QNB) and [[H-3]N-methylscopolamine methyl chloride ([H-3]NMS), to five subtypes of cloned human mAChR was tested. Th e purified inhibitor reduced the binding of [H-3]QNB and/or [H-3]NMS to all subtypes of the mAChR while showing the highest inhibitory effect on the M -5 subtype. The K-d values of the receptors for the antagonists were increa sed in the presence of the inhibitor; however, the B-max values were not ch anged. The effects of the purified inhibitor on the dissociation of [H-3]NM S from the receptors were also investigated. Dissociation of the antagonist was remarkably slowed down by addition of the inhibitor. These findings ma y suggest an allosteric action of the purified inhibitor. In addition, the present study indicates that the presence of mAChR inhibitors is quite comm on in snake venoms. (C) 2000 Academic Press.