A snake venom phospholipase A2 with high affinity for muscarinic acetylcholine receptors acts on guinea pig ileum

A group of small proteins, designated as muscarinic toxins (MTs), have been isolated from the venom of African green mamba (Dendroaspis angusticeps) and documented to bind selectively to individual muscarinic acetylcholine receptor (mAChR) subtypes. These components have less been reported to be isolated from other snake venoms. In this study, we have isolated a snake factor with high affinity for mAChR from the venom of Naja atra (Chinese cobra) by column chromatography on Sephadex G-50, Sephadex G-150, CM-Sepharose Fast Flow and Poros®CM 4.6/100 Perfusion Chromatography Column. The final preparation was homogeneous as determined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and HPLC. The isolated active component, which was designated muscarinic protein (MP), was found to displace [3H]quinuclidinyl benzilate binding to rat cortex synaptosomes in a dose-dependent manner, and the Ki value estimated was 10.1 nM. The isolated MP was determined to have a molecular weight of 13.3 kDa and an N-terminal amino acid sequence of NLYQFKNMIQCTVPSR, which is highly homologous with phospholipase A2 from the venoms of genus Naja. The N. atra MP could hydrolyze phosphatidylcholine in a dose-dependent manner. In guinea-pig ileum, MP produced an onset and dose-dependent contraction, which could be reversed by atropine indicating the involvement of mAChR. The EC50 value of MP for guinea-pig ileum contraction was estimated as 30 nM, and the maximum contraction caused by MP was approximately 43% of that obtained from carbachol. These results seem to suggest that the snake venom phospholipase A2 may not only have high affinity for mAChRs but also have the ability to activate mAChRs. However, it is possible that the toxin caused the contraction in the guinea-pig ileum by inducing acetylcholine release via another mechanism.