Characterization of a new muscarinic toxin from the venom of the Brazilian coral snake Micrurus lemniscatus in rat hippocampus

Aims: We have isolated a new muscarinic protein (MT-MI alpha) from the venom of the Brazilian coral snake Micrurus lemniscatus. Main methods: This small protein, which had a molecular mass of 7,048 Da, shared high sequence homology with three-finger proteins that act on cholinergic receptors. The first 12 amino acid residues of the N-terminal sequence were determined to be: Leu-Ile-Cys-Phe-Ile-Cys-Phe-Ser-Pro-Thr-Ala-His. Key findings: The MT-MI alpha was able to displace the [H-3]QNB binding in the hippocampus of rats. The binding curve in competition experiments with MT-MI alpha was indicative of two types of [H-3]QNB-binding site with pK(i) values of 9.08 +/- 0.67 and 6.17 +/- 0.19, n = 4, suggesting that various muscarinic acetylcholine receptor (mAChR) subtypes may be the target proteins of MT-MI alpha. The MT-MI alpha and the M-1 antagonist pirenzepine caused a dose-dependent block on total [H-3]inositol phosphate accumulation induced by carbachol. The IC50 values for MT-MI alpha and pirenzepine were, respectively, 33.1 and 2.26 nM. Taken together, these studies indicate that the MT-MI alpha has antagonist effect on mAChRs in rat hippocampus. Significance: The results of the present study show, for the first time, that mAChRs function is drastically affected by MT-MI alpha since it not only has affinity for mAChRs but also has the ability to inhibit mAChRs. (C) 2011 Elsevier Inc. All rights reserved.