Effects of Mlx-8, a phospholipase A 2 from Brazilian coralsnake Micrurus lemniscatus venom, on muscarinic acetylcholine receptors in rat hippocampus.

Background: Here, we described the presence of a neurotoxin with phospholipase A(2) activity isolated from Micrurus lemniscatus venom (Mlx-8) with affinity for muscarinic acetylcholine receptors (mAChRs). Methods: The purification, molecular mass determination, partial amino acid sequencing, phospholipase A(2) activity determination, inhibition of the binding of the selective muscarinic ligand [H-3]QNB and inhibition of the total [H-3]inositol phosphate accumulation in rat hippocampus of the Mlx-8 were determined. Results: Thirty-one fractions were collected from HPLC chromatography, and the Mlx-8 toxin was used in this work. The molecular mass of Mlx-8 is 13.628 Da. Edman degradation yielded the following sequence: NLYQFKNMIQCTNTRSWL-DFADYG-CYCGRGGSGT. The Mlx-8 had phospholipase A(2) enzymatic activity. The pK(i) values were determined for Mlx-8 toxin and the M-1 selective muscarinic antagonist pirenzepine in hippocampus membranes via [H-3]QNB competition binding assays. The pKi values obtained from the analysis of Mlx-8 and pirenzepine displacement curves were 7.32 +/- 0.15, n = 4 and 5.84 +/- 0.18, n = 4, respectively. These results indicate that Mlx-8 has affinity for mAChRs. There was no effect on the inhibition ability of the [H-3]QNB binding in hippocampus membranes when 1 mu M Mlx-8 was incubated with 200 mu M DEDA, an inhibitor of phospholipase A(2). This suggests that the inhibition of the phospholipase A(2) activity of the venom did not alter its ability to bind to displace [H-3]QNB binding. In addition, the Mlx-8 toxin caused a blockade of 43.31 +/- 8.86%, n = 3 and 97.42 +/- 2.02%, n = 3 for 0.1 and 1 mu M Mlx-8, respectively, on the total [H-3]inositol phosphate content induced by 10 mu M carbachol. This suggests that Mlx-8 inhibits the intracellular signaling pathway linked to activation of mAChRs in hippocampus. Conclusion: The results of the present work show, for the first time, that muscarinic receptors are also affected by the Mlx-8 toxin, a muscarinic ligand with phospholipase A(2) characteristics, obtained from the venom of the Elapidae snake Micrurus lemniscatus, since this toxin was able to compete with muscarinic ligand [H-3]QNB in hippocampus of rats. In addition, Mlx-8 also blocked the accumulation of total [H-3]inositol phosphate induced by muscarinic agonist carbachol. Thus, Mlx-8 may be a new pharmacological tool for examining muscarinic cholinergic function.