Non-conventional toxin WTX and its disulfide-fixed synthetic fragments: Interaction with nicotinic acetylcholine receptors and reduction of blood pressure

Non-conventional snake venom toxins, such as WTX from the cobra Naja kaouthia, are three-finger proteins containing a fifth disulfide bond in the N-terminal polypeptide loop I and inhibiting α7 and muscle-type nicotinic acetylcholine receptors (nAChRs). Because the central polypeptide loop II of non-conventional toxins plays an important role in their biological activity, we synthesized several WTX loop II fragments with two cysteine residues added at the N- and C-termini and oxidized to form a disulfide bond. The inhibition by peptides of several nAChRs subtypes was investigated using different methods and the effects of peptides on the rat arterial pressure and heart rate were analyzed. The synthetic fragments inhibited α7 and muscle-type nAChRs more potently than WTX. We showed for the first time that WTX and its fragments inhibited α9α10 as well as neuronal α3β2 and α4β2 nAChRs, again the synthetic fragments being more potent than WTX. The loop II fragments reduced blood pressure more potently than WTX in normotensive, awake rats. In connection with this, the WTX cardiovascular effects were analyzed and it was found that toxin very weakly affected parameters of papillary muscle contractions with no influence on aortic ring contractility. The observed effects were not so significant to explain the decrease in BP, the hemodynamic effects of WTX appearing not to result from direct influence on the myocardium and blood vessels. The synthetic fragments of the N- and C-terminal loops I and III were inactive in all tests. Thus, both in inhibition of all analyzed nAChR subtypes and in reduction of blood pressure, fragments of the central loop II were more active than WTX. This appears to be a first indication for three-finger proteins that the fragments of the central loop II are more active than the native toxin.