Noxiustoxin and leiurutoxin III, two homologous peptide toxins with binding properties to synaptosomal membrane K+ channels.

Noxiustoxin (NTX), a short-chain peptide toxin from the scorpion C. noxius, binds to a single class of non-interacting binding sites in brain synaptosomal membranes with a KD = 160-300 nM and a Bmax = 2.2 pmols.mg-1 protein. The K+ channel blocking agents tetraethylammonium, 4-aminopyridine and Charybdotoxin partially inhibit the binding of [125I]NTX, while a variety of toxins targeted against Ca2+ and Na+ channels have no effect, suggesting that NTX interacts with a bona fide K+ channel protein. NTX totally displaces the binding of [125I]Leiurutoxin III, a novel peptide from L. quinquestriatus venom with N-terminal amino acid sequence homologous to that of NTX. These results suggest that both toxins bind to a common receptor site and are promising tools to dissect the molecular mechanisms of channel-toxin interaction.