Synthetic peptides corresponding to the sequence of noxiustoxin indicate that the active site of this K+ channel blocker is located on its amino-terminal portion.

A nonapeptide Thr-Ile-Ile-Asn-Val-Lys-Cys-Thr-Ser (NTX1-9) and a decapeptide Met-Asn-Gly-Lys-Cys-Lys-Cys-Tyr-Asn-Asn (NTX30-39) corresponding to the N-terminal and C-terminal sequences respectively of Noxiustoxin (NTX) were synthesized by the solid phase method of Merrifield (1963). The first synthetic peptide (NTX1-9) was shown to be toxic to mice independently of the route of administration: intraperitoneally, subcutaneously or intraventricularly (100-200 micrograms/20 g mouse weight). The second (NTX30-39) was not toxic even at higher dose (400 micrograms/20 g mouse). When the effects of the peptide NTX1-9 and of the authentic toxin (Noxiustoxin) were studied on the liberation of [3H] 4-aminobutyric acid (3H-GABA) from mouse synaptosomes, both gave essentially the same results, except that peptide NTX1-9 was needed at higher concentration. Synthetic peptide NTX30-39 had no effect in the same preparation at even higher doses. The GABA release produced by toxic peptide NTX1-9 was not affected by tetrodotoxin but was completely abolished by the presence of the K+ ionophore valinomycin, mimicking the effect of native NTX in the same system (Sitges et al., 1986). These results indicate that the toxic active site of Noxiustoxin is possibly located in or near the N-terminal amino acid portion of the molecule.