Charybdotoxin and noxiustoxin, two homologous peptide inhibitors of the K+(Ca2+) channel

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Abstract
We show that noxiustoxin (NTX), like charybdotoxin (CTX) described by others, affects Ca2+‐activated K+ channels of skeletal muscle (K+(Ca2+) channels). Chemical characterization of CTX shows that it is similar to NTX. Although the amino‐terminal amino acid of CTX is not readily available, the molecule was partially sequenced after CNBr cleavage. A decapeptide corresponding to the C‐terminal region of NTX shows 60% homology to that of CTX, maintaining the cysteine residues at the same positions. While CTX blocks the K+(Ca2+) channels with a K d of 1–3 nM, for NTX it is approx. 450 nM. Both peptides can interact simultaneously with the same channel. NTX and CTX promise to be good tools for channel isolation.