A cyclic peptide with high affinity to alpha-bungarotoxin protects mice from the lethal effect of the toxin

Employing a combinatorial phage-peptide library, we previously identified the peptide MRYYESSLKSYPD (designated, library-peptide) that binds the snake toxin alpha-bungarotoxin (alpha-BTX) with a moderate binding constant of 10(-6)M (Balass et al., 1997. Proc. Natl. Acad. Sci. USA 94, 6054-6058). Under the experimental conditions employed, we found that the library-peptide did not protect mice from alpha-BTX lethality when injected concomitantly with the toxin. In order to improve the affinity of the peptide to alpha-BTX, we designed and synthesized the peptide CRYYESSLKSYCD (Met1 and Pro12 were replaced by cysteines), which following oxidation creates a single disulfide bond and forms a cyclic structure. The design of the cyclic peptide was based on our previous NMR analysis of the library-peptide/alpha-BTX complex (Scherf et al., 1997. Proc. Natl. Acad. Sci. USA 94, 6059-6064). The cyclic peptide binds alpha-BTX with affinity two orders of magnitude higher than that of the linear library selected peptide. Whereas the library peptide was ineffective, the cyclic peptide conferred protection from alpha-BTX lethality in mice, even when given 1h after the toxin injection. The cyclic peptide conferred complete protection from alpha-BTX lethality in mice when administered 40min prior to toxin injection. However, experiments with the whole venom of the snake Bungarus multicinctus showed that protection could be achieved only when the cyclic peptide was administered concomitantly with the venom.