A new member of the endothelin/sarafotoxin family of vasoconstrictor peptides, bibrotoxin (BTX), was isolated from the venom of the burrowing aspAtractaspis bibroni by reversed-phase FPLC. The amino acid sequence of BTX differs from SRTX-b in the substitution Ala4 instead of Lys4, which suggests that it represents the peptide isoform of Atractaspis bibroni corresponding to SRTX-b. BTX competed for [125I]ET-1 binding to human ETB-type receptor with a Ki of 3.2 × 10−9 M compared to 4.2 × 10−9 M for SRTX-b. In rat thorax aorta BTX induced vasoconstrictions with a threshold concentration of 3 × 10−8 M compared to 1 × 10−9 for ET-1.
