Bombinin-like peptides with antimicrobial activity from skin secretions of the Asian toad, Bombina orientalis

The structures and hemolytic and bactericidal activities of three bombinin-like peptides, or BLP-1-3, from the skin of Bombina orientalis are described. The peptides were isolated from the skin of B. orientalis and sequenced by tandem mass spectrometry and are amphipathic, cationic peptides of 25-27 amino acids in length. The sequence of the most abundant member (BLP-1) is: Gly-Ile-Gly-Ala-Ser-Ile-Leu-Ser-Ala-Gly-Lys-Ser-Ala-Leu-Lys-Gly-Leu- Ala-Lys-Gly-Leu-Ala-Glu-His-Phe-Ala-Asn-NH2. All three peptides were found to share considerable, but not complete, homology with bombinin, an antimicrobial, hemolytic peptide first isolated by Michl and Csordas (Csordas, A., and Michl, A. (1970) Monatsh. Chem. 101, 182-189) from the skin of Bombina variegata. The BLPs have been assayed for antibiotic and hemolytic activity and found to be more potent than magainin 2 (a related antimicrobial peptide from Xenopus laevis) in their ability to kill bacteria. However, no significant hemolytic activity was found for these peptides which suggests a selectivity for prokaryotic over eukaryotic membranes. The molecular basis for antibacterial activity is presumed to be due to their predicted amphipathic alpha-helical structures which is supported by circular dichroism measurements that found significant helical content (63-69% alpha-helix) in 40% trifluoroethanol. Last, a cDNA library was constructed from the skin of B. orientalis and screened with an oligonucleotide probe complementary to the COOH terminus of BLP-1. Several clones were isolated and sequenced that encode BLP-1 and BLP-3, as well as an additional peptide (BLP-4) that differs by two amino acid substitutions from BLP-3.