Conformational analysis and cytotoxic activities of the frog skin host-defense peptide, hymenochirin-1Pa

Hymenochirin-1Pa (LKLSPKTKDTLKKVLKGAIKGAIAIASMA-NH₂) is a host-defense peptide first isolated from skin secretions of the frog Pseudhymenochirus merlini (Pipidae). A nuclear magnetic resonance structural investigation demonstrates that the peptide has a random coil conformation in water but, in the membrane-mimetic solvent 50% (v/v) trifluoroethanol-water adopts a well-defined conformation characterized by two α-helical domains from residues K6 to G17 and from G21 to M28, with the N-terminal region unfolded. The presence of a GXXXG domain, the most common structural motif found at the interface between interacting trans-membrane helices, between residues 17 and 21, introduces a kink corresponding to a deviation from linearity of 93 ± 31°. Hymenochirin-1Pa shows broad spectrum anti-bacterial activity, including high potency against multidrug-resistant clinical isolates of Staphylococcus aureus, Acinetobacter baumannii, and Stenotrophomonas maltophilia. The peptide also shows high cytotoxic potency against human non-small lung adenocarcinoma A549 cells, breast adenocarcinoma MDA-MB-231 cells, and colorectal adenocarcinoma HT-29 cells but its therapeutic potential as an anti-cancer agent is limited by moderate hemolytic activity against human erythrocytes and lack of selectivity for tumor cells. Increasing cationicity of the peptide by substituting the Asp⁹residue by either l-Lys (K) or d-Lys (k) has relatively minor effects on antimicrobial and anti-tumor potencies but the [D9k] analog is non-hemolytic LC₅₀> 400 μM. Thus, [D9k]hymenochirin-1Pa may serve as a template for the design of non-toxic antimicrobial agents for use against multidrug-resistant pathogenic bacteria.