Determination of the antibacterial and lipopolysaccharide-neutralizing regions of guinea pig neutrophil cathelicidin peptide CAP11.

Previously, we revealed that a cationic antibacterial polypeptide of 11 kDa (CAP11), a member of the cathelicidins isolated from guinea pig neutrophils, exhibits not only potent antibacterial activity but also lipopolysaccharide (LPS)-neutralizing activity. In this study, to determine the biologically active regions of CAP11, we isolated or synthesized the partial peptides of CAP11 and evaluated their antibacterial and LPS-neutralizing activities. Although CAP11 has a unique homodimeric structure with a disulfide bridge, the biological activities of dimeric and monomeric forms of CAP11 were almost the same. Moreover, the G(1)-E-33 peptide of CAP11 showed the same activities as CAP11, whereas the C-terminal region (Y-34 to I-43) possessed no biological activities. In addition, the three 18-mer peptides (G(1)-R-18, T-9-K-26, and L-16-E-13) with overlapping sequences were synthesized, and their activities were determined. The three 18-mer peptides retained the antibacterial activities, and G(1)-R-18 was the most potent. In contrast, the LPS-neutralizing activities of these peptides were markedly reduced. Together, these observations indicate that the active region with antibacterial activity is localized at G(1) to R-18 of CAP11, whereas longer sequences (such as G(1) to E-33) would be required for the expression of LPS -neutralizing activity. Furthermore, the C-terminal region (Y-34 to I-43) and a disulfide bridge are not essential for the antibacterial and LPS-neutralizing activities of CAP11.