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Abstract : CAP11-derived antimicrobial peptides by amino acid substitutions Mammalian myeloid and epithelial cell express several antibacterial peptides (defensins and cathelicidins) that contribute to the innate host defense by killing invaded microorganisms. CAP11 (cationic antibacterial polypeptide of 11 kDa), which was isolated from guinea pig neutrophils, has been shown to possess the potent bactericidal and LPS-neutralizing activities. CAP11 is a homodimer of Gly1-Ile43 joined with one disulfide bond. In this study, we revealed that the dimerization of the peptides is not essential for its biological activity, and 18mer peptide of CAP11 (G1-R18) is the most potent among various peptide derivatives. However, its biological activities were much lower compared with native CAP11. Furthermore, we tried to enhance the biological activities of CAP11 (G1-R18) by substituting amino acids. Two novel peptides, CAP11 (1-18m) and CAP11 (1-18m2), whose hydrophobicity and positive charge were modified by the amino acid substitutions, exhibited more potent bactericidal activities against S. aureus and E. coli, and more strongly inhibited the LPS-binding to CD14+ RAW264.7 cells, compared with CAP11; i.e. the bactericidal and LPS-neutralizing activities of CAP11 (1-18m2) was about 10-fold greater than those of CAP11, and CAP11 (1-18m2) completely killed both methicillin-sensitive S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA) even at 0.1 pg/ml. Thus, the bactericidal and LPS-neutralizing activities of CAP11-derived antibacterial peptides could be enhanced by the modification of their hydrophobicity and positive charge via the amino acid substitutions.