Synthesis and characterization of bactericidal oligopeptides designed on the basis of an insect anti-bacterial peptide

Defensin from a beetle, Allomyrina dichotoma, is known to have anti-bacterial activity against Gram-positive bacteria. This peptide, which comprises 43 amino acid residues, was effective against methicillin-resistant Staphylococcus aureus. We identified the active site of beetle defensin by measuring anti-bacterial activity against S. aureus of 64 overlapping 12-mer peptides with either a free carboxylate or a free amide group at their C-termini. An LCAAHCLAIGRR-NH2 (19L–30R-NH2) fragment showed the greatest activity of the synthetic oligopeptides. The 19L–30R-NH2 fragment was effective against both Gram-positive and Gram-negative bacteria. CD spectra showed that the 19L–30R-NH2 fragment formed an α-helical structure in the lipidic environment. The anti-bacterial effect of the 19L–30R-NH2 fragment was due to its interaction with bacterial membranes, judging from the leakage of liposome-entrapped glucose. Its anti-bacterial activity was increased when certain amino acid residues were replaced. Truncated peptides having had some amino acids removed from the N-terminus of the 19L–30R-NH2 fragment (8–10-mer peptides) still had strong anti-bacterial activity. Deleting some amino acids from the C-terminal region of the fragment dramatically reduced activity, indicating that the C-terminal region of the 19L–30R-NH2 fragment, i.e. RR-NH2, is important for exerting anti-bacterial activity. The AHCLAIGRR-NH2 (22A–30R-NH2) fragment and its analogues exhibited about 3-fold and 9–12-fold higher activity against S. aureus than did the 19L–30R-NH2 fragment, and these analogues were effective against methicillin-resistant S. aureus and Pseudomonas aeruginosa isolated from patients. These oligopeptides showed no haemolytic activity and did not inhibit the growth of murine fibroblast cells.