Antibacterial activity of 15-residue lactoferricin derivatives

Lactoferricins are a class of antibacterial peptides isolated after gastric -pepsin digest of the mammalian iron-chelating-protein lactoferrin. For inv estigation of antibacterial activity, we prepared short synthetic derivativ es of bovine, human, caprine, murine and porcine lactoferricins with 15-ami no-acid residues of high sequence homology. The peptides corresponded to am ino-acid residues 17-31 of the mature bovine lactoferrin. Only the bovine a nd caprine derivatives displayed measurable antibacterial activity, with th e bovine one having a minimal inhibitory concentration of 24 muM and being 10 times more active than the caprine one against Escherichia coli. An alan ine-scan of the bovine lactoferricin derivative was performed to identify s pecific amino acids that were important for the antibacterial activity. We found that neither of the two tryptophan residues (Trp 6 and Trp 8) present in the bovine lactoferricin derivative could be replaced by alanine withou t a major loss of antibacterial activity. The other lactoferricin derivativ es tested contained only one tryptophan residue (Trp 6). Modified human, ca prine and porcine lactoferricin derivatives containing two tryptophan resid ues (Trp 6 and Trp 8) displayed minimal inhibitory concentrations of 74, 17 4 and 219 muM, respectively, which represented up to a six-fold increase in antibacterial activity. The alanine-scan also revealed that the antibacter ial activity was increased when acetamidomethyl-protected cysteine and unpr otected glutamine (Cys 3 and Gln 7) were replaced with alanine. Only the bo vine lactoferricin derivative and a few of its alanine-modified derivatives displayed measurable activity against Staphylococcus aureus.