Increased antibacterial activity of 15-residue murine lactoferricin derivatives

LFM W8 is a synthetic 15-residue lactoferricin derivative (H2N-EKCLRWQWEMRK VGG-COOH), corresponding to residues 16-30 of the mature murine lactoferrin protein except that the asparagine residue in position 8 of the native pep tide is replaced with tryptophan. We have previously reported that the two tryptophan residues in positions 6 and 8 are of crucial importance for the antibacterial activity of many lactoferricin derivatives but, despite fulfi lling this requirement, LFM W8 is inactive against Escherichia coli and Sta phylococcus aureus. In order to solve this puzzle, a quantitative structure -antibacterial activity relationship study of synthetic LFM W8 derivatives was performed by replacing the glutamate residues in positions 1 and 9 with arginine or alanine, and the valine residue in position 13 with tyrosine. The results from the study were analyzed using multivariate data analysis. The derived mathematical model clustered the peptides into distinct groups which reflected their antibacterial activities, pointed out correlations be tween different structural parameters, highlighted the structural parameter s that were important for antibacterial activity, and enabled us to predict the activity of a 15-residue bovine lactoferricin derivative. The results showed that net charge and micelle affinity, as determined from the ratio o f alpha -helicity in sodium dodecyl sulfate micelles and in 1,1,1.3,3,3-hex afluoro-2-propanol, were the most important structural parameters affecting antibacterial activity. The most active derivative, LFM R1,9 W8 Y13, displ ayed a minimal inhibitory concentration of 10 and 12 muM against E. coli an d S. aureus, respectively. This represented more than 50-fold and 40-fold i ncreases in antibacterial activity, respectively, compared with LFM W8.