The interactions of histidine-containing amphipathic helical peptide antibiotics with lipid bilayers - The effects of charges and pH

The alpha-helix of the designed amphipathic peptide antibiotic LAH(4) (KHAL LALALHHLAHLALHLALALKKANH(2)) strongly interacts with phospholipid membranes . The peptide is oriented parallel to the membrane surface under acidic con ditions, but transmembrane at physiological pH (Bechinger, B, (1996) J. Mol . Biol. 263, 768-775). LAH(4) exhibits antibiotic activities against Escher ichia coli and Bacillus subtilis; the peptide does not, however, lyse human red blood cells at bacteriocidal concentrations. The antibiotic activities of LAH(4) are 2 orders of magnitude more pronounced at pH 5 when compared with pH 7,5. Although peptide association at low pH is reduced when compare d with pH 7,5, the release of the fluorophore calcein from large unilamella r 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholine or 1-palmitoyl-2-oleoyl -sn-glycero-3-phosphoglycerol vesicles is more pronounced at pH values wher e LAH(4) adopts an orientation along the membrane surface. The calcein rele ase experiments thereby parallel the results obtained in antibiotic assays, Despite a much higher degree of association, calcein release activity of L AH(4) is significantly decreased for negatively charged membranes. Pronounc ed differences in the interactions of LAH(4) with 1-palmitoyl-2-oleoyl-sn-g lycero-3-phosphoglycerol or 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholi ne membranes also become apparent when the mechanisms of dye release are in vestigated. The results presented in this paper support models in which ant ibiotic activity is caused by detergent-like membrane destabilization, rath er than pore formation by helical peptides in transmembrane alignments.