Membrane lysis by the antibacterial peptide, cecropin B and its analogs cecropins B1 and B3 on lipid bilayers

The natural antibacterial peptide, cecropin B (CB), and it analogs cecropin B1 (CB1), cecropin B2 (CB2) and cecropin B3 (CB3) were synthesized. These peptides were used to investigate their liposome lysis, anti-bacterial and anti-cancer activities by DL₅₀ analysis, lethal concentration (LC) and IC₅₀ measurements. The designed cationic lytic peptides, CB1 or CB2, having extra cationic residues, are less effective in breaking liposomes. and killing bacteria but more effective in lysing cancer cells. The different characteristics of these peptides, CB, CB1, CB2, and CB3, were used to study the morphological changes in the bacterial cell, Klebsiella pneumoniae , and the leukemia cancer cell, HL-60, by scanning electron microscopy and transmission electron microscopy. The results show that the morphological changes of the cells treated with CB are distinguishably different from those treated with CB1 and CB2. The morphological differences may be due to their different modes of action on the cell membranes.; The differences of the characteristics of CB, CB1 and CB3, were further used as the rationale for a study of their efficacy in breaking liposomes with different combinations of phosphatidic acid (PA) and phosphatidylcholine (PC), by binding measurements, liposome lysis activity studies, ESR studies and kinetic studies. Biosensor binding measurements and encapsulating dye leakage studies showed that the higher binding affinity of CB and CB1 to the polar heads of lipids is not necessary for the peptides to be more effective at lysing lipid bilayers, especially when liposomes have a higher PA content. ESR results show that lysis action of CB1 is dependent on its capacity to bind to the lipid bilayers, whereas, there is no evidence of binding from CB3. The mode of membrane lysis of CB/CB1 and CB3 on liposomes was determined as all-or-none by fluorescence quenching study. The kinetic observations show two different permeabilization pathways: multimeric pore formation of CB/CB1 and gross membrane destabilization of CB3. The dose dependence of CB/CB1 on the permeabilization of lipid bilayers is also observed by biosensor. The addition of CB3 to the liposomes of different compositions results in reducing RU and indicates that the lysis of liposomes occurs, but that does not go through the bindings. Based on these results, the possible mechanisms of liposome lysis by CB, CB1 and CB3 are proposed.