Parameters involved in antimicrobial and endotoxin detoxification activities of antimicrobial peptides

Endotoxin [lipopolysaccharide (LPS)] covers more than 90% of the outer monolayer of the outer membrane of Gram-negative bacteria, and it plays a dual role in its pathogenesis: as a protective barrier against antibiotics and as an effector molecule, which is recognized by and activates the innate immune system. The ability of host-defense antimicrobial peptides to bind LPS on intact bacteria and in suspension has been implicated in their antimicrobial and LPS detoxification activities. However, the mechanisms involved and the properties of the peptides that enable them to traverse the LPS barrier or to neutralize LPS endotoxic activity are not yet fully understood. Here we investigated a series of antimicrobial peptides and their analogues with drastically altered sequences and structures, all of which share the same amino acid composition (K 6L 9). The list includes both all- l-amino acid peptides and their diastereomers (composed of both l- and d-amino acids). The peptides were investigated functionally for their antibacterial activity and their ability to block LPS-dependent TNF-alpha secretion by macrophages. Fluorescence spectroscopy and transmission electron microscopy were used to detect their ability to bind LPS and to affect its oligomeric state. Their secondary structure was characterized in solution, in LPS suspension, and in LPS multibilayers by using CD and FTIR spectroscopy. Our data reveal specific biophysical properties of the peptides that are required to kill bacteria and/or to detoxify LPS. Besides shedding light on the mechanisms of these two important functions, the information gathered should assist in the development of AMPs with potent antimicrobial and LPS detoxification activities.