Fluorescent detection of lipopolysaccharide interactions with model membranes.

We have modeled the initial interaction of bacterial lipopolysaccharide (endotoxin) with mammalian cells as consisting of two steps. The first step, adherence, we have previously shown to be ionic in nature and contains the necessary elements to determine the observed cell specificity of lipopolysaccharide interactions. The second step, coalescence, is the hypothetical insertion of the Lipid A component of lipopolysaccharide into the cell membrane lipid bilayer. Using small, unilamellar vesicles composed of phosphatidylcholine to model the cell membrane lipid bilayer, we found that lipopolysaccharide interacted with these vesicles to change the fluidity of the lipid bilayer, as measured by an increase in the fluorescence anisotropy of diphenylhexatriene in the vesicles. Since this increase in diphenylhexatriene anisotropy could not be attributed to a transfer of diphenylhexatriene between non-interacting lipopolysaccharide aggregates and vesicles, we concluded that the lipopolysaccharide aggregate coalesced with the lipid bilayer.