Do endotoxin aggregates intercalate into phospholipid membranes in a nonspecific,hydrophobic manner?

The interaction of endotoxin aggregates with phospholipid liposomes of different composition was investigated applying fluorescence polarization spectroscopy with the fluorophore diphenylhexatriene and the resonance energy transfer technique using N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-PE and N-(Rhodamine B sulfonyl)-PE. Fluorescence polarization data at constant temperature could be interpreted in favor of an intercalation of lipopolysaccharide into phospholipid liposomes even in the absence of Ca²⁺. Intercalation, however, could be clearly excluded from determinations performed as a function of temperature. Experiments employing the resonance energy transfer technique clearly showed that a nonspecific, hydrophobic intercalation of endotoxin aggregates into phospholipid liposomes only takes place in the presence of excess molar concentrations of divalent cations and/or after long-term incubation at elevated temperature (37°C). These findings indicate that under (near) physiological conditions nonspecific intercalation of aggregated lipopolysaccharide into phospholipid membranes represents an unlikely event. The significance of these results for an understanding of the fundamental mechanisms of cell activation by endotoxin is discussed.