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 polariza tion spectroscopy with the fluorophore diphenylhexatriene and the reso nance energy transfer technique using N-(7-nitro-2,1,3-benzoxadiazol-4 -yl)-PE and N-(Rhodamine B sulfonyl)-PE. Fluorescence polarization dat a at constant temperature could be interpreted in favor of an intercal ation of lipopolysaccharide into phospholipid liposomes even in the ab sence of Ca2+. Intercalation, however, could be clearly excluded from determinations performed as a function of temperature. Experiments emp loying the resonance energy transfer technique clearly showed that a n onspecific, hydrophobic intercalation of endotoxin aggregates into pho spholipid liposomes only takes place in the presence of excess molar c oncentrations of divalent cations and/or after long-term incubation at elevated temperature (37 degrees C). These findings indicate that und er (near) physiological conditions nonspecific intercalation of aggreg ated lipopolysaccharide into phospholipid membranes represents an unli kely event. The significance of these results for an understanding of the fundamental mechanisms of cell activation by endotoxin is discusse d.