Changes in phase transition temperature of phospholipids induced by endotoxin.

The effects of endotoxin (lipopolysaccharide from Salmonella minnesota Re 595) on the phase transition temperature (Tm) of various phospholipids were studied. Endotoxin had no effect on the Tm and the width of the phase transition of dipalmitoyl-sn-3-phosphatidylcholine. Endotoxin at 100 micrograms/ml increased the Tm of dipalmitoyl-sn-3-phosphatidylethanolamine by 1.1 degrees C (P less than 0.01) and narrowed the range of transition from 4.5 to 2.6 degrees C; the endotoxin-induced changes in the Tm and the transition range were abolished by the presence of 0.25 mM CaCl2. Endotoxin increased the Tm of dipalmitoyl-sn-3-phosphatidic acid by 1.1 (P less than 0.01), 1.2 (P less than 0.01), and 3.1 (P less than 0.01) degrees C at 25, 50 and 100 micrograms/ml, respectively. Furthermore, the width of phase transition of phosphatidic acid was narrowed from 6.5 to 4.0 degrees C by endotoxin at 100 micrograms/ml. The endotoxin-induced changes in the Tm and the transition range of phosphatidic acid were not affected by the presence of EDTA (1 mM) or CaCl2 (0.05-0.1 mM). These results suggest that endotoxin decreases the fluidity of negatively charged phospholipids such as phosphatidic acid and phosphatidylethanolamine. A change in the physical properties of membrane lipid bilayers induced by endotoxin may have an adverse effect on the function of biological membranes.