Liposomes containing sphingomyelin and cholesterol:: Detergent solubilisation and infrared spectroscopic studies

The solubilisation by Triton X-100 of large unilamellar vesicles consisting of pure egg sphingomyelin (T-m approximate to 39 degrees C) or sphingomyelin:cholesterol mixtures has been tested at various temperatures. For pure sphingomyelin, solubilisation occurs most readily at temperatures just below T-m. In general, egg sphingomyelin is solubilised by Triton X-100 more easily than egg phosphatidylcholine. Mixtures of sphingomyelin and cholesterol are detergent-insoluble under most conditions. Infrared spectroscopy has been applied to explore the interactions of cholesterol and sphingomyelin at the level of the lipid-water interface. Moreover, various cholesterol analogues (cholestane, cholestanone, androstenol) have been used in parallel solubilisation experiments and IR observations. The results show that cholesterol modifies the conformation (or H-bonding properties, or both) of the sphingomyelin polar headgroup, both above and below T-m. Moreover, both the hydroxyl group at C3 and the hydrocarbon chain at C17 of the steroid nucleus appear to be required for insolubility to be detected. Perturbation of the polar environment of the sphingomyelin: cholesterol bilayers by 3M urea makes them soluble in Triton X-100. These results may be related to the observed insolubility of cell membrane 'rafts' in cold detergent.