The role of sphingomyelin in regulating phase coexistence in complex lipid model membranes: competition between ceramide and cholesterol.

The structure, thermotropic phase behavior, dynamic motion and order parameters of bilayer dispersions of egg phosphatidylcholine, egg sphingomyelin, egg ceramide and cholesterol have been determined. The coexistence of gel, liquid-ordered and liquid-disordered structure has been determined by peak fitting analysis of synchrotron X-ray powder patterns. Order parameters and extent of distribution of 16-doxyl-stearic acid spin probe between ordered and disordered environments has been estimated by ESR spectral simulation methods. The presence of ceramide in proportions up to 20 mol% in phosphatidylcholine is characterized by gel-fluid phase coexistence at temperatures up to 46 degrees C depending on the amount of ceramide. Cholesterol tends to destabilize the ceramide-rich domains formed in phosphatidylcholine while sphingomyelin, by formation of stable complexes with ceramide, tends to stabilize these domains. The stability of sphingomyelin-ceramide complexes is evident from the persistence of highly ordered structure probed by ESR spectroscopy and appearance of a sharp wide-angle X-ray reflection at temperatures higher than the gel-fluid transition of ceramide alone in egg phosphatidylcholine bilayers. The competition between ceramide and cholesterol for interaction with sphingomyelin is discussed in terms of control of lipid-mediated signaling pathways by sphingomyelinase and phospholipase A2.