Sphingolipids (galactosylceramide and sulfatide) in lamellar-hexagonal phospholipid please transitions and in membrane fusion

The effects of galactosylceramide (cerebroside) and sulfogalactosylceramide (sulfatide) from bovine brain on the lamellar-to-inverted hexagonal phase transition of dielaidoylphosphatidylethanolamine are examined using differe ntial scanning calorimetry. When mixed with dielaidoylphosphatidylethanolam ine, cerebroside increases the transition temperature (ca. 0.2 degreesC/mol % added cerebroside) and increases the transition DeltaH. Sulfatide increa ses the transition temperature by ca. 0.4 degreesC/mol. % added sulfatide a nd decreases DeltaH. Both lipids are seen to hinder the formation of the no nlamellar phase, although sulfatide is more effective in this respect. When incorporated into vesicles formed by phosphatidylcholine/phosphatidylethan olamine/ cholesterol (2:1:1 mole ratio), which are a good substrate for pho spholipase C and undergo fusion as a consequence of the enzyme activity (Ni eva et al. Biochemistry 1989, 28, 7364), cerebroside at all concentrations and sulfatide at > 5 mol % inhibit enzyme activity and vesicle fusion. Cere broside inhibition of fusion is due not only to a reduced enzyme activity b ut also to the impaired formation of nonlamellar phases. Sulfatide at low c oncentrations (e.g., 1 mol %) enhances phospholipase C activity and vesicle fusion, probably because its net negative charge causes hyperpolarization of the interface, which is known to activate phospholipase C. Under these c onditions, its enzyme-activating effect predominates over its bilayer-stabi lizing properties. Thus, sulfatide at low concentrations is an exception to the rule that amphiphiles hindering the lamellar-hexagonal transition inhi bit both phospholipase C activity and membrane fusion.