Sphingolipids (Galactosylceramide and Sulfatide) in Lamellar−Hexagonal Phospholipid Phase 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 differential scanning calorimetry. When mixed with dielaidoylphosphatidylethanolamine, cerebroside increases the transition temperature (ca. 0.2 °C/mol % added cerebroside) and increases the transition ΔH. Sulfatide increases the transition temperature by ca. 0.4 °C/mol % added sulfatide and decreases ΔH. Both lipids are seen to hinder the formation of the nonlamellar phase, although sulfatide is more effective in this respect. When incorporated into vesicles formed by phosphatidylcholine/phosphatidylethanolamine/cholesterol (2:1:1 mole ratio), which are a good substrate for phospholipase C and undergo fusion as a consequence of the enzyme activity (Nieva et al. Biochemistry <BO>1989</BO>, 28, 7364), cerebroside at all concentrations and sulfatide at >5 mol % inhibit enzyme activity and vesicle fusion. Cerebroside inhibition of fusion is due not only to a reduced enzyme activity but also to the impaired formation of nonlamellar phases. Sulfatide at low concentrations (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 conditions, its enzyme-activating effect predominates over its bilayer-stabilizing properties. Thus, sulfatide at low concentrations is an exception to the rule that amphiphiles hindering the lamellar−hexagonal transition inhibit both phospholipase C activity and membrane fusion.