Haptenic activity of galactosyl ceramide and its topographical distribution on liposomal membranes. Effects of temperature and phospholipid composition.

The relation between the immune reactions of phosphatidylcholine liposomes containing galactosyl ceramide and the physical properties of the glycolipid in membranes was studied. The immune-agglutination of dipalmitoyl phosphatidylcholine liposomes was affected both by reaction temperature and by cholesterol content. Fatty acyl chain length of phosphatidylcholine also influenced the immune-agglutination. The electron spin resonance and calorimetric studies indicated that the fatty acyl chain length of phosphatidylcholine and cholesterol content, as well as temperature, affect the physical properties of galactosyl ceramide in liposomal membranes. In the absence of cholesterol, most galactosyl ceramide molecules were clustered on the phosphatidylcholine liposomes below the chain-melting transition temperature of the phospholipid, whereas they were randomly distributed in the membrane above the transition temperature. Upon addition of cholesterol to the membranes below the chain-melting transition temperature, the number of glycolipid molecules in the cluster phase decreased. Cholesterol increased the ordering of galactosyl ceramide molecules in the phase of random distribution on membranes above the transition temperature. The change in topographical distribution of galactosyl ceramide in membranes was parallel with that of immune-reactivity.