Galactosyl ceramides of the myelin sheath: thermal studies

The thermal properties of several hydrated glycosyl ceramides have been investigated by differential scanning calorimetry in relation to the role of galactosyl ceramides in the myelin sheath of central nerve tissue. Myelin galactosyl ceramides are characterised by the presence of a large proportion of alpha-hydroxy fatty acids which lower the phase transition of this lipid class in comparison to galactosyl ceramides with normal fatty acids of similar chain length. Alteration of the sugar headgroup from galactose to glucose produced no significant change in the transition temperature of the lipid. The presence of a disaccharide headgroup lowered the transition temperature and altered the structure of the endotherm. Increasing concentrations of cholesterol in aqueous dispersions of galactosyl ceramide progressively decreased the enthalpy of the phase transition until, in the presence of 34 mole % cholesterol, it was no longer detectable. Examination of the effects of a range of phospholipids on the phase transition of galactosyl ceramides suggest that the phase behaviour of phospholipid-galactosyl ceramide mixtures is complex. However, only a single endotherm was observed in all the phospholipid-galactosyl ceramide systems examined. This tends to indicate that segregated phases enriched in galactosyl ceramides do not occur in the intact myelin membrane.