Thermotropic behavior and lateral distribution of very long chain sphingolipids

Sphingolipids containing very long acyl chains are abundant in certain specialized tissues and minor components of plasma membranes in most mammalian cells. There are cellular processes in which these sphingolipids are required, and the function seems to be mediated through sphingolipid-rich membrane domains. This study was conducted to explore how very long acyl chains of sphingolipids influence their lateral distribution in membranes. Differential scanning calorimetry showed that 24:0- and 24:1-sphingomyelins, galactosylceramides and glucosylceramides exhibited complex thermotropic behavior and partial miscibility with palmitoyl sphingomyelin. The Tm was decreased by about 20 °C for all 24:1-sphingolipids compared to the corresponding 24:0-sphingolipids. The ability to pack tightly with ordered and extended acyl chains is a necessity for membrane lipids to partition into ordered domains in membranes and thus the 24:1-sphingolipids appeared less likely to do so. Fluorescence quenching measurements showed that the 24:0-sphingolipids formed ordered domains in multicomponent membranes, both as the only sphingolipid and mixed with palmitoyl sphingomyelin. These domains had a high packing density which appeared to hinder the partitioning of sterols into them, as reported by the fluorescent cholesterol analog cholestatrienol. 24:0-SM was, however, better able to accommodate sterol than the glycosphingolipids. The 24:1-sphingolipids could, depending on head group structure, either stabilize or disrupt ordered sphingolipid/cholesterol domains. We conclude that very long chain sphingolipids, when present in biological membranes, may affect the physical properties of or the distribution of sterols between lateral domains. It was also evident that not only the very long acyl chain but also the specific molecular structure of the sphingolipids was of importance for their membrane properties.