Abstract
Sphingomyelins (SM) constitute an important class of phospholipids in the membranes of most eukaryotic cells. In mammalian tissues, SM usually constitute 2β15% of the total organ phospholipid, but certain tissues such as brain, peripheral nervous tissue and ocular lenses have even higher SM contents. Typical properties of SM include their low degree of unsaturation, an asymmetric molecular structure, and their extensive hydrogenβbonding properties. These features are all very important for the structural role of SM in biological membranes. SM interact favorably with cholesterol (and other sterols) and there is an established coβlocalization of SM and cholesterol in the plasma membranes of cells and at the surface of lipoprotein particles. Together they form SM/sterolβrich domains that often are more ordered than the surrounding phase in biological membranes. The growing body of evidence regarding their favorable interaction with sterols indicates that the functional role of SM per se is largely related to being a regulator of cholesterol distribution within cellular membranes and cholesterol homeostasis in cells. Together with other sphingolipids, SM also have an important functional role as precursors of sphingolipid signaling molecules, extensively reviewed elsewhere and not to be discussed here in more detail.