Sphingomyelin
Sphingomyelin (SPH, ˌsfɪŋɡoˈmaɪəlɪn) is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons. It usually consists of phosphocholine and ceramide, or a phosphoethanolamine head group; therefore, sphingomyelins can also be classified as sphingophospholipids. In humans, SPH represents ~85% of all sphingolipids, and typically make up 10–20 mol % of plasma membrane lipids. Sphingomyelin was first isolated by German chemist Johann L.W. Thudicum in the 1880s. The structure of sphingomyelin was first reported in 1927 as N-acyl-sphingosine-1-phosphorylcholine. Sphingomyelin content in mammals ranges from 2 to 15% in most tissues, with higher concentrations found in nerve tissues, red blood cells, and the ocular lenses. Sphingomyelin has significant structural and functional roles in the cell. It is a plasma membrane component and participates in many signaling pathways. The metabolism of sphingomyelin creates many products that play significant roles in the cell. Sphingomyelin consists of a phosphocholine head group, a sphingosine, and a fatty acid. It is one of the few membrane phospholipids not synthesized from glycerol. The sphingosine and fatty acid can collectively be categorized as a ceramide. This composition allows sphingomyelin to play significant roles in signaling pathways: the degradation and synthesis of sphingomyelin produce important second messengers for signal transduction. Sphingomyelin obtained from natural sources, such as eggs or bovine brain, contains fatty acids of various chain length. Sphingomyelin with set chain length, such as palmitoylsphingomyelin with a saturated 16 acyl chain, is available commercially. Ideally, sphingomyelin molecules are shaped like a cylinder, however many molecules of sphingomyelin have a significant chain mismatch (the lengths of the two hydrophobic chains are significantly different). The hydrophobic chains of sphingomyelin tend to be much more saturated than other phospholipids.