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 phosphorylcholine and ceramide. In humans, SPH represents ~85% of all sphingolipids.

Composition
In humans, sphingomyelin is believed to be the only cell membrane phospholipid not derived from glycerol.

Like all sphingolipids, SPH has the ceramide core (sphingosine bonded to a fatty acid via an amide linkage). In addition, it contains one polar head group, which is either phosphocholine or phosphoethanolamine.

Function and location
The function of sphingomyelin remained unclear until recently, when it was found to have a role in signal transduction.

The plasma membrane of cells is highly enriched in sphingomyelin and is considered largely to be found in the exoplasmic leaflet of the cell membrane. However, there is some evidence that there may also be a sphingomyelin pool in the inner leaflet of the membrane. Moreover, neutral sphingomyelinase-2 – an enzyme that breaks down sphingomyelin into ceramide - has been found to localise exclusively to the inner leaflet, further suggesting that there may be sphingomyelin present there.

Abnormalities and associated diseases
Sphingomyelin can accumulate in a rare hereditary disease called Niemann-Pick Disease, types A and B. It is a genetically-inherited disease caused by a deficiency in the enzyme sphingomyelinase, which causes the accumulation of sphingomyelin in spleen, liver, lungs, bone marrow, and brain, causing irreversible neurological damage. Of the two types involving sphingomyelinase, type A occurs in infants. It is characterized by jaundice, an enlarged liver, and profound brain damage. Children with this type rarely live beyond 18 months. Type B involves an enlarged liver and spleen, which usually occurs in the pre-teen years. The brain is not affected. Most patients present with <1% normal levels of the enzyme in comparison to normal levels.

An excess of spingomyelin in the red blood cell membrane (as in abetalipoproteinemia) causes excess lipid accumulation in the outer leaflet of the red blood cell plasma membrane. This results in abnormally shaped red cells called acanthocytes.