Sphingomyelin

Sphingomyelin(SPH) is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath which surrounds some nerve cell axons. It usually consists of phosphorylcholine and ceramide. In humans SPH represents ~85% of all sphingolipids.

Composition

In humans, it is the only membrane phospholipid not derived from glycerol.

Like all sphingolipids, SPH has a 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 function 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 cite journal |author=Linardic, C.M. & Hannun, Y.A. |title=Identification of a distinct pool of sphingomyelin involved in the sphingomyelin cycle.|journal=J. Biol. Chem. |volume=269 |issue=38 |pages=23530-23537 |year=1994 |pmid=8089120 |doi=] cite journal |author=Zhang, P., Jenkins, G.M., Hannun, Y.A. and Obeid, L.M. |title=Expression of neutral sphingomyelinase identifies a distinct pool of sphingomyelin involved in apoptosis.|journal=J. Biol. Chem. |volume=272 |issue=15 |pages=9609-9612 |year=1997 |pmid=9092485 |doi=] . 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. cite journal |author=Tani, M & Hannun, Y.A. |title=Analysis of membrane topology of neutral sphingomyelinase-2|journal=FEBS Letters |volume=581 |issue=7 |pages=1323-1328 |year=2007 |pmid=17349629 |doi=]

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 the 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.

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