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sphingomyelins

Tuesday 15 January 2008

Lipids > sphingolipids > sphingomyelins

Definition: Sphingomyelins are the main subtype of phosphosphingolipids, a subtype of sphingolipids and phospholipids. In humans SPH represents 85% of all sphingolipids.

They have a phosphorylcholine or phosphoroethanolamine molecule esterified to the 1-hydroxy group of the ceramides.

Sphingomyelin (SPH) is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath which surrounds some nerve cell axons.

Sphingomyelin usually consists of phosphorylcholine and ceramide.

Structure

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.

The backbone of sphingomyelin is sphingosine, an amino alcohol formed from palmitate and serine.

The amino terminal is acylated with a by a long-chain acyl CoA to yield ceramide.

Subsequent substitution of the terminal hydroxyl group by phosphatidyl choline forms sphingomyelin.

Functions

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

Sphingomyelin is present in all eukaryotic cell membranes, and is particularly concentrated in the nervous system because sphingomyelin is a major component of myelin, the fatty insulation wrapped around nerve cells by Schwann cells or oligodendrocytes.

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.

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.

Pathology

- Niemann-Pick Disease, types A and B

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.

It 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.

- Multiple Sclerosis is a disease characterised by deterioration of the myelin sheath, leading to impairment of nervous conduction.

See also

- sphingolipids

  • There are three main types of sphingolipids: ceramides, phosphosphingolipids (sphingomyelins), and glycosphingolipids, which differ in the substituents on their head group.