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It is a sphingolipid found in animals’ membranes, particularly the myelin sheath.
It is composed of phosphatidylcholine or phosphatidylethanolamine, a group related to a ceramide (main chain of sphingosine and fatty acid).
It is synthesized in the endoplasmic reticulum and the Golgi trans and is enriched in the plasma membrane with a higher concentration in the external leaflet.
In humans, sphingomyelins comprise almost 85% of all sphingolipids and 10-20% of the total lipids of the plasma membrane.
Sphingomyelins comprise 4 to 18% of all the lipids of the sarcolemmal membrane, with 93% present in the external leaflet.
The sphingomyelins are almost cylindrical, although their alkyl tails often differ in the number of carbons with high saturation.
Its high transition temperature and a greater tendency to form intermolecular hydrogen bonds than similar phospholipids can lead to lateral heterogeneity and the spontaneous formation of microdomains.
In addition, the intermolecular hydrogen bond between sphingomyelin and cholesterol can allow tight packaging in the liquid-ordered phase of “rafts” and favor their formation.
Sphingomyelins contain phosphocholine or phosphoethanolamine as their polar group and are therefore classified together with glycerophospholipids as phospholipids.
Sphingomyelin was isolated for the first time by the German chemist Johann LW 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 eye lenses.
Sphingomyelin has essential structural and functional functions in the cell. It is a component of the plasma membrane and participates in many signaling pathways. The metabolism of sphingomyelin creates many products that play an essential role in the cell.
Physical characteristics
Composition
Sphingomyelin consists of a group of phosphocholine, sphingosine, and fatty acid. It is one of the few membrane phospholipids not synthesized from glycerol.
Sphingosine and fatty acid can be collectively classified as ceramide.
This composition allows sphingomyelin to play an essential role in the signaling pathways: the degradation and synthesis of sphingomyelin produce second important messengers for signal transduction.
Sphingomyelin obtained from natural sources, such as bovine eggs or brain, contains fatty acids of varying chain lengths.
Sphingomyelin with established chain length, such as palmitoylphingomyelin with a chain of 16 saturated acyls, is commercially available.
Properties
Ideally, the sphingomyelin molecules have the shape of a cylinder. However, many sphingomyelin molecules 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.
The temperature of the central transition phase of the sphingomyelins is also higher compared to the phase transition temperature of similar phospholipids, near 37 ° C. This can introduce lateral heterogeneity in the membrane, generating domains in the bilayer membrane.
Sphingomyelin undergoes significant interactions with cholesterol. Cholesterol can eliminate the transition from the liquid to the solid phase in phospholipids.
Because the transition temperature of sphingomyelin is within the physiological temperature ranges, cholesterol can play a significant role in the sphingomyelin phase. Sphingomyelins are also more prone to intermolecular hydrogen bonds than other phospholipids.
Location
Sphingomyelin is synthesized in the endoplasmic reticulum (ER), where it can be found in low amounts, and the Golgi trans. It is enriched in the plasma membrane with a greater concentration in the external prospect than in the internal one.
The Golgi complex represents an intermediate between the endoplasmic reticulum and the plasma membrane, with slightly higher concentrations towards the trans side.
Metabolism
Synthesis
The synthesis of sphingomyelin involves the enzymatic transfer of phosphocholine from phosphatidylcholine to a ceramide.
The first committed step in synthesizing sphingomyelin involves the condensation of L-serine and palmitoyl-CoA. This reaction is catalyzed by serine palmitoyltransferase.
The product of this reaction is reduced, producing dihydrosphingosine. The dihydrosphingosine is subjected to N-acylation followed by desaturation to produce ceramide. Each of these reactions occurs on the cytosolic surface of the endoplasmic reticulum.
The ceramide is transported to the Golgi apparatus, which can be converted into sphingomyelin. Sphingomyelin synthase is responsible for the production of sphingomyelin from ceramide. Diacylglycerol is produced as a by-product when phosphocholine is transferred.
Degradation
The decomposition of sphingomyelin is responsible for initiating many universal signaling pathways. It is hydrolyzed by sphingomyelinases (S-type phospholipases specific for sphingomyelin).
The phosphocholine group is released into the aqueous environment while the ceramide diffuses through the membrane.
Function
Membranes
The membranous myelin sheath that surrounds and electrically isolates many axons from nerve cells is particularly rich in sphingomyelin, suggesting its role as an insulator of nerve fibers.
The plasma membrane of other cells is also abundant in sphingomyelin, although it is mainly found in the endoplasmic leaflet of the cell membrane.
However, there is some evidence that sphingomyelin may also be set in the membrane’s inner leaflet.
In addition, it has been discovered that neutral sphingomyelinase-2, an enzyme that breaks down sphingomyelin in ceramide, is located exclusively in the internal leaflet, suggesting that sphingomyelin may be present there.
Signal transduction
The function of sphingomyelin was not apparent until it was discovered that it had a role in signal transduction. It has been found that sphingomyelin plays a vital role in cell signaling pathways.
The synthesis of sphingomyelin in the plasma membrane by sphingomyelin synthase 2 produces diacylglycerol, a second lipid-soluble messenger that can pass through a signal cascade.
In addition, the degradation of sphingomyelin can produce ceramide that is involved in the apoptotic signaling pathway.
Apoptosis
It has been found that sphingomyelin has a role in cellular apoptosis when hydrolyzed into ceramide. Studies conducted in the late 1990s found that ceramide was produced in various conditions that led to apoptosis.
The hypothesis was then raised that sphingomyelin hydrolysis and ceramide signaling were essential in deciding whether a cell dies.
In the early 2000s, new studies emerged that defined a new role for the hydrolysis of sphingomyelin in apoptosis, which determines when a cell dies and how.
After further experimentation, it has been shown that if the hydrolysis of sphingomyelin occurs at a sufficiently early point in the route, the production of ceramide can influence either the rate and manner of cell death or in releasing blocks in later events.
Lipid rafts
Sphingomyelin and other sphingolipids are associated with lipid microdomains in the plasma membrane known as lipid rafts.
The lipid rafts are characterized by the fact that the lipid molecules are in the orderly phase of lipids, offering more structure and rigidity than the rest of the plasma membrane.
The acyl chains have a low chain movement in the rafts, but the molecules have excellent lateral mobility. This order is due in part to the higher transition temperature of the sphingolipids and the interactions of these lipids with cholesterol.
Cholesterol is a relatively small nonpolar molecule that can fill the space between sphingolipids resulting from large acyl chains.
Lipid rafts are believed to participate in many cellular processes, such as membrane classification and trafficking, signal transduction, and cell polarization. Excess sphingomyelin in lipid rafts can cause insulin resistance.
Due to the specific lipids in these microdomains, lipid rafts can accumulate particular proteins associated with them, thus increasing their specific functions. It has been speculated that lipid rafts are involved in the cascade of cellular apoptosis.
Abnormalities and associated diseases
The biosynthesis of de novo sphingolipids is related to metabolic diseases. However, the mechanism is not yet precise. Sphingolipids are ubiquitous and critical components of biological membranes.
Its biosynthesis begins with soluble precursors in the endoplasmic reticulum and culminates in the Golgi complex and the plasma membrane. The interaction of sphingomyelin, cholesterol, and glycosphingolipid promotes the formation of plasma membrane rafts.
It has been shown that lipid rafts are involved in cell signaling, lipid and protein classification, and membrane trafficking.
It is well known that toll-like receptors, class A and B scavenger receptors, and the insulin receptor are found in lipid rafts.
Of the two types that involve sphingomyelinase, type A occurs in babies. It is characterized by jaundice, enlarged liver, and severe brain damage. Children of this type rarely live beyond 18 months.
Type B involves an enlarged liver and spleen, which usually occurs in the preadolescent years. The brain is unaffected. Most patients have <1% normal enzyme levels compared to normal levels.
As a result of the autoimmune disease multiple sclerosis (MS), the myelin sheath of neuronal cells in the brain and spinal cord is degraded, resulting in the loss of signal transduction capacity.
Patients with multiple sclerosis exhibit favorable regulations of specific cytokines in the cerebrospinal fluid, mainly the tumor necrosis factor-alpha.
This activates sphingomyelinase, an enzyme that catalyzes the hydrolysis of sphingomyelin in ceramide; sphingomyelinase activity has been observed together with cellular apoptosis.
An excess of sphingomyelin in the membrane of red blood cells (as in abetalipoproteinemia) causes a lot of lipid accumulation in the outer leaflet of the plasma membrane of red blood cells.
This results in abnormally shaped red blood cells called acanthocytes. Sphingomyelin is also a reservoir for other sphingolipids.
