Sphingomyeline

Sphingomyeline, commonly written sphingomyelin in English, is a type of sphingophospholipid found predominantly in animal cell membranes, and is especially enriched in myelin sheaths surrounding nerve fibers. It comprises a ceramide backbone to which a phosphorylcholine head group is attached. The ceramide portion consists of a fatty acid amide-linked to sphingosine, while the phosphocholine head is bound via a phosphodiester bond.

Sphingomyeline is synthesized in the Golgi apparatus by sphingomyelin synthase, which transfers a phosphocholine moiety from phosphatidylcholine to ceramide, producing sphingomyelin and diacylglycerol as byproducts. It can be degraded by sphingomyelinases to yield ceramide and phosphocholine; this reaction links sphingomyelin turnover to lipid signaling.

Functionally, sphingomyeline contributes to the structural integrity of membranes, influences membrane fluidity, and supports the formation of lipid rafts that organize signaling molecules. Its abundance in myelin supports the insulating properties of the myelin sheath and efficient nerve conduction.

Clinical and biological significance: disturbances in sphingomyelin metabolism underlie lysosomal storage disorders such as Niemann-Pick disease types A and B, caused by acid sphingomyelinase deficiency and characterized by accumulation of sphingomyelin in lysosomes. In broader signaling contexts, ceramide generated by sphingomyelin hydrolysis acts as a second messenger involved in stress responses and apoptosis. Sphingomyeline levels and metabolism are therefore of interest in studies of neurology and metabolic diseases.