| Sphingomyelin (Fig. 26.10) is found in plasma membranes, subcellular organelles, endoplasmic reticulum, and mitochondria. It comprises 5-20% of the total phospholipids in most cell types, and is mostly localized in the plasma membrane. It is the only sphingolipid that contains phosphorus, and is the major phospholipid of the myelin sheath of nerves. The phosphocholine group in sphingomyelin is transferred to the terminal hydroxyl group of sphingosine from phosphatidylcholine. The fatty acid composition varies, but long-chain fatty acids are common, including lignoceric (C-24:0), cerebronic (2-hydroxylignoceric) and nervonic (24:1) acids.
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