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Plasmalogens are very abundant in the myelin sheaths that insulate the axons of nerve cells. They make up some 80–90% of the myelin membrane phospholipids. In addition to an ethanolamine head group and a long-chain fatty acid attached to the same glycerol phosphate backbone used for phospholipids, plasmalogens contain an unusual fatty alcohol that is attached through an ether linkage (bottom left).

The structure of phosphoglycerides. In most lipids, X is acyl, that is, R—(C═O). In alkyl ethers, present mainly in brain ethanolamine phosphoglycerides (2 to 3%), X is a long-chain hydrocarbon (C₁₆, C₁₈). For plasmalogens, which constitute about 60% of adult human brain PtdEtn, X is 1-alk-1′enyl (that is, —CH═CH—R). Arrows indicate sites of enzymatic hydrolysis of the phosphoglycerides. PLA ₁ , phospholipase A₁; PLA ₂ , phospholipase A₂; PLC, phospholipase C; PLD, phospholipase D. Note that myo-inositol is written in the d-configuration, where the 1′ position is linked to the PtdOH moiety. For polyphosphoinositides, additional phosphate groups are present in the 3, 4 or 5 positions. See Chapter 21 for further detail regarding the stereochemistry of inositol and the use of the turtle representation.

Oxylipin reservoirs in mammalian cells. (a) Membrane-bound phospholipids (PL) are major cellular polyunsaturated fatty acid (PUFA) reservoirs that are cleaved through diverse phospholipase (PLA) enzymes to produce PUFA or fatty acids (FA) and the corresponding lyso PL. PUFAs are subsequently metabolized by various oxygenases (COX, LOX, CYP, sEH) to produce a wide variety of oxylipins. Additionally, oxylipins can be obtained from peroxisome-derived plasmalogens and cholesteryl ester (CE) reservoirs. (b) Lipid droplets are important for lipid storage in the form of triglycerides (TAG) and CEs. Diverse lipases (ATGL, HSL, LDAH, CES1, MGL) hydrolyze TAG and CE from lipid droplets to release PUFA that can be utilized to produce oxylipins. PLA, phospholipase A; PUFA, polyunsaturated fatty acid; ACS, acyl-CoA synthase; CE, cholesterylester; CEH, neutral cholesterol ester hydrolase; FA, fatty acid; lysoPL, lysophospholipid; COX, cyclooxygenase; LOX, lipoxygenase; CYP, cytochrome P450; EPHX1/2, soluble epoxyde hydrolase; CytC, cytochrome C; CES1, carboxylesterase 1; LDAH, lipid droplet–associated hydrolase; TAG, triglyceride; DAG, diacylglycerol