The eicosanoids are oxygenation (oxidation) products of polyunsaturated 20-carbon long-chain fatty acids (eicosa, Greek for “twenty”). They are ubiquitous in the animal kingdom and are also found—together with their precursors—in a variety of plants. They constitute a very large family of compounds that are highly potent and display an extraordinarily wide spectrum of important biologic activities. Thus, their specific receptors, receptor ligands, and enzyme inhibitors, and their plant and fish oil precursors, are therapeutic targets for a growing list of conditions.
* The authors thank Emer M. Smyth, PhD, and Garret A. FitzGerald, MD,for their contributions to previous editions of this chapter.
Arachidonic acid (AA), or 5,8,11,14-eicosatetraenoic acid, the most abundant of the eicosanoid precursors, is a 20-carbon (C20) fatty acid containing four double bonds (designated C20:4–6). The first double bond in AA occurs at 6 carbons from the methyl end, defining AA as an omega-6 fatty acid. AA must first be released or mobilized from the sn-2 position of membrane phospholipids by one or more lipases of the phospholipase A2 (PLA2) type (Figure 18–1) for eicosanoid synthesis to occur. The phospholipase A2 superfamily consists of 16 groups (over 30 isoforms), with at least three classes of phospholipases contributing to arachidonate release from membrane lipids: (1) cytosolic (c) PLA2,
Diverse physical, chemical, inflammatory, and mitogenic stimuli
AA (20:4 cis D5,8,11,14)
FIGURE 18–1 Pathways of arachidonic acid (AA) release and metabolism.
and (2) secretory (s) PLA2, which are calcium-dependent; and (3) calcium-independent (i) PLA2. Chemical and physical stimuli activate the Ca2+-dependent translocation of cPLA2, to the plasma membrane, where it releases arachidonate for metabolism to eicosanoids. In contrast, under nonstimulated conditions, AA liberated by iPLA2 is reincorporated into cell membranes, so there is negligible eicosanoid biosynthesis. While cPLA2 dominates in the acute release of AA, inducible sPLA2 contributes under conditions of sustained or intense stimulation of AA production. AA can also be released from phospholipase C-generated diacylglycerol esters by the action of diacylglycerol and monoacylglycerol lipases.