The pathways for the synthesis of PGs and relatives from membrane lipids are summarized in Figure 1. As will be discussed in more detail in section III.B., when eicosanoid synthesis is stimulated in a cell a fatty acid precursor, usually arachidonic acid, is released from the membrane phospholipids by phospholipase A2. This is the initial and often rate-limiting step in the synthesis of eicosanoids.
Fig1. Eicosanoid biosynthesis. The pathway of the biosynthesis of the main prostanoids from the most abundant substrate, arachidonic acid, is shown. The 5-lipoxygenase (5-LOX) pathway leads to the leukotrienes (LT) through HpETE (hydroxyperoxytetraenoic acid). The action of 12-lipoxygenase on LTA4 leads to lipoxin A4 . Arachidonic acid can also be converted directly to LXA4 through the 15-lipoxygenase pathway. The cyclooxygenase (COX-1 and COX-2) pathway gives rise, through the COX product PGH2, to the prostanoids, prostaglandins, and thromboxanes. Each biologically active prostanoid is produced from PGH2 by a specific synthase. The synthase(s) expressed by a given cell determine which prostanoid(s) the cell will make. TXA2 has a particularly short half-life and its inactive breakdown product, TXB2, is shown.
In cells which produce leukotrienes, the first step in the pathway is catalyzed by lipoxygenase, producing the intermediate hydroperoxyeicosatetraenoic acid. There are two pathways to LXA4, through LTA4 followed by 12-lipoxygenase action or directly through the action of 15-lipoxygenase.
In all cells that produce any of the prostanoids, arachidonic acid (or other polyunsaturated fatty acid) is first acted on by the membrane PGH synthetase complex (PGHS), also known as cyclooxygenase, producing the cyclic endoperoxide intermediates, PGG2 and PGH2. The conversion of PGH2 to any of the PGs (including prostacyclin), or the thromboxanes, will depend on which specific synthase(s) are present in the cell. As indicated above, if 20 carbon polyunsaturated fatty acid substrate for cyclooxygenase has either 3 or 5 double bonds, prostaglandins of the 1 or 3 series, respectively, will result.
The enzymes that convert PGH2 to the other prostanoids shown in Figure 1 each catalyzes a specific reaction and has its own distinguishing characteristics. Prostacyclin synthase (PGIS) is a cytochrome P-450 enzyme that catalyzes the isomerization of PGH2 to PGI2. This is a nonoxidative reaction so, in contrast to other cytochrome P450 enzymes, neither molecular oxygen nor NADPH is required.
There are two forms of PGD synthase (PGDS), one in mast cells and microglia which is gluthione-dependent (H-PGDS) and the other found in brain, male genital organs, and cardiovascular tissues (L-PGDS). L-PGDS is glutathione-independent but uses other sulfhydral compounds.
PGH2 is converted to PGE2 by a glutathione-de pendent microsomal PGE synthase (mPGES-1). mPGES-1 is inducible in several tissues and constituitively expressed in some; it is the form of the enzyme involved in PGE2’s actions in inflammation, pain, and cancer (see sections V and VI). Two other proteins capable of synthesizing PGE2 have been isolated and characterized: a second membrane bound enzyme, mPGES-2; and a soluble protein, cPGES. These activities are both constituitively expressed. Studies with mice in which the gene for mPGES-2 has been inac tivated do not reveal any role for this enzyme in vivo relative to PGE2 synthesis. cPGES is identical to p23, a co-chaperone protein necessary for glucocorticoid receptor function. Whether it also plays a role in nor mal PGE2 synthesis is not yet clear.
PGF2α can arise either from the action of PGH2 9,11-endoperoxide reductase (PGF2α synthase) or that of PGE2 9-ketoreductase. The pathway used in any given cell will depend at least in part on whether the same cell also contains PGE2 synthase.
Thromboxane synthase (TXAS) is a cytochrome P450 enzyme that catalyzes the isomerization of PGH2 to TXA2 but has a mechanism unlike that of PGIS. Each of these enzymes is a subject of intense pharmacological interest, offering the possibility of controlling the production of the prostanoids individually. TXA2 is quite unstable and rapidly breaks down to TXB2, a more stable and biologically inactive metabolite which can be measured in urine to monitor the activity of this pathway.
