First printed in R&D Systems' 1998 Catalog
|Figure 1. An outline of the synthesis of prostaglandins from arachidonate. TXA2 and PGI2 are very unstable, breaking down to the more stable TXB2 and 6-keto-PGF1alpha, which can be measured as an indication of TXA2 and PGI2 production.|
Eicosanoids are a family of compounds derived from polyunsaturated eicosanoic acids. The eicosanoids include prostaglandins, leukotrienes and the intermediate hydroperoxyeicosatetraenoic (HPETE) and hydroxyeicosatetraenoic (HETE) acids (see references 1-5 for reviews). The prostaglandins and leukotrienes act as paracrine and autocrine regulators through a family of transmembrane receptors. They regulate many cell functions and play crucial roles in a variety of physiological and pathophysiological processes, including regulation of smooth muscle contractility and various immune and inflammatory functions.
The prostaglandins are derivatives of prostanoic acid.2 Their nomenclature is based on substituents to the cyclopentane ring of prostanoate (e.g., the "A" group is substituted with alpha,beta unsaturated ketones, "E" with beta-hydroxy ketone and "F" with 1,3 diol), with subscript 1, 2 or 3 indicating the number of unsaturated bonds and sometimes with a subscript alpha or beta indicating the orientation of the C-9 hydroxyl group (projecting below the ring = alpha and above = beta). The most common substrate in humans is arachidonate; it yields the 2 series (2 double bonds) of prostaglandins. The 1 and 3 series of prostaglandins are derived from dihomo-gamma-linoleic acid and eicosapentanoic acid.
The synthesis of prostaglandins begins with the rate-determining hydrolysis of arachidonate (or another 20-carbon polyunsaturated fatty acid) from the 2-position of membrane phospholipids.1-3 The reaction is catalyzed primarily by a receptor-activated cytosolic phospholipase A2.3 The product, non-esterified arachidonate, is the substrate for a cyclooxygenase, which catalyzes cyclization of arachidonate with addition of two oxygen molecules. The product, a hydroperoxide cyclic endoperoxide (PGG2),1, 2 is the substrate for a glutathione-dependent hydroperoxidase, to yield a hydroxy cyclic endoperoxide (PGH2). PGH2 is the substrate for subsequent enzymatic modifications leading to the prostaglandins, PGD2, PGE2, PGF2alpha and prostacyclin (PGI2) and thromboxane A2 (TxA2).1, 2
The cyclooxygenase and the hydroperoxidase are separate enzyme activities on the same protein, called prostaglandin endoperoxide H synthase (PGHS, reviewed in reference 6). There are two PGHSs. PGHS-1 is a constitutive enzyme, while PGHS-2 is an inducible enzyme. (These enzymes are sometimes referred to COX-1 and COX-2, for cyclooxygenase-1 and -2.) PGHSs are each homodimeric, heme-containing, membrane-associated proteins sharing about 60% sequence homology.6 They have essentially identical structures and mechanisms of catalysis, including a "suicide" mechanism that causes them to self destruct after a brief round of catalysis. Thus, supply of arachidonate to PGHSs leads to a "burst" of prostaglandin synthesis. The functions of the two PGHSs must differ, and it has been suggested that PGHS-1 supplies prostaglandins for paracrine or extracellular autocrine functions, while PGHS-2 produces prostaglandins for nuclear autocrine functions.6, 7 Each PGHS is inhibited by non-steroidal anti-inflammatory drugs; most drugs inhibit both, but drugs with specificity for one or the other isoforms have been reported.
There is a large family of G-protein-coupled, seven-transmembrane prostaglandin receptors.4 Complex specificity and regulatory functions arise from the facts that i) the individual receptors have differing and overlapping specificities for individual prostaglandins; ii) the individual receptors are specifically distributed among different cells and tissues; and iii) there are different coupling mechanisms in different cells. With the variety of prostaglandins, this leads to very complicated and incompletely understood patho-physiological functions for most prostaglandins. In general, prostaglandins play key roles in regulation of smooth muscle contractility, in mediation of pain and fever, in regulation of blood pressure and platelet aggregation and in other physiological defense mechanisms, including immune and inflammatory responses. Inhibition of PGHS is the mechanism for much of the analgesic, anti-inflammatory, antipyretic and anti-thrombotic effects of non-steroidial anti-inflammatory drugs. There also is evidence for a role of prostaglandins in certain cancers and allergic reactions.3, 8, 9
In addition to the cyclooxygenase-catalyzed synthesis of prostaglandins, a non-enzymatic, radical-mediated production of prostaglandins has been reported.10 At least some are biologically active, possibly representing one of the mediators of oxidative stress injury and possibly reflecting the level of oxidative stress in an individual.10 8-iso-PGF2alpha is an example of the non-cyclooxygenase prostaglandins.
|Figure 2. An outline of the products of 5-lipoxygenase action of arachidonate. The main products are either LTB4 or LTC4, which is converted to LTD4 and LTE4 by plasma enzyme.|
While the cyclooxygenase-catalyzed addition of two oxygen molecules to an unsaturated fatty acid leads to prostaglandins, a different oxygenation, the lipoxygenase-catalyzed addition of a single molecule of oxygen, leads to leukotrienes.1-3, 5, 11 There is a family of lipoxygenases, each catalyzing addition of oxygen at a specific site, either the 5-, 12- or 15-position of arachidonate, leading to 5-, 12- or 15-HPETE. 5-HPETE is an unstable product that is reduced to either 5-HETE, to the leukotrienes LTA4 and LTB4, or to the sulfidopeptide leukotrienes, LTC4, LTD4 or LTE4.1, 2, 5, 11
Cells tend to secrete either LTB4 or LTC4, though a few secrete comparable amounts of each.5 Activated neutrophils secrete LTB4, which is an inducer of neutrophil chemotaxis, of neutrophil degranulation and of neutrophil-endothelial cell adhesion.2, 5 LTC4, D4 and E4, the "slow reactive substances of anaphylaxis", induce smooth-muscle contraction, increase vascular permeability to proinflammatory cells and enhance mucous secretion in the airways.2, 5 In addition, both types of leukotrienes modulate the cytokine regulation of the immune system. Leukotrienes, like prostaglandins, act through G-protein-coupled, seven-transmembrane receptors.1, 5
While the distribution of 5-lipoxygenase is restricted to certain myeloid cells, LTA4 is secreted, and the enzymes that catalyze the conversions of LTA4 to other leukotrienes are more widely distributed.5 Thus LTA4 can be secreted by one cell and processed to the peptidyl leukotrienes by other types of cells at different sites. This appreciably adds to the overall flexibility of leukotriene regulation and to the difficulty of assessing the physiological actions of the different leukotrienes.