Biochemistry of the Eicosanoids: Cyclooxygenase and Lipoxygenase Products of Polyunsaturated Fatty Acids

Lipids in Modern Nutrition, edited by M. Horisberger and U. Bracco. Nestle Nutrition, Vevey/Raven Press, New York © 1987. Biochemistry of the Eicosanoids: Cyclooxygenase and Lipoxygenase Products of Polyunsaturated Fatty Acids Elisabeth Granstrom Department of Physiological Chemistry, Karolinska Institutet, Stockholm, Sweden Certain polyunsaturated fatty acids can be metabolized by oxygenation into a large family of biologically active substances, the so-called prostanoids or eicosanoids. This family includes the prostaglandins, thromboxanes, prostacyclins, leukotrienes, lipoxins, and also a number of related mono-, di-, and trihydroxy compounds as well as epoxygenated fatty acids. Most of these substances are potent compounds and display a wide variety of effects in many biological systems. Among these are effects on blood pressure, smooth muscle contractility, kidney function, cyclic nucleotide levels, glandular excretion, hormone secretion, platelet aggregation, vessel tone, temperature regu- lation, lipolysis, chemotaxis, immune response, cell growth and differentiation, electrolyte and water balance, etc. In combination with the fact that the eicosanoids are almost ubiquitous in the mammalian body, it is not surprising that these substances are supposed to be involved in many physiological as well as pathologi- cal conditions in the organism. Thus, altered eicosanoid levels are sometimes found to coincide with the appearance of certain symptoms, and normalization occurs upon successful treatment of the condition with drugs known to interfere with eicosanoid biosynthesis or metabolism. The major classes of eicosanoids will be described below, and some of their possible biological roles will be discussed. PRECURSOR FATTY ACIDS The probably most important precursor fatty acid in this field is arachidonic acid, 5,8,11,14-all-cw-eicosatetraenoic acid (20:4to6), which gives rise to prostaglandins (PGs) and thromboxanes (TXs) of the 2-series and leukotrienes (LTs) of the 4-series (the figure indicates the number of double bonds in the product, see below). Other polyunsaturated fatty acids are however also of interest, namely 8,11,14-eicosatrienoic acid (dihomo-7-linolenic acid, 20:3«6), precursor of the 1-series of PGs and TXs and one 3-series of LTs; 5,8,11-eicosatrienoic acid, pre- 59 60 BIOCHEMISTRY OF EICOSANOIDS cursor of the "normal" 3-series of LTs; and 5,8,11,14,17-eicosapentaenoic acid (timnodonic acid, 20:5w3), which gives rise to the PGs and TXs of the 3-series as well as LTs of the 5-series. Recently, however, C22 fatty acids also have come into focus as precursors of correspondingly elongated PGs and TXs and possibly other products as well. For example, adrenic acid (22:4<D6) can be metabolized into biologically active dihomo-PGs. The oxygenation of docosapenta- and doco- sahexaenoic acids (22:5w6 and 22:6o)3) has also attracted attention recently. In general, the precursor fatty acid does not occur free in the unstimulated cell but is esterified to glycerophospholipids almost exclusively in the 2-acyl position. The release of the precursor is considered to be the rate-limiting step in the biosynthesis of PGs and related substances. This hydrolysis can be achieved by a phospholipase A2, or by phospholipase C and then by other lipases, depending on cell type, phospholipid category, and possibly other factors. After release, the parent fatty acid is metabolized via three different types of pathways. The best known of these is catalyzed by the enzyme fatty acid cyclooxygenase and leads to the formation of prostaglandin endoperoxides, which in turn are the immediate precursors of prostaglandins, thromboxanes, and prostacyclins. Other metabolic oxygenations are catalyzed by various lipoxygenases. Several such enzymes with different positional specificity for the introduction of oxygen have been identified. The products of the lipoxygenase pathways include hydroperoxy fatty acids, mono-, di-, and trihydroxy fatty acids, the leukotrienes and the recently discovered lipoxins. Finally, the fatty acid can also be metabolized by the action of various monooxygenases into wl or a>2 hydroxy acids or by epoxidation at the double bonds. Comparatively little is as yet known about the products of these latter pathways. For reviews on the release of the polyunsaturated fatty acids from their phospholipid precursors, see, e.g., refs. 1-3. PROSTAGLANDINS As indicated above, oxygenation can take place at many different positions of the fatty acid. In, for example, arachidonic acid, enzymatic oxygenation can occur at carbon-5, carbon-8, carbon-9, carbon-11, carbon-12, and carbon-15. In the best known metabolic pathway in this field, namely the cyclooxygenase pathway, the enzyme introduces oxygen at carbon-11 in a dioxygenase type reaction, and the resulting ll-peroxy compound then cyclizes to form a 9,11-endoperoxide structure (Fig. 1). Simultaneously, ring closure occurs between carbon-8 and carbon-12, the A14 cis double bond isomerizes to A13 trans, and another oxygen radical attacks at carbon-15. The resulting compound is the first product with the prostanoic acid skeleton (Fig. 2), and its systemic name is, when originating in arachidonic acid, 9a,lla-peroxido-15(5)-hydroperoxy-prosta-5-cw, 13-frans-dienoic acid (trivial name, prostaglandin G2, PGG2) (Fig. 1). The two remaining double bonds of arachidonic acid give rise to the name of the series of compounds originating in this parent molecule, namely the 2-series. The 1-series, originating in dihomo-7-lino- OH OH OH COOH Glutathione COOH COOH leukotriene B; CHCONHCH2COOH NHCOCH2CH2CHCOOH Leukotriene A; Leukotriene C4 JDehydrase 5-Lipoxygenase COOH COOH HOO 12-Lipoxygenase 15-Lipoxygenase 12-Hydroperoxy- OOH eicosatetraenoic acid 15-Hydroperoxy- eicosatetraenoic acid Prostaglandin endoperoxide, PGH, q. 9" , COOH --COOH < ^^ HO' OH OH HO' 6H Prostaglandin E2 Thromboxane A2 Prostacyclin, PGI2 FIG. 1. Simplified scheme of arachidonic acid metabolism, showing some products of the cyclooxygenase and lipoxygenase catalyzed pathways. 62 BIOCHEMISTRY OF EICOSANOIDS -COOH 13 15 17 19 FIG. 2. Structures of prostanoic acid (left) and thrombanoic acid (right). lenic acid, analogously has only one double bond, A13 trans, and the 3-series from timnodonic acid has three double bonds, A5 cis, A13 trans, and A17 cis. Prostaglandin G2 is then converted into the corresponding 15-hydroxy compound by an endoperoxide peroxidase. This product is named prostaglandin H2 (PGH2; Fig. 1). So far it has not been possible to separate the two enzyme activities, the cyclooxygenase and the peroxidase. It is thus currently believed that these two activities are catalyzed by the same enzyme, called prostaglandin endoperoxide synthetase. The two endoperoxides are highly unstable in aqueous medium (half-life about 5 min at 37°C and pH 7.4) and are biologically very potent compounds. Among their biological activities are vasoconstriction, constriction of airways, and induction of platelet aggregation. It is however uncertain how much of their registered effects can be attributed to the endoperoxides themselves, since they are rapidly converted into other compounds, both enzymatically by most tissues and nonenzymatically. The earliest discovered products of endoperoxide metabolism are the "classi- cal" prostaglandins, PGE, PGF, and PGD (Fig. 1). In fact, these compounds of the 1-series were the first ones to be identified in this entire field (4). The prostaglandins originating in arachidonic acid are PGE2 (9-keto-lla,15(5)-dihydroxyprosta-5-cw, 13-frans-dienoic acid), PGF2c (9a,lla,15(S)-trihydroxyprosta-5-cw, 13-fran.s-dienoic acid) and PGD2 (ll-keto-9a,15(S)-dihydroxyprosta-5-c«,13- trans-dienoic acid, an isomer of PGE2) (Fig. 1). At least PGE2 and PGD2 are formed enzymatically from the endoperoxides by the action of specific isomerases. The enzymatic formation of PGF2c< is less certain: the existence of an endoperoxide reductase catalyzing this reaction has not been demonstrated unequivocally. All these three compounds can also be formed nonenzymatically during spontaneous degradation of the endoperoxides in aqueous solution. A large number of important biological activities have been attributed to the prostaglandins. PGE2, for example, is a potent vasodilator and lowers the blood pressure, inhibits the secretion of acid in the gastric mucosa, and also acts as a "cytoprotective" agent in the gastrointestinal tract; PGF2a is a powerful stimulant of the myometrium and also induces regression of the corpus luteum in many ani- mal species; and PGD2 is a potent inhibitor of platelet aggregation. For a recent review on the biochemistry and physiological effects of prostaglandins, see ref. 1. BIOCHEMISTRY OF EICOSANOIDS 63 THROMBOXANES Several other products are also formed from the endoperoxides. One biologically very potent endoperoxide metabolite is formed in large amounts from plate- lets, lung, and spleen, and also by a number of other tissues but in smaller quanti- ties: thromboxane A (Fig. 1). The basic structure of this compound is different from that of the prostaglandins; i.e., it is not a derivative of prostanoic acid but of thrombanoic acid (Fig. 2): it contains an oxane-oxetane ring structure instead of the normal prostaglandin cyclopentane ring. The structures of the side chains are however identical with the prostaglandins. Thromboxane A2 from arachidonic acid is even more unstable (half-life about 30 sec) and biologically potent than its precursor endoperoxides. Among its activities are vasoconstriction, bronchocon- striction, and induction of platelet aggregation. The compound is rapidly hydro- lyzed into a biologically almost inactive substance, thromboxane B2 (TXB2). Thromboxane biosynthesis is usually detected either by rapid bioassay of the vaso- active or proaggregatory effects of TXA2 or by detection by other methods of its degradation product, TXB2. For a recent review of the thromboxanes, see, e.g., ref. 5. PROSTACYCLINS Another potent endoperoxide metabolite is found only in the 2- and 3-series of prostanoids, namely PGI. The structure of PGI2 from arachidonic acid ("prostacyclin") is 9-deoxy-6,9a-epoxy-ll,15-dihydroxyprosta-5,13-dienoic acid (Fig.

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