Eicosanoids lipoxygenase

Compounds 111 having structural features of the dual cyclooxygenase (COX)/5-lipooxygenase (5-LO) inhibitor tepoxalin and the 5-LO inhibitor ABT-761 were prepared. Many of these hybrid compounds are potent COX and 5-LO inhibitors two compounds (111, r =McO, R = R" = R = H, R = NH2, R = Me and r = MeO, R = R = Me, R" = R = H, R = Cl) inhibited eicosanoid biosynthesis in an ex vivo assay, but neither improved on the main deficiency of tepoxalin, duration of 5-LO inhibitory activity (99BMCL979). Compounds 111 inhibit the production of arachidonic acid products associated with 5-lipoxygenase and cyclooxygenase and are useful in the treatment of inflammatory disorders (99USP5925769). 

There are three groups of eicosanoids that are synthesized from C20 eicosanoic acids derived from the essential fatty acids linoleate and a-linolenate, or directly from dietary arachidonate and eicosapentaenoate (Figure 23-5). Arachidonate, usually derived from the 2 position of phospholipids in the plasma membrane by the action of phospholipase Aj (Figure 24-6)—but also from the diet—is the substrate for the synthesis of the PG2, 1X2 series (prostanoids) by the cyclooxygenase pathway, or the LT4 and LX4 series by the lipoxygenase pathway, with the two pathways competing for the arachidonate substrate (Figure 23-5). 

Figure 23-5. The three groups of eicosanoids and their biosynthetic origins. (PG, prostaglandin PGI, prostacyclin TX, thromboxane LT, leukotriene LX, lipoxin , cyclooxygenase pathway , lipoxygenase pathway.) The subscript denotes the total number of double bonds in the molecule and the series to which the compound...

Smith WL, Fitzpatrick FA The eicosanoids Cyclooxygenase, lipoxygenase, and epoxygenase pathways. In Biochemistry of Lipids, Lipoproteins and Membranes. Vance DE, Vance JE (editors). Elsevier, 1996. 

Hyperforin, the major constituent in Hypericum perforatum (St. John s Wort), inhibits the enzymatic activity of 5-lipoxygenase and COX-1 in platelets, acts as a dual inhibitor of 5-lipoxygenase and COX-1, and might have some potential in inflammatory and allergic diseases connected to eicosanoids (32), Several Hypericum species are of medicinal value in Asia and the Pacific. One of these is Hypericum erectum Thunb., the potential of which as a source of 5-lipoxygenase is given here. 

Eicosanoid synthesis. Arachidonic acid is converted by cyclooxygenases into prostaglandins, and thromboxanes. Lipoxygenases convert arachidonic acid into HPETEs, which are then converted to lipoxins, leukotrienes, and 12-HETE (hydroxyeicosatetraenoic acid). Epoxygenases convert arachidonic acid into epoxides. 

Before taking leave of the eicosanoids, 1 need to point out that a second enzyme, 5-lipoxygenase, also metabolizes arachidonic acid. 5-Lipoxygenase initiates the synthesis of the leukotrienes from arachidonic acid. There is a whole family of leukotrienes and these molecules have a spectrum of biological properties. 1 will focus on one important leukotriene, LTB4. 

A summary of the processes for producing the eicosanoids from the polyunsaturated fatty acid, arachidonic acid, is presented in Figure 11.27. The two enzymes separate for synthesising the prostanoids or the leucotrienes are cyclooxygenase and lipoxygenase, respectively. Whether prostanoids or leucotrienes are produced in any given tissue will depend on the relative activities of these two enzymes in that tissue. 

As a result of the action of lipoxygenases [3], hydroxyfatty acids and hydroperoxyfatty acids are formed from arachidonate, from which elimination of water and various conversion reactions give rise to the leukotrienes. The formulae only show one representative from each of the various groups of eicosanoids. 

The eicosanoids, so called because of their derivation from a 20-carbon unsaturated fatty acid, arachidonic acid (eicosatetraenoic acid), are obtained from membrane phospholipids and synthesized de novo at the time of cellular stimulation. Arachidonic acid is cleaved from membrane-bound phosphatidylcholine by the enzyme phospholipase A2. Alternatively, arachidonic acid may be derived by the sequential actions of phospholipase C and diacylglyceryl lipase. Arachidonic acid can then follow either of two enzymatic pathways that result in the production of inflammatory mediators. The pathway initiated by cyclooxygenase (COX) produces prostaglandins the lipoxygenase pathway generates leukotrienes (Fig. 36.2). 

Figure 8.9 Prostaglandins and leukotrienes are potent eicosanoid lipid mediators, derived from phospholipase-released arachidonic acids, that are involved in numerous homeostatic biological functions and inflammation. They are generated by cyclooxygenase isozymes and 5-lipoxygenase, respectively, and their biosynthesis and pharmacological actions are inhibited by clinically relevant nonsteroidal anti-inflammatory drugs.

The cell damage associated with inflammation acts on cell membranes to cause leukocytes to release lysosomal enzymes arachidonic acid is then liberated from precursor compounds, and various eicosanoids are synthesized. As discussed in Chapter 18, the cyclooxygenase (COX) pathway of arachidonate metabolism produces prostaglandins, which have a variety of effects on blood vessels, on nerve endings, and on cells involved in inflammation. The lipoxygenase pathway of arachidonate metabolism yields leukotrienes, which have a powerful chemotactic effect on eosinophils, neutrophils, and macrophages and promote bronchoconstriction and alterations in vascular permeability. 

Inflammation is now recognized as a key process in atherogenesis [Libby, 2002]. The potential for dietary flavonoids to inhibit inflammatory activities is of particular interest. A potential anti-inflammatory feature of the flavonoids is the ability to inhibit the biosynthesis of eicosanoids. Selected phenolic acids and some flavonoids have been shown to inhibit both cyclooxygenase (COX) and 5-lipoxygenase (5-LO) pathways [Nijveldt et al., 2001 Takano-Ishikawa et al., 2006], Epicatechin and related flavonoids have been shown to inhibit the synthesis of pro-inflammatory cytokines in vitro [Sanbongi et al., 1997], and plasma metabolites of catechin and quercetin inhibit the adhesion of monocytes to cultured endothelial cells [Koga and Meydani, 2001]. Silymarin has been shown to inhibit the production of inflammatory cytokines, such as interleukin-1, interferon-, and tumor necrosis factor-a (TNFa), from macrophages and T-cells [Matsuda et al., 2005], Some flavonoids can inhibit neutrophil... 

Leukotriene One of the 20-carbon fatty acid compounds (eicosanoids) formed from arachidonic acid by the lipoxygenase enzyme. Leukotrienes are important in mediating certain allergic and inflammatory responses, especially in respiratory tissues. 

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