Arachidonic acid enzymatic oxidation

Applications of peroxide formation are underrepresented in chiral synthetic chemistry, most likely owing to the limited stability of such intermediates. Lipoxygenases, as prototype biocatalysts for such reactions, display rather limited substrate specificity. However, interesting functionalizations at allylic positions of unsaturated fatty acids can be realized in high regio- and stereoselectivity, when the enzymatic oxidation is coupled to a chemical or enzymatic reduction process. While early work focused on derivatives of arachidonic acid chemical modifications to the carboxylate moiety are possible, provided that a sufficiently hydrophilic functionality remained. By means of this strategy, chiral diendiols are accessible after hydroperoxide reduction (Scheme 9.12) [103,104]. 

Scheme 5. Proposed biosynthesis of allene oxide 64 from 8i -lipoxygenase initiated metabolism of arachidonic acid and subsequent non-enzymatic transformations to racemic cyclopenteone 30 [86]...

The unique characteristic of free peroxyl radicals formed from unsaturated fatty acids is their ability to transform into cyclic radicals. This reaction is of utmost importance because it leads to highly biologically active compounds. Enzymatic oxidation of arachidonic acid catalyzed by COX results in the formation of prostaglandins having various physiopathological... 

Although the acute vasodilator effects, as shown in in vitro studies (see above), may participate in the antihypertensive effects, the reduced blood pressure persisted even 42-48 h after the last administration of quercetin, when the plasma quercetin concentration and its metabolites fell bellow 25% of the peak post-administration levels [43]. Furthermore, the antihypertensive effects of quercetin did not appear to be related to its antioxidant properties since quercetin did not lower the urinary isoprostane F20 excretion, a prostaglandin-like compound produced in a non enzymatic reaction of arachidonic acid in membrane lipids and superoxide, which is currently used as a reliable marker of oxidative stress. The mechanisms involved in the antihypertensive effects and protection from organ damage... 

The benzodioxole ring system is distributed widely in nature and is found in numerous natural products such as safrole and piperonal, as well as a multitude of alkaloids. 1,2-Dioxolanes are intermediates in the arachidonic acid cascade, which is the biochemical pathway from essential fatty acids to prostaglandins and similar hormones. The endoperoxide PGH2 (121) is believed to be formed on initial oxidation of arachidonic acid (120). PGH2 has a half-life of 4-5 minutes and is transformed enzymatically into prostaglandins, prostacyclin and the thromboxanes. These compounds are mediators for the control of platelet aggregation, blood vessel dilation and smooth muscle contraction. 

Prostaglandins are derived from arachidonic acid, a 20-carbon fatty acid with four cis double bonds. Figure 25-11 shows schematically how an enzyme oxidizes and cyclizes arachidonic acid to give the prostaglandin skeleton. One of the functions of aspirin is to inhibit this enzymatic prostaglandin synthesis and alleviate the inflammatory response. 

Without enzymatic catalysis, the endo and bicyclic peroxides (Reaction 46a) usually account for less than a few percent of arachidonic acid oxidation products, and the dominant pathways are formation of exo peroxides (Reaction 45) and polyperoxides (Reaction 46b). 

Finally the lipoperoxidation process can be fully enzymatic and this is carried out by heme-containing cyclooxigenases (prostaglandin-, tromboxan- and prostacyclin-synthases) or ferrous ione-containing lipoxygenases which are oxidized arachidonic acid and another PUFA by means of free radical mechanism [4,5] as can be seen in Figure 2. 

Prostanoids, consisting of prostaglandins (PGs) and thromboxanes (TXs), are members of the lipid mediators derived enzymatically from fatty acids. Arachidonic acid, a C2o essential fatty acid for most mammalians, is freed from the phospholipid molecule by phospholipase A2, which cleaves off the fatty acid precursor. Prostanoids are produced in a wide variety of cells throughout the body from the sequential oxidation of arachidonic acid by cyclooxygenase, PG hydroperoxidase, and a series of prostaglandin synthases (Figure 2.1). 

P-450 arachidonic acid-derived metabolites have been identified in human urine [189]. The renal cytochrome P-450 system is involved in catalyzing the enzymatic transformation of arachidonic acid. This reaction can involve either an epoxygenase system yielding various epoxides or a mono oxygenase system yielding other oxidation products. Cells of the proximal tubule, thick ascending limb of Henle and the collecting duct contain the major amount of cytochrome P-450 enzyme in the kidney. 

The major bioactive products of fatty acid metabolism relevant to atherosclerosis are those that result from enzymatic or non-enzymatic oxidation of polyunsaturated long-chain fatty acids. In most cases, these fatty acids are derived from phospholipase A2-mediated hydrolysis of phospholipids (Chapter 11) in cellular membranes or lipoproteins, or from lysosomal hydrolysis of lipoproteins after internalization by lesional cells. In particular, arachidonic acid is released from cellular membrane phospholipids by arachidonic acid-selective cytosolic phospholipase Aj. In addition, there is evidence that group II secretory phospholipase Aj (Chapter 11) hydrolyzes extracellular lesional lipoproteins, and lysosomal phospholipases and cholesterol esterase release fatty acids from the phospholipids and CE of internalized lipoproteins. Indeed, Goldstein and Brown surmised that at least one aspect of the atherogenicity of LDL may lie in its ability to deliver unsaturated fatty acids, in the form of phospholipids and CE, to lesions (J.L. Goldstein and M.S. Brown, 2001). 

A relatively new class of oxidized arachidonic acid derivatives with potential relevance to atherosclerosis are F2 isoprostanes [24] (Fig. 6) (Chapter 12). These compounds form as a result of non-enzymatic, free-radical attack of the fatty acid moieties of cellular or lipoprotein phospholipids, followed by release of the isoprostanes from the phospholipids by a phospholipase. 8-wo-prostaglandin-F2 may also be formed by the action of COX-1 or -2 in platelets or monocytes, respectively, but the significance of COX-dependent 8-W0-PGF2 formation in vivo is unproven. Fj isoprostanes circulate in the plasma and appear in the urine as free compounds or esterified to phospholipids, and... 

Fig. 6. Three irnportam oxidative pathways for the metabolism of polyunsaturated fatty acids. A. The principal steps in the formation of prostaglandin endoperoxides from arachidonic acid. B. Likely mechanism for the formation of hydroperoxy cis.trans conjugated fatly acids by autooxidalion or by lipoxygena.sc.s. In the latter case, positional as well as stereospecificity arc normally found. The formed hydroperoxides may then be enzymatically transformed into leukotrienes (not shown) or to hydroxy acids. C. The enzymatic sequence in cytochrome P-450 mediated oxygenation. The first electron is donated from NADPH via the f lavoprotein (Fp) NADPH-cytochrome-P-450-reductase. while the second electron comes either from NADPH or NADH. In this way the monooxygenase enzymes inlrrxluce one oxygen from air into the substrate.

Arachidonic acid is the biologically most interesting essential fatty acid of the co6 series. Besides the normal /8-oxidation, the common fate for all fatty acids in the body, arachidonic acid can be enzymatically oxygenated by three different enzyme types (Fig. 6) ... 

Fig. 10. Summary of the enzymatic metabolism of arachidonic acid by cytochrome AaSO. The epoxides presumably all have the as configuration although this has only been shown for the w6 oxide. The epoxides are hydrolysed by epoxide hydrolase into the vicinal diols. Arachidonic acid and the vicinal diols may be further metabolised by wl- and w2-hydroxylation.

Metab. produced during enzymatic NADPH dependant oxidation of arachidonic acid. 

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