5- Hydroperoxyeicosatetraenoic acid

LO catalyses the stereospecific conversion of arachidonic acid (1) to a 5-hydroperoxyeicosatetraenoic acid (5-HPETE) (2). The enzyme contains... 

The hydroperoxides obtained on thermal oxidation of cholesteryl acetate (191e) can be selectively separated by SPE and elution with a polar solvent. After reduction to the corresponding alcohols by NaBH4 and further derivatization to the trimethylsilyl ether, the products can be subjected to GLC with ion-trap MS detection. It can be thus demonstrated with the aid of standards that under the oxidation conditions (160 °C for 90 min) only the 7-position is attacked, leading to the la- and 7/3-hydroperoxy derivatives, while the plausible 4-position remains unscathed . Treatment of erythrocite ghosts with t-BuOOH causes a manyfold content increase of 5-hydroxyeicosatetraenoic acid (5-HETE), 5-hydroperoxyeicosatetraenoic acid (5-HPETE) and 5-oxoeicosatetraenoic acid (5-oxo-ETE) residues of phospholipids. These acids can be separated by HPLC, identified and quantitized by tandem MS . ... 

Figure 12.13. Synthesis of leukotrienes. a Formation of leukotriene A and B from 5-hydroperoxyeicosatetraenoic acid, the product of 5-lipoxygenase, b Successive formation of leukotrienes C, D, and E from leuktriene A. All steps are enzymatically catalyzed.

LTs are produced in the 5-lipoxygenase pathway (Figure 18-25). Their synthesis begins with arachidonic acid obtained from cleavage of the membrane phospholipid pool due to the action of phospholipase A2. Arachidonic acid is converted in a catalytic sequence by 5-lipoxygenase complex and its activating protein to 5-hydroperoxyeicosatetraenoic acid (5-HPETE) and... 

Fig. 6. Biochemical pathway of the metabolism of arachidonic acid into the biologically active leukotrienes. Arachidonic acid released from phospholipids by cytosolic (c) phospholipase Aja is metabolized by 5-lipoxygenase to 5-hydroperoxyeicosatetraenoic acid (5-HpETE) and leukotriene A4 (LTA4) which is then enzymatically converted into leukotriene B4 (LTB4) or conjugated by glutathione to yield leukotriene C4 (LTC4).

The pathway to leukotrienes starts by attack on arachidonate of a lipoxygenase, which adds 02 to C-5, giving 5-hydroperoxyeicosatetraenoic acid (5-HPETE). A dehydration to give the epoxide coupled with isomerization of double bonds gives leukotriene A4. Hydrolysis of the epoxide ring yields leukotriene B4. Transfer of the thiol group of glutathione yields leukotriene C4. Subsequent modifications of the peptide chain (not shown) yield related compounds, leukotrienes D and E. 

Fig. 4. Regulatory targets for leukotriene (LT) inhibition. AA, arachidonic acid FLAP, 5-lipoxygenase activating protein 5-HPETE, 5-hydroperoxyeicosatetraenoic acid.

The first enzymatic transformation in the biosynthesis of leukotrienes is an oxygenation of the fatty acid substrate, by a lipoxygenase, giving a hydroperoxide. Thus, arachidonic acid can be converted to 5-hydroperoxyeicosatetraenoic acid (5-HPETE)... 

Figure 1. Pathways and chemical structures for leukotriene and lipoxin biosynthesis. Enzymes responsible for known catalytic steps are noted in italics. AA, arachidonicacid.5-, 15-LOX, 5-, 15-lipoxygenase. 5-, 15-HETE, 5(5 )-, 15(5)-hydroxyeicosatetraenoicacid.5-, 15-HPETE, 5(5 )-, 15(5 )-hydroperoxyeicosatetraenoic acid. 5(6)-Epoxytetraene, 15(5)-hydroxy-5(6)-epoxy-7,9,13- ra. -ll-c/5-eicosatetraenoic acid. LTA, leukotriene A 5(6)-epoxy-7,9- rafZ5-ll,14-cw-eicosatetraenoic acid. LTB, leukotriene 5(5 ),12(/ )-dihydroxy-6,14-cw-8,10- rfl .y-eicosatetraenoic acid. LTC, peptido-leukotriene C 5(5)-hydroxy-6(/ )-5 -glutathione-7,9-fra 5-ll,14-cw-eicosatetraenoic acid. LTD, peptido-leukotriene D 5(5)-hydroxy-6(/ )-( S-cysteinylglycinyl)-7,9-rra 5-11,14-cA-eicosatetraenoic acid. LTE, peptido-leukotriene E 5(5)-hydroxy-6(/ )-( S-cysteinyl)-7,9-rra 5-ll,14-cw-eicosatetraenoic acid. LXA, lipoxin A 5(5 ),6(/ ), 15(5)-trihydroxy-7,9, 3-trans-11 -cA-eicosatetraenoic acid.

Synthesis of lipoxins can occur by activation of various biosynthetic pathways (Serhan, 1994). Besides derivation from LTA as mentioned above, catalysis by 15-LOX, or by a combination of 5-LOX and 12-LOX, can give rise to various lipoxin isomers. The first pathway to be described was that involving the formation by 15-LOX of 15(5 )-hydroperoxyeicosatetraenoic acid [15(5)-HPETE see Figure 1] followed by the formation of a 5(6)-epoxytetraene intermediate [15(5)-hydroxy-5(6)-epoxy-7,9,13-frans-ll-cw-eicosatetraenoic acid] in a two-step 15-LOX catalytic cycle analogous to that observed with 5-LOX in the biosynthesis of LTA (Serhan, 1994,2002). In addition to these LOX-initiated pathways, more recently a COX-2 dependent synthesis of LXA epimers at the carbon 5 position has been demonstrated. This pathway is... 

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