Arachidonic acid 15 -HPETE from

Platelets have been shown to cooperate with leukocytes in production of chemotactic fectors which the cells are unable to synthesize themselves. 12-HETE from platelets are metabolized by unstimulated neutrophils to 12,20-di-HETE (Marcus et al, 1984,1988). Leukocytes produces increased amounts of leukotrienes, because 12-HPETE produced in activated platelets stimulates the activity of leukocyte 5-lipoxygenase (Maclouf et al, 1982 Romano and Sethan, 1992). Neutrophils convert arachidonic acid released from stimulated platelets to produce 5-HETE and leukotriene B4 (LTB ) (Marcus et al, 1982). Platelet activating fector (PAF) are shown to stimulate production of LTB from tiiese cells (Lin et al, 1982). Platelets can also produce the vasoconstrictor leukotriene C4 (LTC4) from leukotriene A4 (LTA4) synthesized by leukocytes (Maclouf and Murphy, 1988). Activated platelets and neutrophils release PAF (Lynch and Henson, 1986) which is a potent mediator of inflammation and asthma. 

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... 

The HPETEs and HETEs are the primary products of the action of lipoxygenase on arachidonic acid, and from them, by further biochemical transformations, are derived all the remaining transmitters with which this review is concerned. In particular, (5S)-HPETE is the starting point from which are derived all of the most important leukotrienes studied to date. 

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).

Leukotriene biosynthesis depends upon the availability of arachidonic acid (8) as the free carboxylic acid as the 5-LOX substrate, which typically requires the action of cytosolic phospholipase to release arachidonic acid (8) from membrane phospholipids [27]. The name leukotriene was conceived to capture two unique attributes of these molecules. The first relates to those white blood cells derived from the bone marrow that have the capacity to synthesize this class of eicosanoids, for example, the polymorph nuclear leukocyte. The last part of the name refers to the unique chemical structure, a conjugated triene retained within these eicosanoids [29,30]. The first step for the leukotriene biosynthesis is the insertion of molecular oxygen at position 5 of arachidonic add (8) to produce 5-HPETE (17) that can be converted to leukotriene (18) by the second catalytic activity... 

Hepoxylins are metabolites of arachidonic acid which arise from 12-HPETE in tissues such as pancreatic islet cells (where they stimulate glucose-dependent insulin release) and brain (where they appear to have a neuromodulatory role). The structure of the hepoxylins was confirmed by synthesis which also has provided this scarce material for biological investigation. 

The first step in the biosynthesis of eicosanoids from arachidonic acid is generally a lipoxygenation reaction. The resulting hydroperoxides (HPETE s) can undergo reduction to the corresponding alcohols (HETE s). Preparative routes to the 5-, 11-, and 15-HETE s and HPETE s have been developed as oudine below. 

Figure 6.17. Leukotriene formation in neutrophils. Arachidonic acid, which is released from membrane phospholipids by the action of either phospholipase A2 or diacylglycerol lipase (see Fig. 6.13), is oxygenated by 5-lipoxygenase to yield 5 hydroperoxy-6,8,11,14 eicosa-tetraenoic acid (5-HPETE). This is then converted into 5 hydroxy-6,8,11,14 eicosatetra-enoic acid (5-HETE) and leukotriene (LT) A4. LTA4 may then be enzymically converted into LTC4 and LTB4. LTB4 is the major product in activated neutrophils.

The acid-soluble SH-groups in platelets are mainly those of glutathione (GSH). GSH is a cofactor for enzymes such as peroxidase. If feverfew is able to interfere with this cofactor, enzyme function may be impaired. One pathway that may be affected in this way is the metabolism of arachidonic acid (Figure 6.1). In the presence of feverfew extract an increase was found in lipoxygenase product formation and impaired conversion of HPETE to HETE, for which GSH is a cofactor [52]. Inhibition of the liberation of [ " C]arachidonic acid from phospholipids was also found [53], which implies impairment of phospholipase A2 activity and for which SH-groups are thought to be important. 

The other metabolic pathway derived from arachidonic acid is through the activity of the lypoxygenase-12 (2-LOX) (Fig. 5). 2-LOX converts arachidonic acid (AA) to 12-hydroperoxy-5,8,10,14-eicosatetraenoic acid (12-HPETE) that is rapidly reduced by peroxidases to the stable 12-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE). In... 

Enzymatic assay of lipid hydroperoxides A selective assay has been described for lipid hydroperoxides, based on the cyclooxygenase reaction with a sensitivity in the range of 20-200 pmol (Marshall et al., 1985 Pendleton and Lands, 1987 Lands, 1988). The substrate (15-HPETE) is not, however, commercially available and must therefore be synthesized from purified arachidonic acid by the method of Graft (1982) in which soya bean lipoxygenase is used to catalyse the stereospecific oxygenation of arachidonic acid. 

As described in Section 9.16, the leukotrienes, important components in the asthmatic response, are synthesized from arachidonic acid via the hydroperoxide 5-HPETE. Write a stepwise mechanism for the conversion of arachidonic acid to 5-HPETE with O2. 

FIGURE 33-3. Metabolism of arachidonic acid after its release from membrane phospholipids. ASA, aspirin HPETE, hydroperoxyeicosatetraenoic acid NSAlDs, nonsteroidal antiinflammatory drugs PC, prostaglandin. Broken arrow indicates inhibitory effects. 

---