HETEs formation

The formation of nitric oxide in microsomes results in the inhibition of microsomal reductase activity. It has been found that the inhibitory effect of nitric oxide mainly depend on the interaction with cytochrome P-450. NO reversibly reacts with P-450 isoforms to form the P-450-NO complex, but at the same time it irreversibly inactivates the cytochrome P-450 via the modification of its thiol residues [64]. Incubation of microsomes with nitric oxide causes the inhibition of 20-HETE formation from arachidonic acid [65], the generation of reactive oxygen species [66], and the release of catalytically active iron from ferritin [67],... 

Flynn, D.L., Rafferty, M.F., and Doctor, A.M., Inhibition of 5-hydroxyeicosatet-raenoic acid (5-HETE) formation in intact human neutrophils by naturally occurring diarylheptanoids Inhibitory activities of curcuminoids and yakuchinones, Leukot-rienes Med., 22, 357, 1986. 

Several reports have suggested an association between 5-LOX, 5-(S)HETE formation and carcinogenesis. The expression of 5-LOX has been documented in several cancers including prostate, colon, lung, breast, pancreas, bone, brain, and mesothelium (Romano Claria, 2003). Inhibition of 5-LOX enzymatic activity by specific chemical inhibitors blocked the stimulatory effect of arachidonic acid on the growth of prostate cancer cells. Conversely, addition of 5-HETE but not leukotrienes promoted the growth of prostate cancer cells. 

CYP4A inactivator Administration of a single dose of 10-SUYS, a potent and selective mechanism-based CYP4A inactivator, acutely reduced the mean arterial blood pressure as well as the urinary 20-HETE excretion in spontaneously hypertensive rats, consistent with the inactivation of renal 20-HETE formation ". These findings thus suggest that 20-HETE could play an important role in blood pressure regulation in hypertensive states and that the inhibition of its synthesis in these conditions may be of therapeutic benefit ". 

Flynn, D.L., and Rafferty, M.F. (1986) Inhibition of 5-Hy-droxy-eicosatetraenoic acid (5-HETE) Formation in Intact Human Neutrophils by Naturally Occurring DiaryUiep-tanoids Inhibitory Activities of Curcuminoids and Yakuchi-nones. Prostaglandins Leukot. Med. 22,357-360. 

Kragballe, K., DueU, E.A., and Voorhees, J J. (1986) Selective Decrease of 15-Hydroxy-Eiocsatetraenoic Acid (15-HETE) Formation in Uninvolved Psoriatic Demris, Arc/i. Dermatol. 122, 877-880. 

Various flavonoids and related compounds have been tested as inhibitors for cyclooxygenase and lipoxygenase. Some are more effective as lipoxygenase inhibitors, and some more effective as cyclooxygenase inhibitors. Thus, 3, 4 -dihydroxy-flavone and 1,5-dihydroxy-naphthalene are potent inhibitors of 12-HETE formation in rat lung and spleen homogenates. IC50 values are 3.3 X 10 M and 2 X 10 M, respectively [394],... 

Additional hypotheses concerning prostaglandin biosynthesis in P. homomalla resulted from isolation of 11R-HETE (76) from the polar lipid fraction [95]. Apparently, 11R-HETE (76) is also a minor product of incubations of arachidonic acid with acetone powder preparations of P. homomalla [95], In this alternate hypothesis (Scheme 8), an 11-hydroxy or 11-hydroperoxy-8,9-allene oxide intermediate is formed from a sequence of oxidations at C8 and Cll. Opening of the allene oxide to a transient C8 earboeation induces eycli-zation with a consequent addition of water to C15. This proposed pathway leads initially to formation of PGE2 (16 or 38), which following acetylation, elimination of acetic acid from Cl 1-12, and esterification, forms the observed major natural product in the coral, 15-acetoxy methyl PGA2 (36 or 54). Notably, if... 

Inhibition and stimulation of LOX activity occurs as a rule by a free radical mechanism. Riendeau et al. [8] showed that hydroperoxide activation of 5-LOX is product-specific and can be stimulated by 5-HPETE and hydrogen peroxide. NADPH, FAD, Fe2+ ions, and Fe3+(EDTA) complex markedly increased the formation of oxidized products while NADH and 5-HETE were inhibitory. Jones et al. [9] also demonstrated that another hydroperoxide 13(5)-hydroperoxy-9,ll( , Z)-octadecadienoic acid (13-HPOD) (formed by the oxidation of linoleic acid by soybean LOX) activated the inactive ferrous form of the enzyme. These authors suggested that 13-HPOD attached to LOX and affected its activation through the formation of a protein radical. Werz et al. [10] showed that reactive oxygen species produced by xanthine oxidase, granulocytes, or mitochondria activated 5-LOX in the Epstein Barr virus-transformed B-lymphocytes. 

FIGURE 32-7 Sources of free radical formation which may contribute to injury during ischemia-reperfusion. Nitric oxide synthase, the mitochondrial electron-transport chain and metabolism of arachidonic acid are among the likely contributors. CaM, calcium/calmodulin FAD, flavin adenine dinucleotide FMN, flavin mononucleotide HtT, tetrahydrobiopterin HETES, hydroxyeicosatetraenoic acids L, lipid alkoxyl radical LOO, lipid peroxyl radical NO, nitric oxide 0 "2, superoxide radical. 

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.

Additional lipoxygenases are known which oxygenate different positions on the arachidonic acid chain. 12-LO, resulting in the formation of 12-HETE (7), is best known in platelets, while the 15-LO from soybean has been studied in detail for many years [8]. 15-HETE (8) is also produced by mammalian cells the enzymes from neutrophils and particularly rabbit reticulocytes are the best characterized. 

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. 

Besides 12-LOX in platelets, the 5-LOX isoforms are constitutive in neutrophils. Evidences indicate that LOXs are involved in inflammation diseases and in atherosclerosis. 5-LOX is the enzyme that catalyzes the formation of leukotrienes with potential role for leukocytes and platelets interaction and inflammation. After platelet and leukocyte stimulation, products of both COX-1 and 5-LOX pathways increase. COX-1 activity derivatives increase the vascular permeability mediated by prostaglandins and produce platelet aggregation mediated by TXA2. The product of the lipoxygenase pathway, 5-oxo-6,8,1 1,14-eicosatetraenoic acid (5-Oxo-ETE), induces leukocyte chemotaxis and inflammation. 5-Oxo-ETE is formed by the oxidation of 5S-hydroxy-ETE (5-HETE) by 5-hydroxyeicosanoid dehydrogenase (5-HEDH), a microsomal enzyme found in leukocytes and platelets (42). 

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