Arachidonic oxygenation

Metabolism of aromatic amines and BP-7,8-dihydrodiol has been detected during arachidonate oxygenation in intact cells and in cultured trachea (64,58). Exogenous arachidonate was added to... 

The importance of Tyr-385 in aspirin acetylation of Ser-530 suggests that it may play a broader role in interacting with electron-rich centers. In fact, the crystal structure of a catalytically inactive mutant of COX-2 with bound arachidonic acid reveals the arachidonate bound in an inverted, noncanonical orientation in which its carboxylate group is hydrogen-bonded to both Tyr-385 and Ser-530 (38). This conformation is not a productive conformation for arachidonate oxygenation, but... 

Physiology is the study of function. The classical procedure used to define physiological roles is by extirpation, ablation or nerve section to reveal inadequate or inappropriate function in the absence of the postulated mechanism. This approach cannot be used to study the physiological role of arachidonate metabolites since they are not organ-localized like the adrenal steroids or concentrated in specific cells like the adrenergic transmitters. The problem is compounded also by the fact that arachidonate oxygenation is almost a universal phenomenon. Finally the metabolites are not stored like histamine or serotonin but are released immediately upon synthesis. Consequently it is always necessary to initiate synthesis to study release. Thus release is synonymous with synthesis. 

Kockmann, V, Spiehnann, D, Traitler, H and Lagarde, M (1989) Inhibitory effect of stearidonic acid (18 4n-3) on platelet aggregation and arachidonate oxygenation. Lipids, 24, 1004-1007. 

Studies of the biosynthesis of PGE2 from arachidonic acid have shown that all three oxygens come from O2 The enzyme involved prostaglandin endoperoxide syn tliase has cyclooxygenase (COX) activity and catalyzes the reaction of arachidonic acids with O2 to give an endoperoxide (PGG2)... 

Animal cells can modify arachidonic acid and other polyunsaturated fatty acids, in processes often involving cyclization and oxygenation, to produce so-called local hormones that (1) exert their effects at very low concentrations and (2) usually act near their sites of synthesis. These substances include the prostaglandins (PG) (Figure 25.27) as well as thromboxanes (Tx), leukotrienes, and other hydroxyeicosanoic acids. Thromboxanes, discovered in blood platelets (thrombocytes), are cyclic ethers (TxBg is actually a hemiacetal see Figure 25.27) with a hydroxyl group at C-15. 

Any of the collection of oxygenated metabolites of arachidonic acid that are the product of cyclooxygenase, cytochrome P450, or lipoxygenase pathways. 

Lipoxygenases catalyse the regio-specific and stereoselective oxygenation of unsaturated fatty acids. The mammalian enzymes have been detected in human platelets, lung, kidney, testes and white blood cells. The leukotrienes, derived from the enzymatic action of the enzyme on arachidonic acid, have effects on neutrophil migration and aggregation, release of lysosomal enzymes, capillary permeability, induction of pain and smooth muscle contraction (Salmon, 1986). 

Capdevila, J.A., Yadagiri, P., Manna, S. and Falck, J.R. (1986). Absolute configuration of the hydroxyeicosatetraenoic acids (HETEs) formed during catalytic oxygenation of arachidonic acid by microsomal cytochrome P-450. Biochem. Biophys. Res. Commun. 141, 1007-1011. 

Holtzman, M.J., Turk, J. and Pentland, A. (1989). A regiospecific mono-oxygenase with novel stereopreference is the major pathway for arachidonic acid oxygenation in isolated epidermal cells. J. Clin. Invest. 84, 1446-1453. 

Free radicals are by-products of prostaglandin metabolism and may even regulate the activity of the arachidonate pathway. Arachidonic acid, released from lipids as a result of activation of phospholipases by tissue injury or by hormones, may be metabolized by the prostaglandin or leu-kotriene pathways. The peroxidase-catalysed conversion of prostaglandin G2 to prostaglandin H2 (unstable prostanoids) and the mechanism of hydroperoxy fatty acid to the hydroxy fatty acid conversion both yield oxygen radicals, which can be detected by e.s.r. (Rice-Evans et al., 1991). 

Addition of RNA or DNA prior to oxidation of BP by PGH synthase results in substantial nucleic acid binding (17,21). Addition of RNA five minutes after initiation of oxidation leads to no covalent binding (17). This implies that the quinones do not bind to nucleic acid but rather a short-lived intermediate in their formation does. Arachidonic acid oxygenation in ram seminal vesicle microsomes is complete within two min, which suggests that the reactive intermediate is generated concurrently with PGH2 The structures of the nucleic acid adducts have not been elucidated so the identity of the reactive intermediate is unknown. 

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],... 

Schnurr et al. [22] showed that rabbit 15-LOX oxidized beef heart submitochondrial particles to form phospholipid-bound hydroperoxy- and keto-polyenoic fatty acids and induced the oxidative modification of membrane proteins. It was also found that the total oxygen uptake significantly exceeded the formation of oxygenated polyenoic acids supposedly due to the formation of hydroxyl radicals by the reaction of ubiquinone with lipid 15-LOX-derived hydroperoxides. However, it is impossible to agree with this proposal because it is known for a long time [23] that quinones cannot catalyze the formation of hydroxyl radicals by the Fenton reaction. Oxidation of intracellular unsaturated acids (for example, linoleic and arachidonic acids) by lipoxygenases can be suppressed by fatty acid binding proteins [24]. 

Mitochondria, nitric oxide synthase and arachidonic acid metabolism are sources of reactive oxygen species during ischemia—reperfusion injury 568... 

Nitric oxide and peroxynitrite contribute to oxidative damage 569 Production of eicosanoids from polyunsaturated fatty acids such as arachidonic acid may generate reactive oxygen species 570 Brain antioxidant defenses modify ischemia-reperfusion injury 570 Reactive oxygen species may modify both the excitotoxic and the apoptotic components of ischemic brain damage 570... 

Mitochondria, nitric oxide synthase and arachidonic acid metabolism are sources of reactive oxygen species during ischemia-reperfusion injury. ROS generation during ischemia-reperfusion may come from several sources, including NOS activity, mitochondrial electron transport, multiple steps in the metabolism of arachidonic... 

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