Arachidonic acid hydroxylation

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. 

CYP8A1 is the complementary enzyme to CYP5 in that it synthesizes prostacyclin in the arachidonic acid cascade. CYP8B1 catalyzes the steroid 12-alpha hydroxylation in the cholic acid biosynthesis. 

Rodenas et al. [77] studied PMN-stimulated lipid peroxidation of arachidonic acid. As MDA formation was inhibited both with L-arginine (supposedly due to the formation of excess NO) and DTPA (an iron ion chelator), it was concluded that about 40% of peroxidation was initiated by hydroxyl radicals formed via the Fenton reaction and about 60% was mediated by peroxynitrite. However, it should be noted that the probability of hydroxyl radical-initiated lipid peroxidation is very small (see above). Phagocyte-mediated LDL oxidation is considered below. 

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]. 

It has also been shown that the CYP4 family contains a number of natural substrates appear to be arachidonic acid, the prostaglandins, and/or the leukotrienes. For example, CYP4F2 and CYP4F3, isolated from human liver and human leukocytes, respectively, are leukotriene B4 -hydroxylation of arachidonic acid to form 20-hydroxy-5, 8, 11, 14-eicosatetraenoic... 

Kinins, neuropeptides, and histamine are also released at the site of tissue injury, as are complement components, cytokines, and other products of leukocytes and platelets. Stimulation of the neutrophil membranes produces oxygen-derived free radicals. Superoxide anion is formed by the reduction of molecular oxygen, which may stimulate the production of other reactive molecules such as hydrogen peroxide and hydroxyl radicals. The interaction of these substances with arachidonic acid results in the generation of chemotactic substances, thus perpetuating the inflammatory process. 

Prostaglandins are mediators of the inflammatory response and are produced by the action of two activities of the enzyme prostaglandin synthase. The first activity is a cyclooxygenase activity, which adds two oxygen molecules the arachidonic acid. Secondly, the peroxide group from the first step is reduced to a hydroxyl group. 

Lipoxygenase pathway Alternatively, several lipoxygenases can act on arachidonic acid to form 5-HPETE, 12-HPETE and 15-HPETE, which are unstable peroxidated derivatives that are converted to the corresponding hydroxylated derivatives (the HETES), or to leukotrienes or lipoxins, depending on the tissue (Figure 39.3).2... 

Aldehydes The oxidative breakdown of polyunsaturated fatty acids generates a number of aldehydes, which are highly bioactive and well characterized molecules. Aldehydes are longer-lived than free radicals, and are able to attack targets extra or intracellularly (for review see Esterbauer et al., 1991). Among the different aldehydes formed during lipid and LDL oxidation, malondialdehyde and 4-hydroxyl alkenals, in particular 4-hydroxynonenal (4-HNE), have been intensively studied. 4-HNE results from the oxidation of arachidonic acid and, to a lesser extent, linoleic acid (Esterbauer et al., 1991). 4-HNE and related hydroxyalkenals react rapidly... 

The products obtained from the co-6 fatty acids (linoleic acid, y-linolenic acid, and arachidonic acid) by in vivo reactions with strain ALA2 contain diepoxy bicyclic structures, tetrahydrofuranyl rings, and/or trihydroxy groups in their molecules. In contrast to these co-6 PUFAs, substrates classified as co-3 PUFAs (a-linolenic acid, EPA, and DHA) are only converted to hydroxyl THFAs by strain ALA2 with no diepoxy bicyclic or trihydroxy derivatives uncovered to date. Both the hydroxyl groups and cyclic structures derived there from appear to be placed at the same positions on the substrates from the co-carbon termini within each PUFA class, despite differences in carbon chain length and the number of double bonds in the specific PUFA substrates. 

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