Fenton reactions Hydroxylation

Thus, antioxidant effects of nitrite in cured meats appear to be due to the formation of NO. Kanner et al. (1991) also demonstrated antioxidant effects of NO in systems where reactive hydroxyl radicals ( OH) are produced by the iron-catalyzed decomposition of hydrogen peroxide (Fenton reaction). Hydroxyl radical formation was measured as the rate of benzoate hydtoxylation to salicylic acid. Benzoate hydtoxylation catalyzed by cysteine-Fe +, ascorbate - EDTA-Fe, or Fe was significantly decreased by flushing of the reaction mixture with NO. They proposed that NO liganded to ferrous complexes reacted with H2O2 to form nitrous acid, hydroxyl ion, and ferric iron complexes, preventing generation of hydroxyl radicals. 

Floyd, R.A. Zs-Nagy (1984). Formation of long lived hydroxyl free radical adducts of proline and hydroxyproline in a Fenton reaction. Biochimica Bio-physica Acta, 790, 94-7. 

Humans Hydrogen peroxide has been used as an enema or as a cleaning agent for endoscopes and may cause mucosal damage when applied to the surface of the gut wall. Hydrogen peroxide enteritis can mimic an acute ulcerative, ischaemic or pseudomembranous colitis, and ranges from a reversible, clinically inapparent process to an acute, toxic fulminant colitis associated with perforation and death (Bilotta and Waye, 1989). It is conceivable that anecdotal reports of exacerbation of IBD by iron supplementation (Kawai et al. 1992) are mediated by hydroxyl radical production by the Fenton reaction. 

The antiulcer agent rebamipide ((2-(4-chlorobenzoy-lamino)-3-[2(lH)-quinolinon-4-yl]propionic acid) dose-dependently decreased hydroxyl radical signal generated by the Fenton reaction in an e.s.r. study. Rebamipide is active as a hydroxyl radical scavenger and inhibitor of superoxide production by neutrophils (Yoshikawa etal., 1993). 

In the presence of trace amounts of iron, superoxide can then reduce Fe3+ to molecular oxygen and Fe2+. The sum of this reaction (equation 2) plus the Fenton reaction (equation 1) produces molecular oxygen plus hydroxyl radical, plus hydroxyl anion from superoxide and hydrogen peroxide, in the presence of catalytic amounts of iron - the so-called Haber-Weiss reaction (equation 3) (Haber and Weiss, 1934). 

Ferrous iron, by its reaction with hydrogen peroxide in the Fenton reaction, can yield the toxic hydroxyl radical, OH, which will further potentiate oxygen toxicity. 

Reactions of Hydroxyl and Hydroxyl-Like Radicals Produced by the Fenton Reaction with Nucleic Acids... 

For a long time one question remained unanswered the efficiency of the Fenton reaction as the in vivo producer of hydroxyl radicals due to the low rate of Reaction (2) (the rate constant is equal to 42.11 mol 1 s 1 [18]). It is known that under in vitro conditions the rate of Fenton reaction can be sharply enhanced by chelators such as EDTA, but for a long time no effective in vivo chelators have been found. From this point of view new findings obtained by Chen and Schopfer [19] who found that peroxidases catalyze hydroxyl radical formation in plants deserve consideration. These authors showed that horseradish peroxidase (HRP) compound III is a catalyst of the Fenton reaction and that this compound is one to two orders of magnitude more active than Fe EDTA. 

The rate constant for Reaction (3) is in the range of 108 to 1091 mol-1s-1 [20]. Therefore, Reactions (3) and (4) may significantly enhance the concentration of ferrous ions and make Fenton reaction a better competitor with the peroxynitrite-inducible damage [21]. The formation of hydroxyl radicals in the reaction of superoxide with mitochondrial aconitase has... 

The formation of hydroxyl or hydroxyl-like radicals in the reaction of ferrous ions with hydrogen peroxide (the Fenton reaction) is usually considered as a main mechanism of free radical damage. However, Qian and Buettner [172] have recently proposed that at high [02]/ [H202] ratios the formation of reactive oxygen species such as perferryl ion at the oxidation of ferrous ions by dioxygen (Reaction 46) may compete with the Fenton reaction (2) ... 

Of course, superoxide may reduce ferric to ferrous ions and by this again catalyze hydroxyl radical formation. Thus, the oxidation of ferrous ions could be just a futile cycle, leading to the same Fenton reaction. However, the competition between the reduction of ferric ions by superoxide and the oxidation of ferrous ions by dioxygen depends on the one-electron reduction potential of the [Fe3+/Fe2+] pair, which varied from +0.6 to —0.4 V in biological systems [173] and which is difficult to predict.)... 

Iron-stimulated free radical-mediated processes are not limited to the promotion of peroxidative reactions. For example, Pratico et al. [188] demonstrated that erythrocytes are able to modulate platelet reactivity in response to collagen via the release of free iron, which supposedly catalyzes hydroxyl radical formation by the Fenton reaction. This process resulted in an irreversible blood aggregation and could be relevant to the stimulation by iron overload of atherosclerosis and coronary artery disease. 

MF effects on FA relatives and healthy donors. (Fanconi anemia is an autosomal recessive disease associated with the overproduction of free radicals, Chapter 31.) It has been shown earlier [215] that FA leukocytes produce the enhanced amount of hydroxyl or hydroxyl-like free radicals, which are probably formed by the Fenton reaction. It was suggested that MF would be able to accelerate hydroxyl radical production by FA leukocytes. Indeed, we found that MF significantly enhanced luminol-amplified CL produced by non-stimulated and PMA-stimulated FA leukocytes but did not affect at all oxygen radical production by leukocytes from FA relatives and healthy donors (Table 21.3). It is interesting that MF did not also affect the calcium ionophore A23187-stimulated CL by FA leukocytes, indicating the absence of the calcium-mediated mechanism of MF activity, at least for FA leukocytes. 

Iron complexes or microsomal nonheme iron are undoubtedly obligatory components in the microsomal oxidation of many organic compounds mediated by hydroxyl radicals. In 1980, Cohen and Cederbaum [27] suggested that rat liver microsomes oxidized ethanol, methional, 2-keto-4-thiomethylbutyric acid, and dimethylsulfoxide via hydrogen atom abstraction by hydroxyl radicals. Then, Ingelman-Sundberg and Ekstrom [28] assumed that the hydroxylation of aniline by reconstituted microsomal cytochrome P-450 system is mediated by hydroxyl radicals formed in the superoxide-driven Fenton reaction. Similar conclusion has been made for the explanation of inhibitory effects of pyrazole and 4-methylpyrazole on the microsomal oxidation of ethanol and DMSO [29],... 

Thus, superoxide itself is obviously too inert to be a direct initiator of lipid peroxidation. However, it may be converted into some reactive species in superoxide-dependent oxidative processes. It has been suggested that superoxide can initiate lipid peroxidation by reducing ferric into ferrous iron, which is able to catalyze the formation of free hydroxyl radicals via the Fenton reaction. The possibility of hydroxyl-initiated lipid peroxidation was considered in earlier studies. For example, Lai and Piette [8] identified hydroxyl radicals in NADPH-dependent microsomal lipid peroxidation by EPR spectroscopy using the spin-trapping agents DMPO and phenyl-tcrt-butylnitrone. They proposed that hydroxyl radicals are generated by the Fenton reaction between ferrous ions and hydrogen peroxide formed by the dismutation of superoxide. Later on, the formation of hydroxyl radicals was shown in the oxidation of NADPH catalyzed by microsomal NADPH-cytochrome P-450 reductase [9,10]. 

However, subsequent studies demonstrated that the formation of hydroxyl radicals, even if it takes place during lipid peroxidation, is of no real importance. Beloqui and Cederbaum [11] have found that although the glutathione-glutathione peroxidase system suppressed hydroxyl radical generation during the oxidation of 4-methylmercapto-2-oxo-butyrate, it exhibited a much smaller effect on microsomal lipid peroxidation. Therefore, hydroxyl radical formation is apparently unimportant in this process. Other authors also pointed out at an unimportant role of hydroxyl radicals in the initiation of microsomal lipid peroxidation [12 14], For example, it has been shown that Fe(EDTA), a most efficient catalyst of hydroxyl radical formation by the Fenton reaction, inhibited microsomal and liposomal lipid peroxidation, while the weak catalysts of this reaction Fe(ADP) and Fe(ATP) enhanced it [13]. 

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]. 

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