Antioxidants Fenton reaction

Polyakov, N. E., T. V. Leshina et al. (2001c). Carotenoids as scavengers of free radicals in a Fenton reaction, antioxidants or pro-oxidants. Free Rad. Biol. Med. 31 398-404. 

As mentioned above, in contrast to classic antioxidant vitamins E and C, flavonoids are able to inhibit free radical formation as free radical scavengers and the chelators of transition metals. As far as chelators are concerned their inhibitory activity is a consequence of the formation of transition metal complexes incapable of catalyzing the formation of hydroxyl radicals by the Fenton reaction. In addition, as shown below, some of these complexes, for example, iron- and copper-rutin complexes, may acquire additional antioxidant activity. 

Chelators of transition metals, mainly iron and copper, are usually considered as antioxidants because of their ability to inhibit free radical-mediated damaging processes. Actually, the so-called chelating therapy has been in the use probably even earlier than antioxidant therapy because it is an obvious pathway to treat the development of pathologies depending on metal overload (such as calcium overload in atherosclerosis or iron overload in thalassemia) with compounds capable of removing metals from an organism. Understanding of chelators as antioxidants came later when much attention was drawn to the possibility of in vivo hydroxyl radical formation via the Fenton reaction ... 

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. 

Summary of Densitometer Scanning Results of DNA Damage When Subjected to the Fenton Reaction in the Presence of Various Antioxidants (at a concentration of 50 pM each)... 

Flavonoids have the ability to act as antioxidants by a free radical scavenging mechanism with the formation of less reactive flavonoid phenoxyl radicals [Eq. (1) and (2)]. On the other hand, through then-known chelating ability these compounds may inactivate transition metals ions (iron, copper), thereby suppressing the superoxide-driven Fenton Reaction, Eqs. (3) and (4), which is currently believed to be the most important route to activate oxygen species [51]. 

In terms of potential chemical interactions, the effects of NO on ROS-induced injury are multiple, and some effects can be classified as prooxidant and others may be classified as antioxidant still others can be classified as both. In terms of the metal-catalyzed Haber-Weiss reaction, there are two primary effects of -NO. The binding of NO to metal ions will prevent the Fenton reaction and thus results in an antioxidant action. Another important antioxidant action of NO (and its oxidized product N02) is its reaction with hpid radicals, thus resulting in radical chain termination. ... 

Halliwell et al. (55) have described a model that uses hydroxyl radicals generated from Fenton reaction to degrade 2-deoxy-D-ribose. The decomposed products of deoxyribose are 2-thiobarbituric acid-reactive substances (TEARS). If the antioxidant present in the system scavenges hydroxyl radicals generated, deoxyribose is protected and the amount of TEARS produced is less. 

NO was shown by Kanner et al. [120] to inhibit iron-catalysed oxidation reactions by binding to ferrous complexes. It was also shown that NO inhibited the superoxide driven Fenton reaction which, in the presence of iron, generates hydroxyl radical (OH) in vitro. By adding varying amounts of NO to a Fenton reaction process the hydroxylation of benzoic acid was reduced. This demonstrates that depending on the fluxes of the different reactive species, NO may have an antioxidant capability. 

The use of natural antioxidants in supplements can also exhibit pro-oxidant activity under certain conditions such as the concentration and nature of the polyphenolic compounds causing oxidative damage to important cellular components (48). Aqueous extracts and erode polyphenolic fractions of both traditional and green rooibos were evaluated for possible pro-oxidant activity using a Fenton reaction model system containing FeCb-EDTA and H2O2 for the generation of hydroxyl radicals. Pro-oxidant activity was shown for pure aspalathin while the dihydrochalcone and flavonoid contents of the enriched... 

Hydroxyl radical HO was generated by Fenton Reaction. In this case, 500 pL of DIW, 100 pL of 10-fold concentrated PBS, 100 pL of 0.5 pM MCLA solution, 100 pL of antioxidant solution, 100 pL of 50 pM H2O2 solution were mixed in a glass test tube and 100 pL of 5 pM Fe(NH4)2(S04)2 solution was injected into the test tube about 30 s after the start of CL measurement in the same way as above described. 

Recently, research on the biomedical activity of pu-erh tea has focused on its antibacterial, antioxidative, lipid-lowering, and antiobesity effects. Many in vitro studies have shown that pn-erh tea has antioxidative activity. Lin et al. reported that the water extract of pn-erh tea (100 pg/ml) can protect the plasmid DNA from strand breakage indnced by the Fenton reaction as well as the control, regardless of total catechin content. Dnh et al reported that pn-erh tea water extract with less cat-echins (8.01 mg/ml), compared to green tea water extract with more catechins (79.1 mg/ml), could still chelate metal ions, scavenge DPPH radicals, and decrease nitric oxide production in lipopolysaccharide-activated RAW 264.7 macrophages. 

The deoxyribose assay is a simple test tube assay to determine the antioxidant or pro-oxidant properties of test compounds against carbohydrates [151-152]. In the deoxyribose assay, OH are produced from the reaction of H2O2 with Fe2+ (Fenton reaction), where the latter is formed by the ability of ascorbic acid to reduce ferric iron (Fe3+) to ferrous iron (Fe2+) (Eq. 27). 

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