Free radicals damaging reactions

In 1998, Schlotte et al. [259] showed that uric acid inhibited LDL oxidation. However, subsequent studies showed that in the case of copper-initiated LDL oxidation uric acid behaves itself as prooxidant [260,261]. It has been suggested that in this case uric acid enhances LDL oxidation by the reduction of cupric into cuprous ions and that the prooxidant effect of uric acid may be prevented by ascorbate. On the other hand, urate radicals formed during the interaction of uric acid with peroxyl radicals are able to react with other compounds, for example, flavonoids [262], and by that participate in the propagation of free radical damaging reactions. In addition to the inhibition of oxygen radical-mediated processes, uric acid is an effective scavenger of peroxynitrite [263]. 

Ascorbate has multiple antioxidant capacities and may be the most important water-soluble defence against free-radical damage in human plasma. At millimolar concentrations, ascorbate scavenges O2, OH and HOCl (Blake et al., 1983). The latter reaction protects plasma lipids against degradation by activated PMNs. 

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) ... 

The -N02 radical (and also the C03 radical) are of some biological interest (Augusto et al. 2002) because they play some role in the reactions of peroxyni-trite (Chap. 2.4). For example, "N02 oxidizes tyrosine to nitrotyrosine (Prutz et al. 1985b), and the latter has been considered a promoter of free-radical damage in DNA model systems (Prutz 1986). In this context, it may be of interest that CCV reacts with a self-complementary ODN (k = 1.9 x 107 dm3 mol-1 s 1) exclusively at G (by ET) (Chap. 11.2). 

II) Mx+1 can destroy free radicals, being reduced to Mx in the process. The reduction of a redox metal by a free radical can be illustrated as follows Mx + 1 + A- — Mx + A+1, where A- is =Si and =SiCH2. As long as sufficient Mx+1 is available in the system in a well dispersed state, free radical chain reactions are quickly terminated by the destruction of these radicals thus, the rate of oxidative damage is... 

We often want to prevent or retard free-radical reactions. For example, oxygen in the air oxidizes and spoils foods, solvents, and other compounds mostly by free-radical chain reactions. Chemical intermediates may decompose or polymerize by free-radical chain reactions. Even the cells in living systems are damaged by radical reactions, which can lead to aging, cancerous mutations, or cell death. 

It is possible to generate ferryl species by peroxide treatment of ferrous iron ions [18,249], The two-electron oxidation of ferrous (as opposed to ferric) iron does not require the formation of a cation radical, although subsequent reactions may generate hydroxyl radicals. These reactions therefore provide an alternative mechanism to the Fenton reaction for free radical damage associated with low-molecular-weight iron species. In the absence of a protective protein environment, however, such low-molecular-weight ferryl species are unstable and difficult to detect and therefore their existence is controversial [see the review by Koppenol in this volume (Chapter 1)]. 

Koppenol WH (1994) Chemistry of iron and copper in radical reactions. In Rice-Evans CA, Burdon RH (eds) Free radical damage and its control. Elsevier, p 3... 

Chain reactions that form lipid free radicals and lipid peroxides in membranes make a major contribution to ROS-induced injury (Fig. 24.8). An initiator (such as a hydroxyl radical produced locally in the Fenton reaction) begins the chain reaction. It extracts a hydrogen atom, preferably from the double bond of a polyunsaturated fatty acid in a membrane lipid. The chain reaction is propagated when O2 adds to form lipid peroxyl radicals and lipid peroxides. Eventually lipid degradation occurs, forming such products as malondialdehyde (from fatty acids with three or more double bonds), and ethane and pentane (from the w-terminal carbons of 3 and 6 fatty acids, respectively). Malondialdehyde appears in the blood and urine and is used as an indicator of free radical damage. 

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