Radical peroxyl hydroxyl

Antioxidant activity of flavonoids has already been shown about 40 years ago [90,91]. (Early data on antioxidant flavonoid activity are cited in Ref. [92].) Flavonoids are polyphenols, and therefore, their antioxidant activity depends on the reactivity of hydroxyl substituents in hydrogen atom abstraction reactions. As in the case of vitamins E and C, the most studied (and most important) reactions are the reactions with peroxyl radicals [14], hydroxyl radicals [15], and superoxide [16]. 

Similar to some other antioxidants, pyrrolopyrimidines do not contain active free radical scavenging groups such as phenolic or thiolic substituents. At present, at least two different mechanisms of their antioxidant activity have been proposed [307], It was suggested that pyrrolopyrimidines, which are electron donating compounds, can be oxidized by hydroxyl or peroxyl radicals or hydroxylated by cytochrome P-450 forming phenolic metabolite... 

Polyphenols can act as antioxidants by a number of potential pathways. The most important is likely to be by free radical scavenging, in which the polyphenol can break the radical chain reaction. Polyphenols are effective antioxidants in a wide range of chemical oxidation systems, being capable of scavenging peroxyl radicals, alkyl peroxyl radicals, superoxide, hydroxyl radicals, nitric oxide and peroxynitrate in aqueous and organic environments [121]. This activity is due to the ability of donating an H atom from an aromatic hydroxyl group to a free radical, and the major ability of an aromatic structure to support an unpaired electron by delocalization around the 7i-electron system. Phenolic acids... 

R8. Regoli, F., and Winston, G. W., Quantification of total oxidant scavenging capacity of antioxidants for peroxynitrite, peroxyl radicals, and hydroxyl radicals. Toxicol. Appl. Pharmacol. 156, 96-105 (1999). 

Hydroxyl radical may hydroxylate tyrosine to 3,4-dihydroxyphenylalanine (DOPA). DOPAs are the main residues corresponding to protein-bound reducing moieties able to reduce cytochrome c, metal ions, nitro tetrazolium, blue and other substrates (S32). Reduction of metal ions and metalloproteins by protein-bound DOPA may propagate radical reactions by redox cycling of iron and copper ions which may participate in the Fenton reaction (G9). Abstraction of electron (by OH or peroxyl or alkoxyl radicals) leads to the formation of the tyrosyl radical, which is relatively stable due to the resonance effect (interconversion among several equivalent resonant structures). Reaction between two protein-bound tyrosyl radicals may lead to formation of a bityrosine residue which can cross-link proteins. The tyrosyl radical may also react with superoxide, forming tyrosine peroxide (W13) (see sect. 2.6). 

Lipid peroxidation may beinitiated by any primary free radical which has sufficient reactivity to extract a hydrogen atom (Fig. 2.10) from a reactive methylene group of an unsaturated fatty acid. For example, species such as hydroxyl radicals OH, alkoxyl radicals RO peroxyl radicals ROO and alkyl radicals R may be involved. The formation of the initiating species is accompanied by bond rearrangement that results in stabilization by diene conjugate formation. The lipid radical then takes up oxygen to form the peroxyl radical. Peroxyl radicals can... 

Uric acid is a water-soluble antioxidant found in relatively high concentrations in plasma. It inhibits lipid peroxidation by tightly binding iron and copper ions into inactive forms, and by scavenging various oxidants such as hydroxyl radicals, peroxyl radicals, singlet oxygen and hypochlorous acid. By complexing with iron, uric acid stabilizes ascorbic acid in human serum. 

Xanthohumol was able to scavenge a variety of physiological relevant radicals including peroxyl, hydroxyl, and superoxide anion radials more effectively than the known antioxidant Trolox. It was found to inhibit both the constitutive form of cyclooxygenase-1 and, more importantly, the inducible cyclooxygenase-2, which is linked to carcinogenesis. 

Several powerful oxidants are produced during the course of metabolism, in both blood cells and most other cells of the body. These include superoxide (02 ), hydrogen peroxide (H2O2), peroxyl radicals (ROO ), and hydroxyl radicals (OH ). The last is a particularly reactive molecule and can react with proteins, nucleic acids, lipids, and other molecules to alter their structure and produce tissue damage. The reactions listed in Table 52-4 play an important role in forming these oxidants and in disposing of them each of these reactions will now be considered in turn. 

As strong antioxidants and scavengers of superoxide, hydroxyl and peroxyl radicals, tea flavonoids can suppress radical chain reactions and terminate lipid peroxidation (Kumamoto and Sonda, 1998, Yang and Wang, 1993). 

The antioxidant property of ferulic acid and related compounds from rice bran was reported by Kikuzaki et al, (2002). Their results indicated that these compounds elicit their antioxidant function through radical scavenging activity and their affinity with lipid substrates. Another recent study reported by Butterfield et al, (2002) demonstrated that ferulic acid offers antioxidant protection against hydroxyl and peroxyl radical oxidation in synaptosomal and neuronal cell culture systems in vitro. The effect of ferulic acid on blood pressure (BP) was investigated in spontaneously hypertensive rats (SHR). After oral administration of ferulic acid the systolic blood pressure (SBP) decreased in a dose-dependent manner. There was a significant correlation between plasma ferulic acid and changes in the SBP of the tail artery, suggesting... 

BUTTERFIELD D A, MARINA A, JAROSLAW K, ANTONIO s (2002) FeruUc add antioxidant protection against hydroxyl and peroxyl radical oxidation in synaptosomal and neuronal cell culture systems in vitro Structure activity studies. JNutri Biochem, 13(5) 273-81. 

Ascorbate is known to act as a water-soluble antioxidant, reacting rapidly with superoxide, hydroxyl and peroxyl radicals. However, reduced ascorbate can react non-enzymatically with molecular oxygen to produce dehydroascorbate and hydrogen peroxide. Also, ascorbate in the presence of light, hydrogen peroxide and riboflavin, or transition metals (e.g. Fe, Cu " ), can give rise to hydroxyl radicals (Delaye and Tardieu, 1983 Ueno et al., 1987). These phenomena may also be important in oxidative damage to the lens and subsequent cataract formation. 

In Scheme 7, the peroxidic 0-0 bond of the hydroperoxyl group is broken together with /1-scission of the formed alkoxyl radical, and, further, ring closure of alkyl peroxyl diradical may occur. The process generates a hydroxyl radical, methylcarbonyl terminal groups (-CH2-CO-CH3) and dioxetane. The latter is unstable and decomposes into an excited triplet state of formaldehyde and/or excited triplet state of methylcarbonyls (Scheme 8). 

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