Radical peroxyl tyrosine

EPR spectroscopy has demonstrated that the oxidation of the globin occurs ultimately at a tyrosine residue, resulting in the formation of a tyrosine-phenoxyl radical this species is postulated to react subsequently with oxygen to give a tyrosine-peroxyl radical. Studies have shown that both of these species are accessible to components in bulk solution, i.e. they are located on the surface of the protein [47-49]. Ferryl haem protein radical from myoglobin and haemoglobin can react with membranes [33,45,50] and lipoproteins [51-... 

Jonsson M, Lind J, Reitberger T, Eriksen TE, Merenyi G (1993) Free radical combination reactions involving phenoxyl radicals. J Phys Chem 97 8229-8233 Kapoor SK, Gopinathan C (1992) Reactions of halogenated organic peroxyl radicals with various purine derivatives, tyrosine, and thymine a pulse radiolysis study. Int J Chem Kinet 24 1035-1042 Khaikin Gl, Neta P (1995) Formation and reactivity of vinylperoxyl radicals in aqueous solutions. J Phys Chem 99 4549-4553... 

Cyanidin inhibits epidermal growth factor receptor tyrosine kinase (EGF-RTK), a-glycosidase and COX-1 and COX-2. Delphinidin (3,5,7,3, 4, 5 -hexahydroxyflavylium) also inhibits EGF-RTK. Anthocyanidins and anthocyanins can be anti-inflammatory antioxidants by acting as free radical scavengers. Thus, nasunin (delphinidin-3-(j6-coumaroylrutinoside)-5-glucoside) scavenges OH (hydroxyl), 02 (superoxide) and lipid peroxyl radicals and inhibits lipid peroxidation. 

It has been proposed that the second oxidizing equivalent is accepted by tyrosine-103 on the surface of the protein (Ortiz de Montel-lano, 1983) forming a tyrosine phenoxyl radical which rapidly takes up oxygen forming the peroxyl species (Davies, 1990) as depicted in Fig. 4.4. 

Fig. 4.4. Scheme for postulated formation of the tyrosine phenoxyl and peroxyl radical... 

Fig. 7. Oxidation products of proteins. The vertical structure in the middle represents the main peptide chain with amino acid side groups extending horizontally (M2). The a-carbons in the primary chain can be oxidized to form hydroperoxides. Reactions on the right side near the top exemplify oxidation of the primary chain leading to a peroxyl radical. Side chains represented are lysine, methionine, tyrosine, cysteine, and histidine, top to bottom, respectively. Modifications of the side chains and primary chain lead to carbonyl formation and charge modifications. If these reactions are not detoxified by antioxidants, they may propagate chain reactions within the primary chain, leading to fragmentation of the protein. See the text for details, o, represents reaction with oxygen RNS, reactive nitrogen species ROS, reactive oxygen species. Dense dot represents unpaired electron of radical forms.

Lipid oxidation products can interact with proteins and amino acids, and can affect the flavor deterioration and nutritive value of food proteins. Peroxyl radicals are very reactive with labile amino acids (tryptophane, histidine, cysteine, cystine, methionine, lysine and tyrosine), undergoing decarboxylation, decarbonylation and deamination. Methionine is oxidized to a sulfoxide combined cysteine is converted to cystine to form combined thiosulfinate (Figure 11.4). Aldehydes, dialdehydes and epoxides derived from the decomposition of hydroperoxides react with amines to produce imino Schiff bases (R-CH=N-R ). Schiff bases polymerize by aldol condensation producing dimers... 

Aliphatic alkoxyl radicals have reduction potentials of about 1600 mV vs SHE at pH 7 making them better oxidising agents than alkyl peroxyl radicals (E 1000 mV SHE) [130], Phenoxyl radicals usually have even lower reduction potentials, e.g. phenoxyl radical (CsHsO ) with E7 900 mV vj SHE and tocopheroxyl radical with E 500 mV vj SHE [130], and these can also be produced in vivo via the oxidation of phenols, such as the amino acid tyrosine, flavenoids and other phenolic antioxidants (e.g. tocopherols), or via the reduction of quinones. 

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). 

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