The ROS mediated attack of different amino acids results in the formation of different oxidation products such as, tryptophan forms nitrotryptophan, kynurenine, formylkynurinine; Phenylalanine forms 2,3-Dihydroxyphenylalanine, 2-, 3-, and 4-hydroxyphenylalanine; Tyrosine forms 3,4-Dihydroxyphenylalanine, tyrosine-tyrosine cross-linkages, Tyr-O-Tyr, cross-linked nitrotyrosine; Histidine forms 2-Oxohistidine, asparagine, aspartic acid; Arginine forms glutamic semialdehyde; Lysine forms a-Aminoadipic semialdehyde; Proline forms 2-Pyrrolidone, 4- and 5-hydroxyproline pyroglutamic acid, glutamic semialdehyde; threonine forms 2-Amino-3-ketobutyric acid; leucine and valine residues form hydroxyl residues [91]. The ROS induced oxidative damage of amino acid residues such as lysine, proline, threonine and arginine yields carbonyl derivatives. The presence of carbonyl groups in proteins has been considered as the marker of ROS mediated protein oxidation [95]. The other specific markers of protein oxidation are O-tyrosine (a marker for hydroxyl radical) and 3-nitrotyrosine (a marker for RNS). An increase in the levels of protein carbonyls is observed in a number of pathological conditions such as, Alzheimer's disease [96], parkinson's disease [97], muscular dystrophy [98], cataractogenesis [99], Rheumatoid Arthritis [100], diabetes [101], progeria, atherosclerosis, respiratory dystrous syndrome, Werner's syndrome [91], and ageing [96, 102].
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