Protein-peroxy radical

In lipid peroxidation, unsaturated fatty acids are peroxidized in the biological membranes of cells and their organelles. This leads to chain breaks in fatty acids with insertion of hydrophilic groups and cis-trans-iso-merization. Membrane-bound proteins are damaged. Lipid radicals (L ) are transformed into unstable lipid-peroxy radicals (LOO ). Lipid peroxidation markers include malondialdehyde and 4-hydroxynonenal. (18, 38, 79, 97) (s. figs. 2.19 21.12)... 

OH free radicals react with almost all amino-acids. For aliphatic residues, rate constants are correlated with the strength of the X-H bond(X = S, C or N) (1). Thus the reaction is relatively slow with glycine (k = 1.7 x 10 mol 1 s ) and fast with the -SH function of cysteine (k = 1.9 x 10 mol M s i). The thiyl radical formed upon oxidation of cysteine, whose spectral properties are in table 3, is formed but a carbon-centered radical is also present (50, 51). In the presence of oxygen, thiyl radical fixes O2 giving peroxy radicals (52). These radicals may photoisomerize into sulfonyl radicals RS02 (53). In small molecules, disulfide groups can also be oxidized. This reaction was not demonstrated in proteins, but cannot be neglected. A disulfide radical cation is formed (54). Final compounds are not known. 

In investigations of the antioxidant activity of phycocyanin with or without in vitro digestion, peroxy radical scavenging capacity of phycocyanobilin was 2.7-fold greater than that of the native protein (unpublished data). This result coincided with observations of increased peroxy radical scavenging of free phycocyanobilin or trypsin-digested phycocyanin, relative to native phycocyanin [49,50]. 

Thus, radicals are the most reactive and destructive of protein structure, followed by peroxy derivatives, singlet oxygen, and other oxygen compounds. The oxidative reactivity of some of these oxygen species is so high that just contact of the pure compound with paper or cotton fabrics can cause combustion (e.g., superoxide). 

In the field of enzyme catalysis, heme-proteins such as cytochrome P450, for example, exhibit both types of 0-0 bond cleavages in organic hydroperoxides and peroxy acids (178). Heterolytic cleavage of HOOH/ROOH yields H20 or the corresponding alcohol, ROH and a ferryl-oxo intermediate (Scheme 4). Homolytic 0-0 bond cleavage results in the formation of a hydroxyl (HO ) or an alkoxyl (RO ) radical and an iron-bound hydroxyl radical. 

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