Hydroxyl radical lysine

Figure 1.18 Reaction of proline, arginine, and lysine residues with hydroxyl radical results in oxidation of side-chain structures that form carbonyls. Both arginine and proline oxidation result in the same product.

Nonspecific cytochrome P450-mediated oxidation involves enzyme-catalyzed formation of reactive oxygen species (superoxide anions and hydroxyl radicals), which oxidize susceptible amino acids such as proline, arginine, lysine, and histidine. 

As already mentioned, one of the products of action of hydroxyl radicals on proteins is protein hydroperoxides (G6). Valine and lysine residues are particu-larily susceptible to hydroperoxide formation. Reduction of hydroperoxides produces respective hydroxy derivatives of amino acids. Three valine hydroxides derived from hydroperoxides of this amino acid have been characterized structurally as p-hydroxyvaline [(2S)-2-amino-3-hydroxy-3-methyl-butanoic acid], (2S,3S)-y-hydroxyvaline [(2S,3S)-2-amino-3-hydroxymethyl-butanoic acid], and (2S,3R)-y -hydroxyvaline [(2S,3R)-2-amino-3-hydroxymethyl-butanoic acid (Fig. 12). They are suggested to be possible markers of protein peroxidation (F21). 

Aldehyde-protein adducts and hydroxyl radicals also stimulate immunological responses directed against the specific modifications of proteins. High antibody titres have been observed from patients with severe alcoholic liver disease, particularly IgA and IgG autoantibodies. Such antibodies have considerable specificity towards aldehyde-lysine residues. Alcohol consumption markedly increases the hepatic output of very low-density lipoprotein (VLDL), but decreases the low-density lipoprotein (LDL) levels and apolipoprotein B. Ethylation of apo-B-lysine renders LDL immunogenic and accelerates its clearance. Alcoholics have been shown to develop acetaldehyde adducts in apo-B-containing lipoproteins, particularly VLDL. 

Previously, we have examined the formation of amino acid hydroperoxides following exposure to different radical species [100]. We observed that valine was most easily oxidised, but leucine and lysine are also prone to this modification in free solution. Scheme 12 illustrates the mechanism for formation of valine hydroperoxide. However, tertiary structure becomes an important predictor in proteins, where the hydrophobic residues are protected from bulk aqueous radicals, and lysine hydroperoxides are most readily oxidised. Hydroperoxide yield is poor from Fenton-derived oxidants as they are rapidly broken down in the presence of metal ions [101]. Like methionine sulphoxide, hydroperoxides are also subject to repair, in this case via glutathione peroxidase. They can also be effectively reduced to hydroxides, a reaction supported by the addition of hydroxyl radical in the presence of oxygen. Extensive characterisation of the three isomeric forms of valine and leucine hydroxides has been undertaken by Fu et al. [102,103], and therefore will not be discussed further here. 

Oxidation of amino acid residues - Conditions that generate oxygen radicals cause many proteins to undergo mixed-function oxidation of particular residues. Conditions require Fe + and hydroxyl radical, and the amino acids most susceptible to oxidation are lysine, arginine, and proline. E. coli and rat liver each contain a protease that cleaves oxidized glutamine synthetase in vitro, but does not attack the native enzyme. Presumably, other oxidized proteins are also targets for this enzyme. 

Lisinopril, a lysine analogue of enalaprilat, hardly scavenged the superoxide or the hydroxyl radicals in vitro, using an ESR method (Noda et al. 1997). 

Oxidative modification of collagen may be provoked by at least two mechanisms the external impact of different oxidizing substances (hydrogen peroxide or hydroxyl radical) [37] and in-vivo by inflammatory processes [38,39]. The oxidative reaction includes deamination of the amino groups in lysine or hydroxylyline, deguanidation in arginine, etc. The deamination leads to the formation of two aldehydes allysine and hydroxyallysine, which are further processed via two other independent aldehyde-specific pathways [40]. 

Interestingly, the nucleophilic addition of water in the sequence of events giving rise to 41 represents a relevant model system for investigating the mechanism of the generation of DNA-protein cross-links under radical-mediated oxidative conditions [80, 81]. Thus, it was shown that lysine tethered to dGuo via the 5 -hydroxyl group is able to participate in an intramolecular cyclization reaction with the purine base at C-8, subsequent to one electron oxidation [81]. 

Hi. Lysine. Gamma radiolysis of aerated aqueous solution of lysine (94) has been shown, as inferred from iodometric measurements, to give rise to hydroperoxides in a similar yield to that observed for valine and leucine. However, attempts to isolate by HPLC the peroxidic derivatives using the post-column derivatization chemiluminescence detection approach were unsuccessful. This was assumed to be due to the instability of the lysine hydroperoxides under the conditions of HPLC analysis. Indirect evidence for the OH-mediated formation of hydroperoxides was provided by the isolation of four hydroxylated derivatives of lysine as 9-fluoromethyl chloroformate (FMOC) derivatives . Interestingly, NaBILj reduction of the irradiated lysine solutions before FMOC derivatization is accompanied by a notable increase in the yields of hydroxylysine isomers. Among the latter oxidized compounds, 3-hydroxy lysine was characterized by extensive H NMR and ESI-MS measurements whereas one diastereomer of 4-hydroxylysine and the two isomeric forms of 5-hydroxylysine were identified by comparison of their HPLC features as FMOC derivatives with those of authentic samples prepared by chemical synthesis. A reasonable mechanism for the formation of the four different hydroxylysines and, therefore, of related hydroperoxides 98-100, involves initial OH-mediated hydrogen abstraction followed by O2 addition to the carbon-centered radicals 95-97 thus formed and subsequent reduction of the resulting peroxyl radicals (equation 55). 

In trifunctional amino acids, the R radical is connected to an acid or basic radical. Aspartic acid and glutamic acid are acid amino acids, while lysine, histidine, ornithine, citrulline and arginine are basic amino acids. Other trifunctional amino acids have no marked acid or basic character. They have hydroxylated (serine, threonine and tyrosine), thiol or sulfide radicals (cysteine and methionine). 

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