Protein oxidation secondary radical reactions

Hydroperoxides of linoleic and Hnolenic acids also react with sensitive food components, such as carotenes, to form colourless products (lipoxygenase, however, do not directly catalyse the decomposition of carotenes) and induce oxidative changes in sulfur amino acids and proteins. Secondary products of lipid oxidation initiate radical reactions of proteins that lead to a reduction in their nutritional value. 

Among protein oxidation products, formation of protein peroxides should be mentioned. Protein peroxidation occurs as a reaction secondary to free-radical attack on amino acid side groups, effecting a carbon-centered free radical of amino acid formation. Such a radical reacts with the oxygen molecule and produces a hydroperoxide radical ... 

Radicals generated during peroxidation of lipids and proteins show reactivity similar to that of the hydroxyl radical however, their oxidative potentials are lower. It is assumed that the reactive alkoxyl radicals rather than the peroxyl radicals play a part in protein fragmentation secondary to lipid peroxidation process, or protein exposure to organic hydroperoxides (DIO). Reaction of lipid radicals produces protein-lipid covalent bonds and dityrosyl cross-links. Such cross-links were, for example, found in dimerization of Ca2+-ATPase from skeletal muscle sarcoplasmic reticulum. The reaction was carried out in vitro by treatment of sarcoplasmic reticulum membranes with an azo-initiator, 2,2/-azobis(2-amidinopropane) dihydrochloride (AAPH), which generated peroxyl and alkoxyl radicals (V9). 

It is necessary that an antioxidant protects cells at all stages of oxidative stress, and therefore an antioxidant should be able to scavenge the secondary radicals produced by the reaction of primary radicals with biomolecules. Radiation chemists designed methods to study reactions of secondary radicals from amino acids of proteins and base and sugar radicals of DNA with antioxidants.The most commonly employed aromatic amino acid radicals generated by radiation chemical experiments are the indolyl radicals of tryptophan (TRP ), the... 

Protein oxidation is certainly one of the most predominant protein modifications in neurodegeneration. On the other hand, a possible way by which a protein aggregate could lead to neurotoxicity would be via mediation of radical formation due to metabolic impairment of affected cells. This process again leads to secondary protein oxidation and aggregate modification. One of the best-studied proteins is the amyloid peptide, which seems capable of inducing free radical production in Alzheimer s disease [134]. Amyloid peptide is able to bind metals and these are able to produce radicals through the Fenton reaction [135,136]. 

Proteins with a metal ion that can be reduced or oxidized provide additional pathways for reaction of both primary and secondary radicals. The fixed location of the iron within the protein geometry strongly limits its accessibility to primary radicals, small secondary radicals, and radical sites on the host protein. These considerations are especially relevant to myoglobin, the globular protein responsible for meat pigmentation. Several distinctive reactions involving myoglobin have been studied in model and meat systems [18, 19]. 

The health impairing and toxic elfects of oxidation of lipids are due to loss of vitamins, polyenoic fatty acids, and other nutritionally essential components formation of radicals, hydroperoxides, aldehydes, epoxides, dimers, and polymers and participation of the secondary products in initiation of oxidation of proteins and in the Maillard reaction. Dilferent oxysterols have been shown in vitro and in vivo to have atherogenic, mutagenic, carcinogenic, angiotoxic, and cytotoxic properties, as well as the ability to inhibit cholesterol synthesis (Tai et ah, 1999 Wpsowicz, 2002). 

The major cause of deterioration of food products is lipid oxidation, from which low-molecular-weight, off-flavor compounds are formed. This deterioration is often caused by the oxidation of the unsaturated lipids present in foods. Off-flavor compounds are created when the hydroperoxides, formed during the initial oxidation, are degraded into secondary reaction compounds. Free radicals are also formed which can participate in reactions with secondary products and with proteins. Interactions with the latter can result in carbonyl amino... 

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