Amino acids free radical formation

Whenever Compound I is present, the enzyme undergoes slow inactivation by a gradual one-electron reduction of Compound I to Compound II (formal oxidation state of Fe is -i-4) by external electron donors or, in their absence, even by elements of the enzyme molecule itself in this case an amino acid free radical is formed on the tyrosine residue distant from the active site (Tyr 370 in the human enzyme). Compound II is an inactive form of CAT and cannot be recycled by hydrogen peroxide. NADPH prevents formation of Compound II by serving as an electron donor to reduce Compound I. When NADPH is unavailable, the one-electron reduction by an external donor or of the porphyrin TT-cation by an electron from Tyr 370 and formation of Compound II appears to be the next alternative [ 199]. 

The brown pigments which were formed through the oxidative pathway are almost twice those formed through the anaerobic pathway. This effect could be derived from the involvement of H2O2 and oxy--radicals in the aerobic oxidation pathway of ascorbic acid (20,21). The oxy-radicals may generate carbonyls and amino acid free radicals which, by polymerization, could accelerate the formation of more brown pigments. 

Activation of glutamate receptors (in particular, the NMDA type) leads to an intracellular accumulation of Ca2+, which initiates a cascade of alterations resulting in the formation of free oxygen radicals. Glutamate perfusion of the striatum enhances dopamine release, a fact that also might contribute to increases in the formation of hydroxyl radicals. Alternatively, free-radical formation can induce a release of excitatory amino acids, as demonstrated in hippocampal slices (Figure 13.4). 

Fig. 1. a-Oxidation of amino acids. Hydroxyl radical (or other reactive radical) abstracts hydrogen atom from the a-carbon. The C-centered free radical formed may react with other amino acid residues or dimerize in the absence of oxygen, which leads to protein aggregation. In die presence of oxygen the carbon-centered radical forms peroxyl radical. Reduction of peroxyl radical leads to protein hydroperoxide. Decomposition of hydroperoxide leads to formation of carbonyl compounds via either oxidative deamination or oxidative decarboxylation. Oxidation of the new carbonyl group forms a carboxyl group. 

Studies on the non-enzymatic browning of carbohydrates and amino acids evidenced that, besides ionic condensation reactions (3,4), also mechanisms involving free radical formation produce browning compounds in the Maillard reaction (5-7). Because radicals have been detected in model melanoidins (8,9), similar reactions might be involved in the formation of melanoidins during thermal food processing. 

Table II. Influence of Amino Acid Side Chains on Free Radical Formation When Reacted with Glucose...

The use of free-radical reactions for this mode of ring formation has received rather more attention. The preparation of benzo[Z)]thiophenes by pyrolysis of styryl sulfoxides or styryl sulfides undoubtedly proceeds via formation of styrylthiyl radicals and their subsequent intramolecular substitution (Scheme 18a) (75CC704). An analogous example involving an amino radical is provided by the conversion of iV-chloro-iV-methylphenylethylamine to iV-methylindoline on treatment with iron(II) sulfate in concentrated sulfuric acid (Scheme 18b)(66TL2531). 

One type of fatty liver that has been smdied extensively in rats is due to a deficiency of choline, which has therefore been called a lipotropic factor. The antibiotic puromycin, ethionine (a-amino-y-mercaptobu-tyric acid), carbon tetrachloride, chloroform, phosphorus, lead, and arsenic all cause fatty liver and a marked reduction in concentration of VLDL in rats. Choline will not protect the organism against these agents but appears to aid in recovery. The action of carbon tetrachloride probably involves formation of free radicals... 

One of numerous examples of LOX-catalyzed cooxidation reactions is the oxidation and demethylation of amino derivatives of aromatic compounds. Oxidation of such compounds as 4-aminobiphenyl, a component of tobacco smoke, phenothiazine tranquillizers, and others is supposed to be the origin of their damaging effects including reproductive toxicity. Thus, LOX-catalyzed cooxidation of phenothiazine derivatives with hydrogen peroxide resulted in the formation of cation radicals [40]. Soybean LOX and human term placenta LOX catalyzed the free radical-mediated cooxidation of 4-aminobiphenyl to toxic intermediates [41]. It has been suggested that demethylation of aminopyrine by soybean LOX is mediated by the cation radicals and neutral radicals [42]. Similarly, soybean and human term placenta LOXs catalyzed N-demethylation of phenothiazines [43] and derivatives of A,A-dimethylaniline [44] and the formation of glutathione conjugate from ethacrynic acid and p-aminophenol [45,46],... 

In earlier studies the in vitro transition metal-catalyzed oxidation of proteins and the interaction of proteins with free radicals have been studied. In 1983, Levine [1] showed that the oxidative inactivation of enzymes and the oxidative modification of proteins resulted in the formation of protein carbonyl derivatives. These derivatives easily react with dinitrophenyl-hydrazine (DNPH) to form protein hydrazones, which were used for the detection of protein carbonyl content. Using this method and spin-trapping with PBN, it has been demonstrated [2,3] that protein oxidation and inactivation of glutamine synthetase (a key enzyme in the regulation of amino acid metabolism and the brain L-glutamate and y-aminobutyric acid levels) were sharply enhanced during ischemia- and reperfusion-induced injury in gerbil brain. 

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