Reduction of protein radicals by GSH and ascorbate: potential biological significance.
ABSTRACT The oxidation of proteins and other macromolecules by radical species under conditions of oxidative stress can be modulated by antioxidant compounds. Decreased levels of the antioxidants glutathione and ascorbate have been documented in oxidative stress-related diseases. A radical generated on the surface of a protein can: (1) be immediately and fully repaired by direct reaction with an antioxidant; (2) react with dioxygen to form the corresponding peroxyl radical; or (3) undergo intramolecular long range electron transfer to relocate the free electron to another amino acid residue. In pulse radiolysis studies, in vitro production of the initial radical on a protein is conveniently made at a tryptophan residue, and electron transfer often leads ultimately to residence of the unpaired electron on a tyrosine residue. We review here the kinetics data for reactions of the antioxidants glutathione, selenocysteine, and ascorbate with tryptophanyl and tyrosyl radicals as free amino acids in model compounds and proteins. Glutathione repairs a tryptophanyl radical in lysozyme with a rate constant of (1.05±0.05)×10(5) M(-1) s(-1), while ascorbate repairs tryptophanyl and tyrosyl radicals ca. 3 orders of magnitude faster. The in vitro reaction of glutathione with these radicals is too slow to prevent formation of peroxyl radicals, which become reduced by glutathione to hydroperoxides; the resulting glutathione thiyl radical is capable of further radical generation by hydrogen abstraction. Although physiologically not significant, selenoglutathione reduces tyrosyl radicals as fast as ascorbate. The reaction of protein radicals formed on insulin, β-lactoglobulin, pepsin, chymotrypsin and bovine serum albumin with ascorbate is relatively rapid, competes with the reaction with dioxygen, and the relatively innocuous ascorbyl radical is formed. On the basis of these kinetics data, we suggest that reductive repair of protein radicals may contribute to the well-documented depletion of ascorbate in living organisms subjected to oxidative stress.
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ABSTRACT: Low antioxidant levels and oxidative stress due to airway inflammation may be important determinants of asthma severity. The objective of the present study was to determine whether lower antioxidant intake and plasma antioxidant concentrations are associated with more severe asthma. Dietary antioxidant intakes and asthma severity were assessed using questionnaires, and plasma concentrations of ascorbic acid, vitamin E, carotenoids, bilirubin, albumin, uric acid and total antioxidant status were measured in 53 mild-to-moderate and 28 severe asthmatic patients and in 43 nonasthmatic subjects. Vitamin C and carotene intakes were lower in males than females and were particularly low in males with severe asthma. Plasma ascorbic acid was lower in severe (31.9+/-3.6 microM) compared with mild-to-moderate asthmatic (52.3+/-2.6) or control subjects (52.7+/-2.9). Low plasma ascorbic acid (odds ratio (OR) 0.93; 95% confidence interval (CI) 0.9-0.97), bilirubin (OR 0.69; 95% CI 0.51-0.93) and increased plasma cholesterol (OR 1.98; 95% CI 1.05-3.73) were independently associated with severe asthma. Albumin was positively and cholesterol negatively correlated with lung function. Low plasma concentrations of specific antioxidants are associated with more severe asthma. Increased antioxidant intake may help reduce the burden of severe asthma, particularly in males.European Respiratory Journal 09/2005; 26(2):257-64. · 6.36 Impact Factor
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ABSTRACT: The rate constant for the reduction of the tyrosyl radical with selenocysteine has been measured to investigate whether selenocysteine is capable of repair of protein radicals. Tyrosyl radicals, both free in solution and in insulin, were generated by means of pulse radiolysis and laser flash photolysis in aqueous solution. The rate constant for the reaction of free N-acetyl-tyrosyl-amine radicals with selenocysteine is (8 +/- 2) x 10 (8) M (-1) s (-1), and that for tyrosyl radicals in insulin is (1.6 +/- 0.4) x 10 (8) M (-1) s (-1). The rate constant for the reaction of selenoglutathione with the N-acetyl-tyrosyl-amine radical is (5 +/- 2) x 10 (8) M (-1) s (-1). In contrast, cysteine and glutathione react more slowly than their selenium analogues with the tyrosyl radical: the reactions of N-acetyl-tyrosyl-amine radicals with cysteine and glutathione are 3 and 5 orders of magnitude slower, respectively, than those with selenocysteine and selenoglutathione, while those of tyrosyl radicals in insulin are 3 and 2 orders of magnitude slower, respectively.Biochemistry 10/2008; 47(36):9602-7. · 3.38 Impact Factor
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ABSTRACT: Among age-related neurodegenerative diseases, Parkinson's disease (PD) represents the best example for which oxidative stress has been strongly implicated. The etiology of PD remains unknown, yet recent epidemiological studies have linked exposure to environmental agents, including pesticides, with an increased risk of developing the disease. As a result, the environmental hypothesis of PD has developed, which speculates that chemical agents in the environment are capable of producing selective dopaminergic cell death, thus contributing to disease development. The use of environmental agents such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, rotenone, paraquat, dieldrin, and maneb in toxicant-based models of PD has become increasingly popular and provided valuable insight into the neurodegenerative process. Understanding the unique and shared mechanisms by which these environmental agents act as selective dopaminergic toxicants is critical in identifying pathways involved in PD pathogenesis. In this review, we discuss the neurotoxic properties of these compounds with specific focus on the induction of oxidative stress. We highlight landmark studies along with recent advances that support the role of reactive oxygen and reactive nitrogen species from a variety of cellular sources as potent contributors to the neurotoxicity of these environmental agents. Finally, human risk and the implications of these studies in our understanding of PD-related neurodegeneration are discussed.Free Radical Biology and Medicine 07/2008; 44(11):1873-86. · 5.27 Impact Factor