At high concentrations, free radicals and radical-derived, nonradical reactive species are hazardous for living organisms and damage all major cellular constituents. At moderate concentrations, how...

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Free Radicals in the Physiological Control of Cell Function

Living in an oxygenated environment has required the evolution of effective cellular strategies to detect and detoxify metabolites of molecular oxygen known as reactive oxygen species. Here we review evidence that the appropriate and inappropriate production of oxidants, together with the ability of organisms to respond to oxidative stress, is intricately connected to ageing and life span.

Oxidants, oxidative stress and the biology of ageing.

The production of ROS (reactive oxygen species) by mammalian mitochondria is important because it underlies oxidative damage in many pathologies and contributes to retrograde redox signalling from the organelle to the cytosol and nucleus. Superoxide (O2•−) is the proximal mitochondrial ROS, and in the present review I outline the principles that govern O2•− production within the matrix of mammalian mitochondria. The flux of O2•− is related to the concentration of potential electron donors, the local concentration of O2 and the second-order rate constants for the reactions between them. Two modes of operation by isolated mitochondria result in significant O2•− production, predominantly from complex I: (i) when the mitochondria are not making ATP and consequently have a high Δp (protonmotive force) and a reduced CoQ (coenzyme Q) pool; and (ii) when there is a high NADH/NAD+ ratio in the mitochondrial matrix. For mitochondria that are actively making ATP, and consequently have a lower Δp and NADH/NAD+ ratio, the extent of O2•− production is far lower. The generation of O2•− within the mitochondrial matrix depends critically on Δp, the NADH/NAD+ and CoQH2/CoQ ratios and the local O2 concentration, which are all highly variable and difficult to measure in vivo. Consequently, it is not possible to estimate O2•− generation by mitochondria in vivo from O2•−-production rates by isolated mitochondria, and such extrapolations in the literature are misleading. Even so, the description outlined here facilitates the understanding of factors that favour mitochondrial ROS production. There is a clear need to develop better methods to measure mitochondrial O2•− and H2O2 formation in vivo, as uncertainty about these values hampers studies on the role of mitochondrial ROS in pathological oxidative damage and redox signalling.

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How mitochondria produce reactive oxygen species.

Free radicals, metals and antioxidants in oxidative stress-induced cancer

The discovery that mammalian cells have the ability to synthesize the free radical nitric oxide (NO) has stimulated an extraordinary impetus for scientific research in all the fields of biology and medicine. Since its early description as an endothelial-derived relaxing factor, NO has emerged as a fundamental signaling device regulating virtually every critical cellular function, as well as a potent mediator of cellular damage in a wide range of conditions. Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynitrite, produced from the diffusion-controlled reaction between NO and another free radical, the superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins via direct oxidative reactions or via indirect, radical-mediated mechanisms. These reactions trigger cellular responses ranging from subtle modulations of cell signaling to overwhelming oxidative injury, committing cells to necrosis or apoptosis. In vivo, peroxynitrite generation represents a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. Hence, novel pharmacological strategies aimed at removing peroxynitrite might represent powerful therapeutic tools in the future. Evidence supporting these novel roles of NO and peroxynitrite is presented in detail in this review.

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Nitric Oxide and Peroxynitrite in Health and Disease

Publisher Summary This chapter discusses methods to determine carbonyl content in oxidatively modified proteins. The methods described are (1) reduction of the carbonyl group to an alcohol with tritiated borohydride; (2) reaction of the carbonyl group with 2,4-dinitrophenylhydrazine to form the 2,4-dinitrophenylhydrazone; (3) reaction of the carbonyl with fluorescein thiosemicarbazide to form the thiosemicarbazone; and (4) reaction of the carbonyl group with fluorescein amine to form a Schiff base followed by reduction to the secondary amine with cyanoborohydride. Van Poelje and Snell have also quantitated protein-bound pyruvoyl groups through formation of a Schiff base with p-aminobenzoic acid followed by reduction with cyanoborohydride. Although a systematic investigation has not appeared, this method should also be useful in detecting other protein-bound carbonyl groups. Carbonyl content of proteins is expressed as moles carbonyl/mole subunit for purified proteins of known molecular weight. For extracts, the results may be given as nanomoles carbonyl/milligram protein. For a protein having a molecular weight of 50,000, a carbonyl content of 1 mol carbonyl/mol protein corresponds to 20 nmol carbonyl/mg proteins.

Determination of carbonyl content in oxidatively modified proteins.

Publisher Summary Enzymes and structural proteins may be attacked whenever free radicals are generated. As a consequence, oxidative modification of proteins may occur in a variety of physiologic and pathologic processes. Although the distinction is sometimes arbitrary, these modifications may be primary or secondary. Primary modifications occur in metal-catalyzed oxidation, radiation-mediated oxidation, and oxidation by ozone or oxides of nitrogen. Secondary modifications occur when proteins are modified by molecules generated by oxidation of other molecules. One important example is the covalent modification of proteins by hydroxynonenal produced by oxidation of lipids. Carbonyl groups (aldehydes and ketones) may be introduced into proteins by any of these reactions, and the appearance of such carbonyl groups is taken as presumptive evidence of oxidative modification. Assay of carbonyl groups in proteins provides a convenient technique for detecting and quantifying oxidative modification of proteins. This chapter presents new methods for determination of carbonyl content, which are based on the reaction of carbonyl groups with 2,4-dinitrophenylhydrazine to form a 2,4-dinitrophenylhydrazone. The assays provide substantial improvements in both sensitivity and specificity.

Carbonyl assays for determination of oxidatively modified proteins

We summarize here results of studies designed to elucidate basic mechanisms of reactive oxygen (ROS)-mediated oxidation of proteins and free amino acids. These studies have shown that oxidation of proteins can lead to hydroxylation of aromatic groups and aliphatic amino acid side chains, nitration of aromatic amino acid residues, nitrosylation of sulfhydryl groups, sulfoxidation of methionine residues, chlorination of aromatic groups and primary amino groups, and to conversion of some amino acid residues to carbonyl derivatives. Oxidation can lead also to cleavage of the polypeptide chain and to formation of cross-linked protein aggregates. Furthermore, functional groups of proteins can react with oxidation products of polyunsaturated fatty acids and with carbohydrate derivatives (glycation/glycoxidation) to produce inactive derivatives. Highly specific methods have been developed for the detection and assay of the various kinds of protein modifications. Because the generation of carbonyl derivatives occurs by many different mechanisms, the level of carbonyl groups in proteins is widely used as a marker of oxidative protein damage. The level of oxidized proteins increases with aging and in a number of age-related diseases. However, the accumulation of oxidized protein is a complex function of the rates of ROS formation, antioxidant levels, and the ability to proteolytically eliminate oxidized forms of proteins. Thus, the accumulation of oxidized proteins is also dependent upon genetic factors and individual life styles. It is noteworthy that surface-exposed methionine and cysteine residues of proteins are particularly sensitive to oxidation by almost all forms of ROS; however, unlike other kinds of oxidation the oxidation of these sulfur-containing amino acid residues is reversible. It is thus evident that the cyclic oxidation and reduction of the sulfur-containing amino acids may serve as an important antioxidant mechanism, and also that these reversible oxidations may provide an important mechanism for the regulation of some enzyme functions.

Free radical-mediated oxidation of free amino acids and amino acid residues in proteins.

Cysteine and methionine are the two sulfur-containing residues normally found in proteins. Cysteine residues function in the catalytic cycle of many enzymes, and they can form disulfide bonds that contribute to protein structure. In contrast, the specific functions of methionine residues are not known. We propose that methionine residues constitute an important antioxidant defense mechanism. A variety of oxidants react readily with methionine to form methionine sulfoxide, and surface exposed methionine residues create an extremely high concentration of reactant, available as an efficient oxidant scavenger. Reduction back to methionine by methionine sulfoxide reductases would allow the antioxidant system to function catalytically. The effect of hydrogen peroxide exposure upon glutamine synthetase from Escherichia coli was studied as an in vitro model system. Eight of the 16 methionine residues could be oxidized with little effect on catalytic activity of the enzyme. The oxidizable methionine residues were found to be relatively surface exposed, whereas the intact residues were generally buried within the core of the protein. Furthermore, the susceptible residues were physically arranged in an array that guarded the entrance to the active site.

https://www.pnas.org/content/pnas/93/26/15036.full.pdf

Rodney L. Levine

Papers

Determination of carbonyl content in oxidatively modified proteins.

Carbonyl assays for determination of oxidatively modified proteins

Free radical-mediated oxidation of free amino acids and amino acid residues in proteins.

Methionine residues as endogenous antioxidants in proteins

Carbonyl modified proteins in cellular regulation, aging, and disease,