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Biomarkers of Oxidative Damage in Human Disease

¹ Department of Biology, University of Milan, Milan, Italy.
² Department of Neuroscience, University of Siena, Siena, Italy.

^aAddress correspondence to this author at: Department of Biology, University of Milan, via Celoria 26, I-20133 Milan, Italy. Fax 39-02-50314781; e-mail quack{at}unimi.it.

	Abstract

Top Abstract Introduction MDA, HNE, and Acrolein Isoprostanes Glutathione and S... Tyrosine Oxidation, Nitration,... Carbonylated Proteins Oxidative DNA Damage Metabolism of Oxidized... Choice of the Most... Prospects and Challenges References

 
Oxidative/nitrosative stress, a pervasive condition of increased amounts of reactive oxygen/nitrogen species, is now recognized to be a prominent feature of many acute and chronic diseases and even of the normal aging process. However, definitive evidence for this association has often been lacking because of recognized shortcomings with biomarkers and/or methods available to assess oxidative stress status in humans. Emphasis is now being placed on biomarkers of oxidative stress, which are objectively measured and evaluated as indicators of normal biological processes, pathogenic processes, or pharmacologic responses to therapeutic intervention. To be a predictor of disease, a biomarker must be validated. Validation criteria include intrinsic qualities such as specificity, sensitivity, degree of inter- and intraindividual variability, and knowledge of the confounding and modifying factors. In addition, characteristics of the sampling and analytical procedures are of relevance, including constraints and noninvasiveness of sampling, stability of potential biomarkers, and the simplicity, sensitivity, specificity, and speed of the analytical method. Here we discuss some of the more commonly used biomarkers of oxidative/nitrosative damage and include selected examples of human studies.

	Introduction

Top Abstract Introduction MDA, HNE, and Acrolein Isoprostanes Glutathione and S... Tyrosine Oxidation, Nitration,... Carbonylated Proteins Oxidative DNA Damage Metabolism of Oxidized... Choice of the Most... Prospects and Challenges References

 
Increased oxidative/nitrosative stress generally describes a condition in which cellular antioxidant defenses are inadequate to completely inactivate the reactive oxygen species (ROS) ¹ and reactive nitrogen species (RNS) generated because of excessive production of ROS/RNS, loss of antioxidant defenses, or both. A major consequence of oxidative/nitrosative stress is damage to nucleic acid bases, lipids, and proteins, which can severely compromise cell health and viability or induce a variety of cellular responses through generation of secondary reactive species, ultimately leading to cell death by necrosis or apoptosis. Oxidative damage of any of these biomolecules, if unchecked, can theoretically contribute to disease development. Indeed, an increasing amount of evidence suggests that oxidative/nitrosative stress is linked to either the primary or secondary pathophysiologic mechanisms of multiple acute and chronic human diseases (1)(2)(3)(4)(5)(6) (Table 1 ). However, definitive evidence for this association has often been lacking because of recognized shortcomings with methods available to assess oxidative stress status in vivo in humans (7).

Substantial evidence suggests that ROS/RNS participate in the normal aging process as well as in age-related diseases such as atherosclerosis and ophthalmologic and neurodegenerative diseases (8)(9). Furthermore, observations in vitro and in cultured cell systems indicate that oxidative stress contributes to cancer risk by numerous mechanisms that are independent of genotoxicity (10). Recent evidence has further supported the association between the cellular response to oxidants and the mechanisms that regulate longevity. Recent studies suggest that 3 gene products linked to aging—Forkhead transcription factors, the adaptor protein p66Shc, and the histone deacetylase Sir2—are all involved in either regulating the concentrations of intracellular ROS in mammalian cells or increasing oxidative stress resistance (9). It is now well established that biological aging correlates with the accumulation of oxidized biomolecules in most tissues (11)(12)(13). In the study of age-related increases in concentrations of oxidized biomolecules, disparities have been observed between intracellular and extracellular proteins. The concentrations of oxidative markers were found to increase more with age in extracellular proteins than in intracellular proteins (12). This disparity might be explained by a difference in turnover between extracellular (hours to days) and intracellular proteins (minutes to hours). The difference in homeostatic control between extra- and intracellular proteins might also play a role.

The localization and effects of oxidative stress, as well as information regarding the nature of the ROS/RNS, may be gleaned from the analysis of discrete biomarkers of oxidative/nitrosative stress/damage isolated from tissues and biological fluids. Biomarkers are defined as characteristics that can be objectively measured and evaluated as indicators of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention. Several in vitro markers of oxidative/nitrosative stress are available, including ROS/RNS themselves, but most are of limited value in vivo because they lack sensitivity and/or specificity or require invasive methods. Although some ROS/RNS have been directly detected in vitro by electron spin resonance with or without spin-trapping reagents or by chemiluminescence, these direct detection methods are not yet applicable for clinical examination because of the instability of many reactive species and the need for expensive equipment. Furthermore, ROS/RNS are generally too reactive and/or have a half-life too short (even much shorter than seconds) to allow direct measurement in cells/tissues or body fluids. Because molecular products formed from the reaction of ROS/RNS with biomolecules are generally considered more stable than ROS/RNS themselves, most commonly ROS/RNS have been tracked by measuring stable metabolites (e.g., nitrate/nitrite) and/or concentrations of their oxidation target products, including lipid peroxidation end products and oxidized proteins (5)(7)(14)(15)(16)(17)(18)(19)(20). To function as suitable biomarkers of oxidative modifications in relation to disease, it is critical that such oxidation products are stable, can accumulate to detectable concentrations, reflect specific oxidation pathways, and correlate with disease severity, so that they can be used as diagnostic tools.

Oxidative stress-induced peroxidation of membrane lipids can be very damaging because it leads to alterations in the biological properties of the membrane, such as the degree of fluidity, and can lead to inactivation of membrane-bound receptors or enzymes, which in turn may impair normal cellular function and increase tissue permeability. Moreover, lipid peroxidation may contribute to and amplify cellular damage resulting from generation of oxidized products, some of which are chemically reactive and covalently modify critical macromolecules. Products of lipid peroxidation have therefore commonly been used as biomarkers of oxidative stress/damage. Lipid peroxidation generates a variety of relatively stable decomposition end products, mainly ,ß-unsaturated reactive aldehydes, such as malondialdehyde (MDA), 4hydroxy-2-nonenal (HNE), and 2-propenal (acrolein) (21)(22), and isoprostanes (23)(24), which can then be measured in plasma and urine as an indirect index of oxidative stress. Compared with free radicals, the aldehydes are relatively stable and can diffuse within or even escape from the cell and attack targets far from the site of the original event. They therefore are not only end products and remnants of lipid peroxidation processes but also may act as "second cytotoxic messengers" for the primary reactions (25). Some of these aldehydes have been shown to exhibit facile reactivity with various biomolecules, including proteins, DNA, and phospholipids, generating stable products at the end of a series of reactions that are thought to contribute to the pathogenesis of many diseases. Modification of amino acids by ,ß-unsaturated aldehydes occurs mainly on the nucleophilic residues Cys and, to a lesser extent, His and Lys. Lipid hydroperoxides and aldehydes can also be absorbed from the diet and then excreted in urine. It follows that measurements of hydroxy fatty acids in plasma total lipids as well as plasma or urinary MDA and HNE can be confounded by diet and should not be used as an index of whole-body lipid peroxidation unless diet is strictly controlled (26)(27).

Proteins are major targets for ROS/RNS because of their high overall abundance in biological systems and because they are primarily responsible for most functional processes within cells. It has been estimated that proteins can scavenge the majority (50%–75%) of reactive species (ROS/RNS) generated (14). Exposure of proteins to ROS/RNS may alter every level of protein structure from primary to quaternary (if multimeric proteins), causing major physical changes in protein structure. Oxidative damage to proteins is induced either directly by ROS/RNS or indirectly by reaction of secondary byproducts of oxidative stress and can occur via different mechanisms, leading to peptide backbone cleavage, cross-linking, and/or modification of the side chain of virtually every amino acid (11)(14)(28)(29). Most protein damage is irreparable, and oxidative changes of protein structure can have a wide range of downstream functional consequences, such as inhibition of enzymatic and binding activities, increased susceptibility to aggregation and proteolysis, increased or decreased uptake by cells, and altered immunogenicity (28)(30)(31)(32)(33)(34). Individual proteins may display different susceptibilities to oxidative attack, linked to the variable proportions and distributions of sulfhydryl groups, Fe-S clusters, reduced heme moieties, copper prosthetic groups, sequence motifs, and residues exposed on the molecular surface. Oxidative damage to proteins may be important in vivo not only in its own right (affecting, for example, the functions of receptors, enzymes, and transport proteins), but also because it can contribute to secondary damage to other biomolecules; for example, inactivation of DNA repair enzymes and loss of fidelity of damaged DNA polymerases in replicating DNA (1). The major fate of oxidized proteins is catabolism by proteasomal and lysosomal pathways, but some functionally inactive proteins appear to be poorly degraded, form protein aggregates, and accumulate in separate compartments within cells or the extracellular environment (28)(30). The accumulation of such damaged material increases during the normal aging process, may contribute to a range of human pathologies, and is able to act as an inhibitor of the proteasome (35)(36)(37). Because of this decreased capacity for removal of oxidized proteins, the accumulation of misfolded and damaged proteins is accelerated. The vicious circle of decreased proteolysis and accumulation of increasing amounts of oxidatively damaged proteins continues until the protein aggregates cause metabolic dysfunctions or the initiation of apoptotic or necrotic events.

When investigating oxidative/nitrosative damage in relation to disease conditions, what are the most appropriate biomarkers? What are the best ways to measure them? At present, the biomarkers of oxidative/nitrosative stress/damage and the methods used to measure them to determine an individual’s oxidative status in relation to disease conditions frequently vary among studies, making comparisons of study findings difficult. In addition, the validity of many biomarkers remains to be established. Assays that have been developed have several shortcomings related to (a) the limited specificity of the assay itself for the product of oxidative/nitrosative damage being measured; (b) the fact that the analyte being measured is not a specific product of a specific ROS/RNS; (c) the lack of sufficient sensitivity to detect concentrations of the product being measured in healthy individuals, thus not allowing the definition of a reference interval; (d) concentrations of the product being measured being influenced by external factors such as the lipid content of the diet; or (e) the assay being too invasive for in vivo investigations in humans.

In the following sections, we briefly discuss some of the more commonly used biomarkers of oxidative/nitrosative stress and includes selected examples of human studies.

	MDA, HNE, and Acrolein

Top Abstract Introduction MDA, HNE, and Acrolein Isoprostanes Glutathione and S... Tyrosine Oxidation, Nitration,... Carbonylated Proteins Oxidative DNA Damage Metabolism of Oxidized... Choice of the Most... Prospects and Challenges References

 
MDA (Fig. 1 ) is a physiologic ketoaldehyde produced by peroxidative decomposition of unsaturated lipids as a byproduct of arachidonate metabolism. The excess MDA produced as a result of tissue injury can combine with free amino groups of proteins (MDA reacts mainly with Lys residues by Michael addition), producing MDA-modified protein adducts. Modification of proteins by MDA could conceivably alter their biological properties. Moreover, MDA-modified proteins are immunogenic, and autoantibodies against MDA-modified Lys residues have been detected in the sera of rabbits and humans (3). Some studies have reported that the titer of these autoantibodies is associated with the burden of, and may predict progression of, atherosclerosis and myocardial infarction. Higher titers of autoantibodies have also been associated with coronary artery disease (3).

View larger version (9K): [in this window] [in a new window]   Figure 1. Chemical structures of MDA, acrolein, and 4-HNE. For simplicity, stereochemistries are not indicated.

The clinical relevance of the reaction between MDA and proteins is highlighted in atherosclerosis, which is a major cause of coronary heart disease and strokes. MDA-LDL, in addition to oxidized LDL, mediates several proinflammatory and proatherogenic processes, all of which ultimately lead to foam cell generation (38). Plasma MDA concentrations are increased in diabetes mellitus, and MDA can be found in the atherosclerotic plaques promoted by diabetes (39). Adducts of apolipoprotein B-100 Lys residues with MDA and HNE have been characterized extensively in human atherosclerotic lesions (3)(40). Increased MDA concentrations have been found in samples from women with preeclampsia (41), in plasma and breath condensates from asthmatics (42), and in the brains of patients suffering from Parkinson disease (PD), whereas increased thiobarbituric acid-reactive substances (TBARS)—the most prevalent substrate of which is MDA—have been observed in the plasma of patients with amyotrophic lateral sclerosis (ALS) as well as in the brains of persons afflicted with Alzheimer disease (AD) (43).

Acrolein (Fig. 1 ) is present in various environmental sources, the largest source being cigarette smoke. HNE (Fig. 1 ) is a major and toxic aldehyde generated by free radical attack on -6 polyunsaturated fatty acids (arachidonic, linoleic, and linolenic acids) (21) and is considered a second toxic messenger of oxygen free radicals (44)(45). HNE undergoes many reactions with proteins, peptides, phospholipids, and nucleic acids; it therefore has a high biological activity and exhibits numerous cytotoxic, mutagenic, genotoxic, and signal effects (21)(44)(45), including inhibition of protein and DNA synthesis, inactivation of enzymes, stimulation of phospholipase C, reduction of gap-junction communication, stimulation of neutrophil chemotaxis, modulation of platelet aggregation, and modulation of the expression of various genes (21)(22). In addition, HNE may be an important mediator of oxidative stress–induced apoptosis (22), cellular proliferation, and signaling pathways (46). HNE is permanently formed at basal concentrations under physiologic conditions, but its production is greatly enhanced in pathologic conditions related to increased lipid peroxidation. Under physiologic conditions, the cellular concentration of HNE ranges from 0.1 to 3 µmol/L (21)(44). Under conditions of oxidative stress, HNE concentrations are significantly increased in plasma, various organs, and cell types (21)(44)(47). During heavy oxidative stress, e.g., in patients with severe rheumatologic diseases such as rheumatoid arthritis, systemic sclerosis, lupus erythematosus, chronic lymphedema, or chronic renal failure, serum HNE is increased to concentrations up to 3- to 10-fold higher than physiologic concentrations (48). HNE and acrolein are highly reactive toward proteins (in particular, HNE is much more reactive to proteins than to DNA), forming stable covalent adducts with His, Lys, and Cys residues through Michael addition; these adducts are known as advanced lipoxidation end products (ALEs) (49)(50)(51). This process introduces carbonyl groups into proteins.

Numerous studies have demonstrated increased lipid peroxidation in the brains of persons with AD relative to age-matched controls, whereas lipid peroxidation is not a significant feature of normal aging (21)(43)(50)(51)(52). Increased concentrations of HNE and acrolein have been measured in the brains of individuals with mild cognitive impairment (which is considered to be a transition between normal aging and AD) and early AD compared with age-matched controls (53). In particular, binding of the glial glutamate transporter GLT-1 (EAAT2) by HNE is increased in brains of persons with AD (50)(51). Neurofilament proteins are major targets of HNE modification (54). Notably, it has recently been shown that the phosphorylation-dependent adduction/carbonylation of tau protein by HNE promotes and contributes to the generation of major conformational changes associated with neurofibrillary tangles (55). The concentrations of both free and protein-bound HNE are also increased in the brain tissue of PD patients, in the cerebrospinal fluid of ALS patients, and in human atherosclerotic lesions (3)(43)(50)(51)(56). Furthermore, concentrations of acrolein– and HNE–protein adducts are increased in cardiovascular disease (57). Acrolein reacts with Lys residues of apolipoprotein A-I (apoA-I), the major protein of HDL, which plays a critical role in mobilizing cholesterol from artery wall macrophages. Acrolein adducts colocalize with apoA-I in human atherosclerotic lesions. Moreover, the ability of acrolein-modified apoA-I to remove cholesterol from cultured cells is impaired, suggesting that carbonylation might interfere with the normal function of apoA-I in promoting cholesterol removal from artery wall cells, thus playing a critical role in atherogenesis (58). Increased concentrations of HNE–protein adducts have been reported in the lungs of smokers with and without chronic obstructive pulmonary disease (COPD). Notably, HNE concentrations in the pulmonary epithelium, airway endothelium, and particularly, neutrophils of COPD patients were found to be inversely associated with lung function (59). COPD patients also had higher diaphragm concentrations of both protein carbonyls and HNE–protein adducts. Furthermore, a negative correlation was found between carbonyl groups and airway obstruction (i.e., concentrations of reactive carbonyls correlated with disease severity) and between HNE–protein adducts and respiratory muscle strength (i.e., HNE–protein adduct formation correlated with respiratory muscle function) (60).

	Isoprostanes

Top Abstract Introduction MDA, HNE, and Acrolein Isoprostanes Glutathione and S... Tyrosine Oxidation, Nitration,... Carbonylated Proteins Oxidative DNA Damage Metabolism of Oxidized... Choice of the Most... Prospects and Challenges References

 
F₂-Isoprostanes (F₂-IsoPs), isoprostanes containing an F-type prostane ring, are a family of, theoretically, 64 prostaglandin F₂-like compounds generated in vivo, primarily in situ, by nonenzymatic free-radical–catalyzed peroxidation of esterified arachidonic acid and then cleaved and released into the circulation by phospholipases(s) before excretion in the urine as free isoprostanes. Isoprostanes containing alternative ring structures, such as E₂/D₂-IsoPs, A₂/J₂-IsoPs, and thromboxane-like compounds (isothromboxanes), can also be formed by this mechanism. F₂-IsoPs, however, have been the most studied class of isoprostanes and, because of their stability, afford the most accurate measure of oxidative stress. 8-iso-Prostaglandin F₂ (8-iso-PGF₂; Fig. 2 ), also known as iPF₂-III and 15-F_2t-IsoP, is one abundant endogenous F₂-IsoP excreted into the urine of humans and, at present, is the most thoroughly investigated F₂-IsoP. Actually, to date, contradictory results have been reported on the mechanisms leading to F₂-IsoP formation in vitro and in vivo, in humans and in animals, suggesting both enzymatic and nonenzymatic formation. Notably, 8-iso-PGF₂ has also been shown to be formed enzymatically by the action of cyclooxygenase (COX). This has been shown, for example, in isolated rat kidney glomeruli (61) and with isolated COX enzymes(62). Evidence for enzyme-catalyzed formation of 8-iso-PGF₂ is usually based on the inhibition of 8-iso-PGF₂ formation by COX inhibitors such as acetylsalicylic acid, indomethacin, and naproxen (62). Nevertheless, from the knowledge available at present, it can be assumed that the origin of urinary and circulating 8-iso-PGF₂ and other F₂-IsoPs in humans is free-radical–catalyzed peroxidation of arachidonic acid esterified to lipids, but not COX-dependent peroxidation of arachidonate (62). Several of these compounds possess potent biological activity as pulmonary and renal vasoconstrictors and modulators of platelet activation. In general, they have short half-lives. Reports have shown that F₂-IsoPs are authentic, reliable biomarkers of lipid peroxidation and are useful in vivo indicators of oxidative stress in various clinical conditions, such as acute and chronic inflammation, ischemia/reperfusion injury, diabetes, and atherosclerosis (7)(23)(24)(63)(64)(65)(66)(67)(68). F₂-IsoPs have also been used to assess in vivo oxidative response to various drugs, antioxidants, or dietary interventions for their free-radical–scavenging properties (7)(23)(24)(63)(64)(65)(66)(67)(68). Different methods for F₂-IsoP quantification are available (23)(68)(69). In addition to being markers of oxidative stress and antagonists of the action of prostaglandins, they may also exert unique biological effects (63)(66). Notable examples include the disease rhabdomyolysis, in which myoglobin is released into the plasma, oxidizes arachidonic acid in the presence of lipid peroxides, and leads to vasoconstriction in the kidney (70).

View larger version (8K): [in this window] [in a new window]   Figure 2. Chemical structure of 8-iso-PGF2. For simplicity, stereochemistry is not indicated.

A tissue that does not contain isoprostanes is yet to be reported. Isoprostanes have also been found in measurable quantities in most of the biological fluids analyzed, including plasma, urine, synovial fluid, bronchoalveolar fluid, bile, lymph, microdialysis fluid from various organs, and amniotic, pericardial, and seminal fluid, although plasma and urine are the sample types that are commonly analyzed, being the most convenient to obtain and the least invasive (64)(71). In particular, F₂-IsoPs, including 8-iso-PGF₂, are present in human plasma in 2 forms: esterified to lipids and as free acids, with the esterified form being the most abundant, whereas only hydrolyzed isoprostanes are excreted into the urine [for an example, see Ref. (62)].

At present, measurement of F₂-IsoPs is regarded as one of the most reliable approaches for the assessment of oxidative stress status or free-radical–mediated lipid peroxidation in vivo (7)(24)(64)(66)(71). Available data indicate that quantification of F₂-IsoPs in either plasma or urine gives a highly precise and accurate index of oxidative stress (18)(71)(72). Whereas the biological validity of F₂-IsoPs as biomarkers of oxidative stress is well established, it is technically quite complicated to measure F₂-IsoPs and their metabolites in body fluids, and some limitations with respect to their measurement must be taken into account (69). F₂-IsoPs are chemically stable in vivo and ex vivo, but once they generated and released into the circulation, they are rapidly metabolized (although not as rapidly or as extensively as prostaglandins) and eliminated. Their rapid disappearance from plasma may hamper practical application. Current methodologies [gas chromatography–mass spectrometry (GC-MS), GC–tandem MS (GS-MS/MS), liquid chromatography (LC)-MS, LC-MS/MS, enzyme immunoassays, and RIAs] are able to detect the steady-state concentrations of F₂-IsoPs in many tissues and body fluids, even in the basal state, which allows researchers to assess any fluctuation in their concentrations after any degree of oxidant stress or lipid peroxidation in vivo. Several internal standards (¹⁸O- or ²H-labeled analogs of specific isoprostane isomers) are available from commercial sources to quantify the isoprostanes by MS methods.

Alternative approaches have also been developed to quantify F₂-IsoPs by immunologic techniques (RIAs and enzyme immunoassays), and a few immunoassay reagent sets are commercially available (71). A potential drawback of these methods is that limited information is currently available regarding their precision and accuracy. In addition, few data exist comparing F₂-IsoP concentrations measured by immunoassay with MS results. Furthermore, the sensitivity and/or specificity of these assays may vary substantially among manufacturers. However, although MS methods of F₂-IsoP quantification are considered the "gold standard", immunoassays have expanded research in this area because of their low cost and relative ease of use. In addition to commercial immunoassays, some investigators have generated polyclonal antibodies and have developed assays for F₂-IsoP (73). It appears that there is good correlation between these methods and MS.

Different analytical approaches are available for the analysis of isoprostanes, the most sensitive, highly specific, and reliable method being GC with negative-ion chemical ionization (NICI) MS (69)(71). For quantification of lipid peroxidation, measurements of F₂-IsoPs have a clear advantage over currently available methods such as assays for MDA, TBARS, lipid hydroperoxides, or conjugated dienes, which are hampered by various methodologic limitations (64).

F₂-IsoPs are very well suited as biomarkers of oxidative stress for the following reasons: (a) The in vivo formation of isoprostanes increases as a function of oxidative stress (71)(74)(75). (b) They can be measured accurately down to picomolar concentrations with analytical techniques such as GC-MS, GC-MS/MS, LC-MS, LC-MS/MS, or RIA. The first 4 techniques can easily differentiate among the different types of isoprostanes, but they require extensive preparation of the material (e.g., phospholipid extraction and alkaline hydrolysis) and/or expensive instrumentation. RIAs are somewhat easier to perform and are widely available commercially; however, many of these are not able (or have not been shown to be able) to distinguish between the prostanoids and the isoprostanes, much less between the different types of isoprostanes. (c) They are stable in isolated samples of body fluids, including urine and exhaled breath condensates, providing an exceedingly noninvasive route for their measurement. (d) Their measured values do not exhibit diurnal variations and are not affected by lipid content in the diet (76)(77); however, they do vary markedly in clinical and experimental conditions characterized by oxidative stress and closely parallel disease severity. Some diurnal variation in urinary F₂-IsoP excretion does occur within individual humans, although this variation is not present when F₂-IsoPs are evaluated on a group level. Furthermore, although pooled urine samples are likely preferable, F₂-IsoPs determined in urine collected in the morning or in several spot urine samples adequately represent the daily F₂-IsoP excretion (63)(65)(71). (e) They are specific products of peroxidation. (f) They are present in detectable amounts in all healthy tissues and biological fluids, thus allowing definition of a reference interval.

Because of the free-radical–catalyzed conversion of arachidonic acid to isoprostanes, precautions must be taken to avoid artifactual formation during sample storage and processing (69). Blood plasma samples contain considerable amounts of arachidonic acid, mainly esterified to membrane phospholipids. Storage of these samples at –80 °C and addition of antioxidants (e.g., butylated hydroxytoluene and triphenylphosphine) during sample preparation is therefore recommended. Moreover, isoprostanes in blood samples may occur as free fatty acids or esterified to phospholipids or lipoproteins. Thus, one has to distinguish between the two fractions of isoprostanes in human blood, i.e., free and total (free plus esterified). Analysis of the esterified compounds requires hydrolysis to yield the free derivatives. Because urine samples have a very low lipid content, autooxidation is not a problem. Nevertheless, as a precaution, samples should be supplemented with EDTA and 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oyl (4-hydroxy-TEMPO) and stored at –20 °C (69).

Dozens of diseases and experimental conditions with diverse etiologies have been shown to be associated with marked increases in urinary, plasma, and tissue concentrations of F₂-IsoPs (24)(64)(67). However, it has been suggested they should be considered not just mere markers, but also "mediators" of disease, as they evoke important biological responses in virtually every cell type found within the lung. In fact, the isoprostanes may mediate many of the features of the disease states for which they are used as indicators (67). However, we should consider that, in human plasma, the concentration of free 8-iso-PGF₂, the biologically active component, amounts to <10 ng/L (62); at this concentration, it cannot develop considerable biological activity. 8-iso-PGF₂ is generated in substantial amounts in otherwise "normal" individuals exposed to cigarette smoke, allergens, ozone, or hyperoxia and during ventilated ischemia. It is also markedly increased, serving as a biomarker, in the bronchoalveolar lavage (BAL) fluid, plasma, urine, or exhaled breath condensate (a noninvasive method for direct measurement of oxidative stress in the lungs) in several pulmonary diseases such as asthma, COPD, interstitial lung disease, cystic fibrosis, pulmonary hypertension, acute chest syndrome, sickle cell disease, acute lung injury [including acute respiratory distress syndrome (ARDS)], and severe respiratory failure in infants as well as in healthy chronic smokers (24)(42)(64)(78)(79). Systemic and synovial fluid concentrations of 8-iso-PGF₂ are higher in patients with rheumatoid arthritis (23)(64), psoriatic arthritis (64), reactive arthritis (23)(64), and osteoarthritis(64) than in healthy controls. Plasma concentrations are increased in patients with cardiovascular disease, and it has been suggested that this may be a useful biomarker of risk (80). Similarly, several cardiovascular conditions feature marked increases in F₂-IsoP concentrations, including during and after cardiopulmonary bypass (64), renal, cerebral, and myocardial ischemia-reperfusion injury (24), unstable angina (24), heart failure(24)(64), coronary heart disease (24), acute ischemic stroke (24), hypercholesterolemia (23)(24)(64), and atherosclerosis (23)(24)(64). Recently, urinary 8-iso-PGF₂, measured by GC-MS/MS, was found to be a novel, sensitive, and independent risk marker in patients with coronary heart disease, in addition to known risk factors of this pathology, i.e., diabetes mellitus, hypercholesterolemia, hypertension, obesity, and smoking (81). Increased concentrations of 8-iso-PGF₂ have also been found in plasma or urine samples from patients with type 2 diabetes (64). To date, the highest reported concentrations of urinary 8-iso-PGF₂ have been measured, by a validated GC-NICI-MS/MS method, in children with congenital Zellweger syndrome, which is attributable to impaired peroxisomal ß-oxidation of various compounds, including prostaglandins and leukotrienes (82).

Concentrations of F₂- and F₄-isoprostanes [which arise from oxidation of docosahexaenoic acid, an abundant unsaturated fatty acid in the central nervous system (CNS) and are often called neuroprostanes], are increased in the cerebrospinal fluid from AD patients (2)(43)(50)(51)(52)(83)(84) and may even be increased before the development of AD (74)(85), consistent with the view that peroxidation may be an important step in progressive neuronal injury leading to clinically manifested disease (2)(50)(51)(74)(85). Some authors have claimed that urinary and plasma concentrations of isoprostanes are also increased in AD patients to an extent correlated with the degree of cognitive impairment (84)(85), although this was not confirmed in other studies (86)(87). The reason for this discrepancy is possibly because the samples were not taken at the same stage of the disease or because various medications that the patients were taking might have affected the outcome. Because F₂-IsoPs are generated by every cell, peripheral production unrelated to CNS disease could easily confound interpretation of blood or urine concentrations in AD patients. Consequently, comparison of plasma/urine F₂-IsoP concentrations with cerebrospinal fluid concentrations raises serious concerns about the relevance of peripheral F₂-IsoPs to CNS disease. The majority of studies involving AD have shown that isoprostanes are localized in the tissues or in the cerebrospinal fluid (24)(64). Finally, 8-iso-PGF₂ has been found to be a useful marker of oxidative damage and lipid peroxidation in disease states as diverse as multiple sclerosis, systemic lupus erythematosus, several hepatic pathologies (e.g., acute and chronic alcoholic liver disease), and inflammatory diseases (24)(67)(75).

8-Iso-PGF₂ has also been used as a reliable biochemical marker of oxidative stress to assess the in vivo reduction of oxidative stress after pharmacologic treatment at therapeutically relevant doses. For example, nebivolol (a vasodilating and highly selective ß₁-adrenergic receptor antagonist) and olmesartan medoxomil (an angiotensin II subtype-1 receptor antagonist) have been shown to decrease systemic oxidative stress in healthy persons and patients with type 2 diabetes, respectively, as evidenced by decreases in the urinary or plasma concentrations of 8-iso-PGF₂ (88)(89).

	Glutathione and S-Glutathionylated Proteins

Top Abstract Introduction MDA, HNE, and Acrolein Isoprostanes Glutathione and S... Tyrosine Oxidation, Nitration,... Carbonylated Proteins Oxidative DNA Damage Metabolism of Oxidized... Choice of the Most... Prospects and Challenges References

 
Because blood glutathione concentrations may reflect glutathione status in other, less accessible tissues, measurement of both reduced glutathione (GSH) and glutathione disulfide (GSSG; Fig. 3 ) in blood has been considered essential as an index of whole-body GSH status and a useful indicator of oxidative stress status in humans. Several methods have been optimized to identify and quantify glutathione forms in human samples, including spectrophotometric, fluorometric, and bioluminometric assays, often applied to HPLC analysis, as well as the more recently developed GC-MS and HPLC-electrospray ionization-MS techniques (90)(91)(92). Furthermore, a wide variety of methods have been introduced for the determination of GSH and GSSG in human blood, the measurement of which, particularly that of GSSG, could be overestimated if samples are not properly processed (93)(94)(95). In particular, scarce attention is usually paid to sample manipulation and prevention of artifactual GSH oxidation. We have shown that the main artifact results from sample acidification (for protein separation) without prevention of artificial oxidation of –SH groups by blocking with alkylating agents (93)(94)(95). Actually, many published articles reporting concentrations of GSH, GSSG, and S-glutathionylated proteins in blood, both from healthy controls and patients affected by various pathologies, are not artifact free, which makes the conclusions reached in these articles meaningless [reviewed in Refs. (93)(94)(95)]. As a consequence, the notion that some pathophysiologic conditions can alter and/or be influenced by the GSH/GSSG homeostasis of blood still needs to be confirmed.

View larger version (18K): [in this window] [in a new window]   Figure 3. Chemical structures of glutathione, GSSG and S-glutathionylated proteins. For simplicity, stereochemistries are not indicated.

It has been well established that a decrease in GSH concentration may be associated with aging (8) and the pathogenesis of many diseases, including rheumatoid arthritis, ALS, AIDS, AD, alcoholic liver disease, cataract genesis, respiratory distress syndrome, cardiovascular disease, and Werner syndrome (90). Furthermore, there is a drastic depletion in cytoplasmic concentrations of GSH within the substantia nigra of PD patients (96). Depletion of total GSH (GSH + 2 GSSG + protein-bound glutathione) and a decreased GSH:GSSG ratio are indicators of oxidative/nitrosative stress in ischemic brain disease (97), cardiovascular diseases (98), and cancer(99), and decreased concentrations of GSH are consistently observed in both types of diabetes mellitus (90). Low GSH concentrations and a high GSSG:GSH ratio have been measured in blood of patients with various diseases, including breast and lung cancer, coronary heart surgery, and preeclampsia (90)(100). The GSH system is also altered in lung inflammatory conditions. For example, GSH concentrations are increased in the epithelial lining fluid of chronic smokers, whereas they decrease rapidly in patients with mild asthma during an asthma exacerbation. Similarly, GSH concentrations in the epithelial lining fluid are decreased in idiopathic pulmonary fibrosis, asbestosis, acute respiratory distress syndrome, and in HIV-positive patients (101). Total GSH was markedly decreased in older patients with chronic diseases, including cancer and genitourinary, gastrointestinal, cardiovascular, and musculoskeletal diseases (90)(98)(99)(102), the deficit being attributable to lower GSH concentrations and not to higher GSSG. These findings suggested that the decrease in GSH might be used to monitor the severity and progress of the diseases. Conversely, total GSH concentrations are high in the blood of elderly persons who are in excellent physical and mental health (103). Furthermore, a high blood GSH concentration was correlated with long life-span in mice, rats, and healthy elderly humans (102)(103).

Under conditions of moderate oxidative stress, oxidation of Cys residues can lead to the reversible formation of mixed disulfides between protein thiol groups and low–molecular-mass thiols (S-thiolation), particularly with GSH (S-glutathionylation), the most abundant (0.5–10 mmol/L) low–molecular-mass thiol in mammalian cells. Protein S-glutathionylation can directly alter or regulate protein function (redox regulation) and may also have a role in protection from irreversible (terminal) oxidation. S-Glutathionylated proteins accumulate under oxidative/nitrosative stress conditions, but they can be readily reduced to free thiol groups when normal cellular redox status is recovered by glutaredoxins (thioltransferases) or reducing agents. As highlighted above, a characteristic hallmark of many pathophysiologic conditions is a decrease in the GSH:GSSG ratio. When GSSG accumulates in cells, it can undergo disulfide exchange reactions with protein thiols, leading to their S-glutathionylation.

S-Glutathionylated proteins (Fig. 3 ) have been investigated as possible biomarkers of oxidative/nitrosative stress in some human diseases (104)(105), such as renal cell carcinoma and diabetes (5)(104). Significant increases in glutathionylated hemoglobin and glutathionylated actin have been found in the blood and fibroblasts, respectively, of patients with Friedreich ataxia (106). Glutathionylated hemoglobin is also increased in patients with type 1 and type 2 diabetes, hyperlipidemia, and uremia associated with hemodialysis or peritoneal dialysis (104).

	Tyrosine Oxidation, Nitration, and Halogenation

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Analysis of 3-nitrotyrosine (NO₂-Tyr), a stable marker for NO·-derived oxidants), and halogenated Tyr products such as 3-chlorotyrosine (Cl-Tyr) or 3-bromotyrosine (Fig. 4 ) has been performed in various diseases, and several diverse methods have been developed for such measurements (107)(108)(109)(110)(111)(112)(113). The quantitative determination of NO₂-Tyr is hindered by severe methodologic problems, which have been critically and exhaustively discussed in 2 recent reviews (111)(112). The majority of the data available on NO₂-Tyr in tissues and fluids have been derived from antibody-based methods, which however, are usually not rigorously validated. In particular, in most of these works, the reliability of the method is difficult to assess, and its specificity is often not addressed (111). Therefore, such immunologic methods should be considered semiquantitative and the results interpreted accordingly. HPLC with ultraviolet detection does not provide adequate sensitivity or specificity for biological materials. In contrast, HPLC with electrochemical detection, LC-MS/MS, electron capture–negative chemical ionization (EC-NCI) GC-MS, and GC-MS/MS are able to quantify NO₂-Tyr in biological materials and human plasma (110)(114)(115)(116)(117). However, reported concentrations of NO₂-Tyr in normal human plasma vary 30-fold among different studies [reviewed in Refs. (111),(112)]. One possible reason is that NO₂-Tyr can be generated readily ex vivo during sample preparation and analysis as a result of acidification or contribution of proteolytic enzymes by self-digestion.

View larger version (16K): [in this window] [in a new window]   Figure 4. Chemical structures of NO2-Tyr, halogenated tyrosines, and di-Tyr. For simplicity, stereochemistries are not indicated.

Cl-Tyr and 3-bromotyrosine can also be formed artifactually during sample preparation and analysis. In this regard, MS analyses permit the use of a stable, isotopically labeled internal standard that, apart from its heavy isotope, is structurally identical to the target analyte and therefore behaves identically during extraction, processing, and chromatographic analyses. Including such an internal standard corrects for analyte loss during processing and increases the precision of quantitative measurements because it permits evaluation of artifactual production of the analytes that form ex vivo during sample preparation and analysis.

LC-MS/MS and EC-NCI GC-MS were specifically compared as techniques for quantifying Cl-Tyr, 3-bromotyrosine, and NO₂-Tyr in human plasma (110). Artifact generation was shown to be negligible with either technique. EC-NCI GC-MS was found to be 100-fold more sensitive than LC-MS/MS for analyzing authentic Cl-Tyr, 3-bromotyrosine, and NO₂-Tyr and allowed accurate, highly sensitive quantification of all 3 biomarkers in a single, rapid analysis. However, evidence of interference in the GC-MS analysis despite use of an HPLC step for sample purification have suggested that the reported GC-MS methods do not provide accurate values for free NO₂-Tyr (111)(112). The overall discussion on the quantitative determination of both free and protein-bound NO₂-Tyr leads to the conclusions that, at present, only MS/MS-based methods, both GC-MS/MS and LC-MS/MS, provide reliable values for circulating and excreted NO₂-Tyr, with LC-MS/MS being at present considerably less sensitive than GC-MS/MS, and that the basal concentrations obtained by this analytical approach may serve as reference values (111)(112)(113)(117).

Another methodologic problem, in addition to artifactual NO₂-Tyr formation, is considerable interference by coeluting substances, which can be eliminated only by use of MS/MS. On the basis of the data provided by GC-MS/MS and LC-MS/MS, NO₂-Tyr concentrations of 0.5–2.9 nmol/L and protein-associated NO₂-Tyr concentrations of 0.6 pmol/mg of plasma protein or 0.4–1.6 x 1:10⁶, i.e., the molar ratio of NO₂-Tyr to Tyr in plasma proteins, seem to be justified for use as reference intervals in healthy humans (112). Highly specific, interference-free, and accurate quantitative determination of albumin-bound NO₂-Tyr in plasma of healthy humans by GC-MS/MS gave a mean concentration of 24 nmol/L (range, 15–40 nmol/L) and a mean molar ratio of NO₂-Tyr to Tyr of 1.5 x 1:10⁶ (range, 0.5–3.5 x 1:10⁶) (117).

Increased concentrations of stable halogenated Tyr residues have been detected in proteins isolated from atherosclerotic plaques as well as in plasma and airway secretions of patients with asthma, ARDS, and cystic fibrosis, and halogenated Tyr residues are widely used as markers for damage mediated by hypohalous acids (HOCl and HOBr) in these diseases (3)(5)(107)(118)(119)(120)(121)(122). The major products are Cl-Tyr and 3-bromotyrosine, but dihalogenated compounds (3,5-dichlorotyrosine and 3,5-dibromotyrosine; Fig. 4 ) are formed with high excesses of HOCl and HOBr. Dramatic selective enrichment in protein-bound NO₂-Tyr and Cl-Tyr content within apoA-I, the major protein constituent within HDL, recovered from human plasma and atherosclerotic lesions has been demonstrated by proteomic and MS techniques. Analysis of serum also showed that protein-bound NO₂-Tyr and Cl-Tyr concentrations in apoA-I are markedly higher in individuals with established coronary artery disease (122)(123)(124)(125). These observations suggest that increased concentrations of Cl-Tyr and NO₂-Tyr in circulating HDL might represent specific markers for clinically significant atherosclerosis. Remarkably, HDL from human aortic atherosclerotic intima had an 8-fold higher concentration of Cl-Tyr than did plasma HDL. Moreover, the concentration of Cl-Tyr was 13-fold higher in HDL isolated from the plasma of persons with established coronary artery disease than in HDL from the plasma of healthy persons (123). Increased concentrations of Cl-Tyr have also been detected in LDL isolated from human atherosclerotic lesions (40)(126). The mean concentration of protein-bound NO₂-Tyr in HDL isolated from human aortic atherosclerotic intima was 6-fold higher than that in circulating HDL. Moreover, plasma HDL from patients with established coronary artery disease contained twice as much protein-bound NO₂-Tyr as did HDL from plasma of healthy persons (124). It is noteworthy that the concentration of NO₂-Tyr in lesion HDL was very similar to that reported previously for lesion LDL, which contains much higher concentrations of NO₂-Tyr than does circulating LDL (40), indicating that both lipoproteins are nitrated to a similar extent in the human artery wall.

Increased concentrations of nitrated plasma proteins have been associated with predisposition to develop lung injury in premature infants as well as with unfavorable outcome on development of lung injury (31). The clinical relevance of protein Tyr nitration has been emphasized recently by the observation of a strong association between protein-bound NO₂-Tyr concentrations and coronary artery disease risk. Circulating concentrations of protein-bound NO₂-Tyr may serve as an independent biomarker to assess atherosclerosis risk, burden, and incident cardiac events, as well as to monitor the vasculoprotective action of drugs such as statins (hydroxymethylglutaryl-CoA reductase inhibitors) (127)(128).

Patients with lung cancer have significantly higher serum concentrations of nitrated proteins, supporting the presence of oxidative and nitrosative stress (129)(130). Specific locations and targets of Tyr nitration in lung cancer have recently been detailed (131). Increased nitrotyrosine immunostaining is limited mainly to the tumor and not to surrounding healthy tissue or is weakly reactive in different regions of the lung from the same patients with cancer, suggesting a unique environment inside the tumor that may contribute to the disease process. This was noted in squamous cell carcinoma as well as in the well-differentiated adenocarcinoma. Using proteomic and genomic approaches, authors have identified the protein targets (131). Most of the nitrated proteins fall into 4 categories: oxidant defense (such as manganese superoxide dismutase and carbonic anhydrase), energy production (many glycolytic enzymes), structure (such as -actin, - and ß-tubulin, and vimentin), and those involved in apoptosis (annexins).

Tyrosine nitration is one of the earliest markers found in brains from persons affected by AD, in the plaques of brains from persons with multiple sclerosis, and in degenerating upper and lower motor neurons in ALS patients (5)(132). Nitrated -synuclein selectively accumulates in Lewy bodies and protein inclusions in a wide range of pathologies (AD, PD, synucleinopathies, and tauopathies) (5). Nitrated proteins have been evidenced in diverse inflammatory diseases, including ARDS, severe asthma, inflammatory bowel disease, chronic renal failure, rheumatoid arthritis, type 1 and type 2 diabetes, and cystic fibrosis (5). On the other hand, basal protein nitration has been detected under physiologic conditions in most tissues, including plasma and the human pituitary, and some of these nitrated proteins have been identified. Two-dimensional Western blotting and LC-MS/MS analyses have recently been used to detect and characterize 4 nitrated proteins, including actin, in the healthy human pituitary, which participate in neurotransmission, cellular immunity, and cellular structure and motility (133). These data are consistent with the emerging perspective that low-level Tyr nitration may be a physiologic regulator of a signaling pathway (133)(134)(135). Although protein Tyr nitration is a low-yield process in vivo and, under inflammatory conditions, 1 to 5 NO₂-Tyr residues per 10 000 Tyr residues (100–500 µmol/mol) are detected, relatively limited numbers of proteins are preferential targets of nitration, and within these proteins, only one or a few specific Tyr residues can be nitrated (31)(134).

A biomarker worth further development for human use might be o,o'-dityrosine (di-Tyr), which is apparently not metabolized and is also detectable in human urine, in which it is markedly increased in patients with sepsis (136). Its concentration in urine might therefore serve as a noninvasive marker of protein oxidation. Increased concentrations of di-Tyr have been reported in atherosclerosis, in which its accumulation positively correlates with disease severity, and in AD, cystic fibrosis, endstage renal disease, and acute inflammation with or without sepsis (5)(136). Urinary concentrations of di-Tyr are also markedly increased (3- to 7-fold) in children with kwashiorkor (a severe form of protein-energy malnutrition), with or without infection (137). Di-Tyr has also been proposed as a marker of whole-body oxidative stress status such as in atherosclerosis, acute inflammation, and systemic bacterial infections. In this regard, measured di-Tyr concentrations were 100-fold higher in LDL isolated from atherosclerotic lesions than in healthy individuals, and persons suffering from systemic bacterial infections had twice the concentration of di-Tyr in urine than did healthy individuals (40). Moreover, di-Tyr has the advantage of being metabolically stable because, once the 3'–3' carbon–carbon bond is formed, it is released only after enzymatic hydrolysis of the oxidatively modified protein (138).

Several analytical methods have been developed to quantify di-Tyr in vivo and in vitro. These include HPLC with either ultraviolet or fluorescence detection, GC-MS analysis after derivatization, and LC-MS/MS analysis with atmospheric pressure chemical ionization (APCI) or electrospray ionization (139)(140). Unfortunately, HPLC with ultraviolet or fluorescence detection does not have sufficient specificity for the analysis of urine or tissue extracts because several other protein modifications (e.g., the conversion of tryptophan to N-formylkynurenine and the conjugation of HNE with Lys residues) also yield products that exhibit fluorescence characteristics similar to those of di-Tyr. Furthermore, derivatization-based methods may introduce artifactual oxidation product(s) because they require extreme pH values and high temperatures during derivatization. LC-MS–based methods without derivatization may offer a solution to these problems. Recently, an isotope-dilution reversed-phase LC-APCI-MS/MS method and a triple-quadrupole LC-APCI-MS/MS method have been developed for the quantitative determination of di-Tyr in human urine (140)(141). Triple-quadrupole LC-APCI-MS/MS, in particular, is a simple, rapid, sensitive, and reliable method that has a detection limit that is 2.5-fold lower than that of isotope-dilution reversed-phase LC-APCI-MS/MS. Furthermore, it offers a great advantage over other described methods, as it does not require any pretreatment other than centrifugation of the urine sample and addition of the labeled internal standard before injection (140). Actually, reliable quantification of di-Tyr, as well as of NO₂-Tyr, in urine and plasma is attainable only with the MS/MS technology (113)(140)

	Carbonylated Proteins

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Protein carbonyls may be generated by the oxidation of several amino acid side chains (e.g., in Lys, Arg, Pro, and Thr; Fig. 5 ); by the formation of Michael adducts between Lys, His, and Cys residues and ,ß-unsaturated aldehydes, forming ALEs; and by glycation/glycoxidation of Lys amino groups, forming advanced glycation end products (4)(11)(28)(29)(33)(142). The formation of carbonyl compounds is the most general and widely used marker of severe protein oxidation both in vitro and in vivo, with several assays developed for the quantification of these species (15)(16)(143). The chemical stability of protein carbonyls makes them suitable targets for laboratory measurement and is also useful for their storage: their stability during storage for 10 years at –80 °C has been demonstrated (33). As a marker of oxidative damage to proteins, carbonyls have been shown to accumulate during aging, ischemia/reperfusion injury, chronic inflammation, cystic fibrosis, and many of age-related diseases in a variety of organisms (4)(5)(11)(28)(118)(144).

View larger version (17K): [in this window] [in a new window]   Figure 5. Chemical structures of protein carbonyls arising from direct oxidation of amino acid side chains. Shown are 2-pyrrolidone (resulting from direct oxidation of Pro residues), glutamic semialdehyde (resulting from direct oxidation of Arg and Pro residues), -aminoadipic semialdehyde (resulting from direct oxidation of Lys residues), and 2-amino-3-ketobutyric acid (resulting from direct oxidation of Thr residues). For simplicity, stereochemistries are not indicated.

Specific carbonylated proteins have been detected in both the brain tissue and plasma of AD patients (145). The observation of carbonylated proteins in plasma—a body fluid easily obtainable without invasive procedures and, more importantly and unlike brain samples, before the death of the patient—suggests that these oxidized species may be useful as diagnostic biomarkers for (possibly early) AD.

The carbonyl content in plasma proteins (mainly albumin and -globulins) from children with different forms of juvenile chronic arthritis was significantly higher than in healthy children, and more importantly, the carbonyls increased in parallel with the activity of the disease. Correlation between the carbonyl concentration and the activity or the type of chronic juvenile arthritis indicates that plasma protein carbonyl groups are a good marker of inflammatory process activity and may allow the use of carbonyls as a clinical marker of antioxidant barrier impairment in this group of patients as well as for monitoring possible pharmacologic treatments (146).

Plasma concentrations of protein carbonyls, as well as free F₂-IsoPs and protein reduced thiols, differ significantly between chronic kidney disease patients and healthy persons; furthermore, such biomarkers of oxidative/nitrosative stress are significantly higher in patients with diabetes and hypercholesterolemia (147).

Winterbourn et al. (148) determined that protein carbonyl concentrations were increased in both plasma and BAL fluid of patients with severe sepsis or major trauma, which correlated well with measured concentrations of ALEs and with indices of neutrophilia and neutrophil activation. Moreover, patients with acute pancreatitis had significantly increased plasma concentrations of protein carbonyls, which were related to disease severity, thus confirming that this protein modification could be a useful plasma marker of oxidative injury (148).

	Oxidative DNA Damage

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Cellular DNA damage can be caused by ROS generated under different conditions, and several techniques have been developed to measure the oxidatively modified nucleobases in DNA (7)(149)(150). Oxidative DNA damage seems to relate to an increased risk of cancer development later in life (150)(151). DNA subjected to attack by hydroxyl radical generates a wide range of base and sugar modification products (149). Such products can be measured by HPLC, GC-MS, LC-MS, and antibody-based techniques (7)(18)(149)(152). Usually 8-hydroxy-20-deoxyguanosine (8OHdG) is measured as an index of oxidative DNA damage (7)(18)(152). The advantages of and artifacts produced during measurement of 8OHdG in cellular DNA have been reviewed recently (150)(152). Antibody-based methods have also been developed for 8OHdG and are useful for visualization of damage, but they seem likely to be only semiquantitative (7).

DNA can also be damaged by RNS, undergoing mainly nitration and deamination of purines. Methods for the measurement of DNA base nitration and deamination products have been developed but may need more refinement and validation before they can be routinely applied to human materials (7).

None of the analytical methods mentioned above identifies where the oxidative DNA damage is located. Another problem in studying damage to DNA by ROS/RNS is the limited availability of human tissues from which to obtain DNA. Most studies are performed on DNA isolated from lymphocytes or total leukocytes from human blood, and it is assumed (possibly erroneously?) (150) that changes here are reflected in other tissues.

Measurement of 8OHdG in urine has been used to assess "whole-body" oxidative DNA damage. This can be achieved by HPLC and MS techniques (7). However, 8OHdG can arise from degradation of oxidized dGTP in the DNA precursor pool, not just from removal of oxidized guanine residues from DNA by repair processes. Furthermore, there are many other products of oxidative DNA damage. Hence, urinary 8OHdG is a partial measure of damage to guanine residues in DNA and its nucleotide precursor pool, and 8OHdG concentrations may not truly reflect rates of oxidative damage to DNA (149). Nevertheless, the recently completed Biomarkers of Oxidative Stress Study (BOSS), using acute CCl₄ poisoning in rodents as a model for oxidative stress, has demonstrated that 8OHdG in urine is a potential candidate general biomarker of oxidative stress, whereas neither leukocyte DNA-MDA adducts nor DNA-strand breaks resulted from CCl₄ treatment (72).

	Metabolism of Oxidized Biomolecules

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An important question regarding the use of any oxidation product to monitor oxidative stress in vivo is its fate in the body. We therefore examined currently known aspects related to the in vivo metabolism of selected oxidized biomolecules.

The fate of HNE has been investigated both in various isolated cells/organs and cultured cells and in the whole animal following the disposition and biotransformation of exogenous radiolabeled HNE. However, the metabolic pattern of HNE produced within cell membranes or lipoproteins during lipoperoxidation is inevitably very different from that produced by parenteral administration.

HNE is removed mainly by its intracellular metabolism. Mammalian cells possess highly active pathways of HNE metabolism, the HNE degradation rate depending strongly on the cellular concentration and on the initial HNE concentration. The metabolic fate of HNE has been investigated in various mammalian cells and organs, such as hepatocytes, intestinal enterocytes, renal tubular cells, aortic and brain endothelial cells, synovial fibroblasts, neutrophils, thymocytes, the heart, and tumor cells (48). In all cell types investigated, 90%–95% of 100 µmol/L HNE was degraded within 3 min of incubation. At 1 µmol/L HNE, the physiologic serum concentration of 0.1–0.2 µmol/L was already restored after 10–30 s. Current knowledge indicates that the main primary metabolites of HNE are the HNE–GSH adduct, the corresponding carboxylic acid 4-hydroxynonenoic acid (HNA), and the corresponding alcohol 1,4-dihydroxynonene (DHN). The main enzymes involved in the metabolism are therefore glutathione S-transferases (Michael addition of thiols), aldehyde dehydrogenases (NAD+-dependent oxidation of the carbonyl group), and alcohol dehydrogenases (NADH-dependent reduction of the carbonyl group) (48). In most cell types, including hepatocytes, formation of HNE–GSH and HNA is much higher than that of DHN. Additionally, various other enzymes and nonenzymatic reactions should be taken into account for HNE metabolism. Glutathione S-transferase activity is more t