Detection of 8-oxoguanine in cellular DNA using 2,6-diamino-8-oxopurine as an internal standard for high-performance liquid chromatography with electrochemical detection

Elongation and Ligation-Mediated Differential Coding for Label-Free and Locus-Specific Analysis of 8-Oxo-7,8-dihydroguanine in DNA

Oxidative DNA damage is closely associated with the occurrence of numerous human diseases and cancers. 8-Oxo-7,8-dihydroguanine (8-oxoG) is the most prevalent form of DNA damage, and it has become not only an oxidative stress biomarker but also a new epigenetic-like biomarker. However, few approaches are available for the locus-specific detection of 8-oxoG because of the low abundance of 8-oxoG damage in DNA and the limited sensitivity of existing assays. Herein, we demonstrate the elongation and ligation-mediated differential coding for label-free and locus-specific analysis of 8-oxoG in DNA. This assay is very simple without the involvement of any specific labeled probes, complicated steps, and large sample consumption. The utilization of Bsu DNA polymerase can specifically initiate a single-base extension reaction to incorporate dATP into the opposite position of 8-oxoG, endowing this assay with excellent selectivity. The introduction of cascade amplification reaction significantly enhances the sensitivity. The proposed method can monitor 8-oxoG with a limit of detection of 8.21 × 10-19 M (0.82 aM), and it can identify as low as 0.001% 8-oxoG damage from a complex mixture with excessive undamaged DNAs. This method can be further applied to measure 8-oxoG levels in the genomic DNA of human cells under diverse oxidative stress, holding prospect potential in the dynamic monitoring of critical 8-oxoG sites, early clinical diagnosis, and gene damage-related biomedical research.

Construction of a damage site-specific fluorescent biosensor for single-molecule detection of DNA damage

The 8-oxoguanine (8-oxoG) represents the most common DNA damage type, and it has been regarded as the oxidative stress biomarker, but the reported 8-oxoguanine assays are limited by poor specificity and low sensitivity. Herein, we demonstrate the construction of damage site-specific fluorescent biosensor for 8-oxoG assay by integrating single-molecule detection with hyperbranched signal amplification. In this assay, the 8-oxoG damages in DNA can generate free 3' OH with the assistance of formamidopyrimidine DNA glycosylase (Fpg) and polynucleotide kinase (PNK), which subsequently triggers the incorporation of abundant Cy5-labeled dUTPs via terminal deoxynucleotidyl transferase (TDT)-mediated site-specific hyperbranched nucleic acid amplification. After digestion of amplification products with nuclease treatment, abundant mononucleotide Cy5-dUTPs are produced, which will be easily monitored via single-molecule imaging and detection. The introduction of hyperbranched nucleic acid amplification and single-molecule detection can greatly improve the sensitivity to achieve a detection limit of 7.62 × 10^-18 M. This biosensor is highly specific with the capability of discriminating 0.001% 8-oxoG target from the DNA mixture. Moreover, it can be applied for quantitative detection of 8-oxoG damage in genomic DNAs with a detection limit of 0.0017 ng, and even accurately quantifies the absolute number (7025 - 8506) of 8-oxoG damage base in single HeLa cell treated with 150 μM H₂O₂. Importantly, this biosensor can measure the 8-oxoG damage level in different cancer cell lines, facilitating the oxidative damage-associated biomedical researches and clinical diagnosis.

Using DNA devices to track anticancer drug activity

It is beneficial to develop systems that reproduce complex reactions of biological systems while maintaining control over specific factors involved in such processes. We demonstrated a DNA device for following the repair of DNA damage produced by a redox-cycling anticancer drug, beta-lapachone (β-lap). These chips supported ß-lap-induced biological redox cycle and tracked subsequent DNA damage repair activity with redox-modified DNA monolayers on gold. We observed drug-specific changes in square wave voltammetry from these chips at therapeutic ß-lap concentrations of high statistical significance over drug-free control. We also demonstrated a high correlation of this change with the specific ß-lap-induced redox cycle using rational controls. The concentration dependence of ß-lap revealed significant signal changes at levels of high clinical significance as well as sensitivity to sub-lethal levels of ß-lap. Catalase, an enzyme decomposing peroxide, was found to suppress DNA damage at a NQO1/catalase ratio found in healthy cells, but was clearly overcome at a higher NQO1/catalase ratio consistent with cancer cells. We found that it was necessary to reproduce key features of the cellular environment to observe this activity. Thus, this chip-based platform enabled tracking of ß-lap-induced DNA damage repair when biological criteria were met, providing a unique synthetic platform for uncovering activity normally confined to inside cells.

Amelioration of nandrolone decanoate-induced testicular and sperm toxicity in rats by taurine: effects on steroidogenesis, redox and inflammatory cascades, and intrinsic apoptotic pathway

The wide abuse of the anabolic steroid nandrolone decanoate by athletes and adolescents for enhancement of sporting performance and physical appearance may be associated with testicular toxicity and infertility. On the other hand, taurine; a free β-amino acid with remarkable antioxidant activity, is used in taurine-enriched beverages to boost the muscular power of athletes. Therefore, the purpose of this study was to investigate the mechanisms of the possible protective effects of taurine on nandrolone decanoate-induced testicular and sperm toxicity in rats. To achieve this aim, male Wistar rats were randomly distributed into four groups and administered either vehicle, nandrolone decanoate (10mg/kg/week, I.M.), taurine (100mg/kg/day, p.o.) or combination of taurine and nandrolone decanoate, for 8 successive weeks. Results of the present study showed that taurine reversed nandrolone decanoate-induced perturbations in sperm characteristics, normalized serum testosterone level, and restored the activities of the key steroidogenic enzymes; 3β-HSD, and 17β-HSD. Moreover, taurine prevented nandrolone decanoate-induced testicular toxicity and DNA damage by virtue of its antioxidant, anti-inflammatory, and anti-apoptotic effects. This was evidenced by taurine-induced modulation of testicular LDH-x activity, redox markers (MDA, NO, GSH contents, and SOD activity), inflammatory indices (TNF-α, ICAM-1 levels, and MMP-9 gene expression), intrinsic apoptotic pathway (cytochrome c gene expression and caspase-3 content), and oxidative DNA damage markers (8-OHdG level and comet assay). In conclusion, at the biochemical and histological levels, taurine attenuated nandrolone decanoate-induced poor sperm quality and testicular toxicity in rats.

17β-Estradiol and steady-state concentrations of H2O2: antiapoptotic effect in endometrial cells from patients with endometriosis

Increased levels of hydrogen peroxide (H2O2) can initiate protective responses to limit or repair oxidative damage. However, H2O2 signals also fine-tune responses to growth factors and cytokines controlling cell division, differentiation, and proliferation. Because 17β-estradiol (E2) also plays important roles in these processes, and is considered a major risk factor in the development and progression of endometriosis, this study evaluated whether E2 has an antiapoptotic effect on oxidative stress in endometrial cells in combination with steady-state H2O2 levels ([H2O2]ss). Endometrial stromal cells were prepared from the eutopic endometrium of 18 women with and without endometriosis to produce primary cells. These cells were stimulated with E2 for 20h, exposed to [H2O2]ss, and examined for cell viability, proliferation, and apoptosis. The endometrial cells from women with endometriosis maintained the steady state for 120min at high H2O2 concentrations. When they were pretreated with E2 and exposed to [H2O2]ss, a decrease in apoptosis level was observed compared to the control cells (p<0.01). The endometrial cells from patients with endometriosis subjected to both E2 and [H2O2]ss showed increased ERK phosphorylation. These findings suggested that H2O2 is a signaling molecule that downregulates apoptosis in endometrial cells, supporting the fact that endometriosis, albeit a benign disease, shares some features with cancer such as decreased catalase levels. These results link the E2 effects on [H2O2]ss to resistance to apoptosis and progression of endometriosis.

Measurement of oxidatively generated base damage in cellular DNA

This survey focuses on the critical evaluation of the main methods that are currently available for monitoring single and complex oxidatively generated damage to cellular DNA. Among chromatographic methods, HPLC-ESI-MS/MS and to a lesser extent HPLC-ECD which is restricted to a few electroactive nucleobases and nucleosides are appropriate for measuring the formation of single and clustered DNA lesions. Such methods that require optimized protocols for DNA extraction and digestion are sensitive enough for measuring base lesions formed under conditions of severe oxidative stress including exposure to ionizing radiation, UVA light and high intensity UVC laser pulses. In contrast application of GC-MS and HPLC-MS methods that are subject to major drawbacks have been shown to lead to overestimated values of DNA damage. Enzymatic methods that are based on the use of DNA repair glycosylases in order to convert oxidized bases into strand breaks are suitable, even if they are far less specific than HPLC methods, to deal with low levels of single modifications. Several other methods including immunoassays and (32)P-postlabeling methods that are still used suffer from drawbacks and therefore are not recommended. Another difficult topic is the measurement of oxidatively generated clustered DNA lesions that is currently achieved using enzymatic approaches and that would necessitate further investigations.

Influence of iron-overload on DNA damage and its repair in human leukocytes in vitro

Iron is an important element that modulates the production of reactive oxygen species, which are thought to play a causative role in biological processes such as mutagenesis and carcinogenesis. The potential genotoxicity of dietary iron has been seldom studied in human leukocyte and only few reports have investigated in human colon tumor cells. Therefore, DNA damage and repair capacity of human leukocytes were examined using comet assay for screening the potential toxicity of various iron-overloads such as ferric-nitrilotriacetate (Fe-NTA), FeSO(4), hemoglobin and myoglobin, and compared with 200μM of H(2)O(2) and HNE. The iron-overloads tested were not cytotoxic in the range of 10-1000 microM by trypan blue exclusion assay. The exposure of leukocytes to Fe-NTA (500 and 1000 microM), FeSO(4) (250-1000 microM), hemoglobin (10 microM) and myoglobin (250 microM) for 30 min induced significantly higher DNA damage than NC. Treatment with 500 and 1000 microM of Fe-NTA showed a similar genotoxic effect to H(2)O(2), and a significant higher genotoxic effect than HNE. The genotoxicity of FeSO(4) (250-1000 microM), hemoglobin (10 microM) and myoglobin (250 microM) was not significantly different from that of H(2)O(2) and HNE. Iron-overloads generated DNA strand break were rejoined from the first 1h. Their genotoxic effect was not observed at 24h. These data from this study provide additional information on the genotoxicity of iron-overloads and self-repair capacity in human leukocytes.

Oxidation of the sugar moiety of DNA by ionizing radiation or bleomycin could induce the formation of a cluster DNA lesion

Bleomycin, a radiomimetic drug currently used in human cancer therapy, is a well known carcinogen. Its toxicity is mostly attributed to its potentiality to induce DNA double strand breaks likely arising from the formation of two vicinal DNA strand breaks, initiated by C4-hydrogen abstraction on the 2-deoxyribose moiety. In this work we demonstrate that such a hydrogen abstraction reaction is able to induce the formation of a clustered DNA lesion, involving a 3' strand break together with a modified sugar residue exhibiting a reactive alpha,beta-unsaturated aldehyde that further reacts with a proximate cytosine base. The lesion thus produced was detected as a mixture of four isomers by HPLC coupled to tandem mass spectrometry subsequent to DNA extraction and enzymatic digestion. The modified nucleosides that constitute new types of cytosine adducts were identified as the likely two pairs of diastereomers of 6-(2-deoxy-beta-D-erythro-pentofuranosyl)-2-hydroxy-3(3-hydroxy-2-oxopropyl)-2,6-dihydroimidazo[1,2-c]-pyrimidin-5(3H)-one as inferred from mass spectrometry and NMR analyses of the chemically synthesized nucleosides. We demonstrate that bleomycin, and to a minor extent ionizing radiation, are able to induce significant amounts of the cytosine damage in cellular DNA. In addition, the repair kinetic of the lesion in a human lymphocyte cell line is rather slow, with a half-life of 10 h. The 2'-deoxycytidine adducts thus characterized that represent the first example of complex DNA lesions isolated and identified in cellular DNA upon one radical hit are likely to play an important role in the toxicity of bleomycin.

Nuclease P1 digestion/high-performance liquid chromatography, a practical method for DNA quantitation

We have developed a practical method for quantifying DNA. The method is practical in two ways. First, a single enzyme is used to digest the DNA to nucleotides that are then quantified by HPLC under ordinary conditions. Second, the method quantifies DNA even when it is impure. In our method, "nuclease P1/HPLC," the DNA is hydrolyzed by nuclease P1 and the resulting 2'-deoxynucleoside 5'-monophosphates are quantified by HPLC with UV detection. This method was applied to several kinds of genomic DNA in terms of origin and method by which it had been purified. Calf thymus DNA (purified by salt precipitation by the supplier), pig liver DNA (purified by phenolic extraction or by anion-exchange chromatography using a Genomic Tip from Qiagen) and mouse skin DNA (similarly purified) were tested. In some cases a given sample was purified by two of these methods. The values for the amount of DNA by our method were compared with those by three other methods: acid hydrolysis/HPLC (selected as a reference procedure), UV absorbance, and dye binding. Agreement for all DNA samples between the values by our method versus those provided by acid hydrolysis/HPLC was within 10% for amounts of DNA in the 19-54 microg range. In contrast, UV absorbance and the dye-binding assay gave differences up to 30-40% relative to the consistent values furnished by acid hydrolysis and our method. Overall, normalizing the concentrations of the DNA (thymus, liver, skin) by acid hydrolysis/HPLC in 10 samples to values of 1.0 gave the following, relative values and standard deviations: 1.01+/-.07 (nuclease P1/HPLC), 0.8+/-0.17 (dye binding), and 1.1+/-0.1 (UV). Since one cannot assume that any sample of DNA is pure, and determining purity of DNA is difficult, then nuclease P1/HPLC or acid hydrolysis/HPLC is recommended rather than the UV absorbance or dye binding for quantifying DNA whenever an accurate value is important.

Efficient and high fidelity incorporation of dCTP opposite 7,8-dihydro-8-oxodeoxyguanosine by Sulfolobus solfataricus DNA polymerase Dpo4

DNA polymerases insert dATP opposite the oxidative damage product 7,8-dihydro-8-oxodeoxyguanosine (8-oxoG) instead of dCTP, to the extent of >90% with some polymerases. Steady-state kinetics with the Y-family Sulfolobus solfataricus DNA polymerase IV (Dpo4) showed 90-fold higher incorporation efficiency of dCTP > dATP opposite 8-oxoG and 4-fold higher efficiency of extension beyond an 8-oxoG:C pair than an 8-oxoG:A pair. The catalytic efficiency for these events (with dCTP or C) was similar for G and 8-oxoG templates. Mass spectral analysis of extended DNA primers showed >/=95% incorporation of dCTP > dATP opposite 8-oxoG. Pre-steady-state kinetics showed faster rates of dCTP incorporation opposite 8-oxoG than G. The measured K(d)(,dCTP) was 15-fold lower for an oligonucleotide containing 8-oxoG than with G. Extension beyond an 8-oxoG:C pair was similar to G:C and faster than for an 8-oxoG:A pair, in contrast to other polymerases. The E(a) for dCTP insertion opposite 8-oxoG was lower than for opposite G. Crystal structures of Dpo4 complexes with oligonucleotides were solved with C, A, and G nucleoside triphosphates placed opposite 8-oxoG. With ddCTP, dCTP, and dATP the phosphodiester bonds were formed even in the presence of Ca(2+). The 8-oxoG:C pair showed classic Watson-Crick geometry; the 8-oxoG:A pair was in the syn:anti configuration, with the A hybridized in a Hoogsteen pair with 8-oxoG. With dGTP placed opposite 8-oxoG, pairing was not to the 8-oxoG but to the 5' C (and in classic Watson-Crick geometry), consistent with the low frequency of this frameshift event observed in the catalytic assays.

TCDD-induced homologous recombination: the role of the Ah receptor versus oxidative DNA damage

The environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) elicits numerous biological responses including carcinogenicity. The molecular mechanism by which TCDD exerts its tumorigenic effects is unclear, since it does not directly damage DNA. TCDD-initiated toxicity can be mediated by the aryl hydrocarbon receptor (AhR) pathway and/or via increased oxidative stress. DNA damage, including DNA oxidation, can induce DNA double-strand breaks, which can be repaired through homologous recombination. Excessive DNA double-strand breaks may promote aberrant DNA recombination, which can lead to detrimental genetic changes and ultimately to carcinogenesis. TCDD has been shown to induce homologous recombination but the molecular mechanism mediating these events are unknown. To investigate the role of the AhR and oxidative DNA damage in mediating TCDD-induced homologous recombination we used a Chinese hamster ovary (CHO) cell line containing a neo direct repeat recombination substrate (CHO 3-6). CHO 3-6 cells were exposed to TCDD (50, 500 or 1000 pM) in the presence or absence of an AhR antagonists (0.1 microM alpha-naphthoflavone (alpha-NF)) for 6 or 24 h and 2 weeks later homologous recombination frequencies were determined by counting the number of neo expressing, G418-resistant colonies per live cells plated. TCDD-initiated DNA oxidation was determined by measuring the formation of 8-hydroxy-2'-deoxyguanosine via HPLC and electrochemical detection. Exposure to 500 pM TCDD for 24 h significantly increased the frequency of homologous recombination. Southern blot analysis on G418-resistant colonies determined that TCDD induced both conservative gene conversion events and deletion events. DNA oxidation was not increased in cells exposed to TCDD for either 6 or 24 h. However, alpha-naphthoflavone exposure resulted in a significant decrease in TCDD-induced homologous recombination frequency. These results suggest that TCDD-initiated homologous recombination in CHO 3-6 cells is mediated by the AhR and not via increased oxidative stress.

The effects of temperature reduction on gene expression and oxidative stress in skeletal muscle from adult zebrafish

Longevity is inversely proportional to ambient temperature in ectothermic organisms such as fish. However, the mechanism by which reducing temperature over a physiological range increases life span is not known and available data are derived primarily from invertebrates. With a rodent-like longevity and abundant biological resources, the zebrafish is an ideal vertebrate ectothermic model in which to investigate this phenomenon. As an initial approach, the effects of a year-long 10 degrees C reduction in water temperature on global gene expression in tail skeletal muscle from adult zebrafish were determined using an oligonucleotide microarray representing 15,512 genes. Expression levels for approximately 600 genes were up-regulated by 1.7-fold or greater by the reduction in temperature, while a similar number of transcripts were down regulated by more than 1.7-fold. Using gene ontology (GO) classifications for molecular function, two functional groups, "oxygen and reactive oxygen species metabolism" and "response to oxidative stress," were found to be overrepresented among up-regulated genes. Transcripts levels for the genes in these two categories were increased by temperature reduction (TR). However, temperature reduction did not suppress lipid peroxidation potential, protein carbonyl content, or 8-oxoguanine level. Additional studies will be required to further delineate the role of altered gene expression and oxidative stress on the longevity-promoting effects of temperature reduction.

Oxidative stress-induced homologous recombination as a novel mechanism for phenytoin-initiated toxicity

Although the mechanism(s) of phenytoin-initiated toxicity is unknown, phenytoin can be enzymatically bioactivated to a reactive intermediate leading to increased formation of reactive oxygen species, which can damage essential macromolecules, including DNA. The oxidation of DNA can induce DNA double-strand breaks (DSBs), which may be repaired through homologous recombination. Increased levels of DSBs may induce hyper-recombination, leading to deleterious genetic changes. We hypothesize that these genetic changes mediate phenytoin-initiated toxicity. To investigate this hypothesis we used a Chinese hamster ovary cell line containing a neo direct repeat recombination substrate to determine whether phenytoin-initiated DNA oxidation increases homologous recombination. Cells were treated with 0 to 800 microM phenytoin for 5 or 24 h, and homologous recombination frequencies and recombinant product structures were determined. Phenytoin-initiated DNA oxidation was determined by measuring the formation of 8-hydroxy-2'-deoxyguanosine. We demonstrate that phenytoin increases both DNA oxidation and homologous recombination in a concentration- and time-dependent manner. All recombination products analyzed arose via gene conversion without associated crossover. Our data demonstrate that phenytoin-initiated DNA damage can induce homologous recombination, which may be a novel mechanism mediating phenytoin-initiated toxicity.

Oxidative DNA damage: biological significance and methods of analysis

All forms of aerobic life are subjected constantly to oxidant pressure from molecular oxygen and also reactive oxygen species (ROS), produced during the biochemical utilization of O2 and prooxidant stimulation of O2 metabolism. ROS are thought to influence the development of human cancer and more than 50 other human diseases. To prevent oxidative DNA damage (protection) or to reverse damage, thereby preventing mutagenesis and cancer (repair), the aerobic cell possesses antioxidant defense systems and DNA repair mechanisms. During the last 20 years, many analytical techniques have been developed to monitor oxidative DNA base damage. High-performance liquid chromatography-electrochemical detection and gas chromatography-mass spectrometry are the two pioneering contributions to the field. Currently, the arsenal of methods available include the promising high-performance liquid chromatography-tandem mass spectrometry technique, capillary electrophoresis, 32P-postlabeling, fluorescence postlabeling, 3H-postlabeling, antibody-base immunoassays, and assays involving the use of DNA repair glycosylases such as the comet assay, the alkaline elution assay, and the alkaline unwinding method. Recently, the use of liquid chromatography-mass spectrometry has been introduced for the measurement of a number of modified nucleosides in oxidatively damaged DNA. The bulk of available chromatographic methods aimed at measuring individual DNA base lesions require either chemical hydrolysis or enzymatic digestion of oxidized DNA, following extraction from cells or tissues. The effect of experimental conditions (DNA isolation, hydrolysis, and/or derivatization) on the levels of oxidatively modified bases in DNA is enormous and has been studied intensively in the last 10 years.

Free radical-induced damage to DNA: mechanisms and measurement

Free radicals are produced in cells by cellular metabolism and by exogenous agents. These species react with biomolecules in cells, including DNA. The resulting damage to DNA, which is also called oxidative damage to DNA, is implicated in mutagenesis, carcinogenesis, and aging. Mechanisms of damage involve abstractions and addition reactions by free radicals leading to carbon-centered sugar radicals and OH- or H-adduct radicals of heterocyclic bases. Further reactions of these radicals yield numerous products. Various analytical techniques exist for the measurement of oxidative damage to DNA. Techniques that employ gas chromatography (GC) or liquid chromatography (LC) with mass spectrometry (MS) simultaneously measure numerous products, and provide positive identification and accurate quantification. The measurement of multiple products avoids misleading conclusions that might be drawn from the measurement of a single product, because product levels vary depending on reaction conditions and the redox status of cells. In the past, GC/MS was used for the measurement of modified sugar and bases, and DNA-protein cross-links. Recently, methodologies using LC/tandem MS (LC/MS/MS) and LC/MS techniques were introduced for the measurement of modified nucleosides. Artifacts might occur with the use of any of the measurement techniques. The use of proper experimental conditions might avoid artifactual formation of products in DNA. This article reviews mechanistic aspects of oxidative damage to DNA and recent developments in the measurement of this type of damage using chromatographic and mass spectrometric techniques.

Measurement of 8-oxo-2'-deoxyguanosine and 8-oxo-2'-deoxyadenosine in DNA and human urine by high performance liquid chromatography-electrospray tandem mass spectrometry

A method for the determination of 8-oxo-2'-deoxyguanosine and 8-oxo-2'-deoxyadenosine in DNA and urine by High Performance Liquid Chromatography (HPLC)-Tandem Mass Spectrometry is described. For the urine samples there is no sample preparation except for addition of buffer and internal standards followed by redissolvation of precipitate containing 8-oxo-2'-deoxyguanosine and a centrifugation step before the samples are injected onto the HPLC column. The detection limit for 8-oxo-2'-deoxyguanosine and 8-oxo-2'-deoxyadenosine is approximately 0.3 nM corresponding to 7.5 fmol injected. Long runs, that is, > 50 samples, can be analyzed with only minimal loss of sensitivity. The concentrations excreted into urine samples from humans are between 1 and 100 nM for 8-oxo-2'-deoxyguanosine and below 0.3 nM for 8-oxo-2'-deoxyadenosine. In calf thymus DNA levels down to about 1 oxidized guanosine and adenosine per 10(6) unmodified bases can be detected. High levels of 8-oxo-2'-deoxyguanosine were found, 30 per 10(6) 2'-deoxyguanosine, levels of 8-oxo-2'-deoxyadenosine are at or below the detection limit. These findings indicate that High Performance Liquid Chromatography-Tandem Mass Spectrometry is a highly sensitive and specific method for analysis of oxidative DNA modifications in tissue as well as for analysis of excretion of oxidized nucleotides into urine that ensures a minimum artifact formation.

Measurement of 8-hydroxy-2'-deoxyguanosine in DNA by high-performance liquid chromatography-mass spectrometry: comparison with measurement by gas chromatography-mass spectrometry

Measurement of 8-hydroxy-2'-deoxyguanosine (8-OH-dGuo) in DNA by high-performance liquid chromatography/mass spectrometry (LC/MS) was studied. A methodology was developed for separation by LC of 8-OH-dGuo from intact and modified nucleosides in DNA hydrolyzed by a combination of four enzymes: DNase I, phosphodiesterases I and II and alkaline phosphatase. The atmospheric pressure ionization-electrospray process was used for mass spectral measurements. A stable isotope-labeled analog of 8-OH-dGuo was used as an internal standard for quantification by isotope-dilution MS (IDMS). Results showed that LC/IDMS with selected ion-monitoring (SIM) is well suited for identification and quantification of 8-OH-dGuo in DNA at background levels and in damaged DNA. The sensitivity level of LC/IDMS-SIM was found to be comparable to that reported previously using LC-tandem MS (LC/MS/MS). It was found that approximately five lesions per 10(6) DNA bases can be detected using amounts of DNA as low as 2 microgram. The results also suggest that this lesion may be quantified in DNA at levels of one lesion per 10(6) DNA bases, or even lower, when more DNA is used. Up to 50 microgram of DNA per injection were used without adversely affecting the measurements. Gas chromatography/isotope-dilution MS with selected-ion monitoring (GC/IDMS-SIM) was also used to measure this compound in DNA following its removal from DNA by acidic hydrolysis or by hydrolysis with Escherichia coli Fpg protein. The background levels obtained by LC/IDMS-SIM and GC/IDMS-SIM were almost identical. Calf thymus DNA and DNA isolated from cultured HeLa cells were used for this purpose. This indicates that these two techniques can provide similar results in terms of the measurement of 8-OH-dGuo in DNA. In addition, DNA in buffered aqueous solution was damaged by ionizing radiation at different radiation doses and analyzed by LC/IDMS-SIM and GC/IDMS-SIM. Again, similar results were obtained by the two techniques. The sensitivity of GC/MS-SIM for 7,8-dihydro-8-oxoguanine was also examined and found to be much greater than that of LC/MS-SIM and the reported sensitivity of LC/MS/MS for 8-OH-dGuo. Taken together, the results unequivocally show that LC/IDMS-SIM is well suited for sensitive and accurate measurement of 8-OH-dGuo in DNA and that both LC/IDMS-SIM and GC/IDMS-SIM can provide similar results.

Comparison of the levels of 8-hydroxyguanine in DNA as measured by gas chromatography mass spectrometry following hydrolysis of DNA by Escherichia coli Fpg protein or formic acid

8-hydroxyguanine (8-OH-Gua) is one of many lesions generated in DNA by oxidative processes including free radicals. It is the most extensively investigated lesion, due to its miscoding properties and its potential role in mutagenesis, carcinogenesis and aging, and also to the existence of analytical methods using HPLC and gas chromatography mass spectrometry (GC/MS). Some studies raised the possibility of artifacts generated during sample preparation. We investigated several experimental conditions in order to eliminate possible artifacts during the measurement of 8-OH-Gua by GC/MS. Derivatization has been reported to produce artifacts by oxidation of guanine to 8-OH-Gua in acid-hydrolysates of DNA, although the extent of artifacts seems to depend on experimental conditions. For removal of 8-OH-Gua from DNA, we used either formic acid hydrolysis or specific enzymatic hydrolysis with Escherichia coli Fpg protein. Derivatization of enzyme-hydrolysates should not generate additional 8-OH-Gua because of the absence of guanine, which is not released by the enzyme, whereas guanine released by acid may be oxidized to yield 8-OH-Gua. The measurement of 8-OH-Gua in calf thymus DNA by GC/isotope-dilution MS (GC/IDMS) using these two different hydrolyses yielded similar levels of 8-OH-Gua. This indicated that no artifacts occurred during derivatization of acid-hydrolysates of DNA. Pyridine instead of acetonitrile and room temperature were used during derivatization. Pyridine reduced the level of 8-OH-Gua, when compared with acetonitrile, indicating its potential to prevent oxidation. Two different stable-isotope labeled analogs of 8-OH-Gua used as internal standards for GC/IDMS analysis yielded similar results. A comparison of the present results with the results of recent trials by the European Standards Committee for Oxidative DNA Damage (ESCODD) is also presented.

Protection against oxidative damage and cell death by the natural antioxidant ergothioneine

The natural antioxidant ergothioneine (EGT) was tested for its ability to inhibit cell death caused by hydrogen peroxide (H2O2) and to inhibit DNA oxidation by peroxynitrite (ONOO-) in human neuronal hybridoma cell line (N-18-RE-105). High concentrations of EGT (5 mM) were tolerated by the N-18-RE-105 cells. N-acetylcysteine (NAC) was not well tolerated by the cells at concentrations greater than 3 mM (cell viability averaged 50%). Increasing concentrations of EGT increases cell viability in the presence of NAC. EGT at concentrations up to 2 mM weakly improved cell viability in the presence of H2O2. NAC at concentrations up to 2 mM weakly decreased, but not significantly, the viability of the cells. At a higher concentration of 5 mM, NAC weakly protected the neuronal cells against the H2O2-induced cell death. The protection was significantly enhanced by preincubation with EGT. Ergothioneine inhibited ONOO(-)-induced oxidative damage in isolated calf thymus DNA and DNA in N-18-RE-105 cells. The concentration of EGT in human and mammalian tissue has been estimated to be 1-2 mM, which suggests that EGT may serve as a non-toxic thiol buffering antioxidant in vivo and may find applications in pharmaceutical preparations where oxidative stability is desired.

Determination of 8-oxoguanine in human plasma and urine by high-performance liquid chromatography with electrochemical detection

A highly sensitive and selective method for determining 8-oxoguanine in plasma and urine was developed by high-performance liquid chromatography with electrochemical detection. The compound was separated by gradient elution on a C18 reversed-phase column with a mobile phase of acetonitrile and 0.1 M sodium acetate, pH 5.2. 8-Hydroxy-2'-deoxyguanosine was used as internal standard. 8-Oxoguanine was detected electrochemically by setting the potential to +300 mV vs. Pd reference. The sensitivity of the assay was 22 ng/ml with a signal-to-noise ratio of 7:1. The within-day relative standard deviations for 8-oxoguanine quality control samples with concentrations of 3340, 1340 and 84 ng/ml were 3.6, 4.3 and 5.7% for plasma, and 4.1, 4.6 and 6.2% for urine, respectively. The day-to-day relative standard deviations for the same samples were 3.8, 6.8 and 7.1% for plasma, and 3.9, 7.0 and 7.9% for urine, respectively. The method is designed to study the pharmacokinetics and metabolic fate of O6-benzylguanine in a phase I clinical trial. Previously, O6-benzyl-8-oxoguanine was identified as the primary metabolite of O6-benzylguanine in humans. We now demonstrate that 8-oxoguanine is a further metabolite of O6-benzylguanine.

The effect of experimental conditions on the levels of oxidatively modified bases in DNA as measured by gas chromatography-mass spectrometry: how many modified bases are involved? Prepurification or not?

Recently, an artifactual formation of a number of modified DNA bases has been alleged during derivatization of DNA hydrolysates to be analyzed by gas chromatography-mass spectrometry (GC-MS). These modified bases were 8-hydroxyguanine (8-OH-Gua), 5-hydroxycytosine (5-OH-Cyt), 8-hydroxyadenine (8-OH-Ade), 5-hydroxymethyluracil (5-OHMeUra), and 5-formyluracil, which represent only a small percentage of more than 20 modified DNA bases that can be analyzed by GC-MS. However, relevant papers reporting the levels of these modified bases in DNA of various sources have not been cited, and differences in experimental procedures have not been discussed. We investigated the levels of modified bases in calf thymus DNA by GC-MS using derivatization at three different temperatures. The results obtained with GC/isotope-dilution MS showed that the levels of 5-OH-Cyt, 8-OH-Ade, 5-OH-Ura, and 5-OHMeUra were not affected by increasing the derivatization temperature from 23 degrees C to 120 degrees C. The level of 8-OH-Gua was found to be higher at 120 degrees C. However, this level was much lower than those reported previously. Formamidopyrimidines were readily analyzed in contrast to some recent claims. The addition of trifluoroacetic acid (TFA) adversely affected the levels of pyrimidine-derived lesions, suggesting that TFA is not suitable for simultaneous measurement of both pyrimidine- and purine-derived lesions. The data obtained were also compared with those previously published. Our data and this comparison indicate that no artifactual formation of 5-OH-Cyt, 8-OH-Ade, and 5-OHMeUra occurred under our experimental conditions in contrast to recent claims, and no prepurification of DNA hydrolysates by a tedious procedure is necessary for accurate quantification of these compounds. The artifactual formation of 8-OH-Gua can be eliminated by derivatization at room temperature for at least 2 h, without the use of TFA. The results in this article and their comparison with published data indicate that different results may be obtained in different laboratories using different experimental conditions. The data obtained in various laboratories should be compared by discussing all relevant published data and scientific facts, including differences between experimental conditions used in different laboratories.

Discrepancies in the measurement of UVC-induced 8-oxo-2'-deoxyguanosine: implications for the analysis of oxidative DNA damage

Ultraviolet (UV) light-induced indirect, oxidative damage to DNA has received increasing attention with respect to the mutagenic and carcinogenic effects of solar radiation. An oxidative lesion that has raised particular interest because of its qualitative and quantitative importance is 8-oxo-2'-deoxyguanosine. This deoxynucleoside lesion is most frequently measured by high performance liquid chromatography with electrochemical detection (HPLC-EC) following enzymatic hydrolysis of DNA or as the base equivalent, 8-oxoguanine, by gas chromatography-mass spectrometry (GC-MS) following acid hydrolysis of DNA. We have noted a discrepancy in the literature whereby the levels of 8-oxo-2'-deoxyguanosine measured by HPLC-EC in UVC-irradiated DNA are significantly higher than when 8-oxoguanine is measured by GC-MS. By making use of the availability of both HPLC-EC and stable-isotope dilution GC-MS methodologies in our laboratory we have confirmed the discrepancy noted in the literature by parallel analysis of the same UVC-irradiated calf thymus DNA samples. Furthermore, analysis of the UVC-induced product by UV-visible spectrophotometry, voltammetry and its detection by a monoclonal antibody which recognises 8-oxo-2'-deoxyguanosine strongly suggests that the product is indeed 8-oxo-2'-deoxyguanosine. Partial explanation for this discrepancy could be an inordinate resistance of UVC-irradiated DNA to formic acid hydrolysis. However, we cannot completely exclude the possibility that there is a formic acid-labile species which co-elutes with 8-oxo-2'-deoxyguanosine in enzymatically digested UVC-irradiated DNA. Whether this phenomenon is unique to UV-irradiation damage or occurs with other systems that cause oxidative damage to DNA awaits further investigation. Irrespective of the exact mechanism, there will be significant implications for the analysis of oxidative DNA damage.

Repair of iron-induced DNA oxidation by the flavonoid myricetin in primary rat hepatocyte cultures

Oxidative DNA damage and its repair in primary rat hepatocyte cultures was investigated following 4 h of incubation with the toxic iron chelate, ferric nitrilotriacetate (Fe-NTA), in the presence or absence of the potent protective flavonoid myricetin (25-50-100 microM). Seven DNA base oxidation products were quantified in DNA extracts by gas chromatography-mass spectrometry (GC-MS) in selected ion monitoring mode. Concomitantly, DNA repair capacity of hepatocytes was estimated by the release of oxidized-base products into culture media, using the same GC-MS method. A genotoxic effect of Fe-NTA (100 microM) in hepatocytes was evidenced by a severe increase in DNA oxidation over basal levels, with accumulation in cellular DNA of five oxidation products derived from both purines and pyrimidines. This prooxidant effect of iron was also noted by an induction of lipid peroxidation, estimated by free malondialdehyde production. Addition of increasing concentrations of myricetin (25-50-100 microM) simultaneously with iron prevented both lipid peroxidation and accumulation of oxidation products in DNA. Moreover, as an activation of DNA repair pathways, myricetin stimulated the release of DNA oxidation bases into culture media, especially of purine-derived oxidation products. This removal of highly mutagenic oxidation products from DNA of hepatocytes might correspond to an activation of DNA excision-repair enzymes by myricetin. This was verified by RNA blot analysis of DNA polymerase beta gene expression which was induced by myricetin in a dose-dependent manner. This represented a novel and original mechanism of cytoprotection by myricetin against iron-induced genotoxicity via stimulation of DNA repair processes. Since iron-induced DNA damage and inefficient repair in hepatocytes could be related to genotoxicity and most probably to hepatocarcinogenesis, modulation of these processes in vitro by myricetin might be relevant in further prevention of liver cancer derived from iron overload pathologies.

Facts and artifacts in the measurement of oxidative base damage to DNA

This short survey is aimed at critically evaluating the main available methods for measuring oxidative base damage within cellular DNA. Emphasis is placed on separative methods which are currently widely applied. These mostly concern high performance liquid chromatography (HPLC) and gas chromatography (GC) associated with sensitive detection techniques such as electrochemistry (EC) and mass spectrometry (MS). In addition, the comparison is extended to 32p-postlabeling methods, immunoassays and measurement of two main classes of oxidative DNA damage within isolated cells. It may be concluded that the HPLC-electrochemical detection (ECD) method, even if restricted to the measurement of only a few electroactive oxidized bases and nucleosides, is the simplest and safest available method at the moment. In contrast, the more versatile GC-MS method, which requires a HPLC pre-purification step in order to prevent artifactual oxidation of overwhelming normal bases to occur during derivatization, is more tedious and its sensitivity may be questionable. Alternative simpler procedures of background prevention for the GC-MS assay, which, however, remain to be validated, include low-temperature for derivatization and addition of antioxidants to the silylating reagents. Interestingly, similar levels of 8-oxo-7,8-dihydroguanine were found in cellular DNA using HPLC-ECD, HPLC-MS/MS and HPLC/32P-postlabeling methods. However, it should be noted that the level of cellular 8-oxodGuo, thus determined, is on average basis 10-fold higher than that was inferred for more indirect measurement involving the use of DNA repair enzymes with methods on isolated cells. Further efforts should be made to resolve this apparent discrepancy. In addition, the question of the biological validation of the non-invasive measurement of oxidized bases and nucleosides in urine is addressed.