Simultaneous determination of N7-alkylguanines in DNA by isotope-dilution LC-tandem MS coupled with automated solid-phase extraction and its application to a small fish model

Quantification of Intracellular DNA-Protein Cross-Links with N7-Methyl-2'-Deoxyguanosine and Their Contribution to Cytotoxicity

The major product of DNA-methylating agents, N7-methyl-2'-deoxyguanosine (MdG), is a persistent lesion in vivo, but it is not believed to have a large direct physiological impact. However, MdG reacts with histone proteins to form reversible DNA-protein cross-links (DPCMdG), a family of DNA lesions that can significantly threaten cell survival. In this paper, we developed a tandem mass spectrometry method for quantifying the amounts of MdG and DPCMdG in nuclear DNA by taking advantage of their chemical lability and the concurrent release of N7-methylguanine. Using this method, we determined that DPCMdG is formed in less than 1% yield based upon the levels of MdG in methyl methanesulfonate (MMS)-treated HeLa cells. Despite its low chemical yield, DPCMdG contributes to MMS cytotoxicity. Consequently, cells that lack efficient DPC repair by the DPC protease SPRTN are hypersensitive to MMS. This investigation shows that the downstream chemical and biochemical effects of initially formed DNA damage can have significant biological consequences. With respect to MdG formation, the initial DNA lesion is only the beginning.

Recent developments in DNA adduct analysis using liquid chromatography coupled with mass spectrometry

The formation of DNA adducts by genotoxic agents is an early event in cancer development, and it may lead to gene mutations, thereby initiating tumor development. The measurement of DNA adducts can provide critical information about the genotoxic potential of a chemical and its mechanism of carcinogenesis. In recent decades, liquid chromatography coupled with mass spectrometry has become the most important technique for analyzing DNA adducts. The improvements in resolution achievable with new chromatographic separation techniques coupled with the high specificity and sensitivity and wide dynamic range of new mass spectrometry systems have been used for both qualitative and quantitative analyses of DNA adducts. This review discusses the challenges in qualitative and quantitative analyses of DNA adducts by liquid chromatography coupled with mass spectrometry and highlights recent developments towards overcoming the limitations of liquid chromatography coupled with mass spectrometry methods. The key steps and new solutions, such as sample preparation, mass spectrometry fragmentation, and method validation, are summarized. In addition, the fundamental principles and latest advances in DNA adductomic approaches are reviewed.

Emerging Technologies in Mass Spectrometry-Based DNA Adductomics

The measurement of DNA adducts, the covalent modifications of DNA upon the exposure to the environmental and dietary genotoxicants and endogenously produced electrophiles, provides molecular evidence for DNA damage. With the recent improvements in the sensitivity and scanning speed of mass spectrometry (MS) instrumentation, particularly high-resolution MS, it is now feasible to screen for the totality of DNA damage in the human genome through DNA adductomics approaches. Several MS platforms have been used in DNA adductomic analysis, each of which has its strengths and limitations. The loss of 2′-deoxyribose from the modified nucleoside upon collision-induced dissociation is the main transition feature utilized in the screening of DNA adducts. Several advanced data-dependent and data-independent scanning techniques originated from proteomics and metabolomics have been tailored for DNA adductomics. The field of DNA adductomics is an emerging technology in human exposure assessment. As the analytical technology matures and bioinformatics tools become available for analysis of the MS data, DNA adductomics can advance our understanding about the role of chemical exposures in DNA damage and disease risk.

Direct-acting DNA ethylating agents associated with tobacco use primarily originate from the tobacco itself, not combustion

Unburnt tobacco and tobacco smoke contain a variety of carcinogens, exposure to which are causally associated with the incidence of several human cancers. Herein, we used isotope-dilution LC-MS/MS for the quantification of alkylated purines in DNA, following in vitro exposure to aqueous extracts of tobacco itself, and tobacco smoke. Our results demonstrated the presence of direct-acting ethylating agent(s) in unburnt tobacco, which 4.0-6.3 times exceeded that in the particulate phase of sidestream cigarette smoke and 6.8-8.9 times exceeded that in mainstream smoke. Interestingly, particulate phase of sidestream cigarette smoke exhibited higher ethylating potency than that in mainstream smoke. This finding refutes the previous assumptions that the ethylating agent(s) associated with smoking, are derived from cigarette smoke. Indeed, our data show that combustion of tobacco actually decreases the ethylating potency of tobacco. Although the identity of this agent(s) remains unknown, our data suggest that it is highly hydrophilic, and hence likely to be easily extracted by saliva. This would allow intimate contact with the tissues of the oropharyngeal cavity. Taken together, these results have profound implications for tobacco use, in particular for tobacco chewers and passive smokers, whose exposure to ethylating agent(s) is greater than previously thought.

Novel approach to integrated DNA adductomics for the assessment of in vitro and in vivo environmental exposures

Adductomics is expected to be useful in the characterization of the exposome, which is a new paradigm for studying the sum of environmental causes of diseases. DNA adductomics is emerging as a powerful method for detecting DNA adducts, but reliable assays for its widespread, routine use are currently lacking. We propose a novel integrated strategy for the establishment of a DNA adductomic approach, using liquid chromatography-triple quadrupole tandem mass spectrometry (LC-QqQ-MS/MS), operating in constant neutral loss scan mode, screening for both known and unknown DNA adducts in a single injection. The LC-QqQ-MS/MS was optimized using a representative sample of 23 modified 2'-deoxyribonucleosides reflecting a range of biologically relevant DNA lesions. Six internal standards (ISTDs) were evaluated for their ability to normalize, and hence correct, possible variation in peak intensities arising from matrix effects, and the quantities of DNA injected. The results revealed that, with appropriate ISTDs adjustment, any bias can be dramatically reduced from 370 to 8.4%. Identification of the informative DNA adducts was achieved by triggering fragmentation spectra of target ions. The LC-QqQ-MS/MS method was successfully applied to in vitro and in vivo studies to screen for DNA adducts formed following representative environmental exposures: methyl methanesulfonate (MMS) and five N-nitrosamines. Interestingly, five new DNA adducts, induced by MMS, were discovered using our adductomic approach-an added strength. The proposed integrated strategy provides a path forward for DNA adductomics to become a standard method to discover differences in DNA adduct fingerprints between populations exposed to genotoxins, and facilitate the field of exposomics.

Sensitive Detection of 8-Nitroguanine in DNA by Chemical Derivatization Coupled with Online Solid-Phase Extraction LC-MS/MS

8-Nitroguanine (8-nitroG) is a major mutagenic nucleobase lesion generated by peroxynitrite during inflammation and has been used as a potential biomarker to evaluate inflammation-related carcinogenesis. Here, we present an online solid-phase extraction (SPE) LC-MS/MS method with 6-methoxy-2-naphthyl glyoxal hydrate (MTNG) derivatization for a sensitive and precise measurement of 8-nitroG in DNA. Derivatization optimization revealed that an excess of MTNG is required to achieve complete derivatization in DNA hydrolysates (MTNG: 8-nitroG molar ratio of 3740:1). The use of online SPE effectively avoided ion-source contamination from derivatization reagent by washing away all unreacted MTNG before column chromatography and the ionization process in mass spectrometry. With the use of isotope-labeled internal standard, the detection limit was as low as 0.015 nM. Inter- and intraday imprecision was <5.0%. This method was compared to a previous direct LC-MS/MS method without derivatization. The comparison showed an excellent fit and consistency, suggesting that the present method has satisfactory effectiveness and reliability for 8-nitroG analysis. This method was further applied to determine the 8-nitroG in human urine. 8-NitroG was not detectable using LC-MS/MS with derivatization, whereas a significant false-positive signal was detected without derivatization. It highlights the use of MTNG derivatization in 8-nitroG analysis for increasing the method specificity.

Comprehensive analysis of the formation and stability of peroxynitrite-derived 8-nitroguanine by LC-MS/MS: Strategy for the quantitative analysis of cellular 8-nitroguanine

Peroxynitrite is a major oxidizing and nitrating biological agent formed at sites of inflammation. Peroxynitrite can cause DNA damage and is thought to contribute to inflammation-related carcinogenesis. This study describes a sensitive and reliable liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the direct determination of peroxynitrite-derived 8-nitroguanine (8-nitroGua) in DNA hydrolysates. This method exhibited a sensitive detection limit of 3 fmol and inter- and intraday imprecision of <10% and was applied to systemically examine the formation and stability of peroxynitrite-derived 8-nitroGua in different DNA substrates under various conditions. The 8-nitroGua formation was maximal at pH 8. The formation rate of 8-nitroGua in different DNA substrates decreased in the order of monodeoxynucleoside>single-stranded DNA>double-stranded DNA. A stability test revealed that the half-life for the depurination of 8-nitroGua from DNA was short and affected by both the temperature and DNA structure. When present in monodeoxynucleoside, the half-life of 8-nitroGua was estimated to be ~6min at 25°C and 2.3h at ~0°C. In single-stranded DNA, the half-life varied from 1.6h at 37°C to 533h at -20°C, whereas the half-life increased from 2.4h at 37°C to 1115h at -20°C in double-stranded DNA. We demonstrated that the measurement of 8-nitroGua in isolated DNA is not practicable because 8-nitroGua is unstable and lost during DNA extraction from cell. Therefore, we suggest that directly detecting cellular 8-nitroGua following nuclear membrane lysis is an alternative measure of the nitrative damage of nucleic acids, accounting for both DNA and RNA lesions within cells.

Lipid mediators involved in the oxidative stress and antioxidant defence of human lung cancer cells

Background

The oxidative modifications of bioactive macromolecules have important roles in carcinogenesis. Of particular interest are lipid peroxidation products, which are involved in the activation of Nrf2 and endocannabinoids that affect cancer progression.

Methods

In lung cancer tissues (squamous cell lung carcinoma - SCC and adenocarcinoma - AC), the glutathione peroxidase and catalase activity and glutathione level, together with the expression of Nrf2 and its activators/inhibitors were estimated. The oxidative modifications of DNA (8-hydroxy-2′-deoxyguanosine and N7-methylguanine), endocannabinoids (anandamide and 2- arachidonylglyceriol), their receptors (CB1/2, TRV1, GPR55), phospholipid fatty acids (arachidonic, linoleic and docosahexaenoic), and reactive aldehydes (4-hydroxynonenal, 4-oxononenal and malondialdehyde) were determined.

Results

Tumour tissues showed lower antioxidant capacity than healthy tissues, which was accompanied by lower levels of fatty acids and higher levels of reactive aldehydes. Disturbances in antioxidant capacity and enhanced DNA oxidative modifications were observed in 88% of AC patients and 81% of SCC patients. The 4-hydroxynonenal-Histidine adducts were detected in the necrotic and stromal cells in all tumours. These findings were associated with the enhanced Nrf2 activity, especially in AC. The strong difference between the cancer subtypes was evident in the levels of endocannabinoids, with an increase in 89% of SCC and a decrease in 85% of AC patients being observed. Additionally, the increase in the expression of CB1/2 receptors was observed only in 82% of AC, while the expression of VR1 and GPR55 was enhanced in 79% of SCC and 82% of AC patients.

Conclusions

This study shows significant differences in the redox status, Nrf2 pathway and endocannabinoid system between SCC and AC tissues. Understanding the relation between the various lipid mediators and antioxidants in different lung cancer subtypes may be beginning for further research on the effective anticancer therapy.

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AC tissues show lower antioxidant capacity than SCC.

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Nrf2/ARE pathway is activated stronger in AC than in SCC.

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AC exhibits more lipid oxidative modifications than SCC.

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HNE-protein adducts are detected in the necrotic and stromal cells in SCC and AC.

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SCC exhibits increase in endocannabinoids level while in AC reverse effect is observed.

Application of liquid chromatography-tandem mass spectrometry in quantitative bioanalyses of organic molecules in aquatic environment and organisms

Analytical methods using liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) for the simultaneous determination of metabolites or contaminants (or both) in various tissues of aquatic organisms and in the aquatic environment have received increasing attention in the last few years. This review discusses the findings relevant to such procedures published between 2005 and 2015. The aim is to evaluate the advantages, restrictions, and performances of the procedures from sample preparation to mass spectrometry measurement. To support these discussions, a general knowledge on LC-MS/MS is also provided.

Trace analysis of methylated and hydroxymethylated cytosines in DNA by isotope-dilution LC–MS/MS: first evidence of DNA methylation in Caenorhabditis elegans

We developed an online solid-phase extraction LC–MS/MS method to simultaneously measure 5-methyl-2′-deoxycytidine and 5-hydroxymethyl-2′-deoxycytidine in DNA. We demonstrated that 5-methyl-2′-deoxycytidine is present in Caenorhabditis elegans and its level was regulated by decitabine or cadmium in a dose–response manner.

Recent developments in DNA adduct analysis by mass spectrometry: a tool for exposure biomonitoring and identification of hazard for environmental pollutants

DNA adducts represent an important category of biomarkers for detection and exposure surveillance of potential carcinogenic and genotoxic chemicals in the environment. Sensitive and specific analytical methods are required to detect and differentiate low levels of adducts from native DNA from in vivo exposure. In addition to biomonitoring of environmental pollutants, analytical methods have been developed for structural identification of adducts which provides fundamental information for determining the toxic pathway of hazardous chemicals. In order to achieve the required sensitivity, mass spectrometry has been increasingly utilized to quantify adducts at low levels as well as to obtain structural information. Furthermore, separation techniques such as chromatography and capillary electrophoresis can be coupled to mass spectrometry to increase the selectivity. This review will provide an overview of advances in detection of adducted and modified DNA by mass spectrometry with a focus on the analysis of nucleosides since 2007. Instrument advances, sample and instrument considerations, and recent applications will be summarized in the context of hazard assessment. Finally, advances in biomonitoring applying mass spectrometry will be highlighted. Most importantly, the usefulness of DNA adducts measurement and detection will be comprehensively discussed as a tool for assessment of in vitro and in vivo exposure to environmental pollutants.

Effects of dinuclear berenil-platinum(II) complexes on fibroblasts redox status

PURPOSE

Platinum(II) complex anticarcinogenic mechanisms are associated with changes in the cellular redox status of cancer as well as healthy cells. Therefore, the goal of the present study was to investigate oxidative modifications in cellular components following fibroblast exposure to novel dinuclear berenil-platinum(II) complexes.

MATERIAL AND METHOD

ROS levels, antioxidant parameters level/activity, and damage to DNA, lipids, and proteins, including pro-apoptotic and anti-apoptotic factors in human skin fibroblasts following berenil-platinum(II) complex treatments i.e. Pt2(isopropylamine)4(berenil)2, Pt2(piperazine)4(berenil)4, Pt2(2-picoline)4(berenil)2, Pt2(3-picoline)4(berenil)2, and Pt2(4- picoline)4(berenil)2 were examined.

RESULTS

Treatment of fibroblasts with platinum(II) complexes has shown that all compounds enhance total ROS and superoxide anion generation as well as change the activity of antioxidant enzymes such as superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase and decrease in the level of non-enzymatic antioxidants (GSH, vitamin C, E and A). Such a situation is conducive to oxidative stress formation and oxidative modifications of cellular macromolecules and to increase in the expression of proapoptotic proteins. Pt2(isopropylamine)4(berenil)2 elicited the most damage, which resulted in oxidative modification of cellular components. The therapeutic use of this complex would cause considerable side effects in patients, therefore the agent lacks drug potential; however Pt2(piperazine)4(berenil)2 and Pt2(2-picoline)4(berenil)2 exhibited reduced redox and increased apoptotic profiles compared to cisplatin.

CONCLUSION

Results of this paper and preliminary data show that Pt2(2-picoline)4(berenil)2 is less dangers than cisplatin to fibroblasts and more disruptive than cisplatin to breast cancer cell metabolism, and therefore it is a promising candidate for use in future anticancer drug strategies.

Optimization of global DNA methylation measurement by LC-MS/MS and its application in lung cancer patients

Global analyses of DNA methylation contribute important insights into biology and the wide-ranging role of DNA methylation. We describe the use of online solid-phase extraction and isotope-dilution liquid chromatography/tandem mass spectrometry (LC-MS/MS) for the simultaneous measurement of 5-methyl-2'-deoxycytidine (5-medC) and 2'-deoxycytidine (dC) in DNA. With the incorporation of isotope internal standards and online enrichment techniques, the detection limit of this method was estimated to be as low as 0.065 pg which enables human global DNA methylation detection using only picogram amounts of DNA. This method was applied to assess the optimal amounts of enzymes required for DNA digestion regarding an accurate global DNA methylation determination and completeness of digestion and to determine global methylation in human tumor adjacent lung tissue of 79 lung cancer patients. We further determined methylated (N7-methylguanine (N7-meG), O (6)-methylguanine (O (6)-meG), and N3-methyladenine (N3-meA)) and oxidized DNA lesions (8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG)) in lung cancer patients by LC-MS/MS. Optimization experiments revealed that dC was liberated from DNA much more readily than 5-medC by nuclease P1 and alkaline phosphatase (AP) in DNA, which could lead to an error in the global DNA methylation measurement following digestion with insufficient enzymes. Nuclease P1 showed more differential activity for 5-medC and dC than AP. Global DNA methylation levels in adenocarcinoma and squamous cell carcinoma patients were similar in the range of 3.16-4.01 %. Global DNA methylation levels were not affected by smoking and gender and were not correlated with N7-meG or 8-oxodG in lung cancer patients. Levels of O (6)-meG and N3-meA were however found to be undetectable in all lung tissue samples.

Simultaneous quantitative analysis of N3-ethyladenine and N7-ethylguanine in human leukocyte deoxyribonucleic acid by stable isotope dilution capillary liquid chromatography-nanospray ionization tandem mass spectrometry

Cigarette smoke contains ethylating agents which damage DNA producing ethylated DNA adducts, such as N(3)-ethyladenine (3-EtAde), N(7)-ethylguanine (7-EtGua), and regioisomers of ethylthymine. Among them, 3-EtAde and 7-EtGua are present in human urine and their levels are higher in smokers than in nonsmokers. The amount of ethylated DNA adducts in tissue DNA represents the steady-state levels of DNA adducts resulting from the ethylating agent after repair in vivo. In this study, we have developed a highly sensitive, accurate, and quantitative assay for simultaneous detection and quantification of 3-EtAde and 7-EtGua by stable isotope dilution capillary liquid chromatography-nanospray ionization tandem mass spectrometry (capLC-NSI/MS/MS). Under the highly selective reaction monitoring (H-SRM) mode, the detection limit of 3-EtAde and 7-EtGua injected on-column was 5.0 fg (31 amol) and 10 fg (56 amol), respectively. The quantification limit for the entire assay was 50 and 100 fg of 3-EtAde and 7-EtGua, corresponding to 4.7 and 8.6 adducts in 10(9) normal nucleotides, respectively, starting with 20 μg of DNA isolated from <1 mL of blood and injecting an equivalent of 4 μg of DNA on-column. The mean (±SD) levels of 3-EtAde and 7-EtGua in leukocyte DNA from 20 smokers were 16.0±7.8 and 9.7±8.3 in 10(8) normal nucleotides, respectively, which were statistically significantly higher than those of 5.4±2.6 3-EtAde and 0.3±0.8 7-EtGua in 10(8) normal nucleotides from 20 nonsmokers (p<0.0001). The levels of 3-EtAde and 7-EtGua in these 40 leukocyte DNA samples are positively correlated (γ=0.6970, p<0.0001). Furthermore, there are statistically significant associations between the number of cigarettes smoked per day, as well as the smoking index, and the levels of 3-EtAde and 7-EtGua. Levels of 3-EtAde and 7-EtGua are compared to those of ethylthymidine adducts. To our knowledge, this is the first assay for simultaneous quantification of 3-EtAde and 7-EtGua in the same DNA sample and is the first report of 3-EtAde in human DNA. This highly sensitive and specific stable isotope dilution capLC-NSI/MS/MS assay should be useful in measuring 3-EtAde and 7-EtGua in human leukocyte DNA as potential biomarkers for smoking-related cancers.

Quantitation of DNA adducts by stable isotope dilution mass spectrometry

Exposure to endogenous and exogenous chemicals can lead to the formation of structurally modified DNA bases (DNA adducts). If not repaired, these nucleobase lesions can cause polymerase errors during DNA replication, leading to heritable mutations and potentially contributing to the development of cancer. Because of their critical role in cancer initiation, DNA adducts represent mechanism-based biomarkers of carcinogen exposure, and their quantitation is particularly useful for cancer risk assessment. DNA adducts are also valuable in mechanistic studies linking tumorigenic effects of environmental and industrial carcinogens to specific electrophilic species generated from their metabolism. While multiple experimental methodologies have been developed for DNA adduct analysis in biological samples, including immunoassay, HPLC, and ³²P-postlabeling, isotope dilution high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (HPLC-ESI-MS/MS) generally has superior selectivity, sensitivity, accuracy, and reproducibility. As typical DNA adduct concentrations in biological samples are between 0.01-10 adducts per 10⁸ normal nucleotides, ultrasensitive HPLC-ESI-MS/MS methodologies are required for their analysis. Recent developments in analytical separations and biological mass spectrometry, especially nanoflow HPLC, nanospray ionization MS, chip-MS, and high resolution MS, have pushed the limits of analytical HPLC-ESI-MS/MS methodologies for DNA adducts, allowing researchers to accurately measure their concentrations in biological samples from patients treated with DNA alkylating drugs and in populations exposed to carcinogens from urban air, drinking water, cooked food, alcohol, and cigarette smoke.

HPLC-UV, MALDI-TOF-MS and ESI-MS/MS analysis of the mechlorethamine DNA crosslink at a cytosine-cytosine mismatch pair

Background

Mechlorethamine [ClCH₂CH₂N(CH₃)CH₂CH₂Cl], a nitrogen mustard alkylating agent, has been proven to form a DNA interstrand crosslink at a cytosine-cytosine (C-C) mismatch pair using gel electrophoresis. However, the atomic connectivity of this unusual crosslink is unknown.

Methodology/Principal Findings

HPLC-UV, MALDI-TOF-MS, and ESI-MS/MS were used to determine the atomic connectivity of the DNA C-C crosslink formed by mechlorethamine, MALDI-TOF-MS of the HPLC-purified reaction product of mechlorethamine with the DNA duplex d[CTCACACCGTGGTTC]•d[GAACCACCGTGTGAG] (underlined bases are a C-C mismatch pair) indicated formation of an interstrand crosslink at m/z 9222.088 [M−2H+Na]⁺. Following enzymatic digestion of the crosslinked duplex by snake venom phosphodiesterase and calf intestinal phosphatase, ESI-MS/MS indicated the presence of dC-mech-dC [mech = CH₂CH₂N(CH₃)CH₂CH₂] at m/z 269.2 [M]²⁺ (expected m/z 269.6, exact mass 539.27) and its hydrolytic product dC-mech-OH at m/z 329.6 [M]⁺ (expected m/z 329.2). Fragmentation of dC-mech-dC gave product ions at m/z 294.3 and 236.9 [M]⁺, which are both due to loss of the 4-amino group of cytosine (as ammonia), in addition to dC and dC+HN(CH₃)CH = CH₂, respectively. The presence of m/z 269.2 [M]²⁺ and loss of ammonia exclude crosslink formation at cytosine N⁴ or O² and indicate crosslinking through cytosine N³ with formation of two quaternary ammonium ions.

Conclusions

Our results provide an important addition to the literature, as the first example of the use of HPLC and MS for analysis of a DNA adduct at the N³ position of cytosine.

Time course evaluation of N-nitrosodialkylamines-induced DNA alkylation and oxidation in liver of mosquito fish

Here we simultaneously measured N7-alkylguanines and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) in liver of small fish, respectively, to assess the time course of the formation and removal of alkylation and oxidative damage to DNA caused by N-nitrosodialkylamines. Mosquito fish (Gambusia affinis) were killed at various times during (4 days) and post-exposure (16 days) to N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) alone or their combination with concentrations of 10 and 50mg/l. The modified guanine adducts were sensitively and selectively quantitated by isotope-dilution LC-MS/MS methods. During exposure, N7-methylguanine (N7-MeG) and N7-ethylguanine (N7-EtG) in liver DNA increased with the duration and dose of N-nitrosodialkylamine exposure, while 8-oxodG was dose-dependently induced within 1 day. It was found that NDMA formed substantially more N7-alkylated guanines and 8-oxodG than NDEA on the basis of adducts formed per micromolar concentration, suggesting that NDMA can be more easily bioactivated than NDEA to form reactive alkylating agents with the concomitant formation of oxygen radicals. After cessation of exposure, N7-alkylguanines remained elevated for 1 day and then gradually decreased over time but still higher than the background levels, even at day 16 (half-lives of 7-8 days). However, 8-oxodG was excised quickly from liver DNA and returned to the background level within 4 days post-exposure (half-lives less than 2 days). Taken together, this study firstly demonstrated that in addition to alkylation, N-nitrosodialkylamines can concurrently cause oxidative damage to DNA in vivo.

Liquid chromatography-electrospray ionization tandem mass spectrometry analysis of 7-ethylguanine in human liver DNA

Considerable evidence in the literature demonstrates the exposure of humans to an unknown ethylating agent. Previous studies have demonstrated the presence of 7-ethyl-Gua and 3-ethyl-Ade in urine, 7-ethyl-dGuo and O4-ethyl-dThd in human lung, and ethylvaline in hemoglobin. Some studies also report higher levels of ethyl adducts in smokers than in nonsmokers, and there is convincing evidence for an uncharacterized ethylating agent in cigarette smoke. To further investigate this question, we have developed a liquid chromatography-electrospray ionization tandem mass spectrometry-selected reaction monitoring method for analysis of 7-ethyl-Gua in human liver DNA. To our knowledge, there are no previous reports of MS analyses of 7-ethyl-Gua in human tissues. [15N 5]7-Ethyl-Gua was synthesized and used as the internal standard. Human liver DNA was heated to release 7-ethyl-Gua. After partial purification by solid-phase extraction, analysis was carried out using the transition m/z 180 [M+H]+-->m/z 152 [Gua+H]+ for 7-ethyl-Gua and m/z 185-->m/z 157 for the internal standard. The method was accurate and precise. The detection limit was approximately 8-9 fmol/micromol Gua, starting with 1-2 mg of DNA. Clear coeluting peaks for 7-ethyl-Gua and the internal standard were observed in the human liver DNA samples. Twenty-six human liver DNA samples (0.77+/-0.40 mg) were analyzed, and 25 were positive for 7-ethyl-Gua. The mean level of 7-ethyl-Gua was 42.2+/-43.0 fmol/micromol Gua (8.4+/-8.6 adducts per 10(9) nucleotides). These results demonstrate that 7-ethyl-Gua is a common DNA adduct in human liver with likely endogenous sources that require further investigation.

Detection of phosphodiester adducts formed by the reaction of benzo[a]pyrene diol epoxide with 2'-deoxynucleotides using collision-induced dissociation electrospray ionization tandem mass spectrometry

In this study, we investigated the products formed following the reaction of benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide (B[a]PDE) with 2'-deoxynucleoside 3'-monophosphates. The B[a]PDE plus 2'-deoxynucleotide reaction mixtures were purified using solid phase extraction (SPE) and subjected to HPLC with fluorescence detection. Fractions corresponding to reaction product peaks were collected and desalted using SPE prior to analysis for the presence of molecular ions corresponding to m/z 648, 632, 608 and 623 [M-H]- consistent with B[a]PDE adducted (either on the base or phosphate group) 2'-deoxynucleotides of guanine, adenine, cytosine and thymine, respectively, using LC-ESI-MS/MS collision-induced dissociation (CID). Reaction products were identified having CID product ion spectra containing product ions at m/z 452, 436 and 412 [(B[a]Ptriol+base)-H]-, resulting from cleavage of the glycosidic bond between the 2'-deoxyribose and base, corresponding to B[a]PDE adducts of guanine, adenine and cytosine, respectively. Further reaction products were identified having unique CID product ion spectra characteristic of B[a]PDE adduct formation with the phosphate group of the 2'-deoxynucleotide. The presence of product ions at m/z 399 and 497 were observed for all four 2'-deoxynucleotides, corresponding to [(B[a]Ptriol+phosphate)-H]- and [(2'-deoxyribose+phosphate+B[a]Ptriol)-H]-, respectively. In conclusion, this investigation provides the first direct evidence for the formation of phosphodiester adducts by B[a]PDE following reaction with 2'-deoxynucleotides.