Analysis and Identification of 2'-Deoxyadenosine-Derived Adducts in Lung and Liver DNA of F-344 Rats Treated with the Tobacco-Specific Carcinogen 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone and Enantiomers of its Metabolite 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanol

5-(Pyridin-3-yl)-3,4-dihydro-2H-furan-1-ium (NNKFI): a computational study of its physico-chemical properties

Recent work on the diazonium ion metabolite of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNKDI) suggests that 5-(pyridin-3-yl)-3,4-dihydro-2H-furan-1-ium (NNKFI) may form from NNKDI via an intramolecular reaction. NNKDI is an important carcinogen whose role as an alkylating agent has received significant attention. While there is some experimental evidence supporting NNKFI's production in vitro, it has not yet been directly observed. Little is known about NNKFI's structure and reactivity. We report the first in silico examination of this ion. Our study utilized Kohn-Sham density functional theory (B3LYP/6-311G**) and coupled cluster theory (CCSD/6-31G*) to produce energy-optimized structures, vibrational normal modes and molecular orbitals for NNKFI. To gain insight into the chemical properties of this species, we calculated electrostatic potential surfaces, natural population analysis charges and local Fukui indices. We report data and results for NNKFI's cis and trans conformers. Our work confirms C5 as the preferred site for nucleophilic attack in NNKFI. Stretching motions and predicted bond lengths near O1 are consistent with a somewhat weakened carbonyl structure in this ion. Partial charges, electrostatic potential surfaces and local Fukui indices reveal delocalization of cationic charge on the furanium moiety and notable carbocation character at C5.

Comparison of a Tobacco-Specific Carcinogen in Tobacco Cigarette, Electronic Cigarette, and Dual Users

BACKGROUND

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) is known as a lung carcinogen. The objective of this study was to investigate associations of urine NNAL concentrations and smoking status.

METHODS

This was a cross-sectionally designed study based on data from the 2016-2018 Korean National Health and Nutrition Examination Survey. A total of 2,845 participants were classified into past-smoker, electronic cigarette (e-cigar) only, dual-user, and cigarette only smoker groups. All sampling and weight variables were stratified and analysis was conducted accounting for the complex sampling design. Analysis of covariance was used to compare the geometric mean of urine NNAL concentrations and log-transformed urine NNAL level among smoking status with weighted survey design. Post hoc paired comparisons with Bonferroni adjustment was performed according to smoking status.

RESULTS

The estimated geometric mean concentrations of urine NNAL were 1.974 ± 0.091, 14.349 ± 5.218, 89.002 ± 11.444, and 117.597 ± 5.459 pg/mL in past-smoker, e-cigar only, dual-user, and cigarette only smoker groups, respectively. After fully adjusting, log-transformed urine NNAL level was significantly different among groups (P < 0.001). Compared with the past-smoker group, e-cigar only, dual-user, and cigarette only smoker groups showed significantly higher log-transformed urine NNAL concentrations in post hoc test (all P < 0.05).

CONCLUSION

E-cigar only, dual-user, and cigarette only smoker groups showed significantly higher geometric mean concentrations of urine NNAL than the past-smoker group. Conventional cigarette, dual users, and e-cigar users can potentially show harmful health effects from NNAL.

Mass Spectrometry-Based Metabolic Profiling of Urinary Metabolites of N'-Nitrosonornicotine (NNN) in the Rat

The tobacco-specific nitrosamine N'-nitrosonornicotine (NNN) and its close analogue 4-(N-nitrosomethylamino)-1-(3-pyridyl)-1-butanone (NNK) are classified as "carcinogenic to humans" (Group 1) by the International Agency for Research on Cancer. The currently used biomarker to monitor NNN exposure is urinary total NNN (free NNN plus its N-glucuronide). However, total NNN does not provide information about the extent of metabolic activation of NNN as related to its carcinogenicity. Targeted analysis of the major metabolites of NNN in laboratory animals recently led to the identification of N'-nitrosonornicotine-1N-oxide (NNN-N-oxide), a unique metabolite detected in human urine that is specifically formed from NNN. To further investigate NNN urinary metabolites that hold promise as new biomarkers for monitoring NNN exposure, uptake, and/or metabolic activation, we conducted a comprehensive profiling of NNN metabolites in the urine of F344 rats treated with NNN or [pyridine-d4]NNN. Using our optimized high-resolution mass spectrometry (HRMS)-based isotope-labeling method, 46 putative metabolites were identified with robust MS evidence. Out of the 46 candidates, all known major NNN metabolites were identified and structurally confirmed by comparing them to their isotopically labeled standards. More importantly, putative metabolites considered to be exclusively formed from NNN were also identified. The two new representative metabolites─4-(methylthio)-4-(pyridin-3-yl)butanoic acid (23, MPBA) and N-acetyl-S-(5-(pyridin-3-yl)-1H-pyrrol-2-yl)-l-cysteine (24, Py-Pyrrole-Cys-NHAc) ─were identified by comparing them to synthetic standards that were fully characterized by nuclear magnetic resonance and HRMS. They are hypothesized to be formed by NNN α-hydroxylation pathways and thus represent the first potential biomarkers to specifically monitor the uptake plus metabolic activation of NNN in tobacco users.

Hydrophilic interaction chromatography-type sorbent prepared by the modification of methacrylate-base resin with polyethyleneimine for solid-phase extraction of polar compounds

Hydrophilic interaction chromatography (HILIC)-type sorbents were newly developed for the solid-phase extraction (SPE) of polar compounds. Two methacrylate-base resins with different cross-linking monomers and pore properties were synthesized, and three polyethyleneimines (PEIs) with different molecular weights were modified onto each base resin. In both cases, PEIs with a molecular weight of 10,000 (PEI-10,000) exhibited the highest adsorption properties for polar compounds (uracil, uridine, adenosine, cytidine, and guanosine). To control the water-enriched layer at the surface of the PEI-10,000-modified sorbents, the additive amount of PEI-10,000 in the modified reaction was also optimized. When 10 times the amount of PEI-10,000 to each base resin was added, an improvement in adsorption property was observed. Moreover, the use of a nonaqueous sample solution (100% acetonitrile) during the sample loading process drastically improved adsorption, especially for uracil (about 80%) and adenosine (100%). These results indicate that the formation of a strong water-enriched layer at the surface of sorbents with an effective expression of hydrophilic interaction was an important factor in the adsorption properties of polar compounds in HILIC mode-SPE.

DNA Alkylation Damage by Nitrosamines and Relevant DNA Repair Pathways

Nitrosamines occur widespread in food, drinking water, cosmetics, as well as tobacco smoke and can arise endogenously. More recently, nitrosamines have been detected as impurities in various drugs. This is of particular concern as nitrosamines are alkylating agents that are genotoxic and carcinogenic. We first summarize the current knowledge on the different sources and chemical nature of alkylating agents with a focus on relevant nitrosamines. Subsequently, we present the major DNA alkylation adducts induced by nitrosamines upon their metabolic activation by CYP450 monooxygenases. We then describe the DNA repair pathways engaged by the various DNA alkylation adducts, which include base excision repair, direct damage reversal by MGMT and ALKBH, as well as nucleotide excision repair. Their roles in the protection against the genotoxic and carcinogenic effects of nitrosamines are highlighted. Finally, we address DNA translesion synthesis as a DNA damage tolerance mechanism relevant to DNA alkylation adducts.

Multiscale computational investigations of the translesion synthesis bypass of tobacco-derived DNA adducts: critical insights that complement experimental biochemical studies

Among the numerous agents that damage DNA, tobacco products remain one of the most lethal and result in the most diverse set of DNA lesions. This perspective aims to provide an overview of computational work conducted to complement experimental biochemical studies on the mutagenicity of adducts derived from the most potent tobacco carcinogen, namely 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (nicotine-derived nitrosaminoketone or NNK). Lesions ranging from the smallest methylated thymine derivatives to the larger, flexible pyridyloxobutyl (POB) guanine adducts are considered. Insights are obtained from density functional theory (DFT) calculations and molecular dynamics (MD) simulations into the damaged nucleobase and nucleoside structures, the accommodation of the lesions in the active site of key human polymerases, the intrinsic base pairing potentials of the adducts, and dNTP incorporation opposite the lesions. Overall, the computational data provide atomic level information that can rationalize the differential mutagenic properties of tobacco-derived lesions and uncover important insights into the impact of adduct size, nucleobase, position, and chemical composition of the bulky moiety.

Metabolism and DNA Adduct Formation of Tobacco-Specific N-Nitrosamines

The tobacco-specific N-nitrosamines 4-(N-nitrosomethylamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) always occur together and exclusively in tobacco products or in environments contaminated by tobacco smoke. They have been classified as "carcinogenic to humans" by the International Agency for Research on Cancer. In 1998, we published a review of the biochemistry, biology and carcinogenicity of tobacco-specific nitrosamines. Over the past 20 years, considerable progress has been made in our understanding of the mechanisms of metabolism and DNA adduct formation by these two important carcinogens, along with progress on their carcinogenicity and mutagenicity. In this review, we aim to provide an update on the carcinogenicity and mechanisms of the metabolism and DNA interactions of NNK and NNN.

Characterization of adductomic totality of NNK, (R)-NNAL and (S)-NNAL in A/J mice, and their correlations with distinct lung carcinogenicity

Lung cancer is the leading cause of cancer-related deaths. While tobacco use is the main cause, only 10-20% of smokers eventually develop clinical lung cancer. Thus, the ability of lung cancer risk prediction among smokers could transform lung cancer management with early preventive interventions. Given that DNA damage by tobacco carcinogens is the potential root cause of lung carcinogenesis, we characterized the adductomic totality of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (a potent lung carcinogen in tobacco, commonly known as NNK) in the target lung tissues, the liver tissues and the peripheral serum samples in a single-dose NNK-induced lung carcinogenesis A/J mouse model. We also characterized these adductomic totalities from the two enantiomers of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL, the major in vivo metabolite of NNK) given their distinct carcinogenicity in A/J mice. With these adductomic data, we demonstrated that tissue protein adductomics have the highest abundance. We also identified that the adductomic levels at the 8 h time point after carcinogen exposure were among the highest. More importantly, the relationships among these adductomics were characterized with overall strong positive linear correlations, demonstrating the potential of using peripheral serum protein adductomics to reflect DNA adductomics in the target lung tissues. Lastly, we explored the relationships of these adductomics with lung tumor status in A/J mice, providing preliminary but promising evidence of the feasibility of lung cancer risk prediction using peripheral adductomic profiling.

Exposure to Secondhand Smoke and a Tobacco-Specific Carcinogen in Non-Smokers

Background: The International Agency for Research on Cancer classifies 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) as a known carcinogen. This study aimed to investigate the association between exposure to secondhand smoke (SHS) and NNAL concentrations in non-smokers.Methods: This was a cross-sectional study based on data from the 2016 to 2018 Korea National Health and Nutrition Examination Survey. Urine NNAL concentrations were categorized into tertiles of 3,615 study participants who were non-smokers. All sampling and weight variables were stratified, and analyses to account for the complex sampling design were conducted.Results: The overall, male, and female percentages of SHS exposure among non-smokers were 22.4%, 29.2%, and 20.4%, respectively. The geometric means of urine NNAL concentrations were 1.896±0.098 pg/mL and 1.094±0.028 pg/mL in the SHS exposure and non-exposure groups, respectively. After adjusting for confounding variables, in the total group, the geometric mean of urine NNAL concentrations was significantly higher in the SHS exposure group than in the SHS non-exposure group (adjusted P-value <0.001). Compared with the non-exposure group, the adjusted odds ratios (95% confidence intervals) for the highest NNAL tertile group of overall SHS exposure in the total, men, and women groups were 2.44 (1.95–3.05), 1.65 (1.08–2.53), and 2.73 (2.11–3.52), respectively, after full adjustment.Conclusion: The urine NNAL concentration in the SHS exposure group was significantly higher than that in the non-exposure group. Exposure to SHS was associated with a higher risk of elevated urine NNAL concentrations in non-smokers.

Investigation of 2'-Deoxyadenosine-Derived Adducts Specifically Formed in Rat Liver and Lung DNA by N'-Nitrosonornicotine Metabolism

The International Agency for Research on Cancer has classified the tobacco-specific nitrosamines N'-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) as "carcinogenic to humans" (Group 1). To exert its carcinogenicity, NNN requires metabolic activation to form reactive intermediates which alkylate DNA. Previous studies have identified cytochrome P450-catalyzed 2'-hydroxylation and 5'-hydroxylation of NNN as major metabolic pathways, with preferential activation through the 5'-hydroxylation pathway in some cultured human tissues and patas monkeys. So far, the only DNA adducts identified from NNN 5'-hydroxylation in rat tissues are 2-[2-(3-pyridyl)-N-pyrrolidinyl]-2'-deoxyinosine (Py-Py-dI), 6-[2-(3-pyridyl)-N-pyrrolidinyl]-2'-deoxynebularine (Py-Py-dN), and N⁶-[4-hydroxy-1-(pyridine-3-yl)butyl]-2'-deoxyadenosine (N⁶-HPB-dAdo) after reduction. To expand the DNA adduct panel formed by NNN 5'-hydroxylation and identify possible activation biomarkers of NNN metabolism, we investigated the formation of dAdo-derived adducts using a new highly sensitive and specific liquid chromatography-nanoelectrospray ionization-high-resolution tandem mass spectrometry method. Two types of NNN-specific dAdo-derived adducts, N⁶-[5-(3-pyridyl)tetrahydrofuran-2-yl]-2'-deoxyadenosine (N⁶-Py-THF-dAdo) and 6-[2-(3-pyridyl)-N-pyrrolidinyl-5-hydroxy]-2'-deoxynebularine (Py-Py(OH)-dN), were observed for the first time in calf thymus DNA incubated with 5'-acetoxyNNN. More importantly, Py-Py(OH)-dN was also observed in relatively high abundance in the liver and lung DNA of rats treated with racemic NNN in the drinking water for 3 weeks. These new adducts were characterized using authentic synthesized standards. Both NMR and MS data agreed well with the proposed structures of N⁶-Py-THF-dAdo and Py-Py(OH)-dN. Reduction of Py-Py(OH)-dN by NaBH₃CN led to the formation of Py-Py-dN both in vitro and in vivo, which was confirmed by its isotopically labeled internal standard [pyridine-d₄]Py-Py-dN. The NNN-specific dAdo adducts Py-THF-dAdo and Py-Py(OH)-dN formed by NNN 5'-hydroxylation provide a more comprehensive understanding of the mechanism of DNA adduct formation by NNN.

Identification of an N'-Nitrosonornicotine-Specific Deoxyadenosine Adduct in Rat Liver and Lung DNA

The tobacco-specific nitrosamines N'-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) are considered to be two of the most important carcinogens in unburned tobacco and its smoke. They readily cause tumors in laboratory animals and are classified as "carcinogenic to humans" by the International Agency for Research on Cancer. DNA adduct formation by these two carcinogens is believed to play a critical role in tobacco carcinogenesis. Among all the DNA adducts formed by NNN and NNK, 2'-deoxyadenosine (dAdo)-derived adducts have not been fully characterized. In the study reported here, we characterized the formation of N⁶-[4-(3-pyridyl)-4-oxo-1-butyl]-2'-deoxyadenosine (N⁶-POB-dAdo) and its reduced form N⁶-PHB-dAdo formed by NNN 2'-hydroxylation in rat liver and lung DNA. More importantly, we characterized a new dAdo adduct N⁶-[4-hydroxy-1-(pyridine-3-yl)butyl]-2'-deoxyadenosine (N⁶-HPB-dAdo) formed after NaBH₃CN or NaBH₄ reduction both in vitro in calf thymus DNA reacted with 5'-acetoxy-N'-nitrosonornicotine and in vivo in rat liver and lung upon treatment with NNN. This adduct was specifically formed by NNN 5'-hydroxylation. Chemical standards of N⁶-HPB-dAdo and the corresponding isotopically labeled internal standard [pyridine-d₄]N⁶-HPB-dAdo were synthesized using a four-step method. Both NMR and high-resolution mass spectrometry data agreed well with the proposed structure of N⁶-HPB-dAdo. The new adduct coeluted with the synthesized internal standard under various LC conditions. Its product ion patterns of MS² and MS³ transitions were also consistent with the proposed fragmentation patterns. Chromatographic resolution of the two diastereomers of N⁶-HPB-dAdo was successfully achieved. Quantitation suggested a dose-dependent response of the levels of this new adduct in the liver and lung of rats treated with NNN. However, its level was lower than that of 2-[2-(3-pyridyl)-N-pyrrolidinyl]-2'-deoxyinosine, a previously reported dGuo adduct that is also formed from NNN 5'-hydroxylation. The identification of N⁶-HPB-dAdo in this study leads to new insights pertinent to the mechanism of carcinogenesis by NNN and to the development of biomarkers of NNN metabolic activation.

Physico-chemical properties of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) diazonium ion: a theoretical investigation

We have explored the physico-chemical properties of NNK diazonium ion to gain insight into its shape, bonding, charge distribution, and ro-vibrational features. This information is essential if the chemical reactivity and physical properties of this important intermediate are to be understood. NNK diazonium ion is a well-known alkylating agent. Its enzymatic production, its reaction with DNA, and its role in mutagenesis/carcinogenesis have all received significant experimental study. Computational work on the ion, however, is lacking. The species is sufficiently small such that its properties may be probed using sophisticated model chemistries. We present the first in silico investigation of NNK diazonium ion. Kohn–Sham density functional theory (B3LYP/6-311G**) and coupled cluster theory (CCSD/6-31G*) were deployed to obtain energies, geometries, electrostatic potential surfaces, molecular orbitals, and vibrational analyses for several energy-minimized structures. To provide insight into the motion of NNK diazonium ion (NNKDI) in solution, molecular dynamics simulations on the solvated intermediate were undertaken. To explore the initial reactivity of this important electrophile, local Fukui indices and natural population analysis charges were predicted. Analogous ab initio work on propane diazonium ion was also performed. Our vibrational analyses suggest a relatively weak carbon–nitrogen bond and a robust nitrogen–nitrogen interaction. Our condensed Fukui indices show that the terminal nitrogen is a site of significant electrophilicity while our electrostatic predictions yield high values near the formally charged nitrogen and its α carbon.

NNKDI TCTT molecular orbital energies (a), molecular orbitals 46 (HOMO), 47 (LUMO) and 48 (LUMO+1) as calculated at the B3LYP/6-311G** level of theory.

Evaluation of the adsorption properties of nucleobase-modified sorbents for a solid-phase extraction of water-soluble compounds

We developed hydrophilic interaction chromatography (HILIC)-type sorbents modified with nucleobases for solid phase extraction (SPE). The synthesized hydrophilic base resins were modified by each nucleobase (adenine, guanine, and cytosine). The measurement of the amount of water content indicated that each nucleobase-modified sorbent had a water layer. To evaluate the adsorption properties in the HILIC mode, we chose two nucleobases (uracil and adenine) and four nucleosides (uridine, adenosine, cytidine, guanosine) as water-soluble analytes, which were loaded into an SPE cartridge packed with the nucleobase-modified sorbent. Firstly, 95% acetonitrile (ACN) solutions were used in the process of conditioning and sample loading of the above polar analytes. High recoveries of the analytes were observed in each nucleobase-modified sorbent, and the Diol-type sorbent (no modification with any of the nucleobases) did not adsorb each water-soluble analyte. On the basis of this result, a 98% ACN solution was used during the process of conditioning and sample loading to decrease the concentration of water in the sample, which potentially inhibited the formation of hydrogen bonding between each analyte and the modified nucleobase. Considerable improvements of recoveries were observed in Adenine- and Cytosine-modified sorbents. These results were possibly attributed to the effective expression of hydrogen bonding by decreasing water concentration in the sample solution. Although a non-aqueous (100% ACN) sample solution can be expected to obtain higher recoveries compared with the 98% ACN solution, a decrease in recoveries was observed in Adenine-modified sorbent. From these results, the highest adsorption property was observed in Adenine-modified sorbent using 98% ACN as a sample condition, and the combination of this sample condition and sorbent is effective for high adsorption under HILIC condition. Moreover, we also revealed that a balance between the thickness of water layer and the modification amount of nucleobase is important for retention in the HILIC-type sorbent.

Molecular Basis for Metabolic Regioselectivity and Mechanism of Cytochrome P450s toward Carcinogenic 4-(Methylnitrosamino)-(3-pyridyl)-1-butanone

As an abundantly present tobacco component, carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) has also been detected in atmospheric particulate matter, suggesting the ineluctable exposure risk of this contaminant. NNK metabolic activation by cytochrome P450 enzymes (CYPs) is a prerequisite to exerting its genotoxicity, but the metabolic regioselectivity and mechanism are still unknown. Here the binding feature and regioselectivity of CYPs 1A1, 1A2, 2A6, 2A13, 2B6, and 3A4 toward NNK are unraveled through molecular docking and molecular dynamics (MD) simulations. Binding mode analyses reveal that 1A2 and 2B6 have definite preferences for NNK α-methyl hydroxylation, while the other four CYPs preferentially catalyze α-methylene hydroxylation. The binding affinities between NNK and CYPs evaluated by the binding free energies follow the order 2A13 > 2B6 > 1A2 > 2A6 > 1A1 > 3A4. Density functional theory (DFT) calculations are further performed to characterize the mechanism of NNK biotransformation. Results show that the α-hydroxyNNK generated from α-hydroxylation may undergo nonenzymatic decomposition to form genotoxic diazohydroxide and aldehyde, and further oxidation by P450 to yield nitrosamide, which mainly contributes to NNK toxification capacity. Meanwhile the pyridine N-oxidation and denitrosation of C_α-radical intermediate play an important role in detoxifying NNK. Overall, the present study provides the molecular basis for CYP-catalyzed regioselectivity and mechanism of NNK biotransformation, which can enable the identification of metabolites for assessing the health risk of individual NNK exposure.

Effects of pendant-like hydrophilic monomers on the adsorption properties of reversed-phase-type sorbents for solid-phase extraction

Solid-phase extraction (SPE) has been extensively employed as a pretreatment method. In SPE, reversed-phase-type sorbents have been widely applied for the pretreatment of environmental or biological samples. Hydrophilic-lipophilic balance (HLB)-type sorbents, constituting the copolymers used as reversed-phase-type sorbents, have been applied for various sample pretreatment methods. In HLB-type sorbents, the hydrophilic monomer contributes to the improved wettability of sorbents and increase of polar interactions. In this study, three pendant-like hydrophilic monomers, viz. N-vinylpyrrolidone (NVP), 4-acryloylmorpholine (AMO), and 4-vinyl-1,3-dioxolan-2-one (VDO), respectively, exhibiting different Log P values and possibly causing different polar interactions, were selected to improve the adsorption properties of polar compounds, and divinylbenzene (DVB)-based HLB-type sorbents containing each hydrophilic monomer were synthesized and examined. By the optimization of the molar ratio of DVB and the hydrophilic monomer (i.e. HLB), the inert diluent, and the degree of cross-linking, the developed sorbents exhibited higher recoveries for various polar compounds (viz. cytosine, uracil, cytidine, uridine, 2'-deoxycytidine, 2'-deoxyguanosine, adenine, thymidine, adenosine, and 2'-deoxyadenosine) compared to commercially available HLB-type sorbents.

Physical binding of the tobacco smoke carcinogen NNK diazonium ion to the human tumor suppressor gene TP53 Exon 5

The tobacco smoke N-nitrosamine, NNK, is an important carcinogen. It has been shown to induce lung, liver, and pancreatic cancer in animals. Its metabolites are associated with lung cancer in tobacco smokers. Our work focuses upon the physical interaction of NNK diazonium ion with DNA. This species is implicated in the formation of pyridyloxobutyl adducts, reacting with DNA bases and phosphate groups. Past research has investigated the metabolic activation of NNK by various enzymes, subsequent adduct formation with DNA, and the role of these adducts in mutagenesis. We present the first study of the physical interaction of NNK diazonium ion with TP53 (exon 5), a frequently mutated human tumor suppressor gene. We identify physical binding sites found via free energy minimization in computational docking simulations. These structures represent local potential energy minima in this system and suggest plausible sites for adduct formation.

Recent Studies on DNA Adducts Resulting from Human Exposure to Tobacco Smoke

DNA adducts are believed to play a central role in the induction of cancer in cigarette smokers and are proposed as being potential biomarkers of cancer risk. We have summarized research conducted since 2012 on DNA adduct formation in smokers. A variety of DNA adducts derived from various classes of carcinogens, including aromatic amines, polycyclic aromatic hydrocarbons, tobacco-specific nitrosamines, alkylating agents, aldehydes, volatile carcinogens, as well as oxidative damage have been reported. The results are discussed with particular attention to the analytical methods used in those studies. Mass spectrometry-based methods that have higher selectivity and specificity compared to 32P-postlabeling or immunochemical approaches are preferred. Multiple DNA adducts specific to tobacco constituents have also been characterized for the first time in vitro or detected in vivo since 2012, and descriptions of those adducts are included. We also discuss common issues related to measuring DNA adducts in humans, including the development and validation of analytical methods and prevention of artifact formation.

DNA replication studies of N-nitroso compound-induced O6-alkyl-2'-deoxyguanosine lesions in Escherichia coli

N-Nitroso compounds (NOCs) are common DNA-alkylating agents, are abundantly present in food and tobacco, and can also be generated endogenously. Metabolic activation of some NOCs can give rise to carboxymethylation and pyridyloxobutylation/pyridylhydroxybutylation of DNA, which are known to be carcinogenic and can lead to gastrointestinal and lung cancer, respectively. Herein, using the competitive replication and adduct bypass (CRAB) assay, along with MS- and NMR-based approaches, we assessed the cytotoxic and mutagenic properties of three O6-alkyl-2'-deoxyguanosine (O6-alkyl-dG) adducts, i.e. O6-pyridyloxobutyl-dG (O6-POB-dG) and O6-pyridylhydroxybutyl-dG (O6-PHB-dG), derived from tobacco-specific nitrosamines, and O6-carboxymethyl-dG (O6-CM-dG), induced by endogenous N-nitroso compounds. We also investigated two neutral analogs of O6-CM-dG, i.e. O6-aminocarbonylmethyl-dG (O6-ACM-dG) and O6-hydroxyethyl-dG (O6-HOEt-dG). We found that, in Escherichia coli cells, these lesions mildly (O6-POB-dG), moderately (O6-PHB-dG), or strongly (O6-CM-dG, O6-ACM-dG, and O6-HOEt-dG) impede DNA replication. The strong blockage effects of the last three lesions were attributable to the presence of hydrogen-bonding donor(s) located on the alkyl functionality of these lesions. Except for O6-POB-dG, which also induced a low frequency of G → T transversions, all other lesions exclusively stimulated G → A transitions. SOS-induced DNA polymerases played redundant roles in bypassing all the O6-alkyl-dG lesions investigated. DNA polymerase IV (Pol IV) and Pol V, however, were uniquely required for inducing the G → A transition for O6-CM-dG exposure. Together, our study expands our knowledge about the recognition of important NOC-derived O6-alkyl-dG lesions by the E. coli DNA replication machinery.

Quantification of DNA Lesions Induced by 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanol in Mammalian Cells

Quantitative measurement of DNA adducts in carcinogen-exposed cells provides the information about the frequency of formation and the rate of removal of DNA lesions in vivo, which yields insights into the initial events of mutagenesis. Metabolic activation of tobacco-specific nitrosamines, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and its reduction product 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), leads to pyridyloxobutylation and pyridylhydroxybutylation of DNA. In this study, we employed a highly robust nanoflow liquid chromatography-nanoelectrospray ionization-tandem mass spectrometry (nLC-nESI-MS/MS) coupled with the isotope-dilution method for simultaneous quantification of O⁶-[4-(3-pyridyl)-4-hydroxylbut-1-yl]-2'-deoxyguanosine ( O⁶-PHBdG) and O²- and O⁴-[4-(3-pyridyl)-4-hydroxylbut-1-yl]-thymidine ( O²-PHBdT and O⁴-PHBdT). Cultured mammalian cells were exposed to a model pyridylhydroxybutylating agent, 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanol (NNALOAc), followed by DNA extraction, enzymatic digestion, and sample enrichment prior to nLC-nESI-MS/MS quantification. Our results demonstrate, for the first time, that O⁴-PHBdT is quantifiable in cellular DNA and naked DNA upon NNALOAc exposure. We also show that nucleotide excision repair (NER) machinery may counteract the formation of O²-PHBdT and O⁴-PHBdT, and O⁶-alkylguanine DNA alkyltransferase (AGT) may be responsible for the repair of O⁶-PHBdG and O⁴-PHBdT in mammalian cells. Together, our study provides new knowledge about the occurrence and repair of NNAL-induced DNA lesions in mammalian cells.