In vitro alkylation of calf thymus DNA by acrylonitrile. Isolation of cyanoethyl-adducts of guanine and thymine and carboxyethyl-adducts of adenine and cytosine

Quantification of synthetic errors during chemical synthesis of DNA and its suppression by non-canonical nucleosides

Substitutions, insertions, and deletions derived from synthetic oligonucleotides are the hurdles for the synthesis of long DNA such as genomes. We quantified these synthetic errors by next-generation sequencing and revealed that the quality of the enzymatically amplified final combined product depends on the conditions of the preceding solid phase chemical synthesis, which generates the initial pre-amplified fragments. Among all possible substitutions, the G-to-A substitution was the most prominently observed substitution followed by G-to-T, C-to-T, T-to-C, and A-to-G substitutions. The observed error rate for G-to-A substitution was influenced by capping conditions, suggesting that the capping step played a major role in the generation of G-to-A substitution. Because substitutions observed in long DNA were derived from the generation of non-canonical nucleosides during chemical synthesis, non-canonical nucleosides resistant to side reactions could be used as error-proof nucleosides. As an example of such error-proof nucleosides, we evaluated 7-deaza-2´-deoxyguanosine and 8-aza-7-deaza-2´-deoxyguanosine and showed 50-fold decrease in the error rate of G-to-A substitution when phenoxyacetic anhydride was used as capping reagents. This result is the first example that improves the quality of synthesized sequences by using non-canonical nucleosides as error-proof nucleosides. Our results would contribute to the development of highly accurate template DNA synthesis technologies.

THERAPEUTIC OLIGONUCLEOTIDES, IMPURITIES, DEGRADANTS, AND THEIR CHARACTERIZATION BY MASS SPECTROMETRY

Oligonucleotides are an emerging class of drugs that are manufactured by solid-phase synthesis. As a chemical class, they have unique product-related impurities and degradants, characterization of which is an essential step in drug development. The synthesis cycle, impurities produced during the synthesis and degradation products are presented and discussed. The use of liquid chromatography combined with mass spectrometry for characterization and quantification of product-related impurities and degradants is reviewed. In addition, sequence determination of oligonucleotides by gas-phase fragmentation and indirect mass spectrometric methods is discussed. © 2019 John Wiley & Sons Ltd. Mass Spec Rev.

Analysis of DNA Adducts and Mutagenic Potency and Specificity in Rats Exposed to Acrylonitrile

Acrylonitrile (ACN), which is a widely used industrial chemical, induces cancers in multiple organs/tissues of rats by unresolved mechanisms. For this report, evidence for ACN-induced direct/indirect DNA damage and mutagenesis was investigated by assessing the ability of ACN, or its reactive metabolite, 2-cyanoethylene oxide (CEO), to bind to DNA in vitro, to form select DNA adducts [N7-(2'-oxoethyl)guanine, N²,3-ethenoguanine, 1,N⁶-ethenodeoxyadenosine, and 3,N⁴-ethenodeoxycytidine] in vitro and/or in vivo, and to perturb the frequency and spectra of mutations in the hypoxanthine-guanine phosphoribosyltransferase (Hprt) gene in rats exposed to ACN in drinking water. Adducts and frequencies and spectra of Hprt mutations were analyzed using published methods. Treatment of DNA from human TK6 lymphoblastoid cells with [2,3-¹⁴C]-CEO produced dose-dependent binding of ¹⁴C-CEO equivalents, and treatment of DNA from control rat brain/liver with CEO induced dose-related formation of N7-(2'-oxoethyl)guanine. No etheno-DNA adducts were detected in target tissues (brain and forestomach) or nontarget tissues (liver and spleen) in rats exposed to 0, 3, 10, 33, 100, or 300 ppm ACN for up to 105 days or to 0 or 500 ppm ACN for ∼15 months; whereas N7-(2'-oxoethyl)guanine was consistently measured at nonsignificant concentrations near the assay detection limit only in liver of animals exposed to 300 or 500 ppm ACN for ≥2 weeks. Significant dose-related increases in Hprt mutant frequencies occurred in T-lymphocytes from spleens of rats exposed to 33-500 ppm ACN for 4 weeks. Comparisons of "mutagenic potency estimates" for control rats versus rats exposed to 500 ppm ACN for 4 weeks to analogous data from rats/mice treated at a similar age with N-ethyl-N-nitrosourea or 1,3-butadiene suggest that ACN has relatively limited mutagenic effects in rats. Considerable overlap between the sites and types of mutations in ACN-exposed rats and butadiene-exposed rats/mice, but not controls, provides evidence that the carcinogenicity of these epoxide-forming chemicals involves corresponding mutagenic mechanisms.

Synthesis of RNA Containing 5-Hydroxymethyl-, 5-Formyl-, and 5-Carboxycytidine

5-Hydroxymethyl-, 5-formyl-, and 5-carboxy-2'-deoxycytidine are new epigenetic bases (hmdC, fdC, cadC) that were recently discovered in the DNA of higher eukaryotes. The same bases (5-hydroxymethyl-, 5-formyl-, and 5-carboxycytidine; hmC, fC, and caC) have now also been detected in mammalian RNA with a high abundance in mRNA. While DNA phosphoramidites (PAs) that allow the synthesis of xdC-containing oligonucleotides for deeper biological studies are available, the corresponding silyl-protected RNA PAs for fC and caC have not yet been disclosed. Here, we report novel RNA PAs for hmC, fC, and caC that can be used in routine RNA synthesis. The new building blocks are compatible with the canonical PAs and also with themselves, which enables even the synthesis of RNA strands containing all three of these bases. The study will pave the way for detailed physical, biochemical, and biological studies to unravel the function of these non-canonical modifications in RNA.

Acrylonitrile-induced reversible inhibition of uridine uptake by isolated rat intestinal epithelial cells

Isolated rat intestinal epithelial cells (IEC) were used to investigate the mechanism(s) of acrylonitrile (VCN)-induced gastro-intestinal damage. The isolated cells were 93% structurally intact for 60 min, as indicated by trypan blue exclusion. Uridine uptake by isolated IEC was linear from 6-20 min, after which a steady state was reached for up to 40 min. Exposure of isolated IEC to various concentrations of VCN reduced the ability of the cells to take up uridine in a concentration-dependent manner. A concentration of 82 mum VCN inhibited the [(3)H]uridine uptake of the cells by 50% (IU(50)). A time-course study indicated that the maximal inhibition of uridine uptake occurred at 15 min after exposure to VCN. The VCN-induced inhibition of uridine uptake was found to be reversible. IEC exposed to two sublethal doses of VCN (41 and 82 mum) for 15 min regained normal uridine uptake activity within 50 min after removal of VCN. The present study provides a sensitive approach for the detection and evaluation of cytotoxic risk of sublethal doses of the gastro-intestinal toxin VCN using IEC as target cells. The observed in vitro cytotoxicity of sublethal doses of VCN will be used to investigate further the mechanism of VCN-induced gastro-intestinal damage.

Synthesis of oligodeoxynucleotides using fully protected deoxynucleoside 3'-phosphoramidite building blocks and base recognition of oligodeoxynucleotides incorporating N3-cyano-ethylthymine

Oligodeoxynucleotide (ODN) synthesis, which avoids the formation of side products, is of great importance to biochemistry-based technology development. One side reaction of ODN synthesis is the cyanoethylation of the nucleobases. We suppressed this reaction by synthesizing ODNs using fully protected deoxynucleoside 3'-phosphoramidite building blocks, where the remaining reactive nucleobase residues were completely protected with acyl-, diacyl-, and acyl-oxyethylene-type groups. The detailed analysis of cyanoethylation at the nucleobase site showed that N3-protection of the thymine base efficiently suppressed the Michael addition of acrylonitrile. An ODN incorporating N3-cyanoethylthymine was synthesized using the phosphoramidite method, and primer extension reactions involving this ODN template were examined. As a result, the modified thymine produced has been proven to serve as a chain terminator.

A Framework for Human Relevance Analysis of Information on Carcinogenic Modes of Action

The human relevance framework (HRF) outlines a four-part process, beginning with data on the mode of action (MOA) in laboratory animals, for evaluating the human relevance of animal tumors. Drawing on U.S. EPA and IPCS proposals for animal MOA analysis, the HRF expands those analyses to include a systematic evaluation of comparability, or lack of comparability, between the postulated animal MOA and related information from human data sources. The HRF evolved through a series of case studies representing several different MOAs. HRF analyses produced divergent outcomes, some leading to complete risk assessment and others discontinuing the process, according to the data available from animal and human sources. Two case examples call for complete risk assessments. One is the default: When data are insufficient to confidently postulate a MOA for test animals, the animal tumor data are presumed to be relevant for risk assessment and a complete risk assessment is necessary. The other is the product of a data-based finding that the animal MOA is relevant to humans. For the specific MOA and endpoint combinations studied for this article, full risk assessments are necessary for potentially relevant MOAs involving cytotoxicity and cell proliferation in animals and humans (Case Study 6, chloroform) and formation of urinary-tract calculi (Case Study 7, melamine). In other circumstances, when data-based findings for the chemical and endpoint combination studied indicate that the tumor-related animal MOA is unlikely to have a human counterpart, there is little reason to continue the risk assessment for that combination. Similarly, when qualitative considerations identify MOAs specific to the test species or quantitative considerations indicate that the animal MOA is unlikely to occur in humans, such hazard findings are generally conclusive and further risk assessment is not necessary for the endpoint-MOA combination under study. Case examples include a tumor-related protein specific to test animals (Case Study 3, d-limonene), the tumor consequences of hormone suppression typical of laboratory animals but not humans (Case Study 4, atrazine), and chemical-related enhanced hormone clearance rates in animals relative to humans (Case Study 5, phenobarbital). The human relevance analysis is highly specific for the chemical-MOA-tissue-endpoint combination under analysis in any particular case: different tissues, different endpoints, or alternative MOAs for a given chemical may result in different human relevance findings. By providing a systematic approach to using MOA data, the HRF offers a new tool for the scientific community's overall effort to enhance the predictive power, reliability and transparency of cancer risk assessment.

Deoxyribonucleoside phosphoramidites

The detailed preparation of deoxyribonucleoside phosphoramidites bearing a 4-[N-methyl-N-(2,2,2-trifluoroacetyl)amino]butyl group for P(III) protection is presented. The use of this group circumvents nucleobase alkylation during oligonucleotide deprotection. Two syntheses of phosphoramidites starting from either a phosphordichloridite precursor or a bis-(N,N-diisopropylamino)chlorophosphine intermediate are described for the phosphinylation of suitably protected deoxyribonucleosides.

High-quality oligo-RNA synthesis using the new 2′-O-TEM protecting group by selectively quenching the addition of p-tolyl vinyl sulphone to exocyclic amino functions

During the F–-promoted deprotection of the oligo–RNA, synthesized using our 2′-O-(4-tolylsulfonyl)ethoxymethyl (2′-O-TEM) group [Org. Biomol. Chem. 5, 333 (2007)], p-tolyl vinyl sulphone (TVS) is formed as a by-product. The TVS formed has been shown to react with the exocyclic amino functions of adenosine (A), guanosine (G), and cytidine (C) of the fully deprotected oligo–RNA to give undesirable adducts, which are then purified by HPLC and unambiguously characterized by 1H, 13C Heteronuclear Multiple Bond Correlation (HMBC) NMR and mass spectroscopic analysis. The relative nucleophilic reactivities of the nucleobases toward TVS have been found to be the following: N6–A > N4–C > N2–G > > N3–U. This reactivity of TVS toward RNA nucleobases to give various Michael adducts could, however, be suppressed by using various amines as scavengers. Among all these amines, morpholine and piperidine are the most efficient scavenger for TVS, which gave highly pure oligo–RNA even in the crude form and can be used directly in RNA chemical biology studies.Key words: RNA synthesis, RNA alkylation, p-tolyl vinyl sulphone, Michael addition.

Polymorphism of glutathione S-transferases and susceptibility to acrylonitrile and dimethylsulfate in cases of intoxication

In this paper, the difficulties of genetic screening of occupationally exposed subjects for the evaluation of retrospective, and prospective, health risk assessments is illustrated with reference to glutathione S-transferase (GST) function. Individual differences in the magnitude and half-life of adduct levels, derived from background and occupational exposure, are observed largely independently of genetically determined conjugator status. During detoxification, GSTs play a critical role in providing protection against electrophiles and products of oxidative stress. GSTs are a superfamily of enzymes that may have broad and overlapping substrate specificities. Deficiencies of GST isoenzymes may be compensated by the presence of other isoforms and by the use of alternative metabolic pathways. This may be one reason for the abundance of controversial data on GST polymorphisms and adverse health effects.

Comparative chronic toxicity and carcinogenicity of acrylonitrile by drinking water and oral intubation to Spartan Sprague-Dawley rats

Groups of 100 male and 100 female Spartan Sprague-Dawley rats were administered lifetime oral doses of Acrylonitrile (AN) by one of two routes of dosing, either at 0.1 or 10 mg/kg per day, 7 day per week by intubation or continually at 1 or 100 ppm AN in their drinking water. The doses selected were designed to approximate the same daily intake of AN in each of two separate studies, whether by a single bolus dose (intubation) or a more continuous dosing regimen in drinking water. Each study had its own untreated control group of 100 rats per sex. In the drinking water study, the equivalent mean dosage of AN administered to males and females were 0, 0.09, and 0.15 mg/kg per day, respectively, at the 1 ppm level, and 0, 8.0 and 10.7 mg/kg per day, respectively, for 100 ppm dose groups. In both studies, groups of ten rats per sex were sacrificed at 6, 12 and 18 months and at study term. Ophthalmoscopic, hematological, clinical biochemistry, urinalysis and full histopathological exams were performed on control and high dose groups of rats in each study. Similar tests were done in lower dose groups, as required, to define dose-responses of observed effects. All animals were necropsied and underwent microscopic examination of target tissues, including brain, ear canal, stomach, spinal cord and any observable tissue masses. High dose male and female rats in both studies exhibited statistically decreased body weights. Food consumption and water intake were reduced only in the drinking water study. Due to increased deaths in groups of high dose rats of both studies receiving AN, all intubation test groups were terminated after 20 months of treatment. Surviving males and females in the drinking water study were terminated after 22 and 19 months, respectively. Small, sometimes statistically significant, reductions in hemoglobin, hematocrit and erythrocyte count were observed in male and female rats in both high dose (10 mg/kg per day intubation and 100 ppm drinking water) groups from both studies. There were increases in absolute or relative organ weight ratios for liver and adrenal in the high dose intubation study groups, but could not be correlated with AN toxicity in the absence of adverse clinical biochemistry or microscopic findings. Similar organ weight findings were not observed in the drinking water study. Again, there were no changes in clinical biochemistry or microscopic findings in these tissues. Absolute kidney weights were increased in high dose male and female rats in the intubation study and high dose female rats only in the drinking water study. Male and female rats from high dose groups in each study had a higher incidence of palpable masses of the head and the nonglandular stomach and, in females only, the mammary region. In both sexes, treatment-related tumors of the central nervous system (brain, spinal cord), ear canal, and gastrointestinal tract, and in females only, the mammary gland (intubation only) were observed in rats administered either 10 mg/kg per day by intubation or 100 ppm in drinking water. Animals from the intubation study had a substantially higher incidence of AN-related site-specific tumors than did their drinking water study counterparts. While a similar spectrum of tumors was produced by both oral dosing regimens, there were some notable differences in organ-specific incidence of tumors. Astrocytomas of the brain and spinal cord were found at a higher incidence in those rats exposed continuously to AN administered in the drinking water versus bolus dosing by intubation. Conversely, a higher incidence of squamous cell carcinomas/papillomas of the forestomach and adenocarcinomas of the intestine and, in females only, carcinomas of the mammary gland were observed in high dose rats receiving AN by intubation. An increase in the degree of severity of forestomach hyperplasia was observed in all high dose groups of animals, irrespective of mode of administration. These effects were more pronounced, were correlated with a much higher incidence of forestomach tumors, and were identified earlier (12 months) in the intubation study in which there was direct tissue contact with a more concentrated AN solution. Elevations in epidermal cysts in high dose males and females in the intubation study and renal hyperplasia in high dose animals of both sexes in both studies may have a treatment relationship. All other clinical and microscopic findings were considered unremarkable. There were no discernable non-neoplastic effects attributable to treatment in groups of low dose male and female rats given AN by intubation at 0.1 mg/kg per day or 1 ppm in drinking water. The results of this study indicate a consistent spectrum of neoplastic and non-neoplastic effects produced by AN in the same rat strain, whether administered orally by bolus or by continuous dosing in the drinking water. While the spectrum of tumors and target organ toxicity produced was similar, bolus dosing clearly increased tumors associated with the gastrointestinal tract. Neoplasms found in several other tissues were most prominently displayed in groups of more continuously dosed rats.

Mutagenicity, carcinogenicity, and teratogenicity of acrylonitrile

Acrylonitrile (AN) is an important intermediary for the synthesis of a variety of organic products, such as artificial fibres, household articles and resins. Although acute effects are the primary concern for an exposure to AN, potential genotoxic, carcinogenic and teratogenic risks of AN have to be taken seriously in view of the large number of workers employed in such industries and the world-wide population using products containing and possibly liberating AN. An understanding of the effect of acrylonitrile must be based on a characterization of its metabolism as well as of the resulting products and their genotoxic properties. Tests for mutagenicity in bacteria have in general been positive, those in plants and on unscheduled DNA synthesis doubtful, and those on chromosome aberrations in vivo negative. Wherever positive results had been obtained, metabolic activation of AN appeared to be a prerequisite. The extent to which such mutagenic effects are significant in man depends, however, also on the conditions of exposure. It appears from the limited data that the ultimate mutagenic factor(s), such as 2-cyanoethylene oxide, may have little opportunity to act under conditions where people are exposed because it is formed only in small amounts and is rapidly degraded. The carcinogenic action of AN has been evaluated by various agencies and ranged from 'reasonably be anticipated to be a human carcinogen' to 'cannot be excluded', the most recent evaluation being 'possibly carcinogenic to humans'. Animal data that confirm the carcinogenic potential of AN have certain limitations with respect to the choice of species, type of tumors and length of follow up. Epidemiological studies which sometimes, but not always, yielded positive results, encounter the usual difficulties of confounding factors in chemical industries. Exposure of workers to AN should continue to be carefully monitored, but AN would not have to be considered a cancer risk to the population provided limitations on releases from consumer products and guidelines on AN in water and air are enforced. AN is teratogenic in laboratory animals (rat, hamster) at high doses when foetal/embryonic (and maternal) toxicity already is manifest. Pregnant workers should not be exposed to AN. In view of the small concentrations generally encountered outside plants, women not professionally exposed would appear not to be at risk of teratogenic effects due to AN. Future research should concentrate on the elucidation of the different degradation pathways in man and on epidemiological studies in workers including pregnant women, assessing also, if possible, individual exposure by bio-monitoring.

Evaluation of possible genotoxic mechanisms for acrylonitrile tumorigenicity

Acrylonitrile (ACN) exposure is associated with tumors in rat brain, Zymbal gland, and mammary gland. Adducts affecting base pairing were formed in isolated DNA exposed in vitro to the ACN metabolite cyanoethylene oxide (CNEO). DNA from liver, which is not a cancer target organ in ACN-exposed rats, contained low levels of 7-(2-oxoethyl)guanine, and adduct believed not to interfere with base pairing. No adducts have been detected in brain DNA from ACN-exposed rats, suggesting that brain tumors may have arisen by mechanisms other than ACN-DNA reactivity. Genotoxicity assays of ACN have indicated no particular carcinogenic mechanism. Positive reverse mutagenesis in Salmonella typhimurium HisG46 base substitution tester strains by ACN is attributable to CNEO. Other in vitro genotoxicity test assays of ACN have yielded mixed results, without consistent effect of metabolic activation. Some positive genotoxicity data for ACN appear to result from artifacts or from non-DNA-reactive mechanisms. In vivo micronucleus, chromosome aberration, and autoradiographic unscheduled DNA synthesis assays were negative for ACN. The comparative genotoxicity of vinyl chloride and ACN indicates that despite other similarities, they cause rodent tumors by different mechanisms. Also, they absence of ACN-DNA adduct formation in the rat brain suggests the operation of epigenetic mechanisms.

The prebiotic role of adenine: a critical analysis

Adenine plays an essential role in replication in all known living systems today, and is prominent in many other aspects of biochemistry. It occurs among the products of oligomerization of HCN. These circumstances have stimulated the idea that adenine was a component in a replication system that was present at the start of life. Such replicators have included not only RNA, but also a number of simpler RNA-like alternatives which utilize a simpler backbone. Despite these encouraging indicators, a consideration of the chemical properties of adenine reveals reasons that disfavor its participation in such a role. These properties include the following: (1) Adenine synthesis requires HCN concentrations of at least 0.01 M. Such concentrations would be expected only in unique circumstances on the early Earth. Adenine yields are low in prebiotic simulations, and if a subsequent high-temperature hydrolysis step is omitted, the reported yield does not represent adenine itself, but 8-substituted adenines and other derivatives. (2) Adenine is susceptible to hydrolysis (the half life for deamination at 37 degrees C, pH 7, is about 80 years), and to reaction with a variety of simple electrophiles, forming a multiplicity of products. Its accumulation would not be expected over a geological time scale, and its regioselective incorporation into a replicator appears implausible. (3) The adenine-uracil interaction, which involves two hydrogen bonds (rather than three, as in guanine-cytosine pairing) is weak and nonspecific. Pairing of adenine with many other partners has been observed with monomers, synthetic oligonucleotides and in RNA. The hydrogen-bonding properties of adenine appear inadequate for it to function in any specific recognition scheme under the chaotic conditions of a prebiotic soup. New and fundamental discoveries in the chemistry of adenine would be needed to reverse this perception. An alternative and attractive possibility is that some other replicator preceeded RNA (or RNA-like substances) in the origin of life.

Genotoxicity of 2-halosubstituted enals and 2-chloroacrylonitrile in the Ames test and the SOS-chromotest

2-Chloroacrolein and 2-bromoacrolein are very potent direct mutagens not requiring metabolic activation in Salmonella typhimurium strains TA 100 and TA 1535. Mutagenic activities decrease with increasing degree of methyl substitution at carbon atom C-3 of the acrolein moiety from 2-chloroacrolein via 2-chlorocrotonaldehyde to 2-chloro-3,3-dimethylacrolein. With 2-chloroacrylonitrile equivocal results are obtained in strain TA 100 without S9-mix and unequivocal with S9-mix. In the SOS-chromotest the 2-chloroenals are also very strong genotoxins and the structure-activity relationships found in the Ames test are clearly confirmed. 2-Chloroacrylonitrile is not positive in the SOS-chromotest. The mutagenic mechanisms are discussed, and indications are provided that genotoxicity/mutagenicity depends on formation of DNA adducts, e.g., 1,N2-cyclic deoxyguanosine adducts.

Molecular interaction of [2,3-14C] acrylonitrile with DNA in gastric tissue of rat

Acrylonitrile (VCN) is used extensively in polymer industries, and is known to induce gastric cancer following oral administration. A paucity of information exists regarding the mechanism(s) by which acrylonitrile induces gastric neoplasia. The time course for uptake of radioactivity by gastric tissue and covalent binding of [2,3-14C] VCN or its metabolites to gastric DNA were determined following a single oral dose of 46.5 mg/kg. The rates of DNA synthesis and repair, as measured by unscheduled DNA synthesis in the gastric tissue of VCN-treated rats, were also studied. Maximum tissue uptake and covalent binding of radioactivity to gastric DNA were observed at 15 minutes following [2,3-14C] VCN administration. At 6 hours following VCN administration, significant inhibition (37% of control) in gastric replicative DNA synthesis was observed. A rebound followed by an increase (211% of control) in replicative DNA synthesis was observed at 24 hours. A three-fold elevation in unscheduled DNA synthesis was observed at 24 hours following treatment with VCN. These results indicate that VCN or its metabolites irreversibly interact with gastric DNA, causing DNA damage. The results also indicate that the delayed VCN-induced DNA repair, determined as unscheduled DNA synthesis, is inefficient for the removal of the resulting DNA lesions.

In vitro reactions of 2-cyanoethylene oxide with calf thymus DNA

2-cyanoethylene oxide (CEO) is a direct-acting mutagen and the postulated proximate carcinogenic form of acrylonitrile (AN). We have studied the reactions of CEO with 2'-deoxyribonucleosides and in vitro with calf thymus DNA at pH 7.0-7.5 and 37 degrees C for 3 h. Reaction of CEO with dAdo gave 2 adducts, N6-(2-hydroxy-2-carboxyethyl)-dAdo (N6-HOCE-dAdo) (2% yield) and 1,N6-etheno-dAdo (epsilon-dAdo) (11%); reaction with dCyd resulted in the isolation of 3-HOCE-dUrd (22%); reaction with dGuo gave 7-(2-oxoethyl)-Gua (7-OXE-Gua) (31%) and reaction with dThd yielded 3-OXE-dThd (3%). Structural elucidation of adducts was accomplished by ultraviolet spectroscopy, high-field proton NMR spectroscopy and mass spectrometry. Structural confirmation was provided by an accurate mass measurement technique where diagnostic ions in the electron impact mass spectra of trimethylsilyl derivatives were measured to within 0.0007 atomic mass units. The facile Dimroth rearrangement of 1-HOCE-dAdo to N6-HOCE-dAdo and hydrolytic deamination of a dCyd adduct to 3-HOCE-dUrd is postulated to be catalyzed by the hydroxyl group on the 3-carbon side chain of the adduct. Reaction of CEO with calf thymus DNA yielded (nmol/mg DNA) N6-HOCE-dAdo (2); epsilon-dAdo (11); 3-HOCE-dUrd (80); 7-OXE-Gua (110) and 3-OXE-dThd (1). Thus CEO, like its metabolic precursor AN, directly alkylates DNA in vitro but at a much more rapid rate.

Mass spectrometric investigation of the presence of 7-methyl ring-opened guanine derivatives in urine

The involvement of chemical alkylating agents in tumorigenesis and chemotherapy is well established and it was shown that one of the main sites of alkylation is the N-7 of guanine in DNA. Though excision of damaged bases is regarded as one of the repair mechanisms in damaged DNA there is a scarcity of information concerning the excised final metabolites in body fluids. This study attempts to demonstrate the usefulness of CAD MS/MS for the detection of the final metabolite-deformylated ring-opened 7 alkylguanine in urine. Such mass spectrometric methods can be used in biomedical studies.

Pulmonary toxicity of acrylonitrile: covalent interaction and effect on replicative and unscheduled DNA synthesis in the lung

Acrylonitrile (VCN)-induced lung toxicity was studied following a single oral dose (46.5 mg/kg). The mechanism of toxic injury was investigated by assessing the covalent interaction of [2,3-14C]VCN with pulmonary DNA. The effect of the same dose on replicative DNA synthesis and repair in the lungs of treated rats was also investigated. Histologic examination revealed that lungs of VCN-treated animals showed moderate to marked hyperplasia of the Clara cells lining the bronchioles. [14C]Lung tissue uptake was extremely fast, having a maximum at 0.5 h after treatment (150 DPM tissue). Radioactivity in lung tissue declined gradually as a function of time, but was still detected at 72 h after treatment (59 DPM/mg tissue). Covalent binding of [14C]VCN-derived radioactivity to pulmonary DNA was time-dependent, reaching a maximum at 12 h following treatment (61 DPM/mg DNA) and was still detected at 72 h (27 DPM/mg DNA) indicating the incomplete removal of radioactivity covalently bound to DNA. Replicative DNA synthesis in lung tissue was significantly decreased at all time points studied (59, 55 and 72% of control at 0.5, 6 and 24 h, respectively). The DNA repair in the lung was increased by 2-fold at 0.5 h and 1.6-fold at 6 h following VCN oral treatment. The histologic and biochemical results presented in this study provide evidence for the acute genetic toxicity of VCN (and/or its metabolites) in lung tissue following a single oral dose of VCN.

DNA adduct formation by 12 chemicals with populations potentially suitable for molecular epidemiological studies

DNA adduct formation, route of absorption, metabolism and chemistry of 12 hazardous chemicals are reviewed. Methods for adduct detection are also reviewed and approaches to sensitivity and specificity are identified. The selection of these 12 chemicals from the Environmental Protection Agency list of genotoxic chemicals was based on the availability of information and on the availability of populations potentially suitable for molecular epidemiological study. The 12 chemicals include ethylene oxide, styrene, vinyl chloride, epichlorohydrin, propylene oxide, 4,4'-methylenebis-2-chloroaniline, benzidine, benzidine dyes (Direct Blue 6, Direct Black 38 and Direct Brown 95), acrylonitrile and benzyl chloride. While some of these chemicals (styrene and benzyl chloride, possibly Direct Blue 6) give rise to unique DNA adducts, others do not. Potentially confounding factors include mixed exposures in the work place, as well the formation of common DNA adducts. Additional research needs are identified.

Acrylonitrile interaction with testicular DNA in rats

In the present study we report the in vivo interaction of acrylonitrile (VCN) with testicular tissue in rats. Covalent binding of radioactivity to testicular tissue DNA was examined for a period of 72 hr after a single oral dose (46.5 mg/kg) of [2,3-14C] VCN. Maximal covalent binding was observed at 0.5 hr (8.9 mumol VCN equivalent/mol nucleotide). Binding decreased gradually thereafter but was still detected (2.5 mumol VCN equivalent/mol nucleotide) at 72 hr following VCN administration. Further, we examined the effects of VCN on DNA synthesis and repair in the testes of rats following a single oral dose (46.5 mg/kg) of VCN to clarify the impact of the covalent binding observed on the testicular genetic material. A significant decrease in DNA synthesis (80% of control) was observed at 0.5 hr after treatment. At 24 hr following acrylonitrile administration, testicular DNA synthesis was severely inhibited (38% of control). Testicular DNA repair was increased 1.5-fold at 0.5 hr and more than 3.3-fold at 24 hr following treatment with VCN. These results suggest that VCN can act as a multipotent genotoxic agent by alkylating DNA in testicular tissue and may affect the male reproductive function by interfering with testicular DNA synthesis and repair processes.

Mutagenicity of acrylonitrile and its metabolite 2-cyanoethylene oxide in human lymphoblasts in vitro

The mutagenicity of the rat carcinogen acrylonitrile (ACN) and its metabolite 2-cyanoethylene oxide (CNEtO) was assessed in vitro in human lymphoblasts using the heterozygous thymidine kinase (tk) locus as a genetic marker. ACN was tested in both the presence and absence of an Aroclor-induced rat-liver homogenate S9. In the absence of S9, ACN was not mutagenic over the concentration range tested (0.4-1.5 mM X 2 h). In the presence of S9, the mutagenic response of ACN was enhanced, resulting in a significant response at a concentration of 1.4 mM X 2 h. CNEtO, the proposed ultimate mutagenic metabolite of ACN, induced a significant mutagenic response without activation at 100 microM and 150 microM X 2 h. Two phenotypic classes of spontaneous and CNEtO-induced tk-/- mutants were observed; one class of mutants (tkn) had a normal growth rate relative to wild-type while the second class (tks) grew at a slower rate. The molecular nature of these two phenotypic classes was investigated by Southern blot analysis. CNEtO-induced tkn mutant clones (11/12) and 7/9 tkn spontaneous mutants had no detectable alterations in their tk restriction fragment pattern. In contrast, 25/26 tks mutants analyzed (spontaneous and CNEtO-induced) had lost a 14.8-kb polymorphic fragment of the +tk allele.

The isolation and characterization of 2-carboxyethyl adducts following in vitro reaction of acrylic acid with calf thymus DNA and bioassay of acrylic acid in female Hsd:(ICR)Br mice

Reaction of acrylic acid (AA) at pH 7.0 and 37 degrees C for 40 days with 2'-deoxyadenosine (dAdo), 2'-deoxycytidine (dCyd), 2'-deoxyguanosine (dGuo) and thymidine (dThd) resulted in the formation of 2-carboxyethyl (CE) adducts via Michael addition. The alkylated 2'-deoxynucleoside adducts isolated (percent yield after 40 days) were 1-CE-dAdo (5%), N6-CE-dAdo (11%) (via Dimroth rearrangement of 1-CE-dAdo), 3-CE-dCyd (7.5%), 7-CE-Gua (4%), 7,9-bis-CE-Gua (0.9%) (formed by reaction of AA with depurinated 7-CE-Gua during the course of the reaction) and 3-CE-dThd (0.5%). The products isolated following in vitro reaction of AA with calf thymus DNA at pH 7.0 and 37 degrees C for 40 days were (nmol/mg DNA) 1-CE-Ade (9.9), N6-CE-Ade (8.2), 7-CE-Gua (7.2) and 3-CE-Thy (1.9). Compound 3-CE-Cyt was not detected. Thus the adducts formed following in vitro reaction of AA with DNA are identical to those formed by in vitro reaction of the carcinogen beta-propiolactone (BPL) with DNA as reported in an earlier paper. Structures were assigned on the basis of identical UV spectra, Rf values on paper chromatograms and Rt values on HPLC as marker compounds prepared from reactions of BPL with 2'-deoxynucleosides and 2'-deoxynucleotides-5'-monophosphoric acids. AA was assayed for carcinogenic activity by s.c. injection (20 mumol, once a week for 52 weeks) in female Hsd: (ICR)Br mice. Two mice with sarcomas at the site of application were observed out of 30 mice. Malignancies were not observed in solvent and no-treatment controls. The bioassay results reported in this paper and elsewhere in the same strain of mice suggest that AA is a weak carcinogen in female Hsd:(ICR)Br mice.

In vitro transformation of C3H/10T1/2 and NIH/3T3 cells by acrylonitrile and acrylamide

Acrylonitrile (AN) and acrylamide (AM) are carcinogenic in a number of rodent organs and AN is a suspected human carcinogen. We sought to determine whether AN and/or AM could produce morphological transformation in vitro in C3H/10T1/2 and NIH/3T3 mouse fibroblast cells. Both AN and AM induced a dose-dependent cytotoxic effect in C3H/10T1/2 and NIH/3T3 cells and readily transformed both cell lines. Our conclusions are based on the appearance of cells exhibiting a transformed phenotype and growth in soft agar. AN and AM transformed NIH/3T3 cells to a greater extent than C3H/10T1/2 cells. This is the first reported transformation of cells in vitro by AM.

Monitoring exposure to acrylamide by the determination of S-(2-carboxyethyl)cysteine in hydrolyzed hemoglobin by gas chromatography-mass spectrometry

Acrylamide is a potent cumulative neurotoxin in animals and man. In vivo exposure to this electrophile results in the formation of a covalently bound reaction product with cysteine residues in hemoglobin. This adduct yields on acid hydrolysis S-(2-carboxyethyl)cysteine which has been analyzed by capillary gas chromatography with mass spectrometry. Globin isolated from the blood of rats exposed to acrylamide was spiked with an internal standard (globin treated in vitro with d3-acrylamide) and was then hydrolyzed with 6 N HCl. The protein hydrolysate was fractionated on a Dowex 50W H+ ion exchange column and the amino acids in the partially purified extract were determined as N-heptafluorobutyryl methyl esters using an OV-1701 fused silica capillary column. Quantitation was made by chemical ionization (isobutane) selective ion monitoring in which the ions m/z 386 (M-OCH3)+ derived from derivatized S-(2-carboxyethyl)cysteine in the sample and the corresponding ion m/z 389 from the added deuterium-labeled internal standard were monitored. The dose-response relationship between production of hemoglobin adduct and dose of acrylamide (0.1 mg/kg-5 mg/kg) is curved, showing an increasing slope with increasing doses of acrylamide.

[14C]acrylonitrile: preparation via a stable tosylate intermediate and quantitative reaction with amine residues in collagen

A simple, convenient synthetic procedure for [14C]acrylonitrile is described. Na14CN is used as the radioactive starting material. Small (milligram) amounts are converted to 3-[14C]Hydroxypropionitrile by a substitution reaction with 2-chloroethanol. 3-[14C]Hydroxypropionitrile is then tosylated, and the specific activity of this intermediate product is easily determined using its uv extinction coefficient and scintillation counting. [14C]Acrylonitrile is obtained rapidly on distillation by heating the tosylate in the presence of a high boiling tertiary amine base catalyst. The tosylate intermediate can be stored, in contrast to radioactive acrylonitrile, which is unstable. The reaction of acrylonitrile with lysine, hydroxylysine, and histidine residues in human Achilles tendon collagen, as well as chromatographic separation and identification of the carboxyethyl derivatives of these amino acids, is also described.

The influence of N7 substituents on the stability of N7-alkylated guanosines

Guanosine alkylated at the 7-position readily decomposed by imidazole ring opening and depurination. There is a direct correlation between the electron-withdrawing effects, expressed as the Taft substituent constant sigma*, of various alkylsubstituents in 7-position and the respective acid dissociation constants and chemical stabilities of 7-alkylated guanosines. The respective values for the slopes of the regression lines were rho* = +1.89 for imidazole ring opening and rho* = +0.175 for the depurination reaction. Unknown Taft constants can be determined from the sigma*/pKa regression line.