Genetic toxicity of some important epoxides

Epoxides: an underestimated lipid oxidation product

Immense gains in understanding of mechanisms and effects of lipid oxidation have been achieved in the nearly 90 years over which lipid oxidation has been an active research focus. Even so, the substantial questions still being raised about lipid oxidation in this special issue show clearly that missing pieces remain and must be considered for full accounting of this important reaction in any system. In this context, epoxides are spotlighted as a critical overlooked product of lipid autoxidation - underestimated in analysis, underestimated in presence as a functionally active and competitive intermediate and product of lipid oxidation, and underestimated in potential contributions to impact of lipid oxidation on other molecules and cell functions. Logical reasons for ignoring or not finding epoxides are offered in historical development of lipid oxidation knowledge. Reactions generating lipid epoxides in autoxidation are reviewed, limitations in detecting and tracking epoxides are outlined to explain why epoxides may not be detected when they should be present, and justifications for increased research and analysis of epoxides are argued. The main goal is to provide a context for recognizing epoxides as critical products that must be accounted for in determining the state rather than extent of lipid oxidation and in tracking its consequences in oils, foods, personal care products, and tissues. A secondary goal is to stimulate new research using contemporary analyses to fill in the gaps of knowledge about epoxide formation, structure, and reactions in lipid autoxidation.

Biosynthesis of chiral diols from alkenes using metabolically engineered type II methanotroph

Methanotrophs are environmentally friendly microorganisms capable of converting gas to liquid using methane monooxygenases (MMOs). In addition to methane-to-methanol conversion, MMOs catalyze the conversion of alkanes to alcohols and alkenes to epoxides. Herein, the efficacy of epoxidation by type I and II methanotrophs was investigated, and type II methanotrophs were observed to be more efficient in converting alkenes to epoxides. Subsequently, three (Epoxide hydrolase) EHs of different origins were overexpressed in the type II methanotroph Methylosinus trichosporium OB3b to produce 1,2-diols from epoxide. Methylosinus trichosporium OB3b expressing Caulobacter crescentus EH produced the highest amount of (R)-1,2-propanediol (251.5 mg/L) from 1-propene. These results demonstrate the possibility of using methanotrophs as a microbial platform for diol production and the development of a continuous bioreactor for industrial applications.

Glycidamide-induced hypermutation in yeast single-stranded DNA reveals a ubiquitous clock-like mutational motif in humans

Mutagens often prefer specific nucleotides or oligonucleotide motifs that can be revealed by studying the hypermutation spectra in single-stranded (ss) DNA. We utilized a yeast model to explore mutagenesis by glycidamide, a simple epoxide formed endogenously in humans from the environmental toxicant acrylamide. Glycidamide caused ssDNA hypermutation in yeast predominantly in cytosines and adenines. The most frequent mutations in adenines occurred in the nAt→nGt trinucleotide motif. Base substitutions A→G in this motif relied on Rev1 translesion polymerase activity. Inactivating Rev1 did not alter the nAt trinucleotide preference, suggesting it may be an intrinsic specificity of the chemical reaction between glycidamide and adenine in the ssDNA. We found this mutational motif enriched in published sequencing data from glycidamide-treated mouse cells and ubiquitous in human cancers. In cancers, this motif was positively correlated with the single base substitution (SBS) smoking-associated SBS4 signature, with the clock-like signatures SBS1, SBS5, and was strongly correlated with smoking history and with age of tumor donors. Clock-like feature of the motif was also revealed in cells of human skin and brain. Given its pervasiveness, we propose that this mutational motif reflects mutagenic lesions to adenines in ssDNA from a potentially broad range of endogenous and exogenous agents.

Computational Investigations of Reaction Mechanisms and Transformation Products of Olefins with Hypochlorous Acid

Hypochlorous acid (HOCl) as the main component in chlorination and also as the innate immune factor relevant to immune defense has attracted considerable attention. Electrophilic addition reaction of olefins with HOCl, one of the most important prototype of chemical reactions, has been intensively studied for a long time; however, it has not been fully understood yet. In this study, addition reaction mechanisms and transformation products of model olefins with HOCl were systematically investigated by the density functional theory method. The results indicate that the traditionally believed stepwise mechanism with a chloronium-ion intermediate is only suitable for olefins substituted with electron-donating groups (EDGs) and weak electron-withdrawing groups (EWGs) but it is a carbon-cation intermediate that is favorable for EDGs featuring p-π or π-π conjugation with the C═C moiety. Moreover, olefins substituted with moderate or/and strong EWGs prefer the concerted and nucleophilic addition mechanisms, respectively. Epoxide and truncated aldehyde as the main transformation products can be generated from chlorohydrin through a series of reactions involving hypochlorite; however, their generation is kinetically not as feasible as the formation of chlorohydrin. The reactivity of three chlorinating agents (HOCl, Cl₂O, and Cl₂) and the case study of chlorination and degradation of cinnamic acid were also explored. Additionally, APT charge on the double-bond moiety in olefin and energy gap (ΔE) between the highest occupied molecular orbital (HOMO) energy of olefin and the lowest unoccupied molecular orbital (LUMO) energy of HOCl were found to be good parameters to distinguish the regioselectivity of chlorohydrin and reactivity of olefin, respectively. The findings of this work are helpful in further understanding the chlorination reactions of unsaturated compounds and identifying complicated transformation products.

Assessing Alkene Reactivity toward Cytochrome P450-Mediated Epoxidation through Localized Descriptors and Regression Modeling

The prediction of sites of epoxidation by cytochrome P450s during metabolism is particularly important in drug design, as epoxides are capable of alkylating biological macromolecules. Reliable methods are needed to quantitatively predict P450-mediated epoxidation barriers for inclusion in high-throughput screening campaigns alongside protein-ligand docking. Utilizing the fractional occupation number weighted density (FOD) and orbital-weighted Fukui index (f_w⁺) as descriptors of local reactivity and a data set of 36 alkene epoxidation barriers computed with density functional theory (DFT), we developed and validated a multiple linear regression model for the reliable estimation of epoxidation barriers using only substrate structures as input. Using our recommended level of theory (GFN2-xTB//GFN-FF), mean absolute errors in the training and test sets were found to be 0.66 and 0.70 kcal/mol, respectively, with coefficients of determination of ca. 0.80. We demonstrate the utility of this approach on three known substrates of CYP101A1 and further show that this approach is inappropriate for particularly electron-rich alkenes. By employing a modern semiempirical method on force-field-generated geometries, the required descriptors can be calculated on the millisecond timescale per structure, making the approach well suited for incorporation into high-throughput methodologies alongside docking.

Anharmonic effect of the unimolecular dissociation of Glycerol to Glycidol

The unimolecular dissociation rate constants of the dehydration of Glycerol to Glycidol were calculated at the MP2/6–311G(d,p) level using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The anharmonic effect of the reactions was examined by comparing the rate constants at temperatures (700–3000[Formula: see text]K) of the canonical case and total energies (25654–53089[Formula: see text]cm[Formula: see text]) of the microcanonical system. The calculations showed that high temperatures are required for the reaction to proceed. As the temperatures and total energies increased, the rate of reactions increased. However, the growth rate of the unimolecular dissociation rate constants was high and slower both in the canonical and microcanonical systems. Comparative analysis showed that the anharmonic effect was most significant for the reaction [Formula: see text] and least significant for the reaction [Formula: see text]. The anharmonic effect became more significant as the temperatures and total energies increased. Compared with the microcanonical situation, the anharmonic effect of the canonical system was more pronounced.

IQOS: examination of Philip Morris International's claim of reduced exposure

BACKGROUND

New electronic heated tobacco products are being introduced in the global market and are gaining popularity. In 2016, Philip Morris International, Inc. (PMI) submitted a modified risk tobacco product (MRTP) application to the Food and Drug Administration (FDA) to market IQOS in the USA with claims of reduced exposure and reduced risk.

METHODS

We examined PMI's MRTP application, specifically sections on aerosol chemistry and human exposure assessment, to assess the validity of PMI's claims of reduced exposure and risk.

FINDINGS

PMI reported levels for only 40 of 93 harmful and potentially harmful constituents (HPHCs) on FDA's HPHC list in IQOS mainstream aerosol. All substances in PMI's list of 58 constituents (PMI-58) were lower in IQOS emissions compared with mainstream smoke of 3R4F reference cigarettes. However, levels of 56 other constituents, which are not included in the PMI-58 list or FDA's list of HPHCs, were higher in IQOS emissions; 22 were >200% higher and seven were >1000% higher than in 3R4F reference cigarette smoke. PMI's studies also show significantly lower systemic exposure to some HPHCs from use of IQOS compared with smoking combustible cigarettes.

CONCLUSION

PMI's data appear to support PMI's claim that IQOS reduces exposure to HPHCs. However, PMI's data also show significantly higher levels of several substances that are not recognised as HPHCs by the FDA in IQOS emissions compared with combustible cigarette smoke. The impact of these substances on the overall toxicity or harm of IQOS is not known.

Reaction kinetic studies for comparison of mutagenic potency between butadiene monoxide and glycidamide

DNA adducts can be formed from covalent binding of electrophilic reactive compounds to the nucleophilic N- and O-atoms of the biomolecule. The O-sites on DNA, with nucleophilic strength (n) of ca. 2, is recognized as a critical site for mutagenicity. Characterization of the reactivity of electrophilic compounds at the O-sites can be used to predict their mutagenic potency in relative terms. In the present study, reaction kinetic experiments were performed for butadiene monoxide (BM) in accordance with the Swain-Scott relation using model nucleophiles representing N- and O-sites on DNA, and earlier for glycidamide (GA) using a similar approach. The epoxide from the kinetic experiments was trapped by cob(I)alamin, resulting in formation of an alkylcobalamin which was analyzed by liquid chromatography tandem mass spectrometry. The Swain-Scott relationship was used to determine selectivity constant (s) of BM and GA as 0.86 and 1.0, respectively. The rate constant for the reaction at n of 2 was extrapolated to 0.023 and 0.038 M^-1 h^-1 for BM and GA, respectively, implying a higher mutagenic potency per dose unit of GA compared to BM. The reaction kinetic parameters associated with mutagenic potency were also estimated by a density functional theory approach, which were in accordance to the experimental determined values. These types of reaction kinetic measures could be useful in development of a chemical reactivity based prediction tool that could aid in reduction of animal experiments in cancer risk assessment procedures for relative mutagenicity.

Nanotextured alpha Ni(ii)–Co(ii) hydroxides as supercapacitive active phases

Nanotextured layered Ni_1−xCo_x(OH)_2−yCl_y·nH₂O was obtained by epoxide method for all compositions. An optimum supercapacitive performance was recorded for x = 0.83.

The Induction of Gene Mutations and Micronuclei by Oxiranes and Siloranes in Mammalian Cells in vitro

Oxiranes and siloranes are candidate molecules for the development of composite materials with low shrinkage. Since some of these molecules are highly reactive, they could lead to adverse biological effects from underlying genetic mechanisms. Therefore, we analyzed the formation of micronuclei (chromosomal aberrations) and the induction of gene mutations (HPRT assay) in mammalian cells. The numbers of micronuclei induced by the oxirane di(cyclohexene-epoxidemethyl)ether (Eth-Ep) at low concentrations (10 μM) were about five-fold higher than controls. The related compound epoxy cyclohexyl methyl-epoxy cyclo-hexane carboxylate (Est-Ep) was less effective. The activity of diglycidylether of bisphenol A (BADGE) was even lower but similar to the most reactive silorane, di-3,4-epoxy cyclohexylmethyl-dimethyl-silane (DiMe-Sil). No induction of micronuclei was detected in the presence of a rat liver homogenate (S9). Est-Ep and Eth-Ep also induced gene mutations. Our analyses indicated low mutagenic potentials of siloranes; however, some oxiranes induced strong effects at two genetic endpoints.

Biological conversion of gaseous alkenes to liquid chemicals

Industrial gas-to-liquid (GTL) technologies are well developed. They generally employ syngas, require complex infrastructure, and need high capital investment to be economically viable. Alternatively, biological conversion has the potential to be more efficient, and easily deployed to remote areas on relatively small scales for the utilization of otherwise stranded resources. The present study demonstrates a novel biological GTL process in which engineered Escherichia coli converts C2-C4 gaseous alkenes into liquid diols. Diols are versatile industrially important chemicals, used routinely as antifreeze agents, polymer precursors amongst many other applications. Heterologous co-expression of a monooxygenase and an epoxide hydrolase in E. coli allows whole cell conversion of C2-C4 alkenes for the formation of ethylene glycol, 1,2-propanediol, 1,2-butanediol, and 2,3-butanediol at ambient temperature and pressure in one pot. Increasing intracellular NADH supply via addition of formate and a formate dehydrogenase increases ethylene glycol production titers, resulting in an improved productivity of 9mg/L/h and a final titer of 250mg/L. This represents a novel biological method for GTL conversion of alkenes to industrially valuable diols.

One pot synthesis of Mg2Al(OH)6Cl·1.5H2O layered double hydroxides: the epoxide route

Pure Mg2Al(OH)6Cl·1.5H2O layered double hydroxide (LDH) has been synthesized at room temperature by a one-pot method, homogeneously driven by chloride-assisted glycidol rupture (epoxide route). Well-defined nanoplatelet texture was achieved and the LDH crystallization mechanism discussed. Nanoplatelets self-assemble in the form of highly oriented films with excellent optical properties. LDH films exhibited stability toward detaching in aqueous solutions and allowed a fast anionic exchange preserving a high transparency.

(32)P-postlabelling analysis of 1,3-butadiene-induced DNA adducts in vivo and in vitro

Butadiene monoepoxide (BMO), epoxybutanediol (EBD) and diepoxybutane (DEB) are reactive metabolites of 1,3-butadiene (BD), an important industrial chemical classified as a probable human carcinogen. The covalent interactions of these metabolites with DNA lead to the formation of DNA adducts which may induce mutations or other types of DNA damage, resulting in tumour formation. In the present study, two pairs of diastereomeric N-1-BMO-adenine adducts were identified in the reaction of BMO with 2´-deoxyadenosine-5´-monophosphate (5´-dAMP). The major products formed by reacting EBD with 2´-deoxyguanosine-5´-monophosphate (5´-dGMP) were characterized as diastereomeric N-7-(2´,3´,4´-trihydroxybut-1´-yl)-5´-dGMP by UV and electrospray mass spectrometry. The formation of N-7-BMO-guanine adducts (1´-carbon, 60; 2´carbon, 54/10(4) nucleotides) in BMO-treated DNA was about four times higher than that of N-1-BMO-adenine adducts (1´-carbon, 20; 2´-carbon, 8.7/10(4) nucleotides). However, the recovery of N-1-BMO-adenine adducts in DNA (45 ± 5%) was two times higher than that of N-7-guanine adducts (20 ± 4%) by 32P-postlabelling analysis. Using the 32P-postlabelling/ HPLC assay, N-1-BMO-adenine, N-7-BMO-guanine and N-7-EBDguanine adducts were detected in BMO- or DEB-treated DNA and in liver DNA of rats exposed to BD by inhalation. The amount of N-7-EBD-guanine adducts (11/10(8) nucleotides) in rat liver was about three-fold higher than N-7-BMO-guanine adducts (4.0/10(8) nucleotides). The novel finding of N-1-BMO-adenine adducts formed in vivo may contribute to the understanding of the mechanisms of BD carcinogenic action.

The stress response resolution assay. I. Quantitative assessment of environmental agent/condition effects on cellular stress resolution outcomes in epithelium

The events or factors that lead from normal cell function to conditions and diseases such as aging or cancer reflect complex interactions between cells and their environment. Cellular stress responses, a group of processes involved in homeostasis and adaptation to environmental change, contribute to cell survival under stress and can be resolved with damage avoidance or damage tolerance outcomes. To investigate the impact of environmental agents/conditions upon cellular stress response outcomes in epithelium, a novel quantitative assay, the "stress response resolution" (SRR) assay, was developed. The SRR assay consists of pretreatment with a test agent or vehicle followed later by a calibrated stress conditions exposure step (here, using 6-thioguanine). Pilot studies conducted with a spontaneously-immortalized murine mammary epithelial cell line pretreated with vehicle or 20 µg N-ethyl-N-nitrososurea/ml medium for 1 hr, or two hTERT-immortalized human bronchial epithelial cell lines pretreated with vehicle or 100 µM zidovudine/lamivudine for 12 days, found minimal alterations in cell morphology, survival, or cell function through 2 weeks post-exposure. However, when these pretreatments were followed 2 weeks later by exposure to calibrated stress conditions of limited duration (for 4 days), significant alterations in stress resolution were observed in pretreated cells compared with vehicle-treated control cells, with decreased damage avoidance survival outcomes in all cell lines and increased damage tolerance outcomes in two of three cell lines. These pilot study results suggest that sub-cytotoxic pretreatments with chemical mutagens have long-term adverse impact upon the ability of cells to resolve subsequent exposure to environmental stressors.

DNA-protein crosslinks processed by nucleotide excision repair and homologous recombination with base and strand preference in E. coli model system

Bis-electrophiles including dibromoethane and epibromohydrin can react with O(6)-alkylguanine-DNA alkyltransferase (AGT) and form AGT-DNA crosslinks in vitro and in vivo. The presence of human AGT (hAGT) paradoxically increases the mutagenicity and cytotoxicity of bis-electrophiles in cells. Here we establish a bacterial system to study the repair mechanism and cellular responses to DNA-protein crosslinks (DPCs) in vivo. Results show that both nucleotide excision repair (NER) and homologous recombination (HR) pathways can process hAGT-DNA crosslinks with HR playing a dominant role. Mutation spectra show that HR has no strand preference but NER favors processing of the DPCs in the transcribed strand; UvrA, UvrB and Mfd can interfere with small size DPCs but only UvrA can interfere with large size DPCs in the transcribed strand processed by HR. Further, we found that DPCs at TA deoxynucleotide sites are very inefficiently processed by NER and the presence of NER can interfere with these DNA lesions processed by HR. These data indicate that NER and HR can process DPCs cooperatively and competitively and NER processes DPCs with base and strand preference. Therefore, the formation of hAGT-DNA crosslinks can be a plausible and specific system to study the repair mechanism and effects of DPCs precisely in vivo.

Acute and chronic toxicity toward the bacteria Vibrio fischeri of organic narcotics and epoxides: structural alerts for epoxide excess toxicity

The acute and chronic bacterial toxicity of 34 organic compounds comprising 19 baseline narcotics and 15 epoxides has been determined with regard to 30-min bioluminescence and 24-h growth inhibition in terms of EC50 (effective concentration 50%) values employing Vibrio fischeri. For the narcotics, linear regression of log EC50 on log Kow (octanol/water partition coefficient) yields r2 (squared correlation coefficient) and rms (root-mean-square error) values of 0.95 and 0.44 (30-min), and 0.94 and 0.34 (24-h), respectively. Employing the resultant baseline narcosis models, toxicity enhancement (Te) values were derived as a ratio of narcosis-predicted over experimental EC50 for the epoxides. For seven aliphatic epoxides, log Te was below 1 in both assays, indicating narcosis-range toxicity with regard to 30-min bioluminescence and 24-h growth inhibition. Concerning eight nonaliphatic epoxides, log Te values up to 2.4 were observed, reflecting excess toxicity through an enhanced electrophilic reactivity of the compounds. Here, however, the intercorrelation between both assays was very low (r2 = 0.09). The results are discussed in terms of electronic substituent effects activating an SN2-type epoxide reaction with nucleophilic protein sites and side-chain activation offering alternative electrophile-nucleophile reaction routes at side-chain sites, leading to respective structural alerts as indicators of excess toxicity. Surprisingly, 30-min bioluminescence appears to be slightly more sensitive to chemical stress than 24-h growth, which holds both for baseline narcotics and for most of the epoxides. This is also reflected by effective narcosis doses 50%, ED50, of 7.1 mmol/kg (30-min) and 7.7 mmol/kg (24-h) estimated from narcosis theory. Keeping in mind the different end points (bioluminescence vs growth) involved, this finding demonstrates that chronic toxicity is not always more sensitive than acute toxicity, calling for analyses with regard to further respective cases and associated mechanistic causes.

Epoxide and thiirane toxicity in vitro with the ciliates Tetrahymena pyriformis: structural alerts indicating excess toxicity

The 48 h toxicity of 18 organic narcotics, 13 epoxides, and 2 thiiranes toward the ciliates Tetrahymena pyriformis was determined in terms of 50% growth inhibition EC(50). Nominal EC(50) was corrected for volatilization and sorption to quantify the freely dissolved compound fraction in solution. The derived baseline narcosis model served to evaluate toxicity enhancements T(e) as ratios of narcosis-predicted over experimental EC(50) values. Among the nine heterocycles with aliphatic side chains that include two thiiranes, three compounds yielded T(e) > 10, suggesting their covalent binding at nucleophilic protein sites such as -OH, -NHR, and -SH through S(N)2-type ring-opening. As a general trend of this group, T(e) decreases with increasing alkyl group size. Moreover, four of the six nonaliphatic epoxides exerted substantial excess toxicities with T(e) > 10, which could be rationalized by ring-opening activation through negative inductive effect, benzylic stabilization, and phenyl ring H-bonding. By contrast, 1,2 substituted epoxides showed narcosis-level toxicity, despite the opportunity of side-chain Schiff-base formation with protein amino groups. The resulting structural alerts enable an in silico screening of epoxides and thiiranes for their potential to exert excess toxicity. Note that observed differences in T(e) sensitivity between ciliates, bacteria and fish should be taken into account when designing in vitro alternatives to fish toxicity studies.

Alkyltransferase-mediated toxicity of bis-electrophiles in mammalian cells

The primary function of O(6)-alkylguanine-DNA alkyltransferase (AGT) is to maintain genomic integrity in the face of damage by both endogenous and exogenous alkylating agents. However, paradoxically, bacterial and mammalian AGTs have been shown to increase cytotoxicity and mutagenicity of dihaloalkanes and other bis-electrophiles when expressed in bacterial cells. We have extended these studies to mammalian cells using CHO cells that lack AGT expression and CHO cells stably transfected with a plasmid that expresses human AGT. The cytotoxicity of 1,2-dibromoethane, dibromomethane and epibromohydrin was significantly increased by the presence of AGT but cytotoxicity of butadiene diepoxide was not affected. Mutations caused by these agents were assessed using hypoxanthine-guanine phosphoribosyltransferase (HPRT) as a reporter gene. There was a small (c. 2-3-fold) but statistically significant AGT-mediated increase in mutations caused by 1,2-dibromoethane, dibromomethane and epibromohydrin. Analysis of the mutation spectrum induced by 1,2-dibromoethane showed that the presence of AGT also altered the types of mutations with an increase in total base substitution mutants due to a rise in transversions at both G:C and A:T sites. AGT expression also led to mutations arising from the transcribed strand, which were not seen in cells lacking AGT. Although the frequency of deletion mutations was decreased by AGT expression, the formation of large deletions (> or = 3 exons) was increased. This work demonstrates that interaction of AGT with some bis-electrophiles can cause mutagenicity and diminished cell survival in mammalian cells. It is consistent with the hypothesis that DNA-AGT cross-links, which have been characterized in experiments with purified AGT protein and such bis-electrophiles, can be formed in mammalian cells.

Effect of O6-alkylguanine-DNA alkyltransferase on genotoxicity of epihalohydrins

The effect of O(6)-alkylguanine-DNA alkyltransferase (AGT) on the toxicity and mutagenicity of epihalohydrins was studied. AGT is a DNA repair protein that protects cells from agents that produce genotoxic O(6)-alkylguanine lesions by transferring the alkyl group to an internal cysteine residue (Cys(145) in human AGT) in a single-step. This cysteine acceptor site is highly reactive and epihalohydrins reacted readily with AGT at this site with a halide order of reactivity of Br > Cl > F. AGT expression in bacterial cells caused a very large increase in the mutagenicity and cytotoxicity of epibromohydrin. The mutations were almost all G:C to A:T transitions. Epichlorohydrin also augmented AGT-mediated mutagenesis but to a lesser extent than epibromohydrin. In vitro experiments showed that AGT was covalently cross-linked to DNA in the presence of epibromohydrin and that this conjugation occurred predominantly at Cys(145), and to a smaller extent at Cys(150), a less reactive residue also located within the active site pocket. Two pathways yielding the AGT-DNA adduct were found to occur. The predominant mechanism results in an AGT-epihalohydrin intermediate, which, facilitated by the DNA binding properties of AGT, then reacts covalently with DNA. The second pathway involves an initial reactive DNA-epihalohydrin intermediate that subsequently reacts with AGT. Our results show that the paradoxical AGT-mediated increase in genotoxicity which has previously been shown to occur with dihaloalkanes, butadiene diepoxide and nitrogen mustards, also occurs with epihalohydrins and is likely to contribute to their toxicity and mutagenicity.

Evaluation of cancer tests of 1,3-butadiene using internal dose, genotoxic potency, and a multiplicative risk model

In cancer tests with 1,3-butadiene (BD), the mouse is much more sensitive than the rat. This is considered to be related to the metabolism of BD to the epoxide metabolites, 1,2-epoxy-3-butene (EB), 1,2:3,4-diepoxybutane, and 1,2-epoxy-3,4-butanediol. This study evaluates whether the large difference in outcome in cancer tests with BD could be predicted quantitatively on the basis of the concentration over time in blood (AUC) of the epoxide metabolites, their mutagenic potency, and a multiplicative cancer risk model, which has earlier been used for ionizing radiation. Published data on hemoglobin adduct levels from inhalation experiments with BD were used for the estimation of the AUC of the epoxide metabolites in the cancer tests. The estimated AUC of the epoxides were then weighed together to a total genotoxic dose, by using the relative genotoxic potency of the respective epoxide inferred from in vitro hprt mutation assays using EB as standard. The tumor incidences predicted with the risk model on the basis of the total genotoxic dose correlated well with the earlier observed tumor incidences in the cancer tests. The total genotoxic dose that leads to a doubling of the tumor incidences was estimated to be the same in both species, 9 to 10 mmol/Lxh EB-equivalents. The study validates the applicability of the multiplicative cancer risk model to genotoxic chemicals. Furthermore, according to this evaluation, different epoxide metabolites are predominating cancer-initiating agents in the cancer tests with BD, the diepoxide in the mouse, and the monoepoxides in the rat.

Chromosomal aberrations and micronuclei in lymphocytes in relation to alkylation of hemoglobin in workers exposed to ethylene oxide and propylene oxide

Chromosomal aberrations and micronuclei in lymphocytes were measured in workers exposed to propylene oxide in a factory producing alkylated starch, and in workers exposed to ethylene oxide in connection with sterilization of medical equipment. Adduct levels in hemoglobin were determined as a measure of in vivo doses of the two compounds. The levels of hydroxypropylvaline in propylene oxide-exposed workers were correlated in estimated exposure doses. The levels of this adduct in the unexposed group were close to the detection limit of the method. The levels of hydroxyethylvaline, recorded in the propylene oxide-exposed group were consistent with earlier data on hemoglobin alkylation in occupationally unexposed subjects. The adduct measurements revealed increased levels of hydroxyethylvaline in the two subgroups of ethylene oxide-exposed workers, i.e., assemblers with a low and sterilizers with a high exposure. According to expectation the subgroups differed in adduct levels. The results of the cytogenetic study showed that the clastogenic potency of propylene oxide was lower than that of ethylene oxide, since the propylene oxide-exposed individuals had lower frequencies of micronuclei and chromosomal breaks compared to the assemblers despite a lower adduct level in the last group.

Theoretical study of reaction mechanisms of OH radical with toluene 1,2-epoxide/2-methyloxepin

In this study, the reaction mechanism of toluene 1,2-epoxide/2-methyloxepin with OH radical was studied by means of quantum chemical computations performed using B3LYP/6-31G(d,p), B3LYP/6-311G(2df,2p), and BHandHLYP/6-31G(d,p) methods. Ground state, intermediate, and transition states were determined. The results indicated that the 2-methyloxepin, A, isomer is more stable, by 2.4 kcal/mol, than toluene 1,2-epoxide, B. Two reaction pathways were studied, RP-A and RP-B, corresponding to the reaction of OH with toluene 1,2-epoxide and 2-methyloxepin, respectively. The localization of a pre-reactive complex for RP-A is crucial for the accurate estimation of the rate constant, k=1.0x10(-10) cm3 molecule(-1) s(-1), which is in good agreement with that determined experimentally, whereas for RP-B the rate constant is 1.3x10(-14) cm3 molecule(-1) s(-1). Under atmospheric conditions, both pathways yield 6-oxohepta-2,4-dienal as a main product, and from the energetic and kinetic results it was found that RP-A is the preferred pathway. The study of the oxide/oxepin mechanism is relevant because, aside from its relatively high concentration in the troposphere, this compound has carcinogenic and mutagenic properties.

Propylene oxide: genotoxicity profile of a rodent nasal carcinogen

Propylene oxide (PO) is a DNA-reactive genotoxic agent; that is, it reacts with DNA to produce lesions in the genetic material. PO also induces tumors in rodents, although only at high concentrations and at portals of entry. This review of PO's genotoxicity profile is organized according to endpoints measured, that is, nonmutational or mutational endpoints, and as to whether the results were from in vitro or in vivo studies. In addition to results of experimental studies, PO's genotoxicity for humans is assessed by reviewing results of published biomarker studies. The weight of evidence indicates that although it is genotoxic, PO's potency as a DNA-reactive mutagen is weak. Other aspects of PO's overall tissue toxicities are also reviewed, with attention to glutathione (GSH) depletion and its consequences, that is, cell proliferation, death, and necrosis. These toxic tissue responses occur in the same anatomical regions in rodents as do the PO-induced tumors. Furthermore, some of these tissue toxicities can produce effects that may either augment PO's DNA-reactive mutagenicity or be genotoxic in themselves, not dependent on PO's DNA reactivity. Although its DNA reactivity may be a necessary component of PO's overall genotoxicity and rodent carcinogenicity, it is likely not sufficient, and the associated tissue toxicities, which are rate-limiting, also seem to be required. This complex mode of action has implications for estimations of PO's cancer potential in humans, especially at low exposure concentrations.

Effect of gamma irradiation on microbial load and sensory characteristics of aniseed (Pimpinella anisum)

Seeds of anise (Pimpinella anisum) were exposed to doses of 0, 5, 10, 15 and 20kGy in a (60)Co package irradiator. Irradiated and unirradiated samples were stored at room temperature. Microbial populations on seeds, total and inorganic soluble solids in water extract and sensory properties of the latter were evaluated after 0, 6 and 12 months of storage. Results indicated that gamma irradiation reduced the aerobic plate counts of aniseed. Immediately after irradiation, the total soluble solids in an extract of irradiated seeds were greater than those of unirradiated ones. The total soluble solids in an extract of irradiated and un-irradiated seeds increased after 6 and 12 months of storage. There were no significant differences (p>0.05) in inorganic soluble solids between the water extract of irradiated and unirradiated aniseeds. Sensory evaluation indicated that gamma irradiation improved sensory characteristics of aniseed water extract tested immediately after irradiation; however, after 12 months of storage, no significant differences (p>0.05) were found in color, taste or flavor between extract of irradiated and unirradiated seeds.

DNA interstrand cross-linking by epichlorohydrin

Epichlorohydrin (ECH), an important industrial chemical, is a bifunctional alkylating agent with the potential to form DNA cross-links. Occupational exposure to this suspect carcinogen leads to chromosomal aberrations, and ECH has been shown previously to undergo reaction with DNA in vivo and in vitro. We used denaturing polyacrylamide gel electrophoresis to monitor the possible formation of interstrand cross-links within DNA oligomers by ECH and the related compound, epibromohydrin (EBH). Although both compounds did indeed form cross-links between deoxyguanosine residues, EBH was a more efficient cross-linker than ECH. The optimal pH for cross-linking also varied, with ECH more efficient at pH 5.0 and EBH more efficient at pH 7.0. Both agents were relatively flexible in the sequences targeted, with comparable efficiencies for 5'-GGC and 5'GC sites. Furthermore, interstrand cross-linking by the two optical isomers of ECH correlated with their relative cytotoxicities, with R-ECH about twice as potent as S-ECH.

The 5'-GNC site for DNA interstrand cross-linking is conserved for diepoxybutane stereoisomers

The bifunctional alkylating agent 1,2,3,4-diepoxybutane forms interstrand DNA-DNA cross-links between the N7 positions of deoxyguanosine residues on opposite strands of the duplex. For racemic diepoxybutane, these cross-links predominate within 5'-GNC/3'CNG sequences, where N is any nucleotide. We used denaturing polyacrylamide gel electrophoresis (dPAGE) to examine the role of stereochemistry in the cross-linking reaction, subjecting a restriction fragment to cross-linking with S,S-DEB, R,R-DEB, or meso-DEB. DNA cross-links generated by each isomer were isolated by dPAGE, and the sites of cross-linking were identified by sequencing gel analysis of DNA fragments generated by hot piperidine cleavage. We found that the 5'-GNC consensus sequence of racemic DEB is conserved, but the efficiencies of cross-linking vary, with S,S- > R,R- > meso-DEB. These results help explain the observed differences between the biological activities of DEB stereoisomers.

Epoxide adducts at the guanine residue within single-stranded DNA chains: reactivity and stability studies

Emphasis was placed in this work on the assessment of structural and biological features of nucleobase adducts that result from the reaction of DNA with epoxide derivatives. Thus we have prepared and characterized a set of site-specifically modified oligonucleotides at N7-position of a guanine residue, upon reaction with diepoxibutane, with the purpose of further investigating some of their biochemical features. The stability of the lesion-containing DNA fragments has also been investigated and clearly shows that the latter modified oligomers may be used as substrates for in vitro enzymatic assays, aimed at determining the biological effects within cell of these chemically induced DNA damage.

Reaction-kinetic parameters of glycidamide as determinants of mutagenic potency

Values for reaction-kinetic parameters of electrophiles can be used to predict mutagenic potency. One approach employs the Swain-Scott relationship for comparative kinetic studies of electrophilic agents reacting with nucleophiles. In this way glycidamide (GA), the putatively mutagenic/carcinogenic metabolite of acrylamide, was assessed by determining the rates of reaction with different nucleophiles. The rate constants (kNu) were determined using the "supernucleophile" cob(I)alamin [Cbl(I)] as an analytical tool. The Swain-Scott parameters for GA were compared with those of ethylene oxide (EO). The substrate constants, s values, for GA and for EO were found to be 1.0 and 0.93, respectively. The reaction rates at low values of nucleophilic strength (n=1-3), corresponding to oxygens in DNA, were determined to be 2-3.5 times higher for GA compared to EO. GA was also more reactive than EO towards other nucleophiles (n=0-6.4). The mutagenic potency of GA was determined in Chinese hamster ovary cells (hprt mutations in CHO-AA8 cells per dose unit with gamma-radiation as reference standard). The potency of GA was estimated to be about three mutations per 10(5) cells and mMh corresponding to about 40 rad-equ./mMh. A preliminary comparison of the mutagenic potency (per mMh and as rad-equivalents) of GA and EO shows an approximately seven times higher potency for GA. A higher mutagenic potency of GA compared to EO is compatible with expectation from reaction-kinetic data of the two compounds. The data confirmed that GA is not a strong mutagen, which is in line with what is expected for simple oxiranes. The present study shows the value of cob(I)alamin for the determination of reaction-kinetic parameters and their use for prediction of mutagenic potency.