Reaction of epichlorohydrin with 2'-deoxynucleosides: characterization of adducts

Spectral shift supported epichlorohydrin toxicity and the protective role of sage

In this study, the toxicity of epichlorohydrin, a chemical intermediate, was investigated by using Allium cepa L. test material as a bio-indicator. In addition, the protective role of sage leaf extract (Slex) against this toxicity was investigated. Toxicity was handled with the help of physiological (germination percentage, root elongation, and weight gain), cytogenetic (mitotic index = MI, micronucleus = MN, and chromosomal abnormalities = CAs), biochemical (malondialdehyde = MDA, superoxide dismutase = SOD, and catalase = CAT), and anatomical (root meristem cell damages) parameters. A. cepa bulbs were divided into 6 groups (1 control, 5 applications). The bulbs in the control group were treated with tap water, and the bulbs in the application group were treated with epichlorohydrin at a dose of 100 mg/L and Slex at two different doses (190 mg/L and 380 mg/L) and germinated. Germination process was continued uninterruptedly for 72 h in all groups. At the end of the period, physiological parameter measurements were carried out in the bulbs. In addition, root tips were collected and made ready for cytogenetic, biochemical, and anatomical measurements and microscopic observations. As a result, exposure to epichlorohydrin caused statistically significant (p < 0.05) decreases in germination percentage, root length, weight gain, and MI, and statistically significant (p<0.05) increases in MN frequency, CA numbers, MDA level, SOD, and CAT enzyme activities. Epichlorohydrin exposure induced CAs such as fragment, sticky chromosome, unequal distribution of chromatin, reverse polarization, and disordered mitosis in root meristem cells. The toxicity of epichlorohydrin was due to its interaction with DNA, and this interaction was confirmed by the spectral shift in the DNA spectrum. In addition, epichlorohydrin caused anatomical damages such as epidermis cell damage, cortex cell damage, thickening of the cortex cell wall, and flattened cell nuclei in root meristem cells. The application of Slex together with epichlorohydrin decreased the toxicity of epichlorohydrin and again caused statistically significant (p < 0.05) improvements in the values of all the parameters examined. In other words, germination percentage, root length, weight gain, and MI increased again and MN frequency, CAs numbers, MDA level, SOD, and CAT enzyme activities decreased. It was determined that this improvement was even more pronounced at 380 mg/L dose of Slex. As a result, it was determined that epichlorohydrin caused multiple-toxicity for the investigated indicator organism, and Slex had a reducing role in this toxicity. For this reason, Slex should be included in the daily diet as an antioxidant beverage in order to protect from the toxicity of chemical agents exposed in daily life or to reduce their effects.

In vivo epiclorohidrine toxicity: cytogenetic, biochemical, physiological, and anatomical evidences

In this study, the toxic effects of epiclorohidrine (ECh) were investigated in vivo by Allium test. The toxic effects have been investigated in terms of physiological, cytogenetic, anatomical, and biochemical aspects. The changes in germination percentage, weight gain, and root length were investigated as physiological parameter; micronucleus (MN), mitotic index (MI), and chromosomal abnormality (CA) frequencies were as cytogenetic parameter. Oxidative stress indicators such as superoxide dismutase (SOD), catalase (CAT), and malondialdehyde (MDA) were analyzed for biochemical changes and also damages in root tip cells were evaluated as anatomical parameter. It was determined that germination percentage, weight gain, root length, and MI decreased; MN and CA frequencies were increased with the increase of ECh treatment dose. ECh treatment caused significant increase in SOD and CAT enzyme activities and MDA levels and these results indicated a stress formation. A variety of anatomical changes and damages were observed in the root tip cells induced by ECh. In conclusion, the toxic effects of ECh on A. cepa which is a model of eucaryotic cell were investigated in a multi-directional way and serious toxic effects of ECh treatment were determined.

Quantification of the mutagenic potency and repair of glycidol-induced DNA lesions

Glycidol (Gly) is an electrophilic low-molecular weight epoxide that is classified by IARC as probably carcinogenic to humans. Humans might be exposed to Gly from food, e.g. refined vegetable oils, where Gly has been found as a food process contaminant. It is therefore important to investigate and quantify the genotoxicity of Gly as a primary step towards cancer risk assessment of the human exposure. Here, quantification of the mutagenic potency expressed per dose (AUC: area under the concentration-time curve) of Gly has been performed in Chinese hamster ovary (CHO) cells, using the HPRT assay. The dose of Gly was estimated in the cell exposure medium by trapping Gly with a strong nucleophile, cob(I)alamin, to form stable cobalamin adducts for analysis by LC-MS/MS. Gly was stable in the exposure medium during the time for cell treatment, and thus the dose in vitro is the initial concentration×cell treatment time. Gly induced mutations in the hprt-gene at a rate of 0.08±0.01 mutations/10(5) cells/mMh. Through comparison with the effect of ionizing radiation in the same system a relative mutagenic potency of 9.5rad-eq./mMh was obtained, which could be used for comparison of genotoxicity of chemicals and between test systems and also in procedures for quantitative cancer risk assessment. Gly was shown to induce strand breaks, that were repaired by base excision repair. Furthermore, Gly-induced lesions, present during replication, were found to delay the replication fork elongation. From experiments with repair deficient cells, homologous recombination repair and the ERCC1-XPF complex were indicated to be recruited to support in the repair of the damage related to the stalled replication elongation. The type of DNA damage responsible for the mutagenic effect of Gly could not be concluded from the present study.

Exocyclic deoxyadenosine adducts of 1,2,3,4-diepoxybutane: synthesis, structural elucidation, and mechanistic studies

1,2,3,4-Diepoxybutane (DEB) is considered the ultimate carcinogenic metabolite of 1,3-butadiene, an important industrial chemical and environmental pollutant present in urban air. Although it preferentially modifies guanine within DNA, DEB induces a large number of A --> T transversions, suggesting that it forms strongly mispairing lesions at adenine nucleobases. We now report the discovery of three potentially mispairing exocyclic adenine lesions of DEB: N(6),N(6)-(2,3-dihydroxybutan-1,4-diyl)-2'-deoxyadenosine (compound 2), 1,N(6)-(2-hydroxy-3-hydroxymethylpropan-1,3-diyl)-2'-deoxyadenosine (compound 3), and 1,N(6)-(1-hydroxymethyl-2-hydroxypropan-1,3-diyl)-2'-deoxyadenosine (compound 4). The structures and stereochemistry of the novel DEB-dA adducts were determined by a combination of UV and NMR spectroscopy, tandem mass spectrometry, and independent synthesis. We found that synthetic N(6)-(2-hydroxy-3,4-epoxybut-1-yl)-2'-deoxyadenosine (compound 1) representing the product of N(6)-adenine alkylation by DEB spontaneously cyclizes to form 3 under aqueous conditions or 2 under anhydrous conditions in the presence of an organic base. Compound 3 can be interconverted with 4 by a reversible unimolecular pericyclic reaction favoring 4 as a more thermodynamically stable product. Both 3 and 4 are present in double stranded DNA treated with DEB in vitro and in liver DNA of laboratory mice exposed to 1,3-butadiene by inhalation. We propose that in DNA under physiological conditions, DEB alkylates the N-1 position of adenine in DNA to form N1-(2-hydroxy-3,4-epoxybut-1-yl)-adenine adducts, which undergo an S(N)2-type intramolecular nucleophilic substitution and rearrangement to give 3 (minor) and 4 (major). Formation of exocyclic DEB-adenine lesions following exposure to 1,3-butadiene provides a possible mechanism of mutagenesis at the A:T base pairs.

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.

Ring-Opening of 3-beta-D-ribofuranosyl-3,7,8,9-Tetrahydropyrimido [1,2-i]purin-8-ol and preparation of 2-thio- and 2-aza-adenosine derivatives

The adduct 3-beta-D-ribofuranosyl-3,7,8,9-tetrahydropyrimido[1,2-i]purin-8-ol (2), obtained from adenosine and epichlorohydrin, underwent ring fission at basic conditions. The initial ring-opening took place at C2 of the pyrimidine unit resulting in 2-(5-amino-1-beta-D-ribofuranosyl-imidazol-4-yl)-1,4,5,6-tetrahydropyrimidin-5-ol (3). Also the tetrahydropyrimidine ring of 3 could be opened resulting in 5-amino-1-(beta-D-ribofuranosyl)-imidazole-4-(N-3-amino-2-hydroxyl-propyl)-carboxamide (4). In hot acid conditions, 2 was both deglycosylated and ring-opened yielding 2-(5-amino-imidazol-4-yl)-1,4,5,6-tetrahydropyrimidin-5-ol (7) as the final product. When reacting 3 with CS(2) or HNO(2) ring-closure took place and 3-beta-D-ribofuranosyl-3,4,7,8,9-pentahydropyrimido[1,2-i]purin-8-ol-5-thione (5), and 3-beta-D-ribofuranosyl-imidazo[4,5-e]-3,7,8,9-tetrahydropyrimido[1,2-c][1,2,3]triazine-8-ol (6), respectively, were obtained. Also, the pyrimidine ring of the epichlorohydrin adduct with adenine, 10-imino-5,6-dihydro-4H,10H-pyrimido[1,2,3-cd]purin-5-ol (10), underwent ring fission and the product was identified as 3-hydroxy-1,2,3,4-tetrahydroimidazo[1,5-a]pyrimidine-8-carboximidamide (11).

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 metabolism and molecular toxicology of chloroprene

Chloroprene (2-chloro-1,3-butadiene, 1) is oxidised by cytochrome P450 enzymes in mammalian liver microsomes to several metabolites, some of which are reactive towards DNA and are mutagenic. Much less of the metabolite (1-chloroethenyl)oxirane (2a/2b) was formed by human liver microsomes compared with microsomes from Sprague-Dawley rats and B6C3F1 mice. Epoxide (2a/2b) was a substrate for mammalian microsomal epoxide hydrolases, which showed preferential hydrolysis of the (S)-enantiomer (2b). The metabolite 2-chloro-2-ethenyloxirane (3a/3b) was rapidly hydrolysed to 1-hydroxybut-3-en-2-one (4) and in competing processes rearranged to 1-chlorobut-3-en-2-one (5) and 2-chlorobut-3-en-1-al (6). The latter compound isomerised to (Z)-2-chlorobut-2-en-1-al (7). In microsomal preparations from human, rat and mouse liver, compounds 4, 5 and 7 were conjugated by glutathione both in the absence and presence of glutathione transferases. There was no evidence for the formation of a chloroprene diepoxide metabolite in any of the microsomal systems. The major adducts from the reaction of (1-chloroethenyl)oxirane (2a/2b) with calf thymus DNA were identified as N7-(3-chloro-2-hydroxy-3-buten-1-yl)-guanine (20) and N3-(3-chloro-2-hydroxy-3-buten-1-yl)-2'-deoxyuridine (23), with the latter being derived by alkylation at N-3 of 2'-deoxycytidine, followed by deamination. Adducts in DNA were identified by comparison with those derived from individual deoxyribonucleosides. The metabolite (Z)-2-chlorobut-2-en-1-al (7) formed principally two adducts with 2'-deoxyadenosine which were identified as a pair of diastereoisomers of 3-(2'-deoxy-beta-d-ribofuranosyl)-7-(1-hydroxyethyl)-3H-imidazo[2,1-i]purine (25). The chlorine atom of chloroprene thus leads to different intoxication and detoxication profiles compared with those for butadiene and isoprene. The results infer that in vivo oxidations of chloroprene catalysed by cytochrome P450 are more important in rodents, whereas hydrolytic processes catalysed by epoxide hydrolases are more pronounced in humans. The reactivity of chloroprene metabolites towards DNA is important for the toxicology of chloroprene, especially when detoxication is incomplete.

Synthesis and mutagenesis of the butadiene-derived N3 2'-deoxyuridine adducts

1,3-Butadiene is a known carcinogen and mutagen that acts through a variety of metabolic intermediates that react with DNA, forming stable and unstable lesions on dG, dA, dC, and dT. The N3 2'-deoxyuridine adducts are a highly stable, stereoisomeric mixture of adducts derived from the reaction of cytosine with the monoepoxide metabolite of butadiene, followed by spontaneous deamination. In this study, the phosphoramidites and subsequent oligodeoxynucleotides containing the N3 2'-deoxyuridine adducts have been constructed and characterized. Using a single-stranded shuttle vector DNA, the mutagenic potential of these adducts has been tested following replication in mammalian cells. Replication past the N3 2'-deoxyuridine adducts was found to be highly mutagenic with an overall mutation yield of approximately 97%. The major mutations that were observed were C to T transitions and C to A transversions. In vitro, these adducts posed a complete block to both the Klenow fragment of Escherichia coli polymerase I and polymerase epsilon, while these lesions significantly blocked polymerase delta. These data suggested a possible involvement of bypass polymerases in the in vivo replication of these lesions. Overall, these findings indicate that the N3 2'-deoxyuridine adducts are highly mutagenic lesions that may contribute to butadiene-mediated carcinogenesis.

Reaction of epichlorohydrin with adenosine, 2'-deoxyadenosine and calf thymus DNA: identification of adducts

Epichlorohydrin (a probable human carcinogen) was allowed to react with adenosine and the adducts were characterized by NMR and UV spectroscopy, and mass spectrometry. The adduct initially formed was 1-(3-chloro-2-hydroxypropyl)-adenosine, which subsequently ring closures to 1,N(6)-(2-hydroxypropyl)-adenosine at neutral and basic conditions. At acid conditions, the N-1 adduct undergoes a slow deamination to yield 1-(3-chloro-2-hydroxypropyl)-inosine. Minor adducts identified were 7-(3-chloro-2-hydroxypropyl)-adenosine and 3-(3-chloro-2-hydroxypropyl)-adenosine which are easily deglycosylated, and an adduct where the epichlorohydrin residue was attached to the sugar moiety of adenosine. A diadduct, 1,N(6)-(2-hydroxypropyl)-N(6)-(3-chloro-2-hydroxypropyl)-adenosine was also identified. The reaction of epichlorohydrin with calf thymus DNA gave 1,N(6)-(2-hydroxypropyl)-deoxyadenosine and 3-(3-chloro-2-hydroxypropyl)-adenine (major adduct).

Genotoxic effects of ethylene oxide, propylene oxide and epichlorohydrin in humans: update review (1990-2001)

Ethylene oxide (EtO), propylene oxide (PO) and epichlorohydrin (ECH) are important industrial chemicals widely used as intermediates for various synthetic products. EtO and PO are also environmental pollutants. In this review we summarize data published during the period 1990-2001 concerning both the genotoxic and carcinogenic effects of these epoxides in humans. The use of DNA and hemoglobin adducts as biomarkers of exposure and the role of polymorphism, as well as confounding factors, are discussed. We have also included recent in vitro data comprising genotoxic effects induced by EtO, PO and ECH in mammalian cells. The uncertainties regarding cancer risk estimation still persist, in spite of the large database collected.

Specific DNA adducts induced by some mono-substituted epoxides in vitro and in vivo

Alkyl epoxides are important intermediates in the chemical industry. They are also formed in vivo during the detoxification of alkenes. Alkyl epoxides have shown genotoxicity in many toxicology assays which has been associated with their covalent binding to DNA. Here aspects of the formation and properties of DNA adducts, induced by some industrially important alkenes and mono-substituted epoxides are discussed. These include propylene oxide, epichlorohydrin, allyl glycidyl ether and the epoxy metabolites of styrene and butadiene. The major DNA adducts formed by epoxides are 7-substituted guanines, 1- and 3-substituted adenines and 3-substituted cytosines. In addition, styrene oxide and butadiene monoepoxide are able to modify exocyclic sites in the DNA bases, the sites being in the case of styrene oxide N(2)- and O(6)-positions of guanine, N(6)-adenine as well as N(4)-and O(2)-cytosine. In vivo the main adduct is the 7-substituted guanines. The 1-substituted adenines have also shown marked levels, and these adducts should also be targets in biomonitoring of human exposures. Due to its low mutagenicity, 7-substituted guanines are considered as a surrogate marker for other mutagenic lesions, e.g. those of 1-adenine or 3-uracil adducts.

Adducts with haemoglobin and with DNA in epichlorohydrin-exposed rats

Epichlorohydrin (1-chloro-2,3-epoxypropane; ECH) is an important industrial chemical and a carcinogen in experimental animals. The main aims of the present study were to characterize the adduct formation in female Wistar rats and to identify adducts that could potentially be used in human biomonitoring studies. The total binding of radioactivity to haemoglobin in rats administered 0, 0. 11, 0.22, 0.43, or 0.97 mmol [3H]ECH/kg body weight by i.p. injection, and sacrificed 24 h after treatment, was linearly related to a dose up to 0.43 mmol/kg body weight. The binding at the highest dose was higher than predicted by extrapolation from lower doses, indicating saturation of a metabolic process for elimination of ECH. Ion-exchange chromatography of a globin hydrolysate showed one major radioactivity peak corresponding to S-(3-chloro-2-hydroxypropyl)cysteine. The half-life of this adduct was estimated as about 4 days by analysis of globin from rats administered 0.43 mmol/kg body weight and sacrificed after 1, 2 and 9 days. Crosslinking of the adduct, presumably with glutathione, appeared to be the predominant secondary reaction. Hydrolysis of N-(3-chloro-2-hydroxypropyl)valine, the primary reaction product of ECH with N-terminal valine, would give N-(2,3-dihydroxypropyl)valine. A sensitive gas chromatography/mass spectrometry method for the dihydroxypropyl adduct was used to follow its formation and removal after administration of nonlabelled ECH (0.11 mmol/kg body weight). The level of this adduct reached a maximum of about 20 pmol/g globin after a few weeks, corresponding to about 0.1% of the initial binding of ECH to globin. N-7-(3-Chloro-2-hydroxypropyl)guanine was detected in rats administered 0.97 mmol [3H]ECH/kg body weight and sacrificed 6 h after treatment. The adduct levels in haemoglobin and DNA were compared with previously reported adduct levels in male Fischer 344 rats exposed to propylene oxide. Despite its higher chemical reactivity, the capacity of ECH to alkylate macromolecules in vivo was found to be somewhat lower than that of propylene oxide.

3-Chloro-2-hydroxypropylmercapturic acid and α-chlorohydrin as biomarkers of occupational exposure to epichlorohydrin

Until now no urinary biomarker of exposure was available to assess human exposure to epichlorohydrin (ECH). For this purpose the urinary excretion of mercapturic acids and α-chlorohydrin (α-CH), which are potential metabolites of ECH in humans was investigated. This study was undertaken in a chemical plant in which ECH is used in the production of glycidyl ethers. Urine samples were collected from 19 persons at the beginning and at the end of work-shifts and at the morning after the last work-shift. Respiratory air concentrations of ECH were determined by personal air monitoring (PAM) and were found to range from<0.03 to 1.1 mg/m(3) (8 h-TWA, median 0.09, n=23). The determined respiratory exposure to ECH was in all cases below the current occupational exposure limit of 4 mg/m(3) for ECH (8 h-TWA-OEL). In one additional case a dermal exposure to an unknown amount of technical ECH was noted. Urinary metabolites were isolated by ethyl acetate extraction or by lyophilization and determined by GC-MS. In ethyl acetate extracts of acidified urine samples of workers with potential occupational exposure to ECH, 3-chloro-2-hydroxypropylmercapturic acid (CHPMA) was identified with GC-MS and the concentrations measured ranged from<0.05 (detection limit) to 5.35 mmol/mol creatinine. The increase of the CHPMA excretion during the work-shifts, corrected for creatinine excretion, correlated well with the 8 h-TWA respiratory air concentrations of ECH (r(2)=0.94, n=7). For 8 individuals it was possible to assess an urinary half-life for the excretion of CHPMA (2.54±0.94 h). By extrapolating the relation between the ambient air concentrations of ECH and the urinary CHPMA excretions, an excretion of 6.2 mmol CHPMA/mol creatinine (tolerance levels of 95% C.I.: 5.1-7.3) is predicted if ECH exposure is at the level of the current OEL. The urinary excretion of two other known metabolites of ECH in rats, namely α-CH and 2,3-dihydroxypropylmercapturic acid (DHPMA) was also investigated. α-CH was identified in urine of workers exposed to low air concentrations of ECH but DHPMA could only be identified after the dermal exposure to technical ECH. In these latter samples CHPMA and α-CH were determined up to 167 and 6.3 mmol/mol creatinine, respectively. From this investigation it is concluded that urinary excretion of the mercapturic acid CHPMA is an appropriate biomarker of human exposure to ECH. A tentative biological exposure index (BEI) of 6 mmol CHPMA/mol creatinine for ECH during an 8 h work-shift is proposed.