Carcinogenicity of the drinking water mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone in the rat

MX [3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone], a drinking-water carcinogen, does not induce mutations in the liver of cII transgenic medaka (Oryzias latipes)

Mutagenicity assays with Salmonella have shown that 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX), a drinking-water disinfection by-product, is a potent mutagen, accounting for about one-third of the mutagenic potency/potential of chlorinated drinking water. The ability of MX to induce mutations was investigated in the liver of medaka (Oryzias latipes), a small fish model, utilizing the cII transgenic medaka strain that allows detection of in vivo mutations. Methylazoxymethanol acetate (MAMAc), a carcinogen in medaka, served as a positive control. Fish were exposed to MX at 0, 1, 10, or 30 mg/L for 96 h, whereas the MAMAc exposures were for 2 h at 0, 0.1, 1, or 10 mg/L. Both exposures were conducted under static water conditions and with fasted medaka. Following exposure, fish were returned to regular culture conditions to allow mutation expression for 15 or 40 d for MX or for 15 or 32 d for MAMAc. Mutations were not induced in medaka exposed to MX for 96 h. However, a concentration- and time-dependent increase in mutations was observed from the livers of fish exposed to 1 and 10 mg/L MAMAc. In conclusion, mutation induction was not observed in the livers of cII medaka exposed to MX for 96 h; however, studies are planned to examine mutation induction in the gills and skin to explore the possibility that MX-induced DNA damage occurs primarily in the tissues of initial contact.

Induction of oxidative stress in murine cell lines by 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX)

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), the potent bacterial mutagen produced during chlorination of drinking water, was tested for the induction of oxidative stress in two murine cell lines: NIH 3T3 (fibroblasts) and L929 (fibrosarcoma cells). Following 1 h MX treatment at concentrations between 100 and 1000 microM, cellular stress conditions were monitored by measuring reactive oxygen species formation (ROS) and reduced glutathione levels (GSH). The kinetics of ROS formation and GSH depletion was investigated from 10 min to 1 h. MX caused detachment of cells at 1000 microM in L929 cells and at 300 microM in NIH 3T3 cells but the viability of the cells, measured by the trypan blue assay, decreased only by 20 and 7%, respectively, in 1h. MX increased ROS production in L929 cells in a dose-dependent manner, by 120% at 500 microM of MX in 1 h. The maximum ROS production was attained already in 10min. In NIH 3T3 cells, the ROS production was slightly, but not statistically significantly stimulated at 200 microM between 20 and 60 min. Concomitantly, MX decreased the intracellular content of GSH dose-dependently in both cell lines, by 48% in L929 cells at 500 microM of MX and 32% in NIH 3T3 cells at 200 microM of MX in one hour. The majority of this GSH depletion had occurred in 10 min. These findings indicate that MX induces oxidative stress in mammalian cells in vitro though the sensitivity of cells may differ for this effect.

Developmental toxicity evaluation of 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) in Wistar rats

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) is a genotoxic chlorination by-product in drinking water. There is some evidence that it has developmental toxic effects in vitro but its potential to cause developmental effects in vivo is not known. The developmental effects were evaluated in Wistar rats. Rats (22-26 dams per dose group) were administered MX by gavage at the dose levels of 3, 30, or 60 mg/kg in water on gestation days 6-19. Control animals received plain water. Clinical signs, body weight, and food and water consumption were recorded for the dams. On gestation day 20, a cesarean section was performed and the ovaries anduterine contents of the dams were examined and the liver, kidneys, spleen, and thyroid glands weighed. The fetuses of all dose groups were weighed, sexed, and observed for external and skeletal malformations and the fetuses of the two highest dose groups were evaluated for visceral malformations. The highest dose, 60 mg/kg of MX, was slightly toxic to the dams. It decreased the corrected body weight gain of dams by 32% and the water consumption by 16-17%. Kidney and liver weights were slightly increased. MX did not affect the number of implantations nor did it cause any resorptions. The body weights of fetuses were not significantly affected. MX did not cause external malformations or skeletal anomalies. Two fetuses at 60 mg/kg and one fetus at 30 mg/kg had major visceral malformations (persistent truncus arteriosus, diaphragmatic hernia, dilated aorta with a stenosis of pulmonary arteries) and two minor artery abnormalities were observed in those animals. The frequency of unilateral displaced testis was slightly higher (9.2%) in the 60-mg/kg dose group than in controls (1.6%). Since the abnormalities did not form a consistent pattern and occurred most at maternally toxic dose, we conclude that MX can be regarded as non-teratogenic.

p-Nitrophenol and glutathione response in medaka (Oryzias latipes) exposed to MX, a drinking water carcinogen

When chlorine is introduced into public drinking water for disinfection, it can react with organic compounds in surface waters to form toxic by-products such as 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX). We investigated the effect of exposure to MX on cytochrome P450 2E1 (CYP2E1)-like activity and total glutathione (GSH) in the liver of the small fish model, medaka (Oryzias latipes). The multi-site carcinogen methylazoxymethanol acetate (MAMAc) was the positive control compound. Both medaka liver microsome preparations and S-9 fractions catalyzed the hydroxylation of p-nitrophenol (PNP), suggesting CYP2E1-like activity in the medaka. Male medaka exposed for 96 h to the CYP2E1 inducers ethanol and acetone under fasted conditions showed significant increases in PNP-hydroxylation activity. Furthermore, total reduced hepatic GSH was reduced in fish fasted for 96 h, indicating that normal feeding is a factor in maintaining xenobiotic defenses. Exposure to MX and MAMAc induced significant increases in hepatic CYP2E1-like activity, however MX exposure did not alter hepatic GSH levels. These data strengthen the role of the medaka as a suitable species for examining cytochrome P450 and GSH detoxification processes and the role these systems play in chemical carcinogenesis.

Effect of trihalomethane exposure on fetal development

AIMS

To examine the effect of trimester specific and pregnancy average total trihalomethane (TTHM) exposure on infant birth weight, low birth weight, and intrauterine growth retardation in term births, as well as gestational age and preterm delivery in all births.

METHODS

Cross sectional analysis of 56 513 singleton infants born to residents of Massachusetts during 1990. City specific aggregate data were used to estimate maternal exposure to TTHM concentration; individual maternal information was used to adjust for confounding.

RESULTS

Increased pregnancy average and second trimester TTHM exposure were associated with small for gestational age and reductions in birth weight after adjusting for potential confounding variables. Compared to </=60 micro g/l, pregnancy average TTHM exposure over 80 micro g/l was associated with a 32 g reduction in birth weight. There was a 23 g reduction in birth weight in infants born to mothers exposed to greater than 80 micro g/l TTHM during the second trimester. For each 20 micro g/l increase in TTHM, the estimated reduction in birth was 2.8 g for pregnancy average exposure and 2.6 g for second trimester exposure. An increased risk of small for gestational age births was found for pregnancy average (odds ratio (OR) 1.14; 95% CI 1.02 to 1.26) and second trimester (OR 1.13, 95% CI 1.03 to 1.24) TTHM levels greater than 80 micro g/l. There was no evidence of an association between preterm delivery and increased TTHM levels, but there were slight increases in gestational duration associated with TTHM concentrations.

CONCLUSIONS

Maternal exposure to THMs may be associated with fetal growth retardation. Our findings are consistent with most previous work, although we generally found smaller effects of TTHMs on low birth weight and intrauterine growth retardation.

Induction of transitional cell hyperplasia in the urinary bladder and aberrant crypt foci in the colon of rats treated with individual and a mixture of drinking water disinfection by-products

Cancer of the urinary bladder and colon are significant human health concerns. Epidemiological studies have suggested a correlation between these cancers and the chronic consumption of chlorinated surface water containing disinfection by-products (DBPs). The present study was designed to determine if exposure to DBPs would cause preneoplastic or neoplastic lesions in the urinary bladder and colon of rats, and what effect a mixture of DBPs would have on these lesions. Male and female Eker rats were treated via drinking water with low and high concentrations of potassium bromate, 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), chloroform, or bromodichloromethane individually or in a mixture for 10 months. The urinary bladders and colons were examined for the presence of preneoplastic lesions. Cell proliferation in the urothelium was examined using immunohistochemical staining for bromodeoxyuridine. Aberrant crypt foci (ACF), as well as the number of individual crypts in each ACF, were identified and counted microscopically after staining with 0.2% methylene blue. Colon crypt cell proliferation and mitotic index were determined using immunohistochemical staining for proliferating cell nuclear antigen. Labeling indexes for the urinary bladder and colon were calculated based on the percentage of positively labeled cells. Treatment with the high dose of MX caused transitional epithelial hyperplasia and cell proliferation in the rat urinary bladder, and this effect was diminished in the high dose mixture animals. Treatment with 4 individual DBPs, as well as a mixture of them, caused the development of ACF, the putative preneoplastic lesion of colon cancer.

Meta-analysis of studies on individual consumption of chlorinated drinking water and bladder cancer

STUDY OBJECTIVE

To evaluate whether consumption of chlorinated drinking water is associated with bladder cancer.

DESIGN

A bibliographic search was conducted and the authors selected studies evaluating individual consumption of chlorinated drinking water and bladder cancer. The authors extracted from each study risk estimates for intermediate and long term (>40 years) consumption of chlorinated water, stratified by sex when possible, and performed meta-analysis for the two exposure levels. A meta-analysis was also performed of the dose-response regression slopes.

SETTING

Populations in Europe and North America.

PARTICIPANTS

Those included in six case-control studies (6084 incident bladder cancer cases, 10,816 controls) and two cohort studies (124 incident bladder cancer cases) fulfilling the inclusion criteria.

MAIN RESULTS

Ever consumption of chlorinated drinking water was associated with an increased risk of bladder cancer in men (combined OR=1.4, 95%CI 1.1 to 1.9) and women (combined OR=1.2, 95%CI 0.7 to 1.8). The combined OR for mid-term exposure in both genders was 1.1 (95% CI 1.0 to 1.2) and for long term exposure was 1.4 (95%CI 1.2 to 1.7). The combined estimate of the slope for a linear increase in risk was 1.13 (95% CI 1.08 to 1.20) for 20 years and 1.27 (95% CI 1.15 to 1.43) for 40 years of exposure in both sexes.

CONCLUSIONS

This meta-analysis of the best available epidemiological evidence indicates that long term consumption of chlorinated drinking water is associated with bladder cancer, particularly in men. The observed relative risk is only moderately high, but the population attributable risk could be important as the vast majority of the population of industrialised countries is potentially exposed to chlorination byproducts for long time periods.

Disposition of 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5h)-furanone (mx) in b6c3f1 mice and f344 rats

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) is a mutagenic by-product of chlorination of drinking water, particularly where the water contains humic matter. MX has been estimated to account for 50% of the mutagenic activity in some drinking water. A bioassay in rats demonstrated an increased tumor incidence, primarily in liver and thyroid glands. This study was designed to provide disposition/metabolism information in mice to evaluate the necessity of a National Toxicology Program chronic bioassay and to provide data for female rats. Radioactivity was rapidly absorbed and excreted near equally in urine (42-54%) and feces (40-51%) 72 h following oral administration of (14)C-labeled MX at single doses from 0.2 to 20 mg/kg to male and female mice and female rats. A larger percentage (71-73%) of MX-derived radioactivity was excreted in urine after an iv dose (0.2 mg/kg) in both female rats and male mice. Most MX-derived radioactivity was excreted within the first 24 h postdosing. MX was transformed to urinary and biliary metabolites. A major extremely polar urinary metabolite was tentatively identified as 1-hydroxy-1,2,2-ethanetricarboxylic acid. This metabolite is likely transformed from the MX degradation product 2-hydroxy-3-formyl-4-oxo-2-butenoic acid. Oral administration produced highest tissue/blood ratios in the following order: forestomach (>100), glandular stomach, intestine, and kidney. Intravenous administration resulted in high, prolonged levels of radioactivity in blood compared to oral dosing. Therefore, MX disposition appears to be dominated by its chemical reactivity with highest concentrations of radioactivity being found at the site of administration.

A possible new disinfection by-product--2-chloro-5-oxo-3-hexene diacyl chloride (COHC)--in formation of MX by chlorinating model compounds

MX was widely considered as one of the most important disinfection by-products (DPBs) and as the strong carcinogen in the chlorinated drinking water. Some model compounds which have been classified into four families were chlorinated at laboratory conditions. MX was produced only in the chlorinated substituted aromatic aldehydes and amino acids, while a possible new compound (COHC) was found in some substituted aromatic aldehydes, chlorinated substituted aromatic acids and phenols. Through the analysis of the peaks presented in mass spectrum, the composition and structure of the new compound are proposed as 2-chloro-5-oxo-3-hexene diacyl chloride (COHC) which could cause interference in the detection of MX. COHC has also been found in drinking water and extracted chlorinated fulvic acid solution from the sediment and water of Tai Lake, so it should bring about more attention.

Constructing scientific authorities: issue framing of chlorinated disinfection byproducts in public health

The practice of chlorine disinfection of drinking water to reduce microbial risks provides substantial benefits to public health. However, increasing concern around potential risks of cancer associated with exposure to chlorinated disinfection byproducts confuses this issue. This article examines the science agenda regarding chlorinated disinfection byproducts (CDBP) and cancer in Canada and the United States, focusing on the social construction of scientific knowledge claims and evidence. Data for this analysis were obtained from published documents as well as from in-depth interviews with epidemiologists and toxicologists centrally involved with the issue in both countries. Results of the analysis suggest that toxicological scientists want to close the door on the "chloroform issue" due to increasing evidence that chloroform is safe at low doses, because epidemiological scientists can no longer move forward the cancer science until significant improvements can be made in assessing human exposures, and because the scientific foci of research on DBP have shifted accordingly. Further, a distinction emerges in terms of how scientific uncertainties are interpreted when they cross-cut disciplines in the context of human health risk assessment. We suggest this tension reflects a balance of how uncertainty and authorities are managed in a mandated science-policy domain. Sufficient evidence was provided to keep the DBP issue on the regulatory agenda and to generate additional research, yet authorities and concomitant interpretations of uncertainty were contested. Such science generation and contestation inevitably influences complex risk assessment processes with respect to what water-related health risks are addressed and how.

DNA damage induced by hypochlorite and hypobromite with reference to inflammation-associated carcinogenesis

Hypohalites (OCl-, OBr-) are formed at inflammation sites as antimicrobial agents. OCl- is also used for the disinfection of water supplies and the association of drinking chlorinated water with cancer risk is pointed out. In this study, OCl- itself induced 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) formation, while OBr- damaged DNA only when glutathione (GSH) was added. OCl- caused oxidative DNA damage more efficiently than OBr-/GSH. In experiment with 32P-labeled DNA fragments, OCl- strongly caused piperidine-labile sites at guanine residues than piperidine-inert 8-oxodG, whereas OBr-/GSH caused no piperidine-labile sites. Endogenous OCl- may play a role in genotoxicity close to the site of inflammation.

Factors on the formation of disinfection by-products MX, DCA and TCA by chlorination of fulvic acid from lake sediments

[3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone] (MX) and chlorinated acetic acids such as dichlorinated acetic acid (DCA) and trichlorinated acetic acid (TCA) have always been the focus of disinfection by-products (DBPs) studies. In order to find out the influences of reaction time, TOC, chlorine dose, pH and temperature on the formation of MX, DCA and TCA, we extracted fulvic acid (FA) from the sediment of Tai Lake, and conducted simulated chlorination of samples rich in FA. Results showed positive relationship between TOC and the yields of MX, DCA and TCA. But the influences of pH, chlorine dose, reaction time, and temperature are quite complex. The optimal chlorination condition for the formation of MX is pH = 2, T = 45 degrees C, C/Cl2 = 1/4, t = 12 h. Lower pH, longer time, greater chlorine dose can result in greater yield of both DCA and TCA, and there is a strong linear relationship between the formation of DCA and TCA.

Human cell mutagenicity of chlorinated and unchlorinated water and the disinfection byproduct 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX)

Extracts of three water samples--humic acid-enriched water-both peatland water and drinking water, both with and without chlorination were tested for mutagenicity at the tk locus in MCL-5 cells, a line of human B-lymphoblastoid cells that express cytochrome P450 enzymes and microsomal epoxide hydrolase. Our results show that chlorination caused a 5.5-fold increase (P<0.0001) in the mutagenicity of the humic acid-enriched water. The unchlorinated peatland water was mutagenic at the two highest doses (240 and 480 microg equivalent total organic carbon (TOC)/ml), possibly due to polycyclic aromatic hydrocarbons (PAH) that were measured in the peat. In contrast, the chlorinated peatland water was non-mutagenic at low doses, while at the highest dose (240 microg equivalent TOC/ml) the sample was so toxic that an insufficient number of cells survived treatment to allow plating. The chlorinated and unchlorinated drinking water were both non-mutagenic. 3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a potent bacterial mutagen and chlorine-disinfection byproduct, was also tested in MCL-5 cells as well as in two other human B-lymphoblastoid cell-lines, AHH-1 TK+/- and h1A1v2 cells, which differ from each other and from MCL-5 cells in the amounts of cytochrome P450 enzymes they can express. MX was mutagenic to all three cell-lines, but there was no apparent correlation between cytochrome P450 enzyme expression and the mutagenicity of MX. Overall, our results show that samples of chlorinated humic acid-enriched water and MX, a model chlorine-disinfection byproduct, are moderately mutagenic to human cells.

Changes of activity inducing chromosomal aberrations and transformations of chlorinated humic acid

The change of the toxicity of chlorinated water after chlorine injection was examined. For the measurement of toxicity, chromosomal aberration test and transforming test were carried out as indexes to initiating activity and to promoting activity in the carcinogenesis process, respectively. Activity inducing chromosomal aberrations of chlorinated humic acid gradually decreased with time after chlorination. In contrast, activity inducing transformations measured by the two-stage assay gradually increased. Thus, the toxicity that decreases or increases is present in chlorinated water. Furthermore, activity inducing transformations measured by the non-two-stage assay gradually decreased. This direction of change was reverse to that of activity inducing transformations by the two-stage assay and consistent with that of activity inducing chromosomal aberrations. It is speculated that the main reason of decreasing activity inducing transformations by the non-two-stage assay is because initiating activity detected as activity inducing chromosomal aberrations in chlorinated water decreases drastically. Directions of changes of total organic halogen and carbonyl group were qualitatively consistent with that of activity inducing chromosomal aberrations. Directions of changes of chloroform and dichloroacetic acid were qualitatively consistent with that of activity inducing transformations by the two-stage assay. Findings of this study suggest that further research is necessary to compare carcinogenicity of tap water near water purification plant and distant tap water.

Halogenated 2,5-pyrrolidinediones: synthesis, bacterial mutagenicity in Ames tester strain TA-100 and semi-empirical molecular orbital calculations

The chloroimide 3,3-dichloro-4-(dichloromethylene)-2,5-pyrrolidinedione, a tetrachloroitaconimide, is the principal mutagen produced by chlorination of simulated poultry chiller water. It is the second most potent mutagenic disinfection by-product of chlorination ever reported. Six of seven new synthetic analogs of this compound are direct-acting mutagens in Ames tester strain TA-100. Computed energies of the lowest unoccupied molecular orbital (E(LUMO)) and of the radical anion stability (DeltaH(f)(rad)-DeltaH(f)) from MNDO-PM3 for the chloroimides show a quantitative correlation with the Ames TA-100 bacterial mutagenicity values. The molar mutagenicities of these direct acting mutagenic imides having an exocyclic double bond fit the same linear correlation (lnM(m) vs. E(LUMO); lnM(m) vs. DeltaH(f)(rad)--DeltaH(f)) as the chlorinated 2(5H)-furanones, including the potent mutagen MX, 3-chloro-4-(dichloro-methyl)-5-hydroxy-2(5H)-furanone, a by-product of water chlorination and paper bleaching with chlorine. Mutagenicity data for related haloimides having endocyclic double bonds are also given. For the same number of chlorine atoms, the imides with endocyclic double bonds have significantly higher Ames mutagenicity compared to their structural analogs with exocyclic double bonds, but do not follow the same E(LUMO) or DeltaH(f)(rad)-DeltaH(f) correlation as the exocyclic chloroimides and the chlorinated 2(5H)-furanones.

Comparable DNA and chromosome damage in Chinese hamster ovary cells by chlorohydroxyfuranones

Chlorinated drinking water contains several chlorohydroxyfuranone (CHF) by-products whose contribution to cancer risk is not presently known. 3,4-Dichloro-5-hydroxy-2(5H)-furanone (MCA), 3-chloro-4-(chloromethyl)-5-hydroxy-2(5H)-furanone (CMCF), and 3- chloro-4-methyl-5-hydroxy-2(5H)-furanone (MCF) were studied for the induction of DNA damage, using the alkaline single-cell gel (SCG)/comet assay, and for chromosome damage, using sister-chromatid exchange (SCE) and chromosome aberration (CA) tests, in Chinese hamster ovary (CHO) cells. 3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), the known genotoxic chlorination by-product and a rat carcinogen, was used as a reference chemical. The SCG analyses were done using concentrations that caused little or no cytotoxicity compared to that of the concurrent control cultures. In the cytogenetic tests, the CHFs were tested up to maximum cytotoxicity. MX and MCA were the most cytotoxic of the compounds in CHO cells followed by CMCF and MCF. All of the CHFs induced DNA damage, SCEs and CAs (mainly chromatid-type breaks and exchanges) in a concentration-related manner, with the exception that MCA was a weak inducer of SCEs. There were no significant differences between the lowest concentration of MX, MCA, and CMCF to cause DNA damage (SCG assay). Based on comparisons of the slopes of regression lines, MX was somewhat more potent than either MCA or CMCF, and MCF was clearly less potent than the other three compounds in the assay. The order of potency was MX > CMCF > MCA > MCF in inducing SCEs and MX > MCA > CMCF > MCF in inducing CAs. The data show that there are differences in the potency of genotoxicity among the CHFs tested. In many cases, however, the extent of maximum effect observed was comparable between the compounds. The results suggest that besides MX other CHFs should be considered in the evaluation of genotoxic risks associated with the consumption of chlorinated drinking water.

[Chlorination of drinking water in Spain and bladder cancer]

OBJECTIVES

Drinking water chlorination generates trihalomethanes and other by-products with mutagenic and carcinogenic properties in animal experiments. Epidemiological studies have associated trihalomethanes to an increased risk of bladder cancer. We evaluate trihalomethane levels in four Spanish areas and calculate the bladder cancer risk attributable to this exposure.

METHODS

Trihalomethanes have been analysed in 111 drinking water samples from four Spanish areas using gas chromatography. Water utilities were contacted and information on drinking water consumption in Spain has been collected. We reviewed the epidemiological studies that assess the association between bladder cancer risk and exposure to chlorination by-products. Attributable risk was calculated on the basis of these levels, mortality data per area and risk estimates obtained from the literature.

RESULTS

Mediterranean areas present the highest levels of trihalomethanes with 81, 80, 61 and 52 µg/l in Sabadell, Alicante, Barcelona and Manresa respectively. Lower levels are found in Tenerife and Asturias with 7 and 20 µg/l respectively. The bladder cancer attributable risk in high trihalomethane exposure areas may be, on average, around 20%.

CONCLUSIONS

The trihalomethane levels found are high compared to those of other European Union countries. In the high exposure areas, drinking water chlorination may generate a considerable number of bladder cancer cases. These estimations have to be carefully interpreted and verified with more extensive studies.

The comet assay with multiple mouse organs: comparison of comet assay results and carcinogenicity with 208 chemicals selected from the IARC monographs and U.S. NTP Carcinogenicity Database

The comet assay is a microgel electrophoresis technique for detecting DNA damage at the level of the single cell. When this technique is applied to detect genotoxicity in experimental animals, the most important advantage is that DNA lesions can be measured in any organ, regardless of the extent of mitotic activity. The purpose of this article is to summarize the in vivo genotoxicity in eight organs of the mouse of 208 chemicals selected from International Agency for Research on Cancer (IARC) Groups 1, 2A, 2B, 3, and 4, and from the U.S. National Toxicology Program (NTP) Carcinogenicity Database, and to discuss the utility of the comet assay in genetic toxicology. Alkylating agents, amides, aromatic amines, azo compounds, cyclic nitro compounds, hydrazines, halides having reactive halogens, and polycyclic aromatic hydrocarbons were chemicals showing high positive effects in this assay. The responses detected reflected the ability of this assay to detect the fragmentation of DNA molecules produced by DNA single strand breaks induced chemically and those derived from alkali-labile sites developed from alkylated bases and bulky base adducts. The mouse or rat organs exhibiting increased levels of DNA damage were not necessarily the target organs for carcinogenicity. It was rare, in contrast, for the target organs not to show DNA damage. Therefore, organ-specific genotoxicity was necessary but not sufficient for the prediction of organ-specific carcinogenicity. It would be expected that DNA crosslinkers would be difficult to detect by this assay, because of the resulting inhibition of DNA unwinding. The proportion of 10 DNA crosslinkers that was positive, however, was high in the gastrointestinal mucosa, stomach, and colon, but less than 50% in the liver and lung. It was interesting that the genotoxicity of DNA crosslinkers could be detected in the gastrointestinal organs even though the agents were administered intraperitoneally. Chemical carcinogens can be classified as genotoxic (Ames test-positive) and putative nongenotoxic (Ames test-negative) carcinogens. The Ames test is generally used as a first screening method to assess chemical genotoxicity and has provided extensive information on DNA reactivity. Out of 208 chemicals studied, 117 are Ames test-positive rodent carcinogens, 43 are Ames test-negative rodent carcinogens, and 30 are rodent noncarcinogens (which include both Ames test-positive and negative noncarcinogens). High positive response ratio (110/117) for rodent genotoxic carcinogens and a high negative response ratio (6/30) for rodent noncarcinogens were shown in the comet assay. For Ames test-negative rodent carcinogens, less than 50% were positive in the comet assay, suggesting that the assay, which detects DNA lesions, is not suitable for identifying nongenotoxic carcinogens. In the safety evaluation of chemicals, it is important to demonstrate that Ames test-positive agents are not genotoxic in vivo. This assay had a high positive response ratio for rodent genotoxic carcinogens and a high negative response ratio for rodent genotoxic noncarcinogens, suggesting that the comet assay can be used to evaluate the in vivo genotoxicity of in vitro genotoxic chemicals. For chemicals whose in vivo genotoxicity has been tested in multiple organs by the comet assay, published data are summarized with unpublished data and compared with relevant genotoxicity and carcinogenicity data. Because it is clear that no single test is capable of detecting all relevant genotoxic agents, the usual approach should be to carry out a battery of in vitro and in vivo tests for genotoxicity. The conventional micronucleus test in the hematopoietic system is a simple method to assess in vivo clastogenicity of chemicals. Its performance is related to whether a chemical reaches the hematopoietic system. Among 208 chemicals studied (including 165 rodent carcinogens), 54 rodents carcinogens do not induce micronuclei in mouse hematopoietic system despite the positive finding with one or two in vitro tests. Forty-nine of 54 rodent carcinogens that do not induce micronuclei were positive in the comet assay, suggesting that the comet assay can be used as a further in vivo test apart from the cytogenetic assays in hematopoietic cells. In this review, we provide one recommendation for the in vivo comet assay protocol based on our own data.

Lack of modification by environmental estrogenic compounds of thyroid carcinogenesis in ovariectomized rats pretreated with N-bis(2-hydroxypropyl)nitrosamine (DHPN)

The effects of environmental estrogenic compounds, soy isoflavone mixture (SI), genistein (GEN), and nonylphenol (NP), and the possible goitrogen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), on thyroid carcinogenesis were investigated in ovariectomized (OVX) female rats. Five-week-old OVX F344 rats were given a single subcutaneous injection of N-bis(2-hydroxypropyl)nitrosamine (DHPN; 2400 mg / kg, body weight) or vehicle alone. Starting 1 week later, GEN (250 or 25 ppm in diet), SI (400 ppm in diet), NP (250 or 25 ppm in diet), MX (30 ppm, in drinking water), sulfadimethoxine (SDM), a known thyroid tumor-promoter (1000 ppm in drinking water), or beta-estradiol 3-benzoate (EB), a synthetic estrogen (0.5 mg in cholesterol pellet, s.c.) were administered for 12 weeks. SDM and EB were included as positive controls. At sacrifice the major organs including the thyroid, pituitary, liver, kidney, uterus, vagina, brain and pancreas were collected and histopathological observation was performed. Thyroid weights were significantly increased (P < 0. 001) only in the SDM treatment group and pituitary weights were elevated with SDM (P < 0.05) and EB (P < 0.001). Kidney and uterus weights were also significantly increased (P < 0.05) by EB. Histopathologically, proliferative lesions of the thyroid were only observed in the SDM treatment group and of the pituitary in the SDM or EB treatment groups. Renal tubule lesions, uterine squamous metaplasia, vaginal keratinization and telangiectasia of pancreatic islets were also observed with EB. There were no organ weight changes or histopathological lesions in the major organs, including the thyroid, in the GEN, SI, MX or NP treatment groups. Our results thus indicated a lack of modifying effects on thyroid carcinogenesis in female OVX rats, in agreement with our previous finding in males.

Lack of modifying effects of environmental estrogenic compounds on the development of thyroid proliferative lesions in male rats pretreated with N-bis(2-hydroxypropyl)nitrosamine (DHPN)

The modifying effects of various environmental estrogenic compounds on thyroid carcinogenesis were investigated in a rodent two-stage carcinogenesis model. The compounds examined were a soy isoflavone mixture (SI) and genistein (GEN) as phytoestrogens, nonylphenol (NP) as a xenoestrogen, 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) as a thyroid carcinogen and sulfadimethoxine (SDM) as a known thyroid tumor promoter. Five-week-old male F344 rats were given a single subcutaneous injection of N-bis(2-hydroxypropyl)nitrosamine (DHPN; 2800 mg / kg, body weight) or the vehicle alone. Starting one week thereafter, GEN (250 or 25 ppm in diet), SI (400 ppm in diet), NP (250 or 25 ppm in diet), MX (30 ppm, in drinking water) or SDM (1000 ppm in drinking water) was administered for 12 weeks. Major organs including the thyroid, pituitary, liver, kidney, testis, brain and pancreas were weighed and histopathological observation was performed. Thyroid weights were significantly increased (P < 0.001) only in the SDM treatment groups, especially with DHPN pretreatment. Kidney weights were slightly increased in the NP or MX treatment groups, albeit without statistical significance. Histopathologically, thyroid proliferative lesions were only observed in the SDM alone or DHPN + SDM group with significant focal hyperplasias, adenomas and adenocarcinomas limited to the combined treatment case. There were no organ weight changes or histopathological lesions in the major organs including the thyroid in the GEN, SI, NP, and MX treatment groups regardless of DHPN pretreatment. Our results thus indicate that the weakly estrogenic compounds GEN, SI and NP and the environmental rat thyroid carcinogen MX do not exert any modifying effects on thyroid carcinogenesis in rats under the present experimental conditions.

3-Cloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a rat thyroid gland carcinogen, does not affect serum levels of TSH and thyroid hormones

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a chlorine disinfection by-product in drinking water, causes follicular adenomas and carcinomas in thyroid glands of Wistar rats with an unknown mechanism. We evaluated effects of MX on blood thyroid stimulating hormone (TSH), thyroxine (T(4)), triiodothyronine (T(3)), prolactin (PRL) and growth hormone (GH) levels in male and female Wistar rats to assess their role in the tumorigenesis. The levels of TSH, PRL and GH in serum of male rats were not significantly affected by a single dose of 1, 10 or 60 mg/kg of MX administered by gavage 2 h before sampling. In repeated dose experiments MX was administered at dose levels of 1, 10 or 60 mg/kg of MX (40 mg/kg for females) in water by gavage daily for 1 or 3 weeks. Thyroid glands, adrenal glands and the liver were evaluated for morphological changes and cell proliferation activity after staining with proliferating cell nuclear antigen (PCNA). The dose of 60 mg/kg MX was toxic upon repeated administration. Nevertheless, MX did not affect blood TSH and T(4) levels at any time point in either sex. T(3) concentration increased transiently in males (by 37% after week 1) at the highest MX dose but not in females. MX did not change the weights of thyroid glands, their morphology and cell proliferation activity by the end of the week 3. MX did not affect blood PRL levels but decreased GH levels in males at all doses after the first week of MX treatment. The results indicate that MX does not alter blood TSH and thyroid hormone levels in rats, and imply that MX may not cause thyroid follicular cell tumors by TSH-mediated hormonal promotion.

No consistent pattern of mutations in p53 and ras genes in liver tumors of rat treated with the drinking water mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX)

The frequency of point mutations in p53 (exons 4-7) and in Ki-ras, Ha-ras, and N-ras (exons 1 and 2) and the expression of p53 protein were evaluated in the liver tumors of Wistar rats of a 104-week carcinogenicity study on 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a chlorine disinfection by-product in drinking water. Mutations were analyzed in 16 hepatocellular adenomas, 7 hepatocellular carcinomas, 23 cholangiomas, and 2 cholangiocarcinomas of the MX-treated animals and one hepatocellular carcinoma and cholangiocarcinoma in control animals using PCR-SSCP (polymerase chain reaction-single-strand conformation polymorphism) or PCR-TGGE (temperature gradient gel electrophoresis) and direct sequencing. The expression of the p53 protein (wild-type and mutated protein) was detected by immunohistochemistry (CM5 antibody). The expression of p53 and that of the proliferating cell nuclear antigen (PCNA, 19 A2) were also evaluated in livers of female animals exposed to MX for 1 week, 3 weeks, or 18 weeks. Altogether, four mutations were found in p53 in three tumors, in two hepatocellular adenomas, and one cholangiocarcinoma, all in females receiving the highest MX dose (6. 6 mg/kg/day) of the study. Three of the mutations were G:C --> A:T transitions and one was an A:T --> T:A transversion. The mutations were scattered at different codons and positions of the codon. One hepatocellular adenoma contained two p53 mutations. All cholangiomas and cholangiocarcinomas, but no hepatocellular adenomas and carcinomas, overexpressed the p53 protein. MX treatment did not induce p53 expression at any age in the liver or alter the expression of the PCNA in the liver of younger animals. The p53 protein was overexpressed in hyperplastic bile ducts in aged rats but not in bile ducts of younger rats (up to 24 weeks). No mutations were observed in either Ki-ras, Ha-ras, or N-ras of the liver tumors. These data suggest that point mutations in p53, Ki-ras, Ha-ras, and N-ras are not involved in the MX-induced liver carcinogenesis in rats.

Identification of adducts formed in reactions of 2-deoxyadenosine and calf thymus DNA with the bacterial mutagen 3-chloro-4-(chloromethyl)-5-hydroxy-2(5H)-furanone

3-Chloro-4-(chloromethyl)-5-hydroxy-2(5H)-furanone (CMCF) is a strong direct acting bacterial mutagen found in chlorine-disinfected drinking water. We studied the reaction of CMCF with 2-deoxyadenosine in buffered aqueous solutions and found that three main adducts were formed. The adducts were isolated and purified by C18 column chromatography and HPLC, and characterized on the basis of their UV absorbance, fluorescence emission, (1)H and (13)C NMR spectroscopic, and mass spectrometric features. The adducts were identified as 3-(2-deoxy-beta-D-ribofuranosyl)-7H-8-formyl[2, 1-i]pyrimidopurine (pfA-dR), 3-(2-deoxy-beta-D-ribofuranosyl)-7H-8-carboxy[2,1-i]pyrimidopurine++ + (pcA-dR), and 4-(N(6)-2-deoxyadenosinyl)-3-formyl-2-hydroxy-3-butenoic acid (OH-fbaA-dR). In the reactions performed at pH 7.4 and 37 degrees C, the yields of pfA-dR, pcA-dR, and OH-fbaA-dR were 1.1, 6.7, and 5.5 mol %, respectively. The adduct pfA-dR was detected also in calf thymus DNA reacted with CMCF. The yield was about six adducts per 10(5) bases. To elucidate the mechanisms of formation of the adducts, (13)C-3-labeled CMCF was reacted with 2'-deoxyadenosine. The adducts are structurally related to the adducts previously identified in the reactions of structurally analogous chlorohydroxyfuranones with 2-deoxyadenosine.

The drinking water chlorination by-products 3,4-dichloro-5-hydroxy-2[5H]-furanone(mucochloric acid) and 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone do not induce preneoplastic or neoplastic intestinal lesions in F344 rats, balb/ca mice or C57bl/6J-min mice

Epidemiological studies indicate an association between exposure to chlorinated drinking water and risk of intestinal cancer. In order to study this experimentally, we have examined the effects of 3,4-dichloro-5-hydroxy-2[5H]-furanone (mucochloric acid, MCA) and 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX), mutagenic and genotoxic compounds in drinking water, on aberrant crypt foci and tumours in the intestines of male F344 rats and Balb/cA mice, and C57BL/6J-Min (multiple intestinal neoplasia)/+ mice of both sexes, in six independent experiments. In some experiments the effects of MCA and MX on aberrant crypt foci induced by the colon carcinogens 1,2-dimethylhydrazine or its metabolite azoxymethane were also studied. Neither MCA nor MX alone induced aberrant crypt foci or intestinal tumours when given in drinking water. With this route of exposure neither MCA nor MX, when given in combination with 1,2-dimethylhydrazine or azoxymethane, had any effect on the induction or growth of the aberrant crypt foci. Drinking water exposure of MX did not affect the number or growth of aberrant crypt foci or intestinal tumours in the Minl+ mice. When administered intrarectally MCA had a weak inducing effect on aberrant crypt foci in the colons of Balb/cA mice. Exposure to MCA and MX intrarectally apparently promoted the growth of aberrant crypt foci both in rats and mice, increasing the crypt multiplicity, aberrant crypts/aberrant crypt foci. Based on an overall evaluation of these experiments, the intestinal tract, at least in rats and mice, seems not to be a main target organ for effects of MCA or MX on preneoplastic or neoplastic development.

DNA damage induced by the drinking water mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX) in mammalian cells in vitro and in mice

3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX) formed during chlorination of water containing natural organic substances, is a very potent bacterial mutagen. Recently, tumours at multiple sites were reported in rats given MX-containing drinking water. We have investigated the genotoxicity of MX in mammalian cells exposed in vitro and in vivo using alkaline filter elution to detect DNA single-strand breaks and/or alkali-labile sites (SSBs). Concentrations as high as 100 and 300 microM MX were required to induce detectable levels of SSBs in the HL-60 cells. If MX treatment was carried out in the presence of DNA repair inhibitors (AraC plus hydroxyurea), the sensitivity of the assay to detect MX-induced SSBs was increased by a factor of 100. The presence of serum proteins during exposure resulted in a minor reduction of the MX-induced DNA damage in HL-60 cells at the lowest MX concentrations. In primary cultures of testicular cells as well as in resting human peripheral blood mononuclear cells (PBMC), a slightly increased level of SSBs was observed at MX-concentrations above 30 microM, this effect was not further increased by repair inhibitors. In LLC-PK1 renal proximal tubular epithelial cells and in growth stimulated human peripheral PBMC, increased SSBs were detected at MX concentrations as low as low as 3-10 microM and higher using repair inhibitors, and at 10 times higher concentrations without repair inhibitors. No dose dependent DNA damage was detected in the liver, kidney, spleen and colon of male B6C3F1 mice administrated high doses of MX (40 and 80 mg kg-1). Moderately increased and dose dependent SSBs were detected in the liver and kidney in the presence of DNA repair inhibitors during MX treatment, but no such increase was observed in the spleen and colon.

Drinking water disinfection byproducts: review and approach to toxicity evaluation

There is widespread potential for human exposure to disinfection byproducts (DBPs) in drinking water because everyone drinks, bathes, cooks, and cleans with water. The need for clean and safe water led the U.S. Congress to pass the Safe Drinking Water Act more than 20 years ago in 1974. In 1976, chloroform, a trihalomethane (THM) and a principal DBP, was shown to be carcinogenic in rodents. This prompted the U.S. Environmental Protection Agency (U.S. EPA) in 1979 to develop a drinking water rule that would provide guidance on the levels of THMs allowed in drinking water. Further concern was raised by epidemiology studies suggesting a weak association between the consumption of chlorinated drinking water and the occurrence of bladder, colon, and rectal cancer. In 1992 the U.S. EPA initiated a negotiated rulemaking to evaluate the need for additional controls for microbial pathogens and DBPs. The goal was to develop an approach that would reduce the level of exposure from disinfectants and DBPs without undermining the control of microbial pathogens. The product of these deliberations was a proposed stage 1 DBP rule. It was agreed that additional information was necessary on how to optimize the use of disinfectants while maintaining control of pathogens before further controls to reduce exposure beyond stage 1 were warranted. In response to this need, the U.S. EPA developed a 5-year research plan to support the development of the longer term rules to control microbial pathogens and DBPs. A considerable body of toxicologic data has been developed on DBPs that occur in the drinking water, but the main emphasis has been on THMs. Given the complexity of the problem and the need for additional data to support the drinking water DBP rules, the U.S. EPA, the National Institute of Environmental Health Sciences, and the U.S. Army are working together to develop a comprehensive biologic and mechanistic DBP database. Selected DBPs will be tested using 2-year toxicity and carcinogenicity studies in standard rodent models; transgenic mouse models and small fish models; in vitro mechanistic and toxicokinetic studies; and reproductive, immunotoxicity, and developmental studies. The goal is to create a toxicity database that reflects a wide range of DBPs resulting from different disinfection practices. This paper describes the approach developed by these agencies to provide the information needed to make scientifically based regulatory decisions.

Identification of an ethenoformyl adduct formed in the reaction of the potent bacterial mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone with guanosine

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) is a potent direct-acting bacterial mutagen and a rodent carcinogen occurring in chlorine-disinfected drinking water. In this study, we have reacted MX with guanosine, cytidine, thymidine, and calf thymus DNA in aqueous solutions. HPLC analyses of the reaction mixture of MX with guanosine showed that one main product peak was formed. In the reactions of MX with cytidine or thymidine, no product peaks representing base-modified nucleosides could be observed. The product from the MX guanosine reaction mixture was isolated by preparative chromatography on reversed phase C18 columns, and its structure was determined by UV absorbance, 1H and 13C NMR spectroscopy, and mass spectrometry. The product was identified as 3-(beta-D-ribofuranosyl)-7-formylimidazo[1,2-a]purin-9(4H)-one (epsilonfGuo), and the yield for the reaction carried out at pH 7.4 and 37 degrees C was about 0.3 mol %. The adduct could not be observed at the detection limit of five adducts per 10(7) bases in the hydrolysate of the calf thymus DNA reacted with MX. However, this failure does not rule out the possibility that lower amounts of the adduct might be involved in the observed mispairing (adenine incorporated opposite an adducted guanine base) caused by MX in the Salmonella typhimurium strain TA100.

Mutagenicity and amount of chloroform after chlorination of bank filtered lake water

Chlorinated drinking waters produced from humus-rich waters often have a high content of halogenated organic by-products which increases the mutagenicity of drinking water. With in vitro chlorination experiments we studied the formation of chloroform (CHCl3) and mutagenicity of artificially recharged ground water samples. The water samples where obtained from an artificial ground water plant which infiltrates humus-rich lake water through an esker island. The chlorination experiments showed that bank filtration reduced strongly the formation of chloroform and mutagenicity of chlorinated water. Reduction in the amount of chloroform and mutagenicity in chlorinated waters was strongly associated with the decrease in the total content of organic carbon and with the decrease in molecular weight of organic matter during bank filtration.

Reaction of the potent bacterial mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) with 2'-deoxyadenosine and calf thymus DNA: identification of fluorescent propenoformyl derivatives

The potent bacterial mutagen and drinking water disinfection byproduct 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) was reacted with 2'-deoxyadenosine and calf thymus DNA in aqueous solutions at neutral conditions. HPLC analyses of the 2'-deoxyadenosine reaction mixtures showed that two previously unidentified products were formed. The products were isolated by preparative C18 chromatography, and their structures were characterized by UV absorbance, fluorescence emission, 1H and 13C NMR spectroscopy, and mass spectrometry. It was concluded that in both products a propeno bridge had been incorporated between N-1 and N6 of the adenine unit. In one of the products, the propeno bridge carried a formyl group [3-(2'-deoxy-beta-D-ribofuranosyl)-7H-8-formyl[2,1-i]pyrimidopurine++ + (pfA-dR)], and in the other the substituents consisted of a formyl group and a chlorine atom [3-(2'-deoxy-beta-D-ribofuranosyl)-7H-8-formyl-9-chloro[2,1-i]pyrimid opurine (Cl-pfA-dR)]. These novel adducts exhibited fluorescence in the visible region with emission maxima around 460 nm. The yields of the products in reactions performed at pH 7.4 and 37 degrees C were about 0.03 mol %. In reaction of MX with calf thymus DNA, the adduct pfA-dR was formed and its yield was about 0.6 adduct/10(5) nucleotides.

Bromine-, chlorine-, and mixed halogen-substituted 4-methyl-2(5H)-furanones: synthesis and mutagenic effects of halogen and hydroxyl group replacements

The versatility of 4-(hydroxymethyl)-2(5H)-furanone as a starting point for the synthesis of several bromine and mixed halogen analogues of the potent water mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) has been demonstrated. However, in some preparations the yields of desired products were lower for bromine- than chlorine-substituted counterparts. A total of 12 bromine-, chlorine-, and mixed halogen-substituted 4-methyl-2(5H)-furanones were tested repeatedly in 10 independent experiments for levels of Salmonella typhimurium (TA100) mutagenicities. The purpose of these experiments was to determine the mutagenic response to changing halogen content, type, and position as well as to learn the measure of these responses in the presence and absence of the C-5 OH group. Mutagenicities reached levels of 10(3) and 10(2) rev/nmol for all trihalo- and dihalo-4-methyl-5-hydroxy-2(5H)-furanones, respectively, notwithstanding substitutions by bromine or chlorine. Trihalides lacking the C-5 hydroxyl group possessed mutagenicities of the order of 10(2) rev/nmol, while hydroxyl group absence in the dihalides resulted in potency levels of slightly less than 10 rev/ nmol. Pairwise comparisons of compound mutagenicities showed that overall the C-5 H-by-OH replacement and, next in importance, increasing the number of C-6 halogens from one to two resulted in the greatest enhancements of mutagenicities. However, in comparing compound pairs within two different sets of four di- and trihalides, it was observed that replacement of a C-5 H by OH enhanced mutagenicity more for the dihalides than the trihalides indicating that increasing the C-6 halogen number simultaneously with replacing C-5 H by OH results in a nonlinear, additive enhancement. For fewer than half of the compound pairs compared, changing the C-6 halogen from chlorine to bromine resulted in small increases in mutagenicity, and for the remaining compound pairs, no increase could be discerned. This result points to the relative unimportance of only C-6 halogen type as a determinant of mutagenicity. Similarly, no impact on mutagenicity was observed for changing only the halogen type attached to C-3.

Identification of adenine adducts formed in reaction of calf thymus DNA with mutagenic chlorohydroxyfuranones found in drinking water

Calf thymus DNA was reacted with the extremely potent bacterial mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) and the structurally related compounds 3,4-dichloro-5-hydroxy-2(5H)-furanone (MCA) and 3-chloro-4-methyl-5-hydroxy-2(5H)-furanone (MCF). The chromatograms of the HPLC analyses of the DNA hydrolysates showed peaks that represented adducted base moieties. It was possible to establish the structures of the adducts by comparing UV spectra and chromatographical properties of the DNA adducts with known adenosine and 2'-deoxyadenosine adducts. The DNA adduct produced by MX was identified as 3-(2'-deoxyribofuranosyl-N6-adenosinyl)propenal (M1A-dR). It was calculated that 1 nucleotide/10(5) nucleotides was converted to M1A-dR. The same adduct was formed also in the reaction of MX with 2'-deoxyadenosine (yield 0.01%). The M1A-dR adduct may play a role in the mutational events induced by MX in Salmonella typhimurium strain TP2428. The adducts produced in the reactions of MCA and MCF with DNA were identified as 3-(2'-deoxyribofuranosyl)-7-formylimidazo[2,1-i]purine (epsilon cA-dR) and 4-(2'-deoxyribofuranosyl-N6-adenosinyl)-3-formyl-3-butenoic acid (fbaA-dR), respectively. The yield of epsilon cA-dR was 5 adducts/ 10(6) nucleotides and of fbaA-dR 4 adducts/10(5) nucleotides. The biological significance of these adducts is unknown.

Production of unscheduled DNA synthesis in rodent hepatocytes in vitro, but not in vivo, by 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX)

Incubation of both rat and mouse hepatocytes with 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX) in vitro resulted in a dose-dependent increase in unscheduled DNA synthesis (UDS) at sub-cytotoxic concentrations (1-10 microM MX; 20 h incubation). Depletion of glutathione stores by pre-treatment of rat hepatocytes with buthionine sulfoximine did not result in a significant increase in UDS produced by MX. In contrast, MX did not induce UDS in mouse hepatocytes ex vivo either 3 or 16 h following administration of a single oral dose of 100 mg/kg MX. Despite the ability of MX to produce repairable DNA damage, restricted access of MX to the liver may prevent a measurable UDS response in vivo.