Interactions of trichloroethylene with DNA in vitro and with RNA and DNA of various mouse tissues in vivo

Vinyl chloride monomer (VCM) induces high occurrence of neural tube defects in embryonic mouse brain during neurulation

The aim of this study was to explore the direct embryonic teratogenicity of vinyl chloride monomer (VCM), especially the toxic effects on the early development of the nervous system and its underlying mechanisms. Pregnant mice at embryonic day 6.5 (E6.5) were injected with different doses of VCM (200, 400 and 600 mg/kg) and embryos were harvested at E10.5. Our results showed that doses higher than 400 mg/kg of VCM increased the incidence of malformed embryos, especially the neural tube defects (NTDs). In addition, high-dose of VCM decreased mitotic figure counts in the neuroepithelium and enhanced the percentage of cells in G0/G1 phase, while they were reduced in S phase. The more VCM was injected into mice, the fewer positive PCNA cells were seen and the more positive TUNEL cells were observed in the neuroepithelium. Moreover, significant increases in the levels of caspase-3 protein were observed in NTD embryos. Our results demonstrate that during early pregnancy, exposure to doses higher than 400 mg/kg of VCM increases the incidence of malformations and particularly the rate of NTDs. High-dose of VCM inhibits the proliferation of neural cells and induces cell apoptosis, leading to an imbalance in the ratio of proliferation and apoptosis. Meanwhile, the apoptosis of neuroepithelial cells might be accelerated by the activation of the caspase-3 pathway, and it might be a reason for NTDs.

Requirement of DNA repair mechanisms for survival of Burkholderia cepacia G4 upon degradation of trichloroethylene

A Tn5-based mutagenesis strategy was used to generate a collection of trichloroethylene (TCE)-sensitive (TCS) mutants in order to identify repair systems or protective mechanisms that shield Burkholderia cepacia G4 from the toxic effects associated with TCE oxidation. Single Tn5 insertion sites were mapped within open reading frames putatively encoding enzymes involved in DNA repair (UvrB, RuvB, RecA, and RecG) in 7 of the 11 TCS strains obtained (4 of the TCS strains had a single Tn5 insertion within a uvrB homolog). The data revealed that the uvrB-disrupted strains were exceptionally susceptible to killing by TCE oxidation, followed by the recA strain, while the ruvB and recG strains were just slightly more sensitive to TCE than the wild type. The uvrB and recA strains were also extremely sensitive to UV light and, to a lesser extent, to exposure to mitomycin C and H(2)O(2). The data from this study establishes that there is a link between DNA repair and the ability of B. cepacia G4 cells to survive following TCE transformation. A possible role for nucleotide excision repair and recombination repair activities in TCE-damaged cells is discussed.

Increased frequency of micronucleated kidney cells in rats exposed to halogenated anaesthetics

Six halogenated anaesthetics were tested for their ability to induce micronuclei formation in the rat kidney. A statistically significant increase in the frequency of micronucleated cells was detected in rats given a single p.o. dose of 4 mmol/kg of halothane (3.48 x baseline), chloroform (3.32 x baseline), trichloroethylene (3.24 x baseline), sevoflurane (2.98 x baseline), and isoflurane (2.95 x baseline). In contrast, the response was substantially negative in rats given the same dose of enflurane. As compared to controls, rats treated with halothane and trichloroethylene displayed a reduction in the frequency of binucleated cells presumably due to a toxicity-induced inhibition of cellular proliferation. These findings suggest a potential genotoxic activity of halogenated anaesthetics for the rat kidney.

Dose-dependent binding of trichloroethylene to hepatic DNA and protein at low doses in mice

Trichloroethylene (TCE) is a widely used industrial chemical and a low level contaminant of surface and ground water in industrialized areas. It is weakly mutagenic in several test systems and carcinogenic in rodents. However, the mechanism for its carcinogenicity is not known. We investigated the binding of [1,2-14C]TCE ([14C]TCE) to liver DNA and proteins in male B6C3F1 mice at doses more relevant to humans than used previously. The time course for the binding was studied in animals dosed with 4.1 micrograms [14C]TCE/kg body weight (b.w.) and sacrificed between 0.5 and 120 h after i.p. injection. A dose response study was carried out in mice given [14C]TCE at doses between 2 micrograms/kg and 200 mg/kg b.w. and sacrificed 2 h post-treatment. [14C]TCE associated with the DNA and protein extracts was measured using accelerator mass spectrometry. The highest level of protein binding (2.4 ng/g protein) was observed 1 h after the treatment followed by a rapid decline, indicating pronounced instability of the adducts and/or rapid turnover of liver proteins. DNA binding was biphasic with the first peak (75 pg/g DNA) at 4 h. However, the highest binding (120 pg/g DNA) was found between 24 and 72 h after the treatment. Dose response curves were linear for both protein and DNA binding. The binding of TCE metabolites to DNA was ca. 100-fold lower than to proteins when calculated per unit weight of macromolecules and when measured 2 h post-exposure. This study shows that TCE metabolites bind to DNA and proteins in a dose-dependent manner in liver, one of the target organs for its tumorigenicity. Thus, protein and DNA adduct formation should be considered as a factor in the tumorigenesis of TCE.

Trichloroethylene radicals generated by ionizing radiation. An EPR/spin trapping study

Trichloroethylene (TCE) was exposed in the presence of the spin trap N-tert-butyl-alpha-phenyl nitrone (PBN, 0.1 M) to ionizing radiation from two different sources in an attempt to determine the origin of the spin-trapped radicals generating the EPR spectra in precision cut liver slices. TCE samples were irradiated with 18 MeV electrons to a total dose of 1000 Gy in a linear accelerator (LINAC) or exposed to 60Co gamma-rays to total doses of 100 Gy and 1000 Gy. The results show that three PBN adducts were generated during the LINAC radiations. Two of these spin adducts correspond to the addition of carbon-centered radicals to PBN, and the third adduct is consistent with a decomposition product of PBN. The predominant carbon-entered radical yields a PBN adduct that is more stable, persists for over 24 h and has identical hyperfine coupling constants (aN = 1.61 mT, aH beta = 0.325 mT) to the PBN adduct obtained when precision-cut liver slices were exposed to TCE. Gamma radiation (100 Gy) of TCE yields PBN adducts with lower primary nitrogen hyperfine coupling constants (aN = 1.45 mT and aN = 1.54 mT). The results (gamma-radiation) suggest that the carbon-centered radical is formed on a single TCE carbon that is different than the predominant radical formed during LINAC radiations. This difference is confirmed by experiments using 13C-TCE. The results further suggest that, during gamma-radiation of TCE, the radicals are formed by dechlorination at the TCE carbon containing two chlorine atoms. The results obtained during LINAC radiations suggest that the predominant radical is formed by dechlorination at the TCE carbon containing a single chlorine and a single proton. In addition, it is possible that this radical is the initial TCE radical formed during exposure of liver slices to TCE.

Acid-labile adducts to protein can be used as indicators of the cysteine S-conjugate pathway of trichloroethene metabolism

Covalent binding of radiolabel to tissue proteins following [14C]trichloroethene (TRI) exposure has been used as a measure of TRI activation. Gross binding of 14C label does not differentiate between alternate routes of metabolism and can be confounded when there is significant metabolic incorporation of radiolabel. We examined the covalent association of 14C label to hepatic and renal proteins in male F344 rats and B6C3F1 mice following oral treatment with [14C]TRI and three metabolites of TRI: [14C]trichloroacetate (TCA), [14C]dichloroacetate (DCA), and [14C]dichlorovinylcysteine (DCVC) in vivo. Association of radiolabel from [14C]TRI with hepatic proteins reached a maximum at 2 and 4 h in mouse and rat hepatic proteins, respectively. Association of radiolabel with renal proteins reached a maximum at 8 h in both species. An approach was developed based upon formation of protein adducts that release acetate and monochloroacetate (MCA) on acid hydrolysis. These adducts were found to be specifically associated with the activation of DCVC to reactive intermediates. Acetate and MCA were identified by using two different conditions of high-performance liquid chromatography (HPLC) separation with differing selectivity. Diethylmaleate and aminooxyacetic acid pretreatment inhibited the formation of these adducts from TRI, consistent with requirements for glutathione and beta-lyase. No evidence of these adducts was detected following [14C]TCA and [14C]DCA treatment. Renal acid-labile adduct formation from 25 mg/kg DCVC was approximately 12-fold greater in male B6C3F1 mice than in male F344 rats. They accounted for 7.8 and 4.6% of the total adducts to renal protein in rats and mice, respectively. Acid-labile adducts formed from 1000 mg/kg TRI were approximately two times greater in mice than rats. In this case, they accounted for 1.4 and 3.3% of the total adduct formed in renal proteins from TRI (corrected for metabolic incorporation), respectively. This greater dilution of adducts associated with DCVC in renal proteins of the rat suggests that covalent binding of TRI has less specificity for the DCVC pathway in rats than in mice.

Consideration of the target organ toxicity of trichloroethylene in terms of metabolite toxicity and pharmacokinetics

Trichloroethylene (TRI) is readily absorbed into the body through the lungs and gastrointestinal mucosa. Exposure to TRI can occur from contamination of air, water, and food; and this contamination may be sufficient to produce adverse effects in the exposed populations. Elimination of TRI involves two major processes: pulmonary excretion of unchanged TRI and relatively rapid hepatic biotransformation to urinary metabolites. The principal site of metabolism of TRI is the liver, but the lung and possibly other tissues also metabolize TRI, and dichlorovinyl-cysteine (DCVC) is formed in the kidney. Humans appear to metabolize TRI extensively. Both rats and mice also have a considerable capacity to metabolize TRI, and the maximal capacities of the rat versus the mouse appear to be more closely related to relative body surface areas than to body weights. Metabolism is almost linearly related to dose at lower doses, becoming dose dependent at higher doses, and is probably best described overall by Michaelis-Menten kinetics. Major end metabolites are trichloroethanol (TCE), trichloroethanol-glucuronide, and trichloroacetic acid (TCA). Metabolism also produces several possibly reactive intermediate metabolites, including chloral, TRI-epoxide, dichlorovinyl-cysteine (DCVC), dichloroacetyl chloride, dichloroacetic acid (DCA), and chloroform, which is further metabolized to phosgene that may covalently bind extensively to cellular lipids and proteins, and, to a much lesser degree, to DNA. The toxicities associated with TRI exposure are considered to reside in its reactive metabolites. The mutagenic and carcinogenic potential of TRI is also generally thought to be due to reactive intermediate biotransformation products rather than the parent molecule itself, although the biological mechanisms by which specific TRI metabolites exert their toxic activity observed in experimental animals and, in some cases, humans are not known. The binding intensity of TRI metabolites is greater in the liver than in the kidney. Comparative studies of biotransformation of TRI in rats and mice failed to detect any major species or strain differences in metabolism. Quantitative differences in metabolism across species probably result from differences in metabolic rate and enterohepatic recirculation of metabolites. Aging rats have less capacity for microsomal metabolism, as reflected by covalent binding of TRI, than either adult or young rats. This is likely to be the same in other species, including humans. The experimental evidence is consistent with the metabolic pathways for TRI being qualitatively similar in mice, rats, and humans. The formation of the major metabolites--TCE, TCE-glucuronide, and TCA--may be explained by the production of chloral as an intermediate after the initial oxidation of TRI to TRI-epoxide.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of endogenous and exogenous electron donors and trichloroethylene oxidation toxicity on trichloroethylene oxidation by methanotrophic cultures from a groundwater aquifer

Trichloroethylene (TCE)-transforming aquifer methanotrophs were evaluated for the influence of TCE oxidation toxicity and the effect of reductant availability on TCE transformation rates during methane starvation. TCE oxidation at relatively low (6 mg liter-1) TCE concentrations significantly reduced subsequent methane utilization in mixed and pure cultures tested and reduced the number of viable cells in the pure culture Methylomonas sp. strain MM2 by an order of magnitude. Perchloroethylene, tested at the same concentration, had no effect on the cultures. Neither the TCE itself nor the aqueous intermediates were responsible for the toxic effect, and it is suggested that TCE oxidation toxicity may have resulted from reactive intermediates that attacked cellular macromolecules. During starvation, all methanotrophs tested exhibited a decline in TCE transformation rates, and this decline followed exponential decay. Formate, provided as an exogenous electron donor, increased TCE transformation rates in Methylomonas sp. strain MM2, but not in mixed culture MM1 or unidentified isolate, CSC-1. Mixed culture MM2 did not transform TCE after 15 h of starvation, but mixed cultures MM1 and MM3 did. The methanotrophs in mixed cultures MM1 and MM3, and the unidentified isolate CSC-1 that was isolated from mixed culture MM1 contained lipid inclusions, whereas the methanotrophs of mixed culture MM2 and Methylomonas sp. strain MM2 did not. It is proposed that lipid storage granules serve as an endogenous source of electrons for TCE oxidation during methane starvation.

Toxicity of Trichloroethylene to Pseudomonas putida F1 Is Mediated by Toluene Dioxygenase

Trichloroethylene was metabolically activated by toluene dioxygenase to produce toxic effects in Pseudomonas putida F1. Cytotoxicity was indicated by growth inhibition and by the covalent modification of cellular molecules in P. putida F1 exposed to [ 14 C]trichloroethylene. With a toluene dioxygenase mutant, neither growth inhibition nor alkylation of intracellular molecules was observed.

Genetic toxicology of 1,1,2-trichloroethylene

1,1,2-Trichloroethylene (TCE) is a widely used halogenated solvent, produced in hundreds of millions of kg each year for industrial purposes. Occupational and environmental exposure of human populations to TCE has been reported in industrialized areas. Long-term carcinogenicity studies in rodents demonstrate that exposure to high doses of TCE results in the induction of liver and lung tumors in the mouse, and tumors of the kidney and the testis in the rat. An indirect mechanism, based on the stimulation of liver peroxisome proliferation by TCE metabolites, was proposed to explain species differences in TCE hepatocarcinogenicity. Mutagenicity studies indicate that TCE is weakly active both in vitro, where liver microsomes produce electrophilic TCE metabolites, and also in vivo in mouse bone marrow, where high rates of micronuclei, but no structural chromosome aberrations, are found. Among TCE metabolites, trichloroacetic acid was reported to be carcinogenic to mouse liver. Furthermore, both trichloroacetic acid and chloral hydrate were found to be genotoxic in vivo, inducing structural and numerical chromosome abnormalities, respectively.

Lipid peroxidation: a possible mechanism of trichloroethylene-induced nephrotoxicity

The purpose of this study was to investigate whether lipid peroxidation plays a role in (TCE) trichloroethylene-induced nephrotoxicity in mice at different oxygen concentrations. Male NMRI mice (25-30 g) were treated i.p. with TCE in a dosage of 125-1000 mg/kg in sesame oil. To determine the TCE-induced depletion of reduced glutathione (GSH) in the kidney cortex and liver tissue, mice were given 1000 mg/kg TCE i.p., then killed between 0 and 6 h after TCE administration and GSH was measured was non-protein sulfhydryls. In another series of experiments, mice were administered 125 to 1000 mg/kg TCE i.p. with or without a 2 h i.p. pretreatment with 1500 mg/kg L-buthionine-S-R-sulfoximine (BSO). Mice were then exposed to a 10, 15, 20 or 100% oxygen atmosphere for 3 h and lipid peroxidation in vivo was measured as exhalation of ethane. Subsequently, mice were killed and malondialdehyde (MDA) generation was measured in the liver and kidney cortex. Ethane evolution was estimated by gas chromatography and MDA was determined as thiobarbituric acid reactive substances. In a further series of experiments mice were treated in the same manner as for ethane and MDA determination and the changes in blood urea nitrogen (BUN) and accumulation of the organic ion p-aminohippurate (PAH) were determined. PAH accumulation by renal cortical slices were measured as the slice to medium (S/M) ratio. Six hours after administration of 1000 mg/kg TCE to mice, GSH was significantly depleted to about 60% of control in the kidney cortex but not in the liver. Three hours after TCE administration, MDA content in the kidney cortex and ethane exhalation increased in a dose-dependent manner only under a 10% oxygen atmosphere. Under the same experimental conditions, MDA content remained unchanged in the liver. BSO depletion of GSH prior TCE administration induced an increase of the MDA content in the kidney cortex and an increase of the ethane exhalation in vivo. At 10% oxygen concentration, TCE induced a dose-dependent increase in BUN and a dose-dependent decrease of PAH accumulation by the renal cortical slices. Thus, the results of the present study suggest that, under hypoxic conditions, lipid peroxidation plays a role in TCE nephrotoxicity.

Metabolism, toxicity, and carcinogenicity of trichloroethylene

Lifetime cancer or unit risk estimates for TRI have been calculated by the EPA on the basis of metabolized dose-tumor incidence relationships. Previously, it was common practice to directly extrapolate exposure dose-tumor incidence data from laboratory animal studies to predict cancer risks in humans. Such direct species-to-species extrapolations, however, do not take into account potentially important species differences in systemic uptake, tissue distribution, metabolism, deposition at the site(s) of action, and elimination. The consideration and use of pharmacokinetic and metabolic data can significantly reduce, though not eliminate, uncertainties inherent in species-to-species, route-to-route, and high- to low-dose extrapolations. The total amount of TRI metabolized was considered in the most recent EPA Health Assessment Document for Trichloroethylene to be the effective dose (EFD) producing tumors. Exposure dose-metabolism relationships were determined from direct measurement data in inhalation and oral dosing studies in mice and rats. The magnitude of TRI metabolism in these two species closely approximated body surface area. Thus, it was assumed that the amount of TRI metabolized per square meter of surface area was equivalent among species when calculating human equivalent doses from the animal data. Direct measurement data from an inhalation study in humans were used to calculate the amount of TRI metabolized and the unit risk estimate when a person inhales 1 microgram TRI per cubic meter continuously for 24 h. The EPA Cancer Assessment Group (CAG) elected to use this risk estimate for TRI in air, since it was calculated on the basis of a human metabolized dose rather than unit risk estimates based on animal studies. The current survey of literature and ongoing research uncovered no new animal or human studies in which TRI metabolites were directly measured, which would be any more suitable for use in estimating the total metabolized dose of TRI. On the basis of information now available, it is appropriate to continue to use the total amount of TRI metabolized as the EFD producing tumors in the liver. Use of the total amount metabolized represents an important "step in the right direction" in reducing uncertainties in interspecies extrapolations of data on a chemical such as TRI. TRI is believed to be metabolically activated to a reactive intermediate(s), although the identity of the intermediate(s) is unclear. There is evidence that formation of reactive intermediate(s) and TRI hepatotoxicity are directly proportional to the overall extent of TRI metabolism.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolic activity of antipyrine in workers occupationally exposed to trichloroethylene

In order to investigate possible effects of occupational exposure to trichloroethylene (TRI) on the liver cytochrome P-450 dependent monooxygenases, the metabolic activity of salivary antipyrine was determined in workers (I; N = 32) employed in dry-cleaning shops (I-1; N = 17) and in an industrial metal degreasing process (I-2; N = 15). The studies were performed twice: (a) during the working period, (b) and after at least three weeks free of exposure. The control group (II) consisted of 29 subjects with no known exposure to chemicals. Analyses of the solvents used (TRI) showed them to be mixtures. Statistically significant differences were found (P less than 0.01) in antipyrine t1/2 and clearance within the exposed group (Ia:Ib), but not between the exposed (I) and control (II) group. A breakdown of antipyrine pharmacokinetic data by I-1 and I-2 subgroups demonstrated a statistically significant difference in t1/2 (P less than 0.02) and clearance (P less than 0.05) within I-1 subgroup (a:b), in contrast to the I-2 subgroup (a:b). The difference in antipyrine t1/2 between I-1,a and the control group (II) was also statistically significant (P less than 0.05). Although there was no difference in TRI exposure between I-1 and I-2 based on the biological parameters of TRI absorption, the TRI used in I-2 was of higher grade of purity. It can therefore be concluded that TRI itself is not an inducer of liver monooxygenases and that the monooxygenase induction in subgroup I-1 of TRI exposed workers could be due to TRI impurities.

Induction of strand breaks in DNA by trichloroethylene and metabolites in rat and mouse liver in vivo

The ability of trichloroethylene (TCE) and selected metabolites to induce single-strand breaks in hepatic DNA of male B6C3F1 mice and Sprague-Dawley rats in vivo was evaluated using an alkaline unwinding assay. Doses of TCE of 22-30 mmol/kg were required to produce strand breaks in DNA in rats, whereas a dose of 11.4 mmol/kg was sufficient to increase the rate of alkaline unwinding in mice. To assess the importance of TCE metabolism to this response, rats were subjected to pretreatments of ethanol, phenobarbital, TCE, or the appropriate vehicle for 4 days prior to challenge doses of TCE. Phenobarbital and TCE, but not ethanol pretreatments, reduced the dose of TCE required to produce significant increases in single-strand breaks. In another series of experiments, mice and rats were treated with metabolites of TCE. Trichloroacetate, dichloroacetate, and chloral hydrate induced strand breaks in hepatic DNA in a dose-dependent manner in both species. Strand breaks in DNA were observed at doses that produced no observable hepatotoxic effects as measured by serum aspartate aminotransferase and alanine aminotransferase levels. The slopes of the dose-response curves and the order of potency of these metabolites differed significantly between rats and mice, suggesting that different mechanisms of single-strand break induction may be involved in the two species. These data provide a potential explanation for the different sensitivity of mice and rats to the hepatocarcinogenic effects of TCE.

Pharmacokinetic factors and their implication in the induction of mouse liver tumors by halogenated hydrocarbons

The presently available data on pharmacokinetics of halogenated solvents which produce hepatic tumors in B6C3F1 mice, but not in rats, are reviewed. Such compounds are trichloroethylene, perchloroethylene, 1,1,2-trichloroethane, 1,1,2,2-tetrachloroethane, and dichloromethane. It seems likely that higher metabolic rates in mice (compared with other species) may lead to a species-selective toxicity of such compounds. Recurrent cytotoxicity which leads to stimulation of cell replication seems to be a contributing factor in the pathogenesis of mouse liver tumors. However, it is likely that more than one factor contributes to the unique tumor response of the B6C3F1 mouse.

DNA adducts of halogenated hydrocarbons

Although formation of DNA adducts has been postulated for several halomethanes, no chemical identification of such adducts has been performed so far. There is, however, evidence that methyl chloride does not act biologically as a DNA methylating agent. 1,2-Dichloroethane and 1,2-dibromoethane are activated through conjugation with glutathione. There is some evidence for formation on an N-7 adduct of guanine which carries an ethyl-S-cysteinyl moiety. Extensive work has been published on adducts of vinyl chloride, both in vitro and in vivo. The major DNA adduct is 7-(2-oxoethyl)guanine; a minor adduct appears to be N2,3-ethenoguanine. Other "etheno" adducts, i.e., 1,N6-ethenoadenine and 3,N4-ethenocytosine, are readily formed with DNA, vinyl chloride, and a metabolizing system in vitro and with RNA in vivo, but are usually not detected as DNA adducts in vivo. The data on DNA alkylation by vinyl chloride (and vinyl bromide) metabolites are compared with those of structurally related compounds (acrylonitrile, vinyl acetate, vinyl carbamate).

Induction of single-strand breaks in DNA of mice by trichloroethylene and tetrachloroethylene

Trichloroethylene (TRI) (4-10 mmol/kg body wt) and tetrachloroethylene (PER) (4-8 mmol/kg body wt) were given to male mice by i.p. injection. The induction of single-strand breaks (SSB) in DNA of liver, kidney and lung was studied by the DNA unwinding technique. There was a linear increase of the level of SSB in kidney and liver DNA but not in lung DNA 1 h after administration. The damage was completely repaired 24 h after injection. The capability of TRI and PER to induce SSB in liver DNA is compared to that of three other substances, i.e., methyl methanesulfonate (MMS), styrene-7,8-oxide and styrene, which have been studied earlier by the same technique. The potency of the substances for induction of SSB was in the following order: MMS greater than styrene-7,8-oxide greater than styrene greater than PER greater than TRI.

Absorption, elimination and metabolism of trichloroethylene: a quantitative comparison between rats and mice

The absorption, elimination and metabolism of 14C-trichloroethylene (Tri) was studied in adult female Wistar rats and NMRI mice after administration of 200, 20 and 2 mg/kg Tri. Dose-dependent biotransformation of Tri to metabolites was observed in both species. Induction of hepatic mono-oxygenases by phenobarbital or polychlorinated biphenyls resulted in a higher rate of biotransformation after a single oral dose of 200 mg/kg 14C-Tri to rats. An increase in radioactivity covalently bound to liver and kidney macromolecules of induced rats as compared to control rats parallels the toxic effects of Tri on these organs after induction of cytochrome P-450. The urinary metabolites were analysed by h.p.l.c. In both species, 1,1,1-trichlorocompounds (trichloroacetic acid, trichloroethanol and its glucuronide, comprising 88.9-93.5% of the radioactivity excreted in the urine) constituted the main metabolites; in addition, N-(hydroxyacetyl)-aminoethanol (4.1-7.2%), dichloroacetic acid (0.1-2.0%) and oxalic acid (0.7-1.8%) were identified. The pattern of metabolites in the 72 h urine remained constant for each species in the dose range studied and no change was induced by pretreatment. The percentage of radioactivity exhaled as 14CO2 increased with dose in mice, which may indicate dose-dependent formation of dichloroacetic acid and saturation of deactivating mechanisms for reactive intermediates in mice.

Activities of chlorinated ethane and ethylene compounds in the Salmonella/rat microsome mutagenesis and rat hepatocyte/DNA repair assays under vapor phase exposure conditions

Three chlorinated ethane and ethylene solvent products were examined for their genotoxicity in the Salmonella/microsome mutagenesis and hepatocyte primary culture DNA repair assays using vapor phase exposures. The positive control in this study, monochloroethylene (vinyl chloride), induced reversion mutation of Salmonella tester strains TA100 and TA1535 with enhancement by an exogenous activation system and elicited unscheduled DNA synthesis in rat hepatocytes in culture. Exposures to 1,1,1-trichloroethane (methyl chloroform) or 1,1,2-trichloroethylene samples which contained stabilizers resulted in increased recovery of revertant colonies of Salmonella at concentrations causing greater than 96% cell killing. However, these stabilized materials did not induce DNA repair and low-stabilized trichloroethylene did not induce reversion mutation or DNA repair. Exposure of Salmonella tester strains and hepatocytes to highly toxic vapor concentrations of technical grade 1,1,2,2-tetrochloroethylene, low-stabilized and stabilized, increased reversion mutation and elicited DNA repair. Tetrachloroethylene of high purity was not genotoxic. With all of these test products, the presence of an Aroclor-induced rat liver subcellular enzyme preparation in the mutagenesis assay did not have any effect on the results. These observations suggest that stabilizers or unknown impurities normally present at low concentrations in these products are responsible for the positive responses observed at the high exposure concentrations achievable under in vitro test conditions.

Trichloroethylene: an update

The toxicity of tricholoroethylene (TCE) has been summarized in a number of reviews. In this particular update, only the more recent studies that deal with metabolism and carcinogenicity have been examined. In reviewing the more recent publications on metabolism of TCE, we determined that differences exist in its metabolism if low doses are compared with high doses in animals. There may also be a difference in the metabolism of TCE between different species--namely mice, rats, and humans. TCE has not been shown to be a potent carcinogen in rats and it only seems to be a potent carcinogen in one specific strain of mice, namely the B6C3F1 mouse. Epidemiology studies have been rather limited. The number of persons examined so far for chronic toxic effects is small, compared with the enormous size of the work force that is exposed to TCE over prolonged periods. On an empirical basis, the occupational experience with TCE does not suggest that this compound is a potent carcinogen. The risk associated with exposure to trace amount (ppb) concentrations of TCE in water appear to be minimal or perhaps negligible. Because there are differences in metabolism of TCE, it is important that theoretical risks attributed to TCE in the past be reexamined. It is highly possible that in humans, the metabolic pathway leading to the formation of the proximate carcinogen is not activated at low doses, where TCE is excreted by first-order kinetics.

Novel metabolites of trichloroethylene through dechlorination reactions in rats, mice and humans

The excretion and biotransformation of [14C]trichloroethylene (Tri) has been studied in female rats and mice. Seventy-two hours after a single oral dose of 200 mg/kg, rats exhaled 52% and mice 11% of the recovered radioactivity as unchanged Tri, and 1.9% and 6%, respectively, as 14CO2. Rats excreted 41.2% of the recovered radioactivity in the urine, in contrast to mice where urinary activity amounted to 76%. The isolation of urinary metabolites was accomplished by reversed-phase HPLC, using a water-methanol gradient. After chemical derivatization, a combination of radio-GC and GC/MS was used for identification. The metabolites identified in rat urine were: trichloroacetic acid (15.3%); trichloroethanol, free (11.7%) and as the glucuronide (61.9%); dichloroacetic acid (2.0%); oxalic acid (1.3%) and N-(hydroxyacetyl)-aminoethanol (HAAE) (7.2%). In mice, trichloroethanol (free and in several conjugated forms) is the main metabolite of Tri (94.3%), but small amounts of HAAE (4.1%) and oxalic acid (0.7%) are also excreted. Only traces of dichloro- and trichloroacetic acids were found in this species. In human male subjects, HAAE was also identified as a urinary metabolite of Tri after exposure of two volunteers to 200 ppm Tri for 6 hr. The identification of HAAE and oxalic acid as metabolites indicates hydrolytic dechlorination reactions in the metabolism of Tri.

Carcinogenicity study of trichloroethylene, with and without epoxide stabilizers, in mice

Previous analytical studies of industrial samples of trichloroethylene (TRI) have revealed the presence of mutagenic and carcinogenic epoxides which, it was proposed, might be responsible for the carcinogenicity of such samples, as demonstrated with mice in other laboratories. To test this hypothesis, Swiss mice (ICR/HA) of both sexes, bred and kept in SPF conditions, were dosed daily with TRI in corn oil by gavage (males: 2.4 g/kg, females: 1.8 g/kg) with or without the addition of epichlorohydrin (EPC, 0.8%, w/w), 1,2-epoxybutane (BO, 0,8%), or EPC + BO (0.25% + 0,25%) for 18 months. The ensuing observation period terminated at 106 weeks (from start of experiment). Gross and microscopic examination of all organs revealed a statistically significant increase in the incidence of forestomach papillomas and carcinomas after EPC-, BO-, and (EPC + BO)-stabilized samples of TRI, but not after pure, amine base-stabilized TRI. This type of tumor is believed to be induced by the direct alkylating epoxides epichlorohydrin and epoxybutane, whose industrial use in stabilizing chlorinated aliphatic hydrocarbons should be discontinued. No other significant increase in tumor incidences was found. Again, this study does not support the suggestion that trichloroethylene itself is carcinogenic under realistic exposure conditions.