Irreversible binding of 14C-labelled trichloroethylene to mice liver constituents in vivo and in vitro

Toxicological effects of trichloroethylene exposure on immune disorders

Trichloroethylene (TCE) is one of the most common ground water contaminants in USA. Even though recent regulation mandates restricted utilization of TCE, its use is not completely prohibited, especially in industrial and manufacturing processes. The risk of TCE on human health is an ongoing field of study and its implications on certain diseases such as cancer has been recognized and well-documented. However, the link between TCE and immune disorders is still an under-studied area. Studies on the risk of TCE on the immune system is usually focused on certain immune class disorders, but consensus on the impact of TCE on the immune system has not been established. This review presents representative work that investigates the effect of TCE on immune disorders and highlights future opportunities. We attempt to provide a broader perspective of the risks of TCE on the immune system and human health.

Differential immune responses to albumin adducts of reactive intermediates of trichloroethene in MRL+/+ mice

Trichloroethene (TCE) is an industrial degreasing solvent and widespread environmental contaminant. Exposure to TCE is associated with autoimmunity. The mode of action of TCE is via its oxidative metabolism, and most likely, immunotoxicity is mediated via haptenization of macromolecules and subsequent induction of immune responses. To better understand the role of protein haptenization through TCE metabolism, we immunized MRL+/+ mice with albumin adducts of various TCE reactive intermediates. Serum immunoglobulins and cytokine levels were measured to determine immune responses against haptenized albumin. We found antigen-specific IgG responses of the IgG subtypes IgG(1), IgG(2a), and IgG(2b), with IgG(1) predominating. Serum levels of G-CSF were increased in immunized mice, suggesting macrophage activation. Liver histology revealed lymphocyte infiltration in the lobules and the portal area following immunization with formyl-albumin. Our findings suggest that proteins haptenized by metabolites of TCE may act as neo-antigens that can induce humoral immune responses and T cell-mediated hepatitis.

Autoimmune response in MRL+/+ mice following treatment with dichloroacetyl chloride or dichloroacetic anhydride

Dichloroacetyl chloride (DCAC) is formed from trichloroethene (TCE), which is implicated in inducing/accelerating autoimmune response. Due to its potent acylating activity, DCAC may convert proteins to neo-antigens and thus could induce autoimmune responses. Dichloroacetic anhydride (DCAA), which is a similar acylating agent, might also induce autoimmune responses. To evaluate if chloroacylation plays a role in the induction of autoimmunity, we have measured the autoimmune responses following treatment with DCAC or DCAA in autoimmune-prone MRL+/+ mice. Five-week-old female mice were injected intraperitoneally (twice weekly) with 0.2 mmol/kg of DCAC or DCAA in corn oil for 6 weeks. Total serum IgG, IgG1, and IgE levels were significantly increased in DCAC-treated mice as compared to controls. These increases corresponded with increases in DCAC-specific IgG and IgG1 levels. Total serum IgM was decreased in both DCAC- and DCAA-treated mice. Antinuclear antibodies, measured as an indication of systemic autoimmune responses, were increased in both DCAC- and DCAA-treated mice. Of eight Th1/Th2 cytokines measured in the serum, only IL-5 was significantly decreased in both treatment groups. The cytokine secretion patterns of splenic lymphocytes after stimulation with antibodies against CD3 (T cell receptor-mediated signal) and CD28 (costimulatory signal) differed between treatment and control groups. Levels of IL-1, IL-3, IL-6, IFN-gamma, G-CSF, and KC were higher in cultures of stimulated splenocytes from either DCAC- or DCAA-treated mice than from controls. The level of IL-17 was only increased in cultures from DCAC-treated mice. Increased lymphocytic populations were found in the red pulp of spleens following treatment with either DCAC or DCAA. In addition, thickening of the alveolar septa in the lungs of DCAC- or DCAA-treated mice was observed. The lung histopathology in exposed mice was consistent with the symptomology observed in welders exposed to DCAC/phosgene. Thickening was more pronounced in DCAC-treated mice. Our data suggest that DCAC and DCAA elicit autoimmune responses in MRL+/+ mice that might be reflective of their chloroacylation potential in vivo.

Regulatory heme and trichloroethylene intoxication: A possible explanation of the case of "A Civil Action"

In 1998, a amovie entitled "A Civil Action" was released. The movie described the Woburn case, begun in 1982 and concluded in 1990, one of the most famous cases of trichloroethylene pollution. In a small town near Boston, twelve children died of leukemia, which seemed attributable to trichloroethylene contamination of the drinking water. The victims, however, could not win the case, since evidence that the identified chemicals could cause leukemia and other human illnesses was rather sketchy. There have been many cases of trichloroethylene pollution in industrial nations including Japan, therefore, we reconsidered the missing link. Our conclusion is that the disease occurred not by a direct effect of the chemical hazard on biological macromolecules but by an indirect effect through the physiological system such as signal transduction and transcriptional regulation. In 1984, we reported a marked reduction in the regulatory heme pool by trichloroethylene exposure, however, the biological significance was not well understood. Recently, we found that the DNA binding activity of Bach1, a negative regulator of genes, is controlled by heme, the regulation of which seems to explain how leukemia develops. The heterodimer of Bach1 with MafK recognizes Maf recognition elements (MAREs) competing with the erythroid type positive regulator, a complex of NF-E2 with MafK. Bach1/MafK occupies MAREs under lower heme conditions, whereas MAREs are open to NF-E2/MafK along with increasing heme concentration. Since the NF-E2/MafK function is closely related to normal erythroid differentiation, of which disorders such as sideroblastic anemia are often related to neoplasia; i.e., a clonal disorder that can progress to leukemia. Thus, a marked decline in regulatory heme by trichloroethylene intoxication could be one of the pathways to leukemia.

Protein targets of xenobiotic reactive intermediates

Many xenobiotics are metabolically activated to electrophilic intermediates that form covalent adducts with proteins; the mechanism of toxicity is either intrinsic or idiosyncratic in nature. Many intrinsic toxins covalently modify cellular proteins and somehow initiate a sequence of events that leads to toxicity. Major protein adducts of several intrinsic toxins have been identified and demonstrate significant decreases in enzymatic activity. The reactivity of intermediates and subcellular localization of major targets may be important in the toxicity. Idiosyncratic toxicities are mediated through either a metabolic or immune-mediated mechanism. Xenobiotics that cause hypersensitivity/autoimmunity appear to have a limited number of protein targets, which are localized within the subcellular fraction where the electrophile is produced, are highly substituted, and are accessible to the immune system. Metabolic idiosyncratic toxins appear to have limited targets and are localized within a specific subcellular fraction. Identification of protein targets has given us insights into mechanisms of xenobiotic toxicity.

Kinetics of chlorinated hydrocarbon degradation by Methylosinus trichosporium OB3b and toxicity of trichloroethylene

The kinetics of the degradation of trichloroethylene (TCE) and seven other chlorinated aliphatic hydrocarbons by Methylosinus trichosporium OB3b were studied. All experiments were performed with cells grown under copper stress and thus expressing soluble methane monooxygenase. Compounds that were readily degraded included chloroform, trans-1,2-dichloroethylene, and TCE, with Vmax values of 550, 330, and 290 nmol min-1 mg of cells-1, respectively. 1,1-Dichloroethylene was a very poor substrate. TCE was found to be toxic for the cells, and this phenomenon was studied in detail. Addition of activated carbon decreased the acute toxicity of high levels of TCE by adsorption, and slow desorption enabled the cells to partially degrade TCE. TCE was also toxic by inactivating the cells during its conversion. The degree of inactivation was proportional to the amount of TCE degraded; maximum degradation occurred at a concentration of 2 mumol of TCE mg of cells-1. During conversion of [14C]TCE, various proteins became radiolabeled, including the alpha-subunit of the hydroxylase component of soluble methane monooxygenase. This indicated that TCE-mediated inactivation of cells was caused by nonspecific covalent binding of degradation products to cellular proteins.

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)

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.

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)

Extrahepatic organs metabolism of inhaled trichloroethylene

An extrahepatic circulation system for dogs was developed using a portal vein to right femoral vein bypass procedure. This system maintained nearly normal biochemical and physiological parameters, i.e. arterial blood pressure, heart rate, electrocardiogram, leukocyte and erythrocyte count, hematocrit, alkaline phosphatase, blood urea nitrogen, ammonia and creatinine, for 2 h. Thus, the system appears to be a valid technique for investigating extrahepatic metabolism. Dogs were exposed for 1 h to 500, 700 and 1500 ppm of trichloroethylene. Free-trichloroethanol, trichloroacetic acid and conjugated-trichloroethanol appeared in the blood and urine after 30 min of exposure. The amounts of metabolite formed by dogs with hepatic bypass were less than by similarly exposed dogs without hepatic bypasses, specifically 50-80%, 10% and 10-20% for free-trichloroethanol, trichloroacetic acid and conjugated-trichloroethanol, respectively. In addition, trichloroethylene exposure produced a smaller decrease in leukocyte counts in the hepatic bypass dogs than in the non-bypass dogs. This observation may indicate that the liver itself played some role in the elimination or increment of leukocyte counts in the blood.

Biochemical, histological, and ultrastructural changes in rat and mouse liver following the administration of trichloroethylene: possible relevance to species differences in hepatocarcinogenicity

Trichloroethylene (TRI), administered by gavage for 10 consecutive days, at doses of 500 to 1500 mg/kg body wt increased liver weight (175% of control), decreased hepatic DNA concentration (66% of control), and increased the synthesis of DNA (500% of control; as measured by [3H]dT incorporation) in B6C3F1 mice and Alderley Park mice. Similar treatment of Osborne-Mendel rats or Alderley Park rats resulted in smaller increases in liver weight (130% of control) and decreases in DNA concentration (83% of control). No effect of TRI on DNA synthesis was seen in rats. The increased DNA synthesis in the mouse was not apparently due to regenerative hyperplasia since no signs of necrosis were seen. Furthermore the increased [3H]dT incorporation probably represented semiconservative replication of DNA and not repair, since a parallel increase of mitotic figures was observed. Hence, the liver growth noted after TRI administration appears to be due to liver cell enlargement (hypertrophy) in the rat, but both hypertrophy and hyperplasia (cell proliferation) in the mouse. An important observation has been that TRI induced the peroxisomal enzyme activities, catalase, and cyanide-insensitive palmitoyl-CoA oxidation (147 and 786% of control, respectively), in mice but not in rats. Furthermore, increases in peroxisome volume density (up to 1110% of control) were observed in mice receiving TRI. These observations lead us to suggest that the species difference in hepatocarcinogenicity of TRI, seen between the rat and mouse, is possibly due to a species difference in peroxisome proliferation and cell proliferation, the peroxisome proliferation leading to increased reactive oxygen species and DNA damage, and the cell proliferation then acting to promote this lesion.

Species differences in response to trichloroethylene. I. Pharmacokinetics in rats and mice

The elimination of radioactivity in two strains of rats and mice following a single po dose of trichloro[14C]ethylene at dose levels from 10 to 2000 mg/kg has shown a marked dose dependence in rats but not in mice. The metabolism of trichloroethylene in the mouse was linear over the range of doses used, whereas in the rat it became constant and independent of dose at 1000 mg/kg and above. At the 10-mg/kg dosage, both species metabolized trichloroethylene almost completely, 60% of the dose being excreted in urine with only 1 to 4% being eliminated unchanged in expired air in the first 24 hr. At 2000 mg/kg, 78% of the dose was eliminated unchanged in the rat, but only 14% in the mouse. Consequently at high dosages, the mouse was exposed to significantly higher concentrations of trichloroethylene metabolites than the rat. Blood level kinetics of trichloroethylene and its metabolites confirmed a faster rate of metabolism in the mouse than in the rat. Peak concentrations of the metabolites were reached within 2 hr of dosing in the mouse compared to 10 to 12 hr in the rat. The concentrations of both trichloroethanol (4X) and trichloroacetic acid (7X) were significantly higher in the mouse than in the rat. Whereas trichloroethanol was rapidly eliminated from blood, the higher concentrations of trichloroacetic acid were maintained for over 30 hr. The high blood quantities of trichloroethylene-derived trichloroacetic acid are known to induce hepatic peroxisome proliferation in mice but are insufficient to induce this response in rats. These data suggest that trichloroacetic acid blood amounts, peroxisome proliferation, and the link between peroxisomes and liver cancer are the basis of species difference in response to trichloroethylene.

Species differences in response to trichloroethylene. II. Biotransformation in rats and mice

Detailed analysis of urine from two strains of rats and mice dosed po with trichloroethylene at four doses from 10 to 2000 mg/kg failed to detect any major species or strain differences in the metabolism of trichloroethylene. Although a greater proportion of the dose was metabolized in mice than in rats, the relative proportions of the major metabolites were very similar in both strains and were unaffected by the dose amount. Analysis of the same urine samples for minor metabolites failed to establish a major species difference. Small amounts of dichloroacetic acid (less than 1% of the dose) were present in both rat and mouse urine and were not considered significant. Monochloroacetic acid accounted for less than 0.1% of the dose. Daily dosing of trichloroethylene (1000 mg/kg po) for 180 days did not induce the overall metabolism of trichloroethylene but did double the urinary excretion of trichloroacetic acid. This finding was accompanied by an equivalent percentage decrease in the concentration of trichloroethanol. CO2 has been shown to be a major metabolite of trichloroacetic acid, suggesting that this is the source of trichloroethylene-derived CO2. Trichloroacetic acid was also excreted in bile in both rats and mice suggesting possible conjugation of this metabolite in the liver. Very little evidence was found for the formation of chemically reactive species from trichloroethylene in either rats or mice and none that could be the basis of a major species difference. The increased rate of metabolism in the mouse, the resulting high blood concentrations of trichloroacetic acid, and stimulation of hepatic peroxisome proliferation in this species appears to be the major species difference possibly related to tumor formation in the liver. The conjugation of trichloroacetic acid and its metabolism to CO2 may be related to peroxisome proliferation.

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.

Lung injury induced by trichloroethylene

Trichloroethylene (TCE) produced bronchiolar damage when administered to mice. Administration of 2000 mg/kg caused injury in Clara cells of the bronchiolar epithelium, which was observed at 24 h following TCE treatment; increase of the dosage to 2500 mg/kg induced additionally, alterations in alveolar Type II cells of the parenchyma. Specifically, lamellar bodies were reduced in number and microvilli displayed distorted protrusions. The increase in severity of cellular injury with higher dosages of TCE coincided with increased accumulation of pulmonary calcium and lengthened anesthesia recovery times following TCE-induced anesthesia. Time-course studies conducted with 2000 mg/kg demonstrated rapid and marked reduction in pulmonary microsomal cytochrome P-450 content and aryl hydrocarbon hydroxylase activity. Significant decreases were observed as early as 1 h, and the levels were still depressed at 24 h following TCE treatment. Hepatic necrosis was relatively mild at the dosages of TCE examined. These results demonstrate that TCE is pneumotoxic and affects Clara and alveolar Type II cells.

Species differences in carcinogenicity and peroxisome proliferation due to trichloroethylene: a biochemical human hazard assessment

Trichloroethylene (TRI) administered to mice by gavage for 10 consecutive days at doses of 50-2000 mg/kg body weight elicited dose-dependent increases (up to 700% of control values) of hepatic cyanide insensitive palmitoyl CoA oxidation (a marker of peroxisomal beta-oxidation). No effect was seen on catalase; the other peroxisomal marker examined. Similar experiments with rats demonstrated no effect of TRI on either cyanide insensitive palmitoyl CoA oxidation or catalase. A major metabolite of TRI, trichloroacetic acid (TCA) when administered by gavage for 10 consecutive days at doses of 10-200 mg/kg body weight, stimulated hepatic cyanide insensitive palmitoyl CoA oxidation in both mice (up to 500% of control) and rats (up to 650% of control). Again, no effect upon catalase activity was apparent. The kinetics of biotransformation of TRI to TCA in isolated hepatocytes was markedly species dependent. The 'intrinsic clearance' values (Vmax/Km) for TRI in mouse, rat and human hepatocytes were 3.8 X 10(-6), 1.2 X 10(-7) and 3.25 X 10(-8) L/min/10(6) cells respectively. TCA induced peroxisomal beta-oxidation in mouse and rat hepatocytes, but had no effect upon this enzyme activity in cultured human hepatocytes. It is postulated that the species difference in hepatocarcinogenicity of TRI (mouse positive; rat negative) is due to species differences in peroxisome proliferation which in turn is a result of differences in the rate of formation of TCA from TRI. On this basis it is proposed that TRI presents no significant human hepatocarcinogenic hazard since, human hepatocytes produced TCA at a rate even lower than that of the rat, and TCA was not a peroxisome proliferator in human hepatocytes.

Inhibition of delta-aminolevulinic acid dehydratase by trichloroethylene

Male Wistar rats (8 animals/group; 180-200 g) were exposed continuously to trichloroethylene (TRI) for 48 or 240 h or methylchloroform (1,1, 1-trichloroethane: MC for 48 h at 50, 400 and 800 ppm. The inhibition of delta-aminolevulinic acid dehydratase (ALA-D) was examined in liver, blood and bone marrow of naive and phenobarbital pretreated animals exposed to TRI. A clear cut dose-effect relationship between the exposure concentration or duration of exposure and the inhibition of ALA-D activity was seen for rats exposed to TRI. In addition to this finding, significant interaction between TRI exposure and phenobarbital treatment was observed in the inhibition of ALA-D in liver and blood. MC did not produce inhibition. Trichloroacetic acid and trichloroethanol failed to inhibit the ALA-D activity in vitro. It seems that a metabolite(s) of TRI other than the above 2 substances may play a role in the inhibition of ALA-D. The inhibition of ALA-D (38% or 48% of the control in liver or in blood, respectively) observed after the 240 h exposure at 400 ppm to TRI was accompanied by the significant elevation of delta-aminolevulinic acid synthase (186% of the control) in liver and the increase in excretion of delta-aminolevulinic acid in urine (142% of the control). This occurred without an apparent weight loss, liver injury or hematological changes.

Some effects of trichloroethylene on mouse lungs and livers

Repeated administration of trichloroethylene (TCE) to mice by either i.p. injection or by inhalation increased the activity of hepatic microsomal NADPH cytochrome-c reductase. The NADPH cytochrome c reductase activity in microsomes isolated from lungs of animals treated with TCE by inhalation was decreased relative to controls (untreated animals). TCE inhalation was associated with pathologic changes in lungs, but not in livers of the treated animals. The duration of exposure is probably an important factor however, since animals exposed for only 1 hr per day exhibited neither pathologic changes in the lungs nor an alteration of enzyme activity. These findings indicate that inhalation of TCE, without prior treatment with inducers, can enhance activity of the hepatic mixed function oxidase system. The reduced activity of the pulmonary mixed function oxidase system in animals that inhaled TCE may reflect injury to the lungs.

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

The covalent binding of 14C-1,1,2-trichloroethylene (14C-TRI) metabolites to calf thymus DNA in vitro and to RNA and DNA of mouse brain, lung, liver, kidney, spleen, pancreas, and testis after repeated i.p. injections has been studied. Hydrolysates of DNA reacted with 14C-TRI in vitro and hydrolysates of RNA and DNA from selected organs were separated on Aminex A6 for quantitation of alkylation products. The presence of 3,N4-etheno(deoxy)cytidine, 1,N6-etheno(deoxy)adenosine and 1,N6-ethenoadenine was investigated. No radioactivity could be registered in DNA incubated with 14C-TRI in the absence of liver microsomes. Covalent binding of 14C-TRI to DNA took place in the presence of liver microsomes from control mice. The binding was enhanced by 50% if liver microsomes from phenobarbital pretreated mice were used. The radioactivity in DNA reacted with 14C-TRI and microsomes from control mice was eluted in early fractions and together with thymidine. The same two peaks appeared on chromatography of DNA incubated with 14C-TRI and liver microsomes from phenobarbital pretreated mice. In addition, radioactivity was eluted together with 1,N6-ethenoadenine. Radioactivity was registered in RNA and DNA from all of the studied organs after i.p. injections of 14C-TRI. The radioactivity in RNA increased in the order brain less than testis less than pancreas less than kidney less than liver less than lung less than spleen. The radioactivity in DNA increased in the order brain less than kidney less than testis less than lung less than pancreas less than liver less than spleen. Aminex A6 chromatography revealed that the entire radioactivity in RNA from liver and kidney and in DNA from kidney, testis, lung, pancreas, and spleen was due to metabolic incorporation, particularly into guanine and adenine. This finding indicates that the C-C bond in TRI is split, with the formation of C1-fragments, during biotransformation in vivo. In liver DNA, the metabolic incorporation of radioactivity was insignificant. Instead, the dominant part of the radioactivity in liver DNA was eluted in early fractions. The elution profile of radioactivity in liver DNA gave no direct evidence of the formation of TRI-DNA adducts in vivo. No etheno-derivatives were identified as alkylation products of TRI in vivo, which is consistent with current theories of the metabolic fate of TRI.

Dose-dependent induction and suppression of liver mixed-function oxidase system in chlorinated hydrocarbon solvent metabolism

The effect of continuous exposure to trichloroethylene (TRI), tetrachloroethylene (TETRA) or methylchloroform (MC) on the hepatic mixed-function oxidase system (MFOS) was studied in rats by using 10 000 x g supernatant fraction. Exposure to TETRA for 240 h at 200, 100 and 50 ppm enhanced oxidative conversion from TETRA to trichloroacetic acid. When the animals were exposed for 240 h to 200, 400 and 800 ppm, oxidative conversion from MC to trichloroethanol was elevated. However, elevation was less remarkable with the increase of exposure intensities from 400 to 800 ppm. With TRI, MFOS activities were more critically assessed as a function of duration and dose because the variable response in MFOS activity was observed in preliminary studies when rats were exposed to 400 ppm for 240 h. The MFOS activities in rats exposed to TRI at 50, 400 or 800 ppm for 48 h, 72 h, 168 h and 240 h were measured. The MFOS activities were all suppressed after 48-h exposure irrespective of the exposure concentration. After 72--240 h, suppression was superseded by activation at 50 ppm, while continuity of suppressive state was observed at 800 ppm and transitional state was the case of the exposure at 400 ppm. The possibility that epoxide hydratase would be involved in the metabolism of TRI, but not in those of other two chemicals, was also presented. Based on these findings, mathematical models for TRI and TETRA metabolism were established, which can explain hepatotoxicity appearing only after exposure to TRI at 800 ppm for 168 h or more.

Metabolism of [14C]trichloroethylene to 14CO2 and interaction of a metabolite with liver DNA in rats and mice

Male Sprague-Dawley rats and male B6C3F1 mice excreted 5-15% of a tracer dose of [14C]trichloroethylene as 14CO2 within 24 h after ip injection of a single dose in a corn-oil vehicle. The proportion of the dose excreted as CO2 was greater in mice than in rats, but increased in the rats after starvation or pretreatment with phenobarbital. As the dose was increased toward the LD50 level, the proportion excreted as 14CO2 decreased slightly, but this was largely due to increased loss of unchanged trichloroethylene. The excretion of 14CO2 was thus correlated with the expected level of microsomal metabolism of trichloroethylene to an electrophilic intermediate capable of binding to glutathione or macromolecules. Liver protein labeling was observed to be relatively high (10,000-23,000 cpm/mg in the mouse), while DNA labeling was consistently observed to be very low, not allowing identification of any adducts by high-performance liquid chromatography (HPLC). Also, no effect on DNA fragmentation was seen by alkaline sucrose gradient centrifugation after injection of an LD50 dose of trichloroethylene. The ability of trichloroethylene to interact with DNA in vivo was thus observed to be very slight.

Covalent binding of haloethylenes

Halogenated ethylenes are metabolized to reactive intermediates which covalently bind to different cellular targets. Vinyl chloride and vinyl bromide metabolites bind to DNA, preferably to N-7 of deoxyguanosine. With RNA, 1,N6-ethenoadenosine and, 3,N4-ethenocytidine moieties are formed. All the haloethylenes in which this effect has been studied form metabolites capable of alkylating proteins, preferably at free sulfhydryl groups. Also, there is alkylation by haloethylene metabolites of cellular coenzymes. An observed increased exhalation of acetone by rats exposed to different haloethylenes can possibly be explained by alkylation of cytosolic coenzyme A. Such metabolic effects may serve as an indicator for reactive metabolite formation in vivo and should be more investigated.

Increased acetone exhalation induced by metabolites of halogenated C1 and C2 compounds

Rats were exposed, in a closed desiccator jar chamber, to concentrations of various halogenated C1 and C2 compounds at which the metabolizing capacities were saturated (Vmax conditions). Within the exposure period of 50 h concentrations of the xenobiotic and of exhaled acetone were monitored in the gas phase of the system. The quantitative extent of acetone exhalation was dependent on the individual compound examined. Acetone exhalation was stimulated in presence of vinyl chloride, vinyl bromide, vinyl fluoride, vinylidene fluoride, cis- and trans-1,2-dichloroethylene, trichloroethylene, perchloroethylene, methylene chloride, chloroform, carbon tetrachloride and 1,1,2-trichloroethane. No stimulation of acetone exhalation occurred with 1,1,1-trichloroethane and with the reference hydrocarbon n-hexane. Also, acetone exhalation was evoked by infusions of either fluoroacetate or chloroacetate, two anticipated or proven metabolites of some haloethylenes; the infusion rates of which were based on calculations of the metabolic rates of vinylidene fluoride and of vinyl chloride, respectively.

Trichloroethylene vapours do not produce dominant lethal mutations in male mice

Exposure of male mice to trichloroethylene vapours during 24 h at levels or 50, 202 and 450 ppm did not reveal mutagenic effects in the dominant lethal assay. The following parameters were registered and evaluated: Fertilization rate, post-implantation loss, preemplantation loss and dominant lethal mutations.

In vivo covalent binding of organic chemicals to DNA as a quantitative indicator in the process of chemical carcinogenesis

The covalent binding of chemical carcinogens to DNA of mammalian organs is expressed per unit dose, and a 'Covalent-Binding Index', CBI, is defined. CBI for various carcinogens span over 6 orders of magnitude. A similar range is observed for the carcinogenic potency in long-term bioassays on carcinogenicity. For the assessment of a risk from exposure to a carcinogen, the total DNA dmaage can be estimated if the actual dose is also accounted for. A detailed description is given for planning and performing a DNA-binding assay. A complete literature survey on DNA binding in vivo (83 compounds) is given with a calculation of CBI, where possible, 153 compounds are listed where a covalent binding to any biological macromolecule has been shown in vivo or in vitro. Recent, so far unpublished findings with aflatoxin M1, macromolecule-bound aflatoxin B1, diethylstilbestrol, and 1,2-epithiobutyronitrile are included. A comparison of CBI for rat-liver DNA with hepatocarcinogenic potency reveals a surprisingly good quantitative correlation. Refinements for a DNA-binding assay are proposed. Possibilites and limitations in the use of DNA binding in chemical carcinogenesis are discussed extensively.

Vinyl chloride and trichloroethylene: comparison of alkylating effects of metabolites and induction of preneoplastic enzyme deficiencies in rat liver

[1,2-14C] Vinyl chloride and [1,2-14C] trichloroethylene were incubated with rat liver microsomes, NADPH and RNA (from yeast). Whereas trichloroethylene metabolites were irreversibly bound to proteins in microsomal incubations to a higher extent than vinyl chloride metabolites, irreversible binding to RNA was lower for trichloroethylene metabolites. Hydrolysis of the RNA which was reisolated from microsomal incubations with 14C-vinyl chloride or 14C-trichloroethylene and separation of the nucleosides showed different alkylation products arising from vinyl chloride and from trichloroethylene, characteristic for vinyl chloride being formation of 1,N6-ethenoadenosine and 3,N4-enthenocytidine. The different reactivities of metabolites of vinyl chloride and of trichloroethylene prompted a comparison of the oncogenic effects of both compounds against the rat liver cell. Newborn rats were exposed for 10 weeks to 2000 ppm vinyl chloride or trichloroethylene (8 h/day; 5 days/week). After this period livers of the animals were stained for nucleoside-5-triphosphatase. Whereas the vinyl chloride exposed rats showed focal hepatocellular deficiencies in this enzyme, which are supposed to represent an early sign of malignancy, no such changes were induced by trichloroethylene exposure. The data therefore suggest differences between the hepatocarcinogenic activity of vinyl chloride and possible effects of trichloroethylene on the liver.