The cholecystohepatic circulation of trichloroethylene and its metabolites in dogs

In order to examine the cholecystohepatic circulation of trichloroethylene (TRI) and its metabolites, we injected the gallbladder with TRI and its metabolites, i.e. chloral hydrate (CH), free-trichloroethanol (F-TCE), trichloroacetic acid (TCA) and conjugated-trichloroethanol (Conj-TCE), using anesthetized dogs. The absorption rates of water from the gallbladder were 25-30% 2 h after administration for all substances. The absorption rates of substances were 65-70% in the CH, F-TCE and TRI groups, and 40-50% in the Conj-TCE and TCA groups 2 h after the administration. Conj-TCE in the blood absorbed from the gallbladder has a tendency to be directly transported to the venous system rather than to be taken into hepatocytes in the liver. All of the administered substances, in particular, F-TCE might be metabolized to other substances in the gallbladder.     

Biliary excretion of trichloroethylene and its metabolites in dogs

To examine the biliary excretion of trichloroethylene (TRI) and its metabolites, we carried out various experiments with TRI and its metabolites, i.e., chloral hydrate (CH), free-trichloroethanol (F-TCE) and trichloroacetic acid (TCA), using anesthetized dogs. The amount of biliary excretion was significantly increased with the administration of CH and F-TCE, whereas it remained at control levels with the administration of TRI and TCA. The substances excreted into bile were conducted in the form of conjugated-TCE (Conj-TCE) in over 90% of the CH, F-TCE and TRI administration groups. About 95% of these Conj-TCE were conjugated with glucuronic acid. The cumulative excretion ratios of substances and metabolites to dose were 20% for CH and F-TCE, and about 1% for TCA and TRI 2 h after administration.     

Extrahepatic metabolism of chloral hydrate, trichloroethanol and trichloroacetic acid in dogs

To examine the details concerning that part of TRI metabolism which was carried out by the extrahepatic organs, we studied the extrahepatic metabolism of chloral hydrate (CH), free-trichloroethanol (F-TCE) and trichloroacetic acid (TCA) using a method developed in our laboratory. Bypass and non-bypass dogs were given CH, F-TCE and TCA, and we compared the concentrations these substances and their metabolites in the serum and urine of the two groups of animals. In the bypass dogs, F-TCE, TCA and conjugated-trichloroethanol (Conj-TCE) appeared in the blood and urine 30 min. after the CH administration, and TCA and Conj-TCE appeared 30 min. after the F-TCE. All levels of administered substance were higher in bypass dogs than in non-bypass dogs, and the compounds were metabolized in small amounts in the extrahepatic organs compared with the liver. Therefore, administered substances remained at high levels in the serum and were excreted in large amounts in the urine in the form of unchanged substances. The metabolized percentage volumes of CH to TCA in the bypass dogs were 10-20%, and those of F-TCE to TCA were very small, while these percentage values of CH to F-TCE were the same or slightly smaller, respectively. Moreover, trichloroethylene (TRI) acts to decrease the leukocyte count in the blood, but the TRI metabolites described above do not have this function.     

Intestinal absorption of chloral hydrate, free trichloroethanol and trichloroacetic acid in dogs

In order to examine the intestinal absorption of chloral hydrate (CH), free trichloroethanol (F-TCE) and trichloroacetic acid (TCA), an intestinal circulation system in dogs was developed using jejunal, ileal and colonic loops, and solutions of CH, F-TCE and TCA were circulated within them. The concentrations of these substances and their metabolites in the serum, urine, bile and circulates were then measured. In all groups, the fraction of water absorbed from the intestine was about 10% of the administered volume two hours after administration. The absorbed fraction of CH was about 50% in the jejunum and ileum, and about 40% in the colon. The absorbed fraction of F-TCE was about 60% in the jejunum, 50-60% in the ileum and about 40% in the colon, while the figures for TCA were about 40-50% in the jejunum and about 30-40% in the ileum and colon. The combined biliary and urinary excretion ratios of the administered substances and their respective metabolites to the total amounts absorbed from the intestine were about 25-30% for F-TCE, 10-15% for CH and 0.1-0.2% for TCA in all parts of the intestine two hours after administration.     

Intestinal absorption of trichloroethylene in dogs

In order to examine the intestinal absorption of trichloroethylene (TRI), we developed the intestinal circulation system of dogs and administered TRI solution at three concentrations (0.1, 0.25 and 0.5%) to the three parts of the intestinal tract (jejunum, ileum, and colon) of the operated dogs. We measured TRI and its metabolites, free-trichloroethanol, trichloroacetic acid, and conjugated trichloroethanol, in serum or blood, urine, bile and circulating solutions. The absorption rates of TRI from the intestine were 50-70% of the administered volume of TRI 2 hr after administration in all groups, and all parts of intestine readily absorbed TRI. Moreover, there were no significant differences in the absorption rates of TRI and water between the jejunum and ileum, and ileum and colon, respectively. The excretion rates of TRI and its metabolites in urine and bile were very low (0.1-0.4%) compared with the volume of absorbed TRI from the intestine 2 hr after administration in all groups. The high degree of absorption of TRI should be considered when threshold limits for TRI in the drinking water, the surface water, and the ground water are established.     

Kinetics of chloral hydrate and its metabolites in male human volunteers

Chloral hydrate (CH) is a short-lived intermediate in the metabolism of trichloroethylene (TRI). TRI, CH, and two common metabolites, trichloroacetic acid (TCA) and dichloroacetic acid (DCA) have been shown to be hepatocarcinogenic in mice. To better understand the pharmacokinetics of these metabolites of TRI in humans, eight male volunteers, aged 24-39, were administered single doses of 500 or 1,500 mg or a series of three doses of 500 mg given at 48 h intervals, in three separate experiments. Blood and urine were collected over a 7-day period and CH, DCA, TCA, free trichloroethanol (f-TCE), and total trichloroethanol (T-TCE=trichloroethanol and trichloroethanol-glucuronide [TCE-G]) were measured. DCA was detected in blood and urine only in trace quantities (<2 microM). TCA, on the other hand, had the highest plasma concentration and the largest AUC of any metabolite. The TCA elimination curve displayed an unusual concentration-time profile that contained three distinct compartments within the 7-day follow-up period. Previous work in rats has shown that the complex elimination curve for TCA results largely from the enterohepatic circulation of TCE-G and its subsequent conversion to TCA. As a result TCA had a very long residence time and this, in turn, led to a substantial enhancement of peak concentrations following the third dose in the multiple dose experiment. Approximately 59% of the AUC of plasma TCA following CH administration is produced via the enterohepatic circulation of TCE-G. The AUC for f-TCE was found to be positively correlated with serum bilirubin concentrations. This effect was greatest in one subject that was found to have serum bilirubin concentrations at the upper limit of the normal range in all three experiments. The AUC of f-TCE in the plasma of this individual was consistently about twice that of the other seven subjects. The kinetics of the other metabolites of CH was not significantly modified in this individual. These data indicate that individuals with a more impaired capacity for glucuronidation may be very sensitive to the central nervous system depressant effects of high doses of CH, which are commonly attributed to plasma levels of f-TCE.     

The metabolism of trichloroethylene and its metabolites in the perfused liver

The metabolism of trichloroethylene (TRI) and its metabolites, chloral hydrate (CH), trichloroethanol (free-TCE) and trichloroacetic acid (TCA), were examined in the isolated perfused rat liver, to clarify the role of the liver in the metabolism of TRI. TRI was rapidly converted to TCE and TCA by the perfused liver. TCA was produced from TRI about 2.5 times greater than was total-TCE. CH was metabolized to TCE and TCA immediately. TCA was also a dominant metabolite of CH over total-TCE. TCE(free type) was speedily conjugated by the liver. A portion of TCE was converted to TCA. Less than 10% of these metabolites produced by the liver were excreted into the bile. Most of them appeared in the perfusate.     

Trichloroacetic acid in urine as biological exposure equivalent for low exposure concentrations of trichloroethene

A urinary trichloroacetic acid (TCA) concentration of 100 mg/l at the end of the last work shift (8 h/day, 5 days/week) of the week has been established in workers as exposure equivalent for the carcinogenic substance trichloroethene (EKA for TRI) at an exposure concentration of 50 ppm TRI. Due to the continuous reduction of atmospheric TRI concentrations during the last years, the quantitative relation given by the EKA for TRI is revised for exposures to low TRI concentrations. A physiological two-compartment model is presented by which the urinary TCA concentrations are calculated that result from inhaled TRI in humans. The model contains one compartment for trichloroethanol (TCE) and one for TCA. Inhaled TRI is metabolized to TCA and to TCE. The latter is in part further oxidized to TCA. Urinary elimination of TCA is modeled to obey first order kinetics. All required model parameters were taken form the literature. In order to evaluate the model performance on the urinary TCA excretion at low exposure concentrations, predicted urinary TCA concentrations were compared with data obtained in two volunteer studies and in one field study. The model was evaluated at exposure concentrations as low as 12.5 ppm TRI. It is demonstrated that the correlation described by the hitherto used EKA for TRI is also valid at low TRI concentrations. For TRI exposure concentrations of 0.6 and 6 ppm, the resulting urinary TCA concentrations at the end of the last work shift of a week are predicted to be 1.2 and 12 mg/l, respectively.     

The Extent of Dichloroacetate Formation from Trichloroethylene, Chloral Hydrate, Trichloroacetate, and Trichloroethanol in B6C3F1 Mice

Conflicting data have been published related to the formationof dichloroacetate (DCA) from trichloroethylene (TRI), chloralhydrate (CH), or trichloroacetic acid (TCA) in B6C3F1 mice.TCA is usually indicated as the primary metabolic precursorto DCA. Model simulations based on the known pharmacokineticsof TCA and DCA predicted blood concentrations of DCA that were10- to 100-fold lower than previously published reports. BecauseDCA has also been shown to form as an artifact during sampleprocessing, we reevaluated the source of the reported DCA, i.e.,whether it was metabolically derived or formed as an artifact.Male B6C3F1 mice were dosed with TRI, CH, trichloroethanol (TCE),or TCA and metabolic profiles of each were determined. DCA wasnot detected in any of these samples above the assay LOQ of1.9 µM of whole blood. In order to slow the clearanceof DCA, mice were pretreated for 2 weeks with 2 g/liter of DCAin their drinking water. Even under this pretreatment condition,no DCA was detected from a 100 mg/kg iv dose of TCA. Althoughthere is significant uncertainty in the amount of DCA that couldbe generated from TRI or its metabolites, our experimental dataand pharmacokinetic model simulations suggest that DCA is likelyformed as a short-lived intermediate metabolite. However, itsrapid elimination relative to its formation from TCA preventsthe accumulation of measurable amounts of DCA in the blood.     

Metabolism of chloral hydrate in the anoxic perfused liver

The metabolism of chloral hydrate (CH) under anoxic conditions was investigated in the non-recirculating, hemoglobin-free liver perfusion system. CH uptake in the anoxic liver decreased to about 80% of that in the oxygen-supplied liver. The reduction of CH to trichloroethanol (TCE) increased and the oxidation of CH to trichloroacetic acid (TCA) decreased. The TCE/TCA ratio increased; however, the total trichloro compounds, that is TCE and TCA, were not significantly altered by anoxia. Though approximate 14% of the CH infused into the oxygen-supplied liver was changed to substances other than TCE or TCA, the unknown part was a very small portion in the anoxic liver. The decrease in CH uptake, by the anoxic liver, is thought to be equivalent to the decrease of the unknown metabolites. The TCE/TCA ratio under anoxia was also altered by pyruvate or lactate infusion.     

Acute,nonfatal intoxication with trichloroethylene

Nonfatal acute inhalation of trichloroethylene (TRI) at work was described. The subject, male, 54 years old, was drawn unconscious by a metal-degreasing machine and immediately sheltered in intensive care unit. Other than basic life support and common laboratory indices, blood and urine were collected to measure dose and kidney effect parameters such as TRI in blood and urine, trichloroethanol (TCE) and trichloroacetic acid (TCA) in urine, and total urinary proteins (TUP), urinary glutamine synthetase (GS) and urinary N-acetyl-beta-D-glucosaminidase (NAG). Two hours after accident, TRI in blood was 9 mg/l, but after 38 h it was below 1 mg/l. TCE and TCA have a peak 11 and 62 h after poisoning, respectively. Acute renal involvement was revealed by a peak of urinary proteins and enzymes 7 h after exposure with a second peak 74 h after. Seven day after hospitalisation the patient was dismissed with complete recovery. This nonfatal intoxication with TRI shows that the exposure was approximately 150 ppm, three times the ACGIH TLV (50 ppm) and that kidney was the only organ affected. Urinary enzymes, in particular GS, are good indices to monitor transient effects of TRI on the kidney.     

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)     

Ethanol-induced enhancement of trichloroethylene metabolism and hepatotoxicity: difference from the effect of phenobarbital

Male Wistar rats pretreated with ethanol (2.0 g in 80 ml liquid diet/day for 3 weeks) or phenobarbital (PB, 80 mg/kg/day ip for 4 days) were exposed by inhalation to 500, 1000, 2000, 4000, or 8000 ppm trichloroethylene (TRI) for 2 or 8 hr, and the blood concentration of TRI and the urinary concentration of TRI metabolites (trichloroethanol (TCE) and trichloroacetic acid (TCA] were determined at various times. Plasma glutamic-pyruvic transaminase (GPT) activity was measured 22 hr after the end of exposure as an indicator of hepatic damage. Both ethanol and PB enhanced TRI metabolism as evidenced by accelerated disappearance of TRI from the blood and increased excretion of total trichloro compounds (TCE + TCA) in the urine. However, the effects of ethanol and PB were different from each other: ethanol markedly enhanced the metabolism particularly at TRI concentration of 2000 ppm or lower, whereas PB enhanced it only at 4000 ppm or higher. This difference was also reflected in the effect of TRI on liver: ethanol potentiated TRI hepatotoxicity more markedly than did PB when TRI concentration remained 2000 ppm or lower, whereas PB potentiated the toxicity more markedly than ethanol when the concentration was 4000 ppm or higher. It is noteworthy that ethanol potentiated TRI hepatotoxicity at a TRI concentration as low as 500 ppm. The severity of hepatic damage expressed by plasma GPT activity essentially paralleled the urinary excretion rate of total trichloro compounds during and 4 hr after exposure (r = 0.87 to 0.93). Compared between the contribution of concentration and duration of exposure to the toxicity, a higher concentration of TRI tended to cause more severe liver damage to PB-treated rats than did a prolonged period of exposure, whereas the toxicity in ethanol-treated rats was generally more marked in rats exposed to TRI for a longer period than in rats exposed to a higher concentration.     

EFFECT OF ETHANOL ON THE METABOLISM OF TRICHLOROETHYLENE IN THE PERFUSED RAT LIVER

The effect of 2 m M ethanol, a concentration indicative of daily alcohol consumption, was investigated on trichloroethylene (TRI) metabolism in perfused Wistar rat liver. The study consisted of two parts: The first part studied TRI administration with or without ethanol. In the second study chloral hydrate (CH), an intermediate in TRI metabolism, was administered in the absence or presence of ethanol to phenobarbital (PB) treated or non-PB-treated rats. The concentrations of the metabolites, total trichloroethanol (TCE), and trichloroacetic acid (TCA) were measured by gas chromatography and intracellular reduced pyridine nucleotides by surface fluorometry. In the first study, ethanol infusion significantly increased the TCE/TCA ratio, TCE production rate, and percentage of reduced pyridine nucleotides, and decreased TCA production rate without an associated change in the sum of TCE and TCA formation rates. In the second study, ethanol infusion in the absence or presence of PB produced similar significant increases in the TCE/TCA ratio, TCE production rate, and percentage of reduced pyridine nucleotides, accompanied by a decrease in TCA formation. The observed shift in TRI metabolism in the presence of ethanol, from oxidation to TCA to reduction to TCE, suggests that alcohol exerts alterations in hepatic intracellular oxidation-reduction (redox) states.     

Effect of enterohepatic circulation on the pharmacokinetics of chloral hydrate and its metabolites in F344 rats.

Chloral hydrate (CH) is a commonly found disinfection by-product in water purification, a metabolite of trichloroethylene, and a sedative/hypnotic drug. CH and two of its reported metabolites, trichloroacetic acid (TCA) and dichloroacetic acid (DCA), are hepatocarcinogenic in mice. Another metabolite of CH, trichloroethanol (TCE), is also metabolized into TCA, and the enterohepatic circulation (EHC) of TCE maintains a pool of metabolite for the eventual production of TCA. To gain insight on the effects of EHC on the kinetics of CH and on the formation of TCA and DCA, dual cannulated F344 rats were infused with 12, 48, or 192 mg/kg of CH and the blood, bile, urine, and feces were collected over a 48-h period. CH was cleared rapidly (>3000 ml/h/kg) and displayed biphasic elimination kinetics, with the first phase being elimination of the dose and the second phase exhibiting formation rate-limited kinetics relative to its TCE metabolite. The effects of EHC on metabolite kinetics were only significant at the highest dose, resulting in a 44% and 17% decrease in the area under the curve (AUC) of TCA and TCE, respectively. The renal clearance of CH, free TCE (f-TCE), and TCA of 2, 2.7, and 38 ml/h/kg, respectively, indicates an efficient reabsorption mechanism for all of these small chlorinated compounds. DCA was detected at only trace levels (<2 microM) as a metabolite of CH, TCA, or TCE.     

Renal toxicity after chronic inhalation exposure of rats to trichloroethylene

Male Long-Evans rats were exposed to 0 (controls) or 500 ppm trichloroethylene (TRI) for 6 months, 6 h daily, and 5 days a week. The TRI metabolites trichloroethanol (TCE) in blood and trichloroacetic acid (TCA) in urine were measured. Specific parameters related to the renal damage were determined in urine [biomarker for glomerular damage: high molecular weight proteins (HMW), albumin (ALB); for proximal tubular damage: N-acetyl-beta-D-glucosaminidase (NAG), low-molecular-weight-proteins (LMW)]. Significantly increased concentrations of NAG and LMW in urine of exposed rats were detected. No DNA-strand breaks in kidney cells could be detected using the comet assay, and histological examinations were performed. Histological alterations were observed in glomeruli and tubuli of exposed rats. The release of biomarkers for nephrotoxicity suggested alterations preferably in the proximal tubules of the exposed rats.     

Biotransformation of trichloroethylene in collagen gel sandwich cultures of rat hepatocytes

The collagen gel sandwich culture of hepatocytes has been proposed as one of the most suitable culture models available for biotransformation studies of xenobiotics. It is a complex model which imitates the cascade of enzymatic events of in vivo biotransformation and allows investigation of biological endpoints under realistic conditions. The biotransformation of trichloroethylene (TRI) has been studied in this model using rat hepatocytes. Headspace gas chromatographic measurements revealed that hepatocytes, cultured for 4 days in this in vitro system, metabolised TRI into the major oxidative metabolites trichloroacetic acid (TCA) and trichloroethanol (TCE). Cultured hepatocytes were exposed either to TRI, or to TCA and TCE. Endpoints studied were albumin secretion and the cytochrome P450 (CYP)-dependent enzymatic activities ethoxyresorufin O-deethylase (EROD), pentoxyresorufin O-depentylase (PROD) and N-nitrosodimethylamine demethylase (NDMA). The results show that both the parent compound and its metabolites exert specific effects on different CYP-dependent mono-oxygenase activities, as seen in vivo. It is suggested that collagen gel sandwich cultures represent a useful in vitro model for the investigation of metabolism-linked toxicity studies.     

Determination of chloral hydrate metabolites in human plasma by gas chromatography-mass spectrometry

Chloral hydrate (CH) is a widely used sedative. Its pharmacological and toxicological effects are directly related to its metabolism. Prior investigations of CH metabolism have been limited by the lack of analytical techniques sufficiently sensitive to identify and quantify metabolites of CH in biological fluids. In this study a gas chromatography mass spectrometry (GC/MS) method was developed and validated for determining CH and its metabolites, monochloroacetate (MCA), dichloroacetate (DCA), trichloroacetate (TCA) and total trichloroethanol (free and glucuronidated form, TCE and TCE-Glu) in human plasma. Of these, DCA and MCA are newly identified metabolites in humans. The drug, its plasma metabolites and an internal standard, 4-chlorobutyric acid (CBA), were derivatized to their methyl esters by reacting with 12% boron trifluoride-methanol complex (12% BF3-MeOH). The reaction mixture was extracted with methylene chloride and analyzed by GC/MS, using a selected ion monitoring (SIM) mode. The quantitation limits of MCA, DCA, TCA, and TCE were between 0.12 and 7.83 microM. The coefficients of variation were between 0.58 and 14.58% and the bias values ranged between -10.03 and 14.37%. The coefficients of linear regression were between 0.9970 and 0.9996.     

Studies of acute nephrotoxic potential of trichloroethylene in Fischer 344 rats

Group of male Fischer 344 rats, after pretreatment with phenobarbital (80 mg/kg, ip, 3 d), were treated ip in corn oil with 0, 5.5, 11.0, and 22.0 mmol trichloroethylene (TRI) per kg body weight. Urines were collected 24 h after the treatment and the animals were then sacrificed. The nephrotoxicity of TRI was then studied by measuring certain biochemical parameters characteristic of renal injury and its in vivo metabolism by quantitating the TRI principal urinary metabolites. Treatment of rats with TRI up to 11 mmol/kg did not influence any of the measured biochemical parameters of nephrotoxicity. On the other hand, significant increases in the urinary level of N-acetyl-beta-glucose-D-aminidase (NAG) and glucose as well as serum urea nitrogen were observed at 24 h only at the highest dose level (22 mmol/kg) or TRI. Urinary excretions of both trichloroethanol and trichloroacetic acid reached an apparent saturation at the highest dose level of TRI. In inhalation studies, urinary levels of gamma-glutamyltranspeptidase, NAG, glucose, proteins, and serum urea nitrogen were significantly increased at 24 h when rats were exposed to either 1000 or 2000 ppm TRI for 6 h. The capacity of renal cortical slices to accumulate p-aminohippurate was significantly reduced 24 h after the exposure to 22 mmol TRI/kg (ip), or to 1000 or 2000 ppm TRI. These results have demonstrated that TRI exerts its acute nephrotoxic potential at a very high dose level and produces nephrotoxic insult at the proximal tubular and possibly glomerular regions of the rat kidney, whether exposed by inhalation or by an ip route. These data further indicate an involvement of a capacity-limited metabolism in the expression of acute nephrotoxicity due to TRI in Fischer 344 rats.     

Biotransformation of trichloroethene: dose-dependent excretion of 2,2,2-trichloro-metabolites and mercapturic acids in rats and humans after inhalation

 Chronic bioassays with trichloroethene (TRI) demonstrated carcinogenicity in mice (hepatocellular carcinomas) and rats (renal tubular cell adenomas and carcinomas). The chronic toxicity and carcinogenicity is due to bioactivation reactions. TRI is metabolized by cytochrome P450 and by conjugation with glutathione. Glutathione conjugation results in S-(dichlorovinyl) glutathione (DCVG) and is presumed to be the initial biotransformation step resulting in the formation of nephrotoxic metabolites. Enzymes of the mercapturic acid pathway cleave DCVG to the corresponding cysteine S-conjugate, which is, after translocation to the kidney, cleaved by renal cysteine S-conjugate β-lyase to the electrophile chlorothioketene. After N-acetylation, cysteine S-conjugates are also excreted as mercapturic acids in urine. The object of this study was the dose-dependent quantification of the two isomers of N-acetyl-S-(dichlorovinyl)-L-cysteine, trichloroethanol and trichloroacetic acid, as markers for the glutathione- and cytochrome P450-mediated metabolism, respectively, in the urine of humans and rats after exposure to TRI. Three male volunteers and four rats were exposed to 40, 80 and 160 ppm TRI for 6 h. A dose-dependent increase in the excretion of trichloroacetic acid, trichloroethanol and N-acetyl-S-(dichlorovinyl)-L-cysteine after exposure to TRI was found both in humans and rats. Amounts of 3100 μmol trichloroacetic acid+trichloroethanol and 0.45 μmol mercapturic acids were excreted in urine of humans over 48 h after exposure to 160 ppm TRI. The ratio of trichloroacetic acid+trichloroethanol/mercapturic acid excretion was comparable in rats and humans. A slow rate of elimination with urine of N-acetyl-S-(dichlorovinyl)-L-cysteine was observed both in humans and in rats. However, the ratio of the two isomers of N-acetyl-S-(dichlorovinyl)-L-cysteine was different in man and rat. The results confirm the finding of the urinary excretion of mercapturic acids in humans after TRI exposure and suggest the formation of reactive intermediates in the metabolism of TRI after bioactivation by glutathione also in humans. Received: 22 June 1995 / Accepted: 5 October 1995