Metabolism of Chloroform—I. The Metabolism of [14C] Chloroform by Different Species

The Importance of Pharmacokinetic Principles in Characterizing Carcinogenic Thresholds for Naturally Occurring and Synthetic Chemicals

Many chemicals, both man-made and naturally occuring, can elicit a carcinogenic response either as a mutagenic response in vitro or as a frank tumor in vivo. In both instances emphasis has been placed historically on the dose which elicits the response. When the carcinogenic response is related to the pharmacokinetics of the carcinogen taking into account the dose, the frequency of dosing, andthe relative half lives of the carcinogen, its reactive metabolites and any modified macromolecules, a narrow plateau level of modified macro-molecules is found in specific responsive tissues or species of animals. This plateau threshold concept for carcinogen as defined by the pharmacokinetics, has been evaluated for both chloroform and aflatoxin. The carcinogenic risk associated with plateaus can be evaluated experimentally, and perhaps can be evaluated today with the available data at hand.

Problems of drinking water treatment along Ismailia Canal Province, Egypt

The present drinking water purification system in Egypt uses surface water as a raw water supply without a preliminary filtration process. On the other hand, chlorine gas is added as a disinfectant agent in two steps, pre- and post-chlorination. Due to these reasons most of water treatment plants suffer low filtering effectiveness and produce the trihalomethane (THM) species as a chlorination by-product. The Ismailia Canal represents the most distal downstream of the main Nile River. Thus its water contains all the proceeded pollutants discharged into the Nile. In addition, the downstream reaches of the canal act as an agricultural drain during the closing period of the High Dam gates in January and February every year. Moreover, the wide industrial zone along the upstream course of the canal enriches the canal water with high concentrations of heavy metals. The obtained results indicate that the canal gains up to 24.06x10(6) m3 of water from the surrounding shallow aquifer during the closing period of the High Dam gates, while during the rest of the year, the canal acts as an influent stream losing about 99.6x10(6) m3 of its water budget. The reduction of total organic carbon (TOC) and suspended particulate matters (SPMs) should be one of the central goals of any treatment plan to avoid the disinfectants by-products. The combination of sedimentation basins, gravel pre-filtration and slow sand filtration, and underground passage with microbiological oxidation-reduction and adsorption criteria showed good removal of parasites and bacteria and complete elimination of TOC, SPM and heavy metals. Moreover, it reduces the use of disinfectants chemicals and lowers the treatment costs. However, this purification system under the arid climate prevailing in Egypt should be tested and modified prior to application.

Characterization of the metabolic interaction between trihalomethanes and chloroacetic acids using rat liver microsomes

The aim of this study was to investigate the in vitro metabolism of trihalomethanes (THMs) in the presence of trichloroacetic acid (TCA), dichloracetic acid (DCA), monochloroacetic acid (MCA), and 4-methylpyrazole (4-MP) using liver microsomes from male Sprague-Dawley rats. Using the vial equilibration technique, initial experiments were carried out with starting concentrations of approximately 40 ppm THMs and 12-22 mM chloroacetic acids. The results indicated a mutual metabolic inhibition between THMs present as binary or quaternary mixtures. Although DCA and MCA had no influence on THMs, TCA produced a marked inhibition of the metabolism of all THMs: chloroform (CHCl3) (55%), bromodichloromethane (BDCM) (34%), dibromochloromethane (DBCM) (30%), and bromoform (TBM) (23%). The presence of 4-MP also reduced THM metabolism, the importance of which decreased in the following order: CHCl3 > BDCM > DBCM = TBM. In further vial equilibration experiments, using 9-140 ppm as starting concentrations of THMs, enzyme kinetic parameters (i.e., Michaelis constant, K(m), and maximum velocity, V(max)) were determined both in the absence and in the presence of TCA (12.2 mM). Results are consistent with a competitive inhibition between TCA and CHCl3, whereas the metabolic inhibition of BDCM and TMB by TCA was non-competitive. As for DBCM, results suggest a more complex pattern of inhibition. These results suggest that CYP2E1 is involved in the metabolism of THMs as well as in the metabolic interaction between THMs and TCA.

Chloroform: exposure estimation, hazard characterization, and exposure-response analysis

Chloroform has been assessed as a Priority Substance under the Canadian Environmental Protection Act. The general population in Canada is exposed to chloroform principally through inhalation of indoor air, particularly during showering, and through ingestion of tap water. Data on concentrations of chloroform in various media were sufficient to serve as the basis for development of deterministic and probabilistic estimates of exposure for the general population in Canada. On the basis of data acquired principally in studies in experimental animals, chloroform causes hepatic and renal tumors in mice and renal tumors in rats. The weight of evidence indicates that chloroform is likely carcinogenic only at concentrations that induce the obligatory precursor lesions of cytotoxicity and proliferative regenerative response. Since this cytotoxicity is primarily related to rates of formation of reactive, oxidative metabolites, dose response has been characterized in the context of rates of formation of reactive metabolites in the target tissue. Results presented here are from a "hybrid" physiologically based pharmacokinetic (PBPK) animal model that was revised to permit its extension to humans. The relevant measure of exposure response, namely, the mean rate of metabolism in humans associated with a 5% increase in tumor risk (TC05), was estimated on the basis of this PBPK model and compared with tissue dose measures resulting from 24-h multimedia exposure scenarios for Canadians based on midpoint and 95th percentiles for concentrations in outdoor air, indoor air, air in the shower compartment, air in the bathroom after showering, tap water, and food. Nonneoplastic effects observed most consistently at lowest concentrations or doses following repeated exposures of rats and mice to chloroform are cytotoxicity and regenerative proliferation. As for cancer, target organs are the liver and kidney. In addition, chloroform has induced nasal lesions in rats and mice exposed by both inhalation and ingestion at lowest concentrations or doses. The mean rate of metabolism associated with a 5% increase in fatty cysts estimated on the basis of the PBPK model was compared with tissue dose measures resulting from the scenarios already described, and lowest concentrations reported to induce cellular proliferation in the nasal cavities of rats and mice were compared directly with midpoint and 95th percentile estimates of concentrations of chloroform in indoor air in Canada. The degree of confidence in the underlying database and uncertainties in estimates of exposure and in characterization of hazard and dose response are delineated.

An in vitro investigation of the reductive metabolism of chloroform

In hypoxic (1% pO2) and anoxic (0% pO2) incubations of CHCl3 with rat liver microsomes from PB-induced animals, no evidence of formation of monochloromethyl carbene could be found. Dichloromethane was detected as a volatile metabolite of CHCl3 in incubations with rat liver microsomes from PB-induced animals, under different oxygenation conditions (from 0% to 20% pO2). With uninduced microsomes, significant levels of dichloromethane were formed only in hypoxic (1% pO2) or anoxic incubations. The amount of dichloromethane measured was 2-6 times lower than the levels of adducts to the fatty acyl chains (FC) of microsomal phospholipid. The very low rate of dichloromethane formation suggests that the assay of expired dichloromethane is not suitable to detect the reductive metabolism of CHCl3 in vivo.

The regioselective binding of CHCl3 reactive intermediates to microsomal phospholipids

Microsomal phospholipids (PL) are a good target for the reactive intermediates produced by either the oxidative or the reductive biotransformation of CHCl3 (Testai et al. (1990), Toxicol. Appl. Pharmacol. 104, 496-503). In order to preliminarily characterize the different PL with CHCl3 reactive intermediates, two common methods of PL breakdown have been exploited: the acid-catalyzed transmethylation and the enzymatic hydrolysis with phospholipase C. The results indicated that radioactivity derived from the adducts of PL with the oxidation metabolite, phosgene, partitioned preferentially in the aqueous phase (the ratio of aqueous to organic phase radioactivity contents was about 10); the opposite occurred (ratio about 0.1) when the PL adducts were produced by the reductive process metabolites (dichloromethyl radicals). Therefore, the two methods of PL adduct breakdown can be used to detect and quantitate selectively the two reactive intermediates of CHCl3 biotransformation. The use of phospholipase C, which specifically cleaves the bond between the glyceryl-oxygen and the phosphor atom of PL also gave some structural information. Indeed, the radioactivity partitioning in the aqueous phase after enzymatic hydrolysis of CHCl3 oxidation-associated PL adducts, indicated the selective covalent binding of phosgene residues with the PL polar heads. The clear-cut different partition of radioactivity observed after hydrolysis of PL adducts with CHCl3 reduction intermediates, analogously indicated that dichloromethyl radicals were selectively bound to the PL fatty acyl chains. Using this method we could confirm that in in vitro experimental conditions resembling the physiological status of the liver, both metabolic pathways were concurrently active in hepatic microsomes of B6C3F1 mice. Extents of reactive metabolites similar to those found in B6C3F1 mouse liver microsomes, could be measured in Sprague-Dawley rat liver microsomes only after pretreatment of the animals with PB and incubation with higher CHCl3 concentrations. The toxicological implications of these findings are discussed.

Bioactivation of chloroform in hepatic microsomes from rodent strains susceptible or resistant to CHCl3 carcinogenicity

The dependence of adduct formation on oxygen concentration and glutathione (GSH) presence was used to characterize the bioactivation of chloroform in hepatic microsomes of Sprague-Dawley and Osborne-Mendel rats and B6C3F1 and C57Bl/6J mice. Both oxidative and reductive pathways were present in all the animals tested. Oxidative activation, very sensitive to oxygen withdrawal, was the major pathway responsible for the covalent binding to microsomal proteins and lipids at 0.1 mM CHCl3. The relative contribution of either pathway to the covalent binding to microsomal lipids at 5 mM CHCl3 was dependent on the oxygen concentration. At 1% pO2, i.e., in the range of the hepatic physiological oxygenation level, B6C3F1 mouse hepatic microsomes showed an oxidative activation distinctly higher than that of hepatic microsomes of other rodents; on the other hand, reductive activation was present only in B6C3F1 mouse and Osborne-Mendel rat liver microsomes. The reductive intermediates were the only contributors to the covalent binding of CHCl3 equivalents to lipids in the presence of GSH; indeed the reactive intermediates produced by the oxidative pathway were fully scavenged by this compound. These results are discussed with respect to the species specificity of CHCl3 hepatocarcinogenesis.

What is an appropriate measure of exposure when testing drugs for carcinogenicity in rodents?

This discussion paper argues that in the carcinogenicity testing of drugs, biological measures of drug exposure may often be more relevant than the classical pharmacokinetic approaches applicable to reversible pharmacodynamic phenomena. Chemicals that produce tumors in rodents may do so (either directly or after bioactivation) by mechanisms involving inter alia a mutagenic, cytotoxic, or hormone-like effect. Such mechanisms may involve the formation of reactive metabolites of fleeting existence, and these are subject to the principles of irreversible pharmacokinetics. Examples are given of genotoxic and nongenotoxic substances for which the species and target site for tumor formation correlates not with plasma concentration, but with the amount metabolized and/or the rate of metabolism. Other compounds produce tumors in rodents, often in only one sex or species, in association with an exaggerated pharmacodynamic effect, with an increase in liver weight (by one of a diversity of mechanisms) or with hormonal or hormonal-like effects. In such cases the determinant of tumor formation is the degree of disturbance of homeostasis, not the plasma concentration of the parent substance. For drugs, the disturbance in question may have no relevance to the clinical use of the drug. Plasma concentrations of the parent substance are useful in assessing the proportion of an oral dose that is absorbed and the linearity of kinetics over the full dose range and for exploring the differences between dietary and gavage administration. They do not usually, however, provide information of direct relevance to assessment of "exposure" for the purposes of carcinogenic risk assessment.

Multiple activation of chloroform in hepatic microsomes from uninduced B6C3F1 mice

The covalent binding of 14C-label to proteins and lipids was measured after incubation of hepatic microsomes from B6C3F1 mice with different concentrations of [14C]chloroform and oxygen. The effect of reduced glutathione on the covalent binding curves was also investigated. The results indicated that chloroform is activated through three processes: the first, oxidative, shows high affinity for chloroform and low affinity for oxygen; the second, also requiring oxygen, shows low affinity for chloroform and high affinity for oxygen; and the third, showing low affinity for chloroform, is inhibited by oxygen. The covalent binding associated with the oxidative processes is very effectively prevented by GSH. The reactive metabolites formed by the O2-inhibited mechanism are not efficiently scavenged by GSH and presumably are radicals that are produced reductively. The major conclusions which can be drawn from these results are: (i) The anoxic bioactivation of chloroform can cause high levels of covalent binding. This is at variance with the current opinion that the chloroform anoxic bioactivation occurs to a negligible extent. (ii) The damages produced under the usual in vitro experimental conditions by the oxidative biotransformation of chloroform, may be strongly limited by the physiological conditions of the liver. The features of the three processes described may help in understanding the mechanism of toxicity of chloroform.

Development of a physiologically based pharmacokinetic model for chloroform

A physiologically based pharmacokinetic model describing the disposition of chloroform in mice, rats, and humans was developed. This model was designed to facilitate extrapolations from high doses, such as those used in chronic rodent studies, to low doses that humans may be exposed to in the workplace or the environment. Kinetic constants for mice and rats were derived from in vivo experiments. Enzymatic studies conducted with samples of rodent and human tissues provided a rational basis for estimating human in vivo metabolic rate constants. Incorporation of physiological descriptions of the processes of absorption, distribution, metabolism, and excretion allowed extrapolation between different routes of exposure as well. The model was validated by comparing model predictions with experimental data gathered in mice, rats, and humans after inhalation, oral, or intraperitoneal administration of chloroform. Consistent with previous reports, the metabolic activation of chloroform to toxic intermediates was shown to occur most rapidly in the mouse, less rapidly in the rat, and most slowly in humans. Estimates of the "delivered dose" of chloroform metabolites to internal organs sensitive to chloroform toxicity were calculated. This model may be used to develop refined dose estimates for human populations exposed to low levels of chloroform in the environment.

Excretion and tissue disposition of dichloroacetonitrile in rats and mice

The excretion and tissue distribution of [1-14C]dichloroacetonitrile and [2-14C]dichloroacetonitrile were studied in male Fischer 344 rats and male B6C3F1 mice. Three dose levels of dichloroacetonitrile (DCAN) (0.2, 2, or 15 mg/kg) were administered to rats and two dose levels of DCAN (2 or 15 mg/kg) to mice. Daily excreta including exhaled volatiles and radiolabeled carbon dioxide (14CO2) were analyzed for radiolabeled carbon (14C) until greater than 70% of the radioactivity was excreted. At that time the animals were sacrificed and tissues were collected. Tissues and excreta were analyzed for 14C by combustion and liquid scintillation counting. Rats administered [1-14C]DCAN excreted 62 to 73% of the 14C in 6 days, with 42 to 45% in urine, 14 to 20% in feces, and 3 to 8% as CO2. Rats administered [2-14C]DCAN excreted 82 to 86% of the 14C in 48 hr, with 35 to 40% in urine, 33 to 34% as CO2, and 10 to 13% in feces. Mice administered [1-14C]DCAN excreted 83 to 85% of the 14C in 24 hr, with 64 to 70% in urine, 9 to 13% in feces, and 5 to 6% as CO2. Mice administered [2-14C]DCAN excreted 84 to 88% of the 14C in 24 hr with 42 to 43% in urine, 8 to 11% in feces, and 31 to 37% as CO2. Liver tissues retained the most 14C in all studies except the study of [1-14C]DCAN in rats, where blood contained the most 14C. These results indicate that DCAN was absorbed rapidly after oral administration in water. The differences in the route of excretion of [1-14C]DCAN compared to [2-14C]DCAN indicated that the molecule was being cleaved in the body and metabolized by different mechanisms.

Chemically induced nephrotoxicity: role of metabolic activation

Renal xenobiotic metabolism can result in production of electrophiles or free radicals that may covalently bind macromolecules or initiate lipid peroxidation. The mechanisms of renal xenobiotic metabolism may vary in different anatomical regions. Kidney cortex contains a cytochrome P-450 system while medulla contains a prostaglandin endoperoxidase. Recently cysteine conjugated-lyase has been implicated in production of reactive intermediates. Metabolic activation may be amplified by accumulation of xenobiotics within renal cells due to tubular concentrating and/or secretory mechanisms. Additionally, renal xenobiotic detoxicification can occur by conjugation with glucuronide, sulfate or glutathione.

Volatile halocarbons in tap water as a problem in haemodialysis therapy

The concentrations of volatile halocarbons in the tap water of Turku and of Turku University Central Hospital are quite high and are reduced but not eliminated during water treatment at the hospital. Before haemodialysis is started, only trichloromethane is found in the blood of the patients. Two hours later dichlorobromomethane and dibromochloromethane could also be found. These substances are absorbed, and possibly accumulate, in the body or are metabolised and excreted because all their concentrations are lower at the end of dialysis therapy. Therefore, maximum levels for volatile halocarbons in drinking water should be sufficiently low to prevent these substances being detected in body fluids and special care should be taken with hospital water.

Application and results of whole-body autoradiography in distribution studies of organic solvents

With the growing concern for the health hazards of occupational exposure to toxic substances attention has been focused on the organic solvents, which are associated with both deleterious nervous system effects and specific tissue injuries. Relatively little is known about the distribution of organic solvents and their metabolites in the living organism. Knowledge of the specific tissue localizations and retention of solvents and solvent metabolites is of great value in revealing and understanding the sites and mechanisms of organic solvent toxicity. Whole-body autoradiography has been modified and applied to distribution studies of benzene, toluene, m-xylene, styrene, methylene chloride, chloroform, carbon tetrachloride, trichloroethylene and carbon disulfide. The high volatility of these substances has led to the development of cryo-techniques. Whole-body autoradiographic techniques applicable to the study of volatile substances are reviewed. The localizations of nonvolatile solvent metabolites and firmly bound metabolites have also been examined. The obtained results are discussed in relation to toxic effects and evaluated by comparison with other techniques used in distribution studies of organic solvents and their metabolites.

Isopropanol enhancement of carbon tetrachloride metabolism in vivo

We examined the effects of isopropanol (ISOP) pretreatment on the metabolism of 14CCl4 to 14CO2 and CHCl3 exhaled in the breath, to 14C metabolite excreted in 24 hr urine and feces from 0 to 24 hr, and to 14C metabolite bound to liver at 24 hr. Fasted male rats were given 0.1 or 2.0 mmoles 14CCl4/kg. ISOP pretreatment, which markedly enhanced the hepatotoxicity of CCl4, selectively enhanced the rate and total extent of 14CO2 and CHCl3 metabolite exhalation. The pathways of CCl4 metabolism leading to CO2 and CHCl3 metabolite formation may be more relevant to the hepatotoxicity of CCl4 than the pathways leading to urinary, fecal or covalently bound metabolites.

Toxicity of trihalomethanes: I. The acute and subacute toxicity of chloroform, bromodichloromethane, chlorodibromomethane and bromoform in rats

In an acute study, groups of 10 male and 10 female rats were given single oral doses of chloroform, bromodichloromethane (BDCM), chlorodibromomethane (CDBM), or bromoform and were observed for clinical symptoms for the following 14 days. Median lethal doses (LD 50) of the four trihalomethanes were found to be between 848 and 1388 mg/kg. Some groups which survived the treatment for 14 days showed reduced food intake, growth retardation and increased liver and kidney weight. Elevated serum cholesterol levels were observed in the surviving male rats treated with chloroform and CDBM, and in the females treated with chloroform. Decreased liver protein content occurred in male but not female rats fed chloroform and bromoform. In contrast, increase aniline hydroxylase activity was observed in female rats fed chloroform but not bromoform. Hematological values which were altered by the four trihalomethanes were hemoglobin, hematocrit, RBC, WBC, neutrophil and lymphocyte counts. Treatment-related histologic changes were observed in the liver and kidney of rats. These changes were qualitatively and quantitatively similar for the four trihalomethanes. These data indicate that trihalomethanes at large single oral doses can produce a wide range of toxic changes in the rat. In a subacute study, groups of 10 male rats were fed four trihalomethanes at 0, 5, 50 or 500 ppm in their drinking water for 28 days. The growth rate and food intake were not affected by treatment. A slight increase in relative kidney weight was observed in the groups fed 5 ppm chloroform; 500 ppm bromoform, 5 and 500 ppm BDCM. The animals fed the highest dose of chloroform showed decreased neutrophils. Serum biochemical parameters and hepatic microsomal enzyme activities were not altered by any of the four trihalomethanes. No histopathological changes were seen in the tissues examined.

A comparative study on the irreversible binding of labeled halothane trichlorofluoromethane, chloroform, and carbon tetrachloride to hepatic protein and lipids in vitro and in vivo

1) After intraperitoneal injection of labeled CCl4, CHCl3, and halothane in mice, 14C is preferentially bound to liver endoplasmic protein and lipid. A considerable activity is also associated with mitochondrial constituents. Maximal protein binding (nmol/mg): CCl4: 2.8 (0.5 hrs); CHCl3: 11.5 (6 hrs); halothane: 5 (6 hrs). Lipid binding: CCl4: 6.4 (5 min); CHCl3: 8 (4 hrs); halothane: 13.5 (2 hrs). The form of the binding curves in microsomal and mitochondrial protein and lipid differed with the individual haloalkanes. 2) The irreversible (covalent) binding of 14C from labeled haloalkanes in anaerobic suspensions of isolated rabbit liver microsomes and NADPH after 30 min was for protein (lipid) (nmol/mg): CCl4: 15 (58); CHCl3: 3.4 (3.2); halothane: 2.3 (10); trichlorofluoromethane: 6.5 (30). Anerobic incubation favored dehalogenation, but CHCl3 metabolism and irreversible binding requires oxygen. The greatest differences in the in vitro "covalent" binding rates were observed with CHCl3 in rat, mouse, and rabbit. 3) Altered microsomal cytochrome P-450 concentrations in newborn animals, or produced by pretreatment of rats with phenobarbital, 3-methylcholanthrene (MC), or CoCl2 effected similar, but not proportional changes in the rates of irreversible protein and lipid binding. Upon addition of CCl4 the difference of light absorption of reduced liver microsomes from MC-pretreated rats containing cytochrome P-448 appeared at 452 nm. The irreversible binding rate in these microsomes was also increased. The small accleration in irreversible binding in liver microsomes from rats pretreated with isopropanol is not proportional to the high increase of CCl4 toxicity. 4) Practically no binding to added, soluble albumin or RNA was observed in microsomal incubates. However, 14C is bound to the nicotine-adenine dinucleotides of the NADPH system. All haloalkanes produced a similar increase of NADPH oxidation in incubates of rabbit liver microsomes and NADPH.