Nephrotoxicity and hepatotoxicity of chloroform in mice: effect of deuterium substitution

THE USE OF DEUTERIUM ISOTOPE EFFECTS TO PROBE THE ACTIVE SITE PROPERTIES, MECHANISM OF CYTOCHROME P450-CATALYZED REACTIONS, AND MECHANISMS OF METABOLICALLY DEPENDENT TOXICITY

Critical elements from studies that have led to our current understanding of the factors that cause the observed primary deuterium isotope effect, (kH/kD)obs, of most enzymatically mediated reactions to be much smaller than the "true" or intrinsic primary deuterium isotope effect, kH/kD, for the reaction are presented. This new understanding has provided a unique and powerful tool for probing the catalytic and active site properties of enzymes, particularly the cytochromes P450 (P450). Examples are presented that illustrate how the technique has been used to determine kH/kD, and properties such as the catalytic nature of the reactive oxenoid intermediate, prochiral selectivity, the chemical and enzymatic mechanisms of cytochrome P450-catalyzed reactions, and the relative active site size of different P450 isoforms. Examples are also presented of how deuterium isotope effects have been used to probe mechanisms of the formation of reactive metabolites that can cause toxic effects.

Correlation of a specific mitochondrial phospholipid-phosgene adduct with chloroform acute toxicity

The dose and time dependence of formation of a specific adduct between mitochondrial phospholipid and phosgene have been determined in the liver of Sprague-Dawley (SD) rats as well as in the liver and kidney of B6C3F1 mice after dosing with chloroform. Rats were induced with phenobarbital or non-induced. Determination of tissue glutathione (GSH) and of serum markers of hepatotoxicity and nephrotoxicity was also carried out. With dose-dependence experiments, a strong correlation between the formation of the specific phospholipid adduct, GSH depletion and organ toxicity could be evidenced in all the organs studied. With non-induced SD rats, no such effects could be induced up to a dose of 740 mg/kg. Time-course studies with B6C3F1 mice indicated that the specific adduct formation took place at very early times after chloroform dosing and was concurrent with GSH depletion. The adduct formed during even transient GSH depletion (residual level: 30% of control) and persisted after restoration of GSH levels. Following a chloroform dose at the hepatotoxicity threshold (150 mg/kg), the elimination of the adduct in the liver occurred within 24 h and correlated with the recovery of ALT, which was slightly increased (12 times) after treatment. Following a moderately nephrotoxic dose (60 mg/kg), the renal adduct persisted longer than 48 h, when a 100% increase in blood urea nitrogen and a 40% increase in serum creatinine indicated the onset of organ damage. The formation of the adduct in the liver mitochondria of B6C3F1 mice was associated with the decrease of phosphatidyl-ethanolamine (PE), in line with previous results in rat liver indicating that the adduct results from the reaction of phosgene with PE. The adduct levels implicated the reaction of phosgene with about 50% PE molecules in the liver mitochondrial membrane of phenobarbital-induced SD rats and of about 10% PE molecules of the inner mitochondrial membrane of the liver of B6C3F1 mice. The association of this adduct with the toxic effects of chloroform makes it a very good candidate as the primary critical alteration in the sequence of events leading to cell death caused by chloroform.

Comparative characterization of CHCl(3) metabolism and toxicokinetics in rodent strains differently susceptible to chloroform-induced carcinogenicity

A comparative kinetic study in B6C3F1 mice, Osborne-Mendel (OM) and Sprague-Dawley (SD) rats has been undertaken with the major aim to determine the extent of covalent binding of chloroform reactive metabolites produced in vivo through oxidative and/or reductive metabolism in the target organs of chloroform carcinogenicity. Some additional kinetic observations of chloroform biotransformation were also collected comparatively. Expiration of [14C]-CO(2) showed that chloroform metabolism went to saturation in all tested rodent strains. In the B6C3F1 mouse maximal rates of approximately 135 µmol [14C]-CO(2)/kg b.w./h were reached at a dose of approximately 150 mg/kg, while in the two rat strains saturation occurred at a dose of approximately 60 mg/kg, with a maximal rate of approximately 40 µmol [14C]-CO(2)/kg b.w./h. At doses of 150-180 mg/kg b.w., limited differences were found in the distribution and elimination of [14C]-chloroform in the liver and kidney. Species differences have been found in the kinetics of alkali-extractable radioactivity in the blood. The levels of adducts of electrophilic intermediates with the polar heads (PH) of phospholipids (PL) showed a limited variability accross the rodents tested and did not correlate with the species and organ susceptibility to chloroform carcinogenicity. The levels of adducts of radical intermediates with the fatty acyl chains (FC) of PL were much lower than the PH adducts in all the samples analyzed; at the carcinogenicity bioassay doses, statistically significant levels of hepatic FC adducts were present only in the B6C3F1 mouse, where chloroform is hepatocarcinogenic. The observations in the rat kidney were suggestive of the formation of electrophilic reactive metabolites, presumably different from phosgene and associated with an initial chloroform reduction.

In vitro quantitative determination of phospholipid adducts of chloroform intermediates in hepatic and renal microsomes from different rodent strains

We have comparatively studied in vitro the oxidative and reductive pathways of chloroform metabolism in hepatic and renal microsomes of rodent strains used for carcinogenicity testing (B6C3F1 mice, Osborne Mendel and Sprague Dawley rats). To this aim we exploited the regioselective binding of phosgene to phospholipid (PL) polar heads and of dichloromethyl radical to PL fatty acyl chains, using a method based on the chemical transmethylation of PL adducts, followed by phase partitioning of the resulting products (De Biasi et al., 1992). The analysis of results let us to conclude at first that a (14)C label partitioning by 89.2 (±6.5)% or 13.7 (±5.0)% in the aqueous phase is typical of the PL adduct with phosgene (PL-PHOS) or with dichloromethyl radical (PL-RAD), respectively. Metabolism of 0.1 mM CHCl(3) was mainly oxidative in all the samples, being hepatic microsomes more active than renal ones by about one order of magnitude and levels of CHCl(3)-derived PL adducts in B6C3F1 mouse liver microsomes higher than in rat samples. At 5 mM CHCl(3), total levels of PL adducts in renal microsomes reached levels almost similar to those found in liver microsomes. However, while B6C3F1 mouse kidney microsomes produced both reactive metabolites, similarly as the hepatic samples, Osborne Mendel rat kidney microsomes bioactivated CHCl(3) only reductiveiy, producing the radical. The relevance of this finding depends on the fact that phosgene is known to be the major cause of CHCl(3) toxicity, based on data with the rat liver and mouse liver and kidney, while nephrotoxicity in rats occurs with minimal production of COCl(2). Chloroform reductive bioactivation may therefore provide a reasonable explanation for the toxicity of chloroform to the rat kidney. The same finding may be of interest in elucidating the metabolic reasons of the chloroform-induced kidney tumors in Osborne Mendel rats.

Toxicity and cell proliferation in the liver, kidneys and nasal passages of female F-344 rats, induced by chloroform administered by gavage

Dose-response relationships were determined for the induction of cytolethality and regenerative cell proliferation in the established target organs (liver, kidneys, and nasal passages) of female F-344 rats given chloroform daily by gavage. Rats were administered chloroform dissolved in corn oil at doses of 0, 34, 100, 200 or 400 mg/kg/day for 4 consecutive days or for 5 days/wk for 3 wk. Bromodeoxyuridine (BrdU) was administered through an implanted osmotic pump 3.5 days prior to autopsy to label cells in S-phase. Cells in S-phase were visualized immunohistochemically in tissue sections and the labelling index (LI) calculated as the percentage of cells in S-phase. Mild degenerative centrilobular changes and dose-dependent increases in the hepatocyte LI were observed after administration of 100 mg or more chloroform/kg/day. Rats given 200 or 400 mg/kg/day for 4 days or 3 wk had degeneration and necrosis of the proximal tubules of the renal cortex. Regenerating epithelium lining proximal tubules was seen histologically and as an increase in LI. Dose-dependent increases in LI were observed in the kidneys at doses of 100 mg or more chloroform/kg/day at both 4 days and 3 wk. Two distinct treatment-induced responses were observed in specific regions of the olfactory mucosa lining the ethmoid region of the nose. A peripheral lesion was seen at all doses used and included new bone formation, periosteal hypercellularity and increased cell replication. A central lesion was seen at doses of 100 mg or more chloroform/kg/day and was characterized by degeneration of the olfactory epithelium and superficial Bowman's glands. These observations define the dose-response relationships for the liver, kidneys and nasal passages as target organs for chloroform administered by gavage in the female F-344 rat.

Induced regenerative cell proliferation in livers and kidneys of male F-344 rats given chloroform in corn oil by gavage or ad libitum in drinking water

These studies were designed to establish the dose response relationships for the induction of cytolethality and regenerative cell proliferation in the liver and kidneys of male F-344 rats given chloroform by gavage or in drinking water. Rats were administered oral doses of 0, 10, 34, 90 or 180 mg/kg/day chloroform dissolved in corn oil by gavage for 4 days or for 5 days/week for 3 weeks. A second group of rats was given chloroform ad libitum in the drinking water at concentrations of 0, 60, 200, 400, 900 or 1800 ppm for 4 days or 3 weeks. Bromodeoxyuridine (BrdU) was administered via an implanted osmotic pump 3.5 days prior to necropsy to label cells in S-phase. Cells having incorporated BrdU were visualized in tissue sections immunohistochemically and the labelling index (LI) evaluated as the percentage of S-phase cells. Rats treated with 90 or 180 mg/kg/day by gavage for 4 days had mild to moderate degeneration of renal proximal tubules and centrilobular hepatocytes. These alterations were absent or slight after 3 weeks of treatment. LI were increased in the kidney cortex only in the rats treated with 180 mg/kg/day for 4 days. A dose-dependent increase in LI was seen in rat liver after 4 days of treatment with 90 and 180 mg/kg/day by gavage, but the LI remained elevated after 3 weeks of treatment only at the 180 mg/kg/day dose. When chloroform was administered in the drinking water, no microscopic alterations were seen in the kidneys after 4 days of treatment. As a general observation, rats treated for 3 weeks with 200 ppm chloroform and greater had slightly increased numbers of focal areas of regenerating renal proximal tubular epithelium and cell proliferation than were noted in the controls, but no clear dose response relationship was evident. However, the overall renal LI was not increased at any dose or time point. Similarly, only mild hepatocyte vacuolation was observed in rats given 1800 ppm chloroform in the water for 3 weeks with no increase in the hepatic LI at any time point, even though the rats were consuming chloroform at a rate of 106 mg/kg/day at the 1800 ppm drinking water concentration. These data indicate more severe hepatic and renal toxicity when chloroform is administered by gavage than in the drinking water and a different pattern of regenerative proliferation in the kidney.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of chloroform nephrotoxicity. IV. Phenobarbital potentiation of in vitro chloroform metabolism and toxicity in rabbit kidneys

Metabolism of chloroform (CHCl3) by a cytochrome P-450-dependent process to a reactive metabolite may be required to elicit hepatic and renal toxicities. Specific inducers or inhibitors of cytochrome P-450 have been employed frequently as tools to demonstrate this relationship between metabolism and toxicity in the liver. The experiments reported herein were designed to identify the relationship between metabolism and toxicity of CHCl3 in the kidney of rabbits, a species in which renal cytochrome P-450 is induced by phenobarbital. Pretreatment with phenobarbital enhanced the toxic response of renal cortical slices to CHCl3 in vitro as indicated by decreased p-aminohippurate and tetraethylammonium accumulation. Phenobarbital pretreatment also potentiated in vitro 14CHCl3 metabolism to 14CO2 and covalently bound radioactivity in rabbit renal cortical slices and microsomes. Addition of L-cysteine significantly reduced covalent binding in renal microsomes from both phenobarbital-treated and control rabbits and was associated with the formation of the radioactive phosgene-cysteine conjugate 2-oxothiazolidine-4-carboxylic acid (OTZ). Formation of OTZ was enhanced in renal microsomes from phenobarbital-pretreated rabbits. Thus, this in vitro model supports the hypothesis that the kidney metabolizes CHCl3 to the nephrotoxic metabolite, phosgene.

Effect of polybrominated biphenyls, beta-naphthoflavone and phenobarbital on arylhydrocarbon hydroxylase activities and chloroform-induced nephrotoxicity and hepatotoxicity in male C57BL/6J and DBA/2J mice

Administration of chloroform (CHCl3) to male C57/6J (C57) and DBA/2J (DBA) mice produced dose-dependent hepatic and renal damage. Hepatic arylhydrocarbon hydroxylase (AHH) activity was higher in C57 than DBA mice; in kidney, AHH activity was higher in DBA than in C57 mice. CHCl3 caused the same degree of liver damage in both strains of mice; however, nephrotoxicity of CHCl3 was greater in DBA than in C57 mice. Pretreatment of C57 and DBA mice with phenobarbital (PB) markedly increased hepatic AHH activity and hepatotoxicity of CHCl3 in both strains but did not affect renal AHH or nephrotoxicity of CHCl3. Similarly, beta-naphthoflavone (BNF) enhanced AHH activity and CHCl3 hepatotoxicity in C57 mice, but had little effect on nephrotoxicity. BNF did not affect hepatic AHH nor CHCl3-induced hepatic injury in male DBA mice. Pretreatment with polybrominated biphenyl (PBB) enhanced AHH activity in liver and CHCl3 hepatotoxicity in both strains. After PBB, nephrotoxicity of CHCl3 and renal AHH activity were increased in C57 mice whereas PBB did not alter nephrotoxicity or renal AHH in DBA mice. These results suggest that CHCl3-nephrotoxicity is independent of hepatotoxicity.

Sex and strain differences in mouse kidney: Bowman's capsule morphology and susceptibility to chloroform

Chloroform (CHCl3) produces liver damage in male and female mice but renal injury only in male mice. Previous reports demonstrated that the parietal epithelium of Bowman's capsule in female mice is composed of squamous cells, whereas the parietal cells of Bowman's capsule from male mice consist of a mixture of squamous and cuboidal cells or purely cuboidal cells. These data suggested that the factor(s) associated with these histological differences may similarly be associated with factors that determine susceptibility to tubular damage from chloroform. The susceptibility of male mice to chloroform nephrotoxicity varies with animal strain. In the present study, administration of CHCl3 to male C57BL/6J (C57) and male and female DBA/2J (DBA) mice produced dose-dependent kidney tubular damage in male C57 and male DBA mice. However, CHCl3 nephrotoxicity was considerably greater in male DBA mice than in male C57 mice. Female DBA mice failed to develop renal damage following administration of CHCl3. Male DBA mice had a higher percentage of cuboidal parietal epithelial cells in Bowman's capsule whereas male C57 mice had a higher percentage of squamous cells in the parietal epithelium. The parietal epithelium in Bowman's capsule of female DBA mice was predominantly squamous. Since morphological differences between sex and strain of mice occurred in the capsular epithelium, and the differences in susceptibility to CHCl3 appear to be a tubular phenomenon, it is unlikely that there is a causal relationship between the structure of Bowman's capsule and chloroform-induced renal tubular necrosis; rather, these two independent phenomena are regulated by some common factor(s), possibly testosterone.

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.

Nephrotoxicity of chloroform: metabolism to phosgene by the mouse kidney

In this investigation, we have attempted to determine whether chloroform (CHCl3)-induced nephrotoxicity might be due to its metabolism to phosgene (COCl2) in the kidney. We have found that kidney homogenates from DBA/2J male mice in the presence of glutathione metabolize CHCl3 to 2-oxothiazolidine-4-carboxylic acid (OTZ). This product appears to be formed by the initial trapping of COCl2 by two molecules of GSH to form diglutathionyl dithiocarbonate (GSCOSG). Kidney gamma-glutamyl transpeptidase can rapidly metabolize GSCOSG to N-(2-oxothiazolidine-4-carbonyl)-glycine which is then hydrolyzed, possibly by cysteinyl glycinase to OTZ. The finding that deuterium-labeled chloroform (CDCl3) was less nephrotoxic and depleted less renal GSH than did CHCl3 suggests that the metabolism of CHCl3 to COCl2 may also occur in the kidney in vivo and lead to nephrotoxicity.

Mechanism of chloroform nephrotoxicity. II. In vitro evidence for renal metabolism of chloroform in mice

Preincubation of renal cortical slices with chloroform (CHCl3) from male, but not female, mice resulted in a subsequent decrease of the ability of the slices to accumulate the organic ions, p-aminohippurate (PAH) and tetraethylammonium (TEA). These sex-related differences, the time required for manifestation of this effect (60 to 90 min), and the concentration dependency (0 to 50 mumol, 0 to 4 microliter CHCl3) were similar to in vivo observations on CHCl3 nephrotoxicity in mice. Furthermore, an equimolar concentration of deuterated CHCl3 (CDCl3) in vitro was less effective than CHCl3 in decreasing PAH and TEA accumulation in male renal cortical slices. The effects of CHCl3 on PAH and TEA accumulation could be diminished or blocked by preincubation with CHCl3 in the presence of carbon monoxide or at 0 degrees C, respectively. The nephrotoxicity of CHCl3 in vitro was increased in renal cortical slices from male mice pretreated with diethyl maleate. Thus, this in vitro model with mouse renal cortical slices and the sex-related differences in CHCl3 nephrotoxicity suggests that the kidney may metabolize CHCl3 in situ to a nephrotoxic metabolite.