Chemically induced nephrotoxicity: role of metabolic activation

Role of electrostatic potential in the in silico prediction of molecular bioactivation and mutagenesis

Electrostatic potential (ESP) is a useful physicochemical property of a molecule that provides insights into inter- and intramolecular associations, as well as prediction of likely sites of electrophilic and nucleophilic metabolic attack. Knowledge of sites of metabolic attack is of paramount importance in DMPK research since drugs frequently fail in clinical trials due to the formation of bioactivated metabolites which are often difficult to measure experimentally due to their reactive nature and relatively short half-lives. Computational chemistry methods have proven invaluable in recent years as a means to predict and study bioactivated metabolites without the need for chemical syntheses, or testing on experimental animals. Additional molecular properties (heat of formation, heat of solvation and E(LUMO) - E(HOMO)) are discussed in this paper as complementary indicators of the behavior of metabolites in vivo. Five diverse examples are presented (acetaminophen, aniline/phenylamines, imidacloprid, nefazodone and vinyl chloride) which illustrate the utility of this multidimensional approach in predicting bioactivation, and in each case the predicted data agreed with experimental data described in the scientific literature. A further example of the usefulness of calculating ESP, in combination with the molecular properties mentioned above, is provided by an examination of the use of these parameters in providing an explanation for the sites of nucleophilic attack of the nucleic acid cytosine. Exploration of sites of nucleophilic attack of nucleic acids is important as adducts of DNA have the potential to result in mutagenesis.

Dietary honey and ginseng protect against carbon tetrachloride-induced hepatonephrotoxicity in rats

Liver diseases are amongst the most serious health problems in the world today and hepatocellular carcinoma is one of the world's deadliest cancers. The aim of the current study was to evaluate the protective effect of sider honey and/or Korean ginseng extract (KGE) against carbon tetrachloride (CCl(4))-induced hepato-nephrotoxicity in rat. Eighty male Sprague-Dawley (SD) rats were allocated into different groups and over a 4-week period, they orally received honey and/or KGE or were treated either with CCl(4) alone (100 mg/kg b.w) or with CCl(4) after a pretreatment period with honey, KGE or a combination of both. Clinical, clinico-pathological and histopathological evaluations were done and CCl(4)-treated groups were compared with rats receiving no treatment and with rats given honey, KGE or a combination of these substances. The results indicated that oral administration of CCl(4) induced severe hepatic and kidney injury associated with oxidative stress. The combined treatment with CCl(4) plus honey and/or KGE resulted in a significant improvement in all evaluated parameters. This improvement was prominent in the group receiving CCl(4) after combined pretreatment with honey and KGE. Animals receiving honey and/or KGE (without CCl(4)-treatment) were comparable to the control untreated group. It could be concluded that honey and KGE protect SD rats against the severe CCl(4)-induced hepatic and renal toxic effects. Our results suggest that the protective activity of honey and KGE may have been related to their antioxidant properties.

Primary cultures of rabbit renal proximal tubule cells: II. Selected phase I and phase II metabolic capacities

Specific characteristics of cells vary as a function of time in culture. We have determined the stability of selected Phase I and Phase II biotransformation capacities in rabbit renal proximal tubule cells in primary culture. When grown in hormonally-defined medium, proximal tubule cells lost Phase I metabolic capacity. Cytochrome P-450 content and associated mixed-function oxidase activities present in kidney cortex microsomes were not detectable after 14 days in culture. Phase II glutathione-dependent metabolic functions were well retained in cultured cells compared with freshly isolated proximal tubules (FIPT). Cellular total glutathione content was 2.8 mug/mg protein in FIPT compared with approximately 10 mug/mg protein in stable confluent cultures. A higher total glutathione content of 20.6 mug/mg was noted in preconfluent cultures. The glutathione redox state was initially perturbed in FIPT with 37% of the total glutathione present found in its oxidized form. Tubule cells recovered to a normal ratio (6-13% of total glutathione in the oxidized form) while in culture. The glutathione S-transferase activity in 4-day-old cells in culture was reduced to 50% of the 4 U/mg protein level found in FIPT. No appreciable further decline in glutathione S-transferase activity was detected during 15 days in culture. The level of gamma-glutamyl-transpeptidase (a brush-border enzyme necessary for glutathione uptake into proximal tubule cells) declined from 1499 mU/mg protein in homogenates of FIPT to 636 mU/mg in homogenates of 8-day-old cultured cells. A further decline in activity occurred during the next 7 days in culture. In conclusion, although Phase I metabolic functions were diminished in primary cultured rabbit proximal tubule cells, Phase II metabolic functions were retained at levels comparable with FIPT and well above those found in several established kidney cell lines.

Tissue distribution of cytochrome P450 3A (CYP3A) in brushtail possums (Trichosurus vulpecula) exposed to Eucalyptus terpenes

We evaluated the distribution pattern of a specific xenobiotic metabolizing enzyme, cytochrome P450 3A (CYP3A) in the common brushtail possum (Trichosurus vulpecula). Western blot studies using CYP3A antibodies were used to compare CYP3A levels in the intestine, liver, kidney, brain, testes and adrenal gland in possums fed diets with and without a mixture of terpenes. Possums appear to produce at least 3 different CYP3A-like isoforms that are differentially expressed in various tissues. The liver and duodenum produce all three isoforms (CYP3A P1, P2, P3), the jejunum only produces CYP3A P1, the ileum, kidney, testes and adrenal only produce CYP3A P2 and the brain only produces CYP3A P3. Terpene treatment did not alter relative levels of isoforms present in any tissue type. This study is the first to identify the presence and differential expression of several CYP3A-like isoforms in a variety of tissues of a wild mammalian herbivore. Data suggest that CYP3A-like enzymes are not induced by terpenes. However, the wide distribution of CYP3A-like isoforms in a variety of tissues, suggests that these enzymes are an important mechanism for metabolism in possums and may contribute to the high tolerance possums have to a wide range of xenobiotics.

Reduction of carbon tetrachloride-induced nephropathy by melatonin administration

The aim of this study was to investigate possible protective effects of melatonin on carbon tetrachloride (CCl4)-induced renal damage in rats. A total of 24 animals were divided into three equal groups: the control rats received pure olive oil subcutaneously, rats in the second group were injected with CCl4 (0.5 ml kg-1, s.c. in olive oil) and rats in the third group were injected with CCl4 (0.5 ml kg-1) plus melatonin (25 mg kg-1, s.c. in 10% ethanol) every other day for 1 month. At the end of the experimental period, the animals were sacrificed and blood samples were collected. The kidneys were removed and weighed. Urea and creatinine levels were determined in blood samples. Histopathological examination of the kidney was performed using light microscopic methods. Administration of CCl4 significantly increased relative kidney weight (g per 100 g body weight) and decreased serum urea levels compared to controls (p<0.01). Melatonin treatment significantly (p<0.01) reduced relative kidney weight, and it produced a statistically equal (p=0.268) relative weight with the kidneys of control rats. CCl4 administration alone also caused histopathologically prominent damage in the kidney compared to the control group. Glomerular and tubular degeneration, interstitial mononuclear cell infiltration and fibrosis, vascular congestion around the tubules, and interstitial haemorrhage in perivascular areas were observed in the renal cortex and cortico-medullary border. However, the affect of CCl4 on the medulla was limited. Melatonin provided protection against CCl4-induced renal toxicity as was evident by histopathological evaluation. In view of the present findings, it is suggested that melatonin protects kidneys against CCl4 toxicity.

Caffeic acid phenethyl ester protects kidneys against carbon tetrachloride toxicity in rats

Caffeic acid phenethyl ester (CAPE), an active component of propolis produced by honeybees, exhibits antioxidant and anti-inflammatory properties. The aim of this study was to investigate possible protective effects of CAPE on carbon tetrachloride (CCl4)-induced renal damage in rats. A total of 24 animals were divided into three equal groups: the control rats received pure olive oil subcutaneously, rats in the second group were injected with CCl4 (0.5 ml/kg, s.c. in olive oil) and rats in the third group were injected with CCl4 (0.5 ml/kg) plus CAPE (10 micromol/kg, i.p.) every other day for one month. At the end of the experimental period, the animals were sacrificed and blood samples were collected. Serum urea and creatinine levels and renal malondialdehyde (MDA) contents were determined. Histopathological examination of the kidney was also performed using light microscopic methods. It was found that kidney MDA levels were increased significantly following CCl4 exposure and this increase was significantly inhibited by CAPE treatment, while no significant changes were observed in serum urea and creatinine levels. CCl4 administration alone also caused histopathologically prominent damage in the kidney compared to the control group. Glomerular and tubular degeneration, interstitial mononuclear cell infiltration and fibrosis, and vascular congestion in the peritubular blood vessels were observed in the renal cortex. With exception of rare vascular congestions, these histopathological changes were disappeared in rats treated with CCl4 plus CAPE. In view of the present findings, it is suggested that CAPE protects kidneys against CCl4 toxicity.

Long-term effects of a novel phosphorothionate (RPR-II) on detoxifying enzymes in brain, lung, and kidney rats

The effects of a phosphorothionate, 2-butenoic acid-3-(diethoxyphosphinothioyl) methyl ester (RPR-II), on the activities of glutathione S-transferase (GST) and UDP-glucuronyltransferase (UDPGT) and the level of glutathione (GSH) were evaluated in rats after administration of RPR-II at 0.014 (low), 0.028 (medium), and 0.042 (high) mgkg(-1)day(-1) for 90 days and also at 28 days (withdrawal) after stopping treatment. Brain GST activity and GSH level decreased significantly at the high dose on the 45th and 90th days of treatment. Dose- and time-dependent decreases in GST activity and GSH was level were observed in lung at medium and high doses and in kidneys at all three doses on both the 45th and 90th days. UDPGT activity increased significantly in kidneys at the medium and high doses at 45 and 90 days. Brain and lung did not display any significant variations in UDPGT activity when compared with the control. Interestingly, the withdrawal study revealed that the effect was reversible within 28 days of cessation of treatment, when enzyme activity reverted to levels close to those of controls. The study revealed that RPR-II affected the GSH- and GST-dependent detoxification system of the treated tissues of rat and its potential to modulate the enzymes is in the order kidneys>lung>>brain. The present subacute study suggests that RPR-II may bring about physiological upsets by altering GSH- and GST-dependent events in different tissues of exposed organisms.

Chemical-induced nephrotoxicity mediated by glutathione S-conjugate formation

Glutathione conjugation has been identified as an important detoxication reaction. However, several glutathione-dependent bioactivation reactions have been identified. Current knowledge on the mechanisms and the possible biological importance of these reactions is discussed in this article. Vicinal dihaloalkanes are transformed by glutathione S-transferase-catalyzed reactions to mutagenic and nephrotoxic S-(2-haloethyl) glutathione S-conjugates. Electrophilic episulphonium ions are the ultimate reactive intermediates formed and interact with nucleic acids. Several polychlorinated alkenes are bioactivated in a complex, glutathione-dependent pathway. The first step is hepatic glutathione S-conjugate formation followed by cleavage to the corresponding cysteine S-conjugates, and, after translocation to the kidney, metabolism by renal cystein conjugate beta-lyase. Beta-Lyase-dependent metabolism of halovinyl cysteine S-conjugates yields electrophilic thioketenes, whose covalent binding to cellular macromolecules is likely to be responsible for the observed nephrotoxicity of the parent compounds. Finally, hepatic glutathione conjugate formation with hydroquinones and aminophenols yields conjugates that are directed to gamma-glutamyltransferase-rich tissues, such as the kidney, where they cause alkylation or redox cycling reactions, or both, that cause organ-selective damage.

Subacute effects of a phosphorothionate pesticide on mixed function oxidases of Wistar rats

Subacute oral toxicity of a newly developed phosphorothionate insecticide (2-butenoic acid-3-(diethoxy-phosphinothioyl) methyl ester), coded as RPR-2, was studied in male rats by oral (multiple) intubation of low (0.014 mg kg(-1) day(-1)), medium (0.028 mg kg(-1) day(-1)), and high (0.042 mg kg(-1) day(-1)) dose for 90 days. The medium and high dose produced toxic symptoms along-with some mortality (20%) occurred in the high dose treated rats. The medium and high doses caused significant inhibition in cytochrome P-450 activity in liver, lung, kidney and brain tissues at 45 and 90 days. The high dose caused significant decrease in cyt.b5 activity of all the four tissues at 45 and 90 days. Whereas, medium dose brought such effect in liver and lung at 45 and 90 days. Kidney and brain cyt.b5 activity decreased significantly at 90th day due to medium dose. Low dose also caused inhibition in cyt.b5 activity in brain at 90th day. Cytochrome P-450 reductase activity was decreased significantly in liver,

Inhibiting effect of ethinylestradiol/levonorgestrel combination on microsomal enzymatic activities in rat liver and kidney

The aim of the study was to evaluate the effects of two therapeutic combinations of ethinylestradiol (EE) and levonorgestrel (LE), which are used in triphasic contraceptives, on the activities of drug-metabolizing enzymes in rat liver and kidney. Sexually mature female Wistar rats were given 0.03 mg EE and 0.05 mg LE, or 0.03 mg EE and 0.125 mg LE for 6 or 18 sexual cycles, i.e. for 30 or 90 days. EE/LE inhibited not only the metabolic capacity of P450, a protein which directly undergoes suicide inhibition, but also the level of rat liver cytochrome b5 (dependent on the heme pool) as well as the activities of NADPH-cytochrome P450 reductase and NADH-cytochrome b5 reductase in the liver and kidney. The majority of these effects were independent of the gestagen dose and of the duration of treatment, suggesting that estrogen is a predominant inhibiting factor in the EE/LE combination. The study has revealed differences in the enzyme activities between the liver and kidney, which may result from the fact that these organs display different sets of P450 isoforms and, therefore, their monooxygenase systems show distinct capacities to metabolize exogenous steroids.

Repeated dosing with the peroxisome proliferator clofibrate decreases the toxicity of model hepatotoxic agents in male mice

Pretreatment of mice with clofibrate (CFB) has been shown to protect against acetaminophen (APAP) hepatotoxicity. To determine if pretreatment with CFB prevents the toxicity of other model hepatotoxicants, male C57BL6J or CD-1 mice received 500 mg CFB/kg, i.p., daily for 10 days, and then were challenged with either 250 mg bromobenzene (BrB)/kg, 0.025 ml carbon tetrachloride (CCl4)/kg or 0.5 ml chloroform (CHCl3)/kg. Liver and kidney injury was assessed by plasma sorbitol dehydrogenase activity (SDH) and blood urea nitrogen (BUN), respectively and histopathology. Challenge with BrB significantly elevated plasma SDH activity in C57Bl6J mice. This was prevented in CFB pretreated mice receiving the same dose of BrB. Changes in BUN were not detected in either group of BrB treated mice. Similarly, pretreatment of male CD-1 mice with CFB significantly reduced CCl4-induced elevation in plasma SDH activity, with no BUN elevation detected in either group. CFB pretreatment also diminished elevation in plasma SDH activity produced by CHCl3 in CD-1 mice, while BUN was significantly elevated in both groups, indicating that CFB did not protect against CHCl3-induced nephrotoxicity. Histopathological examination of liver and kidney sections confirmed these results. This study shows that mice pretreated with CFB were protected from toxicity at 24 h after challenge with other model hepatotoxic agents besides APAP.

Ketone potentiation of haloalkane-induced hepatotoxicity: CCl4 and ketone treatment on hepatic membrane integrity

Previous results in male Sprague-Dawley rats indicate that acetone (A), methyl ethyl ketone (MEK), and methyl isobutyl ketone (MiBK) pretreatments (3 d, p.o.) at a dosage of 6.8 mmol/kg potentiate CCl4 hepatotoxicity. The potentiation potency profile observed was MiBK > A > MEK. In the present study, male Sprague-Dawley rats were treated for 3 d with 6.8 mmol/kg (p.o.) of A, MEK, or MiBK using Emulphor as vehicle (10 ml/kg). Rats were either killed 18 h after the last pretreatment or treated with CCl4 (prepared in corn oil) and then killed 48 h later. Livers were perfused; purified plasma membrane (PM), sinusoidal (SM) and basal canalicular membrane (BCM) fractions were prepared. Membrane fluidity was monitored by fluorescence polarization using 1,6-diphenyl-1,3,5-hexatriene (DPH) or 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). The following membrane enzymes were measured to monitor membrane purity and treatment effects: 5'-nucleotidase (5N), leucine aminopeptidase (LAP), and alkaline phosphatase (AP). Our results suggest that CCl4 modifies membrane integrity as indicated by a decrease in liver membrane 5N, LAP, and AP activity. CCl4 also increased the fluidity of the lipid and protein portions of the liver membranes as measured by the DPH and TMA-DPH fluorescence probes, respectively. Of the three ketones, only A altered CCl4 effects on plasma membrane enzymes and decreased BCM fluidity. The data only partially support increased susceptibility of liver membranes by ketone pretreatment as a factor implicated in the mechanism of potentiation of CCl4-induced hepatotoxicity.

Biotransformation and membrane transport in nephrotoxicity

The kidney is a frequent target organ for toxic effects of xenobiotics. In recent years, the molecular mechanisms responsible for the selective renal toxicity of many nephrotoxic xenobiotics have been elucidated. Accumulation by renal transport mechanisms, and thus aspects of renal physiology, plays an important role in the renal toxicity of some antibiotics, metals, and agents binding to low molecular weight proteins such as alpha(2u)-globulin. The accumulation by active transport of metabolites formed in other organs is involved in the kidney-specific toxicity of certain polyhaloalkanes, polyhaloalkenes, hydroquinones, and aminophenols. Other xenobiotics are selectively metabolized to reactive electrophiles by enzymes expressed in the kidney. This review summarizes the present knowledge on the mechanistic basis of target organ selectivity of these compounds.

Biotransformation and renal processing of nephrotoxic agents

Nephrotoxicity is often observed as an endpoint in animal toxicity studies. In recent years, the mechanisms of biotransformation, which often provide the basis for renal toxicity, have been elucidated for a variety of compounds. These studies showed that nephrotoxicity of chemicals is either due to accumulation of certain metabolites in the kidney and further bioactivation or due to intrarenal bioactivation of the parent xenobiotic. Both types of mechanisms will be discussed using two relevant samples. The polychlorinated olefin hexachlorobutadiene and other haloolefins cause necrosis of the S-3 segment of the proximal tubules; their nephrotoxicity is dependent on bioactivation reactions. In the liver, hexachlorobutadiene is transformed by conjugation with glutathione to (S-pentachlorobutadienyl)glutathione. This S-conjugate is processed by the enzymes of mercapturic acid formation to give N-acetyl-(S-pentachlorobutadienyl)-L-cysteine, which is accumulated in the proximal tubule cells and deacetylated there to give (S-pentachlorobutadienyl)-L-cysteine. Further bioactivation is catalyzed by renal cysteine conjugate beta-lyase. Both the renal accumulation by the organic anion transporter and the topographical distribution of cysteine conjugate beta-lyase along the nephron are major determinants of organ and cell selectivity. Vinylidene chloride (VDC) is nephrotoxic in mice after inhalation, but not after oral or intraperitoneal administration. The nephrotoxicity of VDC is due to the selective expression of an androgen-dependent cytochrome P450 in the proximal tubules of male mice. This enzyme oxidizes VDC to an electrophile and is not present in female mice, but can be induced be androgen treatment. The observation of nephrotoxicity of VDC after inhalation only is due to the high blood flow to the kidney and thus high concentrations of VDC delivered to the kidney after inhalation. After oral or intraperitoneal application, hepatic first-pass metabolism efficiently reduces the amount of VDC delivered to the kidney. The results demonstrated here demonstrate that prior to in vitro nephrotoxicity screening, toxicokinetics and biotransformation pathways for a chemical have to be elucidated and metabolites have to be included into the testing regimen.

Risk assessment: toxicity from chemical exposure resulting from enhanced expression of CYP2E1

Humans are continuously exposed to a wide variety of xenobiotics either voluntarily or from environmental exposure. Many xenobiotics including pesticides, nitrosamines, polycyclic aromatic hydrocarbons and halogenated hydrocarbons, require bioactivation by P450 enzymes to elicit toxicity. CYP2E1 is considered to be toxicologically important in humans because of its capacity to produce intermediates that promote cytotoxicity and/or carcinogenicity from a number of xenobiotics. Importantly, CYP2E1 is present constitutively and its content can be modulated by a variety of factors including xenobiotics such as alcohol. Because hepatic concentrations of CYP2E1 can vary considerably from one individual to another, the extent of formation of toxic products also varies. Indeed, as hepatic concentrations increase so does the risk of toxicity from chemicals activated by this P450 enzyme. Many chemicals modulate CYP2E1 expression and exposure to one compound may alter the toxicological impact of another. Considering that CYP2E1 content is related to toxicity from chemicals, identifying subjects with elevated levels may lead to minimizing exposure in high risk individuals.

Ketone potentiation of haloalkane-induced hepato- and nephrotoxicity. II. Implication of monooxygenases

Previous results in Sprague-Dawley rats indicate that acetone (A), methyl ethyl ketone (MEK), and methyl isobutyl ketone (MiBK) pretreatment (3 d, po) at dosages of 6.8 and 13.6 mmol/kg potentiate CCl4 hepatotoxicity and CHCl3 nephrotoxicity, respectively. The potentiation potency profile observed was MiBK > A > MEK for liver and A > MEK > or = MiBK for kidney toxicity (Raymond & Plaa, 1995). In the present study, hepatic and renal microsomes from A-, MEK-, and MiBK-pretreated rats (6.8 or 13.6 mmol/kg) were examined for cytochrome P-450 content, substrate-specific monooxygenase activity (aminopyrine and benzphetamine N-demethylase, aniline hydroxylase) and in vitro covalent binding of 14CHCl3 and 14CCl4. Of the three ketones, only MiBK significantly increased P-450 content of liver and renal cortical microsomes. Similarly, 14CCl4 covalent binding under aerobic and anaerobic conditions was significantly increased by MiBK pretreatment only. 14CHCl3 covalent binding by renal cortical microsomes was significantly increased only under aerobic conditions by MiBK pretreatment. MiBK (13.6 mmol/kg) increased (threefold) aminopyrine N-demethylation in both liver and kidney, but only benzphetamine N-demethylation (two-fold, at 6.8 and 13.6 mmol/kg) in liver; A and MEK had no effect on either monooxygenase. All ketones at dosages of 6.8 and 13.6 mmol/kg increased aniline hydroxylation in liver (two-fold) and kidney (fivefold). Comparable profiles for P-450 induction, haloalkane covalent binding, and aminopyrine or benzphetamine N-demethylase activity were observed in liver and kidney microsomes. This profile was consistent with the ketone potentiation potency ranking profile observed in vivo for liver but not kidney injury. These findings affirm the importance of ketone-enhanced bioactivation for potentiation of CCl4 hepatotoxicity but suggest an alternative mechanism for CHCl3 nephrotoxicity.

Protein degradation in kidney proximal tubule cell monolayers

Isolated proximal tubule cells have been labelled with L-[4,5-3H]leucine prior to cell division. Histochemical staining demonstrated the purity of the cultures. The bicarbonate ion or a collagen support was required for cell growth. Different culture growth rates were established by varying these parameters. The proximal tubule marker enzyme, gamma-glutamyl transpeptidase, was expressed throughout the culture period (7-10 days) and the cells undergo a glycolytic shift, shown by an increase in the levels of lactate dehydrogenase. The specific activities of these enzymes were related to the growth conditions. Exponential rates of protein degradation were observed. The uptake of labelled exogenous hepatocyte proteins in proximal tubule cell cultures was completely suppressed in the presence of serum (10%, v/v) showing that endocytosis did not contribute to the observed measurements of intracellular protein degradation. The increased growth rates seen in cultures were accompanied by decreased rates of protein degradation. Use of the inhibitors of proteolysis, leupeptin and ammonium chloride, showed that the decrease was at the lysosomal level. The results suggest that targeting of inhibitors of lysosomal proteolysis, via low-molecular-weight proteins, may be useful in stimulating tubular regeneration in kidney disease.

Different glutathione redox status and lipid peroxidation in the cortex and the medulla of the rat kidney subjected to ischemia-reperfusion stress

When the rat kidney is subjected to ischemia and reperfusion, changes in glutathione content and in lipid peroxidation are produced in the cortex and in the medulla. The cortex shows a decrease in the glutathione content and a higher sensitivity to development of lipid peroxidation, the medulla being less affected. Reperfusion restores the glutathione concentration of the cortex during the first hours of reflow. The lipid peroxidation observed in the cortex and the medulla during reperfusion is higher than in ischemia. The protective role of glutathione and the response of the cortex and the medulla to ischemia-reperfusion injury are discussed.

Bioactivation of halogenated hydrocarbons by cytochrome P4502E1

Numerous halogenated hydrocarbons of the alkane, alkene, and alkyne classes are metabolized by P450 enzymes to products that elicit cytotoxic and/or carcinogenic effects. Such halogenated hydrocarbons include anesthetics (e.g., halothane and enflurane) and industrial solvents (e.g., carbon tetrachloride, chloroform, and vinylidine chloride). Formation of reaction intermediates from these compounds occurs via P450-promoted dehalogenation, reduction, or reductive oxygenation, with certain hydrocarbons undergoing all three reaction types. Of the multiple forms of P450 present in liver microsomes, P4502E1 has been identified as the primary catalyst of hydrocarbon bioactivation in animals and, most likely, in humans as well. As hepatic concentrations of this P450 enzyme are highly inducible by ethanol and similar agents, prior exposure to 2E1-inducing compounds can play a pivotal role in halogenated hydrocarbon toxicity. Considering that metabolism governs the cytotoxicity and carcinogenicity of halogenated hydrocarbons, an understanding of the mechanism(s) underlying 2E1 induction in man becomes all the more important.

Interaction of monocrotophos and its novel thion analogues with microsomal cytochrome P-450: in vivo and in vitro studies in rat

The binding of monocrotophos (MCP) and its two thion analogues (coded as RPR-II and RPR-V) to rat hepatic microsomal cytochrome P-450 (HMC) were investigated in vitro by difference spectroscopy. These three organophosphorus insecticides were found to bind stoichiometrically to HMC with very high affinity (Ks 34-50 microM). RPR-V showed the highest binding affinity followed by RPR-II and MCP. Association of these compounds with HMC occurred within 2 min of addition in the cuvette and therefore, appeared to be tight binding ligands of cytochrome P-450. In vivo studies at equitoxic doses of the three compounds 24 h after oral treatment in rats revealed that they all caused reduction in MC content in liver, lung, kidney and brain, as against induction in cardiac and splenic cytochrome P-450. These in vivo results suggest organ specificity in modulating the microsomal cytochrome P-450 (MC) content of hepatic and extra-hepatic tissues by the three compounds. Apparently, their binding affinity with HMC is strongly correlated with their LD50 value and has a substantial co-relationship with the cytochrome P-450 level in the liver.

Haemorrhages due to defective blood coagulation do not occur in mice and guinea-pigs fed butylated hydroxytoluene, but nephrotoxicity is found in mice

Groups of ten male Slc:ddY mice were fed a purified diet containing butylated hydroxytoluene (BHT) at levels of 0, 1.35, 1.75, 2.28, 2.96, 3.85 or 5.00%. They were kept in cages with soft-wood chips as bedding for 30 days. Groups of five Slc:ddY male mice were kept in cages with stainless-steel wire-mesh bottoms (without wood-chip bedding) and fed BHT at 0, 0.5, 1.0 or 2.0% in the diet for 21 days. Male Crj:Hartley guinea-pigs were given a purified ration containing BHT at levels of 0, 0.125 or 0.25% (five animals per group) for 14 days, or at 0, 0.5, 1.0 or 2.0% for 17 days (six animals per group). When BHT was given to mice housed in the mesh-bottomed cages there were one, one and two deaths during the experiment in the 0.5, 1.0 and 2.0% dose groups, respectively. Lung haemorrhages were observed in these dead mice, but in all other mice and guinea-pigs no haemorrhages were found. Indices of prothrombin time and kaolin-activated partial thromboplastin time were significantly decreased by up to 30 and 40%, respectively, in the mice kept on wood-chip bedding, and by up to 40 and 60% in the mice kept in cages with wire bottoms. In guinea-pigs, the prothrombin index was significantly reduced only in the 1.0% BHT group. We conclude that the BHT-induced lung haemorrhages in mice are not caused by a severe reduction in the coagulation process, as they are in rats, and that BHT does not cause bleeding like that observed in rats. However, dose-related toxic nephrosis was found in mice given 1.35-5.0% BHT in the diet. The nephrotoxic ED50(1 month) was 2.3 g/kg body weight/day. The results suggest that an extremely large dose of BHT can cause renal toxicity in mice.

Captopril does not protect against renal oxidative injury

The free thiol group of captopril is important in the action and metabolism of this drug. It has been postulated that the thiol group may allow captopril to act in a manner similar to glutathione and protect against oxidative injury. This study investigated the ability of captopril, enalaprilat, and N-acetylcysteine to prevent tertbutyl hydroperoxide induced oxidative injury in rat renal homogenates. Lipid peroxidation was significantly increased in homogenates from captopril-treated animals (p < 0.05), and following glutathione depletion this was further enhanced (p < 0.001). Renal glutathione content was significantly reduced by captopril treatment (p < 0.01). These results suggest that captopril does not act as an alternate source of reducing equivalents to glutathione and does not protect against renal oxidative injury in this model.

Haemolytic activity and nephrotoxicity of 2-hydroxy-1,4-naphthoquinone in rats

The short-term toxicity of 2-hydroxy-1,4-naphthoquinone (lawsone) and 2-methyl-1,4-naphthoquinone (menadione) has been compared in rats. 2-Methyl-1,4-naphthoquinone has been shown previously to cause haemolytic anaemia in animals, and this was confirmed in the present experiment. 2-Hydroxyl-1,4-naphthoquinone was found also to cause haemolysis, in a dose-dependent manner, as reflected by decreased blood packed cell volumes and haemoglobin levels and by histopathological changes in spleen, liver and kidney. With both naphthoquinones, the haemolysis was of the oxidative type, characterized by the presence of Heinz bodies within erythrocytes. Haemolysis was the only toxic change identified in rats dosed with 2-methyl-1,4-naphthoquinone. In contrast, 2-hydroxyl-1,4-naphthoquinone was not only a haemolytic agent but also a nephrotoxin, causing renal enlargement, elevated plasma levels of urea and creatinine and histologically-identified tubular necrosis, largely confined to the distal segment of the proximal convoluted tubules. The relationship between the in vivo toxic effects of these naphthoquinones and previously-reported data on their in vitro cytotoxic action is discussed.

Bioactivation of xenobiotics by formation of toxic glutathione conjugates

Evidence has been accumulating that several classes of compounds are converted by glutathione conjugate formation to toxic metabolites. The aim of this review is to summarize the current knowledge on the biosynthesis and toxicity of glutathione S-conjugates derived from halogenated alkanes, halogenated alkenes, and hydroquinones and quinones. Different types of toxic glutathione conjugates have been identified and will be discussed in detail: (i) conjugates which are transformed to electrophilic sulfur mustards, (ii) conjugates which are converted to toxic metabolites in an enzyme-catalyzed multistep mechanism, (iii) conjugates which serve as a transport form for toxic quinones and (iv) reversible glutathione conjugate formation and release of the toxic agent in cell types with lower glutathione concentrations. The kidney is the main, with some compounds the exclusive, target organ for compounds metabolized by pathways (i) to (iii). Selective toxicity to the kidney is easily explained due to the capability of the kidney to accumulate intermediates formed by processing of S-conjugates and to bioactivate these intermediates to toxic metabolites. The influences of other factors participating in the renal susceptibility are discussed.

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)

Histochemical localization of formaldehyde dehydrogenase in the rat

Formaldehyde dehydrogenase (FDH) activity has been demonstrated biochemically in the olfactory and respiratory mucosae and in the liver of the rat, but the cellular localization of this enzyme has not been investigated. A histochemical procedure was developed to permit cellular localization of FDH. This allowed us to examine the relationship between distribution of FDH and formaldehyde-induced toxicity. Cold-processed glycol methacrylate embedded tissues were used to localize FDH activity in the rat respiratory tract, kidney, liver, and brain. Five- or ten-micrometer tissue sections were incubated in a reaction mixture containing formaldehyde (HCHO), glutathione (GSH), NAD+, nitroblue tetrazolium, pyrazole, and disulfiram. A blue formazan precipitate was formed at the site of FDH activity. Epithelial cell cytoplasm of both the respiratory and the olfactory mucosae of the nose stained for FDH, and olfactory sensory cell nuclei were also positive. Underlying Bowman's and seromucous glands were weakly positive. The lung had FDH activity located mainly in the Clara cells of the airways, with only diffuse weak activity in the lung parenchyma. Liver had activity in the cytoplasm of the hepatocytes, while in the kidney FDH was most prominent in the brush border of the P2 segment of the proximal tubules. Brain white matter stained strongly for FDH, while in gray matter only the neuropil exhibited weak activity. Corresponding tissue sections were stained for sulfhydryls; these sections indicated that GSH is likely to be present in all cells with FDH activity. For the respiratory tract these results demonstrate distinct differences between the location of FDH activity and previously reported nonspecific aldehyde dehydrogenase activity in the nose (M. S. Bogdanffy, H. W. Randall, and K. T. Morgan, 1986, Toxicol. Appl. Pharmacol. 82, 560-567). While high aldehyde dehydrogenase activities were found in tissues with low toxicities due to acetaldehyde exposure and vice versa, FDH activity was observed in tissues whether or not they exhibited a toxic response to inhaled HCHO. While not able to account for the localized toxicity of HCHO, the presence of FDH and glutathione in the epithelial layer of the nasal cavity presents a barrier to inhaled formaldehyde at low concentrations and may partially explain the observed nonlinearity of HCHO toxicity.

Activation and detoxication of aminophenols. II. Synthesis and structural elucidation of various thiol addition products of 1,4-benzoquinoneimine and N-acetyl-1,4-benzoquinoneimine

1. Nine thioethers of 4-aminophenol with beta-hydroxyethylmercaptan, ranging from mono- to tetra-substituted thioadducts, were prepared from synthetic 1,4-benzoquinoneimine and characterized by 1H-n.m.r. and u.v. spectroscopy. For each compound, extinction coefficients and pKa values of the amino group were determined. 2. Five thioethers of 4-aminophenol with glutathione (GSH) were prepared and characterized by 1H-n.m.r. and u.v. spectroscopy with their respective extinction coefficients and pKa values. Two further thioadducts were tentatively assigned by their u.v. spectroscopic properties. 3. Reaction products of 1,4-[U-14C]benzoquinoneimine and GSH were studied, indicating formation of 4-amino-2-(glutathione-S-yl)phenol, 4-amino-2,3,6-tris(glutathione-S-yl)phenol as the main products. Formation of glutathione disulphide (GSSG) was not detected. In contrast, N-acetyl-1,4-[U-14C]benzoquinoneimine was partly reduced by GSH and formed only the 2-substituted thioadduct. 4. Investigation of the product orientation in the reductive addition of GSH to 2-(glutathione-S-yl)-1,4-benzoquinoneimine and 3-(glutathione-S-yl)-1,4-benzoquinoneimine, respectively, showed that the 3-substituted derivative formed mainly the 3,5-di-substituted thioadduct, whereas the 2-substituted compound formed mainly the 2,3,6-tri-substituted thioadduct. 5. Formation of thioadducts which autoxidize markedly faster than the parent aminophenol indicates that thioether formation is not an obligatory detoxication process.

Bioactivation of hexachlorobutadiene by glutathione conjugation

Glutathione (GSH) conjugation reactions in the metabolism of hexachlorobutadiene (HCBD), in rats and mice, initiate a series of metabolic events resulting in the formation of reactive intermediates in the proximal tubular cells of the kidney. The GSH S-conjugate 1-(glutathion-S-yl)-1,2,3,4,4-pentachlorobutadiene (GPCB), which is formed by conjugation of HCBD with GSH in the liver, is not reactive and is eliminated from the liver in the bile or plasma, or both. GPCB may be translocated intact to the kidney and processed there by gamma-glutamyl transpeptidase and dipeptidases to the corresponding cysteine S-conjugate. Alternatively, gamma-glutamyl transpeptidase and dipeptidases present in epithelial cells of the bile duct and small intestine may catalyse the conversion of GPCB to cysteine S-conjugates. The kidney concentrates both GSH and cysteine S-conjugates and processes GSH conjugates to cysteine S-conjugates. A substantial fraction of HCBD cysteine S-conjugate thus concentrated in the kidney is metabolized by renal cysteine conjugate beta-lyase to reactive intermediates. The selective formation of reactive intermediates in the kidney most likely accounts for the organ-specific effects of HCBD. Alternatively, cysteine S-conjugates may be acetylated to yield excretable mercapturic acids.

Testosterone-mediated regulation of mouse renal cytochrome P-450 isoenzymes

We have studied the extent to which mouse renal cytochrome P-450 isoenzymes are sexually differentiated, and the factor(s) regulating this dimorphism. Intriguingly, sex differences were not seen in the expression of a single cytochrome P-450 enzyme, but were observed in the expression of all P-450 isoenzymes detectable, encoded by six gene families or sub-families. This effect was mediated by testosterone, which had the capacity to both induce and repress P-450 gene expression, and which was independent of growth hormone. The changes in protein content were mirrored in all but one case by changes in the levels of mRNA, indicating that these genes contain hormone-responsive elements. These findings are consistent with numerous reports of sex differences in the susceptibility of the mouse kidney to the toxic and carcinogenic effects of drugs and environmental chemicals, many of which are metabolized to cytotoxic products by the cytochrome P-450-dependent mono-oxygenases. These data imply that circulating androgen levels will be an important factor in susceptibility of the kidney to toxic or carcinogenic compounds which require metabolic activation.

Renal proximal tubular cells in suspension or in primary culture as in vitro models to study nephrotoxicity

The kidney forms a frequent target for xenobiotic toxicity. The complex biochemical mechanisms underlying nephrotoxicity are best studied in vitro provided that reliable and relevant in vitro models are available. Since most nephrotoxicants affect primarily the cells of the proximal tubules (PTC), much effort has been directed towards the development of in vitro models of PTC. This review focuses on the preparation of PTC and the use of these cells. Discussed are important criteria such as the viability (survival time) of the cells and the parameters to assess toxicity. Recent studies have shown that isolated PTC in suspension are especially suitable for studies on the biochemical mechanisms of 'acute' nephrotoxicity, whereas PTC in primary culture may be used to investigate mechanisms of nephrotoxic damage at very low concentrations, upon prolonged exposure. PTC cultured on porous filter membranes provide new possibilities to study toxicity in relation to cell and transport polarity. Primary cell cultures of human PTC have been set up. Although a further characterization of these systems is needed, recent data indicate their usefulness.

Bioactivation mechanism of L-thiomorpholine-3-carboxylic acid

L-Thiomorpholine-3-carboxylic acid (L-TMC) is a cyclized analog of S-(2-chloroethyl)-L-cysteine, which is cytotoxic in vitro and nephrotoxic in vivo. To determine whether L-TMC may play a role in S-(2-chloroethyl)-L-cysteine-induced toxicity, the cytotoxicity of L-TMC was studied in isolated rat kidney cells. L-TMC produced time- and concentration-dependent cytotoxicity. Probenecid, an inhibitor of the renal anion transport system, and L-alpha-hydroxyisocaproic acid, a substrate for L-amino acid oxidase, inhibited L-TMC-induced cytotoxicity. Rat kidney cytosol catalyzed the metabolism of L-TMC to a product absorbing at 300 nm. The increase in absorbance at 300 nm was accompanied by an increase in oxygen consumption and was inhibited by L-alpha-hydroxyisocaproic acid; moreover, the absorbance of the metabolite was quenched by addition of potassium cyanide or sodium borohydride, which indicated the formation of an imine. When L-TMC was incubated with rat kidney cytosol and sodium borodeuteride was added at the end of the incubation period, analysis by gas chromatography/mass spectrometry of the tert-butyldimethylsilyl ester of L-TMC showed the formation of [2H]TMC, indicating the intermediate formation of the imine 5,6-dihydro-2H-1,4-thiazine-3-carboxylic acid; chemically synthesized TMC imine showed similar behavior. The enzyme responsible for the metabolism of L-TMC was purified from rat kidney and was identified as L-amino acid oxidase. These observations indicate a role for L-amino acid oxidase in the bioactivation and cytotoxicity of L-TMC.

Primary cultures of rabbit renal proximal tubule cells: I. Growth and biochemical characteristics

Before the usefulness of a new in vitro model can be ascertained, the model must be properly defined and characterized. This study presents the growth rate and biochemical characteristics of rabbit renal proximal tubule cells in primary culture over a 2-wk culture period. When grown in a hormonally defined, antibiotic-free medium these cells form confluent monolayer cultures within 7 d after plating. Multicellular dome formation, an indicator of transepithelial solute transport, was expressed after confluent cultures were formed. The activity of the cytosolic enzyme, lactate dehydrogenase, and the lysosomal enzyme, N-acetyl-glucosaminidase, increased 14- and 2-fold during the first 8 d of culture, respectively. In contrast, the activity of a brush border enzyme, alkaline phosphatase, decreased 85% within the first 8 d of culture. Release of these enzyme markers into the culture medium, which are routinely used to measure cytotoxicity, stabilized after 8 d in culture. The ratio of cellular protein to DNA changed according to the state of cellular growth. Values rose from 0.035 mg protein/micrograms DNA in preconfluent cultures to 0.059 mg protein/micrograms DNA in confluent cultures. These results document the characteristics of a primary proximal tubule cell culture system for future studies in in vitro toxicology.

Detection of chemically induced renal injury: the cascade of degenerative morphological and functional changes that follow the primary nephrotoxic insult and evaluation of these changes by in-vitro methods

A diversity of chemicals cause discrete lesions in the kidney by a number of different mechanisms, and similar types of chemicals may give rise to more than one target cell injury. Screening for these lesions in vivo may be unreliable if a single noninvasive or invasive criterion is used. Instead, evidence of nephrotoxicity must be based on an array of tests, applied over a period of time. These should include biochemical, pathological and histochemical tests conducted in tandem. A primary toxic injury to the kidney may give rise to recovery, to permanently altered functional reserve or to a clinically identifiable effect, such as acute or chronic renal failure or malignancy. These clinical effects occur as a result of a cascade of degenerative changes which are a consequence of a primary lesion but also affect other parts of the kidney. A number of factors can modulate the progression of the primary insult to the end-effect. In-vitro nephrotoxicity screening is also difficult, but a rational approach can be based on current understanding of how chemicals target for and damage cells in anatomically well-defined regions of the kidney. In-vitro techniques can provide answers to specific questions about the mechanisms by which chemicals damage these discrete cell types. It is essential that a number of different in-vitro systems be developed in parallel to address the mechanistic aspects and screening of nephrotoxicity properly. Data generated in vitro must be related to the situation in vivo and used to devise reliable noninvasive tests for assessing nephrotoxicity in man.

Mechanism of nephrotoxic action due to organohalogenated compounds

A number of organohalogenated chemicals cause nephrotoxicity in experimental animals and man. Studies in animals have shown that metabolic activation of the chemical is required to produce toxicity. Currently, two major pathways of metabolism, mediated either via cytochrome P-450 or glutathione conjugation, have been implicated. Chloroform is discussed as an example of cytochrome P-450-mediated activation and dihaloethanes and hexachloro-1,3-butadiene as examples of glutathione conjugation followed by activation. Acute human exposure to certain organohalogenated compounds can sometimes result in proximal tubular injury. These intoxications usually occur after either accidental or deliberate ingestion and are rarely occupational. Chronic low-level exposure can occur in the work place, and several biological tests have been developed to detect chronic nephrotoxicity. A few studies have been undertaken of workers exposed to organohalogenated chemicals; these have provided no indication that exposure to these chemicals causes chronic renal damage.

Influence of aging on chemically induced hepatotoxicity: role of age-related changes in metabolism

The effects on hepatotoxicity of age-associated changes in drug metabolism are not always straightforward. In the case of allyl alcohol hepatotoxicity in male rats, there is a good relationship between increased metabolic activation by liver alcohol dehydrogenase and enhanced hepatotoxicity in old age. With regard to two other hepatotoxicants, some tentative conclusions about the role of metabolism can be drawn, but they must be tempered with caution due to gaps in the available information. Acetaminophen-induced hepatotoxicity is reduced in old age, and decreased formation of the toxic intermediate may be the reason. There is a prominent effect of aging on acetaminophen conjugation, a shift from sulfation to glucuronidation, but this change does not affect total clearance. The situation with carbon tetrachloride is difficult to interpret because the final outcome is unaltered hepatotoxicity in old age. Nevertheless, the available data suggest that an age-associated decrease in activation of carbon tetrachloride is counterbalanced by a loss in resistance to lipid peroxidation. These conclusions are summarized in Table 5. Again, it must be emphasized that all of these age-dependent changes in toxicity could be related to effects on other systems that are not necessarily involved in the metabolism of hepatotoxicants. Future research is needed to identify pathways of metabolic activation and detoxification in which age-dependent changes occur that result in significant changes in hepatotoxicity. The entire sequence of events from changes at the molecular level to their sequelae at the level of the cell, tissue and intact animal should be investigated, and the results should be confirmed in more than one mammalian model of aging. The aim would be to identify basic mechanisms that result in increased hazard for the aged liver from exposure to toxic compounds.

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)