Effects of vinyltoluene alone and after pretreatment with polychlorinated biphenyls on hepatocellular morphology and enzyme activities in liver and kidneys in rats

Male Wistar rats were exposed by inhalation to vinyltoluene (300 ppm, 6 h daily, 5 days/week, for 1 or 2 weeks). An intraperitoneal dose of polychlorinated biphenyls (PCBs) (500 mg/kg) was injected 5 days before the inhalation exposure began. Hepatocytes were significantly increased in size after treatment with PCBs alone; this increase in size was caused by the proliferation of smooth endoplasmic reticulum (SER) and the dilatation of interreticular space. After treatment with vinyltoluene, the cell size decreased significantly. This decrease was counterbalanced by pretreatment with PCBs. Both with and without pretreatment with PCBs, the mitochondria were enlarged after exposure to vinyltoluene. Vinyltoluene affected the activity of NADPH-cytochrome c reductase and the content of cytochrome P-450 in the liver or the kidneys only moderately if at all. The activity of hepatic or renal O-deethylases (7-ethoxycoumarin and 7-ethoxyresorufin) and UDPglucuronosyltransferase increased 1,4-fold or more. The activity of epoxide hydrolase in the liver was increased 1.4-fold in 1 week by the inhalation of vinyltluene. A mixture of PCBs was a potent inducer of drug-metabolizing enzymes in the liver. A single i.p. injection of PCBs increased the activity of 7-ethoxyresorufin O-deethylase in the liver by more than 150 times. Exposure to both PCBs and vinyltoluene caused no additive effects on measured activities with the exception of hepatic epoxide hydrolase. The decrease in non-protein sulhydryl concentrations of the liver after exposure to vinyltoluene was accompanied by an increase in the excretion of urinary thioethers. In rats pretreated with PCBs, the effect of vinyltoluene on both of these parameters was less than that caused by the inhalaton of vinyltoluene alone. Our results indicate that the biotransformation of vinyltoluene in rats leads to glutathione conjugates accompanied by concomitant depletion of hepatic glutathione and increased excretion of urinary thioethers. Treatment with PCBs modifies the biotranformation of vinyltoluene in the direction of diol-formation via enhanced activity of epoxide hydrolase.     

The effects of propiconazole on hepatic xenobiotic biotransformation in the rat

Propiconazole, a foliar fungicide used for agricultural purposes was studied for its effects on the hepatic xenobiotic biotransformation in the rat. Rats were given an intraperitoneal injection of 0.1, 1, 10 or 100 mg/kg in corn oil for seven consecutive days. Induction was seen for cytochrome P-450, ethoxyresorufin-O-deethylase, ethoxycoumarin-O-deethylase, aldrin epoxidase, aminopyrine N-demethylase and microsomal expoxide hydrolase activities. Aniline p-hydroxylase and cytosolic glutathione S-transferase activities were unchanged. All responses occurred at only 100 mg/kg, except for that of aminopyrine N-demethylase which also occurred at the 10 mg/kg dose. SDS polyacrylamide gel electrophoresis showed increased staining of a protein band of molecular weight 54,000 corresponding to cytochrome P-450b and/or P-450d. Collectively these results suggest that cytochromes P-450b and P-450d have been induced after exposure of rats to propiconazole.     

Induction of xenobiotic biotransformation by the insecticide chlordimeform, a metabolite 4-chloro-o-toluidine and a structurally related chemical o-toluidine

Chlordimeform, 4-chloro-o-toluidine and o-toluidine have all been found to have carcinogenic properties. Due to an empirical link between such properties and alteration of some biotransformation enzymes, the abilities of these three chemicals to affect cytochrome P-450 mediated biotransformation, epoxide hydrolase and glutathione S-transferase have been examined. Chlordimeform had no effect on the cytochrome P-450 content, aniline p-hydroxylase or glutathione S-transferase activities, but induced ethoxyresorufin-O-deethylase, ethoxycoumarin-O-deethylase and epoxide hydrolase activities and decreased aldrin epoxidase and aminopyrine N-demethylase activities. The metabolite 4-chloro-o-toluidine increased cytochrome P-450, ethoxyresorufin-O-deethylase, ethoxycoumarin-O-deethylase, glutathione S-transferase and epoxide hydrolase activities. o-Toluidine induced cytochrome P-450, ethoxyresorufin-O-deethylase, ethoxycoumarin-O-deethylase, and aldrin epoxidase activities. Ethoxy-resorufin-O-deethylase activity was induced approximately eight times by chlordimeform and 18 times by 4-chloro-o-toluidine and o-toluidine. Induction was seen at 50 mg/kg with chlordimeform and at 10 mg/kg with the other treatments. Chlordimeform increased the 7 alpha and 16 alpha androstenedione hydroxylase pathways. 4-Chloro-o-toluidine increased the 7 alpha, 16 beta and 16 alpha hydroxylase pathways, while o-toluidine increased the 7 alpha, 6 beta, 16 beta and 16 alpha hydroxylase pathways. All three chemicals marginally decreased the testosterone pathways. SDS-PAGE of rat microsomes revealed an increase in a protein band of MW c54,000 for the chlordimeform and 4-chloro-o-toluidine treated groups. Taken together with the increase in ethoxyresorufin-O-deethylase activity these observations are consistent with the induction of hepatic isozyme P-450d. Thus each chemical has been shown to induce various pathways of biotransformation with increases in the P-450c and P-450d specific substrate ethoxyresorufin-O-deethylase being a consistent finding.     

Chronic physical activity: hepatic hypertrophy and increased total biotransformation enzyme activity.

Does chronic voluntary physical activity alter hepatic or intestinal capacities for xenobiotic biotransformation? This question was investigated by comparing biotransformation enzyme activities in liver and small intestine of active and sedentary rats. Male rats allowed unlimited access to a running wheel and fed ad lib. for 6 weeks were weight-matched to sedentary controls; the active rats ate 22% more food than the sedentary rats (P less than 0.05). Active rats ran 2.8 +/- 0.6 miles/day. Liver weights were higher in the active rats (11.2 +/- 0.2 vs 9.8 +/- 0.2 g; P less than 0.05), as were total liver protein, and liver microsomal and cytosolic protein (P less than 0.05). As a result of liver hypertrophy, the active rats showed higher total liver activity of several biotransformation enzymes, including 2-naphthol sulfotransferase, styrene oxide hydrolase, benzphetamine N-demethylase, ethacrynic acid glutathione S-transferase and morphine UDP-glucuronosyltransferase (P less than 0.05). In contrast, there was no detectable difference in total liver N-acetyltransferase activity toward p-aminobenzoic acid, 2-naphthylamine, and 2-amino-fluorene as well as, relative hepatic enzyme activity (expressed per g liver or per mg protein) and total and relative intestinal enzyme activity. We conclude that chronic voluntary physical activity, accompanied by an increased food intake, results in liver hypertrophy and potentially increases total hepatic capacity to biotransform certain xenobiotic chemicals.     

Concomitant induction of microsomal epoxide hydrolase and UDP-glucuronosyltransferase activities by dipyridine compounds.

The coordinated response of the major rat hepatic phase II xenobiotic-metabolizing enzymes following 3-day exposure to diaryl compounds was investigated. Four diaryl compounds containing heterocyclic nitrogen atoms elevated microsomal epoxide hydrolase activity from 2- to 4-fold. Equivalent compounds lacking the heteroatom, when given in the same dosing regimen (75 mg/kg, ig, daily for 3 days), did not induce this or any other drug-metabolizing enzyme activity. Epoxide hydrolase activity closely paralleled UDP-glucuronosyltransferase activity toward three aglycones: 4-nitrophenol (r = 0.87), morphine (r = 0.84), and 1-naphthol (r = 0.78). There was less correlation (r = 0.60) between epoxide hydrolase activity and both UDP-glucuronosyltransferase activity toward testosterone and cytosolic glutathione S-transferase activity. There was no correlation between microsomal epoxide hydrolase activity and cytochrome P-450 or the monooxygenase reaction (4-nitrophenol hydroxylase) preferentially induced by pyridine-containing compounds. Induction of rat hepatic microsomal epoxide hydrolase activity by some pyridine-containing compounds appears coordinately regulated with glucuronidation rather than oxidation enzymes.     

Effect of polybrominated biphenyls on drug metabolizing enzymes in different tissues of C57 mice.

Polybrominated biphenyls (PBBs) are structurally very close to polychlorinated biphenyls (PCBs) which are known to be potent inducers of xenobiotic biotransformation reactions. We have studied the effects of 2 industrial PBB-mixtures, "hexabromobiphenyl" (HBB) and "octabromobiphenyl" (OBB), on enzymes catalyzing drug hydroxylation, epoxide hydration, and conjugation reactions in different tissues of C57 mice. The enzyme activities were measured 10 days after a single i.p. injection of PBBs (75 mg/kg). HBB enhanced the activities of hepatic AHH (1.9-fold), ethoxycoumarin deethylase (5.7-fold), epoxide hydratase (1.5-fold), glutathione S-transferase (1.7-fold) and UDP-glucuronosyltransferase (1.5-fold). In the kidney HBB enhanced the activity of UDP-blucuronosyltransferase 1.5-fold. OBB caused in increase in the activities of liver AHH (1.5-fold), ethoxycoumarin deethylase (2.4-fold) and glutathione S-transferase (1.4-fold). A slight increase was also seen in the activity of UDP-glucuronosyltransferase in digitoninactivated liver microsomes of OBB-treated mice. In the kidney OBB caused a slight but statistically significant decrease in glutathione S-transferase activity. Intraperitoneally injected bromobiphenyls had no effects on these drug metabolizing enzymes in the lung of C57 mice. These results were similar to the effects caused by a mixture of PCBs.     

Antioxidant defenses and lipid peroxidation in the cerebral cortex and hippocampus following acute exposure to malathion and/or zinc chloride

This study investigates the effects of acute exposure to organophosphate insecticide malathion (250 mg/kg, i.p.) and/or ZnCl2 (5 mg/kg, i.p.), with the following parameters: lipid peroxidation and the activity of acetylcholinesterase (AChE), glutathione reductase (GR), glutathione S-transferase (GST), glutathione peroxidase (GPx), glucose-6-phosphate dehydrogenase (G6PDH), and the levels of total glutathione (GSH-t) in the hippocampus and cerebral cortex of female rats. Malathion exposure elicited lipid peroxidation and reduced AChE activity in the cerebral cortex and hippocampus. It also reduced the activity of GR and GST, and increased G6PDH activity in the cerebral cortex, without changing the levels of GSH-t and GPx activity. ZnCl2 exposure reduced AChE activity and caused a mild pro-oxidative effect, since lipid peroxidation was increased in the hippocampus. ZnCl2, individually or in combination with malathion, caused a reduction in GR and GST activity in the cerebral cortex. Malathion and/or ZnCl2 did not change the GSH-t levels. Moreover, ZnCl2 prevented the increase in G6PDH activity caused by malathion. It showed that ZnCl2 had little effect against the changes induced by malathion. In fact, zinc itself produced pro-oxidant action, such as the reduction in the activity of the antioxidant enzymes GR and GST.     

Effect of dietary trans-stilbene oxide on hepatic and renal drug-metabolizing enzyme activities and bromobenzene-induced toxicity in male Sprague-Dawley rats

The effect of dietary trans-stilbene oxide (TSO) on hepatic and renal xenobiotic metabolizing-enzyme activities and bromobenzene-induced toxicity was quantified in adult male Sprague-Dawley rats. Rats were fed a regular diet or the same diet supplemented with 2.5 g TSO/kg diet for 10 days. TSO treatment did not alter hepatic or renal arylhydrocarbon hydroxylase activity, but significantly increased glutathione S-transferase and uridine diphosphoglucuronyl transferase activities in both organs. In addition, TSO increased hepatic, but not renal, epoxide hydrolase activity. The same treatment did not produce adverse effects on renal or hepatic functions, but markedly potentiated bromobenzene hepatotoxicity. A single dose of bromobenzene (0.2 ml/kg) caused a slight increase in serum glutamic pyruvic transaminase (SGPT) activity and minor hepatic necrosis in animals fed the control diet; the same dose of bromobenzene markedly increased SGPT activity and produced severe hepatic necrosis in the TSO-fed animals.     

Effects of model traumatic injury on hepatic drug metabolism in the rat. VI. Major detoxification/toxification pathways

A previously validated small mammal trauma model, hindlimb ischemia secondary to infrarenal aortic ligation in the rat, was utilized to investigate the effects of traumatic injury on two of the major hepatic enzymes of detoxification, glutathione S-transferase and epoxide hydrolase. Hepatic cytosolic glutathione S-transferase activity toward a variety of substrates showed a 26-34% decrease at 24 hr after model injury. Hepatic microsomal epoxide hydrolase activity toward 1,2-epoxy-3-(p-nitrophenoxy)propane was diminished by 53% after model trauma. Both enzymatic activities toward styrene oxide were similarly depressed. The toxicological sequelae of these derangements were illustrated by administering a dose of styrene oxide (300 mg/kg, ip) which was below the threshold dose (350 mg/kg, ip) necessary to produce hepatotoxicity in control animals. Model trauma dramatically enhanced the hepatotoxic effects of the subthreshold dose, as well as the covalent binding of labeled styrene oxide to liver proteins. These findings indicate that traumatic injury renders the animal more susceptible to agents which are detoxified by glutathione S-transferase and epoxide hydrolase. Conversely, model trauma provided almost complete protection from the hepatotoxic effects of a standard dose (200 mg/kg, ip) of bromobenzene. This protection appeared to derive from a post-traumatic alteration of cytochrome P-450 subpopulations that decreased the formation of the potentially toxic 3,4-epoxide metabolite, despite an increase in the cytochrome P-448-mediated generation of the nontoxic 2,3-epoxide. For bromobenzene, the change in cytochrome P-450-mediated activation appeared quantitatively more significant in overall toxicity than the post-traumatic depression of detoxification pathways described above, leading to decreased toxicity in vivo. For other compounds, the combination of post-traumatic influences on cytochrome P-450/P-448 activity and epoxide hydrolase/glutathione S-transferase activities could lead to markedly enhanced toxicity.     

The effect of long-term streptozotocin-induced diabetes on the hepatotoxicity of bromobenzene and carbon tetrachloride and hepatic biotransformation in rats

To exclude the possibility that changes in hepatotoxicity and biotransformation were induced by diabetogen administration, the influence of long-lasting experimental insulin-dependent diabetes on the activities of benzphetamine demethylase, styrene oxide hydrolase, and UDP-glucuronosyl-transferases toward 1-naphthol, diethylstilbestrol, estrone and testosterone, and glutathione S-transferases toward 1-chloro-2,4-dinitrobenzene, ethacrynic acid, and sulfobromophthalein was studied. Adult male Sprague-Dawley rats injected with 45 mg streptozotocin/kg rapidly developed the classical symptoms of diabetes which persisted throughout the 90-day test period. Ketonemia was detectable at 6 but not at either 35 or 90 days after streptozotocin administration. After acute challenge with bromobenzene or carbon tetrachloride (CCl4), aspartate and alanine aminotransferase activities in rats diabetic for 35 and 90 days were markedly higher than those in normal rats, suggesting that diabetes potentiated the hepatotoxicity of these chemicals. Administration of 25 microliters CCl4/kg, ip, to diabetic rats decreased enzyme activities toward benzphetamine, sulfobromophthalein, 1-chloro-2,4-dinitrobenzene, and 1-naphthol. In normal rats, a dose of 400 microliters CCl4/kg, ip, was required to cause similar changes in enzyme activities. Bromobenzene (500 microliters/kg, ip) elicited opposing responses in diabetic and normal rats in N-demethylase activity, in UDP-glucuronosyltransferase activity toward 1-naphthol, estrone, and testosterone, and in glutathione S-transferase activity toward 1-chloro-2,4-dinitrobenzene. Total cytochrome P450 concentrations were reduced by both induction of diabetes and hepatotoxicant challenge. Thus, chronic uncontrolled diabetes alters the response of hepatic xenobiotic biotransformation enzymes in a non-uniform, substrate-dependent manner, independent of initial diabetogen effects. The role of cytochrome P450j in potentiating CCl4 toxicity is discussed.     

Modulatory effect of naringenin on N-methyl-N'-nitro-N-nitrosoguanidine- and saturated sodium chloride-induced gastric carcinogenesis in male Wistar rats

Naringenin is a flavanone that is believed to have many biological actions, including as an anti-oxidant, free radical scavenger and an antiproliferative agent. The global incidence of gastric carcinoma is increasing rapidly, more than for any other cancer. Therefore, in the present study, we tested the effects of naringenin on gastric carcinogenesis induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and saturated sodium chloride (S-NaCl) in rats. Male Wistar rats were divided into five groups and treated over a period of 20 weeks as follows: (i) a control group given corn oil (1 mL/rat, p.o.) daily 20 weeks; (ii) 200 mg/kg, p.o., MNNG on Days 0 and 14 with S-NaCl (1 mL/rat) administered twice a week for the first 3 weeks; (iii) 200 mg/kg, p.o., MNNG on Days 0 and 14, with naringenin (200 mg/kg, p.o., daily) treatment for the entire 20 weeks; (iv) 200 mg/kg, p.o., MNNG on Days 0 and 14, with naringenin treatment (200 mg/kg, p.o., daily) initiated from 6 to 20 weeks; (v) 200 mg/kg, p.o., naringenin alone daily for 20 weeks. In Group II rats in which gastric cancer was inducted with MNNG and S-NaCl, there was a significant increase in hydrogen peroxide and lipid peroxidation levels, with decreases in reduced glutathione, oxidized glutathione, glutathione peroxidase, glutathione reductase and glucose 6-phosphate dehydrogenase. In addition, in Group II rats with gastric cancer, there were significant increases in the activity of cytochrome P450, cytochrome b(5) and NADPH cytochrome c reductase, with concomitant decreases in the activity of the phase II enzymes glutathione S-transferase and UDP-glucuronosyl transferase. Naringenin treatment (Groups III and IV) restored enzyme activity to near control levels. These results indicate that naringenin has a chemopreventive action against MNNG-induced gastric carcinoma in experimental rats.     

Induction of cytosolic and microsomal epoxide hydrolases by the hypolipidaemic compound nafenopin in the mouse liver

The repeated oral administration of nafenopin, a hypolipidaemic compound, at a dose of 100 mg/kg to male C57BL/6, DBA/2, Balb c and C3H mice caused an increase in the specific activity of liver cytosolic epoxide hydrolase, the activity of microsomal epoxide hydrolase was also increased in all except the C3H mice. The dose dependence and the specificity of this induction was investigated in male DBA/2 mice. In the range of 10-200 mg/kg nafenopin the induction of the two hydrolase activities was found to increase with increasing doses of the test compound. Two other cytosolic enzyme activities, lactate dehydrogenase and glutathione S-transferase, remained essentially unchanged within the dose range investigated.     

Effects of mutagenic and non-mutagenic aniline derivatives on rat liver drug-metabolizing enzymes

1. The effect of 4,4'-methylene bis(2-chloroaniline) (MOCA), 4,4'-methylene dianiline (MDA) and 4,4'-sulphonyldianiline (Dapsone) in vivo on xenobiotic biotransformation in male rat liver was studied. 2. Treatment with MOCA or MDA but not Dapsone caused a dose-dependent increase in ethoxyresorufin O-deethylase activity and a concomitant decrease in aldrin epoxidase activity in male rats. 3. Treatment with MOCA or MDA resulted in dose-dependent increases in ethoxycoumarin O-deethylation and epoxide hydrolation, while only MOCA induced cytosolic glutathione S-transferase activity. 4. Treatment with Dapsone resulted in no changes in xenobiotic biotransformation except for the induction of aniline hydroxylation. 5. The results are consistent with the contention that there is a relationship between carcinogenic chemicals and particular alterations in the activities of biotransformation enzymes.     

Differential induction of cytosolic epoxide hydrolase, microsomal epoxide hydrolase, and glutathione S-transferase activities

Three major enzyme systems have been shown to metabolize epoxidized xenobiotics in vertebrate tissues, and this study demonstrates that these enzyme systems can be differentially induced. The cytosolic epoxide hydrolase activity was routinely monitored with trans-beta-ethylstyrene oxide, the microsomal epoxide hydrolase activity with benzo(a)pyrene, 4,5-oxide, and the glutathione S-transferase activity with 2,4-dichloro-4-nitrobenzene. Commonly used inducers of microsomal mixed-function oxidase, microsomal epoxide hydrolase, and cytosolic glutathione S-transferase activities failed to cause significant induction of the cytosolic epoxide hydrolase while leading to the expected induction of the other epoxide metabolizing enzymes. The compounds tested by ip injection into male Swiss-Webster mice included phenobarbital, 3-methylcholanthrene, Aroclor 1254, trans- and cis-stilbene oxides, pregnenolone-16 alpha-carbonitrile, chalcone, and 4-bromochalcone. To determine if there were strain, sex, or species differences, the enzymes were monitored in male C57BL/6 mice, female Swiss-Webster mice, and male Sprague-Dawley rats following ip injection of phenobarbital, 3-methylcholanthrene, and/or pregnenolone-16 alpha-carbonitrile. The time dependence of enzyme induction was followed in Sprague-Dawley rats following trans-stilbene oxide administration. Male Swiss-Webster mice were additionally exposed to dietary alpha-naphthoflavone and 2(3)-tert-butyl-4-hydroxyanisole while male Sprague-Dawley rats were fed 2,6-di-tert-butyl-4-methylphenol. In no case was significant induction of cytosolic epoxide hydrolase activity observed. Dietary di-(2-ethylhexyl)phthalate, 2-ethyl-l-hexanol, and clofibrate proved to be potent inducers of the cytosolic epoxide hydrolase in male Swiss-Webster mice while probucol (a nonperoxisome proliferating hypolipidemic drug) failed to cause significant induction. Data from isoelectric focusing experiments and other data are consistent with the epoxide hydrolase activities induced by 2-ethyl-l-hexanol and clofibrate being due to the same protein that is present in control animals. The lack of induction of the cytosolic epoxide hydrolase by a variety of compounds which were selected to demonstrate induction of other xenobiotic metabolizing enzymes, may indicate that the cytosolic epoxide hydrolase has a constitutive role whereas its induction by clofibrate could be related to some of the pharmacological and/or carcinogenic actions of this drug.     

A pleiotropic response to phenobarbital-type enzyme inducers in the F344/NCr rat. Effects of chemicals of varied structure.

The effects of a number of phenobarbital-type inducers on selected drug-metabolizing enzymes in male F344/NCr rats were determined by measuring specific catalytic activities and/or by measuring the levels of RNA which hybridize with specific probes for the corresponding genes. The effects on hepatic CYP2B1 were assessed by measuring the levels of CYP2B1-specific RNA and benzyloxyresorufin O-dealkylase and testosterone 16 beta-hydroxylase activities. Levels of CYP3A were monitored by measuring the rate of hydroxylation of testosterone at the 6 beta-position. Microsomal epoxide hydrolase activity was determined by measurement of cellular RNA specific for this form and by assaying the hydrolysis of benzo[a]pyrene-4,5-oxide. UDP-glucuronyltransferase activity was assayed by measuring the glucuronidation of 3-hydroxybenz[a]anthracene. Levels of glutathione S-transferase Ya/Yc were measured by quantifying total cellular RNA coding for the proteins. When male F344/NCr rats were administered various doses of phenobarbital or dichlorodiphenyltrichloroethane (DDT), strong correlations between the induction of CYP2B1 and the induction of epoxide hydrolase or UDP-glucuronyltransferase activities were observed. Treatment of rats with barbiturates, hydantoins, halogenated pesticides such as DDT or alpha-hexachlorocyclohexane, 2,4,5,2',4',5'-hexachlorobiphenyl, CYP2B1 inhibitors such as clotrimazole or clonazepam, or such structurally-diverse compounds as 2-hexanone or diallyl sulfide resulted in induction of CYP2B1-mediated enzyme activity and induction of certain other forms of cytochrome P450, microsomal epoxide hydrolase, at least one form of UDP-glucuronyltransferase, and multiple forms of glutathione S-transferase. This suggests that, as a class, compounds which induce CYP2B1 also induce a coordinate hepatic pleiotropic response which includes induction of these other phase I and phase II drug-metabolizing enzymes.     

Activities of hepatic epoxide hydrolase and glutathione S-transferase in rats under the influence of tetramethyl thiuramdisulfide, tetramethyl thiurammonosulfide or dimethyl dithiocarbamate

The epoxide hydrolase (EH) activity in the liver of adult female Wistar rats significantly increased 18 h after the administration by gavage of tetramethyl thiuramdisulfide (TMTD, 1 mmol/kg) or tetramethyl thiurammonosulfide (TMTM, 2 mmol/kg). No increase was observed 5 h after administration of Na-dimethyl dithiocarbamate (Na-DMDTC, 4 mmol/kg). The glutathione S-transferase (GST) activity in the cytosol and microsomes of the liver was slightly enhanced after oral (gavage) administration of TMTD, TMTM or Na-DMDTC (doses up to 4 mmol/kg). In vitro, TMTD, TMTM, and Na-DMDTC significantly enhanced the hepatic activity of EH prepared from adult female Wistar rats. Cytosolic and microsomal GST activities from the liver were significantly raised in vitro by Na-DMDTC. The results have a bearing on the evaluation of the risk to health of these chemicals in the workplace.     

Comparative effects of butylated hydroxyanisole on hepatic in vivo DNA binding and in vitro biotransformation of aflatoxin B1 in the rat and mouse

To determine the mechanisms which mediate species- and treatment-related differences in susceptibility to aflatoxin B1 (AFB), we conducted a comparative study of the effects of dietary butylated hydroxyanisole (BHA) on the hepatic in vivo DNA binding and in vitro biotransformation of AFB in the rat and mouse. Mice are resistant to the hepatocarcinogenic effects of AFB, and BHA pretreatment has been shown to inhibit the carcinogenic effects of AFB in the highly susceptible rat. Rats and mice were fed a control diet or an identical diet containing 0.75% BHA for 10 days. On the 11th day, one-half of the control and BHA animals were administered [3H]AFB (0.25 mg/kg in dimethyl sulfoxide) via intraperitoneal injection. Animals were killed 2 hr later and covalent binding of AFB to hepatic DNA was determined. The remaining animals were killed for preparation of hepatic subcellular fractions used in in vitro assays. BHA treatment resulted in a decrease in in vivo hepatic AFB-DNA adduct formation in mice to 68% of control, but, in rats, treatment decreased AFB-DNA binding to 18% of control. Furthermore, hepatic AFB-DNA binding in control mice was only 1.2% of that measured in control rats. The rate of in vitro activation of AFB to the epoxide was 3.4-fold greater in control mice relative to control rats. BHA pretreatment increased the activation of AFB in mice 3.3-fold, but had no effect on oxidative metabolism in rats. Control mice had 52 times greater glutathione S-transferase (GST) activity toward the AFB-epoxide, but only 2.6 times greater GST activity toward 1-chloro-2,4-dinitrobenzene (CDNB), compared to that of control rats. In mice, BHA did not significantly increase GST activity toward the AFB-epoxide, but increased GST activity toward CDNB 3.1-fold. In rats, BHA increased GST activity toward the AFB-epoxide and CDNB by 3.2- and 2.1-fold, respectively. Epoxide hydrolase activity toward p-nitrostyrene oxide in mice was only 52% of the activity in rats. BHA increased epoxide hydrolase activity 3.8- and 2.5-fold in mice and rats, respectively. These data indicate that mice have high levels of an AFB-epoxide-specific GST activity relative to that of the rat. The rate of formation of the AFB-epoxide and the activity of epoxide hydrolase appear to be relatively unimportant under conditions of high GST activity, whereas elevated GST activity, and thus inactivation of the AFB-epoxide, appears to be the critical component in species- and BHA-induced differences in AFB-DNA adduct formation and, presumably, AFB hepatocarcinogenicity.     

Effects of picloram on xenobiotic biotransformation in rat liver

1. The effect of picloram on model xenobiotic substrate biotransformation in vivo was studied in female and male rat liver. 2. Treatment with picloram had little effect on epoxide hydratase and glutathione S-transferase activity, but caused a dose-dependent increase in ethoxyresorufin-O-deethylase activity and a concomitant decrease in aldrin epoxidase activity in male rats. 3. Treatment of male rats with equivalent doses of 2-acetylaminofluorene, 2-amino-anthracene and picloram induced ethoxyresorufin-O-deethylase activity to the same degree. 4. Treatment of female rats with picloram resulted in dose-dependent increases in ethoxyresorufin and ethoxycoumarin-O-deethylation without decreasing aldrin epoxidase activity. 5. Picloram binds to liver microsomal preparations from rats pretreated with phenobarbitone and/or 3-methylcholanthrene, giving a type I spectrum. 6. The results indicate that picloram is a 3-methylcholanthrene-type inducer, and the implications are discussed.     

Inhibition and recovery of rat hepatic glutathione S-transferase zeta and alteration of tyrosine metabolism following dichloroacetate exposure and withdrawal.

Dichloroacetate (DCA) is an investigational drug for certain metabolic disorders, a by-product of water chlorination and a metabolite of certain industrial solvents and drugs. DCA is biotransformed to glyoxylate by glutathione S-transferase zeta (GSTz1-1), which is identical to maleylacetoacetate isomerase, an enzyme of tyrosine catabolism. Clinically relevant doses of DCA (mg/kg/day) decrease the activity and expression of GSTz1-1, which alters tyrosine metabolism and may cause hepatic and neurological toxicity. The effect of environmental DCA doses (microg/kg/day) on tyrosine metabolism and GSTz1-1 is unknown, as is the time course of recovery from perturbation following subchronic DCA administration. Male Sprague-Dawley rats (200 g) were exposed to 0 microg, 2.5 microg, 250 microg, or 50 mg DCA/kg/day in drinking water for up to 12 weeks. Recovery was followed after the 8-week exposure. GSTz specific activity and protein expression (Western immunoblotting) were decreased in a dose-dependent manner by 12 weeks of exposure. Enzyme activity and expression decreased 95% after a 1-week administration of high-dose DCA. Eight weeks after cessation of high-dose DCA, GSTz activity had returned to control levels. At the 2.5 or 250 microg/kg/day doses, enzyme activity also decreased after 8 weeks' exposure and returned to control levels 1 week after DCA was withdrawn. Urinary excretion of the tyrosine catabolite maleylacetone increased from undetectable amounts in control rats to 60 to 75 microg/kg/24 h in animals exposed to 50 mg/kg/day DCA. The liver/body weight ratio increased in the high-dose group after 8 weeks of DCA. These studies demonstrate that short-term administration of DCA inhibits rat liver GSTz across the wide concentration range to which humans are exposed.     

Effects of Phenoxyherbicides and Glyphosate on the Hepatic and Intestinal Biotransformation Activities in the Rat

Abstract: The effects of phenoxyacid herbicides 2,4-D (2,4-dichlorophenoxyacetic acid) and MCPA (4-chloro-2-methylphenoxyacetic acid), clofibrate, and glyphosate on hepatic and intestinal drug metabolizing enzyme activities were studied in rats intragastrically exposed for 2 weeks. The hepatic ethoxycoumarin O-deethylase activity increased about 2-fold with MCPA. Both 2,4-D and MCPA increased the hepatic epoxide hydrolase activity and decreased the hepatic glutathione S-transferase activity. MCPA also increased the intestinal activities of ethoxycoumarin O-deethylase and epoxide hydrolase. Glyphosate decreased the hepatic level of cytochrome P-450 and monooxygenase activities and the intestinal activity of aryl hydrocarbon hydroxylase. Clofibrate decreased the hepatic activities of UDPglucuronosyltransferase with p-nitrophenol or methylumbelliferone as the substrate. Also 2,4-D decreased the hepatic activity of UDPglucuronosyltransferase with p-nitrophenol as the substrate. MCPA decreased the intestinal activities of UDPglucuronosyltransferase with either p-nitrophenol or methylumbelliferone as the substrate. The results indicate that phenoxyacetic acids, especially MCPA, may have potent effects on the metabolism of xenobiotics. Glyphosate, not chemically related to phenoxyacids, seems to inhibit monooxygenases. Whether these changes are related to the toxicity of these xenobiotics remains to be clarified in further experiments.