Enhancement of microsomal drug hydroxylation and glucuronidation in rat liver by phenobarbital and 3-methylcholanthrene in combination

UDP glucuronosyltransferase and mixed function oxidase activity in microsomes prepared by differential centrifugation and calcium aggregation

Microsomes were prepared from the liver, kidney and lung of phenobarbital or 20-methylcholanthrene treated and control rats with the conventional ultracentrifugation and calcium aggregation methods. The two methods were compared as to the yield of microsomal protein, amount of cytochrome P-450/448 and activity of UDP-GLUCURONOSYLTRANSFERASE, BENZOPYRENE HYDROXYLASE AND P-NITROANISOLE O-demethylase. The absolute amount of cytochrome P-450/448 (nmol/g wet weight), as well as the enzymatic activities dependent on it (nmol produced/g wet weight) did not differ significantly in any tissue of either treated or control animals nor did that of UDPglucuronosyltransferase. However, the ultracentrifugation method resulted in a slightly smaller yield of the hepatic microsomal protein and a correspondingly higher yield of cytochrome P-450/448 per mg protein as well as higher specific enzymatic activities of both the consecutive drug biotransformation reactions studied. The specific activity of UDPglucuronosyltransferase in digitonin treated microsomes was twice as high in the conventional microsomes as in the calcium aggregated microsomes; no differences was found in the trypsin treated microsomes. The specific activity of the hepatic benzpyrene hydroxylase of the benzpyrene treated animals in the calcium harvested microsomes was 55 per cent of that in the ultracentrifugated microsomes.

Effects of Exposure Period on the Developmental Toxicity of 2-Bromopropane in Sprague-Dawley Rats

Recently we reported that 2-bromopropane (2-BP) has maternal toxicity, embryotoxicity, and teratogenicity in Sprague-Dawley rats. The aims of this study are to examine the potential effects of 2-BP administration on pregnant dams and embryo-fetal development, and to investigate the effects of metabolic activation induced by phenobarbital (PB) on developmental toxicities of 2-BP. Pregnant rats received 1000 mg/kg/day subcutaneous 2-BP injections on gestational days (GD) 6 through 10 (Group II and Group IIII) or 11 through 15 (Group IV). Pregnant rats in Group III received an intraperitoneal PB injection once daily at 80 mg/kg/day on GD 3 through 5 for induction of the liver metabolic enzyme system. Control rats received vehicle injections only on GD 6 through 15. All dams underwent caesarean sections on GD 20 and their fetuses were examined for external, visceral, and skeletal abnormalities. Significant adverse effects on pregnant dams and embryo-fetal development were observed in all the treatment groups, and the maternal and embryo-fetal effects of 2-BP observed in Group II were higher than those seen in Group IV. Conversely, maternal and embryo-fetal developmental toxicities observed in Group III were comparable to those seen in Group II. These results suggest that the potential effects of 2-BP on pregnant dams and embryo-fetal development are more likely in the first half of organogenesis (days 6~10 of pregnancy) than in the second half and that the metabolic activation induced by PB pre-treatment did not modify the developmental toxic effects of 2-BP in rats.

Properties of human hepatic UDP-glucuronosyltransferases. Relationship to other inducible enzymes in patients with cholestasis

Glucuronidation of 4-nitrophenol, nopol (a monoterpenoid alcohol) and bilirubin, which in the rat, are catalyzed by three different enzymes, has been examined in liver biopsies from patients with various liver diseases, in particular cholestasis. These different activities were not correlated, which strongly suggests that at least three independently regulated forms of UDP-glucuronosyltransferases were present in the microsomes. Non ionic detergents (Triton X100, Emulgen 911) and deoxycholate produced similar activation (more than 2-fold) of the glucuronidation of 4-nitrophenol. Amphipathic substances, such as CHAPS (3-[3-cholamidopropyl-dimethylammonio]-1-propane sulfonate), and lysophosphatidylcholines maximally increased this UDP-glucuronosyltransferase activity, the most potent being oleoyl lysophosphatidylcholine (4-fold increase). Discriminant analysis of the data revealed no correlation between the three different UDP-glucuronosyltransferase activities and the age or sex of the patients. A good correlation was found on multidimensional analysis between form 1 of the enzyme (4-nitrophenol glucuronidation) and, in decreasing order of magnitude, epoxide hydrolase (measured with benzo(a)pyrene-4,5-oxide as substrate), cytochrome P-450, 7-ethoxycoumarin deethylase, aspartate aminotransferase and gamma-glutamyltransferase (r = 0.89); and between Form 3 of the enzyme (bilirubin glucuronidation) and NADPH cytochrome c reductase, alkaline phosphatase, (r = 0.81). These relationships may reflect the differential variation in enzymatic activities in various hepato-biliary diseases.

Elevated binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin and 3-methylcholanthrene to the Ah receptor in hepatic cytosols from phenobarbital-treated rats and mice

Binding of 3-methylcholanthrene (MC), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and other "MC-type" inducers to cytosolic Ah receptor sites is the first specific step in induction of aryl hydrocarbon hydroxylase (AHH; cytochrome P1-450) by these compounds. [3H]TCDD and [3H]MC were used as radioligands to quantitate and characterize Ah receptor in hepatic cytosols from genetically "responsive" C57BL/6J mice, genetically "nonresponsive" DBA/2J mice, and AHH-inducible Sprague-Dawley rats. Injection of 50-100 mg/kg of phenobarbital (PB) for 3 days more than doubled the concentration of Ah receptor in hepatic cytosol from Sprague-Dawley rats. In C57BL/6J mice, PB injection at 25 mg/kg X 3 days significantly increased (P less than 0.01) the Ah receptor concentration in hepatic cytosol. No cytosolic Ah receptor was detectable in hepatic cytosol from untreated DBA/2J mice, nor did any Ah receptor appear after PB treatment in this "nonresponsive" strain. Although PB significantly elevated Ah receptor in hepatic cytosols of responsive rodents, many previous studies have shown that the maximal level of AHH activity in animals given PB and an "MC-type" inducer simultaneously is additive rather than synergistic. Ah receptor concentrations can be doubled by PB treatment without doubling the subsequent AHH-induction response to "MC-type" compounds. Thus, the cytosolic Ah receptor concentration per se may not be the primary determinant of a given tissue's maximal capacity for AHH induction by "MC-type" compounds.

The metabolism of arachidonic acid in hamster lung microsomes is not completely inhibited by aspirin and indomethacin

Aspirin (100 microM or 1 mM) or indomethacin (10 microM or 100 microM) was incubated with a microsomal preparation of hamster lungs in the presence of NADPH for 10 min. Then 14C-arachidonic acid (20 microM) was added and the incubation was continued for an additional 20 min. The metabolites were extracted with ethyl acetate first at pH 7.4 and then at pH 3.5 and analysed by thin layer chromatography. Both aspirin and indomethacin inhibited dose dependently the formation of all identified prostaglandins, including PGF2 alpha, 6-keto-PGF1 alpha, PGE2 and PGD2. The rate of formation of some unidentified metabolites extracted at pH 7.4 and 3.5 was, however, not changed by aspirin or indomethacin. We have earlier reported that in isolated perfused hamster lungs the formation of all arachidonate metabolites is inhibited by both aspirin and indomethacin. As the present study indicates that in the microsomes of hamster lungs all metabolic pathways of arachidonic acid are not inhibited by aspirin or indomethacin, it is possible that in isolated tissues and in vivo aspirin-like drugs have some other inhibitory effects on arachidonate metabolism than the inhibition of the cyclo-oxygenase enzyme.

UDP-glucuronosyltransferase activity in rat-and hairless mouse skin-microsomes

1. Cutaneous UDP-glucuronosyltransferase activity (E.C.2.4.1.17) was demonstrated in rat- and hairless mouse-skin microsomes using 1-naphthol as substrate. 2. Addition of the detergent Brij 35 increased the activity by approximately twofold in both species. 3. Inhibitor studies demonstrated that under the assay conditions used any UDP-glucuronic acid pyrophosphatase or beta-glucuronidase present did not interfere with the conjugation reaction. 4. Substrate inhibition was observed in hairless mouse-skin preparations and biphasic response to increasing naphthol concentration was seen in rat-skin microsomes. 5. The apparent Km values were considerably lower than those reported for liver. The sp. activity (per mg microsomal protein) in unactivated rat-skin microsomes was about 50% of that reported in unactivated rat-liver microsomes. 6. Pretreatment with 3-methylcholanthrene resulted in a small increase in cutaneous UDP-glucuronosyltransferase activities in both species.

Differential induction of rat liver microsomal UDP-glucuronosyltransferase activites by various inducing agents

The selectivity of various inducers of UDP-glucuronosyltransferase was investigated in rat liver microsomes and compared with their effect on monooxygenase reactions. (1) Similar to 3-methyl-cholanthrene beta-naphthoflavone selectively stimulated the glucuronidation of 1-naphthol and 4-methylumbelliferone (GT1 substrates). (2) In contrast, DDT preferentially enhanced the glucuronidation of morphine, 4-hydroxybiphenyl (GT2 substrates) and bilirubin, similar to phenobarbital. (3) Colfibric acid and bezafibrate selectively enhanced bilirubin glucuronidation without affecting GT1 and GT2 reactions. (4) Similar to ethoxyquin and Aroclor 1254, trans-stilbene oxide enhanced both GT1 and GT2 activities but not bilirubin glucuronidation. (5) In contrast to 3-methylcholanthrene-type inducers which induce both cytochrome P-450MC and GT1, probably through a common receptor protein, ethoxyquin and trans-stilbene oxide markedly induced GT1 reactions without affecting benzo[a]pyrene monooxygenase.

Enhancement of hepatic drug biotransformation by a short-term intermittent turpentine exposure in the rat

An inhalation exposure of male rats to 300 p.p.m. of a commercial turpentine 6 hrs daily 5 days a week for 8 weeks enhanced the activities of drug biotransformation enzymes of liver microsomes considerably. The activities of NADPH cytochrome c reductase and 7-ethoxycoumarin deethylase, and microsomal content of cytochrome P-450 were increased 35-60% during the first weeks of the experiment, but had a tendency to return towards the control values later on. A similar enhancement of activities was also found in liver microsomal epoxide hydratase and UDP glucuronosyltransferase, but these enzyme activities tended to adapt less during the experiment. The turpentine treatment increased the affinity of liver microsomal cytochrome P-450 to alpha-pinene (the main component of the turpentine). The present data suggests that exposure to turpentine is able to modify considerably the biotransformation of drugs.

Carbon disulphide induced activation of liver UDP glucuronosyltransferase in rats pretreated with phenobarbitone

Carbon disulphide (CS2) exposure has been shown to activate the UDP glucuronosyltransferase of liver microsomes in rats pretreated with phenobarbitone. Now the nature of CS2 induced activation of the enzymes has been studied further. Phenobarbitone pretreated rats were exposed to 0.15% CS2 for 2 hrs on two successive days. The activity of UDP glucuronosyltransferase was measured from the liver microsomes after the enzymes was activated by incubation of the microsomes with various concentrations of the detergents Triton X-100, digitonin and cetylpyridinium chloride. The exposed animals showed an increased enzyme activity at all applied concentrations of the detergents; therefore in addition to membrane destruction by CS2 exposure, some other mechanism must also be involved in the CS2 induced activation of liver microsomal UDP glucuronosyltransferase. The changes in membrane lipid-protein interactions with l-anilino-8-naphthalene sulphonate (ANS) were also probed. The CS2 exposed animals had more high-affinity binding sites for ANS in their liver microsomal membranes, and in addition the quantum yield of ANS fluorescence was enhanced by CS2. The changes differed from those found after carbon tetrachloride exposure and suggest that, even if the two drugs have some common effects on microsomes, e.g. UDP glucuronosyltransferase activation, P-450 destruction and lipid peroxidation induction, the changes they cause in the microsomal micro-environment differ.

Long-term effects of single and combined doses of DDT and PCB on drug-metabolizing enzymes in rat liver

Two common environmental pollutants, DDT and a polychlorinated biphenyl (PCB) mixture (Clophen A-50), were administered ip to rats in discrete single doses (160 and 100 mg/kg, respectively) and in combination. All the enzyme activities studied were enhanced by DDT and PCB. the overall drug hydroxylation reactions, and their components, achieved maximal induction in 1 wk. The cytochrome P-450 content of microsomes was increased nearly fourfold and NADPH-cytochrome c reductase activity was enhanced twofold by both compounds. p-Nitroanisole O-demethylase was increased sevenfold by PCB and fourfold by DDT, aryl hydrocarbon hydroxylase threefold by PCB and 1.7-fold by DDT. After 2 wk the activities began to decline. Distinct increases in enzyme activities were still detectable 1 month after a single dose. Epoxide hydratase and UDPglucuronosyltransferase activities were also enhanced in 1 wk (epoxide hydratase 2.5-fold by both compounds, UDPglucuronosyltransferase tenfold by PCB in trypsin-activated microsomes but only threefold by DDT). The disappearance of induction in epoxide hydratase was slower than in the monooxygenases, and UDPglucuronosyltransferase still showed a trend toward increased activity 4 wk after the administration. The DDT-enhanced UDPglucuronosyltransferase activity slightly returned toward the control level. Glutathione S-transferase differed from the microsomal enzymes in that it was already elevated 1 day after the administration of both DDT and PCB. Its activity was only doubled, but the increased activity remained at almost the same level through the whole 1 month period.

Trichloroethylene and 1,1,1-trichloroethane: effects on brain and liver after five days intermittent inhalation

The inhalation exposure of adult male rats to 7.9 mumol/l (200 ppm) of trichloroethylene for 4 days 6 h each day led to an accumulation of the solvent in the perirenal fat 17 h after the last exposure. Exposures to 20 mumol/l (500 ppm) of 1,1,1-trichloroethane caused similar accumulation. The latter rats presented no changes in their behaviour in an open-field test whereas the motor behaviour of the animals exposed to trichloroethylene was more active in comparison to controls 1 h after the exposure on the 4th day. Trace effects of trichloroethylene on emotional behaviour of the same rats could be seen 17 h after the last exposure. Further exposures on the 5th day increased brain, liver, lung and blood contents of trichlorethylene and 1,1,1-trichloroethane. A slight decrease in brain RNA content was found in the 1,1,1-trichloroethane exposed rats while RNA decreased significantly after the fifth day of trichloroethylene inhalation. The exposure to 1,1,1-trichloroethane on the 5th day depressed also the microsomal cytochrome P-450 content in liver of rats whereas trichloroethylene increased the hemochrome content slightly at the same time.

The in vivo toxicity of CS2 to liver microsomes: binding of labelled CS2 and changes of the microsomal enzyme activities

The binding of 35S and 14C labelled CS2 to liver microsomes was studied in control and phenobarbitone pretreated rats 3 and 6 hrs after an intraperitoneal injection. The level of hepatic cytochrome P-450, the activities of epoxide hydratase and UDP-glucuronosyltransferase were analyzed in the same animals. The binding of the sulphur label was considerably higher than that of carbon 3 hrs after the injection, the difference being less evident at 6 hrs.T he measurable P-450 declined after the CS2 injection. It was approximately 40% in the phenobarbitone pretreated rats and 60% in control rats of the values of animals which were not treated with CS2. CS2 did not affect microsomal epoxide hydratase activity, while it increased the measurable activity of UDP-glucuronosyltransferase. The increase was evident 3 hrs after the injection of CS2 in the phenobarbitone pretreated rats. It could also be detected in the control animals 6 hrs after the injection. The present data suggest that the change in the measureble P-450 results from the binding of the metabolite(s) of CS2 to the cytochrome, and its subsequent degradation. The increase in measurable UDP-glycuronosyltransferase activity results probably from the activated perturbation of the structure of microsomal membrane by the metabolites of CS2 in vivo.

Effects of aliphatic chlorohydrocarbons on drug-metabolizing enzymes in rat liver in vivo

1. Various polychlorinated hydrocarbons were administered intragastrically to rats to examine their effects on the biotransformation capacity of the liver. Due to high toxicity, 1,1,2,2-tetrachloroethane and pentachloroethane were given at a dose level equivalent to one quarter of that of CCl4 and the other chlorohydrocarbons (i.e. 2-6 mmol/kg). 2. Carbon tetrachloride at 10-3 mmol/kg was the most active in decreasing cytochrome P-450 content and the overall drug hydroxylation activities in rat liver. 1,1,2,2-Tetrachloroethane was the next most active in decreasing the hepatic drug oxidizing enzymic activities. 3. Expoxide hydratase activity in rat liver declined significantly after CCl4, 1,1,2,2-tetrachloroethane and pentachloroethane administrations. 4. UDP-Glucuronosyltransferase was affected to a lesser extent than the microsomal mono-oxygenase or epoxide hydratase by chlorohydrocarbon treatment.

Enhancement of microsomal monooxygenase, epoxide hydrase and UDPglucuronyltransferase by aldrin, dieldrin and isosafrole administrations in rat liver

The effects of intraperitoneal administration of aldrin (10 mg/kg), dieldrin (10 mg/kg) and isosafrole (50 mg/kg) were investigated on the activities of drug biotransformation enzymes in rat liver. All the compounds studied were found to enhance the activities of microsomal monooxygenase (e.g. p-nitroanisole O-demethylase, aryl hydrocarbon hydroxylase), epoxide hydrase (styrene oxide as substrate) and UDPglucuronyltransferase (p-nitrophenol as aglycone). Dieldrin, the epoxidized derivative aldrin, the eposidized derivative of aldrin, was a more potent inducer than the parent compound itself. NADPH CYTOCHROME C REDUCTASE ACTIVITY WAS EHANCED 2.5-FOLD, P-NITROANISOLE O-demethylase 7-fold, benzpyrene hydroxylase 2-fold, and epoxide hydrase 5-fold after treating rats with dieldrin for 6 days. The increase in activity of the microsomal UDPglucuronyltransferase could be only detected after an in vitro digitonin treatment of microsomal membranes, the enhancement being about 1.5-fold after administering dieldrin for 6 days. The administration of isosafrole to rats increased especially p-nitroanisole O-demethylase activity in liver microsomes (10-fold in 3 days). NADPH cytochrome c reducatase activkty was increased 2-fold, cytochrome P-450 content 1.2-fold, benzpyrene hydrozylase activity 2.5-fold and epoxide hydrase activity 1.2-fold after treatment of rats for 3 days. UDPglucuronyltransferase activity increased 2.2-fold by treating rats for 6 days with isosafrole. This increase was, however, only to be seen in in vitro digitonin-activated microsomes due to the latency of UDPglucuronyltransferase.

Effect of administration route on DDT on acute toxicity and on drug biotransformation in various rodents

DDT was administered to the guinea pig, mouse and rat either ig or ip and to the hamster ig in order to investigate variations in the response of hepatic and duodenal drug-metabolizing enzymes to DDT. The intragastric dose (160 mg/kg) was found to produce gastric bleeding and severe tremor in rats and mice but not in other rodents. The hepatic aryl hydrocarbon hydroxylase activity and cytochrome P-450 concentration decreased after the ig administration of DDT to rats, mice and guinea pigs but in hamsters the activiy of aryl hydrocarbon hydroxylase and cytochrome P-450 concentration increased 12 hr after the dosage. The aryl hydrocarbon hydroxylase activity decreased also in the duodenal mucosa of the rat after the ig administration of DDT. The ip dose had no effects on the hepatic or duodenal monooxygenase system in 12 hr. The UDPglucuronosyltransferase activity was slightly lowered in hepatic microsomes of the rat and mouse after the ig dose of DDT, but the decrease was more profound when measured after in vitro trypsin digestion of microsomes. The trypsin digestion activated the hepatic UDPglucuronosyltransferase in all the species studied, i.e., guinea pig, hamster, mouse and rat (3-, 3-, 5-, and 8-fold, respectively). The duodenal UDPglucuronosyltransferase activity was not affected by DDT administration in any of the species studied. The results suggest that the acute toxic effects of DDT are species-dependent and the administration route is important in DDT toxicity. The hydroxylation step in drug metabolism is more sensitive to DDT than the glucuronidation step.