Drug hydroxylation and glucuronidation in liver microsomes of phenobarbital-treated rats

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.

Glucuronic acid conjugation by hepatic microsomal fractions isolated from streptozotocin-induced diabetic rats

The hepatic glucuronidation of 1-naphthol and 4-nitrophenol (3-methylcholanthrene inducible substrates of glucuronyltransferase, GT 1) was found to be deficient in freshly prepared untreated "native" microsomes from streptozotocin-induced diabetic male rats. The defect was not observed in female rats. Moreover, the glucuronidation of 1-naphthol and 4-nitrophenol was higher in "native" microsomes from male control rats than in those from female controls. This sex difference in the glucuronidation of the GT 1 substrates was abolished by detergent activation of the transferase enzyme in vitro. Streptozotocin treatment did not alter the glucuronidation of paracetamol or phenolphthalein (phenobarbitone inducible substrates for GT 2). This diabetes-induced defect in the glucuronidation of GT 1 substrates was abolished by insulin treatment of the animals and was diminished or completely abolished by detergent activation of the transferase enzyme in vitro. Increased membrane constraint is proposed as the mechanism responsible for the transferase defect. 3-Methylcholanthrene induction abolished the streptozotocin-induced defect in 4-nitrophenol glucuronidation, whereas phenobarbitone did not. This is attributed to the differential effect of these inducers on the microsomal membrane.

Comparison between the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and six other compounds on the vitamin A storage, the UDP-glucuronosyltransferase and the aryl hydrocarbon hydroxylase activity in the rat liver

The effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8- tetrabromodibenzo -p-dioxin ( TBrDD ), 5-chloro-2-(2,4-dichlorophenoxy)phenol (3-Cl- predioxin ) 4,5,6-trichloro-2-(2,4-dichlorophenoxy)phenol (5-Cl- predioxin ), toxaphene, 3-methylcholanthrene (3-MC) and phenobarbital (PB) on the vitamin A storage, UDP-glucuronosyltransferase (UDPGT) and aryl hydrocarbon hydroxylase (AHH) activities in the liver of Sprague-Dawley rats was investigated. Vitamin A was determined as retinol by high pressure liquid chromatography. UDPGT was measured with p-nitrophenol as an aglycone and AHH with 3,4-benzopyrene as a substrate. Both in TCDD- and toxaphene-treated animals a reduced body weight gain was recorded, but no other overt signs of toxicity were seen in this study. Both the concentration and the total amount of hepatic retinol was significantly reduced in TCDD-, 3-MC-, PB- and TBrDD -treated animals. These compounds were also those which gave the most significant enzyme induction as regards the UDPGT and AHH activities. However, the reduction of hepatic retinol caused by these compounds did not correlate with the enzyme activities studied. When compared on a molecular basis, TCDD and TBrDD were in the order of several magnitudes more potent as reducers of hepatic retinol and likewise as enzyme inducers.

The metabolism of 3-benzoylpyridine

3-Benzoylpyridine (3-BP), a decomposition product of the soman antidote, HGG-12 (3-benzoylpyridino(1)-methyl 2'-hydroxyiminomethylpyridino(1')methyl ether dichloride) was rapidly metabolized in the isolated perfused rat liver, giving 3-(alpha-hydroxybenzyl)pyridine and its corresponding glucuronide, 3-benzoylpyridine-N-oxide, and 3-(alpha-hydroxybenzyl)pyridine-N-oxide. The latter is formed both from 3-(alpha-hydroxybenzyl)pyridine and 3-benzoylpyridine-N-oxide. Metabolism of 3-BP studied in rats and dogs in vivo revealed significant species differences. In rat, 80% of 14C-3-BP was excreted as N-oxides and alpha-hydroxybenzyl derivatives in the urine. In dogs, 95% dose was excreted in urine mostly as the glucuronide of 3-(alpha-hydroxybenzyl)pyridine and as the quaternary pyridinium compounds, 3-benzoyl-1-methylpyridinium and 3-(alpha-hydroxybenzyl)-1-methylpyridinium. These latter were hardly detected in rat urine. In contrast to rats, the N-oxides were present only in small amounts in dog urine.

Time-course of effects of toluene on microsomal enzymes in rat liver, kidney and lung during and after inhalation exposure

Inhalation of toluene vapour of 2000 ppm increased the activities of aniline hydroxylase, aminopyrine N-demethylase, aryl hydrocarbon hydroxylase and NADPH-cytochrome c reductase and the concentrations of cytochromes P-450 and b5 in liver microsomes of adult male rats after an exposure period of 1 day or less. Repeated treatments, 8 h daily for 1-16 days, had only a slight further effect. In lung microsomes, the activities of monooxygenases and the concentration of cytochrome P-450 decreased after 6-24 h toluene exposure, but those of cytochrome b5 and NADPH-cytochrome c reductase did not change. In kidney microsomes the changes were mostly insignificant. After discontinuation of exposure the activities of enzymes and the concentrations of cytochromes returned to the control level in 1-4 days. The results obtained resemble the time-courses for the induction of monooxygenases by other inducers. The tissue differences suggest the unequal distribution of various cytochrome P-450 forms and their individual responsiveness to induction in liver, kidneys and lungs.

Induction of aryl hydrocarbon hydroxylase activity in cultured human lymphocytes

Lymphocytes from a non-cancer population (203) of non-smokers, former smokers and smokers were cultured in vitro and assayed for basal and induced AHH activity. A wide range of AHH activities was measured, but there was no indication of separate groups in the distributions of the activities or inducibility. However, smoking slightly increased the basal AHH activity and significantly decreased the inducibility. AHH activity was dependent on the age of the donor, on the season, and correlated with lymphocyte stimulation by mitogens. A lung-cancer population (49 persons) was also tested. They were all smokers, but gave results that were more like those of non-cancer non-smokers, except for the fact that 37% of them gave a basal AHH activity that was too low to be measured, compared to 16% of the non-cancer persons. The observations that many factors, such as season, age and even smoking, affect the AHH activity necessitate a careful control of conditions for the study of the association between AHH inducibility in human lymphocytes and lung cancer. As cancer itself may also have an effect such a study should be done as a prospective study of a selected population.

Tissue concentrations and interaction of zinc with lead toxicity in rabbits

Adult male rabbits were exposed to lead (0.2%), zinc (0.5%) or to both lead and zinc (0.2% and 0.5%) which were given in the drinking water as acetates for 2 weeks or 4 weeks. Blood lead levels in lead exposed rabbits were about 15-fold in comparison with the controls after 2 weeks exposure and remained at this level after further exposure, but when zinc was given with lead the blood lead level doubled from 2 weeks to 4 weeks. In rabbits having both lead and zinc, the cerebral lead concentration was much lower than in rabbits exposed only to the respective amount of lead. No such effect was found in the cerebellum or sciatic nerve. Zinc did not prevent lead accumulation in the parenchymal tissues. Only a rather low induction of drug metabolizing enzyme activities was found in the liver of lead-exposed rabbits and zinc did not modify this induction. The results suggest that, although zinc might delay the lead accumulation in the cerebrum, it has little value in preventing peripheral neuropathy or metabolic alterations caused by lead.

Differences in the response of hepatic and intestinal drug metabolizing enzymes in rats following carbon tetrachloride and/or phenobarbital treatment

The activities of the microsomal drug metabolizing enzymes in the liver and intestinal mucosa of rats were studied after the intraperitoneal administration of carbon tetrachloride and/or subcutaneous phenobarbital administration. The membrane phospholipid content was decreased after carbon tetrachloride treatment indicating destruction in the membrane structure. Aryl hydrocarbon hydroxylase activity was decreased in the liver and intestinal mucosa after treatment with CCl4 alone and in combination with phenobarbital. The CCl4 treatment increased the intestinal epoxide hydratase activity but decreased the activity in the liver. The hepatic UDPglucuronosyltransferase was slightly induced by phenobarbital and the activity was elevated by the CCl4 treatment. In the intestinal mucosa the enhanced UDPglucuronosyltransferase activity was observed only after phenobarbital pretreatment and the activity was decreased by CCl4. These results support the view that epoxide hydratase and UDPglucuronosyltransferase enzymes occupy different locations in the endosplasmic reticulum of intestinal mucosa than of liver.

Hepatic drug metabolism during ethanol ingestion in riboflavin deficient rats

Riboflavin deficiency was induced by feeding rats a riboflavin-deficient diet for 1 month. In order to find out if there are any combined effects of ethanol and riboflavin deficiency on drug metabolism, a group of riboflavin-deficient rats were also given ethanol in their drinking water. At the end of the feeding period, hepatic drug-metabolizing enzyme activities were determined. The hepatic phospholipid and protein contents were the same in rats receiving a standard diet and in those on a riboflavin-deficient diet. However, ethanol ingestion in both groups enhanced significantly the phospholipid content. Ethanol ingestion also markedly enhanced the hepatic cytochrome P-450 concentration in rats fed either a standard or riboflavin-deficient diet. Ethoxycoumarin O-deethylase activity was significantly lower in riboflavin-deficient rat livers than in those of the controls. In both groups ethanol ingestion nearly doubled the activities. Aryl hydrocarbon hydroxylase activity was also significantly decreased during riboflavin deficiency. However, ethanol administration did not change the activities of this enzyme. UDP glucuronosyltransferase activity was slightly lower in riboflavin-deficient rat livers than in those fed a standard diet. No significant decrease was found in the epoxide hydrase activity in the riboflavin-deficient rats. However, the riboflavin-deficient rats had enhanced activity after the ethanol ingestion.

Comparison of the metabolic profiles of benzo[alpha]pyrene obtained from primary cell cultures and subcellular fractions derived from normal and methylcholanthrene-induced rat liver

Hepatocyte primary cell (HPC) cultures derived from either (a) non-induced (normal) or (b) methylcholanthrene (MC)-induced rat liver actively metabolized the carcinogen benzol[alpha]pyrene (BaP) over a 24-h period. In both cases, the BaP metabolites generated were qualitatively similar to those seen in the metabolism of BaP by isolated rat liver microsomal fractions; in addition, unidentified compounds were evident in the chromatographic profile generated by the cultured cells. In cells derived from (a), levels of known metabolites (phenols and diols) increased over the time period studied. On the other hand, in cells derived from (b), levels of diols decreased markedly after 8 h. These results suggest that induction with MC enhances both activation and, to a greater extent, conjugative-detoxification pathways of BaP, so that in cells obtained from (b) the formation of water-soluble metabolites is enhanced and levels of organic soluble metabolites are lower than in cells obtained from (a). Metabolism of BaP in primary cell culture derived from rat liver is thus seen to be similar to in vivo metabolism of the carcinogen, but somewhat in contrast to the in vitro microsomal (subcellular) metabolism of BaP where conjugative-detoxification pathways are virtually inoperative.

Perinatal development of, and effect of chemical pretreatment on, glycine N-acyltransferase activities in liver and kidney of rabbit and rat

The ontogenic development of glycine N-acyltransferase activity was studied in preparations of hepatic and renal mitochondria from the New Zealand White rabbit and the Sprague-Dawley rat. Preparations of hepatic mitochondria from the rat and the rabbit attain adult glycine N-acyltransferase specific activities by birth and 4 weeks of age respectively, whereas mitochondrial preparations from rabbit kidney do not attain adult activity until 4 months of age. Pretreatment of adult rats or immature rabbits with salicylic acid, benzoic acid or phenobarbital had little effect on glycine N-acyltransferase activity in vitro in liver or kidney.

Biochemical and toxicological effects of combined exposure to 1,1,1-trichloroethane and trichloroethylene on rat liver and brain

1. Inhalation exposure of adult male rats to a mixture of 1,1,1-trichloroethane (500 p.p.m.) and trichloroethylene (200 p.p.m.) for four days 6 h daily resulted in an accumulation of 1,1,1-trichloroethane in perirenal fat. Further exposure on the fifth day caused a rapid increase in various organ contents of both solvents with secondary depression of brain RNA. 2. The four-day exposure doubled the RNA content of liver and caused a slight decline in the concentrations of glutathione in liver. 3. The amount of cytochrome P-450 in liver was increased, as well as the overall mono-oxygenase activity, measured with styrene as substrate. During continuing treatment on the fifth day, styrene mono-oxygenase activity decreased, the activity after 6 h being only about 50% of that at the beginning of the fifth day of exposure. 4. UDP-glucuronosyl transferase activity (measured in digitonin-activated microsomes) was doubled by the four-day combined exposure to 1,1,1-trichloroethane and trichloroethylene. 5. The changes during the fifth day of exposure, e.g. rapid increase in the concentrations of solvents in organs, the detection of trichloroethylene in tissues and depression of mono-oxygenase activity, obviously also occurred during the exposures on days 1 to 4 and reverted during each post-exposure period.

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.

Dietary fats and properties of endoplasmic reticulum: II. Dietary lipid induced changes in activities of drug metabolizing enzymes in liver and duodenum of rat

Rats were fed cholesterol, cacao butter, or olive oil diets to determine the effect of dietary lipids on the rate of drug biotransformation in the liver and duodenum. The cholesterol rich diet maintained the hepatic aryl hydrocarbon hydroxylase activity at the same level as did the standard diet. Rats fed olive oil and cacao butter diets showed lower hepatic aryl hydrocarbon hydrorylase activity. The p-nitroanisole O-demethylase activity was doubled in hepatic microsomes of rats fed the high cholesterol diet when compared to rats fed the standard diet. The hepatic uridine diphosphate glucuronosyltransferase activity showed different patterns depending on the in vitro treatment of the microsomal membranes. If the enzyme activity was assayed from the native, untreated microsomes, an increase in the measurable uridine diphosphate glucuronosyl transferase activity was found in rats having cholesterol rich diet. After the in vitro activation of membrane-bound uridine diphosphate glucuronosyltransferase by trypsin, the increase in measurable activity was 10 fold in the group fed the standard diet, 6 fold in group fed cholesterol, 4 fold in group fed cacao butter, and 3 fold in group fed olive oil. Trypsin digestion of microsomes increased the measurable uridine diphosphate glucuronosyltransferase activity less in rats fed diets rich in neutral fats than those fed the standard diet. In the duodenal mucosa, lipid diets decreased the activities of drug hydroxylation and glucuronidation.