Induction Of Drug-metabolizing Enzymes Research Articles

Structure-dependent induction of CYP2B1/2 by 3-methylsulfonyl metabolites of polychlorinated biphenyl congeners in rats

The effects of eleven 3-methylsulfonyl (3-MeSO 2)-metabolites of polychlorinated biphenyl (PCB) congeners (which were reported to remain in Swedish mother's milk and Japanese Yusho patient's tissues) and their two structurally similar 3-MeSO 2-PCBs on the hepatic drug-metabolizing enzyme activities were compared with those of phenobarbital (PB) and 3-methylcholanthrene (3-MC).The induction profile of the drug-metabolizing enzymes, CYP2B1 and CYP2B2 in the hepatic microsomes of rats treated with nine 3-MeSO 2 derivatives, namely 3-MeSO 2-2,4′,5-trichlorobiphenyl, 3-MeSO 2-2,2′,4′,5-tetrachlorobiphenyl (3-MeSO 2-2,2′,4′,5-tetraCB), 3-MeSO 2-2,2′,5,5′-tetraCB, 3-MeSO 2-2,3′,4′,5-tetraCB, 3-MeSO 2-2,2′,3′,4′,5-pentachlorobiphenyl (3-MeSO 2-2,2′,3′,4′,5-pentaCB), 3-MeSO 2-2,2′,4′,5,5′-pentaCB, 3-MeSO 2-2,2′,3′,4′,5,5′-hexachlorobiphenyl (3-MeSO 2-2,2′,3′,4′,5,5′-hexaCB), 3-MeSO 2-2,2′,3′,4′,5,6-hexaCB and 3-MeSO 2-2,2′,4′,5,5′,6-hexaCB, was similar to that of rats treated with PB, but was different from that of rats treated with 3-MC. These findings indicate that 3-MeSO 2 metabolites derived from nine PCBs are PB-type inducers of microsomal drug-metabolizing enzymes. The relative inducing potencies of 3-MeSO 2 derivatives on the hepatic drug-metabolizing enzyme activities differed with the extent of chlorination and the positions of chlorine substituent on the phenyl rings. The results of present study show that the structure-CYP2B1/2 induction relationship exists for the 3-MeSO 2 derivatives studied. The inducing abilities of 3-MeSO 2-2,2′,4′,5-tetraCB and 3-MeSO 2-2,2′,4′,5,5′-pentaCB (2 μmol/kg) on the content of cytochrome P450 were higher than those of 2,3′,4,4′,5-pentaCB ( mono- ortho-substituted PCB) (80 μmol/kg), 3,3′,4,4′-tetraCB (coplanar PCB) (80 μmol/kg) and 3,3′,4,4′,5-pentaCB (coplanar PCB) (0.5 μmol/kg). The inducing effects of the administration of 3-MeSO 2-2,2′,4′,5-tetraCB and 3-MeSO 2-2,2′,4′,5,5′-pentaCB at 2 μmol/kg on the contents of total cytochrome P450, CYP2B1 and CYP2B2 corresponded to those of PB at 431 μmol/kg twice at a 24 h interval. It is noticeable that 3-MeSO 2-2,2′,4′,5-tetraCB and 3-MeSO 2-2,2′,4′,5,5′-pentaCB have highly potent PB-type inducing activity on drug-metabolizing enzyme systems.

Tissue distribution, toxicokinetics and induction of hepatic drug metabolizing enzymes in male rats after a single S.C. dose of 3,4,3′,4′-tetrachlorobiphenyl (PCB-77)

The tissue distribution and toxicokinetics of PCB-77 after a single s.c. dose of 6 mg/kg body weight was investigated in male adult rats as well as the concurrent induction of hepatic drug-metabolizing enzymes. During five days after treatment, the PCB-77 concentrations on fat weight basis in testis and in whole blood were found to be about in the same range as the concentrations in adipose tissue, whereas in liver fat PCB-77 accumulated. EROD and MROD activities were induced by about 8 and 4 times, respectively, during a period of two weeks after treatment. Concentrations in adipose tissue and in liver as well as the hepatic enzyme activities rapidly declined during the investigation period of eight weeks. An elimination half-life for PCB-77 of approximately 7–9 days was estimated.

Induction of Drug-Metabolizing Enzymes by Dioxin

(1997). Induction of Drug-Metabolizing Enzymes by Dioxin. Drug Metabolism Reviews: Vol. 29, No. 4, pp. 1107-1127.

Stereoselectivity in drug metabolism and action: effects of enzyme inhibition and induction.

Many synthetic drugs that contain a chiral center are marketed as racemates. Taking into account the composite nature of chiral rugs has improved our understanding of the dose/effect relationship of these compounds. It is widely accepted that drug metabolism is often responsible for stereoselective disposition and hence factors modulating the activity of drug-metabolizing enzymes can modify this effect. The present article highlights the influence of some of these factors (induction and inhibition of drug-metabolizing enzymes) on stereoselective drug disposition. The latter topic includes an enantiomer/enantiomer interaction that alters drug disposition and drug action.

Induction of CYP1A1 and ALDH-3 in Lymphoid Tissues from Fisher 344 Rats Exposed to 2,3,7,8-Tetrachlorodibenzodioxin (TCDD)

The immune system is a primary target for toxic insult by a number of drugs and environmental chemicals, many of which require activation to toxic metabolites by drug-metabolizing enzymes. We compared the induction of drug-metabolizing enzymes, including cytochrome P450 1A1 (CYP1A1) and aldehyde dehydrogenase (ALDH), in lymphoid tissues of F344 rats following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). ALDH was induced in both the spleen and the thymus after TCDD treatment, with maximal expression at 9 and 15 days, respectively. Thymic microsomal preparations from TCDD-treated animals expressed elevated levels of inducible CYP1A1 as compared to microsomes from the spleens of treated animals or tissues from control rats. TCDD treatment also resulted in increased ethoxyresorufin-O-deethylase (EROD) activity in the thymus. There were no detectable mRNA transcripts for CYP1A1 in peripheral blood or splenic lymphocytes from treated animals; however, CYP1A1 transcripts were induced in isolated thymocytes, whole spleen, and whole thymus.In vitroexposure to TCDD did not result in induction of immunoreactive CYP1A1 in thymocytes unless simultaneously activated with the mitogen, phytohemagluttinin (PHA). Immunohistochemical localization of CYP1A1 in immune tissues indicated that cells other than the lymphoid populations are responsible for the increased CYP1A1 expression. The pattern of CYP1A1 induction was related to the expression of the Ah receptor (AhR) in immune tissues. Western blot analyses demonstrated less AhR present in peripheral blood lymphoid cells and spleen, as compared to whole tissues. These studies indicate that while drug-metabolizing enzymes are present in immune tissues, the induction of enzymes is selective in different lymphoid cells.

Drug metabolizing enzyme induction by benzoquinolines, acridine, and quinacrine; tricyclic aromatic molecules containing a single heterocyclic nitrogen.

Rats were treated with nitrogen-containing phenanthrene (3,4-, 5,6-, or 7,8-benzoquinoline) or anthracene (acridine or quinacrine) derivatives at a dose of 75 mg/kg, daily for 3 days. The hepatic drug metabolizing enzyme response ranged from no induction (quinacrine) through low (5,6-benzoquinoline), intermediate (acridine), and high (3,4-benzoquinoline) magnitude increases of only phase II enzymes, to induction of both phase I and phase II enzymes (7,8-benzoquinoline). The phase I enzyme response of 7,8-benzoquinoline was an induction of CYP1A. All three benzoquinolines, but neither anthracene derivative, elevated NAD(P)H quinone oxidoreductase activity. A similar pattern but of lesser magnitude was seen with glutathione S-transferase activity. 3,4-Benzoquinoline was the only agent to significantly increase microsomal epoxide hydrolase activity (2,3-fold). Both 3,4- and 7,8-benzoquinoline increased UDP-glucuronosyltransferase activity toward 4-nitrophenol (40% and 70%, respectively), but only the 3,4-isomer increased activity toward morphine (75%), diclofenac (75%), and testosterone (23%), and only the 7,8-isomer increased activity toward chloramphenicol (105%). 3,4-Benzoquinoline elevated the hepatic mRNA concentration of UGT2B1 but not UGT1*6. Acridine treatment increased UDP-glucuronosyltransferase activity toward morphine (47%), 1-naphthol (28%), testosterone (19%), and estrone (19%). Quinacrine failed to elevate any UDP-glucuronosyltransferase activity and depressed activities toward testosterone and estrone by 20%. This study shows that some tricyclic aromatic compounds containing a single heterocyclic nitrogen atom have the potential for use as chemoprotective agents based upon their ability to selectively induce only phase II enzymes.

Drug metabolizing enzyme induction by benzoquinolines, acridine, and quinacrine; Tricyclic aromatic molecules containing a single heterocyclic nitrogen

Drug metabolizing enzyme induction by benzoquinolines, acridine, and quinacrine; Tricyclic aromatic molecules containing a single heterocyclic nitrogen

Pesticide-metabolizing enzymes

Pesticides are known to function as substrates, inhibitors and inducers of drug-metabolizing enzymes, with the same compound frequently acting in more than one of these roles. Current studies of phase I metabolism of pesticides include cytochrome P450 (P450) and the flavin-containing monooxygenase (FMO), with particular reference to individual isozymes. In mouse liver, the level of FMO1 is gender dependent, FMO3 is gender specific, while FMO5 appears to be gender independent. The isozyme specificity of methylenedioxyphenyl synergists for induction of P450 in mouse liver involves P450s 1A1, 1A2 and 2B10, including a non-Ah receptor-dependent mechanism for 1A2 induction. The substrate specificity of mouse and human P450 and FMO isozymes is discussed.

Comparative assessment of metabolic enzyme levels in macrophage populations of the F344 rat

The immune system is a direct target for toxic insult by a number of drugs and other chemicals, many of which require activation to toxic metabolites by drug-metabolizing enzymes. We compared the induction of drug-metabolizing enzymes, including cytochrome P450 1A1 (CYP1A1) and aldehyde dehydrogenase (ALDH), which are differentially expressed in various macrophage populations following treatment of F344 rats with the inducer 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD). Kupffer cells, alveolar macrophages and splenic macrophages from TCDD-treated animals expressed elevated levels of inducible CYP1A1 as compared to other macrophage subpopulations or cells from control rats. TCDD treatment also resulted in increased ethoxyresorufin- O-deethylase (EROD) activity and total cytochrome P450 content in tissue-derived macrophages. Immuno-reactive protein and mRNA transcripts for CYP1A1 were not detectable in resident peritoneal macrophages or peripheral blood monocytes. Examination of aromatic hydrocarbon receptor (AhR) levels in macrophage populations suggests that the ability of TCDD to induce metabolic enzymes in specific cell types correlates well with AhR expression. In vivo activation of macrophages, using either Bacillus of Calmette and Guérin, Mycobacterium tuberculosis (BCG) or polyinosinic:polycytidylic acid (Poly I:C), caused no significant alteration in the levels of induction of CYP1A1. ALDH-3 induction was similar in all macrophage populations examined. These studies indicate that macrophages, particularly those from portals of entry, may be induced to produce increased levels of specific enzymes, and the induction is dependent upon their maturational stage rather than their activation state. The metabolism of xenobiotics to toxic intermediates by immune cells and its role in immunosuppression are discussed.

Induction of hepatic drug-metabolizing enzymes by chlornitrofen (CNP) and cnp- amino in rats and mice

The induction of hepatic drug-metabolizing enzymes by chlornitrofen (CNP) and CNP-amino was studied in the liver of male rats and mice. CNP-amino increased the activities of 7-pentoxyresorufin O-depentylase (PROD) and 7-benzyloxyresorufin O-debenzylase (BROD) as CYP2B1-dependent monooxygenase 3.6- and 4.1-fold in rats. On the contrary, these enzyme activities in mice were induced by CNP rather than by CNP-amino. Furthermore, immunoblotting showed that the protein levels of CYP2B subfamily cytoa hrome P450 (P450) in liver microsomes of rats and mice were increased by CNP or CNP-amino. Phase II drug metabolizing enzymes, UDP-glucuronyltransferase (UGT) and glutathione S-transferase (GSC) levels in mice were also significantly increased from 1.4 to 2.6-fold by CNP or CNP-amino. However, neither CNP nor CNP-amino affected UGT and GST in rats. These results suggest that CNP and or CNP-amino induce the P450 isoforms of CYP2B subfamily in the rat and mouse liver, and that the inducibility igdrugmetabolizing enzyme by the compounds is different between rats and mice.

Induction of rat liver drug-metabolizing enzymes by tetrachloroethylene.

The effect of tetrachloroethylene on Phase I and II drug-metabolizing enzymes in rat liver was examined. Rats were treated orally with tetrachloroethylene daily for five days, at doses of 125, 250, 500, 1,000 and 2,000 mg/kg. The higher doses (> 500 mg/kg) of tetrachloroethylene induced the hepatic microsomal 7-pentoxyresorufin O-depentylase and 7-benzyloxyresorufin O-debenzylase activities associated with the CYP2B subfamily. 7-ethoxyresorufin O-deethylase activity was also induced about 2-fold compared with that of control rats at 500, 1,000, and 2,000 mg/kg dose levels of tetrachloroethylene. However, 7-ethoxycoumarin O-deethylase and 7-methoxyresorufin O-demethylase activities were increased significantly at only the 1,000 mg/kg dose level of tetrachloroethylene (1.4- and 1.5-fold). Although other cytochrome P450-mediated monooxygenase activities such as nitrosodimethylamine N-demethylase, aminopyrine N-demethylase and erythromycin N-demethylase were also induced by tetrachloroethylene, the relative induction to control activity was lower than those of 7-pentoxyresorufin O-depentylase and 7-benzyloxyresorufin O-debenzylase. Western immunoblotting showed that the levels of CYP2B1 and CYP2B2 proteins in liver microsomes were increased at doses of 1,000 and 2,000 mg/kg of tetrachloroethylene. In addition to cytochrome P450-mediated monooxygenases, there was significant induction of the Phase II drug-metabolizing enzymes, DT-diaphorase, glutathione S-transferase activities towards 1-chloro-2,4-dinitrobenzene and 1,2-dichloro-4-nitrobenzene, and UDP-glucuronyltransferase activities towards 4-nitrophenol and 7-hydroxycoumarin. The results indicate that tetrachloroethylene induces both Phase I (CYP2B-mediated monooxygenase) and Phase II drug-metabolizing enzymes (DT-diaphorase, glutathione S-transferase and UDP-glucuronyltransferase) in the rat liver.

Induction of hepatic microsomal drug-metabolizing enzymes by methylsulphonyl metabolites of polychlorinated biphenyl congeners in rats

The effect of methylsulphonyl (MeSO 2) metabolites of 2,3′,4′,5-tetrachlorobiphenyl (tetraCB) (IU-70), 2,2′,3′,4′,5-pentachlorobiphenyl (pentaCB) (IU-87), 2,2′,4′,5,5′ -pentaCB (IU-101) and 2,2′,3′,4′,5,5′-hexachlorobiphenyl (hexaCB) (IU-141), on the hepatic microsomal drug-metabolizing enzyme system was investigated in rats. The administration of 3-MeSO 2-2,3′,4′,5-tetraCB (10 μmol/kg), 3-MeSO 2-2,2′,3′,4′,5-pentaCB (0.5 μmol/kg), 3-MeSO 2-2,2′,4′,5,5′-pentaCB (0.5 μmol/kg) and 3-MeSO 2-2,2′,3′,4′,5,5′-hexaCB (2 μmol/kg) to rats significantly increased the contents of cytochromes P-450 and b 5 and the activities of aminopyrine N-demethylase, 7-ethoxycoumarin O-deethylase and benzo[ a]pyrene hydroxylase. From these results, it is suggested that the 3-MeSO 2 derivatives studied are possibly potent phenobarbital-like inducers of microsomal drug-metabolizing enzymes. On the other hand, 4-MeSO 2-2,3′,4′,5-tetraCB, 4-MeSO 2-2,2′,3′,4′,5-pentaCB, 4-MeSO 2-2,2′, 4′,5,5′-pentaCB and 4-MeSO 2-2,2′,3′,4′,5,5′-hexaCB had almost no effect on both cytochrome contents and these enzyme activities. After 96 h, following administration of 2,3′,4′,5-tetraCB, 2,2′,3′,4′,5-pentaCB, 2,2′,4′,5,5′-pentaCB and 2,2′,3′,4′,5,5′-hexaCB

Characterization of hepatic microsomal cytochrome P-450 from rats treated with methylsulphonyl metabolites of polychlorinated biphenyl congeners

The inducing potency of 3-methylsulphonyl(MeSO 2)-2,2′,4′,5,5′-pentachlorobiphenyl (pentaCB), which was one of the major MeSO 2 metabolites of polychlorinated biphenyls (PCBs) present in seal blubber[1], on the hepatic drug metabolizing enzyme activities was examined in comparison with that of the parent compound and phenobarbital (PB). The inducing fashion of the above enzymes and changes in the contents of PB-inducible P-450 forms by 2,3′,4′,5-tetrachlorobiphenyl (tetraCB) (IU-70), 2,2′,3′,4′,5-pentaCB (IU-87), 2,2′,4′,5,5′-pentaCB (IU-101) and 2,2′,3′,4′,5,5′-hexachlorobiphenyl (hexaCB) (IU-141), and their MeSO 2 metabolites were investigated in rats. Administration at various doses (0.2-1.0 μmol/kg) of 3-MeSO 2-2,2′,4′,5,5′-pentaCB produced nearly dose-related increases in the hepatic concentration of this methyl sulphone, in the contents of cytochromes P-450 and b 5, and in activities of aminopyrine N-demethylase, 7-ethoxycoumarin O-deethylase and benzo[ a]pyrene hydroxylase of liver microsomes. Major PB-inducible forms, CYP2B1, CYP2B2, CYP3A2 and CYP2C6 were induced with four PCBs (342 μmol/kg) and their 3-MeSO 2 metabolites (0.5–10 μmol/kg), indicating that 3-MeSO 2 metabolites were strong PB-type inducers of hepatic drug-metabolizing enzymes. 3-MeSO 2-2,2′,4′,5,5′-pentaCB was an especially strong inducer. On the other hand, four PB-inducible forms of cytochrome P-450 were not induced with the 4-MeSO 2 isomers. The relation between liver concentrations of the corresponding 3-MeSO 2 derivatives and induction of four PB-inducible forms of cytochrome P-450 after administration of four PCBs and their 3-MeSO 2 derivatives further confirmed that the 3-MeSO 2 metabolites played an important role in the induction which parent PCB congeners caused on the hepatic drug-metabolizing enzyme system.

Characterization of benzazepine UDP-glucuronosyl-transferases in laboratory animals and man

1. The O-glucuronidation of two dopamine D1 receptor antagonists, Odapipam and Berupipam, were studied in hepatic microsomal fractions from mouse, rat, rabbit, dog, pig, and man using 14C-UDP-glucuronic acid. 2. The influence of pH, detergent, gender, drug-metabolizing enzyme inducers, and age were examined. Detergents like the zwitterionic CHAPS and non-ionic Tween 20, Triton X-100, and Brij 35 stimulated the glucuronidation rate by up to 600% of native activity with the latter being most effective. Both apparent Km and Vmax increased following detergent treatment in rat hepatic microsomes. Less marked activation of UDP glucuronosyltransferase activity was observed with Brij 35 in mouse, rabbit, dog, and pig compared with rat. In contrast, human hepatic microsomes were not stimulated by detergent treatment. 3. Marked species-dependent UDP-glucuronosyltransferase activity were observed for the two compounds. In general, Odapipam exhibited higher Vmax and Km compared with Berupipam with the exception of rabbit where the reverse was true. Similar kinetic parameters were, however, observed in human hepatic microsomes. Highest glucuronidation rate (in general) was observed in mouse followed by dog, pig, rabbit, man, and rat. 4. UGT activity in human livers showed up to a seven-fold variation. Conjugation of each compound were highly correlated (r = 0.92; n = 20) suggesting that identical isoform(s) were involved in this reaction. A significant age-related decrease in UDP-glucuronosyltranferase activity was observed, which partly could be explained by a preponderance in elderly female donor liver samples.

Molecular pharmacological and toxicological studies of drug-metabolizing enzymes

Studies on drug metabolism have opened new fields in the evaluation of drug efficacy and drug safety in experimental animals and humans, especially in the development of new drugs. The author described the history of discoveries of induction and inhibition of drug-metabolizing enzymes as a key point in the development of drug interaction studies. Studies on the sex-related differences in drug-metabolism have been important fields for understanding the sex-related toxicity and efficacy of drugs and their species differences under normal and pathological conditions. Rats are unique animals among experimental animals and humans. The activities of hepatic drug-metabolizing enzymes, especially cytochrome P450 and sulfotransferase, are regulated through the sex-related secretion pattern of growth hormone. The drug-metabolizing enzymes convert drugs into not only inactive metabolites, but produce toxic intermediates which cause mutagenesis, carcinogenesis and drug-caused allergic reactions. The author carried out extensive studies on the metabolic activation of mutagenic-carcinogenic arylamines and demonstrated important roles of O-acetylation and O-sulfation to cause DNA damages by N-hydroxyarylamines. Pharmacogenetic studies on hamster acetyltransferase were described to understand the individual differences in polymorphic acetylation of arylamines and N-hydroxyarylamines in relation to DNA damages. Finally, the author emphasizes important roles of drug metabolism studies for the development of new drugs by showing a prototype, which has multiple metabolic pathways by multiple enzymes and this shows reduced-extents of individual differences, for increasing efficacy and safety in a future clinical drug therapy.

Simultaneous high-performance liquid chromatographic determination of 6β-hydroxycortisol and cortisol in urine with fluorescence detection and its application for estimating hepatic drug-metabolizing enzyme induction

A simple and sensitive high-performance liquid chromatographic method is described for the simultaneous determination of 6β-hydroxycortisol (6β-OHF) and cortisol (F) in urine. Urine (1 ml) containing fludrocortisone as the internal standard is extracted with ethyl acetate. The extract is washed successively with sodium hydroxide solution and water, and subsequently dried under a stream of nitrogen. The residue is redissolved in methanol. The 6β-OHF, F and fludrocortisone in the methanol solution are oxidized by cupric acetate and the resulting glyoxal compounds are converted into fluorescent derivatives with 1,2-diamino-4,5-methylenedioxybenzene (DMB). The DMB derivatives of the corticosteroids are separated within 70 min on a reversed-phase column, L-Column ODS, using stepwise elution with methanol—acetonitrile—0.5 M ammonium acetate and detected fluorimetrically at 350 nm (excitation) and 390 nm (emission). The lower limits of detection for 6β-OHF and F are 1.8 pmol (680 pg) and 2.4 pmol (950 pg)/ml urine (0.6 pmol and 0.8 pmol/100 μl injection volume), respectively, at a signal-to-noise ratio of 3. This method can be applied to the determination of urinary 6β-OHF, and the ratio of 6β-OHF to F in humans and in rhesus monkeys treated orally with phenobarbital as a hepatic drug-metabolizing enzyme inducer.

Expression and induction of drug-metabolizing enzymes in cultured fetal rat hepatocytes

An in vitro experimental model, fetal rat hepatocytes in culture, was metabolically characterized. Several enzymatic activities were expressed in these hepatocytes, namely, testosterone hydroxylations. Hepatocytes cultured up to 3 weeks in the presence of dexamethasone and phenobarbital still expressed some drug-metabolizing enzyme activities (e.g., ECOD). The enzymatic activities were measured both directly on monolayers during culture and on the corresponding harvested and homogenized cells. The results correlate perfectly with each other. The 'on cell' procedure allows us to repeat the assay or to measure several activities on the same cells at different time intervals. The presence of dexamethasone in the culture medium allows the expression and the induction of several cytochrome P450 isoenzymes, namely, those hydroxylating testosterone. This makes the model particularly attractive for induction experiments as well as for metabolic or toxicological studies needing longer treatments.

Subchronic Dietary Exposure to Aroclor 1254 in Rats: Accumulation of PCBs in Liver, Blood, and Adipose Tissue and Its Relationship to Induction of Various Hepatic Drug-Metabolizing Enzymes

Female F344/NCr rats were exposed continuously (7-84 days) or discontinuously (7 days exposure/21 days control diet or 28 days exposure/56 days control diet) to various dietary concentrations (1-100 ppm) of Aroclor 1254. There were dose- and time-dependent increases in PCB levels in liver, blood, and adipose tissue. Following removal of the rats from diet containing Aroclor 1254, there was a relatively rapid decrease in PCB levels, particularly in rats exposed to higher concentrations of Aroclor 1254. In parallel with the alterations in PCB levels observed, the rats showed striking dose- and time-dependent increases in hepatic levels of CYP1A1 and CYP1A2, as determined by various methods [RNA analysis, immunochemical detection, or measurement of the O-dealkylation of methoxyresorufin (CYP1A2) or ethoxyresorufin (CYP1A1)]. In rats removed from the Aroclor 1254 diet, catalytic activity for CYP1A1 as well as RNA levels for both CYP1A1 and CYP1A2 rapidly diminished. In contrast to the high levels of induction of CYP1A1 and CYP1A2 observed, limited induction (<5-fold) of epoxide hydrolase, quinone oxidoreductase, and aldehyde dehydrogenase was detected, even in rats exposed to the highest concentration of Aroclor (100 ppm) for up to 84 days. Furthermore, induction of these non-CYP hepatic drug-metabolizing genes exhibited distinctly different concentration-response curves. The ratios of hepatic CYP1A1 activity to hepatic PCB burden were similar for rats exposed continuously to Aroclor in the diet for 7, 28, or 84 days, and for rats exposed discontinuously (7 days Aroclor/21 days control diet or 28 days Aroclor/56 days control diet). Thus, hepatic PCB levels alone appeared to be reasonably predictive of CYP1A1 levels under a variety of modes of exposure. When the ratio of CYP1A1 activity to adipose or blood PCB concentration was determined, similar ratios were observed for rats exposed continuously for 7, 28, or 84 days. However, lower ratios were observed for rats discontinuously exposed to Aroclor in the diet. These results have important implications with respect to: (a) employing PCB levels in various tissues to predict biological effects, and (b) determining different concentration-response curves for the various biological effects induced by PCBs.

Effects of phenobarbital on drug metabolizing enzyme activities and other biochemical parameters in rats with DL-ethionine-induced liver injury.

Phenobarbital (PB) was orally administered once at a dose of 100 mg/kg to the liver injury model rats treated with DL-ethionine (ET), and the effects of PB on the liver drug metabolizing enzymes (DME) were chiefly examined. Liver weight, liver microsomal protein content, liver aniline 4-hydroxylase (ANH) activity, and aminopyrine N-dimethylase (AMD) activity were markedly increased in the ET-treated rats receiving PB. These findings suggested the induction of DME in the liver. However, the induction pattern of each enzyme was different. AMD activity at 48 hr after dosing of PB in the ET-treated rats was increased in the same degree as that in the control (normal). Whereas, ANH activity at 48 hr after dosing in the ET-treated rats was higher than that in normal rats. Liver lactate dehydrogenase (LDH) activity at 48 hr after dosing in the ET-treated rats was markedly increased, but such induction was not seen in normal rats. These findings indicates that DME in the liver is induced by PB treatment in the ET-treated rats as well as in normal rats, and that the ET-treated rats have a function of physiological adaptation similar to that in normal rats. The induction pattern of liver or serum enzymes in the ET-treated rats receiving PB was different from that in normal rats. Furthermore, the induction pattern of these enzymes in the ET-treated rats receiving PB was different from that in the normal rats, which may be attributed to the difference of localization in liver cells of these enzymes affected by PB.

Role of 2,3,5-Trichlorophenyl Methyl Sulfone, a Metabolite of 1,2,4-Trichlorobenzene, in the Induction of Hepatic Microsomal Drug-Metabolizing Enzymes by 1,2,4-Trichlorobenzene in Rats

2,3,5- and 2,4,5-trichiorophenyl methyl sulfides, 2,3,5- and 2,4,5-trichlorophenyl methyl sulfoxides, and 2,3,5- and 2,4,5-trichlorophenyl methyl sulfones (TCPSO 2Mes) were detected in the urine of rats dosed with 1,2,4-trichlorobenzene (TCB). After the administration of 1,2,4-TCB to rats, swift decreases in concentrations of 1,2,4-TCB in blood, liver, kidneys, and adipose tissue were observed. On the other hand, 2,3,5-TCPSO 2Me appeared in blood, liver, kidneys, and adipose tissue and remained detectable in the blood and the three tissues until 120 hr. The increases in the activities of aminopyrine N-demethylase and aniline hydroxylase and the contents of cytochromes P450 and b 5 in hepatic microsomes produced by 1,2,4-TCB occurred after increases in the hepatic concentration of 2,3,5-TCPSO 2Me. 2,3,5- and 2,4,5-TCPSO 2Me increased the above four parameters in rat liver microsomes. The inducing intensity of 2,3,5-TCPSO 2Me was much higher than that of 2,4,5-TCPSO 2Me. 2,3,5-TCPSO 2Me was considered to be a potent inducer and to play a principal role in the induction by l,2,4-TCB. When l,2,4-TCB was injected ip into bile duct-cannulated rats, little 2,3,5-TCPSO 2Me was detected in blood, liver, kidneys, and adipose tissue. In the antibiotic-pretreated rats dosed with 1,2,4-TCB, 2,3,5-TCPSO 2Me concentrations in the blood and the three tissues markedly decreased. These findings suggest that the formation of methylsulfonyl metabolites from 1,2,4-TCB depends largely upon the metabolism of some precursor(s) excreted in the bile by intestinal microflora. The increasing effects of 1,2,4-TCB administration on the activities of aminopyrine- and aniline-metabolizing enzymes and the contents of cytochromes P450 and b 5 in hepatic microsomes were not observed in the bile duct-cannulated rats. These findings provide evidence that the induction of drug-metabolizing enzymes by 1,2,4-TCB is not due to the action of 1,2,4-TCB itself but is due to its methylsulfonyl metabolite, 2,3,5-TCPSO 2Me.