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PubMed2007

Molecular mechanisms of metabolic resistance to synthetic and natural xenobiotics.

Xenobiotic resistance in insects has evolved predominantly by increasing the metabolic capability of detoxificative systems and/or reducing xenobiotic target site sensitivity. In contrast to the limited range of nucleotide changes that lead to target site insensitivity, many molecular mechanisms lead to enhancements in xenobiotic metabolism. The genomic changes that lead to amplification, overexpression, and coding sequence variation in the three major groups of genes encoding metabolic enzymes, i.e., cytochrome P450 monooxygenases (P450s), esterases, and glutathione-S-transferases (GSTs), are the focus of this review. A substantial number of the adaptive genomic changes associated with insecticide resistance that have been characterized to date are transposon mediated. Several lines of evidence suggest that P450 genes involved in insecticide resistance, and perhaps insecticide detoxification genes in general, may share an evolutionary association with genes involved in allelochemical metabolism. Differences in the selective regime imposed by allelochemicals and insecticides may account for the relative importance of regulatory or structural mutations in conferring resistance.

Animals↗

PubMed2005

Antimycotic drug discovery in the age of genomics.

Genomic-based methodologies are increasingly used at all stages of drug development. The most extensive applications have occurred in early drug discovery stages due to advances in technologies that allow for automated synthesis and characterization of organic compounds, and for high-throughput screening of these molecules against known drug targets. The adaptation of genomic-based methodologies in later stages of drug development presents a more difficult task. In this review we describe how genomics can be used to identify previously uncharacterized pharmacologic actions that provide a basis for the development of new classes of antimycotic agents or for adverse event aversion. Clinically, novel antimycotics are gravely needed. This review provides a perspective on new technologies that will bridge the gap between drug discovery and development that may enable more rapid access to new antimycotic agents.

Animals↗

PubMed2005

Genetics of endometriosis.

UNLABELLED: Endometriosis is a multifactorial disease that can affect up to 10-15% of women in their reproductive age. Epidemiological studies indicate that it is a polygenic disorder with recurrence risks in first-degree relatives of about 5-7%. Thus, the present aim of different research groups is to identify genetic variations in obvious candidate gene that could be associated with an increased susceptibility to endometriosis. The great advancement in molecular biology techniques make this task certainly possible, although particular attention needs to be paid to the study design in order to achieve reliable RESULTS: The data obtained by such studies will allow to expand our knowledge on the pathogenesis of the disease and, more importantly, to develop individualized therapies and prevention strategies to apply at high-risk populations.

Alleles↗

PubMed2000

Activity of detoxication enzymes in rat hepatoma 27 transplanted in different organs.

Activities of enzymes of active oxygen forms detoxication and phase II xenobiotic metabolism were measured in rat hepatoma 27 cells transplanted to different organs. Activity of phase II xenobiotic metabolizing enzymes was higher in hepatoma cells growing subcutaneously than in those transplanted into the liver, while activity of active oxygen forms detoxication enzymes (except catalase) was higher in cells transplanted into the liver. Benz(a)pyrene induced the enzyme activities in hepatoma growing both subcutaneously and in the liver.

Aldehyde Dehydrogenase↗

PubMed1987

The role of conjugation reactions in detoxication.

The role of conjugating enzymes is best understood by looking at the interaction between phase I (mostly cytochromes P-450) and phase II (conjugation) enzymes of drug metabolism. A balance between phase I and II enzymes of detoxication largely determines the disposition to drug toxicity. Reactive electrophilic metabolites, generated by phase I enzymes, are often controlled by GSH-transferases, whereas nucleophilic metabolites such as phenols are controlled by UDP-glucuronosyltransferases (GT) and sulfotransferases. It is more and more recognized that the control of the more stable and more abundant nucleophiles is as important as the control of electrophiles, since the former can be readily converted to electrophiles. For example, phenols and quinols can undergo quinone/quinol redox-cycles with the generation of reactive oxygen species. In the case of benzo(a)pyrene-3,6-quinol toxicity can be prevented by glucuronidation. Conjugating enzymes consist of families of isoenzymes with distinct but overlapping substrate specificity. Rather than dealing with individual isoenzymes, adaptive programs are emphasized by which gene expression of a battery of phase I and II enzymes is turned on by certain types of inducing agents. Mechanistically best known is the program turned on by 3-methylcholanthrene-type inducers which includes enhanced synthesis of certain isoenzymes of cytochrome P-450, GT and probably GSH-transferase. The program may adapt the organism to efficiently detoxify and eliminate aromatic compounds such as benzo(a)pyrene. Evidence is presented that this program exists in both rodents and humans.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

A major inducer of anticarcinogenic protective enzymes from broccoli: isolation and elucidation of structure.

Consumption of vegetables, especially crucifers, reduces the risk of developing cancer. Although the mechanisms of this protection are unclear, feeding of vegetables induces enzymes of xenobiotic metabolism and thereby accelerates the metabolic disposal of xenobiotics. Induction of phase II detoxication enzymes, such as quinone reductase [NAD(P)H:(quinone-acceptor) oxidoreductase, EC 1.6.99.2] and glutathione S-transferases (EC 2.5.1.18) in rodent tissues affords protection against carcinogens and other toxic electrophiles. To determine whether enzyme induction is responsible for the protective properties of vegetables in humans requires isolation of enzyme inducers from these sources. By monitoring quinone reductase induction in cultured murine hepatoma cells as the biological assay, we have isolated and identified (-)-1-isothiocyanato-(4R)-(methylsulfinyl)butane [CH3-SO-(CH2)4-NCS, sulforaphane] as a major and very potent phase II enzyme inducer in SAGA broccoli (Brassica oleracea italica). Sulforaphane is a monofunctional inducer, like other anticarcinogenic isothiocyanates, and induces phase II enzymes selectively without the induction of aryl hydrocarbon receptor-dependent cytochromes P-450 (phase I enzymes). To elucidate the structural features responsible for the high inducer potency of sulforaphane, we synthesized racemic sulforaphane and analogues differing in the oxidation state of sulfur and the number of methylene groups: CH3-SOm-(CH2)n-NCS, where m = 0, 1, or 2 and n = 3, 4, or 5, and measured their inducer potencies in murine hepatoma cells. Sulforaphane is the most potent inducer, and the presence of oxygen on sulfur enhances potency. Sulforaphane and its sulfide and sulfone analogues induced both quinone reductase and glutathione transferase activities in several mouse tissues. The induction of detoxication enzymes by sulforaphane may be a significant component of the anticarcinogenic action of broccoli.

Animals↗

PubMed1994

Suppression of hydroxysteroid sulfotransferase-a gene expression by 3-methylcholanthrene.

The hydroxysteroid sulfotransferases (HSTs) are phase II drug metabolizing enzymes which play a role in xenobiotic detoxication, carcinogen activation, and intratissue hormone modulation. Rat liver contains three principal HST enzymes. Of these, HST-a is most abundant in female rat liver. The phase I drug metabolizing enzyme cytochrome P450 1A1 (CYP1A1) provides a major pathway for polycyclic aromatic hydrocarbon (PAH) carcinogen activation, whereas HST-a, through enzymatic sulfation, offers an alternative pathway for PAH carcinogen activation. Transcription of the rat hepatic CYP1A1 gene is induced in response to treatment with PAH carcinogen 3-methylcholanthrene (3-MC). In view of the potential importance of enzymatic sulfation to the complex process of PAH carcinogenesis, the effect of 3-MC on HST-a gene expression was investigated. Groups of prepubescent (aged approximately 22-30 days) and young adult female (aged approximately 69-84 days) Sprague-Dawley rats were injected ip with corn oil vehicle or with 3-MC (80 mg/kg). After fasting for 24 hr, rats were terminated and hepatic HST-a gene expression was analyzed using slot blot, Northern blot, and Western blot analyses. Initial Northern blot and slot blot analyses demonstrated a substantial suppression of rat hepatic HST-a mRNA in both prepubescent and young adult female rats. Subsequent Northern blot analyses on liver tissue isolated from female rats aged approximately 55 days confirmed CYP1A1 mRNA induction in 3-MC-treated animals and demonstrated a dose-dependent suppression of HST-a mRNA expression of approximately 25, 50, and 75% in rats treated with 10, 25, and 80 mg/kg 3-MC ip, respectively. In contrast, HST-a protein levels remained unaltered 24 hr after 3-MC treatment. A time-course study revealed that hepatic HST-a mRNA levels returned to control levels by 36 hr following 3-MC treatment. These results suggest that xenobiotics such as 3-MC modulate HST-a mRNA expression and that HST-a mRNA suppression after 3-MC treatment may occur at the level of transcription.

Animals↗

PubMed1993

Factors and conditions affecting the glucuronidation of oxazepam.

The aim of the present work was to investigate the impact of disease states and environmental and host factors on the glucuronidation of oxazepam. Glucuronidation represents quantitatively one of the most important metabolic conjugation pathways (phase II) in man for the inactivation and detoxication of xenobiotics and endogenous compounds and the liver is the major site for it to take place. Far less attention has been paid to the conjugation reactions in previous clinical research in this field compared to the immense interest in the oxidative biotransformation pathways (phase I). This fact is mainly due to the latter giving rise to active or reactive metabolites with a toxicological potential. The metabolism of oxazepam expresses exclusively the capacity for glucuronide formation. It was a prerequisite to establish the bioavailability of oxazepam prior to succeeding studies on the oral disposition of the drug. A preparation for intravenous administration was created. Clearance was chosen as measurement of the capacity to glucuronidate oxazepam. Severe decompensated liver disease was associated with a significant decrease in oxazepam clearance, that became even more obvious when corrected for by a diminished binding to plasma proteins. This increase in free fraction of oxazepam was substantial and could mainly be accounted for by low plasma albumin values. The results are in part a settlement with earlier studies on glucuronidation in liver disease and they may undoubtedly be ascribed to the severe degree of liver disease. For the first time it was shown that hypothyroidism led to a decline in the clearance and metabolism of oxazepam and paracetamol that is mainly biotransformed by glucuronidation. It was concluded that the enzymes responsible for glucuronidation in hypothyroidism are under the influence of thyroid hormones as is the case with oxidative enzymes. Further studies focused on the effect of host and environmental factors on glucuronidation. A commercially available very low calorie product for the treatment of obesity resulted in a decrease in oxazepam clearance and a lack of co-factors as a consequence of the low calorie intake was explanatorily proposed. Beta-adrenoceptor antagonists are often prescribed together with other drugs and close knowledge on interactions is mandatory but insufficient in regard of drugs being glucuronidated. Despite the mutual metabolic pathway labetalol exerted no dispositional alterations concerning oxazepam. It was moreover suggested that very elderly subjects between the age of 80 to 94 years had a reduced clearance of oxazepam.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetaminophen↗

PubMed2003

Induction of cytochrome P450, generation of oxidative stress and in vitro cell-transforming and DNA-damaging activities by glucoraphanin, the bioprecursor of the chemopreventive agent sulforaphane found in broccoli.

The reduced cancer risk that appears to be linked to a diet rich in fruits and vegetables has fueled the belief that regular intake of isolated phytochemicals could potentially prevent cancer. In recent years, the glucosinolate metabolites derived from cruciferous vegetables, such as the isothiocyanate sulforaphane in broccoli, have gained much attention as potential cancer chemopreventive agents. The protective effect of sulforaphane, which is liberated from its glucosinolate precursor glucoraphanin (GRP) by myrosinase hydrolysis, is conventionally thought to involve the induction of Phase-II metabolizing enzymes. These Phase-II enzymes are implicated in the detoxication of many carcinogens and reactive oxygen species (ROS), thereby protecting cells against DNA damage and subsequent malignant transformation. While the induction of Phase-II enzymes is usually considered beneficial, in some cases these enzymes also bioactivate several hazardous chemicals. Furthermore, despite its projected benefits, the unknown effect of sulforaphane on Phase-I enzyme systems, which are involved in the bioactivation of a variety of carcinogens, should not be overlooked. Here we show that, in rat lungs, while GRP, the bioprecursor of the chemopreventive agent sulforaphane, slightly induced Phase-II detoxifying enzymes, it powerfully induced Phase-I carcinogen-activating enzymes, including activators of carcinogenic polycyclic aromatic hydrocarbons (PAHs). Concomitant with this Phase-I induction, GRP also over-generated ROS. Additionally, in a cell-transforming assay, GRP facilitated the metabolic activation of the PAH benzo[a]pyrene to reactive carcinogenic forms and in a yeast genotoxicity test it damaged DNA. This suggests that regular administration of GRP could actually increase rather than decrease cancer risk, especially in individuals exposed to environmental mutagens and carcinogens such as those found in tobacco smoke and in certain industrial settings.

Animals↗

PubMed1995

In vitro conjugation of benzene metabolites by human liver: potential influence of interindividual variability on benzene toxicity.

In addition to industrial sources, benzene is present in the environment as a component of cigarette smoke and automobile emissions. Toxicity of benzene most likely results from oxidative metabolism of benzene to reactive products. However, susceptibility to these toxic effects may be related to a balance between activation (phase I) and detoxication (phase II) reactions. In the present study, we have estimated kinetic parameters of the two major detoxication reactions for benzene metabolites--phenol sulfation and hydroquinone glucuronidation--in liver subcellular fractions from 10 humans, and single samples from mice and rats. The extent of oxidative metabolism of benzene by these liver samples has been reported previously. Here, initial rates of phenol sulfation varied 3-fold (range 0.309-0.919 nmol/mg protein/min) among human samples. Measured rates were faster in rats (1.195 nmol/mg protein/min) than in mice (0.458 nmol/mg protein/min). Initial rates of hydroquinone glucuronidation by human samples also varied 3-fold (range 0.101-0.281 nmol/mg protein/min). Hydroquinone glucuronidation was more rapid by mouse microsomes (0.218 nmol/mg protein/min) than by rat microsomes (0.077 nmol/mg protein/min). To integrate interindividual differences in various enzyme activities, a physiological compartmental model was developed that incorporates rates of both conjugation reactions and oxidation reactions. Model equations were solved for steady-state concentrations of phenol and hydroquinone attained in human, mouse and rat blood during continuous exposure to benzene (0.01 microM in blood). Among the 10 human subjects, steady-state concentrations of phenol varied 6-fold (range 0.38-2.17 nM) and steady-state concentrations of hydroquinone varied 5-fold (range 6.66-31.44 nM). Predicted steady-state concentrations of phenol were higher in mice compared with rats (2.28 and 0.83 nM respectively). Likewise, higher steady-state concentrations of hydroquinone were predicted in mice than in rats (42.44 and 17.99 nM respectively). Predicted steady-state concentrations of phenol and hydroquinone in mice were higher than predictions for the 10 human subjects, whereas predicted concentrations for rats fell among the human values. As such, our results underscore the importance of considering the balance between activation and detoxication reactions in the elimination of toxicants. Model simulations suggest that both phase I and phase II pathways influence the relative risk from exposure to benzene.

Adolescent↗

PubMed2004

Regulation of the activity of the human drug metabolizing enzyme arylamine N-acetyltransferase 1: role of genetic and non genetic factors.

Human arylamine N-acetyltransferases 1 and 2 (NAT1 and NAT2) are polymorphic phase II xenobiotic-metabolising enzymes (XME) that acetylate arylamine compounds. They therefore play an important role in the detoxication and/or metabolic activation of certain therapeutic drugs, occupational chemicals and carcinogens. Although the use of the term "xenobiotic" implies that XME form a separate and distinct class of enzymes, possible endogenous substrates may exist. Unlike NAT2, NAT1 is produced in most tissues. Polymorphism at the NAT1 locus has been associated with the existence of at least 26 allelic variants, generating phenotypic variations in terms of NAT1 catalytic activity. This genetic variation affects the acetylator status of individuals, leading to interindividual differences in drug response and predisposition to disease in humans. Recent studies have shown that non-genetic factors may also regulate NAT1 activity at the posttranslational level, with potentially important consequences for drug toxicity. In this mini-review, we summarise what is currently known about the regulation of NAT1 activity by non-genetic factors, including substrates and oxidative stress. Recent findings presented here may account for the genotype/phenotype relationship for the NAT1 locus being less clear-cut than that for human NAT2.

Alleles↗

PubMed2003

The human colonic microflora influences the alterations of xenobiotic-metabolizing enzymes by catechins in male F344 rats.

As other xenobiotics, polyphenols are metabolized both by the endogenous detoxication system and the gut microflora. We hypothesized that the presence of a gut microflora may account for the effect of catechins on phase I and II xenobiotic-metabolizing enzymes and that the human bacterial metabolites may be different from those of a rodent gut microflora. Therefore, the effects of 2% (+)-catechin or 2% (-)-epicatechin were studied in germ free (GF) rats and rats inoculated with the flora of a human volunteer (HFA). In addition, the catechins were administered in ethanol as a vehicle. In the liver, (+)-catechin or (-)-epicatechin decreased the total amount of CYP450 in both GF and HFA rats while the isoenzyme CYP2E1 decreased. In GF rats only, CYP2C11 increased when compared to the rats treated with the vehicle alone. (+)-catechin increased the specific activity of UGT-chloramphenicol in GF rats only and that of cytosolic glutathion-S-transferase (GST) in HFA rats only. In the intestine, (+)-catechin and (-)-epicatechin increased the specific activity of UGT-4-methylumbelliferone in both GF and HFA rats and that of UGT- chloramphenicol in HFA rats only. In conclusion, the presence of a human flora in rats is able to modify the inducing effect of catechins on the UGT and GST activities suggesting the involvement of bacterial metabolites. The alterations on CYP 450 are independent of the presence of a human gut flora.

Administration, Oral↗

PubMed2006

Glutathione-S-transferase pi expression in toxic epidermal necrolysis: a marker of putative oxidative stress in keratinocytes.

BACKGROUND: Toxic epidermal necrolysis (TEN) is a dramatic drug-induced emergency related to extensive destruction of the epidermis. There is evidence that its pathomechanism involves impaired detoxication of xenobiotics. Glutathione-S-transferase pi (GST-pi) is a phase II detoxifying enzyme involved in drug metabolization by human keratinocytes. METHOD: Immunohistochemistry was performed in order to assess the expression of GST-pi in keratinocytes of TEN, other cutaneous adverse drug reactions and bullous pemphigoid. RESULTS: GST-pi was disclosed in the involved epidermis of 16/16 TEN patients. It was present in the cytoplasm of suprabasal keratinocytes. GST-pi was also expressed in the clinically uninvolved skin in a majority (8/12) of TEN patients. By contrast, it was rarely and poorly expressed in the other tested dermatoses. CONCLUSION: The pathomechanism of TEN is not related to an impaired quantitative expression of GST-pi. GST-pi expression is an early event in TEN. As oxidative stress is a major inducer of GST-pi, this mechanism might be involved in TEN. Its GST-pi expression mainly restricted to the suprabasal keratinocytes suggests that the pathomechanisms leading to keratinocyte death in TEN are distinct at different levels of the epidermis.

Adult↗

PubMed2006

Effect of conjugated equine estrogens on oxidative metabolism in middle-aged and elderly postmenopausal women.

The effects of conjugated equine estrogens (CEE) 0.625 mg daily on cytochrome P450 (CYP) were quantified in 12 middle-aged and 13 elderly postmenopausal women at baseline and 6 months later. CYP phenotype was characterized by caffeine (CYP1A2), chlorzoxazone (CYP2E1), dapsone (CYP, N-acetyltransferase 2), dextromethorphan (CYP2D6), and mephenytoin (CYP2C19) metabolism. CEE significantly decreased CYP1A2 (caffeine metabolic ratio: 0.57 +/- 0.20 before, 0.40 +/- 0.20 after, P = .001) and significantly increased CYP2D6 (dextromethorphan/dextrorphan ratio: 0.0116 +/- 0.0143 before, 0.0084 +/- 0.0135 after, P = .022) metabolism. CEE had no overall effect on CYP2C19, CYP2E1, CYP-mediated dapsone metabolism, and N-acetyltransferase 2. The dextromethorphan metabolic ratio decreased only in the seniors. The dapsone recovery ratio decreased in the middle-aged group and increased in the seniors. CEE significantly influenced CYP1A2, CYP2D6, and CYP-mediated dapsone oxidative metabolism but not CYP2C19, CYP2E1, or N-acetyltransferase 2 metabolism in postmenopausal women. Age influenced CYP2D6 metabolism and dapsone hydroxylation.

Aged↗

PubMed2003

Comparative effects and metabolism of two microcystins and nodularin in the brine shrimp Artemia salina.

The toxicity and metabolism of the cyanobacterial toxins microcystin-LR (MCLR), Dhb-microcystin-HtyR and nodularin were investigated in the cysts, nauplii and adults of the brine shrimp Artemia salina. The presence of the phase II detoxication system glutathione S-transferase (sGST) in these stages was shown using different substrates. Exposure of adult A. salina to the toxins led to an elevation of GST activity in vivo. All three toxins were conjugated to glutathione via GST, which has been shown as an initial step of microcystin and nodularin detoxication.

Animals↗

PubMed1988

Aflatoxin B1 carcinogenesis and its relation to DNA adduct formation and adduct persistence in sensitive and resistant salmonid fish.

Rainbow trout (Salmo gairdneri) and coho salmon (oncorhynchus kisutch) were exposed to aflatoxin B1 (AFB1) either by passive embryo uptake or by dietary treatment after hatching and feeding onset. Trout exposed as embryos to an aqueous solution of 0.5 p.p.m. AFB1 for 15 min showed a 62% tumor incidence 12 months later, whereas coho salmon exposed to a similar solution for 30 min showed only a 9% incidence. The difference between salmon and trout response was even greater by dietary AFB1 treatment. Trout exposed for 4 weeks to 20 p.p.b. dietary AFB1 had a 62% tumor response 12 months later, whereas salmon exposed to 40 p.p.b. dietary AFB1 for 4 weeks failed to develop tumors. A 5% tumor incidence was observed in salmon 12 months after 3 weeks exposure to 5000 p.p.b. dietary AFB1, a lethal dose for trout. In addition to a lower tumor incidence when compared to trout, the neoplastic response of salmon to AFB1 is to produce benign hepatic adenomas in contrast to the malignant hepatocellular carcinomas seen in trout. AFB1 metabolism, DNA adduct formation, adduct persistence in vivo and in vitro and cytochrome P-450 isozyme composition were compared in livers of trout and salmon to understand the role of metabolism and initiation in this species difference. AFB1-DNA binding was 7-56 times greater in trout than salmon liver at various times after AFB1 injection, 20 times greater in embryos or in freshly isolated trout hepatocyte preparations after a 1 h incubation with aflatoxin B1, and 18 times greater in trout liver after a three week dietary (80 p.p.b.) exposure. The major AFB1-DNA adduct was 8,9-dihydro-8-(N7-guanyl)-9-hydroxyaflatoxin B1 in both species. Persistence of AFB1-DNA adducts in vivo in liver was high compared to mammalian systems, implying that active enzymatic removal of bulky DNA adducts is low in both species and probably not a factor in their differential response to aflatoxin. Species differences in other phase I and phase II metabolism pathways and in AFB1 elimination were, overall, much less striking than those previously observed for trout fed inhibitors of aflatoxin carcinogenesis. Rates of bile elimination of AFB1 detoxication products, and total excretion of aflatoxins into water after AFB1 exposure, were not significantly different between trout and salmon. Since detoxication differences were not observed, the species difference in AFB1-DNA binding appears to reflect less efficient cytochrome P-450 metabolism of aflatoxin to the reactive 8,9-epoxide in salmon, compared to trout.(ABSTRACT TRUNCATED AT 400 WORDS)

Aflatoxin B1↗

PubMed1999

Phase I and phase II drug-metabolizing enzymes are expressed and heterogeneously distributed in the biliary epithelium.

Tissue expression of drug-metabolizing enzymes influences susceptibility to drugs and carcinogens. Because the biliary epithelium, exposed to bile-borne chemicals, may give rise to drug-induced cholangiopathies and to cholangiocarcinomas, we determined the pattern of expression of drug-metabolizing enzymes in this epithelium. We first demonstrated by blot analyses that biliary epithelial cells (BEC) isolated from human gallbladders display cytochrome P450 (CYP) 1A, 2E1, and 3A, microsomal epoxide hydrolase (mEH), alpha, mu, and pi glutathione S-transferase (GST), transcripts and proteins. We also identified CYP-associated steroid 6beta-hydroxylase activity in BEC. CYP and mEH expression was 5- to 20-fold lower in BEC than in autologous hepatocytes, and further differed by a higher ratio of CYP3A5/CYP3A4, and by CYP1A1 predominance over CYP1A2. alphaGST was highly expressed in both hepatocytes and BEC, while piGST was restricted to BEC. In approximately 50% of individuals, muGST was expressed in hepatocytes and at lower levels in BEC. By using the same antibodies as those used in immunoblots, we could show by immunohistochemistry that CYP2E1, CYP3A, mEH, alpha, mu, and piGST immunoreactivities are expressed and display a heterogeneous distribution in the epithelium lining the entire biliary tract except for small intrahepatic bile ducts that were devoid of CYP3A and alphaGST immunoreactivities. In conclusion, BEC contribute to phase II, and although to a lesser extent than hepatocytes, to phase I biotransformation. The distribution of drug-metabolizing enzymes in BEC suggest that they are heterogeneous in their ability to generate and detoxicate reactive metabolites, which may contribute to specific distributions of cholangiopathies.

Aryl Hydrocarbon Hydroxylases↗

PubMed1997

Effects of administration of the chemoprotective agent oltipraz on CYP1A and CYP2B in rat liver and rat hepatocytes in culture.

The success of oltipraz (OPZ) [5-(2-pyrazinyl)-4-methyl-1,2-dithiole-3-thione] as a chemoprotective agent against aflatoxin B1 (AFB1)-induced hepatocarcinogenesis in the rat is thought to depend principally on its ability to enhance detoxication by inducing phase II enzymes, especially glutathione transferases. However, in primary cultures of human hepatocytes, we recently demonstrated that OPZ also has an important inhibitory effect on the major cytochromes P450 (CYPs) of human hepatic AFB1 metabolism. This has prompted a detailed study of the effect of OPZ on some CYPs involved in metabolism of AFB1 in the rat. Primary cultures of rat hepatocytes behaved similarly to human hepatocytes and responded to OPZ by inhibition of ethoxyresorufin-O-deethylase (EROD) and pentoxyresorufin-O-depentylase (PROD) activities mainly associated, respectively, with CYP1A and CYP2B. A time-course shows that this inhibition is largely reversible, with EROD and PROD activities reaching a minimum at 12 h and tending towards control values within 24 h. As is to be expected, the incubation of isolated microsomes with OPZ also inhibits CYP1A and 2B. The effect of OPZ on CYP1A is not a phenomenon limited to cells in culture, but also occurs in vivo. Using the whole animal, we were able to demonstrate that OPZ also transiently inhibited CYP1A activity in a rat given caffeine, by measuring the amounts of methylxanthines found in the serum. However, microsomes isolated from rats, that had been treated with OPZ in vivo, show no such inhibition, presumably because, since OPZ is a reversible inhibitor, it dissociates and is lost during the course of conventional procedures of microsomal preparation. This explains some earlier failures in studies of isolated microsomes to observe the inhibition of CYPs by OPZ. In addition to inhibiting their enzymatic activity, OPZ is also an inducer of CYP1A and 2B as shown by the increased levels of their mRNAs and of caffeine metabolism in vivo after 24 h or more. It is concluded that the mechanism of chemoprotection by OPZ, of toxic chemical metabolism in the rat, is complex and involves competitive inhibition of activation succeeded by induction of the enzymes of both activation and detoxication.

Aflatoxin B1↗