Increased oxidation of p-nitrophenol and aniline by intact hepatocytes isolated from pyrazole-treated rats

Allyl methyl disulfide (AMDS) prevents N,N-dimethyl formamide-induced liver damage by suppressing oxidative stress and NLRP3 inflammasome activation

N,N-dimethylformamide (DMF), a widely consumed industrial solvent with persistent characteristics, can induce occupational liver damage and pose threats to the general population due to the enormous DMF-containing industrial efflux and emission from indoor facilities. This study was performed to explore the roles of allyl methyl disulfide (AMDS) in liver damage induced by DMF and the underlying mechanisms. AMDS was found to effectively suppress the elevation in the liver weight/body weight ratio and serum aminotransferase activities, and reduce the mortality of mice induced by DMF. In addition, AMDS abrogated DMF-elicited increases in malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) levels and decreases in glutathione (GSH) levels in mouse livers. The increase in macrophage number, mRNA expression of M1 macrophage biomarkers, and protein expression of key components in the NF-κB pathway and NLRP3 inflammasome induced by DMF exposure were all suppressed by AMDS in mouse livers. Furthermore, AMDS inhibited DMF-induced cell damage and NF-κB activation in cocultured AML12 hepatocytes and J774A.1 macrophages. However, AMDS per se did not significantly affect the protein level and activity of CYP2E1. Collectively, these results demonstrate that AMDS effectively ameliorates DMF-induced acute liver damage possibly by suppressing oxidative stress and inactivating the NF-κB pathway and NLRP3 inflammasome.

Caffeic acid abrogates 1,3-dichloro-2-propanol-induced hepatotoxicity by upregulating nuclear erythroid-related factor 2 and downregulating nuclear factor-kappa B

1,3-dichloro-2-propanol is a food-borne contaminant reported to cause liver injury. In this study, we evaluated the protective influence of caffeic acid on 1,3-dichloro-2-propanol-induced hepatotoxicity in rats. Rats were randomized into five groups (A-E). Rats received distilled water or caffeic acid (10 or 20 mg/kg body weight) for 7 days. In addition, rats were challenged with 1,3-dichloro-2-propanol on day 7. Caffeic acid prevented 1,3-dichloro-2-propanol-mediated alterations in alkaline phosphatase, alanine and aspartate aminotransferases, albumin and total bilirubin in the serum of rats. Furthermore, caffeic acid lowered superoxide ion, hydrogen peroxide and cytochrome P2E1 while increasing the activities of superoxide dismutase, catalase and glutathione S-transferase in the liver of 1,3-dichloro-2-propanol-treated rats. Caffeic acid raised the levels of nuclear erythroid-related factor 2 (Nrf-2), protein kinase A and phosphoinositide 3-kinase. Caffeic acid pretreatment annulled 1,3-dichloro-2-propanol-mediated alterations in the oxidative stress biomarkers; caspase-3, glutathione, malondialdehyde, protein carbonyl and fragmented DNA, in the liver of rats. Contrastingly, caffeic acid lowered 1,3-dichloro-2-propanol-mediated increase in the levels of nuclear factor-kappa B (NF-κB), tumour necrosis factor-α, interleukin-1β (IL-1β) and IL-6. In addition, caffeic acid preserved the morphological features of 1,3-dichloro-2-propanol-treated rats. Results from this study revealed that caffeic acid protects against 1,3-dichloro-2-propanol-induced hepatotoxicity by enhancing the cytoprotective enzymes through Nrf-2 while lowering inflammation through NF-κB.

Establishment of a p-nitrophenol oxidation-based assay for the analysis of CYP2E1 activity in intact hepatocytes in vitro

CYP2E1 is a mammalian cytochrome P450 enzyme, which oxidizes a structurally diverse class of endogenous and exogenous (xenobiotic) compounds. Best studied is the role of CYP2E1 in phase I metabolism of xenobiotics including alcohol. CYP2E1 metabolizes ethanol and is active in generating reactive oxygen species (ROS) and subsequent oxidative stress in the hepatic tissues. Several studies have shown and discussed the importance of CYP2E1 in the hepatotoxic actions of alcohol. However, the vast majority assessed the CYP2E1 activity only in isolated microsomes. Here, we aimed to develop and optimize a fast and easy method to assess alcohol-induced CYP2E1 activity in hepatocytes in vitro applying oxidation of para-nitrophenol to para-nitrocatechol as specific substrate probe. Using hepatoma cells with and without stable CYP2E1 expression and primary human hepatocytes, we established specific methodology to assess CYP2E1 catalytic activity and its induction by ethanol in a small number of cells and in a very short time.

Sorghum-based alcoholic beverage, Burukutu, perturbs the redox status of the liver of male rats

The redox status of male rat liver following 28 days consumption of Burukutu was investigated. Twenty rats were randomized into four groups with five rats each. Burukutu consumption at 0.78 g/kg alcohol produced no significant change in the activities of alkaline phosphatase (ALP), alanine aminotransferase (ALT), and aspartate aminotransferase (AST). However, 3.71 and 7.43 g/kg dosages resulted in significant decrease in the activities of ALP, ALT and AST with corresponding increase in serum. The activity of cytochrome P₄₅₀(CYP 2E1) increased significantly in the liver of rats following consumption of Burukutu at all doses investigated. The activities of superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase and glucose 6-phosphate dehydrogenase decreased significantly (P < 0.05) in rats treat with 0.78 g/kg, 3.41 and 7.43 g/kg Burukutu. There was a significant increase in the level of glutathione disulfide (GSSG) with reduction in the levels of glutathione reduced (GSH) and GSH:GSSG. The levels of oxidative stress biomarkers, malondialdehyde, conjugated dienes, lipid hydroperoxides, protein carbonyl and percentage DNA fragmentation, increased significantly (P < 0.05). It is evident from the alterations in the activities of the hepatocellular enzymes, antioxidant enzymes and oxidative stress biomarkers that Burukutu mediated its toxicity through the depletion of the antioxidant enzymes.

Methodology to assay CYP2E1 mixed function oxidase catalytic activity and its induction

The cytochrome P450 mixed function oxidase enzymes are the major catalysts involved in drug metabolism. There are many forms of P450. CYP2E1 metabolizes many toxicologically important compounds including ethanol and is active in generating reactive oxygen species. Since several of the contributions in the common theme series “Role of CYP2E1 and Oxidative/Nitrosative Stress in the Hepatotoxic Actions of Alcohol” discuss CYP2E1, this methodology review describes assays on how CYP2E1 catalytic activity and its induction by ethanol and other inducers can be measured using substrate probes such as the oxidation of para-nitrophenol to para-nitrocatechol and the oxidation of ethanol to acetaldehyde. Approaches to validate that a particular reaction e.g. oxidation of a drug or toxin is catalyzed by CYP2E1 or that induction of that reaction is due to induction of CYP2E1 are important and specific examples using inhibitors of CYP2E1, anti-CYP2E1 IgG or CYP2E1 knockout and knockin mice will be discussed.

Cytochrome P4502E1(CYP2E1) oxidizes ethanol and activates hepatoxins and procarcinogens.

CYP2E1 produces reactive oxygen species during its catalytic cycle.

Methodology to assay CYP2E1 via oxidation of ethanol and p-nitrophenol is reviewed.

Oxidation of these substrates is enhanced after induction of CYP2E1 by ethanol.

Oxidation of these substrates is lowered by CYP2E1 inhibitors, anti-CYP2E1 IgG and in CYP2E1 knockout mice.

Quercetin suppressed CYP2E1-dependent ethanol hepatotoxicity via depleting heme pool and releasing CO

Naturally occuring quercetin protects hepatocytes from ethanol-induced oxidative stress, and heme oxygenase-1 (HO-1) induction and carbon monoxide (CO) metabolite may be implicated in the beneficial effect. However, the precise mechanism by which quercetin counteracts CYP2E1-mediated ethanol hepatotoxicity through HO-1 system is still remained unclear. To explore the potential mechanism, herein, ethanol (4.0 g/kg.bw.) was administrated to rats for 90 days. Our data showed that chronic ethanol over-activated CYP2E1 but suppressed HO-1 with concurrent hepatic oxidative damage, which was partially normalized by quercetin (100mg/kg.bw.). Quercetin (100 μM) induced HO-1 and depleted heme pool when incubated to human hepatocytes. Ethanol-stimulated (100mM) CYP2E1 upregulation was suppressed by quercetin but further enhanced by HO-1 inhibition with resultant heme accumulation. CO scavenging blocked the suppression of quercetin only on CYP2E1 activity. CO donor dose-dependently inactivated CYP2E1 of ethanol-incubated microsome, which was mimicked by HO-1 substrate but abolished by CO scavenger. Thus, CYP2E1-mediated ethanol hepatotoxicity was alleviated by quercetin through HO-1 induction. Depleted heme pool and CO releasing limited protein synthesis and inhibited enzymatic activity of CYP2E1, respectively.

Expression of Cytochromes P-450 2E1, 3A4 and 1A1/1A2 in Growing and Confluent Human HepG2 Hepatoma Cells-Effect of Ethanol

In cultured human hepatoma HepG2 cells, cytochrome (CYP) 1A-associated 7-ethoxyresorufin-O-deethylase (EROD), CYP 3A-associated benzyloxyresorufin O-debenzylase (BROD) and CYP 2E1-associated p-nitrophenol-hydroxylase (PNPH) decreased during time in culture. The enzyme activities in cells at confluence were 35-60% of the activities in cells 24 hours after seeding. Similarly, CYP 3A and CYP 2E1 proteins were present at higher concentrations in growing (G) than in confluent (C) HepG2 cells. CYP 1A1/1A2 protein was not detected, neither in G nor in C HepG2 cells but was strongly induced by 3-methylcholanthrene (3-MC) treatment. Ethanol (EtOH) was shown to increase CYP 2E1 and CYP 3A proteins and CYP 1A1/1A2-, CYP 2E1- and CYP 3A-associated mixed-function oxidase activities (MFOs) in HepG2 cells, as has been previously reported for primary cultures of human hepatocytes. These effects were observed only at the beginning of culture, in growing HepG2 cells, demonstrating the influence of the growth stage of HepG2 cells on their response to EtOH treatment. This is, to our knowledge, the first report on increases in CYP proteins and associated MFOs by EtOH in HepG2 cells. It suggests that growing HepG2 cells provide a useful in vitro model system in which to study the regulation of human CYPs by EtOH.

The protective role of HO-1 and its generated products (CO, bilirubin, and Fe) in ethanol-induced human hepatocyte damage

It has been reported that naturally occurring quercetin exerts hepatoprotective effects through heme oxygenase-1 (HO-1) induction. However, the precise mechanism of how ethanol-associated liver damage is counteracted by quercetin-enhanced HO-1 metabolism still remains unclear. To further decipher the protective role of quercetin on ethanol-induced liver damage, we treated human hepatocytes with quercetin and various (end) products of the HO-1 pathway. Our data clearly showed that quercetin treatment attenuated ethanol-induced damage, whereas hemoglobin and zinc protoporphyrin 9 (ZnPP) abolished such effects. Iron-II aggravated ethanol toxicity and was only partially reduced by quercetin. In contrast, carbon monoxide (CO) dose dependently inhibited ethanol-induced cytochrome P450 2E1 (CYP 2E1) activity and hepatotoxicity but had no influence on CYP 2E1 protein expression. Similarly, hemoglobin dramatically stimulated CYP 2E1 activity but not the protein expression in quercetin- and ethanol-cotreated hepatocytes. ZnPP significantly promoted CYP 2E1 protein expression in the presence and absence of CO treatment but inhibited ethanol-induced CYP 2E1 activation following CO incubation in quercetin- and ethanol-cotreated hepatocytes. These results suggested that quercetin virtually attenuated ethanol-derived oxidative damage via HO-1 induction. Heme degradation and CO release may mediate the protective effects through inhibiting ethanol-induced CYP 2E1 synthesis and enzymatic activity, respectively.

Protective effects of puerarin on carbon tetrachloride-induced hepatotoxicity

Puerarin, the main isoflavone glycoside found in the root of Pueraria lobata, has been used for various medicinal purposes in traditional Chinese medicine for thousands of years. The purpose of this study was to investigate the protective effects of puerarin against hepatotoxicity induced by carbon tetrachloride (CCl4) and the mechanism of its hepatoprotective effect. In mice, pretreatment with puerarin prior to the administration of CCl4 significantly prevented the increased serum enzymatic activity of alanine aspartate aminotransferase and hepatic malondialdehyde formation in a dose-dependent manner. In addition, pretreatment with puerarin significantly prevented both the depletion of reduced glutathione (GSH) content and the decrease in glutathione S-transferase (GST) activity in the liver of CCl4-intoxicated mice. Hepatic GSH levels and GST activity were increased by treatment with puerarin alone. CCl4-induced hepatotoxicity was also prevented, as indicated by liver histopathology. The effects of puerarin on cytochrome P450 (CYP) 2E1, the major isozyme involved in CCl4 bioactivation, were also investigated. Treatment of the mice with puerarin resulted in a significant decrease in the CYP2E1-dependent aniline hydroxylation in a dose-dependent manner. Consistent with these observations, the CYP2E1 protein levels were also lowered. Puerarin exhibited anti-oxidant effects on FeCl2-ascorbate induced lipid peroxidation in mouse liver homogenates, and on superoxide radical scavenging activity. These results suggest that the protective effects of puerarin against the CCl4-induced hepatotoxicity possibly involve mechanisms related to its ability to block CYP-mediated CCl4 bioactivation, induction of GST activity and free radical scavenging effects.

Hepatoprotective and antioxidant effects of the coffee diterpenes kahweol and cafestol on carbon tetrachloride-induced liver damage in mice

The hepatoprotective effects of kahweol and cafestol, coffee-specific diterpenes, on the carbon tetrachloride (CCl(4))-induced liver damage as well as the possible mechanisms involved in these protections were investigated. Pretreatment with kahweol and cafestol prior to the administration of CCl(4) significantly prevented the increase in the serum levels of hepatic enzyme markers (alanine aminotransferase and aspartate aminotransferase) and reduced oxidative stress, such as reduced glutathione content and lipid peroxidation, in the liver in a dose-dependent manner. The histopathological evaluation of the livers also revealed that kahweol and cafestol reduced the incidence of liver lesions induced by CCl(4). Treatment of the mice with kahweol and cafestol also resulted in a significant decrease in the cytochrome P450 2E1 (CYP2E1), the major isozyme involved in CCl(4) bioactivation, specific enzyme activities, such as p-nitrophenol and aniline hydroxylation. Kahweol and cafestol exhibited antioxidant effects on FeCl(2)-ascorbate induced lipid peroxidation in a mouse liver homogenate, and on superoxide radical scavenging activity. These results suggest that the protective effects of kahweol and cafestol against the CCl(4)-induced hepatotoxicity possibly involve mechanisms related to their ability to block the CYP2E1-mediated CCl(4) bioactivation and free radical scavenging effects.

Regulation of Cytochrome P450 2E1 under Hypertonic Environment through TonEBP in Human Hepatocytes

Whereas the liver as well as the other organs are continually exposed to the change of osmotic status, it has never been investigated whether activities and gene expressions of drug-metabolizing enzymes, including cytochromes P450, are dependent on osmotic change in the liver. In the present study, we determined that CYP2E1 is induced under hypertonic environments at a transcriptional level in human primary hepatocytes, as assessed by cDNA microarray and real time-reverse transcription-polymerase chain reaction analyses. Both a protein level and the catalytic activity of CYP2E1 were consistently increased in response to hypertonic conditions. In promoter-reporter assay, it was demonstrated that -586 to -566 in the CYP2E1 5'-flanking region was necessary for 2E1 promoter activation by hypertonic stimulation. It is noteworthy that tonicity-response element (TonE) consensus sequence was found at -578 to -568 in human CYP2E1 5'-flanking region, and electrophoretic mobility shift assay demonstrated the interaction of TonE binding protein (TonEBP) with TonE motif of CYP2E1 promoter. Furthermore, cotransfection of a CYP2E1 promoter construct with wild-type TonEBP expression vector enhanced promoter activity under both isotonic and hypertonic conditions, whereas dominant-negative TonEBP suppressed an induction of CYP2E1 promoter activity. These results indicate that the level of CYP2E1 is induced by hypertonic condition via TonEBP transactivation. The present study suggests that osmotic status may influence individual responses to the substrate of CYP2E1.

Human hepatocytes are protected from ethanol-induced cytotoxicity by DADS via CYP2E1 inhibition

We investigated the protective effects of diallyl disulfide (DADS), a potent inhibitor of cytochrome P450 2E1 (CYP2E1), on ethanol-induced toxicity in human hepatocytes. We found a clear dose-dependent response between ethanol and CYP2E1 activity. The ethanol-dependent CYP2E1 enzyme activity and protein expression, lactate dehydrogenase and aspartate transaminase release, malondialdehyde formation and caspase-3 activity decreased dramatically in the presence of DADS. Furthermore, DADS increased the hepatocellular glutathione (GSH) content and prevented the ethanol-dependent cellular GSH depletion. Our data show that DADS reduces ethanol-induced toxicity in human hepatocytes by reducing CYP2E1 activity and/or stabilizing the cellular GSH content, which might be of therapeutic interest.

Protective Effect of Acteoside on Carbon Tetrachloride-Induced Hepatotoxicity

This study investigated the protective effects of acteoside, a phenylethanoid glycoside, on the carbon tetrachloride-induced hepatotoxicity as well as the possible mechanisms involved in this protection in mice. Pretreatment with acteoside prior to the administration of carbon tetrachloride significantly prevented the increased serum enzymatic activities of alanine and aspartate aminotransferase in a dose-dependent manner. In addition, pretreatment with acteoside significantly prevented the increase in hepatic malondialdehyde formation and the depletion of the reduced glutathione content in the liver of carbon tetrachloride-intoxicated mice. Carbon tetrachloride-induced hepatotoxicity was also essentially prevented, as indicated by a liver histopathologic study. The effects of acteoside on cytochrome P450 (P450) 2E1, the major isozyme involved in carbon tetrachloride bioactivation were also investigated. Treatment of the mice with acteoside resulted in a significant decrease in the P450 2E1-dependent pnitrophenol and aniline hydroxylation in a dose-dependent manner. Consistent with these observations, the P450 2El protein levels were also lower. Acteoside exhibited anti-oxidant effects on FeCl2-ascorbate induced lipid peroxidation in a mouse liver homogenate, and on superoxide radical scavenging activity. These results suggest that the protective effects of acteoside against the carbon tetrachloride-induced hepatotoxicity possibly involve mechanisms related to its ability to block the P450-mediated carbon tetrachloride bioactivation and free radical scavenging effects.

Constitutive and inducible expression of CYP enzymes in immortal hepatocytes derived from SV40 transgenic mice

1. The expression of liver-specific transcription factors and cytochrome P450 (CYP) enzymes have been studied in three new hepatocyte-like cell lines derived from SV Delta 202 transgenic mice: AMH-Delta 202 (adult mouse hepatocytes), TAMH-Delta 202 (tumour-derived adult mouse hepatocytes) and NMH-Delta 202 (newborn mouse hepatocytes). 2. mRNA levels of liver-enriched transcription factors such as D-element binding protein (DBP), liver-enriched transcription activating protein (LAP) and the hepatic nuclear factors (HNF) 1, 2 and 3 in all Delta 202 transgenic hepatocyte lines were similar to those in the wild-type liver and in primary mouse hepatocytes. 3. Analysis of basal CYP activities and testosterone metabolism revealed that Delta 202 cells showed higher similarities to mouse hepatocytes than Hepa 1c1c7 hepatoma cells. All three Delta 202 cell lines exhibited substantial active CYP1A1/2, CYP2A4/5 and CYP3A11 activities and lower levels of CYP2B, CYP2C and CYP2E1 activities. 4. The Delta 202 cells also responded to model inducers. 3-Methylcholanthrene induced CYP1A1/2 (7-ethoxyresorufin O-deethylation); phenobarbital induced CYP2B (7-benzoxyresorufin O-debenzylation), CYP2A4/5 (testosterone 7alpha -hydroxylation) and CYP3A11 (testosterone 6beta -hydroxylation); and rifampicin and dexamethasone induced CYP3A11 activities in the three Delta 202 cell lines, whereas only AMH-Delta 202 cells reproduced to a limited extent the response of CYP2E1 to ethanol observed in hepatocytes. 5. The results suggest that generation of hepatocyte lines from transgenic animals constitutes a successful approach to obtain in vitro models alternative to primary hepatocytes for drug metabolism and CYP inducibility studies.

Strategies and molecular probes to investigate the role of cytochrome P450 in drug metabolism: focus on in vitro studies

Drug metabolism is the major determinant of drug clearance and, because of polymorphic or inducible expression of drug-metabolising cytochrome P450s (CYPs), is the factor most frequently responsible for interindividual differences in pharmacokinetics. A number of well characterised CYP substrates and inhibitors have been identified that allow precise measurements of individual CYP isoforms. Their use, alone or in combination, facilitates the phenotype characterisation of hepatocytes in vitro and in vivo. Two procedures are used for in vitro investigation of the metabolic profile of a drug: incubation with microsomes and incubation with metabolically competent cells. The major limitation of microsomes is that they express phase I activities, but only part of phase II activities, and can only be used for short incubation times. When intact cells are used, gene expression, metabolic pathways, cofactors/enzymes and plasma membrane are largely preserved, but fully differentiated cells such as primary cultured hepatocytes need to be used, since hepatoma cell lines have only very low and partial CYP expression. CYP-engineered cells or their microsomes ('supersomes') have made the identification of the CYPs involved in the metabolism of a drug candidate straightforward and easier. Inhibition of CYP is an undesirable feature for a drug candidate, and needs to be addressed by examining whether the drug candidate inhibits the metabolism of other compounds or whether other compounds inhibit the metabolism of the drug candidate. Such experiments can be conducted both with microsomes and in cells. The major limitation of microsomes is that inhibition parameters may not accurately reflect the situation in vivo, since the contribution of drug transport is not considered. The best picture of a potential drug-drug interaction can be obtained in metabolically competent hepatocytes. Screening of CYP inducers cannot be done in microsomes. It requires the use of a cellular system fully capable of transcribing and translating CYP genes, and can be monitored in vitro as an increase in enzyme mRNA or activity. Human hepatocytes in primary culture respond well to enzyme inducers during the first few days; this ability is lost thereafter. Rat hepatocytes are much less stable and soon become unresponsive to inducers. Hepatoma cell lines respond poorly to inducers, although the induction of a few isoenzymes has been reported. Primary cultured hepatocytes are still the unique in vitro model that allows global examination of the inductive potential of a drug.

p-Nitrophenol and glutathione response in medaka (Oryzias latipes) exposed to MX, a drinking water carcinogen

When chlorine is introduced into public drinking water for disinfection, it can react with organic compounds in surface waters to form toxic by-products such as 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX). We investigated the effect of exposure to MX on cytochrome P450 2E1 (CYP2E1)-like activity and total glutathione (GSH) in the liver of the small fish model, medaka (Oryzias latipes). The multi-site carcinogen methylazoxymethanol acetate (MAMAc) was the positive control compound. Both medaka liver microsome preparations and S-9 fractions catalyzed the hydroxylation of p-nitrophenol (PNP), suggesting CYP2E1-like activity in the medaka. Male medaka exposed for 96 h to the CYP2E1 inducers ethanol and acetone under fasted conditions showed significant increases in PNP-hydroxylation activity. Furthermore, total reduced hepatic GSH was reduced in fish fasted for 96 h, indicating that normal feeding is a factor in maintaining xenobiotic defenses. Exposure to MX and MAMAc induced significant increases in hepatic CYP2E1-like activity, however MX exposure did not alter hepatic GSH levels. These data strengthen the role of the medaka as a suitable species for examining cytochrome P450 and GSH detoxification processes and the role these systems play in chemical carcinogenesis.

Analysis of differential substrate selectivities of CYP2B6 and CYP2E1 by site-directed mutagenesis and molecular modeling

Human CYP2B6 and CYP2E1 were used to investigate the extent to which differential substrate selectivities between cytochrome P450 subfamilies reflect differences in active-site residues as opposed to distinct arrangement of the backbone of the enzymes. Reciprocal CYP2B6 and CYP2E1 mutants at active-site positions 103, 209, 294, 363, 367, and 477 (numbering according to CYP2B6) were characterized using the CYP2B6-selective substrate 7-ethoxy-4-trifluoromethylcoumarin, the CYP2E1-selective substrate p-nitrophenol, and the common substrates 7-ethoxycoumarin, 7-butoxycoumarin, and arachidonic acid. This report is the first to study the active site of CYP2E1 by systematic site-directed mutagenesis. One of the most intriguing findings was that substitution of CYP2E1 Phe-477 with valine from CYP2B6 resulted in significant 7-ethoxy-4-trifluoromethylcoumarin deethylation. Use of three-dimensional models of CYP2B6 and CYP2E1 based on the crystal structure of CYP2C5 suggested that deethylation of 7-ethoxy-4-trifluoromethylcoumarin by CYP2E1 is impeded by van der Waals overlaps with the side chain of Phe-477. Interestingly, none of the CYP2B6 mutants acquired enhanced ability to hydroxylate p-nitrophenol. Substitution of residue 363 in CYP2E1 and CYP2B6 resulted in significant alterations of the metabolite profile for the side chain hydroxylation of 7-butoxycoumarin. Probing of CYP2E1 mutants with arachidonic acid indicated that residues Leu-209 and Phe-477 are critical for substrate orientation in the active site. Overall, the study revealed that differences in the side chains of active-site residues are partially responsible for differential substrate selectivities across cytochrome P450 subfamilies. However, the relative importance of active-site residues appears to be dependent on the structural similarity of the compound to other substrates of the enzyme.

Hepatoprotective effects of 18beta-glycyrrhetinic acid on carbon tetrachloride-induced liver injury: inhibition of cytochrome P450 2E1 expression

The protective effects of 18beta-glycyrrhetinic acid (GA), the aglycone of glycyrrhizin (GL) derived from licorice, on carbon tetrachloride-induced hepatotoxicity and the possible mechanisms involved in this protection were investigated in mice. Pretreatment with GA prior to the administration of carbon tetrachloride significantly prevented an increase in serum alanine, aspartate aminotransferase activity and hepatic lipid peroxidation in a dose-dependent manner. In addition, pretreatment with GA also significantly prevented the depletion of glutathione (GSH) content in the livers of carbon tetrachloride-intoxicated mice. However, reduced hepatic GSH levels and glutathione-S-transferase activities were unaffected by treatment with GA alone. Carbon tetrachloride-induced hepatotoxicity was also prevented, as indicated by a liver histopathologic study. The effects of GA on the cytochrome P450 (P450) 2E1, the major isozyme involved in carbon tetrachloride bioactivation, were also investigated. Treatment of mice with GA resulted in a significant decrease of the P450 2E1-dependent hydroxylation of p-nitrophenol and aniline in a dose-dependent manner. Consistent with these observations, the P450 2E1 expressions were also decreased, as determined by immunoblot analysis. GA also showed antioxidant effects upon FeCl(2)-ascorbate-induced lipid peroxidation in mice liver homogenate and upon superoxide radical scavenging activity. These results show that protective effects of GA against the carbon tetrachloride-induced hepatotoxicity may be due to its ability to block the bioactivation of carbon tetrachloride, primarily by inhibiting the expression and activity of P450 2E1, and its free radical scavenging effects.

Cytochrome P450 expression in human hepatocytes and hepatoma cell lines: molecular mechanisms that determine lower expression in cultured cells

1. Cultured hepatic cells have reduced cytochrome P450 (CYP) activities in comparison with human liver, but the mechanism(s) that underlies this circumstance is not clear. We investigated the causes of this low CYP activity by analysing the activity, protein, mRNA and heterologous nuclear RNA contents of the most important CYPs involved in drug metabolism (1A1, 1A2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5) in cultured human hepatocytes, and in HepG2 and Mz-Hep-1 hepatoma cell lines. 2. After 24 h of culture, hepatocytes retained most of their CYP activities and protein contents, but the mRNA decreased 20-fold. However, the mRNA content of most CYPs in 24-h hepatocytes was still 400-fold higher than in hepatoma cells. When we examined the transcriptional activity of the CYP genes, this decreased during culture time in hepatocytes and it was poor in hepatoma cell lines. 3. We investigated the abundance of key hepatic transcription factors that govern CYP transcription (C/EBP-beta: LAP and LIP, HNF-3alpha, HNF-4alpha, RXR-alpha) and observed that the expression of some factors was altered in the hepatoma cells. 4. In conclusion, the loss of biotransformation activity in cultured hepatic cells is caused by a decrease in CYP transcription, which correlates with an alteration in the expression of key transcription factors.

Protective effect of Platycodi radix on carbon tetrachloride-induced hepatotoxicity

The protective effects of a Platycodi radix (Changkil: CK), the root of Platycodon grandiflorum A. DC (Campanulaceae) on carbon tetrachloride (CC14)-induced hepatotoxicity and the possible mechanisms involved in this protection were investigated in mice. Pretreatment with CK prior to the administration of CC14 significantly prevented the increased serum enzymatic activities of alanine and aspartate aminotransferase in a dose-dependent manner. In addition, pretreatment with CK also significantly prevented the elevation of hepatic malondialdehyde formation and the depletion of reduced glutathione content in the liver of CC14-intoxicated mice. However, hepatic reduced glutathione levels and glutathione S-transferase activities were not affected by treatment with CK alone. CC14-induced hepatotoxicity was also essentially prevented, as indicated by a liver histopathologic study. The effects of CK on the cytochrome P450 (P450) 2E1, the major isozyme involved in CC14 bioactivation were also investigated. Treatment of mice with CK resulted in a significant decrease of P450 2E1-dependent p-nitrophenol and aniline hydroxylation in a dose-dependent manner. CK showed antioxidant effects in FeCl2-ascorbate-induced lipid peroxidation in mice liver homogenate and in superoxide radical scavenging activity. Our results suggest that the protective effects of CK against CC14-induced hepatotoxicity possibly involve mechanisms related to its ability to block P450-mediated CC14 bioactivation and free radical scavenging effects.

Hepatoprotective effects of Platycodon grandiflorum on acetaminophen-induced liver damage in mice

The protective effects of an aqueous extract from the roots of Platycodon grandiflorum A. DC (Campanulaceae), Changkil (CK), on acetaminophen (APAP)-induced hepatotoxicities and the possible protective mechanisms involved were investigated in mice. Pretreatment with CK prior to the administration of APAP significantly prevented the increase in serum alanine aminotransferase and aspartate aminotransferase activity and hepatic lipid peroxidation in a dose-dependent manner. APAP-induced hepatotoxicity was also essentially prevented as evidenced by liver histopathology. Hepatic glutathione levels and glutathione-S-transferase activities were not affected by treatment with CK alone, but pretreatment with CK protected the APAP-induced depletion of hepatic glutathione levels. The effects of CK on cytochrome P450 (P450) 1A2 and 2E1, the major isozymes involved in APAP bioactivation, were investigated. In microsomal incubations, CK effectively inhibited P450 lA2-dependent methoxyresorufin O-deethylase activities and the P450 2E1-dependent p-nitrophenol and aniline hydroxylase. The results suggest that the protective effects of CK against the APAP-induced hepatotoxicity may, at least in part, be due to its ability to block P450-mediated APAP bioactivation.

Expression and induction of a large set of drug-metabolizing enzymes by the highly differentiated human hepatoma cell line BC2

The BC2 cell line derived from the human hepatocarcinoma, HGB, undergoes a spontaneous sharp differentiation process in culture as it becomes confluent, remains stably differentiated for several weeks, and may return to proliferation thereafter under appropriate density conditions. The relevance of the line as an hepatic model has been evaluated. Cells synthesize a large number of plasma proteins, and rates of glycogen and urea synthesis increase with time of confluency and become sensitive to insulin, reflecting the process of differentiation. Differentiated BC2 cells express the most relevant cytochrome P-450 (CYP) isozyme activities (CYP1A1/2, 2A6, 2B6, 2C9, 2E1, and 3A4) and conjugating enzymes (glutathione S-transferase and UDP-glucuronyltransferase) and also respond to model inducers. Methylcholanthrene induced an increase in CYP1A1/2 enzyme activity (eightfold), phenobarbital induced CYP2B6 activity (1.7-fold), and dexamethasone induced CYP3A4 activity (fivefold). In parallel, expression of the most relevant liver-enriched transcription factors, HNF-4, HNF-1, C/EBP-alpha and C/EBP-beta mRNAs, was significantly increased in differentiated cultures. This increase was largest in HNF-1 and HNF-4, which supports the idea that a redifferentiation process towards the hepatic phenotype takes place. BC2 is an hepatic cell line that is able to express most hepatic functions, especially the drug-biotransformation function, far more efficiently than any previously described human hepatoma cell line.

Increased toxicity of cocaine on human hepatocytes induced by ethanol: role of GSH

Increased toxicity of cocaine to human hepatocytes is observed when cells are simultaneously incubated with ethanol. Ethanol might exacerbate cocaine hepatocyte toxicity by three different pathways: a) by increasing the oxidative metabolism of cocaine and hence the oxidative damage; b) by the formation of a more toxic metabolite, namely cocaethylene; or c) by decreasing the defence mechanisms of the cell (i.e. GSH). In the present study, experiments were conducted to investigate the feasibility of these hypotheses. In hepatocytes preincubated for 48 hr with ethanol, neither significant changes in cocaine metabolism nor cytotoxicity were found despite differences in hepatocyte p-nitrophenol hydroxylase (largely CYP2E1 activity). Cocaethylene, the transesterification product of cocaine and ethanol, was found to be more toxic than cocaine for human hepatocytes (3x). However, the small amount formed when human hepatocytes were incubated with cocaine and ethanol would hardly explain the increased toxicity observed. On the other hand, the simultaneous presence of cocaine and ethanol caused a sustained decline in the intracellular GSH content that was larger than that observed in cocaine- or ethanol-treated cultures. Parallel to this phenomenon, a significant increase in lipid peroxidation was observed, as compared to cells treated with equimolar amounts of cocaine, ethanol, or cocaethylene. Finally, depletion of hepatocyte GSH with diethylmaleate down to levels similar to those found in ethanol-treated cells made hepatocytes more susceptible to cocaine. Taken together, the results of this research suggest that by decreasing GSH levels, ethanol makes human hepatocytes more sensitive to cocaine-induced oxidative damage.

Interspecies variations in fatty acid hydroxylations involving cytochromes P450 2E1 and 4A

The liver microsomal fractions of seven mammalian species including rat, dog, monkey, hamster, mouse, gerbil and humans, catalyzed the hydroxylation of saturated (lauric, myristic and palmitic) and unsaturated (oleic and linoleic) fatty acids to the corresponding omega and (omega-1)-hydroxylated derivatives, while stearic acid was not metabolized. Lauric acid was the most efficiently hydroxylated, and the rank of catalytic activity was lauric > myristic > oleic > palmitic > linoleic. Among the mammalian species studied, mouse and hamster presented the highest level of fatty acid omega and (omega-1)-hydroxylases, while the lowest activity was observed in dog and monkey. In all the animal species, the (omega-1)-hydroxylation of fatty acids correlated significantly with the immunodetectable content of CYP2E1 and the 4-nitrophenol hydroxylation activity, known to be mediated by cytochrome P450 2E1. On the contrary, only the omega-hydroxylation of lauric acid slighly correlated with the level of cytochrome P450 4A, while no significant correlation was found with the omega-hydroxylation of the other fatty acids. Furthermore, chemical and immuno-inhibitions of the hydroxylations of fatty acids led to the conclusion that fatty acid (omega-1)-hydroxylase activity is catalyzed by P450 2E1 in all the mammalian species, while the fatty acid omega-hydroxylase activity may be catalyzed by cytochromes P450 from the 4A family. Therefore, lauric acid (omega-1)-hydroxylation along with 4-nitrophenol hydroxylation can be used as a specific and sensitive method to measure the level of CYP2E1 induction in humans and various animals.

Effect of cyanamide on toxicity and glutathione depletion in rat hepatocyte cultures: differences between two dichloropropanol isomers

The effect of aldehyde dehydrogenase inhibition by cyanamide pre-treatment in vitro on dichloropropanol-dependent toxicity and glutathione depletion was investigated in 24 h rat hepatocyte cultures. Cyanamide pre-treatment had no effect on nitrophenol hydroxylase, 7-methoxy-, 7-ethoxy- or 7-benzyloxyresorufin O-dealkylase activities in 24 h cultures from untreated rats, and had no effect on intracellular glutathione content in cultures from untreated rats, or in cultures from isoniazid-treated rats in which cytochrome P4502E1 (CYP2E1) is increased. In cultures from untreated animals the primary alcohol, 2,3-dichloropropanol, was not toxic and did not significantly deplete glutathione. Cyanamide pre-treatment however, potentiated both toxicity and glutathione depletion. Induction of CYP2E1 also potentiated the toxicity of 2,3-dichloropropanol, and in these cultures cyanamide pre-treatment significantly increased both toxicity and glutathione depletion. Cyanamide did not alter the toxicity or glutathione depletion due to the secondary alcohol, 1,3-dichloropropanol, irrespective of CYP2E1 induction. These results indicate that the primary alcohol isomer is metabolised to an aldehyde intermediate which depletes glutathione. Under basal conditions this metabolite appears to be effectively detoxified, but increased CYP2E1 activity and/or decreased aldehyde dehydrogenase activity promotes accumulation of metabolite, and therefore increases glutathione depletion and toxicity.

Inhibition of cytochrome P450 2E1 expression by oleanolic acid: hepatoprotective effects against carbon tetrachloride-induced hepatic injury

The protective effects of oleanolic acid on carbon tetrachloride-induced hepatotoxicities and the possible mechanisms involved in this protection were investigated in mice. Pretreatment with oleanolic acid prior to the administration of carbon tetrachloride significantly prevented the increase in serum alanine aminotransferase and lactate dehydrogenase activity and liver lipid peroxidation in a dose-dependent manner. Hepatic glutathione levels and glutathione-S-transferase activities were not affected by treatment with oleanolic acid alone but pretreatment with oleanolic acid protects carbon tetrachloride-induced depletion of hepatic glutathione levels. The effects of oleanolic acid on the cytochrome P450 (P450) 2E1, the major isozyme involved in carbon tetrachloride bioactivation were investigated. Treatment of mice with oleanolic acid resulted in a significant decrease of P450 2E1-dependent p-nitrophenol and aniline hydroxylation in a dose-dependent manner. Consistent with these observations, the P450 2E1 expressions were also decreased, as determined by immunoblot analysis. These results show that the protective effects of oleanolic acid against the carbon tetrachloride-induced hepatotoxicity may, at least in part, be due to its ability to block bioactivation of carbon tetrachloride mainly by the inhibition of expression and activities of P450 2E1.

Metabolic capacities in cultured human hepatocytes obtained by a new isolating procedure from non-wedge small liver biopsies

A new isolating procedure of human hepatocytes has been developed using two-step collagenase digestion by a non-perfusion procedure (NP) of non-wedge liver biopsies. 1. A yield of 2-7 x 10(6) hepatocytes/g liver, 52-95% viability and 13-75% attachment were obtained from liver biopsies weighing 6-60 g, comparable to that obtained when using the classical perfusion procedure (P) to isolate human hepatocytes from wedge liver samples of 50-150 g. 2. In culture, human hepatocytes obtained by NP remained attached to plastic for up to 5 days and displayed the usual morphological characteristics. Their metabolic capacities, assessed by liver-specific albumin and urea synthesis and by CYP-dependent and conjugation pathways, were equivalent to those of human hepatocytes obtained by P. In addition, they responded adequately to specific CYP inducers, demonstrating that they constitute a model in which human drug metabolism and toxicity studies can be performed.

Irgasan DP 300 (5-chloro-2-(2,4-dichlorophenoxy)-phenol) induces cytochrome P450s and inhibits haem biosynthesis in rat hepatocytes cultured on Matrigel

1. The effect of Irgasan DP 300 (5-chloro-2-(2,4-dichlorophenoxy)phenol) on cytochrome P450 (P450) induction and haem biosynthesis was studied in rat hepatocytes cultured on Matrigel. 2. Irgasan DP 300 significantly induced 7-benzyloxyresorufin O-debenzylase activity, followed by 7-pentoxyresorufin O-depentylase and 7-ethoxyresorufin O-deethylase activities. 4-Nitrophenol hydroxylase, testosterone 6 beta-hydroxylase and methoxyresorufin O-demethylase activities were also slightly increased. The maximum induction of these enzyme activities was obtained at the same concentration of 125 microM in the culture medium. 3. Immunochemical blots using anti-rat cytochrome P450 antibodies revealed that Irgasan DP 300 preferably induced CYP2B1/2 along with a slight increase in 3A. These results indicate that Irgasan DP 300 is a phenobarbital-type inducer. 4. In the absence of exogenous 5-aminolevulinic acid (ALA), slight increases in protoporphyrin IX (2.6-fold) and coproporphyrin III (1.3-fold) were observed in the Irgasan DP 300-treated cultures. In contrast, when 75 microM ALA was present, Irgasan DP 300 (250 microM) caused an extensive accumulation of uroporphyrin I (13-fold). 5. Irgasan DP 300 inhibited rat hepatic uroporphyrinogen III synthase in vitro. 6. These results indicate that Irgasan DP 300 produced accumulation of hydroxymethylbilane in rat hepatocytes by inhibiting uroporphyrinogen III synthase, and consequently an accumulation of uroporphyrin I.

Involvement of cytochrome P4502E1 in the toxicity of dichloropropanol to rat hepatocyte cultures

Hepatocytes were isolated and cultured from untreated rats and rats treated with isoniazid to induce cytochrome P4502E1. Isoniazid selectively increased p-nitrophenol hydroxylase activity in 2-h cultures, and increased the toxicity of both 1,3- and 2,3-dichloropropanol. Isoniazid also increased the rate and extent of glutathione depletion by the dichloropropanols. There was no effect of isoniazid on the toxicity of 1,3-dichloroacetone, precocene II or allyl alcohol. In addition, diethyldithiocarbamate selectively inhibited p-nitrophenol hydroxylase in 2-h cultures from untreated and isoniazid-treated rats, as well as abolishing toxicity of the dichloropropanols. In 24-h cultures from isoniazid-treated rats diethyldithiocarbamate inhibited high affinity MCOD activity by 55% and there was also a small but significant inhibition of precocene II toxicity. These results indicate that isoniazid-inducible P4502E1 can mediate the toxicity of dichloropropanol.

Glucose formation from methylglyoxal in hepatocytes from streptozotocin-induced diabetic mice: the effect of insulin

Acetol and methylglyoxal are intermediates of the intrahepatic metabolism of acetone leading to pyruvate formation. In hepatocytes prepared from fasted streptozotocin-induced diabetic mice, net glucose production could be measured from methylglyoxal but not from acetone or acetol. Insulin increased glucose formation from methylglyoxal in a concentration-dependent manner, whereas it was ineffective when pyruvate was used as substrate. Drug oxidation, as evidenced by p-aminophenol formation from aniline, was enhanced by methylglyoxal, and insulin proved to be stimulatory in this case as well. It is concluded that insulin might be involved in the regulation of glucose formation from methylglyoxal, but its mode of action is not yet clear.

Induction of hepatic cytochrome P4502E1 in rats by acetylsalicylic acid or sodium salicylate

Studies were conducted on the mechanism of the ethanol-inducible cytochrome P450 (cytochrome P4502E1, CYP 2E1) induction by acetylsalicylic acid (ASA) or its metabolite salicylate (SAL). Many exogenous inducers of CYP 2E1 seem to increase CYP 2E1 by post-transcriptional activation without elevation of its mRNA level. Administration of a single high dose of ASA or SAL produces a significant increase in the activity of the hepatic microsomal p-nitrophenol hydroxylase in rats. Pretreatment of ASA-treated rats with a blocker of mRNA transcription, actinomycin D, or a blocker of protein synthesis, cycloheximide, markedly suppressed this enhanced activity of microsomal p-nitrophenol hydroxylase. The CYP 2E1 mRNA levels in livers of control rats and rats treated with ASA or SAL were measured by Northern blot analysis. Significantly elevated CYP 2E1 mRNA levels were measured in livers of treated rats compared with mRNA amounts of the control group. These data suggest that mRNA elevation seems to be characteristic for ASA induction, while other inducing agents show different patterns and mechanisms of activation.

Increased oxidation and decreased conjugation of drugs in the liver caused by starvation. Altered metabolism of certain aromatic compounds and acetone

Starvation causes several changes in the various processes of biotransformation. The focus of this review is on biotransformation of various aromatic and other compounds whose metabolism is catalyzed in phase I by isozymes belonging to the CYP2E1 gene subfamily, while in phase II phenol-UDPGT or conjugation with GSH play a dominant role. The other ways of conjugation are beyond the scope of this review. The reason why this aspect has been chosen is that the capacity of these reactions is profoundly altered by nutritional conditions. There is a balance between the two phases of biotransformation. Therefore, under standard circumstances in a well-fed state the intermediate formed in the course of phase I is converted to a conjugated compound rapidly, as a result of phase II. However, in starvation the pattern of drug metabolism is altered and the balance between the two phases is changed. This alteration of drug metabolism upon starvation is partly connected to the changes of cofactor supplies due to the metabolic state.

Evaluation of the xenobiotic biotransformation capability of six rodent hepatoma cell lines in comparison with rat hepatocytes

Phase I and II activities were examined in six rodent hepatoma cell lines and compared with those of cultured rat hepatocytes both in basal conditions and after exposure to 5 microM methylcholanthrene, 2 mM phenobarbital, and 15 microM beta-naphtoflavone. The metabolic profile of testosterone was also studied. The highest aryl hydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase activities were found in MH1C1 cells. Comparable values for 7-ethoxyresorufin O-deethylase activity, ranging from 21.6 to 42.9 pmol/mg x min, were observed in the hepatocytes and hepatoma cells, except the HTC cells. In contrast, only Fao cells showed 7-pentoxyresorufin O-depentylase activity at levels similar to those of hepatocytes (6.2 +/- 1.0 and 7.4 +/- 1.2 pmol/mg x min, respectively). Rat hepatocytes actively hydroxylated p-nitrophenol, but this activity was not measurable in hepatoma cells. Glutathione transferase activity was maintained in all the hepatoma cell lines at similar levels to those found in hepatocytes (684 +/- 56 nmol/mg x min). The seven hydroxylated metabolites of testosterone produced by cultured hepatocytes were negligible in hepatoma cells. Exposure of cells to inducers revealed that aryl hydrocarbon hydroxylase activity was mainly increased after treatment with 3-methylcholanthrene and beta-naphtoflavone, and the highest values were found in rat hepatocytes followed by MH1C1 and Fao cells. 3-Methylcholanthrene and naphtoflavone treatment also resulted in a marked increase in 7-ethoxyresorufin O-deethylase activity in hepatocytes as well as in H4IIC3, McA-Rh7777, MH1C1, and Fao cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylsalicylic acid--inducer of cytochrome P-450 2E1?

Pretreatment of rats with acetylsalicylic acid or sodium salicylate stimulates the metabolism of dichloromethane to carbon monoxide as measured by the carboxyhemoglobin level in blood. Simultaneous administration of dichloromethane and acetylsalicylic acid or sodium salicylate, respectively, was accompanied by reduced carboxyhemoglobin formation. In liver microsomes of rats pretreated with acetylsalicylic acid the p-nitrophenol hydroxylase activity was increased. It is concluded that (i) cytochrome P-450 2E1 is involved in the metabolic conversion of both dichloromethane and salicylic acid, and (ii) salicylic acid may be an inducer of cytochrome P-450 2E1.

Validation of 4-nitrophenol as an in vitro substrate probe for human liver CYP2E1 using cDNA expression and microsomal kinetic techniques

The involvement of human cytochrome P450 (CYP) 2E1 in the hydroxylation of 4-nitrophenol (4NP) to 4-nitrocatechol (4NC) has been investigated using cDNA expression and liver microsomal kinetic and inhibitor techniques. 4NP hydroxylation by human liver microsomes and cDNA-expressed human CYP2E1 exhibited Michaelis-Menten kinetics; the respective apparent Km values were 30 +/- 7 and 21 microM. Mutual competitive inhibition was observed for 4NP and chlorzoxazone (CZ) (an alternative human CYP2E1 substrate) in liver microsomes, with close similarities between the calculated apparent Km and Ki values for each individual compound. 4NP and CZ hydroxylase activities in microsomes from 18 liver donors varied to a similar extent (3.3- and 3.0-fold, respectively) and 4NP hydroxylase activity correlated significantly (rs > or = 0.75, P < 0.005) with both CZ hydroxylation and immunoreactive CYP2E1 content. The prototypic CYP2E1 inhibitor, diethyldithiocarbamate, was a potent inhibitor of 4NC formation and decreased 4NP hydroxylation by cDNA-expressed CYP2E1 and human liver microsomes in parallel. Probes for other human CYP isoforms namely (alpha-naphthoflavone, coumarin, sulphaphenazole, quinidine, troleandomycin and mephenytoin) caused < 15% inhibition of liver microsomal 4NP hydroxylation. These data confirm that, as in animal species, 4NP hydroxylation is catalysed largely by CYP2E1 in human liver and 4NP may therefore be used as an in vitro substrate probe for the human enzyme.

Influence of the immunization against heterologous alcohol dehydrogenase on liver alcohol dehydrogenase isozymes and alcohol abuse of rats

The possible biochemical and immunological mechanisms of the previously discovered phenomenon of significant and prolonged decrease of alcohol abuse of albino rats after the intravenous injection of the heterologous alcohol dehydrogenase (ADH) were studied. It was found that immunization of rats against horse liver S-ADH result in the appearance of a high titre of antibodies against rat liver ADH-3 isozyme. This isozyme is responsible for the metabolism of 90% of alcohol administrated in vivo. It is shown that immunization against horse liver S-ADH leads also to a significant (2--3-fold) lowering of the activity level of ADH-3 in rat liver. The activity level of the other rat ADH isozymes, ADH-1 and ADH-2, was not found to be changed. The intravenous injection of the specific anti-(horse liver S-ADH) serum, purified by affinity chromatography on immobilized horse liver S-ADH, resulted in a significant decrease of alcohol abuse in alcohol-dependent animals. These results allow us to assume an important role of rat liver ADH-3 in the mechanism of alcohol addiction.

The effect of ellagic acid on xenobiotic metabolism by cytochrome P-450IIE1 and nitrosodimethylamine mutagenicity

Ellagic acid (EA) is an inhibitor of the in vitro mutagenicity of N-nitrosodimethylamine (NDMA) in Salmonella typhimurium strain TA100 using pyrazole-induced rat liver 9000 x g supernatant (S-9). In order to understand this activity, the effect of EA on the metabolic hydroxylation of 4-nitrophenol, a substrate, as is NDMA, for cytochrome P-450IIE1 was studied using pyrazole induced rat S-9 and microsomal protein. It is shown that EA has an inhibitory effect on 4-nitrophenol hydroxylase with both enzyme preparations. This effect on cytochrome P-450IIE1 may be responsible, at least in part, for the inhibition of NDMA mutagenicity by EA.

Accumulation of phenols in isolated hepatocytes after pretreatment with methylglyoxal

The effects of a single intraperitoneal injection of methylglyoxal (50-800 mg/kg body wt.) in mice were investigated in the liver after 24 h. The administration of methylglyoxal (400 mg/kg body wt.) resulted in an increase in aniline hydroxylase activity in liver microsomes. At the same time an accumulation of p-amino-phenol, the hydroxylated product of aniline, was observed in isolated hepatocytes upon addition of aniline similarly to conditions (starvation, diabetes mellitus, pyrazole pretreatment) when aniline hydroxylase was induced. Methylglyoxal also decreased the reduced glutathione content in the liver, while the activity of serum glutamate pyruvate transaminase was increased, suggesting the onset of liver injuries. It is assumed that the increased oxidation of aniline hydroxylase combined with decreased glutathione levels after methylglyoxal treatment favours the formation of potentially hazardous phenol derivatives in the liver.

Ethanol-induced enhancement of cocaine bioactivation and irreversible protein binding: evidence against a role of cytochrome P-450IIE1

Chronic ethanol consumption potentiates cocaine-induced liver injury in rodents. Since cocaine has to be bioactivated by a cytochrome P-450-dependent N-oxidative pathway to exert its hepatotoxic effects, we studied the role of the ethanol-inducible P-450IIE1 for cocaine metabolism. Male Sprague-Dawley rats were pretreated with either a liquid diet containing ethanol (30% of calories) for 4 weeks or injected with pyrazole (200 mg/kg/day, ip, for 3 days). Both agents induced microsomal p-nitrophenol hydroxylation which is a probe for the catalytic activity of P-450IIE1. However, only ethanol, but not pyrazole, increased both microsomal cocaine N-demethylase activity (by 47%) and the extent of irreversible binding of [3H]-cocaine to microsomal proteins (by 100%), which was taken as a quantitative endpoint for the formation of a reactive metabolite. Cocaine N-demethylation and irreversible protein binding of cocaine were not inhibited by P-450IIE1 isozyme-selective substrates, nor was the rate of cocaine metabolism and binding decreased by functionally active polyclonal anti-rat P-450IIE1 antibodies. Furthermore, pyrazole pretreatment sensitized cultured hepatocytes to the glutathione-dependent cytotoxic effects of nontoxic concentrations of cocaine. These results indicate that (a) cocaine is not a major substrate for the ethanol-inducible P-450IIE1, (b) the enhancing effects of ethanol on cocaine bioactivation may be due to induction of other P-450 isoforms, and (c) induction of P-450IIE1 may potentiate cocaine-induced hepatocellular toxicity in vitro independently of cocaine metabolism, e.g., by P-450IIE1-dependent oxidative stress.

Increased catalytic activity of cytochrome P-450IIE1 in pericentral hepatocytes compared to periportal hepatocytes isolated from pyrazole-treated rats

Cytochrome P-450IIE1 is induced by a variety of agents, including acetone, ethanol and pyrazole. Recent studies employing immunohistochemical methods have shown that P-450IIE1 was expressed primarily in the pericentral zone of the liver. In order to evaluate whether catalytic activity of P-450IIE1 is preferentially localized in the pericentral zone of the liver acinus, the oxidation of aniline and p-nitrophenol, two effective substrates for P-450IIE1, by periportal and pericentral hepatocytes isolated from pyrazole-treated rats was determined. Periportal and pericentral hepatocytes were prepared by a digitonin-collagenase procedure; the marker enzymes glutamine synthetase and gamma-glutamyl transpeptidase indicated reasonable separation of the two cell populations. Viability, yield and total cytochrome P-450 content were similar for the periportal and pericentral hepatocytes. Pericentral hepatocytes oxidized aniline and p-nitrophenol at rates that were 2-4-fold greater than periportal hepatocytes under a variety of conditions. Carbon monoxide inhibited the oxidation of the substrates with both preparations and abolished the increased oxidation found with the pericentral hepatocytes. Pyrazole or 4-methylpyrazole, added in vitro, effectively inhibited the oxidation of aniline and p-nitrophenol and prevented the augmented rate of oxidation by the pericentral hepatocytes. Western blots carried out using isolated microsomes revealed a more than 2-fold increase in immunochemical staining with microsomes isolated from the pericentral hepatocytes, which correlated to the 2-4-fold increase in the rate of oxidation of aniline or p-nitrophenol by the pericentral hepatocytes. These results suggest that functional catalytic activity of cytochrome P-450IIE1 is preferentially localized in the pericentral zone of the liver acinus, and that most of the induction by pyrazole of P-450IIE1 appears to occur within the pericentral zone.