Cytochrome P-450-dependent formation of reactive oxygen radicals: isozyme-specific inhibition of P-450-mediated reduction of oxygen and carbon tetrachloride

Aspirin and N-acetylcysteine co-administration markedly inhibit chronic ethanol intake and block relapse binge drinking: Role of neuroinflammation-oxidative stress self-perpetuation

Chronic alcohol intake leads to neuroinflammation and cell injury, proposed to result in alterations that perpetuate alcohol intake and cued relapse. Studies show that brain oxidative stress is consistently associated with alcohol-induced neuroinflammation, and literature implies that oxidative stress and neuroinflammation perpetuate each other. In line with a self-perpetuating mechanism, it is hypothesized that inhibition of either oxidative stress or neuroinflammation could reduce chronic alcohol intake and relapse. The present study conducted on alcohol-preferring rats shows that chronic ethanol intake was inhibited by 50% to 55% by the oral administration of low doses of either the antioxidant N-acetylcysteine (40 mg/kg/d) or the anti-inflammatory aspirin (ASA; 15 mg/kg/d), while the co-administration of both dugs led to a 70% to 75% (P < .001) inhibition of chronic alcohol intake. Following chronic alcohol intake, a prolonged alcohol deprivation, and subsequent alcohol re-access, relapse drinking resulted in blood alcohol levels of 95 to 100 mg/dL in 60 minutes, which were reduced by 60% by either N-acetylcysteine or aspirin and by 85% by the co-administration of both drugs (blood alcohol: 10 to 15 mg/dL; P < .001). Alcohol intake either on the chronic phase or following deprivation and re-access led to a 50% reduction of cortical glutamate transporter GLT-1 levels, while aspirin administration fully returned GLT-1 to normal levels. N-acetylcysteine administration did not alter GLT-1 levels, while N-acetylcysteine may activate the cystine/glutamate transport xCT, presynaptically inhibiting relapse. Overall, the study suggests that a neuroinflammation/oxidative stress self-perpetuation cycle maintains chronic alcohol intake and relapse drinking. The co-administration of anti-inflammatory and antioxidant agents may have translational value in alcohol-use disorders.

Intranasal mesenchymal stem cell secretome administration markedly inhibits alcohol and nicotine self-administration and blocks relapse-intake: mechanism and translational options

BACKGROUND

Chronic consumption of most drugs of abuse leads to brain oxidative stress and neuroinflammation, which inhibit the glutamate transporter GLT-1, proposed to perpetuate drug intake. The present study aimed at inhibiting chronic ethanol and nicotine self-administration and relapse by the non-invasive intranasal administration of antioxidant and anti-inflammatory secretome generated by adipose tissue-derived activated mesenchymal stem cells. The anti-addiction mechanism of stem cell secretome is also addressed.

METHODS

Rats bred for their alcohol preference ingested alcohol chronically or were trained to self-administer nicotine. Secretome of human adipose tissue-derived activated mesenchymal stem cells was administered intranasally to animals, both (i) chronically consuming alcohol or nicotine and (ii) during a protracted deprivation before a drug re-access leading to relapse intake.

RESULTS

The intranasal administration of secretome derived from activated mesenchymal stem cells inhibited chronic self-administration of ethanol or nicotine by 85% and 75%, respectively. Secretome administration further inhibited by 85-90% the relapse "binge" intake that occurs after a protracted drug deprivation followed by a 60-min drug re-access. Secretome administration fully abolished the oxidative stress induced by chronic ethanol or nicotine self-administration, shown by the normalization of the hippocampal oxidized/reduced glutathione ratio, and the neuroinflammation determined by astrocyte and microglial immunofluorescence. Knockdown of the glutamate transporter GLT-1 by the intracerebral administration of an antisense oligonucleotide fully abolished the inhibitory effect of the secretome on ethanol and nicotine intake.

CONCLUSIONS

The non-invasive intranasal administration of secretome generated by human adipose tissue-derived activated mesenchymal stem cells markedly inhibits alcohol and nicotine self-administration, an effect mediated by the glutamate GLT-1 transporter. Translational implications are envisioned.

The relationships between cytochromes P450 and H2O2: Production, reaction, and inhibition

In this review we address the relationship between cytochromes P450 (P450) and H2O2. This association can affect biology in three distinct ways. First, P450s produce H2O2 as a byproduct either during catalysis or when no substrate is present. This reaction, known as uncoupling, releases reactive oxygen species that may have implications in disease. Second, H2O2 is used as an oxygen-donating co-substrate in peroxygenase and peroxidase reactions catalyzed by P450s. This activity has proven to be important mainly in reactions involving prokaryotic P450s, and investigators have harnessed this reaction with the aim of adaptation for industrial use. Third, H2O2-dependent inhibition of human P450s has been studied in our laboratory, demonstrating heme destruction and also the inactivating oxidation of the heme-thiolate ligand to a sulfenic acid (-SOH). This reversible oxidative modification of P450s may have implications in the prevention of uncoupling and may give new insights into the oxidative regulation of these enzymes. Research has elucidated many of the chemical mechanisms involved in the relationship between P450 and H2O2, but the application to biology is difficult to evaluate. Further studies are needed reveal both the harmful and protective natures of reactive oxygen species in an organismal context.

Human recombinant cytochrome P450 enzymes display distinct hydrogen peroxide generating activities during substrate independent NADPH oxidase reactions

Microsomal enzymes generate H2O2 in the presence of NADPH. In this reaction, referred to as "oxidase" activity, H2O2 is generated directly or indirectly via the formation of superoxide anion. In the presence of redox active transition metals, H2O2 can form highly toxic hydroxyl radicals and, depending on the "oxidase" activity of individual cytochrome P450 isoenzymes, this can compromise cellular functioning and contribute to tissue injury. In the present studies, we compared the initial rates of H2O2 generating activity of microsomal preparations containing various human recombinant cytochromes P450s. In the absence of cytochrome P450s the human recombinant NADPH cytochrome P450 reductase (CPR) generated low, but detectable amounts of H2O2 (∼0.04 nmol H2O2/min/100 units of reductase). Significantly greater activity was detected in preparations containing individual cytochrome P450s coexpressed with CPR (from 6.0 nmol H2O2/min/nmol P450 to 0.2 nmol/min/nmol P450); CYP1A1 was the most active, followed by CYP2D6, CYP3A4, CYP2E1, CYP4A11, CYP1A2, and CYP2C subfamily enzymes. H2O2 generating activity of the cytochrome P450s was independent of the ratio of CYP/CPR. Thus, similar H2O2 generating activity was noted with the same cytochrome P450s (CYP3A4, CYP2E1, and CYP2C9) expressed at or near the ratio of CYP/CPR in human liver microsomes (5-7), and when CPR was present in excess (CYP/CPR = 0.2-0.3). Because CYP3A4/5/7 represent up to 40% of total cytochrome P450 in the liver, these data indicate that these enzymes are the major source of H2O2 in human liver microsomes.

Acetaldehyde and parkinsonism: role of CYP450 2E1

The present review update the relationship between acetaldehyde (ACE) and parkinsonism with a specific focus on the role of P450 system and CYP 2E1 isozyme particularly. We have indicated that ACE is able to enhance the parkinsonism induced in mice by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, a neurotoxin able to damage the nigrostriatal dopaminergic pathway. Similarly diethyldithiocarbamate, the main metabolite of disulfiram, a drug widely used to control alcoholism, diallylsulfide (DAS) and phenylisothiocyanate also markedly enhance the toxin-related parkinsonism. All these compounds are substrate/inhibitors of CYP450 2E1 isozyme. The presence of CYP 2E1 has been detected in the dopamine (DA) neurons of rodent Substantia Nigra (SN), but a precise function of the enzyme has not been elucidated yet. By treating CYP 2E1 knockout (KO) mice with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, the SN induced lesion was significantly reduced when compared with the lesion observed in wild-type animals. Several in vivo and in vitro studies led to the conclusion that CYP 2E1 may enhance the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity in mice by increasing free radical production inside the dopaminergic neurons. ACE is a good substrate for CYP 2E1 enzyme as the other substrate-inhibitors and by this way may facilitate the susceptibility of dopaminergic neurons to toxic events. The literature suggests that ethanol and/or disulfiram may be responsible for toxic parkinsonism in human and it indicates that basal ganglia are the major targets of disulfiram toxicity. A very recent study reports that there are a decreased methylation of the CYP 2E1 gene and increased expression of CYP 2E1 mRNA in Parkinson's disease (PD) patient brains. This study suggests that epigenetic variants of this cytochrome contribute to the susceptibility, thus confirming multiples lines of evidence which indicate a link between environmental toxins and PD.

Exacerbation of alcohol-induced oxidative stress in rats by polyunsaturated Fatty acids and iron load

The hypothesis that excessive intake of vegetable oil containing polyunsaturated fatty acids and iron load precipitate alcohol-induced liver damage was investigated in a rat model. In order to elucidate the mechanism underlying this synergism, the serum levels of iron, total protein, serum glutamate pyruvate transaminase, liver thiobarbituric acid reactive substances, and activities of antioxidant enzymes superoxide dismutase, catalase in liver of rats treated with alcohol, polyunsaturated fatty acids and iron per se and in combination were examined. Alcohol was fed to the rats at a level of 10-30% (blood alcohol was maintained between 150-350 mg/dl by using head space gas chromatography), polyunsaturated fatty acids at a level of 15% of diet and carbonyl iron 1.5-2% of diet per se and in combination to different groups for 30 days. Hepatotoxicity was assessed by measuring serum glutamate pyruvate transaminase, which was elevated and serum total protein, which was decreased significantly in rats fed with a combination of alcohol, polyunsaturated fatty acids and iron. It was also associated with increased lipid peroxidation and disruption of antioxidant defense in combination fed rats as compared to rats fed with alcohol or polyunsaturated fatty acids or iron. The present study revealed significant exacerbation of the alcohol-induced oxidative stress in presence of polyunsaturated fatty acids and iron.

Sex-related differences in renal toxicodynamics in rodents

INTRODUCTION

An issue yet to be addressed, in the investigation of the xenobiotic toxicity, is a detailed characterization of the sex differences in toxicological responses. The 'sex issue' is particularly significant in nephrotoxicology as the kidney is a relevant target organ for xenobiotics and few studies have approached this subject in the past. There is a strong need to improve our understanding regarding the influence of sex in toxicology, given their increased requirement to establish the limits of exposure to chemicals in the environment and at work.

AREAS COVERED

In this review, the authors provide the reader with the current knowledge of sex differences in kidney toxicity for rats and mice. To make the review easier to consult, these studies have been organized according to the class of xenobiotic.

EXPERT OPINION

From the analysis of the present knowledge emerges a dramatic need for information on sex differences in xenobiotics toxicity. Although animals are reasonably good predictors of adverse renal effects in patients, there is need to identify alternative methods (e.g. in vitro/ex vivo) to better study sex differences in organ toxicity.

Effects of low-level exposure to xenobiotics present in paints on oxidative stress in workers

Paints are composed of an extensive variety of hazardous substances, such as organic solvents and heavy metals. Biomonitoring is an essential tool for assessing the risk to occupational health. Thus, this study analyzed the levels of biomarkers of exposure for toluene, xylene, styrene, ethylbenzene, and lead, as well as the oxidative stress biomarker alterations in painters of an industry. Lipid peroxidation biomarker (MDA), delta-aminolevulinate dehydratase (ALA-D), nonprotein thyol groups, superoxide dismutase and catalase (CAT) were analyzed in exposed and nonexposed subjects. We estimated which of the paint constituents have the greatest influence on the changes in the biomarkers of oxidative stress in this case of co-exposure. The results demonstrated that despite the fact that all the biomarkers of exposure were below the biological exposure limits, the MDA levels and antioxidant enzyme activities were increased, while nonprotein thyol groups and ALA-D levels were decreased in painters when compared with nonexposed subjects. After statistic test, toluene could be suggested as the principal factor responsible for increased lipid peroxidation and inhibition of ALA-D enzyme; however, further studies on the inhibition of ALA-D enzyme by toluene are necessary.

Genetic polymorphism in CYP2E1: Population distribution of CYP2E1 activity

Cytochrome P-450 2E1 (CYP2E1) is a key enzyme in the metabolic activation of a variety of toxicants including nitrosamines, benzene, vinyl chloride, and halogenated solvents such as trichloroethylene. CYP2E1 is also one of the enzymes that metabolizes ethanol to acetaldehyde, and is induced by recent ethanol ingestion. There is evidence that interindividual variability in the expression and functional activity of this cytochrome (CYP) may be considerable. Genetic polymorphisms in CYP2E1 were identified and linked to altered susceptibility to hepatic cirrhosis induced by ethanol and esophageal and other cancers in some epidemiological studies. Therefore, it is important to evaluate how such polymorphisms affect CYP2E1 function and whether it is possible to construct a population distribution of CYP2E1 activity based upon the known effects of these polymorphisms and their frequency in the population. This analysis is part of the genetic polymorphism database project described in the lead article in this series and followed the approach described in that article (Ginsberg et al., 2009, this issue). Review of the literature found that there are a variety of CYP2E1 variant alleles but the functional significance of these variants is still unclear. Some, but not all, studies suggest that several upstream 5' flanking mutations affect gene expression and response to inducers such as ethanol or obesity. None of the coding-region variants consistently affects enzyme function. Part of the reason for conflicting evidence regarding genotype effect on phenotype may be due to the wide variety of exposures such as ethanol or dietary factors and physiological factors including body weight or diabetes that modulate CYP2E1 expression. In conclusion, evidence is too limited to support the development of a population distribution of CYP2E1 enzyme activity based upon genotypes. Health risk assessments may best rely upon data reporting interindividual variability in CYP2E1 function for input into physiologically based pharmacokinetic (PBPK) models involving CYP2E1 substrates.

Development of oxidative stress by cytochrome P450 induction in rodents is selective for barbiturates and related to loss of pyridine nucleotide-dependent protective systems

Reactive oxygen species (ROS) and oxidative stress have been considered in a variety of disease models, and cytochrome P450 (P450) enzymes have been suggested to be a source of ROS. Induction of P450s by phenobarbital (PB), beta-naphthoflavone (betaNF), or clofibrate in a mouse model increased ROS parameters in the isolated liver microsomes, but isoniazid treatment did not. However, when F(2)-isoprostanes (F(2)-IsoPs) were measured in tissues and urine, PB showed the strongest effect and betaNF had a measurable but weaker effect. The same trend was seen when an Nfr2-based transgene reporter sensitive to ROS was analyzed in the mice. This pattern had been seen earlier with F(2)-IsoPs both in vitro and in vivo with rats (Dostalek, M., Brooks, J. D., Hardy, K. D., Milne, G. L., Moore, M. M., Sharma, S., Morrow, J. D., and Guengerich, F. P. (2007) Mol. Pharmacol. 72, 1419-1424). One possibility for the general in vitro-in vivo discrepancy in oxidative stress found in both mice and rats is that PB treatment might attenuate protective systems. One potential candidate suggested by an mRNA microarray was nicotinamide N-methyltransferase. PB was found to elevate nicotinamide N-methyltransferase activity 3- to 4-fold in mice and rats and to attenuate levels of NAD(+), NADP(+), NADH, and NADPH in both species (20-40%), due to the enhanced excretion of (N-methyl)nicotinamide. PB also down-regulated glutathione peroxidase and glutathione reductase, which together constitute a key enzymatic system that uses NADPH in protecting against oxidative stress. These multiple effects on the protective systems are proposed to be more important than P450 induction in oxidative stress and emphasize the importance of studying in vivo models.

In vivo oxidative damage in rats is associated with barbiturate response but not other cytochrome P450 inducers

Previously published studies have shown that cytochrome P450 (P450) enzyme systems can produce reactive oxygen species and suggest roles of P450s in oxidative stress. However, most of the studies have been done in vitro, and the potential link between P450 induction and in vivo oxidative damage has not been rigorously explored with validated biomarkers. Male Sprague-Dawley rats were pretreated with typical P450 inducers (beta-naphthoflavone, phenobarbital (PB), Aroclor 1254, isoniazid, pregnenolone 16alpha-carbonitrile, and clofibrate) or the general P450 inhibitor 1-aminobenztriazole; induction of P4501A, -2B, -2E, -3A, and -4A subfamily enzymes was confirmed by immunoblotting and the suppression of P450 by 1-aminobenztriazole using spectral analysis. PB and Aroclor 1254 significantly enhanced malondialdehyde and H2O2 generation and NADPH oxidation in vitro and significantly enhanced formation in vivo, in both liver and plasma. Some of the other treatments changed in vitro parameters but none did in vivo. The PB-mediated increases in liver and plasma F2-isoprostanes could be ablated by 1-aminobenztriazole, implicating the PB-induced P450(s) in the F2-isoprostane elevation. The markers of in vivo oxidative stress were influenced mainly by PB and Aroclor 1254, indicative of an oxidative damage response only to barbiturate-type induction and probably related to 2B subfamily enzymes. These studies define the contribution of P450s to oxidative stress in vivo, in that the phenomenon is relatively restricted and most P450s do not contribute substantially.

Hepatic injury induces contrasting response in liver and kidney to chemicals that are metabolically activated: Role of male sex hormone

Injury to liver, resulting in loss of its normal physiological/biochemical functions, may adversely affect a secondary organ. We examined the response of the liver and kidney to chemical substances that require metabolic activation for their toxicities in mice with a preceding liver injury. Carbon tetrachloride treatment 24 h prior to a challenging dose of carbon tetrachloride or acetaminophen decreased the resulting hepatotoxicity both in male and female mice as determined by histopathological examination and increases in serum enzyme activities. In contrast, the renal toxicity of the challenging toxicants was elevated markedly in male, but not in female mice. Partial hepatectomy also induced similar changes in the hepatotoxicity and nephrotoxicity of a challenging toxicant, suggesting that the contrasting response of male liver and kidney was associated with the reduction of the hepatic metabolizing capacity. Carbon tetrachloride pretreatment or partial hepatectomy decreased the hepatic xenobiotic-metabolizing enzyme activities in both sexes but elevated the renal p-nitrophenol hydroxylase, p-nitroanisole O-demethylase and aminopyrine N-demethylase activities significantly only in male mice. Increases in Cyp2e1 and Cyp2b expression were also evident in male kidney. Castration of males or testosterone administration to females diminished the sex-related differences in the renal response to an acute liver injury. The results indicate that reduction of the hepatic metabolizing capacity induced by liver injury may render secondary target organs susceptible to chemical substances activated in these organs. This effect may be sex-specific. It is also suggested that an integrated approach should be taken for proper assessment of chemical hazards.

Protein S-thiolation and depletion of intracellular glutathione in skin fibroblasts exposed to various sources of oxidative stress

It is well established that oxidative stress plays a central role in the onset and progression of over a 100 different diseases. Recently, a growing body of evidence has shown that chemicals/agents as diverse as aromatic compounds, UV radiation and redox-active metals also generate oxy-radicals in vivo and lead to cellular oxidative stress. In this study we have used mouse skin fibroblasts to study the effects of oxidative stress caused by organic aromatic compounds (xylene), UV radiation and redox-active metals. Specifically, we tested the effect of these treatments on intracellular GSH levels, as well as on protein S-thiolation. We show that acute exposure of these diverse set of conditions cause dramatic depletion of the intracellular GSH pool in mouse skin fibroblast cells. We also found evidence of synergistic effects of combined exposure to different sources of oxidative stress. Furthermore, we also found that these treatments also caused significant S-thiolation (protein mixed-disulfide formation) of a 70kDa cytosolic protein.

Contrasting Changes in Phase I and Phase II Metabolism of Acetaminophen in Male Mice Pretreated with Carbon Tetrachloride

Effect of carbon tetrachloride (CCl(4)) pretreatment on the biotransformation and elimination of acetaminophen were examined in male mice. A 24 hr initial dose of CCl(4) (0.05 ml/kg, intraperitioneally) reduced the induction of hepatotoxicity resulting from acetaminophen treatment (350 mg/kg, intraperitoneally) as determined by changes in serum alanine and aspartate aminotransferase, and sorbitol dehydrogenase activities. Acetaminophen and the major metabolites in plasma were monitored for 12 hr following acetaminophen treatment. CCl(4) pretreatment decreased the plasma concentrations of acetaminophen-cysteine and acetaminophen-mercapturate, but acetaminophen-glucuronide and acetaminophen-sulfate were increased significantly. The elimination of the parent drug from plasma was not affected by CCl(4). In urine collected for 24 hr, the concentrations of acetaminophen-sulfate and acetaminophen-glucuronide were increased by 84% and 33%, respectively, whilst acetaminophen-cysteine and acetaminophen-mercapturate were reduced to approximately one third of control. Expression of cytochrome P450 (CYP) isozymes was determined using antibodies of 2E1 and 1A2 as probes. CYP2E1 and 1A2 expressions were decreased significantly by CCl(4). Likewise, CCl(4) treatment reduced the microsomal p-nitrophenol hydroxylase and p-nitroanisole O-demethylase activities to less than one third of control. The results indicate that, although CCl(4) reduces the generation of thioether conjugates of acetaminophen by decreasing the CYP activities, inhibition of the oxidative metabolism of acetaminophen is counterbalanced by the enhancement of conjugate formation via the glucuronide and sulfate pathways, resulting in elimination of the drug at a rate equivalent to that in normal mice. It is suggested that liver injury in patients may not warrant a mandatory reduction of drug doses extensively inactivated via phase II reactions.

Expression and activity of CYP2E1 in circulating lymphocytes are not altered in diabetic individuals

Cytochrome P4502E1 (CYP2E1) plays an important role in ROS production thus favouring accelerated membrane lipid peroxidation. This isoform is strongly expressed in the liver but it can be also found in lymphocytes. As such, lymphocyte may provide a non-invasive accessible pool for screening CYP2E1 expression in man. We have, therefore, analysed CYP2E1 expression and activity in lymphocyte microsomes from 12 healthy controls, 11 type 1 and 12 type 2 diabetic subjects by using Western blot and enzymatic activities. Immunoblotting did not show difference among CYP2E1 protein bands in controls, type 1 and type 2 diabetics. To assess CYP2E1 activity we used the 7-ethoxy-4-trifluoromethylcoumarin (7-EFC), as a fluorescent substrate. The rate of deethylation of 7-EFC from controls did not differ from type 1 and type 2 diabetic subjects. The lack of any difference in CYP2E1 activity also was confirmed by the NADPH-dependent microsomal lipid peroxidation CCL4-induced assay showing similar peroxidation rates among controls and diabetic subjects. The results show that CYP2E1 expression/activity in lymphocytes is not enhanced in diabetes.

Differential effects of diabetes on CYP2E1 and CYP2B4 proteins and associated drug metabolizing enzyme activities in rabbit liver

The effects of diabetes on cytochrome P450 (CYP)-dependent drug metabolizing enzymes are yet to be clarified. The most widely used animals in these studies have been rats, and information on the effects of diabetes on rabbit liver drug metabolizing enzymes have been unavailable until now. In this study, for the first time, a significant induction of liver CYP2E1 is demonstrated via immunoblot analysis in alloxan-induced rabbits. The CYP2E1 content of diabetic microsomes was highly correlated with the activities of liver aniline 4-hydroxylase (r=0.82, p<0.05), and p-nitrophenol hydroxylase (r=0.86, p<0.01), and diabetes increased the activities of the enzymes associated with CYP2E1. The activities of aniline 4-hydroxylase and p-nitrophenol hydroxylase were significantly increased by 1.7 and 1.8-fold, respectively compared to those of control rabbits. In marked contrast, diabetes had no effect on the protein levels of CYP2B4 as determined by immunoblotting and on benzphetamine N-demethylase activity, which is known to be specifically metabolized by CYP2B4 in rabbit liver. The present study demonstrates that diabetes increases the activities of CYP2E1 and associated enzymes but does not change the activity levels of CYP2B4 and associated enzymes in diabetic rabbits. These findings are in contrast to those of mice, hamsters and rats, and that suggest the presence of species-dependent responses of CYP-dependent drug metabolizing enzymes to diabetes.

Influence of a thiazole derivative on ethanol and thermally oxidized sunflower oil-induced oxidative stress

The present work describes the protective influence of the dendrodoine analogue (DA) [4-amino-5-benzoyl-2-(4-methoxy phenylamino) thiazole] on thermally oxidized sunflower oil and ethanol-induced oxidative stress. Ethanol was fed to animals at a level of 20% [(7.9 g/kg body weight (bw)] and thermally oxidized sunflower oil at a level of 15% (15 mL/100 g feed). Hepatotoxicity was assessed by measuring the activity of plasma aspartate transaminase (AST), alkaline phosphatase (ALP) and gamma-glutamyl transferase (GGT), which were elevated in thermally oxidized oil, and ethanol fed rats when compared with normal control rats. Tissue damage was associated with increased lipid peroxidation and disruption in the antioxidant defence mechanism in thermally oxidized oil- and ethanol-fed groups when compared with normal control group. The activity of liver marker enzymes (AST, ALP and GGT) and the level of lipid peroxidation decreased when DA was administered along with ethanol and thermally oxidized oil. The antioxidant status was near normal in DA-administered groups. Thus we propose that DA exerts antioxidant properties by modulating the activity of hepatic marker enzymes, level of lipid peroxidation and antioxidant status.

Effect of lindane on hepatic and brain cytochrome P450s and influence of P450 modulation in lindane induced neurotoxicity

Oral administration of lindane (2.5, 5, 10 and 15 mg/kg, body weight) for 5 days was found to produce a dose-dependent increase in the activity of P450 dependent 7-ethoxyresorufin-O-deethylase (EROD), 7-pentoxyresorufin-O-dealkylase (PROD) and N-nitrosodimethylamine demethylase (NDMA-d) in rat brain and liver. A significant increase in the hepatic and brain P450 monooxygenases was also observed when the duration of exposure of low dose (2.5 mg/kg) of lindane was increased from 5 days to 15 or 21 days. As observed with different doses, the magnitude of induction in the activity of P450 monooxygenases was several fold higher in liver microsomes when compared with the brain. Western blotting studies have indicated that the increase in the P450 enzymes could be due to the increase in the expression of P450 1A1/1A2, 2B1/2B2 and 2E1 isoenzymes. In vitro studies using organic inhibitors specific for individual P450 isoenzymes and antibody inhibition experiments have further demonstrated that the increase in the activity of PROD, EROD and NDMA-d are due to the increase in the levels of P450 2B1/2B2, 1A1/1A2 and 2E1 isoenzymes, respectively. Induction studies have further shown that while pretreatment of 3-methylcholanthrene (MC), an inducer of P4501A1/1A2, did not produce any significant effect in the incidence of lindane induced convulsions, pretreatment with phenobarbital (PB), an inducer of P450 2B1/2B2 or ethanol, an inducer of P450 2E1 catalysed reactions, significantly increased the incidence of lindane induced convulsions. Similarly, when the P450-mediated metabolism of lindane was blocked by cobalt chloride incidence of convulsions was increased in animals treated with lindane indicating that lindane per se or its metabolites formed by PB or ethanol inducible P450 isoenzymes are involved in its neurobehavioral toxicity.

Cytochrome P450 3A conjugation to ubiquitin in a process distinct from classical ubiquitination pathway

We characterize a novel microsome system that forms high-molecular-mass (HMM) CYP3A, CYP2E1, and ubiquitin conjugates, but does not alter CYP4A or most other microsomal proteins. The formation of the HMM bands was observed in hepatic microsomes isolated from rats treated 1 week or more with high doses (50 mg/kg/day) of nicardipine, clotrimazole, or pregnenolone 16alpha-carbonitrile, but not microsomes from control, dexamethasone-, nifedipine-, or diltiazem-treated rats. Extensive washing of the microsomes to remove loosely attached proteins or cytosolic contaminants did not prevent the conjugation reaction. In contrast to prototypical ubiquitination pathways, this reaction did not require addition of ubiquitin, ATP, Mg(2+), or cytosol. Addition of cytosol did result in the degradation of the HMM CYP3A bands in a process that was not blocked by proteasome inhibitors. Immunoprecipitated CYP3A contained HMM ubiquitin. Even so, mass spectrometric analysis of tryptic peptides indicated that the HMM CYP3A was in molar excess to ubiquitin, suggesting that the formation of the HMM CYP3A may have resulted from conjugation to itself or a diffuse pool of ubiquitinated proteins already present in the microsomes. Addition of CYP3A substrates inhibited the formation of the HMM CYP3A and the cytosol-dependent degradation of HMM CYP3A. These results suggest that after extended periods of elevated CYP3A expression, microsomal factors are induced that catalyze the formation of HMM CYP3A conjugates that contain ubiquitin. This conjugation reaction, however, seems to be distinct from the classical ubiquitination pathway but may be related to the substrate-dependent stabilization of CYP3A observed in vivo.

Molecular targets of disulfiram action on song maturation in zebra finches

Disulfiram, an aldehyde dehydrogenase inhibitor, interferes with normal song maturation when applied to brain nucleus HVC of male zebra finches. We present here evidence from Western blots and enzymatic assays showing that known disulfiram targets other than retinaldehyde-specific aldehyde dehydrogenase (zRalDH) are absent in HVC. These findings are consistent with the conclusion that disulfiram disrupts song maturation by interfering with retinoic acid production.

Decrease of hepatic catalase level by treatment with diallyl sulfide and garlic homogenates in rats and mice

Diallyl sulfide (DAS) is a flavor compound derived from garlic and is active in the inhibition of chemically induced cytotoxicity and carcinogenicity in animal models. This study was conducted to examine the effects of the treatment of DAS and garlic homogenates on the activities of catalase, glutathione peroxidase, and superoxide dismutase. Male Sprague-Dawley rats were treated with DAS i.g. at daily doses of 50 or 200 mg/kg for 8 days, causing the hepatic catalase activity to decrease by 55 and 95%, respectively. Such a decrease in hepatic catalase activity was also observed when the DAS treatment was extended to 29 days. Western blot analysis showed that the DAS treatments resulted in corresponding decreases in the liver catalase protein level. No significant change in the catalase activity in the kidney, lung, and brain was observed with the treatments, but a slight decrease in heart catalase activity was observed. These treatments did not cause significant changes in superoxide dismutase and glutathione peroxidase activities in these tissues. Treatment with DAS at a daily dose of 200 mg/kg for 1-7 days resulted in a gradual decrease in the liver catalase activity to 5% of the control level, but it did not decrease the erythrocyte catalase activity. Treatment of rats with fresh garlic homogenates (2 or 4 g/kg, i.g., daily for 7 days) caused a 35% decrease in liver catalase activity. A/J mice treated with DAS and garlic homogenates also showed a decrease in the liver catalase activity. Diallyl sulfone (DASO2), a DAS metabolite, however, did not effectively decrease catalase activity in mice. The catalase activity was not inhibited by either DAS or DASO2 in vitro. The present results demonstrate that treatment with DAS and garlic homogenates decrease the hepatic catalase level in rats and mice.

The alcohol-inducible form of cytochrome P450 (CYP 2E1): role in toxicology and regulation of expression

Cytochrome P450 (CYP) 2E1 catalyzes the metabolism of a wide variety of therapeutic agents, procarcinogens, and low molecular weight solvents. CYP2E1-catalyzed metabolism may cause toxicity or DNA damage through the production of toxic metabolites, oxygen radicals, and lipid peroxidation. CYP2E1 also plays a role in the metabolism of endogenous compounds including fatty acids and ketone bodies. The regulation of CYP2E1 expression is complex, and involves transcriptional, post-transcriptional, translational, and post-translational mechanisms. CYP2E1 is transcriptionally activated in the first few hours after birth. Xenobiotic inducers elevate CYP2E1 protein levels through both increased translational efficiency and stabilization of the protein from degradation, which appears to occur primarily through ubiquitination and proteasomal degradation. CYP2E1 mRNA and protein levels are altered in response to pathophysiologic conditions by hormones including insulin, glucagon, growth hormone, and leptin, and growth factors including epidermal growth factor and hepatocyte growth factor, providing evidence that CYP2E1 expression is under tight homeostatic control.

Resistance to carbon tetrachloride-induced hepatotoxicity in mice which lack CYP2E1 expression

CYP2E1 knockout mice (cyp2e1-/-) were used to investigate the involvement of CYP2E1 in the development of carbon tetrachloride (CCl4)-induced hepatotoxicity. Male cyp2e1-/- and wild-type (cyp2e1+/+) mice were given a single i.p. injection of 1 ml/kg (= 1.59 g/kg) CCl4 and 24 h later liver injury was assessed by elevations of serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities and histopathology. No significant increases in serum ALT and AST activities were observed in cyp2e1-/- mice when compared to wild-type counterparts after CCl4 exposure. No detectable abnormality in liver histology was found in cyp2e1-/- mice after CCl4 exposure. In contrast, CCl4 treatment resulted in 442- and 125-fold increases in serum ALT and AST activities, respectively, in wild-type mice. Consistent with the results of serum ALT and AST activities, severe hepatic damage was noted in livers of wild-type mice, indicating the importance of CYP2E1 in mediating the hepatic damage following CCl4 exposure in these mice. In addition, a dramatic decrease in CYP2E1-catalyzed p-nitrophenol activity and complete loss of immunoreactive CYP2E1 were observed in wild-type mice after CCl4 treatment, suggesting that CYP2E1 was degraded during the process of CCl4-induced hepatotoxicity. These studies conclusively demonstrate that CYP2E1 is the major factor involved in the CCl4-induced hepatotoxicity in mice.

A study on the antioxidant capacities of some benzimidazoles in rat tissues

Seven benzimidazole compounds were synthesized and their in vitro effects on rat liver, lung and kidney microsomal NADPH-dependent lipid peroxidation (LP) levels were determined. The significant decrease in male rat liver microsomal LP level was noted only by the compound 4 at 10(-4) M (20%) and 10(-3) M (40%) concentrations whereas the other compounds were ineffective. In lung, only the compound 6 at 10(-4) M concentration exhibited significant alteration, i.e. 56% increase, in LP level. In kidney, however, apart from the compound 4, all the compounds increased LP level (35-52%) significantly. The classical antioxidant, butylated hydroxy toluene (BHT), at 10(-4) M concentration, significantly decreased LP level about 70%, in all the tissues studied. To clarify the effects of compounds 4 and 6 on LP, the responses of some CYPs, which are active in producing reactive oxygen species, to these compounds were also investigated. The compound 4 at 10(-4) and 10(-3) M concentrations inhibited the hepatic microsomal ethoxyresorufin O-deethylase (EROD) (37 and 65%) and pentoxyresorufin O-depenthylase (PROD) (14 and 62%) enzyme activities significantly. However, it did not alter the hepatic microsomal NADPH-cytochrome P450-reductase activity. BHT, at 10(-3) M concentration, significantly inhibited hepatic microsomal EROD (73%), PROD (62%) and NADPH-cytochrome P450 reductase (17%) enzyme activities. Caffeine (10(-3)M) and SKF 525A (10(-3)M), which are specific inhibitors of EROD and PROD enzyme activities, significantly decreased the enzyme activities 33 and 77%, respectively. Caffeine was unable to alter hepatic microsomal NADPH-cytochrome P450 reductase enzyme activity whereas SKF 525A significantly inhibited (80%) it. In lung and kidney, the compound 6 at 10(-4)M concentration significantly increased EROD (44 and 19%) and PROD (103 and 86%) enzyme activities. However, the elevation of PROD enzyme activity in both tissues was observed to be more pronounced than that of EROD enzyme activity. This compound was ineffective on lung and kidney microsomal P450-reductase enzyme activity. These results reveal that the synthesized benzimidazoles have variable tissue dependent in vitro effects on LP due to their distinct effects on CYP activities but not on NADPH-cytochrome P450 reductase activity in rats.

Effects of tobacco smoke on the gene expression of the Cyp1a, Cyp2b, Cyp2e, and Cyp3a subfamilies in mouse liver and lung: relation to single strand breaks of DNA

Cigarette smoking is a worldwide health problem and is the greatest risk factor for lung cancer. By activating procarcinogens, hepatic and extrahepatic cytochromes P450 can participate in lung carcinogenesis. Tobacco smoke contains numerous cytochrome P450 inducers, substrates, and inhibitors. In the present study we investigated, in male NMRI mice, the effects of cigarette smoke on hepatic and pulmonary cytochrome P450 expression and their possible role in the induction of DNA lesions such as DNA single strand breaks (SSB). Hepatic and pulmonary mouse cytochrome P450 isozymes involved in carcinogenesis (Cyp1a, 2b, 2e, 3a) were differently induced by cigarette smoke. Cyp2e1 mRNA was dramatically enhanced (12.7-fold increase) while Cyp2b10 mRNA remained unchanged and Cyp1a1 was decreased or not detected. Cyp3a protein and mRNA were not detected in lung, suggesting that this isozyme is not expressed in mouse pulmonary tissue. The SSB of DNA increased in lung and liver treated mice. In contrast no modification was observed in lymphocytes that barely expressed cytochromes P450. Cimetidine and propylene glycol reduced SSB of DNA induced by smoking in liver and lung cells. The inhibition (-70%) observed in lung following treatment by propylene glycol, a CYP2E1 inhibitor, suggested that this isozyme is at least in part involved in pulmonary DNA damage induced by tobacco smoke. The high concentration of CYP2E1 function and regulation in mammals suggests that this protein could be involved in pulmonary carcinogenesis in human smokers.

Hepatic cytochrome P450 2E1 is increased in patients with nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH) has multiple etiologic associations, but the pathogenesis is poorly understood. Cytochrome P450 (CYP) 2E1 is induced in the liver of patients who drink alcohol to excess and is important in the pathogenesis of alcoholic liver disease (ALD). We have previously shown that hepatic CYP2E1 is also increased in a rat dietary model of steatohepatitis. The aim of the present study was to test the hypothesis that hepatic CYP2E1 is induced in the liver of patients with NASH, defined on the basis of compatible liver histology and the exclusion of excessive alcohol intake. Sections of paraffin-embedded liver biopsy material from 31 subjects with NASH were evaluated and compared with sections from 10 histologically normal livers and 6 patients with ALD. Hepatic CYP2E1 and CYP3A were detected in liver sections by immunohistochemistry using specific anti-human CYP2E1 and CYP3A antibodies. As expected, normal livers showed CYP2E1 immunostaining confined to a rim, two to three cells thick, around terminal hepatic venule, while livers from alcoholic hepatitis patients showed increased CYP2E1 staining. CYP2E1 immunostaining was also increased in livers from patients with NASH, irrespective of the etiologic association. Further, the pattern of CYP2E1 distribution was similar to ALD, with increased perivenular intensity and more extensive acinar distribution of staining. As in the rat model, the hepatic distribution of CYP2E1 corresponded to that of steatosis. In contrast to CYP2E1, CYP3A immunostaining was decreased in patients with NASH. We conclude that hepatic CYP2E1 is increased in patients with NASH compared with normal livers. Thus, despite many possible etiologic factors for NASH, the pathogenetic mechanisms may be similar and, like alcoholic steatohepatitis, may involve induction of CYP2E1.

Regulation of the hepatic CYP 2E1 gene during chronic alcohol exposure: lack of an ethanol response element in the proximal 5'-flanking sequence

Chronic exposure to ethanol is known to cause a dramatic increase in the level of CYP 2E1 apoprotein. More recently it has been demonstrated that under certain conditions the mRNA encoding cytochrome P450 2E1(CYP 2E1) is inducible; however, the mechanisms by which these increases occur are not well understood. In the current study, DNase I footprinting assays performed on the first kilobase of the CYP 2E1 5'-flanking sequences resulted in the identification of 13 sequence-specific protected regions using rat liver nuclear extracts isolated from either control or ethanol-treated animals. No differences were observed in the DNase I footprint patterns produced by the two different nuclear extracts. In addition, analysis by electrophoretic mobility shift assays (EMSA) revealed that with one exception, there were no differences in the level of binding complexes between the two extracts. However, EMSA analysis with an oligonucleotide to one footprint site (designated Site C) revealed that in nuclear extracts isolated from ethanol-treated animals there was a 2.9-fold increase in this binding complex when compared to control nuclear extracts. This site was previously shown to contain an HNF-1alpha binding site, and here we demonstrate that bacterially expressed HNF-1alpha in footprint assays bind Site C sequences and that HNF-1alpha transactivates the CYP 2E1 promoter in co-transfection experiments with HNF-1alpha expression plasmid and plasmids containing CYP 2E1 promoter sequences coupled to the chloramphenicol acetyl transferase gene. Furthermore, in contrast to the increase observed by EMSA in Site C binding, no increase was detected in the CYP 2E1 transcriptional rate supported by nuclear extracts from ethanol-treated animals over controls using in vitro transcription assays, suggesting that the increase by ethanol in CYP 2E1 transcription is not mediated through the HNF-1alpha site.

Lipid peroxidation and antioxidant systems in rat brain: effect of chronic alcohol consumption

The effect of chronic ethanol exposure, in a liquid diet, on lipid peroxidation and some antioxidant systems of rat brain was investigated. Chronic ethanol administration induced a greater susceptibility to iron/ascorbate-induced lipid peroxidation, estimated as thiobarbituric reactive substances (TBARS) production, in the microsomal fraction, but a lower lipid peroxidation in the total homogenate. Glutathione (GSH) levels as well as GSH peroxidase and GSH reductase were unaffected, while the activity of Cu-Zn superoxide dismutase was decreased and that of catalase increased. Lipid peroxidation experiments performed in the presence of some hydroxyl radical scavengers suggested that a greater OH. generation may be responsible of the greater TBARS production in the microsomal fraction of ethanol treated rats; differently, in total homogenate of control and ethanol rats a relationship was found between the redox state of iron and TBARS production, suggesting that the lower lipid peroxidation in treated rats may depend on a different modulation of the iron redox state.

Putative oxidative metabolites of 1-methyl-6-hydroxy-1,2,3,4-tetrahydro-beta-carboline of potential relevance to the addictive and neurodegenerative consequences of ethanol abuse

Ethanol is metabolized in the brain by catalase/H2O2 to yield acetaldehyde and by an ethanol-inducible form of cytochrome P450 (P450 IIE1) in a reaction that yields oxygen radicals. Within the cytoplasm of serotonergic axon terminals these metabolic pathways together provide conditions for the endogenous synthesis of 1-methyl-6-hydroxy-1,2,3,4-tetrahydro-beta-carboline (1), by reaction of acetaldehyde with unbound 5-hydroxytryptamine (5-HT), and for the oxygen radical-mediated oxidation of this alkaloid. The major initial product of the hydroxyl radical (HO.)-mediated oxidation of 1 in the presence of free glutathione (GSH), a constituent of nerve terminals and axons, is 8-S-glutathionyl-1-methyl-1,2,3,4-tetrahydro-beta-carboline-5,6-dione (6). When administered into the brains of mice, 6 is a potent toxin (LD50 = 2.9 microg) and evokes episodes of hyperactivity and tremor. Compound 6 binds at the GABA(B) receptor and evokes elevated release and turnover of several neurotransmitters. Furthermore, the GABA(B) receptor antagonist phaclofen attenuates the behavioral response caused by intracerebral administration of 6. These observations suggest that 6 might be an inverse agonist at the GABA(B) receptor site. Accordingly, it is speculated that ethanol drinking might potentiate formation of 6 that contributes to elevated release of several neurotransmitters including dopamine (DA) and endogenous opioids in regions of the brain innervated by serotonergic axon terminals. Subsequent interactions of DA and opioids with their receptors might be related to the initial development of dependence on ethanol. Redox cycling of 6 (and of several putative secondary metabolites) in the presence of intraneuronal antioxidants and molecular oxygen to produce elevated fluxes of cytotoxic reduced oxygen species might contribute to the degeneration of serotonergic pathways. Low levels of 5-HT in certain brain regions of the rat predisposes these animals to drink or augments drinking. Accordingly, 6, formed as a result of ethanol metabolism in the cytoplasm of certain serotonergic axon terminals, might contribute to the initial development of dependence on ethanol, by mediating DA and opioid release, and long-term preference and addiction to the fluid as a result of the progressive degeneration of these neurons.

Influence of glutathione on the oxidation of 1-methyl-6-hydroxy-1,2,3,4-tetrahydro-beta-carboline: chemistry of potential relevance to the addictive and neurodegenerative consequences of ethanol use

Recent evidence suggests that intraneuronal metabolism of ethanol by catalase/H2O2 and an ethanol-inducible form of cytochrome P450 together generate acetaldehyde and oxygen radicals including the hydroxyl radical (HO.). Within the cytoplasm of serotonergic neurons, these metabolic processes would thus provide acetaldehyde, which would react with unbound 5-hydroxytryptamine (5-HT) to give 1-methyl-6-hydroxy-1,2,3,4-tetrahydro-beta-carboline (1), known to be formed at elevated levels in the brain following ethanol drinking, and HO. necessary to oxidize this alkaloid. In this study, it is demonstrated that the HO.-mediated oxidation of 1 at physiological pH yields 1-methyl-1,2,3,4-tetrahydro-beta-carboline-5,6-dione (8) that reacts avidly with free glutathione (GSH), a significant constituent of axons and nerve terminals, to give diastereomers of 8-S-glutathionyl-1-methyl-1,2,3,4-tetrahydro-beta-carboline-5,6-dione (9A and 9B). In the presence of free GSH, ascorbic acid, other intraneuronal antioxidants/reductants, and molecular oxygen diastereomers, 9A/9B redox cycle in reactions that generate H2O2 and, via trace transition metal ion catalyzed decomposition of the latter compound, HO.. Further reactions of 9A/9B with GSH and/or HO. generate several additional glutathionyl conjugates that also redox cycle in the presence of intraneuronal reductants and molecular oxygen forming H2O2 and HO.. Thus, intraneuronal formation of 1 and HO. as a consequence of ethanol drinking and resultant endogenous synthesis of 8,9A, and 9B would, based on these in vitro chemical studies, be expected to generate elevated fluxes of H2O2 and HO. leading to oxidative damage to serotonergic axons and nerve terminals and the irreversible loss of GSH, both of which occur in the brain as a consequence of ethanol drinking. Furthermore, deficiencies of 5-HT and loss of certain serotonergic pathways in the brain have been linked to the preference for and addiction to ethanol.

Risk assessment: toxicity from chemical exposure resulting from enhanced expression of CYP2E1

Humans are continuously exposed to a wide variety of xenobiotics either voluntarily or from environmental exposure. Many xenobiotics including pesticides, nitrosamines, polycyclic aromatic hydrocarbons and halogenated hydrocarbons, require bioactivation by P450 enzymes to elicit toxicity. CYP2E1 is considered to be toxicologically important in humans because of its capacity to produce intermediates that promote cytotoxicity and/or carcinogenicity from a number of xenobiotics. Importantly, CYP2E1 is present constitutively and its content can be modulated by a variety of factors including xenobiotics such as alcohol. Because hepatic concentrations of CYP2E1 can vary considerably from one individual to another, the extent of formation of toxic products also varies. Indeed, as hepatic concentrations increase so does the risk of toxicity from chemicals activated by this P450 enzyme. Many chemicals modulate CYP2E1 expression and exposure to one compound may alter the toxicological impact of another. Considering that CYP2E1 content is related to toxicity from chemicals, identifying subjects with elevated levels may lead to minimizing exposure in high risk individuals.

N-Hydroxylation of dapsone by multiple enzymes of cytochrome P450: implications for inhibition of haemotoxicity

1. The adverse reactions associated with the administration of dapsone are believed to be caused by metabolism to its hydroxylamine. Previous reports suggest that CYP3A4 is responsible for this biotransformation [1]. 2. Data presented in this paper illustrate the involvement of more than one cytochrome P450 enzyme in dapsone hydroxylamine formation using human liver microsomes. Eadie-Hofstee plots demonstrated bi-phasic kinetics in several livers. No correlation could be established between hydroxylamine formation and CYP3A concentrations in six human livers (r = -0.47; P = 0.34). 3. Studies with low molecular weight inhibitors illustrate the importance of CYP2C9 and CYP3A in dapsone N-hydroxylation. 4. Differential sensitivity of dapsone N-hydroxylation to selective CYP inhibitors indicated that the contribution of individual CYP enzymes varies between livers. Selective inhibition ranged from 6.8 to 44.1% by 5 microM ketoconazole, and from 24.0 to 68.4% by 100 microM sulphaphenazole. The extent of inhibition, by either ketoconazole or sulphaphenazole was dependent on the CYP3A content of the liver. 5. The levels of expression of these cytochrome P450 enzymes may be an important determinant of individual susceptibility to the toxic effects of dapsone, and may influence the ability of an enzyme inhibitor to block dapsone toxicity in vivo. Because of the inability to produce complete inhibition, selective CYP inhibitors are unlikely to offer any clinical advantage over cimetidine in decreasing dapsone hydroxylamine formation in vivo.

Oxidative stress in isolated rat hepatocytes during naproxen metabolism

Naproxen, a non-steroidal anti-inflammatory drug, induced lipid peroxidation in isolated hepatocytes of rats. The viability of the hepatocytes decreased upon lipid peroxidation, and this effect was accompanied by the formation of high molecular weight protein aggregates in the hepatocytes. Protein aggregation occurred slowly compared with the formation of thiobarbituric acid reactive substances (TBARS). The increase of TBARS was strongly correlated with the decrease of intracellular glutathione. Chemiluminescence was produced from the hepatocyte suspension during naproxen metabolism, and was correlated with the formation of TBARS. These results indicate that lipid peroxidation in the hepatocytes was provoked by reactive oxygens produced in the process of naproxen metabolism.

Oxygen and xenobiotic reductase activities of cytochrome P450

The oxygen reductase and xenobiotic reductase activities of cytochrome P450 (P450) are reviewed. During the oxygen reductase activity of P450, molecular oxygen is reduced to superoxide anion radicals (O2-.) most likely by autooxidation of a P450 ferric-dioxyanion complex. The formation of reactive oxygen species (O2-., hydrogen peroxide, and, notably, hydroxyl free radicals) presents a potential toxication pathway, particularly when effective means of detoxication are lacking. Under anaerobic conditions, P450 may also be involved in the reduction of xenobiotics. During the xenobiotic reductase activity of P450, xenobiotics are reduced by the ferrous xenobiotic complex. After xenobiotic reduction by P450, xenobiotic free radicals are formed that are often capable of reacting directly with tissue macromolecules. Unfortunately, the compounds that are reductively activated by P450 have little structural similarity. The precise molecular mechanism underlying the xenobiotic reductase activity of P450 is, therefore, not yet fully understood. Moreover, description of the molecular mechanisms of xenobiotic and oxygen reduction reactions by P450 is limited by the lack of knowledge of the three-dimensional (3D) structure of the mammalian P450 proteins.

Influence of the xanthine derivative denbufylline and the anti-inflammatory agent nabumetone on microsomal free radical production and lipid peroxidation in rat liver

The influence of denbufylline, nabumetone and its main metabolite BRL 10,720 on iron stimulated lipid peroxidation (LPO), cytochrome P 450 dependent H2O2 and chemiluminescence (CL) production was investigated in rat liver microsomes in vitro (10(-5)-10(-3) M) and in vivo after treatment of rats (5-300 mg/kg b.m. orally on three consecutive days). In rat liver slices the release of thiobarbituric acid reactants (TBAR) was measured after 1 hour of incubation with the drugs. Denbufylline, nabumetone and BRL 10,720 exerted a significant inhibition of iron stimulated LPO in vitro. Nabumetone showed the strongest antioxidative activity, which was also seen in liver slices. These antioxidative effects were not found after in vivo treatment of rats. Denbufylline (10(-3) M) additionally inhibited H2O2 formation and the luminol and lucigenin amplified CL in vitro. Unexpectedly, nabumetone increased H2O2 formation both in vitro and in vivo, but in vitro only lucigenin amplified CL. BRL 10,720 increased microsomal H2O2 production in vivo. Moreover, BRL 10,720 enhanced CL in vitro and in vivo significantly, which is interpreted as an increase of the production of superoxide anion radicals and other reactive oxygen species such as H2O2, but lipid peroxidation in liver microsomes was not enhanced. These results suggest that denbufylline, nabumetone and BRL 10,720 in contrast to the in vitro effects did not exert antioxidative activities after treatment of rats. On the contrary, BRL 10,720 was found to support the formation of reactive oxygen species in liver microsomes.

Characterization of cytochrome P450 2E1 induction in a rat hepatoma FGC-4 cell model by ethanol

The hepatic microsomal ethanol-oxidizing system (MEOS) has been well characterized as an important pathway in ethanol metabolism. Cytochrome P450 2E1 (CYP 2E1), the principal component of MEOS, is ethanol inducible and has been implicated in hepatotoxicity associated with alcohol abuse and exposure to organic solvents. Results of chronic in vivo experiments have shown that ethanol induction of hepatic CYP 2E1 occurs by a two-step mechanism. The first step of induction is associated with low blood alcohol concentrations (BACs) and appears to be post-transcriptional, whereas high BACs observed in step-two induction are associated with increased CYP 2E1 gene transcription. The mechanisms underlying these induction steps are under intense investigation. Progress in this area has been limited due to lack of hepatic cell culture models that express CYP 2E1. We report here an in vitro tissue culture cell model, the FGC-4 hepatoma cell line, that exhibits basal levels of CYP 2E1 apoprotein that are inducible by ethanol treatment. Total cellular RNA and microsomal fractions were isolated from control or ethanol-treated confluent cells, and CYP 2E1 mRNA and apoprotein levels were characterized by northern blot or immunoblot analysis, respectively. Initial experiments on isolated microsomes revealed detectable levels of CYP 2E1 apoprotein in control cells that were induced 5-fold in cells treated with 100 mM ethanol for 24 hr. Concentration-response experiments demonstrated that the maximal 24-hr induction in CYP 2E1 apoprotein level was 5-fold and was attained at a concentration of 10 mM ethanol. Interestingly, while the steady-state mRNA levels encoding CYP 2E1 were detectable, they remained unchanged in identically treated cells. Furthermore, there was no observed increase in CYP 2E1 mRNA levels in an extended time course to 72 hr or at higher alcohol concentrations (up to 1500 mM), providing preliminary evidence that the induction is post-transcriptional. The time course of CYP 2E1 apoprotein induction by exposure to 100 mM ethanol demonstrated maximal induction at 8 hr. Measurement of CYP 2E1 apoprotein levels after removal of ethanol from pretreated cells demonstrated the half-life of the apoprotein to be 12.7 hr, in good agreement with previous reports using primary hepatocytes. The half-life of the induced protein after ethanol removal in the presence of cyclohexamide (10 micrograms/mL) was biphasic with a rapid 1.8 hr first phase followed by a slower 44.7 hr second phase.(ABSTRACT TRUNCATED AT 400 WORDS)

Environment-immune interactions

The need for effective immune function for the maintenance of health has been clearly established in both agriculturally significant animal species and humans. Intensive agricultural practices present production species with numerous disease challenges during the rearing period. Environmental factors represent a ubiquitous, yet frequently manageable, category of immunomodulators that can influence immune performance and ultimately disease susceptibility or resistance. However, strategies for assessing overall immune potential have not been widely implemented for agricultural species. This is in contrast to the use of immune evaluation for human health considerations. Immune assessment relative to environmental-immune interactions can produce benefits in two areas. First, the efficiency of the production operation can be enhanced. Second, the welfare of the animals during the production cycle can be optimized. This paper presents an overview of environmental factors known to influence the immune function of poultry and the opportunities to manage environmental factors to benefit the health of the animals. In addition, the paper discusses the status of immunological assessment for humans and laboratory animals and proposes potential immune assessment panels that could serve as a tool to optimize the environmental management of poultry populations.

Hepatoprotective mechanism of silymarin: no evidence for involvement of cytochrome P450 2E1

The involvement of the alcohol-inducible cytochrome P450 2E1 in the hepatoprotective mechanism of the plant flavonoid extract silymarin, and its main active component silybin, was investigated in isolated hepatocytes. Allyl alcohol toxicity, associated lipid peroxidation and GSH depletion was efficiently counteracted by silymarin (0.01-0.5 mM), and at higher concentrations by silybin. Cell damage by t-butyl hydroperoxide was also prevented by silymarin and silybin, but less efficiently. However, the covalent binding of the acetaminophen intermediate, formed via P450 2E1, was unaffected by addition of the flavonoids. Silybin did not influence microsomal 2E1-catalyzed demethylation of N-nitrosodimethylamine. Neither did demethylation of N-nitrosodimethylamine or aminopyrine by isolated microsomes affect the in vivo administration of silybin. Addition of silymarin or silybin to primary cultures of isolated hepatocytes did not prevent cell damage induced by exposure to the P450 2E1 substrate CCl4. In contrast, the mere presence of low concentrations (25-50 microM) of these compounds was found to inhibit cell attachment to the matrix and eventually resulted in cell damage. We conclude that contrary to earlier reports we found no evidence for an interaction of silymarin or silybin with cytochrome P450 2E1. This suggests that the antioxidant and free radical scavenging properties may account for most of the therapeutic effect of these compounds. The untoward effect of silymarin on cultured cells may have consequences when considering long-term prescription of this therapeutic agent.

Effect of ethanol on cytochrome P450 in the rat brain

After a single dose of ethanol (0.8 ml/kg) administered intraperitoneally, the P450 content of the rat brain increased from 62 +/- 19 to 230 +/- 97 pmol/g (wet weight) of tissue (mean +/- SD, n = 5). Most of this increase could be accounted for by a 10- to 20-fold increase in the olfactory lobes and hypothalamic preoptic area. The P450s were identified by Western blot analysis and by microsequencing of the N-terminal ends after resolution of the proteins on SDS gels. They were identified as P450 2C7, 2C11, 2E1, 4A3, 4A8, and a member of the P450 2D family. In P450 extracted from the brains of control rats, P450 2C and 4A were also detectable but at a much lower concentration. P450 1A1, 2A1, 2B1, or 3A was not detected in the brains of either control or ethanol-treated rats. Oral administration of the same dose of ethanol resulted in a similar increase in the whole brain but smaller effects in the olfactory lobes. This effect of ethanol on the P450 in the brain has implications for the mechanism of toxicity and the development of tolerance to ethanol and for the effects of other drugs and environmental pollutants that act on the central nervous system.

Regulatory mechanisms in the activation of nitrogenous compounds by mammalian cytochrome P-450 isozymes

Metabolic activation of nitrogenous compounds by the cytochrome P-450 system is a highly complex process. Inherent substrate factors, such as basicity, electronic state, lipophilicity, and conformation control binding of the diverse classes of amines to cytochrome P-450. Accommodation of these compounds in the enzyme cavity and proper orientation of the molecules are governed by intrinsic properties of the peptide structure of cytochrome P-450, which may be subject to modification by the action of effectors. On the membrane level, phospholipid might have some impact on substrate binding. On the other hand, bound amine substrate is beneficial to the productive interaction of the electron transport chains with the terminal acceptor, improving economy of the system. Certain amines appear to regulate O2 association with cytochrome P-450 and stabilize the various oxy species formed. Considering the selective prerequisites for oxidative attack by cytochrome P-450 at vulnerable nitrogen centers, many cytotoxic amines belonging to the category of relatively rigid, planar molecules undergo N-oxidative activation by the cytochrome P-450IA subfamily, while more bulky amines with flexible conformation are N-oxygenated preferentially by phenobarbital-inducible cytochromes P-450. Small differences in protein structure between the various cytochrome P-450 subforms might serve to stabilize aminium radicals to permit oxygen rebound. Collectively, the selective regulatory mechanisms operative in the bioactivation of nitrogen-containing compounds appear to be determined largely by the type of substrate used and the isozyme involved in catalysis. With respect to the latter, the interplay of the multiple cytochromes P-450 in the various organs of animal species thus serves to rationalize the differences in the particular selectivities for amine substrates. These are responsible for the mode and/or extent to which activation of nitrogenous compounds, including promutagens and procarcinogens, occurs, and this may explain the tissue-specific response to the tumorigenic action of these agents.

Stimulation by paraquat of microsomal and cytochrome P-450-dependent oxidation of glycerol to formaldehyde

Glycerol can be oxidized to formaldehyde by microsomes in a reaction that is dependent on cytochrome P-450. An oxidant derived from the interaction of H2O2 with iron was responsible for oxidizing the glycerol, with P-450 suggested to be necessary to produce H2O2 and reduce non-haem iron. The effect of paraquat on formaldehyde production from glycerol and whether paraquat could replace P-450 in supporting this reaction were studied. Paraquat increased NADPH-dependent microsomal oxidation of glycerol; the stimulation was inhibited by glutathione, catalase, EDTA and desferrioxamine, but not by superoxide dismutase or hydroxyl-radical scavengers. The paraquat stimulation was also inhibited by inhibitors, substrate and ligand for P-4502E1 (pyrazole-induced P-450 isozyme), as well as by anti-(P-4502E1) IgG. These results suggest that P-450 still played an important role in glycerol oxidation, even in the presence of paraquat. Purified NADPH-cytochrome P-450 reductase did not oxidize glycerol to formaldehyde; some oxidation, however, did occur in the presence of paraquat. Reductase plus P-4502E1 oxidized glycerol, and a large stimulation was observed in the presence of paraquat. Rates in the presence of P-450, reductase and paraquat were more than additive than the sums from the reductase plus P-450 and reductase plus paraquat rates, suggesting synergistic interactions between paraquat and P-450. These results indicate that paraquat increases oxidation of glycerol to formaldehyde by microsomes and reconstituted systems, that H2O2 and iron play a role in the overall reaction, and that paraquat can substitute, in part, for P-450 in supporting oxidation of glycerol. However, cytochrome P-450 is required for elevated rates of formaldehyde production even in the presence of paraquat.

Ethanol-inducible cytochrome P4502E1: genetic polymorphism, regulation, and possible role in the etiology of alcohol-induced liver disease

In the Tsukamoto-French model, ethanol causes an important 10-20-fold induction of ethanol-inducible cytochrome P4502E1 (CYP2E1), mediated through enzyme stabilization and increased rate of gene transcription. The CYP2E1 induction results in a pronounced increase in the rate of NADPH-dependent microsomal lipid peroxidation, an elevation which is not seen after simultaneous administration of the CYP2E1 inhibitor diallylsulfide. Increased amounts of lipid peroxides are seen in plasma and red blood cells of both rats and humans during high ethanol intake. A mechanism for ethanol-dependent liver damage is proposed which involves the CYP2E1-dependent lipid peroxide formation, either directly by its capability to induce NADPH-dependent peroxidation in the microsomal membranes or indirectly by a hypoxia-mediated transformation of xanthine dehydrogenase to xanthine oxidase, in activation of Ito cells and Kupffer cells to yield cytokine and collagen production. The CYP2E1 gene is polymorphic among Caucasians. Four different unrelated or partially linked polymorphisms have been observed. One polymorphism in the 5'-flanking region has been described to be associated with altered enzyme expression in vitro, and the rare allele was found to be less frequent among Swedish patients having lung cancer when compared to two different control groups. Another polymorphism, detectable with Dra I restriction endonuclease fragment length polymorphism (RFLP), was localized to intron 6, and the rare allele was less common among Italian alcoholics with clinical signs of liver cirrhosis, as compared to controls. Several other mutations in the CYP2E1 gene were found to be associated with this allele. However, further research is needed to relate the CYP2E1 gene polymorphism with incidence of liver cirrhosis.

Comparative metabolism of bis(2-methoxyethyl) ether by rat and human hepatic microsomes: formation of 2-methoxyethanol

Rat hepatic microsomes catalysed the NADPH-dependent cleavage of the central ether linkage of bis(2-methoxyethyl) ether (diglyme) yielding 2-methoxyethanol (2ME). Microsomes isolated from phenobarbital- or ethanol-pretreated rats exhibited an increased capacity to cleave diglyme to 2ME. This ethanol-induced increase in 2ME formation was not observed if incubations contained the cytochrome P450IIE1 inhibitor isoniazid. Pretreatment of rats with diglyme significantly increased microsomal P-450 levels, P-450-associated enzyme activities and the conversion of diglyme to 2ME. Following the diglyme pretreatment, an almost 30-fold increase in pentoxyresorufin dealkylase activity (P450IIB1/2) was evident in rat hepatic microsomes. Human hepatic microsomes also catalysed the NADPH-dependent cleavage of diglyme to 2ME. The formation of 2ME from diglyme correlated with the aniline hydroxylase activity (P450IIE1) levels measured in human hepatic microsomes. Studies using microsomes isolated from a cell line transfected with specific human P-450 cDNAs indicate that human CYP2E1 catalyses the conversion of diglyme to 2ME. These results suggest that the central ether linkage of diglyme is cleaved by rat and human P-450 and the specific involvement of hepatic P450IIE1 in this process is implicated.

Generalized cytochrome P450-mediated oxidation and oxygenation reactions in aromatic substrates with activated N-H, O-H, C-H, or S-H substituents

1. The general mechanism of metabolic oxidation of substrates by cytochromes P450 (P450s) appears to consist of sequential one-electron oxidation steps rather than of a single concerted transfer of activated oxygen species from P450 to substrates. 2. In case of the acetanilides paracetamol (PAR), phenacetin (PHEN), and 4-chloro-acetanilide (4-CLAA), the first one-electron oxidation step consists of a hydrogen abstraction from the acetylamino nitrogen and/or from the other side-chain substituent on the aromatic ring. The substrate radicals thus formed delocalize their spin and the respective reactive centres of the substrate radical recombine with a P450 iron-bound hydroxyl radical to either yield oxygenated metabolites, or undergo a second hydrogen abstraction forming dehydrogenated products. By this mechanism, the formation of all known oxidative metabolites of PAR, PHEN, and 4-ClAA can be explained. Furthermore, this mechanism is consistent with all available experimental data on [18O]PAR/PHEN, [2H]PAR, and [14C]PHEN. 3. The oxidative metabolic reactions proposed for the acetanilides PAR, PHEN, and 4-ClAA are used to generalize P450-mediated oxidations of these and other acetanilides, such as analogues of PAR and 2-N-acetyl-aminofluorene. 4. A further generalization of the hydrogen abstraction, spin delocalization, radical recombination concept is derived for other aromatic substrates with abstractable hydrogen atoms, notably those with activated N-H, O-H, C-H, or S-H bonds directly attached to the aromatic nucleus.

Zonation of acetaminophen metabolism and cytochrome P450 2E1-mediated toxicity studied in isolated periportal and perivenous hepatocytes

To study the mechanism of centrilobular damage developing in the centrilobular region after high doses of acetaminophen (APAP), its metabolism and toxicity were compared in periportal and perivenous hepatocytes isolated by digitonin/collagenase perfusion. Contrary to earlier reports, based on perfusions, no evidence for a periportal dominance of APAP sulfation could be observed. Glucuronidation, the dominant pathway of conjugation at high (5 mM) APAP concentration, was faster in perivenous cells. During primary culture, prolonged exposure (> or = 24 hr) to 5 mM APAP damaged perivenous cells, with a higher P450 2E1 level than periportal cells. When cells were isolated from ethanol-pretreated rats, to induce P450 2E1 levels specifically in the perivenous region, perivenous hepatocytes exhibited enhanced APAP vulnerability and extensive glutathione depletion. In contrast, corresponding periportal cells retained good viability. Isoniazid, an inhibitor of cytochrome P450 2E1, protected cells against APAP toxicity and prevented glutathione depletion. Induction of P450 2E1 also caused a 3-fold increase in the covalent binding of reactive intermediates from [14C]APAP, and this increase was mainly confined to perivenous cells. These results indicate that in rat liver there is only slight perivenous zonation of APAP conjugation and suggest that zone-specific APAP activation, mediated by the regional expression of ethanol-inducible cytochrome P450 2E1, is responsible for the characteristic centrilobular liver damage elicited by APAP.