Acetaldehyde as a substrate for ethanol-inducible cytochrome P450 (CYP2E1)

A history of the roles of cytochrome P450 enzymes in the toxicity of drugs

The history of drug metabolism began in the 19th Century and developed slowly. In the mid-20th Century the relationship between drug metabolism and toxicity became appreciated, and the roles of cytochrome P450 (P450) enzymes began to be defined in the 1960s. Today we understand much about the metabolism of drugs and many aspects of safety assessment in the context of a relatively small number of human P450s. P450s affect drug toxicity mainly by either reducing exposure to the parent molecule or, in some cases, by converting the drug into a toxic entity. Some of the factors involved are enzyme induction, enzyme inhibition (both reversible and irreversible), and pharmacogenetics. Issues related to drug toxicity include drug-drug interactions, drug-food interactions, and the roles of chemical moieties of drug candidates in drug discovery and development. The maturation of the field of P450 and drug toxicity has been facilitated by advances in analytical chemistry, computational capability, biochemistry and enzymology, and molecular and cell biology. Problems still arise with P450s and drug toxicity in drug discovery and development, and in the pharmaceutical industry the interaction of scientists in medicinal chemistry, drug metabolism, and safety assessment is critical for success.

Increased Brain Serotonin Rather Than Increased Blood Acetaldehyde as a Common Denominator Behind Alleged Disulfiram-Like Reactions

Several pharmaceutical agents are known to produce ethanol intolerance, which is often depicted as disulfiram-like reaction. As in the case with disulfiram, the underlying mechanism is believed to be the accumulation of acetaldehyde in the blood, due to inhibition of the hepatic aldehyde dehydrogenases, albeit this has not been confirmed in all cases by blood acetaldehyde measurements. Herein, cefamandole, cotrimoxazole, griseofulvin, procarbazine, and propranolol, which are reported to produce a disulfiram-like reaction, as well as disulfiram, were administered to Wistar rats and the hepatic activities of ethanol metabolizing enzymes along with the levels of brain monoamines were determined. Blood acetaldehyde was also evaluated after ethanol administration in rats pretreated with the abovementioned pharmaceutical products. Disulfiram, cefamandole, and procarbazine significantly increased blood acetaldehyde levels after ethanol administration, while on the contrary, cotrimoxazole, griseofulvin, and propranolol had no effect on blood acetaldehyde. Interestingly, all substances used, except disulfiram, increased the levels of brain serotonin. According to our findings, cotrimoxazole, griseofulvin, and propranolol do not produce a typical disulfiram-like reaction, because they do not increase blood acetaldehyde when given together with ethanol. On the other hand, all tested agents share the common property to enhance brain serotonin, whereas a respective effect of ethanol is well established. Hence, the ethanol intolerance produced by these agents, whether blood acetaldehyde concentration is elevated or not, could be the result of a “toxic serotonin syndrome,” as in the case of the concomitant use of serotonin-active medications that provoke clinical manifestations similar to those of a disulfiram reaction.

Cytochrome P450 2E1 and its roles in disease

Cytochrome P450 (P450) 2E1 is the major P450 enzyme involved in ethanol metabolism. That role is shared with two other enzymes that oxidize ethanol, alcohol dehydrogenase and catalase. P450 2E1 is also involved in the bioactivation of a number of low molecular weight cancer suspects, as validated in vivo in mouse models where cancers could be attenuated by deletion of Cyp2e1. P450 2E1 does not have a role in global production of reactive oxygen species but localized roles are possible, e.g. in mitochondria. The structures, conformations, and catalytic mechanisms of P450 2E1 have some unusual features among P450s. The concentration of hepatic P450 varies ≥10-fold among humans, possibly in part due to single nucleotide variants. The level of P450 2E1 may have relevance in the rates of oxidation of drugs, particularly acetaminophen and anesthetics.

Simultaneous phenotyping of CYP2E1 and CYP3A using oral chlorzoxazone and midazolam microdoses

AIMS

Chlorzoxazone is the paradigm marker substrate for CYP2E1 phenotyping in vivo. Because at the commonly used milligram doses (250-750 mg) chlorzoxazone acts as an inhibitor of the CYP3A4/5 marker substrate midazolam, previous attempts failed to combine both drugs in a common phenotyping cocktail. Microdosing chlorzoxazone could circumvent this problem.

METHOD

We enrolled 12 healthy volunteers in a trial investigating the dose-exposure relationship of single ascending chlorzoxazone oral doses over a 10,000-fold range (0.05-500 mg) and assessed the effect of 0.1 and 500 mg of chlorzoxazone on oral midazolam pharmacokinetics (0.003 mg).

RESULTS

Chlorzoxazone area under the concentration-time curve was dose-linear in the dose range between 0.05 and 5 mg. A nonlinear increase occurred with doses ≥50 mg, probably due to saturated presystemic metabolic elimination. While midazolam area under the concentration-time curve increased 2-fold when coadministered with 500 mg of chlorzoxazone, there was no pharmacokinetic interaction between chlorzoxazone and midazolam microdoses.

CONCLUSION

The chlorzoxazone microdose did not interact with the CYP3A marker substrate midazolam, enabling the simultaneous administration in a phenotyping cocktail. This microdose assay is now ready to be further validated and tested as a phenotyping procedure assessing the impact of induction and inhibition of CYP2E1 on chlorzoxazone microdose pharmacokinetics.

Microsomal Ethanol-Oxidizing System: Success Over 50 Years and an Encouraging Future

Fifty years ago, in 1968, the pioneering scientists Charles S. Lieber and Leonore M. DeCarli discovered the capacity for liver microsomes to oxidize ethanol (EtOH) and named it the microsomal ethanol-oxidizing system (MEOS), which revolutionized clinical and experimental alcohol research. The last 50 years of MEOS are now reviewed and highlighted. Since its discovery and as outlined in a plethora of studies, significant insight was gained regarding the fascinating nature of MEOS: (i) MEOS is distinct from alcohol dehydrogenase and catalase, representing a multienzyme complex with cytochrome P450 (CYP) and its preferred isoenzyme CYP 2E1, NADPH-cytochrome P450 reductase, and phospholipids; (ii) it plays a significant role in alcohol metabolism at high alcohol concentrations and after induction due to prolonged alcohol use; (iii) hydroxyl radicals and superoxide radicals promote microsomal EtOH oxidation, assisted by phospholipid peroxides; (iv) new aspects focus on microsomal oxidative stress through generation of reactive oxygen species (ROS), with intermediates such as hydroxyethyl radical, ethoxy radical, acetyl radical, singlet radical, hydroxyl radical, alkoxyl radical, and peroxyl radical; (v) triggered by CYP 2E1, ROS are involved in the initiation and perpetuation of alcoholic liver injury, consequently shifting the previous nutrition-based concept to a clear molecular-based disease; (vi) intestinal CYP 2E1 induction and ROS are involved in endotoxemia, leaky gut, and intestinal microbiome modifications, together with hepatic CYP 2E1 and liver injury; (vii) circulating blood CYP 2E1 exosomes may be of diagnostic value; (viii) circadian rhythms provide high MEOS activities associated with significant alcohol metabolism and potential toxicity risks as a largely neglected topic; and (ix) a variety of genetic animal models are useful and have been applied elucidating mechanistic aspects of MEOS. In essence, MEOS along with its CYP 2E1 component currently explains several mechanistic steps leading to alcoholic liver injury and has a promising future in alcohol research.

The Bidirectional Effect of Defective ALDH2 Polymorphism and Disease Prevention

Despite the role of aldehyde dehydrogenase 2 (ALDH2) in the detoxification of endogenous aldehydes, the defective polymorphism (rs671), which is highly prevalent among East Asians, does not show a serious phenotype, such as congenital abnormality. However, unfavorable and favorable impacts of the variant allele, ALDH2*2, on various disease risks have been reported. The underlying mechanisms are often complicated due to the compensatory aldehyde detoxification systems. As the phenotypes emerge due to overlapping environmental factors (e.g., alcohol intake and tobacco smoke) or individual vulnerabilities (e.g., aging and apolipoprotein E ε4 allele), polymorphism is therefore considered to be important in the field of preventative medicine. For example, it is important to recognize that ALDH2*2 carriers are at a high risk of alcohol drinking-related cancers; however, their drinking habit has less adverse effects on physiological indices, such as blood pressure, body mass index, levels of lipids, and hepatic deviation enzymes in the blood, than in non-ALDH2*2 carriers. Therefore, opportunities to reconsider their excessive drinking habit before adverse events occur can be missed. To perform effective disease prevention, the effects of ALDH2*2 on various diseases and the biological mechanisms should be clarified.

Alcoholic Liver Disease: Alcohol Metabolism, Cascade of Molecular Mechanisms, Cellular Targets, and Clinical Aspects

Alcoholic liver disease is the result of cascade events, which clinically first lead to alcoholic fatty liver, and then mostly via alcoholic steatohepatitis or alcoholic hepatitis potentially to cirrhosis and hepatocellular carcinoma. Pathogenetic events are linked to the metabolism of ethanol and acetaldehyde as its first oxidation product generated via hepatic alcohol dehydrogenase (ADH) and the microsomal ethanol-oxidizing system (MEOS), which depends on cytochrome P450 2E1 (CYP 2E1), and is inducible by chronic alcohol use. MEOS induction accelerates the metabolism of ethanol to acetaldehyde that facilitates organ injury including the liver, and it produces via CYP 2E1 many reactive oxygen species (ROS) such as ethoxy radical, hydroxyethyl radical, acetyl radical, singlet radical, superoxide radical, hydrogen peroxide, hydroxyl radical, alkoxyl radical, and peroxyl radical. These attack hepatocytes, Kupffer cells, stellate cells, and liver sinusoidal endothelial cells, and their signaling mediators such as interleukins, interferons, and growth factors, help to initiate liver injury including fibrosis and cirrhosis in susceptible individuals with specific risk factors. Through CYP 2E1-dependent ROS, more evidence is emerging that alcohol generates lipid peroxides and modifies the intestinal microbiome, thereby stimulating actions of endotoxins produced by intestinal bacteria; lipid peroxides and endotoxins are potential causes that are involved in alcoholic liver injury. Alcohol modifies SIRT1 (Sirtuin-1; derived from Silent mating type Information Regulation) and SIRT2, and most importantly, the innate and adapted immune systems, which may explain the individual differences of injury susceptibility. Metabolic pathways are also influenced by circadian rhythms, specific conditions known from living organisms including plants. Open for discussion is a 5-hit working hypothesis, attempting to define key elements involved in injury progression. In essence, although abundant biochemical mechanisms are proposed for the initiation and perpetuation of liver injury, patients with an alcohol problem benefit from permanent alcohol abstinence alone.

[Importance of an Aldehyde Dehydrogenase 2 Polymorphism in Preventive Medicine]

Unlike genetic alterations in other aldehyde dehydrogenase (ALDH) isozymes, a defective ALDH2 polymorphism (rs671), which is carried by almost half of East Asians, does not show a clear phenotype such as a shortened life span. However, impacts of a defective ALDH2 allele, ALDH2*2, on various disease risks have been reported. As ALDH2 is responsible for the detoxification of endogenous aldehydes, a negative effect of this polymorphism is predicted, but bidirectional effects have been actually observed and the mechanisms underlying such influences are often complex. One reason for this complexity may be the existence of compensatory aldehyde detoxification systems and the secondary effects of these systems. There are many issues to be addressed with regard to the ALDH2 polymorphism in the field of preventive medicine, including the following concerns. First, ALDH2 in the fetal stage plays a role in aldehyde detoxification; therefore, prenatal health effects of environmental aldehyde exposure are of concern for ALDH2*2-carrying fetuses. Second, ALDH2*2 carriers are at high risk of drinking-related cancers. However, their drinking habits result in less worsening of physiological findings, such as energy metabolism index and liver functions, compared with non-ALDH2*2 carriers, and therefore opportunities to detect excessive drinking can be lost. Third, personalized medicine such as personalized prescriptions for ALDH2*2 carriers will be required in the clinical setting, and accumulation of evidence is awaited. Lastly, since the ALDH2 polymorphism is not considered in workers' limits of exposure to aldehydes and their precursors, efforts to lower exposure levels beyond legal standards are required.

Roles of Cytochrome P450 in Metabolism of Ethanol and Carcinogens

Cytochrome P450 (P450) enzymes are involved in the metabolism of carcinogens, as well as drugs, steroids, vitamins, and other classes of chemicals. P450s also oxidize ethanol, in particular P450 2E1. P450 2E1 oxidizes ethanol to acetaldehyde and then to acetic acid, roles also played by alcohol and aldehyde dehydrogenases. The role of P450 2E1 in cancer is complex in that P450 2E1 is also induced by ethanol, P450 2E1 is involved in the bioactivation and detoxication of a number of chemical carcinogens, and ethanol is an inhibitor of P450 2E1. Contrary to some literature, P450 2E1 expression and induction itself does not cause global oxidative stress in vivo, as demonstrated in studies using isoniazid treatment and gene deletion studies with rats and mice. However, a major fraction of P450 2E1 is localized in liver mitochondria instead of the endoplasmic reticulum, and studies with site-directed rat P450 2E1 mutants and natural human P450 2E1 N-terminal variants have shown that P450 2E1 localized in mitochondria is catalytically active and more proficient in producing reactive oxygen species and damage. The role of the mitochondrial oxidative stress in ethanol toxicity is still under investigation, as is the mechanism of altered electron transport to P450s that localize inside mitochondria instead of their typical endoplasmic reticulum environment.

Pharmaceutical Agents Known to Produce Disulfiram-Like Reaction: Effects on Hepatic Ethanol Metabolism and Brain Monoamines

Several pharmaceutical agents produce ethanol intolerance, which is often depicted as disulfiram-like reaction. As in the case with disulfiram, the underlying mechanism is believed to be the accumulation of acetaldehyde in the blood, due to inhibition of the hepatic aldehyde dehydrogenases. In the present study, chloramphenicol, furazolidone, metronidazole, and quinacrine, which are reported to produce a disulfiram-like reaction, as well as disulfiram, were administered to Wistar rats and the hepatic activities of alcohol and aldehyde dehydrogenases (1A1 and 2) were determined. The expression of aldehyde dehydrogenase 2 was further assessed by Western blot analysis, while the levels of brain monoamines were also analyzed. Finally, blood acetaldehyde was evaluated after ethanol administration in rats pretreated with disulfiram, chloramphenicol, or quinacrine. The activity of aldehyde dehydrogenase 2 was inhibited by disulfiram, chloramphenicol, and furazolidone, but not by metronidazole or quinacrine. In addition, although well known for metronidazole, quinacrine also did not increase blood acetaldehyde after ethanol administration. The protein expression of aldehyde dehydrogenase 2 was not affected at all. Interestingly, all substances used, except disulfiram, increased the levels of brain serotonin. According to our findings, metronidazole and quinacrine do not produce a typical disulfiram-like reaction, because they do not inhibit hepatic aldehyde dehydrogenase nor increase blood acetaldehyde. Moreover, all tested agents share the common property to enhance brain serotonin, whereas a respective effect of ethanol is well established. Therefore, the ethanol intolerance produced by these agents, either aldehyde dehydrogenase is inhibited or not, could be the result of a “toxic serotonin syndrome,” as in the case of the concomitant use of serotonin-active medications.

Heme oxygenase 1 protects ethanol-administered liver tissue in Aldh2 knockout mice

A genetic polymorphism of the aldehyde dehydrogenase 2 (​ALDH2) gene, ALDH2*2, encodes an enzymatically defective ALDH2 protein. Recent epidemiological studies suggest that possessing ALDH2*2 is a protective factor for liver tissue in healthy individuals, although these studies lack a mechanistic explanation. Our animal studies have shown the same trend: levels of serum alanine transaminase (ALT), hepatic malondialdehyde (MDA), and hepatic tumor necrosis factor alpha (TNF-α) were lower in Aldh2 knockout (Aldh2(-/-)) mice than in wild-type (Aldh2(+/+)) mice after ethanol administration. To propose a mechanistic hypothesis, residual liver specimens from the previous experiment were analyzed. An anti-oxidative protein, heme oxygenase 1 (HO-1), and an oxidative stress-producing protein, cytochrome P450 2E1 (CYP2E1), were detected at higher levels in Aldh2(-/-) mice than in Aldh2(+/+) mice, regardless of ethanol treatment. Other oxidative stress-related proteins and inflammatory cytokines did not show such a significant difference. To conclude, we propose a protective role of HO-1 in individuals with A​LDH2*2. Our continued studies support the epidemiological finding that possession of ALDH2*2 is a protective factor in the liver of the healthy individual.

Molecular basis of alcoholic fatty liver disease: From incidence to treatment

Alcoholic liver diseases have complex and multiple pathogenic mechanisms but still no effective treatment. Steatosis or alcoholic fatty liver disease (AFLD) has a widespread incidence and is the first step in the progression to more severe stages of alcoholic liver disease, with concomitant increases in morbidity and mortality rates. The ways in which this progression occurs and why some individuals are susceptible are still unanswered scientific questions. Research with animal models and clinical evidence have shown that it is a multifactorial disease that involves interactions between lipid metabolism, inflammation, the immune response and oxidative stress. Each of these pathways provides a better understanding of the pathogenesis of AFLD and contributes to the development of therapeutic strategies. This review emphasizes the importance of research on alcoholic steatosis based on incidence data, key pathogenic mechanisms and therapeutic interventions, and discusses perspectives on the progression of this disease.

Dietary diallyl disulfide supplementation attenuates ethanol-mediated pulmonary vitamin D speciate depletion in C57Bl/6 mice

Background

Slightly more than 5 % of the United States population heavily consumes ethanol, i.e., more than 14 drinks for men and 7 drinks for women a week. Chronic ethanol consumption can result in increased liver disease, reduced recovery from burn injury, and more frequent and severe respiratory infections. Chronic ethanol over-consumption also leads to vitamin D dysmetabolism and depletion. Vitamin D is a fat-soluble pro-hormone that regulates musculoskeletal health, cellular proliferation/differentiation, and innate and adaptive immune response.

Methods

In this study, C57BL/6 mice were fed 20 % ethanol in their water ad libitum for 7 weeks. Some mice were fed either a standard chow or a modified diet containing 0.15 μg/day of diallyl disulfide (DADS). Whole blood, lung tissue, and bronchial alveolar lavage fluid (BALF) were collected at sacrifice and analyzed for 25(OH) D₃, 1,25 (OH)₂D₃, vitamin D receptor VDR, CYP2E1, and CYP27B1 levels.

Results

Ethanol reduced 25(OH) D₃ and 1,25 (OH)₂D₃ in lung tissue and BALF on average 31 %. The largest ethanol-mediated reduction was in the 1,25 (OH)₂D₃ (42 %) measured in the BALF. Dietary supplementation of DADS restored BALF and lung tissue protein of 25(OH) D₃ and 1,25(OH)₂D₃ to control levels. Chronic ethanol consumption also resulted in tissue increases of vitamin D response (VDR) protein, Cyp2E1, and reductions in vitamin D-activating enzyme CYP27B1. All three of these effects were attenuated by dietary supplementation of DADS.

Conclusions

In conclusion, the pulmonary metabolic disturbances mediated by chronic ethanol consumption as measured by 1,25(OH)₂D₃ protein levels, epithelial lining fluid, and lung tissue can be ameliorated by dietary supplementation of DADS in C57BL/6 mice.

Genetic polymorphisms of ADH1B, ADH1C and ALDH2 in Turkish alcoholics: lack of association with alcoholism and alcoholic cirrhosis

No data exists regarding the alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) gene polymorphisms in Turkish alcoholic cirrhotics. We studied the polymorphisms of ADH1B, ADH1C and ALDH2 genes in alcoholic cirrhotics and compared the results with non-cirrhotic alcoholics and healthy volunteers. Overall, 237 subjects were included for the study: 156 alcoholic patients (78 cirrhotics, 78 non-cirrhotic alcoholics) and 81 healthy volunteers. Three different single-nucleotide-polymorphism genotyping methods were used. ADH1C genotyping was performed using a polymerase chain reaction-restriction fragment length polymorphism method. The identified ADH1C genotypes were named according to the presence or absence of the enzyme restriction sites. ADH1B (Arg47Hys) genotyping was performed using the allele specific primer extension method, and ALDH2 (Glu487Lys) genotyping was performed by a multiplex polymerase chain reaction using two allele-specific primer pairs. For ADH1B, the frequency of allele *1 in the cirrhotics, non-cirrhotic alcoholics and healthy volunteers was 97.4%, 94.9% and 99.4%, respectively. For ADH1C, the frequency of allele *1 in the cirrhotics, non-cirrhotic alcoholics and healthy volunteers was 47%, 36.3% and 45%, respectively. There was no statistical difference between the groups for ADH1B and ADH1C (p>0.05). All alcoholic and non-alcoholic subjects (100%) had the allele *1 for ALDH2. The obtained results for ADH1B, ADH1C, and ALDH gene polymorphisms in the present study are similar to the results of Caucasian studies. ADH1B and ADH1C genetic variations are not related to the development of alcoholism or susceptibility to alcoholic cirrhosis. ALDH2 gene has no genetic variation in the Turkish population.

Does alcohol increase breast cancer risk in African-American women? Findings from a case-control study

Background:

Alcohol is an important risk factor for breast cancer in Caucasian women, but the evidence in African-American (AA) women is limited and results are inconclusive.

Methods:

Associations between recent and lifetime drinking and breast cancer risk were evaluated in a large sample of AA women from a case–control study in New York and New Jersey. Multivariable logistic regression models provided odds ratios (ORs) and 95% confidence intervals (CIs).

Results:

There was no association between recent drinking and breast cancer risk, even when stratified by menopausal status or by hormone receptor status. A borderline decreased risk with increased lifetime consumption was found (OR=0.77; 95% CI: 0.58–1.03), which was stronger among women who drank when under 20 years of age (OR=0.65; 95% CI: 0.47–0.89), regardless of menopausal or hormone receptor status.

Conclusion:

Breast cancer risk associated with recent alcohol consumption was not apparent in AA women, while early age drinking seemed to decrease risk. This is the first investigation on recent and lifetime drinking in subgroups and drinking during different age periods in AA women. If findings are replicated, racial differences in biological pathways involving alcohol and its metabolites should be explored.

Cytochrome P450 2E1: its clinical aspects and a brief perspective on the current research scenario

Research on Cytochrome P450 2E1 (CYP2E1), a key enzyme in alcohol metabolism has been very well documented in literature. Besides the involvement of CYP2E1 in alcohol metabolism as illustrated through the studies discussed in the chapter, recent studies have thrown light on several other aspects of CYP2E1 i.e. its extrahepatic expression, its involvement in several diseases and pathophysiological conditions; and CYP2E1 mediated carcinogenesis and modulation of drug efficacy. Studies involving these interesting facets of CYP2E1 have been discussed in the chapter focusing on the recent observations or ongoing studies illustrating the crucial role of CYP2E1 in disease development and drug metabolism.

Multi-step oxidations catalyzed by cytochrome P450 enzymes: Processive vs. distributive kinetics and the issue of carbonyl oxidation in chemical mechanisms

Catalysis of sequential oxidation reactions is not unusual in cytochrome P450 (P450) reactions, not only in steroid metabolism but also with many xenobiotics. One issue is how processive/distributive these reactions are, i.e., how much do the "intermediate" products dissociate. Our work with human P450s 2E1, 2A6, and 19A1 on this subject has revealed a mixture of systems, surprisingly with a more distributive mechanism with an endogenous substrate (P450 19A1) than for some xenobiotics (P450s 2E1, 2A6). One aspect of this research involves carbonyl intermediates, and the choice of catalytic mechanism is linked to the hydration state of the aldehyde. The non-enzymatic rates of hydration and dehydration of carbonyls are not rapid and whether P450s catalyze the reversible hydration is unknown. If carbonyl hydration and dehydration are slow, the mechanism may be set by the carbonyl hydration status.

Distribution of ADH1B, ALDH2, CYP2E1 *6, and CYP2E1 *7B genotypes in Turkish population

The most well-known metabolic pathways from ethanol to acetaldehyde include alcohol dehydrogenase (ADH) and the microsomal ethanol oxidizing system that involves cytochrome P450 2E1 (CYP2E1). Acetaldehyde is further oxidized to acetate by aldehyde dehydrogenase (ALDH). The genetic variation of ADH1B, ALDH2, and CYP2E1 is different among racial populations and cause difference in elimination rates of alcohol. The aim of this study was to determine the polymorphisms of ADH1B (rs1229984; Arg47His), ALDH2 (rs671; Glu487Lys), CYP2E1*6 (rs6413432; T7632A), and CYP2E1*7B (rs6413420; G-71T) in unrelated healthy Turkish population and compare it with other populations. ADH1B and ALDH2 polymorphisms were analyzed with an allele-specific polymerase chain reaction (PCR) assay, and CYP2E1*6 and CYP2E1*7B polymorphisms were genotyped by PCR-restriction fragment length polymorphism method. ADH1B polymorphism analysis yielded the genotype distribution as 83.9% ADH1B*1/1 and 16.1% ADH1B*1/2, and no individuals with ALDH2*1/2 and ALDH2*2/2 genotypes were found in Turkish population. The genotype frequencies for CYP2E1*6 polymorphism were found as 85.3% for homozygote common, 14.1% for heterozygote, and 0.6% for homozygote uncommon. For CYP2E1*7B polymorphism, the genotype frequencies were determined to be 86.5% G/G, 13.5% for G/T; however, no individuals with homozygote uncommon genotype were detected. According to our study results, the genotype distributions of ADH1B, ALDH2, CYP2E1*6, and CYP2E1*7B in Turkish population were similar compared with Caucasian and some European populations, whereas differed significantly from East Asian populations. This study may be useful in epidemiological studies of the influence of ADH1B, ALDH2, CYP2E1*6, and CYP2E1*7B polymorphisms on diseases, including several types of cancer related to alcohol consumption and alcohol dependence.

Oxidation of N-Nitrosoalkylamines by human cytochrome P450 2A6: sequential oxidation to aldehydes and carboxylic acids and analysis of reaction steps

Cytochrome P450 (P450) 2A6 activates nitrosamines, including N,N-dimethylnitrosamine (DMN) and N,N-diethylnitrosamine (DEN), to alkyl diazohydroxides (which are DNA-alkylating agents) and also aldehydes (HCHO from DMN and CH(3)CHO from DEN). The N-dealkylation of DMN had a high intrinsic kinetic deuterium isotope effect ((D)k(app) approximately 10), which was highly expressed in a variety of competitive and non-competitive experiments. The (D)k(app) for DEN was approximately 3 and not expressed in non-competitive experiments. DMN and DEN were also oxidized to HCO(2)H and CH(3)CO(2)H, respectively. In neither case was a lag observed, which was unexpected considering the k(cat) and K(m) parameters measured for oxidation of DMN and DEN to the aldehydes and for oxidation of the aldehydes to the carboxylic acids. Spectral analysis did not indicate strong affinity of the aldehydes for P450 2A6, but pulse-chase experiments showed only limited exchange with added (unlabeled) aldehydes in the oxidations of DMN and DEN to carboxylic acids. Substoichiometric kinetic bursts were observed in the pre-steady-state oxidations of DMN and DEN to aldehydes. A minimal kinetic model was developed that was consistent with all of the observed phenomena and involves a conformational change of P450 2A6 following substrate binding, equilibrium of the P450-substrate complex with a non-productive form, and oxidation of the aldehydes to carboxylic acids in a process that avoids relaxation of the conformation following the first oxidation (i.e. of DMN or DEN to an aldehyde).

Role of oxidative stress in alcohol-induced liver injury

Reactive oxygen species (ROS) are highly reactive molecules that are naturally generated in small amounts during the body's metabolic reactions and can react with and damage complex cellular molecules such as lipids, proteins, or DNA. Acute and chronic ethanol treatments increase the production of ROS, lower cellular antioxidant levels, and enhance oxidative stress in many tissues, especially the liver. Ethanol-induced oxidative stress plays a major role in the mechanisms by which ethanol produces liver injury. Many pathways play a key role in how ethanol induces oxidative stress. This review summarizes some of the leading pathways and discusses the evidence for their contribution to alcohol-induced liver injury. Special emphasis is placed on CYP2E1, which is induced by alcohol and is reactive in metabolizing and activating many hepatotoxins, including ethanol, to reactive products, and in generating ROS.

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.

CYP2E1 and oxidative liver injury by alcohol

Ethanol-induced oxidative stress seems to play a major role in mechanisms by which ethanol causes liver injury. Many pathways have been suggested to contribute to the ability of ethanol to induce a state of oxidative stress. One central pathway seems to be the induction of cytochrome P450 2E1 (CYP2E1) by ethanol. CYP2E1 metabolizes and activates many toxicological substrates, including ethanol, to more reactive, toxic products. Levels of CYP2E1 are elevated under a variety of physiological and pathophysiological conditions and after acute and chronic alcohol treatment. CYP2E1 is also an effective generator of reactive oxygen species such as the superoxide anion radical and hydrogen peroxide and, in the presence of iron catalysts, produces powerful oxidants such as the hydroxyl radical. This review article summarizes some of the biochemical and toxicological properties of CYP2E1 and briefly describes the use of cell lines developed to constitutively express CYP2E1 and CYP2E1 knockout mice in assessing the actions of CYP2E1. Possible therapeutic implications for treatment of alcoholic liver injury by inhibition of CYP2E1 or CYP2E1-dependent oxidative stress will be discussed, followed by some future directions which may help us to understand the actions of CYP2E1 and its role in alcoholic liver injury.

The in vitro metabolism of a pyrethroid insecticide, permethrin, and its hydrolysis products in rats

The in vitro metabolism of permethrin and its hydrolysis products in rats was investigated. Cis- and trans-permethrin were mainly hydrolyzed by liver microsomes, and also by small-intestinal microsomes of rats. trans-Permethrin was much more effectively hydrolyzed than the cis-isomer. When NADPH was added to the incubation mixture of the liver microsomes, three metabolites, 3-phenoxybenzyl alcohol (PBAlc), 3-phenoxybenzaldehyde (PBAld) and 3-phenoxybenzoic acid (PBAcid), were formed. However, only PBAlc was formed by rat liver microsomes in the absence of cofactors. The microsomal activities of rat liver and small intestine were inhibited by bis-p-nitrophenyl phosphate, an inhibitor of carboxylesterase (CES). ES-3 and ES-10, isoforms of the CES 1 family, exhibited significant hydrolytic activities toward trans-permethrin. When PBAlc was incubated with rat liver microsomes in the presence of NADPH, PBAld and PBAcid were formed. The NADPH-linked oxidizing activity was inhibited by SKF 525-A. Rat recombinant cytochrome P450, CYP 2C6 and 3A1, exhibited significant oxidase activities with NADPH. When PBAld was incubated with the microsomes in the presence of NADPH, PBAcid was formed. CYP 1A2, 2B1, 2C6, 2D1 and 3A1 exhibited significant oxidase activities in this reaction. Thus, permethrin was hydrolyzed by CES, and PBAlc formed was oxidized to PBAld and PBAcid by the cytochrome P450 system in rats.

Alcohol use, vascular disease, and lipid-lowering drugs

Many epidemiological and clinical studies have shown that light-to-moderate alcohol (Alc) consumption is associated with reduced risk of coronary heart disease (CHD) and total mortality in middle-aged and elderly men and women. The plausible mechanisms for the putative cardioprotective effects include increased levels of high-density lipoprotein cholesterol, prevention of clot formation, reduced platelet aggregation, promotion of blood clot dissolution, and lowering of plasma lipoprotein (a) concentration. Individuals who need to be treated with lipid-lowering drugs, such as dyslipidemic or CHD patients, may benefit from these effects of Alc. Because hypolipidemic treatment is usually continued for life, an important issue is the suitability of Alc consumption in these patients. In the present review, the beneficial effects of Alc consumption on CHD risk, its side effects, and its safety and suitability when coadministered with hypolipidemic drugs are discussed.

Theoretical Study of the Mechanism of Acetaldehyde Hydroxylation by Compound I of CYP2E1

Recent experimental studies revealed that cytochrome P450 2E1 (CYP2E1) could metabolize not only ethanol but also its primary product, acetaldehyde, accompanying the well-known acetaldehyde dehydrogenases (ALDH) in the metabolism of acetaldehyde. Mechanistic aspects of acetaldehyde hydroxylation by Compound I model active species of CYP2E1 were investigated by means of B3LYP DFT calculations in the present paper. Our study results demonstrate that acetaldehyde hydroxylation by CYP2E1 is in accord with the effectively concerted mechanisms both on the high quartet spin state (HS) and on the low doublet spin state (LS). The rate-limiting step is H-abstraction, and the activation energy is about 11.7 approximately 14.0 kcal/mol on the quartet (doublet) reaction route, which is about one-half to one-third of that needed by methane hydroxylation. The phenomenon that the HS and LS reaction routes are both effectively concerted was shown for the first time to occur in trans-2-phenyl-iso-propylcyclopropane hydroxylation by Kumar et al. (see Figure 7 in the paper of Kumar, D.; de Visser, S. P.; Sharma, P. K.; Cohen, S.; Shaik, S. J. Am. Chem. Soc. 2004, 126, 1907) and was confirmed in our work of acetaldehyde hydroxylation by cytochrome P450. Theoretical exploration of the HS O-rebound barrier degradation is also presented in the present paper on the basis of Shaik's valence bond (VB) model.

Major Cytochrome P450 Enzymes Responsible for Microsomal Aldehyde Oxygenation of 11-Oxo-Δ8-tetrahydrocannabinol and 9-Anthraldehyde in Human Liver

Hepatic microsomes from human liver catalyzed oxidation of the allyl aldehydes such as 11-oxo-Delta(8)-tetrahydrocannabinol and 9-anthraldehyde to the corresponding carboxylic acid metabolites. The oxygenation mechanism was confirmed by GC-MS that molecular oxygen was exclusively incorporated into Delta(8)-tetrahydrocannabinol-11-oic acid and 9-anthracene carboxylic acid formed under oxygen-18 gas. The microsomal aldehyde oxygenase (named MALDO) activities of 11-oxo-Delta(8)-tetrahydrocannabinol and 9-anthraldehyde were significantly inhibited by the antibody against CYP2C and CYP3A, respectively. MALDO activity for 11-oxo-Delta(8)-tetrahydrocannabinol was significantly inhibited by sulfaphenazole whereas that for 9-anthraldehyde was markedly inhibited by troleandomycin, but not by sulfaphenazole. CYP2C9 expressed in human B-lymphoblastoid cells catalyzed efficiently the MALDO activity for 11-oxo-Delta(8)-tetrahydrocannabinol (10.1 nmol/min/nmol P450), while the catalytic activities of other human CYPs expressed in the cells were lesser extents. In MALDO activity for 9-anthraldehyde, CYP3A4 expressed in the cells had the highest catalytic activity (7.72 nmol/min/nmol P450). These results indicate that CYP2C9 and CYP3A4 are major enzymes responsible for the MALDO activity in human liver for 11-oxo-Delta(8)-tetrahydrocannabinol and 9-anthraldehyde, respectively.

Effect of ALDH2 and CYP2E1 gene polymorphisms on drinking behavior and alcoholic liver disease in Japanese male workers

AIMS

We examined the relationships of ALDH2 and CYP2E1 genotypes on drinking behavior and the incidence of alcoholic liver disease in Japanese male workers.

METHODS

Two hundred and eighty-seven Japanese men were selected from one metal company to adjust for similar economic and social backgrounds. Drinking behavior was assessed from a self-assessment questionnaire. Genotypes of ALDH2 and CYP2E1 were analyzed with the polymerase chain reaction-single strand conformation polymorphism and with the polymerase chain reaction-restriction fragment length polymorphism, respectively.

RESULTS

The frequency of the ALDH2 genotype was 55% for typical homozygotes, 42% for heterozygotes, and 4% for atypical homozygotes. The frequency of the CYP2E1 genotype was 62% for c1 homozygotes, 35% for heterozygotes, and 3% for c2 homozygotes. The ALDH2 genotype closely influenced drinking habits, but not the CYP2E1 genotype. Among habitual drinkers, ALDH2 typical homozygotes consumed significantly larger amounts of ethanol than ALDH2 heterozygotes, whereas CYP2E1 genotypes did not influence daily alcohol consumption. Sixteen men (5.6%) were diagnosed with alcoholic liver disease. In terms of ALDH2 genotypes, 12 cases (7.6%) were typical homozygotes and 4 (3.4%) were heterozygotes, whereas the incidence of alcoholic liver disease was not different between c1/c1 homozygotes and c1/c2 heterozygotes. When the interactive contribution of the ALDH2 and CYP2E1 genotypes on drinking behavior and the incidence of alcoholic liver disease were examined, there were no significant differences in the CYP2E1 genotype among the subjects with the same ALDH2 genotype.

CONCLUSION

The ALDH2 genotype is strongly associated with individual alcohol drinking behavior and the development of alcoholic liver disease in Japanese male workers, but the CYP2E1 genotype is not.

The effects of the ALDH2*1/2, CYP2E1 C1/C2 and C/D genotypes on blood ethanol elimination

The effects of CYP2E1 genotypes on the blood ethanol and acetaldehyde levels were investigated in a pair of Japanese volunteers whose ADH2, ADH3 and ALDH2 genotypes were identical but whose CYP2E1 genotypes were different. In the same way, the effects of ALDH2 and ADH2 on the ethanol elimination kinetics were also studied. The predicting 95% confidence bounds determined on regression analysis of the data suggested that after venous injection of ethanol, the blood ethanol and acetaldehyde concentrations in a volunteer normal homozygous for ALDH2 (ALDH2*1/1) were lower than in a heterozygous one (ALDH2*1/2). Also, the blood ethanol and acetaldehyde concentrations in a volunteer with the c2 and C alleles of CYP2E1 (c1/c2 and C/D) were lower than in one without the c2 and C alleles (c1/c1 and D/D). However, there were no significant differences in the blood ethanol and acetaldehyde concentrations between volunteers with ADH2*1 (ADH2*1/1) and without ADH2*1 (ADH2*1/2).

Kinetics of cytochrome P450 2E1-catalyzed oxidation of ethanol to acetic acid via acetaldehyde

The P450 2E1-catalyzed oxidation of ethanol to acetaldehyde is characterized by a kinetic deuterium isotope effect that increases K(m) with no effect on k(cat), and rate-limiting product release has been proposed to account for the lack of an isotope effect on k(cat) (Bell, L. C., and Guengerich, F. P. (1997) J. Biol. Chem. 272, 29643-29651). Acetaldehyde is also a substrate for P450 2E1 oxidation to acetic acid, and k(cat)/K(m) for this reaction is at least 1 order of magnitude greater than that for ethanol oxidation to acetaldehyde. Acetic acid accounts for 90% of the products generated from ethanol in a 10-min reaction, and the contribution of this second oxidation has been overlooked in many previous studies. The noncompetitive intermolecular kinetic hydrogen isotope effects on acetaldehyde oxidation to acetic acid ((H)(k(cat)/K(m))/(D)(k(cat)/K(m)) = 4.5, and (D)k(cat) = 1.5) are comparable with the isotope effects typically observed for ethanol oxidation to acetaldehyde, and k(cat) is similar for both reactions, suggesting a possible common catalytic mechanism. Rapid quench kinetic experiments indicate that acetic acid is formed rapidly from added acetaldehyde (approximately 450 min(-1)) with burst kinetics. Pulse-chase experiments reveal that, at a subsaturating concentration of ethanol, approximately 90% of the acetaldehyde intermediate is directly converted to acetic acid without dissociation from the enzyme active site. Competition experiments suggest that P450 2E1 binds acetic acid and acetaldehyde with relatively high K(d) values, which preclude simple tight binding as an explanation for rate-limiting product release. The existence of a rate-determining step between product formation and release is postulated. Also proposed is a conformational change in P450 2E1 occurring during the course of oxidation and the discrimination of P450 2E1 between acetaldehyde and its hydrated form, the gem-diol. This multistep P450 reaction is characterized by kinetic control of individual reaction steps and by loose binding of all ligands.

Ethanol metabolism, toxicity and genetic polymorphism

The relationships between the individual (and racial) differences in alcohol metabolism and toxicity, and the genetic polymorphism of alcohol dehydrogenase (ADH), aldehyde dehydrogenase (ALDH), and cytochrome P-4502E1(CYPIIE1) were reviewed. In recent studies involving DNA analysis, it was found that a deficiency of the ALDH2 isozyme (ALDH2*2) was responsible for the flushing symptoms as well as other vasomotor symptoms caused by a higher acetaldehyde level after alcohol consumption. Deficiency of ALDH2 activity has been found prevalently only among people of Mongoloid origin, and the deficiency of ALDH2 prevents them from developing alcohol dependence due to the unpleasant physical effects of the flushing symptom. It was reported that Mongoloids such as Japanese and Chinese people carry the enzymatically active (ALDH2*1) subunit and/or the inactive (ALDH2*2) one, and that a low proportion of ALDH2 deficiency (ALDH2*2 allele frequency) was found in alcoholics compared with healthy controls. It was also reported that polymorphism of ALDH2 and/or CYP2E1 may be associated with the susceptibility to alcohol-induced liver injury. Concerning blood ethanol elimination kinetics, it was reported that the c2 gene of CYP2E1 and the ALDH2*1 gene may have greater effects on ethanol and acetaldehyde elimination than the other genotypes, when the blood ethanol level is below 20 m M.

Oxidation kinetics of ethanol by human cytochrome P450 2E1. Rate-limiting product release accounts for effects of isotopic hydrogen substitution and cytochrome b5 on steady-state kinetics

A number of cytochrome P450 (P450) 2E1 substrates are known to show kinetic deuterium isotope effects of approximately 5 on Km (DK = DKm/HKm), but not on kcat, in rat liver microsomes (e.g. N-nitrosodimethylamine, ethanol, and CH2Cl2). We observed DKm values of 3-5 for recombinant human P450 2E1-catalyzed ethanol oxidation. Replacing NADPH and O2 with the oxygen surrogate cumene hydroperoxide yielded similar results. Ferric P450 2E1 reduction was fast (k >1000 min-1) even in the absence of substrate. These results indicate that the basis for the increase in Km is in the latter portion of the catalytic cycle. The intrinsic isotope effect (Dk) for ethanol oxidation was determined (competitively) to be 3.8, indicating that C-H bond cleavage is isotopically sensitive. Pre-steady-state studies showed a burst of product formation (k = 410 min-1), with the burst amplitude corresponding to the P450 concentration. Deuteration of ethanol resulted in an isotope effect of 3.2 on the rate of the burst. We conclude that product release is rate-limiting in the oxidation of ethanol to acetaldehyde by P450 2E1. The steady-state kinetics can be described by a paradigm in which the kcat approximates the rate of product release, and Km is an expression in which the denominator is dominated by the rate of C-H bond breaking.

Metabolism of acetaldehyde to acetate by rat hepatic P-450s: presence of different metabolic pathway from acetaldehyde dehydrogenase system

NADPH-dependent activity of acetaldehyde oxidation was investigated in microsomes by assaying [14C]acetic acid produced from [14C]acetaldehyde with ion-exchange column. Rat hepatic microsomes exhibited acetaldehyde oxidation activity in the presence of NADPH. This activity was induced 2-fold by the treatment of rats with ethanol. We designated this NADPH-dependent oxidation system as microsomal acetaldehyde-oxidizing system (MAOS), to distinguish from the NAD-dependent acetaldehyde oxidation system by acetaldehyde in mitochondria and cytsol. We further investigated essential enzymes contributing to MAOS activity. Acetaldehyde oxidation activity was investigated in eight forms of purified P-450 in a reconstituted system. Cytochrome P-450 (CYP) 2E1 had the highest oxidation activity and CYP1A2 and CYP4A2 had the next highest activity. Other forms had low activity. To assess the contribution of these forms to MAOS activity, immunoblot was done. CYP2E1 was induced 2-fold by ethanol treatment, but CYP1A2 and CYP4A2 were not reflecting the MAOS activity increased by ethanol treatment. These results suggest that CYP2E1 is the essential enzyme in the MAOS of rats.

Effect of the cytochrome P-450IIE1 genotype on ethanol elimination rate in alcoholics and control subjects

We studied an influence of genetic polymorphisms in the cytochrome P-450IIE1 (CYP2E1) gene on ethanol elimination rate in alcoholic patients and healthy subjects. The CYP2E1 genotype was determined by polymerase chain reaction-restriction fragment length polymorphism method for 124 alcoholics and 54 healthy subjects. There was no significant difference in the gene frequency of CYP2E1 between alcoholics and healthy control subjects. Blood ethanol concentrations in the 65 alcoholics on admission ranged from 0.32 to 4.22 mg/ml. In the patients with the c1/c2 genotype, the elimination rate was significantly correlated with blood ethanol concentration. In each of the three genotypes of CYP2E1, the patients were divided into three groups based on ethanol concentrations. The average of the ethanol elimination rate in the patients with c1/c2 having blood ethanol levels of > or = 2.5 mg/ml was significantly higher than the rates in the two other groups of c1/c2. When blood ethanol levels were > or = 2.5 mg/ml, the elimination rate in the patients with c1/c2 was significantly higher than that in those with c1/c1. Regardless of the CYP2E1 genotype, the elimination rate in the alcoholics was higher than that in the control subjects when blood ethanol levels were < 1.0 mg/ml. These results suggest the possibility that the c2 allele of CYP2E1 Influences the rate of ethanol elimination at high ethanol levels. The rate of ethanol elimination was independent of liver disorder judged by serum total bilirubin values.

Polymorphisms in alcohol metabolizing enzyme genes and alcoholic cirrhosis in Japanese patients: a multivariate analysis

Alcohol dehydrogenase (ADH), aldehyde dehydrogenase (ALDH), and P450IIE1 are the primary enzymes that catalyze the conversion of ethanol to acetaldehyde and then to acetate. Genetic polymorphisms have been reported in ADH2, ADH3, ALDH2, and the 5'-flanking region of P450IIEI. In this study, we used multivariate analysis to determine which genetic polymorphisms in alcohol metabolizing enzymes were independently associated with the development of alcoholic cirrhosis. Thirty-four noncirrhotic alcoholic patients, including 27 with fatty liver and 7 with nonspecific changes, and 46 patients with alcoholic liver cirrhosis were studied. Restriction fragment length polymorphisms (RFLPs) in the ADH2 and P450IIE1 genes were detected by digestion of polymerase chain reaction (PCR)-amplified DNA with MaeIII and RsaI, respectively. In the ALDH2 gene, RFLPs were detected by differences in the MboII site after PCR amplification. By multivariate analysis of four significant factors including total alcohol intake, ADH, ALDH, and P450IIE1 using the multiple logistic regression model, genotype ADH2(2)/ADH2(2) (P = .029) and genotype c1/c1 of P450IIE1 (P = .013) were found to be independently associated with alcoholic cirrhosis. The odds ratios for ADH2(2)/ADH2(2) genotype and the type A genotype of P450IIE1 (c1/c1) were 4.600 and 4.006, respectively. These results suggest that ADH2 and P450IIE1 gene polymorphisms may be independently associated with the development of alcoholic liver cirrhosis in Japan.

Recent advances in the metabolism of cannabinoids

This review describes recent advances in the metabolism of cannabinoids. Cannabidiol was metabolized to cannabielsoin, 6 beta-hydroxymethyl-delta 9-tetrahydrocannabinol and an oxepine derivative through epoxide intermediates by hepatic microsomal enzymes containing cytochrome P450 of animals. Cannabidiol inactivated cytochrome P450 UT-2 (CYP2C11) not equal to in male rats and a member of 3A subfamily in mouse liver. These inactivations may be very important because serious drug-drug interactions will occur in the case that cannabidiol is co-administered with drugs which are metabolized mainly by the enzyme system containing these P450 isozymes. A member of cytochrome P450 belonging to 2C subfamily was the major isozymes responsible for the cannabinoid metabolism in many experimental animals and that of 3A subfamily made some contribution to the metabolism of cannabinoids by human hepatic microsomes. Microsomal aldehyde oxygenase, a particular isozyme of cytochrome P450 catalyzing the oxidation of 11-oxo-tetrahydrocannabinol to tetrahydrocannabinol-11-oic acid, was found for the first time by the authors. Cytochrome P450 MUT-2 (CYP2C29) is the major isozyme responsible for the microsomal aldehyde oxygenase activity in mouse hepatic microsomes.

Inhibition of CYP2E1 activity does not abolish pulsatile urine alcohol concentrations during chronic alcohol infusions

Continuous, long-term intragastric infusions of ethanol leads to a two-step induction of hepatic cytochrome P450 2E1 (CYP2E1) that is correlated with blood ethanol concentrations (BECs) and urine alcohol concentrations (UECs). In addition, long-term and continuous ethanol infusion does not produce a steady-state BEC, but results in pulsatile BECs and UECs having peak-peak duration of approximately 6 days and ethanol concentrations ranging from near zero to over 500 mg/dl. In the present study, rats were treated with ethanol (levels reaching 13 g.kg-1.day-1) for 38 days in the presence of compounds reported to block CYP2E1 activity or expression, to study the possible involvement of CYP2E1 in the pulsatile BECs. The inhibitors used were chlormethiazole (CMZ); diallysulfide (DAS); phenethyl isothiocyanate (PET) and dihydrocapsacin (DHC). Hepatic microsomal metabolism of carbon tetrachloride and p-nitrophenol, as well as mean CYP2E1 apoprotein levels, were significantly greater (P < or = 0.05) in ethanol-treated rats than in control rats, whereas rats treated with DAS, CMZ or PET had significantly (P < or = 0.05) reduced p-nitrophenol and carbon tetrachloride metabolism and lower CYP2E1 apoprotein levels compared to those of ethanol controls. UECs were variable in all ethanol-treated groups and there was a typical pulsatile pattern that had a mean interpulse interval (the number of days between the peaks of two consecutive pulses) ranging over 5.4 +/- 0.3-6.0 +/- 0.7 days and a mean amplitude (nadir to peak UEC) of 415 +/- 39-337 +/- 33 mg/dl. None of the putative CYP2E1 blockers altered the pulsatile nature of ethanol in urine. Our results suggest that pulsatile UECs are not the result of variations in the amount of CYP2E1.

CYP2E1 and ALDH2 genotypes and alcohol dependence in Japanese

The genotypes of the CYP2E1 and ALDH2 loci of alcoholic (alcohol dependence) and nonalcoholic (healthy) Japanese were investigated to examine the relationship between the polymorphism of CYP2E1 (C1/C2) and ALDH2 (ALDH2*1/ALDH2*2), and the susceptibility to alcoholism. There was no significant difference in C2 gene frequency between alcoholics (0.19) and nonalcoholics (controls) (0.20), whereas there was a significant difference in ALDH2 allele frequency, suggesting that, in Japanese, the C2 genotype of CYP2E1 may have nothing to do with the risk of developing alcohol dependence. However, the ALDH2*1 allele may influence drinking behavior and the development of alcohol dependence. Furthermore, racial interethnic differences in the frequency of the mutated allele of the CYP2E1 gene (C2) were found, like the ALDH2 gene. Japanese healthy controls showed a significantly higher frequency of the C2 allele than did Swedish healthy controls (0.05; reported by Persson et al., FEBS Lett. 319:207-211, 1993).

CYP2E1 genotypes and serum LAP in Japanese alcoholics

The genotypes of the ALDH2 and CYP2E1 loci of Japanese alcoholic patients were determined to investigate the susceptibility to alcoholic liver injury. In alcoholics with a liver-function disorder, a significant association was observed between the genotypes of the CYP2E1 loci and the serum level of a liver-derived enzyme, LAP. However, there was no significant association between the ALDH2 genotypes and liver dysfunction.

Species differences in the biotransformation of ethyl chloride. I. Cytochrome P450-dependent metabolism

Groups of male and female F-344 rats and B6C3F1 mice were exposed to 15,000 ppm ethyl chloride (monochloroethane, ECL) or to air for 5 days (6 h/day). In this report, features of the P450-dependent ECL metabolism in the animals are described. A concurrent report describes the in vitro and in vivo features of the GSH-dependent ECL metabolism (Fedtke et al. 1994). ECL is oxidatively dechlorinated in an NADPH- and O2-dependent reaction, resulting in the formation of acetaldehyde (AC). The oxidative ECL metabolism rates in microsomal incubations were measured. The results indicated induction of the oxidative ECL metabolism by ECL itself in mice and female rats. The hydroxylation of p-nitrophenol, which was used as an indicator of P450IIE1 activity, was also induced in microsomal incubations from ECL-exposed mice and female rats, but, corresponding to the ECL metabolism, not in male rats. In contrast, catalytic activities related to P450IA and IIB subfamilies were not induced by ECL treatment. Additional experiments with the P450IIE1-specific inhibitor 3-amino-1,2,4-triazole and induction experiments with acetone, phenobarbital and methylcholanthrene confirmed that the isoenzyme mainly involved in the dechlorination reaction is cytochrome P450IIE1. AC was not detected in serum of ECL exposed animals and only slightly enhanced amounts were detected in urine samples from ECL exposed mice, reflecting the high capacities of the AC metabolizing pathways in vivo. The data are discussed with regard to the results of a 2-year bioassay with F-344 rats and B6C3F1 mice exposed to 15,000 ppm ECL (NTP 1989a).(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic polymorphism of cytochrome P450. Functional consequences and possible relationship to disease and alcohol toxicity

The hepatic cytochrome P450 system participates in the oxidative metabolism of numerous endogenous and exogenous compounds. In total several hundred different P450s have been cloned, but it appears that in humans only about 5-10 isoforms account for the major part of drug metabolism. Some of these are polymorphically distributed in the population. Cytochrome P450 2D6 catalyzes the oxidation of over 25 clinically important drugs, eg neuroleptics, antidepressants and lipophilic beta-blockers. Seven % of Caucasians and 1% of Orientals are defective in this enzyme and clearance of drugs metabolized by the enzyme may be substantially decreased in these individuals, with potentially increased risks for side effects caused by the drug treatment. Some individuals are ultrarapid metabolizers and do not achieve therapeutic drug levels at ordinary doses. The molecular genetic basis of these polymorphisms are presented. Methods for genotyping, which can be of predictive value for a more efficient drug therapy, are discussed. Ethanol-inducible cytochrome P450 2E1 (CYP2E1) oxidizes ethanol and acetaldehyde, in addition to over 80 toxicologically important xenobiotics. Furthermore, this isozyme produces reactive oxy radicals which are implicated in the aetiology of alcoholic liver disease. The gene is polymorphic and a mutation in a putative binding site for HNF1, described to affect gene expression, is more rare among subjects with lung cancer as compared to healthy controls. Further studies might give an answer as to whether any of the polymorphic CYP2E1 alleles is associated with the sensitivity to obtain alcoholic liver disease.

Pulsatile blood alcohol and CYP2E1 induction during chronic alcohol infusions in rats

Adult male Sprague-Dawley rats were treated with alcohol for 35 days using a total enteral nutrition model. Intragastric cannulae were inserted into rats and they were infused with a diet designed to promote normal growth in male rats. Alcohol was infused at 35% of total calories for 35 days. Urine and blood alcohol concentrations were determined and found to be pulsatile during continuous alcohol infusion, having values between near zero and greater than 500 mg/dl. Twenty-four-hour urine alcohol concentrations were found to be excellent indicators of blood alcohol concentrations (BACs). Cytochrome P450 CYP2E1 was induced in a two-step manner. Step one occurred at BACs below 250 mg/dl and was characterized by significant (p < or = 0.05) elevations in activities and apoprotein levels with no changes in steady-state mRNA. Step two occurred with BACs greater than 300 mg/dl and resulted in significant (p < or = 0.05) elevations in steady-state mRNA levels. We propose that the pulsatile BACs are caused by an ethanol concentration-dependent regulation of an ethanol metabolizing system, perhaps CYP2E1.