Oxidation of β2-glycoprotein I (β2GPI) by the Hydroxyl Radical Alters Phospholipid Binding and Modulates Recognition by anti- β2GPI Autoantibodies

We investigated whether β2-glycoprotein I (β2GPI), the key antigen in the antiphospholipid syndrome, is susceptible to oxidative modifications by the hydroxyl radical (°OH) that may influence its lipid-binding and antigenic properties. The effects on human and bovine β2GPI of °OH free radicals generated by γ-radiolysis of water with 137Cs were studied. Radiolytic °OH caused a dose-dependent loss of tryptophan, production of dityrosine and carbonyl groups, dimerization and/or extensive aggregation of β2GPI. It ensued a reduction in affinity binding to cardiolipin liposomes and loss of β2GPI-dependent autoanti-body binding to immobilized cardiolipin. Patient anti- β2GPI antibodies (n = 20) segregated into two groups based on the effect in the β2GPIELISA of β2GPI pretreatment with °OH: enhancement (group A, n = 10) or suppression (group B, n = 10) of IgG binding. The avidities of group A antibodies for fluid-phase β2GPI were low but increased in a dose-dependent manner upon β2GPI irradiation, in relation to protein crosslinking. Distinguishing features of group B antibodies included higher avidities for fluid-phase β2GPI that was no longer recognized after °OH treatment, and negative anticardiolipin tests suggesting epitope location near the phospholipid binding site. The °OH scavengers thiourea and mannitol efficiently protected against all above changes. Therefore, oxidative modifications of β2GPI via °OH attack of susceptible amino acids alter phospholipid binding, and modulate recognition by autoantibodies depending on their epitope specificities. These findings may be of clinical relevance for the generation and/or reactivity of anti- β2GPI antibodies.

Susceptibility and exposure biomarkers in people exposed to PAHs from diesel exhaust

Xenobiotic metabolizing enzymes, especially CYP1A1 and GSTM1, are involved in the activation and conjugation of PAHs and are controlled by polymorphic genes. PAHs released from diesel emissions in many cities of the world, especially in developing countries, contribute significantly to the toxic effects of airborne inhalable particles. We have evaluated the gene-environment interaction in Santiago of Chile, studying the contribution of CYP1A1 and GSTM1 polymorphisms on 1-OH-P urinary levels used as the PAHs exposure biomarker. The study was performed on 59 diesel exposed (38 diesel revision workers and 21 subjects working in an urban area as established street vendors) and 44 non-exposed subjects living in a rural area. The 1-OH-P urinary levels of the urban (P=0.043) and rural (P=0.040) populations showed, without considering the genotypes, significant differences between smokers and non-smokers, but no significant differences were found between smokers and non-smokers among the diesel plant workers (P=0.33). Non-smoking subjects of the diesel plants and the urban area showed similar 1-OHP levels (P=0.466) which were significantly higher than those of the subjects living in the rural area (P<0.05). When 1-OH-P levels were related with genotypes, an association was observed for the CYP1A1*2A genotype, so that the diesel-exposed workers carrying the CYP1A1*2A allele showed significantly higher 1-OH-P levels than the subjects from the rural area with the same genotype (P=0.008). On the other hand, there was no significant correlation between urinary 1-OH-P levels and GSTM1 null genotype, although higher levels of the urinary metabolite were found in individuals carrying the combined CYP1A1*2A and GSTM1 null genotype (P=0.055). These results may suggest an association between levels of the exposure biomarker 1-OH-P and presence of the CYP1A1*2A genotype, a potential genetic susceptibility biomarker which might be useful in identifying individuals at higher risk among people exposed to high PAH levels in diesel exhaust.

Influence of genetic polymorphisms of CYP1A1 and GSTM1 on the urinary levels of 1-hydroxypyrene

The aim of the present study is to evaluate the influence of the genetic polymorphism of two enzymes involved in the biotransformation of xenobiotics, cytochrome P450 1A1 (CYP1A1) and glutathione-S-transferase M1 (GSTM1), on the urinary levels of 1-hydroxypyrene (1-OH-P) in workers exposed to polycyclic aromatic hydrocarbons (PAHs) and in unexposed workers (controls). The study group consisted of 30 controls recruited among employees of a service company and 171 PAHs-exposed workers from two electric steel plants and an iron foundry (all males, ranging between 18 and 60 years of age). Determination of airborne PAHs and urinary 1-OH-P was performed by high-performance liquid chromatography (HPLC) with fluorimetric detection. Polymerase chain reaction (PCR)-based restriction fragment length polymorphism (RFLP) was used to determine the genetic polymorphisms of CYP1A1 (CYP1A1*2A and CYP1A1*2B) and GSTM1. No influence of the genetic polymorphism of CYP1A1 and GSTM1 on the urinary levels of 1-OH-P was observed in this study.

Biodistribution study of phosphonolipids: a class of non-viral vectors efficient in mice lung-directed gene transfer

BACKGROUND

A multitude of cationic lipids have been synthesized since they were first proposed for use in gene therapy. Cationic lipids are able to efficiently transfect cells both in vitro and in vivo. Whereas most research groups have focused their investigations on the toxicity of these molecules, and on the location of expression of the DNA transferred by these vectors, little has been done to determine their biodistribution and elimination pathways. Our group has developed a family of cationic lipids termed phosphonolipids. Following a large in vitro screening experiment, we have selected several molecules for in vivo testing, with some of these phosphonolipids forming lipoplexes efficient in transfecting mouse lungs. It was thus of interest to study their fate after intravenous injection.

METHODS

The respective biodistributions of both the GLB43 phosphonolipid and plasmid DNA were investigated and compared with DNA expression sites. Using the optimal conditions determined for phosphonolipids, we followed the gene transfer agent and plasmid DNA distributions versus time by radiolabeling them with (14)C and (32)P, respectively. Otherwise, we performed imaging by radiolabeling plasmid DNA with (99m)Tc.

RESULTS

The lipoplexes appear to be directly located in the lung after administration. Secondly, the plasmid is released mainly into the lungs and the phosphonolipid vector is rapidly degraded. The hydrophilic moiety of the phosphonolipid is eliminated in the urine, as is the free plasmid.

CONCLUSIONS

This study reveals that there are slight differences in the observed results depending on the technique used to label the DNA; secondly, results show that the residence time of phosphonolipids in the mouse body is related to the DNA binding time.

Hypertension after renal transplantation and polymorphism of genes involved in essential hypertension: ACE, AGT, AT1 R and ecNOS

BACKGROUND

Arterial hypertension (HT), secondary to cyclosporine A (CsA) used as main immunosuppressive treatment in renal transplantation (RTx), is very frequent (70%), usually severe and explained mostly by vasoconstriction of the glomerular afferent arteriole with secondary sodium and water retention.

MATERIAL AND METHODS

In a retrospective study, we have analyzed 294 consecutive recipients receiving a first renal cadaveric allograft and all treated with CsA (the majority with triple therapy). We studied, by molecular biology, the polymorphism of genes previously implicated in essential HT such as: angiotensin-converting enzyme (ACE: II, ID and DD), angiotensinogen (AGT: MM, MT and TT), angiotensin II type 1 receptor (AT1-R: AA, AC and CC) and endothelial constitutive nitric oxide synthase (ecNOS: aa, ab and bb), and correlated the data to the prevalence and severity of post-Tx HT. This cohort included 195 (66%) males and 99 females with a mean age of 42 years at time of Tx. The presence and severity of post-Tx HT were indicated by initial persistent blood pressure over 140/90 mmHg with the need for at least one anti-hypertensive drug and by the number of anti-HT medications required to achieve its control.

RESULTS

The distribution of the specific alleles and genotypes for ACE, AGT, AT1-R, and ecNOS was not different in transplant recipients compared to 181 controls. At 5 years post-Tx, the prevalence of HT was 72% (169 out of 235) among functioning grafts. There was no significant difference for ACE, AGT, AT1R and ecNOS genotypes distribution between hypertensive vs non-HT recipients. The number of anti-hypertensive drugs prescribed was not different among ACE, AGT, and AT1-R genotypes. However, the a allele and the non-bb genotype (aa + ab) for ecNOS were significantly (p = 0.001) associated with a less severe HT, needing fewer anti-HT drugs. At 10 years post-Tx, the HT prevalence remained high 78% (67 out of 86) among functioning Tx. However, the limited numbers did not allow further correlation.

CONCLUSIONS

This study produced mainly negative results except for ecNOS-a allele, which seems to protect against severe hypertension. The explanation remains speculative but probably relates to the known cyclosporine-induced upregulation of ecNOS gene and enzyme activity.

Inhibition of CYP3A, CYP1A and CYP2E1 activities by resveratrol and other non volatile red wine components

Resveratrol (RESV), present at concentrations of about 10 microM in red wine, has been found to inhibit events associated with tumor initiation, promotion and progression. The mechanism involved could be the inhibition of activities catalyzed by cytochromes P450 (CYPs), which activate procarcinogens. This led us to investigate the inhibitory effect of RESV on CYP1A, CYP2E1 and CYP3A enzymatic activities and to compare it to that of non volatile compounds present in red wine. Red wine solids (RWS) were prepared by evaporating one volume of red wine to dryness followed by reconstitution with five volumes of buffer (20% natural strength). CYP activities were determined in microsomes from rat liver, human liver or cells containing cDNA-expressed CYPs. Testosterone, chlorzoxazone, and ethoxyresorufin were used as selective substrates for CYP3A, CYP2E1 and CYP1A1/1A2, respectively. RESV and RWS were found to be irreversible (probably mechanism-based) inhibitors for CYP3A4 and non competitive reversible inhibitors for CYP2E1. Their inhibitory potency was assessed using IC(50) values that were found within 4-150 microM for RESV and 0.3-9% natural strength for RWS. Non volatile compounds of other beverages such as white wine, grape juice or Xtra Old Cognac(R) displayed lower inhibitory effect on CYP activities than RWS. When considering the concentration of RESV in red wine (2 microM for 20% natural strength), it appears that RSW inhibitory effect was not only due to RESV, but also to other compounds whose identification would prove to be worthwhile because of their possible chemopreventive properties.

CYP1A1, CYP2E1 and GSTM1 genetic polymorphisms. The effect of single and combined genotypes on lung cancer susceptibility in Chilean people

CYP1A1, CYP2E1 and GSTM1 polymorphisms were evaluated in Chilean healthy controls and lung cancer patients. In the Chilean healthy group, frequencies of CYP1A1 variant alleles for MspI (m2 or CYP1A1*2A) and ile/val (val or CYP1A1*2B) polymorphisms were 0.25 and 0.33, respectively. Frequencies of variant alleles C (CYP2E1*6) and c2 (CYP2E1*5B) for CYP2E1 were 0.21 and 0.16, respectively and frequency for GSTM1(-) was 0.24. The presence of variant alleles for GSTM1, MspI and Ile/val polymorphisms was more frequent in cases than in controls. However, frequencies for the c2 and C alleles were not significantly different in controls and in cases. The estimated relative risk for lung cancer associated to a single mutated allele in CYP1A1, CYP2E1 or GSTM1 was 2.41 for m2, 1.69 for val, 1.16 for C, 0.71 for c2 and 2.46 for GSTM1(-). The estimated relative risk was higher for individuals carrying combined CYP1A1 and GSTM1 mutated alleles (m2/val, OR=6.28; m2/GSTM1(-), OR=3.56) and lower in individuals carrying CYP1A1 and CYP2E1 mutated alleles (m2/C, OR=1.39; m2/c2, OR=2.00; val/C, OR=1.45; val/c2, OR=0.48; not significant). The OR values considering smoking were 4.37 for m2, 4.05 for val, 3.47 for GSTM1(-), 7.38 for m2/val and 3.68 for m2/GSTM1(-), higher values than those observed without any stratification by smoking. Taken together, these findings suggest that Chilean people carrying single or combined GSTM1 and CYP1A1 polymorphisms could be more susceptible to lung cancer induced by environmental pollutants such as polycyclic aromatic hydrocarbons.

[Alcohol-xenobiotic interactions. Role of cytochrome P450 2E1]

Alcohol and xenobiotics share the same oxidative microsomal pathway, which is mainly located in the endoplasmic reticulum of hepatocytes. This pathway involves enzymes that belong to the super family of cytochrome P450 and allows to explain a lot of pharmacokinetic or toxic interactions between alcohol and xenobiotics. Cytochrome P450 2E1 (CYP2E1) is the key enzyme of the microsomal pathway of ethanol oxidation. It is inducible by chronic ethanol consumption and its activity is increased by three to five fold in liver from alcoholics subjects. This induction involves to a lesser extent cytochromes P450 3A4 and 1A2 and contributes to the metabolic tolerance of alcohol and drugs observed in alcoholics. The metabolic tolerance persits several days after ethanol withdrawal. Furthermore, CYP2E1 has a high capacity to activate numerous xenobiotics into toxic or carcinogenic compounds. Drugs currently used such as paracetamol, anesthetics (enflurane, halothane), industrial solvents (benzene or its derivatives), halogenated solvents (CCl4, trichlorethylene) and nitrosamines which are present in food or tobacco smoke are included. Therefore, heavy consumption of alcohol, which results in CYP2E1 induction, increases individual susceptibility to the toxic or carcinogenic effects of these xenobiotics.

Human cytochrome P450 2A6 is the major enzyme involved in the metabolism of three alkoxyethers used as oxyfuels

Methyl t-butyl ether (MTBE), ethyl t-butyl ether (ETBE), and t-amyl methyl ether (TAME) are three alkoxyethers added to gasoline to improve combustion and thereby to reduce the level of carbon monoxide and aromatic hydrocarbons in automobile exhaust. Oxidative demethylation of MTBE and TAME and deethylation of ETBE by CYP enzymes results in the formation of tertiary alcohols and aldehydes, both potentially toxic. The metabolism of these three alkoxyethers was studied in a panel of 12 human liver microsomes. The relatively low apparent Km(1) was 0.25+/-0.17 (mean+/-SD), 0.11+/-0.08 and 0.10+/-0.07 mM and the high apparent Km(2) was 2.9+/-1.8, 5.0+/-2.7 and 1.7+/-1.0 mM for MTBE, ETBE and TAME, respectively. Kinetic data, correlation studies, chemical inhibition and metabolism by heterologously expressed human CYPs support the assertion that the major enzyme involved in MTBE, ETBE and TAME metabolisms is CYP2A6, with a minor contribution of CYP3A4 at low substrate concentration.

Cytochrome CYP2E1 phenotyping and genotyping in the evaluation of health risks from exposure to polluted environments

Humans are exposed to over 70,000 man-made chemicals including drugs, food additives, herbicides, pesticides, and industrial agents. It is well established that environmental chemicals are the cause of numerous human diseases including cancer. In most cases, chemical carcinogens require metabolic activation, which is mainly achieved by P450s enzymes. CYP2E1 is of clinical relevance because it is inducible by ethanol, and it metabolizes many common organic solvents such as benzene, alcohols and halogenated solvents. Therefore, alteration in the level of CYP2E1 might influence the health effects of the environmental pollutants. This hypothesis needs to be validated by epidemiological studies and the objective of the "Biomed-2" project was to develop new tests to assess the individual metabolic capacity of workers exposed to volatile organic compounds in order to predict their occupational risk. In vivo chlorzoxazone 6-hydroxylation was validated as a non-invasive and selective test for the determination of liver CYP2E1 activity. Preliminary data in workers exposed to organic solvents indicated that chlorzoxazone metabolism may be a biomarker of occupational exposure to organic solvents. Other approaches, such as use of salicylate as catalytic probe or measurement of catalytic activity in lymphocytes, were not conclusive. Attempts to use CYP2E1 genotyping for estimating human risks from chemical exposure did not bring convincing data as genetic polymorphism of CYP2E1 could not be clearly related to its catalytic activity.

Oxidation of beta2-glycoprotein I (beta2GPI) by the hydroxyl radical alters phospholipid binding and modulates recognition by anti-beta2GPI autoantibodies

We investigated whether beta2-glycoprotein I (beta2GPI), the key antigen in the antiphospholipid syndrome, is susceptible to oxidative modifications by the hydroxyl radical (*OH) that may influence its lipid-binding and antigenic properties. The effects on human and bovine beta2GPI of *OH free radicals generated by gamma-radiolysis of water with 137Cs were studied. Radiolytic *OH caused a dose-dependent loss of tryptophan, production of dityrosine and carbonyl groups. dimerization and/or extensive aggregation of beta2GPI. It ensued a reduction in affinity binding to cardiolipin liposomes and loss of beta2GPI-dependent autoantibody binding to immobilized cardiolipin. Patient anti-beta2GPI antibodies (n = 20) segregated into two groups based on the effect in the beta2GPI-ELISA of beta2GPI pretreatment with *OH: enhancement (group A, n = 10) or suppression (group B, n = 10) of IgG binding. The avidities of group A antibodies for fluid-phase beta2GPI were low but increased in a dose-dependent manner upon beta2GPI irradiation, in relation to protein crosslinking. Distinguishing features of group B antibodies included higher avidities for fluid-phase beta2GPI that was no longer recognized after *OH treatment, and negative anticardiolipin tests suggesting epitope location near the phospholipid binding site. The *OH scavengers thiourea and mannitol efficiently protected against all above changes. Therefore, oxidative modifications of beta2GPI via *OH attack of susceptible amino acids alter phospholipid binding, and modulate recognition by autoantibodies depending on their epitope specificities. These findings may be of clinical relevance for the generation and/or reactivity of anti-beta2GPI antibodies.

Genetic repeat polymorphism in the regulating region of CYP2E1: frequency and relationship with enzymatic activity in alcoholics

BACKGROUND

Differences in the regulatory region of the CYP2E1 gene could be responsible for the interindividual variation in the cytochrome P-450 2E1 (CYP2E1) involved in ethanol oxidation. Recently, a polymorphic repeat sequence in the human gene was described between -2178 and -1945 base pairs. Its frequency seemed to vary among different ethnic populations, and it was suspected to be related to an increased inducibility to further ethanol intake. In the study reported here, the frequency of this polymorphism was investigated in a white French population. Its relationship with the previously described PstI/RsaI or DraI CYP2E1 polymorphisms, alcoholism, alcoholic liver disease, and inducibility of CYP2E1 by ethanol was examined.

METHODS

The polymorphic region was characterized by polymerase chain reaction in 103 controls, 148 alcoholic subjects without liver diseases, and 98 others with liver cirrhosis. By using in vivo chlorzoxazone (CHZ) metabolism, CYP2E1 phenotype was assessed in 36 non-ethanol-induced subjects (17 controls and 19 withdrawn alcoholics) and in 14 ethanol-induced subjects (10 controls after ingestion of 0.8 g/kg ethanol and four alcoholics with 100 g of daily intake). This phenotype was expressed as the 6-hydroxy CHZ/CHZ ratio.

RESULTS

The rare allele frequency was found to be 1.58% in whites (n = 349). Neither significant association with alcoholism or alcoholic liver diseases, nor relationship with the PstI/RsaI polymorphism, was observed. But the DraI polymorphism was more frequent among the heterozygous subjects when compared with wild-type homozygous ones (p < 0.05). The CYP2E1 phenotype was similar in wild-type homozygotes and in heterozygotes at the constitutive level, as well as after induction with ethanol.

CONCLUSIONS

Our data suggest that CYP2E1 repeat polymorphism does not seem to constitute a major factor for interindividual differences in CYP2E1 expression and susceptibility to alcohol-related disorders in whites.

Adaptation to chronic ethanol administration emphasized by fatty acid hydroxylations in rat liver and kidney microsomes

BACKGROUND

Long-term ethanol consumption in laboratory animals is associated with histological alterations of liver cells and modifications of fatty acid metabolism.

AIM OF THE STUDY

The present study was aimed at investigating the effect of 1- and 2-month chronic treatment of rats with ethanol on the metabolism of two unsaturated (oleic and linoleic) fatty acids in liver and kidney microsomes, in relation to the CYP2E1 enzyme content in both tissues.

METHODS

Rats were fed ethanol (14 g/Kg/d) or dextrose through a permanently implanted gastric cannula, as described in the intragastric feeding rat model for alcoholic liver disease (ALD). CYP2E1 level was immuno-quantified in both liver and kidney microsomes by Western blot, whereas fatty acid omega- and (omega-1)-hydroxylations were measured using HPLC and radiometric analytical methods.

RESULTS

One- and two-month ethanol treatment led to a 3- to 4-fold rise of the CYP2E1 protein in both liver and kidney microsomes. Oleic and linoleic acid (omega-1)-hydroxylations were increased (approximately 3-fold) in liver microsomes after one-month of ethanol administration, but surprisingly such a rise was not observed after a two-month treatment; on the other hand, no effect was observed on the omega-hydroxylations of these fatty acids. Furthermore, as previously described for lauric acid, ethanol intake did not significantly act on the kidney microsome capability to hydroxylate unsaturated fatty acids.

CONCLUSIONS

CYP2E1 is strongly inducible by ethanol and therefore accounts for the tolerance for this hepatotoxicant. Our results support the development of an adaptation process in the liver hydroxylating enzyme system, which occurs between one and two months of ethanol feeding. Although it is usually not appropriate to extrapolate animal findings to humans, rat and human CYP2E1s were observed to have comparable specificities and similar mechanisms of regulation. Thus, the present study allowed the acquirement of detailed information of CYP2E1 activity in patients with severe manifestations of ALD.

Cytochrome P-450 2E1 activity and oxidative stress in alcoholic patients

As cytochrome P-450 2E1 (CYP2E1) induction was related to oxidative stress in experimental models, the aim of this study was to investigate the relationship between CYP2E1 activity and markers of oxidative stress in 40 alcoholic patients entering a rehabilitation programme. Plasma oxidized proteins, lipid peroxides (LPO) and antibodies against hydroxyethyl radical (HER) or malondialdehyde (MDA) adducts were assessed as markers of the production of free radicals, whereas vitamin E levels were evaluated as a marker of the antioxidant defence. CYP2E1 activity was determined by using the 6-hydroxychlorzoxazone:chlorzoxazone blood metabolic ratio, 2 h after drug intake. This ratio was increased by 4-fold in alcoholics, compared to non-alcoholic patients, and was correlated with daily intake of ethanol, carbohydrate-deficient transferrin, and blood alcohol level at the time of admission to hospital. Plasma levels of LPO and oxidized proteins were slightly increased (20%) in alcoholic patients when compared with the control group, whereas those of vitamin E were found to be slightly decreased (by 18%). Antibodies against HER or MDA adducts showed a very significant increase. However, when alcoholic patients were divided into two groups according to low or high CYP2E1 induction, no significant difference was observed in the variation of these parameters, except for anti-HER adducts antibodies. Therefore, our study confirms the main involvement of CYP2E1 in HER production. By contrast, CYP2E1 does not appear to be the main factor responsible for the oxidative stress occurring during human chronic alcoholism. Free radicals from other sources may therefore contribute significantly to the generation of this oxidative stress.

Requirement for omega and (omega;-1)-hydroxylations of fatty acids by human cytochromes P450 2E1 and 4A11

Human liver microsomes and recombinant human P450 have been used as enzyme source in order to better understand the requirement for the optimal rate of omega and (omega;-1)-hydroxylations of fatty acids by cytochromes P450 2E1 and 4A. Three parameters were studied: alkyl chain length, presence and configuration of double bond(s) in the alkyl chain, and involvement of carboxylic function in the fatty acid binding inside the access channel of P450 active site. The total rate of metabolite formation decreased when increasing the alkyl chain length of saturated fatty acids (from C12 to C16), while no hydroxylated metabolite was detected when liver microsomes were incubated with stearic acid. However, unsaturated fatty acids, such as oleic, elaidic and linoleic acids, were omega and (omega;-1)-hydroxylated with an efficiency at least similar to palmitic acid. The (omega;-1)/omega ratio decreased from 2.8 to 1 with lauric, myristic and palmitic acids as substrates, while the reverse was observed for unsaturated C18 fatty acids which are mainly omega-hydroxylated, except for elaidic acid showing a metabolic profile quite similar to those of saturated fatty acids. The double bond configuration did not significantly modify the ability of hydroxylation of fatty acid, while the negatively charged carboxylic group allowed a configuration energetically favourable for omega and (omega;-1)-hydroxylation inside the access channel of active site.

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

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

Ethnic susceptibility to lung cancer: differences in CYP2E1, CYP1A1 and GSTM1 genetic polymorphisms between French Caucasian and Chilean populations

Many investigators have reported an association between genetic polymorphisms of cytochromes P-450 CYP2E1, CYP1A1 or glutathione S-transferase Mu (GSTM1) and susceptibility to lung cancer. However, pronounced interethnic variations have been described in the frequencies of these polymorphisms, especially between Asians and Caucasians. The present study was set up to establish CYP2E1 (c1, c2 and C, D), CYP1A1 (m1, m2 and Ile, Val) and GSTM1 (null) allelic frequencies in Chileans (n = 96) who are an admixture of Native Americans and Caucasians (Spaniards). The rare allele frequencies were found to be 0.15 (c2), 0.21 (C), 0.23 (m2), 0.32 (Val) and 0.21 ('null' genotype). These values are significantly higher than those of Caucasians except for the GSTM1 'null' genotype and suggest differences in susceptibility to lung cancer between both populations.

Chlorzoxazone, a selective probe for phenotyping CYP2E1 in humans

The ability of human cytochromes P450 other than CYP2E1 to catalyse the 6-hydroxylation of chlorzoxazone (6-OH-CHZ) was examined in vitro using human liver microsomal preparations and in vivo using chlorzoxazone as a metabolic probe. Chlorzoxazone 6-hydroxylation activity was significantly correlated with 4-nitrophenol 2-hydroxylase activity and immunodetected CYP2E1 in 14 human liver samples (r = 0.92 and 0.81, P < 0.001, respectively). Conversely, this catalytic activity was not correlated with CYP 3A or CYP1A activities. Diethyldithiocarbamate (DEDTC), a specific CYP2E1 inhibitor, reduced chlorzoxazone 6-hydroxylase activity by 92.3 +/- 7.6% (n = 14 samples) while ketoconazole, a specific CYP3A inhibitor, reduced this activity by 8.6 +/- 6.3% (n = 14). The residual activity following preincubation with DEDTC was significantly correlated with nifedipine oxidation and tamoxifen N-demethylations, both specific to CYP3A (r = 0.76 and 0.68, respectively). Genetically produced pure human CYP2E1 and 3A4 hydroxylated chlorzoxazone with turnover numbers of 19.7 and 0.14 min(-1), respectively. Furthermore, cytochrome b5 stimulated chlorzoxazone 6-hydroxylation. From examination of the relative liver content of CYP2E1 and 3A, it can be asserted that CYP2E1 is the major enzyme involved in chlorzoxazone 6-hydroxylation and that the contribution of CYP3A is very minor. CYP2E1 activity was evaluated by the plasmatic metabolic ratio 6-OH-CHZ/CHZ (CHZ-MR) measured 2 h after ingestion of 500 mg CHZ. Smoker status did not influence the rate of CHZ hydroxylation. The CHZ-MR was 0.30 +/- 0.13 (mean +/- SD) n = 39 non-smokers versus 0.32 +/- 0.15, n = 75 smokers. This result suggests that CYP1A, inducible by cigarette smoking, is not significantly involved in chlorzoxazone hydroxylation. Women exhibited a slightly lower CHZ-MR than men (0.29 +/- 0.15, n = 44 versus 0.34 +/- 0.15 n = 49, respectively). Obesity increased CHZ-MR, especially in non-insulin-dependent diabetic individuals (0.45 +/- 0.21, n = 13 versus 0.30 +/- 0.15, n = 42 control individuals, P = 0.007). Furthermore, exposure of workers to volatile organics in a shoe factory decreased CHZ-MR (0.19 +/- 0.09, n = 10 Mexican workers versus 0.34 +/- 0.12, n = 16 Mexican control individuals, P = 0.001). Concomitant administration of grapefruit juice (known to be an inhibitor of CYP3A4) with chlorzoxazone did not significantly modify the CHZ metabolic ratio: 0.29 +/- 0.1 versus 0.31 +/- 0.1, for nine control individuals without and with grapefruit juice, respectively. In conclusion, all these results demonstrate that chlorzoxazone is a very selective probe for phenotyping CYP2E1 in humans.

Involvement of cytochromes P-450 2E1 and 3A4 in the 5-hydroxylation of salicylate in humans

Hydroxylation of salicylate into 2,3 and 2,5-dihydroxybenzoic acids (2,3-DHBA and 2,5-DHBA) by human liver microsomal preparations was investigated. Kinetic studies demonstrated that salicylate was 5-hydroxylated with two apparent Km: one high-affinity Km of 606 microM and one low-affinity Km greater than 2 mM. Liver microsomes prepared from 15 human samples catalyzed the formation of 2,5-DHBA at metabolic rate of 21.7 +/- 8.5 pmol/mg/min. The formation of 2, 3-DHBA was not P-450 dependent. Formation of 2,5-DHBA was inhibited by 36 +/- 14% following preincubation of microsomes with diethyldithiocarbamate, a mechanism-based selective inhibitor of P-450 2E1. Furthermore, the efficiency of inhibition was significantly correlated with four catalytic activities specific to P-450 2E1, whereas the residual activity was correlated with three P-450 3A4 catalytic activities. Troleandomycin, a mechanism-based inhibitor selective to P-450 3A4, inhibited by 30 +/- 12% the 5-hydroxylation of salicylate, and this inhibition was significantly correlated with nifedipine oxidation, specific to P-450 3A4. The capability of seven recombinant human P-450s to hydroxylate salicylate demonstrated that P-450 2E1 and 3A4 contributed to 2, 5-DHBA formation in approximately equal proportions. The Km values of recombinant P-450 2E1 and 3A4, 280 and 513 microM, respectively, are in the same range as the high-affinity Km measured with human liver microsomes. The plasmatic metabolic ratio 2,5-DHBA/salicylate, measured 2 h after ingestion of 1 g acetylsalicylate, was increased 3-fold in 12 alcoholic patients at the beginning of their withdrawal period versus 15 control subjects. These results confirm that P-450 2E1, inducible by ethanol, is involved in the 5-hydroxylation of salicylate in humans. Furthermore, this ratio was still increased by 2-fold 1 week after ethanol withdrawal. This finding suggests that P-450 3A4, known to be also inducible by alcoholic beverages, plays an important role in this increase, because P-450 2E1 returned to normal levels in less than 3 days after ethanol withdrawal. Finally, in vivo and in vitro data demonstrated that P-450 2E1 and P-450 3A4, both inducible by alcohols, catalyzed the 5-hydroxylation of salicylate.

Liquid chromatography-mass spectrometry and gas chromatography-mass spectrometry of omega- and (omega-1)-hydroxylated metabolites of elaidic and oleic acids in human and rat liver microsomes

In order to characterize the nature of the active site of cytochrome P450 2E1, the metabolism of various fatty acids with cis/trans geometric configurations has been investigated. A system coupling atmospheric pressure chemical ionization-mass spectrometry detection with HPLC separation was developed as an alternative method for the characterization of hydroxylated metabolites of oleic and elaidic acids in rat and human liver microsomes. Oxidation of oleic and elaidic acids led to the formation of two main metabolites which were identified by LC-MS and GC-MS as omega and (omega-1)-hydroxylated (or 17-OH and 18-OH) fatty acids, on the basis of their pseudo-molecular mass and their fragmentation. The assay was accurate and reproducible, with a detection limit of 25 ng per injection, a linear range from 25 to 1128 ng per injection, no recorded interference, intra-day and inter-day precision with variation coefficients <14%. This LC-MS method was validated with oleic acid by using both radiometric and mass spectrometric detections. A significant correlation was found between the two methods in human (r=0.86 and 0.94 with P<0.05 and 0.01) and rat liver microsomes (r=0.90 and 0.85 with P<0.01 and 0.05) for 17-OH and 18-OH metabolites, respectively. HPLC coupled to mass spectrometry for the analysis of hydroxylated metabolites of elaidic acid offers considerable advantages since the method does not require use of a radioactive molecule, completely separates the two hydroxymetabolites, confirms the identification of each metabolite, and is as sensitive as the radiometric analysis method. This method allowed the comparative study of oleic and elaidic acid hydroxylations by both human and rat liver microsomal preparations.

Effects of interferon-alpha on cytochrome P-450 isoforms 1A2 and 3A activities in patients with chronic hepatitis C

BACKGROUND/AIM

The risk of adverse drug interactions with interferon-alpha has been poorly assessed. The aim of our study was to establish whether administration of interferon-alpha at therapeutic doses in patients with chronic hepatitis C may have significant inhibitory effects on other drug metabolism. The study was focused on cytochromes P-450 1A2 and 3A, two major isoforms involved in the metabolism of numerous substrates.

METHODS

Eighteen patients with chronic active hepatitis C requiring an interferon-alpha treatment were studied. Cytochrome P-450 1A2 activity was determined on the basis of an in vivo caffeine metabolism study. Cytochrome P-450 3A activity was determined according to in vivo cortisol metabolism into 6-beta-hydroxycortisol. Both activities were determined 1 month before, at initiation and 1 month after interferon-alpha therapy (3 x 10(6) units, three times a week).

RESULTS

There were no significant differences in the caffeine index (CYP 1A2) and in the 6-beta-hydroxycortisol/free cortisol urinary ratio (CYP 3A) before and after alpha interferon treatment

CONCLUSION

Chronic administration of interferon-alpha at therapeutic doses does not change in vivo cytochrome P-450 1A2 and 3A activities. These results support the suggestion that drugs metabolized by these isoenzymes may be used together with interferon-alpha in patients with chronic hepatitis C without significant risks of drug interactions.

Involvement of cytochrome P450 2E1 in the (omega-1)-hydroxylation of oleic acid in human and rat liver microsomes

In vitro techniques have been used to investigate the nature of microsomal cytochrome P450 involved in the metabolism of oleic acid, a physiological monounsaturated fatty acid. Like lauric acid, which is currently used as a model substrate of fatty acid metabolism, the alkyl chain of oleic acid is hydroxylated on its omega and (omega-1) carbons. The identity of these hydroxylated metabolites was ascertained by GC/MS and LC/MS. The omega/omega-1 ratio of oleic acid metabolites (1.22+/-0.01) was found to be similar to that obtained with lauric acid in rat liver microsomes (1.10+/-0.02), while in human liver microsomes this ratio was 0.75+/-0.5 for lauric acid and 5.2+/-2.6 for oleic acid. After treatment of rats with ethanol or clofibrate, inducers of CYP2E1 and CYP4A, respectively, the hydroxylations of oleic acid were shown to be less inducible than those of lauric acid. Five in vitro approaches were used to identify the P450 isoform(s) responsible for the microsomal (omega-1)-hydroxylation of oleic acid: effect of various inducers in rats, correlation studies between specific P450 catalytic activities in a panel of 25 human liver microsomes, chemical inhibitions, immuno-inhibitions and metabolism by cDNA-expressed human P450 enzymes. From the above results, it can be ascertained that P450 2E1 is the main enzyme involved in the (omega-1)-hydroxylation of oleic acid. Furthermore, the omega-hydroxylation of oleic acid was shown to be mainly catalyzed by P450 4A enzymes in human liver microsomes. The turnover number of (omega-1)-hydroxylation of lauric and oleic acids decreased from 7.8 to 1.5 min(-1), respectively, suggesting that the dodecane alkyl chain allows optimal binding to the active site of CYP2E1.

In vitro interactions between fluoxetine or fluvoxamine and methadone or buprenorphine

Methadone and buprenorphine, widely used in the treatment of opioid abuse, are metabolized by cytochrome P450 3A4, while fluoxetine and fluvoxamine, both selective serotonin reuptake inhibitors, are known to be P450 2D6 and 3A4 inhibitors in vitro. This study deals with the in vitro interactions between methadone or buprenorphine and fluoxetine or fluvoxamine. Fluoxetine inhibited methadone N-demethylation (Ki = 55 microM), but conversely did not inhibit buprenorphine dealkylation. Norfluoxetine inhibited the metabolism of both methadone and buprenorphine metabolisms (Ki 13 and 100 microM, respectively). Fluvoxamine inhibited methadone N-demethylation with a Ki of 7 microM and buprenorphine dealkylation, uncompetitively, with a Ki of 260 microM. Finally, these results suggest that care should be taken when selective serotonin reuptake inhibitors are administered in the treatment of drug craving. This is particularly true in the case of fluvoxamine which is more potent than fluoxetine in inhibiting methadone and buprenorphine metabolism.

Effect of pyrazole and dexamethasone administration on cytochrome P450 2E1 and 3A isoforms in rat liver and kidney: lack of specificity of p-nitrophenol as a substrate of P450 2E1

The induction effects of pyrazole and dexamethasone (known to be specific to P450 2E1 and 3A enzymes, respectively), given alone or simultaneously, were studied in rat liver and kidney microsomes. Pyrazole treatment induced the catalytic activity and the amount of P450 2E1 enzyme in both organs. Immunoreactive P450 2E1 and 4-nitrophenol 2-hydroxylation increased 8- and 13-fold, respectively (versus control), in the kidney, but only 2.4- and 2.7-fold (versus control) in the liver after pyrazole treatment. As assessed by nifedipine oxidation activity, dexamethasone treatment increased the P450 3A catalytic activity approximately 4-fold (versus control) in the liver, but not in the kidney, suggesting that P450 3A was not inducible in the kidney. Pyrazole decreased P450 3A activity in the liver but did not modify it in the kidney. A combination of both chemicals induced both enzymes, but to a lesser extent than treatment with each single chemical compound. Furthermore, the 2-hydroxylation of p-nitrophenol, considered one of the most specific substrates for monitoring the level of P450 2E1, was mediated also by P450 3A, at least in dexamethasone-treated rats. Finally, this experimental work demonstrated that P450 3A induction is organ-specific, and it also demonstrated the lack of specificity of p-nitrophenol as a P450 2E1 substrate.

Inhibition of methadone and buprenorphine N-dealkylations by three HIV-1 protease inhibitors

Ritonavir, indinavir, and saquinavir, all human immunodeficiency virus-1 protease inhibitors with a potent antiviral effect during triple therapy, are extensively metabolized by liver cytochrome P450 3A4. As this P450 isoform is involved in the metabolism of about 50% of drugs, coadministration of protease inhibitors with other drugs may lead to serious effects due to enzyme inhibition. Among these drugs, methadone and buprenorphine, both metabolized by P450 3A4, are potential candidates to drug interactions. In this study, metabolic interactions between these protease inhibitors and methadone or buprenorphine were studied in vitro in a panel of 13 human liver microsomes. Ritonavir was the most potent competitive inhibitor with Ki about 50 and 20 nM for methadone and buprenorphine metabolisms, respectively. Indinavir and saquinavir also inhibited methadone N-demethylation (Ki about 3 and 15 microM, respectively) and buprenorphine N-dealkylation (Ki about 0.8 and 7 microM, respectively). The rank order of inhibition potency against metabolism of methadone and buprenorphine was ritonavir > indinavir > saquinavir. There is obvious potential for clinically significant drug interactions, particularly with ritonavir. In brief, caution should be advised if human immunodeficiency virus-1 protease inhibitors are coadministered with methadone and buprenorphine.

Noninvolvement of CYP2E1 in the (omega-1)-hydroxylation of fatty acids in rat kidney microsomes

Pyrazole, acetone, and ethanol are known to induce cytochrome P450 2E1 (CYP2E1) and fatty acid (omega-1)-hydroxylation in rat liver microsomes. However, the nature of the P450 enzyme involved in this (omega-1)-hydroxylation has not been clearly established in extrahepatic tissues such as kidney. Four enzymatic activities (hydroxylations of chlorzoxazone, 4-nitrophenol, and two fatty acids) were assayed in kidney microsomal preparations of rats treated with CYP2E1 inducers. Per os treatment resulted in large increases (threefold to fivefold) in the chlorzoxazone and 4-nitrophenol hydroxylations, and up to a ninefold increase when ethanol was administered by inhalation. However, neither the omega-hydroxylation nor the (omega-1)-hydroxylation of fatty acids was modified. Immunoinhibition specific to CYP2E1 did not significantly decrease the omega and (omega-1)-lauric acid hydroxylations, while the polyclonal anti-CYP4A1 antibody inhibited in part both the omega- and (omega-1)-hydroxylations. Chemical inhibitions using either CYP2E1 competitive inhibitors (such as chlorzoxazone, DMSO, and ethanol) or P450 mechanism-based inhibitors (such as diethyldithiocarbamate and 17-octadecynoic acid) led to a partial inhibition of the hydroxylations. All these results suggest that fatty acid (omega-1)-hydroxylation, a highly specific probe for CYP2E1 in rat and human liver microsomes, is not mediated by CYP2E1 in rat kidney microsomes. In contrast to liver, where two different P450 enzymes are involved in fatty acid omega- and (omega-1)-hydroxylations, the same P450 enzyme, mainly a member of the CYP4A family, was involved in both hydroxylations in rat renal microsomes.

Both cytochromes P450 2E1 and 3A are involved in the O-hydroxylation of p-nitrophenol, a catalytic activity known to be specific for P450 2E1

4-Nitrophenol 2-hydroxylation activity was previously shown to be mainly catalyzed by P450 2E1 in animal species and humans. As this chemical compound is widely used as an in vitro probe for P450 2E1, this study was carried out to test its catalytic specificity. First, experiments were carried out on liver microsomes and hepatocyte cultures of rat treated with different inducers. Liver microsomes from pyrazole- and dexamethasone-treated rats hydroxylated p-nitrophenol with a metabolic rate increased by 2.5- and 2.7-fold vs control. Dexamethasone treatment increased the hepatic content of P450 3A but not that of P450 2E1. Two specific inhibitors of P450 3A catalytic activities, namely, ketoconazole and troleandomycin (TAO), inhibited up to 50% of 4-nitrophenol hydroxylation in dexamethasone-treated rats but not in controls. Hepatocyte cultures from dexamethasone-treated rats transformed p-nitrophenol into 4-nitrocatechol 7.8 times more than controls. This catalytic activity was inhibited by TAO. Similarly, hepatocyte cultures from pyrazole-treated rats hydroxylated p-nitrophenol with a metabolic ratio increased by about 8-fold vs control. This reaction was inhibited by diethyl dithiocarbamate and dimethyl sulfoxide, both inhibitors of P450 2E1. Second, the capability of human P450s other than P450 2E1 to catalyze the formation of 4-nitrocatechol was examined in a panel of 13 human liver microsomes. Diethyl dithiocarbamate and ketoconazole reduced 4-nitrophenol hydroxylase activity by 77% (+/- 11) and 13% (+/- 16), respectively. Furthermore, the residual activity following diethyl dithiocarbamate inhibition was significantly correlated with seven P450 3A4 catalytic activities. Finally, the use of human cell lines genetically engineered for expression of human P450s demonstrated that P450 2E1 and 3A4 hydroxylated 4-nitrophenol with turnovers of 19.5 and 1.65 min-1, respectively. In conclusion, P450 3A may make a significant contribution to 4-nitrophenol hydroxylase activity in man and rat.

Cytochrome P450 hydroxylation of carbon atoms of the alkyl chain of symmetrical N-nitrosodialkylamines by human liver microsomes

A panel of 14 human liver microsomal preparations metabolized at variable rates three symmetrical nitrosodialkylamines (N-nitroso-dipropyl, dibutyl and diamyl-amines, NDPA, NDBA, NDAA) into aldehydes and hydroxynitrosamines. Formation of linear aldehydes, convenient probes for alpha-hydroxylation of alkyl chain, and production of hydroxy metabolites of NDPA, NDBA and NDAA were simultaneously monitored by two specific HPLC detection methods. The longer the alkyl chain, the smaller the metabolic rate of the alpha-hydroxylation of the alkyl chain and the greater was the metabolic rate of the corresponding (omega-1)-hydroxy metabolite formation. Thus, the (omega-1)-hydroxylation of the alkyl chain was the major metabolic pathway of NDBA and NDAA in so far as it represented 3.3- and 86-fold of the alpha-hydroxylation. The balance between beta- to omega-hydroxylations and alpha-hydroxylation of carbon atoms of the alkyl chain depends upon its length and also upon the specific P450 isoform(s) involved. The hydroxylation site of the alkyl chain by P450 2E1 depends upon its length. For short alkyl chains, the main pathway was alpha-hydroxylation while for long alkyl chains, such as pentyl, (omega-1)-hydroxylation became the major pathway. The rate of alpha-hydroxylation was shown to be correlated with mutagenesis of 5 dialkylnitrosamines, as inferred from literature data, while the (omega-1)-hydroxylation was inversely correlated. Furthermore, other P450s than P450 2E1, such as P450 3A4 and 2C were shown to be involved in the metabolism of nitrosodialkylamines bearing long alkyl chains.

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

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

Cytochrome P450 4A and 2E1 expression in human kidney microsomes

Laurate and arachidonate omega and (omega-1)-hydroxylase activities, cytochrome P450 2E1 (CYP2E1), and CYP4A content were measured in 18 human kidney microsomal samples. The rates of laurate and arachidonate were found to be very different from those measured in human liver samples, with a laurate omega/omega-1 ratio of approximately 22 in human kidney vs 0.75 in human liver. Immunoblot analysis of the 18 human kidney microsomal samples identified 1 CYP4A electrophoretic band, but CYP2E1 was not detectable in human kidney, contrary to liver. Laurate and arachidonate omega-hydroxylase activities were significantly correlated with CYP4A content (r = 0.86 and 0.75, respectively). Polyclonal antirat CYP2E1 antibody did not affect omega-hydroxylase activity, whereas the polyclonal antirat CYP4A1 antibody inhibited it by 60%. These results suggest that, in contrast to other species, human kidney microsomes do not contain significant amounts of CYP2E1, but possess CYP4A and fatty acid omega-hydroxylase activity.

Interaction of methadone with substrates of human hepatic cytochrome P450 3A4

Methadone, a synthetic drug, is one of the most widely used drugs for opiate dependency treatment. This drug has been demonstrated to be extensively metabolized by cytochrome P450 3A4 in human liver microsomes. Thus, the aim of this in vitro study was to determine if methadone is an inhibitor of other P450s characterized by their specific catalytic activities. Enzymatic activities specific to P450 2E1, P450 1A, P450 2B and P450 2C were not inhibited by methadone. Conversely, nifedipine oxidation, mediated by cytochrome P450 3A4, was potently inhibited by methadone by a mixed-type inhibition mechanism with a Ki of 100 microM. Fluvoxamine, a new antidepressant, was shown to be a potent mixed-type inhibitor of methadone N-demethylation with a Ki of 7 microM. Finally, methadone appears to be a mixed-type inhibitor and not a suicide inhibitor of cytochrome P450 3A family. Accordingly, caution should be advised in the clinical use of methadone when other drugs are administered that are able to induce or inhibit P450 3A4, such as rifampicin or nifedipine, diazepam and fluvoxamine.

Involvement of cytochrome P450 3A4 in N-dealkylation of buprenorphine in human liver microsomes

Buprenorphine is a long acting analgesic of the opiate family. Recently, it has been proposed for the opioid dependency treatment at a large scale. The drug is extensively metabolized by the hepatic cytochrome P450 in man, yielding a N-dealkylated metabolite, norbuprenorphine. The specific forms of P450 involved in this oxidative N-demethylation were examined in a panel of 18 human liver microsomal preparations previously characterized with respect to their P450 contents. Buprenorphine was N-dealkylated with an apparent Km of 89 +/- 45 microM (n = 3). The metabolic rates were 3.46 +/- 0.43 nmol/(min x mg of protein). This metabolic pathway was strongly correlated with 6 catalytic activities specific to P450 3A4 and with the immunodetectable P450 3A content of liver microsomal samples (r = 0.87). Buprenorphine metabolism was 62-71% inhibited by three mechanism-based inhibitors (TAO, erythralosamine, gestodene), by nifedipine as competitive inhibitor (Ki = 129 microM) and by ketoconazole 0.6 microM (25% residual activity), all these inhibitors specific to P450 3A. Among 10 heterologously expressed P450s tested, only P450 3A4 was able to dealkylate buprenorphine with a turnover number of 9.6 min(-1). Morever, this catalytic activity was inhibited up to 80% (vs control) by anti-rat P450 3A antibody. Taken together, all these data demonstrate that P450 3A4 is the major enzyme involved in hepatic buprenorphine N-dealkylation.

Induction of liver and kidney CYP1A1/1A2 by caffeine in rat

Caffeine metabolism by hepatic microsomal P450 enzymes is well documented in experimental animals and humans. However, its induction effect on P450 enzymes has not been thoroughly studied. In a preliminary experiment, the time-dependent incubation of 1 mM caffeine with rat hepatocyte culture resulted in an increase of its own metabolic rate. The dose-dependent expression of rat hepatic and renal cytochromes (CYP) 1A1/1A2 was then investigated after per os administration of caffeine. P450 expression was monitored by using specific enzymatic activities and Northern blot analysis. Caffeine caused a dose-dependent elevation of hepatic CYP1A1/1A2 activities in microsomal preparations, which ranged from 1.7- to 6-fold for ethoxyresorufin O-deethylase and 3- to 8.9-fold for methoxy-resorufin O-demethylase according to the dose regimen of 50 and 150 mg caffeine/kg/day for 3 days, respectively. Northern blot analysis demonstrated that caffeine treatment increased liver CYP1A1 and CYP1A2 mRNA levels over the dose regimen of 50-150 mg caffeine/kg/day for 3 days, respectively. The result of this study demonstrates that caffeine increases its own metabolism in a dose-dependent manner and induces CYP1A1/1A2 expression through either transcriptional activation or mRNA stabilization.

Hydroxylation of carbon atoms of the alkyl chain of symmetrical N-nitrosodialkylamines by rat liver microsomes

Liver microsomal preparations from control and treated rats (cytochromes P450 1A, 2B, 3A and 2E1-induced) metabolized at variable metabolic rates three nitrosodialkylamines (N-nitroso-dipropyl, dibutyl and diamyl-amines) into aldehydes and hydroxy-nitrosamines. The longer the alkyl chain, the smaller was the metabolic rate of the alpha-hydroxylation of alkyl chain yielding aldehyde and the greater was the metabolic rate of the corresponding (omega-1)-hydroxyl metabolite formation. Thus, the (omega-1) hydroxylation of the alkyl chain was the major metabolic pathway of N-nitrosodiamylamine (NDAA) so far as it represented 22-fold the alpha-hydroxylation. The balance between beta to omega hydroxylation and alpha-hydroxylation depends upon the alkyl chain length and also on specific P450 isoform induction.

Cytochrome P450 metabolic dealkylation of nine N-nitrosodialkylamines by human liver microsomes

The metabolic dealkylation of nine nitrosodialkylamines, including five symmetrical (nitrosodimethylamine, nitrosodiethylamine, nitrosodipropylamine, nitrosodibutylamine and nitrosodiamylamine) and four asymmetrical nitrosodialkylamines (nitrosomethylethylamine, nitrosomethylpropylamine, nitrosomethylbutylamine and nitrosomethylamylamine), was investigated in 14 samples of human liver microsomes. All these nitrosodialkylamines were dealkytated to aldehydes that were separated by reversed phase HPLC and UV detected as dinitrophenylhydrazones. As the length of the alkyl chain increased from methyl to pentyl, dealkylation of symmetrical nitrosodialkylamines became less efficiently catalyzed by cytochrome P450. Conversely, oxidation of the methyl moiety of asymmetrical nitrosomethylalkylamines increased with the size of the alkyl moiety, while dealkylation of the longer alkyl group decreased. N-Dealkylase activities were significantly correlated with P450 activities measured in human liver microsomes. These catalytic activities involve CYP2A6 (coumarin 7-hydroxylation), CYP2C (mephenytoin 4-hydroxylation and tolbutamide hydroxylation), CYP2D6 (dextromethorphan O-demethylation), CYP2E1 (chlorzoxazone and p-nitrophenol hydroxylation) and CYP3A4 (nifedipine oxidation). By using 10 heterologously expressed P450s, it was shown that nitrosodimethylamine was mainly demethylated by CYP2E1. However, such enzyme specificity was lost with increasing size of the alkyl group. Therefore, the chain length of the alkyl group of nitrosodialkylamines determined the P450 involved in its oxidation. All these results emphasize that the catalytic site of P450 2EI has a geometric configuration such that only small molecules like nitrosodimethylamine fit favorably within the putative active site of the enzyme. Furthermore, there is good evidence that P450s other than P450 2E1, such as P450 2A6, 2C8/2C9/2C19 and 3A4, are involved in the metabolism of nitrosodialkylamines bearing bulky alkyl chains.

Cytochromes P4502E1 and P4501A1 genotypes and susceptibility to cirrhosis or upper aerodigestive tract cancer in alcoholic caucasians

Genetic polymorphisms of various cytochromes P450 have recently been described and could be implicated in the individual susceptibility of alcoholics to ethanol-related diseases. Rsal and Dral polymorphisms of CYP2E1 and Mspl polymorphism of CYP1A1 were studied in 260 controls and 511 alcoholic patients, without any clinical symptoms (n = 202) or with various ethanol-related diseases (n = 309), such as liver cirrhosis (n = 110), esophageal cancer (n = 62), upper aerodigestive tract cancer (n = 96), and other miscellaneous diseases (n = 41). Frequencies of the mutated alleles were found to be 2.5% (Rsal), 7.9% (Dral), and 8.7% (Mspl) in controls; 4%, 14.1%, and 12% in alcoholics without clinical symptoms; and 3.1%, 12.5%, and 11.2% in alcoholics with ethanol-related diseases. The only significant difference was found in the Dral polymorphism, whose frequency was enhanced in alcoholics with (p < 0.05) or without ethanol-related diseases (p < 0.01) when compared with controls. No differences were found between alcoholics without clinical symptoms and alcoholics with cirrhosis, esophageal cancer, or upper aerodigestive tract cancer. However, in liver cirrhosis and in ethanol-related cancers, the rare Dral-C allele was three times less frequent in patients under the age of 45 than in older patients, suggesting a protective role for this allele. In conclusion, our data indicate that the aforementioned mutations do not play a critical role in the development of cirrhosis, esophageal cancer, or upper aerodigestive tract cancers in Caucasians.

Simultaneous radiometric and fluorimetric detection of lauric acid metabolites using high-performance liquid chromatography following esterification with 4-bromomethyl-6,7-dimethoxycoumarin in human and rat liver microsomes

The formation of (omega-1)-hydroxylauric acid from lauric acid (LA) can be used as an indicator of the activity of cytochrome P450 2E1 (CYP2E1) in rat and human liver microsomes. A high-performance liquid chromatographic (HPLC) method that is capable of identifying and measuring the two main metabolites of lauric acid, (omega-1)- and omega-OH-LA, has been developed and used in the study of rat and human liver microsomes. Measurement of the enzymatic activities, based on the esterification of the metabolites and substrate with the fluorescent agent, 4-bromomethyl-6, 7-dimethoxycoumarin, is described using both radiometric and fluorimetric detection methods. Extraction efficiencies of metabolites and residual substrate were calculated using radioactivity and were greater than 85%. The assay is accurate and reproducible and has a detection limit of 75 pg (0.37 pmol). Additionally, a strong correlation between the two techniques was found in both human (r = 0.945, n = 15, p < 0.01) and rat (r = 0.949, n = 18, p < 0.01) livers, for the (omega-1)-hydroxylauric acid.

Human cytochrome P450 2E1 (CYP2E1): from genotype to phenotype

CYP2E1 is involved in the activation of various carcinogens, including N-nitrosamines, which are believed to be important in human carcinogenesis. Humans exhibit wide interindividual variability in levels of CYP2E1 mRNA and protein, which might explain interindividual differences in susceptibility to carcinogens activated by CYP2E1. Such variability could be due either to genetic polymorphisms observed in the CYP2E1 gene (Rsa I in the 5'-flanking region, Dra I in intron 6 and Taq I in intron 7) or to varying inducibility by xenobiotics. The aim of the present study was to establish whether, in a Caucasian population (n = 93), there existed a relationship between allelic forms of the CYP2E1 gene and the phenotype determined in vitro by hepatic ability to 6-hydroxylate chlorzoxazone. Rates of chlorzoxazone-6-hydroxylation were significantly correlated with levels of immunochemically measured CYP2E1 (p < 0.001). CYP1A2, 2C8, 2C9, 2C18, 2D6, 3A4 and 3A5 did not appear to be significantly involved in chlorzoxazone metabolism, whereas the participation of CYP1A1 could not be excluded. Frequencies of the rare alleles for the three polymorphism sites were 2.2% for RsaI, 7.5% for DraI and 8.5% for TaqI. Despite substantial interindividual variations in chlorzoxazone hydroxylase activity, no relationship between any of the three polymorphisms and CYP2E1 activity was established. Therefore, in humans, interindividual variability in CYP2E1 levels is probably due to differing induction levels as a result of environmental factors, or to genetic factors other than those studied in this work.

P450 2E1 expression in liver, kidney, and lung of rats treated with single or combined inducers

Ethanol consumption combined with smoking increase the risk of cancer in many tissues. Such a mechanism implies the involvement of cytochrome P450 alcohol (CYP2E1), which is regulated by numerous xenobiotics. The combination of P450 2E1 inducers (acetone or pyridine) and 3-methylcholanthrene during rat treatment was shown to decrease the liver P450 2E1 content while it enhanced its expression in kidney. It is suggested that this differential tissue response helps explain the organotropy of nitrosamine carcinogenicity.

Analysis of N-nitrosamines by high-performance liquid chromatography with post-column photohydrolysis and colorimetric detection

N-Nitrosamines eluted from reversed-phase HPLC were quantitatively photohydrolysed in a UV photoreactor in aqueous solution to give the nitrite ion which could be determined colorimetrically with the Griess reagent. The chromatographic behavior of N-nitroso compounds (including 19 volatile dialkyl and 7 non-volatile N-nitrosamines) was studied on three octadecylsilane columns. The capacity factor varies linearly with the number of carbons atom of the n-dialkyl chains. N-nitrosamines bearing di-n-alkyl chains with the same number of carbon atoms could be separated with a highly polar mobile phase. The yield of photohydrolysis depends upon pH and time of exposure under UV light. The response was shown to be linear in the 0-200 ng range with a limit of detection of 8 pmoles injected for N-dialkyl nitrosamines. This limit was 20 pmoles for N-nitrosamines bearing two phenyl groups. Although N-nitrosamines could be detected at 230 nm without post-column reaction, such a reaction enhances the specificity of detection in biological matrices such as gastric juice or alcoholic beverages.

Involvement of cytochrome P450 3A4 enzyme in the N-demethylation of methadone in human liver microsomes

Methadone has become one of the most widely used drugs for opiate dependency treatment. This drug is extensively metabolized by the cytochrome P450 hepatic enzyme family in man, yielding an N-demethylated metabolite that cyclizes spontaneously into 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine. The specific forms of cytochrome P450 involved in this oxidative N-demethylation were examined in a panel of 20 human liver microsomal preparations previously characterized with respect to their P450 enzyme contents. Methadone was demethylated with an apparent Km of 545 +/- 258 microM (n = 3). The metabolic rates were 745 +/- 574 pmol/(min.mg of protein). This metabolic pathway was strongly correlated with estradiol 2-hydroxylation, testosterone 6 beta-hydroxylation, nifedipine oxidation, erythromycin N-demethylation, and toremifene N-demethylation, all of these monooxygenase activities being supported by P450 3A4. Furthermore, the total P450 3A content of liver microsomal samples, determined by immuno-quantification using a monoclonal anti-human P450 3A4 antibody, was correlated with methadone demethylation (r = 0.72; p < 0.003). Methadone metabolism was 60-72% inhibited either by three mechanism-based inhibitors of P450 3A4 (gestodene, TAO, and erythralosamine) or by four reversible inhibitors of P450 3A (ketoconazole, dihydroergotamine, quercetin, and diazepam with an apparent Ki of 50 microM) and by two nonspecific inhibitors (metyrapone and SKF-525A). Conversely, quinidine (inhibitor of P450 2D6), 7,8-benzoflavone (inhibitor of P450 1A), or sulfaphenazole (inhibitor of P450 2C) did not significantly inhibit, and may even have activated, methadone metabolism. Four heterologously expressed P450 proteins were able to catalyze the N-demethylation of methadone, namely, P450 2C8, P450 2C18, P450 2D6, and P450 3A4. However, referring to their relative liver content, it can be asserted that P450 3A4 is the major enzyme involved in the N-demethylation of methadone on average. Accordingly, caution should be advised in the clinical use of methadone when other drugs are also administered that induce or inhibit P450 3A4, such as rifampicin or diazepam, respectively.

Effect of the length of alkyl chain on the cytochrome P450 dependent metabolism of N-diakylnitrosamines

Liver microsomes from control and treated rats (P4501A, 2B, 2E1-induced) metabolize at variable metabolic rates eight N-nitroso-di-n-alkylamines, including five symmetrical (N-nitroso-dimethyl, -diethyl, -dipropyl, -dibutyl and -diamyl-amines) and four asymmetrical (N-nitrosomethylethyl, methylpropyl, methylbutyl, and methylamyl-amines), into aldehydes. Thus, the longer the alkyl chain of symmetrical N-nitrosamines, the smaller was the metabolic rate of the corresponding aldehyde formation. The chain length of the alkyl group of N-nitroso-methylalkylamines modified the oxidation of the alkyl moiety: the oxidation by CYP2E1 decreased as the n-alkyl chain length increased and conversely for the oxidation by CYP1A and CYP2B. Finally, the longer the n-alkyl chain length of asymmetrical N-nitrosamines, the greater was the oxidation of methyl groups.

Determination of cytochrome P450 2E1 activity in microsomes by thin-layer chromatography using [2-14C]chlorzoxazone

A thin-layer chromatographic assay was developed as an alternative method for the determination of cytochrome P450 2E1 (CYP2E1) in microsomes using [2-14C]chlorzoxazone. After incubation of microsomes with 0.125 microCi/mmol chlorzoxazone, chlorzoxazone and its single metabolite, 6-hydroxychlorzoxazone, were extracted using chloroform-2-propanol (85:15, v/v) and chromatographed on silica gel 60 F254 plates with acetone-hexane (45:55, v/v) as solvent . The plates were then exposed to X-ray film for 2 days to localize the radiolabelled chlorzoxazone and 6-hydroxychlorzoxazone. The metabolite and substrate regions were scraped and counted in a liquid scintillation analyzer. This method is sensitive enough to determine constitutive and induced CYP2E1 activities in liver or kidney microsomes. The precision of the method was similar to that of the HPLC method. The correlation coefficient between both methods was found to be 0.97 (n = 21). Therefore, the TLC method constitutes a valuable tool for the determination of chlorzoxazone metabolism in microsomes.

Validation of the (omega-1)-hydroxylation of lauric acid as an in vitro substrate probe for human liver CYP2E1

The (omega-1)-hydroxylation of lauric acid (11-OH-LA), a model substrate of fatty acids, was previously shown to be due to CYP2E1 in rat liver microsomes. The present study examined changes in hepatic CYP2E1 content and 11-OH-LA in a panel of 29 human liver microsomes. The 11-OH-LA activity was strongly correlated with the CYP2E1 content, quantitated by immunoblot (r = 0.75) and with four monooxygenase activities known to be mediated by CYP2E1: chlorzoxazone-6-hydroxylation (r = 0.73), 4-nitrophenol hydroxylation (r = 0.84), N-nitrosodimethylamine demethylation (r = 0.79) and n-butanol oxidation (r = 0.73). The (omega-1)-hydroxylation of lauric acid was inhibited by ethanol (Ki = 3.5 mM), acetone (IC50 = 10 mM) dimethylsulfoxide, chlorzoxazone (competitive inhibitors of CYP2E1), diethyldithiocarbamate, and diallylsulfide (both selective mechanism-based inactivators of CYP2E1). The weak value of ethanol Ki on the (omega-1)-hydroxylation of lauric acid suggested that low levels of alcohol could modify fatty acid metabolism in the liver. Furafylline and gestodene, suicide substrates of CYP1A and CYP3A4, respectively, did not modify the 11-hydroxylation of lauric acid. Polyclonal antibody directed against rat CYP2E1 inhibited the formation of 11-OH-LA without affecting 12-OH-LA activity. Taken together, these results suggest that CYP2E1 is involved in the (omega-1)-hydroxylation of lauric acid in human liver microsomes, and omega-hydroxylation is mediated by another enzyme. Finally, the use of yeasts and mammalian cells genetically engineered for expression of 9 human P450s demonstrated that CYP2E1 was the one enzyme involved in the (omega-1)-hydroxylation of lauric acid.