Purification and characterization of an acetone-inducible cytochrome P-450 from hamster liver microsomes

Purification, molecular cloning, heterologous expression and characterization of pig CYP1A2

Porcine cytochrome P450 (CYP) 1A2 was purified to electrophoretic homogeneity from the hepatic microsomes of beta-naphthoflavone-treated male pigs. In a reconstituted system, this enzyme showed a good catalytic activity towards caffeine, acetanilide, and methoxyresorufin, all known markers of mammalian CYP1A2. Using 3'- and 5'-rapid amplification of coding DNA (cDNA) ends (RACE), we amplified from the liver RNA of control pigs a full-length 1827 bp cDNA containing an open reading frame of 1548 bp which encoded a putative CYP1A2 protein of 516 amino acids and an estimated Mr of 58 380 Da. Reverse transcriptase-polymerase chain reaction (RT-PCR) experiments showed that the messenger RNA (mRNA) of CYP1A2 was expressed in liver, heart and nasal mucosa but not in lung, small intestine, kidney and brain. Using the pCW vector containing a N-terminal modified cDNA, pig CYP1A2 was expressed in Escherichia coli. 3-[(3-Chloroamidopropyl)dimethylmmonio]-1-propane-sulfonate (CHAPS)-solubilized E. coli preparations expressing CYP1A2 produced a functionally isoform which, in a reconstituted system, was catalytically active toward ethoxyresorufin and methoxyresorufin showing K(m)'s similar to those obtained with CYP1A2 purified from pig liver or human recombinant CYP1A2. Taken together, these results demonstrate that domestic pigs have a functionally active CYP1A2 gene well expressed in the liver with biochemical properties quite similar to those corresponding to the human enzyme.

Effects of exposure to a mixture of cadmium and chromium on detoxification enzyme (GST, P450-MO) activities in the frog Rana ridibunda

The effect of two heavy metals, namely chromium and cadmium, on GST and P450-MO activities in the liver, kidney and gut of the frog Rana ridibunda, and the use of these detoxification enzyme systems as possible markers of heavy metal contamination of rivers in Northern Greece was investigated. In control animals, the highest P450-MO activity was observed in the liver and the lowest activity in the gut. As regards GST, the liver and kidneys showed the highest activity. Exposure of Rana to either Cr or a mixture of Cr and Cd caused a decrease in liver GST and P450-MO and renal GST activities. An important finding was that both GST and P450-MO activities were negatively correlated with concentrations of both metals in the liver of mixture-exposed animals. The results suggest that the determination of GST and P450-MO activities, in combination, could serve to indicate heavy metal contamination in rivers of Northern Greece.

High-level expression of recombinant rabbit cytochrome P450 2E1 in Escherichia coli C41 and its purification

Cytochrome P450 2E1 (CYP2E1) is of great interest because of its important role in the oxidation of numerous drugs and carcinogens. The yields of CYP2E1 obtained by the traditional recombinant expression systems have been relatively poor. We report here the development of a system for high-level expression of rabbit CYP2E1 in Escherichia coli strain C41 (DE3). Expression of the membrane-bound CYP2E1 by the pLW01-P450 expression plasmid, which utilizes a T7 promoter, is markedly improved by employing E. coli strain C41 (DE3). The pLW01/2E1 expression plasmid was successfully constructed and high-level expression of CYP2E1 was achieved, which ranged between 900 and 1400 nmol (liter culture)(-1). This yield was 9-14-fold higher than other reports of CYP2E1 expression in other E. coli strains. This system provides a highly efficient tool for expressing CYP2E1. An improved purification procedure for the expressed CYP2E1 involving chromatography on diethylaminoethyl cellulose (DE52), Reactive Red-agarose (type 1000-CL), and hydroxyapatite is also reported. This procedure allowed recovery of 45% of the expressed protein and CYP2E1 with a specific content of 14 nmol/mg protein, which showed a single band on a polyacrylamide gel stained with Coomassie brilliant blue.

Stabilization of cytochrome P4502E1 protein by ethanol in primary hamster hepatocyte cultures

We report the effect of ethanol on monooxygenase activities in primary hamster hepatocyte cultures maintained on collagen-coated dishes. The addition of 50mM ethanol to cell cultures both from control and ethanol pretreated animals almost completely maintained, at least for 72hr, the P4502E1-dependent aniline hydroxylase (AnH) activity and the 2E1 immunodetectable apoprotein content at the levels of the corresponding 4-hr plated hepatocytes. On the contrary, other P450-dependent monooxygenase activities, as assayed by testosterone hydroxylases, kept decreasing falling-after 72hr of culture-to the levels of the 4-hr plated hepatocytes. In both cases, in the absence of ethanol, a rapid decline of AnH activities and 2E1 apoprotein contents were also observed, attaining undetectable levels at 72hr. The hybridizable 2E1 mRNA also rapidly declined in both cultures, but such decline was not significantly altered by the presence of 50mM ethanol in the culture medium. Furthermore, we show that P4502E1 in the liver possesses a rapid degradation phase with a half-life of about 6hr. Thus, in the hamster, P4502E1 appears regulated at post-translational level, as in rat, probably by a protein stabilization mechanism.

Cloning and expression of rat CYP2E1 in Saccharomyces cerevisiae: detection of genotoxicity of N-alkylformamides

A cDNA coding for rat cytochrome P450 2E1 was cloned into the multicopy vector pYeDP60 and expressed in haploid RSY6 and diploid RS112 yeast strains of Saccharomyces cerevisiae under control of the GAL10-CYC1 promoter. Spectral and catalytic properties of the expressed 2E1 were examined in whole cells or microsomes of both strains. The level of CYP2E1 obtained in RS112 (200 pmol/mg microsomal protein) was the highest among CYP2E1 produced in the various expression systems. The monooxygenase activity in the microsomes of both strains, measured as aniline hydroxylase, was found comparable to that of control rat hepatic microsomes. In a reconstituted system in the presence of exogenous rat P450 reductase, their activity increased about 10-fold. When exposed to the carcinogen NDMA, a known 2E1 substrate, the recombination frequency determined in the 2E1-expressing RS112 cells was enhanced, in a dose-dependent manner, up to 20-fold. The exposure of the same cells to the hepatotoxic solvents, N-methyl- and N-ethylformamide, resulted in an induction of recombination frequency, which was not observed in the void plasmid containing RS112 cells in the presence of S9 hepatic fractions from pyrazole-induced rats, as a specific exogenous metabolic activation system. These results demonstrate that the 2E1-expressing cells metabolize the two N-alkylformamides to genotoxic intermediates and, therefore, they provide an useful tool to study the bioactivation mechanism of potential P450 2E1 substrates.

Effect of cigarette smoke on the mutagenic activation of environmental carcinogens by rodent liver

In order to assess the effect of cigarette smoke (CS) on metabolic enzymes, male hamsters and rats were exposed for two weeks to smoke produced in a Hamburg type II smoking machine. The livers were then used for Ames liquid incubation and western immunoblot assays. Mutagenic activities of seven heterocyclic amines (HCAs) in Salmonella typhimurium TA98 in the presence of rat or hamster liver S9 were elevated up to 3.7 times above controls (including sham smoke control). Enhancement of mutagenic activities of PhIP and aflatoxin B(1) was observed only in CS-exposed hamster, whereas no significant alteration of mutagenicity was observed with 2-aminofluorene, benzo[a]pyrene, and 3'-hydroxymethyl-N, N-dimethyl-4-aminoazobenzene in strain TA98 or with six N-nitrosodialkylamines in strain TA100. 7,8-Benzoflavone and/or furafylline considerably inhibited the mutagenic activation of IQ and Trp-P-1 in the presence of liver S9 from untreated hamsters and sham smoke- or CS-exposed hamsters and rats, indicating the predominant involvement of hamster cytochrome P450 (CYP) 1A enzymes in the metabolic activation of HCAs. In addition, the data suggest that CS-exposure may selectively induce hepatic CYP1A1/1A2 isoforms. Western immunoblot analyses of liver microsomes using anti-rat CYP antibodies revealed that CS-exposure increased the levels of hamster CYP1A2 (3.9-fold) and rat CYP1A2 (3.0-fold) and CYP1A1, without significant change in the levels of CYP2E1 and CYP2B and 3A isoforms in each species. The presently observed selective induction of HCA activation and CYP isozymes due to CS supports the idea that CS may contribute to enhancing effects on initiation by carcinogens which are metabolically activated by hepatic CYP1A1/1A2. In conjunction with results observed for smokers, the present findings indicate that the hamster is a good animal for studies with CS, and that cigarette smoking in combination with intake of heating protein-rich foods as a life style may markedly contribute to the human carcinogenesis by HCAs.

Purification and characterization of three constitutive cytochrome P-450 isoforms from bovine olfactory epithelium

Three constitutive forms of cytochrome P-450 (P-450s) were isolated from olfactory microsomes of cattle. The purified P-450s, designated P-450bov1, P-450bov2 and P-450bov3, were electrophoretically nearly homogeneous by SDS/PAGE and their apparent relative molecular masses were estimated to be 50000, 53000 and 51000 respectively. As indicated by several criteria including the N-terminal sequence and absorption spectra, the three olfactory forms of P-450 were distinct from each other and from all the other P-450s currently known in cattle. P-450bov1 and P-450bov2 were purified in the low-spin state, whereas P-450bov3 was in the high-spin state. Studies to evaluate, by Western blot analysis, the reactivity of these purified P-450s with antibodies raised against rat hepatic P-450 2E1, 2B, 1A and 3A and rabbit olfactory P-450NMa and P-450NMb showed that P-450bov3 strongly cross-reacted with anti-P-450NMb IgG, and P-450bov1 moderately with anti-P-450NMa IgG. As determined by immunoblots, P-450bov1 and P-450bov3 represented a great portion of the total olfactory P-450. In a reconstituted system with NADPH:cytochrome P-450 reductase and phospholipids, P-450bov1 was more active in the metabolism of xenobiotic compounds (i.e. O-de-ethylation of ethoxycoumarin and N-demethylation of hexamethylphosphoramide) than towards endogenous substrates (testosterone and progesterone). Conversely, P-450bov3 metabolized the xenobiotics at lower rates but exhibited total oxidation rates of the above sex hormones higher than those of P-450bov1. From the comparison of the catalytic, immunochemical and structural properties, it was inferred that P-450bov1 and P-450bov3 are the bovine orthologues of P-450NMa (2A) and P-450NMb (2G1) respectively, the only two olfactory P-450s previously purified from rabbit. P-450bov2, which showed low activity toward some exogenous and endogenous compounds, represents a novel purified olfactory hemoprotein possibly belonging to the 3A subfamily. These results are consistent with a specific presence of catalytically and structurally similar P-450s, at least for the major ones, in the olfactory mucosa of mammals.

Microsomal oxidation of N,N-diethylformamide and its effect on P450-dependent monooxygenases in rat liver

N,N-Diethylformamide (DEF) is a hepatotoxic polar solvent in which metabolism has not been investigated. In this study we examined the following: (a) the oxidative metabolism of DEF using both liver microsomes from rats pretreated with selected P450 inducers and purified P450 enzyme (2B1, 2E1, 2C11); and (b) the effect of administration of DEF and its metabolite, the monoethylformamide (MEF), on induction and/or inhibition of the P450 isoforms in rats. DEF was deethylated by microsomal P450-dependent oxidation forming acetaldehyde and MEF according to Michaelis-Menten kinetic parameters. Microsomes from rats pretreated with acetone and pyrazole (selective P4502E1 inducers) or rats pretreated with dexamethasone and 200 mg/kg DEF were able to deethylate DEF in a biphasic manner, showing a low Km component with a Vmax of about 0.2 nmol/(min.mg of protein) and a Km between 70 microM and 250 microM. The low Km component was not present in control microsomes or in microsomes from rats treated with phenobarbital, beta-naphthoflavone, or clofibrate, where linear Kinetics were observed. The use of purified P4502E1 and 2C11 in a reconstituted system showed that 2E1, which oxidized DEF with a Vmax of 4.5 nmol/(min.nmol of P450) and a Km of 0.7 mM, can partially account for the low Km DEF deethylase, whereas 2C11, which oxidized DEF with a Vmax of 4.8 nmol/(min.nmol of P450) and a Km of 17 mM, might be the high Km deethylase. The purified 2B1 was barely able to deethylate DEF. A confirmation of the role of 2E1 in DEF metabolism was obtained by using various selective inhibitors of P450 isoforms and immunoprecipitation experiments with anti P4502E1 IgG. The low Km component of DEF deethylation in acetoneor pyrazole-induced microsomes was strongly inhibited (approximately 90%) by diethyldithiocarbamate, 4-methylpyrazole, and anti-2E1 IgG, but in 200 mg/kg DEF-induced microsomes the inhibition was partial, suggesting that other P450(s) may be involved. Administration of DEF 200 mg/kg ip for 4 days induced hepatic microsomal P4502E1-dependent aniline hydroxylase, P4502B1/2-linked pentoxyresorufin O-depentylase, 16 beta-testosterone hydroxylase P4503A1/2-associated erythromycin N-demethylase, and 6 beta-testosterone hydroxylase. Alternatively, the same dose regimen of MEF induced only the aniline hydroxylase and depressed the 3A1/2-linked activities. Immunoblot experiments verified these data. These findings indicate that DEF, at low concentrations, is predominantly oxidized by P4502E1 and that this enzyme may be induced in rodents by repeated MEF or DEF treatment, thereby increasing their own metabolism and potentially their cytotoxicity through the formation of ethyl isocyanate.

Antioxidant activity of diethyldithiocarbamate

Diethyldithiocarbamate (DDC), a potent copper chelating agent, has long been used for the treatment of oxygen toxicity to the central nervous system, as an immunomodulator to treat cancer, and in HIV-infected patients. We evaluated the antioxidant properties of DDC, including its scavenging of reactive oxygen species, its reducing properties, its iron-chelating properties, and its protective effects on oxidant-induced damage to brain tissue, protein, human LDL, and DNA. It is found that DDC is a powerful reductant and antioxidant since it scavenges hypochlorous acid, hydroxyl radical and peroxynitrite; it chelates, then oxidizes ferrous ions; it blocks the generation of hydroxyl radicals and inhibits oxidative damage to deoxyribose, protein, DNA, and human LDL. These findings may provide an explanation for the apparent beneficial effects of DDC against oxidative stress-related diseases that have been observed in experimental and clinical studies.

Microsomal metabolism of N,N-diethylacetamide and N,N-dimethylacetamide and their effects on drug-metabolizing enzymes of rat liver

This study was undertaken to investigate: (1) the effect of N,N-diethylacetamide (DEAC) and N,N-dimethylacetamide (DMAC) administration to rats on drug-metabolizing enzymes in the liver; (2) the in vitro dealkylation of DEAC and DMAC by hepatic microsomes from rats treated with various P450 inducers and purified P450 (2B1 and 2E1). DEAC administration at doses of 100-300 mg/kg i.p. for 3 days mostly induced P450 2B1/2-associated hepatic microsomal monooxygenase activities (pentoxyresorufin O-depenthylase and the 16 beta-testosterone hydroxylase) and its own dealkylation (DEAC deethylase activity). P4502E1-linked monooxygenase activities, such as aniline and p-nitrophenol hydroxylases, were not affected. DEAC treatment increased the amount of P4502B1/2 in microsomes in a dose-dependent manner, but depressed the amount of P-4502C11 as assayed by western blotting. DMAC treatment did not alter any microsomal monooxygenases or phase II enzymatic activity. The oxidative metabolism of DEAC and DMAC with control and induced microsomes resulted in the dealkylation of these solvents, giving rise to acetaldehyde and formaldehyde, respectively. The kinetic parameters for these N-dealkylations were investigated. It was found that phenobarbital-, dexamethasone- and DEAC-induced microsomes deethylated DEAC with a Vmax approximately 3-fold of control-, ethanol- or beta-naphtoflavone-induced microsomes, although with a similar affinity; ethanol- or acetone-induced microsomes demethylated DMAC with a Vmax higher than that of control microsomes. In a reconstituted system, the purified P4502B1 dealkylated DEAC, but not DMAC, at the rate of 6.2 nmol/min/nmol P450, whereas purified P4502E1 dealkylated DMAC, but not DEAC, at the rate of 7.9 nmol/min/nmol P450. Oxidation of DEAC and DMAC were markedly inhibited in microsomes from DEAC-treated rats by anti-P4502B1 IgG and in microsomes from acetone-treated rats by anti-P4502E1 IgG, respectively. These results indicate that DMAC and DEAC are predominantly oxidated by different P450 isozymes and that only DEAC, when administered to rat, is capable of altering the expression of the hepatic P450 system. This latter feature could be related to the higher toxicity reported for DEAC.

Molecular cloning and sequence analysis of hamster CYP2E1

Two cDNA clones, pHSj31 and pHSj3, coding for CYP2E1 were isolated from a hamster liver cDNA library. The sequence analyses revealed that they encoded the same polypeptide of 493 amino acid residues (M(r) = 56,616) and differed from the length of their 3'-untranslated region. The deduced amino acid sequence of hamster CYP2E1 showed approx. 90% identities with those of the rats and mice, and approx. 80% identities with those of the rabbits, monkeys and humans. The NH2-terminal 35 deduced amino acid sequences of hamster CYP2E1 were completely identical with purified protein, ha P-450j. The Northern blot analysis showed that CYP2E1 was expressed in livers and to lesser extents in kidneys and lungs.

Acetone-dependent regulation of cytochromes P4502E1 and P4502B1 in rat nasal mucosa

The inducibility and molecular regulation of cytochrome P4502E1 (CYP2E1) has been examined in nasal mucosa of rats after acetone treatment and compared to that of cytochrome P4502B1 (CYP2B1). Twenty-four hours following treatment with acetone (5 mL/kg) for 2 days, the amount of CYP2E1 as well as the rate of microsomal 4-nitrophenol hydroxylase activity had increased by a factor of 2-3, in microsomes isolated from nasal mucosa. The increase in CYP2E1 was accompanied by a corresponding increase of CYP2E1 mRNA, as determined by northern and slot blot analyses. In contrast, hepatic and renal CYP2E1 mRNA, studied in the same rats, did not increase, despite the fact that the amount of CYP2E1 was increased 3- and 5-fold, respectively. The amount of CYP2B1, an isozyme known as acetone-inducible in other tissues, decreased significantly by acetone, as detected by immunoblot analysis. After 48 hr, the amount of CYP2E1 enzyme, the level of CYP2E1 mRNA and the rate of 4-nitrophenol hydroxylase activity had returned to normal levels, whereas in liver and kidneys the immunoreactive protein remained 3-4-fold higher than control. The results indicate that acetone does not regulate CYP2E1 in nasal mucosa by post-translational mechanisms, in contrast to the situation observed in liver and kidneys. This indicates a tissue-specific expression of post-translational regulatory systems responsible for P450 stabilization. Furthermore, nasal CYP2B1 also seems to be regulated in a tissue-specific manner by acetone.