Inhibition of benzo(alpha)pyrene metabolism catalyzed by mouse and hamster lung microsomes.
Induced and constitutive microsomal enzymes of mouse and hamster lungs catalyze both the hydroxylation of benzo(alpha)pyrene and reactions that lead to its irreversible binding to macromolecules. For mouse and hamster, the induced lung hydroxylases have Km values of 1.10 and 0.52 muM, respectively. The induced hydroxylases are strongly inhibited by 7,8-benzoflavone and are stimulated by cyclohexene oxide, an inhibitor of epoxide hydrase. Formation of the macromolecular product by the induced "binding" enzyme follows. Michaelis-Menten kinetics, except for substrate inhibition, and has Km values of 0.52 and 0.25 muM for lung microsomes from mouse and hamster, respectively. These reactions are also inhibited by 7,8-benzoflavone. The reaction catalyzed by the constitutive hydroxylase of mouse lungs is characterized by a brief lag period but proceeds in a linear fashion after the lag. The enzyme requires 60 muM benzo(alpha)pyrene to achieve maximum reaction velocity. Above this concentration, strong substrate inhibition is observed; accurate values for Vmax and Km cannot be derived. The constitutive hydroxylases are moderately inhibited by butylated hydroxytoluene, retinol, cyclohexene oxide, and 7,8-benzoflavone. The product of the constitutive "binding" enzyme is formed in a reaction that follows Michaelis-Menten kinetics. The Km value for enzymes from mouse and hamster lungs are 11.8 and 4.9 muM, respectively. Formation of this product is strongly inhibited by butylated hydroxytoluene and by retinol but not strongly by 7,8-benzoflavone or cyclohexene oxide. Since other evidence indicates that a constitutive enzyme may be involved in carcinogenesis by benzo(alpha)pyrene and since this reaction is inhibited by two known anticarcinogens, we suggest that it may be involved in this process. (+info)
The atherogen 3-methylcholanthrene induces multiple DNA adducts in mouse aortic smooth muscle cells: role of cytochrome P4501B1.
OBJECTIVE: 3-Methylcholanthrene (MC), a polycylic aromatic hydrocarbon, induces atherogenesis in mice fed an atherogenic diet. In this study, we tested the hypothesis that MC would induce DNA adducts in mouse aortic smooth muscle cells (SMCs) and that cytochrome P4501B1 (CYP1B1) plays an important role in the activation of MC to genotoxic intermediates. METHODS: Cultured SMCs were treated with MC or the vehicle dimethyl sulfoxide (DMSO), and DNA was isolated after 24 h. In some experiments, the cells were pre-treated with the CYP1B1 inhibitor 1-ethynylpyrene (EP) prior to exposure to MC. DNA adducts were determined by the 32P-postlabeling assay. Aryl hydrocarbon hydroxylase assay was measured by fluorimetry. RESULTS: MC induced formation of 12 DNA adducts that were not observed in DMSO-treated cells. DNA adduct formation was dose-dependent, with maximum response observed at 3 microM. Pre-treatment of cells with EP dramatically suppressed DNA adduct formation by MC. MC treatment caused induction of CYP1B1, but not CYP1A1. CONCLUSION: The induction of high levels of multiple DNA adducts in SMCs by MC suggests that SMCs have a functional enzymatic machinery capable of metabolically activating MC to genotoxic metabolites. The significant inhibition by EP of MC-induced DNA adduct formation indicated that CYP1B1 was the primary CYP enzyme responsible for formation of genotoxic metabolites that may play a role in the induction of atherosclerosis by MC. (+info)
Development of an assay for aryl hydrocarbon (benzo(a)pyrene) hydroxylase in human peripheral blood monocytes.
An assay has been developed and measurements of aryl hydrocarbon [benzo(a)pyrene] hydroxylase have been made in peripheral blood monocytes from a human population. Treatment with benz(a)anthracene in cell culture increased aryl hydrocarbon hydroxylase activity from 6.5 to 37-fold in monocytes from each of 25 apparently healthy donors. A weak correlation (r = 0.38) was observed between the induction ratios obtained with monocytes and lymphocytes from the same donors. Reproducibilities of the monocyte and lymphocyte assays were comparable. In monocytes, the measurement of basal and induced aryl hydrocarbon hydroxylases activity does not require pretreatment with mitogens as is the case with lymphocytes. Monocytes also exhibit a much wider range of induction ratios than do lymphocytes. (+info)
Mechanism of dioxin action: Ah receptor-mediated increase in promoter accessibility in vivo.
We have analyzed dioxin-inducible, Ah receptor-dependent changes in protein-DNA interactions at the CYP1A1 transcriptional promoter in intact mouse hepatoma cells. Our findings indicate that in uninduced cells, the promoter is inaccessible to its cognate binding proteins, which are known to be expressed constitutively. Dioxin induces, in Ah receptor-dependent fashion, an increase in promoter accessibility, which occurs rapidly and does not require ongoing transcription of the CYP1A1 gene. The change in promoter accessibility is not due to an altered pattern of cytosine methylation at the promoter; it probably reflects a 2,3,7,8-tetrachlorodibenzo-p-dioxin- induced change in the chromatin structure. These findings provide new insight into the mechanism of dioxin action and contribute to a better understanding of the regulation of inducible gene transcription in mammalian cells. (+info)
Regulation of aryl hydrocarbon (benzo-(A)-pyrene) hydroxylase activity in mammalian cells. Induction of hydroxylase activity by N6,O2'-dibutyryl8 adenosine 3':5'-monophosphate and aminophylline.
Treatment of hamster BHK cells with N6,O2'-dibutyryl adenosine 3':5'-monophosphate (Bt2cAMP), aminophylline, theophylline, or papaverine increased the level of aryl hydrocarbon (benzo(a)pyrene) hydrolxylase activity. The highese increase, 100-fold, was obtained with Bt2cAMP plus aminophylline or theophylline. N2,O2-Dibutyryl guanosine 3':5'-monophosphate gave a lower induction than Bt2cAMP. The level of hydroxylase activity started to decrease 6 hours after treatment with the inducer and was reduced to almost the uninduced level after 24 hours. Repeated addition of Bt2cAMP and aminophylline did not prevent this decrease. The hydroxylase can also be induced by treating cells with benz(a)anthracene, and the level of this induced activity was maintained for 24 hours. Aminophylline gave a 2- to 8-fold stimulation of the induction by benz(a)anthracene. The enzyme activity induced by Bt2cAMP, aminophylline, and benz(a)anthracene converted benzo(a)pyrene to similar alkali-extractable metabolities with a fluorescence spectra similar to that of 3-hydroxybenzo(a)pyrene. These induced enzyme activities also showed a similar heat stability. Induction by Bt2cAMP and aminophylline, like induction by benz(a)anthracene, required continued protein synthesis and only an initial period of RNA synthesis. Compared to the benz(a)anthracene-induced hydroxylase with a Km of 4.3 muM, the hydroxylase induced by Bt2cAMP and aminophylline showed a Km of 0.14 muM, and was 100-fold more sensitive to inhibition by 7,8-benzoflavone. Increasing the serum concentration in the culture medium stimulated the induction by aminophylline but did not stimulate induction by benz(a)anthracene. The results indicate that aryl hydrocaarbon (benzo(a)pyrene) hydroxylase can be induced by compounds that increase the level of adenosine 3':5'-monophosphate and that this induction and induced enzyme activity differs from that caused by benz(a)anthracene. (+info)
Activation of 3':5'-cyclic AMP-dependent protein kinase and induction of ornithine decarboxylase as early events in induction of mixed-function oxygenases.
The parenteral administration of a single dose of 3-methylcholanthrene to rats caused an increase in the liver of the concentration of 3', 5'-cAMP and of the activity of cAMP-dependent protein kinase (ATP:protein phosphotransferase, EC 188.8.131.52). These events were followed by an increased activity of ornithine decarboxylase (L-ornithine carboxy-lase, EC 184.108.40.206), the enzyme that controls the biosynthesis of polyamines. Finally, the activity of benzo[a]pyrene hydroxylase, as well as the amount of cytochrome P-448, was increased. Similarly, after the administration of phenobarbital, there was first an increase in the cAMP concentration and in the activity of cAMP-dependent protein kinase, then the induction of ornithine decarboxylase, and finally, an enhanced activity of ethylmorphine N-demethylase and an increased content of cytochrome P-450. These data suggest that the drug-induced processes in liver that increase the activities of the oxidative, and presumably other, drug-metabolizing enzymes include the following sequence of events: (1) increase in cAMP concentration and/or activation of cAMP-dependent protein kinase; (2) induction of ornithine decarboxylase; and, (3) induction of drug-metabolizing enzymes. (+info)
NADH-dependent aryl hydrocarbon hydroxylase in rat liver mitochondrial outer membrane.
NADH-dependent 3,4-benzpyrene hydroxylase activity was detected in the purified mitochondrial outer membrane fraction from the livers of rats treated with 3-methylcholanthrene. The specific activity in the outer membrane fraction is nearly equal to that of microsomes, a level too high to be accounted for only by the microsomal contamination. On the other hand, the NADPH-dependent 3,4-benzpyrene hydroxylase activity in the outer membrane fraction is about 50% of that of microsomes. The ratio of the specific activity of NADPH- to NADH-dependent 3,4-benzpyrene hydroxylase in microsomal fraction was about 3.5, while that of the outer membrane fraction was about 1.5. Moreover, it was found that NADH-dependent 3,4-benzpyrene hydroxylase activity in mitochondrial outer membrane from control rat liver was cyanide-insensitive, while that in microsomes was cyanide-sensitive. These results suggest the presence in the mitochondrial outer membrane fraction of aryl hydrocarbon hydroxylase activity which uses as electron donor NADH nearly to the same extent as NADPH. The hydroxylase system is composed of cyanide-insensitive cytochrome P-450 and is inducible markedly by 3-methylcholanthrene treatment. The probable electron transfer pathways in the mitochondrial outer membrane cytochrome P-450 oxidase system are discussed. (+info)
A relative deficiency of cytochrome P-450 and aryl hydrocarbon [benzo(a)pyrene] hydroxylase in hyperplastic nodules induced by 2-acetylaminofluorene in rat liver.
The concentrations of cytochrome P-450 and the activities of aryl hydrocarbon [benzo(a)pyrene] hydroxylase (AHH) and reduced nicotinamide adenine dinucleotide phosphate-cytochrome c reductase were measured in early (gray-white) and remodeled (brown) hyperplastic nodules induced in the livers of rats with 2-acetylaminofluorene and were compared to the values in control livers and in the liver surrounding the nodules. Cytochrome P-450 content of early (14 weeks) hyperplastic nodules is 30% of the activity of untreated control livers and 48% of the activity of the surrounding liver. AHH activity of the early nodules is 10% of the control activity and 33% of the activity in the surrounding nonnodular liver. Nicotinamide adenine dinucleotide phosphate-cytochrome c reductase activity in the microsomes of early nodules is 76% of the control activity and 78% of the activity in the surrounding liver. In the late remodeled nodules, (22 and 25 weeks), the cytochrome P-450 content is 40% of that of controls and AHH activity is 15% of the control activity. In primary hepatomas induced by 2-acetylaminofluorene, cytochrome P-450 content is 21% of that of controls, AHH activity is 11% of the activity of controls, and reductase is 50% of the control activity. These results, indicating a relative nodule deficiency in some of the cellular components believed to be important in the activation of hepatocarcinogens and hepatotoxins, offer one possible explanation for the relative resistance to carcinogen cytotoxicity of hyperplastic liver nodules. (+info)