The reductive metabolism of halogenated alkanes by liver microsomal cytochrome P450☆
Abstract
Under anaerobic conditions various polyhalogenated alkanes (CCl3-CCl3, HCl2C-CCl3, CF3-CCl3, CCl4, CF3-CHClBr) stimulate the oxidation of NADPH by liver microsomal fractions. The participation of cytochrome P450 in the NADPH oxidation was shown by inducers and inhibitors of the monooxygenase system. The products of the reductive pathway of hexachloroethane were tetrachloroethene (99.5%) and pentachloroethane (0.5%). From pentachloroethane as substrate trichloroethene (96%) and tetrachloroethane (4%) were produced. The stoichiometry of NADPH oxidation and product formation was close to 1:1. There was a synergistic effect in the presence of NADPH and NADH for both hexa- and pentachloroethane. The influence of dioxygen and radical traps (RSH) on the formation of products from hexachloroethane with reduced cytochrome P450 has been investigated. The results indicate the possibility of a reductive in vivo metabolism of polyhalogenated alkanes even at physiological dioxygen concentrations. For the reductive dehalogenation of polyhalogenated alkanes by microsomal cytochrome P450 a reaction scheme is proposed: the reduction proceeds by two subsequent one electron reductions forming first a radical and then a carbanion. The carbanion can form an alkene via β-elimination of chloride.
References (34)
- B.R. Brown et al.
Biochem. Pharmac.
(1977) - P.H. Hernandez et al.
Biochem. Pharmac.
(1967) - H.J. Ahr et al.
Biochem. Pharmac.
(1980) - A.G. Gornall et al.
J. biol. Chem.
(1949) - T. Omura et al.
J. biol. Chem.
(1964) - P. Strittmatter et al.
- A.G. Hildebrandt et al.
Biochem. biophys. Res. Commun.
(1969) - A. Hildebrandt et al.
Archs Biochem. Biophys.
(1971) - O. Pelkonen et al.
FEBS Lett.
(1975) - A. Stier
Biochem. Pharmac.
(1964)
CRC crit. Rev. Tox.
Anesthesiology
Naunyn Schmiedeberg's Arch. Pharmac.
Molec. Pharmac.
J. Biochem.
Molec. Pharmac.
Xenobiotica
Cited by (87)
Hexachloroethane
2014, Encyclopedia of Toxicology: Third EditionHexachloroethane (HCE) as a colorless, crystalline substance may be toxic by variety of routes such as inhalation, ingestion, and dermal contact. It is used in explosives and pyrotechnics industries as well as a vulcanizing and polymer additive agent in the manufacture of smoke candles and grenades. Moreover, it is used in metal refining processes. Effects on the human body may include irritation of the mucous membranes, skin, lung, and cornea; liver and kidney damage; and central nervous system depression. Also, it should be noted that the Integrated Risk Information System has placed HCE on a list of substances that it considers possibly carcinogenic for human, and both the US Occupational Safety and Health Administration permissible exposure limit and the American Conference of Governmental Industrial Hygienists threshold limit value for HCE have been set at a time-weighted average of 1 ppm. HCE may burn with the production of irritating and poisonous gases carbon tetrachloride and tetrachloroethylene. Fires involving HCE may be extinguished with dry chemical, CO2, halogen, water spray, fog, or standard foam. It should be stored in cool, dark areas, away from alkalies and metals.
The phytotoxic effect of C<inf>1</inf>/C<inf>2</inf>-halocarbons and trichloroacetic acid on the steppe plant Artemisia lerchiana
2006, ChemosphereArtemisia lerchiana is a wormwood species of the Central Asian steppe regions, where it completely cover whole areas. For the first time it was possible to show through field experiments that C1/C2 halocarbons (VCHCs), such as chloroform (CHL), tetrachloroethene (PER) and hexachloroethane (HEX), can be taken up by test plants of the species A. lerchiana via the soil/root pathway and metabolised inter alia into trichloroacetic acid (TCA) under semi-aride conditions. At the same time, chlorophyll a fluorescence measurements carried out on the test plants revealed a phytotoxic influence on plant vitality (max. decline in vitality of 52% with application of CHL) and less efficient energy flows in the photosynthesis mechanism of the A. lerchiana test plants. The authors examine possible links between the simultaneous appearance of VCHCs and additional drought stress in the acceleration of desertification processes.
Failure of the standard battery of short-term tests in detecting some rodent and human genotoxic carcinogens
2004, ToxicologyTheoretical reasons and experimental evidence indicate that a no-effect level generally cannot be expected for genotoxic carcinogens; as a consequence, in quantitative risk assessment the capability of distinguishing genotoxic from non-genotoxic carcinogens is of fundamental importance in order to identify relevant levels of human exposure. According to generally accepted guidelines, the standard three-test battery for the detection of genotoxic compounds consists of: (i) an in vitro test for gene mutation in bacteria; (ii) an in vitro test in mammalian cells with cytogenetic evaluation of chromosomal damage and/or a test that detects gene mutations; (iii) an in vivo test for chromosomal damage using rodent hematopoietic cells. This test battery is designed to avoid the risk of false negative results for compounds with genotoxic potential, but it cannot be taken for granted that the risk is completely eliminated. As a matter of fact there are some chemicals, classified by the International Agency for Research on Cancer (IARC) as probably or possibly carcinogenic to humans, which gave consistent negative results in this test battery, and in contrast provided positive results in other not routinely employed genotoxicity assays. The failure of the standard test battery in detecting some genotoxic carcinogens is attributable to several causes, but the principal of them are the following ones: in vitro, the artificial metabolic activity of the liver S9-mix, and the different biotransformation of chemicals in cells of different type and from different animal species; in vivo, the pharmacokinetic behaviour of the test compound, and its possible species-, sex- and tissue-specificity.
Novel anthraquinone derivatives with redox-active functional groups capable of producing free radicals by metabolism: Are free radicals essential for cytotoxicity?
1999, European Journal of Medicinal ChemistryThe mode of action of antitumour anthraquinone derivatives (i.e. mitoxantrone) is not clearly established yet. It includes, among others, intercalation and binding to DNA, bioreduction and aerobic redox cycling. A series of anthraquinone derivatives, with potentially bioreducible groups sited in the side chain, have been synthesized and biologically evaluated. Their redox and cytotoxic activities were screened. Derivatives which bear a 2-(dimethylamino)ethylamino substituent, known to confer high DNA affinity, demonstrated cytotoxicity but not redox activity (beside the anthraquinone reduction). Conversely, derivatives which showed redox activity were not cytotoxic toward the P388 cell line. The results suggest that bioreduction is not the main mode of action in the cytotoxicity of anthraquinones.
Bioactivation of organic nitrates and other nitrovasodilators
1996, Advances in Drug ResearchThis chapter discusses nitric oxide and cyclic guanosine monophosphate as mediators of organic nitrate action and formation of nitric oxide from organic nitrates in cell-free systems. Organic nitrates have been classified as prodrugs releasing the pharmacologically active metabolite nitric oxide. Although several different pathways have been suggested, recent evidence points to a cytochrome P450 system playing an important role in the process of enzymatic nitric oxide generation from organic nitrates. The chapter attempts to answer some unanswered questions such as, “Is cytochrome P450 relevant for the action of organic nitrates in vivo?” and “Is cytochrome P450 involved in nitrate tolerance?” Although there is considerable evidence that cytochrome P450 mediates nitric oxide formation from organic nitrates in vitro, the relevance of this pathway in vivo has still to be determined. It is possible to restore nitric oxide production and inhibit intimal hyperplasia in endothelium-denuded rat carotid arteries by in vivo transfer of the endothelial cell nitric oxide synthase gene. This finding suggests nitric oxide synthase to be a potential target for gene therapy in disease states such as atherosclerosis or restenosis following balloon angioplasty.
The effect of varying halogen substituent patterns on the cytochrome P450 catalysed dehalogenation of 4-halogenated anilines to 4-aminophenol metabolites
1995, Biochemical PharmacologyThe cytochrome P450 catalysed biotransformation of 4-halogenated anilines was studied in vitro with special emphasis on the dehalogenation to 4-aminophenol metabolites. The results demonstrated that a fluorine substituent at the C4 position was more easily eliminated from the aromatic ring than a chloro-, bromo- or iodo-substituent. HPLC analysis of in vitro biotransformation patterns revealed that the dehalogenation of the C4-position was accompanied by formation of non-halogenated 4-aminophenol, without formation of NIH-shifted metabolites. Changes in the apparent Vmaxfor the microsomal oxidative dehalogenation appeared to correlate with the electronegativity of the halogen substituent at C4, the fluorine substituent being the one most easily eliminated. A similar decrease in the rate of dehalogenation from a fluoro- to a chloro- to a bromo- to an iodo-substituent was observed in a system with purified reconstituted cytochrome P450 IIB1, in a tertiair butyl hydroperoxide supported microsomal cytochrome P450 system as well as in a system with microperoxidase 8. This microperoxidase 8 is a haem-based mini-enzyme without a substrate binding site, capable of catalysing cytochrome P450-like reaction chemistry. Together, these results excluded the possibility that the difference in the rate of dehalogenation with a varying C4-halogen substituent arose from a change in the contribution of cytochrome P450 enzymes involved in oxidative dehalogenation with a change in the halogen substituent. Rather, they strongly suggested that the difference was indeed due to an intrinsic electronic parameter of the various C4 halogenated anilines dependent on the type of halogen substituent. Additional in vitro experiments with polyfluorinated anilines demonstrated that elimination of the C4-fluorine substituent became more difficult upon the introduction of additional electron withdrawing fluorine substituents in the aniline-ring. 19F-NMR analysis of the metabolite patterns showed that the observed decrease in 4-aminophenol formation was accompanied by a metabolic switch to 2-aminophenols and N-hydroxyanilines, while products resulting from NIH-type mechanisms were not observed. For a C4chloro-, bromo-, or iodo-substituted 2-fluoroaniline the Vmax for the oxidative dehalogenation was reduced by the additional electron withdrawing fluorine substituent at the C2 position in a similar way. In conclusion, the results of the present study strongly indicate that the possibilities for cytochrome P450 mediated dehalogenation of 4-halogenated anilines to 4-aminophenol metabolites are dependent on: (i) the characteristics of the halogen that has to be eliminated, the most electronegative and smallest halogen being the one most easily eliminated, and (ii) the electron-withdrawing capacities of other substituents in the aromatic ring, electron-withdrawing substituents decreasing the relative rate of the reaction. Together these data lead to the conclusion that the halogen is eliminated as a halogen anion.
- ☆
This work is part of the thesis of Hans J. Ahr at the Universität des Saarlandes, F.R.G.