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Reductive Metabolism of Aromatic Nitro Compounds Including Carcinogens by Rabbit Liver Preparations1

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Reductive Metabolism of Aromatic Nitro Compounds Including Carcinogens by Rabbit Liver Preparations1 [CANCER RESEARCH 46, 1089-1093, March 1986] Reductive Metabolism of Aromatic Nitro Compounds Including Carcinogens by Rabbit Liver Preparations1 Kiyoshi Tatsumi,2 Shigeyuki Kitamura, and Noriko Narai3 Institute of Pharmaceutical Sciences, Hiroshima University School of Medicine, 1-2-3, Kasumi, Minami-ku, Hiroshima 734, Japan ABSTRACT somal and cytosolic fractions of mammalian livers (15, 18, 20, 22-24). Previous studies showed that a cytochrome P-450 sys Reductive metabolism of aromatic nitro compounds was ex tem (23, 25) and xanthine oxidase (23) are involved in the amined with rabbit liver preparations. Under anaerobic condi reduction of nitrobenzene orp-nitrobenzoic acid catalyzed by rat tions, carcinogenic 2-nitrofluorene, 4-nitrobiphenyl, and 1-nitro- liver microsomes and cytosol, respectively. In a preliminary com naphthalene were reduced to the corresponding hydroxylamines munication (18), recently, we demonstrated that a microsomal and amines, whereas the carcinogenic 1-nitropyrene was re cytochrome P-450 system and cytosolic aldehyde oxidase of duced only to the corresponding amine by liver cytosol in the rabbit liver catalyze the reduction of 1-nitropyrene and 2-nitroflu presence of 2-hydroxypyrimidine, an electron donor of aldehyde orene to the corresponding amines. More recently, the partici oxidase. These metabolites were identified unequivocally by pation of cytochrome P-450 in nitroreduction was further con comparing their mass spectra and thin-layer Chromatographie firmed with a reconstituted cytochrome P-450 system from rat behaviors with those of the authentic samples. liver microsomes (20). Both liver microsomes and cytosol catalyzed the reduction of The present study shows the first example of enzymatic these aromatic nitro compounds in varying degrees. The micro- formation of hydroxylamine derivatives from 2-nitrofluorene, 4- somes required reduced pyridine nucleotides for occurrence of nitrobiphenyl, and 1-nitronaphthalene. In addition, the study pro the nitroreductase activities. In this case, reduced nicotinamide vides the first evidence that aldehyde oxidase functions as a adenine dinucleotide phosphate was more effective than reduced major liver enzyme responsible for the reduction of aromatic nitro nicotinamide adenine dinucleotide as an electron donor. The compounds including carcinogens. cytosol by itself exhibited some nitroreductase activities, which were markedly enhanced by addition of an electron donor of aldehyde oxidase, i.e., A/1-methylnicotinamide or 2-hydroxypyr MATERIALS AND METHODS Chemicals. 2-Nitrofluorene, 1-aminopyrene, 2-acetylaminofluorene, imidine. The full activities of the cytosol with the electron donor A/'-methylnicotinamide chloride, 2-hydroxypyrimidine hydrochloride, di- were much higher than those of the microsomes with the reduced phenylamine, phenothiazine, and benzamide were purchased from Tokyo pyridine nucleotide. Purified liver aldehyde oxidase, like the cy Chemical Industry Company, Ltd. 2-Aminofluorene, xanthine, menadi- tosol, exhibited significant nitroreductase activities in the pres one, sodium arsenite, and potassium cyanide were obtained from Nakafai ence of its electron donor. Chemical, Ltd. NADPH, NADH, and chlorpromazine were obtained from These results indicated that cytosolic aldehyde oxidase func Sigma Chemical Company. 4-Nitrobiphenyl and 4-aminobiphenyl were tions as a major enzyme responsible for the reduction of aromatic purchased from Aldrich Chemical Company. 1-Nitronaphthalene and nitro compounds including carcinogens in rabbit liver. bovine serum albumin were purchased from Katayama Chemical Industry Company, Ltd. 1-Aminonaphthalene was obtained from Ishizu Pharma ceutical Company, Ltd., and p-nitrobenzoic acid was from Yoneyama INTRODUCTION Chemical Industries, respectively. Cyproheptadine was donated by Nip pon Merck-Banyu Company, Ltd. Certain aromatic nitro compounds have been proved to be 1-Nitropyrene (mp 148°C)was synthesized by the method of Ristagno carcinogenic (1-5), some of which have been noticed as impor and Shine (26), 2-hydroxylaminofluorene (mp 169-173°C) by that of tant environmental pollutants (5-10). The mechanism of carcin- Poirier eíal. (27), W-acetoxy-2-acetylaminofluorene (mp 176-178°C) by ogenicity is thought to be mediated by the metabolic reduction that of Gutmann and Erickson (28), 1-hydroxylaminonaphthalene (mp of these nitro compounds to reactive hydroxylamine intermedi 76-78°C) by that of Willstätter and Kubli (29), and 4-hydroxylaminobi- ates (11-15). However, previous works showed that the anaer phenyl (mp 178-180°C) by the method of Mäheref al. (30), respectively. obic incubation of carcinogenic 1-nitropyrene (16-20), 2-nitroflu Enzymes. Bovine liver catalase was purchased from Sigma Chemical orene (18, 21), 4-nitrobiphenyl (15), 2-nitronaphthalene (15), and Company. Male albino rabbits weighing 2.0-2.5 kg were used in this 1-nitronaphthalene (15) with mammalian liver preparations led to study. Rabbit liver aldehyde oxidase was purified by the method of the formation of the corresponding amines, with no evidence for Rajagopalan ef a/. (31). Rabbit liver microsomes and cytosol were prepared as follows. The liver was homogenized in 4 volumes of 1.15% the formation of the corresponding hydroxylamines. The reduction of aromatic nitro compounds occurs with micro- KCI, the homogenate was centrifugea for 20 min at 9,000 x g, and the 9,000 x g supernatant fraction was centrifuged for 60 min at 105,000 x g. The microsomal fraction was washed by resuspension in the KCI Received 7/22/85; revised 11/18/85; accepted 11/19/85. solution and resedimentation for 60 min at 105,000 x g. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in Isolation of Metabolites. The incubation mixture consisted of 12 //mol accordance with 18 U.S.C. Section 1734 solely to indicate this fact. of a nitro compound, 120 (¿molof2-hydroxypyrimidine, and liver cytosol 1This work was supported in part by a grant-in-aid from the Ministry of (equivalent to 12 g of liver) in a final volume of 150 ml of 0.1 M phosphate Education, Science, and Culture, Japan. 2 To whom requests for reprints should be addressed. buffer (pH 7.4). The nitro compound was placed in the side arm of the 3 Present address: Faculty of Pharmaceutical Sciences, Setsunan University, Thunberg apparatus separate from the rest of the incubation mixture 45-1, Nagaotogecho, Hirakata-shi, Osaka 573-01, Japan. which was placed in the main tube. The incubation was carried out CANCER RESEARCH VOL. 46 MARCH 1986 1089 Downloaded from cancerres.aacrjournals.org on September 27, 2021. © 1986 American Association for Cancer Research. REDUCTIVE METABOLISM OF AROMATIC NITRO COMPOUNDS anaerobically as follows. The tube was gassed for 5 min with nitrogen, was evaporated to dryness in a vacuum. The residue was dissolved in which was passed through a deoxygenizing solution consisting of 0.5% 50 iil of methanol and then applied to a Gilson 1B high-pressure liquid sodium dithionite and 0.05% sodium 2-anthraquinonesulfonate in 0.4% Chromatograph equipped with a M & S Variactor 311 UV absorption NaOH, evacuated with an aspirator for 10 min, and again gassed with detector. The instrument was fitted with a 15-cm x 4.6-mm inner diameter the nitrogen. After the tube was tightly closed, the reaction was started M & S da column. The Chromatograph was also operated at ambient by mixing a nitro compound of the side arm and all other components of temperature and at a wavelength of 254 nm. In the determination of 4- the body together, and it was continued for 30 min at 37°C. After nitrobiphenyl metabolites, the mobile phase was 0.1 M KH2PO4:CH3CN incubation, the mixture was extracted once with an equal volume of ethyl (3:2), and the flow rate was 1 ml/min. acetate, the ethyl acetate extract was evaporated to dry ness in a In the case of 1-nitropyrene, the incubation mixture, after adding 0.31 vacuum, and the residue was subjected to TLC4. TLC was conducted ,<mol of cyproheptadine as an internal standard, was adjusted to pH 10 on silica gel plates (Kieselgel 60 GF25<; Merck; 0.25 mm thick) and with N NaOH and extracted once with 5 ml of n-heptane. The n-heptane developed in benzene:acetone (7:3), and the chromatograms were vis extract was evaporated to dryness in a vacuum, and the residue, after ualized under UV light (254 nm). Metabolites were eluted with methanol being dissolved in 50 ¿tlof methanol, was applied to a Shimadzu GC- from the TLC plates and subjected to mass spectrometric study. Mass 4CM gas Chromatograph equipped with a hydrogen flame ionizing de spectra were taken with a Shimadzu 7000 mass spectrometer at an tector. The instrument was fitted with a 2-m x 3-mm inner diameter glass ionizing voltage of 70 eV. These procedures were quickly conducted in column packed with 3% Dexsil 400 GC on Chromosorb W. The operating a nitrogen atmosphere and in the dark as much as possible, to preclude conditions were as follows: injection port and detector temperature, the loss of the hydroxylamine derivatives formed. The assay of nitrored- 310°C; column temperature, 260°C. uctase activity described below was also performed under such precau Under the assay conditions, 94% of 2-nitrofluorene, 85% of 4-nitrobi tions. phenyl, and 73% of 1-nitronaphthalene that disappeared were recovered Assay of Nitroreductase Activity. The incubation mixture consisted as the corresponding hydroxylamines and amines,
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