Oxidation of phenylhydroxylamine in aqueous solution: a model for study of the carcinogenic effect of primary aromatic amines.

Abstract

Phenyihydroxylamine is degraded in aqueous phosphate buffers at physiological pH values (6.8-7.4) to give nitrosobenzene, nitrobenzene, and azoxybenzene. The reaction is 02 dependent and subject to general acid and general base catalysis. At pH c 5.8 in cacodylate buffer, it is converted to p-nitrosophenol in addition to nitrosobenzene, nitrobenzene, and azoxybenzene. Nitrobenzene and p-nitrosophenol appear to form directly from phenylhydroxylamine. A common intermediate generated from phenylhydroxylamine and 02 is suggested to account for the formation of nitrobenzene, nitrosobenzene, and p-nitrosophenol and is consistent with kinetic studies and '80-labeling experiments. The results suggest that neither hydrogen peroxide nor superoxide (°2) are involved in the oxidation sequence.Arylhydroxylamines (AHs) are formed metabolically by the action of hepatic mixed-function oxidases on primary arylamines (1, 2). These compounds are of considerable interest because, on further reaction, they produce yet-undefined proximal carcinogens. Evidence supporting the involvement of ionic electrophilic species (1-4) and, alternatively, of free radical species (5-7) have been offered to describe the proximal carcinogen.In preliminary in vitro metabolic studies in mammalian liver preparations with various AHs, their high reactivity has been confirmed and shown in large part not to depend on the presence of active enzyme or any proteinaceous material (8, 9). The chemistry ofthe AHs in several nonaqueous systems (10-12) and at pH extremes (13, 14) has been described; however, these transformations are highly medium dependent and cannot be extrapolated to aqueous systems and in addition, these studies were carried out at high concentrations of AH (>10-2 M) (11), at which the 02 concentration is limiting.Therefore, a more intensive investigation of the reactions of AHs in aqueous environments, particularly in the physiologically relevant pH range is warranted. In this report, we describe the chemistry of PHA in aqueous metal-free cacodylate (pH 5.3-6.8) and phosphate (pH 6.3-7.4) buffers. Although it is not a potent carcinogen, PHA was used because, structurally, it is the simplest representative of this class of compounds. MATERIALS AND METHODSNitrosobenzene (PhNO), p-aminophenol (PAP), p-nitrosophenol (PNP), azoxybenzene (AzB), and nitrobenzene (PhNO2) were from Aldrich and purified before use. PHA was synthesized by reduction of PhNO2 with Zn metal (15) and recrystallized from CH2C12/C5Hl2. Because of the reported lability of hydroxylamines in the presence ofheavy metals (11), the buffers were rendered metal free by extraction with dithizone solution (16). 1802 (99 atom %) and H2180 (95 atom %) were from Prochem Isotopes (Summit, NJ).Reactions were carried out in a 250-ml aspirator bottle fitted with a gas dispersion tube to introduce gas mixtures. The vacuum take offwas sealed with a rubber septum and served as a sampling port. Metal-free buffer (50 ml) was introduced into the reaction vessel, which was flushed with argo...

Mass Spectrometric Characterization of Human Serum Albumin Adducts Formed with N-Oxidized Metabolites of 2-Amino-1-methylphenylimidazo[4,5-b]pyridine in Human Plasma and Hepatocytes

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Trifluoromethyl Vinyl Sulfide: A Building Block for the Synthesis of CF3S-Containing Isoxazolidines

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“…The amino compound 2 likely arises from the expected enzymatic reduction of 6 . The azoxy compound 3 was unexpected and may result from spontaneous oxidation of the arylhydroxylamine 6 to the corresponding nitroso compound during aerobic reaction workup and analysis, 67 followed by condensation of the nitroso with remaining 6 . 57,58 Compound 6 was fluorescent, displaying emission maxima at 442 and 470 nm similar to the helicene 4 , 54 but clearly distinct from the emission spectrum of 6-aminoquinoline 2 (Figure 6B).…”