title: Metabolic oxidation of carcinogenic arylamines by rat, dog, and human hepatic microsomes and by purified flavin-containing and cytochrome P-450 monooxygenases.
abstract: Hepatic N-oxidation and aryl ring oxidation are generally regarded as critical activation and detoxification pathways for arylamine carcinogenesis. In this study, we examined the in vitro hepatic metabolism of the carcinogens, 2-aminofluorene (2-AF) and 2-naphthylamine (2-NA), and the suspected carcinogen, 1-naphthylamine (1-NA), using high-pressure liquid chromatography. Hepatic microsomes from rats, dogs, and humans were shown to catalyze the N-oxidation of 2-AF and of 2-NA, but not of 1-NA; and the rates of 2-AF N-oxidation were 2- to 3-fold greater than the rates of 2-NA N-oxidation. In each species, rates of 1-hydroxylation of 2-NA and 2-hydroxylation of 1-NA were comparable and were 2- to 5-fold greater than 6-hydroxylation of 2-NA or 5- and 7-hydroxylation of 2-AF. Purified rat hepatic monooxygenases, cytochromes P-450UT-A, P-450UT-H, P-450PB-B, P-450PB-D, P-450BNF-B, and P-450ISF/BNF-G but not P-450PB-C or P-450PB/PCN-E, catalyzed several ring oxidations as well as the N-oxidation of 2-AF. Cytochromes P-450PB-B, P-450BNF-B, and P-450ISF/BNF-G were most active; however, only cytochrome P-450ISF/BNF-G, the isosafrole-induced isozyme, catalyzed the N-oxidation of 2-NA. The purified porcine hepatic flavin-containing monooxygenase, which was known to carry out the N-oxidation of 2-AF, was found to catalyze only ring oxidation of 1-NA and 2-NA. No activity for 1-NA N-oxidation was found with any of the purified enzymes. These data support the hypothesis that 1-NA is probably not carcinogenic. Furthermore, carcinogenic arylamines appear to be metabolized similarly in humans and experimental animals and perhaps selectively by a specific form of hepatic cytochrome P-450. Enzyme mechanisms accounting for the observed product distributions were evaluated by Hückel molecular orbital calculations on neutral, free radical, and cation intermediates. A reaction pathway is proposed that involves two consecutive one-electron oxidations to form a paired substrate cation-enzyme hydroxyl anion intermediate that collapses to ring and N-hydroxy products.
mesh_terms: D015057:1-Naphthylamine / Q000378:metabolism*; D015081:2-Naphthylamine / Q000378:metabolism*; D000293:Adolescent; D000328:Adult; D000818:Animals; D002273:Carcinogens / Q000378:metabolism*; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004285:Dogs; D005260:Female; D005415:Flavins / Q000502:physiology*; D005449:Fluorenes / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D008875:Middle Aged; D009281:Naphthalenes / Q000378:metabolism*; D010084:Oxidation-Reduction; D051381:Rats; D011919:Rats, Inbred Strains; D013045:Species Specificity
chemical_list: D002273:Carcinogens; D005415:Flavins; D005449:Fluorenes; D009281:Naphthalenes; C012177:2-aminofluorene; D003577:Cytochrome P-450 Enzyme System; D015057:1-Naphthylamine; D015081:2-Naphthylamine