Metabolic Oxidation of Carcinogenic Arylamines by Rat, Dog, and Human Hepatic Microsomes and by Purified Flavin-containing and Cytochrome P-450 Monooxygenases1

Hepatic A/-oxidation and aryl ring oxidation are generally re garded 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, 1naphthylamine (1-NA), using high-pressure liquid chromatography. Hepatic microsomes from rats, dogs, and humans were shown to catalyze the A/oxidation of 2-AF and of 2-NA, but not of 1-NA; and the rates of 2-AF A/oxidation were 2to 3-fold greater than the rates of 2-NA A/-oxidation. In each species, rates of 1-hydroxylation of 2-NA and 2-hydroxylation of 1-NA were comparable and were 2to 5-fold greater than 6-hydroxylation of 2-NA or 5and 7-hydroxylation of 2-AF. Purified rat hepatic monooxygenases, cytochromes P-450ur-A, P-450ur-H, P-450pB-B, P-450.PB-D,P-45ÛBNF-B, and P-450ISF/BNF-G but not P450pB-cor P-450.PB/PCN-E, catalyzed several ring oxidations as well as the W-oxidation of 2-AF. Cytochromes P-450pB-B,P-450BNF-B, and P-450isF/BNF-G were most active; however, only cytochrome P-45ÃœISF/BNF-G, the isosafrole-induced isozyme, catalyzed the A/ oxidation of 2-NA. The purified porcine hepatic flavin-containing monooxygenase, which was known to carry out the A/-oxidation of 2-AF, was found to catalyze only ring oxidation of 1-NA and 2-NA. No activity for 1-NA A/-oxidation was found with any of the purified enzymes. These data support the hypothesis that 1NA 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 HÃ1⁄4ckel 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 aniÃ3nintermediate that collapses to ring and A/-hydroxy products.