Role of cytochromes P-450 and flavin-containing monooxygenase in the biotransformation of 4-fluoro-N-methylaniline.

In vivo and in vitro biotransformation of secondary aromatic amines was investigated using 4-fluoro-N-methylaniline as the model compound. Attention was focused on the role of cytochromes P-450 and the flavin-containing monooxygenase in formation of the various metabolic products. In vitro studies using microsomal preparations, purified reconstituted cytochromes P-450 IA1 and IIB1 and purified flavin-containing monooxygenase, demonstrated that N-demethylation, N-hydroxylation, and 4-hydroxylation accompanied by dehalogenation were all catalyzed by both the cytochrome P-450 system and the flavin-containing monooxygenase. The turnover rates of the two monooxygenase systems for the various metabolite formations and the reaction pathways involved, were shown to vary significantly. This study provides direct experimental support for the conclusion that the aromatic ring hydroxylation of secondary N-methylamines can be a consequence of flavin-containing monooxygenase catalyzed N-hydroxylation rather than of direct aromatic ring hydroxylation. The results obtained in vitro were compared with data from urine analysis of rats exposed to 4-fluoro-N-methylaniline. In this way it could be demonstrated that the major phase I biotransformation reactions for formation of urine excretable metabolites are (i) the cytochrome P-450-catalyzed N-demethylation followed by aromatic ring hydroxylation of the 4-fluoroaniline formed, and (ii) flavin-containing monooxygenase and cytochrome P-450-dependent formation of defluorinated 4-hydroxy-N-methylaniline.