METABOLISM OF CARCINOGENIC N-NITROSO-N-METHYLANILINE BY PURIFIED CYTOCHROMES P450 2B1 AND P450 2B2

N-Nitroso-N-methylaniline (NMA) is an esophageal carcinogen in the rat , The in vitro enzymatic metabolism of NMA was investigated using cyto chromes P450 2B1 and P450 2B2, isolated from liver microsomes of rats pretreated with phenobarbital (PB), reconstituted with NADPH-cytochrom e P450 reductase and dilauroylphosphatidylcholine. Formaldehyde is pro duced by both cytochromes P350 (P450). NMA is a better substrate for P 450 2B1 than for P450 2B2. The maximal velocity (V-max) values are 3.3 and 1.6 nmol HCHO/min per nmol P450 for P450 2B1 and P450 2B2, respec tively. Beside formation of formaldehyde, aniline and p-aminophenol (p -AP) are found to be metabolites formed from NMA by both P450 isoenzym es. P450 2B1 also affords phenol, while none was found with the P450 2 B2 isoenzyme. Phenol formation presumably arose from direct alpha-C-hy droxylation of NMA via a benzenediazonium ion (BDI) intermediate. The results suggest strongly that P450 2B1 catalyzes both alpha-C-hydroxyl ation and denitrosation of NMA while P450 2B2 catalyzes only denitrosa tion. Therefore, the P450 2B1 isoenzyme participates in the activation of NMA to the ultimate carcinogenic BDI.