M. Stiborova et al., METABOLISM OF CARCINOGENIC N-NITROSO-N-METHYLANILINE BY PURIFIED CYTOCHROMES P450 2B1 AND P450 2B2, Cancer letters, 110(1-2), 1996, pp. 11-17
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.