Theophylline N-demethylations as probes for P4501A1 and P4501A2.

Polycyclic aromatic hydrocarbons present in cigarette smoke induce cytochromes P4501A1 and P4501A2. These isozymes are of toxicological importance because they convert several environmental pollutants to reactive intermediates that form covalent adducts with cellular DNA resulting in mutations and/or malignant transformations. The aim of our research was to investigate whether theophylline metabolites could be used as probes for P4501A1 and P4501A2. It is known that at least two human P450 isozymes are involved in theophylline metabolism. The N-demethylations of theophylline to 3-methylxanthine (3-MX) and 1-methylxanthine (1-MX) appear to be mediated by P4501A1 and/or P4501A2 and the 8-hydroxylation by different isozymes. Theophylline metabolism was measured in liver microsomes from control, benzo(a)pyrene (BP)-, and isosafrole (ISO)-induced rats. Theophylline was also incubated in microsomes prepared from cells expressing high levels of human P4501A1 and P4501A2. A plot of v vs. v/S was linear for 3-MX in the ISO-induced microsomes, but nonlinear for 1-MX, indicating that P4501A2 mainly forms 3-MX, whereas P4501A1 and 1A2 probably mediate 1-MX. A similar nonlinear relation was also obtained in the BP-induced microsomes. Incubation of theophylline in the microsomes from the cells indicated that only 1-MX could be measured in cells expressing P4501A1, and both 1-MX and 3-MX were formed in the P4501A2 cell microsomes. Therefore 1-MX seems to be mediated by P4501A1/P4501A2 and 3-MX specifically by P4501A2, and theophylline N-demethylations can be used as probes for P4501A1 and P4501A2.(ABSTRACT TRUNCATED AT 250 WORDS)