Involvement of CYP2E in 8-hydroxylation of theophylline in mouse hepatic microsomes-difference from its N-demethylations.
We examined which cytochrome P-450 (P-450) species other than CYP1A participates in the oxidative metabolism of theophylline (TP) in mouse hepatic microsomes. Among the three metabolic pathways of TP, only 8-hydroxylation was selectively enhanced by acetone, a potent inducer of CYP2E. We assumed that two P-450 populations with different metabolic ability were involved in this metabolic process, and kinetic analyses revealed that the enhancement was due to the induction of a high-capacity P-450 population. The 8-hydroxylation at a substrate concentration, where most of the total activity was attributed to the catalysis of the high-capacity phase, was markedly impaired by CYP2E inhibitors such as 4-methylpyrazole and aminoacetonitrile, whereas the N-demethylations were little affected by these agents. The activity of TP 8-hydroxylation was significantly correlated with that of p-nitrophenol hydroxylation, a probe for CYP2E, in untreated microsomes. The activities of these oxidative reactions were modified to a similar degree by known enzyme inhibitors with a range of inhibitory potencies and affinity for P-450 isoforms. On the other hand, a relationship between TP N-demethylations and p-nitrophenol hydroxylation was not apparent, but there was a behavioral similarity between the two types of N-demethylations. The results indicated that TP 8-hydroxylation, which accounts for a large portion of TP oxidations, involves CYP2E, and that its N-demethylations are mediated by a common or closely similar P-450 species distinct from CYP2E.
Konishi
H
H
Department of Hospital Pharmacy, Shiga University of Medical Science, Japan.
Morita
K
K
Yamaji
A
A
D000096:Acetone / Q000494:pharmacology; D000607:Aminoacetonitrile / Q000494:pharmacology; D000632:Aminopyrine / Q000378:metabolism; D000818:Animals; D019392:Cytochrome P-450 CYP2E1 / Q000378:metabolism; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D000077604:Fomepizole; D006900:Hydroxylation; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D008862:Microsomes, Liver / Q000201:enzymology* / Q000378:metabolism; D009596:Nitrophenols / Q000378:metabolism; D010078:Oxazines / Q000378:metabolism; D010634:Phenobarbital / Q000494:pharmacology; D011285:Pregnenolone Carbonitrile / Q000494:pharmacology; D011720:Pyrazoles / Q000494:pharmacology; D013806:Theophylline / Q000378:metabolism*
D004791:Enzyme Inhibitors; D009596:Nitrophenols; D010078:Oxazines; D011720:Pyrazoles; D000632:Aminopyrine; D000096:Acetone; D011285:Pregnenolone Carbonitrile; D000607:Aminoacetonitrile; D000077604:Fomepizole; C044943:pentoxyresorufin; D003577:Cytochrome P-450 Enzyme System; D013806:Theophylline; D019392:Cytochrome P-450 CYP2E1; C024836:4-nitrophenol; D010634:Phenobarbital