METABOLISM OF THEOPHYLLINE BY CDNA-EXPRESSED HUMAN CYTOCHROMES P-450

1 Theophylline metabolism was studied using seven human cytochrome P-450 isoforms (CYPs), namely CYP1A1, 1A2, 2A6, 2B6, 2D6, 2E1 and 3A4, and microsomal epoxide hydroxylase (EH), expressed in human B-lymphoblastoid cell lines. 2 At a high theophylline concentration of 10 mM four CYPs (1A1, 1A2, 2D6, 2E1) catalyzed the metabolism of theophylline. 3 Theophylline had the highest affinity (apparent K-m range 0.2-1.0 mM) for the CYP1A subfamily and the kinetics of metabolic formation mediated by CYP1A2 indicated substrate-inhibition (K-i range 9-16 mM). 4 CYP1A2 catalyzed the demethylation of theophylline as well as its hydroxylation, and was associated with the highest intrinsic clearance (1995 1 h(-1) per mol CYP) to 1,3-dimethyluric acid (DMU). Therefore, this isoform can be considered to be the most important enzyme involved in theophylline metabolism in vitro. 5 CYP2E1 was responsible for a relatively high intrinsic clearance by 8-hydroxylation (289 1 h(-1) per mol CYP). The apparent K-m value of this reaction was about 15 mM, suggesting that CYP2E1 may be the low-affinity high-capacity isoform involved in theophylline metabolism. 6 The affinity of theophylline for CYP1A1 was comparable with that of its homologue 1A2. When induced, the participation of CYP1A1 in theophylline metabolism may be important. 7 CYP2D6 played only a minor role and CYP3A4 was not active in the in vitro metabolism of theophylline. 8 Our findings confirm the major role of CYP1A2 in theophylline metabolism and explain why in vivo the elimination kinetics of theophylline are non-linear and in vitro theophylline metabolism by human liver microsomes does not obey monophasic kinetics. 9 The data suggest also that not only tobacco smoking but also chronic alcohol intake may influence theophylline elimination in man as ethanol induces CYP2E1.