Studies were undertaken to determine which human P450 enzymes catalyze the metabolism of theophylline to 1,3-dimethyluric acid (1,3-DU), to facilitate predictions of theophylline drug-drug interactions, and to develop a noninvasive test for human P4501A2. Microsomes from a human cell line transfected individually with human P450 cDNAs for P4501A1, 1A2, 2A6, 2B6, 2C9, 2D6, 2E1, or 3A4 were used to demonstrate that only P4501A2 exhibited catalytic activity for theophylline metabolism to 1,3-DU with high affinity and low capacity (Km = 0.6 mM, Vmax = 37.8, pmol/min/mg), while P4502D6, 2E1, and 3A4 (Km = 14.4, 19.9, and 25.1 mM, respectively, and Vmax = 219.8, 646.4, and 20.8 pmol/min/mg, respectively) exhibited activities with low affinity and variable capacities. Correlations of rates of theophylline 8-hydroxylation to 1,3-DU with other P450 form-specific activities, in a series of ten human liver microsomal preparations, at 5 and 40 mM theophylline concentrations, revealed that at low concentrations the metabolism was catalyzed primarily by P4501A2, while at high substrate concentrations P4502E1 was primarily responsible for catalysis. The results with individually expressed P450s and hepatic microsomal preparations were consistent, indicating that the former system provides a qualitatively accurate reflection of the function of the heterogeneously expressed liver P450s. At pharmacologic theophylline concentrations achieved in vivo, its metabolism must thus be catalyzed primarily by P4501A2.