concentrations of (S)-flurbiprofen from 2 to 300 μM were incubated with CYP2C9 (0.025 μM), CPR (0.05 or 0.1 μM for subsaturating or saturating conditions, respectively), and CYP3A4 (0, 0.025, or 0.05 μM). CYP2C9 was mixed with CYP3A4, allowed to equilibrate on ice for 5 min, after which CPR was added and the ternary mixture allowed to further equilibrate on ice for 5 min. The mixture next was reconstituted in dilauroylphosphatidylcholine (extruded through a 200-nm pore size membrane) and allowed to equilibrate for 5 min. Enzyme mixtures were then added to substrate and buffer and preincubated at 37°C for 5 min before initiation of the reaction. All the experiments were carried out in 50 mM potassium phosphate buffer, pH 7.4, at 37°C. After initiation of the reaction with NADPH (1 mM final concentration), the reactions were allowed to continue for 20 min and then terminated by the addition of 200 μl of 180 ng/ml 2-fluoro-4-biphenyl acetic acid (internal standard) in acetonitrile. To the quenched reaction was then added 40 μl of half-strength phosphoric acid to adjust the pH to ∼3.0. All the experiments were repeated three times on separate days. The amount of 4′-OH-flurbiprofen produced was used as a measure of CYP2C9 activity. Similar experiments were repeated with (S)-naproxen as a substrate (10–1800 μM naproxen concentration range). Likewise, the amount of desmethylnaproxen produced was also used as a measure of CYP2C9 activity.