Contribution of human hepatic cytochrome P450s and steroidogenic CYP17 to the N-demethylation of aminopyrine

1. Aminopyrine N-demethylase activity was determined for 11 forms of human hepatic cytochrome P450s (P450s) expressed in yeast Saccharomyces cerevisia e and for human steroidogenic CYP17 expressed in Escherichia coli. 2. Among the hepatic P450s, the N-demethylation of aminopyrine was catalyse d most efficiently by CYP2C19, followed by CYP2C8, 2D6, 2C18 and 1A2, where as the activity with CYP2E1 was negligible. The kinetics of the N-demethyla tion process by CYP1A2, 2C8, 2C19 and 2D6 were studied by fitting to Michae lis-Menten kinetics by Lineweaver-Burk plots. CYP2C19 exhibited the highest affinity and a high capacity for the aminopyrine N-demethylation. CYP2C8 s howed the highest V-max, followed by CYP2C19, 2D6 and 1A2, whereas the K-m for CYP2Cs, 2D6 and 1A2 were 10-17 times higher than that for CYP2C19. Acco rdingly, the V-max/K-m for CYP2C19 was more than nine times higher than tha t of other P450s. 3. Human steroidogenic CYP17 also catalysed aminopyrine N-demethylation and the activity was comparable with that for CYP3A4 which is a dominant P450 in human liver. The activity was increased 1.5-fold by the addition of cyto chrome b(5), whereas the activity was not affected by the addition of Mg2+. 4. These results suggest that several human hepatic P450s, especially CYP2C 19, and steroidogenic CYP17 exhibit aminopyrine N-demethylase activity.