Abstract

Upon characterization of baculovirus-expressed cytochrome P-450 (CYP) 2C19, it was observed that this enzyme metabolized (+/−) bufuralol to 1′hydroxybufuralol, a reaction previously understood to be selectively catalyzed by CYP2D6. The apparentK_m for this reaction was 36 μM with recombinant CYP2C19, approximately 7-fold higher than for recombinant CYP2D6. The intrinsic clearance for this reaction was 37-fold higher with CYP2D6 than for CYP2C19. The involvement of human CYP1A2 in bufuralol 1′-hydroxylation was also confirmed using the recombinant enzyme. Using S-mephenytoin as an inhibitor, theK_i for inhibition of recombinant CYP2C19-mediated bufuralol hydroxylation was 42 μM, which is the approximate K_m for recombinant CYP2C19-mediated S-mephenytoin metabolism. The classic CYP2D6 inhibitors quinidine and quinine showed no inhibition of CYP2C19-catalyzed bufuralol metabolism at concentrations that abolished CYP2D6-mediated bufuralol metabolism. Ticlopidine, a potent inhibitor of CYP2C19 and CYP2D6, inhibited bufuralol 1′-hydroxylation by each of these enzymes equipotently. In human liver microsomes that are known to be deficient in CYP2D6 activity, it was shown that in the presence of quinidine, the K_m shifted from 14 to 38 μM. This is consistent with the K_mdetermination for recombinant CYP2C19 of 36 μM. In human liver microsomes that have high CYP2D6 and CYP2C19 activity, theK_m shifted to 145 μM in the presence ofS-mephenytoin and quinidine, consistent with theK_m determined for CYP1A2. This data suggests that bufuralol, and possibly other CYP2D6 substrates, have the potential to be metabolized by CYP2C19.