The Role of Cyp2c19 in the Metabolism of (1/2) Bufuralol, the Prototypic Substrate of Cyp2d6

Upon characterization of baculovirus-expressed cytochrome P-450 (CYP) 2C19, it was observed that this enzyme metabolized (1/2) bufuralol to 1*hydroxybufuralol, a reaction previously understood to be selectively catalyzed by CYP2D6. The apparent Km for this reaction was 36 mM 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, the Ki for inhibition of recombinant CYP2C19-mediated bufuralol hydroxylation was 42 mM, which is the approximate Km 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 Km shifted from 14 to 38 mM. This is consistent with the Km determination for recombinant CYP2C19 of 36 mM. In human liver microsomes that have high CYP2D6 and CYP2C19 activity, the Km shifted to 145 mM in the presence of S-mephenytoin and quinidine, consistent with the Km determined for CYP1A2. This data suggests that bufuralol, and possibly other CYP2D6 substrates, have the potential to be metabolized by