The role of CYP2C19 in the metabolism of ( plus /-) bufuralol, the prototypic substrate of CYP2D6

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 apparent K-m for this reaction was 36 mu M 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 K-j for inhibition of recombinant CYP2C19-mediated bufuralol hydroxylation was 42 mu M, 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 mu M. This is consistent with the K-m determination for recombinant CYP2C19 of 36 mu M. In human liver microsomes that have high CYP2D6 and CYP2C19 activity, the K-m shifted to 145 mu M in the presence of S-mephenytoin and quinidine, consistent with the K-m determined for CYP1A2. This data suggests that bufuralol, and possibly other CYP2D6 substrates, have the potential to be metabolized by CYP2C19.