Regioselective hydroxylation of debrisoquine by cytochrome P4502D6: implications for active site modelling

1. Debrisoquine, a prototypic probe substrate for human cytochrome P4502D6 (CYP2D6), is hydroxylated at the alicyclic C4-position by this enzyme. Phen olic metabolites of debrisoquine (5-, 6-, 7- and 8-hydroxygdebrisoquine) ha ve also been reported as in vivo metabolites, but the role of CYP2D6 in the ir formation is unclear. 2. As part of studies to develop a predictive model of the active site of C YP2D6 using pharmacophore and homology modelling techniques, it became impo rtant to determine the precise regioselective hydroxylation of debrisoquine by CYP2D6. 3. Data from studies with human liver microsomes and yeast microsomes conta ining cDNA-derived CYP2D6 demonstrated unequivocally that debrisoquine was hydroxylated by CYP2D6 at each aromatic site in the molecule, as well as at the alicyclic 4-position. The four phenolic metabolites amounted to > 60 % of the total identified products and the pattern of regioselective hydroxy lation (4-HD > 7-HD > 6-HD > 8-HD > 5-HD) was similar in both in vitro syst ems. 4. A pharmacophore model for CYP2D6 indicated that while the hydroxylation of debrisoquine at alternative positions could arise from the substrate ado pting multiple binding orientations, the energy constraints for the aromati c hydroxylations were unfavourable. An alternative proposal involving essen tially a single binding orientation and a mechanism of hydroxylation based on benzylic radical spin delocalization could satisfactorily rationalize al l the hydroxylations of debrisoquine. 5. This latter proposal demonstrates the need to consider the mechanism of oxidation as well as the spatial orientation of the substrate in the develo pment of a predictive model of the active site of CYP2D6.