Changes in the enzymatic properties of CYP2D6 by the substitution of phenylalanine at position 120 by alanine

The functional roles of phenylalanine at position 120 (Phe-120) in the oxidation of bunitrolol (BTL), debrisoquine (DB) and bufuralol (BF) by cytochrome P450 2D6 (CYP2D6) were examined using a yeast cell expression system (Saccharomyces cerevisiae AH-22 strain). The substitution of Phe-120 by alanine markedly increased the activities of enantiomeric BTL4-hydroxylase and DB 4-hydroxylase, whereas it did not remarkably affect BF 1″-hydroxylase activities. Kinetic studies revealed that the substitution of Phe-120 by alanine increased the Km and Vmax values for enantiomeric BTL 4-hydroxylation, but increased only the Vmax value for DB 4-hydroxylation without changing the Km value. Km and Vmax values for BF 1″-hydroxylation were similar between the mutant and the wild-type. The dissociation constants of the mutant calculated from the binding spectra for BTL enantiomers were higher than those of the wild-type, suggesting that the substitution of Phe-120 by alanine decreased the affinity of CYP2D6 for BTL enantiomers. These results indicate that Phe-120 has an important role in the oxidation of substrates by CYP2D6.