Over 50 allelic variants of cytochrome P450 2D6 (CYP2D6) encoding fully functional, reduced-activity, or nonfunctional proteins have been described. Compared with Caucasians, studies in black populations demonstrate a tendency toward slower CYP2D6 activity, attributed in part to the presence of a variant allele associated with reduced activity, the CYP2D6*17 allele. To investigate the kinetic characteristics of this variant protein, expression constructs coding for CYP2D6.1, CYP2D6.2, and CYP2D6.17 gene products were prepared and transfected into mammalian COS-7 and insect (Trichoplusia ni) cells for expression. Microsomal fractions containing the expressed proteins were used to determine the kinetic parameters K(m), V(max), and intrinsic clearance (Cl(int)) for the model substrates dextromethorphan, bufuralol, and debrisoquine. Relative to the wild-type CYP2D6.1 protein expressed in COS-7 cells, CYP2D6.17 exhibited a 2-fold higher K(m) and a 50% reduction in V(max) using dextromethorphan as the substrate. In contrast, no appreciable change in bufuralol K(m) was observed with CYP2D6.17 whereas V(max) was decreased by 50%. When expressed in the baculovirus expression system, CYP2D6.17 exhibited a 6-fold increase in K(m) but no change in V(max) with dextromethorphan as the substrate, a 2-fold higher K(m) and 50% reduction in V(max) with bufuralol, and a 3-fold increase in K(m) and no change in V(max) with debrisoquine relative to CYP2D6.1. These data indicate that CYP2D6.17 exhibits reduced metabolic activity toward all three commonly used CYP2D6 substrates, although specific effects on substrate affinity and turnover demonstrate some substrate dependence.