Catalytic, pharmacological, and molecular criteria have been used to identify cytochrome P450IID1 in mammalian brain (enzyme, P450IID; gene, CYP2D). Sparteine metabolism in canine striatal membranes was shown to be inhibited in a concentration-dependent and stereoselective manner by quinidine (Ki, approximately 51 nM), quinine (Ki, approximately 5.9 microM), and various other known substrates and inhibitors of hepatic P450IID1 activity. In addition, canine striatal P450IID1 was inhibited with high affinity by dopamine uptake blockers, such as (-)-cocaine (Ki, approximately 74 nM), d-amphetamine (Ki, approximately 4.5 microM), and methylphenidate (Ki, approximately 15 microM). Inhibitory constants (Ki) of numerous compounds for inhibition of sparteine metabolism in canine striatal membranes correlated well with (a) Ki values observed in human liver microsomes (r = 0.95), (b) [3H]GBR-12935 binding to P450IID1 in canine striatal membranes (r = 0.85), and (c) the inhibition (IC50) of sparteine metabolism in HepG2 cells expressing human CYP2D6 cDNA (r = 0.93). Moreover, antibodies raised against rat hepatic enzyme inhibited, in a concentration-dependent manner, sparteine metabolism in canine striatal membranes. Enzymatic activity was unevenly distributed throughout the canine brain and ranged from 0.5 to 21 pmol/mg of protein/hr in cerebellum and supraorbital cortex, respectively, with the striatum displaying moderate levels of activity (8 pmol/mg of protein/hr). The polymerase chain reaction was used to amplify cDNA from a human caudate lambda gt11 library encoding exons 6-9 of the human CYP2D6 gene, which revealed, upon sequencing, 100% nucleic acid sequence identity. These data indicate that P450IID1 is expressed centrally and is similar, at the functional and molecular levels, to the human hepatic P450IID1 enzyme. Because the debrisoquine/sparteine mono-oxygenase is a polymorphic enzyme, in which 5-10% of caucasians are deficient in metabolism of various drugs, a genetic difference in human brain metabolism of P450IID1 substrates may possibly lead to differences in drug response and toxicity.