1-(2-Pyrimidinyl)-piperazine (1-PP) is an active metabolite of several psychoactive drugs including buspirone. 1-PP is also the major metabolite in the human circulation and in rat brains following oral administration of buspirone. This study was conducted to identify the enzyme responsible for the metabolic conversion of 1-PP to 5-hydroxy-1-(2-pyrimidinyl)-piperazine (HO-1-PP) in human liver microsomes (HLMs). The product HO-1-PP was quantified by a validated liquid chromatography-tandem mass spectrometry method. In the presence of NADPH, 1-PP (100 microM) was incubated separately with human cDNA-expressed cytochrome P450 isozymes (including CYP2D6, 3A4, 1A2, 2A6, 2C9, 2C19, 2E1, and 2B6) at 37 degrees C. CYP2D6 catalyzed the formation of HO-1-PP from 1-PP. This catalytic activity was >95% inhibited by quinidine, a CYP2D6 inhibitor. HO-1-PP formation rates correlated well with the bufuralol 1-hydroxylase (CYP2D6) activities of individual HLMs. The formation of HO-1-PP followed a Michaelis-Menten kinetics with a K(m) of 171 microM and V(max) of 313 pmol/min x mg protein in HLMs. Collectively, these results indicate that polymorphic CYP2D6 is responsible for the conversion of 1-PP to HO-1-PP.