Identification of human cytochrome P450 isoforms involved in the 7-hydroxylation of chlorpromazine by human liver microsomes

Abstract

Studies to identify the cytochrome P450 (CYP) isoform(s) involved in chlorpromazine 7-hydroxylation were performed using human liver microsomes and cDNA-expressed human CYPs. The kinetics of chlorpromazine 7-hydroxylation in human liver microsomes showed a simple Michaelis-Menten behavior. The apparent K m and V max values were 3.4 ± 1.0 μM and 200.5 ± 83.7 pmol/min/mg, respectively. The chlorpromazine 7-hydroxylase activity in human liver microsomes showed good correlations with desipramine 2-hydroxylase activity ( r = 0.763, p < 0.05), a marker activity for CYP2D6, and phenacetin O-deethylase activity ( r = 0.638, p < 0.05), a marker activity for CYP1A2. Quinidine (an inhibitor of CYP2D6) completely inhibited while α-naphthoflavone (an inhibitor of CYP1A2) marginally inhibited the chlorpromazine 7-hydroxylase activity in a human liver microsomal sample showing high CYP2D6 activity. On the other hand, α-naphthoflavone inhibited the chlorpromazine 7-hydroxylase activity to 55–65% of control in a human liver microsomal sample showing low CYP2D6 activity. Among eleven cDNA-expressed CYPs studied, CYP2D6 and CYP1A2 exhibited significant activity for the chlorpromazine 7-hydroxylation. The K m values for the chlorpromazine 7-hydroxylation of both cDNA-expressed CYP2D6 and CYP1A2 were in agreement with the K m values of human liver microsomes. These results suggest that chlorpromazine 7-hydroxylation is catalyzed mainly by CYP2D6 and partially by CYP1A2.