Evaluation of the contribution of cytochrome P450 3A4 to human liver microsomal bupropion hydroxylation

The purpose of this investigation was to evaluate the role of cytochrome P4 50 (CYP) 3A4 in human liver microsomal bupropion (BUP) hydroxylation. Acros s the BUP concentration range of 0.075 to 12 mM, cDNA-expressed CYP3A4 demo nstrated BUP hydroxylase activity only when incubated with concentrations g reater than or equal to4 mM. When assayed at 12 mM BUP, cDNA-expressed CYP3 A4 catalyzed BUP hydroxylation at a 30-fold lower rate than cDNA-expressed CYP2B6 (0.2 versus 7 pmol/min/pmol of P450). Among a panel of 16 human live r microsomes (HLMs), BUP hydroxylase activity varied 80-fold when assayed a t 500 muM and did not strongly correlate with testosterone 6 beta -hydroxyl ase activity when assayed at 250 muM testosterone (r(2) = 0.39), nor With C YP3A4 protein expression. A selective CYP3A4 inhibitor, troleandomycin (TAO ), did not significantly alter rates of BUP hydroxylation when assayed in a moderate activity HLM at 10 to 2000 muM BUP, as reflected by a similarity in the kinetic parameters of BUP hydroxylation in the absence or presence o f TAO. In addition, the same range of TAO concentrations (0.025-100 muM) th at inhibited testosterone 6 beta -hydroxylation in a concentration-dependen t manner (46-81%) in pooled HLMs produced negligible inhibition (7%) of BUP hydroxylation when assayed at 500 muM BUP. These results suggest that CYP3 A4 does not significantly catalyze BUP hydroxylation. Furthermore, these re sults complement recent data supporting selectivity of BUP hydroxylation fo r CYP2B6 at 500 muM BUP.