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 P450 (CYP) 3A4 in human liver microsomal bupropion (BUP) hydroxylation. Across the BUP concentration range of 0.075 to 12 mM, cDNA-expressed CYP3A4 demonstrated BUP hydroxylase activity only when incubated with concentrations > or =4 mM. When assayed at 12 mM BUP, cDNA-expressed CYP3A4 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 liver microsomes (HLMs), BUP hydroxylase activity varied 80-fold when assayed at 500 microM and did not strongly correlate with testosterone 6beta-hydroxylase activity when assayed at 250 microM testosterone (r(2) = 0.39), nor with CYP3A4 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 microM BUP, as reflected by a similarity in the kinetic parameters of BUP hydroxylation in the absence or presence of TAO. In addition, the same range of TAO concentrations (0.025-100 microM) that inhibited testosterone 6beta-hydroxylation in a concentration-dependent manner (46-81%) in pooled HLMs produced negligible inhibition (7%) of BUP hydroxylation when assayed at 500 microM BUP. These results suggest that CYP3A4 does not significantly catalyze BUP hydroxylation. Furthermore, these results complement recent data supporting selectivity of BUP hydroxylation for CYP2B6 at 500 microM BUP.