TABLE 1.

Kinetic parameters for the formation of 8-hydroxyefavirenz from efavirenz and 4-hydroxybupropion from bupropion in expressed CYP2B6.1 and CYP2B6.6 without and with coexpression of Cyt b5

Kinetic data are presented as mean ± S.D. (n = 3 incubations were performed in duplicate). In vitro Cl_int was calculated as V_max/K_m. Kinetic parameters for the formation of 8-hydroxyefavirenz and 4-hydroxybupropion were estimated by fitting the velocity versus substrate concentrations to the simple single-site Michaelis-Menten equation.

Kinetic Parameters	Without Cyt b5		With Cyt b5		P Value (Comparing with vs. without b5)†
	CYP2B6.1	CYP2B6.6	CYP2B6.1	CYP2B6.6	CYP2B6.1	CYP2B6.6
Efavirenz 8-hydroxylation
Vmax, pmol · min−1 · pmol−1	1.21 ± 0.15	1.61 ± 0.11**	1.02 ± 0.11	0.95 ± 0.02	0.14	0.00002
Km, μM	3.2 ± 1.0	8.8 ± 1.6***	3.4 ± 0.6	6.2 ± 2.6	0.74	0.14
Clint, μl · min−1 · pmol−	0.39 ± 0.08	0.19 ± 0.04**	0.30 ± 0.06	0.17 ± 0.07	0.18	0.69
Bupropion 4-hydroxylation
Vmax, pmol · min−1 · pmol−1	13.37 ± 0.97	18.55 ± 0.82**	15.18 ± 0.99	11.77 ± 1.46**	0.04	0.0002
Km, μM	64.2 ± 13.4	62.6 ± 7.3	90.6 ± 10.3	110.0 ± 29.8	0.02	0.02
Clint, μl · min−1 · pmol−1	0.21 ± 0.03	0.30 ± 0.03**	0.17 ± 0.02	0.11 ± 0.04*	0.07	0.0002

^* P < 0.05,

^** P < 0.01, and

^*** P < 0.001 compared CYP2B6.6 to CYP2B6.1 without and with coexpression of Cyt b5, respectively.

^† Kinetic parameters of CYP2B6.1 with and without Cyt b5 as well as CYP2B6.6 with and without Cyt b5 were also compared.