TABLE 1

Apparent enzyme kinetic parameters of dextromethorphan metabolism in pooled hepatic microsomes from wild-type and Tg-CYP2D6/CYP3A4 mice aged 3, 5, and 8 weeks (N = 4 in each group)

Values are mean ± S.E.M. from triplicate experiments. Incubations were performed in 100 mM phosphate buffer, pH 7.4, at 37°C for 5 min. Dextromethorphan concentrations ranged from 0 to 2000 μM. The rates of DXO and MEM formation were measured by HPLC with fluorescence detection. Michaelis-Menten parameters, K_m and V_max, were estimated by nonlinear regression.

Mouse Liver Microsomes					DXO Formation					MEM Formation
Mouse Liver Microsomes						K_m	V_max	CL_int (V_max/K_m)	K_m	V_max	CL_int (V_max/K_m)
			μM	nmol/min/mg protein	μl/min/mg protein	μM	nmol/min/mg protein	μl/min/mg protein
Male	3 weeks	Wild-type	3.80 ± 0.48	1.19 ± 0.03	313	237 ± 21	2.92 ± 0.12	12.3
		Transgenic	3.60 ± 0.34	3.51 ± 0.06	975	523 ± 38	3.85 ± 0.11	7.36
	5 weeks	Wild-type	3.50 ± 0.46	1.82 ± 0.04	520	503 ± 34	7.68 ± 0.21	15.3
		Transgenic	3.20 ± 0.37	3.54 ± 0.08	1110	534 ± 41	7.06 ± 0.21	13.2
	8 weeks	Wild-type	5.10 ± 0.72	1.17 ± 0.03	229	412 ± 52	2.79 ± 0.16	6.79
		Transgenic	3.10 ± 0.23	2.73 ± 0.03	880	374 ± 36	4.23 ± 0.15	11.3
Female	3 weeks	Wild-type	4.40 ± 0.75	1.35 ± 0.04	307	552 ± 73	4.83 ± 0.25	8.75
		Transgenic	2.50 ± 0.27	2.73 ± 0.05	1090	655 ± 52	3.57 ± 0.11	5.45
	5 weeks	Wild-type	3.70 ± 0.47	2.33 ± 0.05	630	433 ± 46	7.40 ± 0.28	17.1
		Transgenic	3.20 ± 0.43	4.71 ± 0.13	1470	500 ± 54	9.31 ± 0.38	18.6
	8 weeks	Wild-type	4.30 ± 0.46	1.79 ± 0.03	416	354 ± 24	4.54 ± 0.13	12.8
		Transgenic	3.00 ± 0.21	2.79 ± 0.03	930	412 ± 26	4.25 ± 0.12	10.3