Table 2

Comparison of kinetics of glucuronide formation in membrane preparations from HK293 cells stably expressing rat UGT1.1 and UGT2B1

Substrate	Expressed UGT2B1		Expressed UGT1.1
Substrate	ApparentK_M	V_max	V_max/K_M	ApparentK_M	V_max	V_max/K_M
	μM	pmol/min/mg	μl/min/mg (100×)	μM	pmol/min/mg	μl/min/mg (100×)
Buprenorphine	110 ± 26	580 ± 54	530	148, 188	877, 670	593, 356
Norbuprenorphine	340, 135	427, 263	126, 195	119, 170	337, 433	283, 255
Diprenorphine	19, 65	92, 149	484, 229	29, 35	115, 107	397, 306
(−)-Morphine	587, 567	16,780, 15,971	2,860, 2,820	2,800, 2,800	22, 15	0.8, 0.5
Naltrexone	117, 72	439, 567	375, 788	5,200, 4,500	108, 138	2, 3
Naloxone	52, 47	1,813, 3,075	349, 654	191, 134	41, 31	22, 23
Nalorphine	486, 367	13,522, 18,394	2,780, 5,011	2,000, 1,080	113, 114	6, 11

The apparent K_M for UDP-GlcUA was determined previously (7). The apparent K_M values for aglycones were determined using 2.0 mM UDP-GlcUA. Assays were conducted using the pH optimum for each substrate, and linear product formation with respect to time and protein concentration. Values are the results obtained using two different membrane preparations or the means ± SE of results obtained from three different passages of cultured cells. (Limit of detection was 2 pmol/min/mg).