TABLE 3

Kinetic parameters for human UDP-glucuronosyltransferase activities in human liver microsomes and recombinant UGT enzymes

Enzyme^{^a}	Incubation Conditions^{^a}	K_m or S₅₀	V_max	K_si	n	CL_int or CL_max^{^b}
		μM	pmol · min⁻¹ · mg⁻¹	μM		μl · min⁻¹ · mg⁻¹
UGT1A1	HLM	11 ± 0.8	820 ± 20	N.A.	2.5 ± 0.4	40
	HLM + BSA	170 ± 20	1400 ± 70	N.A.	1.8 ± 0.2	4.1
	rUGT	13 ± 0.8	1300 ± 20	N.A.	2.6 ± 0.3	51
	rUGT + BSA	120 ± 4	1700 ± 30	N.A.	2.7 ± 0.2	7.6
UGT1A4	HLM	42 ± 10	1500 ± 300	64 ± 20	N.A.	36
	HLM + BSA	67 ± 10	870 ± 50	N.A.	N.A.	13
	rUGT	15 ± 4	970 ± 200	82 ± 30	N.A.	67
	rUGT + BSA	140 ± 60	5900 ± 2000	490 ± 300	N.A.	43
UGT1A6	HLM	420 ± 50	66,000 ± 3000	15,000 ± 3000	N.A.	160
	HLM + BSA	330 ± 30	47,000 ± 1100	41,000 ± 9000	N.A.	140
	rUGT	570 ± 50	4900 ± 200	13,000 ± 2000	N.A.	8.6
	rUGT + BSA	490 ± 200	5400 ± 1000	17,000 ± 10,000	N.A.	11
UGT1A9	HLM	98 ± 30	1400 ± 300	690 ± 200	N.A.	14
	HLM + BSA	46 ± 4	780 ± 10	N.A.	N.A.	17
	rUGT	200 ± 70	2000 ± 500	330 ± 100	N.A.	10
	rUGT + BSA	63 ± 10	1300 ± 90	3200 ± 1000	N.A.	20
UGT2B7	HLM	610 ± 30	2100 ± 30	N.A.	N.A.	3.5
	HLM + BSA	150 ± 10	4700 ± 50	N.A.	N.A.	32
	rUGT	900 ± 30	900 ± 10	N.A.	N.A.	1.1
	rUGT + BSA	320 ± 10	3100 ± 40	N.A.	N.A.	9.8

K_m, apparent substrate concentration at half-maximal velocity; S₅₀ apparent substrate concentration at half-maximal velocity for substrates exhibiting atypical kinetics; V_max, maximal velocity; K_si, inhibition constant for substrates exhibiting substrate inhibition kinetics; n, Hill coefficient; CL_int, intrinsic clearance; CL_max, maximal clearance; N.A., not applicable.
↵a HLM or rUGTs (0.025 mg/ml) were fully activated with alamethicin (10 μg/ml) and incubated with increasing substrate concentrations in 100 mM Tris-HCl buffer (pH = 7.5) containing 5 mM MgCl₂, 5 mM UDPGA, with or without 2% BSA, as described under Materials and Methods. Values were not corrected for nonspecific binding in incubation and represent mean ± S.E.M. for three to four experiments.
↵b CL_int applies to all UGTs except UGT1A1, whereas CL_max was calculated because ES3-G formation displays atypical kinetics, as described under Materials and Methods.