TABLE 4

SULT enzyme kinetics for five probe substrates in human tissue cytosols

Assays were performed in duplicate on each sample of cytosol for each substrate. Data represent the mean ± S.D. of the values obtained for n tissue cytosols. All the data points were fitted to the Michaelis-Menten equation, with the exception of 4-nitrophenol in the liver and kidney, where substantial substrate inhibition is observed. In these cases, the Michaelis-Menten equation modified for partial substrate inhibition was used (see under Materials and Methods).

Substrate	Liver (n = 28)				Intestine (n = 6)			Kidney (n = 3)
Substrate	K_m	V_max	V_max/K_m	K_i	K_m	V_max	V_max/K_m	K_m	V_max	V_max/K_m	K_i
	μM	pmol/min/mg		μM	μM	pmol/min/mg		μM	pmol/min/mg		μM
4-Nitrophenol	3.9 ± 3.5	1865 ± 1787	478	4.5 ± 5.8	0.9 ± 0.6	33 ± 20	37	1.5 ± 0.8	37 ± 15	25	8.8 ± 7.5
2-Aminophenol	1.3 ± 0.6	590 ± 251	453		2.0 ± 1.3	49 ± 27	25	0.3 ± 0.1	27 ± 16	90
Dopamine	BLQ	BLQ			2.2 ± 1.2	215 ± 180	98	2.4 ± 1.3	10 ± 12	4.2
17β-Estradiol	0.14 ± 0.23	20 ± 10	142		0.03 ± 0.01	7.0 ± 6.1	233	0.08 ± 0.05	0.23 ± 0.02	2.9
DHEA	2.7 ± 0.7	234 ± 86	87		1.3 ± 0.7	15 ± 10.3	12	BLQ	BLQ

BLQ, below the limit of quantification of the assay method used.