Table 2

Kinetic properties of wild-type SULTs 1A3 and 1A1 and site-directed mutant SULT1A3 enzymes toward the substrates dopamine and 4-nitrophenol

Enzyme	Dopamine		4-Nitrophenol
Enzyme	K_m	V_max	V_max/K_m	K_m	V_max	V_max/K_m
	μm	nmol/min/mg		μm	nmol/min/mg
Wild-type SULT1A3	2.2	501	228	1382	426	0.31
SULT1A3 H143Y	5.6	455	81	1469	252	0.17
SULT1A3 E146A	127	187	1.5	6.1	104	17
SULT1A3 H143Y/E146A	87	211	2.4	9.4	148	16
Wild-type SULT1A1	109	37	0.34	3.8	405	107

Data shown are the average of duplicate determinations performed using PAPS at a concentration of 13 μm. Enzyme activities of wild-type SULT1A3 and SULT1A3 H143Y were measured with dopamine concentrations between 0.13 and 26.7 μm, whereas for SULT1A3 E146A and H143Y/E146A mutants and wild-type SULT1A1, the concentration range used was 0.013–1.3 mm. Enzyme activities of wild-type SULT1A3 and SULT1A3 H143Y were measured with 4-nitrophenol concentrations between 0.013 and 1.3 mm, whereas for SULT1A3 E146A and H143Y/E146A mutants and wild-type SULT1A1, the concentration range used was 0.13–26.7 μm.