Enzymatic characterization of human cytosolic sulfotransferases; identification of ST1B2 as a thyroid hormone sulfotransferase

Sulfotransferases (ST) are contained as multiple forms in human tissues with overlapping substrate specificities. To identify a form which contributes to the metabolism of 3, 3',5-triiodothyronine (T₃), the functional properties of human STs were compared using recombinant STs, ST1A3, ST1AS, ST1B2, ST1E4 and ST2A3. ST1B2 showed a high affinity (K(m) 46.2 μM) for T₃ sulfation, whereas ST1A3, ST1A5, ST1E4 and ST2A3 showed high affinities to p-nitrophenol (K(m) 0.4 μM), dopamine (K(m) 7.1 μM), β- estradiol (K(m) 0.3 μM) and dehydroepiandrosterone (K(m) 3.3 μM), respectively. In Western blotting using antibodies raised against an individual ST, hepatic absolute amounts of these STs were determined. The content of ST1B2 in human liver correlated well with T₃ sulfation activities in human liver (r=0.96). These results indicate that ST1B2 is biochemically distinct from other forms of ST, and is involved in the metabolism of T₃ in human. In addition, studies of thermal stability and 2,6-dichloro-4- nitrophenol (DCNP) inhibition showed that ST1B2 was thermostable and more DCNP resistant than other forms of ST. Affinities for a co-factor, phosphoadenosine 5'-phosphosulfate, also differed 9-fold among 5 different forms of ST.