Abstract
It is known that in humans taking soy food, the phytoestrogens, daidzein (DZ) and genistein (GS), exist as sulfates and glucuronides in the plasma and are excreted as conjugates in urine. To investigate which human sulfotransferase (SULT) isoforms participate in the sulfation of these phytoestrogens, the four major cytosolic SULTs, SULT1A1, SULT1A3, SULT1E1, and SULT2A1, occurring in the human liver were bacterially expressed as His-tagged proteins and chromatographically purified to homogeneity in the presence of Tween 20 and glycerol as highly efficient agents for stabilizing the recombinant enzymes. All the SULTs showed sulfating activity toward both DZ and GS. However, kcat/Km values observed indicated that these phytoestrogens were sulfated predominantly by SULT1A1 and SULT1E1 with Km values of 0.3 and 0.7 μM for GS and 1.9 and 3.4 μM for DZ, respectively. DZ and GS strongly inhibited the sulfation of the endogenous substrate, β-estradiol, by SULT1E1 in a non-competitive manner with Ki values of 14 and 7 μM, respectively, suggesting that these phytoestrogens might affect tissue levels of β-estradiol in the human. The phenolic endocrine-disrupting chemicals, bisphenol A (BPA), 4-n-nonylphenol (NP), and 4-t-octylphenol (t-OP), were used as substrates to investigate the possible participation of human SULTs in their metabolism for excretion. High kcat/Km values were observed for the sulfation of BPA by SULT1A1, NP by SULT1A1 and SULT1E1, and t-OP by SULT1E1 and SULT2A1.
