Abstract
Regioselective sulfation of the phytoestrogens daidzein (DZ, 7,4′-dihydroxyisoflavone) and genistein (GS, 5,7,4′-trihydroxyisoflavone) was investigated using human liver cytosol and purified recombinant human sulfotransferase (SULT) isoforms, SULT1A1, SULT1A3, SULT2A1, and SULT1E1. 7-Position-preferential sulfation of DZ and GS was observed in human hepatic cytosols from 3 male and 3 female subjects. Average ratios for 7- to 4′-sulfate formation were 4.5:1 from DZ and 8.4:1 from GS in these human liver cytosols. Apparent Km values for the 7- and 4′-sulfation of DZ and GS by these cytosols were similar and in a range from 0.46 to 0.66 μM. All recombinant human SULTs had activity for 7- and 4′-sulfation of these phytoestrogens except for 7-sulfating activity of SULT1A3. SULT1A1 and SULT1E1 exhibited much higher catalytic efficiency, kcat/Km, for 7- and 4′-sulfation of these substrates than did the other two, SULT1A3 and SULT2A1. SULT1A1 showed Km values of 0.47 and 0.52 μM for the mono-sulfation of DZ and GS, respectively, which were very similar to those of human cytosol. The observed kcat/Km indicated that SULT1A1 catalyzed 7-sulfation of DZ and GS at rates 4.4- and 8.8-fold higher, respectively, than such 4′-sulfation. However, with SULT1E1, catalytic efficiency was very similar for the sulfation of both positions. These data strongly suggest that SULT1A1 plays a major role in monosulfation of the phytoestrogens and determines the regioselectivity of sulfation in human hepatic cytosol. A kinetic study for 7,4′-disulfate formation of DZ and GS from their 7- and 4′-monosulfates indicated that SULT1E1 most efficiently catalyzed both reactions among human SULTs.
