Sulfuryl Transfer: The Catalytic Mechanism of Human Estrogen Sulfotransferase

Abstract

Estrogen sulfotransferase (EST) catalyzes the transfer of the sulfuryl group from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to 17beta-estradiol (E2). The sulfation of E2 prevents it from binding to, and thereby activating, the estrogen receptor. The regulation of EST appears to be causally linked to tumorigenesis in the breast and endometrium. In this study, recombinant human EST is characterized, and the catalytic mechanism of the transfer reaction is investigated in ligand binding and initial rate experiments. The native enzyme is a dimer of 35-kDa subunits. The apparent equilibrium constant for transfer to E2 is (4.5 +/- 0.2) x 10(3) at pH 6.3 and T = 25 +/- 2 degrees C. Initial rate studies provide the kinetic constants for the reaction and suggest a sequential mechanism. E2 is a partial substrate inhibitor (Ki = 80 +/- 5 nM). The binding of two E2 per EST subunit suggests that the partial inhibition occurs through binding at an allosteric site. In addition to providing the dissociation constants for the ligand-enzyme complexes, binding studies demonstrate that each substrate binds independently to the enzyme and that both the E.PAP.E2S and E.PAP.E2 dead-end complexes form. These results strongly suggest a Random Bi Bi mechanism with two dead-end complexes.

Characterization of phase-II conjugation reaction of polycyclic aromatic hydrocarbons in fish species: Unique pyrene metabolism and species specificity observed in fish species

“…However, when inhibited, the velocity decreased to a plateau rather than to zero. This phenomenon was also shown in a study, which observed partial substrate inhibition for human estrogen sulfotransferase (EST) and estradiol (E 2 ) (Zhang et al, 1998). This indicated that one or more of the kinetic parameters for the reaction were being titrated from one value to another.…”

“…This indicated that one or more of the kinetic parameters for the reaction were being titrated from one value to another. In the study by Zhang et al (1998), two E 2 were bound to each EST, suggesting that partial inhibition occurred through binding at an allosteric site. From this report, we assumed that HPyS utilized the same mechanism.…”

Intratumoral estrogen sulfotransferase induction contributes to the anti-breast cancer effects of the dithiocarbamate derivative TM208

“…Therefore, we tested whether the induction of EST plays a role in the anti-breast cancer effect of TM208. EST shows the highest affinity for E2 and estrone [15] , whereas other SULTs, such as SULT1A1 and SULT2A1, exhibit some affinities for E2 and estrone, which are only found at non-physiological high concentrations [14,41] . Not counting EST, SULT1A1 was reported to play the most crucial role in the sulfation of intratumoral estrogens by its contribution to the conjugation of the metabolites of E2 (such as 4-OH-E2 and 2-methoxy-E2) metabolized by CYP1B1 and 1A1 [14,42] .…”

“…However, we once investigated the effect of TM208 on SULT1A1 and SULT2A1 and found that TM208 induced the expression of SULT1A1 more significantly than that of SULT2A1 in both MCF-7 cells and MCF-7 xenograft tumors, but all of the observed increases were less than 4-fold (Figure S1-S2). However, the affinity of estrogen is much higher for EST than for SULT1A1, the K m of which is at the nano-or micromolar level, respectively [15] . Therefore, EST is responsible for the sulfation of intratumoral activated E2, which is why we investigated EST rather than other SULT isoforms in this paper.…”

“…The main SULT isoforms that catalyze the sulfation of estrogen in the human body include SULT1E1, SULT1A1, SULT2A1, and SULT2B1, as well as others [8,9,11,14] . Among these isoforms, estrogen sulfotransferase (EST, SULT1E1) plays the most important role in estrogen inactivation under physiological conditions due to its high affinity for estrogens [15] . In most previous studies, the intratumoral estrogen concentrations were expressed as pg/g tissue or pmol/g tissue, which can be approximately calculated at the "nanomolar" (physiological) level even if they are much higher than those found in normal mammary tissues [6,7] or plasma [4,5] .…”