Potent Inhibition of Human Sulfotransferase 1A1 by 17α-Ethinylestradiol: Role of 3′-Phosphoadenosine 5′-Phosphosulfate Binding and Structural Rearrangements in Regulating Inhibition and Activity

Abstract

ABSTRACT:Sulfotransferase (SULT) 1A1 is the major drug/xenobiotic-conjugating SULT isoform in human liver because of its broad substrate reactivity and high expression level. SULT1A1 sulfates estrogens with low micromolar K m values consistent with its affinity for sulfation of many small phenolic compounds. Binding studies showed the unexpected ability of 17␣-ethinylestradiol (EE2) to bind and inhibit SULT1A1 activity toward p-nitrophenol and ␤-naphthol at low nanomolar concentrations, whereas EE2 was not sulfated until significantly higher concentrations were reached. EE2 had a K i of 10 nM for inhibiting p-nitrophenol and ␤-naphthol sulfation and inhibited 17␤-estradiol (E2) sulfation in intact human MCF-7 breast cancer cells with a K i of 19 nM. In contrast, the K m for EE2 sulfation by SULT1A1 was 700 nM. The K d for EE2 binding of pure SULT1A1 was 0.5 ؎ 0.15 M; however, the K d for EE2 binding to the SULT1A1-PAP complex was >100-fold lower (4.3 ؎ 1.7 nM). The K d for E2 binding to SULT1A1 changed from 2.3 ؎ 0.9 to 1.2 ؎ 0.56 M in the presence of PAP. Docking studies with E2 indicate that E2 binds in a competent orientation in the resolved structure of SULT1A1 in the both presence and absence of 3-phosphoadenosine 5-phosphosulfate (PAPS). However, EE2 binds in a catalytically competent orientation in the absence of PAPS but in a noncompetent orientation via formation of a charge interaction with Tyr108 if PAPS is bound first. In conclusion, EE2 is a potent inhibitor, but not a substrate, of SULT1A1 at low nanomolar concentrations, indicating the possibility of drug-drug interactions during contraceptive therapy.

Structural plasticity in the human cytosolic sulfotransferase dimer and its role in substrate selectivity and catalysis

“…9) [68]. This observation suggests the location of Loop 1 is a critical factor in SULT substrate binding.…”

“…A number of SULT inhibitor/substrate interactions likely exist that are yet to be identified. Our laboratory has recently described the potent inhibition of hSULT1A1 by 17a-ethinylestradiol, a common female contraceptive agent [68]. The effect of this inhibition on the metabolism of other drugs remains to be evaluated.…”

“…These shifts can positively or negatively alter the ability of some substrates to bind to the active site and are key in defining SULT substrate specificity. To take full advantage of the conservation between isoforms, the most complete "open" crystal structure (PDBID 1J99) has been used as the base model for the open architecture of other SULT isoforms [68]. Insight can be gained using this approach.…”

“…The mobility of these specific loop regions can make structure-based prediction of SULT substrates problematic. In silico prediction of SULT/substrate interactions is not always predictive of their in vitro interactions [68,105]. Rather than relying on fixed crystal structures and homology models to describe SULT activities, researchers have taken notice of the influence of dynamics on SULT/substrate interactions and attempted to describe its impact on SULT activity in a more direct manner.…”

The potent inhibition of human cytosolic sulfotransferase 1A1 by 17α-ethinylestradiol is due to interactions with isoleucine 89 on loop 1

“…Understanding possible DDI with the different SULT isoforms will help predict adverse drug effects and increase our knowledge about metabolism within the human body. Our laboratory has previously described one potential mechanism for adverse drug effects involving interactions between EE2 and SULT1A1 [25]. SULT1A1, which is found in various tissues throughout the human body, including the liver [26], breast [27], gastrointestinal tract [28], and platelets [29], is known to sulfate a broad range of compounds including small phenols [1-naphthol (1-NA), p-nitrophenol (pNP)], thyroid hormones, and estrogens [30].…”