The potent inhibition of human SULT1A1 by 17α‐ethinylestradiol (EE2) is due to interactions with ILE89 in loop 1 (1141.12)

Abstract

Drug‐drug interactions (DDI) are a concern for the 17% of women in the United States taking birth control pills. Competition for metabolism with phase I and II drug metabolizing enzymes is one cause of these DDI. The object of this study was to determine the mechanism of SULT1A1 inhibition by EE2. SULT1A1 is the major drug/xenobiotic conjugating isoform found mainly in liver and the GI tract, but is also widely expressed at varying levels in other tissues including breast and brain. SULT1A1 conjugates a broad range of xenobiotics and endogenous compounds such as small phenols, estrogens, and acetaminophen. The Km of EE2 for SULT1A1 is 3.5‐fold lower than the Km of 17β‐estradiol (E2). In silico docking of EE2 to SULT1A1 crystal structures and homology models suggested that EE2 may inhibit SULT1A1. The inhibition constant range for E2, 2‐naphthol, and p‐nitrophenol was 9‐19nM, a physiological range for potent inhibition of SULT1A1. We were interested in the mechanism by which EE2 could inhibit SULT1A1 at low nanomolar concentrations while EE2 is not sulfated until assay concentrations are 30‐fold higher. Therefore, further analysis of the docking studies identified two amino acids that may contribute to EE2 inhibition, TYR169 and ILE89. Mutants of each residue were made, TYR169PHE (Y169F) and ILE89ALA (I89A), and tested for sulfation by and inhibition of EE2. Both mutants displayed the same Km for E2; however, the Km of EE2 for SULT1A1‐Y169F increased 3‐fold while the Km for SULT1A1‐I89A increased 10‐fold. The SULT1A1‐Y169F mutant retained the ability of EE2 to inhibit substrate sulfation with a slightly higher Ki although still in the nanomolar range, whereas SULT1A1‐I89A inhibition by EE2 was almost completely abolished at the concentrations tested. These data provide a better understanding of the SULT1A1 active site and indicate that the potent inhibition of SULT1A1 by low nanomolar EE2 concentrations may be a source of DDI in women using EE2 containing contraceptives.Grant Funding Source: Supported by NIH GM38953