TY - JOUR T1 - HETEROTROPIC MODULATION OF SULFOTRANSFERASE 2A1 ACTIVITY BY CELECOXIB: PRODUCT RATIO SWITCHING OF ETHYNYLESTRADIOL SULFATION JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 1260 LP - 1264 DO - 10.1124/dmd.32.11.1260 VL - 32 IS - 11 AU - Donghui Cui AU - Catherine L. Booth-Genthe AU - Edward Carlini AU - Brian Carr AU - Michael L. Schrag Y1 - 2004/11/01 UR - http://dmd.aspetjournals.org/content/32/11/1260.abstract N2 - The major sulfated product of 17α-ethynylestradiol (EE) after incubations with 3′-phosphoadenosine-5′-phosphosulfate and recombinant human sulfotransferase 2A1 (SULT2A1), or liver cytosol, is the 3-O-sulfate of EE. However, when celecoxib is also present in the incubation, sulfation is switched (in a concentration-dependent manner) from the 3-O-position to the 17β-O-position of ethynylestradiol. In incubations with recombinant SULT2A1, increasing concentrations of celecoxib decreased the Vmax of 3-O-sulfate product formation by 3- to 4-fold, with no major change in the Km value. For 17β-O-sulfate formation, increasing concentrations of celecoxib resulted in an 8-fold decrease in the Km and a 7-fold increase in Vmax. Celecoxib not only modulated the regioselectivity of the enzyme, but also activated the enzyme such that total sulfated product exceeded product formation by the native enzyme, 3- to 4-fold (at 250 μM celecoxib). Finally, IC50 values obtained by varying celecoxib concentrations (0–250 μM) at fixed concentrations of EE showed that 3-O-sulfation was inhibited by celecoxib to the same extent, independent of the concentration of EE. In addition, the apparent kinetic constant for celecoxib (as measured by EE 17β-O-sulfation) decreased 2-fold in the presence of high concentrations of EE, consistent with the potential for celecoxib to bind to either the enzyme-EE complex or to free enzyme. Taken as a whole, these data suggest that celecoxib is acting as a heterotropic modulator of SULT2A1 activity, most likely involving a separate noncompetitive binding site. The American Society for Pharmacology and Experimental Therapeutics ER -