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Assay Method Information

Assay Name:  Binding Assay
Description:  Recombinant ER-α or ER-β ligand binding domain (LBD) was combined with [3H]E2 (PerkinElmer, Waltham, Mass.) in buffer A (10 mM Tris, pH 7.4, 1.5 mM disodium EDTA, 0.25 M sucrose, 10 mM sodium molybdate, 1 mM PMSF) to determine the equilibrium dissociation constant (Kd) of [3H]E2. Protein was incubated with increasing concentrations of [3H]E2 with and without a high concentration of unlabeled E2 at 4° C. for 18h in order to determine total and non-specific binding. Non-specific binding was then subtracted from total binding to determine specific binding. Ligand binding curves were analyzed by nonlinear regression with one site saturation to determine the Kd of E2 (ER-α: 0.65 nM; ER-β: 1.83 nM). In addition, the concentration of [3H]E2 required to saturate ER-α and ER-β LBD was determined to be 1-3 nM.Increasing concentrations of two β-SERMs (14m and 12u) (range: 10−11 to 10−6 M) were incubated with [3H]E2 (1-2 nM) and ER LBD using the conditions described above. Following incubation, plates were harvested with GF/B filters on the Unifilter-96 Harvester (PerkinElmer) and washed three times with ice-cold buffer B (50 mM Tris, pH 7.2). The filter plates were dried at room temperature, then Microscint-O cocktail was added to each well and the filter plates were sealed with TopSeal-A. Radioactivity was counted in a TopCount® NXT Microplate Scintillation Counter using the settings for [3H] in Microscint cocktail (PerkinElmer).The specific binding of [3H]E2 at each concentration of compound was determined by subtracting the nonspecific binding of [3H]E2 (determined by incubating with 10−6 M unlabeled E2) and expressing it as a percentage of the specific binding in the absence of compound. The concentration of compound that reduced the specific binding of [3H]E2 by 50% (IC50) was determined by computer-fitting the data with SigmaPlot and non-linear regression with the four parameter logistic curve. The equilibrium binding constant (Ki) of each compound was then calculated by: Ki=Kd×IC50/(Kd+L), where Kd is the equilibrium dissociation constant of [3H]E2, and L is the concentration of [3H]E2.
Affinity data for this assay
 

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Last update November 1, 2007
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