The estrogen receptor relative binding affinities of 188 natural and xenochemicals: Structural diversity of ligands

We have utilized a validated (standardized) estrogen receptor (ER) competit ive-binding assay to determine the ER affinity for a large, structurally di verse group of chemicals. Uteri from ovariectomized Sprague-Dawley rats wer e the ER source for the competitive-binding assay. Initially, test chemical s were screened at high concentrations to determine whether a chemical comp eted with [H-3]-estradiol for the ER. Test chemicals that exhibited affinit y for the ER in the first tier were subsequently assayed using a wide range of concentrations to characterize the binding curve and to determine each chemical's IC50 and relative binding affinity (RBA) values. Overall, we ass ayed 188 chemicals, covering a 1 x 10(6)-fold range of RBAs from several di fferent chemical or use categories, including steroidal estrogens, syntheti c estrogens, antiestrogens, other miscellaneous steroids, alkylphenols, dip henyl derivatives, organochlorines, pesticides, alkylhydroxybenzoate preser vatives (parabens), phthalates, benzophenone compounds, and a number of oth er miscellaneous chemicals. Of the 188 chemicals tested, 100 bound to the E R while 88 were non-binders. Included in the 100 chemicals that bound to th e ER were 4-benzyloxyphenol, 2,4-dihydroxybenzophenone, and 2,2'-methyleneb is(4-chlorophenol), compounds that have not been shown previously to bind t he ER. It was also evident that certain structural features, such as an ove rall ring structure, were important for ER binding. The current study provi des the most structurally diverse ER RBA data set with the widest range of RBA values published to date.