Structural requirements of para-alkylphenols to bind to estrogen receptor

Octyl- and nonylphenols in the environment have been proposed to function a s estrogens. To gain insight into their structural essentials in binding to the estrogen receptor, a series of phenols with saturated alkyl groups at the para position, HO-C6H4-CnH2n+1 (n = 0-12), were examined for their abil ity to displace [H-3]17 beta-estradiol in the recombinant human estrogen re ceptor, which was expressed in Sf9 cells using the vaculovirus expression s ystem. All tested para-alkylphenols were found to bind fully to the estroge n receptors in a dose-dependent manner. The interaction of alkylphenols wit h the receptor became stronger with increase in the number of the alkyl car bons and the activity was maximized with n = 9 of nonylphenol. Phenol (a = 0) exhibited weak but full binding to the receptor, whereas anisole with a protected phenolic hydroxyl group was completely inactive. Also, alkanes su ch as n-octane, 2,2,4-trimethylpentane corresponding to tert-octane, and n- nonane exhibited no binding. The results indicate that the binding of para- alkylphenols to the estrogen receptor is due to the effect of covalent bond ing of two constituents of the phenol and alkyl groups, which correspond to the A-ring and hydrophobic moiety of the steroid structure, respectively. When alkylphenols were examined for their receptor binding conformation by H-1-NMR measurements and ab initio molecular orbital calculations, it was s uggested that nonbranched alkyl groups are in an extended conformation, whi le branched alkyl groups are in a folded conformation. These results sugges t that branched and nonbranched alkyl moieties of alkylphenols interact dif ferently with the lipophilic ligand binding cavity of the estrogen receptor when compared to the binding of 17 beta-estradiol.