Interactions of sterols with antiestrogen-binding sites: structural requirements for high-affinity binding.
Animal and human tissues contain a microsomal protein that binds nonsteroidal antiestrogens with high affinity and specificity. The functions of these binding sites and the identity of their natural ligands are unknown. Following a report that certain sterols inhibit [3H]tamoxifen binding to this site, we attempted to define the structural requirements for maximal inhibition of [3H]tamoxifen binding to rat liver antiestrogen-binding sites. Our studies identified 5 alpha-cholestan-3 beta-ol-7-one (7-ketocholestanol) as the most potent sterol, having an inhibitory activity that was 12% that of unlabeled tamoxifen and an equilibrium dissociation constant of 6.3 nM. Structural features that appeared important for the inhibitory activity of this sterol include the presence of i) a hydrocarbon side chain at C17; ii) an oxygen function at C7; iii) a hydroxyl group at C3; and iv) the absence of a double-bond between C5 and C6. Saturation analysis and kinetic studies of [3H]tamoxifen binding in the presence of varying concentrations of 7-ketocholestanol clearly indicated that this sterol competed directly with tamoxifen for the antiestrogen-binding site. Unlike tamoxifen, this sterol did not bind to the estrogen receptor. These features make 7-ketocholestanol a potentially valuable tool for studying the properties and functions of this site.
Hwang
P L
PL
Department of Physiology, National University of Singapore.
Matin
A
A
D000818:Animals; D002855:Chromatography, Thin Layer; D004965:Estrogen Antagonists / Q000378:metabolism*; D005260:Female; D007700:Kinetics; D008024:Ligands; D008099:Liver / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D011960:Receptors, Estrogen / Q000378:metabolism*; D013261:Sterols / Q000378:metabolism*; D013629:Tamoxifen / Q000378:metabolism
D004965:Estrogen Antagonists; D008024:Ligands; D011960:Receptors, Estrogen; D013261:Sterols; D013629:Tamoxifen