Unsaturated fatty acids as endogenous inhibitors of tamoxifen binding to anti-oestrogen-binding sites.
It is known that triphenylethylene anti-oestrogens such as tamoxifen bind to specific high-affinity anti-oestrogen-binding sites, which are distinct from oestrogen receptors. These binding sites are widely distributed in human and animal tissues, but their function and endogenous ligands are unknown. By using [3H]tamoxifen and a rat liver microsomal fraction, a radio-ligand-binding assay was developed in an attempt to identify endogenous ligands for the anti-oestrogen-binding sites in the rat. An ether extract of rat serum inhibited [3H]tamoxifen binding to rat liver binding sites in a dose-dependent manner. Identification of the active serum constituents that inhibited [3H]tamoxifen binding was achieved by g.l.c.-mass spectrometry after preliminary purification of a rat serum extract by silica-gel t.l.c. Three unsaturated fatty acids (oleic, linoleic and arachidonic) accounted for about 50% of the total inhibiting activity of the serum extract. The concentrations of these fatty acids required to inhibit [3H]tamoxifen binding were in the range of 10-100 microM, comparable with those found in the rat circulation under physiological conditions. Saturated fatty acids present in rat serum (palmitic and stearic) did not inhibit [3H]tamoxifen binding. A survey of other fatty acids revealed that, in general, unsaturated fatty acids were far more potent than saturated fatty acids in inhibiting [3H]tamoxifen binding. These studies demonstrate that unsaturated fatty acids are quantitatively the most important circulating inhibitors of [3H]tamoxifen binding to the anti-oestrogen-binding sites. The biological significance of their interaction with these sites, however, remains to be clarified.
D000818:Animals; D001665:Binding Sites; D002850:Chromatography, Gel; D002855:Chromatography, Thin Layer; D005231:Fatty Acids, Unsaturated / Q000494:pharmacology*; D005260:Female; D066298:In Vitro Techniques; D007700:Kinetics; D008024:Ligands; D008099:Liver / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D011955:Receptors, Drug; D011960:Receptors, Estrogen / Q000187:drug effects / Q000378:metabolism*; D013629:Tamoxifen / Q000037:antagonists & inhibitors*
D005231:Fatty Acids, Unsaturated; D008024:Ligands; D011955:Receptors, Drug; D011960:Receptors, Estrogen; D013629:Tamoxifen