HR & Ayalon N. Affinity of rabbit uterine oestradiol receptor for phytooestrogens and its use in a competitive protein-binding radioassay for plasma coumestrol

Summary. The non-steroidal phyto-oestrogens, coumestrol and genistein, were able to compete with oestradiol-17\\g=b\\(Oe2) for binding sites on a macromolecular component of the uterine cytosol from 6-day\\x=req-\\ pregnant rabbits. Binding affinity for these compounds was related to their reported oestrogenic potency: approximately one part by weight of Oe2, seventy of coumestrol and 175 of genistein produced equivalent inhibition of the uptake in vitro of [3H]Oe2 by this uterine receptor. Biochanin-A, formononetin, 12-O-methylcoumestrol, sativol and medicagol did not significantly inhibit Oe2 binding, suggesting that free hydroxyl groups in both position 7 and 12 ( = 4') of coumestans and isoflavones are essential for effective interaction with the oestrogen receptor. The 7- and 12-methoxy-coumestans and isoflavones tested appear to be pro-oestrogens, able to bind to the uterine receptor only after O-demethylation in vivo. A rapid competitive protein-binding radioassay for phyto-oestrogens in the blood that makes use of the uterine cytosol receptor is described. Its useful range is 0\\m=.\\5to 40 ng for coumestrol and 2\\m=.\\5to 200 ng for genistein. Prior chromatographic separation is required to discriminate between plant-derived and endogenous steroidal oestrogens. Coumestrol was present in the blood ofgoats (2\\m=.\\0to 3\\m=.\\9ng/ml) after feeding alfalfa hay at a rate supplying 12 mg coumestrol/24 hr/animal.