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AFFINITY OF RABBIT UTERINE OESTRADIOL RECEPTOR FOR PHYTO-OESTROGENS 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-17β (Oe₂) for binding sites on a macromolecular component of the uterine cytosol from 6-day-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 0·5 to 40 ng for coumestrol and 2·5 to 200 ng for genistein. Prior chromatographic separation is required to discriminate between plant-derived and endogenous steroidal oestrogens. Coumestrol was present in the blood of goats (2·0 to 3·9 ng/ml) after feeding alfalfa hay at a rate supplying 12 mg coumestrol/24 hr/animal.

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