Estrone and estriol, as well as 17(β- estradiol, can induce the synthesis of an estrogenspecific uterine protein under appropriate in vitro conditions. Quantitatively, the induction for all three steroids closely parallels the specific uptake of the steroid into the nuclear fraction. This in turn appears to closely correlate with the relative affinity for the cytoplasmic estrogen binding protein (17β-estradiol > estriol > estrone). For 50% of maximal induced protein response or for half saturation of nuclear or cytosol binding sites, approximately three times as much estrone as estradiol and two times as much estriol as estradiol (estradiol = 2 × 10-9M) is required. Estriol is not metabolized to estradiol to any degree; metabolism of estrone to estradiol does not account for either the in vitro response or nuclear localization. It is concluded that estrone, in its own right, is a biologically active estrogen.
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