Oestrogen Receptors in Chick Oviduct

Macromolecular components with properties of oestrogen receptors have been identified in the 0.5 M KCl nuclear soluble, the nuclear insoluble and the cytosol fractions of laying hen and immature (2–4 weeks, untreated by hormone) chicken oviduct. In the 0.5 M KCl extract of laying hen oviduct nuclei, a receptor, of protein nature according to the effects of enzymic treatments, has been identified. It exhibits high affinity for oestradiol with an apparent equilibrium association constant K A = 4 · 10 9 M −1 at 4°C. The binding of [ 3 H] oestradiol is abolished by 1 μM oestriol, oestrone and diethylstilboestrol, but not by the same concentration of progesterone, testosterone, and cortisol. Sucrose gradient ultracentrifugation studies in the presence of 0.5 M KCl indicate a sedimentation coefficient of 4.3 S, and there is partial aggregation in low‐ionic‐strength medium. The estimated number of binding sites per nucleus is about 5000, as calculated from DNA content of chick diploid genome. Most of the binding sites were found to be occupied by endogenous oestrogen(s). Oestradiol dissociates from the receptor according to an apparent two‐step mechanism. The half‐life time for the faster dissociation step is 18 h at 0 °C, 25 min at 20°C and 10 min at 30°C, and for the slower one is 180 h, 115 min and 60 min, respectively. In the 0.5 M KCl extract of immature chicken oviduct nuclei, there are approximately 500 receptor sites per nucleus; their affinity for oestradiol is the same as in the case of laying hen soluble nuclear receptor. After repeated extractions of nuclei with 0.5 M KCl medium, a substantial quantity of oestrogen binding sites remains in the residual fraction. Binding characteristics of this insoluble nuclear receptor resemble those of the soluble nuclear receptor: high affinity for oestradiol ( K A = 7 · 10 8 M −1 at 37 °C) and specificity for oestrogens. The estimated number of binding sites are approximately 2000/cell for laying hen, and approximately 1000/cell for immature chicken. In the high‐speed supernatant fraction of laying hen oviduct homogenates, an oestrogen receptor is also present, but its concentration is low (≤ 100 sites/cell) and at the limits of sensitivity of the methods used. In the cytosol of immature chicken oviduct, there are approximately 2500 oestradiol receptor sites per cell.