An oestradiol-binding macromolecule was observed in the left Müllerian duct of the 15-day female chick embryo. The embryonic receptor binds oestradiol with a high affinity and low capacity, having a Kd of 3.2 × 10(-9)M and a maximal number of sites of 5.45 fmol/10(6) cells in the left Müllerian duct. The receptor is protein in nature, as suggested by its susceptibility to proteolysis; in addition, it is organ- and steroid-specific. Judging by glycerol-gradient analysis, the hormone receptors in the cytosol are present in 8S and 4.5S forms, and the 8S form could be dissociated into a 4.5S form in the presence of 0.5M-KCl. A 4.5-6S receptor could be extracted from the nuclei. Under physiological salt conditions, the embryonic receptors bind to DNA-cellulose and can be eluted when the salt concentration is increased to 0.5M-KCl. Determination by isoelectric focusing indicates that the isoelectric point is 5.8 for the 8S and 6.9 for the 4.5S receptor.
