A cytoplasmic high affinity estrogen-binding protein in the embryonic chicken liver.

1. Embryonic chicken liver contains cytoplasmic estrogen-binding sites with high affinity for estradiol (Kd = 1 X 10(-9) M) and with low capacity (200-1100 binding sites/cell, dependeing on the age of the embryo). 2. Only estrogenic hormones are bound with high affinity. 3. At least the estrogen-bindin moiety of these binding sites is a protein. 4. By agarose gel chromatography an apparent molecular weight of 50000-60000 daltons was determined, very similar to that of the nuclear estrogen-binding protein. 5. In low salt the cytoplasmic binding sites exhibit a strong tendency to aggregate, which is abolished by mild trypsination. 6. A single injection of estradiol into the egg seems to cause a translocation of the cytoplasmic binding protein to the nucleus. 7. During the embryonic development the number of binding sites increases up to the age of 19 days. Thereafter it decreases again reaching a constant low level soon after hatching. 8. Data are presented to show that this low level of binding sites can not be due solely to a degradation or inhibition in vitro by a component eventually present in the liver homogenate of hatched animals.