An Insoluble Receptor for Oestrogens in the “Residual” Nuclear Proteins of Chick‐Liver

When chick liver nuclei are incubated at 37°C with [ 3 H]oestradiol in buffer or in liver cytosol, the majority of the incorporated radioactivity is not extractable by high or low salt solutions or by the usual detergents. Total extraction is obtained only by using polar organic solvents, such as methanol. This was also observed when liver minces were incubated with the radioactive hormone and the nuclei were prepared subsequently. The hormone recovered in this insoluble fraction of the nuclei is bound to a protein component, as determined by treatment with various hydrolytic enzymes. This protein binds oestrogens specifically (oestradiol with a K d ∼ 1 nM at 37°C) and is found in the liver but not in the lung nor the kidney. There is a larger number of oestradiol binding sites in the “residual” nuclear fraction of oestradiol‐treated chicks or laying hens than in immature or male chickens.