Ontogeny of Estradiol-Binding Sites in Rat Brain. II. Characteristics of a Neonatal Binding Macromolecule

Abstract

Fetal and neonatal rat brain cytosols contain high concentrations of macromolecules which stereospecifically bind tritiated estradiol in vitro. Certain physical-chemical properties of this perinatal binding system were characterized and compared with properties of the soluble cytoplasmic estradiol-binding protein present in adult female hypothalamus and uterus. The estimated total estradiol-binding capacity of the neonatal brain cytosol, although saturable, is relatively high: 4000 pmoles estradiol per g cytosol protein, approximately 200-fold greater than the binding capacity of adult hypothalamic cytosol previously reported by Eisenfeld. Some other features of the neonatal binding substance which differ from those of the adult binding protein include: 1) migration as an approximately “4S” peak in a 5–20% sucrose density gradient; 2) failure to bind diethylstilbesterol; 3) absence of regional concentration within the brain; 4) failure to bind in vitro to rat DNA in the presence of 0.14M NaCl. While differing in the above respects from the adult binding protein, these neonatal molecules are similar, and possibly identical, to an estradiol-binding protein present in fetal and neonatal rat plasma. However, estradiol binding in the brain is not due to blood contamination, since it is present in large amounts after perfusion has removed all detectable blood contamination. Experiments in which binding material can be extracted from the neonatal brain by procedures which do not remove the adult protein indicate that the infant binding sites might be localized in some extracellular compartment of the neonatal brain. (Endocrinology93: 1129, 1973)