[Physico-chemical properties and amino acid composition of a highly purified preparation of a specific estrogen-binding protein of the rat liver].

The structure and properties of an unusual estrogen-binding protein (UEBP) from male rat liver that was purified by affinity adsorption chromatography was studied. A high degree of purity of UEBP (greater than 99%) associated with appreciable microheterogeneity was demonstrated. The latter seems to be due to partial proteolysis of the protein at the N-end in the course of the isolation procedure. The purified UEBP molecules have the following characteristics: Mr = 31 000 (data from SDS-PAAG electrophoresis), sedimentation coefficient 3.75 S, Stockes'radius 25.6 A, friction coefficient ratio 1.11. The protein absorbance maximum in the UV region lies at 276 nm; extinction coefficient--26, alpha-helix content is 25-30%. The value of equilibrium association constant for estradiol is 5 X 10(7) M-1. Estriol (greater than 100%) and, in a weaker degree, estrone and testosterone (approximately 10%) compete for the binding sites with [3H]estradiol; androsterone has no competitive effect. The amino acid composition of UEBP was determined. The protein was shown to possess a great number of residues carrying hydrophobic side groups (34.4%) and more acidic amino acids over basic ones as well as a low content of cystein, threonine and histidine.