Estradiol binding serum proteins in normal and immunized rabbits and their physiological significance.

Characterization of serum proteins, which selectively bind estradiol for the regulation of its effective concentration, showed that the binding proteins differ in the normal and estradiol immunized rabbits. The estradiol binding protein in the control serum had a molecular weight of 65,000; Stokes radius 31 Å; frictional ratio 1.28 and sedimentation coefficient of 4.6 S while that in the immune serum had a molecular weight of 160,000; Stokes value 54 Å; frictional ratio 1.26 and sedimentation constant of 6.9 S. Binding affinity and binding capacity of the binding proteins in the sera of control and immunized rabbits showed that the former is a low affinity and high capacity protein with an association constant of 1.67 × 10⁻⁶ M⁻¹ and the number of binding sites 1.125 × 10⁻⁵ mol/ml serum, while the latter had an association constant of 1.56 × 10⁻⁹ M⁻¹ with 3 × 10⁻⁹ mol/ml serum binding sites. The difference in the nature of binding proteins was further authenticated by their relative mobilities on electrophoresis. The data suggested that estradiol binding serum proteins in the immunized rabbit are different from those in the control and this, in turn, has marked implications for the availability of hormone at the target site.