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Steroid-Protein Interactions

XVII. INFLUENCE OF SOLVENT ENVIRONMENT ON INTERACTION BETWEEN HUMAN ₁-ACID GLYCOPROTEIN AND PROGESTERONE

From the ¹ From the Biochemistry Department, University of Louisville, School of Medicine, Reynolds Building, Louisville, Kentucky 40208

The apparent binding affinity between progesterone and ₁-acid glycoprotein (AAG) is increased manifoldly by neutral salts, such as Na₂SO₄, (NH₄)₂SO₄, or NaCl, which generally stabilize the conformational structure of globular proteins; other perturbants which are known to destabilize globular proteins, e.g. LiBr, CaCl₂, or urea, decrease the apparent stability of the complex. The intrinsic viscosity of AAG in 3 m CaCl₂ and in 4 m NaCl is greater and smaller, respectively, than in water; this is in agreement with the assumption of a conformational change toward a random coil and a more compact structure, respectively, in the two salt solutions. The order of efficiency of the perturbants in increasing or decreasing the apparent progesterone-AAG affinity is similar to the Hofmeister series.

The affinity-increasing NaCl enhances the apparent association constant of the progesterone-AAG complex and does not affect the number of binding sites; the affinity-decreasing LiBr reduces both the apparent association constant and the average number of binding sites. Similar enhancement of the apparent binding affinity by 4 m NaCl was also observed for the AAG complexes of testosterone, cortexone (deoxycorticosterone), corticosterone, cortisol, estrone, and estradiol. Thermodynamic data indicate that the interaction of AAG with the ⁴-3-ketosteroids is associated with a negative free energy change, composed of a negative enthalpy change and a positive entropy change. The progesterone-binding affinity is independent of the presence of sialic acid in the AAG molecule.

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