STEROID-PROTEIN INTERACTIONS .21. METAL ION INHIBITION OF ASSOCIATION BETWEEN PROGESTERONE AND ALPHA1-ACID GLYCOPROTEIN

The binding affinity of α1-acid glycoprotein (AAG, orosomucoid) for progesterone was determined by equilibrium dialysis at 4 and 37 °. Evaluation of number of binding sites and association constants according to Scatchard's procedure gave one primary site, n1, and a number of secondary sites of considerably lower affinity. The association constants, k1, were found to be 9.0 × 105 m-1 and 3.5 × 105 m-1 at 4 and 37 °, respectively. The apparent changes of free energy, ΔF °, and of enthalpy, gDH °, were negative, whereas a positive entropy change, ΔS °, was found for the progesterone-AAG complex. The progesterone-AAG interaction was inhibited by Hg2+ > Ag+ > Cu+ > Fe2+. The inhibition of the complex formation by the four metal ions was analyzed by Scatchard plots and was interpreted as most likely noncompetitive. The glycoprotein could be protected by EDTA from the action of the metal ions. Moreover, the inhibitory effect of the metal ions once established, was found to be reversible when EDTA was added to a maximally inhibited system. © 1969.