Steroid protein interactions. XXVIII. The isoelectric point and pH stability of the progesterone-binding globulin

Isoelectric focusing of progesterone-binding globulin of pregnant guinea pig serum occured at pI = 2.8. This result was obtained when the binding activity of the focused protein fractions was measured by equilibrium dialysis. Reliance on localization of the radioprogesterone alone may lead to erroneous results presumably due to dissociation of the uncharged progesterone molecule from the protein during the electrophoretic migration into the acidic milieu. This assumption was corroborated by binding assays over the pH range from 2 to 11. Almost complete dissociation occurred at pH 4 and below; however, return to neutral pH restored over 80% of the binding activity. The pH gradient of the electrofocusing column was improved by adding aspartic and glutamic acid to the pH 3–5 Ampholines.