Steroid-protein interactions. XXIII. Nonidentity of cortisol-binding globulin and progesterone-binding globulin in guinea pig serum

Abstract

Exposure of pregnant guinea pig serum to heat and low pH decreases the activity of the cortisol-binding globulin (CBG) to a greater extent than the progesterone-binding activity. The inactivation is irreversible. Hydroxylapatite chromatography separates the two binding proteins from the bulk of other serum proteins; the progesterone binder emerges before CBG. Separation of the progesterone-binding protein from CBG is obtained by Sephadex G-200 filtration. The progesterone binder migrates in the electric field as a globulin, somewhat more slowly than CBG. The affinity constant of the progesterone complex with the progesterone-binding globulin (PBG) is approximately two orders of magnitude greater than that of the cortisol CBG complex. The molecular weights of the guinea pig PBG and CBG, assessed by gel filtration, are of the order of 100,000 and 50,000, respectively.