Progesterone-binding Plasma Protein of Pregnant Guinea Pig

The progesterone-binding plasma protein (PBP) of the pregnant guinea pig has been purified to homogeneity. The molecular weight, 77,500 (as determined by equilibrium sedimentation and sodium dodecyl sulfate polyacrylamide gel electrophoresis), the s20,w0, 4.5, the optical extinction coefficient at 280 nm E1 mg/ml1 cm, 0.49, the pHi, 3.6 (isoelectro-focusing), the Stokes radius (47 A), and the partial specific volume (0.683) were obtained. The PBP chemical composition is characterized by a very high (48.7%) carbohydrate content. One molecule of PBP binds 1 molecule of progesterone, with an association constant Ka = 9.108 m-1 at 4° measured at equilibrium. Ka for testosterone is 1.6 107 m-1 whereas the affinity for cortisol could not be studied because it is less than 106 m-1. PBP binds with high affinity various C21 compounds among which are 5α-pregnan-3,20-dione, 20α-hydroxy-pregn-4-en-3-one, and 21-hydroxy-pregn-4-en-3,20-dione. PBP, found in the maternal plasma, is not detected in the fetus nor in the umbilical vein or arteries. PBP could not be induced in nonpregnant animals by administering large doses of estrogen or progesterone, thus raising the possibility that it is synthesized in an organ present only during pregnancy (placenta?). No protein similar to PBP was detected n pregnant rats or women.