Isolation and characterisation of a progesterone- and testosterone-binding globulin from pregnant guinea pig serum

A progesterone- and testosterone-binding globulin, has been isolated from a pool of pregnant guinea pig serum, using DEAE-Sephadex chromatography, gel filtration on Sephadex G-200 and preparative polyacrylamide gel electrophoresis. This globulin appeared to be heterogeneous on ion exchange chromatography, isoelectric focusing and equilibrium ultracentrifugation, indicating a quarternary structure sensitive to concentration and to ionic environment. In line with the steroid-binding serum proteins so far described progesterone-binding globulin is a glycoprotein with a carbohydrate content of 42%. Extrapolated sedimentation and diffusion coefficients were 4.52 S and 5.1.x 10(-7) cm2/s, respectively. A partial specific volume of 0.678 cm3/g has been calculated from the chemical composition. A molecular weight of 82,00 was obtained from equilibrium centrifugation experiments in the presence of o0I1 sodium dodecyl sulfate. The electrophloretic mobility at pH 8.6 corresponds to that of an a,lpha1globulin; isoelectric points were 4.4 and 3.5; Stokes radius, 4.9 nm; frictional ratio, 1.74; extinction coefficient at 280 nm, 7.3. Pure progesterone-binding globulin showed an association constant at 4.0 degrees of 14 x 10(9)M-(1) for progesterone. The affinity for testosterone was lower being 8.2 x 10(7) M(-1). The number of high-affinity binding sites was determined to 1.0 or both steroids.