Abstract
After in vitro labeling with radioactive progesterone, a steroid macromolecule complex was isolated from the cytosol fraction of pregnant guinea pig uterus. The binding protein was purified about 200-fold by chromatographies on hydroxylapatite, DEAE-cellulose and Sephadex G-200. The following molecular characteristics were assessed for the binding protein: Sedimentation coefficient 5 S, Stokes radius 5.5 nm, apparent molecular weight 106,000, pI 2.5, and association constant for progesterone 3.2 × 10 9 l/mole. Relative affinity for ligands was as follows: progesterone>5 α-pregnane-3,20-dione>deoxycorticosterone>5β-pregnane-3,20-dione>20β-hydroxy-4-pregnen-3-one> 5α-dihydrotestosterone>testosterone. No significant affinity for pregnenolone, 17α-hydroxyprogesterone, cortisol or estradiol was detected. Neither the crude cytosolic steroid-macromolecule complex nor the partially purified progesterone-binding protein formed aggregates in a low salt medium. No splitting of the macromolecule into subunits was observed in high ionic media, either.
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