Progester one and Corticosterone Binding in Rabbit Uterine Cytosols1

The steroid hormone-binding proteins in rabbit uterine cytosols and blood plasma have been investigated. Uterine cytosols prepared from ovariectomized estrogen-primed animals contained a 7.8S “receptor” which bound progesterone but not corticosterone. A 4.3S progesterone- and a 4.1S corticosterone-binding protein were also present in the cytosols. Rabbit blood plasma contains a protein (CBG) which binds both progesterone and corticosteroids and has a sedimentation coefficient of about 4S. The possibility that the cytosol corticosterone binder was derived from blood present in the estrogen-stimulated uterus was investigated by in situ perfusion. However, the moles of corticosterone binding sites read from Scatchard plots were found to be the same in cytosols from either perfused or unperfused uteri. The intrinsic association constant of the high affinity rabbit progesterone receptor calculated from Scatchard plots was 7.8 ± 0.7 × 108M-1. In the presence of 0.3M KC1 the 7.8S progesterone receptor appeared to dissociate into 4S subunits, in agreement with reports from other laboratories. However, Scatchard plots of equilibrium dialysis data revealed a loss of high affinity binding in the presence of KCl, and a 4S subunit was not seen on recentrifugation of isolated 8S receptor in sucrose gradients containing 0.3M KCl. The properties and behavior of the uterine progesterone receptor are very similar in at least two species, the rabbit and the guinea pig. (Endocrinology93: 74, 1973)