The postnatal development of the retina in the normal and rodless CBA mouse: A light and electron microscopic study

The question of the identity of the high-affinity binding sites for progesterone with those of the corticosteroid-binding globulin (CBG) was investigated in pregnancy serum of man, rabbit, rat and mouse. Chromatographic techniques failed to separate a potential progesterone-binding macromolecule from CBG in any of the sera. Competition studies with enriched fractions containing added ³H] progesterone and ¹⁴C] cortisol strongly bound to protein showed displacement of ³H] progesterone by an excess of unlabeled cortisol, and vice versa. It is concluded that CBG is the principal progesterone binder in the pregnancy sera investigated and that no other protein with high affinity for progesterone is present in significant quantity. In contrast, the progesterone-binding globulin (PBG) in the serum of the pregnant guinea pig is distinct from CBG and has been separated from it. The PBG differed from the uterine cytosol receptor for progesterone in its sedimentation coefficient (sucrose gradient centrifugation) and its molecular size (gel filtration). With both the uterine receptor and PBG, norethynodrel displaced radiolabeled progesterone from the specific binding sites. This is in contrast to analogous binding systems involving other steroid hormones and other species, where the synthetic hormonal agent interacts strongly with the receptor protein of the target tissue, but not with the high-affinity binder in the serum.