title: Search for a progesterone-binding protein in secretory granules of the ovine corpus luteum.
abstract: The hypothesis that electron-dense granules present in the corpus luteum contain progesterone in a protein-bound form was examined using differential and sucrose gradient centrifugation. Ovine luteal tissue was fractionated by differential centrifugation at 1,000 X g (P1 pellet), 10,000 X g (P2), and 82,000 X g (P3; supernatant S3). Samples of P2, P3, and S3 were further fractionated o 20-40% (P2 and P3) or 5-25% (S3) sucrose gradients and examined for progesterone-binding activity by measuring the progesterone content and/or the specific binding of [3H]progesterone of sucrose gradient samples. In addition, saturation binding assays were performed with steroid-free samples of P2 and S3. Saturable binding of progesterone was not found in P2, the fraction containing electron-dense granules. In S3, two progesterone-binding proteins with sedimentation rates of 3.2S and 8.6S and an affinity of 7.1 X 10(5) M-1 for progesterone were detected. The sedimentation behavior of these proteins was distinct from that of ovine plasma transcortin, a 4S protein. The view that a binding protein is released into the interstitial fluid during the exocytosis of granules was examined by measuring the progesterone-binding activity of protein released by slices of corpus luteum in vitro. No binding activity was found. The results of this investigation do not support the hypothesis that putative progesterone-secreting granules observed in luteal tissue contain a binding protein
mesh_terms: D000510:Alpha-Globulins / Q000032:analysis*; D000818:Animals; D002499:Centrifugation, Density Gradient; D003338:Corpus Luteum / Q000166:cytology*; D003594:Cytoplasmic Granules / Q000032:analysis*; D003600:Cytosol / Q000378:metabolism; D005260:Female; D011247:Pregnancy; D011374:Progesterone / Q000378:metabolism; D011376:Progesterone-Binding Globulin / Q000032:analysis*; D012756:Sheep
chemical_list: D000510:Alpha-Globulins; D011376:Progesterone-Binding Globulin; D011374:Progesterone