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Characteristics of high affinity progesterone binding to rat placental cytosol.

An exchange assay has been developed to study the progesterone receptor from rat placental cytosol prepared on the 12th day of pregnancy. Specific binding of progesterone was optimal in a buffered medium containing 10 mm Tris, 30% (vol/vol) glycerol, and 1 mm dithiothreitol, pH 7.8, at 4 C for 18–24 h. There was no detectable degradation in binding up to at least 43 h of incubation under these conditions. However, binding rapidly declined at elevated temperatures. Exposure of placental cytosol to N-ethymaleimide eliminated specific binding without affecting nonspecific binding, whereas exposure to HgCl2 decreased specific binding by 91% but enhanced nonspecific binding 324%. Scatchard analysis of [3H]progesterone binding to receptor yielded a Kd of 3.4 ± 0.3 nm and 0.25 ± 0.04 pmol binding sites/mg placental cytosol protein or 11.3 ± 1.8 pmol/placenta (mean ± sem for eight experiments). Several structurally similar corticosteroids were weak competitive inhibitors of progesterone binding. The Ki was 46.0 ±...