Characterization of specific steroid binding in human amnion at term

In human fetal membranes (amnion and chorion) the appearance, during the last few weeks of pregnancy, of a protein which binds progesterone has been suggested as playing an important role in the inset of labor. The purpose of the present study was to quantitatively characterize specific binding in fetal membranes. Amnion, rather than chorion, was studied because it could be obtained free from contamination by other tissues. The cytosol (105,000 X g supernatant) was used after being extracted for 1 h with dextran-coated charcoal. The specific binding of cortisol in the cytosol, determined with a charcoal assay, was stable after storage at -20 degrees C or -60 degrees C and after heating for 1 h at 37 degrees C. It was reversible, and showed high-affinity, KD = 1 +/- 0.5 nM (mean +/- SEM, n = 6) and a large number of sites were found, 1497 +/- 666 fmoles/mg protein (mean +/- SEM, n = 6). In contrast, progesterone binding in the cytosol had a 10-fold lower affinity, KD = 12 +/- 5 nM (mean +/- SEM, n = 5) with 802 +/- 246 fmol bound/mg protein (mean +/- SEM, n = 5). A number of steroids were examined for their ability to compete with cortisol binding. The following were found to express affinities relative to cortisol which was considered to be 100: corticosterone (100), progesterone (10), cortisone (8), 5 alpha-pregnane-3, 20-dione (2), and 20 alpha-hydroxy-4-pregnen-3-one (less than 1). No competition was found for dexamethasone, betamethasone, triamcinolone, triamcinolone acetonide, R5020, medroxyprogesterone acetate, estrone or estradiol. These studies indicate that the steroid binding protein associated with human amnion at term exhibits a higher affinity for a cortisol and corticosterone than progesterone. Thus it is perhaps involved in the metabolism of glucocorticoids by the membranes, rather than in the local withdrawal of progesterone, as proposed by previous investigators.