Characterization of Specific Steroid Binding in Human Amnion at Term

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Abstract
In human fetal membranes (amnion and chorion) the appearance, during the last few weeks of pregnancy, of a protein which binds progesterone may play an important role in the onset of labor. Specific binding in fetal membranes was quantitatively characterized. Amnion, rather than chorion, was studied because it could be obtained free from contamination by other tissues. The cytosol (105,000 g supernant) was used after being extracted for 1 h with dextran-coated characoal. Specific binding of cortisol in the cytosol, determined with a charcoal assay, was stable after storage at -20 or -60.degree. C and after heating for 1 h at 37.degree. C. It was reversible, and showed high-affinity, Kd = 1 .+-. 0.5 nM (mean .+-. SEM [standard error of the mean], n = 6) and a large number of sites were found, 1497 .+-. 666 fmol/mg protein (mean .+-. SEM, n = 6). Progesterone binding in the cytosol had a 10 times 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 (< 1). No competition was found for dexamethasone, betamethasone, triamcinolone, triamcinolone acetonide, R5020 [2-(1-methyl-4-piperidyl)-2-phenylglutarimide], medroxyprogesterone acetate, estrone or estradiol. Steroid binding protein associated with human amnion at term exhibits a higher affinity for cortisol and corticosterone than progesterone. It may be involved in the metabolism of glucocorticoids by the membranes, rather than in the local withdrawal of progesterone, as previously proposed.

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