Characterization of specific steroid binding in human amnion at term

Area of Science:

• Reproductive biology
• Endocrinology
• Biochemistry

Background:

• Human fetal membranes play a role in labor onset.
• A progesterone-binding protein in these membranes has been implicated in labor initiation.

Purpose of the Study:

• To quantitatively characterize specific steroid binding in human fetal membranes.
• To investigate the binding affinity and characteristics of steroid-binding proteins in the amnion.

Main Methods:

• Cytosol from human amnion was prepared and extracted.
• Specific binding of cortisol and progesterone was measured using a charcoal assay.
• Steroid competition assays were performed to determine binding affinities.

Main Results:

• The amnion cytosol exhibited high-affinity binding for cortisol (KD = 1 nM) and a large number of binding sites.
• Progesterone binding showed a 10-fold lower affinity (KD = 12 nM) compared to cortisol.
• Cortisol binding was stable, reversible, and showed higher affinity for cortisol and corticosterone than progesterone.

Conclusions:

• The steroid-binding protein in human amnion at term has a higher affinity for cortisol and corticosterone than progesterone.
• This suggests a potential role in glucocorticoid metabolism rather than progesterone withdrawal in labor onset.
• Findings challenge previous hypotheses regarding the protein's function in initiating labor.