[Study on the progesterone receptor in human chorion and placenta at different stages of gestation]

Characteristics of progesterone receptor (PR) in human chorion and placenta were studied at different stages of gestation. The cytosol was labeled with 3H-R5020 at 0 degree C for 24 hr. The dissociation constant and the concentration of progestin binding site were determined by the dextran-coated charcoal method. The affinity of PR for ATP was evaluated on ATP-Sepharose column chromatography. It had been reported that the ability of PR to bind to ATP paralleled that to chromatin. PR was characterized by an analytical isoelectric focusing. Scatchard analysis demonstrated progestin binding of high affinity (Kd = 13.4 +/- 4.3 nM) (mean +/- SD) and low capacity (Bmax. = 478 +/- 275 fmol/mg protein) (mean +/- SD). And the level of PR in placenta did not change at various stages during gestation. The PR up to 32 weeks of gestation showed high affinity for ATP (activated form) following activation. This affinity, however, decreased significantly (non-activated form) after 33 weeks of gestation. The PR was focused as three peaks at p16.3, 6.0 and 5.3. And the peak at p15.3 was increased in term placenta. These results suggest that the action of progesterone in human placenta might be regulated by the qualitative change rather than by the quantitative change in its cytosolic PR in late pregnancy.