The inhibitory effect of cytosolic fraction of human decidua on prostaglandin synthesis.

To elucidate the mechanism of suppression of prostaglandin (PG) production in decidua in early pregnancy, the PG synthetase activity of decidua and mid-secretory endometrium was studied. The microsomal fractions and their supernatants were prepared from the tissue by ultracentrifugation at 105,000 g. The standard incubation mixture consisted of the microsomal fraction and 14C arachidonic acid with cofactors, with incubation being carried out for 10 minutes at 37 degrees C. After extraction, the radioactivity of PGE2 was measured and PG synthetase activity was assayed. The apparent Km value for PG synthetase in decidua was 4.6 +/- 0.14 x 10(-6) M (n = 4), whereas that in endometrium was 4.6 +/- 1.18 x 10(-6) M (n = 3). Subsequently, kinetic studies on PG synthetase inhibitor in decidua were carried out. When sheep seminal vesicle was used as an enzyme source, the decidual supernatant showed competitive inhibition. The inhibitory substance in decidua was inactivated after incubation for 15 minutes at 65 degrees C. It seems likely that the suppression of PG biosynthesis in human decidua in early pregnancy is not due to the difference in PG synthetase found in decidua and in endometrium, but due to the existence of PG synthetase inhibitor in decidua.