PROSTAGLANDIN ENDOPEROXIDE-H SYNTHASE (PGHS) ACTIVITY AND IMMUNOREACTIVE PGHS-1 AND PGHS-2 LEVELS IN HUMAN AMNION THROUGHOUT GESTATION, AT TERM, AND DURING LABOR

Prostaglandins (PGs) are of primary importance in the initiation and m aintenance of labor in women. A major intrauterine source of prostagla ndins is the amnion, which synthesizes increased amounts of PGE(2) at term labor. Because PG endoperoxide-H synthase (PGHS) catalyzes the ra te-limiting step of PG synthesis from arachidonic acid, we investigate d the changes in amniotic PGHS specific activity during gestation and at term and preterm labor. Also, we determined the level of immunoreac tive PGHS protein in the amnion to evaluate the mechanisms by which PG HS activity may be regulated. PGHS specific activity, measured as the amount of PGE(2) produced by amnion microsomes under optimal condition s, was 18.2 +/- 3.7 pg PGE(2)/mu g protein.min (mean +/- SE; n = 19) a t term (37-42 weeks gestation) before the spontaneous onset of labor. PGHS specific activity was significantly higher after spontaneous term labor (38.9 +/- 6.0 pg PGE(2)/mu g protein.min; n = 19; P < 0.05). Am nion samples from preterm (<36 weeks gestation) nonlaboring patients c ontained low levels of PGHS specific activity (5.9 +/- 1.8 pg PGE(2)/m u g protein.min; n = 9), which increased significantly with spontaneou s preterm labor (28.3 +/- 6.8 pg PGE(2)/mu g protein.min; n = 10; P < 0.05). Longitudinal analysis of the data showed that PGHS specific act ivity was low in the first and second trimesters of gestation, but inc reased dramatically before labor onset at term. We detected PGHS prote in in all microsomal samples, with an antiovine PGHS antibody recogniz ing both PGHS-1 and -2 isoforms of the enzyme. However, there was no c orrelation between PGHS specific activity and the amount of immunoreac tive PGHS protein. Using an antibody specific for PGHS-2, we detected immunoreactive protein in only 9 of the 25 tissues examined and found no correlation between PGHS specific activity and the amount of PGHS-2 protein. These results suggest that 1) PGHS specific activity in the amnion increases sharply before the onset of labor at term; 2) further increases in specific activity occur during term and preterm labor; a nd 3) the specific activity of PGHS in the amnion is not related direc tly to the amount of immunoreactive enzyme protein.