Changes in activity of cytosolic phospholipase A₂ in human amnion at parturition

OBJECTIVE: The purpose of this study was to determine whether increased cytosolic phospholipase A₂ activity mediated arachidonic acid mobilization for prostaglandin synthesis in amnion at parturition. STUDY DESIGN: Amnion was collected immediately after delivery from four groups of patients: preterm (<37 weeks) with no labor or labor and term (>37 weeks) with no labor or labor and stored at -70°C. Tissues were homogenized and centrifuged for 1 hour at 100,000g, and cytosol was assayed for cytosolic phospholipase A₂ activity with use of carbon 14-labeled 1-stearoyl-2 arachidonyl phosphatidylcholine plus 10 μmol/L unlabeled substrate and 5 mmol/L calcium in 10 mmol/L N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid, pH 7.4. Incubations were performed in duplicate ± 10 μmol/L arachidonyl trifluoromethyl ketone, a specific inhibitor of cytosolic phospholipase A₂ activity, at 30°C for 45 minutes. RESULTS: Total cytosolic phospholipase A₂ activity (in picomoles of arachidonic acid per minute per milligram of protein) calculated as the difference between the activity in the presence and absence of arachidonyl trifluoromethyl ketone was (mean ± SE) as follows: preterm no labor (n = 7) 8.94 ± 3.08, preterm with labor (n = 6) 6.79 ± 2.31, term no labor (n = 7) 14.85 ± 1.66, and term with labor (n = 5) 5.51 ± 1.52. Enzyme activity increased with gestational age and was highest in the term no labor group. A significant decrease in cytosolic phospholipase A₂ activity occurred with labor (p < 0.05). The greatest decrease in activity was in the term group (p < 0.05). CONCLUSION: Total cellular cytosolic phospholipase A₂ activity in amnion is highest in anticipation of labor but during labor total activity is depleted, resulting in the low activity measured after delivery of the placenta. The substrate specificity and changes in amnion total cytosolic phospholipase A₂ activity with labor strongly suggests a role in mediation of arachidonic acid mobilization and prostaglandin synthesis at labor.