Estimation of phospholipase A2 activity in guinea-pig endometrium on days 7 and 16 of the estrous cycle.

Phospholipase A2 (PLA2) activity was detected in both the microsomal fraction and the supernatant fraction of guinea-pig endometrial homogenates following centrifugation at 100,000 x g for 60 min. Between 85% and 95% of the PLA2 activity was detected in the microsomal fraction. The PLA2 enzymes in both fractions had a pH optimum of 8.0. The microsomal PLA2 required 7 mM Ca2+ and the supernatant fraction PLA2 required 2 mM Ca2+ for optimum activity. The activities of the microsomal PLA2 and supernatant fraction PLA2 increased significantly from 662.6 +/- 74.5 and 42.0 +/- 7.6 pmol/mg protein/10 min respectively on Day 7 to 960.4 +/- 106.5 and 79.5 +/- 8.7 pmol/mg protein/10 min respectively on Day 16. There was, therefore, a 1.5- to 1.9-fold stimulation in the initial rate of PLA2 activity between Days 7 and 16 of the cycle. The activities of microsomal PLA2 in guinea-pig endometrium are more than adequate on both Days 7 and 16 of the cycle to account for the amounts of prostaglandin (PG)F2 alpha released from the guinea-pig uterus. It is considered that the activation of microsomal (presumably membrane-bound) PLA2 by increasing free intracellular Ca2+ levels is probably of more importance than the absolute activities of PLA2 in controlling the supply of free arachidonic acid for PGF2 alpha synthesis by the guinea-pig endometrium during the estrous cycle.