PHOSPHOLIPASE-A2 ACTIVITY IN ENDOMETRIUM FROM EARLY PREGNANT AND NONPREGNANT EWES

In the ewe, synthesis of the luteolytic factor , prostaglandin F2alpha , increases from day 13 to the end of the estrous cycle. Availability of free arachidonic acid is usually the rate-limiting step in prostagl andin biosynthesis. Phospholipase A2 (PLA2) may be the key enzyme for the hydrolysis of arachidonic acid from membrane-bound phospholipids. To investigate uterine PLA2 activity during the estrous cycle and earl y pregnancy, we monitored the release of [C-14]oleic acid from the sub strate 1-palmitoyl-2-[C-14]oleoyl-phosphorylcholine by homogenates and cytosolic fractions of endometrium from ewes on days 12, 14 and 16 of the estrous cycle or pregnancy. We observed that PLA2 activity droppe d by 58% (p<0.02) in day-16 pregnant endometrium compared to day-16 no n-pregnant endometrium. We then investigated whether the reduced PLA2 activity was due to induction of a specific inhibitor. The PLA2-inhibi tor activity was determined by monitoring the inhibition of release of [C-14]oleic acid from the radioactive substrate by porcine pancreatic PLA2. Inhibition by endometrial homogenates of pregnant animals of th e control enzyme activity was 27% and only 14% by cyclic ones. Inhibit ion was dose-dependent and was as high as 53%. (p<0.01) with 1 mg prot ein from pregnant endometrial homogenates. Endometrial PLA2 behaved as a Michaelian enzyme in the endometrium of day-16 cyclic ewes (Km = 79 .4 mumol/l). Furthermore, the inhibitory activity from pregnant endome trium had characteristics of competitive inhibition. Our results sugge st that inhibition of endometrial PLA2 activity could occur in early p regnant ewes.