ATP-MEDIATED ACTIVATION OF CA2-INDEPENDENT PHOSPHOLIPASE A(2) IN SECRETORY GRANULAR MEMBRANES FROM RAT PAROTID-GLAND()

We characterized the Ca2+-independent, membrane-associated phospholipa se A(2) (PLA(2)) from rat parotid secretory granules. Among four phosp hatidylcholine species with different fatty acyl (palmitoyl, oleoyl, l inoleoyl, and arachidonoyl) groups at the sn-2 position, 2-arachidonoy l-phosphatidylcholine was the preferred substrate, Such specificity wa s also apparent even when 2-arachidonoyl-phosphatidylcholine coexisted with another species. The various well-documented inhibitors of PLA(2 )s, bromo enol lactone, arachidonyl trifluoromethyl ketone, methyl ara chidonyl fluorophosphate, and diisopropyl fluorophosphate, did not inh ibit granular PLA, activity. The granular PLA(2) was activated markedl y by ATP, and to a lesser extent by GTP and ATP gamma S, GTP also part ially suppressed the ATP-mediated activation. UTP, CTP, GTP gamma S, a nd the hydrolyzed products of ATP and GTP showed little activation of the enzyme. Neither addition of K-252a nor depletion of Mg2+ affected ATP-mediated activation, Although this enzyme was located in the granu lar membranes, the granular soluble contents or BSA were required for the full activity and full ATP-mediated activation. These results sugg ested that the PLA(2) located in granular membranes may participate in the liberation of arachidonic acid in parotid cells and be regulated through a mechanism mediated by ATP.