ATP-mediated activation of Ca2+-independent phospholipase A2 in secretory granular membranes from rat parotid gland.

We characterized the Ca2+-independent, membrane-associated phospholipase A2 (PLA2) from rat parotid secretory granules. Among four phosphatidylcholine species with different fatty acyl (palmitoyl, oleoyl, linoleoyl, and arachidonoyl) groups at the sn-2 position, 2-arachidonoyl-phosphatidylcholine was the preferred substrate. Such specificity was also apparent even when 2-arachidonoyl-phosphatidylcholine coexisted with another species. The various well-documented inhibitors of PLA2s, bromoenol lactone, arachidonyl trifluoromethyl ketone, methyl arachidonyl fluorophosphate, and diisopropyl fluorophosphate, did not inhibit granular PLA2 activity. The granular PLA2 was activated markedly by ATP, and to a lesser extent by GTP and ATPgammaS. GTP also partially suppressed the ATP-mediated activation. UTP, CTP, GTPgammaS, and 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 granular membranes, the granular soluble contents or BSA were required for the full activity and full ATP-mediated activation. These results suggested that the PLA2 located in granular membranes may participate in the liberation of arachidonic acid in parotid cells and be regulated through a mechanism mediated by ATP.