Possible effector of Ca₂4-independent phospholipase a ₂, activity in secretory granules from rat parotid gland

To clarify the regulatory mechanism of the Ca²⁺-independent phospholipase A₂ (iPLA₂)activity in parotid secretory granules, the effects of various compounds were investigated. The granular iPLA₂ activity was not inhibited by either bromoenoJ lactone (BEL) or 5,5′-dithiobis(2-nitrobenzoic acid), which are potent inhibitors of other well-known iPLA₂S such as myocardial iPLA₂ ATP markedly activated the granular iPLA as well as most iPLAs. Derivatives of ATP (e.g. ATPy S and GTP) were less effective than ATP. Neither addition of K252a (a potent protein kinase inhibitor) nor depletion of Mg²⁺ affected the ATP-mediated activation. The granular iPLA₂ activity was located only in granular membranes, but separation of granular membranes from the granular soluble fraction decreased the activity and ATP-mediated activation of iPLA ₂ When the granular soluble fraction was returned to the incubation mixture, both the activity and the ATPmediated activation of iPLAp in the granular membranes were recovered. BSA mimicked the effects of the granular soluble fraction. The addition of free fatty acid reduced the iPLA₂ activity. These findings suggest that the granular iPLA₂ is regulated through a mechanism mediated directly by ATP. The granular soluble proteins may contribute to the removal of the products from the enzyme protein.