Calcium is sufficient but not necessary for activation of sheep platelet cytosolic phospholipase A₂

In this study we demonstrate that: (1) although the major phospholipase A₂ present in sheep platelets is activated by calcium ions, it can effectively catalyze hydrolysis of the sn-2 ester linkage in phospholipids in the absence of calcium: (2) expression of calcium-independent phospholipase A₂ activity can be induced by NACl utilizing purified (but not crude) cytosolic enzyme; and (3) calcium-independent phospholipase A₂ activity is regulated by a reconstitutable cytosolic protein. Collectively, these results underscore the fundamental catalytic differences between extracellular and intracellular calcium-dependent phospholipases A₂ and demonstrate that calcium is sufficient, but not necessary, for the activation of this class of intracellular phospholipases A₂.