PLATELET SECRETORY PHOSPHOLIPASE A(2) FAILS TO INDUCE RABBIT PLATELETACTIVATION AND TO RELEASE ARACHIDONIC-ACID IN CONTRAST WITH VENOM PHOSPHOLIPASES A(2)

The ability of platelet secretory phospholipase A(2) (sPLA(2)) to indu ce platelet activation was investigated. sPLA(2) (group II) contained in an activated platelet supernatant, as well as high concentrations o f purified recombinant platelet sPLA(2), failed to induce platelet act ivation. Furthermore, sPLA(2) did not modify platelet activation induc ed by various agonists. The possible relationship between the failure of this enzyme to induce platelet activation and its origin (mammalian ) or its structural group (group II) was then investigated, using panc reatic PLA(2)s (group I) and venom PLA(2)s from groups I, II and III, All venom PLA(2)s induced platelet activation that was accompanied by the liberation of arachidonic acid and was abolished by aspirin. In co ntrast, as observed for platelet sPLA(2), enzymes from hog or bovine p ancreas were unable to induce platelet activation even when used at hi gh concentrations. Interestingly, PLA(2) able to induce platelet activ ation efficiently hydrolyse phosphatidylcholine, while those inactive on platelets did not. Taken together, these results suggest that the c atalytic activity of added PLA(2) is necessary but not sufficient to i nduce platelet activation. Moreover, the ability of PLA(2) to induce p latelet activation is not related to its structural group (I, II, III) but rather to its origin (venom vs. mammalian) and capacity to hydrol yse phosphatidylcholine, the major phospholipid of the outer leaflet o f the plasma membrane.