Lipid Peroxide Overcomes the Inability of Platelet Secretory Phospholipase A₂ to Hydrolyze Membrane Phospholipids in Rabbit Platelets

Abstract

The present study investigated the effect of lipid peroxide on the ability of group IIA secretory phospholipase A₂ (IIAsPLA₂) to hydrolyze platelet membrane phospholipids. The treatment of rabbit platelets with tert-butyl hydroperoxide (BHP) and FeSO₄, generated malondialdehyde, an index of lipid peroxidation, and slightly induced arachidonic acid liberation and lysophosphatidylcholine formation. Further addition of IIAsPLA₂ purified from rabbit platelets synergistically enhanced the liberation and the formation induced by the oxidizing reagents, although the enzyme alone did not. When the IIAsPLA₂ was pretreated with heparin, the enhancement was not observed. The combination of IIAsPLA₂ with linoleic acid hydroperoxide and FeSO₄ also caused synergistic arachidonic acid liberation. Furthermore, IIAsPLA₂ enhanced thromboxane B₂ generation and platelet aggregation induced by BHP and FeSO₄. The synergistic aggregation was sensitive to indomethacin. With a membrane fraction as a substrate, IIAsPLA₂ caused arachidonic acid liberation, which was enhanced in the presence of BHP and FeSO₄. These results suggest that modification of membrane phospholipids by oxidizing reagents increases the accessibility of the membrane to platelet IIAsPLA₂, and sequential enhancement of arachidonic acid liberation may contribute to the propagation of oxidative stress-induced cellular injury.