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.
The effect of lipid peroxide on the hydrolytic action of cytosolic phospholipase A2 (cPLA2) in rabbit platelets was investigated. Ionomycin‐stimulated arachidonic acid liberation and lysophosphatidylcholine formation were significantly potentiated when platelets were pretreated with tert‐butyl hydroperoxide (BHP) and FeSO4, and then washed. Under the conditions, oxidizing reagents did not enhance the increase in cPLA2 activity by ionomycin or the basal activity in unslimulated cells. Furthermore, the treatment of a platelet lysate with BHP and FeSO4 did not affect Ca2+‐induced translocation of cPLA2 to the membranes. However, with a membrane fraction, arachidonic acid liberation catalyzed by the partially purified cPLA2 was synergistically enhanced by BHP and FeSO4. These results suggest that oxidative stress may potentiate the hydrolytic action of cPLA2 on membrane phospholipids without an influence on the processes leading to the enzyme activation.
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