Detection in Human Platelets of Phospholipase A2 Activity Which Preferentially Hydrolyzes an Arachidonoyl Residue1

Abstract

It has been generally considered that highly specific liberation of arachidonic acid is induced upon the stimulation of the platelets, although the molecular mechanism of the regulation of its action has not been well understood. An aim of the present study is to clarify the role of phospholipase A₂ in the arachidonic acid metabolism within human platelets. Phosphatidylcholine or phosphatidylethanolamine with arachidonate at the sn-2 position of glycerol was cleaved efficiently by phospholipase A₂ activity in homogenates as well as in the cytoplasmic fraction of human platelets, leading to the selective liberation of free arachidonate, whereas phospholipids with linoleate were hardly hydrolyzed under the same conditions. Double-reciprocal plots of kinetic data further strengthened the conclu sion that human platelet phospholipase A₂ showed high selectivity for arachidonoyl residue. This enzyme may play a crucial role in the intracellular metabolism of the arachidonate of phospholipids.