Phospholipase A2 activity specific for phosphatidic acid. A possible mechanism for the production of arachidonic acid in platelets.

Activation of platelets induces the formation of phosphatidic acid, which results from the combined actions of a phosphatidylinositol-specific phospholipase C and a 1,2-diacylglycerol kinase. It has been proposed that this phosphatidic acid leads in some way to the subsequent production of arachidonic acid. The present studies reveal the existence of a particulate phospholipase Az enzyme that appears to act selectively on phosphatidic acid to produce arachidonic acid. This enzyme has been assayed by degradation of exogenously added [32P]phosphatidic acid to [32P]lysophosphatidic acid or by the conversion of exogenously added 2-[14C]arachidonylphosphatidic acid to ['4C]arachidonic acid. The degradation of phosphatidic acid to lysophosphatidic acid is optimal at pH 7.0. Activity is maximal at 10 p~ Caz+ and is completely inhibited by ethyleneglycol bis(aminoether) N,N,N,N-tetraacetic acid. The rate of [32P]lysophosphatidic acid formation is 2-4 nmol/min/ mg of protein and is linear up to 12 p~ of exogenously added [3aP]phosphatidic acid. This phospholipase Az activity is relatively specific for phosphatidic acid as compared to other major phospholipids. Detergents such as sodium deoxycholate and Triton X-100 inhibit the formation of lysophosphatidic acid. This activity is thus distinctly different from that of other platelet phospholipase Az activities which preferentially degrade phosphatidylethanolamine and phosphatidylcholine and which require alkaline pH values, millimolar concentrations of Ca2+, and deoxycholate for maximal activation. Quinacrine, an inhibitor of the production of arachidonic acid in stimulated platelets, inhibits this phosphatidic acid-specific activity in a calcium-dependent manner. We propose that this phosphatidic acid-specific phospholipase Az might play an important role in the formation of arachidonic acid in stimulated platelets.