EFFECT OF 15-HYDROPEROXY-ARACHIDONIC ACID ON PLATELET FUNCTIONS E VERICEL and M. LAGARDE Labororoire d'H~rnoh~oloflle, Facult~ Alexis Carrel 6V372. l..~n Cedcx 2 and In~erm L' 63. in~titut Pa,~teur, L)'on. France. INTRODUCTION Endothelial cells generate a strong inhibitor of platelet aggregation called prostacyc- lin. TM This compound is produced from arachidonic acid via prostaglandin endoperox- ides. '+ Various hydroperoxides of fatty acids and mainly 15-hydroperoxy-arachidonic acid (15-HPETE) have been described as potent inhibitors of prostacyclin synthe- tase. l+'ts It has been shown that prostaglandins are synthesized during platelet aggrega- tion, to and more recently, that platelets produce thromboxane A2, a very potent aggre- gating agent, from arachidonic acid. 6 12-Hydroperoxy-arachidonic acid (12-H PETE) produced by platelet lipoxygenase has been found to inhibit platelet thromboxane synthetase. ~ We report here the effect of 15-HPETE on platelet functions and on arachidonic acid metabolism by these cells. METHODS 15-HPETE was synthesized from arachidonic acid by soybean lipoxidase ~'~ and puri- fied by thin-layer chromatography on silica gel plates (hexane/diethyl ether/acetic acid, 60:40:1, as eluent). After purification, this hydroperoxide was measured by its absorb- ance at 234 nm and by an enzymic method using oxidation of NADPH." Human platelets were isolated from their plasma as previously described.' 0 15-H PETE in ethanol (less than 1:'200) was added to platelets simultaneously with aggregating agents. Platelet aggregation was performed according to the turbidimetric method of Born.t Metabolism of exogenous arachidonic acid by platelets was studied with a radio- chemical technique.t t RESULTS AND DISCUSSION Platelet aggregation induced by sodium arachidonate was inhibited by 15-H PETE in a dose dependent manner. Figure l shows the aggregation induced by two different con- centrations of arachidonate. The effects of either prostaglandin H2 (PGH2) or its 9-methano-analogue on platelets were also counteracted by 15-HPETE (Fig. 2). The inhibition of human platelet aggregation by hydroperoxides was already men- tioned by using linoleic acid hydroperoxide, t~ However, these data were obtained with utilization of 10 times more of linoleic acid hydroperoxide to block the aggregation. Results of Figs. 1 and 2 suggest that 15-HPETE did not act against cyclooxygenase activity. Besides, platelet aggregation induced by thrombin, an agent which does not need PGH2 and thromboxane A2 to aggregate platelets, was not inhibited by 15-HPETE (Fig. 3~. ~.v 2o~.+4 ~, 553