TRIENE PROSTAGLANDINS - PROSTACYCLIN AND THROMBOXANE BIOSYNTHESIS AND UNIQUE BIOLOGICAL PROPERTIES

Platelets enzymatically convert prostaglandin H3(PGH3) into thromboxane A3. Both PGH2 and thromboxane A2 aggregate human platelet-rich plasma. In contrast, PGH3 and thromboxane A3 do not. PGH3 and thromboxane A3 increase platelet cyclic AMP in platelet-rich plasma and thereby (i) inhibit aggregation by other agonists, (ii) block the ADP-induced release reaction, and (iii) suppress platelet phospholipase-A2 activity or events leading to its activation. PGI3 (Δ17-prostacyclin; synthesized from PGH3 by blood vessel enzyme) and PGI2 (prostacyclin) exert similar effects. Both compounds are potent coronary relaxants that also inhibit aggregation in human platelet-rich plasma and increase platelet adenylate cyclase activity. Radioactive eicosapentaenoate and arachidonate are readily and comparably acylated into platelet phospholipids. In addition, stimulation of prelabeled platelets with thrombin releases comparable amounts of eicosapentaenoate and arachidonate, respectively. Although eicosapentaenoic acid is a relatively poor substrate for platelet cyclooxygenase, it appears to have a high binding affinity and thereby inhibits arachidonic acid conversion by platelet cyclooxygenase and lipoxygenase. It is therefore possible that the triene prostaglandins are potential antithrombotic agents because their precursor fatty acids, as well as their transformation products, PGH3 thromboxane A3, and PGI3 are capable of interfering with aggregation of platelets in platelet-rich plasma.