Thrombin arachidonic acid release

The other sources of the arachidonic acid released by platelets in response to thrombin are phosphatidylcholine and phosphatidylethanolamine (Table 1.1). However, each of these phospholipids exists as three species in platelets, namely as the diacyl-, 1-0-alkyl-, or 1-0-alk-l enyl- forms (Table 1.2). Table 1.2 also shows that l-O-alkyl-2-acyl-PC and l-O-alk-l enyl-2-acyl-PE are enriched in arachidonic acid relative to the other classes of PC or PE, giving rise to the idea that they could serve as important sources of arachidonic acid in stimulated platelets. Purdon and Smith " prelabelled platelets with pH]-arachidonic acid and followed the changes in the different species of PC and PE following thrombin stimulation. It was found that while there was a decrease in radiolabel of both 1,2-diacyl-PC and 1,2-diacyl-PE at all times studied, there was no decrease in the other species of PC or PE, and, indeed, radiolabel in l-O-alkyl-2-acyl-PC and l-0-alk-l -enyl-2-acyl-PE increased at later time points (3-5 min) after thrombin. The thrombin-induced incorporation of arachidonic acid in plasmalogen PE was observed previously by others. Purdon and Smith " concluded that, upon stimulation of human platelets, arachidonic acid is released from both 1,2-diacyl-PC and 1,2-diacyl-PE for metabolism by cyclo-oxygenase and lipoxygenase, while certain other pools of phosphatidylcholine and phosphatidylethanolamine act to collect arachidonic acid. 

Figure 1.2 suggests a mechanism for arachidonic acid release in thrombin-stimulated human blood platelets. Phospholipase A2 acts selectively on 1-acyl-2-arachidonoyl-phospholipids, of which the most important quantitatively is phosphatidylcholine, to liberate arachidonic acid and produce lysophospholi-pids. These lysophospholipids are subsequently hydrolysed in part by lysophospholipase to produce mainly glycerophosphocholine and smaller... 

The mechanism by which thrombin activates phospholipase is presently unknown. Touqui et have provided evidence for the presence of the phospholipase A2 inhibitor, lipocortin, in blood platelets and have suggested that its phosphorylation during the platelets response to thrombin relieves inhibition and allows arachidonic acid release. On the other hand, Apitz-Castro d al. have suggested that phosphatidic acid (made available subsequent to phospholipase C acting on phosphoinositides) changes the state of organization of the phospholipids in the platelet membrane making them more susceptible to attack by phospholipase A2. 

Thrombin is by far the most efficient stimulus in releasing arachidonic acid from platelets and consequently has been the most studied. Collagen is also able to release substantial amounts of arachidonic acid by a mechanism that seems to depend on positive feedback by formed prostaglandin endoperoxides but most other platelet agonists, such as ADP, serotonin and platelet activating factor, are weak inducers of arachidonic acid release. 

Murayama T, Ui M (1985) Differential susceptibility to islet-activating protein, pertussis toxin, of multiple effects of thrombin on 3T3 fibroblasts including adenylate cyclase inhibition and arachidonic acid release. J Biol Chem 260 7226-7233... 

Arachidonic acid is not present in significant amounts in tissues as the free acid but is stored as a fatty acid at the sn-2 position of phospholipids. Prostaglandin biosynthesis is initiated by the interaction of a stimulus with the cell surface. Depending on the cell type, the stimulus can take the form of a hormone, such as angiotensin II or antidiuretic hormone, or a protease such as thrombin (involved in blood clotting), or both hormone and protease. These agents bind to a specific receptor that activates a phospholipase A2 that specifically releases the arachidonic acid from a phospholipid such as phosphatidylcholine. The release of arachidonic acid by phospholipase A2 is believed to be the rate-limiting step for the biosynthesis of eicosanoids. 

Thrombin, thromboxane A2 and exposed collagen cause release of arachidonic acid from platelet membrane. 

Mahadevappa VG, and Holub BJ. (1986). Diglyceride pathway is a minor source o( released arachidonic acid in thrombin-stimulated platelets. Biochem. Biophys. Res. Commun. 134,1327-1333. 

FIGURE 9.98 Events taking place in the plasma membrane on stimulation of a cell. Phospha tidylinositol (Pi) and more hi ly phosphoiylated versions of this lipid account for 2 to 8% of the lipids of the plasma membrane of eukaryotic cdIs. The inositol 1,4,5 triphosphate (fP3) moiety of phosphatidyl inositol- 4,8-di phosphate may be hydiulyitec from this lipid immediately after the cell is stimulated. For example, the stimulation of platelets by thrombin or the islets of the pancreas by glucose is followed by the release of 1P3 into the cytoplasm. In some cells, arachidonic acid is hydrolyzed from l-acyl-2-arachidonyl-glyceroL which can Support a burst of prostaglandin. synthesis. 

The stimulation of platelets to form a blood dot involves thromboxane production. The sequence of events resulting in the aggregation of platelets is summarized in Figure 9.100. The initial event of cell stimulation is the reaction of thrombin with the platelet surface, which provokes the release of arachidonic acid, as mentioned in the Vitamin K section. Arachidonic acid is converted to thromboxane A2. The TXAj, in turn, is released from the platelet. It travels a short distance, within the bloodstream, and binds to its receptor on the surface of nearby platelets. 

Filopodia formation and microparticle formation are results of calcium signaling. Filopodia are little feet, extending from the platelet, which allow the platelet to make better contact with the outside environment. Microparticles are tiny vesicles (phospholipid spheres) that are shed from the platelet, allowing the aforementioned docking proteins easier access to a phospholipid surface, and vastly enhancing the rate of activation of further molecules of thrombin. The generation of free arachidonic acid, and consequent formation and release of thromboxane A2 in the surrounding blood, provokes the activation of nearby platelets. 

Mahadevappa VG, Holub BJ. Diacylglycerol lipase pathway is a minor source of released arachidonic acid in thrombin-stimulated human platelets. Biochem Biophys Res Commun 1986 134 1327-1333. Maiese K, Holloway HH, Larson DM. Soncrant TT. Effect of acute and chronic arecoline treatment on... 

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