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B Brune and V Ullrich
Faculty of Biology, University of Konstanz, Federal Republic of Germany.
12-Hydroperoxy-5,8,10,14-eicosatetraenoic acid (12-HPETE) as well as several other fatty acid hydroperoxides are potent inhibitors of platelet activation. 12-HPETE but not 12-hydroxy-5,8,10,14- eicosatetraenoic acid blocks the U46619- and the thrombin-triggered aggregation of aspirin-treated platelets, dose dependently. 12-HPETE suppresses thromboxane production by inhibiting platelet cyclooxygenase and stimulates its own production by increasing lipoxygenase activity, although this effect does not explain the inhibitory activity of 12- HPETE during the initial phase of cell activation. The inhibitory effect is related to altered calcium homeostasis during platelet activation. 12-HPETE inhibits calcium release from intracellular stores and modifies the influx of extracellular calcium. The inhibitory effect on calcium mobilization is explained by activation of soluble guanylate cyclase. These inhibitory properties are shared by sodium nitroprusside, a compound known to activate soluble guanylate cyclase. Fatty acid hydroperoxides, especially 12-HPETE, produce a rapid and dose-dependent activation of soluble guanylate cyclase, using intact human platelets as a detection system. Activation of the enzyme shows a position isomer specificity, with 12-HPETE being the most potent activator. The generation of the labile lipoxygenase product 12-HPETE during platelet activation may modulate platelet reactivity by increasing cyclic GMP. This pathway may contribute to a physiological feedback mechanism to limit the size of a growing platelet plug.
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