Receptor‐Mediated Regulation of Leukotriene C 4 Synthase Activity in Human Platelets

Human platelets possess a specific membrane‐bound leukotriene (LT) C 4 synthase, which catalyzes the conversion of LTA 4 to LTC 4 . Stimulation of the receptors for thrombin, collagen or thromboxane A 2 provoked inhibition of this enzyme, as judged by suppressed transformation of exogenous LTA 4 to LTC 4 . Similarly, direct activation of protein kinase (PK) C with nanomolar concentrations of 4β‐phorbol 12‐myristate 13‐acetate (PMA) inhibited the production of LTC 4 . Kinetic studies demonstrated that the inhibition induced by thrombin and PMA was non‐competitive. Elevation of intracellular cAMP levels with carbacyclin did not affect basal LTC 4 formation, but abolished the attenuation of platelet LTC 4 synthase activity induced by the thromboxane receptor agonist U‐46619. The unselective protein kinase inhibitor staurosporine prevented both receptor‐mediated and PMA‐induced suppression of LTC 4 formation. In contrast, two selective PKC inhibitors, Ro 31–8220 and GF 109203X, reversed the inhibitory effect provoked by PMA, but failed to prevent thrombin‐induced inhibition. Furthermore, the protein tyrosine phosphatase inhibitor, sodium orthovanadate, induced dose‐dependent inhibition of LTC 4 production in platelet sonicates. In conclusion, receptor‐mediated activation of human platelets leads to decreased LTC 4 synthase activity via phosphoregulation. Although the present results demonstrate that platelet LTC 4 synthase can be regulated via PKC‐dependent events, alternative mechanisms appears to be involved in the physiological regulation of this enzyme. The findings suggest the possible importance of protein tyrosine phophorylations in this process.