An anti-platelet agent, OPC-29030, inhibits translocation of 12-lipoxygenase and 12-hydroxyeicosatetraenoic acid production in human platelets

1 In human platelets, arachidonic acid is mainly metabolized by the two enz yme systems; cyclooxygenase and 12-lipoxygenase. Cyclo-oxygenase produces p rostaglandin H-2 which is further converted to thromboxane B-2. 12-Lipoxyge nase synthesizes 12(S)-hydroperoxyeicosatetraenoic acid which is reduced to 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE). 2 An anti-platelet compound, OPC-29030, dose-dependently inhibited 12(S)-HE TE production with an IC50 of 0.06 +/- 0.01 mu M, but not synthesis of thro mboxane Bz in human platelets. Although the compound suppressed 12(S)-HETE production in human platelets, cytosolic 12-lipoxygenase activity was not i nhibited up to 10 mu M. Essentially identical data were obtained with a 12- lipoxygenase of human erythroleukaemia cells which had megakaryocyte/platel et-like properties. 3 OPC-29030 also suppressed production of S(S)-HETE, a 5-lipoxygenase produ ct, in rat basophilic leukaemia cells without inhibiting enzyme activity. I t has been shown that 5-lipoxygenase binds to membrane 5-lipoxygenase-activ ating protein (FLAP) to produce S(S)-HETE, and thus FLAP inhibitor suppress es cellular S(S)-HETE production. 4 A FLAP inhibitor, L-655,238, suppressed platelet IZ(S)-HETE production, b ut had no effect on the 12-lipoxygenase activity. 5 Western blot analysis showed that platelet 12-lipoxygenase translocated f rom cytosol to membranes upon thrombin stimulation, and OPC-29030 suppresse d this process in a dose-dependent manner. 6 These results suggest that the 12-lipoxygenase of human platelets binds t o FLAP or a similar protein, and OPC-29030 suppresses IZ(S)-HETE production by inhibiting a certain step of the 12-lipoxygenase translocation.