15-hydroxy-5,8,11,13-eicosatetraenoic Acid: A Potent and Selective Inhibitor of Platelet Lipoxygenase

The functional interrelationships between the cyclooxygenase and lipoxygenase systems for arachidonic acid metabolism in platelets have not yet been clarified. Although a number of specific inhibitors of the cyclooxygenase, such as aspirin and indomethacin, have been described, few inhibitors of the lipoxygenase have been found. Several hydroxy and hydroperoxy derivatives of arachidonic acid were prepared and purified by high performance liquid chromatography, and their structures were confirmed by gas chromatography-mass spectrometry. The effect of these compounds on oxygenation of [1-14C]arachidonic acid by platelet cyclooxygenase and lipoxygenase enzymes was monitored both with an oxygen electrode and by analysis of radioactive products formed. It was found that 15-hydroxy-5,8,11,13-eicosatetraenoic acid selectively inhibited platelet lipoxygenase activity at micromolar concentrations (I50 = 8 microM) without inhibiting the cyclooxygenase. The 15-hydroperoxy analog, although more potent, was less selective, whereas neither 12-hydroxy-5,8,10,14-eicosatetraenoic acid nor 12-hydroxy-9-octadecenoic acid (ricinoleic acid) appeared to affect either enzyme. The high degree of selectivity of the 15-hydroxy derivative of eicosatetraenoic acid makes it the most suitable inhibitor so far discovered for studying the functions of the platelet lipoxygenase system.