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Arachidonate 5-lipoxygenase purified from porcine leukocytes transformed arachidonic acid to 5-hydroperoxy-6,8,11,14-eicosatetraenoic acid. By the leukotriene A synthase activity of the same enzyme the product was further metabolized to leukotriene A4 (actually detected as 6-trans-leukotriene B4, 12-epi-6-trans-leukotriene B4, and 5,6-dihydroxy-7,9,11,14-eicosatetraenoic acids). The enzyme was incubated with [10-DR-3H]- or [10-LS-3H]-labeled arachidonic acid, and 6-trans-LTB4 and its 12-epimer were analyzed. More than 90% of 10-DR-hydrogen was lost while about 100% of 10-LS-hydrogen was retained, indicating a stereospecific hydrogen elimination from C-10 during the formation of leukotriene A4.
Lipoxygenases react with hydroperoxy fatty acids and catalyze dioxygenase or dehydrase (leukotriene A4 (LTA4) synthase) types of reactions. In the present investigation we studied the mechanism of reaction of the purified porcine leukocyte 12S-lipoxygenase with 15S-hydroperoxyeicosatetraenoic acid (15S-HPETE). Oxygen-18 labeling experiments with GC-MS analysis were used to distinguish dioxygenase and leukotriene synthase activities of the enzyme; 8S,15S-DiHPETE and 14R,15S-DiHPETE were formed by oxygenation, and a series of 8,15- and 14,15-diols were formed via enzymatic synthesis of 14,15-LTA4 and nonenzymatic hydrolysis of the epoxide. 10D-3H- and 10L-3H-labeled substrates were used to study the stereospecificity of the C-10 hydrogen abstraction in the synthesis of these products. Formation of 14,15-DiHPETE and 14,15-LTA4 was associated with stereoselective abstraction of hydrogen from the 10L position of 15S-HPETE. The same type of measurements on the 8S,15S-DiHPETE product indicated a variable (50-250%) retention of the 10L-3H label, and a consistent 90% retention of the 10D-3H. In contrast, the synthesis of 8S,15S-DiHPETE by the soybean lipoxygenase was associated with the expected stereoselective abstraction of the 10D hydrogen. It appears that the porcine leukocyte 12S-lipoxygenase synthesizes 8S,15S-DiHPETE by a different mechanism.