ω‐Oxidation of cysteine‐containing leukotrienes by rat‐liver microsomes

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Abstract
Leukotriene E4 was metabolized to two polar products by rat liver microsomes. These products were characterized by physico‐chemical and chemical techniques. The chemical structures, (5S, 6R)‐5,20‐dihydroxy‐6S‐cysteinyl‐7,9‐trans‐11,14‐cis‐icosatetraenoic acid (ω‐hydroxy‐leukotriene E4) and (5S, 6R)‐5‐hydroxy‐6S‐cysteinyl‐7,9‐trans‐11,14‐cis‐icosatetraen‐1,20‐dioic acid (ω‐carboxy‐leukotriene E4) suggested that leukotriene E4 was transformed by an ω‐hydroxylase and ω‐hydroxyleukotriene E dehydrogenase in sequence. N‐Acetyl‐leukotriene E4 was also transformed by these enzymes, but at a rate six times lower than leukotriene E4. The products formed from N‐acetylleukotriene E4 were characterized as being N‐acetyl‐ω‐hydroxy‐leukotriene E4 and N‐acetyl‐ω‐carboxy‐leukotriene E4. Other substrates were 11‐trans‐leukotriene E4 and N‐acetyl‐11‐trans‐leukotriene E4. In contrast, leukotrienes C4 and D4 were not converted into ω‐oxidized metabolites. The leukotriene E ω‐hydroxylase reaction required NADPH and molecular oxygen as cofactors, and was most rapidly catalyzed by liver microsomes. Liver cytosol, fortified with NAD+, converted ω‐hydroxyleukotriene E4 and N‐acetyl‐ω‐hydroxy‐leukotriene E4 into ω‐carboxy metabolites. Microsomes contained at least 18 times less ω‐hydroxy‐leukotriene E dehydrogenase activity than did cytosol. Liver microsomes supplemented with acetyl‐coenzyme A converted ω‐hydroxy and ω‐carboxy‐leukotriene E4 into the corresponding N‐acetyl derivatives. The novel enzyme, leukotriene E ω‐hydroxylase, which is described here is distinct from a previously described leukotriene B ω‐hydroxylase based on substrate competition and kinetic data.