ω‐Oxidation of Lipoxin B₄ by Rat Liver: Identification of an ω‐Carboxy Metabolite of Lipoxin B₄

Lipoxin B₄ (LXB₄) is metabolized to 20‐hydroxy‐LXB₄ by rat liver microsomes. The ω‐hydroxylation requires both molecular oxygen and NADPH, and is inhibited by carbon monoxide, indicating involvement of a cytochrome P‐450 (P‐450). This is supported by inhibition of the reaction by antibodies raised against NADPH–P‐450 reductase. The P‐450 appears to be the one responsible for leukotriene B₄ω‐hydroxylation, because leukotriene B₄ inhibits the formation of 20‐hydroxy‐LXB₄ and LXB₄ blocks the leukotriene B₄ω‐hydroxylase activity in microsomes. Incubation of 20‐hydroxy‐LXB₄ with both rat liver cytosol and NAD⁺ leads to formation of a more polar metabolite on high‐performance liquid chromatography. The metabolite is identified as 20‐carboxy‐LXB₄, a novel metabolite of LXB₄, based on analyses by ultraviolet spectrometry and by gas chromatography/mass spectrometry. The 20‐carboxy‐LXB₄‐forming activity is localized in cytosol, with an optimal pH of 8.5. The activity is dependent on NAD⁺, but NADP⁺ can not replace NAD⁺. The reaction is inhibited by pyrazole and 4‐methylpyrazole, inhibitors of alcohol dehydrogenase, and by substrates of the enzyme such as ethanol and 20‐hydroxy‐leukotriene B₄. Disulfiram, an inhibitor of aldehyde dehydrogenase, also blocks the 20‐carboxy‐LXB₄ formation. These observations suggest that both alcohol dehydrogenase and aldehyde dehydrogenase participate in the oxidation of 20‐hydroxy‐LXB₄ to 20‐carboxy‐LXB₄.