OMEGA-OXIDATION OF LIPOXIN B-4 BY RAT-LIVER - IDENTIFICATION OF AN OMEGA-CARBOXY METABOLITE OF LIPOXIN B-4

Lipoxin B-4 (LXB(4)) is metabolized to 20-hydroxy-LXB(4) by rat liver microsomes. The omega-hydrox ylation 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 B-4 omega-hydroxylation, because leukotriene B-4 inhibits the formation of 20-hydroxy-LXB(4) and LXB(4) blocks the leukotriene B-4 omega-hydroxylase activity in microsomes. Incubation of 20-hydroxy-LXB(4) 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-LXB(4), a novel metabolite of LXB(4), based on analyses by ultraviolet spectrometry and by gas chromatography/mass spectrometry. The 20-carboxy-LXB(4)-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 B-4. Disulfiram, an inhibitor of aldehyde dehydrogenase, also blocks the 20-carboxy-LXB(4) formation. These observations suggest that both alcohol dehydrogenase and aldehyde dehydrogenase participate in the oxidation of 20-hydroxy-LXB(4) to 20-carboxy-LXB(4).