Hepatic microsomal ω-oxidation of leukotriene B4

In this study the authors investigated the metabolism of LTB4 by rat hepatic microsomes. 3H-LTB4 (9 μM) was incubated for 20 min in the presence3 of oxygen, NADPH and liver microsomes (1.5 mg). Incubation was followed by extraction and analysis by HPLC. Metabolite identification was based on cochromatography with reference standards. At least three radioactive peaks were observed; two of which were ω-oxidation products 20-OH-LTB4 and 20-COOH-LTB4. The rate of formation of 20-OH-LTB4 (14.6 nmoles/min/mg protein) was higher than that of 20-COOH-LTB4 (2.5 nmoles/min/mg protein). The third radioactive peak remains unidentified. Product formation was negligible with boiled microsomes. LTB4 ω-hydroxylase activity required NADPH and oxygen, was linear with respect to incubation time and protein, and was maximal at pH 7.4. Enzyme activity was inhibited (>90%) by carbon monoxide, SKF 525A (1 mM), but was not affected by α-naphthoflavone. Phenobarbital (PB, 80 mg/kg for 3 days), AROCLOR 1254 (100 mg/kg for 3 days) or 3-methylcholanthrene (40 mg/kg for 3 days) administration to rats resulted in only slight (<20%) or no increase in LTB4 ω-hydroxylase activity. However, the formation of the unidentified peak was increased (>100%) by PB treatment. These results suggest that ω-oxidation is the major pathway for biotransformation of LTB4 in liver microsomes and that this reaction is mediated by cytochrome P-450