Hepatic microsomal metabolism of leukotriene B₄ in rats: Biochemical characterization, effect of inducers, and age- and sex-dependent differences

Abstract

1. The cytochrome P-450-dependent metabolism of leukotriene B₄ (LTB₄) by rat hepatic microsomes was characterized. Hepatic microsomes were found to metabolize LTB₄ to 20-hydroxy-LTB₄ and 20-carboxy:LTB_4. The rate of formation of 20-hydroxy-LTB₄ (14.6 pmol/min per mg protein) was 5.8-fold higher than that of 20-carboxy-LTB₄ (2.5 pmol/min per mg protein).

2. LTB₄ ω-hydroxylase activity required NADPH and oxygen indicating that the reaction is mediated by a mono-oxygenase system. The ω-hydroxylase activity was optimal at pH 7.4 and product formation was linear with respect to time of incubation and protein concentration. The reaction was significantly inhibited by carbon monoxide (89%), SKF 525-A (1 mM), and metyrapone (0.1 mM) whereas α-naphthoflavone had only marginal inhibitory effects. The apparent K_m and V_max of LTB₄ ω-hydroxylase were 4 μ and 19.6 pmol/min per mg protein, respectively.

3. Ontogenic studies revealed that LTB₄ ω-hydroxylase activity was low in 4-day-old rats and that there was a steady increase in enzyme activity as the animal matured.

4. Phenobarbital, 3-methylcholanthrene or Aroclor 1254 treatment of rats did not induce LTB₄ ω-hydroxylase activity whereas clofibrate resulted in 61% induction in enzyme activity. No significant sex-dependent differences were observed.

5. It is concluded that hepatic metabolism of LTB₄ may afford an effective mechanism for limiting many of the pro-inflammatory effects of circulating leukotrienes.