Metabolism of Leukotriene B 4 via Reduction Into Dihydro‐Leukotriene B 4 in Human Monocytes, Alveolar Macrophages, and U‐937 Cells

The biological effects of leukotriene B 4 (LTB 4 ) within the microenvironment are controlled by rapid inactivation. In this regard human granulocytes convert LTB 4 into ω‐oxidated products (20‐OH‐LTB 4 and 20‐COOH‐LTB 4 ); moreover, we recently described the formation of unpolar metabolites of LTB 4 in human tonsillar and lung macrophages. By means of high performance liquid chromatography (HPLC) we identified the main metabolite of LTB 4 as dihydro‐LTB 4 (5,12‐dihydroxyeicosatrienoic acid). Studies on a lymphocyte (74–78%), monocyte (19–22%), and basophil (<4%) containing cell fraction isolated from peripheral blood as well as peripheral monocytes purified by elutriation centrifugation revealed evidence that these cells metabolize LTB 4 to a very low degree if incubated immediately after isolation. However, after culture for 24–72 h these cells showed a strongly increased capacity to metabolize LTB 4 . The pattern of metabolites in this cell fraction was identical to bronchoalveolar macrophages (purity >95%). Similarly, the LTB 4 ‐reductase was expressed in differentiated human monocytic U‐937 cells almost 5–7 h after the addition of dimethylsulfoxide (1.3%) or phorbol‐myristate‐acetate (16 nM). The expression of this pathway was blocked in the presence of cycloheximide (10 μg/ml) whereas actinomycin (3.8 μg/ml) had no effects. Dihydro‐LTB 4 was further metabolized by granulocytes probably via ω‐oxidation; therefore, several metabolites could be detected by radioactive high performance liquid chromatography (HPLC) after incubation of bronchoalveolar cells consisting of macrophages and granulocytes with 3 H‐LTB 4 . Our data provide evidence for a unique role of macrophages to control the level of LTB 4 by generation as well as metabolism into dihydro‐LTB 4 .