Properties of leukotriene B4 20-hydroxylase from polymorphonuclear leukocytes

Human polymorphonuclear leukocytes (PMNL) convert arachidonic acid (20:4) to a number of dihydroxy metabolites, including leukotriene B4 (LTB4) 5S,12R-dihydroxy-6,8,10,14-EEEZ-icosatetraenoic acid (isomer-1), 5S,12S-dihydroxy-6,8,10,14-EEEZ-icosatetraenoic acid, 5S,12S-dihydroxy-6,8,10,14-EZEZ-icosatetraenoic acid (5S,12S-dh-20:4), 5,6-dihydroxy-7,9,11,14-icosatetraenoic acid, and 5,15-dihydroxy-6,8,11,13-icosatetraenoic acid. LTB4 was synthesized rapidly after stimulation of PMNL with the divalent cation ionophore, A23187, but its concentration rapidly declined after about 4 min, in contrast to the other dihydroxy metabolites of 20:4 whose concentrations remained stable for at least 20 min. The amounts of polar metabolites (identified primarily as 20-hydroxy-LTB4) increased steadily with time up to 20 min. These results suggest that LTB4 may be specifically converted to its 20-hydroxy metabolite by PMNL. We prepared 3H- and 14C-labeled analogs of the dihydroxyicosatetraenoic acid metabolites described above by incubation of labeled 20:4 with PMNL. Although all of these substances were metabolized to some extent by human PMNL, LTB4 (apparent Km, 1.0 microM) was metabolized the most rapidly, followed by 5S,12S-dh-20:4 (apparent Km, 2.4 microM) and isomer-1 (apparent Km, 4.8 microM). All three substrates were shown by mass spectrometry to be converted to their 20-hydroxy metabolites. LTB4 was also metabolized to its omega-carboxy derivative. Human mononuclear leukocytes and rabbit PMNL metabolized LTB4 very slowly, whereas rat PMNL metabolized this substrate at about one-sixth the rate of human PMNL. These results demonstrate that human PMNL contain an omega-hydroxylase that specifically converts LTB4 to its 20-hydroxy metabolite. This enzyme may be important for the regulation of LTB4 levels in vivo.