Metabolism of arachidonic acid by peripheral and elicited rat polymorphonuclear leukocytes. Formation of 18- and 19-oxygenated dihydro metabolites of leukotriene B4.

Abstract

Previous studies have shown that leukotriene B4 is metabolized by polymorphonuclear leukocytes (PMNL) by a 20-hydroxylase, a 19-hydroxylase, and a reductase. We have now identified for the first time LTB4 metabolites formed by a combination of the reductase and omega-oxidation pathways. We have also discovered that rat PMNL metabolize LTB4 by a novel pathway to 18-hydroxy products. Dihydro metabolites of LTB4 have formerly been reported only after incubation of exogenous LTB4 with PMNL, but we have now shown that they are formed to the same extent from endogenous arachidonic acid after stimulation of PMNL with the ionophore, A23187. The following metabolites have been identified after incubation of either LTB4 or arachidonic acid with rat PMNL: 10,11-dihydro-LTB4, 10,11-dihydro-12-epi-LTB4, 10,11-dihydro-12-oxo-LTB4, 19-hydroxy-LTB4, 19-hydroxy-10,11-dihydro-LTB4, 19-oxo-10,11-dihydro-LTB4, 18-hydroxy-LTB4, 18-hydroxy-10,11-dihydro-LTB4, and 18-hydroxy-10,11-dihydro-12-oxo-LTB4. Negligible amounts of 20-hydroxylated products were formed. Incubation of PMNL with 10,11-dihydro-LTB4 resulted in the formation of all of the above dihydro metabolites. However, none of the omega-oxidized metabolites of LTB4 was further metabolized to a significant extent when incubated with PMNL, possibly at least partially because they were not substrates for a specific LTB4 uptake mechanism. We found that the biosynthesis and metabolism of LTB4 is considerably enhanced in PMNL from an inflammatory site (carrageenan-induced pleurisy) compared with peripheral PMNL. When arachidonic acid was the substrate, the greatest increase was observed for products formed by the reductase pathway, which were about eight times higher in pleural PMNL. The rates of formation of both LTA hydrolase and omega-hydroxylase products were about three times higher, whereas the total amounts of 5-lipoxygenase products were about twice as high in pleural PMNL. The amounts of products formed by the above enzymatic pathways reached maximal levels about 4-6 h after injection of carrageenan and then declined.