Novel 3-hydroxylated leukotriene b4 metabolites from ethanol-treated rat hepatocytes

Negative ion electrospray tandem mass spectrometric structural characterization of leukotriene B4 (LTB 4) and LTB 4-derived metabolites

The low energy collision induced dissociation (CID) of the carboxylate anions generated by electrospray ionization of leukotriene B4 (LTB4) and 16 of its metabolites was studied in a tandem quadrupole mass spectrometer. LTB4 is a biologically active lipid mediator whose activity is terminated by metabolism into a wide variety of structural variants. The collision-induced dissociation spectra of the carboxylate anions revealed structurally informative ions whose formation was determined by the position of hydroxyl substituents and double bonds present in the LTB4 metabolite. Major ions resulted from charge remote α-hydroxy fragmentation or charge directed α-hydroxy fragmentation. The conjugated triene moiety present in some metabolites was proposed to undergo cyclization to a 1,3-cyclohexadiene structure prior to charge remote or charge driven a-hydroxy fragmentation. The mechanisms responsible for all major ions observed in the CID spectra were studied using stable isotope labeled analogs of the LTB4 metabolites. In general, the collision-induced decomposition of carboxylate anions produced unique spectra for all LTB4 derived metabolites. The observed decomposition product ions from the carboxylate anion could be useful in developing assays for these molecules in biological fluids.

Metabolism of 6-trans-isomers of leukotriene B4 in cultured hepatoma cells and in human polymorphonuclear leukocytes. Identification of a delta 6-reductase metabolic pathway

The intermediate metabolic events which degrade hydroxy polyunsaturated fatty acids is largely unknown. Such molecules are common products of lipid peroxidation and lipoxygenase catalyzed oxidation of arachidonic acid. Metabolism of two 5,12-dihydroxyeicosatetraenoic acids, 6-trans-LTB4 (leukotriene B4), and 6-trans-12-epi-LTB4 was studied in HepG2 cells (a human-derived hepatoma cell line). Extensive metabolism was observed with a major metabolite identified as 4-hydroxy-6-dodecenoic acid for both epimers. Incubation of 6-trans-LTB4 epimers at shorter times revealed the formation of intermediate metabolites, including 6-hydroxy-4,8-tetradecadienoic acid and 8-hydroxy-4,6,10-hexadecatrienoic acid suggesting beta-oxidation as the major pathway leading to the formation of the common terminal metabolite. Two additional metabolites were structurally elucidated as 5-oxo-6,7-dihydro-LTB4 and 6,7-dihydro-LTB4 which have not been previously described. Formation of 5-oxo-6,7-dihydro-LTB4 and 6,7-dihydro-LTB4 were also observed during metabolism of 6-trans-12-epi-LTB4 in human polymorphonuclear leukocytes. Of particular interest is the metabolism of these compounds by beta-oxidation from the carboxyl terminus, a process which is not observed with leukotriene B4 or leukotriene C4. Identification of these metabolites suggested the operation of the 5-hydroxyeicosanoid dehydrogenase pathway followed by a delta 6-reductase metabolic pathway which has not been previously described. This pathway of beta-oxidation may limit the activity of various 5,12-diHETEs including nonenzymatic hydrolysis products of LTA4 and also the recently described B4-isoleukotrienes.