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.