Negative ion electrospray tandem mass spectrometric structural characterization of leukotriene B4 (LTB4) and LTB4-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.