Collision-induced dissociation of carboxylate anions from derivatized 5-lipoxygenase metabolites of arachidonic acid.

The low-energy collision-induced dissociations (CID) of carboxylate anions derived from pentafluorobenzyl ester, trimethylsilyl ether derivatives of four arachidonic acid metabolites of the 5-lipoxygenase pathway have been determined. These molecules include leukotriene B4 (LTB4), a potent chemotactic factor for the human neutrophil; 20-carboxy-LTB4, an inactive metabolite; 5-hydroxyeicosatetraenoic acid (5-HETE), a useful marker of 5-lipoxygenase activity within cells; and 5-hydroxyeicosanoic acid (5-HEA), which has been previously used for the quantitation of leukotriene E4. The carboxylate anion of 5-HEA (m/z 399) was found to decompose by the loss of trimethylsilanol as well as the loss of 146 u corresponding to the loss of trimethylsilanol followed by acrolein, a process specific for 5-hydroxy-containing saturated fatty acids. The loss of trimethylsilanol by a remote site mechanism is the major transition observed for the 5-HETE carboxylate anion (m/z 391). The ion formed (m/z 301) further decomposes by the loss of CO2 (m/z 257). The loss of trimethylsilanol is also seen at m/z 389 after collisional activation of the carboxylate anion of LTB4 (m/z 479) by a complex charge-driven mechanism, not the remote site fragmentation mechanism as expected. The loss of an olefinic proton possibly from carbon-7 is involved as well as an oxygen atom derived from the carboxylic acid moiety. The loss of two trimethylsilanol neutral molecules gives rise to ions seen at m/z 299. Isotopic labeling studies revealed that two isobaric ions are present at m/z 299. Both of these ions involve the loss of trimethylsilanol from the carbon-12 position according to remote site mechanisms, but only one has lost the olefinic proton at carbon-7 and, therefore, likely originates from the further decomposition of the ion (m/z 389) described above. An additional ion seen at m/z 317 is attributed to the loss of trimethylsilyl ether (TMS-O-TMS) following a charge-driven mechanism involving the oxygen atom at carbon-12. The 20-carboxy-LTB4 carboxylate anion (m/z 689) decomposes primarily through the loss of one and two trimethylsilanol moieties, but the base peak (m/z 491) is due to the loss of pentafluorobenzyl alcohol. This ion, likely a ketene, further gives rise to three ions by the sequential loss of one and two trimethylsilanols and TMS-O-TMS. All collision-induced decompositions of the carboxylate anions of these eicosanoids are characterized by losses of small neutral molecules from the derivatizing groups (TMS and pentafluorobenzyl) and little fragmentation of the carbon backbone.