Electrospray ionization and tandem mass spectrometry of cysteinyl eicosanoids: Leukotriene C4and FOG7

Abstract
The cysteinyl leukotrienes, LTC4, LTD4 and LTE4, and the recently described cysteinyl eicosanoid, 5-oxo-7-glutathionyl-8,11,14-eicosatrienoic acid (FOG7) have been analyzed by tandem mass spectrometry. Both [M−H]− and [M+H]+ ions were produced by electrospray ionization and collision-induced dissociation of these molecular ion species were studied using both an ion trap and a triple quadrupole instrument. Product ion spectra obtained were characteristic of the structure of the cysteinyl leukotrienes and mechanisms of ion formation were investigated by using deuterium-labeled analogs. The product ion spectrum obtained following collision-induced dissociation of the [M−H]− anion from FOG7 was devoid of significant structural information and further studies of collision activation of the [M+H]+ spectrum were therefore examined. Positive ion MS3 spectra obtained in the ion trap from the γ-glutamate cleavage products of FOG7 and its derivative (d7-FOG7) afforded an abundant ion not observed in spectra generated from the cysteinyl leukotrienes. Formation of this fragment ion likely occurred via a McLafferty-type rearrangement to afford cleavage of the C6–C7 bond adjacent to the sulfur atom and was valuable for the identification of the structure of FOG7 and defining the biosynthetic pathway as a 1,4-Michael addition of glutathione to 5-oxo-eicosatetraenoic acid (5-oxo-ETE).