A fragmentation-based method for the differentiation of glutathione conjugates by high-resolution mass spectrometry with electrospray ionization

Abstract

Idiosyncratic reactions are one of the major causes of drug treatment limitations or market withdrawals, and likely involve the formation of reactive metabolites. Because of their unstable nature, reactive species are usually discovered as stable conjugates by glutathione (GSH) trapping rather than by direct detection. The GSH conjugates are then detected by neutral loss scanning of 129Da or precursor ion scanning at m/z 272, but the conjugation sites can only be identified by comparison with reference standards. In the present study, the fragmentation behaviors of 52 GSH conjugates belonging to five structural classes (aliphatic, aromatic, benzylic, disulfide, and thioester) were investigated in both positive and negative electrospray ionization modes by high-resolution mass spectrometry with suitable collision energies such that the relative abundance of the parent ion was approximately 50% of the most abundant product ion. Several structural-diagnostic fragmentations were identified: aliphatic conjugates gave i/j-type ions upon cleavage of the CS bond between the drug and GSH, and d/k-type ions formed by the cleavage of the cysteinyl CS bond, with approximately equal intensity, in both positive and negative modes, whereas aromatic conjugates only possessed d/k-type ions, and benzylic conjugates primarily yielded i/j-type ions. Disulfide conjugates typically produced dehydrogenated GS− fragment ions ([i-2H]-type) in negative mode, and thioester conjugates displayed sequential losses of pyroglutamic acid and water ([e-H2O]-type) in positive mode. A fragmentation-based method was thus established to facilitate the discrimination of these five classes of GSH conjugates, thereby providing insight into the bioactivation mechanisms and supporting lead optimization.