[N^ε-(γ-glutamyl) lysine] as a potential biomarker in neurological diseases: New detection method and fragmentation pathways

Protein aggregates are characteristic of a number of diseases of the central nervous system such as diseases of polyQ expansion. Covalent bonds formed by the action of transglutaminase are thought to participate in the stabilization of these aggregates. Transglutaminase catalyzes the formation of cross-links between the side chains of glutaminyl and lysyl residues of polypeptides. Identification of the isodipeptide N^ε-(γ- glutamyl) lysine (iEK) in terminal proteolytic digests of neuronal aggregates would demonstrate participation of transglutaminase in neurological diseases. In order to identify and quantify the iEK present in the brain of patients with neurological disease, a method combining liquid chromatography and multistep mass spectrometry was developed. Because isobaric peptides of iEK could be present in the digest of aggregated proteins, the choice of fragment diagnostic ions was crucial. These ions were identified by mass spectrometry on sodiated iEK, which was derivatized on the carboxylic functions and terminal amines in order to improve sensitivity. Deuterated molecules as well as ¹³C₆- and ¹⁵N₂-isotopomers were used to derive filiations in the multistep fragmentations. The main fragmentation patterns have been identified, so that two ions (m/z 396 [MH - 56-42 u] ⁺ and 350 [MH - 56-88 u]⁺) are shown to be adequate markers for quantitation experiments. In order to gain a better understanding of the fragmentation processes, detailed quantum chemical calculations have been performed at levels which are expected to provide good accuracy. A thorough study has been carried out with a reduced model in which only the 'active' part of the molecule is retained. This allowed obtaining full mechanistic details on the pathways leading to a number of observed fragments. In particular, it has been shown that losses of 87 and 88 u from A⁺ = [MH - 56 u] ⁺ are competitive. Computations on the entire derivatized isodipeptide have been used to validate the use of the smaller model in order to obtain reliable energetics and mechanisms.