The early glycation products of the Maillard reaction: mass spectrometric characterization of novel imidazolidinones derived from an opioid pentapeptide and glucose

Abstract

Glucose-substituted imidazolidinones related to the endogenous opioid peptide leucine-enkephalin have been investigated using fast atom bombardment tandem mass spectrometry (FAB-MS/MS) and electrospray ionization tandem mass spectrometry (ESI-MS/MS). In addition to Amadori compounds, the studied imidazolidinones represent a novel type of the early glycation products formed in the Maillard reaction. To obtain insight into the fragmentation behavior of these carbohydratepeptide adducts, we also studied synthetic precursors of the glucose-substituted imidazolidinones as well as the corresponding isopropylidene derivatives. The collision-induced dissociation (CID) spectra of [M H] ions of all these imidazolidinones have been compared. Detailed analysis showed that fragmentation of each compound generates two ions at m/z 566 and m/z 598 which are characteristic and undoubtedly confirm the imidazolidinone-type structure. These two significant ions were identified as the M 10 and M 42 modifications of the N-terminus of the parent opioid pentapeptide effected by the carbohydrate moiety. Furthermore, the ion at m/z 178 is identified as the M 42 modification of the immonium ion of the N-terminal amino acid (tyrosine) also effected by the carbohydrate moiety. They can be used as diagnostic ions for imidazolidinone-type compounds in studying the Maillard reaction. Thus, we have demonstrated the utility of FAB-MS/MS and ESI-MS/MS in the structural determination and identification of such novel peptide-carbohydrate adducts, useful in understanding the details of the mechanism of non-enzymatic glycation in vivo. To investigate the early glycation products, in our recent studies we used carbohydrate-peptide esters in which the sugar moiety (D-glucose, D-mannose, or D-galactose) is linked through its C-6 hydroxy group to the C-terminal carboxy group of the endogenous opioid pentapeptide leucine-enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH) as model compounds for the study of the Maillard reaction in vitro.12±14 We found for the first time that, depending on the reaction conditions, in addition to Amadori rearrangement, an alternative pathway for the carbohydrate-induced