Glycation of lysine-containing dipeptides

Protein glycation through Maillard reaction (MR) is a fundamental reaction both in foods and in the human body. The first step of the reaction is the formation of Amadori product (AP) that is converted into intermediate and advanced MR products during reaction development. Although the MR is not an enzymatic reaction, a certain degree of specificity in the glycation site has been observed. In the present study, we have monitored the glycation of different lysine-containing dipeptides to evaluate the influence on the NH₂ reactivity of the neighboring amino acid. Lysine dipeptides were reacted with glucose, galactose, lactose and maltose. The formation and identification of glycated compounds were monitored by mass spectrometry (MALDI-TOF and ESI-MS/MS) and by HPLC of their Fmoc derivatives. MS/MS analysis showed that the glucose APs formed on dipeptides have a characteristic fragmentation pattern: the fragment at [M - 84]⁺ due to the formation of pyrylium and furylium ion is mainly present in the monoglucosylated form, while the [M - 162]⁺ and the [M - 324]⁺ are more evident in the fragmentation pattern of the diglucosylated forms. The nature of the vicinal amino acids strongly af fects lysine reactivity towards the different carbohydrates: the presence of hydrophobic residues such as IIe, Leu, Phe strongly increases lysine reactivity. Contrasting results were obtained with basic residues. The Lys-Arg dipeptide was among the most reactive while the Lys-Lys was not.