Fructose-induced N-terminal glycation of enkephalins and related peptides.

The formation of glycation products in model systems consisting of fructose and the endogenous opioid peptides not containing lysine residue, such as Leu-enkephalin (Tyr-Gly-Gly-Phe-Leu) and Met-enkephalin (Tyr-Gly-Gly-Phe-Met), or of their fragments, Tyr-Gly-Gly-Phe and Tyr-Gly-Gly, was examined. N-(2-Deoxy-aldos-2-yl)-peptides (Heyns compounds) as well as diastereoisomeric imidazolidinone compounds were identified as reaction products of N-terminal amino group glycation for each of the peptides studied. The structure of the glycation products and relative configuration of C-2 substituents on the imidazolidinone ring in diastereoisomers were determined by NMR experiments. The chemical and enzymatic stability of the fructose-derived glycated products of Leu- and Met-enkephalin was studied in phosphate-buffered saline (pH 7.4) and in human serum at 37 degrees C. The obtained results revealed that glycation increases the stability of the parent peptide to enzymatic degradation. As a result of different configuration at the newly formed stereogenic center, large stability differences in the 2S* and 2R* isomers of the imidazolidinone compounds were observed.