Preferential recognition of Amadori-rich lysine residues by serum antibodies in diabetes mellitus: role of protein glycation in the disease process.

This study analyzes effect of glycation on proteins rich in lysine residues as hyperglycemia induced protein glycation has been mainly reported in diabetes mellitus at the intrachain lysine residues leading to the formation of Amadori modified proteins. We have studied the effect of glucose on poly-l-lysine (PLL), a homopolymer of lysine residues. Levels of Amadori products in the glycated PLL were evaluated by fructosamine assay and the presence of 5-hydroxymethylfurfural (HMF) in the glycated PLL was analyzed by thiobarbituric acid assay. Fluorescence and FT-IR spectroscopy were applied to characterize the modified PLL. Binding characteristics of experimentally induced antibodies against glycated PLL and the presence of antibodies against glycated PLL in the sera of diabetes patients was evaluated by solid phase enzyme immunoassays. The fructosamine assay showed significantly high yield of early glycation (Amadori) products in the glycated PLL, which was confirmed by increased yield of HMF from Amadori products of glycated PLL. Loss in fluorescence intensity and appearance of a new band corresponding to Amadori products were observed in FT-IR spectrum of the glycated PLL. Glycated PLL was found to be highly immunogenic in rabbits as compared to the native form. Serum antibodies from diabetes patients showed appreciably high recognition of the glycated PLL. The results conclusively show the glycation induced damage to the lysine molecules and specific recognition of Amadori-lysine residues by serum antibodies from diabetes patients. The glycated lysine residues may serve as a diagnostic biomarker for early glycation process in diabetes mellitus.