Quantitative screening of advanced glycation endproducts in cellular and extracellular proteins by tandem mass spectrometry

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Abstract
Glycation of proteins forms fructosamines and advanced glycation endproducts. Glycation adducts may be risk markers and risk factors of disease development. We measured the concentrations of the early glycation adduct fructosyl-lysine and 12 advanced glycation endproducts by liquid chromatography with tandem mass spectrometric detection. Underivatized analytes were detected free in physiological fluids and in enzymic hydrolysates of cellular and extracellular proteins. Hydroimidazolones were the most important glycation biomarkers quantitatively; monolysyl adducts (Nε-carboxymethyl-lysine and Nε-1-carboxyethyl-lysine) were found in moderate amounts, and bis(lysyl)imidazolium cross-links and pentosidine in lowest amounts. Quantitative screening showed high levels of advanced glycation endproducts in cellular protein and moderate levels in protein of blood plasma. Glycation adduct accumulation in tissues depended on the particular adduct and tissue type. Low levels of free advanced glycation endproducts were found in blood plasma and levels were 10–100-fold higher in urine. Advanced glycation endproduct residues were increased in blood plasma and at sites of vascular complications development in experimental diabetes; renal glomeruli, retina and peripheral nerve. In clinical uraemia, the concentrations of plasma protein advanced glycation endproduct residues increased 1–7-fold and free adduct concentrations increased up to 50-fold. Comprehensive screening of glycation adducts revealed the relative and quantitative importance of α-oxoaldehyde-derived advanced glycation endproducts in physiological modification of proteins–particularly hydroimidazolones, the efficient renal clearance of free adducts, and the marked increases of glycation adducts in diabetes and uraemia–particularly free advanced glycation endproducts in uraemia. Increased levels of these advanced glycation endproducts were associated with vascular complications in diabetes and uraemia.