Use of 13C labeling and NMR spectroscopy for the investigation of degradation pathways of Amadori compounds

Plasma Proteins Modified by Advanced Glycation End Products (AGEs) Reveal Site-specific Susceptibilities to Glycemic Control in Patients with Type 2 Diabetes

Protein glycation refers to the reversible reaction between aldoses (or ketoses) and amino groups yielding relatively stable Amadori (or Heyns) products. Consecutive oxidative cleavage reactions of these products or the reaction of amino groups with other reactive substances (e.g. α-dicarbonyls) yield advanced glycation end products (AGEs) that can alter the structures and functions of proteins. AGEs have been identified in all organisms, and their contents appear to rise with some diseases, such as diabetes and obesity. Here, we report a pilot study using highly sensitive and specific proteomics approach to identify and quantify AGE modification sites in plasma proteins by reversed phase HPLC mass spectrometry in tryptic plasma digests. In total, 19 AGE modification sites corresponding to 11 proteins were identified in patients with type 2 diabetes mellitus under poor glycemic control. The modification degrees of 15 modification sites did not differ among cohorts of normoglycemic lean or obese and type 2 diabetes mellitus patients under good and poor glycemic control. The contents of two amide-AGEs in human serum albumin and apolipoprotein A-II were significantly higher in patients with poor glycemic control, although the plasma levels of both proteins were similar among all plasma samples. These two modification sites might be useful to predict long term, AGE-related complications in diabetic patients, such as impaired vision, increased arterial stiffness, or decreased kidney function.

Oxidative degradation of N(ε)-fructosylamine-substituted peptides in heated aqueous systems

Glycation, or non-enzymatic glycosylation, is a common protein modification formed by reactions between reducing sugars (i.e. aldoses and ketoses) with protein amino groups. Resulting Amadori and Heyns compounds, respectively, can be oxidatively degraded yielding a structurally heterogeneous group of advanced glycation end-products. We have studied this process in aqueous conditions at 95 °C in terms of appearing products and their formation kinetics in the presence or absence of reactive oxygen species (ROS)-generating systems (iron(II) sulfate). RP-HPLC-ESI-MS revealed 20 products, 12 of which were confirmed after synthesis by identical retention times and fragmentation patterns. These products accumulated during the incubation period of 4 h (N(ε)-carboxymethyl-, N(ε)-formyl- and N(ε)-methyl lysine) or appeared intermediately (2-aminoadipic semialdehyde, N(ε)-ethanalyl lysine). Acidic and basic amino acid residues near the glycation site and elevated ROS levels in the reaction mixture had significant effects on both product formation and degradation kinetics.