Determination of N-epsilon-(carboxymethyl)lysine in foods and related systems

The sensitive and specific determination of advanced glycation end products (AGES) is of considerable interest because these compounds have been associated with pro-oxidative and proinflammatory effects in vivo. AGES form when carbonyl compounds, such as glucose and its oxidation products, glyoxal and methylglyoxal, react with the epsilon-amino group of lysine and the guanidino group of arginine to give structures including N-epsilon-(carboxymethyl)lysine (CML), N-epsilon-(carboxyethyl)lysine, and hydroimidazolones. CML is frequently used as a marker for AGES in general. It exists in both the free or peptide-bound forms. Analysis of CML involves its extraction from the food (including protein hydrolysis to release any peptide-bound adduct) and determination by immunochemical or instrumental means. Various factors must be considered at each step of the analysis. Extraction, hydrolysis, and sample clean-up are all less straight forward for food samples, compared to plasma and tissue. The immunochemical and instrumental methods all have their advantages and disadvantages, and no perfect method exists. Currently, different procedures are being used in different laboratories, and there is an urgent need to compare, improve, and validate methods.