Liquid chromatographic method for the quantitative determination of Nϵ-carboxymethyllysine in human plasma proteins

The modification of the lysine moieties of proteins to Nϵ-carboxymethyllysine (CML) is supposed to play a major role in the development of long-term complications in patients with diabetes mellitus. This paper presents an analytical method for the quantitative determination of CML in plasma proteins, which could be used for studying the development of diabetic complications. The method is based on isolating proteins from plasma by precipitation with trichloroacetic acid and hydrolysing these under acidic conditions (6 M hydrochloric acid at 110 °C for 20 h) to the individual amino acids. After hydrolysis, CML is derivatised along with the other amino acids to 9-fluorenylmethoxycarbonyl (FMOC) derivatives, which are subsequently separated by reversed-phase column liquid chromatography using a 150 mm × 4.6 mm C8 column and a mobile phase of 25 mM potassium phosphate buffer (pH 2.0) and acetonitrile (80:20 (v/v)) and detected using fluorescence detection (excitation at 260 nm and emission at 310 nm). Quantification of the protein-bound CML content of a plasma sample is achieved using standard addition. The impact of several aspects of the sample preparation and chromatography on method performance is discussed. Method evaluation results are reported and show that this method is capable of determining CML with good accuracy and precision (below 10%) in the relevant concentration range (1–10 μg/ml), with a limit of detection of 0.2 μg/ml.