Formation of /Nε-(carboxymethyI)lysine and loss of lysine in casein glucose-Fatty acid model systems

Advanced glycation end-products (AGEs) and advanced lipoxidation 7nd-products (ALEs) form when proteins are heated with reducing sugar or lipid. Nε(Carboxymethyl)lysine (CML) is the most commonly studied AGE/ALE in foods, but the relative importance of dietary sugar and lipid as its precursors is uncertain. The aim of this stud°y was to determine the relative amounts of CML formed from fatty acid and glucose in a model food system. Model systems were prepared by heating casein (3.2%) with glucose or fatty acid (oleic, linoleic, linolenic, or arachidonic acid) (200 mM) or a mixture of glucose and linolenic acid (200 mM of each precursor) at 95 °C for up to 8 h. CML was determined by ultrapressure liquid chromatography-tandem mass spectrometry. The amount of CML formed from casein and glucose incubated at 95 °C for 8 h was 15-fold higher than that obtained when casein was heated with arachidonic acid under the same conditions. However, the loss of lysine in the casein-arachidonic acid incubations was 83% compared to 54% loss in the casein-glucose incubations. The loss of lysine in casein-fatty acid model systems increased with degree of unsaturation of the fatty acid. The formation of lipid peroxidation products during oxidation of fatty acids might be a potent factor for loss of lysine in the casein-fatty acid systems. © 2009 American Chemical Society