Free and protein-bound Nε-carboxymethyllysine and Nε-carboxyethyllysine in fish muscle: Biological variation and effects of heat treatment

•The levels of free and protein-bound AGEs in fish muscle were investigated.•High biological variation in the levels of AGEs was found among individual fish.•Heating had little or no effect on the levels of free AGEs in fish muscle.•Heating significantly accelerated the formation of protein-bound AGEs.

Nε-Carboxymethyllysine (CML) and Nε-carboxyethyllysine (CEL) are typical advanced glycation end-products (AGEs) found in foods, which have been linked to various health risks. Little is known about AGEs formation in fish muscle and the variability in AGEs formation from one animal to another. In this study, free CML and CEL (glycated amino acids) and their protein-bound forms (protein glycation adducts) in fresh grass carp (Ctenopharyngodon idellus) and catfish (Clarias leather) muscle before and after heating (100 °C, 5, 10, 30 min) were determined by an HPLC-MS/MS method. High biological variation in CML and CEL levels was found between individual fish, particularly for CEL in catfish muscle [n = 21, free CEL 0.18–30.1(6.50 ± 7.19) mg/kg; protein-bound CEL 0.48–8.63 (3.08 ± 2.70) mg/kg]. Heating resulted in great increase of protein-bound CML (2.1–10.8 fold increase) and CEL (27%–242% increase) in fish muscle, but had little or no effect on free CML and CEL contents. Simple kinetic functions did not fit well for the formation rate of protein-bound AGEs during heating, although zero-order reaction fitted very well for some individual fish, which further indicated the complexity of AGEs formation and the strong impact of biological variation of individual fish on AGEs; formation.