Cysteine Mediated Formation of N-ε-Carboxymethyllysine (CML) on Proteins

The reaction of glyoxal with nucleophilic amino acids was monitored for β-casein as well as β-lactoglobulin. As predicted from previous experiments with hippuryl amino acids, a measurable decrease of arginine can be found in the thiol-free β-casein, while the lysine content remained almost unchanged. For β-lactoglobulin, the incubation with glyoxal led to a slight decrease in the lysine content, while the arginine residues remained unmodified. Here, in accordance with nucleophilicity, it is suggested, that mainly cystein residues react with glyoxal. In solutions containing β-casein with or without glutathione, the effects were less pronounced and regarding the lysine and arginine content, the influence of thiols could hardly be recorded on a significant level. However, comparing the CML levels in the different incubations, it becomes obvious, that glutathione is favouring CML formation in a concentration depended manner. Therefore, the use of CML as an indicator, e.g. for the Maillard reaction, must be related to the composition of the reaction system.