Triosidines: novel Maillard reaction products and cross-links from the reaction of triose sugars with lysine and arginine residues

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Abstract
The role of the highly reactive triose sugars glyceraldehyde and glyceraldehyde-3-phosphate in protein cross-linking and other amino acid modifications during the Maillard reaction was investigated. From the incubation of glyceraldehyde with N (alpha)-acetyl-L-lysine and N (alpha)-acetyl-L-arginine, we isolated four new Maillard reaction pyridinium compounds named 'triosidines'. Two of them, 'lys-hydroxy-triosidine' [1-(5-amino-5-carboxypentyl)-3-[(5-amino-5-carboxypentylamino)methyl]-5-hydroxypyridinium] and 'arg-hydroxy-triosidine' [2-(4-amino-4-carboxybutylamino)-8-(5-amino-5-carboxypentyl)-6-hydroxy-3,4-dihydro-pyrido[2,3-d]pyrimidin-8-ium] are fluorescent, UV-active Lys-Lys and Lys-Arg cross-links respectively. Their structures were identified by NMR and MS. In addition, two UV-active lysine adducts, 'trihydroxy-triosidine' [1-(5-amino-5-carboxypentyl)-3,4-dihydroxy-5-(hydroxymethyl)pyridinium] and 'triosidine carbaldehyde' [1-(5-amino-5-carboxypentyl)-3-formylpyridinium] were tentatively identified by MS. All structures involve six sugar-derived carbons as part of the heterocyclic ring. Of the two novel cross-links, only arg-hydroxy-triosidine was formed by glyceraldehyde-3-phosphate, an intermediate metabolite of the glycolytic pathway. Lys-hydroxy-triosidine and arg-hydroxy-triosidine were detected in human and porcine corneas treated with glyceraldehyde. The HPLC-fluorescence identification was confirmed by MS. Triosidines were also formed from dihydroxyacetone, a widely used artificial sun-tanning agent. Triosidines are expected to be useful tools in tissue engineering, where the utilization of highly reactive sugars is needed to stabilize the loose matrix. In addition, they are expected to be present in selected biological conditions, such as on consumption of a high fructose diet, and syndromes associated with high glyceraldehyde excretion, such as Fanconi Syndrome, fructose-1,6-diphosphatase deficiency and tyrosinaemia.