Formation Mechanisms of Melanoidins and Fluorescent Pyridinium Compounds as Advanced Glycation End Products

The formation mechanisms of melanoidins as advanced glycation end products (AGEs) have not been resolved. Blue and red pigments generated in the D-xylose–glycine reaction system are postulated to be intermediate oligomers in the generation of melanoidins. A novel blue pigment, designated blue-M5, was identified as a similar structure to blue-M1 except for the side chain of two dihydroxypropyl groups. Blue pigments were also generated in the D-glucose–glycine and D-xylose–β-alanine reaction systems as well as in the D-xylose–glycine reaction system. Blue pigments by the Maillard reaction might be formed by the decarboxylation of two molecules of pyrrolopyrrole-2-carbaldehydes (PPA). PPA, composed of a side chain of a dihydroxypropyl group, was identified as a precursor of blue pigments. In fact, blue-M5 was generated by the incubation of PPA alone. Blue pigments, which involved pyrrolopyrrole structures, were readily changed to brown polymers. Glyceraldehyde-derived pyridinium (GLAP) compound, a glyceraldehyde-derived fluorescent AGE, and lysyl-pyrropyridine, a 3-deoxyglucosone-derived fluorescent AGE, were detected at higher levels in the plasma proteins and the tail tendon collagen of streptozotocin-induced diabetic rats compared to normal rats. GLAP and lysyl-pyrropyridine, therefore, might be related to the progression of diabetic complications.