Aldoses and ketoses react with aliphatic primary and secondary amines (including amino acids and proteins) to form /V-glyco-sides, which readily dehydrate to the respective Schiff base by the Maillard reaction, as shown in Figure 1-7 (reactions i and ii). The aldose Schiff base spontaneously undergoes an Amadori rearrangement at C-l and C-2, giving a substituted 1-amino-l-deoxyketose (reaction iii); a ketose Schiff base will rearrange to a substituted 2-amino-2-deoxyal-dose. These sugar amines undergo additional very complex reactions, leading to highly reactive dicarbonyls (such as 3-deoxy-d-gluco-sone), cross-linking of proteins (as in reaction iv), fluorescent compounds and brown pig-
Advanced Glycation End Products
Figure 1-7. Initiation of the Maillard reaction of amines with aldoses. The aminoketose undergoes various reactions, including conversion to a highly reactive dicarbonyl compound (3-deoxy-o-glucosone). When the amino group is from a protein, the reaction may result in cross-linked proteins.
ments, and to low molecular weight compounds, some of which are useful flavoring agents. The Maillard and subsequent reactions occur in food systems such as powdered or evaporated milk during processing or storage, giving off-white colors and decreasing the nutritive value of the proteins. Loss of lysine accounts for only part of the decrease in nutritive value. Although the Maillard complex of reactions has been extensively studied, the reactions are understood only in part. Realization that these reactions occur under physiological conditions in vivo has come more recently, and this is now an active area of research (Baynes and Monnier, 1989; Brownlee, 1995). The reaction of glucose with hemoglobin was discovered first. Plasma glucose reacts with hemoglobins via the Maillard reaction, and the modified protein, detected by gel electrophoresis, is an indicator of plasma glucose levels in diabetics over the life span of the erythrocytes. The term glycated protein is used to distinguish these Maillard-derived, carbohydrate-modified proteins from true glycosylated proteins (glycoproteins).
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