Abstract
The chemistry of Maillard or browning reactions of glycated proteins was studied using the model compound, N alpha-formyl-N epsilon-fructoselysine (fFL), an analog of glycated lysine residues in protein. Incubation of fFL (15 mM) at physiological pH and temperature in 0.2 M phosphate buffer resulted in formation of N epsilon-carboxymethyllysine (CML) in about 40% yield after 15 days. CML was formed by oxidative cleavage of fFL between C-2 and C-3 of the carbohydrate chain and erythronic acid (EA) was identified as the split product formed in the reaction. Neither CML nor EA was formed from fFL under a nitrogen atmosphere. The rate of formation of CML was dependent on phosphate concentration in the incubation mixture and the reaction was shown to occur by a free radical mechanism. CML was also identified by amino acid analysis in hydrolysates of both poly-L-lysine and bovine pancreatic ribonuclease glycated in phosphate buffer under air. CML was also detected in human lens proteins and tissue collagens by HPLC and the identification was confirmed by gas chromatography/mass spectroscopy. The presence of both CML and EA in human urine suggests that they are formed by degradation of glycated proteins in vivo. The browning of fFL incubation mixtures proceeded to a greater extent under a nitrogen versus an air atmosphere, suggesting that oxidative degradation of Amadori adducts to form CML may limit the browning reactions of glycated proteins. Since the reaction products, CML and EA, are relatively inert, both chemically and metabolically, oxidative cleavage of Amadori adducts may have a role in limiting the consequences of protein glycation in the body.
Highlights
The chemistryof Maillard or browning reactionosf fluorescent compounds [3, 4]
0.2 M phosphate buffer resulted in formation of lV- of tissue proteins [8].Concomitant structural and functional carboxymethyllysine (CML) in about 40% yield after alterations of tissueprotein via the Maillard reaction are
CML was formed by oxidative cleavage of thought to contribute to the development of pathologies asfFL between C-2 and C-3 of the carbohydrate chain sociated with both diabetes and aging, such as blindness, renal and erythronic acid (EA) was identified a s, the split product formed in the reaction
Summary
37 "C, but only lysine and a second major product were detected on the amino acid analyzer. The formation of mannose and glucose by the reverse reaction was suggested by the cluster of peaks at about 1000 s in Fig. 4A (labeled HEXOSE), which had retention times and mass spectreaxpected of a mixture of anomers and conformers of acetylated methylhexosides. To confirm this identification, an aliquot of the fFL incubation mixture was reduced with NaBH4 priorto GC/MS analysis and, asshown, both mannitol andglucitolwere identified as major reaction products. Oxidative degradation of fFL toCML proceeds only in the presence of oxygen, whilethe formation of lysine by reversal of the Amadori rearrangement occurs at similar rates under air and nitrogen.
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