The Maillard reaction, or nonenzymatic browning, proceeds in vivo, and the resulting protein modifications (advanced glycation end products) have been associated with various pathologies. Despite intensive research only very few structures have been established in vivo. We report here for the first time N 6 -{2-[(5-amino-5-carboxypentyl)amino]-2-oxoethyl}lysine (GOLA) and N 6 -glycoloyllysine (GALA) as prototypes for novel amide protein modifications produced by reducing sugars. Their identity was confirmed by independent synthesis and coupled liquid chromatography/mass spectrometry. Model reactions with N ␣ -t-butoxycarbonyl-lysine showed that glyoxal and glycolaldehyde are immediate precursors, and reaction pathways are directly linked to N ⑀ -carboxymethyllysine via glyoxal-imine structures. GOLA, the amide cross-link, and 1,3-bis(5-amino-5-carboxypentyl)imidazolium salt (GOLD), the imidazolium cross-link, share a common intermediate. The ratio of GOLA to GOLD is greater when glyoxal levels are low at constant lysine concentrations. GOLA and GALA formation from the Amadori product of glucose and lysine depends directly upon oxidation. With the advanced glycation end product inhibitors aminoguanidine and pyridoxamine we were able to dissect oxidative fragmentation of the Amadori product as a second mechanism of GOLA formation exactly coinciding with N ⑀ -carboxymethyllysine synthesis. In contrast, the formation of GALA appears to depend solely upon glyoxal-imines. After enzymatic hydrolysis GOLA was found at 66 pmol/mg of brunescent lens protein. This suggests amide protein modifications as important markers of pathophysiological processes.

With this work, we report on further insights into the chemistry of 1-deoxy-D-erythro-hexo-2,3-diulose (1-deoxyglucosone, 1-DG). This alpha-dicarbonyl plays an important role as a highly reactive intermediate in the Maillard chemistry of hexoses. Degradation of 1-DG in the presence of the amino acid l-alanine led to the formation of several products. Lactic acid and glyceric acid were found to be major degradation products. Their formation was dependent on the presence of oxygen. Therefore, a mechanism is postulated based on oxidation leading to a tricarbonyl intermediate. Carbonyl cleavage of this structure should then give rise to carboxylic acids. This mechanism was supported by the isotope distribution observed during degradation of different (13)C-labeled D-glucose isotopomers. Furthermore, we identified 3,5-dihydroxy-6-methyl-2,3-dihydro-4H-pyran-4-one (gamma-pyranone) to be capable of rehydration forming 1-DG to a minor extent and therefore leading to the same degradation products. The formation of carboxylic acids from gamma-pyranone was also dependent on the presence of oxygen in agreement with the postulated oxidative fragmentation. Finally, we investigated the formation of aldehydes expected as retro-aldol products formed within the degradation of 1-DG. Results seemed to rule out this reaction as an important degradation pathway under the conditions investigated herein.

No abstract

No abstract

No abstract

No abstract

Synthesis of 1-Deoxy-D-erythro-hexo-2,3-diulose, a Major Hexose Maillard Intermediate.-The title compound (V) is synthesized by reaction of the protected erythronolactone (I) with ethoxyvinyllithium (II) followed by transformation of the extremely acid labile vinylether (III) into the carbonyl derivative (IV) which is deprotected under mild controllable conditions. The major isomer of the title compound in solution adopts the 2 C 5 -chair conformation. Compound (V) is a much more potent protein modifier than 3-deoxy-D-erythro-hexos-2-ulose. -(GLOMB, MARCUS A.; PFAHLER, CHRISTOPH; Carbohydr. Res. 329 (2000) 3, 515-523; Inst.