In human lens proteins, advanced glycation endproducts (AGEs) originate from the reaction of glycating agents, e.g., vitamin C and glucose. AGEs have been considered to play a significant role in lens aging and cataract formation. Although several AGEs have been detected in the human lens, the contribution of individual glycating agents to their formation remains unclear. A highly sensitive liquid chromatography–tandem mass spectrometry multimethod was developed that allowed us to quantitate 21 protein modifications in normal and cataractous lenses, respectively. N6-Carboxymethyl lysine, N6-carboxyethyl lysine, N7-carboxyethyl arginine, methylglyoxal hydroimidazolone 1, and N6-lactoyl lysine were found to be the major Maillard protein modifications among these AGEs. The novel vitamin C specific amide AGEs, N6-xylonyl and N6-lyxonyl lysine, but also AGEs from glyoxal were detected, albeit in minor quantities. Among the 21 modifications, AGEs from the Amadori product (derived from the reaction of glucose and lysine) and methylglyoxal were dominant.

Numerous investigations concerning Maillard degradation of carbohydrates clearly depict the important impact of α-dicarbonyl compounds on changes occurring during preparation of food or physiological processes in vivo. To study the formation of these reactive intermediates during degradation of maltose in the presence of lysine, α-dicarbonyl compounds were isolated, identified and quantified after reaction with o-phenylenediamine to form their stable quinoxaline derivatives. Maltosone and 1,4-dideoxyglucosone were synthesized and incubated independently with lysine to investigate follow-up products and to gain further insights into the complex degradation mechanisms. Glyoxylic acid as a dicarbonyl structure and 5,6-dihydroxy-2,3-dioxohexanal as a 1,2,3-tricarbonyl compound were established as novel Maillard degradation products of maltose. Conducted experiments unequivocally demonstrated that inter- and intramolecular redox reactions are of major importance during degradation of disaccharides. 1,4-Dideoxyglucosone, 1-lysino-1,4-dideoxyglucosone, 5,6-dihydroxy-2,3-dioxohexanal, 3,4-dideoxypentosone and glyoxylic acid were found to be the central intermediates involved in the redox chemistry. With the present study we deliver a comprehensive overview on the mechanisms behind α-dicarbonyl compounds evolving from Maillard degradation of maltose.

The Maillard reaction network with focus on the chemistry of dicarbonyl structures causes considerable interest of research groups in food chemistry and medical science, respectively. Dicarbonyl compounds are well established as the central intermediates in the nonenzymatic browning reaction and have been verified to be responsible for advanced glycation endproduct (AGE) formation. A multitude of Maillard dicarbonyls covering the range of the intact carbon backbone down to C3 and C2 fragments were detected in several carbohydrate systems, for example, in glucose, maltose, or ascorbic acid reactions. By definition, dicarbonyls with a C2-C5 carbon backbone must originate by fission of the original carbon skeleton. The present review deals with the five major mechanisms reported in the literature for dicarbonyl decomposition: (i) retro-aldol fragmentation, (ii) hydrolytic α-dicarbonyl cleavage, (iii) oxidative α-dicarbonyl cleavage, (iv) hydrolytic β-dicarbonyl cleavage, and (v) amine-induced β-dicarbonyl cleavage.

A novel species of amides formed from degradation of one of the most important key intermediates in Maillard hexose chemistry-1-deoxyhexo-2,3-diulose-was investigated. In 1-deoxyhexo-2,3-diulose/N(alpha)-t-BOC-lysine reaction mixtures four amides, N(epsilon)-acetyl lysine, N(epsilon)-formyl lysine, N(epsilon)-lactoyl lysine and N(epsilon)-glycerinyl lysine, were identified and their structures verified by authentic reference standards. Amides and corresponding carboxylic acids (acetic acid, formic acid, lactic acid and glyceric acid) accumulated over time. Both N(epsilon)-lysine amides and carboxylic acids were thus determined as stable Maillard end products. Results of model incubations suggested the synthesis of amides to be mechanistically closely related to the formation of their corresponding carboxylic acids by beta-dicarbonyl cleavage. Due to the different chemical properties of all the compounds monitored, various analytical strategies had to be carried out (LC-MS(2), GC-MS, GC-FID, enzymatic determination).

SummaryProteins in basement membrane (BM) are long‐lived and accumulate chemical modifications during aging; advanced glycation endproduct (AGE) formation is one such modification. The human lens capsule is a BM secreted by lens epithelial cells. In this study, we have investigated the effect of aging and cataracts on the AGE levels in the human lens capsule and determined their role in the epithelial‐to‐mesenchymal transition (EMT) of lens epithelial cells. EMT occurs during posterior capsule opacification (PCO), also known as secondary cataract formation. We found age‐dependent increases in several AGEs and significantly higher levels in cataractous lens capsules than in normal lens capsules measured by LC‐MS/MS. The TGFβ2‐mediated upregulation of the mRNA levels (by qPCR) of EMT‐associated proteins was significantly enhanced in cells cultured on AGE‐modified BM and human lens capsule compared with those on unmodified proteins. Such responses were also observed for TGFβ1. In the human capsular bag model of PCO, the AGE content of the capsule proteins was correlated with the synthesis of TGFβ2‐mediated α‐smooth muscle actin (αSMA). Taken together, our data imply that AGEs in the lens capsule promote the TGFβ2‐mediated fibrosis of lens epithelial cells during PCO and suggest that AGEs in BMs could have a broader role in aging and diabetes‐associated fibrosis.

Background: Advanced glycation end products (AGEs) play a prominent role in various pathophysiologies. Results: A new class of lysine amide modifications was established in vivo. Conclusion: Non-enzymatic Maillard mechanisms participate on amide-AGE formation pathways in vivo. Significance: Plasma levels of the amide-AGEs were in the same range similar to other established lysine modifications and suggest a comparable impact of non-enzymatic biochemistry on pathophysiologies in the human organism.

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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.