Novel α-Oxoamide Advanced-Glycation Endproducts within the N6-Carboxymethyl Lysine and N6-Carboxyethyl Lysine Reaction Cascades.

The highly reactive α-dicarbonyl compounds glyoxal and methylglyoxal are major precursors of posttranslational protein modifications in vivo. Model incubations of N2-t-Boc-lysine and either glyoxal or methylglyoxal were used to further elucidate the underlying mechanisms of the N6-carboxymethyl lysine and N6-carboxyethyl lysine reaction cascades. After independent synthesis of the authentic reference standards, we were able to detect N6-glyoxylyl lysine and N6-pyruvoyl lysine for the first time by HPLC-MS2 analyses. These two novel amide advanced-glycation endproducts were exclusively formed under aerated conditions, suggesting that they were potent markers for oxidative stress. Analogous to the well-known Strecker degradation pathway, leading from amino acids to Strecker acids, the oxidation of an enaminol intermediate is suggested to be the key mechanistic step. A highly sensitive workup for the determination of AGEs in tissues was developed. In support of our hypothesis, the levels of N6-glyoxylyl lysine and N6-pyruvoyl lysine in rat livers indeed correlated with liver cirrhosis and aging.