Application of an in vitro antioxidant assay to solvent fractions isolated from bread crust, bread crumb, and flour, respectively, revealed the highest antioxidative potential for the dark brown, ethanol solubles of the crust, whereas corresponding crumb and flour fractions showed only minor activities. To investigate whether these browning products may also act as antioxidants in biological systems, their modulating activity on detoxification enzymes was investigated as a functional parameter in intestinal Caco-2 cells. The bread crust and, in particular, the intensely brown, ethanolic crust fraction induced a significantly elevated glutathione S-transferase (GST) activity and a decreased phase I NADPH-cytochrome c reductase (CCR) activity compared to crumb-exposed cells. Antioxidant screening of Maillard-type model mixtures, followed by structure determination, revealed the pyrrolinone reductones 1 and 2 as the key antioxidants formed from the hexose-derived acetylformoin and N(alpha)-acetyl-L-lysine methyl ester or glycine methyl ester, chosen as model substances to mimic nonenzymatic browning reactions with the lysine side chain or the N terminus of proteins, respectively. Quantitation of protein-bound pyrrolinone reductonyl-lysine, abbreviated pronyl-lysine, revealed high amounts in the bread crust (62.2 mg/kg), low amounts in the crumb (8.0 mg/kg), and the absence of this compound in untreated flour. Exposing Caco-2 cells for 48 h to either synthetically pronylated albumin or purified pronyl-glycine (3) significantly increased phase II GST activity by 12 or 34%, respectively, thus demonstrating for the first time that "pronylated" proteins as part of bread crust melanoidins act as monofunctional inducers of GST, serving as a functional parameter of an antioxidant, chemopreventive activity in vitro.

Metabolic transit data on food-borne advanced MRPs (Maillard reaction products) termed melanoidins are yet not completely elucidated and it is still an open question whether isolated melanoidin structures undergo metabolic biotransformation and subsequently cause physiological effects in vivo. Advanced MRPs, acting as premelanoidins, and melanoidins are formed under severe heat treatment of foods and are ingested with the habitual diet at considerable amounts. Metabolic transit data are known for Amadori compounds classified as early MRPs, like, e.g., fructose-lysine. For rats and humans, the percentages of ingested free versus protein-bound fructose-lysine excreted in the urine were found within ranges of 60–80% and 3–10%, respectively. Balance studies on free advanced MRPs are still lacking, but protein-bound low-molecular-weight premelanoidins and high-molecular-weight melanoidins have already been investigated in animal experiments using ¹⁴C-tracer isotopes. The amount of ingested radioactivity absorbed and excreted in the urine was found at levels ranging from 16 to 30% and from 1 to 5% for premelanoidins and melanoidins, respectively. These different metabolic transit data of premelanoidins and melanoidins can be explained by the following mechanisms involved: (i) intestinal degradation by digestive and microbial enzymes; (ii) absorption of these compounds or their degradates, and (iii) tissue retention. Structure specific in vivo effects have been identified for protein-bound premelanoidins on intestinal microbial activity, xenobiotic biotransformation enzymes and further glycation reactions. The latter are hypothesized to be involved in the aging process and in the course of different diseases. Further investigations are needed to clarify synergistic in vivo effects of dietary ingested melanoidins and endogenously formed glycation products.

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Enhanced plasma AGE levels in vegetarians in comparison to omnivores are herein presented for the first time. Mechanisms of AGE elevation and potential pathophysiological relevance of this finding are to be elucidated in prospective studies.