Reference Number: 127
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 NR-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. Quantification 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 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.
SIGNIFICANCE OF THIS STUDY
The development of the brown color that occurs during baking, roasting or frying occurs due a mechanism called Maillard reaction which is a chemical reaction that occurs between amino acids and carbohydrates resulting in compounds called melanoid which imparts the brown colour. The brown crust we see on sourdough loaves is a perfect example of maillard reaction. The brown crust is often associated in consumers’ minds with a delicious and desirable loaf and although the sensory aspects are well established, not many papers have looked into the physiological implications of this phenomenon. The current paper is one such study which explores the antioxidant potential of sourdough bread crust and the respective compounds that are released during the Maillard reaction. The current study was the first to identify a novel anti-oxidant that is concentrated in the crust called pronyl-lysine which was found 8 times more in the crust than in the crumb. Pronyl-lysine is formed by the reaction of the protein-bound amino acid L-lysine and starch as well as reducing sugars in the presence of heat. The current paper showed pronyl-lysine had the ability to increase the levels of specific enzymes called glutathione S-transferase, which have been shown in previous studies to play a role in cancer prevention. The current analysis was performed on human intestinal cell lines that were grown under strict laboratory conditions and the current findings are yet to be tested on humans or animals. The study also found that the antioxidant is likely to be more abundant when bread is broken down into smaller pieces and baked, as with stuffing, because the smaller pieces contain more surface area on which these reactions can occur in comparison to larger bread products