Chapter 5
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Radical Induced Formation of D-Glucosone from Amadori Compounds R. Liedke and K. Eichner Institut für Lebensmittelchemie der Universität Münster, Corrensstrasse, 45, D-48149 Münster, Germany
Amadori rearrangement products (ARP) are the first stable intermediates in the Maillard reaction between reducing sugars and amino acids. The oxidative decomposition of ARPs leads to the formation of osones like D-glucosone. The radical induced formation of D-glucosone in model systems containing ARPs is dependent on the chosen reaction parameters like pH, temperature, and water activity. Our investigations showed that, in general, the formation of D-glucosone is increased in the presence of transition metals; whereas, the yield of the D— glucosone is greatly reduced by elimination of the metal ions (addition of EDTA). Under these conditions the non-oxidative enolization reactions leading to the deoxyosones gain importance. The assumed radical pathway could be confirmed by EPR and spin trapping of radicals with DMPO. Currently, oxidative pathways in the Maillard reaction are being discussed for their relevance in medicine and health (autoxidative glycosylation of tissue protein, diabetes, aging).
© 2002 American Chemical Society Morello et al.; Free Radicals in Food ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 28, 2016 | http://pubs.acs.org Publication Date: March 4, 2002 | doi: 10.1021/bk-2002-0807.ch005
70 Radical reaction mechanisms during the early Maillard reaction were first detected by Namiki et al. (/, 2). He identified N,AT-dialkylpyrazine-cation radicals that originated from the primary Schiff base formed by reaction between glucose and amino acids. The glycolaldehyde alkylimine formed by a reverse aldol reaction of the Schiff base leads to a dialkylpyrazinium radical cation after self-condensation. The formation of dialkylpyrazinium radical cations, which could be detected by EPR spectrometry, represents an alternative pathway of the Maillard reaction; it starts at the very beginning of the reaction, well before the formation of Amadori rearrangement products and depends on the pH value; it starts around pH 7 and increases up to pH 11. In a similar way, EPR spectroscopy of orange-brown melanoidins, which were isolated from heated aqueous solutions of bovine serum albumin and glycolaldehyde, revealed the protein-bound 1,4-bis(5-amino-5-carboxy-1 pentyl)pyrazinium radical cation (CROSSPY) as a previously unknown type of cross-linking of proteins in vivo and during food processing (J). CROSSPY could be found in wheat bread crust, roasted cocoa, as well as coffee beans. On the other side, glyoxal may be formed by oxidation of the above mentioned glycolaldehyde alkylimine; the glyoxal can react with the amino groups of amino acids (for example, the e-amino group of protein-bound lysine) to form carboxymethyl-lysine (CML). Furthermore, there are different pathways for the oxidative decomposition of Amadori products. Ahmed et al. (4) described the formation of N-e-carboxymethyl-lysine (CML) by oxidation of e-fructose-lysine which is promoted in the presence of iron and by increasing the pH value. CML can be formed during food processing as well as in vivo. On the other side, the enaminol form of Amadori compounds can be oxidized through a radical mechanism, especially in the presence of heavy metal ions like copper (5); as shown in Figure 1, glucosone is formed in this way, alternatively to the formation of the 3- and 1-desoxyosones via 1,2- and 2,3 enolization of Amadori compounds, respectively. The reactive oxygen species (ROS) formed during autoxidation of Amadori compounds (superoxide- and hydroxyl-radicals) may cause oxidative damage of proteins in vivo (6) and may be responsible for the "oxidative stress". But glucosone is also formed during thermal food processing where the Maillard reaction takes place. Furthermore, glucose and other a-hydroxyaldehydes or ahydroxyketones may undergo radical induced oxidation processes leading to very reactive glycosones and ROS which may damage proteins (6, 7). According to Wells-Knecht et al. (8) also glyoxal is formed during autoxidation of glucose; as already mentioned, it may cause crosslinking of proteins and formation of CML. Moreover, glucosone is formed during irradiation of glucose and fructose (9, 10).
Morello et al.; Free Radicals in Food ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
Morello et al.; Free Radicals in Food ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
H + H2O
+ Cu(ll) >-Cu(l)/H+
+ Cu(ll) >• Cu(l)/H+
•
glucosone
R-C-(f H
H +
R—C—(p=N—R'
ft
H
R-C=(p-N—R'
9"
H
R-C-^-N-R*
2
aminoacid
H N-R'
H
R-C-
