Cross-link, protein, Maillard reaction

Armstrong attributed the increased resistance of dentin matrix to proteolysis to the blockage of susceptible sites by covalently bound carbohydrate. Later it became clear that the Maillard reaction induces the formation of covalent bonds (cross-links) between protein molecules, accounting for such resistance as well. The presence of non-degradable matrix proteins inhibits mineral dissolution (Chapter 2). In addition, both brown pigments and cross-linked proteins inhibit the production of extracellular polysaccharides by cariogenic streptococci (Kobayashi et al., 1990). 

K. Nakamura, T. Hasegawa, Y. Fukunaga, and K. Ienaga, Crosslines A and B as candidates for the fluorophores in age- and diabetes-related cross-linked proteins, and their diacetates produced by Maillard reaction of -/V-acety 1-L-lysi nc with D-glucose,. /. Chem. Soc., Chem. Commun., 1992, 992-994. 

For a Maillard reaction involving glucose the major dicarbonyl intermediate is 3-deoxy-D-erythro-hexose-ulose (also known as 3-deoxyglucosone). Ascorbic acid can also produce many dicarbonyl compounds (Taqui-Khan, 1967 Kurata et al., 1973 Kurata and Fujimaki, 1976 Martell, 1980) via the Maillard reaction including 3-deoxy-D-erythro-hexose-ulose (Hirsch et al., 1992). Ascorbic acid can also lead to 3-deoxy-D-glycero-pentose-2-ulose and D-glycero-pentose-2-ulose. Aldehydic products of the Maillard reaction are able to cross-link proteins. 

Several markers for the Maillard reaction have been described in the literature. For example, the product initially formed between glucose and lysine is partly transformed into furosine (Heyns et ah, 1968) on acid hydrolysis. Conversely, the fluorescent amino acid pentosidine (Sell and Monnier, 1989) is an advanced glycation endproduct (AGE) and may form covalent bonds between proteins (cross-linking). Furthermore, the Maillard reaction leads to an increase in characteristic fluorescence (excitation 370 nm, emission 440 nm) (Monnier et ah, 1984 Pongor et ah, 1984). 

The aim of this study was to find proof that the Maillard reaction can take place in demineralized dentin. Markers for the initial and advanced Maillard reaction as well as physiological cross-links were investigated after incubation of bovine dentin with glucose. Changes in susceptibility to protein-degrading enzymes and in fluorescence specific for the reaction were additionally measured. 

In the present investigation we sought to clarify the purported relation between dentin caries and matrix protein modifications such as the Maillard reaction and cross-linking. 

Aside from the Maillard reaction, other covalent modifications of amino acids and proteins are possible within the caries lesion, which merit future investigation. For example, certain oral microorganisms excrete y-glutamyl transferases. These enzymes catalyse the formation of cross-links between glutamic acid and lysine residues of proteins. In addition, N-acyl amino acids are present in plaque, which adsorb to mineral surfaces. 

F. Gerum, M. O. Lederer, and T. Severin, Cross-linking of proteins by Maillard processes Model reaction of an Amadori compound with W -acctyl-l -arginine, in F. 1998, 409. 

E. B. Frye, T. P. Degenhardt, S. R. Thorpe, and J. W. Baynes, Role of the Maillard reaction in aging tissue proteins Advanced glycation end product-dependent increase in imi-dazolium cross-links in human lens, J. Biol. Chem., 1998, 273, 18714—18719. 

As the Maillard reaction progresses, there can be cross-linking between protein species and the formation of polymeric species. The formation of these species is expected to further modify the viscosity and gelation properties of the proteins. 

Nagaraj, R. H., Shipanova, I. N., and Faust, F. M. 1996. Protein cross-linking by the Maillard reaction. Isolation, characterization, and in vivo detection of a lysine-lysine cross-link derived from methylglyoxal./. Biol. Chem. 271 19338-45. 

The main classes of Maillard reaction end products of practical interest inelude volatile heterocyclic molecules responsible for aromas of thermally processed foods colored oligomeric and polymeric molecules that determine non-enzymatic browning of thermally processed foods protein and lipid cross-linking and other advanced glycation end products in vivo, which are implicated in pathogenesis of comphcations in diabetes and aging. Here we only consider the contribution of D-finctosamines implicated in the formation of these end products. A number of reviews containing detailed considerations of the formation of volatiles, melanoidins, and... 

In heat treated or stored food products several amino acids are not fully available because of derivatization or crosslinking reactions. Since 30 years furosine is known as a useful indicator of early Maillard reaction which is applied in food science, nutrition and medical biochemistry. Recently more sensitive analytical methods for furosine determination are available which have again increased the attractivity of this important indicator. Lately, N -carboxymethyllysine (CML) became available as another marker of special interest, because CML is a more useful indicator of the advanced heat damage by Maillard reaction than furosine. In addition, CML has the advantage to indicate reactions of lysine with ascorbic acid or ketoses such as fructose. Indicators for protein oxidation of sulfur amino acids are methionine sulfoxide and cysteic acid. An established marker for cross-linking reactions is lysinoalanine, which also indicates protein damages due to processing under alkaline conditions. Other markers formed as a consequence of alkaline treatment are D-amino acids. 

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