Protein modification, Maillard reaction

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. 

Glomb MA and Monnier VM (1995) Mechanism of protein modification by glyoxal and glycolaldehyde, reactive intermediates of the Maillard reaction. J Biol Chem 270, 10017-10026. 

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. 

Oxidation modifications such as carbonylation, thiol oxidation, and aromatic hydroxylation, and Maillard glycation (the reaction of sugars with amino acid side chains) are the protein modifications most frequently reported in foodstuffs that have been subjected to thermal processing. However, condensations and eliminations of side chains or peptide backbone breakdown have also been described (95). 

Uchida, K., Khor, O.T., Oya, T., Osawa, T., Yasuda, Y., and Miyata, T. 1997. Protein modification by a Maillard reaction intermediate methylglyoxal. Immunochemical detection of fluorescent 5-methylimidazolone derivatives in vivo. FEBS Lett 410 313-318. 

Numerous undesirable reactions that result in organoleptic, nutritional and functional deterioration may occur in food proteins during processing and storage. These include the non-enzymatic or Maillard reactions, transamidation condensation reactions with dehydroalanine forming crosslinks, and carbonyl amine interactions, all of which may involve the free e-amino group of lysine (11,23). To minimize these reactions a significant volume of work has been done on the protective modification of the e-NH2 of lysine by formylation, acetylation, propionylation (26) or reductive dimethylation (10,11). 

Bjamason and Carpenter (38, 52, 53) studied the use of formylation, acetylation, and propionylation for blocking amino groups in food proteins. Many of these chemical modifications could be easily applied on a commercial scale. The formyl and acetyl derivatives were nutritionally utilized at least partially. The propionylated lysine was not utilized however the propionylated lactalbumin was partially utilized (Table VII). The acylation procedure lowered considerably the extent of the Maillard reaction. Previous investigations, in support of the observations of Bjamason and Carpenter, have shown deacylase in the kidney (51). 

Modification of protein by reductive methylation of amino groups with formaldehyde/NaBH4 retards Maillard reactions. The resultant methyl derivative, depending on the degree of substitution, is less accessible to proteolysis (Fig. 1.47). Hence, its value from a nutritional/physiological point of view is under investigation. 

Modification of food proteins with reducing sugars through Maillard reaction has been evaluated by various analytical methods determination of free amino groups, amino acid analysis after acid hydrolysis, detection of brownish pigments and fluorescent compounds. Some Maillard products have also been analyzed immunochemically using specific antibodies. 

Advanced glycation end-products (AGEs) are a heterogenous group of modifications on proteins that are formed as a result of non-enzymatic reactions, including the Maillard reaction. The formation of AGEs is accelerated under diabetic conditions due to chronic hyperglycemia, oxidative stress and diminished clearance of AGE... 

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