Antioxidant properties amino acids

Besides all the sensory and texturizing properties, GA has interesting antioxidant properties such as an efficient capacity for deactivation of excited electronic states and moderated radical scavenging capacity. There is increasing experimental evidence that associate the antioxidant function with its protein fraction, mainly by amino acid residues such as histidine, tyrosine and lysine, which are generally considered as antioxidants molecules (Marcuse, 1960,1962 Park et al., 2005). 

Yang J, Alemany LB, Driver J, Hartgerink JD, Barron AR (2007a) Fullerene-derivatized amino acids Synthesis, characterization, antioxidant properties, and solid-phase peptide synthesis. Chem. Eur. J. 13 2530-2545. 

Vitamin C is essential for the formation of collagen, the principal structural protein in skin, bone, tendons, and ligaments, being a cofactor in the hydroxylation of the amino acids proline to 4-hydroxyproline, and of lysine to 5-hydroxylysine. These hydroxyamino acids account for up to 25% of the collagen structure. Vitamin C is also associated with some other hydroxylation reactions, e.g. the hydroxylation of tyrosine to dopa (dihydroxyphenylalanine) in the pathway to catecholamines (see Box 15.3). Deficiency leads to scurvy, a condition characterized by muscular pain, skin lesions, fragile blood vessels, bleeding gums, and tooth loss. Vitamin C also has valuable antioxidant properties (see Box 9.2), and these are exploited commercially in the food industries. 

The electronic property of the amino acid on the C-terminus also has an effect on antioxidant activity (Li et al., 2011), that is, the larger the electronic property, the higher is the activity. The C-terminus is a polar position that is thus affected by its electrostatic potential, to some extent therefore, the amino acids Trp, Glu, Leu, lie, Met, Val, Tyr, etc. are suitable at the C-terminus. Some researchers have speculated that the identity of the amino acid on the C-terminus would play an important role in its activity. Suetsuna (2000) separated and identified a radical scavenging peptide, Tyr-Phe-Tyr-Pro-Glu-Leu, from casein hydrolysate, and it was confirmed that the Glu-Leu on the C-terminus mainly contributed to its antioxidant activity. Kim et al. (2009) speculated that the hydrophobic property of the amino acid on the C-terminus, for example, Val and Leu, had a distinct effect on the activity, as determined from the analysis of antioxidative peptides derived from venison hydrolysate. 

Chen et al. (1996) investigated 28 synthetic peptides that were based on the antioxidative peptide Leu-Leu-Pro-His-His. They found that the deletion of the C-terminus His decreased the activity, whereas the deletion of the N-terminus Leu had no effect therefore, it seemed that the C-terminus was related to the antioxidative activity of the peptide, while the N-terminus had no effect. Saito et al. (2003) pointed out that two Tyr-containing tripeptides (Tyr-X-Tyr) showed higher activity than did two His-containing tripeptides (His-X-His). According to the structural characteristics, the hydrophobicity of an amino acid on the N-terminus is the most important property for activity with respect to the same amino acid in the central position in addition, Tyr on the N-terminus has stronger hydrophobicity than His. Therefore, it can be predicted that Tyr-X-Tyr has higher activity than His-X-His. 

Gelatin peptides have repeated unique Gly-Pro-Ala sequence in their structure, and it is presumed that the observed antioxidative properties of gelatin peptides can be associated with their unique amino acid compositions. 

The value of antioxidant protection by way of natural food sources has been pointed out in the literature with considerable frequency. Among the components of soy flour known to have some antioxidant properties are isoflavones and phospholipids. Amino acids and peptides in soybean flour also possess some antioxidant activity. There also may be some antioxidant impact from aromatic amines and sulfhydryl compounds. 

When melanoidins are formed, many factors come into play. Here, several aspects will be considered (cfi ref. 466) differences between sugars and between amino acids, the molar ratio of amino acid to sugar, use of peptides and of hydrolysates, sugar-amino acid mixtures versus MRPs, properties of MRPs, and the mechanisms of antioxidant activity. 

Wu, H. C., Chen, H. M., and Shiau, C. Y. 2003. Free amino acids and peptides as related to antioxidant properties in protein hydrolysates of mackerel (Scomber austrias-icus). Food Res. Int., 36, 949-957. 

Interestingly, some Maillard reaction products have antioxidant activity (reviewed in ref. [29]), the Maillard reaction products from the amino acid-sugar combination of histidine and glucose having stronger antioxidant properties than other combinations [30]. 

Allwood and Plane (85) found that the rate of photodegradation of vitamin A palmitate infusion was reduced by up to 50% in the presence of some amino acids mixtures compared to those prepared in a dextrose infusion. The nature of the protective effect of the amino acids was not elucidated in this report. However, it may be attributable to the antioxidant and the photon absorbing properties of these amino acids. 

Aside from general caveats that apply to many in vitro models, there is one concern specific to research on AGs and cisplatin. Their mechanisms of toxicity involve the formation of ROS and are therefore sensitive to the antioxidant capacity of the incubation medium. This parameter is rarely controlled and different standard media contain varying amounts of redox-active amino acids, glutathione, or other compounds. Even the common pH indicator phenol red is an important contributor to the total antioxidant capacity of cell and tissue culture media [97]. Thus, the antioxidative capacity of the incubation media may differ from the endolymph and comparable media need to be used to compare therapeutic efficiencies. The concern applies not only to studies of ototoxic mechanisms of these drugs but perhaps even more also to attempts to identify protective treatments which most frequently include therapeutics with antioxidant properties. 

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