The synthesis of the most active fragments of peptides is possible when identifying the oxygen radical absorbance capacity (ORAC-FL) of component amino acids in these parts of peptides. Thus, the highest value of oxygen radical absorbance capacity was determined for Trp, and high ORAC-FL values were identified for Tyr and Met. This aspect indicates the significant and rather high antioxidant activity of these amino acids.
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The lower antioxidant activity was typical of Cys, His, and Phe. Other amino acids, including Arg, Asn, Gln, and Asp among them, did not demonstrate any antioxidant activity. The explanation of the activity for Trp and Tyr is based on the ability of the indolic and phenolic groups to perform as hydrogen donors. As a result, stable indoyl and phenoxyl radicals become formed. In its turn, Met is oxidised, but Cys provides the sulfur hydrogen. Thus, the radical scavenging activity, in this case, is explained by the presence of Trp, Tyr, Met, and His in peptides.
The synthesis process and the focus on radical scavenging activity for three sequences of active peptides indicated that the sequence Trp-Tyr-Ser- Leu-Ala-Met-Ala-Ala-Ser-Asp-Ile had high antioxidant activity. The radical scavenging activity was about 1.7- and 4-fold lower than it is of the flavonoid quercetin and p-coumaric acid, but antioxidant activity was higher than the activity of butylated hydroxyanisole (BHA), which is a synthetic antioxidant with adverse effects.
Consequently, BHA is proposed to be replaced with natural antioxidants. In order to improve food antioxidant properties and prevent the oxidation reaction, such natural antioxidants as hydrolysates from whey proteins can be utilised, and they can be obtained from cheese whey, for example. Comparing protein hydrolysates to antioxidants from plants, it is important to accentuate their extra nutritional value, but their antigenicity should also be considered before using them in food.
The capacity of this sequence is explained by the presence of Trp, Tyr, and Met. It is important to note that the antioxidant activity of amino acids can be weakened because of the peptidic bond and structural peptide conformation, and radical scavenging activity can be decreased when several amino acids are present because of antagonist effects.
Other two sequences also indicated the radical scavenging activity because of the presence of Met and Tyr, but the antioxidant activity of these amino acids was even higher when they were measured separately. Moreover, the antioxidant activity of these amino acids was improved because of the peptidic bond and structural peptide conformation in contrast to the analysis of the previous sequence. Thus, both synergistic and antagonistic effects can be observed when several amino acids are present, influencing the radical scavenging activity.
The authors of the article conclude that this study is the first one to discuss the antioxidant properties of peptides that are taken from whey proteins with the help of enzymatic hydrolysis. In order to obtain antioxidant hydrolysates, Corolase PP is suitable to be used.
The tandem mass spectrometry (HPLC-MS/MS) was used to identify 42 peptides. Among them, the focus of the authors was on the peptide Trp-Tyr-Ser- Leu-Ala-Met-Ala-Ala-Ser-Asp-Ile because of its high antioxidant activity that was greater than the capacity of BHA. Furthermore, the researchers accentuated specific synergistic and antagonistic effects on the antioxidant activity that resulted from the peptide conformation of the amino acids. Therefore, further research is required in order to examine these effects in detail with the focus on the antioxidant activity.