# The Structure-Activity Relationship of the Antioxidant Peptides from Natural Proteins
## 2 The Mechanism of the Antioxidant and the Evaluating Methods of Antioxidant Capacity  
During the organism's metabolism, reactive oxygen species (ROS) or free radicals are naturally produced by oxidation reactions through breathing. Living organisms have developed their own antioxidant defenses against excessive amounts of ROS. When their limited efficiencies cannot prevent all the oxidative damages related to environmental conditions (_i.e._, UV light, non-equilibrated food and pollution), the excessive amounts of ROS finally cause an oxidative stress condition, which results in many non-communicable chronic diseases, such as diabetes, atherosclerosis, arthritis and cancer [27; 28; 29]. In addition, lipid peroxidation caused by ROS is one of the main causes of the degradation of lipids in food or cosmetic matrices [30]. Natural antioxidants have been found to possess the ability to effectively prevent the damage caused by ROS [31]. Therefore, there is a growing interest in exploring various antioxidant natural compounds, such as peptides derived from natural proteins. Antioxidant capacity is used to evaluate the efficiency of natural antioxidants via two main chemical mechanisms. Free radical scavenging is the main role of antioxidants, which would \begin{table}
\begin{tabular}{l l l l} \hline \hline \multicolumn{1}{c}{**Source Protein**} & \multicolumn{1}{c}{**Assays**} & \multicolumn{1}{c}{**Sequence**} & \multicolumn{1}{c}{**Ref. \({}^{*}\)**} \\ \hline Chicken egg white & ORAC & AEERYP, DEDTQAMP & [9] \\ \hline Egg white & oxygen radical scavenging, DPPH & DHTKE, MDDAHL, & [10] \\  & radical scavenging. & FFGEN & \\ \hline Rice residue protein & DPPH and ABTS radical scavenging, & RPNYTDA, TSQLSDQ, & [11] \\  & FRAP-Fe\({}^{3+}\) reducing assay. & TRICDPEF, NFHPQ & \\ \hline Grass carp & DPPH radical, hydroxyl radical, & \\ _(Ctenopharyngeal idella)_ skin & ABTS radical scavenging, Inhibiting & PYSFK, GFCPEL, GGRP & [12] \\  & lipid peroxidation. & & \\ \hline Bluefin leatherjacket skin (_Nuxodon_ _septentriomalis_) & DPPH, HO, O\({}^{2-}\) radical scavenging. & GSGGL, GPGGFI, FIGP & [6] \\ \hline Palm kernel cake proteins & DPPH radical scavenging, Metal & GIFE LPWRPATNVF & [13] \\ \hline Blood clam (_Tegillarca granosa_) muscle & lipid peroxidation, radical scavenging activity. & WPP & [14] \\ \hline Sweet potato & OH radical scavenging. & YYIVS & [15] \\ \hline Crocceine croaker & DPPH, superoxide, ABTS and (_Pseudosciena croca_) muscle & hydraz radical scavenging, lipid peroxidation. & WLMSA, VLWEE, \\ \hline Spotless smoothhound (_Mustelus griscus_) muscle & Hydroxyl, ABTS, superoxide radical scavenging. & GISHR, ELLI, KFPE, \\ \hline Bluefin leatherjacket (_Nuxodon_ _septentriomalis_) heads & DPPH, hydroxyl, ABTS, superoxide & WEGPK, GPP, GVPLT & [18] \\ \hline Hemp (_Camabis sativa L.) seed & DPPH radicals scavenging. & PSLPA, WVYY & [19] \\ \hline Chickpea protein & DPPH radicals scavenging. & VGDI, DHG & [20] \\ \hline Marine _Spien brevimana_ mantle & DPPH radicals scavenging, lipid peroxidation. & I/LNI/LCCN & [21] \\ \hline _Sphyrna lewini_ muscle & ABTS, DPPH radicals scavenging. & WDR, PYFNK & [22] \\ \hline Tilapia (_Oreochromis nelicticus_) gelatin & Hydroxyl radicals scavenging. & LSGYGP & [23] \\ \hline Corn gluten meal & DPPH, ABTS, and hydroxyl radicals scavenging. & LLPF & [24] \\ \hline Oyster (_Saccostra cucultata_) & DPPH radicals scavenging, & LANAK, & \\  & Inhibiting human colon carcinoma & PSLVGRPVGKLTL, & [25] \\  & (HT-29) cell lines. & VKVLEHEHVL & \\ \hline Corp & Preventing hepatic fibrosis. & LLPF, FLPE & [26] \\ \hline \hline \end{tabular} Note: ref. \({}^{*}\) -reference.  
\end{table} 
Table 1: Sources, evaluation assays, and amino acid sequences of antioxidant peptides. be responsible for inhibiting electron migration (EM) and hydrogen atom transfer (HAT).