## 2 Results and Discussion
### Antioxidant Activity
#### 2.4.1 Radical Scavenging Activity  
DPPPH\(\bullet\) Scavenging Activity  
DPPPH\(\bullet\) shows maximal absorbance at 517 nm in its oxidized form, and the absorbance wears off with the free radical accepting an electron [3]. As shown in Figure 8A, eight isolated peptides (TJP1 to TJP8) showed strong DPPH\(\bullet\) scavenging activities and there was also a positive correlation between the concentration and the radical-scavenging activity. The half elimination ratio (EC\({}_{50}\)) values of TJP3, TJP4, and TJP8 were 0.902, 0.626, and 0.663 mg/mL, respectively, and TJP4 exhibited the highest  
Figure 10: Lipid peroxidation inhibition assays of eight isolated peptides (TJP1 to TJP8) from protein hydrolysate of hairtail (_T. japonica_s) muscle. All data are presented as the mean \(\pm\) SD of triplicate results. Figure 9: Reducing power of eight isolated peptides (TJP1 to TJP8) from protein hydrolysate of hairtail (_T. japonica_s) muscle. All data are presented as the mean \(\pm\) SD of triplicate results. DPPH\(\bullet\) scavenging ability among eight isolated peptides, but its activity was still lower than that of the positive control of glutathione (GSH) at the same concentration. The EC\({}_{50}\) value of TJP4 was lower than those of most antioxidant peptides from protein hydrolyses of loach (PSYV: 17.0 mg/mL) [39], miuay croaker (GIEWA: 0.78 mg/mL) [6], scalloped hammerhead cartilage (GPE: 2.43 mg/mL; GARGPQ: 2.66 mg/mL; GFTGPGFNNG: 1.99 mg/mL) [12], blue mussel (YPPAK: 2.62 mg/mL) [26], blueinf leatherjacket (GPP: 1.927 mg/mL; WEGPK: 4.438 mg/mL; GVPLT: 4.541 mg/mL) [8;14], skate carlages (FIMGPY: 2.60 mg/mL; GPAGDY: 3.48 mg/mL; IVAGPQ: 3.93 mg/mL) [24], grass carp skin (GFGPL: 2.249 mg/mL; VGGRP: 2.937 mg/mL) [40], and salmon pectoral fin (TTANIEDRR: 2.503 mg/mL) [41]. However, the EC\({}_{50}\) value of TJP4 was higher than those of peptides from protein hydrolyses of Chinese leek (GSQ: 0.61 mg/mL) [42], miuay croaker (FPYLRH: 0.51 mg/mL) [6], grass carp skin (HFGBPFH: 0.20 mg/mL) [40], and skate muscle (APPTAYAQS: 0.614 mg/mL; NWDMEKIWD 0.289 mg/mL) [10]. Therefore, these data indicated that eight isolated peptides (TJP1 to TJP8), especially TJP3, TJP4, and TJP8 had the strong capacity to contribute an electron or hydrogen radical for suppressing the DPPH\(\bullet\) reaction. HO\(\bullet\) Scavenging Activity  
HO\(\bullet\) is a highly reactive radical to the organism and only can be eliminated by endogenous and dietary antioxidants. Therefore, it is the ideal assay for searching the radical scavenging agent of organism. The abilities of eight isolated peptides (TJP1 to TJP8) were investigated, and the dose-related effects were observed at different peptide concentrations ranging from 0 to 10.0 mg/mL (Figure 8B). EC\({}_{50}\) values of TJP3, TJP4, and TJP8 were 1.740, 2.378, and 2.498 mg/mL, respectively, and TJP3 exhibited the highest HO\(\bullet\) scavenging ability among all isolated peptides at the same concentration. EC\({}_{50}\) value of TJP3 was lower than those of peptides from protein hydrolyses of blueinf leatherjacket heads (WEGPK: 5.567 mg/mL; GPP: 2.385 mg/mL; GVPLT: 4.149 mg/mL) [8], weatherfish loach (PSYV: 2.64 mg/mL) [39], skate carlides (FIMGPY: 3.04 mg/mL; GPAGDY: 3.92 mg/mL; IVAGPQ: 5.03 mg/mL) [24], and grass carp skin (PYSFK: 2.283 mg/mL; VGGRP: 2.055 mg/mL) [40]. However, the EC\({}_{50}\) value of TJP3 was higher than those of antioxidant peptides from protein hydrolyses of miuay croaker (FPYLRH: 0.68 mg/mL; GIEWA: 0.71 mg/mL) [6], spotless smoothness cartilage (GAERP: 0.25 mg/mL; GEREANVM: 0.34 mg/mL; AEVG: 0.06 mg/mL) [3], blue mussel (YPPAK: 0.228 mg/mL) [26], giant squid (NGLEGLK: 0.313 mg/mL; NADFGLNIGLEGLA: 0.612 mg/mL) [43], longer eel (LGLNGDDVN: 0.687 mg/mL) [44], and skate muscle (APPTAYAQS: 0.390 mg/mL; NWDMEKIWD 0.176 mg/mL) [10]. TJP3, TJP4, and TJP8 showed strong HO\(\bullet\) scavenging ability, which indicated that it could serve as a HO\(\bullet\) scavenger for decreasing or eliminating the damage caused by HO\(\bullet\) in food industries and biological systems.