## 2 Results and Discussion
### Antioxidant Activity
#### 2.3.1 Radical Scavenging Activity
In addition, HO\(\bullet\) cannot be eliminated by an enzymatic reaction, and scavenging it from organisms using antioxidants, such as glutathione, sulforaphene, a-tocopherol, curcumin, and ascorbic acid is the only mean to protect important cellular structures from its damage [22, 47]. Therefore, four isolated APs, especially RSHP-B, showed a high scavenging ability on HO\(\bullet\) and it might have the possibility to serve as a scavenger to weaken the HO\(\bullet\) damage in biological systems. The O\({}_{2}^{-}\bullet\) scavenging activities of four isolated APs (RSHP-A-D) were studied, and the dose-effect relations were observed when their concentrations gradually increased from 0.1 to 5.0 mg/mL (Figure 6C). The EC\({}_{50}\) values of RSHP-A, RSHP-B, RSHP-C, and RSHP-D were 0.08, 0.17, 0.15, and 0.16 mg/mL, respectively (Table 1). RSHP-A showed stronger O\({}_{2}^{-}\bullet\) scavenging activity than RSHP-B, RSHP-C, and RSHP-D did. The EC\({}_{50}\) value of RSHP-A was lower than those of APs from protein hydrolysates of skipjack tuna bone (GADIVA: 0.52 mg/mL; GAEGFIF: 0.48 mg/mL) [40], miuity croaker swim bladder (FPYLRH: 0.34 mg/mL; GIEWA: 0.30 mg/mL) [39], hairtail muscle (KA: 2.08 mg/mL; AKG: 2.54 mg/mL; IYG: 1.36 mg/mL) [44], mussel sauce (HFGDPFH: 0.20 mg/mL) [43], _Mytilus coruscus_ (SLPIGLIMIAM: 0.32 mg/mL) [48], round scad (HDHDPVC: 0.27 mg/mL; HEKVC: 0.24 mg/mL) [49], miuuy croaker muscle (FWKVV: 0.29 mg/mL; FMPLH: 0.15 mg/mL) [9], croceine croaker muscle (YLMR: 0.45 mg/mL; VIYEE: 0.69 mg/mL; MILMR: 0.99 mg/mL) [34, 50], and skate cartilage (FIMGPY: 1.61 mg/mL; GPAGDY: 1.66 mg/mL; IVAGPQ: 1.82 mg/mL) [21]. However, the EC\({}_{50}\) value of RSHP-A was higher than those of the APs from protein hydrolysates of blue mussel (YPPAK: 0.072 mg/mL) [3] and monkfish muscle (LMGQW: 0.042 mg/mL) [51]. O\({}_{2}^{-}\bullet\) is the most common free radical that is generated _in vivo_, and it can promote oxidative reaction to generate hydrogen peroxide and hydroxyl radical [22]. O\({}_{2}^{-}\bullet\) and its derivative radicals seriously threaten the bioactive biomacromolecules and cause different diseases of vital organs of the human body [1, 30]. For example, O\({}_{2}^{-}\bullet\) plays crucial roles in cardiovascular and neurodegenerative diseases, as well as in cancer [47]. In organism, SOD can timely catalyze O\({}_{2}^{-}\bullet\) into uninjurious hydrogen peroxide and oxygen under normal conditions. Subsequently, RSHP-A, RSHP-B, RSHP-C, and RSHP-D have high O\({}_{2}^{-}\bullet\) scavenging activity and might be applied to eliminate the O\({}_{2}^{-}\bullet\) damage together with SOD in biological systems. The abilities of four isolated APs (RSHP-A-D) to scavenge ABTS\({}^{+}\bullet\) in comparison with ascorbic acid were investigated, and dose-related effects were observed at peptide concentrations that ranged from 0.1 to 5.0 mg/mL (Figure 6D). The EC\({}_{50}\) values of RSHP-A, RSHP-B, RSHP-C, and RSHP-D were 0.15, 0.11, 0.19, and 0.18 mg/mL, respectively (Table 1). RSHP-B showed the strongest scavenging activity among four isolated APs at all tested concentrations. The EC\({}_{50}\) value of RSHP-B was lower than those of APs from the protein hydrolyses of croceine croaker muscle (VLYEE: 0.31 mg/mL; YLMSR: 0.42 mg/mL; MILMR: 0.89 mg/mL) [34], hairtail muscle (KA: 1.65 mg/mL; AKG: 0.83 mg/mL; IYG: 0.59 mg/mL) [44], scalloped hammerhead muscle (WDR: 0.34 mg/mL; LDK (0.19 mg/mL) [37; 38], salmon (FLNEFLFHV: 1.55 mg/mL) [3], grass carp skin (GFSPL: 0.33 mg/mL; VGGRP: 0.47 mg/mL) [36], skipjack tuna bone (GADIVA: 0.41 mg/mL; GAEGFIF: 0.21 mg/mL) [40], blueinf leatherjacket head (WEGPK: 5.41 mg/mL; GPP: 2.47 mg/mL; GVPLT: 3.12 mg/mL) [30], and skate cartilage (FIMGPY: 1.04 mg/mL; GPAGDY: 0.77 mg/mL; IVAGPG: 1.29 mg/mL) [20]. ABTS\({}^{+}\bullet\) scavenging assay has been successfully used to evaluate the capacity of dietary antioxidant ingredients, such as polyphenols, polysaccharides, and peptides based on the special chemical properties of formed free radicals [22; 38].