## 2 Results and Discussion
### Antioxidant Activity
#### 2.3.1 Radical Scavenging Activity
\begin{table}
\begin{tabular}{l c c c c} \hline  & \multicolumn{4}{c}{**EC\({}_{50}\) (mg/mL)**} \\ \cline{2-5}  & **DPPH\(\bullet\)** & **HO\(\bullet\)** & **O\({}_{2}^{\bullet}\)** & **ABTS\({}^{+}\bullet\)** \\ \hline F7-P1 & \(1.01\pm 0.09\)\({}^{\text{a}}\) & \(3.22\pm 0.21\)\({}^{\text{a}}\) & \(3.98\pm 0.26\)\({}^{\text{a}}\) & \(2.12\pm 0.16\)\({}^{\text{a}}\) \\ F7-P2 & \(1.46\pm 0.12\)\({}^{\text{b}}\) & \(2.13\pm 0.15\)\({}^{\text{b}}\) & \(2.96\pm 0.19\)\({}^{\text{b}}\) & \(4.51\pm 0.24\)\({}^{\text{b}}\) \\ F7-P3 & \(0.80\pm 0.09\)\({}^{\text{a}}\) & \(0.81\pm 0.05\)\({}^{\text{c}}\) & \(0.91\pm 0.08\)\({}^{\text{c}}\) & \(0.86\pm 0.05\)\({}^{\text{c}}\) \\ F7-P4 & \(4.63\pm 0.21\)\({}^{\text{c}}\) & \(2.27\pm 0.14\)\({}^{\text{b}}\) & \(1.73\pm 0.11\)\({}^{\text{d}}\) & \(3.27\pm 0.27\)\({}^{\text{d}}\) \\ F7-P5 & \(3.02\pm 0.19\)\({}^{\text{d}}\) & \(0.66\pm 0.08\)\({}^{\text{c}}\) & \(0.80\pm 0.06\)\({}^{\text{c}}\) & \(1.07\pm 0.10\)\({}^{\text{c}}\) \\ F7-P6 & \(3.65\pm 0.30\)\({}^{\text{e}}\) & \(2.11\pm 0.16\)\({}^{\text{b}}\) & \(7.21\pm 0.27\)\({}^{\text{e}}\) & \(3.86\pm 0.19\)\({}^{\text{e}}\) \\ F7-P7 & \(7.93\pm 0.52\)\({}^{\text{f}}\) & \(5.24\pm 0.32\)\({}^{\text{d}}\) & \(9.83\pm 0.35\)\({}^{\text{f}}\) & \(8.54\pm 0.43\)\({}^{\text{f}}\) \\ F7-P8 & \(0.72\pm 0.06\)\({}^{\text{a}}\) & \(0.88\pm 0.09\)\({}^{\text{c}}\) & \(0.73\pm 0.06\)\({}^{\text{c}}\) & \(0.82\pm 0.04\)\({}^{\text{c}}\) \\ \hline \end{tabular} All data are presented as the mean \(\pm\) SD of triplicate results. \({}^{\text{a-f}}\) Values with same letters indicate no significant difference of different sample at same radicals (\(p>0.05\)).  
\end{table} 
Table 2: EC\({}_{50}\) values of eight antioxidant peptides (F7-P1 to F7-P8) from collagen hydrolysate fraction (F7) of Spanish mackerel (_S. nipindius_) skin on four kinds of free radicals. \(\text{O}_{2}^{-}\bullet\) is a main cause of oxidative stress because it can facilitate oxidative reaction to bring forth hydrogen peroxide and HO\(\bullet\) to attack susceptible biological targets including lipids, proteins, and nucleic acids, and alter their physiological function [13]. The data in Figure 3C indicated eight isolated collagen peptides (F7-P1 to F7-P8) that could positively scavene \(\text{O}_{2}^{-}\bullet\) in a dose-effect relation. As presented in Table 2, EC\({}_{50}\) values of F7-P3 (0.91 \(\pm\) 0.08 mg/mL), F7-P5 (0.80 \(\pm\) 0.06 mg/mL), and F7-P8 (0.73 \(\pm\) 0.06 mg/mL) were less than 1.0 mg/mL. Those data were significantly lower than the other six isolated collagen peptides (\(p<0.05\)). However, the \(\text{O}_{2}^{-}\bullet\) scavenging activity of eight isolated collagen peptides (F7-P1 to F7-P8) were lower than that of GSH at the tested concentrations. EC\({}_{50}\) values of F7-P3 (0.91 \(\pm\) 0.08 mg/mL), F7-P5 (0.80 \(\pm\) 0.06 mg/mL), and F7-P8 (0.73 \(\pm\) 0.06 mg/mL) were lower than those of APs from croceireoaker muscle (MILMR: 0.993 mg/mL) [3], cartilage of _Raja porosa_ (GPAGDY: 1.66 mg/mL; FIMGPY: 1.61 mg/mL; IVAGPQ: 1.82 mg/mL) [5], bluefin leatherjacket heads (WEGPK: 3.223 mg/mL; GPP: 4.668 mg/mL; GVPLT: 2.881 mg/mL) [12], and swim bladders of miuuy croaker (FYKWP: 1.92 mg/mL; FTGMD: 3.04 mg/mL; YLPYA: 3.61 mg/mL; GFYAA: 3.03 mg/mL; FSGLR: 3.35 mg/mL; VPDDD:4.11 mg/mL) [1]. In living organisms, superoxide dismutase (SOD) protects the cell from the deleterious effects of superoxides. Therefore, eight isolated collagen peptides (F7-P1 to F7-P8), especially F7-P3, F7-P5, and F7-P8 might be used as the \(\text{O}_{2}^{-}\bullet\) scavenger to help SOD to reduce \(\text{O}_{2}^{-}\bullet\) damage in cells. ABTS\({}^{+}\bullet\) Scavenging Activity  
ABTS\({}^{+}\bullet\) scavenging assay is widely applied to detect the anti-radical ability of peptides [2]. In the measurement process, ABTS is oxidized by potassium persulfate to generate the blue/green ABTS\({}^{+}\bullet\) with an absorption maximum of 734 nm.