## 2 Results and Discussion
### Antioxidant Activity
#### 2.3.1 Radical Scavenging Activity  
DPPH\(\bullet\) Scavenging Activity  
DPPH, when reacted with an antioxidant, can generate the hydrazine DPPH, and the purple DPPH solution, with the maximum absorption wavelength 515 to 528 nm, will change to yellow [20; 21]. As shown in Figure 3A, eight isolated collagen peptides (F7-P1 to F7-P8) scavenged DPPH\(\bullet\) in a concentration-dependent manner, but they showed lower activity than the positive control (glutathione (GSH)). The EC\({}_{50}\) values of F7-P3 and F7-P8 were \(0.80\pm 0.09\) mg/mL and \(0.72\pm 0.06\) mg/mL, respectively, which were significantly lower than the EC\({}_{50}\) values of other six collagen peptides (\(p<0.05\)). Moreover, EC\({}_{50}\) values of F7-P3 and F7-P8 were significantly less than those of peptides from blue mussel (PYSFK-1.575 mg/mL; YPPAK: 2.62 mg/mL; FLNEFLHV: 4.950 mg/mL) [7; 22], bluefin leather jacket heads (GPP: 1.927 mg/mL; GVPLT: 4.541 mg/mL; WEGPK: 4.438 mg/mL) [11], salmon pectoral fin (TTANIEDRR: 2.503 mg/mL) [23], loach (PSYV: 17.0 mg/mL) [24], skins of grass carp (GFGPL: 2.249 mg/mL; VGGRP: 2.937 mg/mL) [25], swim bladders of miuuy croaker (FTGMD: 2.22 mg/mL; YLPYA: 3.63 mg/mL; GFYAA: 5.02 mg/mL; FSGLR: 4.01 mg/mL; VPDDD: 2.87 mg/mL) [1], and cartilage of _Raja porosa_ (GPAGDY: 3.48 mg/mL; IVAGPQ: 3.93 mg/mL, FIMGPY: 2.60 mg/mL) [5]. Those results indicated that eight isolated collagen peptides (F7-P1 to F7-P8) from Spanish mackerel skins, especially F7-P3 and F7-P8 can work as hydrogen donors and radical scavengers to inhibit the DPPH\(\bullet\) chain reaction. HO\(\bullet\) Scavenging Activity  
As the most ROS, HO\(\bullet\) is involved in the induction of lipid peroxidation in the biological membrane, and it also has the ability to initiate carcinogenesis, mutagenesis, and cytotoxicity due to its effective reaction with biomacromolecules [7]. Figure 3B shows that eight isolated collagen peptides (F7-P1 to F7-P8) could effectively clear HO\(\bullet\) in a dose-response relationship, and the HO\(\bullet\) scavenging activities of F7-P3, F7-P5, and F7-P8 were the significantly better that those of the other six collagen peptides (\(p<0.05\)), but weaker than that of GSH at the tested concentrations. EC\({}_{50}\) values of F7-P3, F7-P5, and F7-P8 were \(0.81\pm 0.05\), \(0.66\pm 0.08\), and \(0.88\pm 0.09\) mg/mL, respectively. Those data were significantly lower than those of the other five collagen peptides (\(p<0.05\)) (Table 2). Furthermore, EC\({}_{50}\) values F7-P3, F7-P5, and F7-P8 were less than those of peptides from bluefin leather jacket heads (WEGPK: 5.567 mg/mL; GPP: 2.385 mg/mL; GVPLT: 4.149 mg/mL) [12], weatherfish loach (PSYV: 2.64 mg/mL) [24], skins of grass carp (PYSFK: 2.283 mg/mL; GFGPL: 1.612 mg/mL; VGGRP: 2.055 mg/mL) [25], swim bladders of miuuy croaker (FYKWP: 1.45 mg/mL; FTGMD: 2.31 mg/mL; YLPYA: 2.90 mg/mL; GFYAA: 2.35 mg/mL; FSGLR: 2.45 mg/mL; VPDDD: 2.85 mg/mL) [1], and cartilage of _Raja porosa_ (FIMGPY: 3.02 mg/mL; HYAGPQ: 5.03 mg/mL) [5]. Those data showed that eight isolated collagen peptides (F7-P1 to F7-P8) from Spanish mackerel skin could effectively scavenge HO\(\bullet\) to lessen or eliminate injury caused by HO\(\bullet\) in organisms.