## 2 Results and Discussion
### Antioxidant Activity
#### 2.4.1 Radical Scavenging Activity  
DPPH is a stable and cell-permeable radical that is commonly applied to measure the antioxidant ability of peptides, which serve as hydrogen donor or free radical scavenger. The reaction of DPPH with an antioxidant peptide produces the corresponding hydrazine, DPPH2, with the color of the reaction solution changing from purple (absorbance at around 520 nm) to yellow [44]. As shown in Figure 5A, five antioxidant peptides (STP1-STP5) showed strong DPPH radical scavenging activities with a positive correlation between the concentration and the activity, but their activity was still lower than that of the positive control of GSH at the same concentration. The EC\({}_{50}\) value of STP5 was 0.30 mg/mL, which was significantly lower than those of STP1 (2.49 mg/mL), STP2 (0.57 mg/mL), STP3 (1.93 mg/mL), and STP4 (1.66 mg/mL), respectively. In addition, The EC\({}_{50}\) values of STP2 and STP5 were lower than those of most antioxidant peptides from protein hydrolyses of salmon pectoral fin (TTANIEDRR: 2.503 mg/mL) [45], skate cartilages (FIMGPY: 2.60 mg/mL; GPAGDY: 3.48 mg/mL; IVAGPQ: 3.93 mg/mL) [37], scalloped hammerhead cartilage (GPE: 2.43 mg/mL; GARGPQ: 2.66 mg/mL; GFTGPGPFING: 1.99 mg/mL) [46], swim bladders of miuiv croaker (GIEWA: 0.78 mg/mL) [47], and blue mussel (YPPAK: 2.62 mg/mL) [48]. However, the EC\({}_{50}\) values of STP2 and STP5 were higher than those of peptides from protein hydrolyses of grass carp skin (HFGBPFFH: 0.20 mg/mL) [49] and skate muscle (NWDMEKIWD 0.289 mg/mL) [50]. Therefore, five gelatin peptides (STP1 to STP5), especially STP2 and STP5, had a strong ability to serve as a hydrogen donor or free radical scavenger for preventing the DPPH radical reaction. Hydroxyl radicals are able to instantaneously attack and unselectively oxidize biomacromolecules, initiating the process of oxidative stress in an organism. Therefore, it is one of the important ways to search for antioxidant agents. As presented in Figure 5B, five peptides (STP1-STP5) showed dose-related effects in the scavenging activity of hydroxyl radicals at peptide concentrations ranging from 0 to 5.0 mg/mL. The EC\({}_{50}\) values of STP1, STP2, STP3, STP4, and STP5 were 1.21, 0.25, 0.64, 0.49, and 0.32 mg/mL, respectively, and STP2 exhibited the highest hydroxyl radical scavenging ability among all isolated peptides at the same concentration, but its activity was still lower than that of the positive control of GSH. The EC\({}_{50}\) values of STP2 and STP5 were lower than those of most peptides from protein hydrolyses of longer eel (LGLNGDDVN: 0.687 mg/mL) [51], giant squid (NADFGLNGLEGLA: 0.612 mg/mL) [52], swim bladders of miuiv croaker (FPYLRH: 0.68 mg/mL; GIEWA: 0.71 mg/mL) [47], grass carp skin (PYSFK: 2.283 mg/mL; VGGRP: 2.055 mg/mL) [49], and weatherfish loach (PSYV: 2.64 mg/mL) [53]. Nevertheless, EC\({}_{50}\) values of STP2 and STP5 were still higher than those of antioxidant peptides from spotless smoothhound cartilage (AEVG: 0.06 mg/mL) [54], blue mussel (YPPAK: 0.228 mg/mL) [48], and skate muscle (NWDMEKIWD 0.176 mg/mL) [50]. The data indicated that STP2 and STP5 could act as a scavenger to decrease the hydroxyl radical damage in biological systems. Superoxide anion Radical Scavenging Assay  
Superoxide anion radical can undergo fenton-chemistry and produce the highly reactive hydroxyl radical to inactivate enzymes with an iron-sulfur cluster, initiate lipid peroxidation, and react with carbonyl compounds to generate toxic peroxy radicals. Superoxide anion radicals are catalyzed into hydrogen peroxide and oxygen by superoxide dismutases (SOD) in an organism. Figure 5C indicates that five antioxidant peptides (STP1-STP5) showed strong superoxide anion radical scavenging activities in a dose-effect manner with EC\({}_{50}\) values of 1.48, 0.52, 0.67, 1.22, and 0.48 mg/mL, respectively, but their activities were still lower than that of GSH at a concentration ranging from 0.1 to 5.0 mg/mL.