## 2 Results and Discussion
### Antioxidant Activity  
#### 2.3.1 Radical Scavenging Activity  
The activity curves on DPPH\(\bullet\) scavenging of four isolated APs (RSHP-A-D) are shown in Figure 6A, and a positive correlation between the APs concentrations and their radical-scavenging activities was observed. The EC\({}_{50}\) value of RSHP-B was 1.90 mg/mL, which was significantly (\(p<0.05\)) higher than those of RSHP-A (4.61 mg/mL), RSHP-C (3.69 mg/mL), and RSHP-D (4.01 mg/mL), respectively (Table 1). Furthermore, the EC\({}_{50}\) value of RSHP-B was lower than those of the APs from protein hydrolysates of croceine croaker muscle (YLMSR: 2.74 mg/mL; MILMR: 5.01 mg/mL) [34], loach (PSYV: 17.0 mg/mL) [35], blue mussel (TTANIEDRR: 2.50 mg/mL) [31], salmon (FLNEFLHV: 4.95 mg/mL) [3], blueinf leatherjacket head (WEGPK: 4.44 mg/mL; GVPL: 4.54 mg/mL) [30], grass carp skin (GFGPL: 2.25 mg/mL; VGGRP: 2.94 mg/mL) [36], skate cartilage (FIMGPY: 2.60 mg/mL; GPAGDY: 3.48 mg/mL; IVAGPQ: 3.93 mg/mL) [21], and scalloped hammerhead muscle (WDR: 3.63 mg/mL; PYFNK: 4.11 mg/mL; LDK: 3.06 mg/mL) [37; 38]. However, the EC\({}_{50}\) value of RSHP-B was higher than those of APs from protein hydrolysates of miuuy croaker swim bladder (FPYLRH: 0.51 mg/mL; GIEWA: 0.78 mg/mL) [39], blue mussel (PIIYVWK: 0.71 mg/mL; FSVVPSPK: 0.94 mg/mL) [31], skipjack tuna bone (GADIVA: 0.57 mg/mL; GAEGFIF: 0.30 mg/mL) [40], grass carp skin (PYSFK: 1.58 mg/mL) [36], and blueinf leatherjacket skin (GSGGL: 0.41 mg/mL; GPGGFI: 0.19 mg/mL; FIGP: 0.12 mg/mL) [41]. The radical-quenching activities of APs are due to their ability to participate in the single electron transfer reaction [1]. The present results illustrated that RSHP-B have a strong ability to act as a free radical scavenger or hydrogen donor to transform DPPH\(\bullet\) into harmless compounds and to prevent the single electron transfer reaction. The HO\(\bullet\) scavenging activity of four isolated APs (RSHP-A-D) was dose-dependent at the test concentrations (Figure 6B). RSHP-B exhibited the highest HO\(\bullet\) scavenging activity, with an EC\({}_{50}\) value of 0.45 mg/mL, which was significantly (\(p<0.05\)) stronger than those of RSHP-A (0.76 mg/mL), RSHP-C  
Figure 6: DPPH\(\bullet\) (**A**), HO\(\bullet\) (**B**), O\({}_{2}^{-}\bullet\) (**C**), and ABTS\({}^{+}\bullet\) (**D**) scavenging activities of RSHP-A, RSHP-B, RSHP-C, and RSHP-D from protein hydrolysate of red stingray (_D. akajei_) cartilages. All data are presented as mean \(\pm\) SD (\(n=3\)). (0.70 mg/mL), and RSHP-D (1.30 mg/mL), respectively (Table 1). The EC\({}_{50}\) value of RSHP-B was lower than those of APs from the protein hydrolysates of copper eel (LGLNLGDDVN: 0.69 mg/mL) [42], weatherfish loach (PSYV: 2.64 mg/mL) [35], miuity croaker muscle (FWKVV: 0.97 mg/mL; FMPLH: 0.80 mg/mL) [9], mussel sauce (HFGDPFH: 0.50 mg/mL) [43], blue mussel (YPPAK 0.23 mg/mL) [2], hairtail muscle (KA: 1.74 mg/mL; AKG: 2.38 mg/mL; IYG: 2.50 mg/mL) [44], grass carp (PSKYEPFV: 2.86 mg/mL; PYSFK: 2.28 mg/mL; GFGPL: 1.61 mg/mL; VGGRP: 2.06 mg/mL) [36, 45], miuuy croaker swim bladder (FPYLRH: 0.68 mg/mL; GIEWA: 0.71 mg/mL) [39], giant squid (NADFGLNLGELA: 0.61 mg/mL) [43], and skate cartilage (FIMGPY: 3.04 mg/mL; GPAGDY: 3.92 mg/mL; IVAGPQ: 5.03 mg/mL) [21]. However, the EC\({}_{50}\) value of RSHP-B was higher than those of the APs from protein hydrolysates of scalloped hammerhead muscle (PYFNK: 0.24 mg/mL; LDK: 0.17 mg/mL) [37, 38], skipjack tuna bone (GADIVA: 0.25 mg/mL; GAEGFIF: 0.32 mg/mL) [40], blue mussel (YPPAK: 0.23 mg/mL) [3], blueinf leather jacket skin (GSGGL: 0.18 mg/mL; GPGGFI: 0.09 mg/mL; FIGP: 0.07 mg/mL) [41], and giant squid (NGLEGLK: 0.31 mg/mL) [43]. HO\(\bullet\) is one of the most deleterious intracellular ROS and it can cause severe damage to biomacromolecules, including carbohydrates, nucleic acids (mutations), lipids (lipid peroxidation), and amino acids (e.g., conversion of Phe to m-Tyrosine and o-Tyrosine), which further causes a series of chronic diseases [46].