## 3 Results and Discussion
### Antioxidant Activity
#### 3.4.1 Radical Scavenging Activity  
DPPH Radical Scavenging Activity  
Figure 6A indicated that six isolated APs (P1 to P6) can positively influence the DPPH radical scavenging rates when the concentrations of APs ranged from 0.05 to 5.0 mg/mL. Among six isolated APs, the EC\({}_{50}\) value of P2 (0.31 mg/mL), P4 (0.33 mg/mL), P5 (0.93 mg/mL)), and P6 (0.46 mg/mL) was less than 1.0 mg/mL (Table 2). Moreover, the EC\({}_{50}\) value of P2 and P4 did not show a significant difference  
\begin{table}
\begin{tabular}{l c c c} \hline \hline
**No.** & **RT (min)** & **Theoretical Mass/Observed Mass (Da)** & **Amino Acid Sequence** \\ \hline P1 & 9.05 & 375.37/375.39 & VEE \\ P2 & 11.36 & 783.89/783.90 & WMFDW \\ P3 & 12.63 & 788.84/788.85 & DAGPYGPI \\ P4 & 13.25 & 652.76/652.79 & WMGPY \\ P5 & 16.78 & 861.94/861.95 & ERGPLGPH \\ P6 & 17.22 & 503.57/503.58 & EMGPA \\ \hline \hline \end{tabular}
\end{table}
Table 1: Retention time (RT), molecular mass (Da), and amino acid sequences of six isolated antioxidant peptides (APs) (P1 to P6) from the protein hydrolysate of skipjack tuna heads.  
Figure 5: Mass spectrogram of P1 (**A**), P2 (**B**), P3 (**C**), P4 (**D**), P5 (**E**), and P6 (**F**) from the hydrolysate of skipjack tuna (_K. pelamis_) heads.  
with the positive control of GSH (\(p>0.05\)). The EC\({}_{50}\) values of P2 and P4 were lower than those of most APs from protein hydrolysates of aquatics and their processing by-products, such as blue mussel (_M. edulis_) (YPPAK: 2.62 mg/mL) [31], skate cartilages (FIMGPY: 2.60 mg/mL; GPAGDY: 3.48 mg/mL; IVAGPQ: 3.93 mg/mL) [3], the pectoral fin of salmon (TTANIEDRR: 2.50 mg/mL) [36], spanish mackerel skins (PFGPD: 0.80 mg/mL; PYGAKG: 3.02 mg/mL; YGPM: 0.72 mg/mL) [37], _Tergillarca granosa_ muscle (MDLFTE: 0.53 mg/mL; WPDP: 0.36 mg/mL) [34], miuuy croaker swim bladders (FPYLRH: 0.78 mg/mL; GIEWA: 0.78 mg/mL) [8], croceine croaker muscle (YLMSR: 1.35 mg/mL) [38], and loach (PSYV: 17.0 mg/mL) [39]. Nevertheless, the EC\({}_{50}\) values of P2 and P4 were greater than those of APs from the hydrolysates of grass carp skin (HFGBPFFH: 0.20 mg/mL) [40], bluefin leatherjacket skin (FIGP: 0.12 mg/mL) [12], and skate muscle (NWDMEKIWD: 0.29 mg/mL) [41]. Then, six isolated APs (P1 to P6), especially P2 and P4, have a strong ability to inhibit the DPPH radical reaction through donating hydrogens or scavenging free radicals.  
Hydroxyl Radical Scavenging Activity  
Figure 6B revealed that P1, P2, P3, P4, P5, and P6 can concentration-dependently scavenge hydroxyl radicals with EC\({}_{50}\) values of 2.43, 0.30, 1.71, 0.43, 0.81, and 0.52 mg/mL, respectively (Table 2). P2 revealed the highest radical scavenging ability among six isolated APs, but its EC\({}_{50}\) value was none the less significantly less than that of GSH (0.12 mg/mL) (\(p<0.05\)). All the more important, the EC\({}_{50}\) value of P2 was lower than those of most APs from hydrolysates of the muscle of croceine croaker (YLMSR: 0.35 mg/mL) [38], miuuy croaker swim bladders (GFYA: 2.35 mg/mL; FSGLR: 2.45 mg/mL) [8], weatherfish loach (PSYV: 2.64 mg/mL) [39], conger eel (LGLNGDDVN: 0.69 mg/mL) [42], bluefin leatherjacket (GPP: 2.36 mg/mL) [6], giant squid (NADFGLNEGLLA: 0.61 mg/mL) [43], hairtail muscle (IYG: 2.50 mg/mL; AKG: 2.38 mg/mL; KA: 1.74 mg/mL) [44], and grass carp skin (PYSFK: 2.28 mg/mL; VGGRP: 2.06 mg/mL) [40]. Nevertheless, the EC\({}_{50}\) value of P2 was still greater than those of APs from bluefin leatherjacket skin (FIGP: 0.07 mg/mL) [12], _Sphyrna lewini_ muscle (SAP: 0.17 mg/mL; PYFNA: 0.24 mg/mL) [32; 33], and monkfish muscle (EWPAQ: 0.27 mg/mL; FLHRP: 0.11 mg/mL; LMGQW: 0.04 mg/mL) [45]. In the body, hydroxyl radicals can give rise to the process of oxidative stress to unselectively attack and oxidize biomacromolecules, and further cause some chronic diseases. The results indicated that P2 can serve as a hydroxyl radical eliminator to eliminate its injury in biological systems.