## 2 Results and Discussion
### Amino Acid Composition of SBP-III-3  
The bioactivities of protein hydrolysates and peptides were directly influenced by the amino acid compositions. [PERSON], [PERSON], [PERSON], and [PERSON] reported that Gly residue may contribute significantly to antioxidant activity since the single hydrogen atom in the side chain of Gly serves as a proton-donorand neutralizes active free radical species [38]. [PERSON], [PERSON], and [PERSON] also reported that the single hydrogen atom of Gly residue could provide a high flexibility to the peptide backbone and positively influence the antioxidant properties [39]. [PERSON] et al. reported that hydrophobic amino acid residues including Ala, Leu, and Met played an important role in scavenging free radicals because the large hydrophobic group could help them make contact with hydrophobic radical species [10]. The pyrimidine ring of the Pro residue can increase the flexibility of the peptides and also be capable of quenching singlet oxygen due to its low ionization potential [38]. As shown in Table 2, the contents of Gly, Ala, and Pro residues in SBP-III-3 were 35.10%, 13.46%, and 11.28%, respectively, which reached to 59.84% of the total content of amino acids. In addition, the aromatic amino acid, such as Phe, Trp, His, and Tyr, with imidazole groups were also proved to have the ability to quench free radicals by direct electron transfer [11], and acid and basic amino acid residues, including Glu and Asp, were identified to have strong abilities to chelate metal ions, as well as scavenge HO\(\bullet\). Therefore, SBP-III-3 was rich in amino acids with antioxidant activity, which should be one of the important factors of its high antioxidant activity.  
Amino acids were also found to play a key role in the regulatory metabolism involved in muscular activity. [PERSON] et al. reported that feeding horses with an amino acid-based supplement every day might support muscle development during exercise and promote exercise metabolism and recovery [40]. The essential amino acid (EAA) ingestion during the exercise could attenuate the degradation of myofibrillar protein, thereby enhancing the exercise capability [41]. The EAA content of SBP-III-3 is 15.7%, which might be beneficial to its anti-fatigue capability. Glu residue was proved to have positive influence to the nervous system and would also be beneficial during exercise [42]. [PERSON] et al. reported that Asp residue was advantageous in the oxidative deamination and could induce the blood ammonia concentration and postpone the occurrence of fatigue [43]. Therefore, SBP-III-3 contains 6.86% Glu residue and 4.54% Asp residue, suggesting that it might have a potential anti-fatigue effect. Furthermore, [PERSON] et al. reported that the content of amino acid residues, especially Gly, Ala, Val, Thr, Ile, Ser, and Tyr in the plasma will quickly reduce during an endurance test [44]. Table 3 showed that SBP-III-3 contained 57.72% of the above amino acid residues, which was higher than that of loach peptides (32.2%) with antioxidant activity and anti-fatigue effect [1]. The data indicated that these amino acids could enhance the exercise capability of SBP-III-3. In addition, the result of amino acid composition provided a basis for the good antioxidant and anti-fatigue capacities of SBP-III-3.