## 4 Discussion  
Previous reports indicated that low molecular weight peptides were better than high molecular weight peptides in donating hydrogen atoms for ABTSe+ reduction, i.e., the low molecular weight peptide fractions could have the superior ABTSe+ scavenging ability to the high molecular weight peptide fractions [12]. For example, [PERSON] et al. [13] indicated that the smaller-sized peptides (<3 kDa) had a stronger potency as ABTSe+ scavengers when compared with the larger-sized peptides (>3 kDa). [PERSON] et al. [14] demonstrated that smaller peptides (<3 kDa) from shortin scad (Decapterus macrosoma) muscle protein hydrolysate possessed stronger antioxidant activity than the larger peptides (>3 kDa). [PERSON] et al. [15] showed that the corn germ protein hydrolysate fraction with the highest ABTSe+ scavenging activity was the fraction containing low molecular weight peptides. In this study, SIM protein hydrolysates exhibited stronger antioxidant activity than the crude protein. In particular, the protamex-treated (SPr) and the alkaline protease-treated (SAI) hydrolysates had the strongest ABTS free radicals scavenging activity. Moreover, the inhibition activities of SPr and SAI were also the highest, at the concentration of 2.5 mg/mL, and their inhibition rates were 74.15 \(\pm\) 0.68% and 56.38 \(\pm\) 1.51%, respectively, with the IC\({}_{50}\) values of 1.007 mg/mL and 2.130 mg/mL, respectively. According to Table 1, SPr and Sal contained much more small peptides (<0.5 kDa), up to 69.28 \(\pm\) 0.11% and 79.97 \(\pm\) 0.07%, respectively, compared with other hydrolysates. This means that the present observation was similar to previous reports, confirming that lower molecular weight peptides could tend to provide greater ABTS radical scavenging activity than higher molecular weight peptides.  
On the other hand, one of the most crucial factors that affects the bioactivity of protein hydrolysates is that the species of the used enzyme, depending on which functional groups the enzyme cleave to, in turn, increases bioactivity. For example, the peptides containing Pro, Gly, Ala, Val and Leu in their peptides or having Arg at the third amino acid position next to the N-terminus exhibited strong antioxidant activity [16; 17]. The peptides with branched chains (such as Lys, Phe, Tyr and Trp) and cationic residues were considered to be strong alpha-amylase inhibitors [18]. Gly or Phe at the N-terminal and Phe or Leu at the C-terminal were considered to be important for alpha-amylase inhibition [19]. [PERSON] et al. [20] indicated that the alcalase-digested Vigna unguiculata hydrolysate had the highest DPP-IV inhibition (IC\({}_{50}\) = 0.58 mg soluble protein /mL). [PERSON] et al. [21] compared alcalase- and pepsin-treated oilseed protein hydrolysates and demonstrated that alcalase-treated whey protein hydrolysate exhibited the strongest DPP-IV inhibition (43.9 \(\pm\) 6.0% at 10 mg/mL). [PERSON] et al. [22] showed that the alcalase-derived hydrolysate from walnut (_Juglas regia_ L.) protein exhibited better DPP-IV inhibitory activity of 33.90% (at 0.50 mg/mL). The present data showed that the alkaline protease-treated (SAI) hydrolysates also had stronger DPP-IV inhibitory activity. This suggests that alkaline protease could be a good choice for generating DPP-IV inhibitory hydrolysate. This was probably attributed to the fact that the cleavage sites of alkaline proteases are mainly peptide bonds with aromatic or hydrophobic amino acids on the carboxyl side hydrophobic N-terminal, and the hydrophobic N-terminal such as tryptophan, and proline or alanine were important features of DPP-IV inhibitory peptides [23]. Moreover, small peptides could be absorbed via mucosa of the small intestine [24], so the fractions with molecular weight <3 kDa are suitable for functional food applications.