## 3 Discussion  
At present, there is still lack of sufficient evidence to illuminate the relationship between the structure characteristics of peptides and their antioxidant properties to each mechanistic action. In general, molecular size, hydrophobicity, amino acid composition, and sequence are believed to play an essential role in antioxidant activity of peptides [20]. Figure 9: The effects of MCPE-A, MCPE-B and MCPE-C on the levels of dismutase (T-SOD) (**A**), catalase (CAT) (**B**), glutathione peroxidase (GSH-Px) (**C**), glutathione reductase (GSH-Rx) (**D**), and content of malondialdehydehyde (MDA) (**E**) in H\({}_{2}\)O\({}_{2}\)-induced HepG2 cells. All data are presented as the mean \(\pm\) SD of triplicate results. \({}^{\#}p\) < 0.05 vs. the control group. \({}^{*}p\) < 0.05 vs. the H\({}_{2}\)O\({}_{2}\) treated group. Acidic and basic amino acids play a critical role in metal ion chelating and HO\(\bullet\) scavenging activities of peptide, which is related to carboxyl and amino groups in their side chains [3; 24]. Similar results were also reported by [PERSON] et al. who found that basic (arginine, Arg) and acidic (Glu and aspartic acid (Asp)) amino acid residues in the sequences of NTDGSTDYGILQINSR and LDEPDPLI were critical for their antioxidant activities [44]. Therefore, Glu should be one favorable factor for the antioxidant activities of MCPE-A, MCPE-B, and MCPE-C; and Arg might have positive effect on the antioxidant activities of MCPE-A and MCPE-B. [PERSON] et al. reported that glycine (Gly) residue might contribute significantly to antioxidant activity since the single hydrogen atom in the side chain of Gly serves as protons, and thus neutralises active free radical species [45]. [PERSON], [PERSON], & [PERSON] reported that the single hydrogen atom of the Gly could provide a high flexibility to the peptide backbone and positively influence the antioxidant properties [46]. Therefore, Gly residues in MCPE-A, MCPE-B, and MCPE-C might help the peptides easier to contact with the free radicals and donate proton to terminate the oxidation reaction. Hydrophobic amino acids including proline (Pro) and Met in peptide sequences are believed to play an important role in scavenging free radicals because their large hydrophobic group can help them to facilitate the contacts with hydrophobic radical species [47]. Pyrrolidine ring of Pro could increase the flexibility of peptides and also be capable of quenching singlet oxygen due to its low ionization potential [3; 48]. [PERSON], [PERSON], [PERSON], & [PERSON] proved that Pro played an important role in the antioxidative activity of Pro-His-His [49]. [PERSON], [PERSON], [PERSON], & [PERSON] confirmed that tryptophan (Trp), Tyr, and Met showed the highest antioxidant activity among all the amino acids [50]. Therefore, Met residues might be the important contributor for the antioxidant activity of MCPE-B, and Pro residues might be the important contributor for the antioxidant activity of MCPE-A. In addition, antioxidant activities of peptides highly rely on their molecular size, and shorter size peptides especially peptides with 2-10 amino acid residues are deemed to obtain stronger radical scavenging and lipid peroxidation inhibition activities than their parent native proteins and long-chain peptides [3; 51]. In the test, MCPE-A, MCPE-B, and MCPE-C exhibited good antioxidant activities on radical scavenging and lipid peroxidation inhibition assays, which suggested that shorter size of MCPE-A, MCPE-B, and MCPE-C could interact more effectively and easily with free radicals and inhibit the propagation cycles of lipid peroxidation in the radical scavenging and lipid peroxidation model system [32]. But, in the experiment, MCPE-A showed the strongest O\({}_{2}^{-}\bullet\) scavenging activity, MCPE-B showed the strongest scavenging activity on DPPH\(\bullet\), ABTS\({}^{+}\bullet\), and lipid peroxidation inhibition activities, and MCPE-C exhibited the highest HO\(\bullet\) scavenging activity among all the samples and fractions.