## 4 Experimental Section
### Antioxidant Activity
#### 4.5.2 The HepG2 Cell Culture and Cytotoxicity Assay  
The HepG2 cells were cultured in Dulbecco's Modified Eagle's (DMEM) medium contained 10% FBS supplemented with 2 mM L-Glu and 1% penicillin-streptomycin solution at 37 \({}^{\circ}\)C and 5% CO\({}_{2}\) atmosphere. MTT test was used to measure the cytotoxicity of samples and the assay method was performed on the previous method [53]. After 24 h incubation in a 96-well plate (7 \(\times\) 10\({}^{3}\) cells/well), the HepG2 cells were cultured at the presence of different concentrations of peptide solution for 12 h. After that, the wells were washed twice with PBS and the MTT with final concentration of 0.5 mg/mL was added for an additional 4 h period. After that the formazan crystals formed by active cells were dissolved in 150 \(\upmu\)L of DMSO. The absorbance at 570 nm was measured and the cell viability was calculated by the following equation:  
\[\text{Cell viability}=(\text{A}_{\text{sample}}/\text{A}_{\text{control}}) \times 100\%\]  
5.3 H\({}_{2}\)O\({}_{2}\)-Induced Oxidative Damage and the Protection by MCPE-A, MCPE-B, and MCPE-C on HepG2 Cells  
The assay was performed on the previous method described by [PERSON] et al. [53]. The HepG2 cells were seeded on a 96-well plate with the density of 6 \(\times\) 10\({}^{4}\) cells/well. After culturing for 24 h, the supernatant was aspirated and H\({}_{2}\)O\({}_{2}\) (final concentration of 100, 200, 300, 400, 500, and 600 mM) was added into the experimental group and sequentially incubated for 24 h. MTT test was applied to determine the cell viability as described in Section 4.5.2, and the optimal H\({}_{2}\)O\({}_{2}\) concentration was confirmed when the cell viability was close to 50%.  
The oxidative damaged HepG2 cells under the H\({}_{2}\)O\({}_{2}\)-induced optimal conditions were used to test the protective effects of MCPE-A, MCPE-B, and MCPE-C on the damaged cells. The peptides were dissolved in the DMEM medium with concentrations of 10.0, 25.0, and 50.0 \(\upmu\)g/mL. The selected HepG2 cells were grown (6 \(\times\) 10\({}^{4}\) cells/well) in a 96-well plate for 24 h. Then the supernatant was aspirated and 100 \(\upmu\)L of peptide samples were added into the protection groups respectively incubating for 8 h. After removing peptide samples, H\({}_{2}\)O\({}_{2}\) was added into the damage and protection groups with the optimal concentration and sequentially incubated for optimal induced incubation time. Finally, the cell viability was measured.  
5.4 Determination of the Levels of Antioxidant Enzymes in H\({}_{2}\)O\({}_{2}\)-Induced HepG2 Cells  
The assay was performed on the previous method described by [PERSON] et al. [53]. HepG2 cells were cultured in 6-well plates (1 \(\times\) 10\({}^{6}\) cells/well). The isolated peptides (final concentration of 10.0, 25.0, and 50.0 \(\upmu\)g/mL) were added into the protection groups. At last the damage and protection groups were induced by H\({}_{2}\)O\({}_{2}\). Subsequently, 500 mL of cell lysis buffer was added into each well on ice lysed for 30 min and centrifuged at 12,000 \(\times\) \(g\), 4 \({}^{\circ}\)C for 10 min. The resulted liquid supernatant was followed cold standby at 4 \({}^{\circ}\)C (the indicators should be measured in 6 h). The levels of T-SOD, CAT, GSHPx, GSH-Rx, and MDA were measured using assay kits according to the protocols of manufacturer. Protein concentrations were determined using the bicinchoninic acid (BCA) method to normalize the levels of T-SOD, CAT, GSH-Px, GSH-Rx, and MDA. The results were expressed as units of enzymatic activity per milligram of protein (U/mg prot).