## 2 Results and Discussion
### Antioxidant Activity
#### Cytotoxicityicity of MCPE-A, MCPE-B, and MCPE-C in HepG2 Cells  
Cytotoxicity of compound measured by 3-(4,5-Dimethyl-2-thiazol)-2,5-diphenyl-2H-tetrazol-bromide (MTT) method is an important index for drug discovery, and it can be used to screen cytotoxic compounds for developing the anticancer pharmaceuticals or compound without cytotoxic effects before investing in their development as other pharmaceuticals. Cytotoxic effects of MCPE-A, MCPE-B, and MCPE-C at the concentrations of 10.0 ug/mL, 25.0 ug/mL, and 50.0 ug/mL were determined in HepG2 cells by the MTT assay. As shown in Figure 7, MCPE-A, MCPE-B, and MCPE-C didn't exhibited significantly cytotoxic effects on HepG2 cells compared to the control (no peptide treatment) at the tested concentrations for 24 h treatment (\(p<0.05\)). The result indicated that MCPE-A, MCPE-B, and MCPE-C from cartilage protein hydrolysate of spotless smoothhound might be the candidate compounds for antioxidant drugs. Protection of MCPE-A, MCPE-B, and MCPE-C on H\({}_{2}\)O\({}_{2}\)-Induced Oxidative Damage HepG2 Cells  
H\({}_{2}\)O\({}_{2}\) is a well-known powerful oxidizer and hepatotoxic chemical, and can be directly converted to hydroxyl radical and oxygen free radical. Therefore, H\({}_{2}\)O\({}_{2}\) is often used to establish the oxidative  
Figure 6: Lipid peroxidation inhibition assays of MCPE-A, MCPE-B and MCPE-C from cartilage protein hydrolysate of spotless smoothhound. All data are presented as the mean \(\pm\) SD of triplicate results. Figure 7: Cytotoxicityicity of MCPE-A, MCPE-B, and MCPE-C in HepG2 cells at concentrations of 10.0 ug/mL, 25.0 ug/mL, and 50.0 ug/mL, respectively. stress model in different cells to evaluate the antioxidant activity of compounds. As shown in Figure 8A, the viability of HepG2 cells gradually reduced by H\({}_{2}\)O\({}_{2}\) with the concentration increasing. The cell viability achieved 49.57% \(\pm\) 3.21% of control group at the H\({}_{2}\)O\({}_{2}\) concentration of 300 \(\upmu\)M for induced 24 h. Hence, the H\({}_{2}\)O\({}_{2}\) concentration of 300 \(\upmu\)M was selected for the following assays of peptides oxidative stress activity evaluation. Figure 8B showed the effects of MCPE-A, MCPE-B, and MCPE-C on H\({}_{2}\)O\({}_{2}\)-induced oxidative damage HepG2 cells, and the cell viability of MCPE-A, MCPE-B, and MCPE-C treated groups were improved comparing with the H\({}_{2}\)O\({}_{2}\) treated group without peptides. It's worth noting that the MCPE-A treated group increased the HepG2 cell viability to 78.39% \(\pm\) 2.74% and 85.14% \(\pm\) 3.57% at the concentrations of 25.0 and 50.0 \(\upmu\)g/mL, respectively, which were significantly higher than that of H\({}_{2}\)O\({}_{2}\)-induced oxidative damage group (\(p\) < 0.01). The present data indicated that MCPE-A, MCPE-B, and MCPE-C have strong protective effects on H\({}_{2}\)O\({}_{2}\)-induced oxidative damage HepG2 cells, especially at the high concentrations. 4.8 Effects of MCPE-A, MCPE-B, and MCPE-C on the Antioxidant Enzymes in Oxidative Damage HepG2 Cells  
Uncontrolled generation reactive oxygen species (ROS) are involved in a number of human disease states, including diabetes and cancer due to disturbance in cellular and molecular processes including cell growth, differentiation, and proliferation. Antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), and glutathione reductase (GSH-Rx) are critical for maintaining optimal cellular and systemic health and wellbeing through stabilizing, or deactivating free radicals before they attack cellular components: SOD converts superoxide to H\({}_{2}\)O\({}_{2}\), CAT converts H\({}_{2}\)O\({}_{2}\) to H\({}_{2}\)O, GSH-Px promotes GSH elimination of H\({}_{2}\)O\({}_{2}\), and GSH-Rx reduces glutathione disulfide (GSSG) to GSH. In addition, malonaldehyde (MDA) is the product of cell lipid oxidation which can reflect the status of oxidative stress in cells.