## Results and Discussion
### Lipid Peroxidation Inhibition Assay
#### Effects of MMP-4, MMP-7, and MMP-12 on the levels of HOX in oxidative damage HepG2 Cells  
Figure 12A-C showed the effects of MMP-4, MMP-7, and MMP-12 on the levels of intracellular antioxidant enzymes (SOD, CAT, and GSH-Px). Compared with the normal control group, the levels of SOD, CAT, and GSH-Px were significantly decreased by H\({}_{2}\)O\({}_{2}\) in the HepG2 cells, which indicated that the oxidative stress badly damaged the intracellular antioxidant enzymes of HepG2. It is exciting that the SOD, CAT, and GSH-Px activities of HepG2 cells incubated by MMP-4, MMP-7, and MMP-12 increased in a dose-dependent manner and were significantly higher than the H\({}_{2}\)O\({}_{2}\) damaged group (\(p<0.05\)). In addition, the HepG2 cell groups incubated by MMP-12 showed the highest levels of SOD, CAT, and GSH-Px at the same tested concentrations, and followed by the group incubated by MMP-4. Figure 12D indicated that the MDA content (21.63 \(\pm\) 0.81 nM/mg prot) of HepG2 cells in the damage group was significantly (\(p<0.05\)) increased compared with that of the control group (9.32 \(\pm\) 0.35 nM/mg prot). At the concentration of 10.0, 50.0, and 100 \(\upmu\)M, the MDA content of MMP-12 incubated group was 18.8 \(\pm\) 0.56, 16.46 \(\pm\) 0.74, and 12.43 \(\pm\) 0.62 nM/mg prot, which was lower than those of MMP-4, MMP-7, and the H\({}_{2}\)O\({}_{2}\) treated groups. Therefore, MMP-4, MMP-7, and MMP-12 could reduce the oxidative stress injury by the restoration of endogenous antioxidant and the decrease of lipid peroxidation.  
Figure 11: Effects of MMP-4, MMP-7, and MMP-12 on the levels of ROS in oxidative damage HepG2 cells at the concentration of 10, 50, and 100 \(\upmu\)M. The data are presented as the mean \(\pm\) SD (\(n=3\)). \"\"\"\(p<0.001\)_versus_ the blank control group; \"\"\"\(p<0.001\)_versus_ the H\({}_{2}\)O\({}_{2}\) treated group.  
Intracellular antioxidant enzymes, such as SOD, CAT, and GSH-Px, are the most important factors for preserving cellular and systemic health through holding back the radicals attacking cellular  
Figure 12: The effects of MMP-4, MMP-7, and MMP-12 on the levels of intracellular antioxidant enzymes (SOD (**A**), CAT (**B**), and GSH-Px (**C**)) and content of MDA (**D**) in H\({}_{2}\)O\({}_{2}\)-induced HepG2 cells. All data are presented as the mean \(\pm\) SD (\(n\) = 3). \(\widehat{\texttt{\#}}\)\(p\)\(<\) 0.001 _versus_ the control group; \({}^{*}\)\(p\)\(<\) 0.05, \({}^{**}\)\(p\)\(<\) 0.01, and \({}^{***}\)\(p\)\(<\) 0.001 _versus_ the H\({}_{2}\)O\({}_{2}\) treated group.  
membrane and components. MDA is the oxidative metabolite of cell lipid oxidation, and the reduction of oxidative damage in cells was accompanied by the decrease of oxidative metabolites, which give an index to the degree of oxidative damage in cells. [PERSON] et al. reported that KTFCGRH from croaker (_Otolithes ruber_) muscle protein hydrolysate could increase an endurance capacity and facilitate recovery from oxidative stress because the peptide could efficiently quench radicals, inhibit the lipid peroxidation and DNA damage, and improve the activities of endogenous cellular antioxidant enzymes (CAT, GST and SOD) [45]. [PERSON] et al. reported that loach peptide (LP) can increase an endurance capacity and facilitate recovery from fatigue of mice because LP can increase the levels of blood glucose and liver glycogen, decrease the levels of lactic acid and blood urea nitrogen, and improve the activities of endogenous cellular antioxidant enzymes (SOD, CAT, and GSH-Px) [35]. The present results indicated that three isolated peptides (MMP-4, MMP-7, and MMP-12) have similar activity to protect H\({}_{2}\)O\({}_{2}\)-induced HepG2 cells from oxidative stress by enhancing endogenous antioxidant enzyme defense system.