# Enzymatic Degradation of _Gracilariopsis lemaneiformis_
## 2 Results and Discussion
### GDP Alleviated \(H_{2}\)O\({}_{2}\)-Induced Oxidative Injury in HFL1 Cells  
In vitro cell assays for screening antioxidants are vital since the mechanism of the antioxidants on human health benefits goes beyond their antioxidant ability, and their bioavailability, cytotoxicity, and other bioactivities also matter [40]. Cell viability determination through the MTT method was performed to evaluate the cytotoxicity and antioxidant capacity of GDP2, GDP3, and GDP4 on H\({}_{2}\)O\({}_{2}\)-induced oxidative damage in HFL1 cells. HFL1 cells treated with different concentrations (6.25-400 ug/mL) of GDPs (GDP2, GDP3, and GDP4) for 24 h significantly increased the cell viability compared with the control (Figure 2), showing that GDPs had no cytotoxicity to HFL1 cells. This finding was also observed in seaweed polysaccharides from _S. fusiforme_, which improved the cell viability of the HaCaT cells [41]. H\({}_{2}\)O\({}_{2}\)-induced oxidative stress has been widely used in investigating the antioxidant abilities of various bioactive substances [42]. The cell viability of HFL1 cells decreased to approximately one-fourth of that of normal cells after exposure to 200 uM H\({}_{2}\)O\({}_{2}\) for 6 h, indicating that the establishment of the H\({}_{2}\)O\({}_{2}\)-induced oxidative damage model was successful (Figure 3). Moreover, Figure 3 suggests that pretreatment with GDPs can significantly improve the cell viability of oxidative damaged HFL1 cells. Compared with the negative control group (only H\({}_{2}\)O\({}_{2}\) was added), the cell viability of GDP2, GDP3, and GDP4 treatment groups significantly increased by 167.48%, 125.06%, and 132.00% at a concentration of 6.25 ug/mL, respectively (Figure 3), indicating that GDPs possessed excellent cytoprotection effect on oxidative damaged HFL1 cells. Notably, GDP2 was the most effective, followed by GDP4 and GDP3, which was in accordance with their DPPH scavenging ability, and the best cytoprotection effect of GDP2 may be ascribed to its better DPPH free radical scavenging ability (Table 4). Moreover, the cell viability of the HFL1 cells increased gradually with GDPs concentration in a dose-dependent manner. The cell viability in GDP2, GDP3, and GDP4 groups increased 2.6, 1.9, and 2.2 times, respectively, at a concentration of 400 ug/mL, and no significant difference was among the 100, 200, and 400 mg/L treatments. Thus, 200 ug/mL of GDP was selected for the further investigation of the antioxidant activities of GDP on HFL1 cells.  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Samples** & **Scheme** & \begin{tabular}{c} **Hydroxyl Radical** \\ **Scavenging Rate/\%** \\ \end{tabular} & \begin{tabular}{c} **DPPH Radical** \\ **Scavenging Rate/\%** \\ \end{tabular} & \
\begin{tabular}{c} **Reducing** \\ **Capacity** \\ \end{tabular} &
\begin{tabular}{c} **Total Antioxidant** \\ **Capacity** \\ \end{tabular} \\ \hline GDP1 & A\({}_{1}\)B\({}_{1}\)C\({}_{1}\) & 52.20 \(\pm\) 0.82b & 29.73 \(\pm\) 0.71b & 0.117 \(\pm\) 0.002c & 0.325 \(\pm\) 0.001d \\ GDP2 & A\({}_{1}\)B\({}_{1}\)C\({}_{3}\) & 39.92 \(\pm\) 0.63d & 32.68 \(\pm\) 0.75a & 0.125 \(\pm\) 0.004b & 0.334 \(\pm\) 0.002c \\ GDP3 & A\({}_{2}\)B\({}_{2}\)C\({}_{2}\) & 61.99 \(\pm\) 2.44a & 24.28 \(\pm\) 1.08d & 0.129 \(\pm\) 0.002b & 0.345 \(\pm\) 0.002b \\ GDP4 & A\({}_{3}\)B\({}_{3}\)C\({}_{3}\) & **44.27** \(\pm\) 0.16c & 26.15 \(\pm\) 0.23c & 0.140 \(\pm\) 0.001a & 0.355 \(\pm\) 0.002a \\ \hline \hline \end{tabular}
\end{table}
Table 4: Results of verification experiment for the antioxidant activities of GDPs obtained at optimal conditions. Data represent means \(\pm\) SD (n = 3), and different letters in the same column indicate significant difference (\(p\) < 0.05).