## 3 Results and Discussion
### Cytotoxicity and the Prevention of Mitochondrial Dysfunction by the Samples  
As shown in Figure 2, AW and AF were not cytotoxic to human fibroblasts and even increased mitochondrial enzyme activities in a concentration-dependent manner. In general, most plant extracts have shown wide ranges of cytotoxicity, inducing the death of up to 60-70% of cells in MTT experiments at high dosages. In contrast, in the present study, both the AW and AF improved mitochondrial activities as the treatment concentrations increased to 0.3% (\(w/v\)) and remained relatively constant, similar to a plateau, at higher dosages. Thus, the extracts from _Aloe vera_ leaf skin have the ability to increase the enzymatic activities of mitochondrial dehydrogenase. Specifically, AF substantially increased the mitochondrial enzyme activities of human fibroblasts up to 163.50 \(\pm\) 2.21% of the control, while their activities were increased to ca. 122.31 \(\pm\) 3.54% by AW at a concentration of 0.3%, which also showed a concentration dependence. Upon treatment with concentrations greater than 0.3%, no further increase in enzyme activity was observed but no serious decrease was observed. This result indicates that AF improves mitochondrial activities and is not cytotoxic, at even higher concentrations. Quercetin in the extracts could play an important role in increasing mitochondrial activities and exerts a synergistic effect with other bioactive substances in the extracts, especially in the AF, compared to the AW. As shown in Figure 2, AW and AF may increase mitochondrial enzyme activities. Therefore, Figure 3 shows whether AF altered the mitochondrial activities of human skin fibroblasts by comparing the succinate-tetrazolium reductase activities of UVb-irradiated human fibroblasts because UVb light is known to be the main factor caused to skin photodaging [5]. A total of 38.10 \(\pm\) 2.83% decrease in mitochondrial reductase activities only in being exposed to UVb light was observed compared to the control cells (not exposed to UVb). After treatment with 10 \(\upmu\)g/mL quercetin, the activity of this enzyme recovered up to 92.31 \(\pm\) 3.45% of the control level in UVb-irradiated skin fibroblasts, while the enzyme activities were increased up to 112.57 \(\pm\) 5.25% and 109.30 \(\pm\) 1.41% after the addition of 0.3% and 0.5% AW, respectively.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Component** & **Range (ug/mL)** & **Equation for Regression Line** & **R2** & **SD Intercept** & **SD Slope** \\ \hline AW & 298.5–3825.1 & Y = 1.534X + 59.0595 & 0.9997 & 38.35 & 0.19 \\ AF & 273.4–3489.2 & Y = 1.207X + 35.8621 & 0.9996 & 36.41 & 0.10 \\ \hline \hline \end{tabular}  
X, concentration (ug/mL); Y, peak area; R2; correlation coefficient.  
\end{table}
Table 1: Linearity for quercetin analyzed.  
to 135.11 \(\pm\) 6.36%, but a slight decrease to 133.08 \(\pm\) 4.24% was observed in treating 0.5% of AF, whose pattern was somewhat different from that of the AW. However, AF shows the ability to protect against mitochondrial damage caused by UV\({}_{\text{b}}\) light, indicating that AF inhibits the skin aging process by increasing mitochondrial enzyme activities. This interesting effect was speculated to be attributed to the synergistic effects of quercetin and bioactive substances in the AF or AW because a recent study has shown that quercetin plays an important role in increasing mitochondrial activities [41]. In the present study, AF containing the same contents of quercetin also showed higher activities than quercetin itself.