Dioscorin, the storage protein of yam (Dioscorea batatas Decne) tuber (which is different from dioscorine found in tubers of Dioscorea hirsuta), was purified to homogeneity after DE-52 ion exchange column according to the methods of Hou et al. (J. Agric. Food Chem. 1999, 47, 2168-2172). A single band of 32 kDa dioscorin was obtained on a sodium dodecyl sulfate-polyacrylamide gel electrophoresis gel with 2-mercaptoethanol treatment. This purified dioscorin was shown by spectrophotometric method to have scavenging activity against 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical in a pH-dependent manner. There is a positive correlation between scavenging effects against DPPH (8-46%) and amounts of 32 kDa dioscorin (5.97-47.80 nmol) added in Tris-HCl buffer (pH 7.9), which are comparable to those of glutathione at the same concentrations. Using electron paramagnetic resonance (EPR) spectrometry for DPPH radical detection, it was found that the intensities of the EPR signal were decreased by 28.6 and 57 nmol of 32 kDa dioscorin in Tris-HCl buffer (pH 7.9) more than in distilled water compared to controls. EPR spectrometry was also used for hydroxyl radical detection. It was found that 32 kDa dioscorin could capture hydroxyl radical, and the intensities of the EPR signal were significantly decreased dose-dependently by 1.79-14.32 nmol of 32 kDa dioscorin (r = 0.975) compared to the control. It is suggested that 32 kDa dioscorin, the storage protein of yam tuber, may play a role as antioxidant in tubers and may be beneficial for health when people take it as a food additive or consume yam tubers.
Dioscorin, the tuber storage protein of yam, was reported to have immunomodulatory activity in RAW264.7 murine macrophage cell lines ( Food and Chemical Toxicology , 2007 , 45 , 2312 -2318 ). However, the immunomodulatory function of dioscorin after being ingested was not elucidated in vivo. Hence, BALB/c mice were given oral dioscorin (2.5 and 20 mg/kg/day) once a day for 21-days. Lymphocyte subpopulation changes in the peripheral blood and splenocytes, stimulation in phagocytosis of the polymorphonuclear cell (PMN) and monocytes, the natural killer (NK) cell cytotoxicity, the splenocyte proliferation, and cytokine secretions in the presence of PHA were determined. The number changes of Peyer's patches and secreted IgA (sIgA) in the feces were determined. Oral dioscorin for 21-days increased the subpopulation in natural killer cells (CD49(+)) and/or B cells (CD19(+)), elevated the phagocytosis of PMN (p < 0.01) and MON (p < 0.01), and the NK cell cytotoxic activity (p < 0.05), and stimulated splenocyte proliferations in the presence of LPS or PHA (p < 0.05) in comparison with those of the control. Cytokines of INF-gamma, IL-4, and IL-10 secretions, the numbers of Peyer's patches, and sIgA in the feces showed higher levels in oral dioscorin and significant difference to those of the control (p < 0.001). These results suggested that dioscorin exhibited systemic and mucosal immunomodulatory activities after being ingested in vivo.
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