Dioscorea spp.: Comprehensive Review of Antioxidant Properties and Their Relation to Phytochemicals and Health Benefits

Dioscorea, consisting of over 600 species, is the most important genus in the Dioscoreaceae family; however, the practically used plants, which are commonly called yam, are restricted to a remarkably smaller number of species. Numerous studies have reported the high nutritional value of yam, particularly as an alternative source of starch and some important micronutrients. Several Dioscorea species are widely used for various medicinal purposes as well. In many studies, the bioactivities and health benefits of Dioscorea extracts and other preparations have been related to the presence of phytochemicals, which possess antioxidant properties; they are related mainly to radical-scavenging capacity in chemical assays and positive effects on the endogenous antioxidant system in cell-based and in vivo assays. Considering the increasing number of publications on this topic and the absence of comprehensive and focused review papers on antioxidant potential, this article summarizes the results of studies on the antioxidant properties of Dioscorea spp. and their relation to phytochemicals and health benefits. A comprehensive survey of the published articles has revealed that the majority of studies have been performed with plant tubers (rhizomes, roots), while reports on leaves are rather scarce. In general, leaf extracts demonstrated stronger antioxidant potential than tuber preparations. This may be related to the differences in phytochemical composition: saponins, phenanthrenes and, for some pigment-rich species (purple yams), anthocyanins are important constituents in tubers, while phenolic acids and flavonoids are characteristic phytochemicals in the leaves. The review may assist in explaining ethnopharmacological knowledge on the health benefits of Dioscorea plants and their preparations; moreover, it may foster further studies of poorly investigated species, as well as their wider application in developing new functional foods and nutraceuticals.

Isolation and characterization of a new cytotoxic dihydrophenanthrene from Dioscorea membranacea rhizomes and its activity against five human cancer cell lines

ETHNOPHARMACOLOGICAL RELEVANCE

The rhizomes of Dioscorea membranacea Pierre (DM) have been used as ingredients in anticancer herbal formulations in Thai traditional medicine (TTM). Thus, the aim of this study was to investigate the active constituents of DM for cytotoxic activity in order to support its TTM use.

MATERIALS AND METHODS

A bioassay-guided isolation procedure was used to separate the cytotoxic constituents from ethanolic extract of Dioscorea membranacea rhizomes by testing against five human cancer cell lines, i.e. large cell lung carcinoma, COR-L23; liver cancer cells, HepG2; prostate cancer cells, PC3; breast cancer cells MCF-7; cervical cancer cells, Hela; and one normal human lung cell line (MRC 5) using the SRB assay.

RESULTS

Two known dihydrophenanthrene compounds [2,4 dimethoxy-5,6-dihydroxy-9,10-dihydrophenanthrene (1) and 5-hydroxy-2,4,6-trimethoxy-9,10-dihydrophenanthrene (2)], and a new dihydrophenanthrene compound, 5,6,2 -trihydroxy 3,4-methoxy, 9,10-dihydrophenanthrene (3) were isolated and fully characterized. 1 showed the highest cytotoxic activity against COR-L23, MCF-7 and PC3 cell lines (IC₅₀=14.89, 17.49 and 19.04 µM, respectively), and 2 showed selective cytotoxic activity against PC3 (IC₅₀=23.54 µM). The new compound 3 showed selective cytotoxic activity against only MCF-7 cells (IC₅₀=31.41 µM). Interestingly the crude extract of DM was much less toxic to the normal cell line (MRC-5) (IC₅₀>50 µg/ml) compared to the five cancer cell lines, (IC50 value ranged between 6 and 29 µg/ml).

CONCLUSION

The phytochemicals isolated from DM may serve as lead compounds for the design of new anti-cancer agents with better selective cytotoxic indices.