Anti-allergic substances from the rhizomes of Dioscorea membranacea

Abstract

Extracts of five species of Thai medicinal plants, locally known as Hua-Khao-Yen, were screened for anti-allergic activities using RBL-2H3 cells. Of the five species studied, the ethanolic extract of Dioscorea membranacea exhibited potent inhibitory activity against β-hexosaminidase release as a marker of degranulation in RBL-2H3 cells, with an IC 50 value of 37.5 μg/mL. Eight compounds were isolated from this crude ethanolic extract, [two naphthofuranoxepins ( 1, 2), one phenanthraquinone ( 3), three steroids ( 4– 6), and two steroidal saponins ( 7, 8)], and tested for their anti-allergic activities. The results showed that dioscorealide B ( 2) possessed the highest activity with an IC 50 value of 5.7 μM, followed by dioscoreanone ( 3, IC 50 = 7.7 μM), dioscorealide A ( 1, IC 50 = 27.9 μM), and diosgenin ( 9, IC 50 = 29.9 μM). Structure–activity relationship studies of naphthofuranoxepins on anti-allergic activity revealed that the hydroxylation at position 8 conferred higher activity than methoxylation. For diosgenin derivatives, the aglycone was found to possess higher activity than the diglucosylated molecule; whereas substitution with rhamnoglucosides apparently results in loss of activity. Furthermore, effects of dioscorealide A, dioscorealide B, and dioscoreanone on antigen-induced release of TNF-α and IL-4 in the late phase reaction were also examined.