Anti-allergic substances from the rhizomes of Dioscorea membranacea.

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 beta-hexosaminidase release as a marker of degranulation in RBL-2H3 cells, with an IC(50) value of 37.5microg/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 5.7microM, followed by dioscoreanone (3, IC(50)=7.7microM), dioscorealide A (1, IC(50)=27.9microM), and diosgenin (9, IC(50)=29.9microM). 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-alpha and IL-4 in the late phase reaction were also examined.