Investigation of the antioxidant activity of African potato (Hypoxis hemerocallidea).

African potato (AP) is widely used as an immune booster for the treatment of various ailments. The norlignan glycoside hypoxoside, a major phytoconstituent of AP, its aglycon rooperol, and an aqueous preparation of lyophilized AP corms were screened for in vitro antioxidant activity using the DPPH (1,1-diphenyl-2-picryl hydrazine) and FRAP (ferric reducing ability of plasma) tests. Inhibition of quinolinic acid (QA) induced lipid peroxidation in rat liver tissue was studied in vitro using the thiobarbituric assay (TBA). Superoxide free radical scavenging activity was determined by the nitroblue tetrazolium assay. An isocratic HPLC method was developed to quantitatively determine both hypoxoside and rooperol concurrently. While rooperol and AP extracts reduced QA-induced lipid peroxidation in rat liver homogenates and significantly scavenged the superoxide anion at pharmacological doses, in comparison, hypoxoside was virtually devoid of activity. Since hypoxoside is converted to rooperol in vivo following administration of AP, the results indicate that the hypoxoside component in AP could have value as an antioxidant prodrug.