Antioxidant Activity of Rhubarb ( Rheum rhabarbarum L. ) Extract and Its Main Component Emodin

Freeze-dried rhubarb was fractionated using column chromatography with organic solvents (various ratios of water/methanol and acetone). The seven fractions obtained were examined for antioxidant activity using a gas chromatography/malonaldehyde (GC/MA) assay. Emodin (6-methyl-1,3,8-trihydroxyanthraquinone) in each fraction was analyzed by HPLC with a diode array detector. The amounts of emodin found in the seven fractions ranged from 22.4 ± 0.35 μg/g of dried rhubarb (100% methanol eluate) to 0.33 ± 0.01 μg/g (acetone eluate). Emodin was not detected in the fractions eluted with water/methanol mixtures. Among the seven fractions, the fraction eluted with 100% methanol showed the greatest antioxidant activity and contained the highest level of emodin. This fraction exhibited antioxidant activities by 75.4 ± 13.7% at the level of 50 μg/mL, 92.8 ± 0.6% at the level of 100 μg/mL, and 93.9 ± 1.1% at the level of 500 μg/mL. These levels of antioxidant activity were comparable to those of known antioxidants α-tocopherol and BHT. The strong relationship between amounts of emodin in the fractions and antioxidant activity was observed.