Inhibition of Xanthine Oxidase by Rhodiola crenulata Extracts and Their Phytochemicals

Using a fractionation technique, four phytochemicals were isolated from Rhodiola crenulata extracts. These compounds were identified as 4'-hydroxyacetophenone (4-HAP), epicatechin-(4 beta,8)-epicatechin gallate (B2-3'-O-gallate), salidroside, and p-tyrosol using mass spectrometry and nuclear magnetic resonance spectroscopy. The inhibition of xanthine oxidase (XO) activity by these purified compounds was then evaluated and compared to that of a known XO inhibitor (allopurinol; IC50 = 12.21 +/- 0.27 mu M). Both 4-HAP and B2-3'-O-gallate showed an XO inhibitory effect, for which the half maximal inhibitory concentration (IC50) values were 15.62 +/- 1.19 and 24.24 +/- 1.80 mu M, respectively. However, salidroside and p-tyrosol did not show significant inhibitory effects on XO at 30 mu M. Furthermore, an inhibition kinetics study indicated that 4-HAP and B2-3'-O-gallate are mixed competitive inhibitors. The inhibition constants (K-i) of 4-HAP and B2-3'-O-gallate were 8.41 +/- 1.03 and 6.16 +/- 1.56 mu M, respectively. These results suggest that 4-HAP and B2-3'-O-gallate are potent XO inhibitors.