Inhibition of xanthine oxidase by some Chinese medicinal plants used to treat gout

The enzyme xanthine oxidase catalyses the oxidation of hypoxanthine to xanthine and then to uric acid, which plays a crucial role in gout. A total of 122 traditional Chinese medicinal plants, selected according to the clinical efficacy and prescription frequency for the treatment of gout and other hyperuricemia-related disorders, have been evaluated for the enzyme inhibitory activity. Among the 122 methanol extracts derived from these species, 69 were shown to be inhibitory at 100 microg/ml, with 29 having greater than 50% inhibition. As to the equal amount of water extracts, 40 were disclosed to be active at 100 microg/ml, with 13 possessing more than 50% inhibition. At 50 microg/ml, 58 methanol and 24 water extracts exhibited inhibitory activity, with 15 of the former and two of the latter showing greater than 50% inhibition. The most active was the methanol extract of the twig of Cinnamomum cassia (Lauraceae) (IC(50), 18 microg/ml), which was followed immediately by those of the flower of Chrysanthemum indicum (Asteraceae) (IC(50), 22 microg/ml) and the leaves of Lycopus europaeus (Lamiatae) (IC(50), 26 microg/ml). Among the water extracts, the strongest inhibition of the enzyme was observed with that of the rhizome of Polygonum cuspidatum (Polygonaceae) (IC(50), 38 microg/ml). The IC(50) value of allopurinol used as a positive control was 1.06 microg/ml. The study demonstrated that the effects for these medicinal plants used for the gout treatment were based, at least in part, on the xanthine oxidase inhibitory action.