Effects of Aquilaria crassna on Xanthine Oxidase Activity In Vitro and Hyperuricemic Mice

Antihyperuricemic effect Aquilaria crassna leaf extract has been commercially advertised to markedly reduce uric acid level after drinking with hot water in Thailand, whereas its experimental evidence has not been published yet. The present study was therefore conducted to investigate pharmacological activities of the water extract of A. crassna leaves, focusing on its inhibitory effect on xanthine oxidase activity in vitro and antihyperuricemic effect in plasma uric acid level in oxonate-induced hyperuricemic mice. Moreover, the residual xanthine oxidase inhibitory effect in the liver of mice treated with the extract was also determined. We found that A. crassna inhibited xanthine oxidase activity in with IC50 of 1.35 +/- 0.03 mg/ml. Lineweaver-Burk analysis showed that the inhibition of xanthine oxidase was non-competitive with Ki of 1.72 mg/ml. Oral administration of 3000 mg/kg of A. crassna extract significantly prevented the increase of uric acid level induced by potassium oxonate in mice, however, the lower doses (500 and 1000 mg/kg) showed no effect. Livers of mice treated with the extract at the doses of 1000 and 3000 mg/kg significantly decreased the production of uric acid through inhibition on xanthine oxidase activity approximately 47.2% and 63.6%, as compared with untreated hyperuricemic mice (P<0.01). However, the concentrations of A. crassna of which exhibit antihyperuricemic effect observed in the study were much higher than that of recommend dosage written in the commercial, therefore, we concluded that A. crassna leaves extract could not inhibit xanthine oxidase activity and the effect on urate excretion in the kidney by long term administration the extract is further needed.