Effects of various flavonoids on xanthine oxidase activities in vitro and on plasma uric acid levels in oxonate-induced rats

Abstract

Xanthine oxidase (XO) is the key enzyme that catalyzes the oxidation of xanthine and hypoxanthine into uric acid, and plays an important role in producing hyperuricemia and gout [1]. It has been shown in the literature that flavonoids showed potent XO inhibitory activities in vitro [2]. However, less information about the in vivo hypouricemic activities of flavonoids are available. In the present study it was therefore of interest to investigate if the in vitro XO inhibitory activities of various flavonoids can be correlated with hypouricemic effects in vivo. Eight flavonoids including flavones, flavonols, and flavanones were used to inhibit the XO activity in vitro. The aglycones luteolin, apigenin, kaempferol and quercetin were shown to have potent xanthine oxidase inhibitory activities with IC50 values less than 5µM. Glycosides such as luteolin-7-O-glucoside and rutin showed weaker activities with IC50 value greater than 20µM. Eriodictyol and naringenin showed low activities with IC50 value greater than 50µM. All compounds were administered orally to oxonate-induced hyperuricemic rats at doses of 50 and 100mg/kg. None of the tested flavonoids elicited any hypouricemia in vivo after oral administration. This lack of effect might be due to the low intestinal absorption of the flavonoids or the first pass effect through the liver.