onion, a potent inhibitor of xantine oxidase

onion (allium cepa) contains high levels of flavonoids. although there are many studies indicating the inhibitory effects of flavonoids on xanthine oxidase, there is no report on the effect of onion on this enzyme. therefore, in the present study, the inhibitory effects of onion on xanthine oxidase are investigated. fresh filtered juice of onion was prepared and its inhibitory effect on guinea pig liver and bovine milk xanthine oxidase activity was assayed spectrophotometrically using xanthine as substrate. in addition, the effects of hydromethanolic extract of the powdered onion and its major flavonoid, quercetin, were also studied. the juice caused more than 80% inhibition on both guinea pig and bovine milk xanthine oxidase. the extract also resulted in a marked inhibition on guinea pig liver (ic50=10 mg/ml) and bovine milk (ic50=13 mg/ml) xanthine oxidase activities. quercetin exerted its inhibitory effect on bovine milk xanthine oxidase through a linear mixed-type (ki= 0.06±0.04 and ki=0.22±0.16 mm), whereas, the guinea pig enzyme was inhibited in a competitive manner (ki=0.11±0.02 mm). in conclusion, consumption of onion as a staple vegetable with a potent inhibitory effect on xanthine oxidase not only could be useful in some diseases such as gout, but also may result in some interactions with those drugs that are metabolized by xanthine oxidase.