Evaluation of Xanthine Oxidase Inhibitory Potential and In vivo Hypouricemic Activity of Dimocarpus longan Lour. Extracts

BACKGROUND: Longan is a fruit tree known to contain many phenolic components, which are capable of protecting people from oxidative damage through an anti-inflammatory mechanism. It may be also worthwhile to study the effect on lowering uric acid activity. MATERIALS AND METHODS: This study investigates the lowering of uric acid using longan extracts, including flowers, pericarps, seeds, leaves, and twigs, on potassium-oxonate-induced hyperuricemia mice and its inhibitory actions against xanthine oxidase (XO) activities. RESULTS: The findings revealed that ethyl acetate fraction of longan extracts exhibited strong XO-inhibitory activity, and the flower extracts (IC(50) = 115.8 μg/mL) revealed more potent XO-inhibitory activity to those of pericarps (118.9 μg/mL), twigs (125.3 μg/mL), seeds (262.5 μg/mL), and leaves (331.1 μg/mL) in vitro. In addition, different dosages of longan extract (50–100 mg/kg) were administered to hyperuricemic mice. The lowering effect of longan extracts on uric acid at 75 mg/kg markedly reduced plasma uric acid levels in decreasing order: Flowers (80%) > seeds (72%) > pericarps (64%) > twigs (59%) > leaves (41%), compared with allopurinol (89%). Finally, 10 isolated phytochemicals from longan flowers were then examined in vitro. The results indicated that proanthocyanidin A2 and acetonylgeraniin A significantly inhibited XO activity in vitro. This is the first report providing new insights into the urate-reducing effect of phenolic dimer and hydrolyzable tannin, which can be developed to potential hypouricemic agents. SUMMARY: Longan flower extracts possess more potent XO-inhibitory activity than pericarps, twigs, seeds, and leaves in vitro. The lowering effect of longan flowers and seeds extracts markedly reduced plasma uric acid levels as compared to allopurinol in vivo. The extract proanthocyanidin A2 and acetonylgeraniin A were demonstrated potent XO inhibitory activity in vitro. Abbreviations used: PO: Potassium-oxonate, XO: xanthine oxidase, HE: n-hexane, EA: ethyl acetate, i.p.: intraperitoneal, PBS: phosphate-buffered saline, AP: allopurinol, P(UA): plasma uric acid.