| Summary: | <i>Toona sinensis</i> leaf is used as a seasonal vegetable in Korea. A 70% ethanol extract of these leaves exhibited potent xanthine oxidase (XO) inhibition, with a 50% inhibitory concentration (IC<sub>50</sub>) of 78.4 µM. To investigate the compounds responsible for this effect, bioassay-guided purification led to the isolation of five constituents, identified as quercetin-3-<i>O</i>-rutinoside, quercetin-3-<i>O</i>-β-<span style="font-variant: small-caps;">d</span>-glucopyranoside, 1,2,3,4,6-penta-<i>O</i>-galloyl-β-<span style="font-variant: small-caps;">d</span>-glucopyranose (compound <b>3</b>), quercetin-3-<i>O</i>-α-<span style="font-variant: small-caps;">l</span>-rhamnopyranoside, and kaempferol-3-<i>O</i>-α-<span style="font-variant: small-caps;">l</span>-rhamnopyranoside. Compound <b>3</b> showed the most potent inhibition of XO, with an IC<sub>50</sub> of 2.8 µM. This was similar to that of allopurinol (IC<sub>50</sub> = 2.3 µM), which is used clinically to treat hyperuricemia. Kinetic analyses found that compound <b>3</b> was a reversible noncompetitive XO inhibitor. In vivo, the <i>T. sinensis</i> leaf extract (300 mg/kg), or compound <b>3</b> (40 mg/kg), significantly decreased serum uric acid levels in rats with potassium oxonate-induced hyperuricemia. Furthermore, ultraperformance liquid chromatography-quadrupole time-of-flight mass spectrometry analysis identified a high level of compound <b>3</b> in the leaf extract. These findings suggest that <i>T. sinensis</i> leaves could be developed to produce nutraceutical preparations.
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