Effects of Toona sinensis Leaf Extract and Its Chemical Constituents on Xanthine Oxidase Activity and Serum Uric Acid Levels in Potassium Oxonate-Induced Hyperuricemic Rats

Abstract

Toona sinensis 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 (IC₅₀) 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-O-rutinoside, quercetin-3-O-β-d-glucopyranoside, 1,2,3,4,6-penta-O-galloyl-β-d-glucopyranose (compound 3), quercetin-3-O-α-l-rhamnopyranoside, and kaempferol-3-O-α-l-rhamnopyranoside. Compound 3 showed the most potent inhibition of XO, with an IC₅₀ of 2.8 µM. This was similar to that of allopurinol (IC₅₀ = 2.3 µM), which is used clinically to treat hyperuricemia. Kinetic analyses found that compound 3 was a reversible noncompetitive XO inhibitor. In vivo, the T. sinensis leaf extract (300 mg/kg), or compound 3 (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 3 in the leaf extract. These findings suggest that T. sinensis leaves could be developed to produce nutraceutical preparations.

Keywords: Toona sinensis; hyperuricemia; ultraperformance liquid chromatography-quadrupole time-of-flight mass spectrometry; uric acid; xanthine oxidase.

Figure 1

Chemical structures of isolated compounds (1−5) from the leaves of T. sinensis.

Figure 2

(A) Inhibitory effects of compounds (1−5) on the activity of xanthine oxidase (XO) for the oxidation of xanthine to uric acid. (B) Catalytic activity of XO as a function of enzyme concentration at different concentrations of compound 3. (C) Lineweaver–Burk plots were constructed for the inhibition of XO by compound 3. The plot is expressed as 1/velocity versus 1/xanthine (S) with or without an inhibitor in the reaction solutions. (D) Dixon plots of XO inhibition by compound 3. The graphical symbols are substrate concentrations (50 μM, ●; 100 μM, ○; 200 μM, ▼).

Figure 3

Effects of 70% EtOH T. sinensis leaves (TSE) and compound 3 on serum uric acid levels in potassium oxonate (PO)-induced hyperuricemic rats. NC: normal control group; PO: potassium oxonate-induced hyperuricemia group; TSE-300: 300 mg/kg 70% EtOH TSE; AP-10: 10 mg/kg allopurinol. Data are expressed as the mean ± SEM (n = 6); ^### p < 0.001 versus the NC group; * p < 0.05 and *** p < 0.005 versus the PO group.

Figure 4

Representative chromatograms of 70% EtOH extract from the leaves of T. sinensis: (A) Photodiode array (PDA) chromatogram and (B) total ion current-base peak intensity (TIC-BPI) chromatogram.