Ultrafiltration coupled with liquid chromatography-mass spectrometry (LC-MS) was established to screen xanthine oxidase (XO) inhibitors from Pueraria lobata root extract. Four compounds were screened out and identified as puerarin, daidzin, daidzein and genistein with half-maximal inhibitory concentration (IC50) values of 30.8, 5.31, 14.5 and 3.02 µg mL−1 on XO, respectively. The interactions between these compounds and XO were investigated by fluorescence spectroscopic method. The hydrogen peroxide induced oxidative stress model of human normal gastric epithelial cell lines (GES-1) was used to investigate the protections on injured cell. As a result, four XO inhibitors exhibited protective effects without cytotoxicity. With the increased concentrations of four inhibitors, cell viability was higher with decreased mortality rate, the decrease of superoxide dismutase activity, leakage of lactate dehydrogenase and increase of intracellular superoxide anion production induced by hydrogen peroxide were restrained. It showed that these four XO inhibitors could effectively enhance cell viability and protect injury of GES-1 cells from oxidative stress.

Lotus leaf is gaining growing popularity due to its various benefits and widely usage. In this paper, ten flavonoids in lotus leaf extract were analyzed by high performance liquid chromatography (HPLC). Centrifugal ultrafiltration combined liquid chromatography was used to screen alpha-amylase inhibitors from ten flavonoids mixture and the binding degrees ranged from 2.34 to 94.1%. The alpha-amylase inhibition and the 1,1-diphenyl-2-picrylhydrazyl (DPPH) antioxidant activity of ten flavonoids were verified as well. Apigenin, kaempferol and isorhamnetin with relatively higher binding degrees showed higher inhibition on alpha-amylase as well. The interactions between these flavonoids and alpha-amylase were investigated by spectroscopic method. As a result, the fluorescence quenching could be considered as static quenching because the quenching rate constant (Kq) values were higher than 2.0 × 1010 L mol -1 s-1. The binding constants (log10 Ka) of ten flavonoids were in the range of 5.971 to 7.417 L mol-1. The hydrogenation of C2=C3 double bond of apigenin and quercetin decreased the affinity for alpha-amylase (1.92- and 2.82-fold). The hydroxylation on 3 and 3' position decreased the affinity for alpha-amylase. Moreover, the glycosylation with different sugar moiety improved in varying degrees of affinity. The hydrogen bond force might be important in the binding between alpha-amylase and flavonoids.

In this study, centrifugal ultrafiltration coupled with high performance liquid chromatography-mass spectrometry was utilized to screen and identify xanthine oxidase inhibitors from Puerariae flos extract. The experimental conditions of centrifugal ultrafiltration including xanthine oxidase concentration, incubation time, pH and temperature were optimized. At the optimum condition (xanthine oxidase concentration: 30.0 μg mL-1, incubation time: 20 min, pH 7.0 and temperature: 25 ºC), four compounds were successfully screened from P. flos extract and identified as tectoridin, daidzin, ononin and biochanin A. The yields of tectoridin, daidzin, ononin and biochanin A were 0.231, 0.117, 0.303 and 0.089 g from 50.0 g crude P. flos samples. The inhibitory activities of these compounds were verified by xanthine oxidase inhibition assays. The experimental half maximal inhibitory concentration (IC50) values of tectoridin, daidzin, ononin and biochanin A were 88.5, 85.1, 88.8 and 87.0 μmol L-1, and the binding degree of them were 5.70, 8.28, 6.31 and 37.83% at the optimum condition, respectively. The proposed method provided a rapid and effective way to screen and analyze active compounds from natural products.