Effect of luteolin on xanthine oxidase: inhibition kinetics and interaction mechanism merging with docking simulation

Inhibitory Effects of Apium graveolens on Xanthine Oxidase Activity and Serum Uric Acid Levels in Hyperuricemic Mice

Celery (Apium graveolens) is traditionally used to treat rheumatism and cardiovascular disorders. Hyperuricemia is considered as a predisposing factor for gout and is also suggested to be associated with coronary artery disease. In the present study, the effect of hydroalcoholic extracts from A. graveolens (AGE) against potassium oxonate (PO)-induced hyperuricemia was investigated in mice. AGE (250, 500, and 1,000 mg/kg) or allopurinol (5 mg/kg, as positive control) were orally administrated 1 h after PO injection (250 mg/kg, ip) for two weeks. After that, the serum uric acid level and hepatic xanthine dehydrogenase (XDH) and xanthine oxidase (XO) activities were measured. In addition, the antioxidant activity of AGE was determined by assessment of hepatic lipid peroxidation, in vivo and the ferric reducing/antioxidant power assay, in vitro. The extract exhibited good capacity to reduce ferric ion to ferrous ion with mean value of 63.8±8.5 μmol/g. The data also showed that oxonate treatment produced a significant increase in serum uric acid level (4.6 vs. 2.3 mg/ dL, P<0.001), liver XO/XDH activities (P<0.01 and P<0.001, respectively), and hepatic lipid peroxides levels (about two fold, P<0.01), compared to the healthy mice. AGE significantly decreased the serum uric acid level, hepatic XO/XDH activities, and lipid peroxidation, in a dose-dependent manner. Oral administration of 1,000 mg/kg AGE for two weeks reversed the elevated serum uric acid level (2.7 vs. 4.6 mg/dL, P<0.001) and significantly inhibited liver XO/XDH activities (P<0.001) and diminished hepatic lipid peroxidation (0.45 vs. 0.82 nmol/mg protein, P<0.05), compared with hyperuricemic mice. AGE (1,000 mg/kg) per se did not significantly modify these parameters. Our results demonstrated that AGE could reduce the serum uric acid level via inhibition of hepatic XDH/XO and indicated its potential utility as an effective hypouricemic bioactive agent or functional food.

Phosphorescent inner filter effect-based sensing of xanthine oxidase and its inhibitors with Mn-doped ZnS quantum dots

Large bandgap semiconductor ZnS QDs (Mn-doped) were explored for inner filter effect-based sensing of xanthine oxidase and its inhibitors, due to the maximum spectral overlap between the absorption of uric acid (the enzymatic product of xanthine oxidase) and the excitation of Mn-doped ZnS QDs.

Management of hyperuricemia through dietary polyphenols as a natural medicament: A comprehensive review

Hyperuricemia, a condition due to high serum uric acid level and is notorious to health. It is considered to be a potent risk factor for gout and dramatically associated in the development of many chronic diseases such as malignant tumor, cardiovascular disorders and renal failure. Modern innovative medicinal and therapeutic interventions are underlying these days to combat hyperuricemia. Previously reported studies revealed the significant impact of dietary polyphenols (e.g. anthocyanins, phenolic acids, flavonoids etc.) against hyperurecemia disorder. Dietary plant polyphenols, unlike anti- hyperuricemic agents, are not reported to have any side effects in curing hyperuricemia. The current comprehensive review figure outs the use of dietary polyphenols as a natural remedy for the management of hyperuricemia. The sources, affiliated pathways, mode of actions and factors affecting their efficiency to prevent hyperuricemia are deeply discussed in this article. Additionally, limitations and suggestions regarding previously reported studies are also highlighted.

Xanthine oxidase inhibitory activity of natural and hemisynthetic flavonoids from Gardenia oudiepe (Rubiaceae) in vitro and molecular docking studies

Xanthine oxidase (XO), an enzyme widely distributed among mammalian tissues, is associated with the oxidation of xanthine and hypoxanthine to form uric acid. Reactive oxygen species are also released during this process, leading to oxidative damages and to the pathology called gout. Available treatments mainly based on allopurinol cause serious side effects. Natural products such as flavonoids may represent an alternative. Thus, a series of polymethoxyflavones isolated and hemisynthesized from the bud exudates of Gardenia oudiepe has been evaluated for in vitro XO inhibitory activity. Compounds 1, 2 and 3 were more active than the reference inhibitor, Allopurinol (IC₅₀ = 0.25 ± 0.004 μM) with IC₅₀ values of (0.004 ± 0.001) μM, (0.05 ± 0.01) μM and (0.09 ± 0.003) μM, respectively. Structure-activity relationships were established. Additionally, a molecular docking study using MOE™ tool was carried out to establish the binding mode of the most active flavones with the enzyme, showing important interactions with its catalytic residues. These promising results, suggest the use of these compounds as potential leads for the design and development of novel XO inhibitors.

Quercetin as a tyrosinase inhibitor: Inhibitory activity, conformational change and mechanism

Quercetin, a flavonoid compound, was found to inhibit both monophenolase and diphenolase activities of tyrosinase, and its inhibition against diphenolase activity was in a reversible and competitive manner with an IC₅₀ value of (3.08±0.74)×10^-5molL^-1. Quercetin bound to tyrosinase driven by hydrophobic interaction, thereby resulted in a conformational change of tyrosinase and its intrinsic fluorescence quenching. Tyrosinase had one binding site for quercetin with the binding constant in the order of magnitude of 10⁴Lmol^-1. The molecular docking revealed that quercetin bound to the active site of tyrosinase and chelated a copper with the 3', 4'-dihydroxy groups. It can be deduced that the chelation may prevent the entrance of substrate and then inhibit the catalytic activity of tyrosinase. These findings may be helpful to understand the inhibition mechanism of quercetin on tyrosinase and functional research of quercetin in the treatment of pigmentation disorders.

Mechanism and kinetics of tyrosinase inhibition by glycolic acid: a study using conventional spectroscopy methods and hydrogen/deuterium exchange coupling with mass spectrometry

Tyrosinase is an enzyme that promotes enzymatic browning of fruits and vegetables, thereby reducing product quality.

Enhanced antioxidant activity, antibacterial activity and hypoglycemic effect of luteolin by complexation with manganese(ii) and its inhibition kinetics on xanthine oxidase

A coordination compound with enhanced biological activities was synthesized with luteolin and manganese(ii) cation. The inhibition effect assay found that the complex reversibly inhibited xanthine oxidase in a competitive manner.

Myricetin inhibits the generation of superoxide anion by reduced form of xanthine oxidase

Myricetin, a plant-derived flavonol, was found to inhibit the formation of uric acid in a mixed-type manner with IC₅₀ value of (8.66±0.03)×10^-6molL^-1 and more potently inhibit the generation of superoxide anion (O₂^-) catalysed by xanthine oxidase (XOD) with IC₅₀ value of (4.55±0.02)×10^-6molL^-1. Inhibiting O₂^- generation by myricetin may be attributed to the reduced form of XOD with a substantially higher reduction potential for FADH/FADH₂ couple. Moreover, molecular docking verified that myricetin bound to the site around isoalloxazine ring in the flavin adenine dinucleotide (FAD) domain to block the diffusion of O₂^- out of the FAD site, resulting in the transfer of another electron from FADH₂ to O₂^- to form hydrogen peroxide. This study has provided new insight into the role of myricetin in inhibiting XOD catalysis, which may be beneficial to improve myricetin's potential application in functional foods.

Screening of Potential Xanthine Oxidase Inhibitors in Gnaphalium hypoleucum DC. by Immobilized Metal Affinity Chromatography and Ultrafiltration-Ultra Performance Liquid Chromatography-Mass Spectrometry

In this study, a new method based on immobilized metal affinity chromatography (IMAC) combined with ultrafiltration-ultra performance liquid chromatography-mass spectrometry (UF-UPLC-MS) was developed for discovering ligands for xanthine oxidase (XO) in Gnaphalium hypoleucum DC., a folk medicine used in China for the treatment of gout. By IMAC, the high flavonoid content of G. hypoleucum could be determined rapidly and efficiently. UF-UPLC-MS was used to select the bound xanthine oxidase ligands in the mixture and identify them. Finally, two flavonoids, luteolin-4′-O-glucoside and luteolin, were successfully screened and identified as the candidate XO inhibitors of G. hypoleucum. They were evaluated in vitro for XO inhibitory activity and their interaction mechanism was studied coupled with molecular simulations. The results were in favor of the hypothesis that the flavonoids of G. hypoleucum might be the active content for gout treatment by inhibiting XO.

Effect of Soy Sauce on Serum Uric Acid Levels in Hyperuricemic Rats and Identification of Flazin as a Potent Xanthine Oxidase Inhibitor

This is the first report on the ability of soy sauce to effectively reduce the serum uric acid levels and xanthine oxidase (XOD) activities of hyperuricemic rats. Soy sauce was partitioned sequentially into ethyl acetate and water fractions. The ethyl acetate fraction with strong XOD inhibition effect was purified further. On the basis of xanthine oxidase inhibitory (XOI) activity-guided purification, nine compounds including 3,4-dihydroxy ethyl cinnamate, diisobutyl terephthalate, harman, daidzein, flazin, catechol, thymine, genistein, and uracil were obtained. It was the first time that 3,4-dihydroxy ethyl cinnamate and diisobutyl terephthalate had been identified from soy sauce. Flazin with hydroxymethyl furan ketone group at C-1 and carboxyl at C-3 exhibited the strongest XOI activity (IC50 = 0.51 ± 0.05 mM). According to fluorescence quenching and molecular docking experiments, flazin could enter into the catalytic center of XOD to interact with Lys1045, Gln1194, and Arg912 mainly by hydrophobic forces and hydrogen bonds. Flazin, catechol, and genistein not only were potent XOD inhibitors but also held certain antioxidant activities. According to ADME (absorption, distribution, metabolism, and excretion) simulation in silico, flazin had good oral bioavailability in vivo.

The inhibitory kinetics and mechanism of dietary vitamins D3and B2on xanthine oxidase

Dietary guidelines to promote health are usually based on the patterns’ prediction on disease risk of foods and nutrients.

Different modes of inhibition for organic acids on polyphenoloxidase

It is still unclear whether the inhibitory effect of organic acid on polyphenoloxidase (PPO) is due to the reversible inhibition or decrease of pH. In this study, cinnamic acid, citric acid and malic acid inhibited PPO in different modes. Results showed that the inhibition by cinnamic acid resulted from reversible inhibition, while the decrease of pH was the main cause for citric acid and malic acid. The kinetic results showed that cinnamic acid reversibly inhibited PPO in a mixed-type manner. Fluorescence emission spectra indicated that cinnamic acid might interact with PPO and quench its intrinsic fluorescence, while the decrease of the fluorescence intensity induced by citric acid or malic acid was due to the acid-pH. Cinnamic acid bound to PPO and induced the rearrangement of secondary structure. Molecular docking result revealed cinnamic acid inserted into the hydrophobic cavity of PPO by forming π-π stacking.

Bioassay-Guided Isolation and Identification of Xanthine Oxidase Inhibitory Constituents from the Leaves of Perilla frutescens

Activity-directed fractionation and purification processes were employed to identify xanthine oxidase (XO) inhibitory compounds from the leaves of Perilla frutescens. The total extract was evaluated in vitro on XO inhibitory activity and in vivo in an experimental model with potassium oxonate-induced hyperuricemia in mice which was used to evaluate anti-hyperuricemic activity. The crude extract showed expressive urate-lowering activity results. Solvent partitioning of the total extract followed by macroporous resin column chromatography of the n-butanol extract yielded four extracts and eluted parts. Among them, only the 70% ethanol eluted part of the n-butanol extract showed strong activity and therefore was subjected to separation and purification using various chromatographic techniques. Five compounds showing potent activity were identified by comparing their spectral data with literature values to be caffeic acid, vinyl caffeate, rosmarinic acid, methyl rosmarinate, and apigenin. These results indicate that pending further study, these compounds could be used as novel natural product agents for the treatment of hyperuricemia.

Dietary Flavonoids as Xanthine Oxidase Inhibitors: Structure-Affinity and Structure-Activity Relationships

The flavonoid family has been reported to possess a high potential for inhibition of xanthine oxidase (XO). This study concerned the structural aspects of inhibitory activities and binding affinities of flavonoids as XO inhibitors. The result indicated that the hydrophobic interaction was important in the binding of flavonoids to XO, and the XO inhibitory ability increased generally with increasing affinities within the class of flavones and flavonols. The planar structure and the C2═C3 double bonds of flavonoids were advantageous for binding to XO and for XO inhibition. Both the hydroxylation on ring B and the substitution at C3 were unfavorable for XO inhibition more profoundly than their XO affinity. The methylation greatly reduced the inhibition (0.75-3.07 times) but hardly affected the affinity. The bulky sugar substitutions of flavonoids decreased the inhibition (1.69-1.99 times) and lowered the affinities (4.20-9.22 times) to different degrees depending on the conjunction site.

Evaluation of the antimutagenic, antigenotoxic, and antioxidant activities of Eriobotrya japonica leaves

CONTEXT

The leaves of Eriobotrya japonica (Thunb.) Lindl. (Rosaceae) are used in traditional medicine to treat inflammatory diseases. However, information about the antigenotoxic and antioxidant properties of its leaves remains to be elucidated.

OBJECTIVE

The objective of this work was to evaluate the mutagenic/antimutagenic, genotoxic/antigenotoxic, and antioxidant potentials of aqueous and total oligomers flavonoid (TOF) extracts from E. japonica.

MATERIALS AND METHODS

The mutagenic/antimutagenic and genotoxic/antigenotoxic potentials of extracts (50, 250, and 500 µg/plate) were evaluated, respectively, by the Ames test with 48 h incubation and the SOS chromotest test with 2 h incubation. The antioxidant capacity of these extracts (ranging from 50 to 700 µg/mL) was tested using xanthine/xanthine oxidase and the deoxyribose assays.

RESULTS

Eriobotrya japonica extracts showed neither mutagenic nor genotoxic effect. The highest protective effect against methyl methanesulfonate and 2-aminoanthracene was obtained in the presence of aqueous extract, with IC50 values of 80 and 140 µg/plate, respectively, against S. typhimurium TA104. Moreover, this extract (500 µg/plate) was also able to reduce significantly the genotoxicity induced by nitrofurantoin and aflatoxin B1 with IC50 values of 140 and 240 µg/assay, respectively. Likewise, aqueous and TOF extracts inhibited xanthine oxidase and superoxide anion formation with IC50 values ranging from 45 to 95 and from 70 to 90 µg/mL, respectively. However, TOF extract is more efficient in inhibiting hydroxyl radical and chelating iron ion with IC50 values of 140 and 400 µg/mL, respectively, when compared with the aqueous extract.

CONCLUSION

Eriobotrya japonica prevents the genotoxicity of some carcinogenic substances probably thanks to its antioxidant capacities.

Novel insights into the inhibitory mechanism of kaempferol on xanthine oxidase

Xanthine oxidase (XO), a key enzyme in purine catabolism, is widely distributed in human tissues. It can catalyze xanthine to generate uric acid and cause hyperuricemia and gout. Inhibition kinetics assay showed that kaempferol inhibited XO activity reversibly in a competitive manner. Strong fluorescence quenching and conformational changes of XO were found due to the formation of a kaempferol-XO complex, which was driven mainly by hydrophobic forces. The molecular docking further revealed that kaempferol inserted into the hydrophobic cavity of XO to interact with some amino acid residues. The main inhibition mechanism of kaempferol on XO activity may be due to the insertion of kaempferol into the active site of XO occupying the catalytic center of the enzyme to avoid the entrance of the substrate and inducing conformational changes of XO. In addition, luteolin exhibited a stronger synergistic effect with kaempferol than did morin at the lower concentration.

Potential toxicity of phthalic acid esters plasticizer: interaction of dimethyl phthalate with trypsin in vitro

Dimethyl phthalate (DMP) is widely used as a plasticizer in industrial processes and has been reported to possess potential toxicity to the human body. In this study, the interaction between DMP and trypsin in vitro was investigated. The results of fluorescence, UV–vis, circular dichroism, and Fourier transform infrared spectra along with cyclic voltammetric measurements indicated that the remarkable fluorescence quenching and conformational changes of trypsin resulted from the formation of a DMP–trypsin complex, which was driven mainly by hydrophobic interactions. The molecular docking and trypsin activity assay showed that DMP primarily interacted with the catalytic triad of trypsin and led to the inhibition of trypsin activity. The dimensions of the individual trypsin molecules were found to become larger after binding with DMP by atomic force microscopy imaging. This study offers a comprehensive picture of DMP–trypsin interaction, which is expected to provide insights into the toxicological effect of DMP.

Evaluation of the Antihyperuricemic Activity of Phytochemicals from Davallia formosana by Enzyme Assay and Hyperuricemic Mice Model

Abnormal serum urate levels are recognized as a critical factor in the progression of several chronic diseases. To evaluate the antihyperuricemic effect of Davallia formosana, the inhibitory activities of 15 isolated phytochemicals, including five novel compounds of 6,8-dihydroxychromone-7-C-β-d-glucopyranoside (1), 6,8,3′,4′-tetrahydroxyflavanone-7-C-β-d-glucopyranoside (2), 6,8,4′-trihydroxyflavanone-7-C-β-d-glucopyranoside (3), 8-(2-pyrrolidinone-5-yl)-catechin-3-O-β-d-allopyranoside (4), and epiphyllocoumarin-3-O-β-d-allopyranoside (5), were examined against xanthine oxidase (XOD) and in a potassium oxonate-(PTO-) induced acute hyperuricemic mice model. The results indicated that compounds 3 and 5 significantly inhibited XOD activity in vitro and reduced serum uric acid levels in vivo. This is the first report providing new insights into the antihyperuricemic activities of flavonoid glycosides which can possibly be developed into potential hypouricemic agents.