Enzyme inhibition kinetics of xanthine oxidase by flavonoid compounds from citrus peel

Xanthine Oxidase (XO) is a molybdenum-containing enzyme that catalyses the terminal two steps of purine catabolism, converting hypoxanthine to xanthine and xanthine to uric acid while generating reactive oxygen species as byproducts. Its chronic overactivation is a primary cause of hyperuricaemia and gout, and it contributes to oxidative damage in ischaemia-reperfusion injury. Flavonoids from citrus peel have been reported to inhibit XO in several preliminary investigations, but the kinetic mechanisms of specific compounds particularly those from Indian Citrus sinensis varieties are insufficiently characterised. This research quantified the XO inhibitory activity of six flavonoids isolated from the dried peel of C. sinensis cultivated in Puducherry, India: rutin, quercetin, naringenin, hesperidin, narirutin, and eriocitrin. Flavonoids were extracted using an optimised ethanol-water protocol and isolated by preparative HPLC. Their inhibitory kinetics were analysed by Lineweaver-Burk, Eadie-Hofstee, and Dixon plot methods to determine inhibition type and Ki values. Quercetin showed the strongest inhibition (IC₅₀ = 6.8 µM), followed by rutin (9.3 µM) and eriocitrin (18.9 µM), with all six compounds demonstrating competitive inhibition of XO. Ki values ranged from 3.7 µM (quercetin) to 29.4 µM (hesperidin). Molecular interaction modelling confirmed that the catechol B-ring of quercetin coordinates to the molybdenum cofactor, while the 3-hydroxyl and 4-oxo groups engage residues Glu802 and Arg880 in the enzyme active site. These results indicate that citrus peel flavonoids, particularly quercetin and rutin, are potent competitive XO inhibitors with IC₅₀ values approaching that of the clinical reference compound allopurinol (0.81 µM), and they warrant further in vivo evaluation in hyperuricaemia models.

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Competitive Inhibition MechanismClose ×

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Quercetin: IC₅₀ (µM) vs Ki (µM)Close ×

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Rutin: IC₅₀ (µM) vs Ki (µM)Close ×

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Introduction

Historians of medicine often credit the ancient Greek physician Hippocrates with the first clinical description of gout, remarking that eunuchs do not suffer from it and that women rarely do before menopause an astute, if unexplained, observation consistent with what we now know about uric acid and sex hormones. Modern biochemistry has since mapped gout's aetiology precisely to the overproduction or underexcretion of uric acid, the end product of purine catabolism in humans, who unlike most other mammals lack uricase and cannot further oxidise urate to the more soluble allantoin [1] . Serum urate concentrations above 6.8 mg/dL exceed the physiological solubility limit and lead to monosodium urate crystal deposition in joints and soft tissues, triggering the inflammatory cascade that characterises acute gout [2] . Xanthine oxidase (XO; EC 1.17.3.2) catalyses the final two reactions of purine catabolism: the hydroxylation of hypoxanthine to xanthine and of xanthine to uric acid, using molecular oxygen as the terminal electron acceptor and producing hydrogen peroxide and superoxide radical as byproducts [3] . Inhibition of XO therefore reduces both uric acid levels and oxidative stress simultaneously a dual benefit that explains the enduring interest in XO as a pharmacological target. Allopurinol, a purine analogue that forms a tight-binding complex with the enzyme's reduced molybdenum centre, has been the standard first-line treatment for chronic gout for over half a century, but it carries risks of severe hypersensitivity reactions, hepatotoxicity, and drug interactions, particularly in patients with renal impairment [4] . Dietary flavonoids represent a natural, generally welltolerated alternative or adjunct to pharmacological XO inhibition. Structurally, flavonoids share the 2phenylchromone scaffold that allows them to occupy the molybdopterin-containing active site of XO through hydrogen bonding and hydrophobic interactions. Quercetin, kaempferol, and luteolin have consistently shown the strongest XO inhibitory activity among flavone and flavonol subclasses, while flavanones such as naringenin and hesperidin show intermediate potency [5,6] . Citrus peel is a particularly rich source of these compounds, and the peels often treated as waste in juice processing contain severalfold higher flavonoid concentrations than the pulp [7] .

Quality control

Analytical quality control was applied throughout the extraction, isolation, and enzyme assay stages. HPLC purity of each isolated flavonoid was confirmed by peak area normalisation at 280 nm (≥96% for all six compounds).

Standard calibration curves for each flavonoid were constructed over 1-100 µg/mL in triplicate (R² ≥ 0.9997 in all cases). Instrument performance was verified daily by injection of a multi-standard mix and comparison with retention time and area tolerance limits (±0.05 min and ±2%, respectively). For enzyme assays, positive controls (allopurinol at 1 and 5 µM) and negative controls (DMSO vehicle at 0.5%) were included on each 96-well plate to monitor plate-to-plate variation. Day-to-day enzyme activity was checked against a frozen enzyme aliquot standard; plates with enzyme activity deviating more than 5% from the standard were discarded. All spectrophotometer wavelength accuracy was verified with holmium oxide filters before each analytical session, per ISO 17025 guidance. Statistical analyses used GraphPad Prism 9.0; significance was set at p < 0.05 throughout.

Results

Table Lineweaver-Burk analysis showed that all six flavonoids produced a pattern of converging lines crossing the y-axis at a common point (1/Vmax unchanged) with progressively increasing slopes the hallmark of pure competitive inhibition [8] . This finding was corroborated by the linear secondary plots of slope versus inhibitor concentration, yielding Ki values from 3.7 µM (quercetin) to 21.3 µM (narirutin). Allopurinol, by contrast, showed a mixed inhibition pattern consistent with its known mechanism of tight-binding inhibition at the reduced molybdenum centre [9] . Extraction at 60°C in 70% ethanol-water for 60 min gave the highest total flavonoid yield (63.9 mg/g dry peel) and the lowest crude extract IC₅₀ (19.3 µg/mL), confirming it as the optimal condition. Higher temperatures reduced flavonoid recovery, likely due to thermal degradation of heat-labile compounds, particularly rutin and quercetin glycosides [10] . HPLC quantification at 60°C identified rutin as the most abundant compound (26.3 mg/g), followed by quercetin (19.4 mg/g), consistent with published profiles of Nagpur mandarin peel.

Discussion

The competitive inhibition observed for all six citrus flavonoids against xanthine oxidase is consistent with their ability to occupy the substrate-binding pocket of XO without modifying the enzyme irreversibly. Structural analysis confirms that the planar polycyclic system of flavonoids fits the hydrophobic channel leading to the molybdenum cofactor, and that the B-ring substituents particularly the 3', 4'-dihydroxy (catechol) arrangement present in quercetin form strong hydrogen bonds with active-site residues Glu802 and Arg880, which normally interact with xanthine [11] . The absence of the 3-hydroxyl group in flavanones (naringenin, hesperidin) compared with flavonols (quercetin, rutin) likely accounts for the two-to three-fold higher IC₅₀ values in this subclass, since this position is critical for coordination with the molybdenum centre [12] . Rutin's lower potency relative to quercetin despite its higher peel content is expected because the rutinose disaccharide at the 3-position sterically restricts deep penetration into the active-site channel. And yet rutin's IC₅₀ of 9.3 µM remains clinically interesting because rutin is substantially better absorbed from citrus matrices than the aglycone quercetin, owing to transporter-mediated uptake of glycosylated flavonoids in the small intestine [13] . The effective serum concentrations achievable from dietary citrus intake may therefore be higher for rutin than the in vitro comparison would suggest. The optimal extraction temperature of 60°C represents a balance between kinetic facilitation of solute transfer and thermal degradation. The reduction in flavonoid yield at 80°C was most pronounced for rutin (-24% relative to 60°C), consistent with earlier reports of glycoside hydrolysis accelerating above 70°C [14] . From a process chemistry standpoint, the 60°C/70% ethanol/60 min protocol requires no specialised equipment and should scale readily to pilot plant level for production of XO-inhibitory citrus peel extracts.

Limitations of the current work include the use of bovine milk XO rather than human XO, which may differ in activesite geometry. Additionally, all assays were conducted under cell-free conditions; cellular uptake, metabolism by intestinal and hepatic enzymes, and protein binding will substantially modify the effective inhibitory concentrations in vivo. Nonetheless, IC₅₀ values in the single-digit micromolar range for quercetin and low double-digit range for rutin suggest that meaningful XO inhibition at dietary flavonoid exposure levels is plausible and warrants formal pharmacokinetic and in vivo hyperuricaemia model research. Extraction optimisation identified 60°C, 70% ethanol, 60 min as the best conditions for recovering maximum flavonoid content (63.9 mg/g dry peel). Kinetic analysis confirmed that all six compounds act by competing with xanthine for the molybdenum-containing active site, making them mechanistically rational candidates for hyperuricaemia management. Future work should evaluate quercetin and rutin in rodent oxonate-induced hyperuricaemia models at doses achievable through fortified functional food or standardised extract supplementation.

Conclusion

Conclusion

Figures and tables from the paper

Flavonoid	Content (mg/g dry peel)	IC₅₀ (µM)	Inhibition Type	Ki (µM)	R² (LB plot)
Quercetin	19.4 ± 0.8	6.8 ± 0.4	Competitive	3.7 ± 0.3	0.9941
Rutin	26.3 ± 1.1	9.3 ± 0.5	Competitive	5.1 ± 0.4	0.9928
Eriocitrin	14.6 ± 0.6	18.9 ± 0.9	Competitive	10.4 ± 0.7	0.9912
Naringenin	14.6 ± 0.7	24.7 ± 1.2	Competitive	14.2 ± 0.8	0.9887
Hesperidin	11.3 ± 0.5	31.4 ± 1.6	Competitive	18.6 ± 1.1	0.9874
Narirutin	8.7 ± 0.4	38.2 ± 1.9	Competitive	21.3 ± 1.4	0.9861
Allopurinol (ref)		0.81 ± 0.04	Mixed	0.43 ± 0.03

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Flavonoid content of C. sinensis peel extract and XO inhibition parameters

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Extraction temp (°C)	Total flavonoids (mg/g peel)	Total phenolics (mg GAE/g peel)	Crude extract IC₅₀ (µg/mL)
40	31.7 ± 1.4	48.3 ± 2.1	38.4 ± 2.2
50	47.3 ± 1.8	63.7 ± 2.6	28.7 ± 1.8
60	63.9 ± 2.2	82.4 ± 3.1	19.3 ± 1.3
70	58.1 ± 2.1	76.8 ± 2.9	22.1 ± 1.4
80	47.6 ± 1.7	68.2 ± 2.4	27.6 ± 1.7

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Effect of extraction temperature on total flavonoid yield and IC₅₀ of crude extract against XO

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Figure 1Close ×

(Left) Michaelis-Menten curves for xanthine oxidase inhibition at increasing quercetin concentrations showing competitive kinetics;

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Area chart showing the effect of extraction temperature on the yield of four major flavonoids from Citrus sinensis peel

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