Xanthine oxidase

ssDNA was purchased from Sangon Biotech Co., Ltd. (Shanghai, China), and the sequence is as follows: 5′-NH₂-(CH)₆-GGT CCG CTT GCT CTC GC-3′. Melamine (≥99%), potassium chloride (KCl, 99.5%), sodium chloride (NaCl, 99.9%), hydrochloric acid (HCl, 36%~38%), N-hydroxysuccinimide (NHS), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC), FeSO₄·7H₂O (99%~101%), H₂O₂ (30%), MB, chitosan, acetic acid, and xanthine were all obtained from Aladdin (Shanghai, China). Xanthine oxidase was purchased from Shanghai Yuanye Bio-Technology Co., Ltd. 5-tert-Butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO) was purchased from Dojindo Laboratories (Japan). BMPO was dissolved in ultra-pure water and was used as the trapping agent to radicals. The final concentration of BMPO in samples was 10 mM. Phosphate buffer solution (PBS) was prepared by mixing Na₂HPO₄ and KH₂PO₄. The mixture of a series concentration of Fe²⁺ and H₂O₂ (the concentration ratio was 1:6) promoted the formation of ^•OH. ¹O₂ was obtained by the reaction of NaClO and H₂O₂ (1 mM NaClO and 1 mM H₂O₂). O₂^•− was generated by a mixture of 100 µM xanthine and 22 mU Xanthine oxidase. All chemicals were analytical reagents, and all solutions were prepared with ultra-pure water.

Standards of kaempferol, myricetin, chlorogenic acid, caffeic acid, p-coumaric acid, resveratrol, luteolin, gallic acid, and ellagic acid were purchased from Shanghai Yuanye Bio-Technology Co., Ltd. (Shanghai, China). The purity of all standards was required to be greater than or equal to 98%. Potassium oxonate, xanthine oxidase and 2,6-dihydroxypurine were purchased from Shanghai Yuanye Bio-Technology Co., Ltd. (Shanghai, China). Xanthine sodium salt (XSS) and allopurinol (APL) were purchased from Sigma Aldrich Inc. (St. Louis, MO, USA). A uric acid (UA) test kit, xanthine oxidase (XOD) assay kit (Colorimetric method), urea assay kit (BUN), and creatinine (Cr) assay kit (sarcosine oxidase) were purchased from Nanjing Jiancheng Bioengineering Institute (Nanjing, China).

Xanthine, Xanthine Oxidase, and Standards (rutin, malic acid, citric acid, tartaric acid, succinic acid, lactic acid, oxalate, fumarate, glucose, fructose, and sucrose) were purchased from Yuanye (Yuanye Bio-Tech, Shanghai, China). Single Element Standard Solution Fe (1,000 μg/ml) and K (100 μg/ml) were purchased from the National Center (Guobiao, Beijing, China). The acetonitrile and methanol for the high-performance liquid chromatography (HPLC) were from Merck (Merck KGaA, Darmstadt, Germany). The 2,2′-azinobis(3-ethylbenzothiaziline-6-sulfonate (ABTS) was obtained from Solabio (Solabio Sci&Tech, Beijing, China).