Differential pulse voltammetric oxidation of polyriboxanthylic acid at the pyrolytic graphite electrode and a method for detection and determination of traces of xanthine and xanthosine-5'- monophosphate in polyxanthylic acid

Poly(X) (polyriboxanthylic acid) gives up to two differential pulse voltammetric oxidation peaks (peaks I and II) at the PGE. Xanthine, which appears to be a trace contaminant of commercial poly(X) samples, exhibits a differential pulse voltammetric oxidation peak at more negative potentials than the peaks of poly(X). Xanthosine-5'-monophosphate also gives up to two differential pulse voltammetric oxidation peaks at the PGE. An analytical method has been developed to determine trace amounts of xanthine and xanthosine-5'-monophosphate in poly(X) samples based on differential pulse voltammetry.     

Linear-sweep voltammetry and the simultaneous determination of xanthine and xanthosine at the glassy carbon electrode

Linear sweep voltammetry of xanthine and xanthosine has been studied at a sweep rate of 10mVs⁻¹ in phosphate buffers with different pH values. Based on the linear relation of peak current versus concentration, the simultaneous determination of both the compounds was carried out.     

Electrochemical determination of adenine and adenosine: adsorption of adenine and adenosine at the pyrolytic graphite electrode

Adenine and adenosine are polarographically reducible from aqueous solution at pH 4.7 at the DME at the same E(1 2 ) and are also voltammetrically oxidizable at the PGE, but at different potentials, adenosine at higher potentials. Competitive adsorption of both compounds at the PGE results in a decrease in the scanning voltammetric oxidation peak of adenine in the presence of adenosine, reaching a constant value when the concentration of the latter is above 6 mM. In mixtures, the sum of the two is obtained by polarography at the DME. Solid adenosine is then added to the solution and the adenine is determined by voltamrnetry at the PGE.     

Amperometric sensor for hypoxanthine and xanthine based on the detection of uric acid

The construction and response of an immobilized enzyme modified electrode as an amperometric sensor is described. Xanthine oxidase was adsorbed on a carbon paste electrode and physically entrapped with a semipermeable membrane. Uric acid, the product of the enzymatic reaction, was oxidized electrochemically at +0.4 V vs. Ag/AgCl, yielding a steady-state current directly related to the bulk concentration of the substrate. Hypoxanthine and xanthine were determined in the range 5–100 μM at Ph 7.2 with good precision. Interferences are discussed.     

Semiconductor laser fluorimetry for enzyme and enzymatic assays

Near-infrared semiconductor laser fluorimetry is applied to assays of xanthine and xanthine oxidase. The fluorescence of indocyanine green in the near-infrared region is quenched by hydrogen peroxide. Xanthine is converted to uric acid by xanthine oxidase, in a reaction which also produces hydrogen peroxide; xanthine can be determined by measuring the decrease in fluorescence intensity of the dye added to the sample solution. The calibration graph for xanthine is linear from 5 × 10^?5 M to 5 × 10^?7 M. The enzyme activity can also be determined.     

用聚苯胺膜从牛乳中直接分离黄嘌呤氧化酶

酶电极;新鲜牛乳;固定分离;用聚苯胺膜从牛乳中直接分离黄嘌呤氧化酶     

Double-walled carbon nanotube based carbon paste electrode as xanthine biosensor

We describe the first usage of a double walled carbon nanotube (DWCNT) modified carbon paste electrode as biosensor transducer. Xanthine was chosen as a substrate for evaluation of the electrode performance. Proper amount of DWCNT and xanthine oxidase enzyme were mixed with proper amount of graphite and mineral oil for attaining the xanthine biosensor. Results were compared with previous work that includes multi-walled carbon nanotube and single-wall carbon nanotube based carbon paste electrode xanthine biosensors. A linearity was obtained in the concentration range between 2–50 μM xanthine under the response time of 150 s with the equation of y?=?0.0441x + 0.2013 and RSD value of 4.20%. This system was applied to the determination of xanthine in canned tuna fish samples and recovery was calculated as 99.20%?±?0.07.     

Electrochemical Investigation on Kinetics of Xanthine Metabolism and Inhibition Effect of Febuxostat on Xanthine Oxidase Activity

Xanthine is a significant biomolecule and its concentration level in urine and blood plasma is an indicator of specified pathological states. Here, a new sensing platform was designed, which showed excellent analytical performance for xanthine. Importantly, it is the first time to investigate the kinetics of xanthine metabolic reaction by electrochemical method. The results demonstrated that the conversion of xanthine to uric acid completely conformed to the Michaelis-Menten kinetics. Furthermore, we also studied the inhibitory effect of febuxostat on xanthine oxidase activity detailed. As expected, the work may offer potential value for researchers in the treatment of hyperuricemia and gout.     

Oxidation of Protopine at a Pyrolytic Graphite Electrode Using Cyclic and Square‐Wave Voltammetry

This paper describes oxidation of the isoquinoline alkaloid, protopine (PR) at a pyrolytic graphite electrode (PGE) using cyclic and square‐wave voltammetry. In the alkaline range (pH 7.5–10.5) of a Britton–Robinson (B–R) buffer, a PR oxidation can be observed as a well‐developed voltammetric peak around +0.9 V (vs. Ag|AgCl|3 M KCl). With increasing pH of the B–R buffer, the PR peak is shifted to less positive potentials. The acquired voltammetric data suggest that PR strongly adsorbs onto the surface of the pyrolytic graphite where it is subjected to irreversible electrochemical oxidation in its uncharged free (tricyclic) base form. The results are discussed in connection with the electrochemical oxidation of other isoquinoline alkaloids and the potential applications of these data.     

Synthesis of 5′,9-anhydro-3-(β-d-ribofuranosyl)xanthine, and 3,5′-anhydro-xanthosine as potential anti-hepatitis C virus agents

5′,9-Anhydro-3-(β-d-ribofuranosyl)xanthine and 3,5′-anhydro-xanthosine were prepared as potential anti-hepatitis C virus (HCV) agents from uridine and xanthosine, respectively.     

Simultaneous Trace Electrochemical Determination of Xanthine Theophylline and Theobromine with a Novel Sensor Based on a Composite Including Metal Oxide Nanoparticle Multi-walled Carbon Nanotube and Nano-Na-montmorillonite Clay

We report the simultaneous determination of purine molecules with biological significance on pencil graphite electrode (PGE) modified with a composite solution including NiO nanoparticles, multi-walled carbon nanotube (MWCNT), and natural nano-Na-montmorillonite clay (NNaM) using DPV technique. The novel sensor, NiO/MWCNT/NNaM/PGE, achieved simultaneous determination of xanthine, theophylline, and theobromine at the detection limits 0.077 μM, 0.361 μM, and 0.458 μM with the linear working ranges 0.5–150 μM, 5–200 μM, and 5–250 μM in Britton-Robinson buffer at pH 2.0, respectively. The sensor revealed excellent performance for the simultaneous determination of XT, TP, and TB in three real-world samples.     

Chelate Adsorption for Trace Voltammetric Determination of Xanthosine 5′-Monophosphate and Xanthosine 5′-Diphosphate

Square-wave cathodic adsorptive stripping voltammetry based on adsorptive accumulation is a very sensitive technique for the trace determination of xanthosine 5′-monophosphate (5′-XMP) and xanthosine 5′-diphosphate (5′-XDP). The determination is based on the strong interaction of the adsorption of xanthosine phosphate compounds on a mercury electrode surface, forming Hg(II)-xanthate. The cathodic reduction of the accumulated Hg(II)-xanthate complex provides the basis for direct stripping measurements of the investigated biological compounds at subnanomolar concentration levels. Moreover, controlled adsorptive accumulation of the Cu(II) complex of xanthosine phosphate is also reported to assay trace amounts of xanthosine phosphate. The height of the sharp chelate peak of adsorbed Cu(II)-xanthate, coupled with the flat baseline, facilitates measurements at nanomolar and submicromolar concentration levels. The adsorption and the redox behaviour of the investigated complexes are indicated by cyclic voltammetry. Experimental and instrumental parameters for the quantitative determination were optimized. Statistical analysis of the calibration curve data is also included.     

A combined computational and experimental study of the hydrogen-bonded dimers of xanthine and hypoxanthine

In addition to uracil, the noncanonical nucleobases xanthine and hypoxanthine are important lesions that are formed from the canonical bases when a cell is under oxidative stress. It is known that they lead to point mutations; however, more detailed information about their ability to form hydrogen-bonded complexes is not available. In the present paper such information is obtained by a combined experimental and theoretical approach. Accurate association constants of xanthosine and inosine dimers are determined by concentration dependent 1H NMR experiments, and a structural characterization of individual complexes formed in solution is performed through measurements under slow exchange conditions at very low temperatures. An interpretation of the experimental data concerning complex geometries becomes possible through a comparison of measured and computed NMR chemical shifts. Further qualitative insights into the hydrogen bonding abilities of xanthine and hypoxanthine are obtained by a theoretical characterization of all possible pairing modes of xanthine and hypoxanthine dimers and by a comparison with simplified model systems. The influence of a polar medium on the bonding properties is also estimated and the importance of the various effects is discussed. Our analysis shows to what extent secondary electronic and electrostatic effects influence the hydrogen bonding properties of xanthine and hypoxanthine in the gas phase and in polar solvents.     

Assay for guanase in blood serum by flow injection analysis with fluorescence detection

A sensitive assay for guanase activity in human serum (10 μl) is described. Xanthine, formed enzymatically from the substrate guanine, is determined in a flow system in which columns of immobilized xanthine oxidase, uricase and horseradish peroxidase are connected in that sequence in the flow line. Hydrogen peroxide formed in the enzymatic conversion of xanthine is measured fluorimetrically by reaction with 3-(p-hydroxyphenyl)-proprionic acid in the system. Linear calibration graphs are obtained for 0.52-500 pmol of xanthine in the 20-μl sample injected. This method permits the assay of guanase activity in the sera of healthy persons and patients with hepatitis.     

Amperometric biosensor for xanthine with supramolecular architecture

Xanthine oxidase modified with 1-adamantanyl residues was supramolecularly immobilized on Au electrodes coated with Au nanoparticles coated with a perthiolated beta-cyclodextrin polymer; the analytical response of the electrode toward xanthine was evaluated.     

Highly Sensitive Differential Pulse and Square Wave Voltammetric Methods for Determination of Strontium Ranelate in Bulk and Pharmaceutical Dosage Form

The voltammetric behavior of Strontium Ranelate (SR) was studied using Cyclic (CV), differential pulse (DPV) and square wave (SWV) voltammetry. CV showed two well‐defined, irreversible, diffusion‐controlled anodic peaks using Britton‐Robinson buffer, pH 2.0 at Pencil graphite (PGE), Carbon paste (CPE) and glassy carbon (GCE) electrodes. The peak current‐concentration relationship was rectilinear over the range 1.0–10.0, 1.0–11.25 and 2.5–24.0 µg/mL at PGE, CPE and GCE respectively, with a minimum detectability of 0.17, 0.24 and 0.39 µg/mL for peak 1 and 0.19, 0.27 and 0.51 µg/mL for peak 2. Recoveries showed the high accuracy of the method; 99.8 %, 99.5 % and 99.7 % at PGE, CPE and GCE respectively for peak 1 and 100.1 %, 99.9 % and 99.7 % at PGE, CPE and GCE respectively for peak 2. Hence DPV and SWV were conducted for the quantitative determination of SR in its pure and pharmaceutical dosage form. the method was validated and the results were in good agreement with those obtained from the reported method.     

X-ray crystal structure of arsenite-inhibited xanthine oxidase: μ-sulfido,μ-oxo double bridge between molybdenum and arsenic in the active site

Xanthine oxidoreductase is a molybdenum-containing enzyme that catalyzes the hydroxylation reaction of sp(2)-hybridized carbon centers of a variety of substrates, including purines, aldehydes, and other heterocyclic compounds. The complex of arsenite-inhibited xanthine oxidase has been characterized previously by UV-vis, electron paramagnetic resonance, and X-ray absorption spectroscopy (XAS), and the catalytically essential sulfido ligand of the square-pyrimidal molybdenum center has been suggested to be involved in arsenite binding through either a μ-sulfido,μ-oxo double bridge or a single μ-sulfido bridge. However, this is contrary to the crystallographically observed single μ-oxo bridge between molybdenum and arsenic in the desulfo form of aldehyde oxidoreductase from Desulfovibrio gigas (an enzyme closely related to xanthine oxidase), whose molybdenum center has an oxo ligand replacing the catalytically essential sulfur, as seen in the functional form of xanthine oxidase. Here we use X-ray crystallography to characterize the molybdenum center of arsenite-inhibited xanthine oxidase and solve the structures of the oxidized and reduced inhibition complexes at 1.82 and 2.11 ? resolution, respectively. We observe μ-sulfido,μ-oxo double bridges between molybdenum and arsenic in the active sites of both complexes. Arsenic is four-coordinate with a distorted trigonal-pyramidal geometry in the oxidized complex and three-coordinate with a distorted trigonal-planar geometry in the reduced complex. The doubly bridged binding mode is in agreement with previous XAS data indicating that the catalytically essential sulfur is also essential for the high affinity of reduced xanthine oxidoreductase for arsenite.     

Effect of iron chelates on luminol chemiluminescence in the presence of xanthine oxidase

Xanthine oxidase, in catalysing the oxidation of hypoxanthine to uric acid, produces hydrogen peroxide. Chemiluminescence is produced by oxidation of luminol by reactive hydroxyl radicals formed from H₂O₂ by Fe-EDTA and similar complexes. The concentrations of the various components in the active reagent are optimized in order to obtain a constant chemiluminescent signal of high intensity. The effect of chelate structure on chemiluminescence generation is studied, and a structure-activity relationship is deduced. The detection limit for xanthine oxidase is 5 pg.     

Non‐enzymatic Electrochemical Sensor for the Simultaneous Determination of Xanthine,its Methyl Derivatives Theophylline and Caffeine as well as its Metabolite Uric Acid

Xanthine and its methyl derivatives, theophylline and caffeine are purines which find important roles in biological systems. The simultaneous voltammetric behaviour of these purines has been studied on a glassy carbon electrode modified with an electropolymerised film of para amino benzene sulfonic acid. Well defined and well separated peaks were obtained for the oxidation of xanthine, theophylline and caffeine on the polymer modified electrode in the square wave mode. The experimental requirements to obtain the best results for individual as well as simultaneous determination were optimised. The signal for the electro‐oxidation was found to be free of interferences from each other in the range 0.9 – 100 μM in the case of xanthine and from 10–100 μM in the case of theophylline and caffeine with detection limits 0.35 μM, 7.02 μM and 11.95 μM respectively. The simultaneous determination of uric acid, the final metabolic product of xanthine oxidation in biological systems could also be accomplished along with xanthine, theophylline and caffeine atphysiological pH. The mechanistic aspects of the electro‐oxidation on the polymer modified electrode was also studied using linear sweep voltammetry. Chronoamperometry was employed to determine the diffusion coefficient of these xanthines. The developed sensor has been successfully demonstrated to be suitable for the determination of these compounds in real samples without much pre‐treatment.     

Xanthine and hypoxanthine sensors based on xanthine oxidase immobilized in poly(mercapto-p-benzoquinone) film

The construction and performance of an enzyme electrode as an amperometric sensor of xanthine and hypoxanthine is described. Xanthine oxidase has been immobilized in a conductive redox polymer, poly(mercapto-p-benzoquinone), by means of an electropolymerization of mercaptohydroquinone in the presence of the enzyme. An Au-electroplated glassy carbon electrode coated with the resulting polymer film functioned well as a direct response type of sensor, where the polymer chain served as a conductive molecular chain between the active sites in the enzyme and the substrate electrode. Response characteristics as well as kinetic parameters have been evaluated.