Hypoxanthine, concentrations

The principle of antioxidant detection is shown in Fig. 17.3. Superoxide was enzymatically produced and dismutated spontaneously to oxygen and H202. Under controlled conditions of superoxide generation such as air saturation of the buffer, optimal hypoxanthine concentration (100 pM) and XOD activity (50mU ml-1) a steady-state superoxide level could be obtained for several min (580-680 s). Since these steady-state superoxide concentrations can be detected by the cyt c-modified gold electrode, the antioxidate activity can be quantified from the response of the sensor electrode by the percentage of the current decrease. 

In patients suffering from leukaemia and renal failure who are being treated with allopurinol the xanthine concentration may rise to values between 8S and 230 mg/1. In these patients the hypoxanthine concentration can be between 13 and 43 mg/1. 

Whole brain homogenates have been found to contain very low levels or to be devoid of either of the molybdenum hydroxylases [92, 107, 108, 120]. However, more specific assays for xanthine oxidase gave values of 2-20 nmol xanthine transformed/mg per h for homogenates of cortex or whole brain from mouse, rat, guinea-pig, rabbit and cow [46, 121], Less activity was detected in the cerebellum [121] and, again, the cranial capillary endothelial cells were found to be enriched in xanthine oxidase [46], Brain hypoxanthine concentrations are reported to rise during ischaemia due to increased ATP breakdown, and Betz [46] proposed that brain capillaries may be susceptible to damage... 

Luong and Male (1992) developed a biosensor system to measure the hypoxanthine concentration ratio as an indicator of fish freshness. Hy-poxanthine is the autodegradation product formed from adenosine 5 -triphosphate in fish tissue and is responsible for the bitter off-taste characteristic of fish which has lost its freshness. 

Pulse radiolytic investigations of the hypoxan-thine-xanthine-uric acid system 0.8 is after the generation of electron pulses (800 KeV, 4 ns) showed transient species produced by action of HO radical on hypoxanthine (Santamaria et al. 1984). The rate of formation of the transient depends on hypoxanthine concentration. The radical decay leads directly to the formation of xanthine. Another radical was produced by oxidation of xanthine by HO in less than 1.6 ps. Such a reaction was xanthine concentration dependent. The decay of this radical after ca. 400 ps did not lead directly to the formation of uric acid. Santamaria et al. (1984) suppose that its disproportionation occurs through another transient, which could be a dimer. 

In a control series and in some deficiency cases together with a number of their relatives plasma hypoxanthine concentration was measured in addition to the and lactate levels before and... 

H2O) SP, syringe pump HC, holding coil W, waste A, air S, sample PBS, phosphate buffer solution, EEC, electrochemical flow cell (internal volume 200 yl). (b) Stripping voltammograms for (a-i) 0.05-10 mmol/l hypoxanthine. Inset log 1 vs. log hypoxanthine concentration calibration plot. Reproduced from Ref [13] with permission from Elsevier... 

The higher hypoxanthine concentration results in increased activity because of the high Km of the mutant enzyme for hypoxanthine. In addition, the mutant HGPRT has minimal activity at PP-ribose-P concentrations below 2.5 mM, whereas substantial activity is observed at PP-ribose-P concentrations from 2.5 mM to 10.0 mM. The curves predicted from a mixture of normal and mutant hemolysates at low and high hypoxanthine are illustrated by the dashed lines. 

Ara-A-5 -monophosphate [29984-33-6] (ara-AMP), C2QH24N OyP, is more water-soluble than ara-A, and therefore can be used in higher dosage during the first hours of treatment of viral infections. Ara-AMP has been shown to decrease virion-associated DNA polymerase concentrations in ground squirrels carrying ground squirrel hepatitis vims. The hypoxanthine derivative, ara-HxMP [54656-49-4] (24) is more water-soluble, appears to have a similar antiviral spectmm to ara-A, and is considerably less toxic (48). 

Scott et al. [12] provided some experimental evidence supporting equation (27). The mixture contained uracil, hypoxanthine, guanine and cytosine, each present in the mobile phase at a concentration of 14 mg/1. The column employed was Im long, 1.5 mm I.D., packed with a pellicular cation exchange resin and operated at a flow rate of 0.3 ml/min. 

Anti-gout Drugs. Figure 1 Xanthine oxidase-catalyzed reactions. Xanthine oxidase converts hypoxanthine to xanthine and xanthine to uric acid, respectively. Hypoxanthine and xanthine are more soluble than uric acid. Xanthine oxidase also converts the uricostatic drug allopurinol to alloxanthine. Allopurinol and hypoxanthine are isomers that differ from each other in the substitution of positions 7 and 8 of the purine ring system. Although allopurinol is converted to alloxanthine by xanthine oxidase, allopurinol is also a xanthine oxidase inhibitor. Specifically, at low concentrations, allopurinol acts as a competitive inhibitor, and at high concentrations it acts as a noncompetitive inhibitor. Alloxanthine is a noncompetitive xanthine oxidase inhibitor. XOD xanthine oxidase. 

Allopurinol is an analog of hypoxanthine and is converted to alloxanthine by XOD. Both allopurinol and hypoxanthine inhibit XOD (Fig. 1). Alloxanthine is a noncompetitive inhibitor of XOD as is allopurinol at high concentrations. At low concentrations, allopurinol is a competitive inhibitor of XOD. As a result of XOD inhibition, the formation of the poorly soluble... 

Most patients in the United States are treated with allopurinol, which usually is effective if the dosage is titrated appropriately. The drug and its primary active metabolite, oxypurinol, reduce serum uric acid concentrations by inhibiting the enzyme xanthine oxidase, thereby blocking the oxidation of hypoxanthine and xanthine to uric acid. 

As in the case in the analysis of food samples, the introduction of relatively inexpensive MS detectors for GC has had a substantial impact on the determination of methylxanthines by GC. For example, in 1990, Benchekroun published a paper in which a GC-MS method for the quantitation of tri-, di-, and monmethylxanthines and uric acid from hepatocyte incubation media was described.55 The method described allows for the measurement of the concentration of 14 methylxanthines and methyluric acid metabolites of methylxanthines. In other studies, GC-MS has also been used. Two examples from the recent literature are studies by Simek and Lartigue-Mattei, respectively.58 57 In the first case, GC-MS using an ion trap detector was used to provide confirmatory data to support a microbore HPLC technique. TMS derivatives of the compounds of interest were formed and separated on a 25 m DB-% column directly coupled to the ion trap detector. In the second example, allopurinol, oxypurinol, hypoxanthine, and xanthine were assayed simultaneously using GC-MS. 

XOD is one of the most complex flavoproteins and is composed of two identical and catalytically independent subunits each subunit contains one molybdenium center, two iron sulfur centers, and flavine adenine dinucleotide. The enzyme activity is due to a complicated interaction of FAD, molybdenium, iron, and labile sulfur moieties at or near the active site [260], It can be used to detect xanthine and hypoxanthine by immobilizing xanthine oxidase on a glassy carbon paste electrode [261], The elements are based on the chronoamperometric monitoring of the current that occurs due to the oxidation of the hydrogen peroxide which liberates during the enzymatic reaction. The biosensor showed linear dependence in the concentration range between 5.0 X 10 7 and 4.0 X 10-5M for xanthine and 2.0 X 10 5 and 8.0 X 10 5M for hypoxanthine, respectively. The detection limit values were estimated as 1.0 X 10 7 M for xanthine and 5.3 X 10-6M for hypoxanthine, respectively. Li used DNA to embed xanthine oxidase and obtained the electrochemical response of FAD and molybdenum center of xanthine oxidase [262], Moreover, the enzyme keeps its native catalytic activity to hypoxanthine in the DNA film. So the biosensor for hypoxanthine can be based on... 

Allopurinol and its major metabolite, oxypurinol, are xanthine oxidase inhibitors and impair the conversion of hypoxanthine to xanthine and xanthine to uric acid. Allopurinol also lowers the intracellular concentration of PRPP. Because of the long half-life of its metabolite, allopurinol can be given once daily orally. It is typically initiated at a dose of 100 mg/day and increased by 100 mg/day at 1-week intervals to achieve a serum uric acid level of 6 mg/dL or less. Serum levels can be checked about 1 week after starting therapy or modifying the dose. Although typical doses are 100 to 300 mg daily, occasionally doses of 600 to 800 mg/day are necessary. The dose should be reduced in patients with renal insufficiency (200 mg/day for CLcr 60 mL/min or less, and 100 mg/day for CLcr 30 mL/min or less). 

Upon purification of the XDH from C. purinolyticum, a separate Se-labeled peak appeared eluting from a DEAE sepharose column. This second peak also appeared to contain a flavin based on UV-visible spectrum. This peak did not use xanthine as a substrate for the reduction of artificial electron acceptors (2,6 dichlor-oindophenol, DCIP), and based on this altered specificity this fraction was further studied. Subsequent purification and analysis showed the enzyme complex consisted of four subunits, and contained molybdenum, iron selenium, and FAD. The most unique property of this enzyme lies in its substrate specificity. Purine, hypoxanthine (6-OH purine), and 2-OH purine were all found to serve as reductants in the presence of DCIP, yet xanthine was not a substrate at any concentration tested. The enzyme was named purine hydroxylase to differentiate it from similar enzymes that use xanthine as a substrate. To date, this is the only enzyme in the molybdenum hydroxylase family (including aldehyde oxidoreductases) that does not hydroxylate the 8-position of the purine ring. This unique substrate specificity, coupled with the studies of Andreesen on purine fermentation pathways, suggests that xanthine is the key intermediate that is broken down in a selenium-dependent purine fermentation pathway. ... 

The decrease in the concentration of IMP preceded a simultaneous increase in the concentrations of both inosine and hypoxanthine in meat samples. (From Tikk et ah, 2006)... 

Among the methods used, the determination of selected of analytes in vitreous humor (and of potassium in particular, based on the observation that its concentration progressively increases in this substrate after death) has been often adopted in the attempt to reduce imprecision of the estimate. Recently, for example, Munoz et al. [142] have developed an HPLC method for the determination of hypoxanthine, another substance whose concentration has been found to increase after death in vitreous humor. Separation was carried out under RP conditions using a mobile phase of KH2PO4 0.05M (pH 3) containing 1% (v/v) methanol at a flow rate of 1.5mL/min. UV spectra were recorded in the range 200-400 nm. Based on the analysis of samples collected at different PMIs, the authors found that about 53% of the variation in the data is explained by PMI. 

Allopurinol, in contrast to the uricosuric drugs, reduces serum urate levels through a competitive inhibition of uric acid synthesis rather than by impairing renal urate reabsorption. This action is accomplished by inhibiting xanthine oxidase, the enzyme involved in the metabolism of hypoxanthine and xanthine to uric acid. After enzyme inhibition, the urinary and blood concentrations of uric acid are greatly reduced and there is a simultaneous increase in the excretion of the more soluble uric acid precursors, xanthine and hypoxanthine. 

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