Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:1.17.3.2 (xanthine oxidase)
8,383 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

1. An electron-spin-resonance signal with g( parallel)2.08 and g( perpendicular)2.00 is observed by the rapid-freezing technique during the oxidation of substrates by molecular oxygen catalysed by xanthine oxidase at pH10. 2. The intensity of this signal is shown to depend on oxygen rather than on enzyme concentration, indicating that it is due to an oxygen free radical and not to the enzyme. 3. The same species is shown to be produced in the reaction at pH10 between hydrogen peroxide and periodate ions. Studies with this system have facilitated comparison of the properties of the oxygen radical with data in the literature on the products of pulse radiolysis of oxygenated water over a wide pH range. 4. It is concluded that the species observed is the superoxide ion, O(2) (-), and that the stability of this ion is greatly increased in alkaline solution. A mechanism explaining the alkaline stability is proposed. 5. The importance of O(2) (-) in the enzymic reaction is discussed.
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PMID:Electron-spin-resonance evidence for enzymic reduction of oxygen to a free radical, the superoxide ion. 430 73

Studies have been carried out of effects of 17O substitution on a Mo(V) e.p.r. signal from xanthine oxidase, known as Very Rapid. This transient signal is believed to represent an intermediate in enzymic turnover. When Very Rapid was developed from enzyme equilibrate with 17O-enriched water, strong coupling of Mo(V) to a single oxygen atom was observed, with A(17O)1,2,3 1.34, 1.40, 1.36 mT. The isotropic character of the splittings is interpreted as favouring a structure of the type Mo--O--C. The rate of exchange with water of the oxygen atom detected in the signal was studied. In oxidized enzyme, which contains a terminal oxygen ligand, the exchange rate constant was 2--4 h-1 (pH 5.9--7.8 and about 20 degrees C). However, if the exchange was allowed to take place whilst the enzyme was turning over a substrate, then the process occurred within a few seconds. The present and previous results are interpreted as favouring an enzymic mechanism in which a terminal oxygen ligand reacts, as a nucleophile, with a substrate carbonium ion. To complete the reaction, product liberation, by hydrolysis of the enzyme-bound species, occurs in such a way as to cleave the Mo--O bond, thus explaining the fast oxygen exchange in the presence of the substrate.
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PMID:Oxygen-17 splitting of the very rapid molybdenum(V) e.p.r. signal from xanthine oxidase. Rate of exchange with water of the coupled oxygen atom. 626 85

The effect of using [17O]water (24-50% enriched) as solvent on the Mo(V) electron paramagnetic resonance spectra of different reduced forms of xanthine oxidase has been investigated. All the Mo(V) signals are affected. Procedures are described, based on the use of difference spectral techniques, that facilitate interpretation of such spectra. The number of coupled oxygen atoms may be determined by estimation of the fraction of the spectrum that remains unchanged by the isotope at a known enrichment. For a species having two coupled oxygen atoms, the use of two different isotope enrichments permits elimination from the difference spectra of the contribution of the two singly substituted species. From the application of these methods, it is concluded that not only the strength of the hyperfine coupling of oxygen ligands of molybdenum but also their number and their exchangeability with the solvent vary from one reduced form of the enzyme to another. The inhibited species from active xanthine oxidase has been studied in the most detail. It has two weakly coupled oxygen atoms [A(17O)av = 0.1-0.2 mT] that do not exchange with the solvent. A cyclic structure is proposed for this species in which two oxygen ligands of molybdenum are bonded to the carbon of the formaldehyde or other alcohol or aldehyde molecule that reacted in producing the signal. Structures of the other signal-giving species from active xanthine oxidase (Very Rapid and Rapid types 1 and 2) are discussed, as is corresponding information on species from the desulfo enzyme and from sulfite oxidase.
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PMID:Numbers and exchangeability with water of oxygen-17 atoms coupled to molybdenum (V) in different reduced forms of xanthine oxidase. 629 49

The catalysis by iron of the formation of reactive oxygen species in biological systems has been well documented. In this present study, we have investigated the hypothesis that iron-catalyzed formation of hydroxyl radical (.OH) from superoxide anion radical (O-.2) and H2O2 requires the availability of at least one iron coordination site that is open or occupied by a readily dissociable ligand such as water. This hypothesis was tested by measuring the catalytic activity of 12 different iron chelates using hypoxanthine and xanthine oxidase to generate O-.2. In these same chelates, we also determined the presence or absence of coordinated water by UV-visible spectroscopy and 1H NMR relaxation measurements. Of all chelates tested, only Fe3+ coordinated to diethylenetriamine pentaacetic acid; ethylenediamine di(o-hydroxyphenylacetic acid), phytate, and Desferal lacked coordination water; and only these four complexes failed to produce hydroxyl radical. Separate determinations of the two redox half-reactions involved (i.e. Fe3+ + O-.2----Fe2+ + O2 and Fe2+ + H2O2----Fe3+ + .OH + OH-) indicate that an available coordination site is necessary for the latter (Fenton) reaction. This principle governing iron reactivity may help advance our understanding of the mechanism of oxidative damage in biological systems and may also permit the design of more effective chelators for the control of iron in biological systems.
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PMID:Iron-catalyzed hydroxyl radical formation. Stringent requirement for free iron coordination site. 632 33

Formamide is a substrate of xanthine oxidase. At pH 8.2 and 1.14 mM-O2, Vmax.(app.) is 3.1 s-1 and Km (app.) is 0.7 M. Mo(V) e.p.r. signals obtained by treating the enzyme with formamide were studied, and these provide new information about the ligation of molybdenum in the enzyme and about the enzymic mechanism. The substrate is the first compound that is not a nitrogen-containing heterocycle to give a Very Rapid signal. This supports the hypothesis that the Very Rapid signal, though it is not detectable with all substrates, represents an essential intermediate in turnover. Formamide also gives the Inhibited signal and is the first non-aldehyde substrate to do so. The Rapid type 1 signal obtained in the presence of formamide was examined in H2O enriched with 2H or with 17O. The single oxygen atom detectable in the signal is shown to be strongly and anisotropically coupled. This indicates that this atom remains as an oxo ligand of molybdenum in this signal-giving species. Other structural features of this species are discussed.
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PMID:Formamide as a substrate of xanthine oxidase. 633 8

Polymorphonuclear leukocytes and other inflammatory cells release superoxide anion and additional oxidant species following stimulation. Corneal endothelial cells were exposed to a flux of chemically generated superoxide anion (oxygen-free radical) produced by the combination of 1 mM hypoxanthine and 0.06 U/ml xanthine oxidase. Exposure of endothelial cells to the combination of hypoxanthine and xanthine oxidase resulted in anatomic disruption of the cells with interference in the function of endothelial water movement and resultant swelling of the corneal stroma. Catalase reduced the corneal swelling caused by exposure of endothelium to the oxygen-free radical generating system, whereas superoxide dismutase, ascorbic acid, D-mannitol, and ethanol did not prevent damage. The data suggest that hydrogen peroxide produced during the dismutation reaction of the superoxide anion is one of the toxic species, whereas the superoxide anion itself and the hydroxyl-free radical probably do not participate. The data suggest that corneal endothelial cells are susceptible to physiologic and anatomic damage induced by the products of reactive oxygen species, which, from previous studies, are known to be generated by inflammatory cells. The development of therapeutic modalities directed at the prevention of damage produced by hydrogen peroxide and other oxidant species may be of benefit in reducing corneal endothelial cell damage secondary to ocular inflammatory disease processes.
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PMID:Hydrogen peroxide-mediated corneal endothelial damage. Induction by oxygen free radical. 643 89

Evidence is presented for a sensitive method useful for the detection of hydroxyl free radical generation in various systems. The methodology employs high pressure liquid chromatography with electrochemical detection (LCED) for the quantification and identification of the hydroxylation products from the reaction of OH with both phenol and salicylate. A detection limit of less than 1 pmol for the hydroxylation products has been achieved with electrochemical detector responses linear over at least three orders of magnitude. Detection and quantitation of the hydroxylation products obtained and formed during OH generation from biologically meaningful systems have been demonstrated. The three systems utilized were ADP/FE(II)/H2O/, hypoxanthine/xanthine oxidase plus chelated iron, and UV photolysis of H2O2.
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PMID:Sensitive assay of hydroxyl free radical formation utilizing high pressure liquid chromatography with electrochemical detection of phenol and salicylate hydroxylation products. 653 May 10

We studied the cerebral effects of oxygen-derived free radicals generated from the xanthine oxidase/hypoxanthine/ADP-Fe3+ system. Xanthine oxidase/hypoxanthine/ADP-Fe3+ solution (0.1 ml) was infused into caudate putamen, and brain was frozen rapidly in situ. Brain water and sodium content increased concomitant with decreased potassium content at 24 hours and 48 hours after the infusion. The degree of brain edema and injury depended on the dose of xanthine oxidase. Spongy neuropil and neuronal cytoplasmic vacuoles were seen at 2 hours, with an infiltration by polymorphonuclear leukocytes at 24 hours, followed by lipid-laden macrophages and reactive astrocytes. Leakage of fluorescent dye into neuropil was seen at 2 hours, but not later. These data suggest that oxygen-derived free radicals damage endothelial cells of the blood-brain barrier; the brain injury is characterized by edema and by structural damage of neurons and glia.
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PMID:Brain injury, edema, and vascular permeability changes induced by oxygen-derived free radicals. 654 10

A specific and highly sensitive fluorimetric method was developed for the determination of 6-mercaptopurine (6MP) in serum. The method is based on the enzymatic oxidation of 6MP with xanthine oxidase to the oxypurine, followed by oxidation with acidic chromate to the corresponding 6-sulfonate. The fluorescent product has excitation and emission maxima at 330 and 400 nm, respectively. The limit of sensitivity was approximately 22 pg/ml for 6MP in water. The sensitivity limit for 6MP in serum containing azathioprine was approximately 2.2 ng/ml. The rate constants for conversion of 6MP into the final product (6-thiouric acid) and the apparent Michaelis constant were also determined by a nonlinear regression analysis based on the integrated Michaelis-Menten equation using the ultraviolet absorbance data and the simplified complementary tristimulus colorimetry.
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PMID:The use of xanthine oxidase for a specific and sensitive fluorimetric determination of 6-mercaptopurine in serum. 654 26

Acyclovir [9-[(2-hydroxyethoxy)methyl]guanine] is an acyclic guanine nucleoside analogue that is widely used clinically as an antiherpetic agent. Its limited absorption in humans after oral administration prompted the search for prodrugs. A congener, referred to as 6- deoxyacyclovir [2-amino-9-[(2-hydroxyethoxy)methyl]-9H-purine], was synthesized and found to be 18 times more water soluble than was acyclovir. Surprisingly, this congener was readily oxidized to acyclovir by xanthine oxidase (EC 1.2.3.2). It was also oxidized by aldehyde oxidase (EC 1.2.3.1) largely to 8-hydroxy-6- deoxyacyclovir [2-amino-8-hydroxy-9-[(2-hydroxyethoxy)methyl]-9H-purine] and then to 8- hydroxyacyclovir [2-amino-6,8-dihydroxy-9[(2-hydroxyethoxy)methyl]-9H-purine]. 6- Deoxyacyclovir and the major products of its oxidation by aldehyde oxidase lacked appreciable activity against herpes simplex type I in vitro. On the basis of these results, it was apparent that the success of 6- deoxyacyclovir as a prodrug in vivo would depend upon how well its desired activation by xanthine oxidase competed with the nonactivating oxidations by aldehyde oxidase. In rats dosed orally with 6- deoxyacyclovir , absorption was extensive and the major urinary metabolite was acyclovir. In two human volunteers, urinary excretions of acyclovir were 5-6 times greater than those typically observed after administration of equivalent doses of acyclovir itself. The areas under the plasma concentration-time curves for acyclovir were also 5-6 times greater. Plasma levels of acyclovir peaked soon after ingestion of the prodrug, indicating rapid absorption and metabolic conversion. These results suggested that 6- deoxyacyclovir might have clinical usefulness as a prodrug of acyclovir suitable for oral administration.
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PMID:6-Deoxyacyclovir: a xanthine oxidase-activated prodrug of acyclovir. 658 47


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