Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P47989 (xanthine oxidase)
 8,633 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Rabbit liver metallothionein-1 (Mr 6500), which contains zinc and/or cadmium ions, appears to scavenge free hydroxyl (.OH) and superoxide (O-.2) radicals produced by the xanthine/xanthine oxidase reaction much more effectively than bovine serum albumin (Mr 65 000) which was used as a control. Kinetic competition studies between metallothionein and either a spin trap for .OH or ferricytochrome c for O-.2 radicals, gave bimolecular rate constants of the order of kOH/MT approximately equal to 10(12) M-1 X s-1 and kO-2/MT approximately equal to 5 X 10(5) M-1 X s-1, respectively. The former value suggests that all 20 cysteine sulfur atoms are involved in this quenching process and that they all act in the diffusion control limit. The aerobic radiolysis of an aqueous solution of metallothionein, generating O-.2 and .OH radicals, induced metal ion loss and thiolate oxidation. These effects could be reversed by incubation of the irradiated protein with reduced glutathione and the appropriate bivalent metal ion. Metallothionein appears to be an extraordinarily efficient .OH radical scavenger even when compared to proteins 10-50-times its molecular weight. Moreover, hydroxyl radical damage to metallothionein appears to occur at the metal-thiolate clusters, which may be repaired in the cell by reduced glutathione. Metallothionein has the characteristics of a sacrificial but renewable cellular target for .OH-mediated cellular damage.
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PMID:Possible role for metallothionein in protection against radiation-induced oxidative stress. Kinetics and mechanism of its reaction with superoxide and hydroxyl radicals. 298 55

The reactive species involved in the cell lysis during ultraviolet irradiation of Ehrlich ascitic carcinoma cells in the presence of red hair melanin (RHM) were investigated by determining 51Cr release from labeled cells. Cysteine at 1 mM in the presence of RHM increased the cell lysis during the incubation in the dark as well as during irradiation; this lysis was enhanced by superoxide dismutase (SOD). Catalase abolished the dark reaction and inhibited the cysteine-induced increase of cell lysis during irradiation. The cell lysis by the superoxide-generating xanthine oxidase system was not significantly increased by SOD, but was significantly decreased by nitroblue tetrazolium and completely abolished by catalase. The cell lysis induced by the supernatants obtained from the suspensions of RHM either irradiated alone or with cysteine was abolished by catalase. Sediments of irradiated RHM when incubated in the dark with the cells did not release 51Cr. Irradiation of the cells in the presence of the same sediments produced lysis which was not inhibited by catalase. These studies suggest that superoxide per se is not toxic to the cells, but the H2O2 formed by dismutation of superoxide produces cell lysis either directly or by generating OH through Fenton-type reactions. A large part of the cell lysis seen during irradiation of cells in the presence of RHM is not due to H2O2, but may possibly be due to the melanin free radicals formed during irradiation.
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PMID:Role of superoxide and hydrogen peroxide in cell lysis during irradiation in vitro of Ehrlich ascitic carcinoma cells in the presence of melanin. 299 Jun 46

3 alpha-Hydroxysteroid dehydrogenase (EC 1.1.1.50) from Pseudomonas testosterone was inactivated by superoxide radicals generated by the aerobic xanthine oxidase reaction. Superoxide dismutase, NAD+, bovine serum albumin and histidine and cysteine as free amino acids partially protected the enzyme from inactivation. NADH-binding properties were determined by fluorescence spectroscopy, and no variation was found between native enzyme and the unmodified fraction of the partly inactivated one. The fluorescence emission maximum for the completely inactivated enzyme was shifted 10 nm to a longer wavelength when compared with the native one, and it seems possible that the modification of histidine and cysteine residues by superoxide radicals causes the conformational change of the enzyme and the consequent loss of catalytic activity.
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PMID:Inactivation of 3 alpha-hydroxysteroid dehydrogenase by superoxide radicals. Modification of histidine and cysteine residues causes the conformational change. 300 70

The flavoprotein nitroreductases NADPH:cytochrome P-450 reductase and xanthine oxidase catalyzed the cofactor-dependent anaerobic nitro group reduction and covalent binding to protein sulfhydryl groups of the 5-nitroimidazole substrate ronidazole [1-methyl-5-nitroimidazole-2-yl)-methyl carbamate). Studies with variously radiolabeled ronidazole molecules demonstrated that the imidazole ring was intact while greater than 80% of the C-4 3H and 2-carbamoyl group were lost from the covalently bound product. The stoichiometry of cofactor consumption during the enzyme-catalyzed reduction of the substrate could not be determined, so a model nitroreductase system which utilized dithionite as the reductant and agarose-immobilized cysteine as the target for alkylation was developed. Two moles of dithionite was consumed per mole of substrate for maximal reduction of uv absorbance due to the nitro group, for maximal release of C-4 3H, and for maximal covalent binding to agarose-immobilized cysteine. These results indicate that four electrons are required for the reductive activation of the substrate, consistent with formation of a hydroxylamine reactive intermediate. Covalent binding of variously radiolabeled substrate molecules after dithionite reduction exhibited the same labeling pattern as flavoprotein-catalyzed covalent binding, suggesting that covalent binding is mediated by the same species in both chemical and biological systems. The data are consistent with a mechanism where the substrate undergoes four-electron reduction to form a hydroxylamine, which is susceptible to nucleophilic attack at C-4. When water attacks C-4, the 2-carbamoyl group can eliminate to form a Michael-like acceptor which adds thiols at the 2-methylene position.
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PMID:Mechanism of reductive activation of a 5-nitroimidazole by flavoproteins: model studies with dithionite. 312 79

Cultured canine gastric chief cells exposed to a toxic oxygen metabolite-generating system (xanthine plus xanthine oxidase) demonstrated minimal cytolysis, suggesting that these cells have important endogenous antioxidant mechanisms. We have quantified the role of glutathione for protection against toxic oxygen metabolites by measuring cell lysis by lactate dehydrogenase release after variable depletion and repletion of cellular glutathione content. In the absence of exogenous oxidant stress, the glutathione content of chief cells can be depleted to less than 0.2 nmol total glutathione/micrograms DNA or 22% of control without cell lysis over 5 h. However, when challenged with the oxygen metabolite-generating system, cytolysis was greatly enhanced by glutathione depletion. Oxygen metabolite-mediated cytolysis after glutathione depletion was inhibited by exogenous catalase, thiourea, and deferoximine, but not superoxide dismutase or mannitol. These data suggested that hydrogen peroxide and hydroxyl radical mediated cytolysis in glutathione-depleted chief cells. If a substrate for glutathione synthesis, N-acetyl-L-cysteine, was provided to the depleted cells for 1 h before challenge with the oxygen radical-generating system, cell lysis was markedly decreased. However, if glutathione synthesis was blocked during the repletion period by buthionine sulfoximine, protection was not restored. The data supported an important role for glutathione as an endogenous antioxidant, which modulated the sensitivity of cultured chief cells to toxic oxygen metabolite injury.
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PMID:Glutathione modulates toxic oxygen metabolite injury of canine chief cell monolayers in primary culture. 327 18

The role of nonprotein thiols (NPSH) in the enzymatic reduction of the nitro function in 2-nitroimidazoles (2-NI) has been investigated. The addition of NPSH has been shown previously to protect cells from the hypoxic cytotoxicity of 2-NI, whereas depletion of NPSH enhances the hypoxic cytotoxicity. In this report, we have investigated the effects of thiol depleting agents, N-ethylmaleimide (NEM) and diethyl maleate (DEM), on the enzymatic reduction of the nitro group. Cytosolic and microsomal fractions of rat hepatic tissue and xanthine oxidase were employed as sources of nitro reductases. Addition of NPSH caused an enhancement in the reduction of the nitro group of 2-NI; cysteine was significantly more effective than glutathione (GSH) in stimulating the enzymatic reduction. The reduction of the nitro function was decreased markedly in the presence of NEM or DEM. Addition of cysteine or GSH reversed the inhibition with NEM. Both NEM and DEM also attenuated the enhancement of reduction observed after the addition of NPSH. These results suggest that the addition of NPSH facilities the reduction of the nitro function to the reduced intermediates that may be inactivated by an excess of NPSH, whereas the depletion of NPSH allows the accumulation of the toxic nitro radicals causing increased cytotoxicity.
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PMID:Role of nonprotein thiols in enzymatic reduction of 2-nitroimidazoles. 333 46

Molybdenum cofactor (mocofactor) is extracted efficiently, free of impurities and in high concentrations, by acid treatment of xanthine oxidase and subsequent incubation of the precipitate with phosphate buffer containing EDTA, molybdate and oxygen. It is suggested that cofactor is bound to the enzyme via hydrophobic forces as well as via an oxygen-sensitive mechanism. Upon extraction, the capability to complement the apo nitrate reductase of Neurospora crassa nit-1 can be conserved only in the total absence of oxygen. Cysteine and glutathione were shown to protect efficiently free mocofactor from oxidation. Two species of active mocofactor, probably a molybdoform and a demolybdoform, could be separated by means of reversed-phase HPLC with a mobile phase of 5 mM sodium citrate at a pH of 6.5. The mode of interaction between either of these species with thiol reagents is discussed.
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PMID:Extraction and purification of molybdenum cofactor from milk xanthine oxidase. 369 96

There is evidence that oxygen-derived free radicals may play a role in myocardial ischaemic and reperfusion injury. Major sources of O2 free radicals formation during ischaemia and reperfusion are: the enzyme xanthine oxidase, activated neutrophils and the myocardial mitochondria. However, in the heart there are defense mechanisms against the toxic oxygen metabolites. They include the enzyme superoxide dismutase, catalase and glutathione peroxidase plus endogenous antioxidants like vitamin E, ascorbic acid and cysteine. We have investigated in the isolated rabbit hearts the effects of ischaemia and reperfusion on these defence mechanisms. 90 min of ischaemia and/or hypoxia induced a significant reduction of mitochondrial superoxide dismutase, and of reduced glutathione/oxidized glutathione ratio which was further declined after reperfusion indicating that an oxidative stress has occurred. These alterations are associated with massive tissue and mitochondrial calcium accumulation, loss of mitochondrial function and severe membrane damage. The effects of vitamin E on these parameters have been investigated. Administration of 1.1 mg of dl-alpha-tocopherol acetate showed a protective effect on mitochondrial function but it failed to improve the recovery of mechanical function during reperfusion.
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PMID:Role of oxygen in myocardial ischaemic and reperfusion damage: effect of alpha-tocopherol. 384 29

Two hereditary disorders of sulfur amino acid metabolism, beta-mercaptolactate-cysteine disulfideuria and sulfite oxidase deficiency, were described twenty years ago. Other examples of these disorders have been limited to about 5 of each in the world literature since then. Reasons for the apparent rarity of these conditions are discussed and the analytical procedures to identify them are reviewed. The detection of the first depends on the positive result of a cyanide-nitroprusside test followed by positive identification of the specific mixed disulfide. The enzyme mercaptopyruvate sulfur transferase has been shown to be deficient. In the second disorder of sulfite oxidase deficiency, the clinical presentation with progressive dystonia and dislocated lenses in an infant should suggest further laboratory investigations for this disorder which would not be detected by conventional laboratory screening procedures. Laboratory diagnosis can be obtained by use of the Merckoquant sulfite test on a fresh urine sample. Quantitative thiosulfate and taurine measurements can also be made. Positive identification of the specific amino acid S-sulfo-L-cysteine should also be made. The enzyme sulfite oxidase is missing from such organs as liver, kidney and brain. This latter condition may also be associated with xanthinuria. For this combined disorder of sulfite oxidase and xanthine oxidase, a deficiency of a molybdenum-containing cofactor has been demonstrated.
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PMID:A review of the clinical presentation and laboratory findings in two uncommon hereditary disorders of sulfur amino acid metabolism, beta-mercaptolactate cysteine disulfideuria and sulfite oxidase deficiency. 388 41

1. The presence of xanthine was required for the inhibition of bovine milk xanthine oxidase by o-iodosobenzoate, iodoacetamide, hydrogen peroxide or p-chloromercuribenzoate. 2. Inactivation by p-chloromercuribenzoate was very rapid, was reversed by cysteine and was less in the presence of FAD. Lineweaver-Burk plots showed that the inactivation by p-chloromercuribenzoate was competitive with substrate. 3. Inactivation by o-iodosobenzoate, iodoacetamide or hydrogen peroxide could not be reversed by cysteine or xanthine. However, the presence of xanthine during the incubation with inhibitor protected the enzyme against o-iodosobenzoate but not against iodoacetamide or hydrogen peroxide. 4. p-Chloromercuribenzoate protected the enzyme against inactivation by hydrogen peroxide.
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PMID:Xanthine oxidase inactivation by reagents that modify thiol groups. 562 93


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