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)

The generation of hydroxyl radicals by the xanthine-xanthine oxidase reaction (C. Beauchamp and I. Fridovich (1970) J. Biol. Chem. 245, 4641-1616) has been shown to be increased by iron-saturated lactoferrin isolated from pig neutrophils. Hydroxyl radical production, measured by EPR spin trapping and by ethylene production from alpha-keto-gamma-methiol butyric acid, has been demonstrated to be produced by a Fenton-type Haber-Weiss reaction catalysed by lactoferrin. The possibility that lactoferrin catalyses such a reaction in vivo is considered.
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PMID:Enhanced production of hydroxyl radicals by the xanthine-xanthine oxidase reaction in the presence of lactoferrin. 628 Jul 74

The relaxation behavior of the EPR signals of MoV, FAD semiquinone, and the reduced Fe/S I center was measured in the presence and absence of other paramagnetic centers in milk xanthine oxidase. Specific pairs of prosthetic groups were rendered paramagnetic by poising the native enzyme or its desulfo glycol inhibited derivative at appropriate potentials and pH values. Magnetic interactions were found between the following species: Mo--Fe/S I (100-fold increase in microwave power required to saturate the MoV EPR signal at 103 K when Fe/S I is reduced as opposed to oxidized), FAD--Fe/S I and FAD--Fe/S II (70-fold increase in power required to saturate the FADH.EPR signal at 173 K when either Fe/S center is reduced), and Fe/S I--Fe/S II (2.5-fold increase in power to saturate the reduced Fe/S I EPR signal at 20 K when Fe/S II is reduced). The Mo--Fe/S I interaction was also detected as a reduced Fe/S I induced splitting of the MoV EPR spectrum at 30 K. No splittings of the FADH. or Fe/S center spectra were detected. No magnetic interactions were found between FAD and Mo or between Mo and Fe/S II. These results, together with those of Coffman & Buettner [Coffman, R. E., & Buettner, G. R. (1979) J. Phys. Chem. 83, 2392-2400], were used to estimate the following approximate distances between the electron carrying prosthetic groups of milk xamthine oxidase: Mo--Fe/S I, 11 +/- 3 A; Fe/S I-Fe/S II, 15 +/- 4 A; FAD-Fe/S I, 16 +/- 4 A; FAD-Fe/S II, 16 +/- 4 A. A model for the arrangement of these groups within the xanthine oxidase molecule is suggested.
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PMID:Magnetic interactions in milk xanthine oxidase. 628 13

Rabbit liver aldehyde oxidase (AO), like milk xanthine oxidase (XO) and chicken liver xanthine dehydrogenase (XDH), possesses the following prosthetic groups: FAD, a functional Mo center, and two spectroscopically distinct iron-sulfur centers, one with gav less than 2.0 (termed Fe/S I) and the other with gav greater than 2.0 (termed Fe/S II) in the reduced enzyme. EPR spectra for the Mov species were found to be nearly identical in AO and XO for a number of enzyme complexes, and the midpoint reduction potentials for functional MoVI/MoV (-359 mV) and MoV/MoVI (-351 mV) were nearly the same in all three enzymes (50 mM phosphate, pH 7.8). A strong magnetic interaction between MoV and reduced Fe/S I, previously detected in XO and XDH, was also found in AO. No MoV-Fe/S II interaction could be detected in AO (nor in XO). In contrast, the order of reduction of Fe/S I and Fe/S II, as measured from their midpoint potentials, is reversed in AO (Em = -207 and -310 mV, respectively) as compared to XO (Em = -280 and -245 mV, respectively) in phosphate buffer at pH 7.8. The oxidized-reduced extinction coefficients at 450 and 550 nm for the two centers are also apparently reversed in AO and XO. Although magnetic interaction between FAD and one or both reduced Fe/S centers has been detected in both AO and XO, no magnetic interaction between the two reduced Fe/S centers themselves was found in AO (although such interaction has been seen in XO). The average FAD reduction potential is substantially more positive in AO (Em for FAD/FADH., -258 mV; FADH./FADH2, -212 mV at pH 7.8) than in XO or XDH. It can be concluded that although the properties and immediate environment of the functional Mo center are conserved in the three Mo hydroxylase enzymes, and all three enzymes possess the same set of prosthetic groups, the properties of the groups which transfer electrons from the Mo to the ultimate electron acceptor can vary substantially in AO, XO, and XDH.
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PMID:Properties of the prosthetic groups of rabbit liver aldehyde oxidase: a comparison of molybdenum hydroxylase enzymes. 628 79

The binding of arsenite to the molybdenum center of milk xanthine oxidase is re-examined. The Kd for the arsenite complex has been determined to be 24 microM from equilibrium binding studies and this value has been confirmed by determination of the association and dissociation rate constants for the interaction of arsenite with xanthine oxidase. Formation of the complex is not prevented by prior reaction of the enzyme with thiol reagents such as 5,5'-dithiobis-(2-nitrobenzoic acid) or methyl methanethiosulfonate. Binding of arsenite to the enzyme perturbs both the oxidation-reduction potentials and the electron paramagnetic resonance signal of the molybdenum center observed after partial reduction of the enzyme with sodium dithionite. The EPR signal of the partially reduced arsenite-complexed enzyme is further modified in two different ways by the addition of xanthine or salicylate. Other purine and pteridine substrates and products for the enzyme yield EPR signals indistinguishable from that generated by xanthine, whereas aromatic aldehydes and carboxylic acids give signals similar to that observed in the presence of salicylate. It is thus clear that while arsenite prevents enzyme turnover, it does not preclude binding of substrate and product molecules. Binding of arsenite at the molybdenum center of xanthine oxidase does not disturb the oxidation-reduction potentials of the iron-sulfur centers of the enzyme, but evidence is presented to suggest that the midpoint potential of the FAD site is decreased by approximately 15 mV. A structure for the arsenite complex is proposed to provide a framework in which to interpret the EPR signals in a quantitative fashion.
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PMID:The interaction of arsenite with xanthine oxidase. 630 Jan 1

Hydroxyl radicals are generated in the hypoxanthine-xanthine oxidase system in the presence of iron-saturated transferrin isolated from human serum. This has been demonstrated by colorimetrically measuring the hydroxylation of salicylic acid and by EPR using the spin trap DMPO (5,5-dimethyl-1-pyrroline-N-oxide). A Fenton-type Harber-Weiss reaction catalyzed by transferrin is proposed.
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PMID:Superoxide-dependent formation of hydroxyl radical catalyzed by transferrin. 630 16

The interaction of xanthine oxidase with the substrate analog 8-bromoxanthine has been examined in an effort to determine the nature of interaction of purines with the active site of the enzyme. It is found that 8-bromoxanthine is an inhibitor of xanthine oxidase with a Ki of approximately 400 microM; inhibition is uncompetitive with respect to xanthine and noncompetitive with respect to molecular oxygen. While 8-bromoxanthine has only a slight effect on the reaction of reduced enzyme with oxygen, it dramatically slows the rate of enzyme reduction by xanthine, suggesting that inhibition does involve the interaction of 8-bromoxanthine with the molybdenum center of the enzyme. KD determinations for binding of 8-bromoxanthine to oxidized and reduced xanthine oxidase indicate that the inhibitor binds preferentially to the fully reduced form of the molybdenum center (MoIV), with dissociation constants of 1.5 mM and 18 microM for oxidized and reduced enzyme, respectively. This preferential binding to the reduced form of the enzyme is manifested in a significant increase in the oxidation-reduction potentials of the molybdenum center as determined by potentiometric titrations with 8-bromoxanthine complexed with xanthine oxidase. The shape of the Mov EPR signal observed in the course of these titrations as well as a comparison with results of reductive titrations and KD determinations with uric acid and xanthine indicate that 8-bromoxanthine interacts with the molybdenum center of xanthine oxidase in a way that is typical of purine substrates and products, despite the presence of the bulky Br group. The inhibitor thus has a potential as a probe of enzyme-substrate interactions, particularly using the technique of x-ray absorption spectroscopy.
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PMID:The inhibition of xanthine oxidase by 8-bromoxanthine. 631 3

Isolated erythrocyte membranes incubated with xanthine, xanthine oxidase, and Fe(III) underwent lipid peroxidation, as indicated by the thiobarbituric acid reaction and iodometric determination of hydroperoxides. In detergent-free medium (phosphate buffered saline) peroxidation was inhibited by superoxide dismutase, catalase, and EDTA; but was promoted by OH. scavangers, eg. mannitol. Generation of OH. in the system via iron-catalyzed reduction of H2O2 by O-2 was demonstrated by EPR spectrometry using spin trapping. In membranes treated with Triton X-100 lipid peroxidation was stimulated by EDTA and suppressed by OH. traps. This and other evidence suggests that OH. in the medium was an effective initiator of lipid peroxidation in detergent-dispersed membranes, but not in intact membranes.
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PMID:Superoxide and hydrogen peroxide-dependent lipid peroxidation in intact and triton-dispersed erythrocyte membranes. 632 49

Very little is known of the metabolism of copper on a molecular level. For example, there is no evidence of an oxidative breakdown of Cu(I)-thionein leading to Cu(II). Thus it was of interest to use L- and D-amino-acid oxidases, amino oxidase and galactose oxidase to control the oxidation of Cu(I)-thionein by enzymically generated H2O2. In the presence of these enzymes Cu(II) was generated in each case. In a more detailed study the Cu(I)-thiolate chromophores of Cu-thionein were oxidized in the presence of xanthine oxidase as deduced from spectrometrical measurements using EPR and circular dichroism. Unlike Cu2Zn2-superoxide dismutase catalase inhibited the oxidative cleavage, suggesting peroxide as the actual oxidizing agent. Possibly there is an enzymic oxidative pathway for the generation of biologically important Cu(II).
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PMID:Oxidation of Cu(I)-thionein by enzymically generated H2O2. 654 80

A series of potentiometric titrations of xanthine oxidase have been performed at room temperature in the pH range 6.1-9.9. Reduction of the two Fe/S centers was monitored by CD, and that of the FAD and Mo center by EPR. The Fe/S centers behave as centers having a protonable group whose pKa changes with reduction state (E = -344 mV, pKo = 6.4, and pKr = 8.1 for Fe/S I; E = -249 mV, pKo = 6.4, and pKr = 8.0 for Fe/S II). The flavin and the two types of molybdenum centers show varying behavior, but, in all cases, electron addition is accompanied by protonation. The sequence for FAD is reduction, protonation, reduction, protonation with E1 = -398 mV, E2 = -240 mV, pK1 = 9.5, pK2 = 7.4. For "rapid" molybdenum, the sequence is protonation, reduction, protonation, reduction with E1 = -369 mV, E2 = -301 mV, pK1 = 7.9, pK2 = 8.4; and for slow molybdenum, protonation, reduction, protonation with E1 = 320 mV, E2 = -477 mV, pK1 = 7.5, pK2 = 9.5. Comparison to data obtained previously at cryogenic temperatures (Cammack, R., Barber, M. J., and Bray, R. C. (1976) Biochem. J. 157, 469-468 and Barber, M. J., and Seigel, L. M. (1982) in Flavins and Flavoproteins (Massey, V., and Williams, C. H., eds) pp. 796-804, Elsevier/North-Holland, New York) showed the centers to have significant temperature dependence, which calls for a re-evaluation of conclusions reached using cryogenic techniques (e.g. rapid-freeze). The optical absorbance characteristics of the enzyme were also investigated and a possible absorbance for molybdenum was suggested.
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PMID:The room temperature potentiometry of xanthine oxidase. pH-dependent redox behavior of the flavin, molybdenum, and iron-sulfur centers. 689 74

Anaerobic addition of lumazine (2,4-dihydroxypteridine) to xanthine oxidase leads to a rapid bleaching of the enzyme with concomitant formation of a long wave-length species with maximum absorbance at 650 nm. At the conclusion of the reaction there is a net increase in absorbance at wavelengths greater than 550 nm. A spectrally similar species is obtained when violapterin is added to dithionite-reduced enzyme. This long wavelength absorbance is not accompanied by an EPR signal characteristic of flavin neutral radical and it is produced equally well in flavin-free xanthine oxidase. Its production is eliminated by the inhibitors, cyanide and allopurinol, which react at the molybdenum center. We conclude that this new species is a charge transfer complex between Mo(IV) and the product of the reaction, violapterin (2,4,7-trihydroxypteridine). A similar, though less intense, absorbance at 650 nm is also obtained when lumazine is bound to the reduced enzyme. These Mo(IV)-pteridine charge transfer compounds are optically active and exhibit intense circular dichroism centered at 630 nm.
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PMID:Charge transfer complexes between pteridine substrates and the active center molybdenum of xanthine oxidase. 689 5


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