SEARCH · Search PubMed

Results for “Xanthines”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3Linked to original sources

PubMed1987

Does the xanthine-xanthine oxidase system alter contractile behaviour of vascular smooth muscle?

Since xanthine oxidase (XOD) preparations are usually applied as crystalline suspensions in ammonium sulphate solution, we studied separately the contractile responses of excised rabbit carotid arteries to the free radical generating system of xanthine-XOD and ammonium ions. Using dialyzed XOD (0.1-0.2 U/ml) the contractile pattern of the xanthine-XOD system was characterized by initial contraction followed by relaxation. Identical contractile behaviour was observed when xanthine was omitted from that system. These contractile responses were indistinguishable from those obtained with ammonium sulphate (10-20 mM). The threshold concentration of ammonium sulphate for inducing contraction was 1-2 mM. Both Ca++-free solution and verapamil only slightly inhibited the contractions produced by XOD and ammonium sulphate. No contractile effect was observed with dialyzed XOD. We conclude that the contractile effects of the xanthine-XOD system on vascular smooth muscle are due to simultaneously applied ammonium ions, and our findings emphasize the need for further purification of XOD preparations when investigating their effects on smooth muscle.

Ammonia↗

PubMed1990

Luminol chemiluminescence using xanthine and hypoxanthine as xanthine oxidase substrates.

Luminol chemiluminescence induced by the xanthine or hypoxanthine-O2-xanthine oxidase system is analyzed and compared. Characteristics of the light emission curves were examined considering the conventional reaction scheme for the oxidation of both substrates in the presence of xanthine oxidase. The ratio of the areas of the rate of superoxide production during substrate oxidation to uric acid. The O2-. to uric acid ratio for each substrate can account for differences in xanthine and hypoxanthine-supported light emission, since uric acid is a strong inhibitor of O2-.-dependent luminol chemiluminescence. These results are consistent with a free radical scavenging role for uric acid. A similar but weaker scavenging effect of xanthine may also contribute to the observed differences in chemiluminescent yields between both substrates.

Animals↗

PubMed1990

Lack of conversion of xanthine dehydrogenase to xanthine oxidase during warm renal ischemia.

Irreversible transformation of xanthine dehydrogenase (XDH) to xanthine oxidase (XO) during ischemia was determined measuring XDH and total enzyme activity in kidneys before and after 60 min of clamp of the renal pedicle. Tissue levels of adenine nucleotides, xanthine and hypoxanthine were used as indicators of ischemia. After 60 min of clamping, ATP levels decreased by 72% with respect to controls whereas xanthine and hypoxanthine progressively reached tissue concentrations of 732 +/- 49 and 979 +/- 15 nmol.g tissue-1, respectively. Both total and XDH activities in ischemic kidneys (30 +/- 15 and 19 +/- 1 nmol.min-1.g tissue-1) were significantly lower than in controls when expressed on a tissue weight basis. The fraction of enzyme in the XDH form was however unchanged indicating that the reduction of the nucleotide pool is not accompanied by induction of the type-O activity of xanthine oxidase.

Adenosine Diphosphate↗

PubMed1989

Simultaneous assay of hypoxanthine, xanthine and allopurinol by high-performance liquid chromatography and activation of immobilized xanthine oxidase as an enzyme reactor.

A selective and sensitive assay of substrates (hypoxanthine, xanthine and allopurinol) of xanthine oxidase by reversed-phase liquid chromatography coupled with the use of immobilized enzyme reactors is described. These compounds were oxidized by immobilized xanthine oxidase and produced hydrogen peroxide, which was determined fluorometrically using immobilized peroxidase and p-hydroxyphenylacetic acid. The detection limits of hypoxanthine, xanthine and allopurinol were approximately 50, 120 and 130 pg per injection, respectively. Immobilized xanthine oxidase inhibited by oxipurinol during the assay was reactivated by 2,6-dichlorophenolindophenol and could be used for a long period without a significant activity loss. These methods were applied to plasma and urine samples.

Allopurinol↗

PubMed1991

A xanthinuric family--the proposita having immunologically reactive xanthine oxidase but no xanthine oxidase activity.

The antibody was raised against purified human liver xanthine oxidase in a rabbit. In a xanthinuric patient, the double immunodiffusion method demonstrated the existence of an immunologically reactive duodenal mucosa xanthine oxidase which did not possess xanthine oxidase activity. These results indicated that xanthine oxidase protein is abnormal in structure and/or amino acid sequence.

Female↗

PubMed1986

In vitro damage to rat lens by xanthine-xanthine oxidase: protection by ascorbate.

The potential of ascorbic acid acting against the toxic effects of active oxygen species on the lens has been studied. The active species of oxygen were generated by the action of xanthine oxidase on xanthine. Rat lenses incubated in medium containing xanthine and xanthine oxidase were physiologically damaged, as evidenced by the decrease in the ability of the tissue to accumulate rubidium or alpha-aminoisobutyric acid against a concentration gradient. The pressure of ascorbate in the medium protected against the tissue damage. One of the functions of high ascorbate in the aqueous humor of many primates including human beings may, therefore, be to protect the lens and other surrounding tissues against the toxic effects of active oxygen derivatives produced in situ under ambient, as well as under photochemical, conditions.

Aminoisobutyric Acids↗

PubMed2005

NADH oxidase activity of rat liver xanthine dehydrogenase and xanthine oxidase-contribution for damage mechanisms.

The involvement of xanthine oxidase (XO) in some reactive oxygen species (ROS) -mediated diseases has been proposed as a result of the generation of O*- and H2O2 during hypoxanthine and xanthine oxidation. In this study, it was shown that purified rat liver XO and xanthine dehydrogenase (XD) catalyse the NADH oxidation, generating O*- and inducing the peroxidation of liposomes, in a NADH and enzyme concentration-dependent manner. Comparatively to equimolar concentrations of xanthine, a higher peroxidation extent is observed in the presence of NADH. In addition, the peroxidation extent induced by XD is higher than that observed with XO. The in vivo-predominant dehydrogenase is, therefore, intrinsically efficient at generating ROS, without requiring the conversion to XO. Our results suggest that, in those pathological conditions where an increase on NADH concentration occurs, the NADH oxidation catalysed by XD may constitute an important pathway for ROS-mediated tissue injuries.

Aldehyde Oxidase↗

PubMedDate not supplied

Chemiluminescence measurements of xanthine oxidase and xanthine dehydrogenase activity in four types of cardiovascular cell.

The activity and location of xanthine oxidase (EC.1.2.3.2.) and xanthine dehydrogenase (EC.1.2.1.37) have been measured using luminol-enhanced chemiluminescence in four types of cell from the cardiovascular system (neonatal and adult rat cardiac myocytes, rat aortic vascular smooth muscle cells, rat cardiac fibroblasts and human umbilical vein endothelial cells). The detection system developed was both rapid and reproducible and could be used on sub-milligram quantities of cells. Xanthine oxidase was located primarily in cells derived from the vasculature and especially in endothelial cells, as was xanthine dehydrogenase. Only neonatal myocytes had more dehydrogenase activity than oxidase. The significance of the location and activity of these enzymes is discussed in relation to the pathology of myocardial ischaemia, arrhythmogenesis and microvascular disorders.

Animals↗

PubMed1976

Oxidation of N-methyl substituted hypoxanthines, xanthines, purine-6,8-diones and the corresponding 6-thioxo derivatives by bovine milk xanthine oxidase.

1. The oxidation of six series of purines (hypoxanthines, xanthines, purine-6,8-diones and the corresponding 6-thioxo derivatives) by a highly purified bovine milk xanthine oxidase (EC 1.2.3.2) has been studied, using a variety of N-methyl derivatives. 2. N-Methyl substituents can either enhance or reduce enzymic rates. Enhancement is ascribed to blockade of groups which mediate unfavorable modes of binding of substrate to enzyme. Introduction of N-methyl groups can also inhibit enzymic oxidation, either by occluding essential binding groups or by preventing spontaneous or enzyme-induced tautomerisation processes, which create suitable binding sites in the substrates. 3. In all purines which are rapidly attacked by xanthine oxidase, proper attachment to the active center is mediated by the groupings (3) NH, (9) N or (3) N, (9) NH. 4. Reduced rates usually express lowered substrate affinity, which finds its expression in weak competitive inhibition of xanthine oxidation.

Animals↗

PubMed1985

Inhibitory action of quercetin on xanthine oxidase and xanthine dehydrogenase activity.

Quercetin is an equally good inhibitor of xanthine oxidase (type O, oxygen-reducing enzyme) and xanthine dehydrogenase (type D, NAD+-reducing enzyme) activity of a preparation of the xanthine-oxidizing enzyme partially purified from rat liver. The inhibition seems competitive with the oxidase form and non-competitive (mixed-type) with the dehydrogenase form of the enzyme. These inhibitory properties should be referred to the flavonoid structure of quercetin rather than to its antioxidant power. The antioxidant properties of quercetin and its inhibitory effect on the xanthine-oxidizing enzyme are discussed with reference to hyperuricemic and ischemic states.

Animals↗

PubMed1989

Proteolytic conversion of xanthine dehydrogenase to xanthine oxidase: evidence against a role for calcium-activated protease (calpain).

The present study tested the hypothesis that calpain is responsible for the limited proteolytic conversion of xanthine dehydrogenase (XD) to xanthine oxidase (XO). We compared the effects of various proteases on the activity and molecular weight of a purified preparation of xanthine dehydrogenase from rat liver. In agreement with previous reports, trypsin treatment produced a complete conversion of XD to XO accompanied by a limited proteolysis of XDH from an Mr of 140 kD to an Mr of 90 kD. Treatment with calpain I or calpain II did not produce a conversion from XD to XO nor did it result in partial proteolysis of the enzyme. Similarly, trypsin treatment partially degraded a reversibly oxidized form of xanthine dehydrogenase while calpain I or calpain II were ineffective. The possibility that an endogenous inhibitor prevented the proteolysis of XDH by calpain I or II was excluded by verifying that brain spectrin, a known calpain substrate, was degraded under the same incubation conditions. The results indicate that calpain is not likely to be responsible for the in vivo conversion of XD to XO under pathological conditions.

Animals↗

PubMed2001

Tautomeric energetics of xanthine oxidase substrates: xanthine, 2-oxo-6-methylpurine, and lumazine.

The stability of the tautomers of each of the three important substrates of xanthine oxidase, xanthine, 2-oxo-6-methylpurine, and lumazine, was examined by quantum mechanical calculations. The geometries of these tautomers were optimized at the AM1, Hartree-Fock (HF/6-31G), and hybrid Hartree-Fock/density functional theory (B3LYP/6-31G(d)) levels of theory. The single point energies of some of the more stable tautomers for each of the substrates were calculated at the B3LYP/6-311 +G(2d,p) level of theory. The Conductor Polarized Continuum Model (CPCM) was used to evaluate the solvent effects on the relative stabilities of these tautomers. The calculations clearly identify the lowest energy tautomeric form for xanthine and lumazine. On the other hand, there appear to be three tautomers for 2-oxo-6-methylpurine, with only minor energetic differences in vacuo. In water, however, only one of them predominates. The lowest energy tautomers presumably represent the predominant tautomeric forms at the molybdenum center of xanthine oxidase during catalysis. Implications of these computational results are discussed in the context of enzyme catalysis.

Catalysis↗

PubMed1978

Magnetic coupling of the molybdenum and iron-sulphur centres in xanthine oxidase and xanthine dehydrogenases.

Magnetic interaction between molybdenum and one of the iron-sulphur centres in milk xanthine oxidase [Lowe, Lynden-Bell & Bray (1972) Biochem. J. 130, 239-249] was studied further, with particular reference to the newly discovered Mo(V) e.p.r.(electron-paramagnetic-resonance) signal, Resting II [Lowe, Barber, Pawlik & Bray (1976) Biochem. J. 155, 81-85]. E.p.r. measurements at 35GHz near to 4.2K showed that the interaction has the same sign at all molybdenum orientations and is ferromagnetic. The predicted splitting of the e.p.r. signal from the reduced iron-sulphur centre, Fe/S I, was observed, Providing positive identification of this as the other interacting species. Chemical modification of the molybdenum environment in xanthine oxidase can change the size of the interaction severalfold, but interaction always remains approximately isotropic. The interaction in turkey liver xanthine dehydrogenase is indistinguishable from that in the oxidase. However, a bacterial xanthine dehydrogenase with different iron-sulphur centres shows rather larger interaction. Guanidinium chloride disturbs the iron-sulphur centres of the oxidase, and when this occurs there is a parallel and relatively small change in the interaction. Removal of flavin from the molecule, or raising the pH to 12.0, changes the interaction slightly without affecting the chromophores themselves. It is concluded that the Fe/S I centre and the Mo are at least 1.0nm and probably nearer 2.5nm apart, and that the conformation of the protein between them is relatively stable up to pH 12.

Animals↗

PubMed1978

Electron-paramagnetic-resonance spectroscopy of complexes of xanthine oxidase with xanthine and uric acid.

Molybdenum(V) e.p.r. signals from reduced functional milk xanthine oxidase molecules (the Rapid signals), obtained in the presence of purine substrates and products, were further investigated [cf. Bray & Vänngård, (1969) Biochem. J. 114, 725-734; Pick & Bray (1969) Biochem. J. 114, 735-742]. Xanthine forms two complexes with the enzyme that are believed to correspond to different orientations of the substrate molecule in the active site. Only one complex appears to undergo the catalytic reaction. Non-productive complexes, analogous to theone with xanthine, are not formed by 1-methylxanthine or purine. Uric acid forms more than one e.p.r.-detectable complex, one of which is analogous to the non-productive xanthine complex. The computer program used for handing the e.p.r. data is described briefly.

Animals↗

PubMed1989

Differences in protein structure of xanthine dehydrogenase and xanthine oxidase revealed by reconstitution with flavin active site probes.

The native flavin, FAD, was removed from chicken liver xanthine dehydrogenase and milk xanthine oxidase by incubation with CaCl2. The deflavoenzymes, still retaining their molybdopterin and iron-sulfur prosthetic groups, were reconstituted with a series of FAD derivatives containing chemically reactive or environmentally sensitive substituents in the isoalloxazine ring system. The reconstituted enzymes containing these artificial flavins were all catalytically active. With both the chicken liver dehydrogenase and the milk oxidase, the flavin 8-position was found to be freely accessible to solvent. The flavin 6-position was also freely accessible to solvent in milk xanthine oxidase, but was significantly less exposed to solvent in the chicken liver dehydrogenase. Pronounced differences in protein structure surrounding the bound flavin were indicated by the spectral properties of the two enzymes reconstituted with flavins containing ionizable -OH or -SH substituents at the flavin 6- or 8-positions. Milk xanthine oxidase either displayed no preference for binding of the neutral or anionic flavin (8-OH-FAD) or a slight preference for the anionic form of the flavin (6-hydroxy-FAD, 6-mercapto-FAD, and possibly 8-mercapto-FAD). On the other hand, the chicken liver dehydrogenase had a dramatic preference for binding the neutral (protonated) forms of all four flavins, perturbing the pK of the ionizable substituent greater than or equal to 4 pH units. These results imply the existence of a strong negative charge in the flavin binding site of the dehydrogenase, which is absent in the oxidase.

Animals↗

PubMed1987

12-O-tetradecanoylphorbol-13-acetate-dependent induction of xanthine dehydrogenase and conversion to xanthine oxidase in murine epidermis.

Both xanthine dehydrogenase (XD) and xanthine oxidase (XO) catalyze the conversion of hypoxanthine to xanthine, and xanthine to uric acid. Topical application of a promoting dose of 12-O-tetradecanoylphorbol-13-acetate (TPA) to the dorsal skin of female SENCAR mice resulted in a 3.0-3.5-fold elevation of epidermal XO specific activity. Epidermal XO specific activity was maximally elevated 48-96 h after TPA treatment, and required 11 days to return to control levels. Although TPA increased the XO/(XD + XO) ratio from 0.45 to 0.7, the conversion of preexisting XD to XO could not solely account for the TPA-dependent elevation in XO specific activity since control XD plus XO activity was less than just the XO activity in TPA-treated epidermis. Topical application of cycloheximide simultaneously with, or 12 h after, TPA treatment inhibited the TPA-dependent increases in the XO/(XD + XO) ratio and XO specific activities. Collectively, these results suggest that the increased XO activity detected following TPA treatment is the consequence of TPA-induced XD synthesis, and a conversion of existing and newly synthesized XD to XO. In addition, the in vivo promoting activities of analogues of TPA could be correlated with their abilities to elevate XO activity (TPA greater than phorbol-12,13-dibenzoate much greater than 4-O-methyl-TPA = phorbol).

Animals↗

PubMed1996

[Xanthine oxidase (xanthine dehydrogenase)].

Xanthine oxidase (xanthine dehydrogenase) is composed of two identical subunits of approximately 150,000 daltons. Each subunit contains four oxdation-reduction active cofactors/monomers. In vivo, the enzyme exists mostly as the dehydrogenase type (the NAD-dependent type). The cDNA has been cloned from human liver, and the amino acid sequence has been determined. As xanthine oxidase seems to produce superoxide in postischemic reperfusion, the relation between the superoxide and postischemic tissue injury has been discussed. It has also been reported that inhibition of xanthine oxidase by allopurinol may cause severe 6-mercaptopurine toxicity.

DNA↗

PubMed1995

Flavoprotein structure and mechanism. 4. Xanthine oxidase and xanthine dehydrogenase.

Xanthine oxidase and xanthine dehydrogenase are enzymes involved in the metabolism of purines and pyrimidines in various organisms. Their relationship to one another has been the subject of considerable debate, primarily because of their proposed roles in ischemia/reperfusion damage in tissues. Differences in the kinetics and oxidation-reduction behavior of the two forms are accounted for by the presence in the dehydrogenase of a binding site for NAD+, as well as a substantially lower reduction potential for the flavin FADH./FADH2 couple of the dehydrogenase relative to the oxidase. This review presents recent advances of our understanding of the biochemistry and molecular biology of these systems, including a model for the overall morphology of xanthine oxidizing enzymes. The evidence that the two enzymes represent alternate forms of the same gene product, in some cases reversibly interconvertible between one another, is discussed.

Animals↗