Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:1.17.3.2 (xanthine oxidase)
 8,383 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

When xanthine oxidase was prepared from fresh raw cow's milk in the presence of dithioerythritol, 94% of its xanthine-oxidizing activity was found as a dehydrogenase type. The enzyme was reversibly converted to an oxidase type when dithioerythritol was removed. The conversion was ascribable to the oxidation of sulfhydryl groups of the enzyme by oxygen. The two forms of the enzyme gave the same visible spectrum, but the dehydrogenase form alone gave a characteristic difference spectrum upon addition of NAD+. NADH served as a good electron donor for the dehydrogenase form of the enzyme but not for the oxidase form. When xanthine was used as an electron donor, the overall rate of p-benzoquinone reduction was the same for the oxidase and dehydrogenase forms, but the proportion of one-electron flux from the enzyme to p-benzoquinone was considerably greater in the reaction of the dehydrogenase form than in that of the oxidase form.
 ...
PMID:Preparation of bovine milk xanthine oxidase as a dehydrogenase form. 695 93

The effects of 2-iodosobenzoic acid, 4-chloromercuribenzoate, 5,5'-dithiobis-(2-nitrobenzoic acid) and tetraethylthioperoxydicarbonic diamide (disulphiram) on the NAD+-dependent activity of xanthine oxidoreductase from rat liver were investigated. Only disulphiram converted the NAD+-dependent activity into the O2-dependent activity quantitatively, without changing the xanthine hydroxylation rate. The modification process was a first-order reaction with respect to time (min) and disulphiram concentration (microM). The kinetic data showed that modification of single thiol group is sufficient for loss of the enzymic activity towards NAD+ as electron acceptor. The complete protection afforded by NAD+ against the action of disulphiram suggests that the essential thiol group may be involved in binding of NAD+ to the xanthine oxidoreductase molecule.
 ...
PMID:Involvement of a single thiol group in the conversion of the NAD+-dependent activity of rat liver xanthine oxidoreductase to the O2-dependent activity. 696 18

We describe an enzymic, one-step kinetic method for determination of guanine deaminase (guanase, EC 3.5.4.3) in serum with a centrifugal analyzer. A combined enzyme-substrate system consists of the enzymes xanthine oxidase, catalase, and aldehyde dehydrogenase, the coenzyme NAD+, the substrate guanine, and ethanol in tris(hydroxymethyl)methylamine buffer, with KCl added as activator for aldehyde dehydrogenase. The method requires only 40 microL of sample. Guanase activity in 28 samples can be determined within 10 min by setting a 4-min lag period. The increase in absorbance at 340 nm is linearly proportional to the activity of guanase to 60 U/L. Within-run precision (CV) was 1.32 to 4.50% over the range studied. Day-to-day precision corresponds to CVs of 4.8 to 7.2% over the same range of guanase activity. The reference interval, as calculated from data on 25 healthy humans, was 0 to 1.02 U/L. The enzymic automated method shows good correlation with Caraway's (Clin. Chem. 12: 187, 1966) method (r = 0.949).
 ...
PMID:Kinetic measurement of guanine deaminase in serum with a centrifugal analyzer. 747 21

Hypoxia-induced hepatocyte injury results not only from ATP depletion but also from reductive stress and oxygen activation. Thus the NADH/NAD+ ratio was markedly increased in isolated hepatocytes maintained under 95% N2/5% CO2 in Krebs-Henseleit buffer well before plasma membrane disruption occurred. Glycolytic nutrients fructose, dihydroxyacetone or glyceraldehyde prevented cytotoxicity, restored the NADH/NAD+ ratio, and prevented complete ATP depletion. However, the NADH generating nutrients sorbitol, xylitol, glycerol and beta-hydroxybutyrate enhanced hypoxic cytotoxicity even though ATP depletion was not affected. On the other hand, NADH oxidising metabolic intermediates oxaloacetate or acetoacetate prevented hypoxic cytotoxicity but did not affect ATP depletion. Restoring the cellular NADH/NAD+ ratio with the artificial electron acceptors dichlorophenolindophenol and Methylene blue also prevented hypoxic injury and partly restored ATP levels. Ethanol which further increased the cellular NADH/NAD+ ratio increased by hypoxia also markedly increased toxicity whereas acetaldehyde which restored the normal cellular NADH/NAD+ ratio, prevented toxicity even though hypoxia induced ATP depletion was little affected by ethanol or acetaldehyde. The viability of hypoxic hepatocytes is therefore more dependent on the maintenance of normal redox homeostasis than ATP levels. GSH may buffer these redox changes as hypoxia caused cell injury much sooner with GSH depleted hepatocytes. Hypoxia also caused an intracellular release of free iron and cytotoxicity was prevented by desferoxamine. Furthermore, increasing the cellular NADH/NAD+ ratio markedly increased the intracellular release of iron. Hypoxia-induced hepatocyte injury was also prevented by oxypurinol, a xanthine oxidase inhibitor. Polyphenolic antioxidants or the superoxide dismutase mimic, TEMPO partly prevented cytotoxicity suggesting that reactive oxygen species contributed to the cytotoxicity. The above results suggests that hypoxia induced hepatocyte injury results from sustained reductive stress and oxygen activation.
 ...
PMID:Modulating hypoxia-induced hepatocyte injury by affecting intracellular redox state. 748 48

By correlating lactate/pyruvate ratios and ATP levels, cytotoxicity induced by the mitochondrial respiratory inhibitors or hypoxia:reoxygenation injury can be attributed not only to ATP depletion but also to reductive stress and oxygen activation. Thus hypoxia, cyanide or antimycin markedly increases reductive stress, non-heme Fe release and H2O2 formation in hepatocytes. Cytotoxicity was partly prevented with the ferric chelator desferoxamine, the xanthine oxidase inhibitor oxypurinol and the hydrogen peroxide scavenger glutathione. No lipid peroxidation could be detected and phenolic anti-oxidants had little effect. However, polyphenolic antioxidants or the superoxide dismutase mimics TEMPO or TEMPOL partly prevented cytotoxicity. Furthermore, increasing the hepatocyte NADH/NAD+ ratio with NADH generating compounds such as ethanol, glycerol, or beta-hydroxybutyrate markedly increased cytotoxicity (prevented by desferoxamine) and further increased the intracellular release of non-heme iron. Cytotoxicity could be prevented by glycolytic substrates (eg. fructose, dihydroxyacetone, glyceraldehyde) or the NADH utilising substrates acetoacetate or acetaldehyde which decreased the reductive stress and prevented intracellular iron release. These results suggest that liver injury resulting from insufficient respiration involves reductive stress which releases intracellular Fe, converts xanthine dehydrogenase to xanthine oxidase and causes mitochondrial oxygen activation. The cell's antioxidant defences are compromised and ATP catabolism contributes to oxygen activation.
 ...
PMID:Hepatocyte injury resulting from the inhibition of mitochondrial respiration at low oxygen concentrations involves reductive stress and oxygen activation. 758 49

The ability of O2 metabolites derived from the xanthine-xanthine oxidase system to inhibit mitochondrial function was examined using freshly isolated rat liver mitochondria. Under 2,4-dinitrophenol-uncoupled conditions, mitochondria exposed to free radicals exhibited a significant decrease in O2 consumption supported by NAD(+)-linked substrates, but showed almost no change in O2 consumption in the presence of succinate and ascorbate. Oxidative stress caused the loss of intramitochondrial nicotinamide nucleotides, and addition of NAD+ fully prevented any fall in O2 consumption with NAD(+)-linked substrates. The activity of electron-transfer complex I (NADH oxidase and NADH-cytochrome c oxidoreductase) and the energy-dependent reduction of NAD+ by succinate were unaltered by oxidative stress. Exposure to free radicals also had an uncoupling effect at all three coupling sites. The degree of mitochondrial swelling was closely correlated with the inhibition of State-3 oxidation of site-I substrates and with the increase in State-4 oxidation of succinate. The immunosuppressive agent cyclosporin A completely prevented the mitochondrial damage induced by oxygen free radicals (swelling, Ca2+ release, sucrose trapping, uncoupling and selective inhibition of the mitochondrial respiration of site-I substrates). The same protective effect was found when Ca2+ cycling was prevented, either by chelating Ca2+ with EGTA or by inhibiting Ca2+ reuptake with Ruthenium Red. These findings suggest that the deleterious effect of free radicals on mitochondria in the present experimental system was triggered by the cyclosporin A-sensitive and Ca(2+)-dependent membrane transition, and not by direct impairment of the mitochondrial inner-membrane enzymes.
 ...
PMID:Oxidative damage to mitochondria is mediated by the Ca(2+)-dependent inner-membrane permeability transition. 769 Oct 56

Reactive oxygen species (ROS) generated from xanthine oxidase (XO) play an important role in ischemia-induced injury. We hypothesize that XO and xanthine dehydrogenase (XDH) are released into the circulation with ischemia reperfusion to the human liver and intestine. Blood was drawn from a patient, before and at intervals after an aortic cross-clamp procedure. Plasma was incubated in the presence of xanthine, with NAD+ (for XD +XO) and without NAD+ (for XO). The amount of urate formed was quantified using a high-performance liquid chromatograph (HPLC). The calculated XDH+XO and XO activity increased from 1.88 and 1.66 microU/mg protein, respectively, before the cross clamp to 3.77 and 3.11 microU/mg, respectively, 7 minutes after reperfusion to the superior mesenteric, celiac, and right renal artery beds. The release of a significant biological source of ROS may explain the damage to lung or heart observed after ischemia to the human liver and intestine.
 ...
PMID:Circulating xanthine oxidase in human ischemia reperfusion. 771 6

When cultures of pancreatic islet cells are exposed to the nitric oxide donor sodium nitroprusside, to enzymatically generated reactive oxygen intermediates or to streptozotocin cell lysis occurs after 4-12 h. We report here that a heat shock at 43 degrees C for 90 min reduces cell lysis from nitric oxide (0.45 mM sodium nitroprusside) by 70%, from reactive oxygen intermediates (12 mU xanthine oxidase and 0.05 mM hypoxanthine) by 80% and from streptozotocin (1.5 mM) by 90%. Heat shock induced resistance was observed immediately after termination of the 90 min culture at 43 degrees C and correlated with enhanced expression of hsp70. The occurrence of DNA strand breaks, a major early consequence of nitric oxide, reactive oxygen intermediates, or streptozotocin action, was not suppressed by heat shock treatment. However, the depletion of NAD+, the major cause of radical induced islet cell death, was suppressed after heat shock (P < 0.01). We conclude that pancreatic islet cells can rapidly activate defence mechanisms against nitric oxide, reactive oxygen intermediates and streptozotocin by culture at 43 degrees C. Islet cell survival is due to the prevention of lethal NAD+ depletion during DNA repair, probably by slowing down poly(ADP-ribose)polymerase activation.
 ...
PMID:Heat shock induces resistance in rat pancreatic islet cells against nitric oxide, oxygen radicals and streptozotocin toxicity in vitro. 776 24

Despite extensive studies on streptozotocin, alloxan and nitric oxide toxicity in pancreatic islets the mechanism of oxygen radical induced islet cell death has not been determined. The present study shows at the level of single cells that following exposure to oxygen radicals generated from xanthine oxidase DNA strand breaks occur in cell nuclei within 5-60 min and precede cell death by several hours. Similar kinetics were seen when treating islet cells with the alkylating agent streptozotocin. Immunofluorescence studies demonstrated the endogenous formation of ADP-ribose polymers in nearly all islet cell nuclei within minutes of treatment with xanthine oxidase, indicating activation of the enzyme poly(ADP-ribose) polymerase (PARP). Concomitantly, cellular NAD+ depletion was noted. Nicotinamide largely prevented NAD+ depletion and in parallel resulted in islet cell survival. These findings identify islet cell nuclear DNA as a primary target of oxygen radical toxicity and suggest related pathways of oxygen radical, nitric oxide and streptozotocin toxicity.
 ...
PMID:Analysis of oxygen radical toxicity in pancreatic islets at the single cell level. 784 Sep 1

Xanthine dehydrogenase (XDH) is an important precursor to the oxygen radical producing enzyme xanthine oxidase (XO). We found that the apparent activity of rabbit myocardial XDH increased from 2 +/- 1 to 50 +/- 3 microU/g (P < 0.05) following extraction of tissue homogenate with butanol. Further studies suggested that the basis for this observation was a high molecular weight compound which consumes the XDH cofactor, NAD+. Addition of myocardial homogenate to exogenous NAD+ resulted in depletion of NAD+ and concomitant formation of an additional compound (peak A). Both NAD+ consumption and peak A formation were abrogated by prior extraction of homogenate with butanol. Separation of myocardial homogenate by Sephadex chromatography revealed a high molecular weight compound which suppressed activity of purified milk XDH but not xanthine oxidase (XO). This activity co-eluted with the ability of myocardial homogenate to consume added NAD+ and form peak A. The NAD(+)-consuming activity was heat and acid-labile. In addition, nicotinamide was both a product and an inhibitor of the NADase activity, consistent with the existence of a previously described myocardial glycohydrolase. Extraction of tissue with butanol may be necessary to detect low levels of XDH activity in vitro.
 ...
PMID:Suppression of rabbit myocardial xanthine dehydrogenase activity by an endogenous compound. 800 74


<< Previous 1 2 3 4 5 Next >>