Rat Liver Xanthine Oxidase Research Articles

Xanthine oxidase inhibitory properties and anti-inflammatory activity of 2-amino-5-alkylidene-thiazol-4-ones

Thirty 2-amino-5-alkylidene-thiazol-4-ones were assayed for inhibitory activity against commercial enzyme xanthine oxidase (XO) in vitro and XO in rat liver homogenate as well as for anti-inflammatory response on human peripheral blood mononuclear cells (PBMCs). 4-((2-Benzylamino-4-oxothiazol-5(4H)-ylidene)-methyl)benzonitrile showed the most potent inhibitory effect against commercial XO (IC50=17.16μg/mL) as well as against rat liver XO (IC50=24.50μg/mL). All compounds containing the 4-cyanobenzylidene group or (indol-3-yl)methylene group at the position 5 of thiazol-4-one moiety were moderately potent inhibitors of commercial XO. The assayed compounds were docked into the crystal structures of XO enzyme complexes with three diverse inhibitors (PDB codes: 1FIQ, 1VDV, and 1V97) using OEDocking software. Our results strongly point to a correlation between the data on inhibitory activity against commercial XO and data on antioxidant activity of studied compounds, screened using a lipid peroxidation (LP) method. 2-(Benzylamino)-5-((thiophen-2-yl)methylene)thiazol-4(5H)-one showed the highest anti-inflammatory response on PBMCs, exerted most probably through the NF-κB inhibition. Studied 2-amino-5-alkylidene-thiazol-4-ones obey the “Rule of five” and meet all criteria for good solubility and permeability.

Activity and stability of rat liver xanthine oxidase in the presence of pyridine

In the present study, rat liver xanthine oxidase activity and its thermostability in the presence of pyridine were investigated. The activity of the enzyme was determined by following the formation of uric acid spectrophotometrically. The thermal stability of the enzyme was studied in the presence of 0.0%–2.0% of pyridine in Sorenson’s buffer. Thermal stability parameters (half-life, inactivation constant, and activation energies for enzyme inactivation), thermodynamic constants (ΔH*, ΔS*, and ΔG*) and the kinetic parameters (Kmand Vmax), were determined in pyridine-free and pyridine-containing buffer solution. A dramatic reduction was observed in xanthine oxidase activity in the presence of pyridine. However, the pyridine-treated enzyme showed a marked enhancement in thermal stability compared with the native enzyme. The ΔG values for the enzyme activity in the presence of pyridine were found to be about 1.5-fold larger than that calculated for the native enzyme, indicating that the enzyme becomes kinetically more stable in the presence of pyridine. The Kmvalue for xanthine oxidase in the presence of 0.5% pyridine increased by 4.8-fold compared with the enzyme in the pyridine-free buffer solution; however, there was 1.8-fold reduction in the Vmaxvalue in the hydro-organic solution compared with the enzyme activity in the buffer solution. As the stability of enzymes is one of the most difficult problems in protein chemistry, this thermostability property of xanthine oxidase could be of great value in developing novel strategies to improve and expand its application in various areas.

Monochloramine produces reactive oxygen species in liver by converting xanthine dehydrogenase into xanthine oxidase

In the present study, we assessed the influence of monochloramine (NH 2Cl) on the conversion of xanthine dehydrogenase (XD) into xanthine oxidase (XO) in rat liver in vitro. When incubated with the partially purified cytosolic fraction from rat liver, NH 2Cl (2.5–20 μM) dose-dependently enhanced XO activity concomitant with a decrease in XD activity, implying that NH 2Cl can convert XD into the reactive oxygen species (ROS) producing form XO. The NH 2Cl (5 μM)-induced XD/XO interconversion in the rat liver cytosol was completely inhibited when added in combination with an inhibitor of NH 2Cl methionine (25 μM). A sulfhydryl reducing agent, dithiothreitol at concentrations of 0.1, 1 and 5 mM also dose-dependently reversed the NH 2Cl (5 μM)-induced XD/XO interconversion. These imply that NH 2Cl itself acts on the XD/XO interconversion, and that this conversion occurs at the cysteine residues in XD. Furthermore, using the fluorescent probe 2′,7′-dichlorodihydrofluorescein diacetate, it was found that NH 2Cl could increase ROS generation in the cytoplasm of rat primary hepatocyte cultures, and that this increase might be reversed by an XO inhibitor, allopurinol. These results suggest that NH 2Cl has the potential to convert XD into XO in the liver, which in turn may induce the ROS generation in this region.

Disulfide S-Monoxides Convert Xanthine Dehydrogenase into Oxidase in Rat Liver Cytosol More Potently Than Their Respective Disulfides

Xanthine oxidase (XO)/xanthine dehydrogenase (XD) oxidizes oxypurines to uric acid, with only the XO form producing reactive oxygen species. In the present study, the effects of cystamine S-monoxide and cystine S-monoxide (disulfide S-monoxides) on the conversion of XD to XO in rat liver were examined. A partially purified enzyme fraction from the rat liver was incubated with xanthine in the presence or absence of NAD+, and the uric acid formed was measured by HPLC. Under basal conditions, XO activity represented about 15% of the total XO plus XD activity. Cystamine S-monoxide and cystine S-monoxide converted XD into XO in a dose-dependent manner, and the concentrations required to increase XO activity by 50% were approximately 1 and 2 microM, respectively. Their respective thiols (cysteamine and cysteine) and disulfides (cystamine and cystine) up to 10 microM showed weak or no effects on the activities of XO and XD and their conversion. Experiments utilizing a sulfhydryl reducing reagent (dithiothreitol) and sulfhydryl modifiers (4,4'-dithiodipyridine and 1-fluoro-2,4-dinitrobenzene) indicated that disulfide S-monoxides-induced conversion of XD to XO occurs via disulfide bridge formation in XD, but not the modification of sulfhydryl groups. These results suggest that disulfide S-monoxides have the potential to increase the generation of reactive oxygen species through the conversion of XD to XO in liver.

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 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 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.

Hypouricemic Effects of Acacetin and 4,5-O-Dicaffeoylquinic Acid Methyl Ester on Serum Uric Acid Levels in Potassium Oxonate-Pretreated Rats

The effects of acacetin (1) and 4,5-O-dicaffeoylquinic acid methyl ester (2), compounds contained in the flowers of Chrysanthemum sinense SABINE, on the serum uric acid level were investigated using the rats pretreated with the uricase inhibitor potassium oxonate as an animal model for hyperuricemia. When administered per orally at doses of 20 and 50 mg/kg, 1 reduced the serum uric acid level by 49.9 and 63.9%, respectively and 2 reduced the level by 31.2 and 44.4%, respectively. On the other hand, when the same doses were given intraperitoneally, both of compounds also exhibited a dose-dependent and more marked reduction of the serum uric acid level (% reduction at 20 and 50 mg/kg were 63.0 and 95.1% in 1, respectively and 66.9 and 86.5% in 2, respectively). Furthermore, the compounds 1 and 2 inhibited the rat liver xanthine oxidase activity with IC(50) values of 2.22 muM and 5.27 muM, respectively. These results demonstrated the hypouricemic action of 1 and 2, which may be attributable to their xanthine oxidase inhibitory activity.

Purification of Rat Liver Xanthine Oxidase and Xanthine Dehydrogenase by Affinity Chromatography on Benzamidine-Sepharose

The oxidase form of xanthine dehydrogenase (XO; EC 1.1.3.22) has been purified approximately 200-fold from rat liver extracts using a three-step process of heat treatment, ammonium sulfate precipitation, and chromatography on benzamidine-Sepharose. The purified enzyme showed only minor contamination when analyzed by gel electrophoresis under either native or sodium dodecyl sulfate (SDS)-denatured conditions and appears to be intact based on its subunit size on SDS–polyacrylamide gel electrophoresis, its N-terminal amino acid sequence, and its ability to be converted to the NAD-dependent dehydrogenase form (XD; EC 1.1.1.204) by incubation with dithiothreitol. Isoelectric focusing analysis showed that the purified enzyme consists of two major, enzymatically active isoforms with average pIvalues of 6.13 and 6.23 and a minor enzymatically active isoform with an average pIvalue of 6.07. A similar purification of XD was achieved by preincubating the partially purified oxidase with dithiothreitol prior to affinity chromatography on benzamidine-Sepharose. The effects of benzamidine on the kinetic properties of purified rat XO were characterized at pH 8 and 9 and were compared to those of bovine milk XO. Benzamidine was found to be a weak competitive inhibitor of the purified rat enzyme withKivalues of 30 and 10 mMat pH 8 and 9, respectively. In contrast, theKivalues for benzamidine with bovine XO were more than 10-fold greater. The findings presented in this study show that benzamidine is a competitive inhibitor of XO and that affinity chromatography on benzamidine-Sepharose provides a simple, rapid, and effective means of purifying both the oxidase and dehydrogenase forms of rat XO.

Reaction of 5-ethynyluracil with rat liver xanthine oxidase.

5-Ethynyluracil is a time-dependent and tight binding inhibitor of xanthine oxidase. The maximal value of the first-order rate constant for onset of inhibition is 0.01 s-1, and the concentration of 5-ethynyluracil which gives one-half of this value is 190 microM. Because the t1/2 for formation of active enzyme from inhibited enzyme is greater than 30 h in the absence of NADH, inhibition of xanthine oxidase by 5-ethynyluracil is functionally irreversible. One equivalent of 5-[2-14C]ethynyluracil/equivalent of active enzyme is required for complete inhibition. Allopurinol (100 microM), a potent inhibitor of xanthine oxidase, and cyanide (5 mM), an inactivator of the enzyme, do not abolish the binding of 5-[2-14C]ethynyluracil to the enzyme. Because radiolabel is released from 5-[2-14C]ethynyluracil-treated enzyme by treatment with 6 M guanidine HCl, a stable covalent bond is not formed between the inhibitor and the enzyme. However, the radiolabel released from inhibited enzyme is not 5-ethynyluracil. Moreover, NADH restores catalytic activity to the inhibited enzyme and displaces the radiolabel as 5-acetyluracil. Thermal denaturation of 5-ethynyluracil-inhibited xanthine oxidase results in the release of approximately equal amounts of 5-acetyluracil and a more hydrophilic product. Consequently, the 5-ethynyluracil-xanthine oxidase complex yields different degradation products of 5-ethynyluracil under different denaturation conditions. Seven uracil analogues with 5-substituents were tested as time-dependent inhibitors of xanthine oxidase. 5-Ethynyluracil is the only uracil analogue that potently inhibited xanthine oxidase. The reactivity of these uracil derivatives with sulfite was also determined. 5-Ethynyluracil is many fold more susceptible to nonenzymatic nucleophilic addition of sulfite than are the other analogues. Thus, the potency of these uracil analogues as inhibitors of xanthine oxidase is related to the nonenzymatic reactivity of the analogues with sulfite.

Conversion of xanthine dehydrogenase into xanthine oxidase in rat liver and plasma at the onset of reperfusion after ischemia

The aim of this study was to test whether conversion of xanthine dehydrogenase into xanthine oxidase as induced by fasting, ischemia of the liver or both is an in vivo process or only occurs in vitro in homogenates. For this purpose, the conversion rate of xanthine dehydrogenase into xanthine oxidase was studied in liver homogenates obtained from rats after normal feeding or 24 hr of fasting followed or not by 2 hr of ischemia of the liver. In fed rats, the conversion rate of xanthine dehydrogenase into xanthine oxidase was studied as well in liver homogenates after different periods of reperfusion after 2 hr of ischemia. Homogenization was carried out under strictly controlled conditions, after which the supernatants were incubated at 37 degrees C in buffer for 0 to 5 hr. Enzyme activities were assayed spectrophotometrically by measuring urate production at 295 nm. Conversion started only after 2 to 3 hr of incubation of supernatants of control fed livers, whereas conversion started immediately after 24 hr of fasting. The percentage oxidase activity of total xanthine oxidoreductase activity in ischemic livers from fed animals was slightly higher (26.7% +/- 1.7%; p < 0.05) than in control livers (19.3% +/- 1.4%), whereas the percent oxidase activity in ischemic livers from fasted animals (16.7% +/- 1.0%) was not different from that in control animals (16.8% +/- 1.1%). Ischemia for 2 hr caused in vitro a substantial increase in the conversion rate in supernatants of livers of fed and fasted rats as compared with their controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Conversion of xanthine dehydrogenase into xanthine oxidase in rat liver and plasma at the onset of reperfusion after ischemia

The aim of this study was to test whether conversion of xanthine dehydrogenase into xanthine oxidase as induced by fasting, ischemia of the liver or both is an in vivo process or only occurs in vitro in homogenates. For this purpose, the conversion rate of xanthine dehydrogenase into xanthine oxidase was studied in liver homogenates obtained from rats after normal feeding or 24 hr of fasting followed or not by 2 hr of ischemia of the liver. In fed rats, the conversion rate of xanthine dehydrogenase into xanthine oxidase was studied as well in liver homogenates after different periods of reperfusion after 2 hr of ischemia. Homogenization was carried out under strictly controlled conditions, after which the supernatants were incubated at 37° C in buffer for 0 to 5 hr. Enzyme activities were assayed spectrophotometrically by measuring urate production at 295 nm. Conversion started only after 2 to 3 hr of incubation of supernatants of control fed livers, whereas conversion started immediately after 24 hr of fasting. The percentage oxidase activity of total xanthine oxidoreductase activity in ischemic livers from fed animals was slightly higher (26.7% ± 1.7%; p<0.05) than in control livers (19.3% ± 1.4%), whereas the percent oxidase activity in ischemic livers from fasted animals (16.7% ± 1.0%) was not different from that in control animals (16.8% ± 1.1%). Ischemia for 2 hr caused in vitro a substantial increase in the conversion rate in supernatants of livers of fed and fasted rats as compared with their controls. Furthermore, the appearance of xanthine dehydrogenase and xanthine oxidase in the blood during reperfusion up to 60 min after 2 hr ischemia of the liver was studied. The enzyme started to appear in the blood after 5 min of reperfusion, predominantly as xanthine dehydrogenase. Very rapid conversion of xanthine dehydrogenase into xanthine oxidase was observed in the plasma. We conclude that fasting and 2-hr in vivo ischemia of rat liver can affect the conversion rate of xanthine dehydrogenase into xanthine oxidase during homogenization but not in vivo. Therefore initial tissue injury due to in vivo ischemia and reperfusion in liver is not caused by conversion of xanthine dehydrogenase into xanthine oxidase. It cannot be ruled out that, after leakage of the enzyme to the circulation, additional damage is induced. (HEPATOLOGY 1994;19:1488–1495.)

Normothermic liver ischemia in rats: xanthine oxidase is not the main source of oxygen free radicals.

We studied the effect of allopurinol (ALL) on the activity of xanthine dehydrogenase (XDH), xanthine oxidase (XOX), superoxide dismutase (SOD), and catalase (CAT) in rat liver during ischemia followed by 60 min of reperfusion. We induced 60-min ischemia in the median and left lobes by clamping the hepatic artery and portal branches. The percentage XOX relative to total oxidase activity increased significantly in the control group, from 10% during the stabilization period to 18% after 60 min of reperfusion. The XDH activity decreased during reperfusion. Activity of both XDH and XOX was almost completely blocked by ALL. The activity of SOD and CAT did not differ significantly between the ALL group and controls after 60 min of reperfusion. ALL treatment did not affect liver injury parameters, as concentrations of lactate dehydrogenase (LDH) and alanine transferase (ALT) increased in plasma after ischemia, both in controls and in the ALL-treated group. We concluded that ischemia promotes conversion of XDH to XOX during reperfusion. XOX may not be the main source of free radical production, since intracellular scavengers (SOD and CAT) did not differ significantly between controls and the ALL-treated group, despite the fact that ALL blocked XOX activity completely.

Effect of glutathione depletion on the conversion of xanthine dehydrogenase to oxidase in rat liver

The ability of endogenous glutathione (GSH) to modify the activity of the enzyme xanthine oxidase (XO) in rat liver was investigated. The effect of hepatic GSH depletion on the conversion of xanthine dehydrogenase (XDH) (EC 1.1.1.204) to XO (EC 1.1.3.22) was determined 10 min after i.p. administration of different amounts of diethylmaleate to fasted rats. After administration of 400 mg/kg, total hepatic non-protein GSH (reduced + oxidized GSH) decreased significantly to 14% of controls. In this condition the level of oxidized GSH was unchanged and no lipid peroxidation was observed, while a significant increase of reversible XO and a minor increase of the irreversible form of the enzyme was detected.

Enhanced activity of the free radical producing enzyme xanthine oxidase in hypoxic rat liver. Regulation and pathophysiologic significance.

It has been widely proposed that conversion of xanthine dehydrogenase (XDH) to its free radical-producing form, xanthine oxidase (XOD), underlies ischemic/reperfusion injury, although the relationship of this conversion to hypoxia and its physiologic control have not been defined. This study details the time course and control of this enzymatic interconversion. In a functionally intact, isolated perfused rat liver model, mean % XOD activity increased as a function of both the duration (25 to 45% in 3 h) and degree (r = 0.97) of hypoxia. This process was markedly accelerated in ischemic liver by an overnight fast (45 vs. 30% at 2 h), and by imposing a short period of in vivo ischemia (cardiopulmonary arrest 72%). Moreover, only under these conditions was there a significant rise in the XOD activity due to the conformationally altered XDH molecule (XODc, 18%), as well as concomitant morphologic injury. Neither circulating white blood cells nor thrombosis appeared to contribute to the effects of in vivo ischemia on enzyme conversion. Thus, it is apparent that conversion to the free radical-producing state, with high levels of XOD activity and concurrent cellular injury, can be achieved during a relatively short period of hypoxia under certain well-defined physiologic conditions, in a time course consistent with its purported role in modulating reperfusion injury. These data also suggest that the premorbid condition of organ donors (e.g., nutritional status and relative state of hypoxia) is important in achieving optimal organ preservation.

Decreased activity of xanthine oxidase in rat liver by feeding with di-(2-ethylhexyl)phthalate and clofibrate.

(1990). Decreased Activity of Xanthine Oxidase in Rat Liver by Feeding with Di-(2-ethylhexyl)phthalate and Clofibrate. Agricultural and Biological Chemistry: Vol. 54, No. 10, pp. 2745-2746.

Mechanisms of conversion of xanthine dehydrogenase to xanthine oxidase in ischemic rat liver and kidney.

Previous studies have proposed and supported a role for the proteolytic, irreversible conversion of xanthine dehydrogenase to xanthine oxidase (XO) in postischemic injury in a wide variety of organs. A second mechanism of conversion, due to sulfhydryl modification and reversible with dithiothreitol (DTT), is potentially important but has not been well investigated. In this study rat liver and kidney were found to produce significant amounts of DTT-reversible XO during normothermic global ischemia. Formation of reversible XO precedes that of irreversible XO by approximately 0.5 h with a strong correlation (r = 0.92) existing between the rate of irreversible XO formation and the concentration of reversible XO. The formation of reversible XO is preceded by a depletion of glutathione with concentrations of glutathione during ischemia correlating (r = 0.85) with the observed concentration of reversible XO. While a large increase in the extent of liver damage occurs concurrently with conversion in an in vivo liver model of liver ischemia, an ischemia-reperfusion regimen (1 h of ischemia plus 0.5 h of reperfusion) that resulted in no conversion caused significant elevations in serum glutamic pyruvic transaminase and serum glutamic-oxaloacetic transaminase. Rats depleted of XO by tungsten dieting release 65% less enzyme after the same insult, suggesting that endogenous XO may also participate in the damage process independent of any conversion.

Ultracytochemischer Nachweis von Enzymen durch Reduktion von Kaliumhexacyanoferrat (III): I. Eine Methode zum Nachweis der Xanthinoxidase

Xanthine oxidase is a flavoprotein which directly catalyses the oxidation of xanthine and hypoxanthine by oxygen or by potassium ferricyanide as an artificial acceptor of protons. In doing so, the potassium ferricyanide is reduced into potassium ferrocyanide which in the presence of manganese(II)ions leads to the manganese(II)ferrocyanide which is insoluble in water and in organic solvents. The latter is deposited on the areas with enzyme activity and marks them under the electron microscope. After the detection of the xanthine oxidase in rat liver on ultrathin non-contrasted sections, it was observed that the fine granular reaction product was deposited only on the peroxisomes of the hepatocytes. A greater quantity of the reaction product is deposited on the outer membrane and the matrix and a smaller one on the nucleoid of these cell organelles. No deposition of the reaction product was observed on the other cell structures. The method can be used for the study of purine metabolism on the cellular level as well as for the specific ultracytochemical detection of the peroxisomes.

Effect of Dietary Protein and Iron on the Fractional Turnover Rate of Rat Liver Xanthine Oxidase

Rat liver xanthine oxidase activity is regulated in response to dietary protein and iron. To investigate whether the change in activity was mediated by a change in the rate of protein degradation, we measured the fractional turnover rate using the double-isotope technique with [3H]- and [14C]leucine and calculated the apparent half-life of xanthine oxidase in rats fed diets containing either 20 or 5% casein with either 35 or 5 mg iron/kg diet. Under control conditions, xanthine oxidase had an apparent half-life of 4.8 d and approximately 65% of the enzyme subunits were active. Rats fed diets with low dietary protein had lower xanthine oxidase activity, but the enzyme had a slower fractional turnover rate, resulting in an apparent half-life of 6.4 d, and only 15–20% of the enzyme was active. The apparent half-life of xanthine oxidase increased to 7.5 d in rats fed diets with low dietary iron, but dietary iron did not affect the specific activity of the enzyme or the percentage of active subunits. These results suggest that the loss of enzyme activity is not due to loss of enzyme protein by increased degradation, but rather to inactivation of the enzyme.

Imprinting by the neonatal testis: a mechanism for sexual dimorphism of rat liver xanthine oxidase.

We have shown that adult rat liver xanthine oxidase exhibits sexual dimorphism.1 Males have greater enzyme activity by comparison to females. The increased enzyme activity in the male corresponds to a greater mass of xanthine oxidase protein.2 Immature animals of both sexes have similar enzyme activity.KeywordsSexual DimorphismXanthine OxidaseXanthine Oxidase ActivityMale CastrateAndrogen TherapyThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Enzymic conversion of rat liver xanthine oxidase from dehydrogenase (D form) to oxidase (O form)

Enzymic conversion of rat liver xanthine oxidase from dehydrogenase (D form) to oxidase (O form)

Reduction of benzydamine N-oxide by rat liver xanthine oxidase.

Evidence is presented that a purified xanthine oxidase preparation, like milk xanthine oxidase, is responsible for the xanthine-dependent reduction of benzydamine N-oxide. Rat liver preparations contain enzymes which catalyze the anaerobic reduction of benzydamine N-oxide by either reduced nicotinamide adenine dinucleotide (phosphate) NAD (P) H or xanthine. In crude enzyme preparations, the enzymatic reduction seems to involve xanthine oxidase and/or cytochrome P-450 because allopurinol, an inhibitor of xanthine oxidase, and n-octylamine, an inhibitor of aliphatic tertiary amine N-oxide reductase (cytochrome P-450) block the reduction of benzydamine N-oxide in crude enzyme preparations of rat liver. Moreover, the enzymatic system exhibited dual pH optima of 7.4 and 9.0 for reductions dependent on NADPH and xanthine, respectively. An enzyme preparation which did not contain cytochrome P-450 was isolated from rat liver and purified 186-fold in terms of xanthine oxidase activity. Xanthine oxidase activity and xanthine-dependent benzydamine N-oxide reduction activity were copurified in parallel.