Serum xanthine oxidase in jaundice

Febuxostat inhibition of endothelial-bound XO: implications for targeting vascular ROS production

Xanthine oxidase (XO) is a critical source of reactive oxygen species (ROS) that contribute to vascular inflammation. Binding of XO to vascular endothelial cell glycosaminoglycans (GAGs) results in significant resistance to inhibition by traditional pyrazolopyrimidine-based inhibitors such as allopurinol. Therefore, we compared the extent of XO inhibition (free and GAG-bound) by allopurinol to that by febuxostat, a newly approved nonpurine XO-specific inhibitor. In solution, febuxostat was 1000-fold more potent than allopurinol at inhibiting XO-dependent uric acid formation (IC₅₀= 1.8 nM vs 2.9 μM). Association of XO with heparin-Sepharose 6B (HS6B-XO) had minimal effect on the inhibition of uric acid formation by febuxostat (IC₅₀= 4.4 nM) while further limiting the effect of allopurinol (IC₅₀= 64 μM). Kinetic analysis of febuxostat inhibition revealed K(i) values of 0.96 (free) and 0.92 nM (HS6B-XO), confirming equivalent inhibition for both free and GAG-immobilized enzyme. When XO was bound to endothelial cell GAGs, complete enzyme inhibition was observed with 25 nM febuxostat, whereas no more than 80% inhibition was seen with either allopurinol or oxypurinol, even at concentrations above those tolerated clinically. The superior potency for inhibition of endothelium-associated XO is predictive of a significant role for febuxostat in investigating pathological states in which XO-derived ROS are contributive and traditional XO inhibitors are only slightly effective.

Antioxidant activities of Melittis melissophyllum L. (Lamiaceae)

Extracts of Melittis melissophyllum leaves in ether, chloroform, ethyl acetate, n-butanol and water were evaporated to dryness and dissolved in 50% ethanol to make 10% (w/v) solutions. The potential protective action of the extracts was assessed by the corresponding in vitro and in vivo tests. In the in vitro experiments extracts were tested as potential scavengers of free radicals (DPPH, O₂·⁻, NO, and OH radicals), as well as inhibitors of liposomal peroxidation (LPx). The results obtained show that all extracts (exept n-BuOH extract) are good scavengers of radicals and reduce LPx intensity in liposomes, which points to their protective (antioxidant) activity. In vivo experiments were concerned with antioxidant systems (activities of GSHPx, GSHR, Px, CAT, XOD, GSH content and intensity of LPx) in liver homogenate and blood-hemolysate of experimental animals after their treatment with extracts of M. melissophyllum leaves, or in combination with CCl₄. On the basis of the results obtained it can be concluded that the examined extracts have protective (antioxidative) effect and this antioxidative behaviour is more pronounced in liver than in blood-hemolysate. The reason is probably the fact that liver contains other enzymatic systems, which can also participate in the antioxidative mechanism. Of all the extracts the H₂O one showed the highest protective activity.

Nitro-oleic acid, a novel and irreversible inhibitor of xanthine oxidoreductase

Xanthine oxidoreductase (XOR) generates proinflammatory oxidants and secondary nitrating species, with inhibition of XOR proving beneficial in a variety of disorders. Electrophilic nitrated fatty acid derivatives, such as nitro-oleic acid (OA-NO2), display anti-inflammatory effects with pleiotropic properties. Nitro-oleic acid inhibits XOR activity in a concentration-dependent manner with an IC50 of 0.6 microM, limiting both purine oxidation and formation of superoxide (O2.). Enzyme inhibition by OA-NO2 is not reversed by thiol reagents, including glutathione, beta-mercaptoethanol, and dithiothreitol. Structure-function studies indicate that the carboxylic acid moiety, nitration at the 9 or 10 olefinic carbon, and unsaturation is required for XOR inhibition. Enzyme turnover and competitive reactivation studies reveal inhibition of electron transfer reactions at the molybdenum cofactor accounts for OA-NO2-induced inhibition. Importantly, OA-NO2 more potently inhibits cell-associated XOR-dependent O2. production than does allopurinol. Combined, these data establish a novel role for OA-NO2 in the inhibition of XOR-derived oxidant formation.

Inherited metabolic disease: prospects for the future in both basic and clinical research

The birth of a child with an inherited disorder is often the beginning of a life-long problem for the whole family. About 8.5% of paediatric deaths and 4.7% of paediatric hospital admissions are due to autosomal and sex-linked recessive diseases. These figures are likely to be erroneously low because of incomplete ascertainment. The inherited metabolic diseases therefore merit study on economic as well as humanitarian grounds. Investigations of the disorders of purine metabolism have been conducted for more than a century and a half in the borderland between biochemistry and medicine, illuminating both disciplines and reflecting their separate developments. These studies are a general model for work in other branches of human intermediary metabolism. It is hoped that the basic study of the inborn errors or metabolism will expand our knowledge of the defective gene and of its product, the enzyme protein. Clinical studies should aim to improve the prenatal, postnatal and carrier-state diagnosis of these disorders, and to improve their treatment by methods which can be made practicable and generally available at the clinical level. There may be some hope for enzyme replacement in certain circumstances. The prospect for genetic modification at the clinical level is almost infinitely far away, where many would say that is should remain.

Moderate hypoxia induces xanthine oxidoreductase activity in arterial endothelial cells

Xanthine oxidoreductase (XOR) activity has been previously noted to be responsive to changes in O2 tension. While prior studies have focused on the extremes (0-3% and 95-100%) of O2 tensions, we report the influence of 10% O2 on endothelial cell XOR, a concentration resembling modest arterial hypoxia commonly found in patients with chronic cardiopulmonary diseases. Exposure of bovine aortic endothelial cells to 10% O2 increased XOR mRNA and protein abundance by 50%. Concomitantly, there was a 3-fold increase in XOR activity, XOR-dependent reactive oxygen species production, and cellular export of active enzyme. Although increases in mRNA and immunoreactive protein levels were observed, inhibition of transcription, translation, or protein degradation did not significantly alter cellular XOR specific activity, suggesting only modest contributions to 10% O2-induced effects. Exposure to 10% O2 did not increase cellular HIF-1alpha protein levels and hypoxia mimics did not alter XOR activity. Treatment of control cells with adenosine resulted in increased XOR activity similar to hypoxia. Exposure to the adenosine receptor agonist NECA increased enzymatic activity 4-fold while 8SPT, an adenosine receptor antagonist, reduced hypoxic induction of XOR activity approximately 50%. Combined, these data reveal that moderate hypoxia significantly enhances endothelial XOR specific activity, release, and XOR-derived reactive oxygen species generation. These effects appear to be mediated in part via adenosine-dependent processes.

Structure and function of xanthine oxidoreductase: where are we now?

Xanthine oxidoreductase (XOR) is a complex molybdoflavoenzyme, present in milk and many other tissues, which has been studied for over 100 years. While it is generally recognized as a key enzyme in purine catabolism, its structural complexity and specialized tissue distribution suggest other functions that have never been fully identified. The publication, just over 20 years ago, of a hypothesis implicating XOR in ischemia-reperfusion injury focused research attention on the enzyme and its ability to generate reactive oxygen species (ROS). Since that time a great deal more information has been obtained concerning the tissue distribution, structure, and enzymology of XOR, particularly the human enzyme. XOR is subject to both pre- and post-translational control by a range of mechanisms in response to hormones, cytokines, and oxygen tension. Of special interest has been the finding that XOR can catalyze the reduction of nitrates and nitrites to nitric oxide (NO), acting as a source of both NO and peroxynitrite. The concept of a widely distributed and highly regulated enzyme capable of generating both ROS and NO is intriguing in both physiological and pathological contexts. The details of these recent findings, their pathophysiological implications, and the requirements for future research are addressed in this review.

Oxygen radical inhibition of nitric oxide-dependent vascular function in sickle cell disease

Plasma xanthine oxidase (XO) activity was defined as a source of enhanced vascular superoxide (O(2)( *-)) and hydrogen peroxide (H(2)O(2)) production in both sickle cell disease (SCD) patients and knockout-transgenic SCD mice. There was a significant increase in the plasma XO activity of SCD patients that was similarly reflected in the SCD mouse model. Western blot and enzymatic analysis of liver tissue from SCD mice revealed decreased XO content. Hematoxylin and eosin staining of liver tissue of knockout-transgenic SCD mice indicated extensive hepatocellular injury that was accompanied by increased plasma content of the liver enzyme alanine aminotransferase. Immunocytochemical and enzymatic analysis of XO in thoracic aorta and liver tissue of SCD mice showed increased vessel wall and decreased liver XO, with XO concentrated on and in vascular luminal cells. Steady-state rates of vascular O(2)( *-) production, as indicated by coelenterazine chemiluminescence, were significantly increased, and nitric oxide (( *)NO)-dependent vasorelaxation of aortic ring segments was severely impaired in SCD mice, implying oxidative inactivation of ( *)NO. Pretreatment of aortic vessels with the superoxide dismutase mimetic manganese 5,10,15,20-tetrakis(N-ethylpyridinium-2-yl)porphyrin markedly decreased O(2)( small middle dot-) levels and significantly restored acetylcholine-dependent relaxation, whereas catalase had no effect. These data reveal that episodes of intrahepatic hypoxia-reoxygenation associated with SCD can induce the release of XO into the circulation from the liver. This circulating XO can then bind avidly to vessel luminal cells and impair vascular function by creating an oxidative milieu and catalytically consuming (*)NO via O(2)( small middle dot-)-dependent mechanisms.

Serum xanthine oxidase in human liver disease

OBJECTIVES

High concentrations of serum xanthine oxidase (XO) have been reported during human liver disease and hepatocyte injury in experimental settings. However, it is unclear whether this elevation reflects hepatocyte necrosis or has a different meaning.

METHODS

The serum level of XO in 64 patients with chronic liver disease (17 patients with cirrhosis, 30 with chronic hepatitis, and 17 with cholestatic disorders) and in 12 control subjects was determined by a competitive ELISA. Conventional serum markers of liver damage were assessed in all patients, and grading and staging were scored in the chronic hepatitis group according to Knodell.

RESULTS

The XO serum levels were significantly higher in the patients than in the controls. The differences were also significant when controls were compared to patients with chronic hepatitis and cholestatic disorders separately, but not when compared to the cirrhosis group. Patients with cholestatic disorders had XO values higher than those of patients with cirrhosis or chronic hepatitis. XO levels did not correlate with stage and grade in chronic hepatitis group. We found a weak but significant positive correlation in patients between XO serum level and gamma-glutamyl transpeptidase (r = 0.37). This correlation was stronger when chronic hepatitis (r = 0.42) and, especially cholestatic disorders (r = 0.71), were separately tested, but was absent in the cirrhosis group. The XO values positively correlated with alkaline phosphatase in patients with cholestatic disorders. A level of serum XO >32 microg/ml specifically identified cholestatic disorders in our study population.

CONCLUSIONS

A marked elevation of serum XO in patients with chronic liver disease seems to reflect the presence of cholestasis. No correlation between XO levels and histological or serum evidence of hepatocyte necrosis was found in these patients.

Determination of xanthine oxidoreductase forms: influence of reaction conditions

Xanthine oxidoreductase (XOR) has been implicated in ischaemia-reperfusion injury, and increases in this enzyme have been found in plasma of patients with different illnesses. The catalytic concentrations of the XOR forms found in plasma, using various reaction conditions, greatly differ in the related literature. We studied the effect of the assay conditions on the xanthine oxidation rate catalysed by the XOR forms. Our results demonstrate inhibition of XOR by the reaction products and a time-dependent decrease in the reaction rates of XOR forms. Substrate consumption and inhibition by the products did not account for this decrease. Determination at 60 min incubation leads to catalytic concentrations up to 80% lower for the XOR forms than those obtained at 10 min. We conclude that elimination of the reaction products (NADH, H(2)O(2) and O(2)) from the reaction mixture, and short incubation times, are necessary for accurate measurement of the XOR activities.

Determination of xanthine oxidase in human serum by a competitive enzyme-linked immunosorbent assay (ELISA)

Xanthine oxidase was purified from human milk and used to immunise rabbits. A competitive immunoenzymatic assay with purified enzyme and rabbit antiserum was optimised to measure xanthine oxidase in human serum, the lowest detectable amount being 0.03 pmol of enzymatic protein. Thus, the test (i) is sensitive enough to determine xanthine oxidase in human serum, being more sensitive than the spectrophotometric method, (ii) it is more convenient for clinical laboratories than other sensitive tests and (iii) it has the advantage over the enzyme activity-based assays of also detecting inactive enzyme molecules. A competitive enzyme-linked immunosorbent assay (ELISA) was used to measure the serum xanthine oxidase level in healthy donors and in patients with liver diseases, and it was found that any concentration below 1 mg/L is in the normal range.

Binding of xanthine oxidase to vascular endothelium. Kinetic characterization and oxidative impairment of nitric oxide-dependent signaling

Concentrations of up to 1.5 milliunits/ml xanthine oxidase (XO) (1.1 micrograms/ml) are found circulating in plasma during diverse inflammatory events. The saturable, high affinity binding of extracellular XO to vascular endothelium and the effects of cell binding on both XO catalytic activity and differentiated vascular cell function are reported herein. Xanthine oxidase purified from bovine cream bound specifically and with high affinity (Kd = 6 nM) at 4 degreesC to bovine aortic endothelial cells, increasing cell XO specific activity up to 10-fold. Xanthine oxidase-cell binding was not inhibited by serum or albumin and was partially inhibited by the addition of heparin. Pretreatment of endothelial cells with chondroitinase, but not heparinase or heparitinase, diminished endothelial binding by approximately 50%, suggesting association with chondroitin sulfate proteoglycans. Analysis of rates of superoxide production by soluble and cell-bound XO revealed that endothelial binding did not alter the percentage of univalent reduction of oxygen to superoxide. Comparison of the extent of CuZn-SOD inhibition of native and succinoylated cytochrome c reduction by cell-bound XO indicated that XO-dependent superoxide production was occurring in a cell compartment inaccessible to CuZn-SOD. This was further supported by the observation of a shift of exogenously added XO from extracellular binding sites to intracellular compartments, as indicated by both protease-reversible cell binding and immunocytochemical localization studies. Endothelium-bound XO also inhibited nitric oxide-dependent cGMP production by smooth muscle cell co-cultures in an SOD-resistant manner. This data supports the concept that circulating XO can bind to vascular cells, impairing cell function via oxidative mechanisms, and explains how vascular XO activity diminishes vasodilatory responses to acetylcholine in hypercholesterolemic rabbits and atherosclerotic humans. The ubiquity of cell-XO binding and endocytosis as a fundamental mechanism of oxidative tissue injury is also affirmed by the significant extent of XO binding to human vascular endothelial cells, rat lung type 2 alveolar epthelial cells, and fibroblasts.

Determination of human plasma xanthine oxidase activity by high-performance liquid chromatography

An assay for human plasma xanthine oxidase activity was developed with pterin as the substrate and the separation of product (isoxanthopterin) by high-performance liquid chromatography with a fluorescence detector. The reaction mixture consists of 60 microliters of plasma and 240 microliters of 0.2 M Tris-HCl buffer (pH 9.0) containing 113 microM pterin. With this assay, the activity of plasma xanthine oxidase could be easily determined despite its low activity. As a result, it could be demonstrated that the intravenous administration of heparin or the oral administration of ethanol did not increase plasma xanthine oxidase activity in normal subjects, and also that plasma xanthine oxidase activity was higher in patients with hepatitis C virus infection than in healthy subjects or patients with gout. In addition, a single patient with von Gierke's disease showed a marked increase in the plasma activity of this enzyme, relative to that apparent in normal subjects.

Xanthine oxidase activity in the circulation of rats following hemorrhagic shock

Reactive oxygen metabolites generated from xanthine oxidase play an important role in the pathogenesis of ischemia-induced tissue injury. In a hemorrhagic shock model of ischemia-reperfusion, the intracellular enzyme xanthine oxidase was released into the vasculature. This intravascular source of superoxide (O2.-) and hydrogen peroxide (H2O2) interacted reversibly with glycosaminoglycans of vascular endothelium and markedly concentrated xanthine oxidase at cell surfaces, enhancing its ability to produce extensive damage to remote tissues. Rats were made hypotensive by hemorrhage, maintained for 2h, and reinfused with shed blood. Blood samples were obtained prior to hemorrhage and 15, 30, 60, and 90 min after reperfusion for determination of xanthine oxidase (XO), lactate dehydrogenase (LDH), and alanine transaminase (AST). These enzymes were not significantly elevated in control animals. Reperfusion after hemorrhage-induced ischemia resulted in significantly elevated AST and LDH in both low heparin (100 U/h) and high heparin (1000 U/h) groups. Xanthine oxidase was detected in the circulation only after 90 min reperfusion in the low heparin group and was elevated during the entire reperfusion period in the high heparin group. Studies with cultured vascular endothelium showed significant heparin-reversible binding of XO to cellular glycosaminoglycans. These results suggest that XO can gain access to the circulation following ischemia, where it then binds to the vascular endothelial cells to produce site-specific oxidant injury to organs remote from the site of XO release.

Regulation of xanthine dehydrogenase and xanthine oxidase activity and gene expression in cultured rat pulmonary endothelial cells

The central importance of xanthine dehydrogenase (XDH) and xanthine oxidase (XO) in the pathobiochemistry of a number of clinical disorders underscores the need for a comprehensive understanding of the regulation of their expression. This study was undertaken to examine the effects of cytokines on XDH/XO activity and gene expression in pulmonary endothelial cells. The results indicate that IFN-gamma is a potent inducer of XDH/XO activity in rat lung endothelial cells derived from both the microvasculature (LMVC) and the pulmonary artery. In contrast, interferon-alpha/beta, tumor necrosis factor-alpha, interleukin-1 or -6, lipopolysaccharide and phorbol myristate acetate have no demonstrable effect. The increase in XDH/XO activity requires new protein synthesis. By Northern analysis, IFN-gamma markedly increases the level of the 5.0-kb XDH/XO mRNA in LMVC. The increase is due, in part, to increased transcription rate of the XDH/XO gene. Transcriptional activation does not require new protein synthesis. The physiologic relevance of these observations was evaluated by administering IFN-gamma to rats. Intraperitoneal administration leads to an increased XDH/XO activity and XDH/XO mRNA level in rat lungs. In sum, IFN-gamma is a potent and biologically relevant inducer of XDH/XO expression; the major site of upregulation occurs at the transcriptional level.

Spectrophotometric assay of xanthine oxidase with 2,2'-azino-di(3-ethylbenzthiazoline-6-sulphonate) (ABTS) as chromogen

A new method for the determination of xanthine oxidase activity, based on the oxidation of 2,2'-azino-di(3-ethylbenzthiazoline-6-sulphonate) (ABTS) by use of uricase and peroxidase, is described. The absorbance increase of the oxidized form of ABTS, measured after 10 min at 410 nm is proportional to xanthine oxidase activity. The method is sensitive, precise (CV below 8.3%), and linear up to 20 U/l. The analytical recovery of the ABTS-method was quantitative. Comparison with the UV and colorimetric NBT-method gave good correlation (r greater than or equal to 0.984). Reference values for serum xanthine oxidase activities determined with the new ABTS-method on 83 healthy persons are 0 to 1.20 U/l.

Purine nucleoside and purine base concentrations in bovine thyroid and plasma

Concentrations of eight purine nucleosides and bases in bovine thyroid and plasma were determined by high pressure liquid chromatography. Plasma purines were metabolized to uric acid in the absence of inhibitors. The concentrations of these purines were 10-100 times greater in thyroid tissue than in plasma.

Urine xanthine oxidase activity in urinary tract infection

Xanthine oxidase (XO) activity was found to be negligible in sterile human urines (less than 480 units, as presently defined, per litre). Significant XO activity was found in all urines containing more than 10(5) bacteria/ml, except for urines infected with Staphylococcus aureus, in which XO activity ranged from 347 to 714 units per litre. Plasma XO is not transferred to the urine, as demonstrated by the negligible XO activity found in sterile urines from patients with raised plasma XO activity. Determination of urinary XO activity is a suitable procedure for the detection of urinary tract infection.

Elevation of serum xanthine oxidase activity following halothane anesthesia in man

Halothane, but not methoxyflurane, was found to cause specific hepatocellular damage, the hepatotoxicity being prompt but transient. The hepatotoxicity was demonstrated by the elevation in the serum activity of xanthine oxidase, a highly sensitive marker for acute liver damage.

[Adenosine deaminase activity in human erythrocytes and lymphocytes]

A radiochromatographic method is described for measuring enzymatic activity of adenosine deaminase in human erythrocytes and lymphocytes. [8-14C]-adenosine is converted into inosine and hypoxanthine; after chromatographic separation of the products, the radioactivity is determined. The kinetic properties of the enzyme have been studied. The Km values for the erythrocyte and lymphocyte enzymes are higher as compared with purified deaminase. Optimum conditions for substrate concentration for assay were established. The mean normal activity (+/- S.E. of mean) is: for erythrocytes, 494 +/- 61; nmol min-1 ml-1; for lymphocytes- 147 +/- 0.18 nmol min-1 10(6) cellules. The mean values are higher than that given by other methods working at a lower (non-staurating) substrate concentration.

Bovine milk intake and xanthine oxidase activity in blood serum

Xanthine oxidase activity in blood serum was measured by a sensitive radio-enzymatic assay. Pigs receiving 7.6 liters of milk daily for 100 days did not show any detectable enzymatic activity in their blood Xanthine oxidase activity in blood serum of 25 human volunteers had an average of 6.7 milliunits per liter with a range of 0 to 34.6 milliunits per liter. Neither a causal nor statistically significant relationship existed between xanthine oxidase activity in blood and average daily milk consumption, age, or sex.

Elevation of serum xanthine oxidase following halothane anesthesia in the rat

Halothan anesthesia was found to be hapatotoxic in the rat, as demonstrated by a significant elevation of serum xanthine oxidase (SXO) level. SXO appeared to be more sensitive marker of liver damage than serum glutamic oxalacetic transaminase. SXO was found to be elevated also following exposure to relative hypoxia.