Xanthine Oxidoreductase Activity Research Articles

Consistent antioxidant and antihypertensive effects of oral sodium nitrite in DOCA-salt hypertension

Hypertension is a common disease that includes oxidative stress as a major feature, and oxidative stress impairs physiological nitric oxide (NO) activity promoting cardiovascular pathophysiological mechanisms. While inorganic nitrite and nitrate are now recognized as relevant sources of NO after their bioactivation by enzymatic and non-enzymatic pathways, thus lowering blood pressure, mounting evidence suggests that sodium nitrite also exerts antioxidant effects. Here we show for the first time that sodium nitrite exerts consistent systemic and vascular antioxidant and antihypertensive effects in the deoxycorticosterone-salt (DOCA-salt) hypertension model. This is particularly important because increased oxidative stress plays a major role in the DOCA-salt hypertension model, which is less dependent on activation of the renin-angiotensin system than other hypertension models. Indeed, antihypertensive effects of oral nitrite were associated with increased plasma nitrite and nitrate concentrations, and completely blunted hypertension-induced increases in plasma 8-isoprostane and lipid peroxide levels, in vascular reactive oxygen species, in vascular NADPH oxidase activity, and in vascular xanthine oxidoreductase activity. Together, these findings provide evidence that the oral administration of sodium nitrite consistently decreases the blood pressure in association with major antioxidant effects in experimental hypertension.

A new paradigm for XOR-catalyzed reactive species generation in the endothelium.

A plethora of vascular pathology is associated with inflammation, hypoxia and elevated rates of reactive species generation. A critical source of these reactive species is the purine catabolizing enzyme xanthine oxidoreductase (XOR) as numerous reports over the past 30 years have demonstrated XOR inhibition to be salutary. Despite this long standing association between increased vascular XOR activity and negative clinical outcomes, recent reports reveal a new paradigm whereby the enzymatic activity of XOR mediates beneficial outcomes by catalyzing the one electron reduction of nitrite (NO2(-)) to nitric oxide (NO) when NO2(-) and/or nitrate (NO3(-)) levels are enhanced either via dietary or pharmacologic means. These observations seemingly countervail numerous reports of improved outcomes in similar models upon XOR inhibition in the absence of NO2(-) treatment affirming the need for a more clear understanding of the mechanisms underpinning the product identity of XOR. To establish the micro-environmental conditions requisite for in vivo XOR-catalyzed oxidant and NO production, this review assesses the impact of pH, O2 tension, enzyme-endothelial interactions, substrate concentrations and catalytic differences between xanthine oxidase (XO) and xanthine dehydrogenase (XDH). As such, it reveals critical information necessary to distinguish if pursuit of NO2(-) supplementation will afford greater benefit than inhibition strategies and thus enhance the efficacy of current approaches to treat vascular pathology.

Dispelling dogma and misconceptions regarding the most pharmacologically targetable source of reactive species in inflammatory disease, xanthine oxidoreductase.

Xanthine oxidoreductase (XOR), the molybdoflavin enzyme responsible for the terminal steps of purine degradation in humans, is also recognized as a significant source of reactive species contributory to inflammatory disease. In animal models and clinical studies, inhibition of XOR has resulted in diminution of symptoms and enhancement of function in a number of pathologies including heart failure, diabetes, sickle cell anemia, hypertension and ischemia-reperfusion injury. For decades, XOR involvement in pathologic processes has been established by salutary outcomes attained from treatment with the XOR inhibitor allopurinol. This has served to frame a working dogma that elevation of XOR-specific activity is associated with enhanced rates of reactive species generation that mediate negative outcomes. While adherence to this narrowly focused practice of designating elevated XOR activity to be "bad" has produced some benefit, it has also led to significant underdevelopment of the processes mediating XOR regulation, identification of alternative reactants and products as well as micro-environmental factors that alter enzymatic activity. This is exemplified by recent reports: (1) identifying XOR as a nitrite reductase and thus a source of beneficial nitric oxide ((•)NO) under in vivo conditions similar to those where XOR inhibition has been assumed an optimal treatment choice, (2) describing XOR-derived uric acid (UA) as a critical pro-inflammatory mediator in vascular and metabolic disease and (3) ascribing an antioxidant/protective role for XOR-derived UA. When taken together, these proposed and countervailing functions of XOR affirm the need for a more comprehensive evaluation of product formation as well as the factors that govern product identity. As such, this review will critically evaluate XOR-catalyzed oxidant, (•)NO and UA formation as well as identify factors that mediate their production, inhibition and the resultant impact on inflammatory disease.

Metabolomic markers of altered nucleotide metabolism in early stage adenocarcinoma.

Adenocarcinoma, a type of non-small cell lung cancer, is the most frequently diagnosed lung cancer and the leading cause of lung cancer mortality in the United States. It is well documented that biochemical changes occur early in the transition from normal to cancer cells, but the extent to which these alterations affect tumorigenesis in adenocarcinoma remains largely unknown. Herein, we describe the application of mass spectrometry and multivariate statistical analysis in one of the largest biomarker research studies to date aimed at distinguishing metabolic differences between malignant and nonmalignant lung tissue. Gas chromatography time-of-flight mass spectrometry was used to measure 462 metabolites in 39 malignant and nonmalignant lung tissue pairs from current or former smokers with early stage (stage IA-IB) adenocarcinoma. Statistical mixed effects models, orthogonal partial least squares discriminant analysis and network integration, were used to identify key cancer-associated metabolic perturbations in adenocarcinoma compared with nonmalignant tissue. Cancer-associated biochemical alterations were characterized by (i) decreased glucose levels, consistent with the Warburg effect, (ii) changes in cellular redox status highlighted by elevations in cysteine and antioxidants, alpha- and gamma-tocopherol, (iii) elevations in nucleotide metabolites 5,6-dihydrouracil and xanthine suggestive of increased dihydropyrimidine dehydrogenase and xanthine oxidoreductase activity, (iv) increased 5'-deoxy-5'-methylthioadenosine levels indicative of reduced purine salvage and increased de novo purine synthesis, and (v) coordinated elevations in glutamate and UDP-N-acetylglucosamine suggesting increased protein glycosylation. The present study revealed distinct metabolic perturbations associated with early stage lung adenocarcinoma, which may provide candidate molecular targets for personalizing therapeutic interventions and treatment efficacy monitoring.

Cyclin-dependent kinase five mediates activation of lung xanthine oxidoreductase in response to hypoxia.

BackgroundXanthine oxidoreductase (XOR) is involved in oxidative metabolism of purines and is a source of reactive oxygen species (ROS). As such, XOR has been implicated in oxidant-mediated injury in multiple cardiopulmonary diseases. XOR enzyme activity is regulated, in part, via a phosphorylation-dependent, post-translational mechanism, although the kinase(s) responsible for such hyperactivation are unknown.Methods and ResultsUsing an in silico approach, we identified a cyclin-dependent kinase 5 (CDK5) consensus motif adjacent to the XOR flavin adenine dinucleotide (FAD) binding domain. CDK5 is a proline-directed serine/threonine kinase historically linked to neural development and injury. We tested the hypothesis that CDK5 and its activators are mediators of hypoxia-induced hyperactivation of XOR in pulmonary microvascular endothelial cells (EC) and the intact murine lung. Using complementary molecular and pharmacologic approaches, we demonstrated that hypoxia significantly increased CDK5 activity in EC. This was coincident with increased expression of the CDK5 activators, cyclin-dependent kinase 5 activator 1 (CDK5r1 or p35/p25), and decreased expression of the CDK5 inhibitory peptide, p10. Expression of p35/p25 was necessary for XOR hyperactivation. Further, CDK5 physically associated with XOR and was necessary and sufficient for XOR phosphorylation and hyperactivation both in vitro and in vivo. XOR hyperactivation required the target threonine (T222) within the CDK5-consensus motif.Conclusions and SignificanceThese results indicate that p35/CDK5-mediated phosphorylation of T222 is required for hypoxia-induced XOR hyperactivation in the lung. Recognizing the contribution of XOR to oxidative injury in cardiopulmonary disease, these observations identify p35/CDK5 as novel regulators of XOR and potential modifiers of ROS-mediated injury.

Skeletal muscle as an endogenous nitrate reservoir

The nitric oxide synthase (NOS) family of enzymes form nitric oxide (NO) from arginine in the presence of oxygen. At reduced oxygen availability NO is also generated from nitrate in a two step process by bacterial and mammalian molybdopterin proteins, and also directly from nitrite by a variety of five-coordinated ferrous hemoproteins. The mammalian NO cycle also involves direct oxidation of NO to nitrite, and both NO and nitrite to nitrate by oxy-ferrous hemoproteins. The liver and blood are considered the sites of active mammalian NO metabolism and nitrite and nitrate concentrations in the liver and blood of several mammalian species, including human, have been determined. However, the large tissue mass of skeletal muscle had not been generally considered in the analysis of the NO cycle, in spite of its long-known presence of significant levels of active neuronal NOS (nNOS or NOS1). We hypothesized that skeletal muscle participates in the NO cycle and, due to its NO oxidizing heme protein, oxymyoglobin has high concentrations of nitrate ions. We measured nitrite and nitrate concentrations in rat and mouse leg skeletal muscle and found unusually high concentrations of nitrate but similar levels of nitrite, when compared to the liver. The nitrate reservoir in muscle is easily accessible via the bloodstream and therefore nitrate is available for transport to internal organs where it can be reduced to nitrite and NO. Nitrate levels in skeletal muscle and blood in nNOS−/− mice were dramatically lower when compared with controls, which support further our hypothesis. Although the nitrate reductase activity of xanthine oxidoreductase in muscle is less than that of liver, the residual activity in muscle could be very important in view of its total mass and the high basal level of nitrate. We suggest that skeletal muscle participates in overall NO metabolism, serving as a nitrate reservoir, for direct formation of nitrite and NO, and for determining levels of nitrate in other organs.

Xanthine Oxidoreductase Function Contributes to Normal Wound Healing.

Chronic, nonhealing wounds result in patient morbidity and disability. Reactive oxygen species (ROS) and nitric oxide (NO) are both required for normal wound repair, and derangements of these result in impaired healing. Xanthine oxidoreductase (XOR) has the unique capacity to produce both ROS and NO. We hypothesize that XOR contributes to normal wound healing. Cutaneous wounds were created in C57Bl6 mice. XOR was inhibited with dietary tungsten or allopurinol. Topical hydrogen peroxide (H2O2, 0.15%) or allopurinol (30 μg) was applied to wounds every other day. Wounds were monitored until closure or collected at d 5 to assess XOR expression and activity, cell proliferation and histology. The effects of XOR, nitrite, H2O2 and allopurinol on keratinocyte cell (KC) and endothelial cell (EC) behavior were assessed. We identified XOR expression and activity in the skin and wound edges as well as granulation tissue. Cultured human KCs also expressed XOR. Tungsten significantly inhibited XOR activity and impaired healing with reduced ROS production with reduced angiogenesis and KC proliferation. The expression and activity of other tungsten-sensitive enzymes were minimal in the wound tissues. Oral allopurinol did not reduce XOR activity or alter wound healing but topical allopurinol significantly reduced XOR activity and delayed healing. Topical H2O2 restored wound healing in tungsten-fed mice. In vitro, nitrite and H2O2 both stimulated KC and EC proliferation and EC migration. These studies demonstrate for the first time that XOR is abundant in wounds and participates in normal wound healing through effects on ROS production.

Xanthine oxidoreductase reference values in platelet-poor plasma and platelets in healthy volunteers.

Introduction. Xanthine oxidoreductase (XOR) is an enzyme belonging to the class of hydroxylases. XOR is stated, inter alia, in the kidneys, liver, and small intestine as well as in leukocytes and platelets and endothelial cells of capillaries. Its main role is to participate in the conversion of hypoxanthine to xanthine and the uric acid. It occurs in two isoforms: dehydrogenase (XD) and oxidase (XO), which is considered one of the sources of reactive oxygen species. Aim of the Study. Determination of reference values of xanthine oxidoreductase activity in PPP and platelets. Materials and Methods. Study group consisted of 70 healthy volunteers. The isoform activities of xanthine oxidoreductase were determined by kinetic spectrophotometry. Results. A statistically significant difference between the activity of the XOR in PPP and platelets (P < 0.001). The highest activity of XO was found in both PPP and blood platelets. Significant differences between the activity of the various isoforms in PPP (P = 0.0032) and platelets (P < 0.001) were also found. Conclusions. The healthy volunteers showed the highest activity XO (prooxidant) and the lowest XD (antioxidant), which indicates a slight oxidative stress and confirmed physiological effects of XOR.

Evaluation of neuronal protective effects of xanthine oxidoreductase inhibitors on severe whole-brain ischemia in mouse model and analysis of xanthine oxidoreductase activity in the mouse brain.

Global cerebral ischemia and reperfusion (I/R) often result in high mortality. Free radicals play an important role in global cerebral I/R. Xanthine oxidoreductase (XOR) inhibitors, such as allopurinol, have been reported to protect tissues from damage caused by reactive oxygen species (ROS) by inhibiting its production through XOR inhibition. The recently introduced XOR inhibitor febuxostat, which is a more potent inhibitor than allopurinol, is expected to decrease free radical production more effectively. Here, we analyzed the effects of allopurinol and febuxostat in decreasing global severe cerebral I/R damage in mice. Mice were divided into three groups: a placebo group, an allopurinol group, and a febuxostat group. Pathological examinations, which were performed in each group in the CA1 and CA2 regions of the hippocampus 4 days after I/R surgery, revealed that there was a decrease in the number of neuronal cells in the 14-min occlusion model in both regions and that drugs that were administered to prevent this damage were not effective. The enzymatic activity was extremely low in the mouse brain, and XOR could not be detected in the nonischemic and ischemic mice brains with western blot analyses. Thus, one of the reasons for the decreased effectiveness of XOR inhibitors in controlling severe whole-brain ischemia in a mouse model was the low levels of expression of XOR in the mouse brain.

ChemInform Abstract: Xanthine Oxidoreductase‐Catalyzed Reactive Species Generation: A Process in Critical Need of Reevaluation

AbstractReview: 53 refs.

Remote ischemic preconditioning differentially affects NADPH oxidase isoforms during hepatic ischemia–reperfusion

AimsWe investigated the function of major superoxide-generating enzymes after remote ischemic preconditioning (IPC) and hepatic ischemia–reperfusion (IR), with the specific aim of assessing the importance of the most relevant NADPH oxidase (NOX) isoforms, NOX2 and NOX4, in the mechanism of action. Main methods60-min partial liver ischemia was induced in Sprague–Dawley rats in the presence or absence of remote IPC (2×10-min limb IR), and hepatic microcirculatory variables were determined through intravital video microscopy and lightguide spectrophotometry during reperfusion. Inflammatory enzyme activities (myeloperoxidase (MPO) and xanthine oxidoreductase (XOR)), cytokine production (TNF-α and HMGB-1), liver necroenzyme levels (AST, ALT and LDH) and NOX2 and NOX4 protein expression changes (Western blot analysis) were assayed biochemically. Key findingsIn this setting, remote IPC significantly decreased the IR-induced hepatic NOX2 expression, but the NOX4 expression remained unchanged. The remote IPC provided significant, but incomplete protection against the leukocyte–endothelial cell interactions and flow deterioration. Hepatocellular damage (AST, ALT and LDH release), cytokine levels, and XOR and MPO activities also diminished. SignificanceRemote IPC limited the IR-induced microcirculatory dysfunction, but the protective effect did not affect all NOX homologs (at least not NOX4). The residual damage and inflammatory activation could well be linked to the unchanging NOX4 activity.

Protective effects of l-alpha-glycerylphosphorylcholine on ischaemia–reperfusion-induced inflammatory reactions

Choline-containing dietary phospholipids, including phosphatidylcholine (PC), may function as anti-inflammatory substances, but the mechanism remains largely unknown. We investigated the effects of L-alpha-glycerylphosphorylcholine (GPC), a deacylated PC derivative, in a rodent model of small intestinal ischaemia-reperfusion (IR) injury. Anaesthetized Sprague-Dawley rats were divided into control, mesenteric IR (45 min mesenteric artery occlusion, followed by 180 min reperfusion), IR with GPC pretreatment (16.56 mg kg⁻¹ GPC i.v., 5 min prior to ischaemia) or IR with GPC post-treatment (16.56 mg kg⁻¹ GPC i.v., 5 min prior to reperfusion) groups. Macrohaemodynamics and microhaemodynamic parameters were measured; intestinal inflammatory markers (xanthine oxidoreductase activity, superoxide and nitrotyrosine levels) and liver ATP contents were determined. The IR challenge reduced the intestinal intramural red blood cell velocity, increased the mesenteric vascular resistance, the tissue xanthine oxidoreductase activity, the superoxide production, and the nitrotyrosine levels, and the ATP content of the liver was decreased. Exogenous GPC attenuated the macro- and microcirculatory dysfunction and provided significant protection against the radical production resulting from the IR stress. The GPC pretreatment alleviated the hepatic ATP depletion, the reductions in the mean arterial pressure and superior mesenteric artery flow, and similarly to the post-treatments with GPC, also decreased the xanthine oxidoreductase activity, the intestinal superoxide production, the nitrotyrosine level, and normalized the microcirculatory dysfunction. These data demonstrate the effectiveness of GPC therapies and provide indirect evidence that the anti-inflammatory effects of PC could be linked to a reaction involving the polar part of the molecule.

Structural and Functional Insights into the Catalytic Inactivity of the Major Fraction of Buffalo Milk Xanthine Oxidoreductase

BackgroundXanthine oxidoreductase (XOR) existing in two interconvertible forms, xanthine dehydrogenase (XDH) and xanthine oxidase (XO), catabolises xanthine to uric acid that is further broken down to antioxidative agent allantoin. XOR also produces free radicals serving as second messenger and microbicidal agent. Large variation in the XO activity has been observed among various species. Both hypo and hyper activity of XOR leads to pathophysiological conditions. Given the important nutritional role of buffalo milk in human health especially in south Asia, it is crucial to understand the functional properties of buffalo XOR and the underlying structural basis of variations in comparison to other species.Methods and FindingsBuffalo XO activity of 0.75 U/mg was almost half of cattle XO activity. Enzymatic efficiency (k cat/K m) of 0.11 sec−1 µM−1 of buffalo XO was 8–10 times smaller than that of cattle XO. Buffalo XOR also showed lower antibacterial activity than cattle XOR. A CD value (Δε430 nm) of 46,000 M−1 cm−1 suggested occupancy of 77.4% at Fe/S I centre. Buffalo XOR contained 0.31 molybdenum atom/subunit of which 48% existed in active sulfo form. The active form of XO in buffalo was only 16% in comparison to ∼30% in cattle. Sequencing revealed 97.4% similarity between buffalo and cattle XOR. FAD domain was least conserved, while metal binding domains (Fe/S and Molybdenum) were highly conserved. Homology modelling of buffalo XOR showed several variations occurring in clusters, especially close to FAD binding pocket which could affect NAD+ entry in the FAD centre. The difference in XO activity seems to be originating from cofactor deficiency, especially molybdenum.ConclusionA major fraction of buffalo milk XOR exists in a catalytically inactive form due to high content of demolybdo and desulfo forms. Lower Fe/S content and structural factors might be contributing to lower enzymatic efficiency of buffalo XOR in a minor way.

Inhibition of xanthine oxidase by the aldehyde oxidase inhibitor raloxifene: Implications for identifying molybdopterin nitrite reductases

Sources of nitric oxide alternative to nitric oxide synthases are gaining significant traction as crucial mediators of vessel function under hypoxic inflammatory conditions. For example, capacity to catalyze the one electron reduction of nitrite (NO2-) to ·NO has been reported for hemoglobin, myoglobin and molybdopterin-containing enzymes including xanthine oxidoreductase (XOR) and aldehyde oxidase (AO). For XOR and AO, use of selective inhibition strategies is therefore crucial when attempting to assign relative contributions to nitrite-mediated ·NO formation in cells and tissue. To this end, XOR inhibition has been accomplished with application of classic pyrazolopyrimidine-based inhibitors allo/oxypurinol or the newly FDA-approved XOR-specific inhibitor, Uloric® (febuxostat). Likewise, raloxifene, an estrogen receptor antagonist, has been identified as a potent (Ki=1.0nM) inhibitor of AO. Herein, we characterize the inhibition kinetics of raloxifene for XOR and describe the resultant effects on inhibiting XO-catalyzed ·NO formation. Exposure of purified XO to raloxifene (PBS, pH 7.4) resulted in a dose-dependent (12.5–100μM) inhibition of xanthine oxidation to uric acid. Dixon plot analysis revealed a competitive inhibition process with a Ki=13μM. This inhibitory process was more effective under acidic pH; similar to values encountered under hypoxic/inflammatory conditions. In addition, raloxifene also inhibited anoxic XO-catalyzed reduction of NO2- to NO (EC50=64μM). In contrast to having no effect on XO-catalyzed uric acid production, the AO inhibitor menadione demonstrated potent inhibition of XO-catalyzed NO2- reduction (EC50=60nM); somewhat similar to the XO-specific inhibitor, febuxostat (EC50=4nM). Importantly, febuxostat was found to be a very poor inhibitor of human AO (EC50=613μM) suggesting its usefulness for validating XO-dependent contributions to NO2- reduction in biological systems. Combined, these data indicate care should be taken when choosing inhibition strategies as well as inhibitor concentrations when assigning relative NO2- reductase activity of AO and XOR.

L-alpha-glycerylphosphorylcholine reduces the microcirculatory dysfunction and nicotinamide adenine dinucleotide phosphate-oxidase type 4 induction after partial hepatic ischemia in rats

BackgroundWe set out to investigate the microcirculatory consequences of hepatic ischemia–reperfusion (IR) injury and the effects of L-alpha-glycerylphosphorylcholine (GPC), a deacylated phospholipid derivative, on postischemic hepatocellular damage, with special emphasis on the expression of nicotinamide adenine dinucleotide phosphate oxidase type 4 (NOX4), which is predominantly expressed in hepatic microvessels. Materials and methodsAnesthetized male Sprague–Dawley rats were subjected to 60-min ischemia of the left liver lobes and 180-min reperfusion, with or without GPC treatment (50 mg/kg intravenously 5 min before reperfusion, n = 6 each). A third group (n = 6) served as saline-treated control. Noninvasive online examination of the hepatic microcirculation was performed hourly by means of modified spectrometry. Plasma tumor necrosis factor (TNF-α), high-mobility group box 1 protein (HMGB1), plasma aspartate aminotransferase, alanine aminotransferase and lactate dehydrogenase levels, tissue xanthine oxidoreductase (XOR) and myeloperoxidase (MPO) activities, and expressions of NOX2 and NOX4 proteins were determined. ResultsLiver IR resulted in significant increases in NOX2 and NOX4 expressions and XOR and MPO activities, and approximately 2-fold increases in the levels of the inflammatory cytokines TNF-α and HMGB1. The microvascular blood flow and tissue oxygen saturation decreased by ∼20% from control values. GPC administration ameliorated the postischemic microcirculatory deterioration and reduced the liver necroenzyme levels significantly; the NOX4 expression, MPO activity, and HMGB1 level were also decreased, whereas the NOX2 expression, TNF-α level, and XOR activity were not influenced by GPC pretreatment. ConclusionsNOX4 activation is a decisive component in the IR-induced microcirculatory dysfunction. Exogenous GPC ameliorates the inflammatory activation, and preserves the postischemic microvascular perfusion and liver functions, these effects being associated with a reduced hepatic expression of NOX4.

Fatty acid nitroalkenes ameliorate glucose intolerance and pulmonary hypertension in high-fat diet-induced obesity

Obesity is a risk factor for diabetes and cardiovascular diseases, with the incidence of these disorders becoming epidemic. Pathogenic responses to obesity have been ascribed to adipose tissue (AT) dysfunction that promotes bioactive mediator secretion from visceral AT and the initiation of pro-inflammatory events that induce oxidative stress and tissue dysfunction. Current understanding supports that suppressing pro-inflammatory and oxidative events promotes improved metabolic and cardiovascular function. In this regard, electrophilic nitro-fatty acids display pleiotropic anti-inflammatory signalling actions. It was hypothesized that high-fat diet (HFD)-induced inflammatory and metabolic responses, manifested by loss of glucose tolerance and vascular dysfunction, would be attenuated by systemic administration of nitrooctadecenoic acid (OA-NO2). Male C57BL/6j mice subjected to a HFD for 20 weeks displayed increased adiposity, fasting glucose, and insulin levels, which led to glucose intolerance and pulmonary hypertension, characterized by increased right ventricular (RV) end-systolic pressure (RVESP) and pulmonary vascular resistance (PVR). This was associated with increased lung xanthine oxidoreductase (XO) activity, macrophage infiltration, and enhanced expression of pro-inflammatory cytokines. Left ventricular (LV) end-diastolic pressure remained unaltered, indicating that the HFD produces pulmonary vascular remodelling, rather than LV dysfunction and pulmonary venous hypertension. Administration of OA-NO2 for the final 6.5 weeks of HFD improved glucose tolerance and significantly attenuated HFD-induced RVESP, PVR, RV hypertrophy, lung XO activity, oxidative stress, and pro-inflammatory pulmonary cytokine levels. These observations support that the pleiotropic signalling actions of electrophilic fatty acids represent a therapeutic strategy for limiting the complex pathogenic responses instigated by obesity.

The significance of serum xanthine oxidoreductase in patients with nonalcoholic fatty liver disease.

Serum uric acid levels are significantly associated with nonalcoholic fatty liver disease (NAFLD). Xanthine oxidoreductase (XOR) is the key enzyme that catalyzes the formation of uric acid. The aim of this study was to investigate the association between serum XOR activity and NAFLD. In this cross-sectional study, serum XOR activity was measured by enzyme-linked immunosorbent assay in 129 patients with NAFLD and 71 controls. Serum XOR activity was markedly higher in patients with NAFLD than in the controls (p < 0.001). Serum XOR activity positively correlated with markers of liver injury and indices of oxidative stress. Moreover, patients with high XOR activity had a higher prevalence of metabolic syndrome. These data show that serum XOR activity is significantly associated with NAFLD.

Uric Acid Secretion from Adipose Tissue and Its Increase in Obesity

Obesity is often accompanied by hyperuricemia. However, purine metabolism in various tissues, especially regarding uric acid production, has not been fully elucidated. Here we report, using mouse models, that adipose tissue could produce and secrete uric acid through xanthine oxidoreductase (XOR) and that the production was enhanced in obesity. Plasma uric acid was elevated in obese mice and attenuated by administration of the XOR inhibitor febuxostat. Adipose tissue was one of major organs that had abundant expression and activities of XOR, and adipose tissues in obese mice had higher XOR activities than those in control mice. 3T3-L1 and mouse primary mature adipocytes produced and secreted uric acid into culture medium. The secretion was inhibited by febuxostat in a dose-dependent manner or by gene knockdown of XOR. Surgical ischemia in adipose tissue increased local uric acid production and secretion via XOR, with a subsequent increase in circulating uric acid levels. Uric acid secretion from whole adipose tissue was increased in obese mice, and uric acid secretion from 3T3-L1 adipocytes was increased under hypoxia. Our results suggest that purine catabolism in adipose tissue could be enhanced in obesity.

Nitroso-Redox Imbalance Affects Cardiac Structure and Function

Cardiovascular diseases (CVDs) and their treatment pose a huge economic and social burden in Western societies and in the developing world (1). Thus, there is significant interest in developing new therapeutic targets, and it is of particular interest that an old class of drugs, inhibitors of xanthine oxidoreductase (XO), may be repositioned to treat heart failure and left ventricular (LV) dysfunction. In this context, there is growing awareness that nitroso-redox balance may represent a new therapeutic target for heart failure (2). However, whether or not XO inhibitors fully address nitroso-redox imbalance in all patients remains controversial. The hypothesis that XO inhibition could treat heart failure was tested in the OPT-CHF trial (3). In that study, the primary endpoint was not statistically different in the population of symptomatic heart failure patients enrolled; however patients with hyperuricemia appeared to be a responsive population. While hyperuricemia is classically associated with gout and nephropathy, it is clearly also a biomarker in heart failure populations (4-6). Uric acid (UA) is the end product of purine metabolism, involving the conversion of hypoxanthine to xanthine and then to UA in reactions catalyzed by XO (Figure 1) (7). In most mammals including rodents, UA is further degraded to allantoin via the enzyme urate oxidase (UO), resulting in low UA serum levels. In humans and great apes, however, this final step does not occur because of a single point mutation inactivating UO, resulting in substantially higher serum UA levels. Whether UA plays beneficial or deleterious roles remains controversial, and some arguments can be made that the impact of UA on vascular tone have played an evolutionary role (4). However, in hypertension, type 2 diabetes mellitus, coronary artery disease (CAD), heart failure, and chronic kidney disease (CKD), high UA levels correlate with an increased risk of stroke and CVDs (4,6,8-10) Figure 1 Increased xanthine oxidase activity and nitroso-redox imbalance As UA levels represent a biomarker of XO activity, the OPT-CHF results suggest that patients with elevated XO activity responded preferentially (3). XO is a key oxidase participating in nitroso-redox imbalance in the heart, producing superoxide as a byproduct of purine metabolism (Figure 1). Thus, the epidemiological association of high UA levels with worse prognosis in a wide cohort of patients with CVDs likely reflects the fact that UA levels rise with increased XO activity (4-10).

Hydralazine and Organic Nitrates Restore Impaired Excitation-Contraction Coupling by Reducing Calcium Leak Associated with Nitroso-Redox Imbalance*

Although the combined use of hydralazine and isosorbide dinitrate confers important clinical benefits in patients with heart failure, the underlying mechanism of action is still controversial. We used two models of nitroso-redox imbalance, neuronal NO synthase-deficient (NOS1(-/-)) mice and spontaneously hypertensive heart failure rats, to test the hypothesis that hydralazine (HYD) alone or in combination with nitroglycerin (NTG) or isosorbide dinitrate restores Ca(2+) cycling and contractile performance and controls superoxide production in isolated cardiomyocytes. The response to increased pacing frequency was depressed in NOS1(-/-) compared with wild type myocytes. Both sarcomere length shortening and intracellular Ca(2+) transient (Δ[Ca(2+)]i) responses in NOS1(-/-) cardiomyocytes were augmented by HYD in a dose-dependent manner. NTG alone did not affect myocyte shortening but reduced Δ[Ca(2+)]i across the range of pacing frequencies and increased myofilament Ca(2+) sensitivity thereby enhancing contractile efficiency. Similar results were seen in failing myocytes from the heart failure rat model. HYD alone or in combination with NTG reduced sarcoplasmic reticulum (SR) leak, improved SR Ca(2+) reuptake, and restored SR Ca(2+) content. HYD and NTG at low concentrations (1 μm), scavenged superoxide in isolated cardiomyocytes, whereas in cardiac homogenates, NTG inhibited xanthine oxidoreductase activity and scavenged NADPH oxidase-dependent superoxide more efficiently than HYD. Together, these results revealed that by reducing SR Ca(2+) leak, HYD improves Ca(2+) cycling and contractility impaired by nitroso-redox imbalance, and NTG enhanced contractile efficiency, restoring cardiac excitation-contraction coupling.