The role of the [2Fe-2s] cluster centers in xanthine oxidoreductase

Management of hypertension addressing hyperuricaemia: introduction of nano-based approaches

Uric acid (UA) levels in blood serum have been associated with hypertension, indicating a potential causal relationship between high serum UA levels and the progression of hypertension. Therefore, the reduction of serum UA level is considered a potential strategy for lowering and mitigating blood pressure. If an individual is at risk of developing or already manifesting elevated blood pressure, this intervention could be an integral part of a comprehensive treatment plan. By addressing hyperuricaemia, practitioners may subsidize the optimization of blood pressure regulation, which illustrates the importance of addressing UA levels as a valuable strategy within the broader context of hypertension management. In this analysis, we outlined the operational principles of effective xanthine oxidase inhibitors for the treatment of hyperuricaemia and hypertension, along with an exploration of the contribution of nanotechnology to this field.

Triazole derivatives as potential xanthine oxidase inhibitors: Design, enzyme inhibition potential, and docking studies

Considerable ingenuity has been shown in the recent years in the discovery of novel xanthine oxidase (XO) inhibitors that fall outside the purine scaffold. The triazole nucleus has been the cornerstone for the development of many enzyme inhibitors for the clinical management of several diseases, where hyperuricemia is one of them. Here, we give a critical overview of significant research on triazole-based XO inhibitors, with respect to their design, synthesis, inhibition potential, toxicity, and docking studies, done till now. Based on these literature findings, we can expect a burst of modifications on triazole-based scaffolds in the near future by targeting XO, which will treat hyperuricemics, that is, painful conditions like gout that at present are hard to deal with.

Lipoylation is dependent on the ferredoxin FDX1 and dispensable under hypoxia in human cells

Iron-sulfur clusters (ISC) are essential cofactors that participate in electron transfer, environmental sensing, and catalysis. Amongst the most ancient ISC-containing proteins are the ferredoxin (FDX) family of electron carriers. Humans have two FDXs- FDX1 and FDX2, both of which are localized to mitochondria, and the latter of which is itself important for ISC synthesis. We have previously shown that hypoxia can eliminate the requirement for some components of the ISC biosynthetic pathway, but FDXs were not included in that study. Here, we report that FDX1, but not FDX2, is dispensable under 1% O2 in cultured human cells. We find that FDX1 is essential for production of the lipoic acid cofactor, which is synthesized by the ISC-containing enzyme lipoyl synthase. While hypoxia can rescue the growth phenotype of either FDX1 or lipoyl synthase KO cells, lipoylation in these same cells is not rescued, arguing against an alternative biosynthetic route or salvage pathway for lipoate in hypoxia. Our work reveals the divergent roles of FDX1 and FDX2 in mitochondria, identifies a role for FDX1 in lipoate synthesis, and suggests that loss of lipoic acid can be tolerated under low oxygen tensions in cell culture.

Mechanisms of nucleic acid degradation and high hydrostatic pressure tolerance of a novel deep-sea wall-less bacterium

Wall-less bacteria are broadly distributed in diverse habitats. They evolved from a common ancestor within the Firmicutes phylum through reductive evolution. Here, we report the cultivation, characterization, and polyphasic taxonomic analysis of the novel free-living wall-less bacterium, Hujiaoplasma nucleasis zrk29. We demonstrated that strain zrk29 had a strong ability to degrade DNA and RNA both under laboratory conditions and in the deep sea. We found that nucleic acids induced strain zrk29 to release chronic bacteriophages which supported strain zrk29 and other marine bacteria to metabolize nucleic acids without lysing host cells. We also showed that strain zrk29 tolerated high hydrostatic pressure via two pathways: (i) by transporting cations into its cells to increase intracellular osmotic pressure and (ii) by adjusting the unsaturated fatty acid chain content in its cell membrane phospholipids to increase cell membrane fluidity. This study extends our understanding of free-living wall-less bacteria and provides a useful model to explore the unique adaptation mechanisms of deep-sea microbes. IMPORTANCE The unique physiology and survival strategies of the Tenericutes bacterium-a typical wall-less bacterium-have fascinated scientists and the public, especially in extreme deep-sea environments where there is high hydrostatic pressure (HHP) and limited availability of nutrients. Here, we have isolated a novel free-living Tenericutes strain from deep-sea sediment and have found that it metabolizes nucleic acids with the support of chronic bacteriophages. This Tenericutes strain tolerates HHP stress by increasing intracellular osmotic pressure and the unsaturated fatty acid chain content of phospholipids in its cell membrane. Our results provide insights into the unique physiology of deep-sea free-living Tenericutes bacteria and highlight the significant role that chronic bacteriophages play in assisting wall-less bacteria to adapt to harsh conditions.

XDH and XO Research and Drug Discovery-Personal History

The author will outline the research history of the main issues addressed in this paper. The author has worked on this research himself. XDH, which is responsible for purine degradation, is present in various organisms. However, conversion to XO only occurs in mammals. The molecular mechanism of this conversion was elucidated in this study. The physiological and pathological significance of this conversion is presented. Finally, enzyme inhibitors were successfully developed, two of which are used as therapeutic agents for gout. Their wide application potential is also discussed.

A heterodimeric hyaluronate lyase secreted by the activated sludge bacterium Haliscomenobacter hydrossis

Haliscomenobacter hydrossis is a filamentous bacterium common in activated sludge. The bacterium was found to utilize hyaluronic acid, and hyaluronate lyase activity was detected in its culture. However, no hyaluronate lyase gene was found in the genome, suggesting the bacterium secretes a novel hyaluronate lyase. The purified enzyme exhibited two bands on SDS-PAGE and a single peak on gel filtration chromatography, suggesting a heterodimeric composition. N-terminal amino acid sequence and mass spectrometric analyses suggested that the subunits are molybdopterin-binding and [2Fe-2S]-binding subunits of a xanthine oxidase family protein. The presence of the cofactors was confirmed using spectrometric analysis. Oxidase activity was not detected, revealing that the enzyme is not an oxidase but a hyaluronate lyase. Nuclear magnetic resonance analysis of the enzymatic digest revealed that the enzyme breaks hyaluronic acid to 3-(4-deoxy-β-d-gluc-4-enuronosyl)-N-acetyl-d-glucosamine. As hyaluronate lyases (EC 4.2.2.1) are monomeric or trimeric, the enzyme is the first heterodimeric hyaluronate lyase.

Mutation of OsSAC3, Encoding the Xanthine Dehydrogenase, Caused Early Senescence in Rice

In both animals and higher plants, xanthine dehydrogenase is a highly conserved housekeeping enzyme in purine degradation where it oxidizes hypoxanthine to xanthine and xanthine to uric acid. Previous reports demonstrated that xanthine dehydrogenase played a vital role in N metabolism and stress response. Is xanthine dehydrogenase involved in regulating leaf senescence? A recessive early senescence mutant with excess sugar accumulation, ossac3, was isolated previously by screening the EMS-induced mutant library. Here, we show that xanthine dehydrogenase not only plays a role in N metabolism but also involved in regulating carbon metabolism in rice. Based on map-based cloning, OsSAC3 was identified, which encodes the xanthine dehydrogenase. OsSAC3 was constitutively expressed in all examined tissues and the OsSAC3 protein located in the cytoplasm. Transcriptional analysis revealed purine metabolism, chlorophyll metabolism, photosynthesis, sugar metabolism and redox balance were affected in the ossac3 mutant. Moreover, carbohydrate distribution was changed, leading to the accumulation of sucrose and starch in the leaves containing ossac3 on account of decreased expression of OsSWEET3a, OsSWEET6a and OsSWEET14 and oxidized inactivation of starch degradation enzymes in ossac3. These results indicated that OsSAC3 played a vital role in leaf senescence by regulating carbon metabolism in rice.

An interference-free new xanthine biosensor based on immobilized enzyme-nanogold conjugate on carbon nanotube doped poly(3,4-Ethylenedioxythiophene) composite film

A new design of biosensor based on polymeric nano(bio)composite has been proposed for the selective detection of xanthine to be used in the clinical analysis as well as food quality control. The xanthine oxidoreductase (XOR) gene ofPseudomonas aerogenosastrain CEBP1 wascloned to obtainpurifiedenzyme through affinity chromatography. fMWCNTdoped PEDOTwas electrodeposited on the working electrodeto enhance the sensitivity and selectivity of the biosensor. Bio-synthesized gold nanoparticles conjugated XOR (Au-XOR) was covalently immobilized on the polymeric nanocomposite. The enzymatic activity was enhanced 1.12 times with increased substrate affinity. The surface morphology and structural properties of the polymeric layer were investigated using SEM, FESEM, TEM. Electrochemical characteristics were performed by cyclic voltammetry, differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy. Xanthine was oxidized (pH 7.0) on the uniquely designed polymeric nano(bio)composite modified electrode at a lower anodic potential of + 0.446 V vs. Ag/AgCl (3 M NaCl)at optimized DPV conditions. The simple, newly designed Au-XOR/fMWCNT-PEDOT/GCE exhibited interference-free reproducibility and stability (∼4 months) with excellent sensitivity of 16.075 µA.µM^-1.cm^-2for the quantification of xanthine in biological samples such as blood, tissue, urine. The applicability of thebiosensor was validatedby comparing the sensing results for the real biological fluidic solutions with HPLC data (RE = 0.5-3.1%).

Human and rodent red blood cells do not demonstrate xanthine oxidase activity or XO-catalyzed nitrite reduction to NO

A number of molybdopterin enzymes, including xanthine oxidoreductase (XOR), aldehyde oxidase (AO), sulfite oxidase (SO), and mitochondrial amidoxime reducing component (mARC), have been identified as nitrate and nitrite reductases. Of these enzymes, XOR has been the most extensively studied and reported to be a substantive source of nitric oxide (NO) under inflammatory/hypoxic conditions that limit the catalytic activity of the canonical NOS pathway. It has also been postulated that XOR nitrite reductase activity extends to red blood cell (RBCs) membranes where it has been immunohistochemically identified. These findings, when combined with countervailing reports of XOR activity in RBCs, incentivized our current study to critically evaluate XOR protein abundance/enzymatic activity in/on RBCs from human, mouse, and rat sources. Using various protein concentrations of RBC homogenates for both human and rodent samples, neither XOR protein nor enzymatic activity (xanthine → uric acid) was detectable. In addition, potential loading of RBC-associated glycosaminoglycans (GAGs) by exposing RBC preparations to purified XO before washing did not solicit detectable enzymatic activity (xanthine → uric acid) or alter NO generation profiles. To ensure these observations extended to absence of XOR-mediated contributions to overall RBC-associated nitrite reduction, we examined the nitrite reductase activity of washed and lysed RBC preparations via enhanced chemiluminescence in the presence or absence of the XOR-specific inhibitor febuxostat (Uloric®). Neither addition of inhibitor nor the presence of the XOR substrate xanthine significantly altered the rates of nitrite reduction to NO by RBC preparations from either human or rodent sources confirming the absence of XO enzymatic activity. Furthermore, examination of the influence of the age (young cells vs. old cells) of human RBCs on XO activity also failed to demonstrate detectable XO protein. Combined, these data suggest: 1) that XO does not contribute to nitrite reduction in/on human and rodent erythrocytes, 2) care should be taken to validate immuno-detectable XO by demonstrating enzymatic activity, and 3) XO does not associate with human erythrocytic glycosaminoglycans or participate in nonspecific binding.

Classical Xanthinuria in Nine Israeli Families and Two Isolated Cases from Germany: Molecular, Biochemical and Population Genetics Aspects

Classical xanthinuria is a rare autosomal recessive metabolic disorder caused by variants in the XDH (type I) or MOCOS (type II) genes. Thirteen Israeli kindred (five Jewish and eight Arab) and two isolated cases from Germany were studied between the years 1997 and 2013. Four and a branch of a fifth of these families were previously described. Here, we reported the demographic, clinical, molecular and biochemical characterizations of the remaining cases. Seven out of 20 affected individuals (35%) presented with xanthinuria-related symptoms of varied severity. Among the 10 distinct variants identified, six were novel: c.449G>T (p.(Cys150Phe)), c.1434G>A (p.(Trp478*)), c.1871C>G (p.(Ser624*)) and c.913del (p.(Leu305fs*1)) in the XDH gene and c.1046C>T (p.(Thr349Ileu)) and c.1771C>T (p.(Pro591Ser)) in the MOCOS gene. Heterologous protein expression studies revealed that the p.Cys150Phe variant within the Fe/S-I cluster-binding site impairs XDH biogenesis, the p.Thr349Ileu variant in the NifS-like domain of MOCOS affects protein stability and cysteine desulfurase activity, while the p.Pro591Ser and a previously described p.Arg776Cys variant in the C-terminal domain affect Molybdenum cofactor binding. Based on the results of haplotype analyses and historical genealogy findings, the potential dispersion of the identified variants is discussed. As far as we are aware, this is the largest cohort of xanthinuria cases described so far, substantially expanding the repertoire of pathogenic variants, characterizing structurally and functionally essential amino acid residues in the XDH and MOCOS proteins and addressing the population genetic aspects of classical xanthinuria.

Iron: An Essential Element of Cancer Metabolism

Cancer cells undergo considerable metabolic changes to foster uncontrolled proliferation in a hostile environment characterized by nutrient deprivation, poor vascularization and immune infiltration. While metabolic reprogramming has been recognized as a hallmark of cancer, the role of micronutrients in shaping these adaptations remains scarcely investigated. In particular, the broad electron-transferring abilities of iron make it a versatile cofactor that is involved in a myriad of biochemical reactions vital to cellular homeostasis, including cell respiration and DNA replication. In cancer patients, systemic iron metabolism is commonly altered. Moreover, cancer cells deploy diverse mechanisms to increase iron bioavailability to fuel tumor growth. Although iron itself can readily participate in redox reactions enabling vital processes, its reactivity also gives rise to reactive oxygen species (ROS). Hence, cancer cells further rely on antioxidant mechanisms to withstand such stress. The present review provides an overview of the common alterations of iron metabolism occurring in cancer and the mechanisms through which iron promotes tumor growth.

Toxicoproteomic Profiling of hPXR Transgenic Mice Treated with Rifampicin and Isoniazid

Tuberculosis is a global health threat that affects millions of people every year, and treatment-limiting toxicity remains a considerable source of treatment failure. Recent reports have characterized the nature of hPXR-mediated hepatotoxicity and the systemic toxicity of antitubercular drugs. The antitubercular drug isoniazid plays a role in such pathologic states as acute intermittent porphyria, anemia, hepatotoxicity, hypercoagulable states (deep vein thrombosis, pulmonary embolism, or ischemic stroke), pellagra (vitamin B₃ deficiency), peripheral neuropathy, and vitamin B₆ deficiency. However, the mechanisms by which isoniazid administration leads to these states are unclear. To elucidate the mechanism of rifampicin- and isoniazid-induced liver and systemic injury, we performed tandem mass tag mass spectrometry-based proteomic screening of mPxr^-/- and hPXR mice treated with combinations of rifampicin and isoniazid. Proteomic profiling analysis suggested that the hPXR liver proteome is affected by antitubercular therapy to disrupt [Fe-S] cluster assembly machinery, [2Fe-2S] cluster-containing proteins, cytochrome P450 enzymes, heme biosynthesis, homocysteine catabolism, oxidative stress responses, vitamin B₃ metabolism, and vitamin B₆ metabolism. These novel findings provide insight into the etiology of some of these processes and potential targets for subsequent investigations. Data are available via ProteomeXchange with identifier PXD019505.

Targeted knock-in mice expressing the oxidase-fixed form of xanthine oxidoreductase favor tumor growth

Xanthine oxidoreductase has been implicated in cancer. Nonetheless, the role played by its two convertible forms, xanthine dehydrogenase (XDH) and oxidase (XO) during tumorigenesis is not understood. Here we produce XDH-stable and XO-locked knock-in (ki) mice to address this question. After tumor transfer, XO ki mice show strongly increased tumor growth compared to wild type (WT) and XDH ki mice. Hematopoietic XO expression is responsible for this effect. After macrophage depletion, tumor growth is reduced. Adoptive transfer of XO-ki macrophages in WT mice increases tumor growth. In vitro, XO ki macrophages produce higher levels of reactive oxygen species (ROS) responsible for the increased Tregs observed in the tumors. Blocking ROS in vivo slows down tumor growth. Collectively, these results indicate that the balance of XO/XDH plays an important role in immune surveillance of tumor development. Strategies that inhibit the XO form specifically may be valuable in controlling cancer growth.

The roles of the convertible forms, xanthine dehydrogenase (XDH) and xanthine oxidase (XO) during tumorigenesis is not known. Here, the authors develop XDH-stable and XO-locked knock-in (ki) mice and show increased tumor growth in XO ki mice, via macrophage-mediated immunoregulatory responses.

In Silico and 3D QSAR Studies of Natural Based Derivatives as Xanthine Oxidase Inhibitors

BACKGROUND

A large number of disorders and their symptoms emerge from deficiency or overproduction of specific metabolites has drawn the attention for the discovery of new therapeutic agents for the treatment of disorders. Various approaches such as computational drug design have provided the new methodology for the selection and evaluation of target protein and the lead compound mechanistically. For instance, the overproduction of xanthine oxidase causes the accumulation of uric acid which can prompt gout.

OBJECTIVE

In the present study we critically discussed the various techniques such as 3-D QSAR and molecular docking for the study of the natural based xanthine oxidase inhibitors with their mechanistic insight into the interaction of xanthine oxidase and various natural leads.

CONCLUSION

The computational studies of deferent natural compounds were discussed as a result the flavonoids, anthraquinones, xanthones shown the remarkable inhibitory potential for xanthine oxidase inhibition moreover the flavonoids such as hesperidin and rutin were found as promising candidates for further exploration.

Diminishing Inflammation by Reducing Oxidant Generation: Nitrated Fatty Acid-Mediated Inactivation of Xanthine Oxidoreductase

Inhibition of xanthine oxidoreductase (XOR) has proven beneficial in a plethora of inflammatory disease processes due to a net reduction in pro-inflammatory oxidants and secondary nitrating species. Electrophilic nitrated fatty acid derivatives, such as nitro-oleic acid (OA-NO₂) are also noted to display a broad spectrum of anti-inflammatory effects via interaction with critical signaling pathways. An alternative process in which nitrated fatty acids may extend anti-inflammatory actions is via inactivation of XOR, a process that is more effective than allo/oxypurinol-mediated inhibition. Herein, we describe the molecular aspects of nitrated fatty acid-associated inactivation of XOR, identify specificity via structure function relationships and discuss XOR as a crucial component of the anti-inflammatory portfolio of nitrated fatty acids.

Regulation of xanthine dehydrogensase gene expression and uric acid production in human airway epithelial cells

Introduction

The airway epithelium is a physical and immunological barrier that protects the pulmonary system from inhaled environmental insults. Uric acid has been detected in the respiratory tract and can function as an antioxidant or damage associated molecular pattern. We have demonstrated that human airway epithelial cells are a source of uric acid. Our hypothesis is that uric acid production by airway epithelial cells is induced by environmental stimuli associated with chronic respiratory diseases. We therefore examined how airway epithelial cells regulate uric acid production.

Materials and methods

Allergen and cigarette smoke mouse models were performed using house dust mite (HDM) and cigarette smoke exposure, respectively, with outcome measurements of lung uric acid levels. Primary human airway epithelial cells isolated from clinically diagnosed patients with asthma and chronic obstructive pulmonary disease (COPD) were grown in submerged cultures and compared to age-matched healthy controls for uric acid release. HBEC-6KT cells, a human airway epithelial cell line, were grown under submerged monolayer conditions for mechanistic and gene expression studies.

Results

HDM, but not cigarette smoke exposure, stimulated uric acid production in vivo and in vitro. Primary human airway epithelial cells from asthma, but not COPD patients, displayed elevated levels of extracellular uric acid in culture. In HBEC-6KT, production of uric acid was sensitive to the xanthine dehydrogenase (XDH) inhibitor, allopurinol, and the ATP Binding Cassette C4 (ABCC4) inhibitor, MK-571. Lastly, the pro-inflammatory cytokine combination of TNF-α and IFN-γ elevated extracellular uric acid levels and XDH gene expression in HBEC-6KT cells.

Conclusions

Our results suggest that the active production of uric acid from human airway epithelial cells may be intrinsically altered in asthma and be further induced by pro-inflammatory cytokines.

Enhanced XOR activity in eNOS-deficient mice: Effects on the nitrate-nitrite-NO pathway and ROS homeostasis

Xanthine oxidoreductase (XOR) is generally known as the final enzyme in purine metabolism and as a source of reactive oxygen species (ROS). In addition, this enzyme has been suggested to mediate nitric oxide (NO) formation via reduction of inorganic nitrate and nitrite. This NO synthase (NOS)-independent pathway for NO generation is of particular importance during certain conditions when NO bioavailability is diminished due to reduced activity of endothelial NOS (eNOS) or increased oxidative stress, including aging and cardiovascular disease. The exact interplay between NOS- and XOR-derived NO generation is not fully elucidated yet. The aim of the present study was to investigate if eNOS deficiency is associated with changes in XOR expression and activity and the possible impact on nitrite, NO and ROS homeostasis. Plasma levels of nitrate and nitrite were similar between eNOS deficient (eNOS^-/-) and wildtype (wt) mice. XOR activity was upregulated in eNOS^-/- compared with wt, but not in nNOS^-/-, iNOS^-/- or wt mice treated with the non-selective NOS inhibitor L-NAME. Following an acute dose of nitrate, plasma nitrite increased more in eNOS^-/- compared with wt, and this augmented response was abolished by the selective XOR inhibitor febuxostat. Livers from eNOS^-/- displayed higher nitrite reducing capacity compared with wt, and this effect was attenuated by febuxostat. Dietary supplementation with nitrate increased XOR expression and activity, but concomitantly reduced superoxide generation. The latter effect was also seen in vitro after nitrite administration. Treatment with febuxostat elevated blood pressure in eNOS^-/-, but not in wt mice. A high dose of dietary nitrate reduced blood pressure in naïve eNOS^-/- mice, and again this effect was abolished by febuxostat. In conclusion, eNOS deficiency is associated with an upregulation of XOR facilitating the nitrate-nitrite-NO pathway and decreasing the generation of ROS. This interplay between XOR and eNOS is proposed to play a significant role in NO homeostasis and blood pressure regulation.

The C-terminal peptide plays a role in the formation of an intermediate form during the transition between xanthine dehydrogenase and xanthine oxidase

Mammalian xanthine oxidoreductase can exist in both dehydrogenase and oxidase forms. Conversion between the two is implicated in such diverse processes as lactation, anti-bacterial activity, reperfusion injury and a growing number of diseases. We have constructed a variant of the rat liver enzyme that lacks the carboxy-terminal amino acids 1316-1331; it appears to assume an intermediate form, exhibiting a mixture of dehydrogenase and oxidase activities. The purified variant protein retained ~ 50-70% of oxidase activity even after prolonged dithiothreitol treatment, supporting a previous prediction that the C-terminal region plays a role in the dehydrogenase to oxidase conversion. In the crystal structure of the protein variant, most of the enzyme stays in an oxidase conformation. After 15 min of incubation with a high concentration of NADH, however, the corresponding X-ray structures showed a dehydrogenase-type conformation. On the other hand, disulfide formation between Cys535 and Cys992, which can clearly be seen in the electron density map of the crystal structure of the variant after removal of dithiothreitol, goes in parallel with the complete conversion to oxidase, resulting in structural changes identical to those observed upon proteolytic cleavage of the linker peptide. These results indicate that the dehydrogenase-oxidase transformation occurs rather readily and the insertion of the C-terminal peptide into the active site cavity of its subunit stabilizes the dehydrogenase form. We propose that the intermediate form can be generated (e.g. in endothelial cells) upon interaction of the C-terminal peptide portion of the enzyme with other proteins or the cell membrane.

DATABASE

Coordinate sets and structure factors for the four crystal structures reported in the present study have been deposited in the Protein Data Bank under the identification numbers 4YRW, 4YTZ, 4YSW, and 4YTY.

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.

Mechanism of porcine liver xanthine oxidoreductase mediated N-oxide reduction of cyadox as revealed by docking and mutagenesis studies

Xanthine oxidoreductase (XOR) is a cytoplasmic molybdenum-containing oxidoreductase, catalyzing both endogenous purines and exogenous compounds. It is suggested that XOR in porcine hepatocytes catalyzes the N-oxide reduction of quinoxaline 1,4-di-N-oxides (QdNOs). To elucidate the molecular mechanism underlying this metabolism, the cDNA of porcine XOR was cloned and heterologously expressed in Spodoptera frugiperda insect cells. The bovine XOR, showing sequence identity of 91% to porcine XOR, was employed as template for homology modeling. By docking cyadox, a representative compound of QdNOs, into porcine XOR model, eight amino acid residues, Gly47, Asn352, Ser360, Arg427, Asp430, Asp431, Ser1227 and Lys1230, were located at distances of less than 4Å to cyadox. Site-directed mutagenesis was performed to analyze their catalytic functions. Compared with wild type porcine XOR, G47A, S360P, D431A, S1227A, and K1230A displayed altered kinetic parameters in cyadox reduction, similarly to that in xanthine oxidation, indicating these mutations influenced electron-donating process of xanthine before subsequent electron transfer to cyadox to fulfill the N-oxide reduction. Differently, R427E and D430H, both located in the 424-434 loop, exhibited a much lower K(m) and a decreased V(max) respectively in cyadox reduction. Arg427 may be related to the substrate binding of porcine XOR to cyadox, and Asp430 is suggested to be involved in the transfer of electron to cyadox. This study initially reveals the possible catalytic mechanism of porcine XOR in cyadox metabolism, providing with novel insights into the structure-function relationship of XOR in the reduction of exogenous di-N-oxides.

Xanthine oxidoreductase-catalyzed reactive species generation: A process in critical need of reevaluation

Nearly 30 years have passed since the discovery of xanthine oxidoreductase (XOR) as a critical source of reactive species in ischemia/reperfusion injury. Since then, numerous inflammatory disease processes have been associated with elevated XOR activity and allied reactive species formation solidifying the ideology that enhancement of XOR activity equates to negative clinical outcomes. However, recent evidence may shatter this paradigm by describing a nitrate/nitrite reductase capacity for XOR whereby XOR may be considered a crucial source of beneficial (•)NO under ischemic/hypoxic/acidic conditions; settings similar to those that limit the functional capacity of nitric oxide synthase. Herein, we review XOR-catalyzed reactive species generation and identify key microenvironmental factors whose interplay impacts the identity of the reactive species (oxidants vs. (•)NO) produced. In doing so, we redefine existing dogma and shed new light on an enzyme that has weathered the evolutionary process not as gadfly but a crucial component in the maintenance of homeostasis.

Structure and evolution of vertebrate aldehyde oxidases: from gene duplication to gene suppression

Aldehyde oxidases (AOXs) and xanthine dehydrogenases (XDHs) belong to the family of molybdo-flavoenzymes. Although AOXs are not identifiable in fungi, these enzymes are represented in certain protists and the majority of plants and vertebrates. The physiological functions and substrates of AOXs are unknown. Nevertheless, AOXs are major drug metabolizing enzymes, oxidizing a wide range of aromatic aldehydes and heterocyclic compounds of medical/toxicological importance. Using genome sequencing data, we predict the structures of AOX genes and pseudogenes, reconstructing their evolution. Fishes are the most primitive organisms with an AOX gene (AOXα), originating from the duplication of an ancestral XDH. Further evolution of fishes resulted in the duplication of AOXα into AOXβ and successive pseudogenization of AOXα. AOXβ is maintained in amphibians and it is the likely precursors of reptilian, avian, and mammalian AOX1. Amphibian AOXγ is a duplication of AOXβ and the likely ancestor of reptilian and avian AOX2, which, in turn, gave rise to mammalian AOX3L1. Subsequent gene duplications generated the two mammalian genes, AOX3 and AOX4. The evolution of mammalian AOX genes is dominated by pseudogenization and deletion events. Our analysis is relevant from a structural point of view, as it provides information on the residues characterizing the three domains of each mammalian AOX isoenzyme. We cloned the cDNAs encoding the AOX proteins of guinea pig and cynomolgus monkeys, two unique species as to the evolution of this enzyme family. We identify chimeric RNAs from the human AOX3 and AOX3L1 pseudogenes with potential to encode a novel microRNA.

Xanthine oxidoreductase-catalyzed reduction of nitrite to nitric oxide: insights regarding where, when and how

Numerous inflammatory disorders are associated with elevated levels of xanthine oxidoreductase (XOR) and allied enhancement of reactive species formation contributory to systemic pathology. Despite a long standing association between increased XOR activity and negative clinical outcomes, recent reports describe a paradigm shift where XOR mediates beneficial actions by catalyzing the reduction of NO2(-) to NO. While provocative, these observations contradict reports of improved outcomes in similar models upon XOR inhibition as well as reports revealing strict anoxia as a requisite for XOR-mediated NO formation. To garner a more clear understanding of conditions necessary for in vivo XOR-catalyzed NO production, this review critically analyzes the impact of O2 tension, pH, substrate concentrations, glycoaminoglycan docking and inhibition strategies on the nitrite reductase activity of XOR and reveals a hypoxic milieu where this process may be operative. As such, information herein serves to link recent reports in which XOR activity has been identified as mediating the beneficial outcomes resulting from nitrite supplementation to a microenvironmental setting where XOR can serve as substantial source of NO.

Protein conformational gating of enzymatic activity in xanthine oxidoreductase

In mammals, xanthine oxidoreductase can exist as xanthine dehydrogenase (XDH) and xanthine oxidase (XO). The two enzymes possess common redox active cofactors, which form an electron transfer (ET) pathway terminated by a flavin cofactor. In spite of identical protein primary structures, the redox potential difference between XDH and XO for the flavin semiquinone/hydroquinone pair (E(sq/hq)) is ~170 mV, a striking difference. The former greatly prefers NAD(+) as ultimate substrate for ET from the iron-sulfur cluster FeS-II via flavin while the latter only accepts dioxygen. In XDH (without NAD(+)), however, the redox potential of the electron donor FeS-II is 180 mV higher than that for the acceptor flavin, yielding an energetically uphill ET. On the basis of new 1.65, 2.3, 1.9, and 2.2 Å resolution crystal structures for XDH, XO, the NAD(+)- and NADH-complexed XDH, E(sq/hq) were calculated to better understand how the enzyme activates an ET from FeS-II to flavin. The majority of the E(sq/hq) difference between XDH and XO originates from a conformational change in the loop at positions 423-433 near the flavin binding site, causing the differences in stability of the semiquinone state. There was no large conformational change observed in response to NAD(+) binding at XDH. Instead, the positive charge of the NAD(+) ring, deprotonation of Asp429, and capping of the bulk surface of the flavin by the NAD(+) molecule all contribute to altering E(sq/hq) upon NAD(+) binding to XDH.

Both human ferredoxins 1 and 2 and ferredoxin reductase are important for iron-sulfur cluster biogenesis

Ferredoxins are iron-sulfur proteins that have been studied for decades because of their role in facilitating the monooxygenase reactions catalyzed by p450 enzymes. More recently, studies in bacteria and yeast have demonstrated important roles for ferredoxin and ferredoxin reductase in iron-sulfur cluster assembly. The human genome contains two homologous ferredoxins, ferredoxin 1 (FDX1) and ferredoxin 2 (FDX2--formerly known as ferredoxin 1L). More recently, the roles of these two human ferredoxins in iron-sulfur cluster assembly were assessed, and it was concluded that FDX1 was important solely for its interaction with p450 enzymes to synthesize mitochondrial steroid precursors, whereas FDX2 was used for synthesis of iron-sulfur clusters, but not steroidogenesis. To further assess the role of the FDX-FDXR system in mammalian iron-sulfur cluster biogenesis, we performed siRNA studies on FDX1 and FDX2, on several human cell lines, using oligonucleotides identical to those previously used, along with new oligonucleotides that specifically targeted each gene. We concluded that both FDX1 and FDX2 were important in iron-sulfur cluster biogenesis. Loss of FDX1 activity disrupted activity of iron-sulfur cluster enzymes and cellular iron homeostasis, causing mitochondrial iron overload and cytosolic iron depletion. Moreover, knockdown of the sole human ferredoxin reductase, FDXR, diminished iron-sulfur cluster assembly and caused mitochondrial iron overload in conjunction with cytosolic depletion. Our studies suggest that interference with any of the three related genes, FDX1, FDX2 or FDXR, disrupts iron-sulfur cluster assembly and maintenance of normal cytosolic and mitochondrial iron homeostasis.

Effect of mitochondrial complex I inhibition on Fe-S cluster protein activity

Iron-sulfur (Fe-S) clusters are small inorganic cofactors formed by tetrahedral coordination of iron atoms with sulfur groups. Present in numerous proteins, these clusters are involved in key biological processes such as electron transfer, metabolic and regulatory processes, DNA synthesis and repair and protein structure stabilization. Fe-S clusters are synthesized mainly in the mitochondrion, where they are directly incorporated into mitochondrial Fe-S cluster-containing proteins or exported for cytoplasmic and nuclear cluster-protein assembly. In this study, we tested the hypothesis that inhibition of mitochondrial complex I by rotenone decreases Fe-S cluster synthesis and cluster content and activity of Fe-S cluster-containing enzymes. Inhibition of complex I resulted in decreased activity of three Fe-S cluster-containing enzymes: mitochondrial and cytosolic aconitases and xanthine oxidase. In addition, the Fe-S cluster content of glutamine phosphoribosyl pyrophosphate amidotransferase and mitochondrial aconitase was dramatically decreased. The reduction in cytosolic aconitase activity was associated with an increase in iron regulatory protein (IRP) mRNA binding activity and with an increase in the cytoplasmic labile iron pool. Since IRP activity post-transcriptionally regulates the expression of iron import proteins, Fe-S cluster inhibition may result in a false iron deficiency signal. Given that inhibition of complex I and iron accumulation are hallmarks of idiopathic Parkinson's disease, the findings reported here may have relevance for understanding the pathophysiology of this disease.

Cytosolic iron-sulfur cluster assembly (CIA) system: factors, mechanism, and relevance to cellular iron regulation

FeS cluster biogenesis is an essential process in virtually all forms of life. Complex protein machineries that are conserved from bacteria through higher eukaryotes facilitate assembly of the FeS cofactor in proteins. In the last several years, significant strides have been made in our understanding of FeS cluster assembly and the functional overlap of this process with cellular iron homeostasis. This minireview summarizes the present understanding of the cytosolic iron-sulfur cluster assembly (CIA) system in eukaryotes, with a focus on information gained from studies in budding yeast and mammalian systems.

Iron-sulfur proteins in health and disease

Iron-sulfur (Fe/S) proteins are a class of ubiquitous components that assist in vital and diverse biochemical tasks in virtually every living cell. These tasks include respiration, iron homeostasis and gene expression. The past decade has led to the discovery of novel Fe/S proteins and insights into how their Fe/S cofactors are formed and incorporated into apoproteins. This review summarizes our current knowledge of mammalian Fe/S proteins, diseases related to deficiencies in these proteins and on disorders stemming from their defective biogenesis. Understanding both the physiological functions of Fe/S proteins and how Fe/S clusters are formed will undoubtedly enhance our ability to identify and treat known disorders of Fe/S cluster biogenesis and to recognize hitherto undescribed Fe/S cluster-related diseases.

Mechanism of transition from xanthine dehydrogenase to xanthine oxidase: effect of guanidine-HCL or urea on the activity

Mammalian xanthine oxidoreductase can be converted from the dehydrogenase to the oxidase form, either reversibly by formation of disulfide bridges or irreversibly by proteolytic cleavage within the xanthine oxidoreductase protein molecule. A tightly packed amino acid cluster stabilizes the dehydrogenase form, and disruption of this cluster is accompanied with rearrangement of the active site loop. Here, we show that the conversion occurs in the presence of guanidine-HCl or urea. We propose that xanthine dehydrogenase and oxidase are in a thermodynamic equilibrium that can be shifted by disruption of the amino acid cluster with a denaturant.

Functional characterization of human xanthine oxidase allelic variants

OBJECTIVE

Xanthine oxidase (XO) catalyzes the oxidation of endogenous and exogenous purines and pyrimidines. In this study, we speculated that individual variations in XO activity are caused by genetic variations in the XO gene.

METHODS

To investigate the genetic variations in XO in 96 Japanese participants, denaturing high-performance liquid chromatography was used. To assess the effects of these variations on enzymatic activity, wild-type XO and 21 types of variant XO--including those in the database and those just discovered--were transiently expressed in COS-7 cells.

RESULTS

Three nonsynonymous single nucleotide polymorphisms, including 514G>A (Gly172Arg), 3326A>C (Asp1109Thr), and 3662A>G (His1221Arg) were identified in Japanese participants. Functional characterization of 21 XO variants showed a deficiency in enzyme activity in two variants (Arg149Cys and Thr910Lys); low activity (intrinsic clearance, CLint: 22-69% compared with the wild-type) in six variants (Pro555Ser, Arg607Gln, Thr623Ile, Asn909Lys, Pro1150Arg, and Cys1318Tyr); and high activity (CLint: approximately two-fold higher than that in the wild-type) in two variants (Ile703Val and His1221Arg).

CONCLUSION

These results suggest that several single nucleotide polymorphisms in the XO gene are involved in individual variations in XO activity. In addition, such findings will be useful to identify xanthinuria patients.

Spectroscopic and biochemical studies on protein variants of quinaldine 4-oxidase: Role of E736 in catalysis and effects of serine ligands on the FeSI and FeSII clusters

Quinaldine 4-oxidase (Qox), which catalyzes the hydroxylation of quinaldine to 1H-4-oxoquinaldine, is a heterotrimeric (LMS)2 molybdo-iron/sulfur flavoprotein belonging to the xanthine oxidase family. Variants of Qox were generated by site-directed mutagenesis. Replacement in the large subunit at E736, which is presumed to be located close to the molybdenum, by aspartate (QoxLE736D) resulted in a marked decrease in kcat app for quinaldine, while Km app was largely unaffected. Although a minor reduction of the glutamine substituted variant QoxLE736Q by quinaldine occurred, its activity was below detection, indicating that the carboxylate group of E736 is crucial for catalysis. Replacement of cysteine ligands C40, C45, or C60 (FeSII) and of the C120 or C154 ligands to FeSI in the small subunit of Qox by serine led to decreased iron contents of the protein preparations. Substitutions C40S and C45S (Fe1 of FeSII) suppressed the characteristic FeSII EPR signals and significantly reduced catalytic activity. In QoxSC154S (Fe1 of FeSI), the g-factor components of FeSI were drastically changed. In contrast, Qox proteins with substitutions of C48 and C60 (Fe2 of FeSII), and of the C120 ligand at Fe2 of FeSI, retained considerable activity and showed less pronounced changes in their EPR parameters. Taken together, the properties of the Qox variants suggest that Fe1 of both FeSI and FeSII are the reducible iron sites, whereas the Fe2 ions remain in the ferric state. The location of the reducible iron sites of FeSI and FeSII appears to be conserved in enzymes of the xanthine oxidase family.

High-level expression and characterization of Zea mays cytokinin oxidase/dehydrogenase in Yarrowia lipolytica

Cytokinin oxidase/dehydrogenase (CKO/CKX) is a flavoenzyme, which irreversibly inactivates cytokinins by severing the isoprenoid side chain from the adenine/adenosine moiety. There are several genes coding for the enzyme in maize (Zea mays). A Z. mays CKO1 cDNA was cloned in the yeast Yarrowia lipolytica to achieve heterologous protein expression. The recombinant ZmCKO1 was recovered from cultures of transformed yeasts and purified using several chromatographic steps. The enzyme was obtained as a homogeneous protein in a remarkably high-yield and its molecular and kinetic properties were characterized. The enzyme showed a molecular mass of 69 kDa, pI was 6.3. Neutral sugar content of the molecule was 22%. Absorption and fluorescence spectra were in accordance with the presence of FAD as a cofactor. Peptide mass fingerprinting using MALDI-MS correctly assigned the enzyme in MSDB protein database. The enzyme showed a relatively high degree of thermostability (T50=55 degrees C for 30 min incubation). The following pH optimum and K(m) values were determined for natural substrates (measured in the oxidase mode): pH 8.0 for isopentenyl adenine (K(m)=0.5 microM), pH 7.6 for isopentenyl adenosine (K(m)=1.9 microM), pH 7.9 for zeatin (K(m)=1.5 microM) and pH 7.3 for zeatin riboside (K(m)=2.0 microM). ZmCKO1, functioning in the oxidase mode, catalyzes the production of one molecule of H2O2 per one molecule of cytokinin substrate. This finding represents clear evidence for the existence of dual enzyme functionality (oxygen serves as a cosubstrate in the absence of better electron acceptors).

Drug-Metabolizing Ability of Molybdenum Hydroxylases

Molybdenum hydroxylases, which include aldehyde oxidase and xanthine oxidoreductase, are involved in the metabolism of some medicines in humans. They exhibit oxidase activity towards various heterocyclic compounds and aldehydes. The liver cytosol of various mammals also exhibits a significant reductase activity toward nitro, sulfoxide, N-oxide and other moieties, catalyzed by aldehyde oxidase. There is considerable variability of aldehyde oxidase activity in liver cytosol of mammals: humans show the highest activity, rats and mice show low activity, and dogs have no detectable activity. On the other hand, xanthine oxidoreductase activity is present widely among species. Interindividual variation of aldehyde oxidase activity is present in humans. Drug-drug interactions associated with aldehyde oxidase and xanthine oxidoreductase are of potential clinical significance. Drug metabolizing ability of molybdenum hydroxylases and the variation of the activity are described in this review.

Study of the spin-spin interactions between the metal centers of Desulfovibrio gigas aldehyde oxidoreductase: identification of the reducible sites of the [2Fe-2S]1+,2+ clusters

The aldehyde oxidoreductase from Desulfovibrio gigas belongs to the family of molybdenum hydroxylases. Besides a molybdenum cofactor which constitutes their active site, these enzymes contain two [2Fe-2S](2+,1+) clusters which are believed to transfer the electrons provided by the substrate to an acceptor which is either a FAD group or an electron-transferring protein. When the three metal centers of D. gigas AOR are simultaneously paramagnetic, splittings due to intercenter spin-spin interactions are visible when the EPR spectra are recorded at low temperatures. By studying quantitatively these interactions with a model based on the X-ray crystal structure, which takes into consideration the interactions between the magnetic moments carried by all the metal sites of the system, it is possible to determine the location of the reducible sites of the [2Fe-2S] clusters. When combined with the electron-transfer pathways proposed on the basis of the X-ray crystal structure, the results provide a detailed description of the electron-transfer system of D. gigas AOR.

Vascular physiology and pathology of circulating xanthine oxidoreductase: from nucleotide sequence to functional enzyme

The evolutionarily conserved, cofactor-dependent, enzyme xanthine oxidoreductase exists in both cell-associated and circulatory forms. The exact role of the circulating form is not known; however, several putative physiological and pathological functions have been suggested that range from purine catabolism to a mediator of acute respiratory distress syndrome. Regulation of gene expression, cofactor synthesis and insertion, post-translational conversion, entry into the circulation, and putative physiological and pathological roles for human circulating xanthine oxidoreductase are discussed.

Regulation of xanthine oxidoreductase by intracellular iron

Xanthine oxidoreductase (XOR) may produce reactive oxygen species and play a role in ischemia-reperfusion injury. Because tissue iron levels increase after ischemia, and because XOR contains functionally critical iron-sulfur clusters, we studied the effects of intracellular iron on XOR expression. Ferric ammonium citrate and FeSO(4) elevated intracellular iron levels and increased XOR activity up to twofold in mouse fibroblast and human bronchial epithelial cells. Iron increased XOR protein and mRNA levels, whereas protein and RNA synthesis inhibitors abolished the induction of XOR activity. A human XOR promoter construct (nucleotides +42 to -1937) was not induced by iron in human embryonic kidney cells. Hydroxyl radical scavengers did not block induction of XOR activity by iron. Iron chelation by deferoxamine (DFO) decreased XOR activity but did not lower endogenous XOR protein or mRNA levels. Furthermore, DFO reduced the activity of overexpressed human XOR but not the amount of immunoreactive protein. Our data show that XOR activity is transcriptionally induced by iron but posttranslationally inactivated by iron chelation.

Crystal structures of the active and alloxanthine-inhibited forms of xanthine dehydrogenase from Rhodobacter capsulatus

Xanthine dehydrogenase (XDH), a complex molybdo/iron-sulfur/flavoprotein, catalyzes the oxidation of hypoxanthine to xanthine followed by oxidation of xanthine to uric acid with concomitant reduction of NAD+. The 2.7 A resolution structure of Rhodobacter capsulatus XDH reveals that the bacterial and bovine XDH have highly similar folds despite differences in subunit composition. The NAD+ binding pocket of the bacterial XDH resembles that of the dehydrogenase form of the bovine enzyme rather than that of the oxidase form, which reduces O(2) instead of NAD+. The drug allopurinol is used to treat XDH-catalyzed uric acid build-up occurring in gout or during cancer chemotherapy. As a hypoxanthine analog, it is oxidized to alloxanthine, which cannot be further oxidized but acts as a tight binding inhibitor of XDH. The 3.0 A resolution structure of the XDH-alloxanthine complex shows direct coordination of alloxanthine to the molybdenum via a nitrogen atom. These results provide a starting point for the rational design of new XDH inhibitors.