Cloning and expression in vitro of human xanthine dehydrogenase/oxidase

Transcriptome analysis indicates the mechanisms of BmNPV resistance in Bombyx mori midgut

Bombyx mori nucleopolyhedrovirus (BmNPV) caused serious economic losses in sericulture. Analyzing the molecular mechanism of silkworms (B. mori) resistance to BmNPV is of great significance for the prevention and control of silkworm virus diseases and the biological control of agricultural lepidopteran pests. In order to clarify the defense mechanisms of silkworms against BmNPV, we constructed a near isogenic line BC8 with high resistance to BmNPV through the highly BmNPV-resistant strain NB and the highly BmNPV-susceptible strain 306. In this study, RNA-Seq technique was used to analyze the transcriptome level differences in the midgut of BC8 and 306 following BmNPV infection. A total of 1350 DEGs were identified. Clustering analysis showed that these genes could be divided into 8 clusters with different expression patterns. Functional annotations based on GO and KEGG analysis indicated that they were involved in various metabolism pathways. Finally, 32 BmNPV defense responsive genes were screened. They were involved in metabolism, reactive oxygen species (ROS), signal transduction and immune response, and insect hormones. The further verification shows that HSP70 should participate in resistance responses of anti-BmNPV. These findings have paved the way in further functional characterization of candidate genes and subsequently can be used in breeding of BmNPV resistance dominant silkworms.

Mutations associated with functional disorder of xanthine oxidoreductase and hereditary xanthinuria in humans

Xanthine oxidoreductase (XOR) catalyzes the conversion of hypoxanthine to xanthine and xanthine to uric acid with concomitant reduction of either NAD⁺ or O₂. The enzyme is a target of drugs to treat hyperuricemia, gout and reactive oxygen-related diseases. Human diseases associated with genetically determined dysfunction of XOR are termed xanthinuria, because of the excretion of xanthine in urine. Xanthinuria is classified into two subtypes, type I and type II. Type I xanthinuria involves XOR deficiency due to genetic defect of XOR, whereas type II xanthinuria involves dual deficiency of XOR and aldehyde oxidase (AO, a molybdoflavo enzyme similar to XOR) due to genetic defect in the molybdenum cofactor sulfurase. Molybdenum cofactor deficiency is associated with triple deficiency of XOR, AO and sulfite oxidase, due to defective synthesis of molybdopterin, which is a precursor of molybdenum cofactor for all three enzymes. The present review focuses on mutation or chemical modification studies of mammalian XOR, as well as on XOR mutations identified in humans, aimed at understanding the reaction mechanism of XOR and the relevance of mutated XORs as models to estimate the possible side effects of clinical application of XOR inhibitors.

Xanthine oxidoreductase: a journey from purine metabolism to cardiovascular excitation-contraction coupling

Xanthine oxidoreductase (XOR) is a ubiquitous complex cytosolic molybdoflavoprotein which controls the rate limiting step of purine catabolism by converting xanthine to uric acid. It is known that optimum concentrations of uric acid (UA) and reactive oxygen species (ROS) are necessary for normal functioning of the body. The ability of XOR to perform detoxification reactions, and to synthesize UA and reactive oxygen species (ROS) makes it a versatile intra- and extra-cellular protective "housekeeping enzyme". It is also an important component of the innate immune system. The enzyme is a target of drugs against gout and hyperuricemia and the protein is of major interest as it is associated with ischemia reperfusion (I/R) injury, vascular disorders in diabetes, cardiovascular disorders, adipogenesis, metabolic syndrome, cancer, and many other disease conditions. Xanthine oxidoreductase in conjugation with antibodies has been shown to have an anti-tumor effect due to its ability to produce ROS, which in turn reduces the growth of cancer tissues. Apart from this, XOR in association with nitric oxide synthase also participates in myocardial excitation-contraction coupling. Although XOR was discovered over 100 years ago, its physiological and pathophysiological roles are still not clearly elucidated. In this review, various physiological and pathophysiological functional aspects of XOR and its association with various forms of cancer are discussed in detail.

Xanthine oxidoreductase - clinical significance in colorectal cancer and in vitro expression of the protein in human colon cancer cells

Xanthine oxidoreductase (XOR) is a key enzyme in degradation of DNA and RNA, and has previously been shown to be decreased in aggressive breast and gastric cancer. In this study, XOR expression was assessed in tissue microarray specimens of 478 patients with colorectal cancer and related to clinical parameters. In addition, we performed in vitro studies of XOR activity, protein and mRNA in colon cancer cells (Caco-2). Results from the tissue expression analyses show that XOR was decreased in 62% and undetectable in 22% of the tumours as compared to normal tissue. Loss of XOR was associated with poor grade of differentiation (p=0.006) and advanced Dukes stage (p=0.03). In multivariate survival analysis, XOR was a prognostic factor (p=0.008), independent of Dukes stage, histological grade, age and tumour location. The in vitro analyses show that XOR is not measurable in undifferentiated Caco-2 cells, but appears and increases with differentiation. We conclude that XOR expression is associated with histological grade of differentiation and extent of disease in colorectal cancer, and it provides significant prognostic information independently of established factors.

Pivotal role of xanthine oxidase in the initiation of tubulointerstitial renal injury in rats with hyperlipidemia

Hyperlipidemia can induce or aggravate renal tubulointerstitial injury. Experiments in a complex rat model with chronic glomerulonephritis and long-standing, coexisting hyperlipidemia suggested that induction of xanthine oxidase (XO), with increased oxygen radical generation, is involved in aggravation of tubulointerstitial injury. To separate the role of XO in the initial events of lipid-mediated tubulointerstitial injury, short-term experiments with diet-induced hyperlipidemia over 21 and 35 days were performed in otherwise healthy rats. XO expression in relation to the antioxidant enzymes was examined in the cortical tubulointerstitium (TIS) and proximal tubules (PT). Subsequent experiments with XO inhibition were performed, examining tubulointerstitial infiltration with ED1-positive cells and expression of adhesion molecules and monocyte chemoattractant protein-1 (MCP-1) as indicators of early injurious events. Hyperlipidemia increased XO activity in TIS by 40 and 86%, and in PT by 28 and 90% at days 21 and 35, compared with controls on regular diet. This increased activity was associated with increased reactive oxygen species. Among the antioxidant enzymes, glutathione peroxidase activity increased in TIS by 40% and in PT by 90%. Histological evaluation showed a three-fold increase in ED1-positive cells and increased MCP-1 and vascular cell adhesion molecule-1 (VCAM-1) expression at day 35 in the TIS. Inhibition of XO prevented tubulointerstitial ED1 cell infiltration, together with a decreased expression of MCP-1 and VCAM-1. These results point to an important role for XO in the early stage of hyperlipidemia-associated renal injury, mediating macrophage infiltration by a putatively redox-dependent upregulation of MCP-1 and VCAM-1.

The aldehyde oxidase gene cluster in mice and rats. Aldehyde oxidase homologue 3, a novel member of the molybdo-flavoenzyme family with selective expression in the olfactory mucosa

Mammalian molybdo-flavoenzymes are oxidases requiring FAD and molybdopterin (molybdenum cofactor) for their catalytic activity. This family of proteins was thought to consist of four members, xanthine oxidoreductase, aldehyde oxidase 1 (AOX1), and the aldehyde oxidase homologues 1 and 2 (AOH1 and AOH2, respectively). Whereas the first two enzymes are present in humans and various other mammalian species, the last two proteins have been described only in mice. Here, we report on the identification, in both mice and rats, of a novel molybdo-flavoenzyme, AOH3. In addition, we have cloned the cDNAs coding for rat AOH1 and AOH2, demonstrating that this animal species has the same complement of molybdo-flavoproteins as the mouse. The AOH3 cDNA is characterized by remarkable similarity to AOX1, AOH1, AOH2, and xanthine oxidoreductase cDNAs. Mouse AOH3 is selectively expressed in Bowman's glands of the olfactory mucosa, although small amounts of the corresponding mRNA are present also in the skin. In the former location, two alternatively spliced forms of the AOH3 transcript with different 3'-untranslated regions were identified. The general properties of AOH3 were determined by purification of mouse AOH3 from the olfactory mucosa. The enzyme possesses aldehyde oxidase activity and oxidizes, albeit with low efficiency, exogenous substrates that are recognized by AOH1 and AOX1. The Aoh3 gene maps to mouse chromosome 1 band c1 and rat chromosome 7 in close proximity to the Aox1, Aoh1, and Aoh2 loci and has an exon/intron structure almost identical to that of the other molybdo-flavoenzyme genes in the two species.

Purification and partial characterisation of camel milk xanthine oxidoreductase

Xanthine oxidoreductase (XOR) was purified in the presence of dithiothrietol from camel milk with yields of up to 22.2mg/l that were comparable to those obtained from bovine and human milk sources. On SDS-PAGE, the freshly purified camel milk XOR had a protein flavin (A280/A450) ratio of 5.3 +/- 0.4 and appeared homogenous with a single major band of approximately Mr 145.3 KDa. Surprisingly, in all the batches (n = 8) purified camel milk XOR showed no detectable activity towards xanthine or NADH. The molybdenum content of camel XOR was comparable to human and goat milk enzymes. After resulphuration, camel milk XOR gave a specific activity of 1.1 nmol/min/mg and 13.0 nmol/min/mg enzyme towards pterin (fluorimetric assay) and xanthine (spectrophotometric assay) respectively. This activity was markedly lower than that of human, bovine and goat enzymes obtained under the same conditions. These findings suggest that the molybdo-form of camel enzyme is totally under desulpho inactive form. It is possible that camel neonates are equipped with an enzymic system that reactivates XOR in their gut and consequently generates antibacterial reactive oxygen species.

Posttranslational inactivation of human xanthine oxidoreductase by oxygen under standard cell culture conditions

Xanthine oxidoreductase (XOR) catalyzes the final reactions of purine catabolism and may account for cell damage by producing reactive oxygen metabolites in cells reoxygenated after hypoxia. We found a three- to eightfold higher XOR activity in cultured human bronchial epithelial cells exposed to hypoxia (0.5-3% O2) compared with cells grown in normoxia (21% O2) but no difference in XOR protein or mRNA. XOR promoter constructs failed to respond to hypoxia. The cellular XOR activity at 3% O2 returned to basal levels when the cells were returned to 21% O2, and hyperoxia (95% O2) abolished enzyme activity with no change in XOR protein. Our data suggest reversible enzyme inactivation by oxygen or its metabolites. NADH was normally oxidized by the oxygen-inactivated enzyme, which rules out damage to the flavin adenine dinucleotide cofactor. Hydrogen peroxide partially inactivated the molybdenum center of XOR, as shown by a parallel decrease in XOR-catalyzed xanthine oxidation and dichlorophenolindophenol reduction. We conclude that the transcription or translation of XOR is not influenced by hypoxia or hyperoxia. Instead, the molybdenum center of XOR is posttranslationally inactivated by oxygen metabolites in "normal" (21% O2) cell culture atmosphere. This inactivation is reversed in hypoxia and accounts for the apparent induction.

Mammalian molybdo-flavoenzymes, an expanding family of proteins: structure, genetics, regulation, function and pathophysiology

The molybdo-flavoenzymes are structurally related proteins that require a molybdopterin cofactor and FAD for their catalytic activity. In mammals, four enzymes are known: xanthine oxidoreductase, aldehyde oxidase and two recently described mouse proteins known as aldehyde oxidase homologue 1 and aldehyde oxidase homologue 2. The present review article summarizes current knowledge on the structure, enzymology, genetics, regulation and pathophysiology of mammalian molybdo-flavoenzymes. Molybdo-flavoenzymes are structurally complex oxidoreductases with an equally complex mechanism of catalysis. Our knowledge has greatly increased due to the recent crystallization of two xanthine oxidoreductases and the determination of the amino acid sequences of many members of the family. The evolution of molybdo-flavoenzymes can now be traced, given the availability of the structures of the corresponding genes in many organisms. The genes coding for molybdo-flavoenzymes are expressed in a cell-specific fashion and are controlled by endogenous and exogenous stimuli. The recent cloning of the genes involved in the biosynthesis of the molybdenum cofactor has increased our knowledge on the assembly of the apo-forms of molybdo-flavoproteins into the corresponding holo-forms. Xanthine oxidoreductase is the key enzyme in the catabolism of purines, although recent data suggest that the physiological function of this enzyme is more complex than previously assumed. The enzyme has been implicated in such diverse pathological situations as organ ischaemia, inflammation and infection. At present, very little is known about the pathophysiological relevance of aldehyde oxidase, aldehyde oxidase homologue 1 and aldehyde oxidase homologue 2, which do not as yet have an accepted endogenous substrate.

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.

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.

Inducible nitric oxide synthase, but not xanthine oxidase, is highly expressed in interstitial pneumonias and granulomatous diseases of human lung

We assessed the distribution and expression of inducible nitric oxide synthase (i-NOS), endothelial nitric oxide synthase (e-NOS), and xanthine oxidase (XAO) in usual interstitial pneumonia, desquamative interstitial pneumonia, and granulomatous diseases. The material consisted of biopsy specimens from 5 healthy subjects (nonsmokers), 9 patients with usual interstitial pneumonia, 11 with desquamative interstitial pneumonia, 14 with sarcoidosis, and 8 with extrinsic allergic alveolitis. i-NOS was expressed intensively in inflammatory but not infibrotic lesions. It was expressed most prominently in alveolar macrophages and alveolar epithelium of all disorders and in the granulomas of sarcoidosis and extrinsic allergic alveolitis. In contrast with i-NOS, e-NOS was expressed prominently in control lung tissue samples but also in granulomas of sarcoidosis and extrinsic allergic alveolitis. Reverse transcription-polymerase chain reaction performed on bronchoalveolar lavage fluid samples from patients with sarcoidosis or usual interstitial pneumonia andfrom healthy subjects indicated positivity for XAO, but immunohistochemical analysis in samples from healthy lung and all parenchymal lung disorders showed no immunoreactivity for XAO. i-NOS has an important role in the pathogenesis of interstitial lung diseases, being up-regulated during the inflammatory but not during the fibrotic disease stage.

Molecular cloning of a cDNA coding for feline liver xanthine dehydrogenase

A cDNA coding for feline liver xanthine dehydrogenase (XDH, EC 1.1.204) was amplified by RT-PCR and cloned for determining the sequence. The clones contained an open reading frame of 4002 base pairs encoding 1333 amino acid residues. The calculated molecular weight and isoelectric point were approximately 146 kDa and 7.0. Comparison of the deduced amino acid sequences indicated remarkable high homology, i.e., the amino acid residues of feline XDH shared approximately 90%, 87%, 87% and 86% identity with those of human, bovine, rat and mouse, respectively. The anino acid sequences of two putative iron-sulfur centers, one NAD binding site and one molybdenum binding site were well conserved among mammalian animals.

Human xanthine dehydrogenase cDNA sequence and protein in an atypical case of type I xanthinuria in comparison with normal subjects

To investigate the properties of xanthine dehydrogenase/xanthine oxidase (XDH/XO) deficiency in a patient with atypical type I xanthinuria, as indicated by oxypurine data, a cDNA sequence encoding XDH, XDH/XO immunoblot analysis and a competitive PCR assay were performed, and the results were compared with those of normal subjects. The xanthine dehydrogenase cDNA sequence of the patient was consistent with the controls, while immunologically reactive 150 kD XDH/XO protein was not present in the xanthinuric duodenal mucosa, unlike the control duodenal mucosa. In addition, a decrease in XDH/XO messenger RNA was found by competitive PCR. These results suggest that atypical type I xanthinuria is due to a decrease in messenger RNA of XDH/XO. Furthermore, it was considered that this decrease could explain the normal plasma level and near normal urinary excretion of hypoxanthine seen in this case of xanthinuria, though XDH/XO activity and protein were not detected spectrophotometrically and immunologically, respectively.

Cellular distribution, metabolism and regulation of the xanthine oxidoreductase enzyme system

Xanthine oxidase (EC 1.1.3.22) and xanthine dehydrogenase (EC 1.1.1. 204) are both members of the molybdenum hydroxylase flavoprotein family and represent different forms of the same gene product. The two enzyme forms and their reactions are often referred to as xanthine oxidoreductase (XOR) activity. Physiologically, XOR is known as the rate-limiting enzyme in purine catabolism but has also been shown to be able to metabolize a number of other physiological compounds. Recent studies have also demonstrated its ability to metabolize xenobiotics, including a number of anticancer compounds, to their active metabolites. During the past 10 years, evidence has mounted to support a role for XOR in the pathophysiology of inflammatory diseases and atherosclerosis as well as its previously determined role in ischemia-reperfusion injury. While significant progress has recently been made in our understanding of the physiological and biochemical nature of this enzyme system, considerable work still needs to be done. This paper will review some of the more recent work characterizing the interactions and the factors that influence the interactions of XOR with various physiological and xenobiotic compounds.

Cloning of the cDNAs coding for two novel molybdo-flavoproteins showing high similarity with aldehyde oxidase and xanthine oxidoreductase

The cDNAs coding for two novel mouse molybdo-flavoproteins, AOH1 and AOH2 (aldehyde oxidase homolog 1 and 2), were isolated. The AOH1 and AOH2 cDNAs code for polypeptides of 1336 amino acids. The two proteins have similar primary structure and show striking amino acid identity with aldehyde oxidase and xanthine oxidoreductase, two other molybdo-flavoenzymes. AOH1 and AOH2 contain consensus sequences for a molybdopterin-binding site and two distinct 2Fe-2S redox centers. In its native conformation, AOH1 has a molecular weight consistent with a homotetrameric structure. Transfection of the AOH1 and AOH2 cDNAs results in the production of proteins with phenanthridine but not hypoxanthine oxidizing activity. Furthermore, the AOH1 protein has benzaldehyde oxidizing activity with electrophoretic characteristics identical to those of a previously identified aldehyde oxidase isoenzyme (Holmes, R. S. (1979) Biochem. Genet. 17, 517-528). The AOH1 transcript is expressed in the hepatocytes of the adult and fetal liver and in spermatogonia. In liver, the AOH1 protein is synthesized in a gender-specific fashion. The expression of AOH2 is limited to keratinized epithelia and the basal layer of the epidermis and hair folliculi. The selective cell and tissue distribution of AOH1 and AOH2 mRNAs is consistent with the localization of the respective protein products.

Nuclear factor Y activates the human xanthine oxidoreductase gene promoter

To study the regulation of the human xanthine oxidoreductase (XOR) gene, we cloned 1.9 kb of the promoter region. In reporter gene assays, a construct encompassing nucleotides between 142 to +42 conferred maximal basal activity of the XOR promoter in 293T cells, in comparison with shorter (-92 to +42) or longer (up to -1937 to +42) constructs. The promoter activity was low in NIH-3T3 cells. The most active construct contained a putative CCAAT motif at -119 to -123. Electrophoretic mobility shift assays showed that this sequence binds the ubiquitous nuclear factor Y (NF-Y). Mutation of the CCAAT motif (CTGAT) abolished the NF-Y binding and considerably reduced the promoter activity. Our data suggest an important functional role for NF-Y in the transcriptional activation of the human XOR gene.

XDH gene mutation is the underlying cause of classical xanthinuria: a second report

BACKGROUND

Classical xanthinuria is a rare autosomal recessive disorder characterized by excessive excretion of xanthine in urine. Type I disease results from the isolated deficiency of xanthine dehydrogenase (XDH), and type II results from dual deficiency of XDH and aldehyde oxidase. The XDH gene has been cloned and localized to chromosome 2p22-23. The aim of this study was to characterize the molecular basis of classical xanthinuria in an Iranian-Jewish family.

METHODS

The apparently unrelated parents originated from a community in which consanguineous marriages are common. Subtyping xanthinuria was attempted by homozygosity mapping using microsatellite markers D2S352, D2S367, and D2S2374 in the vicinity of the XDH gene. Mutation detection was accomplished by PCR-SSCP screening of all 36 exons and exon-intron junctions of the XDH gene, followed by direct sequencing and confirmation of sequence alteration by restriction analysis.

RESULTS

The index case was homozygous for all three microsatellite markers analyzed. The expected frequency of this genotype in a control population was 0. 0002. These results suggested that xanthinuria in the patient is linked to the XDH gene. Consequently, a 1658insC mutation in exon 16 of the XDH gene was identified. The 1658insC mutation was not detected in 65 control DNA samples.

CONCLUSION

A molecular approach to the diagnosis of classical xanthinuria type I in a female patient with profound hypouricemia is described. Linkage of xanthinuria to the XDH locus was demonstrated by homozygosity mapping, and a 1658insC mutation, predicting a truncated inactive XDH protein, was identified. These results reinforce the notion that mutations in the XDH gene are the underlying cause of classical xanthinuria type I.

A review and proposed nomenclature for major proteins of the milk-fat globule membrane

The characteristics and possible functions of the most abundant proteins associated with the bovine milk-fat globule membrane are reviewed. Under the auspices of the Milk Protein Nomenclature Committee of the ADSA, a revised nomenclature for the major membrane proteins is proposed and discussed in relation to earlier schemes. We recommend that proteins be assigned specific names as they are identified by molecular cloning and sequencing techniques. The practice of identifying proteins according to their Mr, electrophoretic mobility, or staining characteristics should be discontinued, except for uncharacterized proteins. The properties and amino acid sequences of the following proteins are discussed in detail: MUC1, xanthine dehydrogenase/oxidase, CD36, butyrophilin, adipophilin, periodic acid Schiff 6/7 (PAS 6/7), and fatty acid binding protein. In addition, a compilation of less abundant proteins associated with the bovine milk-fat globule membrane is presented.

The mouse aldehyde oxidase gene: molecular cloning, chromosomal mapping and functional characterization of the 5'-flanking region

In this article, we report on the chromosome mapping and molecular cloning of the genetic locus encoding the mouse molybdo-iron/sulfur-flavoprotein aldehyde oxidase. The aldehyde oxidase locus maps to mouse chromosome 1 band C1-C2, as determined by fluorescence in situ hybridization experiments conducted on metaphase chromosomes. The gene is approximately 83 kb long and consists of 35 exons. The exon/intron boundaries are perfectly conserved relative to the corresponding human homolog and almost completely conserved relative to the mouse xanthine oxidoreductase gene. This further supports the concept that the aldehyde oxidase and xanthine oxidoreductase loci evolved from the same ancestral precursor by a gene duplication event. The position of a major transcription start site was defined by primer extension and RNase mapping analysis. The 5'-flanking region of the mouse aldehyde oxidase gene contains a functional and orientation-dependent promoter as well as several putative binding sites for known cell-specific and general transcription factors. Deletion analysis of the 5'-flanking region defines an approximately 470 bp DNA stretch which is necessary and sufficient for the transcription of the mouse aldehyde oxidase gene.

Molecular cloning of retinal oxidase/aldehyde oxidase cDNAs from rabbit and mouse livers and functional expression of recombinant mouse retinal oxidase cDNA in Escherichia coli

Retinal oxidase (EC 1.2.3.11) is a molybdenum-containing flavoenzyme with high enzymatic activity as to retinoic acid synthesis. In this study, we provide direct evidence that retinal oxidase is identical to aldehyde oxidase (EC 1.2.3.1) by cDNA cloning. Retinal oxidase and aldehyde oxidase, purified from rabbit liver cytosol using the original methods, showed completely identical HPLC patterns and amino acid sequences for three corresponding polypeptides (103 amino residues). The primary structural information obtained from the cleaved polypeptides permitted molecular cloning of the full-length cDNA of rabbit liver retinal oxidase (aldehyde oxidase). We also cloned and sequenced the full-length cDNA of mouse retinal oxidase. The cDNAs of rabbit and mouse retinal oxidase have a common sequence approximately 4.6 kb long, comprising 4-kb coding regions. The open reading frames of the cDNAs predict single polypeptides of 1334 and 1333 amino acids; the calculated minimum molecular mass of each is approximately 147,000. Northern blot analysis showed that the rabbit retinal oxidase mRNA was widely expressed in tissues. Finally, we successfully constructed a prokaryotic expression system for mouse retinal oxidase. The purified recombinant retinal oxidase from Escherichia coli showed a typical spectrum of aldehyde oxidases and a lower Km (3.8 microM) for retinal and a higher Vmax (807 nmol/min/mg protein) for retinoic acid synthesis than those of rabbit retinal oxidase (8 microM and 496 nmol/min/mg protein). This represents the first eukaryotic molybdenum-containing flavoprotein to be expressed in an active form in a prokaryotic system.

Role of xanthine oxidase in passive Heymann nephritis in rats

Passive Heymann nephritis (PHN) in rats is a model of human membranous nephropathy characterized by formation of subepithelial immune deposits in the glomerular capillary wall and complement activation. Oxygen radicals have been implicated in the subsequent glomerular damage which leads to proteinuria. This study examines the involvement of xanthine oxidase in this process. Xanthine oxidase activity was increased nearly twofold in glomeruli isolated 1 and 12 d after induction of PHN, and this was associated with increased glomerular superoxide anion generation. Analysis of glomerular samples by Northern and Western blotting revealed no quantitative changes in xanthine oxidoreductase expression in PHN, suggesting conversion of xanthine dehydrogenase to the oxidase form as the cause of increased activity. Treatment of rats with tungsten, an inhibitor of xanthine oxidase, before induction of PHN resulted in a marked decrease in glomerular xanthine oxidase activity and superoxide anion generation, and decreased proteinuria by 80% (day 12: 423+/-245 mg/d in PHN versus 78+/-53 mg/d in tungsten-treated PHN animals, P < 0.01). These findings point to a pivotal role of xanthine oxidase in the pathophysiology of PHN and could be of importance in the therapy of human membranous nephropathy.

Irreversible conversion of xanthine dehydrogenase into xanthine oxidase by a mitochondrial protease

Irreversible conversion of xanthine dehydrogenase (XDH) to its oxygen free radical producing oxidase (XO) form occurs through an uncharacterized proteolytic process, which was studied in human liver. Upon incubation of fresh unfrozen liver cytosol, XDH remained intact. When recombinant human XDH was coincubated with subcellular fractions of human liver, the mitochondrial intermembrane space was shown to contain a heat-labile activity that converted XDH irreversibly to XO. This activity is resistant to inhibitors of all major groups of proteases. We postulate that this novel type of proteolytic enzyme is released into the cytosol upon mitochondrial damage.

Altered specificity mutations define residues essential for substrate positioning in xanthine dehydrogenase

We describe the sequence changes of a number of mutations of the Aspergillus nidulans xanthine dehydrogenase (XDH). We have located the amino acids affected by these changes in the three-dimensional (3D) structure of aldehyde oxido-reductase (MOP) from Desulfovibrio gigas, related to eukaryotic XDHs. Of these, two are loss of function mutations, mapping, respectively, in the molybdenum-pterin co-factor (MoCo) domain and in the domain involved in substrate recognition. Changes in two amino acids result in resistance to the irreversible inhibitor allopurinol. In Arg911 two different changes, conserved among all XDHs and MOP but not in other aldehyde oxidases (AO), change the position of hydroxylation of the analogue 2-hydroxypurine from C-8 to C-6. A number of changes affect residues adjacent to the molybdenum or its ligands. Arg911 is positioned in the substrate pocket in a way that it can account for the positioning of purine substrates in relation to the MoCo reactive center, together with a glutamate residue, universally conserved among the XDHs (Glu833).

Xanthine dehydrogenase from the phototrophic purple bacterium Rhodobacter capsulatus is more similar to its eukaryotic counterparts than to prokaryotic molybdenum enzymes

Fourteen Rhodobacter capsulatus mutants unable to grow with xanthine as sole nitrogen source were isolated by random Tn5 mutagenesis. Five of these Tn5 insertions were mapped within two adjacent chromosomal EcoRI fragments hybridizing to oligonucleotides synthesized according to conserved amino acid sequences of eukaryotic xanthine dehydrogenases. DNA sequence analysis of this region revealed two open reading frames, designated xdhA and xdhB, encoding xanthine dehydrogenase. The deduced amino acid sequence of XDHA contains binding sites for two [2Fe-2S] clusters and FAD, whereas XDHB is predicted to contain the molybdopterin cofactor. In contrast to R. capsulatus, these three cofactor binding sites reside within a single polypeptide chain in eukaryotic xanthine dehydrogenases. The amino acid sequence of xanthine dehydrogenase from R. capsulatus showed a higher degree of similarity to eukaryotic xanthine dehydrogenases than to the xanthine dehydrogenase-related aldehyde oxidoreductase from Desulphovibrio gigas. The expression of an xdhA-lacZ fusion was induced when hypoxanthine or xanthine was added as sole nitrogen source. Mutations in nifR1 (ntrC) and nifR4 (rpoN, encoding sigma54) had no influence on xdh gene expression. A putative activator sensing the availability of substrate seems to respond to xanthine but not to hypoxanthine. The transcriptional start site of xdhA was mapped by primer extension analysis. Comparison with known promoter elements revealed no significant homology. Xanthine dehydrogenase from R. capsulatus was purified to homogeneity. The enzyme consists of two subunits with molecular masses of 85 kDa and 50 kDa respectively. N-terminal amino acid sequencing of both subunits confirmed the predicted start codons. The molecular mass of the native enzyme was determined to be 275 kDa, indicating an alpha2beta2-subunit structure. Analysis of the molybdenum cofactor of xanthine dehydrogenase from R. capsulatus revealed that it contains the molybdopterin cofactor and not a molybdopterin dinucleotide derivative.

Hypoxia regulates xanthine dehydrogenase activity at pre- and posttranslational levels

Hypoxia increases the activity of xanthine oxidase (XO) and its precursor, xanthine dehydrogenase (XDH), but the mechanism of regulation is unclear. In hypoxic Swiss 3T3 cells, an early (0-24 h) cycloheximide-insensitive increase in XO-XDH activity, coupled with a lack of increase in de novo XO-XDH synthesis (immunoprecipitation) or mRNA levels (quantitative RT-PCR), demonstrated a posttranslational effect of hypoxia. Similarly, hyperoxia decreased XO-XDH activity faster than could be accounted for by cessation of XO-XDH protein synthesis. In further support of a posttranslational effect, cells transfected with a constitutively driven XDH construct displayed an exaggerated increase in activity in hypoxia but no increase in activity in hyperoxia. However, more prolonged exposure to hypoxia (24-48 h) induced an increase in XO-XDH mRNA levels and de novo XO-XDH protein synthesis, suggesting an additional pretranslational effect. Finally, hypoxic induction of XO-XDH activity was found to be cell-type-restricted. We conclude that control of XO-XDH levels by oxygen tension is a complex process which involves several points of regulation.

Identification of two mutations in human xanthine dehydrogenase gene responsible for classical type I xanthinuria

Hereditary xanthinuria is classified into three categories. Classical xanthinuria type I lacks only xanthine dehydrogenase activity, while type II and molybdenum cofactor deficiency also lack one or two additional enzyme activities. In the present study, we examined four individuals with classical xanthinuria to discover the cause of the enzyme deficiency at the molecular level. One subject had a C to T base substitution at nucleotide 682 that should cause a CGA (Arg) to TGA (Ter) nonsense substitution at codon 228. The duodenal mucosa from the subject had no xanthine dehydrogenase protein while the mRNA level was not reduced. The two subjects who were siblings with type I xanthinuria were homozygous concerning this mutation, while another subject was found to contain the same mutation in a heterozygous state. The last subject who was also with type I xanthinuria had a deletion of C at nucleotide 2567 in cDNA that should generate a termination codon from nucleotide 2783. This subject was homozygous for the mutation and the level of mRNA in the duodenal mucosa from the subject was not reduced. Thus, in three subjects with type I xanthinuria, the primary genetic defects were confirmed to be in the xanthine dehydrogenase gene.