Overexpression of a GmCnx1 gene enhanced activity of nitrate reductase and aldehyde oxidase, and boosted mosaic virus resistance in soybean

Molybdenum cofactor (Moco) is required for the activities of Moco-dependant enzymes. Cofactor for nitrate reductase and xanthine dehydrogenase (Cnx1) is known to be involved in the biosynthesis of Moco in plants. In this work, a soybean (Glycine max L.) Cnx1 gene (GmCnx1) was transferred into soybean using Agrobacterium tumefaciens-mediated transformation method. Twenty seven positive transgenic soybean plants were identified by coating leaves with phosphinothricin, bar protein quick dip stick and PCR analysis. Moreover, Southern blot analysis was carried out to confirm the insertion of GmCnx1 gene. Furthermore, expression of GmCnx1 gene in leaf and root of all transgenic lines increased 1.04-2.12 and 1.55-3.89 folds, respectively, as compared to wild type with GmCnx1 gene and in line 10 , 22 showing the highest expression. The activities of Moco-related enzymes viz nitrate reductase (NR) and aldehydeoxidase (AO) of T1 generation plants revealed that the best line among the GmCnx1 transgenic plants accumulated 4.25 μg g(-1) h(-1) and 30 pmol L(-1), respectively (approximately 2.6-fold and 3.9-fold higher than non-transgenic control plants).In addition, overexpression ofGmCnx1boosted the resistance to various strains of soybean mosaic virus (SMV). DAS-ELISA analysis further revealed that infection rate of GmCnx1 transgenic plants were generally lower than those of non-transgenic plants among two different virus strains tested. Taken together, this study showed that overexpression of a GmCnx1 gene enhanced NR and AO activities and SMV resistance, suggesting its important role in soybean genetic improvement.

Xanthine dehydrogenase AtXDH1 from Arabidopsis thaliana is a potent producer of superoxide anions via its NADH oxidase activity

Xanthine dehydrogenase AtXDH1 from Arabidopsis thaliana is a key enzyme in purine degradation where it oxidizes hypoxanthine to xanthine and xanthine to uric acid. Electrons released from these substrates are either transferred to NAD(+) or to molecular oxygen, thereby yielding NADH or superoxide, respectively. By an alternative activity, AtXDH1 is capable of oxidizing NADH with concomitant formation of NAD(+) and superoxide. Here we demonstrate that in comparison to the specific activity with xanthine as substrate, the specific activity of recombinant AtXDH1 with NADH as substrate is about 15-times higher accompanied by a doubling in superoxide production. The observation that NAD(+) inhibits NADH oxidase activity of AtXDH1 while NADH suppresses NAD(+)-dependent xanthine oxidation indicates that both NAD(+) and NADH compete for the same binding-site and that both sub-activities are not expressed at the same time. Rather, each sub-activity is determined by specific conditions such as the availability of substrates and co-substrates, which allows regulation of superoxide production by AtXDH1. Since AtXDH1 exhibits the most pronounced NADH oxidase activity among all xanthine dehydrogenase proteins studied thus far, our results imply that in particular by its NADH oxidase activity AtXDH1 is an efficient producer of superoxide also in vivo.

[Relationship between hyperuricemia and metabolic syndrome]

Metabolic syndrome is a cluster of cardiovascular risk factors such as hypertriglyceridemia, hypertension, insulin resistance, based on visceral fat accumulation. Hyperuricemia is also thought as a one of the complications of metabolic syndrome. Hyperinsulinemia caused by insulin resistance induces the low excretion type hyperuricemia. In contrast visceral fat accumulation itself causes the hypersynthetic type hyperuricemia through increased fatty acid influx into the liver. Recently hyperuricemia is suggested to play a causal role for the metabolic syndrome. Xanthine oxido-reductase, a key enzyme of uric acid metabolism was indicated as one of regulatory factors in adipocyte differentiation. These studies may shed a new light on the understanding of the relationship between hyperuricemia and metabolic syndrome.

Nitrite confers protection against myocardial infarction: role of xanthine oxidoreductase, NADPH oxidase and K(ATP) channels

Reduction of nitrite to nitric oxide during ischemia protects the heart against injury from ischemia/reperfusion. However the optimal dose of nitrite and the mechanisms underlying nitrite-induced cardioprotection are not known. We determined the ability of nitrite and nitrate to confer protection against myocardial infarction in two rat models of ischemia/reperfusion injury and the role of xanthine oxidoreductase, NADPH oxidase, nitric oxide synthase and K(ATP) channels in mediating nitrite-induced cardioprotection. In vivo and in vitro rat models of myocardial ischemia/reperfusion injury were used to cause infarction. Hearts (n=6/group) were treated with nitrite or nitrate for 15 min prior to 30 min regional ischemia and 180 min reperfusion. Xanthine oxidoreductase activity was measured after 15 min aerobic perfusion and 30 min ischemia. Nitrite reduced myocardial necrosis and decline in ventricular function following ischemia/reperfusion in the intact and isolated rat heart in a dose- or concentration-dependent manner with an optimal dose of 4 mg/kg in vivo and concentration of 10 microM in vitro. Nitrate had no effect on protection. Reduction in infarction by nitrite was abolished by the inhibition of flavoprotein reductases and the molybdenum site of xanthine oxidoreductase and was associated with an increase in activity of xanthine dehydrogenase and xanthine oxidase during ischemia. Inhibition of nitric oxide synthase had no effect on nitrite-induced cardioprotection. Inhibition of NADPH oxidase and K(ATP) channels abolished nitrite-induced cardioprotection. Nitrite but not nitrate protects against infarction by a mechanism involving xanthine oxidoreductase, NADPH oxidase and K(ATP) channels.

The mobA gene is required for assimilatory and respiratory nitrate reduction but not xanthine dehydrogenase activity in Pseudomonas aeruginosa

The requirement for the mobA gene in key assimilatory and respiratory nitrogen metabolism of Pseudomonas aeruginosa PAO1 was investigated by mutational analysis of PA3030 (mobA; MoCo guanylating enzyme), PA1779 (nasA; assimilatory nitrate reductase), and PA3875 (narG; respiratory nitrate reductase). The mobA mutant was deficient in both assimilatory and respiratory nitrate reductase activities, whereas xanthine dehydrogenase activity remained unaffected. Thus, P. aeruginosa requires both the molybdopterin (MPT) and molybdopterin guanine dinucleotide (MGD) forms of the molybdenum cofactor for a complete spectrum of nitrogen metabolism, and one form cannot substitute for the other. Regulation studies using a Phi(PA3030-lacZGm) reporter strain suggest that expression of mobA is not influenced by the type of nitrogen source or by anaerobiosis, whereas assimilatory nitrate reductase activity was detected only in the presence of nitrate.

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.

Unique amino acids cluster for switching from the dehydrogenase to oxidase form of xanthine oxidoreductase

In mammals, xanthine oxidoreductase is synthesized as a dehydrogenase (XDH) but can be readily converted to its oxidase form (XO) either by proteolysis or modification of cysteine residues. The crystal structures of bovine milk XDH and XO demonstrated that atoms in the highly charged active-site loop (Gln-423-Lys-433) around the FAD cofactor underwent large dislocations during the conversion, blocking the approach of the NAD+ substrate to FAD in the XO form as well as changing the electrostatic environment around FAD. Here we identify a unique cluster of amino acids that plays a dual role by forming the core of a relay system for the XDH/XO transition and by gating a solvent channel leading toward the FAD ring. A more detailed structural comparison and site-directed mutagenesis analysis experiments showed that Phe-549, Arg-335, Trp-336, and Arg-427 sit at the center of a relay system that transmits modifications of the linker peptide by cysteine oxidation or proteolytic cleavage to the active-site loop (Gln-423-Lys-433). The tight interactions of these residues are crucial in the stabilization of the XDH conformation and for keeping the solvent channel closed. Both oxidative and proteolytic generation of XO effectively leads to the removal of Phe-549 from the cluster causing a reorientation of the bulky side chain of Trp-336, which then in turn forces a dislocation of Arg-427, an amino acid located in the active-site loop. The conformational change also opens the gate for the solvent channel, making it easier for oxygen to reach the reduced FAD in XO.

A new route to peroxynitrite: a role for xanthine oxidoreductase

Peroxynitrite, a potent oxidising, nitrating and hydroxylating agent, results from the reaction of nitric oxide with superoxide. We show that peroxynitrite can be produced by the action of a single enzyme, xanthine oxidoreductase (XOR), in the presence of inorganic nitrite, molecular oxygen and a reducing agent, such as pterin. The effects of oxygen concentration on peroxynitrite production have been examined. The physiologically predominant dehydrogenase form of the enzyme is more effective than the oxidase form under aerobic conditions. It is proposed that XOR-derived peroxynitrite fulfils a bactericidal role in milk and in the digestive tract.

Structure and expression of tandemly duplicated xanthine dehydrogenase genes of the silkworm (Bombyx mori)

Xanthine dehydrogenase (XDH) is a molybdoenzyme which catalyses oxidation of xanthine and hypoxanthine to uric acid. We isolated genomic clones of silkworm (Bombyx mori) XDH genes (BmXDH1 and BmXDH2). The BmXDH2 gene is located upstream from the BmXDH1 gene and they show a tandemly duplicated structure. Both BmXDH genes were expressed in the fat body and Malpighian tubules, whereas only the BmXDH1 gene was expressed in the midgut. Phylogenetic analysis indicates that BmXDH gene duplication occurred after the divergence of the silkworm and dipteran species. Intron insertion site comparison shows that some introns were lost during insect XDH gene evolution.

A reappraisal of xanthine dehydrogenase and oxidase in hypoxic reperfusion injury: the role of NADH as an electron donor

Xanthine oxidase (XO) is conventionally known as a generator of reactive oxygen species (ROS) which contribute to hypoxic-reperfusion injury in tissues. However, this role for human XO is disputed due to its distinctive lack of activity towards xanthine, and the failure of allopurinol to suppress reperfusion injury. In this paper, we have employed native gel electrophoresis together with activity staining to investigate the role human xanthine dehydrogenase (XD) and XO in hypoxic reperfusion injury. This approach has provided information which cannot be obtained by conventional spectrophotometric assays. We found that both XD and XO of human umbilical vein endothelial cells (HUVECs) and lymphoblastic leukaemic cells (CEMs) catalysed ROS generation by oxidising NADH, but not hypoxanthine. The conversion of XD to XO was observed in both HUVECs and CEMs in response to hypoxia, although the level of conversion varied. Purified human milk XD generated ROS more efficiently in the presence of NADH than in the presence of hypoxanthine. This NADH oxidising activity was blocked by the FAD site inhibitor, diphenyleneiodonium (DPI), but was not suppressible by the molybdenum site inhibitor, allopurinol. However, in the presence of both DPI and allopurinol the activities of XD/XO were completely blocked with either NADH or hypoxanthine as substrates. We conclude that both human XD and XO can oxidise NADH to generate ROS. Therefore, the conversion of XD to XO is not necessary for post-ischaemic ROS generation. The hypoxic-reperfusion injury hypothesis should be reappraised to take into account the important role played by XD and XO in oxidising NADH to yield ROS.

[The role of xanthine dehydrogenase (xanthine oxidase) in ischemia-reperfusion injury in rat kidney]

Xanthine dehydrogenase (XDH) and xanthine oxidase (XO) are enzymes involved in the metabolism of purines in various organisms. XO produces superoxide radicals, suggesting that is responsible for tissue ischemia-reperfusion injury. To test this notion further studies were performed on rat kidneys and the time course of changes in purine nucleotides, oxypurines and XDH and XO activity was determined. At 24 hours after reperfusion subsequent to 30-minute ischemia, serum creatinine increased to 0.83 +/- 0.74 mg/dl from 0.28 +/- 0.06 mg/dl (the level prior to ischemia, the control). Renal ATP and ADP contents were reduced after ischemia lasting for 30 minutes and restored 10 minutes after reperfusion following 30 minutes of ischemia. The renal AMP content increased after 30 minutes of ischemia and recovered within 10 minutes after reperfusion. The total adenine nucleotide (TAN) content was reduced gradually during ischemia-reperfusion in the rat kidney. Although the energy charge was reduced following 30 minutes of ischemia, it was restored to the control level 10 minutes following reperfusion. Hypoxanthine (HX) and xanthine (X), which had accumulated at 30 minutes after ischemia, were reduced to the control levels 10 minutes after reperfusion. There were no significant changes in the pre-ischemia values of total XDH and XO activities or XDH/XO ratio during the period nor at various time intervals (up to 24 hours) during reperfusion. It was shown that HX and X accumulate without significant conversion of XDH to XO during ischemia. Therefore the putative role of XO in ischemia-reperfusion injury seems to more complex than initially predicted.

[Xanthine oxidase (xanthine dehydrogenase)]

Xanthine oxidase (xanthine dehydrogenase) is composed of two identical subunits of approximately 150,000 daltons. Each subunit contains four oxdation-reduction active cofactors/monomers. In vivo, the enzyme exists mostly as the dehydrogenase type (the NAD-dependent type). The cDNA has been cloned from human liver, and the amino acid sequence has been determined. As xanthine oxidase seems to produce superoxide in postischemic reperfusion, the relation between the superoxide and postischemic tissue injury has been discussed. It has also been reported that inhibition of xanthine oxidase by allopurinol may cause severe 6-mercaptopurine toxicity.

Role of xanthine oxidase and its inhibitor in hypoxia: reoxygenation injury

OBJECTIVE

This article reviews the biochemistry and function of xanthine dehydrogenase (XDH) and xanthine oxidase (XO) as well as their role in hypoxia-reoxygenation injury. Possible benefits of XO blockade are discussed.

METHODOLOGY

The available literature was reviewed.

RESULTS

It is evident that relatively high activities of XO are restricted to a few organs in man. Because positive effects of XO blockade with allopurinol have been reported even in organs containing relatively low activities of XO, two other possible favorable actions of allopurinol are mentioned. First it may act as an oxygen radical scavenger, and second, it may augment the adenine nucleotides and, hence, adenosine triphosphate concentration in the cell.

CONCLUSIONS

XDH and XO may play an important role in a series of pathophysiologic conditions. Their role in hypoxia-reoxygenation injury has been critically reviewed. However, care should be exercised in starting randomized trials to prevent hypoxia-reoxygenation injury with allopurinol, especially in newborn infants.

SPECULATION

XDH and XO are released from the liver during hypoxic conditions, for instance, and consequently, they may reach a number of organs via the circulation.

Cloning and expression in vitro of human xanthine dehydrogenase/oxidase

To study the expression of human xanthine dehydrogenase/oxidase (hXDH/XO), we cloned the cDNA covering its complete coding sequence and characterized it by translation in vitro in rabbit reticulocyte lysates and by transient expression in COS-1 cells. Two specific protein products with approximate molecular masses of 150 and 130 kDa were detected in both expression systems. These products are compatible with the molecular sizes of XDH/XO, and these peptides also showed immunoreactivity with polyclonal anti-hXDH antibodies. Significant XDH/XO enzyme activity (277 +/- 54 pmol/min per mg of protein) was measured in lysates of transfected COS cells, whereas in control transfections the activities were below the detection limit of our assay (0.2 pmol/min per mg of protein). The COS cells expressed the enzyme predominantly (89.8 +/- 0.3%) in the dehydrogenase form.

Antifreeze protein does not confer cold tolerance to transgenic Drosophila melanogaster

Fish antifreeze proteins (AFPs) have been reported by some researchers to prolong the viability of tissues, organs, and embryos under hypothermic conditions, while others have observed no such effect or even AFP-mediated cryotoxicity. We examined the influence of Type III AFP from Atlantic wolffish on cold tolerance in a whole animal model system, transgenic Drosophila. The activity of the AFP, transgenically expressed under the transcriptional control of the female-specific yp1 and yp2 promoters and secreted into fly hemolymph, was confirmed through thermal hysteresis and differential scanning calorimetry measurements as well as through observations of ice crystal morphology. In cold exposure trials, at 0 degrees C and at -7 degrees C, transgenic adult flies of both sexes exhibited greater survival than nontransgenic controls even though the antifreeze was only produced in females. We attribute these observations to the expression of the xanthine dehydrogenase marker gene used to identify transgenics, rather than the production of AFP. Type III AFP therefore appears unable to enhance survival of adult Drosophila under hypothermic conditions.

Dietary nicotinamide supplementation increases xanthine oxidoreductase activity in the kidney and heart but not liver of obese Zucker rats

The conversion of xanthine dehydrogenase to xanthine oxidase that produces oxygen radicals has been implicated in the ischemic injury to the myocardium and to the kidney. Xanthine dehydrogenase uses NAD as the electron acceptor to catalyze a reaction which does not produce any oxygen free radicals and may depress the conversion of xanthine dehydrogenase to xanthine oxidase. Nicotinamide is the preferred precursor for NAD. This study was conducted to examine the effect of an 18% casein diet supplemented with 0.5% nicotinamide on the activity of oxidoreductase and its two enzyme forms, xanthine dehydrogenase and xanthine oxidase, in kidney, heart and liver of female obese Zucker rats that spontaneously develop glomerulosclerosis, cardiomegaly and fatty liver. Lean litter mates were used as controls. Nicotinamide supplementation had no effect on the activities of these enzyme forms in the liver of either obese rats or lean rats. Obese rats fed the nicotinamide supplemented diet had higher activities of these enzyme forms in kidneys and hearts than unsupplemented diet fed obese rats, but this difference was not observed in lean rats. In unsupplemented rats, xanthine oxidase activity in the kidney was greater in lean rats than obese rats. Thus, the abnormalities observed in obese rats are unlikely attributable to the xanthine oxidase-mediated oxidant stress.

The conversion of xanthine dehydrogenase to xanthine oxidase and the role of the enzyme in reperfusion injury

Although mammalian xanthine oxidase exists originally as a dehydrogenase form in freshly prepared samples, it is converted to an oxidase form during purification, either irreversibly by proteolysis or reversibly by sulfhydryl oxidation of the protein molecule. However, avoiding proteolysis the mammalian enzyme can be purified as an interconvertible form and thus can be used to compare directly the properties of xanthine dehydrogenase and the oxidase derived from the same enzyme molecule. The cDNAs encoding the enzyme have been cloned from several sources, and structural information is becoming available. The most significant difference between the two forms is the protein conformation around FAD, which changes the redox potential of the flavin and the reactivity of FAD with the electron acceptors, NAD and molecular oxygen. The flavin semiquinone is thermodynamically stable in xanthine dehydrogenase, but is unstable in xanthine oxidase. Detailed analyses by stopped-flow techniques suggest that the flavin semiquinone reacts with oxygen to form superoxide anion while the fully reduced flavin reacts to form hydrogen peroxide. Although xanthine dehydrogenase can produce greater amounts of superoxide anion than xanthine oxidase during xanthine-oxygen turnover, it seems to be physiologically insignificant because NAD inhibits almost completely the formation of superoxide anion. Although the involvement of this enzyme in reperfusion injury has been proposed, this seems to be more complex than originally envisaged and still remains to be established.

The role of xanthine oxidase and xanthine dehydrogenase in skin ischemia

The importance of sequential events which lead to skin necrosis has significant implications in trauma, vascular injury, and wound healing. In this series of experiments, we tested the hypothesis that xanthine oxidase (XO) activity was increased along an ischemic gradient of a skin flap and that the XO enzyme activity correlated with an increase in neutrophils. There were two animal groups in which the skin flaps were raised and assayed at 0, 1, or 6 hr. In the other group, they were created as bipedicle flaps for 7 days, before the distal attachment was divided and the tissue assayed. In the acutely raised flaps, some animals were treated with the XO inhibitor, allopurinol. Xanthine dehydrogenase (XD) and XO activity was measured with a fluorometric pterin assay and neutrophil concentration was measured using a myeloperoxidase marker. In this model, there was consistent skin necrosis in the distal end of the skin flap (48 +/- 8%). The data showed that both XD and XO activity in the distal ends was statistically significantly increased over the sham control or proximal ends of the skin flaps at 1 hr (P < 0.05). XO activity remained elevated in the distal ends at 6 hr. Allopurinol significantly reduced the neutrophil concentrations in the distal ends of the skin flaps when compared to untreated animals (P < 0.05). Moreover, allopurinol reduced skin necrosis to 12 +/- 1%. Preconditioning of the skin flap reduced the XO activity to sham control levels. The observations implicate XO activity as source of free radical injury in skin necrosis seen in random skin flaps.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement of xanthine dehydrogenase mediated mitomycin C metabolism by dicumarol

These studies examined the effect of dicumarol on xanthine dehydrogenase (XDH), an enzyme recently shown to bioreduce mitomycin C. Dicumarol, which has previously been shown to inhibit xanthine oxidase (XO), inhibited both XDH and XO mediated conversion of xanthine to uric acid but potentiated the metabolism of mitomycin C by XDH and XO. Formation of 2,7-diaminomitosene following mitomycin C bioactivation by XDH was increased 3-fold aerobically and 4-fold hypoxically when 20 microM dicumarol was included in the reaction mixture. XO mediated metabolism of mitomycin C hypoxically was increased approximately 50% by the inclusion of dicumarol.

Role of oxygen radicals in tourniquet-related ischemia-reperfusion injury of human patients

In the current study we evaluated effluent blood from extremities of human patients undergoing reconstructive surgical treatment which is routinely accompanied by upper extremity exsanguination and application of a tourniquet. Following tourniquet release (reperfusion), there were immediate increases in the plasma levels of xanthine oxidase activity, uric acid, and histamine. Xanthine dehydrogenase activity was not detectable. Plasma also contained products consistent with the formation of oxygen-derived free radicals, namely hemoglobin and fluorescent compounds. Our data indicate in humans that ischemia-reperfusion events are associated with the appearance of xanthine oxidase activity and its products in the plasma effluent.

High postmortem levels of hypoxanthine in the vitreous humor of premature babies with respiratory distress syndrome

To test whether or not premature babies at risk for retinopathy of prematurity have elevated hypoxanthine levels in the eye, the vitreous humor of 13 premature babies who died of severe respiratory distress syndrome and lung failure, was analyzed for hypoxanthine. Their hypoxanthine level was 459 +/- 171 mumol/L (mean +/- SD) compared with 54 +/- 71 mumol/L in seven newborn babies who died suddenly (P less than .001). In 53 adults who died suddenly, the hypoxanthine concentration was 136 +/- 119 mumol/L (P less than .001 when compared with babies with respiratory distress syndrome). Babies with respiratory distress syndrome underwent a significantly longer period with arterial PO2 levels less than 40 mm Hg (5.3 kPa) and they required supplementation with 100% oxygen significantly longer than control babies. The hypoxanthine concentration was correlated with the time during which the arterial PO2 was lower than 40 mm Hg (5.3 kPa) before death, and a significant positive correlation was found (R = .59, P less than .12). The study shows that high levels of hypoxanthine are found in vitreous humor of premature babies with respiratory distress syndrome. Because hypoxanthine is a potential oxygen radical generator and premature babies might have lower levels of antioxidants than full-term babies, it is suggested that the hypoxanthine accumulation in the eyes of premature babies with respiratory distress syndrome could play a pathogenetic role in the development of retinopathy of prematurity.