Studies of the reductive half-reaction of milk xanthine dehydrogenase

Catalase increases ethanol oxidation through the purine catabolism in rat liver

Hepatic ethanol oxidation increases according to its concentration and is raised to near-saturation levels of alcohol dehydrogenase (ADH); therefore, re-oxidation of NADH becomes rate limiting in ethanol metabolism by the liver. Adenosine is able to increase liver ethanol oxidation in both in vivo and in vitro conditions; the enhancement being related with the capacity of the nucleoside to accelerate the transport of cytoplasmic reducing equivalents to mitochondria, by modifying the subcellular distribution of the malate-aspartate shuttle components. In the present study, we explored the putative effects of adenosine and other purines on liver ethanol oxidation mediated by non-ADH pathways. Using the model of high precision-cut rat liver slices, a pronounced increase of ethanol oxidation was found in liver slices incubated with various intermediates of the purine degradation pathway, from adenosine to uric acid (175-230%, over controls). Of these, urate had the strongest (230%), whereas xanthine had the less pronounced effect (178% over controls). The enhancement was not abolished by 4-methylpyrazole, indicating that the effect was independent of alcohol dehydrogenase. Conversely, aminotriazole, a catalase inhibitor, completely abolished the effect, pointing out that this enhanced ethanol oxidation is mediated by catalase activity. It is concluded that the H₂O₂ needed for catalase activity is derived from the oxidation of (hypo)xanthine by xanthine oxidase and the oxidation of urate by uricase. The present and previous data led us to propose that, depending on the metabolic conditions, adenosine might be able to stimulate the metabolism of ethanol through different pathways.

"Repurposing" of Xanthine Oxidoreductase as a Nitrite Reductase: A New Paradigm for Therapeutic Targeting in Hypertension

SIGNIFICANCE

In contrast to nitric oxide (NO), which has well-established, important effects in regulation of cardiovascular homeostasis, its oxidative metabolite nitrite has, until recently, been considered to be of minor functional significance.

RECENT ADVANCES

However, this view of nitrite has been radically revised over the past 10 years with evidence now supporting a critical role for this anion as a storage form of NO.

CRITICAL ISSUES

Importantly, while hypoxia and acidosis have been shown to play a pivotal role in the generation of nitrite to NO, a number of mammalian nitrite reductases have been identified that facilitate the reduction of nitrite. Critically, these nitrite reductases have been demonstrated to operate under physiological pH conditions and in normoxia, extending the functional remit of this anion from an ischemic mediator to an important regulator of physiology. One particular nitrite reductase that has been shown to operate under a wide range of environmental conditions is the enzyme xanthine oxidoreductase (XOR).

FUTURE DIRECTIONS

In this review, we discuss the evidence supporting a role for XOR as a nitrite reductase while focusing particularly on its function in hypertension. In addition, we discuss the potential merit in exploiting this activity of XOR in the therapeutics of hypertension.

Nitrite reductase activity of rat and human xanthine oxidase, xanthine dehydrogenase, and aldehyde oxidase: evaluation of their contribution to NO formation in vivo

Nitrite is presently considered a NO "storage form" that can be made available, through its one-electron reduction, to maintain NO formation under hypoxia/anoxia. The molybdoenzymes xanthine oxidase/dehydrogenase (XO/XD) and aldehyde oxidase (AO) are two of the most promising mammalian nitrite reductases, and in this work, we characterized NO formation by rat and human XO/XD and AO. This is the first characterization of human enzymes, and our results support the employment of rat liver enzymes as suitable models of the human counterparts. A comprehensive kinetic characterization of the effect of pH on XO and AO-catalyzed nitrite reduction showed that the enzyme's specificity constant for nitrite increase 8-fold, while the Km(NO2(-)) decrease 6-fold, when the pH decreases from 7.4 to 6.3. These results demonstrate that the ability of XO/AO to trigger NO formation would be greatly enhanced under the acidic conditions characteristic of ischemia. The dioxygen inhibition was quantified, and the Ki(O2) values found (24.3-48.8 μM) suggest that in vivo NO formation would be fine-tuned by dioxygen availability. The potential in vivo relative physiological relevance of XO/XD/AO-dependent pathways of NO formation was evaluated using HepG2 and HMEC cell lines subjected to hypoxia. NO formation by the cells was found to be pH-, nitrite-, and dioxygen-dependent, and the relative contribution of XO/XD plus AO was found to be as high as 50%. Collectively, our results supported the possibility that XO/XD and AO can contribute to NO generation under hypoxia inside a living human cell. Furthermore, the molecular mechanism of XO/AO-catalyzed nitrite reduction was revised.

Electron transferases

The flavin isoalloxazine ring in electron transferases functions in a redox capacity, being able to take up electrons from a donor to subsequently deliver them to an acceptor. The main characteristics of these flavoproteins, including their unique ability to mediate obligatory processes of two-electron transfers with those involving single-electron transfer, are here described. To illustrate the versatility of these proteins, the acquired knowledge of the function of the two electron transferases involved in the cyanobacterial photosynthetic electron transfer from photosystem I to NADP(+) is presented. Many aspects of their biochemistry and biophysics have been extensively characterized using site-directed mutagenesis, steady-state and transient kinetics, spectroscopy, calorimetry, X-ray crystallography, electron paramagnetic resonance, and computational methods.

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.

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.

The role of active site glutamate residues in catalysis of Rhodobacter capsulatus xanthine dehydrogenase

Xanthine dehydrogenase (XDH) from the bacterium Rhodobacter capsulatus catalyzes the hydroxylation of xanthine to uric acid with NAD+ as the electron acceptor. R. capsulatus XDH forms an (alphabeta)2 heterotetramer and is highly homologous to homodimeric eukaryotic xanthine oxidoreductases. Here we first describe reductive titration and steady state kinetics on recombinant wild-type R. capsulatus XDH purified from Escherichia coli, and we then proceed to evaluate the catalytic importance of the active site residues Glu-232 and Glu-730. The steady state and rapid reaction kinetics of an E232A variant exhibited a significant decrease in both kcat and kred as well as increased Km and Kd values as compared with the wild-type protein. No activity was determined for the E730A, E730Q, E730R, and E730D variants in either the steady state or rapid reaction experiments, indicating at least a 10(7) decrease in catalytic effectiveness for this variant. This result is fully consistent with the proposed role of this residue as an active site base that initiates catalysis.

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.

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.

Selenium-dependent metabolism of purines: A selenium-dependent purine hydroxylase and xanthine dehydrogenase were purified from Clostridium purinolyticum and characterized

During purification of the selenium-dependent xanthine dehydrogenase (XDH) from Clostridium purinolyticum, another hydroxylase was uncovered that also contained selenium and exhibited similar spectral properties. This enzyme was purified to homogeneity. It uses purine, 2OH-purine, and hypoxanthine as substrates, and based on its substrate specificity, this selenoenzyme is termed purine hydroxylase (PH). The product of hydroxylation of purine by PH is xanthine. A concomitant release of selenium from the enzyme and loss of catalytic activity on treatment with cyanide indicates that selenium is essential for PH activity. Selenium-dependent XDH, also purified from C. purinolyticum, was found to be insensitive to oxygen during purification and to use both potassium ferricyanide and 2,6-dichloroindophenol as electron acceptors. Selenium is required for the xanthine-dependent reduction of 2, 6-dichloroindophenol by XDH. Kinetic analyses of both enzymes revealed that xanthine is the preferred substrate for XDH and purine and hypoxanthine are preferred by PH. This characterization of these selenium-requiring hydroxylases involved in the interconversion of purines describes an extension of the pathway for purine fermentation in the purinolytic clostridia.

The roles of two amino acid residues in the active site of L-lactate monooxygenase. Mutation of arginine 187 to methionine and histidine 240 to glutamine

Lactate monooxygenase (LMO) catalyzes the conversion of L-lactate to acetate, CO(2), and water with the incorporation of molecular oxygen. Arginine 187 of LMO is highly conserved within the family of L-alpha-hydroxyacid oxidizing enzymes (Lê, K. H. D., and Lederer, F. (1991) J. Biol. Chem. 266, 20877-20881). By comparison with the equivalent residue in flavocytochrome b(2) from Saccharomyces cerevisiae (Pike, A. D., Chapman, S. K, Manson, F. D. C,. Reid, G. A. , Gondry, M., and Lederer, F. (1996) in Flavins and Flavoproteins (Stevenson, K. J., Massey, V., and Williams, C. H., Jr., eds) pp. 571-574, University of Calgary Press, Calgary, AB, Canada), arginine 187 might be expected to have an important role in catalytic efficiency and substrate binding in LMO. Histidine 240 is predicted to be close to the substrate binding site of LMO, although it is not conserved within the enzyme family. Arginine 187 has been replaced with methionine (R187M), and histidine 240 has been replaced with glutamine (H240Q). L-Lactate oxidation by R187M is very slow. The binding of L-lactate to the mutant enzyme appears to be very weak, as is the binding of oxalate, a transition state analogue. The binding of pyruvate to the reduced enzyme is also very weak, resulting in complete uncoupling of enzyme turnover, with H(2)O(2) and pyruvate as the final products. In addition, anionic forms of the flavin are unstable. The K(d) for sulfite is increased nearly 400-fold by this mutation. The semiquinone form of R187M is also thermodynamically unstable, although the overall midpoint potential for the two-electron reduction of R187M is only 34 mV lower than for the wild-type enzyme. H240Q more closely resembles the wild-type enzyme. The steady-state activity of H240Q is completely coupled. The k(cat) is similar to that for the wild-type enzyme.

Role of the flavin midpoint potential and NAD binding in determining NAD versus oxygen reactivity of xanthine oxidoreductase

Xanthine oxidoreductase from bovine milk can be prepared in two interconvertible forms, xanthine oxidase (XO) and xanthine dehydrogenase (XDH), depending on the number of protein cysteines versus cystines. Enzyme forms differ in respect to their oxidizing substrates; XDH prefers NAD to molecular oxygen, whereas XO only reacts significantly with oxygen. The preference for oxidizing substrate is partially explained by thermodynamics. Unlike XDH, the midpoint potential of the FAD, the center at which oxygen and NAD react, is too high in XO to efficiently reduce NAD (Hunt, J., Massey, V., Dunham, W.R., and Sands, R.H. (1993) J. Biol. Chem. 268, 18685-18691). To distinguish between changes in thermodynamics and in substrate binding, samples of both XO and XDH have been prepared in which the native FAD has been replaced with an FAD analog of different redox potential, 1-deaza-FAD or 8-CN-FAD. Reductive titrations indicate that both 1-deaza-XO and 1-deaza-XDH have a flavin midpoint potential similar to native XDH and that 8-CN-XO and 8-CN-XDH each have a flavin potential higher than XO. Both the low potential 1-deaza-XO and the high potential 8-CN-XDH contain essentially no xanthine/NAD activity. However, 1-deaza-XDH does exhibit xanthine/NAD activity, and 8-CN-XO has normal xanthine/oxygen activity. The binding of NAD to oxidized XO and XDH was investigated by ultrafiltration and isothermal titration calorimetry. The Kd for the binding of NAD to XDH was determined to be 280 +/- 145 microM by ultrafiltration and 160 +/- 40 microM by isothermal titration calorimetry. No evidence for the binding of NAD to XO by either method could be obtained. A low flavin midpoint potential is necessary but not sufficient for dehydrogenase activity.

The reductive half-reaction of xanthine oxidase. Reaction with aldehyde substrates and identification of the catalytically labile oxygen

The kinetics of xanthine oxidase has been investigated with the aim of addressing several outstanding questions concerning the reaction mechanism of the enzyme. Steady-state and rapid kinetic studies with the substrate 2,5-dihydroxybenzaldehyde demonstrated that (kcat/Km)app and kred/Kd exhibit comparable bell-shaped pH dependence with pKa values of 6.4 +/- 0.2 and 8.4 +/- 0.2, with the lower pKa assigned to an active-site residue of xanthine oxidase (possibly Glu-1261, by analogy to Glu-869 in the crystallographically known aldehyde oxidase from Desulfovibrio gigas) and the higher pKa to substrate. Early steps in the catalytic sequence have been investigated by following the reaction of the oxidized enzyme with a second aldehyde substrate, 2-aminopteridine-6-aldehyde. The absence of a well defined acid limb in this pH profile and other data indicate that this complex represents an Eox.S rather than Ered.P complex (i.e. no chemistry requiring the active-site base has taken place in forming the long wavelength-absorbing complex seen with this substrate). It appears that xanthine oxidase (and by inference, the closely related aldehyde oxidases) hydroxylates both aromatic heterocycles and aldehydes by a mechanism involving base-assisted catalysis. Single-turnover experiments following incorporation of 17O into the molybdenum center of the enzyme demonstrated that a single oxygen atom is incorporated at a site that gives rise to strong hyperfine coupling to the unpaired electron spin of the metal in the MoV oxidation state. By analogy to the hyperfine interactions seen in a homologous series of molybdenum model compounds, we conclude that this strongly coupled, catalytically labile site represents a metal-coordinated hydroxide rather than the Mo=O group and that this Mo-OH represents the oxygen that is incorporated into product in the course of catalysis.

Xanthine oxidoreductase catalyses the reduction of nitrates and nitrite to nitric oxide under hypoxic conditions

Xanthine oxidoreductase (XOR) catalyses the reduction of the therapeutic organic nitrate, nitroglycerin (glyceryl trinitrate, GTN), as well as inorganic nitrate and nitrite, to nitric oxide (NO) under hypoxic conditions in the presence of NADH. Generation of nitric oxide is not detectable under normoxic conditions and is inhibited by the molybdenum site-specific inhibitors, oxypurinol and (-)BOF 4272. These enzymic reactions provide a mechanism for generation of NO under hypoxic conditions where nitric oxide synthase does not function, suggesting a vasodilatory role in ischaemia.

The oxidative half-reaction of xanthine dehydrogenase with NAD; reaction kinetics and steady-state mechanism

The reaction between reduced xanthine dehydrogenase (XDH) from bovine milk and NAD has been studied in detail. An understanding of this reaction is necessary for a complete description of XDH turnover with its presumed natural electron acceptor and to address the preference of XDH for NAD over oxygen as a substrate. The reaction between pre-reduced XDH and NAD was studied by stopped-flow spectrophotometry. The reaction was found to involve two rounds of oxidation with 2 eq of NAD. The first round goes to completion, and the second round reaches a slightly disfavored equilibrium. Rapid binding of NAD with an apparent Kd of 25 +/- 2 microM is followed by NAD reduction at a rate constant of 130 +/- 13 s-1. NADH dissociation at a rate constant of 42 +/- 12 s-1 completes a round of oxidation. These steps have been successfully tested and modeled to repeat themselves in the second round of oxidation. The association rate constant for NAD binding was estimated to be much greater than any rate constant measured in the oxidation by molecular oxygen, thus explaining how NAD competes with oxygen for reducing equivalents. Rate constants for NAD reduction and NADH dissociation are respectively 21- and 7-fold greater than kcat, indicating that the reductive half-reaction of the enzyme by xanthine is mostly rate-limiting in xanthine/NAD turnover. A steady-state mechanism for XDH is discussed.

Kinetic isotope effects and electron transfer in the reduction of xanthine oxidoreductase with 4-hydroxypyrimidine. A comparison between oxidase and dehydrogenase forms

Isolated from bovine milk, xanthine oxidase (XO) and xanthine dehydrogenase (XDH) are two interconvertible forms of the same protein, differing in the number of protein cysteines versus cystines. Most differences between XO and XDH are localized to the FAD center, the site at which the oxidizing substrates NAD and molecular oxygen react. A comparative study of the reduction of XO and XDH has been performed to assess differences in reactivity of the molybdopterin site, as well as subsequent electron-transfer events from molybdenum to 2Fe/2S and FAD centers. The compound 4-hydroxypyrimidine (4-OH-P) was chosen as reducing substrate because its higher Km value raised the possibility of binding weak enough to measure kinetically, and its high kcat value could allow detection of intramolecular electron-transfer reactions. As measured by stopped flow spectrophotometry, XO and XDH react with the first equivalent of 4-OH-P via similar mechanisms, differing in the magnitude of rate and dissociation constants. Using [2-2H]4-OH-P as substrate, a D(k/Kd) isotope effect of 1.9 to 2.3 suggests that movement of the hydrogen abstracted from substrate appreciably limits the rate of initial enzyme reduction from Mo(VI) to Mo(IV). Monitoring the visible spectrum of the enzymes, the first observed step is reduction of a single 2Fe/2S center and presumably re-oxidation of Mo(IV) to Mo(V). This suggests a common pathway for electron transfer involving reduction of a 2Fe/2S center prior to reduction of the second 2Fe/2S and FAD centers. Rates of the first electron transfer from molybdenum to the 2Fe/2S center are rapid, 290 s-1 with XO and 180 s-1 with XDH, and are consistent with rates measured by flash photolysis (Walker, M. C., Hazzard, J. T., Tollin, G., and Edmondson, D. E. (1991) Biochemistry 30, 5912-5917) allowing discrete observation of the electron-transfer reactions that occur during turnover. This step also exhibits a modest primary kinetic isotope effect of 1.5 to 1.6 when [2-2H]4-OH-P is used, possibly due to deprotonation of the molybdenum center prior to electron transfer. A second one-electron transfer, presumably oxidizing Mo(V) to Mo(VI), follows in a step coincident with product dissociation, consistent with a role for product release in controlling electron transfer events. The kinetics of this complex system are described and interpreted quantitatively in models that are consistent with all the data.

NADH oxidase activity of human xanthine oxidoreductase--generation of superoxide anion

Human xanthine oxidase was purified from breast milk. The dehydrogenase form of the enzyme, which predominates in most mammalian tissues, catalyses the oxidation of NADH by oxygen, generating superoxide anion significantly faster than does the oxidase form. The corresponding forms of bovine enzyme behave very similarly. The steady-state kinetics of NADH oxidation and superoxide production, including inhibition by NAD, by the dehydrogenase forms of both enzymes, are analysed in terms of a model involving two-stage recycling of oxidised enzyme. Established inhibitors of xanthine oxidoreductases (allopurinol oxypurinol, amflutizole and BOF 4272), which block all other reducing substrates, were ineffective in the case of NADH. Diphenyleneiodonium, on the other hand, was a powerful inhibitor of NADH oxidation. The potential involvement of reactive oxygen species arising from NADH oxidation by xanthine oxidoreductase in ischaemia-reperfusion injury and other disease states, as well as in normal signal transduction, is discusssed.

The reaction of reduced xanthine dehydrogenase with molecular oxygen. Reaction kinetics and measurement of superoxide radical

Xanthine dehydrogenase (XDH) from bovine milk contains significant activity in xanthine/oxygen turnover assays. The oxidative half-reaction of XDH with molecular oxygen has been studied in detail, at 25 degrees C, pH 7.5, to determine the basis of the preference of XDH for NAD over oxygen as oxidizing substrate. Spectral changes of XDH accompanying oxidation were followed by stopped-flow spectrophotometry. The amount of superoxide radicals formed during oxidation was investigated to assess the ability of XDH to catalyze production of oxygen radicals. Reduced XDH reacts with oxygen in at least 4 bi-molecular steps, with 1.7-1.9 mol of superoxide per mol of XDH formed from the last 2 electrons oxidized. A model is discussed in which the flavin hydroquinone transfers electrons to oxygen to produce hydrogen peroxide at a rate constant of at least 72,000 M-1 s-1, whereas flavin semiquinone reduces oxygen to form superoxide as slow as 16 M-1 s-1. Steady-state kinetics of xanthine/oxygen and NADH/oxygen turnover of XDH were determined to have kcat values of 2.1 +/- 0.1 and 2.5 +/- 0.9 s-1, respectively, at 25 degrees C, pH 7.5. XDH is therefore capable of catalyzing the formation of reduced oxygen species at one-third the rate of xanthine/NAD turnover, 6.3 s-1 (Hunt, J., and Massey, V. (1992) J. Biol. Chem. 267, 21479-21485), in the absence of NAD. As XDH contains a significant and intrinsic xanthine oxidase activity, estimates of relative amounts of XO and XDH based solely upon turnover assays must be made with caution. Initial-rate assays containing varying amounts of xanthine, NAD, and oxygen indicate that at 100% oxygen saturation, NADH formation is only inhibited at concentrations of xanthine and NAD below Km for each substrate.

The molybdenum centers of xanthine oxidase and xanthine dehydrogenase. Determination of the spectral change associated with reduction from the Mo(VI) to the Mo(IV) state

The UV-visible absorbance change associated with reduction of the molybdenum centers of xanthine oxidase and xanthine dehydrogenase has been determined using a double-difference technique. At pH 8.5, the Mo(VI) minus Mo(IV) difference spectrum seen with xanthine oxidase exhibits a positive feature at 420 nm, having an extinction change of approximately 3,000 M-1 cm-1 as well as evidence for a negative feature below 340 nm. In xanthine oxidase this change is found to exhibit a marked pH dependence, implicating protonation/deprotonation events associated with changes in the molybdenum oxidation state. Application of the double-difference protocol to the respective circular dichroism spectra of xanthine oxidase and xanthine dehydrogenase reveals appreciable CD changes at 420 and 580 nm associated with the reduction of the molybdenum center. The present results demonstrate a direct spectroscopic handle on the molybdenum centers of both xanthine oxidase and xanthine dehydrogenase.