Uric acid formation is driven by crosstalk between skeletal muscle and other cell types

Hyperuricemia is implicated in numerous pathologies, but the mechanisms underlying uric acid production are poorly understood. Using a combination of mouse studies, cell culture studies, and human serum samples, we sought to determine the cellular source of uric acid. In mice, fasting and glucocorticoid treatment increased serum uric acid and uric acid release from ex vivo-incubated skeletal muscle. In vitro, glucocorticoids and the transcription factor FoxO3 increased purine nucleotide degradation and purine release from differentiated muscle cells, which coincided with the transcriptional upregulation of AMP deaminase 3, a rate-limiting enzyme in adenine nucleotide degradation. Heavy isotope tracing during coculture experiments revealed that oxidation of muscle purines to uric acid required their transfer from muscle cells to a cell type that expresses xanthine oxidoreductase, such as endothelial cells. Last, in healthy women, matched for age and body composition, serum uric acid was greater in individuals scoring below average on standard physical function assessments. Together, these studies reveal skeletal muscle purine degradation is an underlying driver of uric acid production, with the final step of uric acid production occurring primarily in a nonmuscle cell type. This suggests that skeletal muscle fiber purine degradation may represent a therapeutic target to reduce serum uric acid and treat numerous pathologies.

Design, synthesis, and evaluation of the in vitro activity of novel dual inhibitors of XOR and URAT1 containing a benzoic acid group

Xanthine oxidoreductase (XOR) and uric acid transporter 1 (URAT1) are involved in the production and reabsorption of uric acid, respectively. However, the currently available individual XOR- or URAT1-targeted drugs have limited efficacy. Thus, strategies for combining XOR inhibitors with uricosuric drugs have been developed. Previous virtual screening identified Compounds 1-5 as hits for the potential dual inhibition of XOR/URAT1. Nevertheless, in vitro experiments yielded unsatisfactory results. The first round of optimization work on those hits was performed, and two series of compounds were designed and synthesized. Compounds of the A series exerted moderate inhibitory effects on URAT1, but extremely weak inhibitory effects on XOR. Compounds of the B series exerted strong inhibitory effects on both XOR and URAT1. B5 exhibited the greatest inhibitory activity, with similar inhibitory effects on XOR and URAT1. The half maximal inhibitory concentration (IC50 ) of XOR was 0.012 ± 0.001 μM, equivalent to that of febuxostat (IC50  = 0.010 ± 0.001 μM). The IC50 of URAT1 was 30.24 ± 3.46 μM, equivalent to that of benzbromarone (IC50  = 24.89 ± 7.53 μM). Through this optimization, the in vitro activity of most compounds of the A and B series against XOR and URAT1 was significantly improved versus that of the hits. Compound B5 should be further investigated.

Diurnal Variations in Serum Uric Acid, Xanthine, and Xanthine Oxidoreductase Activity in Male Patients with Coronary Artery Disease

Hyperuricemia is influenced by diet and can cause gout. Whether it is a potential risk factor for cardiovascular disease (CVD) remains controversial, and the mechanism is unclear. Similar to CVDs, gout attacks occur more frequently in the morning and at night. A possible reason for this is the diurnal variation in uric acid (UA), However, scientific data regarding this variation in patients with CVD are not available. Thus, we aimed to investigate diurnal variations in serum levels of UA and plasma levels of xanthine, hypoxanthine, and xanthine oxidoreductase (XOR) activity, which were measured at 18:00, 6:00, and 12:00 in male patients with coronary artery disease. Thirty eligible patients participated in the study. UA and xanthine levels significantly increased from 18:00 to 6:00 but significantly decreased from 6:00 to 12:00. By contrast, XOR activity significantly increased both from 18:00 to 6:00 and 6:00 to 12:00. Furthermore, the rates of increase in UA and xanthine levels from night to morning were significantly and positively correlated. In conclusion, UA and xanthine showed similar diurnal variations, whereas XOR activity showed different diurnal variations. The morning UA surge could be due to UA production. The mechanism involved XOR activity, but other factors were also considered.

Possible Use of Non-purine Selective Xanthine Oxidoreductase Inhibitors for Prevention of Exercise-induced Acute Kidney Injury Associated with Renal Hypouricemia

Exercise-induced acute kidney injury (EIAKI) is frequently complicated with renal hypouricemia (RHUC). In patients with RHUC, limiting anaerobic exercise can prevent EIAKI. However, it is challenging to reduce exercise intensity in athletes. We herein report a 16-year-old Japanese football player with familial RHUC with compound heterozygous mutations in urate transporter 1 (URAT1) who presented with recurrent EIAKI. As prophylaxis (hydration during exercise) could not prevent EIAKI, febuxostat was initiated. EIAKI was not observed for 16 months despite exercising intensively. Hence, non-purine-selective xanthine oxidoreductase inhibitors may decrease the incidence of EIAKI in athletes with RHUC.

Xanthine Oxidase-A Personal History

A personal perspective is provided regarding the work in several laboratories, including the author's, that has established the reaction mechanism of xanthine oxidase and related enzymes.

Protein S-glutathionylation decreases superoxide/hydrogen peroxide production xanthine oxidoreductase

Our group has found that protein S-glutathionylation serves as an important feedback inhibitor for superoxide (O₂^●-)/hydrogen peroxide (H₂O₂) production by several mitochondrial dehydrogenases. Since cytoplasmic oxidases can also serve as important reactive oxygen species (ROS) sources, we hypothesized that glutathionylation can also inhibit O₂^●-/H₂O₂ by these enzymes. We first focused our attention on using a purified xanthine oxidase (XO) of bacterial origin to discern if glutathionylation can shut down ROS production by this enzyme. Incubating XO in glutathione disulfide (GSSG) at a final concentration of 1 mM did not significantly alter ROS production. Additionally, incubating samples in up to 10 mM GSSG increased ROS production. However, diamide and disulfiram titrations in the presence of 1 mM GSH revealed that both glutathionylation catalysts were able to abolish O₂^●-/H₂O₂ by XO. Exposure of XO to glutaredoxin-1 (GRX1) and GSSG did not alter the rate of O₂^●-/H₂O₂ production. However, incubation with GSH and purified glutathione S-transferase (GST) almost abolished ROS production by XO. Similar results were collected with rat liver cytoplasm. Indeed, diamide and disulfiram significantly decreased ROS production by xanthine oxidoreductase (XOR). Additionally, incubating the cytoplasm in GSH and GST led to a significant decrease in XOR activity. Immunoblot analyses revealed that immunoreactive bands corresponding to XOR were glutathionylated by diamide. Collectively, our findings demonstrate for the first time that cytoplasmic ROS sources, such as XOR, can also be inhibited by glutathionylation and these reactions are enzymatically mediated by GST. Additionally, we found that bacterial XO is also a target for glutathionylation.

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

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

Plasma Xanthine Oxidoreductase Activity Is Associated with a High Risk of Cardiovascular Disease in a General Japanese Population

The purpose of this study was to investigate the association between xanthine oxidoreductase (XOR) activity and a high risk of cardiovascular disease (CVD) in a general Japanese population. The Iwate Tohoku Medical Megabank Organization pooled individual participant data from a general population-based cohort study in Iwate prefecture. The cardiovascular risk was calculated using the Framingham Risk Score (FRS). A total of 1605 of the 1631 participants (98.4%) had detectable XOR activity. Multiple regression analysis demonstrated that XOR activity was independently associated with body mass index (β = 0.26, p < 0.001), diabetes (β = 0.09, p < 0.001), dyslipidemia (β = 0.08, p = 0.001), and uric acid (β = 0.13, p < 0.001). Multivariate analysis showed that the highest quartile of XOR activity was associated with a high risk for CVD (FRS ≥ 15) after adjustment for baseline characteristics (OR 2.93, 95% CI 1.16-7.40). The area under the receiver operating characteristic curves of the FRS with XOR activity was 0.81 (p = 0.008). XOR activity is associated with a high risk for CVD, suggesting that high XOR activity may indicate cardiovascular risk in a general Japanese population.

Purine Metabolite Signatures and Type 2 Diabetes: Innocent Bystanders or Actionable Items?

PURPOSE OF REVIEW

This article reviews evidence linking cardiometabolic conditions with changes in purine metabolites, including increased serum uric acid (sUA), and discusses intervention studies that investigated the therapeutic relevance of these associations.

RECENT FINDINGS

Metabolic and epidemiological findings support a correlation between sUA and circulating levels of other purines with insulin resistance (IR) and risk factors for cardiovascular disease (CVD). In addition, increased activity of xanthine oxidoreductase (XOR), the rate-limiting enzyme for UA production, has been detected in tissues targeted by obesity. Yet, inhibition of XOR in pre-clinical and clinical studies generally failed to support a causal role for excess sUA in IR and CVD. The lack of efficacy of XOR inhibitors strongly suggests that UA is a marker of, rather than a direct contributory factor for, cardiometabolic diseases. Validation of the function of other purines will require a paradigm shift, from a "UA-centric" view to a more granular assessment of the entire purine network and its interaction with other pathways.

Metabolic engineering of Escherichia coli cell factory for highly active xanthine dehydrogenase production

The aim of this work was to demonstrate the first proof-of-concept for the use of ab initio-aided assembly strategy intensifying in vivo biosynthesis process to construct Escherichia coli cell factory overproducing highly active xanthine dehydrogenase (XDH). Three global regulator (IscS, TusA and NarJ) and four chaperone proteins (DsbA, DsbB, NifS and XdhC) were overexpressed to aid the formation and ordered assembly of three redox center cofactors of Rhodobacter capsulatus XDH in E. coli. The NifS, IscS and DsbB enhanced the specific activity of RcXDH by 30%, 94% and 49%, respectively. The combinatorial expression of NarJ and IscS synergistically increased the specific activity by 129% and enhanced the total enzyme activity by a remarkable 3.9-fold. The crude enzyme showed nearly the same coupling efficiency of electron transfer and product formation as previously purified XDHs, indicating an integrity and efficient assembly of highly active XDH.

Complementation at the maroon-like eye-color locus of Drosophila melanogaster

Two "allelic" Drosophila melanogaster mutants which are deficient in xanthine dehydrogenase can complement one another in heterozygotes. This complementation is due to the production of small amounts of xanthine dehydrogenase, enough of which is present to restore the normal eye color. However, not enough of the enzyme is present to produce normal amounts of the enzyme products, or to reduce the accumulation of the enzyme substrates to levels found in wild-type flies.

REGULATION OF XANTHINE DEHYDROGENASE IN CHICK LIVER. EFFECT OF STARVATION AND OF ADMINISTRATION OF PURINES AND PURINE NUCLEOSIDES

1. The xanthine-dehydrogenase activity of chick liver, expressed per mg. of nitrogen, is increased during starvation. 2. Administration of inosine and possibly of adenine has a comparable effect on the xanthine dehydrogenase, and also induces an elevation of the total quantity of enzyme. Hypoxanthine, xanthine, guanine, xanthosine, guanosine and adenosine are ineffective. Cortisone is equally ineffective. 3. The administration of puromycin abolishes the effect of inosine and reduces that of starvation. It is concluded that inosine induces an increased synthesis of xanthine dehydrogenase, whereas during starvation the enzyme is spared with respect to other liver proteins. 4. The hypothesis is formulated that chick-liver xanthine dehydrogenase is an adaptive enzyme, its activity being regulated by inosine or by one of its metabolites.

Molybdenum EXAFS of the Desulfovibrio gigas Mo(2Fe-2S) protein--structural similarity to "desulfo" xanthine dehydrogenase

The molybdenum EXAFS of the Mo(2Fe-2S) protein from Desulfovibrio gigas has been examined using fluorescence detection and synchrotron radiation. In the oxidized form the molybdenum environment is found to contain two terminal oxo groups and two long (2.47 A) Mo-S bonds. Evidence was also found for an oxygen or nitrogen donor ligand at 1.90 A. Addition of dithionite to the oxidized enzyme results in loss of a terminal oxo group, perhaps due to protonation. In addition, a 0.1 A contraction in the Mo-S bond lengths is observed. The behavior of both oxidized and dithionite-treated forms is similar to that observed previously with "desulfo" xanthine oxidase.

Enhanced xanthine oxidase activity in mice treated with interferon and interferon inducers

Administration to mice of either interferon (IFN) or IFN inducers resulted in a marked increase of xanthine oxidase (XO) activity in different organs. Dose response studies revealed that serum XO was increased by administration of polyinosylic-polycyticylic acid (poly I-C) at doses as low as 0.1 mg/kg. In view of the well known ability of XO to generate superoxide radicals it is suggested that its induction might play a role in several biological effects of IFN.

Molybdopterin in carbon monoxide oxidase from carboxydotrophic bacteria

The carbon monoxide oxidases (COXs) purified from the carboxydotrophic bacteria Pseudomonas carboxydohydrogena and Pseudomonas carboxydoflava were found to be molybdenum hydroxylases, identical in cofactor composition and spectral properties to the recently characterized enzyme from Pseudomonas carboxydovorans (O. Meyer, J. Biol. Chem. 257:1333-1341, 1982). All three enzymes exhibited a cofactor composition of two flavin adenine dinucleotides, two molybdenums, eight irons and eight labile sulfides per dimeric molecule, typical for molybdenum-containing iron-sulfur flavoproteins. The millimolar extinction coefficient of the COXs at 450 nm was 72 (per two flavin adenine dinucleotides), a value similar to that of milk xanthine oxidase and chicken liver xanthine dehydrogenase at 450 nm. That molybdopterin, the novel prosthetic group of the molybdenum cofactor of a variety of molybdoenzymes (J. Johnson and K. V. Rajagopalan, Proc. Natl. Acad. Sci. U.S.A. 79:6856-6860, 1982) is also a constituent of COXs from carboxydotrophic bacteria is indicated by the formation of identical fluorescent cofactor derivatives, by complementation of the nitrate reductase activity in extracts of Neurospora crassa nit-l, and by the presence of organic phosphate additional to flavin adenine dinucleotides. Molybdopterin is tightly but noncovalently bound to the protein. COX, sulfite oxidase, xanthine oxidase, and xanthine dehydrogenase each contains 2 mol of molybdopterin per mol of enzyme. The presence of a trichloroacetic acid-releasable, so-far-unidentified, phosphorous-containing moiety in COX is suggested by the results of phosphate analysis.

The pterin of the molybdenum cofactor

The molybdenum cofactor common to a variety of molybdoenzymes has been shown to contain a novel pterin. The pterin has been isolated from sulfite oxidase from several sources, xanthine-oxidizing enzymes from milk and chicken liver, and nitrate reductase of Chlorella vulgaris after denaturation of the proteins in the presence of I2. Investigation of the anionic nature of the isolated pterin has revealed that it is a monophosphate ester susceptible to cleavage by alkaline phosphatase. Quantitative analyses have shown that one molecule of the pterin phosphate is associated with each molybdenum atom in sulfite oxidase. Studies to date have shown that the pterin is present in a reduced form in sulfite oxidase and xanthine dehydrogenase, and that in situ oxidation of the pterin leads to inactivation of sulfite oxidase.

Xanthine Dehydrogenase: Differences in Activity among Drosophila Strains

In Drosophila melanogaster, mutants at two loci are known to lack detectable amounts of xanthine dehydrogenase activity. These are the maroon-like eye-color locus on the X chromosome and the rosy-eye-color locus on the third chromosome (52+/-). A survey was made of the xanthine dehydrogenase content of 98 wild-type strains of D. melanogaster. One strain with 25 percent of the xanthine dehydrogenase activity found in normal flies is described. Strains with high xanthine dehydrogenase activity have also been obtained by selection.

Xanthine Dehydrogenase in Drosophila : Detection of Isozymes

Two or possibly three isozymes of the enzyme, xanthine dehydrogenase, have been detected in wildtype Drosophila melanogaster. by starchgel electrophoresis. Two genes, rosy and maroon-like, may control these isozymes by producing two different polypeptides that assemble in groups of three or four.

Convenient Assay of Xanthine Dehydrogenase in Single Drosophila melanogaster

Each fruit fly is homogenized in buffer and treated with charcoal. The enzyme is subsequently assayed by following the conversion of 2-amino-4-hydroxyptridine to isoxanthopterin in a sensitive fluorometer.