Studies on Xanthine Oxidase: The Dynamics of the Oxidase System

Natural mutations of human XDH promote the nitrite (NO2-)-reductase capacity of xanthine oxidoreductase: A novel mechanism to promote redox health?

Several rare genetic variations of human XDH have been shown to alter xanthine oxidoreductase (XOR) activity leading to impaired purine catabolism. However, XOR is a multi-functional enzyme that depending upon the environmental conditions also expresses oxidase activity leading to both O2·- and H2O2 and nitrite (NO2-) reductase activity leading to nitric oxide (·NO). Since these products express important, and often diametrically opposite, biological activity, consideration of the impact of XOR mutations in the context of each aspect of the biochemical activity of the enzyme is needed to determine the potential full impact of these variants. Herein, we show that known naturally occurring hXDH mutations do not have a uniform impact upon the biochemical activity of the enzyme in terms of uric acid (UA), reactive oxygen species (ROS) and nitric oxide ·NO formation. We show that the His1221Arg mutant, in the presence of xanthine, increases UA, O2·- and NO generation compared to the WT, whilst the Ile703Val increases UA and ·NO formation, but not O2·-. We speculate that this change in the balance of activity of the enzyme is likely to endow those carrying these mutations with a harmful or protective influence over health that may explain the current equipoise underlying the perceived importance of XDH mutations. We also show that, in presence of inorganic NO2-, XOR-driven O2·- production is substantially reduced. We suggest that targeting enzyme activity to enhance the NO2--reductase profile in those carrying such mutations may provide novel therapeutic options, particularly in cardiovascular disease.

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.

Application of EPR and related methods to molybdenum-containing enzymes

A description is provided of the contributions made to our understanding of molybdenum-containing enzymes through the application of electron paramagnetic resonance spectroscopy and related methods, by way of illustrating how these can be applied to better understand enzyme structure and function. An emphasis is placed on the use of EPR to identify both the coordination environment of the molybdenum coordination sphere as well as the structures of paramagnetic intermediates observed transiently in the course of reaction that have led to the elucidation of reaction mechanism.

Uric acid and allopurinol aggravate absence epileptic activity in Wistar Albino Glaxo Rijswijk rats

Uric acid has a role in several physiological and pathophysiological processes. For example, uric acid may facilitate seizure generalization while reducing uric acid level may evoke anticonvulsant/antiepileptic effects. Allopurinol blocks the activity of xanthine oxidase, by which allopurinol inhibits catabolism of hypoxanthine to xanthine and uric acid and, as a consequence, decreases the level of uric acid. Although the modulation of serum uric acid level is a widely used strategy in the treatment of certain diseases, our knowledge regarding the effects of uric acid on epileptic activity is far from complete. Thus, the main aim of this study was the investigation of the effect of uric acid on absence epileptic seizures (spike-wave discharges: SWDs) in a model of human absence epilepsy, the Wistar Albino Glaxo/Rijswijk (WAG/Rij) rat. We investigated the influence of intraperitoneally (i.p.) injected uric acid (100 mg/kg and 200 mg/kg), allopurinol (50 mg/kg and 100 mg/kg), a cyclooxygenase 1 and 2 (COX-1 and COX-2) inhibitor indomethacin (10 mg/kg) and inosine (500 mg/kg) alone and the combined application of allopurinol (50 mg/kg) with uric acid (100 mg/kg) or inosine (500 mg/kg) as well as indomethacin (10 mg/kg) with uric acid (100 mg/kg) and inosine (500 mg/kg) with uric acid (100 mg/kg) on absence epileptic activity. We demonstrated that both uric acid and allopurinol alone significantly increased the number of SWDs whereas indomethacin abolished the uric acid-evoked increase in SWD number. Our results suggest that uric acid and allopurinol have proepileptic effects in WAG/Rij rats.

"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.

Electronic structure contributions to reactivity in xanthine oxidase family enzymes

We review the xanthine oxidase (XO) family of pyranopterin molybdenum enzymes with a specific emphasis on electronic structure contributions to reactivity. In addition to xanthine and aldehyde oxidoreductases, which catalyze the two-electron oxidation of aromatic heterocycles and aldehyde substrates, this mini-review highlights recent work on the closely related carbon monoxide dehydrogenase (CODH) that catalyzes the oxidation of CO using a unique Mo-Cu heterobimetallic active site. A primary focus of this mini-review relates to how spectroscopy and computational methods have been used to develop an understanding of critical relationships between geometric structure, electronic structure, and catalytic function.

Mechanism of Substrate and Inhibitor Binding of Rhodobacter capsulatus Xanthine Dehydrogenase

Rhodobacter capsulatus xanthine dehydrogenase (XDH) is an (alphabeta)(2) heterotetrameric cytoplasmic enzyme that resembles eukaryotic xanthine oxidoreductases in respect to both amino acid sequence and structural fold. To obtain a detailed understanding of the mechanism of substrate and inhibitor binding at the active site, we solved crystal structures of R. capsulatus XDH in the presence of its substrates hypoxanthine, xanthine, and the inhibitor pterin-6-aldehyde using either the inactive desulfo form of the enzyme or an active site mutant (E(B)232Q) to prevent substrate turnover. The hypoxanthine- and xanthine-bound structures reveal the orientation of both substrates at the active site and show the importance of residue Glu(B)-232 for substrate positioning. The oxygen atom at the C-6 position of both substrates is oriented toward Arg(B)-310 in the active site. Thus the substrates bind in an orientation opposite to the one seen in the structure of the reduced enzyme with the inhibitor oxypurinol. The tightness of the substrates in the active site suggests that the intermediate products must exit the binding pocket to allow first the attack of the C-2, followed by oxidation of the C-8 atom to form the final product uric acid. Structural studies of pterin-6-aldehyde, a potent inhibitor of R. capsulatus XDH, contribute further to the understanding of the relative positioning of inhibitors and substrates in the binding pocket. Steady state kinetics reveal a competitive inhibition pattern with a K(i) of 103.57 +/- 18.96 nm for pterin-6-aldehyde.

The action of cyanide and other respiratory inhibitors on xanthine oxidase

It was stated by Dixon and Thurlow (1925) that xanthine oxidase is not inhibited by cyanide. They found that concentrations of cyanide up to M/100 did not produce any inhibition of either the uptake of oxygen or the reduction of methylene blue by hypoxanthine in presence of the enzyme. Some inhibition was, however, observed with higher concentrations of cyanide. During the course of some other work on xanthine oxidase, Dixon and Lemberg, using the methylene blue technique, observed in 1931 (unpublished) that even small concentrations of cyanide completely inactivated the oxidase, provided that it was incubated with the cyanide for some time at 37° before testing. Similar results were later obtained by Keilin (unpublished) for O 2 uptake. A reinvestigation of the action of cyanide on the xanthine oxidase was therefore desirable, and the results obtained are described in the present paper. It has been found that the action of cyanide on this oxidase shows a number of peculiar features which distinguish it from the other cases of cyanide inhibition hitherto known.

A re-evaluation of the tissue distribution and physiology of xanthine oxidoreductase

Xanthine oxidoreductase is an enzyme which has the unusual property that it can exist in a dehydrogenase form which uses NAD+ and an oxidase form which uses oxygen as electron acceptor. Both forms have a high affinity for hypoxanthine and xanthine as substrates. In addition, conversion of one form to the other may occur under different conditions. The exact function of the enzyme is still unknown but it seems to play a role in purine catabolism, detoxification of xenobiotics and antioxidant capacity by producing urate. The oxidase form produces reactive oxygen species and, therefore, the enzyme is thought to be involved in various pathological processes such as tissue injury due to ischaemia followed by reperfusion, but its role is still a matter of debate. The present review summarizes information that has become available about the enzyme. Interpretations of contradictory findings are presented in order to reduce confusion that still exists with respect to the role of this enzyme in physiology and pathology.

Substrate inhibition of xanthine oxidase and its influence on superoxide radical production

The influence of substrate inhibition on xanthine oxidase-intramolecular electron transport was studied by steady-state kinetic analysis. Experiments with hypoxanthine and xanthine up to 900 microM indicated an inhibition pattern which fitted an equation of the general form nu 0 = nu max . [S]/(Km + a[S] + b[S]2/Ki). Univalent electron flux to oxygen was favored at substrate concentrations above 50 microM. This augmentation of univalent flux percentage that appeared at a high substrate concentration was greater for hypoxanthine that xanthine and at pH 8.3 than at 9.5. Our results support a mechanism of inhibition in which a substrate-reduced enzyme, non-productive Michaelis complex was formed. It is possible that this non-productive complex favored the univalent pathway of enzyme reoxidation (superoxide production) by increasing the midpoint redox potential of the molybdenum active site.

Studies on the specificity toward aldehyde substrates and steady-state kinetics of xanthine oxidase

The aldehyde specificity of xanthine oxidase (xanthine:oxygen oxidoreductase, EC 1.2.3.2) has been reinvestigated. The biogenic aldehydes and succinate semialdehyde are reasonable substrates for xanthine oxidase. Pyrophosphate, which binds to xanthine oxidase, does not seem to affect significantly the enzyme's catalytic activity. The steady-state parameters for the oxidation of several substrates by xanthine oxidase and oxygen have been determined. Formaldehyde differs from xanthine and other aldehydes in phi 2, the parameter describing the reaction with oxygen. Substrate inhibition has been studied at high concentrations of xanthine with oxygen as the electron acceptor. The inhibition is hyperbolic and uncompetitive with respect to oxygen. This is possibly due to rate-limiting product release from molybdenum(IV) being slower than from molybdenum(VI).

Xanthine dehydrogenase activity in the clawed frog, Xenopus laevis

Xanthine dehydrogenase (XDH; EC 1.2.1.37) activity in the clawed frog, Xenopus laevis, was detected in kidney tissue homogenates, but not in skin, liver, ovaries or gut tissues. The enzyme migrated as a single band of activity on both polyacrylamide and starch gel electropherograms, exhibited substrate inhibition, and did not appear developmentally until feeding larval stages. The tissue specificity, post-fertilization stage of appearance and single isozymic form make this a useful enzyme marker for further study concerning its developmental appearance and maintenance as a kidney-specific protein.