Uric acid enzyme-catalyzed oxidation

Xanthine dehydrogenase, XDH [E.C. 1.1.1.204], and the closely related isoenzyme xanthine oxidoreductase, XOR [E.C. 1.1.3.22], are molybdopterin oxidoreductive enzymes. The enzymes catalyze oxidation of a C-substrate atom by two electrons. In human body the enzymes convert the nucleic acid metabolite hypoxanthine into xanthine and xanthine into uric acid. Fig. 10.6 The reductive rate constant of XDH,... 

The uric acid formed from nucleic acid degradation of endogenous or exogenous nucleic acids is excreted as such without chemical alteration in higher apes and man. (In the dalmatian uric acid is partly oxidized and partly excreted.) In all other mammals, uric acid is further oxidized to yield allantoin, and uricase is the enzyme that catalyzes that reaction. In amphibians, fish, and many invertebrates, two more enzymes exist one involved in the oxidation of allantoin to allantoic acid (allantoinase), the other catalyzing the splitting of allantoic acid to yield urea and glyoxylic acid. 

The answer is c. (Hardman, pp 649—650.) Acute hyperuricemia, which often occurs in patients who are treated with cytotoxic drugs for neoplasic disorders, can lead to the deposition of urate crystals in the kidneys and their collecting ducts. This can produce partial or complete obstruction of the collecting ducts, renal pelvis, or ureter. Allopurinol and its primary metabolite, alloxanthine, are inhibitors of xanthine oxidase, an enzyme that catalyzes the oxidation of hypo xanthine and xanthine to uric acid. The use of allopurinol in patients at risk can markedly reduce the likelihood that they will develop acute uric acid nephropathy. 

Xanthine oxidoreductase (XOR) is a molybdenum-containing complex homodimeric 300-kDa cytosolic enzyme. Each subunit contains a molybdopterin cofactor, two nonidentical iron-sulfur centers, and FAD (89). The enzyme has an important physiologic role in the oxidative metabolism of purines, e.g., it catalyzes the sequence of reactions that convert hypoxanthine to xanthine then to uric acid (Fig. 4.36). 

Purine oxidation. The oxidation of purines and purine derivatives is catalyzed by xanthine oxidase. For example, the enzyme oxidizes hypoxanthine to xanthine and thence uric acid (Fig. 4.34). Xanthine oxidase also catalyzes the oxidation of foreign compounds, such as the nitrogen heterocycle phthalazine (Fig. 4.35). This compound is also a substrate for aldehyde oxidase, giving the same product. 

NAD+. They catalyze the hydroxylation of purines. Equation (50) shows the xanthine oxidase-catalyzed oxidation of xanthine to uric acid. Xanthine oxidase is probably the most well-studied molybdenum enzyme. There is good evidence that the molybdenum is the site for binding and reduction of xanthine. The enzyme contains MoVI in the resting form, while MoIV and Mov are implicated during catalysis. 

Mercaptopurine and thioguanine are both given orally (Table 55-3) and excreted mainly in the urine. However, 6-MP is converted to an inactive metabolite (6-thiouric acid) by an oxidation catalyzed by xanthine oxidase, whereas 6-TG requires deamination before it is metabolized by this enzyme. This factor is important because the purine analog allopurinol, a potent xanthine oxidase inhibitor, is frequently used with chemotherapy in hematologic cancers to prevent hyperuricemia after tumor cell lysis. It does this by blocking purine oxidation, allowing excretion of cellular purines that are relatively more soluble than uric acid. Nephrotoxicity and acute gout produced by excessive uric acid are thereby prevented. Simultaneous therapy with allopurinol and 6-MP results in excessive toxicity unless the dose of mercaptopurine is reduced to 25% of the usual level. This effect does not occur with 6-TG, which can be used in full doses with allopurinol. 

The cofactors of both xanthine and aldehyde oxidases belong to the LMoVI(S)(0) subfamily (see Section IV). However, inactive dioxo forms, LMovi(0)2, of both xanthine and aldehyde oxidase are known. These dioxo forms do not catalyze oxidation of the respective substrates of these enzymes. The Mov/Molv redox potential for the inactive bis(oxido) form of xanthine oxidase differs from the oxido-sulfido form by -30 mV (bovine xanthine oxidase) and -lOOmV (chicken liver xanthine oxidase) [91]. Although the difference is small, given the xanthine/uric acid reduction potential (-360 mV), it is possible that the Mov/MoIV couple (-433 mV) of the chicken-liver xanthine oxidase bis(ox-ido) form impedes the effective oxidation of xanthine for redox reasons alone. However, the bis(oxido) form of bovine xanthine oxidase (with a reduction potential of -386 mV) should be able to oxidize xanthine, since the redox potential, and hence the thermodynamic driving force, is sufficient for activity [91,92,99]. As substrate oxidation does not occur, the chemical differences between the bis(oxido) and oxido-sulfido (Movl) forms must be critical to the dramatic difference in activity (see Section VI.E.l). 

Xanthine oxidase is able to bind allopurinol and catalyze one oxidation, converting it to a compound that is similar to xanthine. However, after that conversion, the enzyme is trapped in an inactive oxidation state and can t carry out its normal function of forming uric acid. Additionally, allopurinol inhibits the de novo ( new, from other compounds not recycled) synthesis of purines, further decreasing the amount of uric acid formed in the blood. 

Xanthine oxidase catalyzes the oxidation of hypox-anthine and xanthine to uric acid. Xanthine oxidase is a complex metalloflavoprotein containing one molybdenum, one FAD and two iron-sulfur centers of the ferredoxine type in each of its two independent subunits. Usually, the enzyme is isolated from cow s milk. The enzyme is inhibited by allopurinol and related compounds. The production of uric acid from the substrate (xanthine) can be determined by measuring the change in optical density in the UV range. 

The xanthine oxidoreductases are large, complex molybdo-flavoproteins with roles in the catabolism of purines, for example, oxidizing hypoxanthine to xanthine and xanthine to uric acid (equation 9). Xanthine oxidase can also catalyze the reduction of nitrate to nitrite (or in the presence superoxide, peroxynitrite) and the reduction of nitrite to nitric oxide. Peroxynitrite, a powerfiil and destructive oxidant, has been implicated in diseases such as arthritis, atherosclerosis, multiple sclerosis, and Alzheimer s and Parkinson s diseases. The microbicidal role of milk and intestinal xanthine oxidase may also involve the generation of peroxynitrite in the gut. The high levels of the enzyme in the mammary glands of pregnant... 

This enzyme catalyzes die oxidation of xanthine to uric acid ... 

Xanthine dehydrogenase The enz)une is used in the catabolism of the purine ring. It catalyzes the NAE>-dependent oxidation of xanthine to uric acid. The enzyme also contains FAD and molybdenum. A fraction of the enzyme normally occurs in the body as xanthine oxidase, which represents an altered form of the enzyme. Xanthine oxidase uses O2 as an oxidant, rather than NAD. Xanthine oxidase converts xanthine to uric acid, and O2 to HOOH and the hydroxyl radical. 

In a commercially available assay, serum NTP catalyzes the hydrolysis of IMP to yield inosine, which is then converted to hypoxanthine by purine-nucleoside phosphorylase (EC 2.4.2.1). Hypoxanthine is oxidized to urate with xanthine oxidase (EC 1.2.3.2). Two moles of hydrogen peroxide are produced for each mole of hypoxanthine liberated and converted to uric acid. The formation rate of hydrogen peroxide is monitored by a spectrophotometer at 510nm by the oxidation of a chromogenic system. The effect of ALPs on IMP is inhibited by p-glycerophosphate. This material is substrate for ALP but not for NTP, and by forming substrate complexes with the former enzyme, it reduces the proportion of the total ALP activity that is directed to the hydrolysis of the NTP substrate, IMP. ... 

In most cells, more than 90% of the oxygen utilized is consumed in the respiratory chain that is coupled to the production of ATP. However, electron transport and oxygen utilization occur in a variety of other reactions, including those catalyzed by oxidases or oxygenases. Xanthine oxidase, an enzyme involved in purine catabolism (Chapter 27), catalyzes the oxidation of hypoxanthine to xanthine, and of xanthine to uric acid. In these reactions, reducing equivalents are transferred via FAD, and Fe and Mo " ", while the oxygen is converted to superoxide anion (O2) ... 

Laccase also catalyzes the 02-dependent oxidation of ascorbic acid, ferrocyanide, iodide, and uric acid. These reactions have been utilized to eliminate electrochemical interferences in amperometric hydrogen peroxide detection at membrane-covered enzyme electrodes (Wollen-berger et al., 1986). The capacity of the laccase membrane to oxidize ferrocyanide has been characterized by anodic oxidation of ferrocyanide at +0.4 V (Fig. 62). When a fresh enzyme membrane is used, a current signal appears only at substrate concentrations above 5 mmolA the current increases linearly with increasing concentration. This threshold concentration decreases with increasing membrane age until the remaining enzyme activity is too low for complete substrate oxidation. 

The enzyme used for the assay of uric acid is uricase (urate oxidase, EC 1.7.3.3) which catalyzes the oxidation of uric acid to allantoin, CO2, and H2O2 ... 

Aldehyde oxidase is structurally and chemically similar to xanthine oxidase, and both enzymes exhibit a similar distribution between tissues and share many common substrates, despite clear differences in certain catalytic properties. Thus, although both enzymes catalyze the oxidation of hypoxanthine to xanthine, conversion of the latter to uric acid is accomplished only by xanthine oxidase. 

Fig. 8.19. Allopurinol is a suicide inhibitor of xanthine oxidase. A. Xanthine oxidase catalyzes the oxidation of hyrpoxanthine to xanthine, and xanthine to uric acid (urate) in the pathway for degradation of purine nucleotides. B. The oxidations are performed by a molybdenum-oxo-sul-fide coordination complex in the active site that complexes with the group being oxidized. Oxygen is donated from water. The enzyme can work either as an oxidase (O2 accepts the 2e and is reduced to H2O2) or as a dehydrogenase (NAD accepts the 2e and is reduced to NADH.) C. Xanthine oxidase is able to perform the first oxidation step and convert allopurinol to alloxanthine (oxypurinol). As a result, the enzyme has committed suicide the oxypurinol remains bound in the molybdenum coordination sphere, where it prevents the next step of the reaction.

The pterin-containing molybdenum enzymes catalyze particularly the oxidation ofC-H compounds [56, 57], For example, xanthine oxidase transforms xanthine into uric acid according to the equation ... 

The enzymic oxidation of uric acids with peroxidase (type VIII from horseradish peroxidase) has recently been studied and critically compared to the electrochemical oxidation. Peroxidase was studied because this enzyme will oxidize not only uric acid but also most of its N-methyl derivatives. Typical spectra of uric acid obtaine d during the peroxidase-catalyzed oxidation of uric acid are shown in Figure 27. Curve 1 is the spectrum of uric acid. Addition of hydrogen peroxide and peroxidase causes the initial rapid decrease of the spectrum because of dilution. However, it is quite clear from Figure 27... 

For comparitive purposes, it is worthwhile reviewing the information available from purely biochemical studies of the peroxidase-catalyzed oxidation of uric acids. In fact, more detailed studies have been carried out on the uricase-catalyzed oxidation of uric acid. " However, this enzyme is virtually specific for uric acid and its mechanism has not been studied using the electrochemical approach outlined earlier for the case of the peroxidase enzymes. 

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