Xanthine oxidase EC

It has been recognized for about 80 years that milk contains an enzyme capable of oxidizing aldehydes and purines. The enzyme is now generally referred to as xanthine oxidase (XO) milk is a very good source of XO, at 

Isolation. Numerous methods have been developed for the purification of XO from milk since the enzyme is concentrated in the MFGM, in which it is one of the principal proteins, all methods employ cream as starting material, use a dissociating agent to liberate XO from membrane lipoproteins and some form of chromatography for further purification. 

Milk XO has a molecular weight of c. 300kDa and consists of two subunits. The pH optimum is about 8.5 and the enzyme requires flavin adenine dinucleotide (FAD), Fe, Mo and an acid-labile compound as co-factors cows deficient in Mo have low XO activity. The amino acid composition of XO has been determined by a number of workers at least five genetic polymorphic forms have been reported. 

which can excite stable triplet oxygen ( Oj), is a pro-oxidant. Milk which undergoes spontaneous rancidity contains about 10 times the normal level of XO, and spontaneous oxidation can be induced in normal milk by the addition of XO to about four times normal levels. Heat-denatured or flavin-free enzyme is ineffective and the susceptibility of unsaturated fatty acids to oxidation increases with the degree of unsaturation. 

It has been suggested that XO from homogenized milk enters the vascular system and may be involved in atherosclerosis via oxidation of plasmalo-gens (Appendix 3B) in cell membranes. However, the experimental evidence in support of this view is very weak and the hypothesis has been disclaimed (see Farkye, 1992). 

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An EPR signal, characteristic for the superoxide radical, was observed by the rapid-freezing technique in the oxidation at pH 10 of xanthine by dioxygen catalysed by xanthine oxidase (EC 1,2.3.2) The enzymatic reduction of dioxygen by aldehyde oxidase (EC 1.2.3.1) produces also the superoxide radical. 

Hypoxanthine, xanthine HsO, Oj Xanthine oxidase (EC 1.2.3.2) 7.8 Xanthine, uric acid... 

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. ... 

Watanabe et al. (1986) developed a sequence sensor for the successive assay of hypoxanthine (HX) and inosine (HXR) by arranging nucleoside phosphorylase (EC 2.4.2.1) and xanthine oxidase (EC 1.2.3.2) in spatially separated layers in front of an oxygen probe. Nucleoside phosphorylase was... 

Other oxidoreductases that can play a major or less important role in drug metabolism are hemoglobin, monoamine oxidases (EC 1.4.3.4 MAO-A and MAO-B), which are essentially mitochondrial enzymes, the cytosolic molybdenum hydroxylases (xanthine oxidase, EC 1.1.3.22 xanthine dehydrogenase, EC 1.1.1.204 and aldehyde oxidase, EC 1.2.3.1), d the broad group of copper-containing amine oxidases (EC 1.4.3.6) (36-39). 

Xanthine oxidase (EC 1.2.3.2) catalyzes the formation of uric acid, an end-product of purine catabolism. The mammalian enzyme is a metalloflavoprotein composed of two subunits containing molybdenum, FAD and Fe/S clusters as prosthetic groups in a ratio of 1 1 4 per subunit (1). Besides its endogenous metabolic function, xanthine oxidase is also active toward a wide spectrum of oxidizable xenobiotic substrates. Although some cestodes and trematodes produce trace amounts of uric acid (16), the presence of xanthine oxidase activity in these organisms has not been demonstrated. Xanthine oxidase was found in the cytosolic fractions of the nematodes Ancylostoma ceylanicum and Nippostrongylus brasiliensis (17), but its activity toward xenobiotic substrates was not tested. 

Dioxide(l-), superoxide, (O2J (O2) is mainly produced in biological systems through one-electron reduction of triplet O2 mediated by enzymes (Fig. 2). In brain, xanthine oxidase (EC... 

Xanthine oxidase (EC 1.2.3.2). Failure to convert xanthine to uric acid (see Purine degradation). Xanthine therefore replaces uric acid as end product of purine metabolism. Urinary xanthine greatly increased. Urinary uric acid abnormally low. Xanthine calculi tend to form in renal tract. 

Molybdoenzymns At present, 6 oligomeric oxi-doreductases are known, which contain Mo as an essential constituent 1. Nitrogenase (see) 2. Nitrate reductase, EC 1.6.6.3 (see) 3. Xanthine oxidase, EC 1.2.3.2 (see), from animals and bacteria 4. Aldehyde oxidase, EC 1.2.3.1 from animal liver, which catalyses the reaction R-CHO + HjO R-COOH + 2H + 2e 5. Sulfite oxidase, EC 1.18.3.1 from mammalian and bird liver (Af, 114,000 2 subunits), which catalyses the reaction S03 + HjO-> SO/ + 2H + 2e this enzyme also contains a b5-like cytochrome and passes electrons directly to cytochrome c in the respiratory chain and 6. Formate dehydrogenase, EC 1.2.1.2, a membrane-bound protein from E. coli, containing one atom each of molybdenum and selenium, one heme group and nonheme iron-sulfur centers. It is NAD -dependent and catalyses the reaction HCOO + NAD CO2 -I- NADH. 

Aerobic P. d. The amino groups of adenine and guanine are removed hydrolytically by specific deaminases, which attack the free bases, the nucleosides or the nucleotides (Fig.). Uric acid is then produced by the action of xanthine oxidase (EC 1.2.3.2), which is the key enzyme of aerobic P.d. In humans and apes, the uric acid is excreted largely unchanged. In most reptiles and mammals, it is oxidized to allantoin by uricase (EC 1.73.3) (uricolysis). 

Due to the presence of an intermediate electron carrier in the incubation medium, activity of both xanthine oxidoreductase and xanthine oxidase (EC 1.1.3.22) are detected. 

The molybdoenzyme, xanthine oxidoreductase is a homodimer of 150 kDa subunits and exists in two interconvertible forms, dehydrogenase (EC 1.1.1.204) and xanthine oxidase (EC 1.1.3.22). Reduction of oxygen by either form of the enzyme yields superoxide radical anion and hydrogen peroxide with xanthine or hypoxanthine as substrates. Xanthine dehydrogenase preferentially reduces NAD whereas xanthine oxidase does not reduce NAD preferring molecular oxygen. 

Cu (2-10 p.M) catalysed the conversion of xanthine dehydrogenase (EC 1.1.1.204) to xanthine oxidase (EC 1.1.3.22) which was prevented by oleic acid, arachidonic acid, eicosapentaenoic acid and docosahexaenoic add (50-200 iM) as shown in the perfused rabbit Uver (Fujita et al. 1995). 

Both normoxic and hypoxic bovine pulmonary microvascular endothelial cells constitutively released xanthine oxidase (EC 1.1.3.22) activity into their culture media (Partridge et al. 1992). Incubation of hypoxic or normoxic bovine pulmonary microvascular endothelial cells with oxygenated medium (95% O2) stimulated the release of xanthine oxidase activity into the extracellular medium within 5 min. The xanthine oxidase activity could not be detected in the oxygenated medium after 60 min incubation with 95 % Oj. Oestradiol (10 iM) almost completely inhibited xanthine oxidase and dehydrogenase activities in both normoxic and hypoxic rat pulmonary microvascular endothelial cells (Kayyali et al. 1999). Dexamethasone increased the activity of xanthine oxidase, and also induced xanthine oxidase promoter activation in... 

DERiKS and Vreeling-SindelArovA 2002). Xanthine oxidase (EC 1.1.3.22) activity was not found in Kupffer cells and sinusoidal endothelial cells. 

Xanthine dehydrogenase (EC 1.1.1.204), the enzymatic precursor of xanthine oxidase (EC 1.1.3.22) reacts with doxorubicin via a two-electron reduction (Yee and Pritsos 1997). This reduction is different from the modified and more extensively studied form xanthine oxidase, which reacts with doxorubicin via a one-electron reduction. Under hypoxic conditions, the formation of large quantities of 7-deoxydoxorubicin aglycone, a deactivation product of doxorubicin metabolism, may serve to moderate the antineoplastic activity of doxorubicin. Under aerobic conditions, however, xanthine dehydrogenase activation led to a greater rate of formation of oxygen radicals than xanthine oxidase thereby possibty potentiating the cytotoxicity of doxorubicin to aerobic tumour cells. 

Xanthine oxidase (EC 1.2.3.2) catalyzes the irreversible oxidation of hypoxanthine and xanthine to uric acid. Recently, we have shown that rat liver xanthine oxidase enzyme activity is, in part, dependent on both age-and sex-specific differences. Immaturity in both sexes, adult females and pubertal male castrates demonstrate a basal or feminine pattern of enzyme activity. Androgen is required in the pubescent period for the full expression of hepatic xanthine oxidase activity in the adult male. The effect of androgen exposure on hepatic enzyme activity, however, remains speculative. 

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