Molybdenum hydroxylase xanthine oxidase

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). 

As indicated in Fig. 25-18, free adenine released from catabolism of nucleic acids can be deaminated hydrolytically to hypoxanthine, and guanine can be deaminated to xanthine.328 The molybdenum-containing xanthine oxidase (Chapter 16) oxidizes hypoxanthine to xanthine and the latter on to uric acid. Some Clostridia convert purine or hypoxanthine to xanthine by the action of a selenium-containing purine hydroxylase.3283 Another reaction of xanthine occurring in some plants is conversion to the trimethylated derivative caffeine. 328b One of the physiological effects of caffeine in animals is inhibition of pyrimidine synthesis.329 However, the effect most sought by coffee drinkers may be an increase in blood pressure caused by occupancy of adenosine receptors by caffeine.330... 

Molybdenum Hydroxylases (Aldehyde Oxidase, Xanthine Oxidase) Oxidations Purines, pteridine, methrotrexate, quinolones, 6-deoxycyclovir... 

The phase I reactions are mediated primarily by liver enzymes such as cytochrome P450 (CYP450), FAD-containing mono-oxygenase (FMO), monoamine oxidase (MAO), molybdenum hydroxylase (aldehyde oxidase/xanthine oxidase AO/XO), aldo-ketoreductase (AKR), epoxide hydrolase (EH), and esterase. 

A pyranopterin cofactor, consisting of a bicyclic pterin fused to a monocyclic pyran ring, is known to directly coordinate the molybdenum of xanthine oxidase through a dithiolene side chain. The pyranopterin cofactor probably does not directly participate in the catalytic sequence of molybdenum hydroxylases but has been implicated in mediating electron transfer to other redox-active cofactors and/or modulating the reduction potential of the molybdenum. 

Another Mossbauer study on molybdenum hydroxylases was performed on a nonenriched sample of milk xanthine oxidase (219), and an unusually large AEq (3.2 mm/s at 175 K) was also observed for the ferrous site of one of the clusters. 

Xanthine oxidase (XO) was the first enzyme studied from the family of enzymes now known as the molybdenum hydroxylases (HiUe 1999). XO, which catalyzes the hydroxylation of xanthine to uric acid is abundant in cow s milk and contains several cofactors, including FAD, two Fe-S centers, and a molybdenum cofactor, all of which are required for activity (Massey and Harris 1997). Purified XO has been shown to use xanthine, hypoxan-thine, and several aldehydes as substrates in the reduction of methylene blue (Booth 1938), used as an electron acceptor. Early studies also noted that cyanide was inhibitory but could only inactivate XO during preincubation, not during the reaction with xanthine (Dixon 1927). The target of cyanide inactivation was identified to be a labile sulfur atom, termed the cyanolyzable sulfur (Wahl and Rajagopalan 1982), which is also required for enzyme activity. 

The monooxygenase group of enzymes includes the non-P450 hydroxylases which catalyze the insertion of a hydroxyl group to replace a hydrogen atom at a saturated carbon [6-8] and the non-heme-dependent oxygenases such as the flavin-molybdenum-cobalt-dependent xanthine oxidase and aldehyde oxidase... 

Fe2S2] clusters are part of the molybdenum containing hydroxylases. Typically, apart from molybdenum and two EPR-distinct iron-sulfur centres there can be FAD as additional cofactor. In Chlostridium purinolyticum a selenium-dependent purine hydroxylase has been characterized as molybdenum hydroxylase. The EPR of the respective desulfo molybdenum (V) signal indicated that the Mo-ligands should differ from those of the well known mammalian corollary xanthine oxidase.197 For the bacterial molybdenum hydroxylase quinoline oxidoreductase from Pseudomonas putida an expression system was developed in order to be able to construct protein mutants for detailed analysis. EPR was used to control the correct insertion of the cofactors, specifically of the two [Fe2S2] clusters.198... 

The rationale for studies on flavin semiquinone metal interactions stems from the presence of flavin coenzymes which participate in electron transfer in a number of metalloflavoproteins. Iron-containing redox centers such as the heme and nonheme iron sulfur prosthetic groups (Fe2/S2, Fe+ZS, or the rubredoxin-type of iron center) constitute the more common type of metal donor-acceptor found in metalloflavoproteins, although molybdenum is encountered in the molybdenum hydroxylases (e.g. xanthine oxidase, aldehyde dehydrogenase). 

In addition to these classical aromatic ring hydroxylations, many nitrogen heterocycles are substrates for molybdenum-containing enzymes, such as xanthine oxidase and aldehyde oxidase, which are present in the hepatic cytosolic fractions from various animal species. The molybdenum hydroxylases (B-75MI10902) catalyze the oxidation of electron-deficient carbons in aromatic nitrogen heterocycles. The reactions catalyzed by these enzymes are generally represented by equations (2) and (3). 

Coughlan, M. P. 1980. Aldehyde oxidase, xanthine oxidase and xanthine dehydrogenase. Hydroxylases containing molybdenum, iron-sulphur and flavin. In Molybdenum and Molybdenum-Containing Enzymes. M.P. Coughlan (Editor). Pergamon Press, Oxford, pp. 119-185. 

Of all of the molybdenum enzymes, mammalian xanthine oxidase/dehydrogenase has been the most studied (Figure 15). These studies, along with those of other members of this relatively large class of hydroxylases (Table la-c), suggest that all molybdenum enzymes that catalyze hydroxylation of C—H bonds contain a common structural motif. This motif is unique in high-valent molybdenum chem-... 

There are a number of molybdenum-containing enzymes, but those that are important in carbon oxidation of xenobiotics are aldehyde oxidase (AO) and xanthine oxidase (XO), also referred to as molybdenum hydroxylases (Figure 10.7). Both enzymes catalyze the oxidation of a wide range of aldehydes and N-heterocycles. The name aldehyde oxidase is somewhat misleading, however, because oxidation of heteroaromatics is more significant. The differences in substrate specificities between... 

The molybdenum-containing oxidoreductases that catalyze Eq. (1) have been variously termed molybdenum hydroxylases (6), oxotransferases (7), and oxo-type molybdenum enzymes (8). Molybdenum hydroxylase aptly describes the conversion of xanthine to uric acid, but the name seems less appropriate for the reactions catalyzed by sulfite oxidase and nitrate reductase oxotransferase implies that the function of these enzymes is to transfer oxo groups, even though relatively little is known about their actual mechanism of action and the name oxo-type molybdenum enzyme recognizes both the apparent oxo transfer chemistry of Eq. (1) and the fact that the molybdenum atom in each of these enzymes contains at least one terminal oxo group. In this chapter, we shall refer to these enzymes as pterin-containing molybdenum enzymes because a 6-substituted pterin appears to be a common chemical feature of all of the enzymes. 

The cytosolic molybdenum hydroxylases, namely aldehyde oxidase and xanthine oxidoreductase, which exist in a dehydrogenase form (XDH) and an oxidase form (XO). 

Other enzymes may also be involved in the oxidation of aldehydes, particularly aldehyde oxidase and xanthine oxidase, which belong to the molybdenum hydroxylases. These enzymes are primarily cytosolic, although microsomal aldehyde oxidase activity has been detected. They are flavoproteins, containing FAD and also molybdenum, and the oxygen incorporated is derived from water rather than molecular oxygen. Aldehyde oxidase and xanthine oxidase in fact oxidize a wide variety of substrates, both aldehydes and nitrogen containing heterocycles such as caffeine and purines (see below). Aldehyde... 

AO/XO Aldehyde oxidase/xanthine oxidase (molybdenum hydroxylases)... 

There are probably more publications relating to xanthine oxidase than to any other enzyme studied, certainly more than those pertaining to aldehyde oxidase. This is presumably because the former enzyme is easily accessible from cow s milk rather than from animal tissue. It is not the purpose of this review to include all the data amassed on xanthine oxidase, as this has been fully covered in recent reviews [8, 12, 13]. Furthermore, most of our own work has been concerned with aldehyde oxidase. Thus, this report compares the properties of the molybdenum hydroxylases, where possible, in terms of distribution, substrate and inhibitor specificity and mechanism of oxidation. 

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