Aldehyde Oxidase and Xanthine Dehydrogenase

These two enzymes are found in the cytosol of the liver (and other tissues) and also oxidize substrates (Panoutsopoulos et ah, 2004 Rajagopalan, 1997), with a very different stoichiometry and mechanism  

The two electrons may be transferred to an oxidized pyridine nucleotide or to O2 (yielding H2O2). 

These enzymes contain flavin, a molybdenum center, and Fe-S clusters in the ratio 1 1 4. The pathway of electron flow is from Mo to Fe-S to FAD. 

FIGURE 2.7 Oxidation of A-methyl, 4-phenyl-l,2,5,6-tetrahydropyridine by MAO (Chiba et al., 1985). 

the Mo center is involved in the oxidation of the substrate. In the mechanism, the source of the oxygen atom inserted into the substrate is H2O instead of O2 (as with P450 and FMO). The list of substrates includes some endogenous and xenobiotic aldehydes, and various heterocycles, including purines, pyridines, pyrimidines, pteridines, and others. Xanthine is a substrate for xanthine dehydrogenase but not aldehyde oxidase purines are substrates for both. 

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Fe prosthetic groups.282 283 A group of aldehyde oxidases and xanthine dehydrogenases also contain molybdenum as well as iron (Chapter 16). In every case the metal ions are bound independently of the flavin.2833... 

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. 

Any discussion of the mechanism of xanthine oxidase should attempt to incorporate the special features of xanthine oxidase (and xanthine dehydrogenase and aldehyde oxidase) which are not present, for example, in sulfite oxidase. There are two such features at least (a) the involvement of two protons rather than the one found for sulfite oxidase, and (b) the presence of the cyanolyzable sulfur atom. The mechanistic features discussed so far involve the abstraction of two electrons and a proton. This means that a carbonium ion is generated, which could undergo attack by a nucleophile. Thus, the presence of a nucleophile at the active site could lead to the formation of a covalent intermediate that will break down to give the products.1032 The nucleophile could either be the cyanolyzable sulfur atom or a group associated with the second proton. A possible scheme is shown in Figure 41. 

In addition to liver aldehyde dehydrogenase, a number of other enzymes present in the soluble fraction of liver homogenates will oxidize aldehydes and certain N-heterocyclic compounds. Among these are aldehyde oxidase and xanthine oxidase (see below), both flavoprotein enzymes containing molybdenum. These enzymes catalyze the oxidation of aldehydes formed by the deamination of endogenous amines by amine oxidases. 

The bioconversion of alcohols to aldehydes and ketones is catalyzed by soluble alcohol dehydrogenases present in the liver and other tissues. NAD is required os a coenzyme, although NADP also may serve as a coenzyme. The reaction catalyzed by alcohol dehydrogenase is reversible but often proceeds to the right because the aldehyde formed is fuithcr oxidized to the acid. Several aldehyde dehydrogenases, including aldehyde oxidase and xanthine oxidase, carry out the oxidation of aldehydes to their corresponding acids." - ... 

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

In addition to these more-or-less well characterized proteins, iron is known to be bound to certain flavoproteins such as succinic dehydrogenase (20), aldehyde oxidase (27), xanthine oxidase (22) and dihydrooro-tate dehydrogenase (23). Iron is present and functional in non-heme segments of the electron transport chain but again no real structural information is at hand (24). 

Long recognized as an essential element for the growth of plants, molybdenum has never been directly demonstrated as a necessary animal nutrient. Nevertheless, it is found in several enzymes of the human body, as well as in 30 or more additional enzymes of bacteria and plants.632 Aldehyde oxidases,633 xanthine oxidase of liver and the related xanthine dehydrogenase, catalyze the reactions of Eqs. 16-58 and 16-59 and contain molybdenum that is essential for catalytic activity. Xanthine oxidase also contains two Fe2S2 clusters and bound FAD. The enzymes can also... 

Aldehyde Oxidase. Besides xanthine oxidase, liver contains a more specific aldehyde oxidase. This enzyme does not oxidize reduced DPN or purines, but does convert a large number of aliphatic and aromatic aldehydes to the corresponding acids. This enzyme contains molybdenum and an iron protoporphyrin in addition to FAD. It is completely reduced by aldehydes, whereas aldehydes bleach xanthine oxidase only partially (compared with hydrosulfite). Aldehyde dehydrogenase oxidizes DPNH at no more than 1 per cent of the rate of aldehyde oxidation. Cytochrome c, oxygen, and dyes all serve as oxidants. 

It is the coenzyme of xanthine oxidase, aldehyde oxidase and other aerobic dehydrogenases. Like riboflavin and FMN, FAD is universally present in the biosphere. It is reddish-yellow in colour but, like FMN and riboflavin, its solutions are a yellow-green. 

The following enzymes have been shown to be flavoproteins D-amino acid oxidases, Z-amino acid oxidases, xanthine oxidases, aldehyde oxidases, the glucose dehydrogenase, notatin, the diamine oxidase, and fumaric hydrogenase. Details concerning these enzymes may be found in the review articles of TheorelP and Krebs. 

Of the mammalian enzymes, the sulphite oxidase of bovine liver has only recently been discovered to contain molybdenum (15). The better known molybdenum enzymes, xanthine oxidase from cows milk (31) and aldehyde oxidase from rabbit liver (16) are closely related to one another as they are to the xanthine dehydrogenases from chicken liver (17) and from bacteria (18). 

So little is known about molybdenum enzymes other than milk xanthine oxidase that there is little to be said by way of general conclusions. In all cases where there is direct evidence (except possibly for xanthine dehydrogenase from Micrococcus lactilyticus) it seems that molybdenum in the enzymes does have a redox function in catalysis. For the xanthine oxidases and dehydrogenases and for aldehyde oxidase, the metal is concerned in interaction of the enzymes with reducing substrates. However, for nitrate reductase it is apparently in interaction with the oxidizing substrate that the metal is involved. In nitrogenase the role of molybdenum is still quite uncertain. 

R is an electron-donor substrate such as purine or xanthine and A is an electron acceptor such as 02 or NAD+. It is thought that the in vivo mammalian form of xanthine oxidase uses NAD+ as acceptor and is therefore, more appropriately named xanthine dehydrogenase. No evidence exists for a dehydrogenase form of aldehyde oxidase. The specificities of xanthine oxidase and aldehyde oxidase have been extensively catalogued (96), and the mechanism and properties of these enzymes have been reviewed (97, 98). 

Xanthine oxidase, xanthine dehydrogenase and aldehyde oxidase 658... 

These enzymes catalyze the two-electron oxidation of purines, aldehydes and pyrimidines, sulfite, formate and nicotinic acid in the general reaction shown in equation (49). These enzymes show some differences in properties. Xanthine oxidase, xanthine dehydrogenase and aldehyde oxidase all have relatively low redox potentials and a unique cyanolyzable sulfur atom, and so will be discussed together. 

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