Xanthine oxidase reaction mechanism

Mechanisms of action of 30, 31 and 32 were probed in various cell lines. Phthalides 30, 31 and 32 prevented cortical neuronal cell death and inhibited the release of several injury surrogate biomarkers induced by KC1 [335], A -methyl-D-aspartate (NMDA) [335], AA [336] and hypoxia/hypoglycemia [337-339], These effects appeared to be related to an increase in NO and PGI2 release from neuronal [340, 341] and cerebral endothelial cells [342], Phthalides 30, 31 and 32 reduced superoxide anion production in a xanthine-xanthine oxidase reaction system [343]. The three phthalides also decreased intracellular Ca2+ level in cortical neuronal cells [344]. Furthermore, phthalides 30, 31 and 32 ameliorated the abnormal activities of several mitochondrial respiratory chain complexes induced by MCAO [322] and those of mitochondrial ATPase induced by hypoxia/hypoglycemia in cortical neuronal cells [345],... 

A current overall picture of the reaction mechanism of xanthine oxidase, which differs substantially from one proposed earlier (87) is as follows. The enzyme is presumed to have two independent catalytic units, though this has not so far been proved rigorously. Reducing substrates are bound at molybdenum and reduce this from Mo(VI) both to Mo(V) and to Mo (IV). Reducing equivalents are then transferred by intramolecular reactions from molybdenum to iron-sulphur and also, either directly or via this, to flavin. Oxidizing substrates as a class, seem capable of reacting with all three types of centre in the enzyme. Thus, oxygen reacts predominantly with flavin, phenazine methosulphate... 

The initial electrochemical and biological oxidation with xanthine oxidase are essentially identical. However, electrochemically 2,8-dioxyadenine the final product in the presence of xanthine oxidase is much more readily oxidizable than adenine 59) so that considerable further oxidation occurs. To the authors knowledge, 2,8-dioxyadenine is not a major metabolite of adenine in man or other higher organisms. Accordingly, it is likely that other enzymes accomplish further degradation of 2,8-dioxyadenine. The relationship between the products so formed and the mechanism of the reaction to the related electrochemical processes has yet to be studied. 

The mechanism of iron-initiated superoxide-dependent lipid peroxidation has been extensively studied by Aust and his coworkers [15-18]. It was found that superoxide produced by xanthine oxidase initiated lipid peroxidation, but this reaction was not inhibited by hydroxyl radical scavengers and, therefore the formation of hydroxyl radicals was unimportant. Lipid peroxidation depended on the Fe3+/Fe2+ ratio, with 50 50 as the optimal value [19]. Superoxide supposedly stimulated peroxidation both by reducing ferric ions and oxidizing ferrous ions. As superoxide is able to release iron from ferritin, superoxide-promoted lipid peroxidation can probably proceed under in vivo conditions [16,20]. 

Figure 17.4 Reaction mechanism proposed for xanthine oxidase. (From Hille, 2005. Copyright 2005, with permission from Elsevier.)...

The bis(l,2-enedithiolate) complexes discussed closely resemble the metal centers found in the dmso reductase family of Mo enzymes and in the tungsten enzymes. The reactivity of mono(l,2-enedithiolate) complexes remains a continuing challenge as synthetic chemists pursue accurate models for the xanthine oxidase and sulfite oxidase families of metal sites. New 1,2-dithiolate ligands [70,71] and complexes are needed to demonstrate ligand effects to help elucidation reaction mechanism. 

Mechanisms of action for the metal centers in acetylene hydratase, polysulfide reductase, and formate dehydrogenase have been briefly described in Sections VI.A and VLB. The discussion, in each case, was relatively straightforward insofar as the natures of these reactions lend themselves to simple mechanistic proposals. The mechanism by which the metal centers function in most of the other Mo and W enzymes is not as obvious. We elect to discuss mechanistic roles for the molybdenum centers in xanthine oxidase, sulfite oxidase, and dmso reductase. These enzymes are representative members of each large class of molybdenum enzymes, and the large body of literature on each enzyme makes detailed discussion possible. 

Possible Xanthine Oxidase Mechanism. The proposed reaction mechanism, (Figure 27), which must still be regarded as a working hypothesis, entails metal binding of substrate, metal-assisted activation of water, CEPT, and stabilization of the hydrosulfido ligand. 

The conversion of NO to HNO can proceed by several mechanisms, including formal reduction by metalloenzymes such as superoxide dismutase (SOD) (83-85) and xanthine oxidase (XO) (86) or reductants such as flavins (87) and ubiquinol (88). The reaction of 5-nitrosthiols, which would be formed initially upon NO biosynthesis, with excess thiols also releases HNO (89-92). 

Mondal, M. S., and Mitra, S., 1994, Kinetics and thermodynamics of the molecular mechanism of the reductive half-reaction of xanthine oxidase. Biochemistry 33 10305nl0312. 

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