Xanthine oxidase/hypoxanthine system

Not only Oj " and HO, but also alkyl or alkoxyl radicals (R ) were formed when saccharides such as glucose, fructose and sucrose were added into the xanthine oxidase/hypoxanthine system containing iron (Luo et al. 2001). The generated amount of R depended on the kind and concentration of saccharides added into this system. In the absence of saccharides no R were detected, indication that there is an interaction between the saccharide molecules and the free radicals generated from the iron containing hypoxanthine/xanthine oxidase system. 

Anti-gout Drugs. Figure 1 Xanthine oxidase-catalyzed reactions. Xanthine oxidase converts hypoxanthine to xanthine and xanthine to uric acid, respectively. Hypoxanthine and xanthine are more soluble than uric acid. Xanthine oxidase also converts the uricostatic drug allopurinol to alloxanthine. Allopurinol and hypoxanthine are isomers that differ from each other in the substitution of positions 7 and 8 of the purine ring system. Although allopurinol is converted to alloxanthine by xanthine oxidase, allopurinol is also a xanthine oxidase inhibitor. Specifically, at low concentrations, allopurinol acts as a competitive inhibitor, and at high concentrations it acts as a noncompetitive inhibitor. Alloxanthine is a noncompetitive xanthine oxidase inhibitor. XOD xanthine oxidase. 

Ascorbic Acid (AA). Experimental (SI, S7, S24) and clinical (B13, B17) studies have provided some evidence for the concept that oxidative stress is the common pathway for the initiation of AP (B14). The most abundant endogenous antioxidant in the aqueous phase is ascorbic acid (AA), a bioactive form of vitamin C, which scavenges oxygen-derived free radicals produced by activated neutrophils and the hypoxanthine-xanthine oxidase system (D12). Scott et al. 

The major studies in this field are the work of one group102 using mainly in vitro techniques. The action of xanthine oxidase (XO), a milk enzyme, converts purine through hypoxanthine and xanthine to uric acid, in which the order of oxidation is seen to be the same as for base substitution of 2,6,8-trichloropurine. It was first held that the nucleophilic reagent responsible was water, which added across the double bonds and resulted in dihydropurines which then lost hydrogen, so restoring the aromaticity of the system.103 This view has now been modified and hydroxyl-ions are assumed to be the nucleophiles.104 As purine contains no substituents, directive influences are supplied by the dipolar forms extant at the time of reaction, of which 59 represents... 

A complete hypoxanthine-xanthine oxidase system should give a final A q of about 0.65, corresponding to 150-200 nmol of hydroxyl-ated product. Formation of hydroxylated products can be inhibited almost completely by superoxide dismutase, catalase, or the iron chelator desferrioxamine (Richmond et al., 1981). 

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. ... 

Aruoma, O.I., Halliwell, B., and Dizdaroglu, M. (1989) Iron ion-dependent modification of bases in DNA by the superoxide radical-generating system hypoxanthine/xanthine oxidase. J. Biol. Chem., 264, 13024-13028. 

Unno, T., Sugimoto, A., and Kakuda, T. 2000. Scavenging effect of tea catechins and their epimers on superoxide anion radicals generated by a hypoxanthine and xanthine oxidase system. J. Sci. FoodAgric. 80 601-6. 

Hypoxanthine/xanthine oxidase system Fenton reaction system HjOz/NaOH/DMSO system Suppressed the superoxide, hydroxyl, and methyl radicals [115, 118J... 

The antioxidant activity of alizarin was established in four different assays (1) suppression of light emission in the p-iodophenol enhanced chemiluminescent assay, (2) scavenging of superoxide anion (02 -) in a hypoxanthine-xanthine oxidase system, (3) protection of rat liver microsomes from lipid peroxidation by ADP/iron(II) ions, and (4) protection of bromobenzene-intoxicated mice from liver injury in vivo [141]. Alizarin was compared with Trolox (water soluble vitamin E), the flavonoid baicalin and green tea proanthocyanidins. In assay (1) the activity of alizarin was 76% of that of Trolox. In assay (2) the inhibition of 02 -induced chemiluminescence was 40%, 32%, 23% and 14% for Trolox, alizarin, green tea polyphenols and baicalin respectively. Alizarin was not significantly active in the lipid peroxidation assay but after baicalin the most active compound in the in vivo assay. This shows again the difficulty in the evaluation of antioxidant activity and the differences between in vitro and in vivo assays [141]. 

The xanthine oxidase system consisted of 20 p,g (60 nAf) of xanthine oxidase and 1 mAf hypoxanthine. 

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