Xanthine oxidase reaction

Spin-trapping experiments connected with biological superoxide production have also been used to examine the biochemistry of anti-tumour antibiotics bleomycin (Sugiura and Kikuchi, 1978) and mitomycin C (Lown et al., 1978), as well as the effect of iron concentration on xanthine oxidase reactions (Buettner et al., 1978), and the photochemistry of melanins (Felix et al., 1978). 

Nagano, T., and Fridovich, 1. (1985). The co-oxidation of ammonia to nitrite during the aerobic xanthine oxidase reaction. Arch. Bixhem. Biophys. 241, 596-601. 

Xia, M., Dempski, R., and Hille, R., 1999, The reductive half-reaction of xanthine oxidase. Reaction with aldehyde substrates and identification of the catalytically labile oxygen, J. Biol. Chem. 274 3323iB330. 

Because the enzyme substrate and the products are unstable, it is difficult to measure the disappearance of substrate or the formation of products as is usual in enzymatic assays. Routine assays for SOD usually employ an indirect assay in which one unit of enzyme activity is defined as the amount of enzyme that inhibits the reaction of 02 with the indicator by 50%. The most frequently used method for measuring SOD activity employs the xanthine/xanthine oxidase reaction for... 

The enzymic-catalyzed reduction of cytochrome cox using the xanthine - xanthine oxidase reaction led to the assumption that Oi- was the active reducing agent (133). Myoglobin and carboanhydrase were able... 

Experimental evidence for the participation of the flavin moiety in Oi- formation in nonenzymic reactions was presented by Ballou et al. (151). During the oxidation of different reduced flavins by molecular oxygen substantial yields of Oi- were obtained. Tetra-acetyl riboflavin proved most appropriate as a model compound (153). The respective EPR parameters for Oi- were in close agreement with those obtained in the KIO4—H2O2 system, in the xanthine oxidase reaction (149), and in the oxidation of the reduced tetra-acetyl riboflavin. They also agree well with the EPR data obtained earlier (154—156). The time course of the appearance and decay of Oi- produced by the reaction of tetra-acetyl riboflavin is shown in Fig. 24. 

Fig. 26 (149). EPR signals of O - obtained both enzymically and non-enzymically. Recording at —170 °C, rapid freezing technique (152) a) xanthine—xanthine oxidase reaction. Note the molybdenum signals (overmodulated). The sloping base line is attributed to an iron signal b) non-enzymically induced Og- from H2O2 and NaIC>4, pH 9.9 c) same as (b) but at pH 13.2...

All the inhibitions reported so far indicate that there is no known specific inhibitor for the manganese superoxide dismutase. The enzyme from B. stearothermophilus was found to be inhibited by cacodylate (Thornalley et al., 1982) however, the inhibition could only be observed in the xanthine/xanthine oxidase assay and not in the pulse radiolysis assay. No evidence was obtained that cacodylate could be competing with the enzyme for because little or no inhibition was observed when the copper/zinc rather than the manganese enzyme was assayed in the presence of cacodylate. Further investigations revealed that the inhibitory effect is due to a cacodylate anion radical produced by the interaction of hydroxyl radicals (generated by the xanthine/xanthine oxidase reaction) and cacodylate anions. A radical of pamoic acid (4,4 -methylenebis-(3-... 

The autoxidation of hydroxylamine to nitrite also involves a radical chain process (Kono, 1978), and the reaction is carried out at high pH. The assay was originally utilized by Elstner and Heupel (1976) who initiated the autoxidation by O2 generated by the xanthine/xanthine oxidase reaction. Nitrite formation was determined colorimetrically at 530 nm by diazo coupling with a-naphthylamine and superoxide dismutase was found to inhibit end product formation. Kono (1978) developed the assay further by utilizing nitroblue tetrazolium as the... 

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],... 

Billiar and co-workers (53) have proposed a calibration method based upon the stoichiometry of the xanthine oxidase reaction ... 

Holtzman s method (54) circumvents the problems associated with the Billiar method by including catalase in the hypoxanthine-xanthine oxidase reaction chain according to the following scheme ... 

Finally, and perhaps most importantly, for nitrate reductase and sulfite oxidase, we have to inquire about the number of electrons transferred from the enzyme to the substrate, or vice versa. This brings us to the question of the valence states between which molybdenum in the enzymes cycles in the turnover processes. For both of these enzymes, the overall reaction is a two-electron process, as is the xanthine oxidase reaction. For xanthine oxidase, the evidence (Olson et al, 1974) favors xanthine reducing Mo(VI) directly to... 

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