Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:1.10.3.3 (ascorbate oxidase)
 778 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The reactions of the monodehydroascorbate radical (As.-) with various biological molecules were investigated by pulse radiolysis. As.- reacted with both fully reduced and semiquinone forms of hepatic NADH-cytochrome b5 reductase with second-order rate constants of 4.3 x 10(6) and 3.7 x 10(5) M-1 s-1, respectively, at pH 7.0. In contrast, no reaction of As.- with ferrous cytochrome b5 could be detected by pulse radiolysis, whereas the oxidation of cytochrome b5 by As.- was observed by ascorbate-ascorbate oxidase method. This suggests that the rate constant of As.- with the ferrous cytochrome b5 must be several orders in magnitude smaller than that of the disproportionation of As.-. On the other hand, As.- reduced Fe3+EDTA with a second-order rate constant of 4.0 x 10(6) M-1 s-1 but did not reduce ferric hemoproteins such as metmyoglobin, methemoglobin, and cytochrome b5 by either the pulse radiolysis or the ascorbate-ascorbate oxidase method.
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PMID:Kinetic behavior of the monodehydroascorbate radical studied by pulse radiolysis. 188 18

The formation of ascorbate radicals, identified by ESR experiments, was observed in the ascorbate peroxidase reaction by thyroid microsomes. The steady-state concentration of ascorbate radicals decreased in the presence of NADH. The oxidation of NADH was followed optically. Using the ascorbic acid oxidase system, NADH-dependent electron transport in thyroid microsomes was examined. Ascorbate radicals competed with bound cytochrome b5 for the reaction with reduced NADH-cytochrome b5 reductase. The NADH-ascorbate radical reductase activity of thyroid microsomes was calculated to be 0.17 mumol/mg.s at 3.3 microM ascorbate radicals. Kinetic results show that the properties of NADH-cytochrome b5 reductase in thyroid microsomes were similar to those of the enzyme in liver microsomes. The formation of ascorbate radicals by thyroid microsomes was stimulated by the addition of thyroxine, and the stimulation was decreased also by NADH. The thyroxine-mediated oxidation of ascorbate is explained in terms of consecutive one-electron transfers initiated by bound thyroid peroxidase. These results, along with those described in our previous paper (M. Nakamura, I. Yamazaki, and S. Ohtaki, 1990, J. Biochem. 108, 804-810), support the idea that ascorbate protects thyroid hormones from oxidative degradation through the NADH-cytochrome b5 reductase system.
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PMID:Formation and reduction of ascorbate radicals by hog thyroid microsomes. 839 46