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)

Competitive inhibition by phenolic compounds of the ascorbic acid oxidation reaction catalyzed by ascorbate oxidase was investigated at pH 7.0 and 23.0 degrees C. Inhibition of p-nitrophenol is pH dependent over the range 5.0-8.0, with inhibitor binding favored at higher pH. Bulky substituents on the phenol nucleus reduce or prevent the inhibitory effect. The presence of phenol affects the binding characteristics of azide to the trinuclear cluster of the enzyme. In particular, binding of azide to type 2 copper is prevented, and the affinity of azide to type 3 copper is reduced. In addition, reduction of type 1 copper is observed upon prolonged incubation of ascorbate oxidase with excess phenol and azide, but not with phenol alone. It is proposed that binding of phenolic inhibitors occurs at or near the site where the substrate (ascorbate) binds. NMR relaxation measurements of the protons of phenols in the presence of ascorbate oxidase show paramagnetic effects due to the proximity of the bound inhibitor to a copper center, likely type 1 copper. Copper-proton distance estimates between this paramagnetic center and p-cresol or p-nitrophenol bound to ascorbate oxidase are between 4.4 and 5.9 A.
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PMID:Inhibition of ascorbate oxidase by phenolic compounds. Enzymatic and spectroscopic studies. 912 5

The effect of various flavonoids upon the ascorbate radical lifetime was investigated by ESR spectroscopy. The radical was generated via the reaction between ascorbic acid and ascorbate oxidase, the ascorbate radical being detected. The inclusion of the flavonoids in the ascorbic acid-ascorbate oxidase reaction mixture affected both the initial intensity of the ascorbate radical and its lifetime. Of the natural sources tested, Pycnogenol prolonged the ascorbate radical lifetime to the greatest extent, from a control value of 20 min to a maximum of 80 min with 200 micrograms/ml Pycnogenol. The flavonoids could either be regenerating ascorbic acid from ascorbate, or interacting with ascorbate oxidase, thus preventing ascorbic acid binding. When p-cresol, a known ascorbate oxidase inhibitor was added to the ascorbic acid-ascorbate oxidase reaction mixture, the ascorbate radical signal intensity was dramatically reduced and did not display the time-dependent decay observed with the flavonoids. This indicates that a direct interaction between the flavonoids and ascorbate radical occurs. Some of the flavonoids tested; myricetin, polyphenon and theaflavin appeared to compete with ascorbic acid for ascorbate oxidase, as they displayed saturation behaviour. By modifying the experimental conditions the myricetin radical was detected, thus confirming the direct interaction between myricetin and ascorbate oxidase.
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PMID:ESR studies of vitamin C regeneration, order of reactivity of natural source phytochemical preparations. 967 60