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)

Paramagnetic nitroxyl spin labels have potential clinical utility as contrast agents in proton magnetic resonance imaging. Reduction of the nitroxyl moiety in vivo results in the formation of the diamagnetic hydroxylamine, which lacks contrast-enhancing activity. Bioreduction is therefore an important determinant of the imaging behavior of these agents. Both enzymatic and nonenzymatic reduction mechanisms have been suggested for nitroxyl spin labels. This study examines the nonenzymatic mechanisms in rat liver and kidney, mammalian tissues that demonstrate high reducing activity. Protein-free preparations, obtained by heat precipitation or ultrafiltration of rat liver and kidney homogenates, were used to test piperidine and pyrrolidine nitroxyl spin-label derivatives, for which imaging properties and bioreduction had previously been examined. For the piperidine derivative, the initial reduction rates in ultrafiltrates and supernatant fluids were 25-60% of those in whole liver and kidney homogenates. However, the pyrrolidine derivative was reduced at rates much slower than those in whole tissue homogenates. The reduction in whole tissue homogenates was NADPH-dependent, while reduction in ultrafiltrates was unaffected by the addition of NADPH. Preincubation of the ultrafiltrates and supernatant fluids with ascorbic acid oxidase caused almost complete inhibition of the reduction. The reduction rates of these nitroxyl derivatives were determined in ascorbic acid solution; second order rate constants were 0.45 +/- 0.04 and 0.0042 +/- 0.001 mM-1 min-1 for the piperidine and pyrrolidine derivatives, respectively. The concentrations of ascorbic acid in the supernatant fluids and ultrafiltrates of rat liver and kidney were then predicted from the observed reduction rates and found to be virtually identical with those from spectrophotometric determinations.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Nonenzymatic bioreduction in rat liver and kidney of nitroxyl spin labels, potential contrast agents in magnetic resonance imaging. 288 71

Extracellular brain ascorbate fluctuates with neuronal activity. There is previous evidence that the release of ascorbate is triggered by the re-uptake of neuronally released glutamate. This hypothesis predicts that drugs which block the release and re-uptake of glutamate will also block the release of ascorbate. In the present experiments we have used a novel dialysis electrode which allows continuous monitoring of physiologically induced ascorbate release from the striatum in freely moving rats. An infusion of the enzyme ascorbic acid oxidase abolished the increase in oxidation current in response to tail-pinch, which identified it as an ascorbate current. Perfusion with tetrodotoxin reduced the response to 25% and with CdCl2 to 4% of control. Perfusion with the uptake blocker L-trans-pyrrolidine-2,4-di-carboxylate reduced the response to 24% of control. A neuroprotective function for this coupling of ascorbate and glutamate release is discussed.
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PMID:The physiologically induced release of ascorbate in rat brain is dependent on impulse traffic, calcium influx and glutamate uptake. 781 14

Biological applications of stable nitroxyl radicals, NR, include their use as contrast agents for magnetic resonance imaging, spin labels, superoxide dismutase mimics, and antioxidants. The rapid reduction of NR in biological samples into hydroxylamines (HA) significantly limits their application. In turn, reoxidation of HA back to the NR has been used for detection of reactive oxygen species (ROS). In this work comparative studies of the reduction of pyrrolidine, imidazoline, and imidazolidine NR by ascorbate were performed taking advantage of recently synthesized tetraethyl-substituted NR with much higher stability toward reduction both in vitro and in vivo. Surprisingly, these NR kept 10-50% of initial intensity of electron paramagnetic resonance signal for about 1 h in the presence of 100-fold excess of ascorbate. To explain these data, reoxidation of the corresponding HA by ascorbate radical and dehydroascorbic acid back to the NR was proposed. This hypothesis was supported by direct measurement of the NR appearance from the HA on ascorbate radical generation by ascorbate oxidase, or in the presence of the dehydroascorbic acid. The reversible reaction between NR and ascorbate was observed for the various types of NR, and the rate constants for direct and reverse reactions were determined. The equilibrium constants for one-electron reduction of the tetraethyl-substituted NR by ascorbate were found to be in the range from 2.65x10(-6) to 10(-5) which is significantly lower than corresponding values for the tetramethyl-substituted NR (more or about 10(-4)). This explains the establishment of an EPR-detectable quasi-equilibrium level of tetraethyl-substituted NR in the presence of an excess of ascorbate. The redox reactions of the NR-HA couple in ascorbate-containing media were found to be significantly affected by glutathione (GSH). This effect was attributed to the reduction of ascorbate radicals by GSH, and the rate constant of this reaction was found to be equal to 10 M-1 s-1. In summary, the data provide new insight into the redox chemistry of NR and HA, and significantly affect interpretation and strategy of their use as redox- and ROS-sensitive probes, or as antioxidants.
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PMID:Reversible reduction of nitroxides to hydroxylamines: roles for ascorbate and glutathione. 1721 Apr 53