Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: DrugBank:EXPT00568 (ascorbate)
 23,072 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

A 37,000 X g supernatant fraction prepared from fat lung homogenate demonstrated a 2- to 3-fold increase in guanylate cyclase activity after incubation at 30 degrees for 30 min (preincubation). Treatment of the supernatant fraction with Triton X-100 increased activity to approximately the same extent as preincubation, but would not increase the activity after preincubation. By chromatography on Sepharose 2B, before and after preincubation, it was demonstrated that the increase in activity was only associated with the soluble guanylate cyclase, and not the particulate enzyme. Activation by preincubation required O2. It was completely inhibited by thiols such as 2-mercaptoethanol, and by bovine serum albumin, KCN, and sodium diethyldithiocarbamate. These inhibitors suggested a copper requirement for activation, and this was confirmed by demonstrating that 20 to 60 muM CuCl2 could relieve the inhibition by 0.1 mM sodium diethyldithiocarbamate. 2-Mercaptoethanol inhibition could also be reversed by removal of the thiol on a Sephadex G-25 column, however, this treatment partially activated the enzyme. Addition of 2-mercaptoethanol to a preincubated preparation would not reverse the activation. H2O2 was found to activate guanylate cyclase, either by its generation in the lung supernatant with glucose oxidase and glucose, or by its addition to a preparation in which the catalase was inhibited with KCN. KCN or bovine serum albumin was able to partially inhibit activation by glucose oxidase plus glucose, however, larger amounts of glucose oxidase could overcome that inhibition, indicating a catalytic role for Cu2+ at low H2O2 concentrations. No direct evidence for H2O2 formation during preincubation could be found, however, indirect evidence was obtained by the spectrophotometric detection of choleglobin formation from hemoglobin present in the lung supernatant fluid. The H2O2 is believed to result from the reaction of oxyhemoglobin with ascorbate.
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PMID:Activation of soluble guanylate cyclase from rat lung by incubation or by hydrogen peroxide. 1 60

The mechanism of the enzymic reaction of an iron-containing superoxide dismutase purified from the marine bacterium Photobacterium leiognathi was studied by using pulse radiolysis. Measurements of activity were done with two different preparations of enzyme containing either 1.6 or 1.15 g-atom of iron/mol. In both cases, identical values of the second-order rate constant for reaction between superoxide dismutase and the superoxide ion in the pH range 6.2-9.0 (k=5.5 X 10(8) M-1-S-1 at pH 8.0) were found. As with the bovine erythrocuprein, there was no evidence for substrate saturation. The effects of reducing agents (H2O2, sodium ascorbate or CO2 radicals) on the visible and the electron-paramagnetic-resonance spectra of the superoxide dismutase containing 1.6 g-atom of ferric iron/mol indicate that this enzyme contains two different types of iron. Turnover experiments demonstrate that only that fraction of the ferric iron that is reduced by H2O2 is involved in the catalysis, being alternately oxidized and reduced by O2; both the oxidation and the reduction steps have a rate constant equal to that measured under turnover conditions. These results are interpreted by assuming that the superoxide dismutase isolated from the organism contains 1 g-atom of catalytic iron/mol and a variable amount of non-catalytic iron. This interpretation is discused in relation to the stoicheiometry reported for iron-containing superoxide dismutases prepared from several other organisms.
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PMID:A pulse-radiolysis study of the catalytic mechanism of the iron-containing superoxide dismutase from Photobacterium leiognathi. 1 40

Dehydroascorbate, an electron affinic metabolite of vitamin C, sensitized Ehrlich ascites tumor cells, in vivo, to radiation and was selectively toxic to V79 Chinese hamster lung cells under hypoxic conditions (without radiation). The radiosensitization may involve both the electron affinic nature of dehydroascorbate as well as its ability to oxidize the intracellular NAD(P)H and non-protein sulfhydryl. Dehydroascorbate's oxidation of NAD(P)H required higher concentrations than other sulfhydryl oxidants such as N-ethylmaleimide and diamide. The oxidation of NAD(P)H by dehydroascorbate could be reversed by glucose. Hypoxic cell radiosensitization of V79 cells in tissue culture by dehydroascorbate could not be easily demonstrated because of the rapid breakdown and appreciable cytotoxicity of the drug at high concentration. The cytotoxicity was found to occur with both high and low densities of V79 cells. With low cell densities small amounts of oxygen did not reduce the cytotoxicity of dehydroascorbate, but virtually eliminated the cytotoxicity of nitroaromatic electron affinic compounds (metronidazole and Ro-07-0582). The cytotoxicity to dense cell suspensions was found to depend upon the type of buffer included in the reaction medium. The maximum cytotoxicity was obtained in buffer free saline. The reduced form of dehydroascorbate, vitamin C, was found to be toxic only under aerobic conditions. The aerobic cytotoxicity could be prevented by the addition of catalase to the growth medium or by an increase in cell density, suggesting it was caused entirely by the production of H2O2 from the oxidation of vitamin C.
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PMID:Toxicity, radiation sensitivity modification, and metabolic effects of dehydroascorbate and ascorbate in mammalian cells. 2 85

1. Chloroplasts washed with Cl--free, low-salt media (pH 8) containing EDTA, show virtually no DCMU-insensitive silicomolybdate reduction. The activity is readily restored when 10 mM Cl- is added to the reaction mixture. Very similar results were obtained with the other Photosystem II electron acceptor 2,5-dimethylquinone (with dibromothymoquinone), with the Photosystem I electron acceptor FMN, and also with ferricyanide which accepts electrons from both photosystems. 2. Strong Cl--dependence of Hill activity was observed invariably at all pH values tested (5.5--8.3) and in chloroplasts from three different plants: spinach, tobacco and corn (mesophyll). 3. In the absence of added Cl- the functionally Cl--depleted chloroplasts are able to oxidize, through Photosystem II, artificial reductants such as catechol, diphenylcarbazide, ascorbate and H2O2 at rates which are 4--12 times faster than the rate of the residual Hill reaction. 4. The Cl--concentration dependence of Hill activity with dimethylquinone as an electron acceptor is kinetically consistent with the typical enzyme activation mechanism: E(inactive) + Cl- in equilibrium E . Cl- (active), and the apparent activation constant (0.9 mM at pH 7.2) is unchanged by chloroplast fragmentation. 5. The initial phase of the development of inhibition of water oxidation in Cl--depleted chloroplasts during the dark incubation with NH2OH (1/2 H2SO4) is 5 times slower when the incubation medium contains Cl- than when the medium contains NH2OH alone or NH2OH plus acetate ion. (Acetate is shown to be ineffective in stimulating O2 evolution).
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PMID:The role of chloride ion in photosystem II. I. Effects of chloride ion on photosystem II electron transport and on hydroxylamine inhibition. 2 90

L-ascorbic acid (LAA) augmented cGMP many-fold in highly purified human peripheral blood lymphocytes. The cGMP response occurred within 10 sec and persisted for at least 60 min. D-ascorbic acid (DAA) and dehydroascorbic acid (DHAA) were also equally active in enhancing cGMP concentrations but metabolic precursors of ascorbic acid and other inorganic acids did not increase cGMP levels. Determination of the amount of DHAA contaminating the LAA precluded the possibility that it was solely responsible for the enhanced cGMP levels. The sodium or calcium salts of ascorbic acid did not increase cGMP concentrations. If these neutralized preparations were acidified, increased cGMP concentrations were then noted. In broken cell preparations, LAA, DAA, and DHAA and to a lesser extent sodium ascorbate (NaA) enhanced guanylate cyclase activity while neither inhibited cAMP or cGMP phosphodiesterase (PDE) activity. The possible role of H2O2, fatty acid liberation, prostaglandin production, oxidizing-reducing agents, and free radical formation in mediating the effects of ascorbic acid on cGMP levels were evaluated, but none of these potential mechanisms were definitively proven to be a required intermediary for the cGMP enhancing activity of ascorbic acid. LAA, DHAA or NaA did not induce lymphocyte transformation or modulate lectin-induced mitogenesis.
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PMID:Effects of ascorbic acid and sodium ascorbate on cyclic nucleotide metabolism in human lymphocytes. 3 16

Iron--EDTA was shown to catalyse OH. production from H2O2 and ascorbate by a mechanism largely independent of superoxide. When ascorbate and superoxide were both present, the ascorbate mechanism was more important than superoxide as a source of OH., and would appear to be more significantly biologically.
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PMID:Comparison of superoxide with other reducing agents in the biological production of hydroxyl radicals. 4 21

The degradation of DNA by bleomycin was studied in the absence and in the presence of added reducing agents, including 2-mercaptoethanol, dithiothreitol, reduced nicotinamide adenine dinucleotide phosphate, H2O2, and ascorbate, and in the presence of a superoxide anion generating system consisting of xanthine oxidase and hypoxanthine. In all cases, breakage of DNA was inhibited by low concentrations of chelators; where examined in detail, deferoxamine mesylate was considerably more potent than (ethylenedinitrilo)tetraacetic acid. Iron was found to be present in significant quantities in all reaction mixtures. Thus, the pattern of inhibition observed is attributed to the involvement of contaminating iron in the degradation of DNA by bleomycin. Cu(II), Zn(II), and Co(II) inhibit degradation of DNA by bleomycin and Fe(II) in the absence of added reducing agents. A model is proposed in which the degradation of DNA in these systems is dependent on the oxidation of an Fe(II)-bleomycin-DNA complex.
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PMID:Effect of chelating agents and metal ions on the degradation of DNA by bleomycin. 8 Feb 26

The production of superoxide radicals (O2-) by human granulocytes after phagocytosis of latex particles has been studied as a function of cell, latex, and cytochrome c concentration. Under the chosen conditions, latex phagocytosis stimulated the reduction of cytochrome c by O2- sevenfold to 420 mumoles per 10-10 granulocytes per hour. At high cytochrome c concentrations, this value increased to 800 to 1,000 mumoles per 10-10 granulocytes per hour. These amounts suggest that O2- plays an important role in the formation of H2O2 during the phagocytic process. We propose that the extra oxygen consumed in the respiratory burst is first reduced to O2-, possibly by ascorbate, before being converted to H2O2. No stimulation of the superoxide radical formation was found in the phagocytizing granulocytes of two patients with chronic granulomatous disease, and intermediate values were found for the heterozygotes of this deficiency.
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PMID:Quantitative aspects of the production of superoxide radicals by phagocytizing human granulocytes. 16 83

The reaction of oxyhaemoglobin and acetylphenylhydrazine, which results in haemoglobin denaturation and precipitation, was found to be influenced by H202 and superoxide (O2-.) generated during the reaction. By analysing the different haemoglobin oxidation products, it was found that by influencing the rate at which oxyhaemoglobin was oxidized, H2O2 accelerated the overall haemoglobin breakdown, and O2-. inhibited it. By adding GSH (reduced glutathione) or ascorbate, it was possible to slow down the rates of both oxyhaemoglobin oxidation and O2-. production, and the overall rate of haemoglobin breakdown. These results are compatible with a mechanism involving production of the acetylphenylhydrazyl free radical, and with GSH, ascorbate and O2-. acting as radical scavengers and preventing its further reactions. The reaction produced choleglobin, as well as acetylphenyldiazine and methaemoglobin, which combined to form a haemichrome. The haemichrome was less stable and precipitated first. It was also less stable than the haemichrome formed by direct reaction of acetylphenyldiazine with methaemoglobin, and it is proposed that this is because the methaemoglobin produced from oxyhaemoglobin and acetylphenylhydrazine was modified by the free radicals and H2O2 produced in the reaction.
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PMID:Mechanism of oxyhaemoglobin breakdown on reaction with acetylphenylhydrazine. 21 Jul 65

It seems that superoxide dismutase plays the key role in protecting aerobes against O2 toxicity, but there is a whole range of ancillary mechanisms: enzymes to remove H2O2 (catalase, peroxidases) and hence to control formation of .OH from O2, which requires H2O2; antioxidants (ascorbate, GSH, alpha-tocopherol, carotenoids), which also react with singlet oxygen and/or .OH and often inhibit lipid peroxidation and last, but not least in animals, glutathione peroxidase, which controls the rate of lipid peroxidation. These mechanisms cope well at normal O2 concentrations but are insufficient at higher levels.
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PMID:Biochemical mechanisms accounting for the toxic action of oxygen on living organisms: the key role of superoxide dismutase. 35 40


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