Handbook of Vitamin C Research

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158Borut Poljsak and John G. Ionescuautooxidation reactions within the cell produce superoxide by addition of an electron tomolecular oxygen (Anderson and Phillips 1999). It was reported that O 2-˙ is produced fromascorbate (AH - ) autooxidation by dioxygen (Scarpa 1983), yet is has been shown (Buettner1993) that aerobic oxidation of ascorbate strictly requires a metal catalyst (Figure 1).Figure 2. The prevention of mercury II induced ascorbate oxidation by reduced glutathione.Figure 3. The prevention of cupper II induced ascorbate oxidation by reduced glutathione.
Pro-Oxidant vs. Antioxidant Effects of Vitamin C 159Antioxidants, which are reducing agents, capable of reacting with molecular oxygen (e.g.ascorbic acid) will generate superoxide radicals under aerobic conditions. This will dismutateto H 2 O 2 that can enter cells and react with superoxide or reduced metal ions to form highlydamaging hydroxyl radicals (Anderson and Phillips 1999). Even though H 2 O 2 production fromascorbate and dioxygen is thermodynamically favored, a direct oxidation of ascorbate bydioxygen does not occur. Thus, the spin restriction of dioxygen is a kinetic barrier thatprevents the oxidation of organic biomolecules regardless of thermodynamic considerations(Miller et al. 1990). In the absence of transitional metals, the rate constant for the reaction ofdioxygen with ascorbate has been reported to be 6x10 –7 s -1 , which results in an observedsecond rate order constant of approximately 2x10 -3 M -1 s -1 . Mitochondrial respiration keepsO 2 0-10 M in the cell while intracellular concentration of GSH 1 mM (Buettner 1993).In the study of Poljsak et al. (2005) it was demonstrated that hydroxyl radicals are generatedfollowing the interaction of Cr(VI) with ascorbic acid in vitro. This is believed to involve thereduction of the metal ion by ascorbic acid, followed by the reduction of oxygen toH 2 O 2 /HO • . Stearns et al. (1995) found out that the reduction of Cr(VI) by ascorbate underphysiological conditions produced Cr(V) and carbon-based radicals as intermediates which reactedwith DNA to produce Cr-DNA adducts and DNA single-strand breaks, respectively. Lowconcentrations of ascorbate enhance oxygen radical activity whilst high concentrationsscavenge hydroxyl radicals, singlet oxygen and lipid peroxides.A study by Anderson et al. (1997) examined DNA damage in lymphocytes with cometassay. Vitamin C supplementation significantly elevated plasma vitamin C concentration, buthad no effect on oxidative DNA damage either with or without an ex vivo hydrogen peroxidechallenge. However, a statistically significant increase in bleomycin-induced aberrations wasfound after vitamin C supplementation. In the study of Green et al. (1994) vitamin C acted asa pro-oxidant when added to isolated lymphocytes in vitro. Addition of vitamin C to purifiedDNA or isolated nuclei in the presence of redox active metal ions in vitro results in singlestrandbreaks and base modifications such as 8-oxodG (Drouin et al. 1996; Fischer-Nielsenet al.1992, Hu and Shih 1997).The results of Sugiyama et al. (Sugiyama et al. 1991a; Sugiyama1991b; Sugiyama 1991c) on Chinese hamster V-79 cells showed increased cytotoxicity of Cr(VI)in ascorbic acid pretreated cells.Other reports on the effect of ascorbic acid alone on different types of cultured cells areconfusing and contradictory (Witenberg et al. 1999; Barroso et al. 1997; Sakagami et al.2000; Satoh et al. 1998; Blankenship et al. 1997). Furthermore, vitamin C may be able topromote metal ion-dependent hydroxyl radical production in biological fluids, but only undercertain unphysiological conditions (Smith et al. 1997; Winterbourn, 1981).Addition of Fe 2+ (but not Fe 3+ ) and H 2 O 2 to human serum results in rapid generation ofhydroxyl radical (Fenton reaction, Figure 4). On the other side, addition of ascorbate to Fe 3+ -loaded serum leads to an immediate increase of superoxide generation, through ascorbateautooxidation (Figure 5) (Ionescu 2002).
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ContentsPrefaceChapter IChapter IIC
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PrefaceThe 6-carbon lactone known a
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Prefaceixbalance of antioxidant and
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Prefacexiprovided or is contradicto
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PrefacexiiiChapter IX - Background:
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Prefacexvdetoxification defence sys
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Prefacexviiprogressed faster than i
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In: Handbook of Vitamin C Research
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Human Specific Vitamin C Metabolism
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mgmgmgHuman Specific Vitamin C Meta
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88Ana I. Haza, Almudena García and
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90Figure 1.Ana I. Haza, Almudena Ga
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94Figure 3.Ana I. Haza, Almudena Ga
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100Ana I. Haza, Almudena García an
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Page 149 and 150: Vitamin C: Dietary Requirements, Di
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Page 175 and 176: Pro-Oxidant vs. Antioxidant Effects
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208Magdalena Stevanović and Dragan
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210Magdalena Stevanović and Dragan
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Molecular Bases of the Cellular Han
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Vitamin C: Daily Requirements, Diet
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262Alan M. Preston, Luis Vázquez Q
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Percent of Study Group Selecting Di
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The Role of Vitamin C in Human Repr
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300Mustafa NazıroğluKeywords: Vit
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302Mustafa NazıroğluNO is synthes
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304Mustafa Nazıroğlu‗recycling
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306Mustafa Nazıroğluagents, such
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308Mustafa NazıroğluFuture Direct
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310Mustafa Nazıroğlu[21] Basu, TK
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312Mustafa Nazıroğlu[59] Cengiz M
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314Samar Al Sayegh Petkovšek and B
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(mgg -1 )320Samar Al Sayegh Petkov
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330Antonio J. López-Farré, José
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Vitamin C in the Treatment of Endot
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Vitamin C Intake by Japanese Patien
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Reciprocal Effects of Ascorbate on
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378Akihito Ishigamicontrols (12,13)
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380Akihito IshigamiFigure 3. Vitami
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382Akihito IshigamiFuture of the SM
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The Importance of Food Processing o
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IndexAA , 249abdominal cramps, 243a
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Index 391atherosclerotic plaque, 23
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Index 393catalase, 6, 13, 169, 181,
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Index 395cyclooxygenase-2, 337, 341
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Index 397endometrium, 286, 294endon
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Index 399GCS, 18gel, x, 87, 92, 121
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Index 401hyperglycaemia, 223hypergl
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Index 403kinetics, 71, 120, 124, 14
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Index 405metastases, 182metastasis,
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Index 407nitrate, 14, 88, 179, 234,
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Index 409phosphodiesterase, 344, 35
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Index 411recovery, vii, 1, 5, 7, 8,
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Index 413steady state, 63, 76, 78st
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Index 415UV, 156, 191, 201, 204, 22
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