One-electron oxidation and reduction reactions of vitamin C derivatives: 6-bromo- and 6-chloro-6-deoxy-ascorbic acid

Chemical Transport Knockout for Oxidized Vitamin C, Dehydroascorbic Acid, Reveals Its Functions in vivo

Despite its transport by glucose transporters (GLUTs) in vitro, it is unknown whether dehydroascorbic acid (oxidized vitamin C, DHA) has any in vivo function. To investigate, we created a chemical transport knockout model using the vitamin C analog 6-bromo-ascorbate. This analog is transported on sodium-dependent vitamin C transporters but its oxidized form, 6-bromo-dehydroascorbic acid, is not transported by GLUTs. Mice (gulo^−/−) unable to synthesize ascorbate (vitamin C) were raised on 6-bromo-ascorbate. Despite normal survival, centrifugation of blood produced hemolysis secondary to near absence of red blood cell (RBC) ascorbate/6-bromo-ascorbate. Key findings with clinical implications were that RBCs in vitro transported dehydroascorbic acid but not bromo-dehydroascorbic acid; RBC ascorbate in vivo was obtained only via DHA transport; ascorbate via DHA transport in vivo was necessary for RBC structural integrity; and internal RBC ascorbate was essential to maintain ascorbate plasma concentrations in vitro/in vivo.

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Red cells in vivo obtain vitamin C (ascorbate) by dehydroascorbic acid transport.

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Red blood cell ascorbate is necessary to maintain red blood cell structural integrity.

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Red blood cell ascorbate maintains external plasma ascorbate concentrations in vivo by transmembrane electron transfer.

In animals and humans, it is unknown whether the oxidized form of vitamin C, termed dehydroascorbic acid, has a physiologic purpose. Using a mouse model and a custom-synthesized vitamin C analog, we show that red blood cells obtain their vitamin C by transport of dehydroascorbic acid, instead of vitamin C itself. The transported material is reduced inside and has at least two physiologic functions. One is to maintain structural integrity of red blood cells, and the other is to maintain vitamin C in the liquid part of blood, plasma.

Chemical repair activity of free radical scavenger edaravone: reduction reactions with dGMP hydroxyl radical adducts and suppression of base lesions and AP sites on irradiated plasmid DNA

Reactions of edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one) with deoxyguanosine monophosphate (dGMP) hydroxyl radical adducts were investigated by pulse radiolysis technique. Edaravone was found to reduce the dGMP hydroxyl radical adducts through electron transfer reactions. The rate constants of the reactions were greater than 4 × 10(8) dm(3) mol(-1) s(-1) and similar to those of the reactions of ascorbic acid, which is a representative antioxidant. Yields of single-strand breaks, base lesions, and abasic sites produced in pUC18 plasmid DNA by gamma ray irradiation in the presence of low concentrations (10-1000 μmol dm(-3)) of edaravone were also quantified, and the chemical repair activity of edaravone was estimated by a method recently developed by the authors. By comparing suppression efficiencies to the induction of each DNA lesion, it was found that base lesions and abasic sites were suppressed by the chemical repair activity of edaravone, although the suppression of single-strand breaks was not very effective. This phenomenon was attributed to the chemical repair activity of edaravone toward base lesions and abasic sites. However, the chemical repair activity of edaravone for base lesions was lower than that of ascorbic acid.

Generation of Hydroxyl Radicals from Dissolved Transition Metals in Surrogate Lung Fluid Solutions

Epidemiological research has linked exposure to atmospheric particulate matter (PM) to several adverse health effects, including cardiovascular and pulmonary morbidity and mortality. Despite these links, the mechanisms by which PM causes adverse health effects are poorly understood. The generation of hydroxyl radical (.OH) and other reactive oxygen species (ROS) through transition metal-mediated pathways is one of the main hypotheses for PM toxicity. In order to better understand the ability of particulate transition metals to produce ROS, we have quantified the amounts of .OH produced from dissolved iron and copper in a cell-free, surrogate lung fluid (SLF). We also examined how two important biological molecules, citrate and ascorbate, affect the generation of .OH by these metals. We have found that Fe(II) and Fe(III) produce little .OH in the absence of ascorbate and citrate, but that they efficiently make .OH in the presence of ascorbate and this is further enhanced when citrate is also added. In the presence of ascorbate, with or without citrate, the oxidation state of iron makes little difference on the amount of .OH formed after 24 hours. In the case of Cu(II), the production of .OH is greatly enhanced in the presence of ascorbate, but is inhibited by the addition of citrate. The mechanism for this effect is unclear, but appears to involve formation of a citrate-copper complex that is apparently less reactive than free, aquated copper in either the generation of HOOH or in the Fenton-like reaction of copper with HOOH to make .OH. By quantifying the amount of .OH that Fe and Cu can produce in surrogate lung fluid, we have provided a first step into being able to predict the amounts of .OH that can be produced in the human lung from exposure to PM containing known amounts of transition metals.

Effect of cisplatin and 6-bromo-6-deoxy-L-ascorbic acid on some biochemical and functional parameters in mice

The results of the present study demonstrate that 6-bromo-6-deoxy-L-ascorbic acid (6-BrAA), an antioxidative derivative of ascorbic acid, is capable of lowering the toxicity of cisplatin, cis-diaminedichloroplatinum (cis-DDP), an anticancerogenic drug. The biological aspects and pharmacological significance of a combined treatment of these two substances were investigated in a mouse model. The results indicate that the effectiveness of 6-BrAA on biological response(s) is strongly dependent on the dose of cis-DDP. Injection of 10 mg/kg body weight (bw) of cis-DDP following pretreatment with 6-BrAA (480 mg/kg bw) enhances the tissue-protecting effect of 6-BrAA and reduces, to some extent, the ensuing nephro-, liver and spleen toxicity. On the other hand, 6-BrAA in animals treated with a higher dose of cis-DDP (15 mg/kg bw) leads to exacerbation of the toxic cis-DDP effect and concurrent loss of the protective potential of 6-BrAA with respect to tissue damage. The exact mechanism(s) of 6-BrAA protection and exacerbation of the toxic cis-DDP effect is unclear, although scavenging or generating of free radicals may play an important role. The results obtained may be of importance in planning the rational use of cis-DDP and 6-BrAA administration in the potential treatment of cancer.

Positron-labeled antioxidant 6-deoxy-6-[18F]fluoro-L-ascorbic acid: increased uptake in transient global ischemic rat brain

The in vivo uptake and distribution of 6-deoxy-6-[18F]fluoro-L-ascorbic acid (18F-DFA) were investigated in rat brains following postischemic reperfusion. Global cerebral ischemia was induced in male Wistar rats for 20 min by occlusion of four major arteries. Two time points were chosen for 18F-DFA injection to rats subjected to cerebral ischemia, at the start of recirculation and 5 days following recirculation. The rats were then killed at 2 h after tail-vein administration of 18F-DFA and tissue radioactivity concentration was determined. Increased uptake of radioactivity in particular brain regions, including the cerebral cortex, hypothalamus, and amygdala following injection of 18F-DFA, compared to the sham-operated control, was observed 5 days after reperfusion. Similar results were also obtained in in vitro experiments using brain slices. Abnormal in vivo accumulation of 45Ca, a marker of regional postischemic injury, was observed in these brain regions in tissue dissection experiments. Furthermore, metabolite analysis of nonradioactive DFA using 19F-NMR showed that DFA remained intact in the postischemic reperfusion brain. The present results indicate that 18F-DFA increasingly accumulates in damaged regions of postischemic reperfusion brain.

6-Amino-6-deoxyascorbic acid induces apoptosis in human tumor cells

6-Amino-6-deoxyascorbic acid was found to inhibit human tumor cell growth. The antitumor effect depends on the tumor type and concentration of the acid. After cell treatment with 6-amino-6-deoxyascorbic acid, drastic morphological changes were found. Although image analysis did not show a difference in p53 and c-myc gene expression, the appearance of chromatin aggregation and DNA fragmentation points to apoptosis or programmed cell death.