Ascorbic acid, redox cycling, lipid peroxidation, and the binding of dopamine receptor antagonists

Redox cycling of quinones reduced by ascorbic acid

In aqueous solutions containing ascorbic acid and O₂, many quinones undergo reduction to the semiquinone followed by reoxidation. This redox cycling mediates the oxidation of ascorbic acid and the reduction of O₂ to superoxide and ultimately hydrogen peroxide. For that reason, redox cycling has attracted attention as a source of reactive oxygen species. This redox cycling is paradoxical, however, because the one-equivalent reduction potentials of the reactants are unfavorable, so the concentrations of the products, monodehydroascorbate and superoxide, must be kept extremely low. Disproportionation is not fast enough to eliminate these products. We have investigated the mechanism of ascorbate-driven redox cycling by monitoring the redox status of the quinone and the rate of redox cycling in parallel. Evidence is presented for a mechanism in which monodehydroascorbate is oxidized by the semiquinone. The result is that cycling of the semiquinone and hydroquinone mediates a rapid disproportionation of monodehydroascorbate. This mechanism accounts for the dependence of the redox cycling rate on quinone and ascorbate concentrations as well as on the reduction potential of the quinone. Therefore, it predicts how fast ascorbate-driven redox cycling will generate hydrogen peroxide under a variety of conditions and with different quinones.

Dietary vitamin C reduced mercury contents in the tissues of juvenile olive flounder (Paralichthys olivaceus) exposed with and without mercury

A 2×3 factorial design was employed to evaluate the effects of dietary vitamin C (l-ascorblyl-2-monophosphate, C2MP) levels on growth and tissue mercury (Hg) accumulations in juvenile olive flounder, Paralichthys olivaceus. Six experimental diets with two levels of mercuric chloride (0 or 20mg HgCl2/kg diet) and three levels of vitamin C (0, 100, or 200mg C2MP/kg diet) were added to the basal diet. At the end of 6 weeks feeding trial, in presence or absence of dietary Hg, fish body weight gain, specific growth rate, feed efficiency, protein efficiency ratio and whole body lipid content were increased in a dose-dependent manner as dietary vitamin C level increased in the diets. Interestingly, fish fed 100 or 200mg C2MP/kg diets showed significant interactive effects on reducing Hg content in kidney tissue. These results revealed that dietary vitamin C as 100 or 200mg C2MP/kg diet had protective effect against Hg accumulation in juvenile olive flounder.

Micronutrients and Risk of Parkinson's Disease: A Systematic Review

Parkinson’s disease (PD) is the second most common neurodegenerative disorder. Although the precise pathogenetic mechanisms of PD remain undetermined, there appears to be both genetic and environmental factors that contribute to the risk of developing PD. With regard to environmental risk factors, there has been significant interest related to the role of diet, nutrition, and nutrients on the onset and progression of PD. As the current treatments are predominantly focused on symptomatic management, efforts must be directed toward prevention of the PD and identification of potentially modifiable risk and preventive factors. This comprehensive review gives an overview of studies examining the role of micronutrients in PD, and provides guidance on the value of the reported outcomes.

Extracellular dopamine induces the oxidative toxicity of SH-SY5Y cells

Dopamine-induced neuronal cytotoxicity has been proposed as a leading pathological mechanism underlying many neuronal degenerative disorders including Parkinson disease. Various hypotheses have been proposed including oxidative stress and dopamine (DA)-induced intracellular signal disorder via DA D1 and D2 receptors. The exact mechanism involved in this process is far from clear. In this study, employing a neuronal blastoma cell line, SH-SY5Y, we tried to elucidate the roles of these different suggested mechanisms in this pathological process. The results showed that DA induced cell toxicity in a dose- and time-dependent way. Selective D1 and D2 DA receptor antagonist could not block the cytotoxic effects, whereas reductive reagent ascorbic acid but not GSH could effectively rescue the cell death, suggesting that DA-induced cell toxicity was caused by an extracellular oxidative stress. This was further supported by the enhancing effects of DA transporter blocker, GBR, which could increase the cell death when pretreated. Finally, ascorbic acid could also protect SY5Y cells from DA-induced cellular apoptotic signal changes including PARP and P53. Our studies suggested that DA exerted its cytotoxic effects via an extracellular metabolism, whereas intracellular transportation could reduce its oxidative stress. Cytotoxicity effects induced by extracellular DA could be protected by reductive agents as ascorbic acid. These results help to broaden our understanding of the mechanisms of DA-induced cell death and may provide potentially therapeutical alternative for the neurodegenerative disorders.

Nutrition and the risk for Parkinson's disease: review of the literature

The etiology of Parkinson's disease (PD), the second most common movement disorder, is still unclear. A genetic vulnerability, even in idiopathic PD seems likely. Additional factors like endo- and exotoxins are proposed to contribute to the induction and in some cases possibly acceleration of the disorder. Among the epidemiological risk factors dietary components are being broadly discussed. Moreover, there is a growing awareness of the population concerning possibly preventive dietary habits. However, dietary factors are difficult to assess. This review gives an overview on epidemiological studies addressing a possible relation of dietary compounds and the risk for PD.

Intake of vitamin E, vitamin C, and carotenoids and the risk of Parkinson's disease: a meta-analysis

We studied the effect of vitamin C, vitamin E, and beta carotene intake on the risk of Parkinson's disease (PD). We did a systematic review and meta-analysis of observational studies published between 1966 and March 2005 searching MEDLINE, EMBASE, and the Cochrane Library. Eight studies were identified (six case-control, one cohort, and one cross-sectional). We found that dietary intake of vitamin E protects against PD. This protective influence was seen with both moderate intake (relative risk 0.81, 95% CI 0.67-0.98) and high intake (0.78, 0.57-1.06) of vitamin E, although the possible benefit associated with high intake of vitamin E was not significant. The studies did not suggest any protective effects associated with vitamin C or beta carotene. We conclude that dietary vitamin E may have a neuroprotective effect attenuating the risk of PD. These results require confirmation in randomised controlled trials.

pH-induced alteration and oxidative destruction of heme in purified chromaffin granule cytochrome b(561): implications for the oxidative stress in catecholaminergic neurons

The transmembrane hemoprotein, cytochrome b(561) (b(561)), in the neuroendocrine secretory vesicles is shown to shuttle electrons from the cytosolic ascorbate (Asc) to the intravesicular matrix to provide reducing equivalents for the dopamine beta-monooxygenase (DbetaM) reaction. Intravesicular Asc may also play a role in relieving catecholamine-induced oxidative stress in catecholaminergic neurons. In the present study, we have examined the alteration of purified oxidized b(561) (b(561,ox)) under mild alkaline conditions to probe the structural and functional characteristics of the protein, using UV-vis and EPR spectroscopic and kinetic techniques. Our results show that low spin heme in oxidized b(561) (b(561,ox)) readily transforms to an altered high spin form and then slowly to an Asc nonreducible form, in a pH-, temperature-, and time-dependent manner, which can be described by single-exponential rate equations, A(t) = A(o)(1- e (-kt)) and A(t) = A(o)e(-kt), respectively. More than half of the Asc nonreducible altered b(561) could be converted back to the native b(561) by pH adjustment followed by dithionite reduction, suggesting the reversibility of the process. The heme center of the transformed Asc nonreducible protein is completely bleached instantaneously by dithionite in the presence of atmospheric oxygen, which appears to be mediated by molecular oxygen and/or hydrogen peroxide. These results demonstrate that the heme centers of the protein are susceptible to the pH-induced alteration and oxidative destruction, raising some questions regarding the proposed one alkaline labile, two-heme model of b(561) [Tsubaki, M.; Nakayama, M.; Okuyama, E.; Ichikawa, Y. (1997) J. Biol. Chem. 272, 23206-23210]. The pH-induced alteration and the destruction of heme under oxidative conditions may play a significant role in the amplification of oxidative stress in catecholaminergic neurons.

Interaction of nitric oxide donors and ascorbic acid on D-[3H] aspartate efflux from rat striatal slices

There are conflicting reports in the literature concerning the neuroprotective effect of ascorbic acid on excitotoxic processes in which excessive glutamate release and nitric oxide are supposed to be major factors. To study the influence of ascorbate on the nitric oxide modulated glutamate release rat striatal slices, preloaded with the tritiated glutamate analog D-aspartate, were used. The high potassium-induced efflux of D-[3H]aspartate was concentration dependently stimulated by the nitric oxide donors sodium nitroprusside, S-nitroso-N-acetylpenicillamine (SNAP) or 5-amino 3-morpholinyl-1,2,3-oxadiazolium chloride (SIN-1), as well as by solutions of gaseous nitric oxide and, interestingly, by cyanide. Only the stimulation of D-[3H]aspartate release by SNAP and nitroprusside was affected by ascorbate in terms of a highly significant potentiation. Ascorbate was shown to exert its effect primarily by influencing the decomposition of these nitric oxide donors rather than by a direct interaction of ascorbate with nitric monoxide on glutamate release.

Dietary antioxidants and other dietary factors in the etiology of Parkinson's disease

It has been suggested that dietary antioxidants reduce Parkinson's disease (PD) risk by neutralizing free radicals, thus preventing injury to neurons in the substantia nigra. This case-control study examined the possible role of long-term dietary antioxidant intake in PD etiology. Cases (n = 57) were males 45-79 years old with at least two cardinal signs of PD and no evidence of other forms of parkinsonism or dementia. Age-matched friend controls (n = 50) were chosen from lists provided by the cases. Usual dietary intake 20 years ago, including vitamins E and C and carotenoids, was assessed by a 102-item food frequency questionnaire. Odds ratios and 95% confidence intervals were calculated using conditional logistic regression. Antioxidant intake, adjusted for age, education, smoking, rural living, and total energy intake, was not associated with reduced PD risk. Trends toward greater PD risk were associated with higher intakes of vitamin C and carotenoids, especially xanthophylls, reflecting higher intakes by PD cases of fruit and certain vegetables. Intakes of sweet foods, including fruit, were associated with higher PD risk, suggesting that the observed trends may be due to a preference for sweet foods. This study does not provide support for a protective effect of long-term dietary antioxidant intake on PD risk.

Ascorbate-induced free radical toxicity to isolated islet cells

Ascorbate is known to be cytotoxic by generating free oxygen radicals, a property shared with the diabetogenic drug alloxan. Some reports indicate that also ascorbate may be diabetogenic. In this study the cytotoxicity of ascorbate on isolated mouse islet cells has been investigated. Ascorbate (0.5-2.0 mmol/l) induced a concentration-dependent increase of trypan blue uptake by the cells, indicating an increase of membrane permeability to the dye. Trypan blue uptake induced by 2.0 mmol/l ascorbate was inhibited by concomitant incubation of the cells with 200 mg/l superoxide dismutase, 200 mg/l catalase, 3.0 mmol/l cytochrome-c or 50 mumol diethylenetriaminepentacetic acid (DTPA), but not by 50 mmol/l D-mannitol. The results indicate that ascorbate is cytotoxic to islet cells by metal-catalysed free radical generation.

Identification of a dopamine-binding protein on the membrane of the human platelet

The binding of [3H]dopamine to platelet membranes has been examined in an attempt to identify the putative dopamine-uptake mechanism of the platelet. [3H]Dopamine has been shown to bind to a 42,000 Da glycoprotein in platelet membrane with high affinity (Kd = 22.6 nM) and binding of [3H]dopamine was competed for by dopamine, molecules with catechol moieties, 5-hydroxytryptamine, GSH and ascorbic acid. Differences in pharmacological profile and molecular mass suggest that [3H]dopamine does not bind to a known receptor, a neuronal-type dopamine transporter or the platelet 5-hydroxytryptamine-uptake site. It is proposed that this novel binding site for dopamine, which has been purified 1000-fold from particulate platelet membrane, is likely to be a component of the dopamine-uptake mechanism of the human platelet.

Modulation of collagen synthesis by growth factors: the role of ascorbate-stimulated lipid peroxidation

Ascorbic acid has been shown to stimulate collagen synthesis through induction of lipid peroxidation leading to increased transcription of the collagen genes. The mechanism by which lipid peroxidation stimulates collagen transcription is unknown; however, an alteration of cell membranes may affect the activity of serum growth factors leading to a change in gene expression. To test this hypothesis, we treated dermal fibroblasts with transforming growth factor-beta (TGF-beta), epidermal growth factor (EGF), interleukin-1 (IL-1), platelet-derived growth factor (PDGF), or fibroblast growth factor (FGF) in the presence of lipid peroxidation stimulating (200 microM) and nonstimulating (1 microM) concentrations of ascorbic acid. EGF and IL-1 had no effect on collagen synthesis at either concentration of ascorbic acid. FGF affected collagen synthesis only in the presence of 200 microM ascorbic acid, producing both a stimulation (0.4-2 ng/ml) and an inhibition (greater than 50 ng/ml). PDGF and TGF-beta stimulated collagen synthesis in the presence of both concentrations of ascorbic acid, with TGF-beta producing an 11-fold increase in collagen synthesis in the presence of ascorbate. This synergism produced by the combination of ascorbic acid and TGF-beta was inhibitable by the lipid peroxidation inhibitor, propyl gallate. These results indicate that regulation of collagen synthesis by ascorbic acid does not occur through altering the response to EGF or Il-1. Ascorbate has no effect on PDGF but the effects of TGF-beta and FGF on collagen synthesis appear to be sensitive to lipid peroxidation.

d-alpha-tocopherol inhibits collagen alpha 1(I) gene expression in cultured human fibroblasts. Modulation of constitutive collagen gene expression by lipid peroxidation

Ascorbic acid stimulates collagen gene transcription in cultured fibroblasts, and this effect is mediated through the induction of lipid peroxidation by ascorbic acid. Quiescent cultured fibroblasts in the absence of ascorbic acid have a high constitutive level of collagen production, but the mechanisms of collagen gene regulation in this unstimulated state are not known. Because lipid peroxidation also occurs in normal cells, we wondered if lipid peroxidation plays a role in the regulation of basal collagen gene expression. Inhibition of lipid peroxidation in cultured human fibroblasts with d-alpha-tocopherol or methylene blue decreased the synthesis of collagen, the steady-state levels of procollagen alpha 1(I) mRNA and the transcription of the procollagen alpha 1(I) gene. This effect on collagen gene expression was selective and not associated with cellular toxicity. Thus, these experiments suggest a role for lipid peroxidation in the modulation of constitutive collagen gene expression.

Accumulation of noradrenaline and its oxidation products by cultured rodent astrocytes

The accumulation of [3H]noradrenaline ([3H]NA) and its oxidation products was studied in primary cultures of cerebral astrocytes. Astroglial accumulation of radiolabeled catecholamine ([3H] NA and oxidation products) was enhanced by manganese or iron, but it was inhibited by unlabeled NA, dopamine or ascorbate. Tissue: medium ratios of radioactivity increased as extracellular [3H]NA was oxidized. When extracellular oxidation was prevented by ascorbate, as confirmed by high performance liquid chromatography with electrochemical detection, either ouabain pretreatment or nominally Na(+)-free incubation medium inhibited approximately one-half of specific [3H]NA accumulation by rat (but not mouse) astrocytes. These observations suggest that neurological responses to trace metals and ascorbate may arise from the effects of these agents on the clearance of extracellular catecholamines. Astrocytes can accumulate oxidation products of NA more rapidly than they take up NA itself, but ascorbate at physiological concentrations prevents the oxidation process in extracellular fluid. Furthermore, in the presence of ascorbate, Na(+)-dependent transport mediates a significant component of NA accumulation in rat astrocytes.

Lipid peroxidation effects of a novel iron compound, ferric maltol. A comparison with ferrous sulphate

Lipid peroxidation effects of ferric maltol have been compared with those of ferrous sulphate both in lecithin liposomes and in brush border and mitochondrial membranes prepared from rat small intestine. Ferrous sulphate, but not ferric maltol, initiated peroxidation in liposomes as measured by conjugated diene production, but, with 500 microM ascorbic acid present, both caused intense peroxidation which was inhibitable by N2, tocopherol, maltol and ferrous chelators, but not by OH or H2O2 scavengers. The rate of peroxidation increased with ferrous sulphate concentration up to 100 microM but was independent of ferric maltol concentration between 5-500 microM. Material eluted from rat small intestine contained a reducing factor, similar in size to ascorbic acid, capable of generating ferrous ions from ferric maltol and initiating peroxidation. Peroxidation in mitochondrial membranes appeared unaffected by addition of iron whilst that in brush border membranes was detectable only in the presence of iron. At iron concentrations of 100 microM and above ferric maltol produced less liposomal peroxidation than ferrous sulphate. Maltol itself may delay recycling of Fe3+ to Fe2+. Thus ferric maltol could provide a less toxic alternative to ferrous salts in the oral treatment of iron-deficiency.

High-affinity cannabinoid binding site: regulation by ions, ascorbic acid, and nucleotides

The high-affinity cannabinoid site in rat brain is an integral component of brain membranes that recognizes cannabinoids with inhibitory constants (Ki) in the nanomolar range. To clarify its physiological role, we studied the regulation of [3H]5'-trimethylammonium delta 8-tetrahydrocannabinol ([3H]TMA) binding. The site is inhibited by heavy metal ions, such as La3+, at low micromolar concentrations; divalent cations, such as Ca2+ and Mg2+, inhibit [3H]TMA binding, though at somewhat higher concentrations. In contrast, [3H]TMA binding is stimulated by Fe2+, Cu2+, and Hg2+ ions. Ascorbic acid and its analogs are also stimulators of cannabinoid binding at low micromolar concentrations. Stimulation of [3H]TMA binding by ascorbate or ions is dependent upon molecular oxygen, but is not inhibited by metabolic poisons. Metabolically stable nucleoside triphosphate analogs enhance [3H]TMA binding by different mechanisms, with hydrolysis of a high-energy phosphate bond apparently requisite for these influences. These results suggest that the cannabinoid binding site is associated with a nucleotide-utilizing protein possessing multiple regulatory subsites.

Ascorbic acid inhibits [3H]SCH-23390 binding to striatal dopamine D1 receptors

The present study describes the inhibition of [3H]SCH-23390 binding to striatal dopamine D1 receptors in the presence of ascorbic acid. Specific [3H]SCH-23390 binding was maximally inhibited by 0.1 mM ascorbic acid. As determined by Scatchard analysis the binding in the presence of 0.01, 0.1, or 10 mM ascorbic acid was consonant with non-competitive inhibition with a 26%, 38%, or 19% decrease, respectively, in the maximal number of binding sites; the affinity of these binding sites was not affected. Inhibition of [3H]SCH-23390 binding by ascorbic acid was reversible; striatal homogenates incubated with 0.1 mM ascorbic acid and subsequently washed free of ascorbic acid had the same Scatchard parameters as untreated preparations.