Two distinct uptake mechanisms for ascorbate and dehydroascorbate in human lymphoblasts and their interaction with glucose

Review: Targeting therapeutics against glutathione depletion in diabetes and its complications

Glutathione (GSH) is the most abundant intracellular antioxidant, the dysregulation of which is widely implicated in disease states. There is in vitro and clinical evidence that abnormal glutathione status is involved in β-cell dysfunction and in the pathogenesis of long-term complications of diabetes. Interest has developed in the potential for therapeutic modification of glutathione status in the treatment of diabetes. There is evidence which supports the use of glutathione pro-drugs, lipoic acid and vitamin supplementation but further studies are required before these enter widespread use. Studies into the role of oxidative stress in diabetes rely heavily on the ability to measure glutathione, which has been a problematic analyte to measure in the laboratory. New electrochemical methods being developed should speed up the rate at which data can be accumulated and will help define clinical utility for its measurement.

Dehydroascorbic acid-induced endoplasmic reticulum stress and leptin resistance in neuronal cells

Due to its anti-obesity effects, an adipocyte-derived hormone, leptin, has become important for the treatment of obesity. However, most obese subjects are in a state of leptin resistance, and endoplasmic reticulum (ER) stress is suggested to be involved in the pathophysiology of leptin resistance. Dehydroascorbic acid (DHAA), an oxidized form of vitamin C, was found to be increased in diabetes. In the present study, we investigated the possible effects of DHAA on the activation of ER stress and leptin resistance. A human neuroblastoma cell line, stably transfected with the Ob-Rb leptin receptor (SH-SY5Y-ObRb), was treated with DHAA. We found that DHAA upregulated ER stress-related genes such as GRP78, CHOP, and spliced XBP1. Moreover, leptin-induced STAT3 phosphorylation was hindered by DHAA. These results suggested that increases in the levels of DHAA might be harmful to neurons, contributing to defective leptin-responsive signaling.

Ascorbate attenuates pulmonary emphysema by inhibiting tobacco smoke and Rtp801-triggered lung protein modification and proteolysis

Cigarette smoking causes emphysema, a fatal disease involving extensive structural and functional damage of the lung. Using a guinea pig model and human lung cells, we show that oxidant(s) present in tobacco smoke not only cause direct oxidative damage of lung proteins, contributing to the major share of lung injury, but also activate Rtp801, a key proinflammatory cellular factor involved in tobacco smoke-induced lung damage. Rtp801 triggers nuclear factor κB and consequent inducible NOS (iNOS)-mediated overproduction of NO, which in combination with excess superoxide produced during Rtp801 activation, contribute to increased oxido-nitrosative stress and lung protein nitration. However, lung-specific inhibition of iNOS with a iNOS-specific inhibitor, N6-(1-iminoethyl)-L-lysine, dihydrochloride (L-NIL) solely restricts lung protein nitration but fails to prevent or reverse the major tobacco smoke-induced oxidative lung injury. In comparison, the dietary antioxidant, ascorbate or vitamin C, can substantially prevent such damage by inhibiting both tobacco smoke-induced lung protein oxidation as well as activation of pulmonary Rtp801 and consequent iNOS/NO-induced nitration of lung proteins, that otherwise lead to increased proteolysis of such oxidized or nitrated proteins by endogenous lung proteases, resulting in emphysematous lung damage. Vitamin C also restricts the up-regulation of matrix-metalloproteinase-9, the major lung protease involved in the proteolysis of such modified lung proteins during tobacco smoke-induced emphysema. Overall, our findings implicate tobacco-smoke oxidant(s) as the primary etiopathogenic factor behind both the noncellular and cellular damage mechanisms governing emphysematous lung injury and demonstrate the potential of vitamin C to accomplish holistic prevention of such damage.

Non-transferrin iron reduction and uptake are regulated by transmembrane ascorbate cycling in K562 cells

K562 erythroleukemia cells import non-transferrin-bound iron (NTBI) by an incompletely understood process that requires initial iron reduction. The mechanism of NTBI ferrireduction remains unknown but probably involves transplasma membrane electron transport. We here provide evidence for a novel mechanism of NTBI reduction and uptake by K562 cells that utilizes transplasma membrane ascorbate cycling. Incubation of cells with dehydroascorbic acid, but not ascorbate, resulted in (i) accumulation of intracellular ascorbate that was blocked by the glucose transporter inhibitor, cytochalasin B, and (ii) subsequent release of micromolar concentrations of ascorbate into the external medium via a route that was sensitive to the anion channel inhibitor, 4,4'-diisothiocyanatostilbene-2,2'-disulfonate. Ascorbate-deficient control cells demonstrated low levels of ferric citrate reduction. However, incubation of the cells with dehydroascorbic acid resulted in a dose-dependent stimulation of both iron reduction and uptake from radiolabeled [(55)Fe]ferric citrate. This stimulation was abrogated by ascorbate oxidase treatment, suggesting dependence on direct chemical reduction by ascorbate. These results support a novel model of NTBI reduction and uptake by K562 cells in which uptake is preceded by reduction of iron by extracellular ascorbate, the latter of which is subsequently regenerated by transplasma membrane ascorbate cycling.

Translational control of the ascorbic acid transporter SVCT2 in human platelets

Reactive oxygen species (ROS) and redox state have emerged as physiological mediators, controlling blood coagulation and thrombosis. The redox balance is obviously linked to the presence of antioxidants; in particular, vitamin C appears to be a key modulator of platelet oxidative state, since these cells physiologically accumulate ascorbic acid and, moreover, platelet ascorbate plays a role during aggregation. Here, we showed that platelets could compensate for fluctuations in ascorbate levels by modulating the expression of the Na+-dependent transporter SVCT2. Furthermore, the use of anucleated cells demonstrated, for the first time, that SVCT2 expression could be regulated at the translational level. The control of ascorbic acid uptake, through regulation of its carrier, was not only related to substrate availability, but it also occurred during platelet activation, which was accompanied by vitamin C deprivation and alteration in the redox state. Finally, we showed that changes in intracellular ascorbic acid content had physiological relevance, since they modulate the surface sulfhydryl content and the thrombus viscoelastic properties. Beside its role during aggregation, vitamin C may also have important effects during postaggregatory events.

The contribution of ascorbic acid and dehydroascorbic acid to the protective role of pleura during inflammatory reactions

It is well-known that parapneumonic effusions lead to the formation of inflammatory exudates which contain an increasing amount of inflammatory cells, especially polymorphonuclear. At these pathological conditions characterized by oxidative stress, ascorbic acid (AA) plays an important role in quenching free radicals, so that it could protect neutrophils and mesothelial cells from oxidative damage. Besides that ascorbic acid and its metabolite dehydroascorbic acid (DHA) alters the sheep visceral and parietal pleura permeability. More specific ascorbic acid as well as dehydroascorbic acid decreases the permeability of pleura after addition on apical and basolateral side in both visceral and parietal pleurae. It seems that, AA and DHA have an opposite action upon pleura from that of the inflammatory mediators, like VEGF, which increases the permeability of pleura and causes mesothelial barrier dysfunction. The decrease of pleura permeability induced by AA and DHA suggest the hypothesis that AA and/or its metabolite DHA during inflammatory reactions not only protects mesothelial cells from oxidative damage, but also contributes to maintaining the mesothelial barrier function. Consequently, the inflammatory pleural fluid may be trapped in pleural space and the inflammation may be restricted, and have extension avoided.

Dehydroascorbate transport in human chondrocytes is regulated by hypoxia and is a physiologically relevant source of ascorbic acid in the joint

OBJECTIVE

To evaluate the dehydroascorbate (DHA) transport mechanisms in human chondrocytes.

METHODS

The transport of L-(14)C-DHA in human chondrocytes was analyzed under various conditions, including the use of RNA interference (RNAi), to determine the role of glucose transporter 1 (GLUT-1) and GLUT-3 in L-14C-DHA transport and to evaluate the effects of physiologically relevant oxygen tensions on L-14C-DHA transport. In order to estimate the contributions of reduced ascorbic acid (AA) and DHA to intracellular ascorbic acid (Asc), the quantities of AA and DHA were measured in synovial fluid samples from osteoarthritis (OA) patients and compared with the reported levels in rheumatoid arthritis (RA) patients.

RESULTS

DHA transport in human chondrocytes was glucose-sensitive, temperature-dependent, cytochalasin B-inhibitable, modestly stereoselective for L-DHA, and up-regulated by low oxygen tension. Based on the RNAi results, GLUT-1 mediated, at least in part, the uptake of DHA, whereas GLUT-3 had a minimal effect on DHA transport. DHA constituted a mean 8% of the total Asc in the synovial fluid of OA joints, in contrast to 80% of the reported total Asc in RA joints.

CONCLUSION

We provide the first evidence that chondrocytes transport DHA via the GLUTs and that this transport mechanism is modestly selective for L-DHA. In the setting of up-regulated DHA transport at low oxygen tensions, DHA would contribute 26% of the total intracellular Asc in OA chondrocytes and 94% of that in RA chondrocytes. These results demonstrate that DHA is a physiologically relevant source of Asc for chondrocytes, particularly in the setting of an inflammatory arthritis, such as RA.

Vitamin C metabolomic mapping in experimental diabetes with 6-deoxy-6-fluoro-ascorbic acid and high resolution 19F-nuclear magnetic resonance spectroscopy

Metabolomic mapping is an emerging discipline geared at providing information on a large number of metabolites as a complement to genomics and proteomics. Here we have probed ascorbic acid homeostasis and degradation in diabetes using 6-deoxy-6-fluoro ascorbic acid (F-ASA) and 750 MHz (19)F-nuclear magnetic resonance (NMR) spectroscopy with proton decoupling In vitro, Cu(2+)-mediated degradation of F-ASA revealed the formation of 4 major stable degradation products at 24 hours. However, when normal or diabetics rats were injected with F-ASA intraperitoneally (IP) for 4 days, up to 20 fluorine-labeled compounds were observed in the urine. Their composition resembled, in part, metal catalyzed degradation of F-ASA and was not explained by spontaneous degradation in the urine. Diabetes led to a dramatic increase in urinary F-ASA loss and a relative decrease in most other urinary F-compounds. Diabetes tilted F-ASA homeostasis toward oxidation in liver (P <.01), kidney (P <.01), spleen (P <.01), and plasma (P <.01), but tended to decrease oxidation in brain, adrenal glands, and heart. Surprisingly, however, besides the major oxidation product fluoro-dehydroascorbic acid (F-DHA), no F-ASA advanced catabolites were detected in tissues at 5 micromol/L sensitivity. These findings not only confirm the key role of the kidney in diabetes-mediated loss of ascorbic acid, but demonstrate that only selected tissues are prone to increased oxidation in diabetes. While the structure of most degradation products needs to be established, the method illustrates the power of high resolution (19)F-NMR spectroscopy for the mapping of complex metabolomic pathways in disease states.

Reduction of the endoplasmic reticulum accompanies the oxidative damage of diabetes mellitus

The endoplasmic reticulum (ER), similary to other subcompartments of the eukaryotic cell possesses a relatively oxidizing environment. The special milieu of ER lumen is important for many ER-specific processes (redox protein folding, glycoprotein synthesis, quality control of secreted proteins, antigen presentation, etc.). Despite of the vital importance of redox regulation in the ER, we have a surprisingly fragmented knowledge about the mechanisms responsible for the ER redox balance. Moreover, new observations on disulfide bridge synthesis and on glutathione functions urge us to revise our recent theories based on many indirect and in vitro results. We have also very little information about the effects of different pathological conditions on the thiol metabolism and redox folding in the ER. Examining the role of molecular chaperones in the cellular pathology of diabetes mellitus we found that the ER redox environment shifted to a more reducing state, which was followed by changes of the thiol metabolism and structural-functional changes of the protein machinery involved in the redox folding process in diabetes. The possible consequences of these unexpected changes are also discussed.

Are diabetic neuropathy, retinopathy and nephropathy caused by hyperglycemic exclusion of dehydroascorbate uptake by glucose transporters?

Vitamin C exists in two major forms. The charged form, ascorbic acid (AA), is taken up into cells via sodium-dependent facilitated transport. The uncharged form, dehydroascorbate (DHA), enters cells via glucose transporters (GLUT) and is then converted back to AA within these cells. Cell types such as certain endothelial and epithelial cells as well as neurons that are particularly prone to damage during diabetes tend to be those that appear to be dependent on GLUT transport of DHA rather than sodium-dependent AA uptake. We hypothesize that diabetic neuropathies, nephropathies and retinopathies develop in part by exclusion of DHA uptake by GLUT transporters when blood glucose levels rise above normal. AA plays a central role in the antioxidant defense system. Exclusion of DHA from cells by hyperglycemia would deprive the cells of the central antioxidant, worsening the hyperglycemia-induced oxidative stress level. Moreover, AA participates in many cellular oxidation-reduction reactions including hydroxylation of polypeptide lysine and proline residues and dopamine that are required for collagen production and metabolism and storage of catecholamines in neurons. Increase in the oxidative stress level and metabolic perturbations can be expected in any tissue or cell type that relies exclusively or mainly on GLUT for co-transport of glucose and DHA including neurons, epithelial cells, and vascular tissues. On the other hand, since DHA represents a significant proportion of total serum ascorbate, by increasing total plasma ascorbate concentrations during hyperglycemia, it should be possible to correct the increase in the oxidative stress level and metabolic perturbations, thereby sparing diabetic patients many of their complications.

Recycling processes of cellular ascorbate generate oxidative stress in pancreatic tissues in in vitro system

Ascorbate is a reducing agent, which is also known to oxidize cellular components. Our proposed mechanism of the oxidative action is as follows: Ascorbate is concentrated in the pancreas and is leaked in adverse conditions, and oxidized to dehydroascorbate. The dehydroascorbate is carried into cells by a glucose transporter (GLUT) and reduced back to ascorbate. The reduction processes take electrons from other cellular components. Ascorbate or dehydroascorbate treatment elevated thiobarbituric acid-reactive substance (TBARS) concentrations in pancreas. The elevations in TBARS concentrations were blocked by cytochalasin B, a GLUT inhibitor. To confirm further the prooxidative action, changes in glutathione content were quantified. Glutathione concentrations were lower in ascorbate- or dehydroascorbate-treated groups. The ascorbate-induced decrease in glutathione was blocked by cytochalasin B. To prevent oxidation of ascorbate to dehydroascorbate, glutathione was added to the medium. The ascorbate plus glutathione and dehydroascorbate plus glutathione groups showed lower TBARS concentrations than those of the ascorbate and dehydroascorbate groups, respectively. There were changes in the morphology of Langerhans islets following ascorbate treatment, which disappeared following treatment with ascorbate plus cyto-chalasin B. The observations indicate that ascorbate generates oxidative stress and affects the structure of islets.

Dehydroascorbic acid uptake by coronary artery smooth muscle: effect of intracellular acidification

Dehydroascorbic acid (DHAA) enters cells via Na(+)-independent glucose transporters (GLUT) and is converted to ascorbate. However, we found that Na(+) removal inhibited [(14)C]DHAA uptake by smooth-muscle cells cultured from pig coronary artery. The uptake was examined for 2-12 min at 10-200 microM DHAA in either the presence of 134 mM Na(+) or in its absence (N-methyl D-glucamine, choline or sucrose replaced Na(+)). This inhibition of DHAA uptake by Na(+) removal was paradoxical because it was inhibited by 2-deoxyglucose and cytochalasin B, as expected of transport via the GLUT pathway. We tested the hypothesis that this paradox resulted from an inefficient intracellular reduction of [(14)C]DHAA into [(14)C]ascorbate upon intracellular acidosis caused by the Na(+) removal. Consistent with this hypothesis: (i) the Na(+)/H(+)-exchange inhibitors ethylisopropyl amiloride and cariporide also decreased the uptake, (ii) Na(+) removal and Na(+)/H(+)-exchange inhibitors lowered cytosolic pH, with the decrease being larger in 12 min than in 2 min, and (iii) less of the cellular (14)C was present as ascorbate (determined by HPLC) in cells in Na(+)-free buffer than in those in Na(+)-containing buffer. This inability to obtain ascorbate from extracellular DHAA may be detrimental to the coronary artery under hypoxia-induced acidosis during ischaemia/reperfusion.

Hyperglycemia-induced ascorbic acid deficiency promotes endothelial dysfunction and the development of atherosclerosis

Dehydroascorbic acid, the oxidized form of vitamin C, is transported into mammalian cells via facilitative glucose transporters and hyperglycemia inhibits this process by competitive inhibition. This inhibited transport may promote oxidative stress and contribute to the increase in atherosclerotic cardiovascular disease observed in patients with diabetes mellitus. This review explores the importance of this proposed mechanism in light of current research. For example, recent reports suggest that administration of antioxidants, such as vitamin C, may slow atherogenesis by improving endothelium-dependent vasodilation in individuals with abnormal glucose and lipid metabolism, perhaps by preventing the oxidation of nitric oxide, an important regulator of vasomotor tone. Endothelial dysfunction plays a key role in the development of atherosclerosis and endothelial cells may be particularly affected by hyperglycemia-induced ascorbic acid deficiency as they line the interior of blood vessels. In addition, we discuss evidence of several other mechanisms by which vitamin C status may affect the development of atherosclerotic cardiovascular disease, particularly its inverse relationship to multiple cardiovascular disease risk factors and indicators. Given these factors, vitamin C administration is recommended during periods of both acute and chronic hyperglycemia to help preserve endothelial function.

Physiological amounts of ascorbate potentiate phorbol ester-induced nuclear-binding of AP-1 transcription factor in cells of macrophagic lineage

The aim of the reported research was to assess the potential modulatory effect exerted by physiological amounts of ascorbate complexed or not to iron on activator protein 1 (AP-1) nuclear binding. The metal-vitamin complex was shown able to strongly potentiate AP-1 binding as induced by phorbol 12-myristate 13-acetate (PMA). Such enhancing activity by ascorbate was not observed on PMA-dependent induction of another redox-sensitive transcription factor nuclear factor kappaB (NF-kappaB). Experiments performed in the presence of the metal chelator desferrioxamine (DFO) clearly indicated that ascorbate rather than iron was responsible for the potentiation of PMA effect. The composition of AP-1 heterodimers revealed c-Jun, Jun D, and c-Fos as the major subunits upon PMA +/- ascorbate stimulation. The change in AP-1 components consequent to such stimuli was mainly dependent upon new synthesis. In fact, protein synthesis inhibitor cycloheximide (CHX) prevented the stimulation of AP-1 nuclear binding due to PMA and ascorbate plus PMA. Further, the vitamin was able to amplify the PMA-dependent induction of p38 and pJNK. Thus, a fine modulation of critical thiols by the vitamin along the MAPK pathway is conceivable.

Vitamin C transport in human lens epithelial cells: evidence for the presence of SVCT2

Vitamin C [ascorbic acid (AA)] is an important antioxidant present in m M amounts in the aqueous humor. Recently, two specific transporters for vitamin C (SVCT1, SVCT2) have been cloned in the rat and the human. The aim of the present study was to characterize vitamin C transport in an immortalized human lens epithelial cell line (HLE-B3). AA uptake was linear for 120 min in experiments conducted with 14C AA + 40 microM unlabelled AA. Uptake was measured at varying AA concentrations (0.04-1 m M) in Na+-containing and Na+-free buffers for 30 min at 37 degrees C. Effect of potential inhibitors of AA transport was also examined. Presence (or absence) of SVCT1 and SVCT2 was studied by RT-PCR of HLE-B3 poly (A)+ RNA using gene specific primers. Uptake studies revealed that AA uptake was highly Na+-dependent and exhibited saturation. Na+-dependent 14C-AA uptake was strongly inhibited (85-90%) by 10 m M unlabelled AA. Incubation of HLE-B3 cells with cAMP (0.1 m M), cytocholasin B (0.1 m M) and phorbol dibutyrate (1 microM) resulted in partial inhibition (36-51%) of AA uptake. Under similar conditions, D -glucose (10 m M) and staurosporine (0.1 microM) had no effect. RT-PCR showed the presence of SVCT2 while SVCT1 could not be amplified. Exposure to the chemical oxidant tert-butylhydroperoxide (TBH) up-regulated SVCT2 gene expression in HLE-B3 cells. Our data suggest that Na+-dependent transport of AA in normal lens epithelium is most likely mediated by SVCT2 rather than by SVCT1. This transport system may be subject to regulation by oxidant stress and by various second messenger signals.

Oxidative stress induced by ascorbate causes neuronal damage in an in vitro system

Of particular physiological interest, ascorbate, the ionized form of ascorbic acid, possesses strong reducing properties. However, it has been shown to induce oxidative stress and lead to apoptosis under certain experimental conditions. Ascorbate in the brain is released during hypoxia, including stroke, and is subsequently oxidized in plasma. The oxidized product (dehydroascorbate) is transported into neurons via a glucose transporter (GLUT) during a reperfusion period. The dehydroascorbate taken up by cells is reduced to ascorbate by both enzymatic and non-enzymatic processes, and the ascorbate is stored in cells. This reduction process causes an oxidative stress, due to coupling of redox reactions, which can induce cellular damage and trigger apoptosis. Ascorbate treatment decreased cellular glutathione (GSH) content, and increased the rates of lipid peroxide production in rat cortical slices. Wortmannin, a specific inhibitor of phosphatidylinositol (PI)-3-kinase (a key enzyme in GLUT translocation), prevented the ascorbate induced-decrease of GSH content, and suppressed ascorbate-induced lipid peroxide production. However, wortmannin was ineffective in reducing hydrogen peroxide (H(2)O(2))-induced oxidative stress. The oxidative stress caused ceramide accumulation, which was proportionally changed with lipid peroxides when the cortical slices were treated with ascorbate. These differential effects support the hypothesis that GLUT efficiently transports the dehydroascorbate into neurons, causing oxidative stress.

Protein-disulfide isomerase- and protein thiol-dependent dehydroascorbate reduction and ascorbate accumulation in the lumen of the endoplasmic reticulum

The transport and intraluminal reduction of dehydroascorbate was investigated in microsomal vesicles from various tissues. The highest rates of transport and intraluminal isotope accumulation (using radiolabeled compound and a rapid filtration technique) were found in hepatic microsomes. These microsomes contain the highest amount of protein-disulfide isomerase, which is known to have a dehydroascorbate reductase activity. The steady-state level of intraluminal isotope accumulation was more than 2-fold higher in hepatic microsomes prepared from spontaneously diabetic BioBreeding/Worcester rats and was very low in fetal hepatic microsomes although the initial rate of transport was not changed. In these microsomes, the amount of protein-disulfide isomerase was similar, but the availability of protein thiols was different and correlated with dehydroascorbate uptake. The increased isotope accumulation was accompanied by a higher rate of dehydroascorbate reduction and increased protein thiol oxidation in microsomes from diabetic animals. The results suggest that both the activity of protein-disulfide isomerase and the availability of protein thiols as reducing equivalents can play a crucial role in the accumulation of ascorbate in the lumen of the endoplasmic reticulum. These findings also support the fact that dehydroascorbate can act as an oxidant in the protein-disulfide isomerase-catalyzed protein disulfide formation.

Dehydroascorbic acid

Dehydroascorbic acid (DHA) is an important, interesting but somewhat enigmatic compound in biological systems. DHA has many unique properties that set it apart from ascorbic acid (AA), and DHA has functions that may be very important beyond that in the AA:DHA cycle. Future studies should help to better clarify chemical activity of DHA and related products that form from DHA, as well as to highlight the role DHA plays in normal cellular homeostasis.

Dehydroascorbic acid uptake in a human keratinocyte cell line (HaCaT) is glutathione-independent

Vitamin C plays an important role in neutralizing toxic free radicals formed during oxidative metabolism or UV exposure of human skin. This study was performed to investigate the mechanisms that regulate the homoeostasis of vitamin C in HaCaT cells by identifying the events involved in the transport and in the reduction of dehydroascorbic acid. Dehydroascorbic acid accumulated to a greater extent and faster compared with ascorbic acid; its transport appeared to be mediated by hexose transporters and was entirely distinct from ascorbic acid transport. Dehydroascorbate reductase activity was unaffected by glutathione depletion, although it was sensitive to thiol protein reagents. These observations, as well as the subcellular distribution of this enzymic activity and the cofactor specificity, indicate that thioredoxin reductase and lipoamide dehydrogenase play an important role in this reduction process. HaCaT cells were able to enhance their dehydroascorbic acid reductase activity in response to oxidative stress.

Interaction of respiratory burst and uptake of dehydroascorbic acid in differentiated HL-60 cells

HL-60 cells differentiated with DMSO increased their rates of uptake of ascorbate when they were activated with PMA. The rates observed after this activation were essentially the same as those with dehydroascorbic acid as the original transport substrate. The effect of activation was sensitive to the antioxidant enzymes superoxide dismutase and catalase. When ascorbate was oxidized in situ by chemical or enzymic oxidation, the rates of uptake were similar to those after activation of the cells by phorbol ester; however, in the latter case the extracellular vitamin remained largely in the reduced form and there was very little loss by degradation, whereas after immediate oxidation no more reduced ascorbate could be found outside the cells after a few minutes and a significant part of the total vitamin was lost. The generation of superoxide by xanthine/xanthine oxidase stimulated the uptake of ascorbate much less than the activation by phorbol ester; H(2)O(2) was even less effective. Stimulation of the uptake by phorbol ester was also insensitive to GSH, in contrast with stimulation by the chemical oxidation of ascorbate. Stimulation of ascorbate uptake by phorbol ester was sensitive to the respiratory-burst inhibitor diphenyliodonium as well as the protein kinase C inhibitor staurosporine, indicating the respiratory burst as the cause of stimulation. Activation of the cells by the phorbol ester also stimulated the uptake of dehydroascorbate as the original substrate, in a manner insensitive to antioxidants or inhibitors of the respiratory burst. In all cases the intracellular vitamin was completely in the reduced form. Kinetic characterization by the calculation of maximal velocities and apparent K(m) values and assaying for the dependence of uptake rates on the ionic milieu and for inhibition by glucose analogues and inhibitors of glucose transport revealed that after treatment with phorbol ester the uptake of total vitamin C in differentiated HL-60 cells was largely due to the low-affinity high-capacity glucose transporter. In contrast, in non-stimulated cells reduced ascorbate was taken up by the Na(+)-dependent high-affinity low-capacity ascorbate transporter. This change was probably due to the oxidation of ascorbate and, simultaneously, the recruitment of additional transporter molecules to the cell surface.

Preferential uptake and accumulation of oxidized vitamin C by THP-1 monocytic cells

THP-1 cells preferentially accumulate vitamin C in its oxidized form. The uptake displays first-order kinetics and leads to a build-up of an outward concentration gradient which is stable in the absence of extracellular vitamin. The transport is faster than reduction by extracellular glutathione or by added cytosolic extract, and glutathione-depleted cells show the same uptake rates as control cells. In addition, energy depletion or oxidation of intracellular sulfhydryls does not inhibit accumulation of ascorbate. The accumulation, however, always occurs in the reduced form. The affinity for dehydroascorbate is lower (Km 450 microM vs 60 microM) than for reduced ascorbate, but the maximal rate is more than 30 times higher (581 compared to 19 pmol.min-1 per 106 cells), and it is independent of sodium, whereas the uptake of ascorbate is not. The sodium gradient also allows accumulation of reduced ascorbate. Inhibitors of glucose transport by the GLUT-1 transporter also inhibit uptake of dehydroascorbate (DHA), but there are some inconsistencies, because the Ki-values are higher than reported for the isolated transporter and one inhibitor (deoxyglucose) is noncompetitive. The preferential uptake of the dehydro-form of the vitamin may be useful for situations where this short-lived metabolite is formed by oxidation in the environment.

Plasma and platelet ascorbate pools and lipid peroxidation in insulin-dependent diabetes mellitus

BACKGROUND

As diabetes mellitus represents a situation in which production of peroxides is increased, the aim of this study was to investigate the relationship between plasma and platelet levels of ascorbic acid (AA)/dehydroascorbic acid (DHA) and those of malonyldialdehyde (MDA), an indirect marker of lipoperoxides, both assayed using high-performance liquid chromatography (HPLC), in 59 patients with insulin-dependent diabetes mellitus (IDDM) compared with 51 healthy control subjects matched for sex, age, smoking habits, as well as for dietary intake of energy, alcohol and vitamin C.

RESULTS

Mean plasma and platelet MDA were significantly higher in the patients affected with IDDM than in control subjects. Moreover, the diabetic group was characterized by a huge decrease in plasma AA [8.45 +/- 5.5 mumol L-1 (SD) vs. 33.4 +/- 7.6 mumol L-1, P = 0.0001], mirrored by a significant increase in plasma DHA (11.9 +/- 3.9 mumol L-1 vs. 3.9 +/- 2.5 mumol L-1, P = 0.0001). No detectable DHA was observed in the platelets from both diabetic and control subjects, whereas AA was significantly increased in platelets from diabetic patients compared with control subjects (42.6 +/- 7.4 vs. 34.8 +/- 5.1 nmol 10(-9) platelets, P = 0.0001). Platelet AA in the diabetic group was significantly inversely correlated with glycated haemoglobin (r = -0.34; P = 0.04) and directly with plasma AA (r = 0.39; P = 0.02), the sum of plasma AA + DHA (r = 0.44; P = 0.009) and with platelet MDA (r = 0.38; P = 0.02).

CONCLUSION

(a) The ratio plasma AA/DHA is significantly lowered in IDDM in association with an increase in MDA levels; (b) only AA is detected in platelets, being augmented in the diabetic group; (c) plasma ascorbate depletion does not reflect platelet levels of AA; and, finally, (d) metabolic control, as well as intracellular lipoperoxides, modulates platelet AA in IDDM.

Dehydroascorbate and ascorbate transport in rat liver microsomal vesicles

Ascorbate and dehydroascorbate transport was investigated in rat liver microsomal vesicles using radiolabeled compounds and a rapid filtration method. The uptake of both compounds was time- and temperature-dependent, and saturable. Ascorbate uptake did not reach complete equilibrium, it had low affinity and high capacity. Ascorbate influx could not be inhibited by glucose, dehydroascorbate, or glucose transport inhibitors (phloretin, cytochalasin B) but it was reduced by the anion transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and by the alkylating agent N-ethylmaleimide. Ascorbate uptake could be stimulated by ferric iron and could be diminished by reducing agents (dithiothreitol, reduced glutathione). In contrast, dehydroascorbate uptake exceeded the level of passive equilibrium, it had high affinity and low capacity. Glucose cis inhibited and trans stimulated the uptake. Glucose transport inhibitors were also effective. The presence of intravesicular reducing compounds increased, while extravesicular reducing environment decreased dehydroascorbate influx. Our results suggest that dehydroascorbate transport is preferred in hepatic endoplasmic reticulum and it is mediated by a GLUT-type transporter. The intravesicular reduction of dehydroascorbate leads to the accumulation of ascorbate and contributes to the low intraluminal reduced/oxidized glutathione ratio.