Static and Dynamic Stabilization of the Shoulder After Total Scapulectomy in an Electric Burn Patient

ABSTRACT

An otherwise healthy 49-year-old man experienced a high-voltage electrical injury to the left shoulder resulting in total scapulectomy, partial calviculectomy, and a substantial soft tissue defect. The majority of the muscles around his shoulder were debrided because of necrosis, with only the pectoralis and latissimus dorsi muscles remaining attached to the humerus. Surprisingly, the patient's brachial plexus remained intact, and his left elbow, wrist, and hand function were preserved. A novel combination of 3 static and dynamic suspension techniques were used to stabilize his shoulder and prevent traction injury to the brachial plexus. Postoperative follow-up at 1 year demonstrated excellent stability of his reconstructed shoulder, which allowed him to ambulate independently and return to employment.

Mechanical Testing of Artificial Vessels and Tissues for Photoplethysmography Phantoms

Various studies have looked at the efficiency of artificial vessel and tissue networks in the study of photoplethysmography (PPG) in an effort to better understand the origin of various morphological features present in the signal. Whilst there are all reasonable attempts made to replicate geometrical features such as vessel depth, vessel wall thickness and diameter etc., not many studies have attempted to replicate the mechanical properties such as vessel elasticity and tissue compressibility. This study reports two methods for tissue mechanical testing for the analysis of vessel elasticity and tissue compressibility. A two-part polydimethylsiloxane (PDMS) was used as a base material for both tissue and vessel construction, and the properties altered by changing the curing component ratio. Tissue compression properties were investigated using an industrially calibrated materials testing device using the protocol from the ASTM 0575-91 testing method. Vessel elasticity was investigated using a custom method and apparatus to report vessel diameter and length change simultaneously. Tissue compressive properties proved reasonably easy to replicate through catalyst alteration, however the vessel elasticity properties were found to be higher than expected at all reasonable catalyst ratios. The property of hyper-elasticity was observed in the artificial vessels though, leading to the conclusion that alternative material recipes or construction methods may be needed to correctly replicate the expected mechanical characteristics. Clinical Relevance- The latest generation of health monitoring devices, especially those that are wearable and used widely by individuals wishing to monitor their health daily are becoming smarter and more sophisticated in their functionality. The majority of such devices use photoplethysmography (PPG) as their primary monitoring technique. Being able to replicate the PPG in a phantom allows the continued study and development of devices, and to improve their functionality without the continued need for extensive user-testing.

The Use of Biologic Wound Agents in Pediatric Reconstructions

The indications for using biologic wound agents have expanded greatly since first being employed for acute burn management. The majority of the literature details the use of said agents in the adult population; however, there is little representation regarding their uses for reconstructing defects typically observed in the pediatric population. Ironically, children, and to a lesser extent adolescents, greatly benefit from their use given the reduced skin laxity and amount of surrounding tissue available for locoregional tissue transfer when compared with adults. Herein, we detail the use of acellular and cellular biologic wound agents in the pediatric population.

Altered synaptic glutamate homeostasis contributes to cognitive decline in young APP/PSEN1 mice

Non-convulsive epileptiform activity is a common and under-studied comorbidity of Alzheimer’s disease that may significantly contribute to onset of clinical symptoms independently of other neuropathological features such as β-amyloid deposition. We used repeated treatment with low dose kainic acid (KA) to trigger subthreshold epileptiform activity in young (less than 6 months) wild-type (WT) and APP/PSEN1 mice to test the role of disruption to the glutamatergic system in epileptiform activity changes and the development of memory deficits. Short-term repeated low-dose KA (five daily treatments with 5 mg/kg, IP) impaired long-term potentiation in hippocampus of APP/PSEN1 but not WT mice. Long-term repeated low-dose KA (fourteen weeks of bi-weekly treatment with 7.5–10 mg/kg) led to high mortality in APP/PSEN1 mice. KA treatment also impaired memory retention in the APP/PSEN1 mice in a Morris water maze task under cognitively challenging reversal learning conditions where the platform was moved to a new location. Four weeks of bi-weekly treatment with 5 mg/kg KA also increased abnormal spike activity in APP/PSEN1 and not WT mice but did not impact sleep/wake behavioral states. These findings suggest that hyperexcitability in Alzheimer’s disease may indeed be an early contributor to cognitive decline that is independent of heavy β-amyloid-plaque load, which is absent in APP/PSEN1 mice under 6 months of age.

Pilot investigation of anterior fontanelle photoplethysmographic signals and their suitability in estimating arterial oxygen saturation

There is a need for more reliable, non-invasive and alternative measurement sites for the monitoring of arterial blood oxygen saturation in critically ill newborns at times of peripheral compromise. A pilot investigation on 14 Intensive Care Unit (ICU) newborns was conducted utilizing custom-made reflectance photoplethysmographic (PPG) sensors placed at the fontanelle and foot. The results suggest that the fontanelle is sensitive to changes in saturation, where saturation values obtained from the custom sensor were compared against commercial pulse oximeter values and results from a blood gas analyzer, however careful placement of the sensor at the fontanelle is an issue that needs further investigation.

Optical analysis of lithium carbonate: towards the development of a portable lithium blood level analyzer for bipolar disorder patients

Lithium medication is the gold standard of treatment in Bipolar Disorder patients, preventing and reducing mood swings and suicidality. However, despite its effectiveness, it is a potentially hazardous drug requiring regular monitoring of blood levels to ensure toxic levels are not reached. This paper describes the first steps towards developing a new portable device that can be used by Bipolar Disorder patients to facilitate the analysis of lithium blood levels at home. Solutions of lithium carbonate have been optically fingerprinted using a high-end spectrophotometer. Preliminary measurements indicate that while the visible to near infrared region of the absorption spectra fall heavily within the water band, measurements in the Ultraviolet region show a strong distinction between different lithium concentrations. The optical spectra of Lithium in the 220 nm to 230 nm region demonstrated the ability to differentiate between concentrations representing those found in patients.

The Concise Guide to PHARMACOLOGY 2013/14: overview

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties from the IUPHAR database. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. This compilation of the major pharmacological targets is divided into seven areas of focus: G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors & Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and GRAC and provides a permanent, citable, point-in-time record that will survive database updates.

Investigation of photoplethysmographs from the anterior fontanelle of neonates

Photoplethysmography (PPG) signals have been investigated at a new anatomical site, the anterior fontanelle (ANTF), on the hypothesis that blood supply at this location is preferentially preserved during cases of poor peripheral circulation which might cause the commercial pulse oximeters to fail to estimate accurately arterial blood oxygen saturation (SpO2). Two custom built reflectance PPG sensors have been developed, one for placement on the fontanelle and one on the periphery (foot). A PPG processing system and software were also developed to process the raw PPG signals and to estimate SpO2. A pilot study on sixteen babies, (9 male, 7 female) with a median age of 15.5 d (interquartile range = 46.8 d) and a median weight of 3.15 kg (SD = 0.93 kg), on a neonatal intensive care unit (NICU) has been carried out. PPG signals from the ANTF were of good quality and high signal-to-noise ratio. The amplitudes of the ANTF PPGs were found to be sensitive to changes in amplitude when amplitudes were observed at the reference PPG site. Bland-Altman analysis of the gold standard blood gas analysis reveals that all three sensors are inaccurate at SaO2 < 85-90 %, but the ANTF sensor shows better mean difference than the commercial device.

Esophageal SpO2 measurements from a pediatric burns-patient: a case study

Pulse oximetry is being used in everyday clinical practice in anesthesia utilizing peripheral saturation sensors. However, it may be unreliable in certain clinical situations such as peripheral hypoperfusion. Similar situations occur in burns patients and more importantly burns to extremities which limit the sites available for measurement of peripheral oxygen saturation (SpO2). To overcome these limitations, the esophagus has been investigated as an alternative measurement site, as perfusion may be preferentially preserved centrally. A miniaturized reflectance esophageal saturation (SpO2 probe has been constructed utilizing infrared and red photodiodes and a photodetector. Our case study was aimed at evaluating the reliability of esophageal pulse oximetry in a major burns infant. Measurable photoplethysmographic (PPG) traces and SpO2 values were obtained in the neonatal esophagus. It was found that the esophageal pulse oximeter results were in good agreement with oxygen saturation measurements obtained by a commercial ear lobe pulse oximeter. This study suggests that the esophagus can be used as an alternative site for monitoring arterial blood oxygen saturation by pulse oximetry in burned infants.

Photoplethysmographic and SpO(2) readings from the neonatal anterior fontanelle: a case study

There is a need for a more reliable, non-invasive and alternative measurement site for the monitoring of arterial blood oxygen saturation in critically ill neonates when peripheral perfusion is poor. The anterior fontanelle, a unique anatomical feature of the neonate, has been presented as an alternative site for the estimation of arterial blood oxygen saturation (SpO(2)). A new fontanelle photoplethysmographic sensor and processing system has been developed to investigate fontanelle photoplethysmographic (PPG) signals and estimate SpO(2) values at this anatomical location. Preliminary clinical trials have shown that good quality PPG signals with large amplitudes and high signal to noise ratio can be obtained from the neonatal fontanelle. The estimation of SpO(2) values from the fontanelle were in broad agreement with a commercial foot pulse oximeter.

Development of an optoelectronic sensor for the investigation of photoplethysmographic signals from the anterior fontanel of the newborn

There is a need for more reliable, non-invasive and alternative measurement sites for the monitoring of arterial blood oxygen saturation in critically ill newborns at times of peripheral compromise. The anterior fontanelle, a unique anatomical feature of the newborn, has been presented as an alternative site for the estimation of oxygen saturation. A multi-wavelength non-invasive optoelectronic sensor has been designed and developed for the investigation of photoplethysmographic (PPG) signals and blood oxygen saturation values from the fontanelle. In vivo thermal tests of the optical sensor show that under normal operating conditions the heating at the skin surface was negligible (<0.1 °C). Good quality PPGs with large amplitudes and high signal to noise ratio were recorded at all three (red, infrared and green) wavelengths prior to clinical measurements.

Low vitamin C and increased oxidative stress and cell death in mice that lack the sodium-dependent vitamin C transporter SVCT2

The sodium-dependent vitamin C transporter (SVCT2) is responsible for the transport of vitamin C into cells in multiple organs, from either the blood or the cerebrospinal fluid. Mice null for SVCT2 (SVCT2(-/-)) do not survive past birth but the cause of death has not yet been ascertained. After mating of SVCT2(+/-) males and SVCT2(+/-) females, fewer SVCT2(-/-) and SVCT2(+/-) progeny were observed than would be expected according to Mendelian ratios. Vitamin C levels in SVCT2(-/-), SVCT2(+/-), and SVCT2(+/+) were genotype-dependent. SVCT2(-/-) fetuses had significantly lower vitamin C levels than littermates in placenta, cortex, and lung, but not in liver (the site of vitamin C synthesis). Low vitamin C levels in placenta and cortex were associated with elevations in several markers of oxidative stress: malondialdehyde, isoketals, F(2)-isoprostanes, and F(4)-neuroprostanes. Oxidative stress was not elevated in fetal SVCT2(-/-) lung tissue despite low vitamin C levels. In addition to the expected severe hemorrhage in cortex, we also found hemorrhage in the brain stem, which was accompanied by cell loss. We found evidence of increased apoptosis in SVCT2(-/-) mice and disruption of the basement membrane in fetal brain. Together these data show that SVCT2 is critical for maintaining vitamin C levels in fetal and placental tissues and that the lack of SVCT2, and the resulting low vitamin C levels, results in fetal death and, in SVCT2(-/-) mice that survive the gestation period, in oxidative stress and cell death.

Unexpected cardiovascular collapse from massive air embolism during endoscopic retrograde cholangiopancreatography

A 72 year-old woman with cholangiocarcinoma presented for endoscopic retrograde cholangio pancreatography (ERCP) for diagnostic intraductal endoscopy under GETA. During the technically difficult procedure the patient became suddenly hypoxic, hypotensive, bradycardic, and progressed to PEA code (ETCO2 5 mmHg). ACLS was initiated. Transesophageal echo demonstrated massive right heart air accumulation; abdominal X-Ray showed air filled bile ducts. Central access was obtained, a pulmonary artery catheter floated, and 30 ml of air aspirated from the RV. Within 5 minutes pulses returned; the patient was transferred to the ICU. MRI revealed two watershed infarcts in the right frontal lobe. The patient fully recovered and returned a month later for an uneventful ERCP.

Antioxidants and cognitive training interact to affect oxidative stress and memory in APP/PSEN1 mice

The present study investigated the relationships among oxidative stress, beta-amyloid and cognitive abilities in the APP/PSEN1 double-transgenic mouse model of Alzheimer's disease. In two experiments, long-term dietary supplements were given to aged APP/PSEN1 mice containing vitamin C alone (1 g/kg diet; Experiment 1) or in combination with a high (750 IU/kg diet, Experiments 1 and 2) or lower (400 IU/kg diet, Experiment 2) dose of vitamin E. Oxidative stress, measured by F(4)-neuroprostanes or malondialdehyde, was elevated in cortex of control-fed APP/PSEN1 mice and reduced to wild-type levels by vitamin supplementation. High-dose vitamin E with C was less effective at reducing oxidative stress than vitamin C alone or the low vitamin E+C diet combination. The high-dose combination also impaired water maze performance in mice of both genotypes. In Experiment 2, the lower vitamin E+C treatment attenuated spatial memory deficits in APP/PSEN1 mice and improved performance in wild-type mice in the water maze. Amyloid deposition was not reduced by antioxidant supplementation in either experiment.

Vitamin C deficiency increases basal exploratory activity but decreases scopolamine-induced activity in APP/PSEN1 transgenic mice

Vitamin C is a powerful antioxidant and its levels are decreased in Alzheimer's patients. Even sub-clinical vitamin C deficiency could impact disease development. To investigate this principle we crossed APP/PSEN1 transgenic mice with Gulo knockout mice unable to synthesize their own vitamin C. Experimental mice were maintained from 6 weeks of age on standard (0.33 g/L) or reduced (0.099 g/L) levels of vitamin C and then assessed for changes in behavior and neuropathology. APP/PSEN1 mice showed impaired spatial learning in the Barnes maze and water maze that was not further impacted by vitamin C level. However, long-term decreased vitamin C levels led to hyperactivity in transgenic mice, with altered locomotor habituation and increased omission errors in the Barnes maze. Decreased vitamin C also led to increased oxidative stress. Transgenic mice were more susceptible to the activity-enhancing effects of scopolamine and low vitamin C attenuated these effects in both genotypes. These data indicate an interaction between the cholinergic system and vitamin C that could be important given the cholinergic degeneration associated with Alzheimer's disease.

Ascorbic acid attenuates scopolamine-induced spatial learning deficits in the water maze

Vitamin C (ascorbate) has important antioxidant functions that can help protect against oxidative stress in the brain and damage associated with neurodegenerative disorders such as Alzheimer's disease. When administered parenterally ascorbate can bypass saturable uptake mechanisms in the gut and thus higher tissue concentrations can be achieved than by oral administration. In the present study we show that ascorbate (125 mg/kg) administered intraperitoneally (i.p.) 1-h before testing, partially attenuated scopolamine-induced (1 mg/kg i.p.) cognitive deficits in Morris water maze performance in young mice. Cumulative search error, but not escape latency nor path length, was significantly improved during acquisition in ascorbate plus scopolamine-treated mice although performance did not equal that of control mice. During the probe trial, scopolamine led to increased search error and chance level of time spent in the platform quadrant, whereas mice pre-treated with ascorbate prior to scopolamine did not differ from control mice on these measures. Ascorbate had no effect on unimpaired, control mice and neither did it reduce the peripheral, activity-increasing effects of scopolamine. Ascorbate alone increased acetylcholinesterase activity in the medial forebrain area but had no effect in cortex or striatum. This change, and its action against the amnestic effects of the muscarinic antagonist scopolamine, suggest that ascorbate may be acting in part via altered cholinergic signaling. However, further investigation is necessary to isolate the cognition-enhancing effects of ascorbate.

Vitamin C reduces spatial learning deficits in middle-aged and very old APP/PSEN1 transgenic and wild-type mice

Alzheimer's disease is a progressive and fatal neurodegenerative disease characterized by a build up of amyloid beta (Abeta) deposits, elevated oxidative stress, and deterioration of the cholinergic system. The present study investigated short-term cognitive-enhancing effects of acute intraperitoneal (i.p.) Vitamin C (ascorbate) treatment in APP/PSEN1 mice, a mouse model of Alzheimer's disease. Middle-aged (12 months) and very old (24 months) APP/PSEN1 bigenic and wild-type mice were treated with ascorbate (125 mg/kg i.p.) or the vehicle 1 h before testing on Y-maze spontaneous alternation and Morris water maze tasks. Very old mice performed more poorly on cognitive tasks than middle-aged mice. Ascorbate treatment improved Y-maze alternation rates and swim accuracy in the water maze in both wild-type and APP/PSEN1 mice. Abeta deposits and oxidative stress both increased with age, and acetylcholinesterase (AChE) activity was significantly reduced in APP/PSEN1 compared to wild-type mice. However, the short course of acute ascorbate treatment did not alter Alzheimer-like neuropathological features of plaque deposition, oxidative stress, or AChE activity. These data suggest that ascorbate may have noötropic functions when administered parenterally in high doses and that the mode of action is via an acute, pharmacological-like mechanism that likely modulates neurotransmitter function.

GSH is required to recycle ascorbic acid in cultured liver cell lines

Liver is the site of ascorbic acid synthesis in most mammals. As human liver cannot synthesize ascorbate de novo, it may differ from liver of other species in the capacity or mechanism for ascorbate recycling from its oxidized forms. Therefore, we compared the ability of cultured liver-derived cells from humans (HepG2 cells) and rats (H4IIE cells) to take up and reduce dehydroascorbic acid (DHA) to ascorbate. Neither cell type contained appreciable amounts of ascorbate in culture, but both rapidly took up and reduced DHA to ascorbate. Intracellular ascorbate accumulated to concentrations of 10-20 mM following loading with DHA. The capacity of HepG2 cells to take up and reduce DHA to ascorbate was more than twice that of H4IIE cells. In both cell types, DHA reduction lowered glutathione (GSH) concentrations and was inhibited by prior depletion of GSH with diethyl maleate, buthionine sulfoximine, and phenylarsine oxide. NADPH-dependent DHA reduction due to thioredoxin reductase occurred in overnight-dialyzed extracts of both cell types. These results show that cells derived from rat liver synthesize little ascorbate in culture, that cultured human-derived liver cells have a greater capacity for DHA reduction than do rat-derived liver cells, but that both cell types rely largely on GSH- or NADPH-dependent mechanisms for ascorbate recycling from DHA.

Selenium spares ascorbate and alpha-tocopherol in cultured liver cell lines under oxidant stress

The selenoenzyme thioredoxin reductase (TR) can recycle ascorbic acid, which in turn can recycle alpha-tocopherol. Therefore, we evaluated the role of selenium in ascorbic acid recycling and in protection against oxidant-induced loss of alpha-tocopherol in cultured liver cells. Treatment of HepG2 or H4IIE cultured liver cells for 48 h with sodium selenite (0-116 nmol/l) tripled the activity of the selenoenzyme TR, measured as aurothioglucose-sensitive dehydroascorbic acid (DHA) reduction. However, selenium did not increase the ability of H4IIE cells to take up and reduce 2 mM DHA, despite a 25% increase in ascorbate-dependent ferricyanide reduction (which reflects cellular ascorbate recycling). Nonetheless, selenium supplements both spared ascorbate in overnight cultures of H4IIE cells, and prevented loss of cellular alpha-tocopherol in response to an oxidant stress induced by either ferricyanide or diazobenzene sulfonate. Whereas TR contributes little to ascorbate recycling in H4IIE cells, selenium spares ascorbate in culture and alpha-tocopherol in response to an oxidant stress.

Mechanisms of ascorbic acid recycling in human erythrocytes

Vitamin C, or ascorbic acid, is efficiently recycled from its oxidized forms by human erythrocytes. In this work the dependence of this recycling on reduced glutathione (GSH) was evaluated with regard to activation of the pentose cycle and to changes in pyridine nucleotide concentrations. The two-electron-oxidized form of ascorbic acid, dehydroascorbic acid (DHA) was rapidly taken up by erythrocytes and reduced to ascorbate, which reached intracellular concentrations as high as 2 mM. In the absence of D-glucose, DHA caused dose-dependent decreases in erythrocyte GSH, NADPH, and NADH concentrations. In the presence of 5 mM D-glucose, GSH and NADH concentrations were maintained, but those of NADPH decreased. Reduction of extracellular ferricyanide by erythrocytes, which reflects intracellular ascorbate recycling, was also enhanced by D-glucose, and ferricyanide activated the pentose cycle. Diethylmaleate at concentrations up to 1 mM was found to specifically deplete erythrocyte GSH by 75-90% without causing oxidant stress in the cells. Such GSH-depleted erythrocytes showed parallel decreases in their ability to take up and reduce DHA to ascorbate, and to reduce extracellular ferricyanide. These results show that DHA reduction involves GSH-dependent activation of D-glucose metabolism in the pentose cycle, but that in the absence of D-glucose DHA reduction can also utilize NADH.

Plant catechols prevent lipid peroxidation in human plasma and erythrocytes

The antioxidant activity of several plant catechol derivatives was tested in buffer, plasma, and human erythrocytes. In buffer, chlorogenic acid (CGA), caffeic acid (CA), and dihydrocaffeic acid (DCA) reduced ferric iron equally well in the ferric reducing antioxidant power (FRAP) assay. Low concentrations of the polyphenols enhanced the ability of plasma to reduce ferric iron by about 10%. In plasma, lipid hydroperoxide and F2-isoprostane formation induced by a water-soluble free radical initiator were reduced by CGA at concentrations as low as 20 microM. During incubation at 37 degrees C, human erythrocytes took up DCA, but not CGA, and intracellular DCA enhanced the ability of erythrocytes to reduce extracellular ferricyanide. When intact erythrocytes were exposed to oxidant stress generated by liposomes containing small amounts of lipid hydroperoxides, extracellular CGA at a concentration of 5 microM decreased both lipid peroxidation in the liposomes, and spared alpha-tocopherol in erythrocyte membranes. These results suggest that the catechol structure of these compounds convey the antioxidant effect in plasma and in erythrocytes.

Requirement for GSH in recycling of ascorbic acid in endothelial cells

Ascorbic acid may be involved in the defense against oxidant stress in endothelial cells. Such a role requires that the cells effectively recycle the vitamin from its oxidized forms. In this work, we studied the ability of cultured bovine aortic endothelial cells (BAECs) to take up and reduce dehydroascorbic acid (DHA) to ascorbate, as well as the dependence of ascorbate recycling on intracellular GSH. BAECs took up and reduced DHA to ascorbate much more readily than they took up ascorbate. Although BAECs in culture did not contain ascorbate, ascorbate accumulated to concentrations of 2-3 mM in BAECs following incubation with 400 microM DHA. Extracellular ferricyanide oxidized intracellular ascorbate, which was recycled by the cells. Reduction of DHA, either when added to the cells or when generated in response to ferricyanide, caused significant decreases in intracellular GSH concentrations. Depletion of intracellular GSH with 1-chloro-2,4-dinitrobenzene, diethylmaleate, and diamide almost abolished the ability of the cells to reduce DHA to ascorbate. DHA reduction by thioredoxin reductase was evident in dialyzed cell extracts, but occurred at rates far lower than direct GSH reduction of DHA. These results suggest that maximal rates of DHA reduction, and thus recycling of ascorbate from DHA, are dependent upon GSH in these cells.

Recycling of the ascorbate free radical by human erythrocyte membranes

Reduction of the ascorbate free radical (AFR) at the plasma membrane provides an efficient mechanism to preserve the vitamin in a location where it can recycle alpha-tocopherol and thus prevent lipid peroxidation. Erythrocyte ghost membranes have been shown to oxidize NADH in the presence of the AFR. We report that this activity derives from an AFR reductase because it spares ascorbate from oxidation by ascorbate oxidase, and because ghost membranes decrease steady-state concentrations of the AFR in a protein- and NADH-dependent manner. The AFR reductase has a high apparent affinity for both NADH and the AFR (< 2 microM). When measured in open ghosts, the reductase is comprised of an inner membrane activity (both substrate sites on the cytosolic membrane face) and a trans-membrane activity that mediates extracellular AFR reduction using intracellular NADH. However, the trans-membrane activity constitutes only about 12% of the total measured in ghosts. Ghost AFR reductase activity can also be differentiated from NADH-dependent ferricyanide reductase(s) by its sensitivity to the detergent Triton X-100 and insensitivity to enzymatic digestion with cathepsin D. This NADH-dependent AFR reductase could serve to recycle ascorbic acid at a crucial site on the inner face of the plasma membrane.

Glucose uptake and metabolism by cultured human skeletal muscle cells: rate-limiting steps

To use primary cultures of human skeletal muscle cells to establish defects in glucose metabolism that underlie clinical insulin resistance, it is necessary to define the rate-determining steps in glucose metabolism and to improve the insulin response attained in previous studies. We modified experimental conditions to achieve an insulin effect on 3-O-methylglucose transport that was more than twofold over basal. Glucose phosphorylation by hexokinase limits glucose metabolism in these cells, because the apparent Michaelis-Menten constant of coupled glucose transport and phosphorylation is intermediate between that of transport and that of the hexokinase and because rates of 2-deoxyglucose uptake and phosphorylation are less than those of glucose. The latter reflects a preference of hexokinase for glucose over 2-deoxyglucose. Cellular NAD(P)H autofluorescence, measured using two-photon excitation microscopy, is both sensitive to insulin and indicative of additional distal control steps in glucose metabolism. Whereas the predominant effect of insulin in human skeletal muscle cells is to enhance glucose transport, phosphorylation, and steps beyond, it also determines the overall rate of glucose metabolism.

Combined selenium and vitamin E deficiency causes fatal myopathy in guinea pigs

Selenium and vitamin E deficiencies were studied as part of an evaluation of oxidant defenses in guinea pigs. Male guinea pigs (100-120 g) were fed a control diet (C) or the diet without selenium (0 Se), without vitamin E (0 E), or without either selenium or vitamin E (0 Se-0 E). Between d 30 and 35, 7 of 13 guinea pigs fed the 0 Se-0 E diet were euthanized because of severe weakness of their extremities. No guinea pigs in the other diet groups developed weakness. Guinea pigs from each group were killed on d 37. Selenium deficiency and vitamin E deficiency were verified by measurement of glutathione peroxidase and alpha-tocopherol. Creatine phophokinase (CPK) activity was greater than controls in both groups fed vitamin E-deficient diets, but the increase was greater in the 0 Se-0 E group than in the 0 E group. Muscle F(2)-isoprostanes were greater than controls in both groups fed vitamin E-deficient diets with the level in the 0 Se-0 E group greater than that in the 0 E group. Histologic muscle necrosis was severe in the 0 Se-0 E group, minimal in the 0 E group and absent from other groups. The diets used in this study induced selenium and vitamin E deficiencies in guinea pigs. The study demonstrates that combined selenium and vitamin E deficiency results in a fatal myopathy in guinea pigs that is associated with lipid peroxidation in the affected muscle. This nutritional myopathy is much more severe than the myopathy that occurs with vitamin E deficiency alone.

Mitochondrial Uptake and Recycling of Ascorbic Acid

Mitochondria generate reactive oxygen species as by-products of oxidative metabolism. Since ascorbic acid can scavenge such destructive species, we studied the ability of mitochondria from rat liver and muscle to take up, recycle, and oxidize ascorbate. Freshly prepared mitochondria contain ascorbate, as do mitoplasts that lack the outer mitochondrial membrane. Both mitochondria and mitoplasts rapidly take up oxidized ascorbate as dehydroascorbic acid and reduce it to ascorbate. Ascorbate concentrations in mitochondria and mitoplasts rise into the low millimolar range during dehydroascorbic acid uptake, although uptake and reduction is opposed by ascorbate efflux. Mitochondrial dehydroascorbic acid reduction depends mainly on GSH, but mitochondrial thioredoxin reductase may also contribute. Reactive oxygen species generated within mitochondria oxidize ascorbate more readily than they do GSH and alpha-tocopherol. These results show that mitochondria can recycle ascorbate, which in turn might help to prevent deleterious effects of oxidant stress in the organelle.

Nitrite uptake and metabolism and oxidant stress in human erythrocytes

Nitric oxide, when released into the bloodstream, is quickly scavenged by Hb in erythrocytes or oxidized to nitrite. Nitrite can also enter erythrocytes and oxidize Hb. The goals of this work were to determine the mechanism of erythrocyte nitrite uptake and whether this uptake causes oxidant stress in these cells. Erythrocytes took up 0.8 mM nitrite with a half-time of 11 min. Nitrite uptake was sensitive to temperature and to the pH and ionic composition of the medium but was not inhibited by the specific anion-exchange inhibitor DIDS. About 25% of nitrite uptake occurred on the sodium-dependent phosphate transporter and the rest as diffusion of nitrous acid or other species across the plasma membrane. Methemoglobin formation increased in proportion to the intracellular nitrite concentration. Nitrite reacted with erythrocyte ascorbate, but ascorbate loading of cells decreased nitrite-induced methemoglobin formation only at high nitrite concentrations. In conclusion, nitrite rapidly enters erythrocytes and reacts with oxyhemoglobin but does not exert a strong oxidant stress on these cells.

Ascorbate-dependent protection of human erythrocytes against oxidant stress generated by extracellular diazobenzene sulfonate

Diazobenzene sulfonic acid (DABS) has been used to label thiols and amino groups on cell-surface proteins. However, we found that in addition to inhibiting an ascorbate-dependent trans-plasma membrane oxidoreductase in human erythrocytes, it also depleted alpha-tocopherol severely in the cell membrane. When erythrocytes were loaded with ascorbate, DABS-dependent loss of alpha-tocopherol was decreased, despite little change in intracellular ascorbate content. Sparing of alpha-tocopherol also was seen in erythrocyte ghosts resealed to contain ascorbate, although this was accompanied by loss of intravesicular ascorbate, probably due to the inability of ghosts to recycle ascorbate. A transmembrane transfer of electrons from ascorbate was confirmed by electron paramagnetic resonance spectroscopy, in which extracellular DABS was found to generate the ascorbate free radical within cells. When the membrane content of alpha-tocopherol was decreased to 20% of the initial value by DABS treatment, lipid peroxidation ensued, manifest by generation of F(2)-isoprostanes in the cell membranes. Intracellular ascorbate also strongly protected against F(2)-isoprostane formation. These results show that DABS causes an oxidant stress at the membrane surface that is transmitted within the cell, in part by an alpha-tocopherol-dependent mechanism, and that ascorbate recycling of alpha-tocopherol can protect against loss of alpha-tocopherol and the ensuing lipid peroxidation.

How does ascorbic acid prevent endothelial dysfunction?

Human coronary and peripheral arteries show endothelial dysfunction in a variety of conditions, including atherosclerosis, hypercholesterolemia, smoking, and hypertension. This dysfunction manifests as a loss of endothelium-dependent vasodilation to acetylcholine infusion or sheer stress, and is typically associated with decreased generation of nitric oxide (NO) by the endothelium. Vitamin C, or ascorbic acid, when acutely infused or chronically ingested, improves the defective endothelium-dependent vasodilation present in these clinical conditions. The mechanism of the ascorbic acid effect is unknown, although it has been attributed to an antioxidant function of the vitamin to enhance the synthesis or prevent the breakdown of NO. In this review, multiple mechanisms are considered that might account for the ability of ascorbate to preserve NO. These include ascorbate-induced decreases in low-density lipoprotein (LDL) oxidation, scavenging of intracellular superoxide, release of NO from circulating or tissue S-nitrosothiols, direct reduction of nitrite to NO, and activation of either endothelial NO synthase or smooth muscle guanylate cyclase. The ability of ascorbic acid supplements to enhance defective endothelial function in human diseases provides a rationale for use of such supplements in these conditions. However, it is first necessary to determine which of the many plausible mechanisms account for the effect, and to ensure that undesirable toxic effects are not present.

Extracellular reduction of the ascorbate free radical by human erythrocytes

We investigated the possibility that human erythrocytes can reduce extracellular ascorbate free radical (AFR). When the AFR was generated from ascorbate by ascorbate oxidase, intact cells slowed the loss of extracellular ascorbate, an effect that could not be explained by changes in enzyme activity or by release of ascorbate from the cells. If cells preserve extracellular ascorbate by regenerating it from the AFR, then they should decrease the steady-state concentration of the AFR. This was confirmed directly by electron paramagnetic resonance spectroscopy, in which the steady-state extracellular AFR signal varied inversely with the cell concentration and was a saturable function of the absolute AFR concentration. Treatment of cells N-ethylmaleimide (2 mM) impaired their ability both to preserve extracellular ascorbate, and to decrease the extracellular AFR concentration. These results suggest that erythrocytes spare extracellular ascorbate by enhancing recycling of the AFR, which could help to maintain extracellular concentrations of the vitamin.

Troglitazone protects human erythrocytes from oxidant damage

The antidiabetic drug troglitazone contains the active chromanol ring of alpha-tocopherol, which should give it antioxidant properties within cells. In these studies, the antioxidant effects of troglitazone were tested in human erythrocytes and in their ghosts. Troglitazone bound to erythrocyte ghosts in a linear manner and was retained even after centrifugation washes. In response to an oxidant stress generated by a water-soluble free radical initiator, troglitazone that was bound to erythrocyte ghosts was oxidized, but induced a lag-phase in the disappearance of endogenous alpha-tocopherol and in the appearance of lipid hydroperoxides. Troglitazone also delayed loss of endogenous alpha-tocopherol and hemolysis in washed intact erythrocytes in response to free radical-induced extracellular oxidant stress. To mimic exposure of erythrocytes to lipid hydroperoxides in vivo, erythrocytes were incubated with phospholipid liposomes that contained small amounts of preformed lipid hydroperoxides. This induced an oxidant stress in both the liposomes and cells. Troglitazone in concentrations above 4 microM almost completely prevented further appearance of lipid hydroperoxides in the liposomes, and also completely preserved alpha-tocopherol in the erythrocytes. The present results suggest that troglitazone will help to prevent peroxidative damage to erythrocytes in areas of excessive oxidant stress in the vascular bed.

Uptake and antioxidant effects of ergothioneine in human erythrocytes

Ergothioneine is a fungal metabolite that may have antioxidant functions in mammalian cells. Although it accumulates to low millimolar concentrations in liver and other tissues, it is not thought to be taken up by mature erythrocytes. During a study of the function of ergothioneine as an antioxidant in human erythrocytes, we found that these cells do take up ergothioneine from the surrounding medium. Ergothioneine concentrations in freshly prepared erythrocytes were 2-9-fold higher than in plasma from the same donor. Slow but progressive accumulation of ergothioneine to about 125% of basal levels was observed in erythrocytes over a 4 h incubation. After a 2 h incubation, intracellular ergothioneine concentrations rose on addition of increasing amounts of ergothioneine to the incubation medium, although saturation was not evident in cells from all donors. Both initial levels and rates of ergothioneine uptake varied in erythrocytes from different donors. Intracellular ergothioneine was stable to depletion of GSH by N-ethylmaleimide and to a more severe oxidant stress induced by hydrogen peroxide in the presence of catalase. These results show that human erythrocytes do take up ergothioneine; however, the GSH results do not support an antioxidant role for ergothioneine in erythrocytes.

Ascorbate-dependent electron transfer across the human erythrocyte membrane

Reduction of extracellular ferricyanide by intact cells reflects the activity of an as yet unidentified trans-plasma membrane oxidoreductase. In human erythrocytes, this activity was found to be limited by the ability of the cells to recycle intracellular ascorbic acid, its primary trans-membrane electron donor. Ascorbate-dependent ferricyanide reduction by erythrocytes was partially inhibited by reaction of one or more cell-surface sulfhydryls with p-chloromercuribenzene sulfonic acid, an effect that persisted in resealed ghosts prepared from such treated cells. However, treatment of intact cells with the sulfhydryl reagent had no effect on NADH-dependent ferricyanide or ferricytochrome c reductase activities of open ghosts prepared from treated cells. When cytosol-free ghosts were resealed to contain trypsin or pronase, ascorbate-dependent reduction of extravesicular ferricyanide was doubled, whereas NADH-dependent ferricyanide and ferricytochrome c reduction were decreased by proteolytic digestion. The trans-membrane ascorbate-dependent activity was also found to be inhibited by reaction of sulfhydryls on its cytoplasmic face. These results show that the trans-membrane ferricyanide oxidoreductase is limited by the ability of erythrocytes to recycle intracellular ascorbate, that it does not involve the endofacial NADH-dependent cytochrome b(5) reductase system, and that it is a trans-membrane protein that contains sensitive sulfhydryl groups on both membrane faces.

Functional interaction between the N- and C-terminal halves of human hexokinase II

Mammalian hexokinases (HKs) I-III are composed of two highly homologous approximately 50-kDa halves. Studies of HKI indicate that the C-terminal half of the molecule is active and is sensitive to inhibition by glucose 6-phosphate (G6P), whereas the N-terminal half binds G6P but is devoid of catalytic activity. In contrast, both the N- and C-terminal halves of HKII (N-HKII and C-HKII, respectively) are catalytically active, and when expressed as discrete proteins both are inhibited by G6P. However, C-HKII has a significantly higher Ki for G6P (KiG6P) than N-HKII. We here address the question of whether the high KiG6P of the C-terminal half (C-half) of HKII is decreased by interaction with the N-terminal half (N-half) in the context of the intact enzyme. A chimeric protein consisting of the N-half of HKI and the C-half of HKII was prepared. Because the N-half of HKI is unable to phosphorylate glucose, the catalytic activity of this chimeric enzyme depends entirely on the C-HKII component. The KiG6P of this chimeric enzyme is similar to that of HKI and is significantly lower than that of C-HKII. When a conserved amino acid (Asp209) required for glucose binding is mutated in the N-half of this chimeric protein, a significantly higher KiG6P (similar to that of C-HKII) is observed. However, mutation of a second conserved amino acid (Ser155), also involved in catalysis but not required for glucose binding, does not increase the KiG6P of the chimeric enzyme. This resembles the behavior of HKII, in which a D209A mutation results in an increase in the KiG6P of the enzyme, whereas a S155A mutation does not. These results suggest an interaction in which glucose binding by the N-half causes the activity of the C-half to be regulated by significantly lower concentrations of G6P.

Role of ascorbic acid in transferrin-independent reduction and uptake of iron by U-937 cells

The role of ascorbic acid in transferrin-independent ferric iron reduction and uptake was evaluated in cultured U-937 monocytic cells. Uptake of 55Fe by U-937 cells was doubled by 100 microM extracellular ascorbate, and by pre-incubation of cells with 100 microM dehydroascorbic acid, the two-electron-oxidized form of ascorbate. Reduction of extracellular ferric citrate also was enhanced by loading the cells with dehydroascorbic acid. Dehydroascorbic acid was taken up rapidly by the cells and reduced to ascorbate, such that the latter reached intracellular concentrations as high as 6 mM. However, some ascorbate did escape the cells and could be detected at concentrations of up to 1 microM in the incubation medium. Further, addition of ascorbate oxidase almost reversed the effects of dehydroascorbic acid on both 55Fe uptake and ferric citrate reduction. Thus, it is likely that extracellular ascorbate reduced ferric to ferrous iron, which was then taken up by the cells. This hypothesis also was supported by the finding that during loading with ferric citrate, only extracellular ascorbate increased the pool of intracellular ferrous iron that could be chelated with cell-penetrant ferrous iron chelators. In contrast to its inhibition of ascorbate-dependent ferric iron reduction, ascorbate oxidase was without effect on ascorbate-dependent reduction of extracellular ferricyanide. This indicates that the cells use different mechanisms for reduction of ferric iron and ferricyanide. Therefore, extracellular ascorbate derived from cells can enhance transferrin-independent iron uptake by reducing ferric to ferrous iron, but intracellular ascorbate neither contributes to this reduction nor modifies the redox status of intracellular free iron.

Is ascorbic acid an antioxidant for the plasma membrane?

Ascorbic acid, or vitamin C, is a primary antioxidant in plasma and within cells, but it can also interact with the plasma membrane by donating electrons to the alpha-tocopheroxyl radical and a trans-plasma membrane oxidoreductase activity. Ascorbate-derived reducing capacity is thus transmitted both into and across the plasma membrane. Recycling of alpha-tocopherol by ascorbate helps to protect membrane lipids from peroxidation. However, neither the mechanism nor function of the ascorbate-dependent oxidoreductase activity is known. This activity has typically been studied using extracellular ferricyanide as an electron acceptor. Whereas an NADH:ferricyanide reductase activity is evident in open membranes, ascorbate is the preferred electron donor within cells. The oxidoreductase may be a single membrane-spanning protein or may only partially span the membrane as part of a trans-membrane electron transport chain composed of a cytochrome or even hydrophobic antioxidants such as alpha-tocopherol or ubiquinol-10. Further studies are needed to elucidate the structural components, mechanism, and physiological significance of this activity. Proposed functions for the oxidoreductase include stimulation of cell growth, reduction of the ascorbate free radical outside cells, recycling of alpha-tocopherol, reduction of lipid hydroperoxides, and reduction of ferric iron prior to iron uptake by a transferrin-independent pathway.

Vitamin C recycling and function in human monocytic U-937 cells

The uptake, recycling, and function of ascorbic acid was evaluated in cultured U-937 monocytic cells. Dehydroascorbic acid, the two-electron oxidized form of the vitamin, was taken up on the glucose transporter and reduced to ascorbate to a much greater extent than ascorbate itself was accumulated by the cells. In contrast to dehydroascorbic acid, ascorbate entered the cells on a sodium- and energy-dependent transporter. Intracellular ascorbate enhanced the transfer of electrons across the cell membrane to extracellular ferricyanide. Rates of ascorbate-dependent ferricyanide reduction were saturable, fivefold greater than basal rates, and facilitated by intracellular recycling of ascorbate. Whereas reduction of dehydroascorbic acid concentrations above 400 microM consumed reduced glutathione (GSH), even severe GSH depletion by 1-chloro-2,4-dinitrobenzene was without effect on the ability of the cells to reduce concentrations of dehydroascorbic acid likely to be in the physiologic range (< 200 microM). Dialyzed cytosolic fractions from U-937 cells reduced dehydroascorbic acid to ascorbate in an NADPH-dependent manner that appeared due to thioredoxin reductase. However, thioredoxin reductase did not account for the bulk of dehydroascorbic acid reduction, since its activity was also decreased by treatment of intact cells with 1-chloro-2,4-dinitrobenzene. Thus, U-937 cells loaded with dehydroascorbic acid accumulate ascorbate against a concentration gradient via a mechanism that is not dependent on GSH or NADPH, and this ascorbate can serve as the major source of electrons for transfer across the plasma membrane to extracellular ferricyanide.

Synthesis and biological activity of oxo-7H-benzo[e]perimidine-4-carboxylic acid derivatives as potent, nonpeptide corticotropin releasing factor (CRF) receptor antagonists

A novel series of derivatives of oxo-7H-benzo[e]perimidine-4-carboxylic acid (I) potently displaced radioligand binding of 125I-CRF to both CRF1 and CRF2 receptors. The members of this series antagonized CRF-stimulated cAMP formation and CRF-stimulated corticotropin release from rat pituitary in vivo. These are the first nonpeptide antagonists to show activity at both CRF1 and CRF2 receptors.

Inhibition of aldose reductase in human erythrocytes by vitamin C

Ascorbic acid, or vitamin C, has been reported to lower erythrocyte sorbitol concentrations, and present studies were performed to determine the mechanism of this effect. Incubation of erythrocytes with increasing concentrations of glucose (5-40 mM) progressively increased erythrocyte sorbitol contents, reflecting increased flux through aldose reductase. At extracellular concentrations of 90 microM, both ascorbic acid and its oxidized form, dehydroascorbate, decreased intracellular sorbitol by 25 and 45%, respectively. This inhibition was not dependent on the extracellular glucose concentration, or on erythrocyte contents of free NADPH or GSH. To test for a direct effect of ascorbate on aldose reductase, erythrocyte hemolysates were prepared and supplemented with 100 microM NADPH. Hemolysates reduced glucose to sorbitol in a dose-dependent manner that was inhibited with a Ki of 120 microM by the aldose reductase inhibitor tetramethylene glutaric acid. Above 100 microM, ascorbic acid also lowered hemolysate sorbitol generation by about 30%. Studies with ascorbic acid derivatives showed that the reducing capacity of ascorbic acid was not required for inhibition of sorbitol production from glucose in erythrocyte hemolysates. These results show that high, but physiologic, concentrations of ascorbic acid can directly inhibit erythrocyte aldose reductase, and provide a rationale for the use of oral vitamin C supplements in diabetes.

Ascorbate 6-palmitate protects human erythrocytes from oxidative damage

Lipid-soluble antioxidants, such as alpha-tocopherol, protect cell membranes from oxidant damage. In this work we sought to determine whether the amphipathic derivative of ascorbate, ascorbate 6-palmitate, is retained in the cell membrane of intact erythrocytes, and whether it helps to protect the cells against peroxidative damage. We found that ascorbate 6-palmitate binding to erythrocytes was dose-dependent, and that the derivative was retained during the multiple wash steps required for preparation of ghost membranes. Ascorbate 6-palmitate remained on the extracellular surface of the cells, because it was susceptible to oxidation or removal by several cell-impermeant agents. When bound to the surface of erythrocytes, ascorbate 6-palmitate reduced ferricyanide, an effect that was associated with generation of an ascorbyl free radical signal on EPR spectroscopy. Erythrocyte-bound ascorbate 6-palmitate protected membrane alpha-tocopherol from oxidation by both ferricyanide and a water-soluble free radical initiator, suggesting that the derivative either reacted directly with the exogenously added oxidant, or that it was able to recycle the alpha-tocopheroxyl radical to alpha-tocopherol in the cell membrane. Ascorbate 6-palmitate also partially protected cis-parinaric acid from oxidation when this fluorescent fatty acid was intercalated into the membrane of intact cells. These results show that an amphipathic ascorbate derivative is retained on the exterior cell surface of human erythrocytes, where it helps to protect the membrane from oxidant damage originating outside the cells.

Reduction of the ascorbyl free radical to ascorbate by thioredoxin reductase

Recycling of ascorbic acid from its oxidized forms is required to maintain intracellular stores of the vitamin in most cells. Since the ubiquitous selenoenzyme thioredoxin reductase can recycle dehydroascorbic acid to ascorbate, we investigated the possibility that the enzyme can also reduce the one-electron-oxidized ascorbyl free radical to ascorbate. Purified rat liver thioredoxin reductase catalyzed the disappearance of NADPH in the presence of low micromolar concentrations of the ascorbyl free radical that were generated from ascorbate by ascorbate oxidase, and this effect was markedly stimulated by selenocystine. Dehydroascorbic acid is generated by dismutation of the ascorbyl free radical, and thioredoxin reductase can reduce dehydroascorbic acid to ascorbate. However, control studies showed that the amounts of dehydroascorbic acid generated under the assay conditions used were too low to account for the observed loss of NADPH. Electron paramagnetic resonance spectroscopy directly confirmed that the reductase decreased steady-state ascorbyl free radical concentrations, as expected if thioredoxin reductase reduces the ascorbyl free radical. Dialyzed cytosol from rat liver homogenates also catalyzed NADPH-dependent reduction of the ascorbyl free radical. Specificity for thioredoxin reductase was indicated by loss of activity in dialyzed cytosol prepared from livers of selenium-deficient rats, by inhibition with aurothioglucose at concentrations selective for thioredoxin reductase, and by stimulation with selenocystine. Microsomal fractions prepared from rat liver showed substantial NADH-dependent ascorbyl free radical reduction that was not sensitive to selenium depletion. These results suggest that thioredoxin reductase can function as a cytosolic ascorbyl free radical reductase that may complement cellular ascorbate recycling by membrane-bound NADH-dependent reductases.

Enzyme-dependent ascorbate recycling in human erythrocytes: role of thioredoxin reductase

Human erythrocytes efficiently reduce dehydroascorbic acid (DHA) to ascorbate, which helps to maintain the ascorbate content of blood. Whereas erythrocyte DHA reduction is thought to occur primarily through a direct chemical reaction with GSH, this work addresses the role of enzyme-mediated DHA reduction by these cells. The ability of intact erythrocytes to recycle DHA to ascorbate, estimated as DHA-dependent ferricyanide reduction, was decreased in parallel with GSH depletion by glutathione-S-transferase substrates. In contrast, the sulfhydryl reagent phenylarsine oxide inhibited DHA reduction to a much greater extent than it decreased GSH in intact cells. DHA reduction in excess of that due to a direct chemical reaction with GSH was also observed in freshly prepared hemolysates. Hemolysates likewise showed NADPH-dependent reduction of DHA that appeared due to thioredoxin reductase, because this activity was inhibited 68% by 10 microM aurothioglucose, doubled by 5 microM E. coli thioredoxin, and had an apparent Km for DHA (1.5 mM) similar to that of purified thioredoxin reductase. Additionally, aurothioglucose-sensitive, NADPH-dependent DHA reductase activity was decreased 80% in hemolysates prepared from phenylarsine oxide-treated cells. GSH-dependent DHA reduction in hemolysates was more than 10-fold that of NADPH-dependent reduction. Nonetheless, the ability of phenylarsine oxide to decrease DHA reduction in intact cells with little effect on GSH suggests that enzymes, such as thioredoxin reductase, may contribute more to this activity than previously considered.

Erythrocyte defenses against hydrogen peroxide: the role of ascorbic acid

Ascorbate has been reported to increase intracellular hydrogen peroxide (H2O2) generation in human erythrocytes. In the present work, the basis for this prooxidant effect of the vitamin was investigated in the context of erythrocyte defenses against H2O2. Ascorbate added to erythrocytes caused a dose-dependent increase in intracellular H2O2, which was measured as inactivation of endogenous catalase in the presence of 3-amino-1,2,4-triazole (aminotriazole). Ascorbate-induced catalase inactivation was not observed when only the intracellular ascorbate concentration was increased, when cells were incubated with ascorbate in plasma, or when extracellular Fe3+ was chelated. Together, these results suggest that the observed ascorbate-induced H2O2 generation is due to Fe3+-catalyzed oxidation of extracellular, as opposed to intracellular, ascorbate by molecular oxygen. Rather than generate an oxidant stress in erythrocytes, ascorbate was one of the most sensitive intracellular antioxidants to H2O2 coming from outside the cells. On the other hand, intracellular ascorbate contributed little to the detoxification of H2O2, which was found to be mediated by both catalase and by the GSH system.

Similarities in the metabolism of alloxan and dehydroascorbate in human erythrocytes

The beta-cell toxin alloxan is reduced within cells to dialuric acid, which may then decompose to release damaging reactive oxygen species. We tested whether such redox cycling of alloxan occurs in the human erythrocyte, a cell with stronger antioxidant defenses than beta-cells. Erythrocytes incubated with increasing concentrations of alloxan progressively accumulated dialuric acid, as measured directly by HPLC with electrochemical detection. At concentrations up to 2 mM, alloxan decreased cellular GSH slightly, but did not affect erythrocyte contents of ascorbate or alpha-tocopherol. Intracellular H2O2 generation, measured as inhibition of endogenous catalase activity in the presence of 3-amino-1,2,4-triazole (aminotriazole), was decreased by alloxan. Despite its failure to induce significant oxidant stress in erythrocytes, 2 mM of alloxan doubled the activity of the hexose monophosphate pathway (HMP). This likely reflected consumption of reducing equivalents during reduction of alloxan to dialuric acid. Alloxan pretreatment enhanced the ability of erythrocytes to reduce extracellular ferricyanide while protecting alpha-tocopherol in the cell membrane from oxidation by ferricyanide. Ninhydrin, a hydrophobic derivative of alloxan, showed similar effects, but caused progressive GSH depletion and cell lysis at concentrations above 50 microM. The ability of alloxan to enhance ferricyanide reduction and to spare alpha-tocopherol suggests that dialuric acid or other reducing species within the cells can protect or recycle alpha-tocopherol and donate electrons to a transmembrane transfer process. This behavior resembles that observed for the dehydroascorbate (DHA)/ascorbate pair, and leads to the unexpected conclusion that alloxan increases the reducing capacity of the erythrocyte.

Erythrocyte ascorbate recycling: antioxidant effects in blood

Ascorbic acid is an important antioxidant in human plasma, but requires efficient recycling from its oxidized forms to avoid irreversible loss. Human erythrocytes prevented oxidation of ascorbate in autologous plasma, an effect that required recycling of ascorbate within the cells. Erythrocytes had a high capacity to take up dehydroascorbate, the two-electron oxidized product of ascorbate, and to reduce it to ascorbate. Uptake and conversion of dehydroascorbate to ascorbate was saturable, was half-maximal at 400 microM dehydroascorbate, and achieved a maximal intracellular ascorbate concentration of 1.5 mM. In the presence of 100 microM dehydroascorbate, erythrocytes had the capacity to regenerate a 35 microM ascorbate concentration in blood every 3 min. Ascorbate recycling from DHA required intracellular GSH. Depletion of erythrocyte GSH by more than 50% with diamide did not acutely affect the cellular ascorbate content, but did impair the subsequent ability of GSH-depleted cells to recycle dehydroascorbate to ascorbate. Whereas erythrocyte ascorbate recycling was coupled to GSH, an overwhelming extracellular oxidant stress depleted both ascorbate and alpha-tocopherol before the GSH content of cells fell appreciably. Recycled ascorbate was released from cells into plasma, but at a rate less than one tenth that of dehydroascorbate uptake and conversion to ascorbate. Nonetheless, ascorbate released from cells protected endogenous alpha-tocopherol in human LDL from oxidation by a water soluble free radical initiator. These results suggests that recycling of ascorbate in erythrocytes helps to maintain the antioxidant reserve of whole blood.

Protection and recycling of alpha-tocopherol in human erythrocytes by intracellular ascorbic acid

Ascorbic acid can recycle alpha-tocopherol from the tocopheroxyl free radical in lipid bilayers and in micelles, but such recycling has not been demonstrated to occur across cell membranes. In this work the ability of intracellular ascorbate to protect and to recycle alpha-tocopherol in intact human erythrocytes and erythrocyte ghosts was investigated. In erythrocytes that were 80% depleted of intracellular ascorbate by treatment with the nitroxide Tempol, both 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) and ferricyanide oxidized alpha-tocopherol to a greater extent than in cells not depleted of ascorbate. In contrast, in erythrocytes in which the intracellular ascorbate concentration had been increased by loading with dehydroascorbate, loss of alpha-tocopherol was less with both oxidants than in control cells. Protection against AAPH-induced oxidation of alpha-tocopherol was not prevented by extracellular ascorbate oxidase, indicating that the protection was due to intracellular and not to extracellular ascorbate. Incubation of erythrocytes with lecithin liposomes also generated an oxidant stress, which caused lipid peroxidation in the liposomes and depleted erythrocyte alpha-tocopherol, leading to hemolysis. Ascorbate loading of the erythrocytes delayed liposome oxidation and decreased loss of alpha-tocopherol from both cells and from alpha-tocopherol-loaded liposomes. When erythrocyte ghosts were resealed to contain ascorbate and challenged with free radicals generated by AAPH outside the ghosts, intravesicular ascorbate was totally depleted over 1 h of incubation, whereas alpha-tocopherol decreased only after ascorbate was substantially oxidized. These results suggest that ascorbate within the erythrocyte protects alpha-tocopherol in the cell membrane by a direct recycling mechanism.

Ascorbate function and metabolism in the human erythrocyte

Ascorbic acid, or vitamin C, is an important antioxidant in plasma, where it consumes oxygen free radicals and helps to preserve alpha-tocopherol (vitamin E) in lipoproteins. Erythrocytes, as the most plentiful cell in blood, help to preserve ascorbate in the blood plasma. In contrast to nucleated cells, which avidly concentrate ascorbate, the erythrocyte ascorbate concentration is the same as that in plasma. Erythrocytes nonetheless have a high capacity to regenerate the vitamin from its two electron-oxidized form, dehydroascorbic acid (DHA). DHA is rapidly taken up by these cells on the abundant glucose transport protein, GLUT1. Intracellular DHA is rapidly reduced to ascorbate by GSH in a direct chemical reaction, although enzyme-dependent mechanisms involving both glutaredoxin and thioredoxin reductase have also been demonstrated. Ascorbate, which carries a negative charge at physiologic pH, enters and leaves the cells slowly. Nonetheless, this slow release of ascorbate from erythrocytes can preserve both the plasma concentration of the vitamin, and prevent oxidation of alpha-tocopherol in low-density lipoprotein. In addition, intracellular ascorbate can spare and possibly recycle alpha-tocopherol in the erythrocyte membrane. In turn, alpha-tocopherol protects the cell membrane from lipid peroxidation. The ability of erythrocytes to recycle ascorbate, coupled with the ability of ascorbate to protect alpha-tocopherol in the cell membrane and in lipoproteins, provides a potentially important mechanism for preventing lipid peroxidative damage in areas of inflammation in the vascular bed, such as those involved with atherosclerosis.

Reduction of dehydroascorbate to ascorbate by the selenoenzyme thioredoxin reductase

Recycling of ascorbate from its oxidized forms is essential to maintain stores of the vitamin in human cells. Whereas reduction of dehydroascorbate to ascorbate is thought to be largely GSH-dependent, we reconsidered the possibility that the selenium-dependent thioredoxin system might contribute to ascorbate regeneration. We found that purified rat liver thioredoxin reductase functions as an NADPH-dependent dehydroascorbate reductase, with an apparent Km of 2. 5 mM for dehydroascorbate, and a kcat of 90 min-1. Addition of 2.8 microM purified rat liver thioredoxin lowered the apparent Km to 0.7 mM, without affecting the turnover (kcat of 71 min-1). Since thioredoxin reductase requires selenium, we tested the physiologic importance of this enzyme for dehydroascorbate reduction in livers from control and selenium-deficient rats. Selenium deficiency lowered liver thioredoxin reductase activity by 88%, glutathione peroxidase activity by 99%, and ascorbate content by 33%, but did not affect GSH content. NADPH-dependent dehydroascorbate reductase activity due to thioredoxin reductase, on the basis of inhibition by aurothioglucose, was decreased 88% in dialyzed liver cytosolic fractions from selenium-deficient rats. GSH-dependent dehydroascorbate reductase activity in liver cytosol was variable, but typically 2-3-fold that of NADPH-dependent activity. These results show that the thioredoxin system can reduce dehydroascorbate, and that this function is required for maintenance of liver ascorbate content.

Liposomes containing alpha-tocopherol and ascorbate are protected from an external oxidant stress

The interaction between alpha-tocopherol and ascorbate in protecting membrane lipids from peroxidation was studied in unilamellar liposomes in which alpha-tocopherol was incorporated into the liposomal membrane, and ascorbate was trapped within the vesicles. Extravesicular ferricyanide was reduced by ascorbate-derived electrons, and this was enhanced by the presence of alpha-tocopherol in the lipid bilayer. When a water-soluble free radical initiator was added to the outside of liposomes, intravesicular ascorbate prevented oxidation of alpha-tocopherol, and this effect was associated with complete protection against peroxidation of membrane lipids. These results suggest that ascorbate-dependent recycling of alpha-tocopherol can protect biological membranes from peroxidation by oxidants originating across the membrane bilayer from ascorbate.