An improved method for determination of L-ascorbic acid and L-dehydroascorbic acid in blood plasma

Changes in the antioxidant status in leaves of Solanum species in response to elicitor from Phytophthora infestans

Three Solanum genotypes with various polygenic resistance levels to the oomycete pathogen Phytophthora infestans (Mont.) De Bary were studied for their antioxidant response to the pathogen culture filtrate (CF). Detached plant leaves were treated with CF for 6, 18 and 30 h, and assayed for changes in hydrogen peroxide content, total ascorbate and glutathione pools and redox ratios (reduced form to total pool), as well as for changes in activities of ascorbate peroxidase, glutathione reductase and glutathione-S-transferase. In CF treated leaves of non-host resistant S. nigrum var. gigantea and field resistant S. tuberosum cv Bzura, the H(2)O(2) content did not change in comparison to water treated control leaves, whereas in the susceptible S. tuberosum clone H-8105 it decreased below the control level. In CF treated leaves of all genotypes, the total ascorbate pools were relatively unaltered and their redox ratio changed only transiently. In Bzura leaves the total glutathione content increased earlier than in the two other genotypes. The glutathione redox ratio remained rather stable, except for the susceptible clone H-8105, where it decreased transiently by about 42%. The relative increases in activity of all the studied enzymes were the highest in the susceptible clone H-8105. The results are discussed in the light of oxidative processes occurring in CF treated leaves. We conclude that stringent control of pro- and anti-oxidant reactions bringing the H(2)O(2) and/or cellular redox state to the threshold level is decisive for deployment of an effective defense strategy.

Measurement of reduced, oxidized and total ascorbate content in plants

Ascorbate is one of the major antioxidant metabolites in plant tissues. This protocol describes a microplate-adapted colorimetric ascorbate assay, in which ferric ion is reduced by ascorbate to the ferrous ion. The ferrous ion reacts with alpha-alpha'-bipyridl to form a complex with characteristic absorbance at 525 nm. With the chemical reduction of any dehydroascorbate (DHA) in a sample, total ascorbate can be assayed using the alpha-alpha'-bipyridl method, and DHA can be estimated by subtracting the reduced portion from the total ascorbate pool. The assay is performed in microcentrifuge tubes and assessed in a 96-well plate reader. Reduced ascorbate, DHA and total ascorbate of at least 64 experimental samples can be analyzed easily in 1 d.

Environmental contamination of chrysotile asbestos and its toxic effects on antioxidative system of Lemna gibba

Asbestos was monitored in various plant samples around an asbestos cement factory. Asbestos residue was found on the surface of all plant samples monitored. Based on asbestos concentration found in different plant samples during monitoring and on the property of asbestos to cause reactive oxygen species-mediated oxidative stress in animal models, laboratory experiments were conducted to assess the toxicity of chrysotile asbestos on an aquatic macrophyte, duckweed (Lemna gibba.). L. gibba plants were exposed to four concentrations (0.5, 1.0, 2.0, and 5.0 microg/mL) of chrysotile asbestos under laboratory conditions, and alterations in the glutathione and ascorbate antioxidative system were estimated at postexposure days 7, 14, 21, and 28 in order to assess changes in their level as suitable biomarkers of chrysotile contamination. Chrysotile exposure caused a decrease in total and reduced glutathione and an enhancement in the oxidized glutathione as well as the reduced/oxidized glutathione ratio. An increase in ascorbate pool size, and reduced as well as oxidized ascorbate was found to be accompanied by a decrease in the ratio of reduced/oxidized ascorbate. Alteration in the glutathione and ascorbate level might be considered as a biomarker of exposure to an unsafe environment because these are essential compounds of the general antioxidative strategy to overcome oxidative stress due to environmental constraints. Because an increase in the oxidation rate of antioxidants weakens cellular defenses and indicates a precarious state, they could constitute indicators of toxicity.

Antioxidative defence of old growth beech (Fagus sylvatica) under double ambient O3 concentrations in a free-air exposure system

In this study the influence of chronic free-air ozone exposure and of different meteorological conditions in the very dry year 2003 and the more humid year 2004 on the antioxidative system in sun and shade leaves of adult FAGUS SYLVATICA trees were investigated. Contents of ascorbate, glutathione, and alpha-tocopherol, as well as chloroplast pigments were determined under ambient (1 x O(3)) and double ambient (2 x O(3)) ozone concentrations. Ozone affected the antioxidative system in June and July, causing lower ascorbate contents in the apoplastic space, a more oxidized redox state of ascorbate and glutathione and an increase in pigment contents predominantly in the shade crown. For all measured parameters significant differences between the years were observed. In 2004 the redox state of ascorbate and glutathione was in a more reduced state and leaf contents of alpha-tocopherol, pigments of the xanthophyll cycle, beta-carotene, lutein, neoxanthin, and alpha-carotene were lower compared to 2003. Contents of total glutathione and chlorophyll a + b were increased in the second year. These results indicate a strong influence of the drought conditions in 2003 on the antioxidative system of beech overruling the ozone effects. Shade leaves showed lower contents of ascorbate in both years and the redox states of ascorbate and glutathione were more oxidized compared to sun leaves. Contents of photoprotective and accessory pigments generally were enhanced and the de-epoxidation state of the xanthophyll cycle was lower in the shade compared to the sun crown. Exhibiting less antioxidants shade leaves seem to be more sensitive against ozone than sun leaves.

Oxidant stress and antioxidant status among patients with nonalcoholic fatty liver disease (NAFLD)

BACKGROUND

One of the major pathogenic mechanisms for progression of nonalcoholic fatty liver disease (NAFLD) is oxidative stress. Recently, many studies have demonstrated the role of oxidative stress in NAFLD however, studies describing the antioxidant status in these patients are lacking.

AIM

To study the levels of oxidative stress and antioxidant status among patients with NAFLD.

PATIENTS AND METHODS

It was a prospective study in which 29 patients with NAFLD, 25 diseased controls with chronic viral hepatitis, and 23 healthy controls were enrolled. Apart from standard biochemical parameters, lipid peroxidation products were measured as thiobarbituric acid reactive substances. As measures of antioxidant capacity, superoxide dismutase, vitamin C levels and ferric reducing ability of plasma were measured.

RESULTS

Level of thiobarbituric acid reactive substances was significantly higher among NAFLD patients as compared with diseased [4.7 nmol/mL (1.0 to 10.2) vs. 2.4 nmol/mL (0.8 to 10.7); P=0.02] or healthy controls [4.7 nmol/mL (1.0 to 10.2) vs. 1.8 nmol/mL (0.5 to 4.1); P=0.0001]. FRAP was found to be significantly higher in patients with NAFLD as compared with healthy controls [450.3 (197.6 to 733.3) vs. 340.8 (141.6 to 697.5) mumol Fe liberated; P=0.04], even though it was similar between NAFLD and diseased controls. Among NAFLD patients, there was no significant correlation between histological grading or staging and levels of pro and antioxidants.

CONCLUSIONS

Products of lipid peroxidation are significantly increased among patients with NAFLD as compared with chronic viral hepatitis or healthy controls. Larger studies and newer markers of oxidative stress are required to clarify the association between oxidative stress and histological severity in NAFLD.

Physiological and biochemical stress responses in grassland species are influenced by both early-season ozone exposure and interspecific competition

The effects of two-year early season ozone exposure on physiological and biochemical stress response were investigated in model plant communities. Achillea millefolium and Veronica chamaedrys target plants were grown in monocultures and in mixed cultures with Poa pratensis (phytometer) and exposed in open-top chambers over two years for five weeks to charcoal-filtered (CF) air plus 25 nl l(-1) O3 (control) and non-filtered (NF) air plus 50 nl l(-1) O3. Significant O3 effects were detected in different physiological and biochemical parameters, evidencing interspecific differences in metabolic stress responses and a strong influence of the competition factor. O3 induced strong oxidative effects in Achillea irrespective to the different growth modality. Veronica showed less O3-induced effects in monoculture than when grown in competition with the phytometer. Poa exhibited a different behaviour against O3 depending on the species in competition, showing an overall higher sensitivity to O3 when in mixture with Achillea.

The role of ozone flux and antioxidants in the suppression of ozone injury by elevated CO2 in soybean

The projected rise in atmospheric CO2 concentration is expected to increase growth and yield of many agricultural crops. The magnitude of this stimulus will partly depend on interactions with other components of the atmosphere such as tropospheric O3. Elevated CO2 concentrations often lessen the deleterious effects of O3, but the mechanisms responsible for this response have received little direct examination. Previous studies have indicated that protection against O3 injury by elevated CO2 can be attributed to reduced O3 uptake, while other studies suggest that CO2 effects on anti-oxidant metabolism might also be involved. The aim of this experiment was to test further the roles of O3 flux and antioxidant metabolism in the suppression of O3 injury by elevated CO2. In a two-year experiment, soybean [Glycine max (L.) Merr.] was exposed from emergence to maturity to charcoal-filtered air or charcoal-filtered air plus a range of O3 concentrations in combination with ambient or approximately twice-ambient CO2 concentrations in open-top field chambers. Experimental manipulation of O3 concentrations and estimates of plant O3 uptake indicated that equivalent O3 fluxes that suppressed net photosynthesis, growth, and yield at ambient concentrations of CO2 were generally much less detrimental to plants treated concurrently with elevated CO2. These responses appeared unrelated to treatment effects on superoxide dismutase, glutathione reductase, and peroxidase activities and glutathione concentration. Total ascorbic acid concentration increased by 28-72% in lower canopy leaves in response to elevated CO2 and O3 but not in upper canopy leaves. Increasing concentrations of atmospheric CO2 will likely ameliorate O3 damage to many crops due to reduced O3 uptake, increased carbon assimilation, and possibly as yet undetermined additional factors. The results of this study further suggest that elevated CO2 may increase the threshold O3 flux for biomass and yield loss in soybean.

Using a suppression subtractive library-based approach to identify tobacco genes regulated in response to short-term sulphur deficit

Monitoring expression at the transcriptional level is an essential first step for the functional analysis of plant genes. Genes encoding proteins directly involved in sulphur metabolism constitute only a small fraction of all the genes affected by sulphur deficiency stress. Transcriptional responses to various periods of sulphur deprivation have been extensively studied in Arabidopsis thaliana; however, no corresponding data are available for Solanaceae sp. To address this problem, a subtractive library-based approach to search for tobacco genes regulated by a short-term sulphur starvation has been adopted. In this work, 38 genes were identified, of which 22 were regulated positively and 16 were regulated negatively. The transcript levels of the representative genes were monitored in four parts of the plants (mature and immature leaves, stems, and roots), which exhibited differential sulphur deficiency. Interestingly, some genes exhibit different regulation of expression in different parts of the plants. Database analysis allowed assignment of the potential function for many of the identified genes; however, the functions of a small number of genes strongly regulated by sulphur starvation remain unknown. The genes were grouped into nine functional categories, each including both up- and down-regulated genes. The possible links between the identified regulated genes and sulphur metabolism are considered, and compared where possible with expression patterns in Arabidopsis thaliana. Although no obvious regulatory genes were identified, the genes encoding proteins of unknown function remain as potential components of the regulatory processes.

A critical role of gastric mucosal ascorbic acid in the progression of acute gastric mucosal lesions induced by compound 48/80 in rats

AIM: To study the role of gastric mucosal ascorbic acid (AA) in the progression of acute gastric mucosal lesions induced by compound 48/80 (C48/80), a mast cell degranulator, in rats.

METHODS: C48/80 (0.75 mg/kg) was intraperitoneally injected to fasted Wistar rats. Oral administration of AA (10, 50 or 100 mg/kg) was performed 0.5 h after C48/80 treatment. Determinations for gastric mucosal lesion severity and blood flow, and assays for gastric mucosal total AA, reduced AA, oxidized AA, vitamin E, thiobarbituric acid reactive substances (TBARS), adherent mucus, nitrite/nitrate (NO_x), non-protein SH (NPSH), and myeloperoxidase (MPO), and serum total AA, reduced AA, oxidized AA, and NO_x were conducted 0.5 and 3 h after C48/80 treatment.

RESULTS: Gastric mucosal lesions occurred 0.5 h after C48/80 treatment and progressed at 3 h. Gastric mucosal blood flow decreased 0.5 h after C48/80 treatment but the decrease was recovered at 3 h. Gastric mucosal total AA, reduced AA, vitamin E, and adherent mucus concentrations decreased 3 h after C48/80 treatment. Gastric mucosal oxidized AA concentration remained unchanged after C48/80 treatment. Gastric mucosal NPSH concentration decreased 0.5 h after C48/80 treatment, but the decrease was recovered at 3 h. Gastric mucosal TBARS concentration and MPO activity increased 0.5 h after C48/80 treatment and further increased at 3 h. Serum total AA and reduced AA concentrations increased 0.5 h after C48/80 treatment and further increased at 3 h, while serum oxidized AA concentration increased at 0.5 h. Serum and gastric mucosal NOx concentrations increased 3 h after C48/80 treatment. AA administration to C48/80-treated rats at 0.5 h after the treatment prevented the gastric mucosal lesion progression and the changes in gastric mucosal total AA, reduced AA, vitamin E, adherent mucus, NOx, and TBARS concentrations and MPO activity and serum NOx concentration found at 3 h after the treatment dose-dependently. The AA administration to C48/80-treated rats caused further increases in serum total AA and reduced AA concentrations at 3 h after the treatment dose-dependently.

CONCLUSION: Gastric mucosal AA plays a critical role in the progression of C48/80-induced acute gastric mucosal lesions in rats.

Duodenal ascorbate and ferric reductase in human iron deficiency

BACKGROUND

The first step in iron absorption requires the reduction of ferric iron to ferrous iron, a change that is catalyzed by duodenal ferric reductase. Iron deficiency is associated with high iron absorption, high ferric reductase activity, and high duodenal ascorbate concentrations in experimental animals, but it is not known whether a relation between reductase and ascorbate is evident in humans.

OBJECTIVE

The objective of the study was to assess the relation between ferric reductase activity in human duodenal biopsy specimens and ascorbate concentrations in iron-replete and iron-deficient subjects.

DESIGN

Patients and control subjects were overnight-fasted adults presenting sequentially for upper gastrointestinal endoscopic investigation. Ferric reductase activity in duodenal biopsy specimens was assayed by using nitroblue tetrazolium. Ascorbate was assayed in duodenal biopsy specimens and plasma.

RESULTS

Iron-deficient patients had significantly higher reductase activity (n = 6-9; P < 0.05) and duodenal (n = 20; P < 0.001) and plasma (n = 6; P < 0.001) ascorbate concentrations than did control subjects. Incubation of biopsy specimens with dehydroascorbate (to boost cellular ascorbate) increased reductase activity in the tissues that initially had normal activity (n = 9; P < 0.01) but inhibited reductase activity in the tissues that already had high reductase activity (n = 13; P < 0.001).

CONCLUSIONS

Iron deficiency in humans is associated with increased duodenal ascorbate concentrations. This finding suggests that increased reductase activity is partly due to an increase in this substrate for duodenal cytochrome b reductase 1.

Vitamin C treatment reduces transcapillary escape rate of albumin in type 1 diabetes

BACKGROUND: Latent tissue scurvy in clinical and experimental ascorbic acid (AA) deficiency resembles diabetic microangiopathy as reflected in capillary hyperperfusion and increased transcapillary escape rate (TER), von Willebrand factor (vWf), and capillary fragility (CF). We aimed to investigate the possible clinical impact of the low plasma AA level observed in type 1 diabetes mellitus. METHODS: Twenty normoalbuminuric patients with retinopathy were included in a randomized, double-blinded, placebo-controlled 6-month study with AA (6 g/day; n=10, mean age+/-S.D. 35.3+/-8.5 years, diabetes duration 17.2+/-3.3 years) or placebo (n=10; mean age 37.8+/-6.6 years, diabetes duration 18.4+/-1.7 years). RESULTS: An inverse correlation between log plasma AA (HPLC method) and TER of (125)I-labelled albumin was found (r=-0.66, p=0.002). TER decreased in the AA-treated group from 6.8+/-1.0 to 5.4+/-1.5%/h compared to the unchanged placebo group (from 5.6+/-1.5 to 5.9+/-1.1%/h; p=0.031). The CF (capillary resistance test) decreased in the AA group compared to the placebo group (p=0.028), while vWf, fibrinogen and fibronectin levels, and glomerular filtration rate (clearance of (125)I-iothalamate) remained unchanged. CONCLUSIONS: The results suggest that a low plasma AA level might be associated (pathogenetic?) with development of microangiopathy in type 1 diabetes mellitus and points to a possible treatment modality.

The methanol method for the quantification of ascorbic acid and dehydroascorbic acid in biological samples

We present a fast to perform spectrophotometric method for the quantification of ascorbic acid and its oxidized form dehydroascorbic acid in biological samples. The assay detects a chromophore formed during the reaction of dehydroascorbic acid with methanol in phosphate/citrate buffer. This reaction can also be employed for the determination of ascorbate (vitamin C) in the presence of ascorbate oxidase. The major advantage of the developed protocol for the determination of both forms of vitamin C is a simple spectrophotometrical single end point determination. It is demonstrated that the methanol method is an improvement compared with a commercially available test kit for the determination of vitamin C. Using the methanol method, a dose-dependent increase in intracellular ascorbic acid was determined upon incubation of L-929 cells and RAW 264.7 macrophages with increasing concentrations of extracellular ascorbate. In blood serum, vitamin C was determined at concentrations between 46 and 97 microM. Supplementation with different amounts of ascorbate showed satisfying recovery. In L-929 cells, even unphysiologically high amounts of reactive nitrogen species were unable to completely oxidize intracellular vitamin C.

Short-term ascorbic acid deficiency induced oxidative stress in the retinas of young Guinea pigs

We examined whether short-term ascorbic acid deficiency induces oxidative stress in the retinas of young guinea pigs. Four-week-old guinea pigs were given a scorbutic diet (20 g/animal/day) with and without adequate ascorbic acid (400 mg/animal/day) in drinking water for 3 weeks. The serum concentrations of the reduced form of ascorbic acid and the oxidized form of ascorbic acid in the deficient group were 14.1 and 4.1%, respectively, of those in the adequate group. The retinal contents of the reduced form of ascorbic acid and the oxidized form of ascorbic acid in the deficient group were 6.4 and 27.3%, respectively, of those in the adequate group. The retinal content of thiobarbituric acid-reactive substances, an index of lipid peroxidation, was 1.9-fold higher in the deficient group than in the adequate group. Retinal reduced glutathione and vitamin E contents in the deficient group were 70.1 and 69.4%, respectively, of those in the adequate group. This ascorbic acid deficiency did not affect serum thiobarbituric acid-reactive substances and reduced glutathione concentrations but increased serum vitamin E concentration. These results indicate that short-term ascorbic acid deficiency induces oxidative stress in the retinas of young guinea pigs without disrupting systemic antioxidant status.

Role of antioxidant defense during different stages of preadult life cycle in European corn borer (Ostrinia nubilalis, Hubn.): Diapause and metamorphosis

Antioxidant enzymes, total glutathione (GSH), and ascorbic acid (ASA) were determined in whole body homogenates of nondiapausing larvae, diapausing larvae during the diapausing period (October, December, and February), and in pupae emerged from both diapausing and nondiapausing larvae of the European corn borer (Ostrinia nubilalis, Hubn., Lepidoptera: Pyralidae). The activities of catalase, selenium nondependent glutathione peroxidase (GPx), and glutathione-S-transferase (GST), as well as the content of GSH and ASA, were found to vary throughout the larval diapause. Compared to diapausing larvae, nondiapausing larvae were higher in levels of catalase, GPx, GST, and dehydroascorbate reductase (DHAR) activity. GSH content was also increased. However, nondiapausing larvae contained less ASA than diapausing ones. Pupae had higher GPx and GST activity and an increased ASA content compared to larvae. The pupae emerged from nondiapausing larvae had higher GST, glutathione reductase (GR), and DHAR activities, but lower GPx activity and ASA content than those emerged from diapausing larvae. Correlation analysis revealed differences in the way the antioxidant level is equilibrated for a particular stage and developmental pattern. The results suggest that cellular antioxidants are involved in both the protection of cells and the regulation of redox levels during the pre-adult stages of Ostrinia nubilalis. Arch. Insect Biochem. Physiol. 55:79-89, 2004.

Effect of dietary ascorbyl-2-phosphate on immune function after transport to a feeding facility

Effects of dietary ascorbyl-2-phosphate on immune function after a 210-km trip were measured in 18 Holstein heifers. After transport on d 0, 10 g of ascorbyl-2-phosphate each were added to the diets of 10 heifers, whereas eight heifers were fed a control diet. Plasma cortisol concentrations increased by an average of 25.6 microgram/ml on d 0 following transport, but by d 7 after transport had decreased to pretransport levels. Average daily gain was lower in heifers fed ascorbyl-2-phosphate from d 28 to 49 d after transport, but did not differ over the entire study. Feeding ascorbyl-2-phosphate maintained plasma ascorbate concentrations on d 7 post-transport, which decreased in control heifers. Plasma keyhole limpet hemocyanin antibody titers were significantly higher in control heifers from d 7 to 49. Mononuclear leukocyte proliferation responses were decreased on d 0 in lymphocytes stimulated by mitogens, with pokeweed mitogen-stimulated cells showing less of a response than cells stimulated by the other mitogens. In the absence of mitogens, dietary ascorbyl-2-phosphate increased basal 3H-methyl thymidine incorporation by cultured lymphocytes. Across diets and mitogens, lymphocytes treated with cortisol showed decreased 3H-methyl thymidine incorporation. Transportation acted as a stressor, as evidenced by the increased plasma cortisol levels at d 0 immediately after transport, but immunological effects were not apparent by d 7. Feeding ascorbyl-2-phosphate maintained plasma ascorbate concentrations on d 7, but had negative effects on immune responses posttransport.

Ozone differentially affects physiological and biochemical responses of two clover species; Trifolium repens and Trifolium pratense

The effect of acute ozone exposure (150 ppb for 3 h) on two clover species, white clover (Trifolium repens L.) and red clover (Trifolium pratense L.) was investigated through the analysis of 10 different physiological and biochemical parameters. Twenty-four hours after fumigation, visible symptoms of injury on leaves were observed only in red clover, but from the biochemical point of view, both species revealed significant ozone-induced modifications. A decrease in the photosynthetic efficiency as well as an increase in the de-epoxidation index and a decrease in the redox state of ascorbate were detected only in T. pratense leaves; no significant change in pigment content was found in either of the two species. On the other hand, both white and red clover showed, although to different extents, significant decreases in the activities of the antioxidant enzymes peroxidase and ascorbate peroxidase. Multivariate statistical analysis revealed not only that ozone affects both species, but also that they differentially respond to the pollutant, confirming the higher sensitivity of Trifolium pratense to ozone exposure.

Ozone tolerance in snap bean is associated with elevated ascorbic acid in the leaf apoplast

Ascorbic acid (AA) in the leaf apoplast has the potential to limit ozone injury by participating in reactions that detoxify ozone and reactive oxygen intermediates and thus prevent plasma membrane damage. Genotypes of snap bean (Phaseolus vulgaris L) were compared in controlled environments and in open-top field chambers to assess the relationship between extracellular AA content and ozone tolerance. Vacuum infiltration methods were employed to separate leaf AA into extracellular and intracellular fractions. For plants grown in controlled environments at low ozone concentration (4 nmol mol-1 ozone), leaf apoplast AA was significantly higher in tolerant genotypes (300-400 nmol g-1 FW) compared with sensitive genotypes (approximately 50 nmol g-1 FW), evidence that ozone tolerance is associated with elevated extracellular AA. For the open top chamber study, plants were grown in pots under charcoal-filtered air (CF) conditions and then either maintained under CF conditions (29 nmol mol-1 ozone) or exposed to elevated ozone (67 nmol mol-1 ozone). Following an 8-day treatment period, leaf apoplast AA was in the range of 100-190 nmol g-1 FW for all genotypes, but no relationship was observed between apoplast AA content and ozone tolerance. The contrasting results in the two studies demonstrated a potential limitation in the interpretation of extracellular AA data. Apoplast AA levels presumably reflect the steady-state condition between supply from the cytoplasm and utilization within the cell wall. The capacity to detoxify ozone in the extracellular space may be underestimated under elevated ozone conditions where the dynamics of AA supply and utilization are not adequately represented by a steady-state measurement.

Ascorbate transport from the apoplast to the symplast in intact leaves

Infiltration of reduced ascorbate (ASC) into the leaves of Betula pendula Roth and subsequent measurement of its loss therein after incubation allowed us to follow ascorbate transport from apoplast to symplast in intact leaves. All of the ascorbate extracted from the native apoplast was in fully oxidized form, dehydroascorbate (DHA). When 5 mM of ASC was infiltrated into the leaves, its intense decay occurred, but only 55% of ASC lost was recovered in apoplast as DHA. When ASC was added to the freshly extracted intercellular washing fluid (IWF), ASC oxidation occurred as well. However, all oxidized ASC was recovered as DHA, indicating that further decomposition of DHA did not occur. Similarly, all of the ASC infiltrated into the leaves was found therein either as ASC or DHA after incubation of leaves for up to 60 min. On this base the ascorbate infiltrated into the leaves and not recovered in the IWF was interpreted as ascorbate taken up into the symplast. The calculated uptake rates of ascorbate at different ASC concentrations followed saturation kinetics with the maximum uptake rate of 300 nmol m-2 plasma membrane (PM) area min-1 and Michaelis constant of 12.8 mM. The uptake of ascorbate was significantly inhibited by the addition of dithiothreitol or by PM H+ ATPase inhibitor erythrosin B. Thus, our results support the previous observations that DHA is preferably transported from the apoplastic to the cytoplasmic side of the membrane and show that this process is dependent upon PM proton gradient.

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.

Dehydroascorbic acid reduction in several tissues and cultured hepatocytes of the chicken

Dehydroascorbic acid, the oxidized form of ascorbic acid, is rapidly reduced to ascorbate in living organs (ascorbate recycling). We examined the GSH-dependent dehydroascorbate reductase activity in several tissues of the chicken. The activity was highest in the liver, and second highest in the brain. The activity was localized in the cytosol fraction of the liver. We subsequently examined the dehydroascorbate reduction in separated chicken hepatocytes. The cellular ascorbate concentration was elevated in dehydroascorbate-treated cells. It is thought that hepatocytes incorporated external dehydroascorbate and converted it into ascorbate. These findings suggest that the liver plays an important role in ascorbate recycling by the chicken.

Changes in ascorbic acid content in primary cultured rat hepatocytes exposed to 2,2'-azobis (2-amidinopropane) dihydrochloride, a radical initiator

Changes in ascorbate content in primary cultured rat hepatocytes exposed to oxidative stress derived from water soluble radical initiator 2,2'-azobis (2-amidinopropane) dihydrochloride (AAPH) were examined. Cells were exposed to 0.05 and 5 mg/ml of AAPH as 'mild' and severe' oxidative stresses, respectively. Lipid peroxidation in hepatocytes was induced by 'severe' oxidative stress, but not by 'mild' oxidative stress. Ascorbate decreased at 6 hr after administration of both mild' and severe' oxidative stresses, and recovered to the control level after a further 6 hr. In cells treated with 'severe oxidative stress, however, total ascorbate (reduced form plus oxidized form) had increased 24 hr after administration. These results indicated that consumption alone did not account for the increase of ascorbate in hepatocytes under oxidative stress.

Degradation of dehydroascorbic acid in parenteral nutrition mixtures

The degradation of ascorbic acid (AA) stored in parenteral nutrition (PN) regimens is initially by oxidation, catalysed by trace elements, in particular copper. After prolonged storage the concentration of AA remains relatively constant, with little variation, due to the lack of available oxygen. The initial degradation product is dehydroascorbic acid (DHAA). This is generated in an anaerobic environment, and is hypothesised to degrade by hydrolysis. It is the purpose of this investigation to ascertain the effect of temperature and trace elements on the anaerobic degradation of DHAA, and to identify the kinetics of the reaction. A stability-indicating reversed-phase HPLC assay was used. The column contained C(18) reverse-phase packing (Luna), mean diameter 5 microm. The column dimensions were 15 cm long with an internal diameter of 0.4 cm. The mobile phase consisted of methanol: phosphate buffer (pH 7.8: 0.067 mol dm(-3)) at a ratio of 40: 60 (v/v) and also included Cetrimide (mixed alkyltrimethylammonium bromide) (0.05 mol dm(-3)) as an ion pair reagent. The flow rate was 0.7 ml min(-1) and detection was by ultra-violet light absorption at 278 nm. This assay was used to monitor the degradation rate of DHAA in PN mixtures with and without trace elements over a range of temperatures (5-35 degrees C). Results indicated a first order reaction that was temperature-dependent but trace elements independent.

Ascorbate, glutathione and related enzymes in chloroplasts of tomato leaves infected by Botrytis cinerea

The effect of Botrytis cinerea infection on the total pool sizes of ascorbate, glutathione and their redox status, the activities of ascorbate-glutathione cycle related enzymes, APX, DHAR, GR as well as GST and GSH-Px activities in chloroplasts isolated from tomato leaves were studied. An accelerated chlorophyll loss in chloroplasts from infected leaves was observed. The AA content was not changed considerably but a burst in DHA formation, up to 319% of control, and a significant prooxidative shift in the ascorbate redox status were found in chloroplasts from infected leaves. A massive progressive decrease in GSH and total glutathione contents as well as GR activity increase were found after infection. However, similar but less-pronounced glutathione-related changes were observed in control suggesting that they could be at least partly the result of B. cinerea induced promotion of senescence and not a specific response to pathogen infection. As shown by the analysis of ascorbate-glutathione cycle related enzyme activities as well as correlations between the two antioxidants and enzymes responsible for keeping them in the reduced state GSH is the limiting factor for the whole cycle operation in chloroplasts. This is in accordance with our previous results for the whole-cell extracts.

Doxorubicin toxicity to the skin: possibility of protection with antioxidants enriched yeast

The possibility of skin protection against doxorubicin toxicity was examined after oral antioxidative pretreatment of the rats with yeast supplemented with selenium and vitamins E, C and A for 15 days. The activity and level of antioxidative defense components were monitored in the skin and blood 48 h after i.v. applied doxorubicin. In the blood, increased glutathione peroxidase activity in the erythrocytes, and amounts of vitamin E and glutathione in the plasma were found after the antioxidative treatment. It also led to an increase of the reductive capacity in the skin (increased thioredoxin reductase activity and reduced glutathione level). Doxorubicin alone, depleted reductive capacity, i.e. decreased the activity of thioredoxin reductase in the skin, as well as the content of reduced glutathione both in the skin and blood plasma. Depletion of reductive capacity represents one of the first harmful doxorubicin effects to the skin at the time when the changes of other antioxidative enzyme activities were not detectable. Reductive capacity in the skin of animals given antioxidative pretreatment was maintained elevated upon doxorubicin application in comparison with the corresponding control. Oral supplementation with antioxidants thus prevents toxic effects of doxorubicin in the skin and may contribute to the alleviation of its secondary cytotoxicity during the chemotherapy.

Redox state and peroxidase system in sunflower plants exposed to ozone

Sunflower plants subjected to a short-term fumigation with O(3) (150 ppb for 4 h repeated for 4 days) exhibited an increase in total ascorbate content, accompanied by a marked oxidation of ascorbate, leading to a decrease in its redox state, either at intracellular or extracellular level. O(3) exposure induced a rise in free extracellular peroxidase (POD) activity, assayed by syringaldazine as electron donor, as well as in the ionically and covalently cell wall bound PODs. On the contrary, the activity of both extracellular and intracellular guaiacol-POD did not show significant changes as a consequence of the pollutant exposure. The stimulation of syringaldazine-POD activities may be related to the effect of ozone on the growth of the cells, inducing an early senescence through the activation or acceleration of lignification processes. Beside, the reduced plasticity of the cell wall may oppose an unspecific mechanical resistance against the abiotic stress induced by the ozone exposure.

Oxidation products of uric acid and ascorbic acid in preterm infants with chronic lung disease

Allantoin, the oxidation product of uric acid (UA), can be used as an in vivo marker of free radical generation. The aims of the present study were to evaluate the allantoin changes in plasma and bronchoalveolar lavage fluid (BALF) as well as to examine plasma levels of ascorbic acid (AA) and its oxidation product, dehydroascorbic acid (DHAA), in infants with or without chronic lung disease (CLD) during the first week of life. The study population was 20 infants of 24-30 weeks gestation, comprising 10 who subsequently developed CLD and 10 without CLD. In the CLD infants, the plasma allantoin/UA ratio showed a significant increase after day 1 and continued to increase gradually to reach a peak on day 6 (6.5 +/- 4.1% for CLD and 2.1 +/- 0.9% for non-CLD infants). The allantoin/UA ratio in BALF was also higher in CLD infants and the difference reached significance on days 4-6 (41.2 +/- 15.8% for CLD and 11.7 +/- 9.9% for non-CLD infants). In contrast to allantoin, the plasma DHAA/AA ratio did not differ between the 2 groups throughout the study period. Our findings that the allantoin/UA ratios were significantly higher in CLD than non-CLD infants not only in plasma but also in BALF, and that the intergroup differences of this ratio in both plasma and BALF was more prominent in the latter half of the first week of life further confirm our previous speculation that oxygen radicals are involved in the development of neonatal CLD.

Ascorbate levels in red blood cells and urine in patients with sickle cell anemia

Ascorbic acid can be important in sickle cell anemia (SCA) because significant oxidative stress occurs in the disease. Ascorbate could contribute to reduction of the increased oxygen free radicals generated in sickle red blood cells (SRBC) and to the recycling of vitamin E in the cells, while renal loss could contribute to the low plasma levels. Evaluation of red blood cell (RBC) and urine ascorbate in SCA has not been reported. Results showed (1) ascorbate levels in SRBC were similar to those in normals; (2) urine ascorbate excretion was increased in 36% of patients; (3) plasma levels of ascorbate were decreased.

CONCLUSIONS

(1) Ascorbate is present in SRBC, most likely due to ascorbate recycling, despite increased free-radical generation. (2) The increase in renal excretion may contribute to the low plasma levels of ascorbate. (3) The presence of ample ascorbate in SRBC and decreased plasma ascorbate suggests that ascorbate movement across the SRBC membrane may differ from normal RBC.

Antioxidant status, erythrocyte membrane lipid peroxidation and osmotic fragility in malignant lymphoma patients

We studied erythrocyte and leukocyte superoxide dismutase and catalase activities, erythrocyte malondialdehyde (MDA) and osmotic fragility and plasma L-ascorbic acid and L-dehydroascorbic acid levels in adult patients with acute lymphoblastic leukemia (ALL), Hodgkin's disease (HD) and non-Hodgkin's lymphoma (NHL) before and after treatment. SOD activity was elevated in leukocytes of ALL and HD patients before treatment, and borderlike-significantly elevated in leukocytes of the same patients after treatment in comparison to the control subjects. SOD activity was not changed in NHL patients before or after chemotherapy. Erythrocyte superoxide dismutase and catalase activities were elevated in the three groups of lymphomas before and after treatment. MDA level and osmotic fragility of red blood cells of patients with lymphomas were increased before and after treatment in comparison to the control group. Plasma L-ascorbic acid concentrations were decreased, whereas L-dehydroascorbic acid concentrations were increased in ALL, HD and NHL patients before and after treatment. There were also significant differences in the activities of the antioxidant enzymes, concentrations of antioxidants, MDA and osmotic fragility in the most of the malignant lymphoma patients. The present data suggest that hematological complications and autoimmune hemolytic anemia might be attributed to the oxidative stress produced by malignant lymphomas.

The biosynthesis of erythroascorbate in Saccharomyces cerevisiae and its role as an antioxidant

This study investigated the ability of the yeast Saccharomyces cerevisiae to synthesize ascorbate and its 5-carbon analogue erythroascorbate from a variety of precursors, and their importance as antioxidants in this organism. Studies of ascorbate and analogues in micro-organisms have been reported previously, but their function as antioxidants have been largely ignored. Ascorbate and erythroascorbate concentrations in yeast extracts were measured spectrophotometrically, and their levels and identity were checked using liquid chromatography-electrospray mass spectrometry. The yeast was readily able to synthesize ascorbate from L-galactono-1,4-lactone or erythroascorbate from D-arabinose and D-arabino-1,4-lactone, whereas L-gulono-1,4-lactone was a much poorer substrate for ascorbate biosynthesis. In untreated cells, the concentration of ascorbate-like compounds was below the level of detection of the methods of analysis used in this study (approximately 0.1 mM). Intracellular ascorbate and erythroascorbate were oxidized at high concentrations of tert-butylhydroperoxide, but not hydrogen peroxide. Their synthesis was not increased in response to low levels of stress, however, and preloading with erythroascorbate did not protect glutathione levels during oxidative stress. This study provides new information on the metabolism of ascorbate and erythroascorbate in S. cerevisiae, and suggests that erythroascorbate is of limited importance as an antioxidant in S. cerevisiae.

Ascorbic acid metabolism in pea seedlings. A comparison of D-glucosone, L-sorbosone, and L-galactono-1,4-lactone as ascorbate precursors

L-Ascorbic acid (AsA) accumulates in pea (Pisum sativum L.) seedlings during germination, with the most rapid phase of accumulation coinciding with radicle emergence. Monodehydroascorbate reductase and dehydroascorbic acid reductase were active in the embryonic axes before AsA accumulation started, whereas AsA oxidase and AsA peroxidase activities increased in parallel with AsA. Excised embryonic axes were used to investigate the osone pathway of AsA biosynthesis, in which D-glucosone and L-sorbosone are the proposed intermediates. [U-14C]Glucosone was incorporated into AsA and inhibited the incorporation of [U-14C]glucose (Glc) into AsA. A higher D-glucosone concentration (5 mM) inhibited AsA accumulation. L-Sorbosone did not affect AsA pool size but caused a small inhibition in the incorporation of [U-14C]Glc into AsA. Oxidase and dehydrogenase activities capable of converting Glc or Glc-6-phosphate to glucosone were not detected in embryonic axis extracts. The osones are therefore unlikely to be physiological intermediates of AsA biosynthesis. L-Galactono-1,4-lactone, recently proposed as the AsA precursor (G.L. Wheeler, M.A. Jones, N. Smirnoff [1998] Nature 393: 365-369), was readily converted to AsA by pea embryonic axes. Although L-galactono-1,4-lactone did not inhibit [14C]Glc incorporation into AsA, this does not mean that it is not a precursor, because competition between endogenous and exogenous pools was minimized by its very small pool size and rapid metabolism.

Homocysteine and alpha-lipoic acid regulate p44/42 MAP kinase phosphorylation in NIH/3T3 cells

Biological thiols can regulate cell signal transduction. The effects of two biothiols, homocysteine (Hcy), a risk factor for cardiovascular disease, and alpha-lipoic acid (alphaLA), a therapeutic antioxidant, on p44/42 mitogen-activated protein kinases (MAPK) phosphorylation were examined in NIH/3T3 fibroblasts. Cells grown in serum-containing media had constitutive levels of MAPK phosphorylation as determined by Western blot analysis using the phospho-specific MAPK antibody. Treatment of cells with 20 microM Hcy for 0-60 min resulted in a transient enhancement of MAPK phosphorylation. In contrast, 20 microM alphaLA inhibited serum-mediated phosphorylation of MAPK. The differential effects of these two thiols are not due to their redox states as oxidized Hcy (Hcy thiolactone) enhanced MAPK phosphorylation. The effect of alphaLA appears to be serum-dependent because Hcy or alphaLA treatment of serum-deprived cells activated MAPK phosphorylation. Thus, alphaLA and Hcy can either induce common signal transduction pathways or differentially modulate MAPK phosphorylation, depending on the state of the cell. This relationship may be important to understand how some biothiols are associated with pathogenic events while others offer potential as therapeutic agents.

Protection by vitamin C of loss of vitamin E in cultured rat hepatocytes

Results from in vivo studies of the capacity of vitamin C to spare and/or recycle vitamin E are equivocal. While some in vitro and membrane models reveal an interaction between vitamins C and E, the characterization of this relationship in biologically relevant systems is lacking. Thus, we investigated this relationship using hepatocytes isolated from 3- to 6-month-old male Sprague-Dawley rats. Cells were incubated for 18-20 h in medium supplemented with 0.1-4 mM ascorbic acid. The loss of alpha-tocopherol and the formation of its primary oxidized metabolite, alpha-tocopherolquinone, was determined by HPLC. Levels of alpha-tocopherol in hepatocytes incubated without ascorbic acid declined from 390 to 35 pmol/mg protein; hepatocyte ascorbic acid levels declined from 9 to 0.5 nmol/mg protein. alpha-Tocopherolquinone was undetectable in freshly isolated hepatocytes but following incubation in ascorbate-free medium reached 10 pmol/mg protein. The formation of alpha-tocopherolquinone was not detected in hepatocytes incubated with ascorbic acid. Dehydroascorbic acid (DHA) levels represented 10-20% of the total ascorbate content in freshly isolated hepatocytes but after 3 h incubation the proportion of DHA increased to 50%; after 18-20 h incubation DHA was undetectable. Hepatocytes incubated with 1.0, 2.0, 2.5, or 4.0 mM ascorbic acid lost significantly less alpha-tocopherol (62, 69, 67, and 56%, respectively) than unsupplemented controls (90%). Twelve percent of the alpha-tocopherol lost from hepatocytes during incubation was detected in the medium of cells incubated with ascorbic acid, but vitamin E was undetectable in the medium of cells incubated without ascorbic acid. These results demonstrate an interaction between vitamins C and E in cell culture and are not inconsistent with a potential recycling of oxidized alpha-tocopherol by ascorbic acid.

Transport of ascorbic acid in gastric epithelial cells in vitro

The normal human stomach contains high concentrations of ascorbic acid in both the mucosa and gastric juice, but the mechanism of ascorbic acid transport in the stomach is unknown. To understand more, ascorbic acid accumulation in gastric epithelial cell lines was investigated. Ascorbic acid was transported into gastric epithelial cells (Kato III and AGS cell lines) and accumulated up to eight-fold against a concentration gradient, as measured by high-performance liquid chromatography with electrochemical detection. Kinetic analysis using both non-radioactive and radioactive sources of ascorbic acid showed that ascorbic acid accumulation was mediated by one saturable concentration-dependent transport system with a Km of 3-11 micromol/l and Vmax of 0.8-0.9 nmol/10(8) cells/min. These data suggest that ascorbic acid uptake in gastric mucosal cells may be facilitated by a high-affinity saturable transport activity. Loss of intracellular ascorbic acid from Kato III and AGS cells was slower than seen in vivo which may limit the usefulness of these cell lines as a physiological model for the secretion of mucosal ascorbic acid into gastric juice.

Bicarbonate promotes a cleavage of lactone ring of dehydroascorbate

The half-life of dehydroascorbate (DHA) in human plasma is only a few minutes. This DHA disappearance is caused by a cleavage of lactone ring. Similarly, when DHA was incubated in Dulbecco's modified Eagle's medium (D-MEM), which stood in atmosphere of 5% CO2-95% air, the rapid transformation of DHA into 2,3-diketogulonate (2,3-DKG) is also observed. These observations suggest that both human plasma and D-MEM contain a common component, which promotes the hydrolysis of DHA. In the present study, this component was identified to be bicarbonate which acts as a general base catalyst. Direct evidence for this mechanism was obtained as follows: (1) significant hydrolysis of DHA in the bicarbonate-free D-MEM (pH 7.40) was not observed; (2) hydrolysis of DHA in Tris-HCl buffer at constant pH (7.4) increases with increasing bicarbonate concentration; and (3) significant hydrolysis of DHA in the decarbonated ultrafiltrate of plasma was not observed. These results suggest that DHA hydrolysis may be controlled by the variation of CO2 pressure in circulating blood.

Cysteine and ascorbate loss in the diabetic rat lens prior to hydration changes

PURPOSE

Glutathione (GSH) loss precedes vacuole formation in the diabetic rat lens, but the cause of this loss is not known. Cysteine availability is a rate limiting factor to glutathione biosynthesis in rat and human lenses but its concentration is not known; therefore free cysteine was measured prior to lens hydration in the diabetic rat lens. GSH can regenerate ascorbate from dehydroascorbate within the lens and potentially modulate the ascorbate pool; therefore ascorbate loss is also a possibility that has not been examined previously.

METHODS

Diabetes was induced in Wistar rats to provide a slowly progressing model of cortical cataract. Age-matched control rats were injected with buffer vehicle only. Lens condition was monitored by binocular slit-lamp microscope after pupil dilation. Lens cysteine and glutathione were measured in the same lens, while ascorbate and total ascorbate (ascorbate + dehydroascorbate) of the contralateral lens were quantified by high performance liquid chromatography electrochemical detection. The 1- and 2-week periods of diabetes were chosen as they both preceded lens hydration changes and Na+/K+ changes, to avoid leakage due to ruptured cell membranes.

RESULTS

Lens weights were not significantly different compared to controls at either the 1- or 2-week periods, and lenses were completely free of initial vacuole formation. Lens GSH concentration was diminished by 72% compared with controls after 1 week of diabetes and 74% after 2 weeks of diabetes. Lens free cysteine was decreased by 62% and 78% compared with controls after 1 and 2 weeks of diabetes, respectively. Total lens ascorbate concentration was decreased by 34% after 1 week of diabetes and 48% after 2 weeks of diabetes. Dehydroascorbate levels represented less than 10% of the total lens ascorbate pool in all experimental groups. GSH and ascorbate concentration were correlated after 1 week of diabetes (p < 0.005) and after 2 weeks of diabetes (p < 0.001). GSH and cysteine concentration were also correlated after 1 week of diabetes (p < 0.001) and after 2 weeks of diabetes (p < 0.001).

CONCLUSIONS

Decreased free cysteine, in the diabetic rat lens, precedes hydration changes and vacuole formation, contributing to decreased glutathione content. While cysteine was not abundant in the lens, its concentration is greater than previously supposed. The lens ascorbate pool was also diminished prior to lens hydration.

Seasonal variation in the antioxidant defense system of the brain of the ground squirrel (Citellus citellus) and response to low temperature compared with rat

Seasonal variation in the activity of antioxidant enzymes (superoxide dismutase (EC 1.15.1.1.; SOD), catalase (EC 1.11.1.6; CAT), glutathione peroxidase (EC 1.11.1.9; GSH-Px), glutathione reductase (EC 1.6.4.2; GR), glutathione-S-transferase (EC 2.5.1.18; GST) and low-molecular-weight antioxidants: ascorbic acid (AsA), vitamin E (VIT E) and glutathione (CSH+GSSG) were examined in the brain of the ground squirrels (Citellus citellus) maintained at 30 degrees C during the whole year. The highest activity (per mg protein) of antioxidant defense (AD) enzymes was found in the spring and was much lower in the summer. A further decrease in activity of CAT, GSH-Px and GST was observed in the winter. The highest levels of AsA and glutathione were recorded in winter in comparison with spring and summer. AD system in the brain of the ground squirrel and rates (maintained at thermoneutrality) exposed to low temperature (4 degrees C) for 3, 6 or 24 hr during the summer was studied as well. Summer was chosen as a period of stable euthermia for ground squirrels and in thermoregulation similar to rats. Consumption of free fatty acid and glucose during the acute exposure to low temperature was found to be species specific. In the ground squirrel, an increase in the specific activities of SOD, after 3, 6 and 24 hr, CAT after 3 and 6 hr and GR after 6 hr of exposure to low temperature was detected. When activities were expressed in U/g wet mass, an increase of SOD after 3, 6 and 24 hr (P < 0.02, P < 0.02, P < 0.005) and CAT and GSH-Px 3 hr (P < 0.01) upon exposure to low temperature was observed. In the rats, no changes in the specific activities of these enzymes after exposure to low temperature were recorded and only an increase in GST activity (U/g wet mass) after 6 hr exposure was registered. Low-molecular-weight AD components in both animal species were unchanged upon short-term exposure to low temperature. The species-specific differences in brain AD between the rats and the ground squirrels after short exposure to low temperature may be ascribed to seasonal changes of the brain activity in the latter.

Extracellular glutathione peroxidase and ascorbic acid in aqueous humor and serum of patients operated on for cataract

Patients operated on for cataract (32 men/75 women, aged 50-93 years) were studied with respect to antioxidative agents in aqueous humor and serum. Extracellular glutathione peroxidase (eGSHPx) was demonstrated in aqueous humor for the first time by a radioimmunoassay, the concentration of eGSHPx being 0.66(0.18) mg/l (mean(S.D.)). The concentration of eGSHPx in serum was 3.81(0.84) mg/l, and its level in aqueous humor was 18(7)% of that level. Serum selenium had positive correlations with both serum eGSHPx (r = 0.34, P < 0.001) and aqueous humor eGSHPx (r = 0.25, P = 0.011). However, there was no relation between the concentrations of eGSHPx in aqueous humor and in serum, suggesting that the maintenance of eGSHPx levels in the two fluids is controlled by different mechanisms beside selenium status. There was an inverse correlation between age and serum eGSHPx but not with aqueous humor eGSHPx. The concentration of ascorbic acid in aqueous humor was 2.04(0.58) mmol/l, and it was closely correlated to the level of ascorbic acid in serum (0.052(0.032) mmol/l), r = 0.58 (P < 0.001). The ratio between the level of ascorbic acid in aqueous humor and that in serum was 39(17). There was no significant difference among patients with nuclear (n = 39), cortical (n = 20), posterior-subcapsular (n = 23) or mixed (n = 23) lens opacity with respect to levels of eGSHPx and ascorbic acid in serum and aqueous humor. Since serum ascorbic acid is related to ascorbic acid intake, its association to aqueous humor ascorbic acid indicates that dietary habits are important for maintaining that level which could play an important role in protecting ocular tissue against oxidative damage. The role of eGSHPx secreted into aqueous humor in the oxidant defence system needs further study.

Determination of ascorbic and dehydroascorbic acids in blood plasma and serum by liquid chromatography

A liquid-chromatography (LC) method with ultraviolet detection for measuring ascorbic (AA) and dehydroascorbic acid (DHA) in human blood and serum was studied. The method used an ODS reversed-phase column and cetyltrimethyl ammonium bromide as an ion-pairing agent. AA was measured before and after the reduction of DHA with dithiothreitol. The absence of interferences resulting from hemolysis products was verified and also the stability of the ascorbic acid in metaphosphoric acid extracts. The analytical parameters, linearity (1-80 micrograms/ml), accuracy (recovery, 96.7-100.7%) and precision (C.V. = 3.1%), show that the method is reliable and adequate for measuring the total vitamin C content in serum and plasma.