Ascorbic acid regeneration by bovine iris-ciliary body

Antioxidant delivery pathways in the anterior eye

Tissues in the anterior segment of the eye are particular vulnerable to oxidative stress. To minimise oxidative stress, ocular tissues utilise a range of antioxidant defence systems which include nonenzymatic and enzymatic antioxidants in combination with repair and chaperone systems. However, as we age our antioxidant defence systems are overwhelmed resulting in increased oxidative stress and damage to tissues of the eye and the onset of various ocular pathologies such as corneal opacities, lens cataracts, and glaucoma. While it is well established that nonenzymatic antioxidants such as ascorbic acid and glutathione are important in protecting ocular tissues from oxidative stress, less is known about the delivery mechanisms used to accumulate these endogenous antioxidants in the different tissues of the eye. This review aims to summarise what is currently known about the antioxidant transport pathways in the anterior eye and how a deeper understanding of these transport systems with respect to ocular physiology could be used to increase antioxidant levels and delay the onset of eye diseases.

Differential effects of ascorbate on endothelium-derived hyperpolarizing factor (EDHF)-mediated vasodilatation in the bovine ciliary vascular bed and coronary artery

1. The ability of ascorbate to inhibit endothelium-derived hyperpolarizing factor (EDHF)-mediated vasodilatation was compared in the bovine perfused ciliary vascular bed and isolated rings of coronary artery. 2. Acetylcholine-induced, EDHF-mediated vasodilatation of the ciliary circulation was blocked following inclusion of ascorbate (50 micro M, 120 min) in the perfusion fluid. The blockade was highly selective since ascorbate had no effect on the vasodilator actions of the K(ATP) channel opener, levcromakalim, nor on the tonic vasodepressor action of basally released nitric oxide. 3. The possibility that concentration of ascorbate by the ciliary body was a prerequisite for blockade to occur was ruled out, since EDHF was still blocked when the anterior and posterior chambers were continuously flushed with Krebs solution or when both the aqueous and vitreous humour were drained. 4. Ascorbate at 50 micro M failed to affect bradykinin- or acetylcholine-induced, EDHF-mediated vasodilatation in rings of bovine coronary artery. Raising the concentration to 3 mM did produce blockade of EDHF, but this was nonselective, since vasodilator responses to endothelium-derived nitric oxide were also inhibited. 5. Thus, ascorbate (50 micro M) is not a universal blocker of EDHF. Whether its ability to block in the bovine ciliary circulation, but not in the coronary artery, is due to differences in the nature of EDHF at the two sites, differences in vessel size (resistance arterioles versus conduit artery), the presence or absence of flow, or to some other factor remains to be determined.

Glutathione-dependent ascorbate recycling activity of rat serum albumin

An efficient regeneration of vitamin C (ascorbate) from its oxidized byproduct, dehydroascorbate (DHAA), is necessary to maintain sufficient tissue levels of the reduced form of the vitamin. Additionally, the recycling may be more significant in mammals, such as guinea pigs and humans, who have lost the ability to synthesize ascorbate de novo, than it is in most other mammals who have retained the ability to synthesize the vitamin from glucose. Both a chemical and an enzymatic reduction of DHAA to ascorbate have been proposed. Several reports have appeared in which proteins, including thioltransferase, protein disulfide isomerase, and 3-alpha-hydroxysteroid dehydrogenase, characterized for other activities have been identified as having DHAA reductase activity in vitro. Whether these previously characterized proteins catalyze the reduction of DHAA in vivo is unclear. In the present study, a 66 kD protein was purified strictly on the basis of its DHAA-reductase activity and was identified as rat serum albumin. The protein was further characterized and results support the suggestion that serum albumin acts as an antioxidant and exerts a significant glutathione-dependent DHAA-reductase activity that may be important in the physiologic recycling of ascorbic acid.

Ascorbic acid recycling in Nb2 lymphoma cells: implications for tumor progression

Analysis of cultured rat "Nb2 lymphoma" cell lines, showing different degrees of malignant progression, can lead to identification of phenotypic changes associated with this phenomenon in T-cell cancers. In the present study we have compared the metastatic sublines, Nb2-11 and Nb2-SFJCD1, with regard to ascorbate and glutathione recycling, important processes in cellular protection from oxidative stresses. Whereas the Nb2-11 subline is prolactin (PRL)-dependent, the genetically related Nb2-SFJCD1 subline is growth factor-independent and shows more chromosomal alterations, indicative of more advanced progression. The Nb2-SFJCD1 cells, compared to the Nb2-11 cells, were less sensitive to toxic effects of dehydroascorbate, a potentially toxic oxidation product of ascorbate. Results were consistent with a significantly higher production of reducing equivalents (e.g., NADPH, GSH) and an accelerated reduction of dehydroascorbate by homogenates of Nb2-SFJCD1 cells. However, the increased resistance was apparently not directly related to the cellular uptake and reduction of dehydroascorbate by whole cells, which was similar in both cell lines. Observations indicate that Nb2 lymphoma cells, in their progression to malignancy, can acquire an enhanced capability to protect themselves from oxidative damage assisting them in withstanding the oxidative stress that anti-neoplastic drugs can cause. The adaptation may also be a mechanism that is utilized by tumor cells in suppressing apoptosis and other protective cellular functions facilitating, or potentiating, a tumor cell's ability to become more metastatic. However, the mechanism leading to this augmented capacity of Nb2 lymphoma cells to resist oxidative stress in not known and is the subject for further study.

Tear fluid content of electrochemically active components including water soluble antioxidants

PURPOSE

To describe the content of water-soluble antioxidants in tear fluid.

METHODS

We collected tear fluid from healthy subjects, either into borosilicate glass tubing or by absorption onto Schirmer strips. High pressure liquid chromatography with electrochemical detection provided data on several components in a single assay. The system was sufficiently sensitive to provide reliable values for components present in tear fluid collected at normal (basal) or stimulated rates of secretion.

RESULTS

Tear fluid at basal secretion was found to contain four compounds often considered to function in biology as antioxidants. Ascorbic acid (AA) is found at 665 microM, tyrosine at 45 microM and glutathione (GSH) at 107 microM. Cysteine (48 microM) and uric acid (328 microM) are reported for the first time; the latter is somewhat controversial as a physiologically active antioxidant. One peak on the chromatogram was consistently present but has not been identified. During stimulation of flow by brief inhalation of ammonium hydroxide fumes, the concentration of each compound was lower.

CONCLUSION

This work demonstrates the presence of electro-chemically active components that might function as antioxidants at the anterior surface of the cornea against potential damage from radiation, oxygen toxicity, abrasion and environmental chemicals.

Vitamin C crosses the blood-brain barrier in the oxidized form through the glucose transporters

Vitamin C concentrations in the brain exceed those in blood by 10-fold. In both tissues, the vitamin is present primarily in the reduced form, ascorbic acid. We identified the chemical form of vitamin C that readily crosses the blood-brain barrier, and the mechanism of this process. Ascorbic acid was not able to cross the blood-brain barrier in our studies. In contrast, the oxidized form of vitamin C, dehydroascorbic acid (oxidized ascorbic acid), readily entered the brain and was retained in the brain tissue in the form of ascorbic acid. Transport of dehydroascorbic acid into the brain was inhibited by d-glucose, but not by l-glucose. The facilitative glucose transporter, GLUT1, is expressed on endothelial cells at the blood-brain barrier, and is responsible for glucose entry into the brain. This study provides evidence showing that GLUT1 also transports dehydroascorbic acid into the brain. The findings define the transport of dehydroascorbic acid by GLUT1 as a mechanism by which the brain acquires vitamin C, and point to the oxidation of ascorbic acid as a potentially important regulatory step in accumulation of the vitamin by the brain. These results have implications for increasing antioxidant potential in the central nervous system.

Efficient transport and accumulation of vitamin C in HL-60 cells depleted of glutathione

Human myeloid leukemia cells (HL-60) transport only the oxidized form of vitamin C (dehydroascorbic acid) and accumulate the vitamin in the reduced form, ascorbic acid. We performed a detailed study of the role of glutathione in the intracellular trapping/accumulation of ascorbic acid in HL-60 cells. Uptake studies using HL-60 cells depleted of glutathione by treatment with L-buthionine-(S,R) sulfoximine and diethyl maleate, revealed no changes in the cells' ability to transport dehydroascorbic acid and accumulate ascorbic acid. Similar transport and accumulation rates were obtained using HL-60 cells containing intracellular glutathione concentrations from 6 mM to 1 microM. HL-60 cells, containing as little as 5 microM glutathione, were able to accumulate up to 150 mM ascorbic acid intracellularly when incubated with dehydroascorbic acid. Glutathione was capable of reducing dehydroascorbic acid by a direct chemical reaction, but only when present in a greater than 10-fold stoichiometric excess over dehydroascorbic acid. The accumulation of ascorbic acid by HL-60 cells was strongly temperature-dependent and was very inefficient at 16 degrees C. On the other hand, the direct chemical reduction of dehydroascorbic acid by excess glutathione proceeded efficiently at temperatures of 16 degrees C. Our data indicate that glutathione-dependent reductases in HL-60 cells are not responsible for the ability of these cells to accumulate millimolar concentrations of ascorbic acid. These findings indicate that alternative enzymatic mechanisms are involved in the cellular reduction of dehydroascorbic acid.

Increased uptake and accumulation of vitamin C in human immunodeficiency virus 1-infected hematopoietic cell lines

Vitamin C (ascorbic acid) is required for normal host defense and functions importantly in cellular redox systems. To define the interrelationship between human immunodeficiency virus (HIV) infection and vitamin C flux at the cellular level, we analyzed vitamin C uptake and its effects on virus production and cellular proliferation in HIV-infected and uninfected human lymphoid, myeloid, and mononuclear phagocyte cell lines. Chronic or acute infection of these cell lines by HIV-1 led to increased expression of glucose transporter 1, associated with increased transport and accumulation of vitamin C. Infected cells also showed increased transport of glucose analogs. Exposure to vitamin C had a complex effect on cell proliferation and viral production. Low concentrations of vitamin C increased or decreased cell proliferation depending on the cell line and either had no effect or caused increased viral production. Exposure to high concentrations of vitamin C preferentially decreased the proliferation and survival of the HIV-infected cells and caused decreased viral production. These findings indicate that HIV infection in lymphocytic, monocytic, and myeloid cell lines leads to increased expression of glucose transporter 1 and consequent increased cellular vitamin C uptake. High concentrations of vitamin C were preferentially toxic to HIV-infected host defense cell lines in vitro.

Purification, cloning and expression of dehydroascorbic acid-reducing activity from human neutrophils: identification as glutaredoxin

Dehydroascorbic acid-reducing activity in normal human neutrophil lysates was characterized and identified by activity-based purification and measurement of newly synthesized ascorbate by HPLC. The initial reducing activity was non-dialysable and could not be accounted for by the activity of glutathione as a reducing agent. The reducing activity was purified to homogeneity as an 11 kDa protein. The protein had a specific activity of 3 mumol/min per mg of protein and was glutathione dependent. Kinetic experiments showed that the protein had a K(m) for glutathione of 2.0 mM and a K(m) for dehydroascorbic acid of 250 microM. Dehydroascorbic acid reduction by the purified protein was pH dependent and was maximal at pH 7.5. Peptide fragments from the purified protein were analysed for amino acid sequence and the protein was identified as glutaredoxin. By using degenerate oligonucleotides based on the amino acid sequence, glutaredoxin was cloned from a human neutrophil library. Expressed purified glutaredoxin displayed reducing activity and kinetics that were indistinguishable from those of native purified enzyme. Several approaches indicated that glutaredoxin was responsible for the most of the protein-mediated dehydroascorbic acid reduction in lysates. From protein purification data, glutaredoxin was responsible for at least 47% of the initial reducing activity. Dehydroascorbic acid reduction was at least 5-fold greater in neutrophil lysates than in myeloid tumour cell lysates, and glutaredoxin was detected in normal neutrophil lysates but not in myeloid tumour cell lysates by Western blotting. Glutaredoxin inhibitors inhibited dehydroascorbic acid reduction in neutrophil lysates as much as 80%. These findings indicate that glutaredoxin plays a major role in dehydroascorbic acid reduction in normal human neutrophil lysates, and represent the first identification of dehydroascorbic acid reductase in human tissue by activity-based purification.

Resolution of the facilitated transport of dehydroascorbic acid from its intracellular accumulation as ascorbic acid

We performed a detailed kinetic analysis of the uptake of dehydroascorbic acid by HL-60 cells under experimental conditions that enabled the differentiation of dehydroascorbic acid transport from the intracellular reduction/accumulation of ascorbic acid. Immunoblotting and immunolocalization experiments identified GLUT1 as the main glucose transporter expressed in the HL-60 cells. Kinetic analysis allowed the identification of a single functional activity involved in the transport of dehydroascorbic acid in the HL-60 cells. Transport was inhibited in a competitive manner by both 3-O-methyl-D-glucose and 2-deoxy-D-glucose. In turn, dehydroascorbic acid competitively inhibited the transport of both sugars. A second functional component identified in experiments measuring the accumulation of ascorbic acid appears to be associated with the intracellular reduction of dehydroascorbic acid to ascorbic acid and is not directly involved in the transport of dehydroascorbic acid via GLUT1. Transport of dehydroascorbic acid by HL-60 cells was independent of the presence of external Na+, whereas the intracellular accumulation of ascorbic acid was found to be a Na(+)-sensitive process. Thus, the transport of dehydroascorbic acid via glucose transporters is a Na(+)-independent process which is kinetically and biologically separable from the reduction of dehydroascorbic acid to ascorbic acid and its subsequent intracellular accumulation.

Analysis of water-soluble antioxidants by high-pressure liquid chromatography

The measurement of endogenous substances that function as biological antioxidants is of importance because the values obtained might be an index of future health. We quantified three water-soluble antioxidants by high-pressure liquid chromatography with electrochemical detection (h.p.l.c.-e.c.). Current-voltage relationships made at various settings of the D2 porous graphite electrode help to identify ascorbic acid, glutathione and uric acid. The antioxidants are determined simultaneously and without need for derivatization. The method is seen to be useful for comparison of normal rat liver with liver that had undergone oxidative stress through ischaemia. Antioxidant levels in liver, kidney, pancreas and intestinal mucosa are presented and compared with literature values. Endogenous contents of oxidized forms of ascorbic acid and glutathione become apparent following exposure of tissue samples to a strong reductant such as 2-mercapthoethanol.

Cerebral metabolism of oxidized ascorbate

The brain has a high level of ascorbic acid which is thought to act as a reducing agent, e.g. in protecting tissues against oxidative stress. The mechanism by which ascorbate is maintained in the useful, reduced state in the CNS is evaluated herein. Cerebrum from rat or calf was minced and homogenized in buffer. The endogenous levels of ascorbic acid, dehydro-L-ascorbic acid (DHAA) and reduced glutathione (GSH) were determined by HPLC with coulometric electrochemical detection. We also quantitated tissue capacity to regenerate ascorbic acid from DHAA, which is a product of electron transfer reactions of ascorbic acid. The homogenate was fractionated by centrifugation in steps up to 110,000 x g and dialyzed free of low molecular weight components. The activity for reducing DHAA was approximately equal in the various supernatants; resuspended pellets had little activity. The active component has several properties of a protein, including being precipitated by solid ammonium sulfate addition to the tissue extract; most activity appeared in the 40-80% saturated fraction. The activity was stable up to a temperature of 80 degrees C, but was lost at 95 degrees C. The protein was digested by trypsin. The results suggest that a cytosolic component of cerebrum regenerates ascorbic acid in a step that preferentially uses GSH and NADPH as reducing cofactors. At least one form of DHAA reductase exists in brain.