Glutathione deficiency increases hepatic ascorbic acid synthesis in adult mice

Restoration of the cellular thiol status of peritoneal macrophages from CAPD patients by the flavonoids silibinin and silymarin

During continuous ambulatory peritoneal dialysis (CAPD) the peritoneal immune cells, mainly macrophages, are highly compromised by multiple factors including oxidative stress, resulting in a loss of functional activity. One reason for the increase of inflammatory reactions could be an imbalance in the thiol-disulfide status. Here, the possible protective effects of the antioxidant flavonoid complex silymarin and its major component silibinin on the cellular thiol status were investigated. Peritoneal macrophages from dialysis fluid of 30 CAPD patients were treated with silymarin or silibinin up to 35 days. A time-dependent increase of intracellular thiols was observed with a nearly linear increment up to 2.5-fold after 96 hours, reaching a maximum of 3.5-fold after 20 days of culture. Surface-located thiols were also elevated. The stabilization of the cellular thiol status was followed by an improvement of phagocytosis and the degree of maturation as well as significant changes in the synthesis of IL-6 and IL-1ra. Furthermore, the treatment of peritoneal macrophages with flavonoids in combination with cysteine donors resulted in a shortened and more efficient time course of thiol normalization as well as in a further increased phagocytosis. In addition, GSH-depletion in thiol-deficient media simulating CAPD procedures led to intracellular thiol deficiency similar to the in vivo situation. It is concluded that treatment with milk thistle extracts silymarin and silibinin alone or, more effectively in combination with cysteine donors, provide a benefit for peritoneal macrophages of CAPD-patients due to a normalization and activation of the cellular thiol status followed by a restoration of specific functional capabilities.

Minor role of oxidative stress during intermediate phase of acute pancreatitis in rats

Reactive oxygen species have been implicated in the pathogenesis of acute pancreatitis. Few studies have focused on the loss of endogenous antioxidants and molecular oxidative damage. Two acute pancreatitis models in rats; taurocholate (3% intraductal infusion) and cerulein (10 microg/kg/h), were used to study markers of oxidative stress: Glutathione, ascorbic acid, and their oxidized forms (glutathione disulfide and dehydroascorbic acid), malondialdehyde, and 4-hydroxynoneal in plasma and pancreas, as well as 7-hydro-8-oxo-2'-deoxyguanosine in pancreas. In both models, pancreatic glutathione depleted by 36-46% and pancreatic ascorbic acid depleted by 36-40% (p <.05). In the taurocholate model, plasma glutathione was depleted by 34% (p <.05), but there were no significant changes in plasma ascorbic acid or in plasma and pancreas dehydroascorbic acid, malondialdehyde, and 4-hydroxynoneal, and no significant changes in the pancreas glutathione disulfide/glutathione ratio. While pancreas glutathione disulfide/glutathione ratio increased in the cerulein model, there were no significant changes in plasma glutathione, plasma, or pancreas ascorbic acid, dehydroascorbic acid, 4-hydroxynoneal, and malondialdehyde, or in pancreas 7-hydro-8-oxo-2'-deoxyguanosine. Reactive oxygen species have a minor role in the intermediate stages of pancreatitis models.

Knockout of the mouse glutamate cysteine ligase catalytic subunit (Gclc) gene: embryonic lethal when homozygous, and proposed model for moderate glutathione deficiency when heterozygous

The biosynthesis of reduced glutathione (GSH) is carried out by the enzymes gamma-glutamylcysteine synthetase (GCL) and GSH synthetase. GCL is the rate-limiting step and represents a heterodimeric enzyme comprised of a catalytic subunit (GCLC) and a ("regulatory"), or modifier, subunit (GCLM). The nonhomologous Gclc and Gclm genes are located on mouse chromosomes 9 and 3, respectively. GCLC owns the catalytic activity, whereas GCLM enhances the enzyme activity by lowering the K(m) for glutamate and increasing the K(i) to GSH inhibition. Humans have been identified with one or two defective GCLC alleles and show low GSH levels. As an initial first step toward understanding the role of GSH in cellular redox homeostasis, we have targeted a disruption of the mouse Gclc gene. The Gclc(-/-) homozygous knockout animal dies before gestational day 13, whereas the Gclc(+/-) heterozygote is viable and fertile. The Gclc(+/-) mouse exhibits a gene-dose decrease in the GCLC protein and GCL activity, but only about a 20% diminution in GSH levels and a compensatory increase of approximately 30% in ascorbate-as compared with that in Gclc(+/+) wild-type littermates. These data show a reciprocal action between falling GSH concentrations and rising ascorbate levels. Therefore, the Gclc(+/-) mouse may be a useful genetic model for mild endogenous oxidative stress.

Selective activation of p38 MAPK cascade and mitotic arrest caused by low level oxidative stress

Apoptosis induced by high level oxidative stress accompanies diverse cellular biochemical events including activation of the stress signal cascades of JNK and NF-kappaB. We report here selective activation of p38 MAPK cascade and mitotic arrest under a low level oxidative stress that lacks apoptosis induction. U937 human lymphoid cells treated with low dose (0.02 mm) H(2)O(2) rapidly caused p38 MAPK cascade activation detectable by phosphorylation of MKK3/6, p38 MAPK, activating transcription factor-2, and cAMP-responsive element-binding protein, leaving the JNK and NF-kappaB cascades unaffected. The p38 kinase activation was sustained for 24 h under the low level stress conditions and led to formation of polyploid nuclei. N-Acetyl-l-cysteine, a precursor of anti-oxidant glutathione, canceled both p38 MAPK activation and abnormal cell cycle progression, whereas blockage of the kinase by specific inhibitor SB203580 allowed the appearance of apoptotic cells. Thus, mimicking the effects of nocodazole, the low level oxidative stimulus caused inhibition of cell division in the M phase through p38 MAPK activation. The kinase cascade may serve as a primary transducer of cytoplasmic oxidative signals to nucleus for stress-relieving gene expression and cell cycle control before apoptosis-inducing signals are transduced. This is the first report demonstrating that oxidative stress can participate in cell cycle control by induction of a signal cascade.

Plasma L-5-oxoproline carbon and nitrogen kinetics in healthy young adults

L-5-oxoproline (OP), an intermediate of the gamma-glutamyl cycle of glutathione synthesis and degradation, may serve as a probe for the state of glutathione kinetics. We explored the whole-body carbon and nitrogen kinetics of OP in five male healthy subjects (75.2 kg; 181 cm; 26 y) after a 5-d adaptation to an adequate L-amino acid-based diet (160 mg N x kg(-1) x d(-1); 188 kJ x kg(-1) x d(-1)), using a crossover design. On day 6 of the diet period, we carried out an 8-h tracer protocol (3 h fast; 5 h fed; 2/3 of daily nitrogen intake) with intravenous infusion of L-[1-(13)C]oxoproline and L-[3, 3-(2)H]cysteine or, in randomized order, on the second occasion, L-[(15)N]oxoproline and L-[3,3-(2)H]cysteine. Plasma OP was isolated by cation exchange and after addition of internal standards (DL-[(2)H(3)]-5-oxoproline; L-[(15)N, U-(13)C(5)]-5-oxoproline; DL-[(2)H(3)]-glutamic acid) derivatized to form TBDMS esters and measured by gas chromatography/mass spectrometry. Plasma OP concentration did not differ between fed and fasted state (fast: 59. 4 +/- 8.3; fed 59.2 +/- 8.9 nmol/mL). (13)C- and (15)N OP flux during the fasted and fed state were 19 +/- 3.6, 21.2 +/- 3.2, and 22.6 +/- 3.9, 25.8 +/- 4.3 micromol x kg(-1) x 30 min(-1), respectively. OP oxidation was 15.6 +/- 3.6 and 17.9 +/- 3.5 micromol x kg(-1) x 30 min(-1), in fasting and feeding, respectively, (P < 0.05). More than 80% of the plasma flux was oxidized. These findings are compared with the published literature on GSH turnover in plasma of human subjects and underscore the need to define more completely the dynamic aspects of glutathione metabolism and of the intermediates of the gamma-glutamyl cycle.

The role of glutathione in p-aminophenol-induced nephrotoxicity in the mouse

p-Aminophenol (PAP) produces nephrotoxicity in rats through a mechanism presumably involving oxidation and conjugation with glutathione (GSH). Recently it was found that PAP also causes nephrotoxicity in mice as evidenced by elevated blood urea nitrogen (BUN) and serum creatinine levels. The objective of this study was to further investigate the mechanism and elucidate the role of GSH in PAP-induced nephrotoxicity in the mouse. Male C57BL/6 mice injected i.p. with various doses of PAP were sacrificed at 12 hr for measurement of BUN and serum creatinine levels and determination of the extent of renal cortical nonprotein sulfhydryl (NPSH) and GSH depletion. PAP depleted renal cortical NPSH content in a dose- and time-dependent manner. Depletion of NPSH in mouse kidney did not occur at PAP doses below 600 mg/kg. Buthionine sulfoximine, an inhibitor of GSH synthesis, decreased nephrotoxicity. Ascorbate, a reducing agent, prevented PAP-induced nephrotoxicity and attenuated renal cortical NPSH depletion. However, acivicin and aminooxyacetic acid, inhibitors of gamma-glutamyltranspeptidase and beta-lyase, respectively, did not prevent toxicity in the mouse. Piperonyl butoxide, an inhibitor of cytochrome P-450 enzymes, enhanced nephrotoxicity and renal cysteine depletion but not GSH depletion. The results suggest that PAP-induced nephrotoxicity in the mouse may involve oxidation and formation of a GSH conjugate.

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

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

Topical application of alpha-tocopherol modulates the antioxidant network and diminishes ultraviolet-induced oxidative damage in murine skin

The aim of this study was to investigate the effects of topical alpha-tocopherol application on epidermal and dermal antioxidants and its ability to prevent ultraviolet (UV)-induced oxidative damage. Hairless mice received topical applications of alpha-tocopherol 24 h before a single, acute UV irradiation (10 x minimal erythemal dose). The four major antioxidant enzymes (catalase, superoxide dismutase, glutathione reductase and glutathione peroxidase), hydrophilic and lipophilic antioxidants, and lipid hydroperoxides, markers of oxidative damage, were assayed in both epidermis and dermis of hairless mice. Topical alpha-tocopherol treatment increased dermal superoxide dismutase activity by 30% (P < 0.01) and protected epidermal glutathione peroxidase and superoxide dismutase from depletion after UV irradiation. Total and reduced glutathione levels in the epidermis increased by 50% after the topical treatment (P < 0.05), as did dermal ascorbate levels (by 40%: P < 0.01). The topical treatment increased alpha-tocopherol levels both in the epidermis (62-fold) and the dermis (22-fold: P < 0.001 in each layer). Furthermore, alpha-tocopherol treatment significantly reduced the formation of epidermal lipid hydroperoxides after UV irradiation (P < 0.05). These results demonstrate that topical administration of alpha-tocopherol protects cutaneous tissues against oxidative damage induced by UV irradiation in vivo, and suggest that the underlying mechanism of this effect involves the up-regulation of a network of enzymatic and non-enzymatic antioxidants.

Regulation of prostaglandin biosynthesis in vivo by glutathione

Intraperitoneal administration of urate crystals to mice reduced subsequent macrophage conversion of arachidonic acid (AA) to prostaglandins (PGs) and 12-hydroxyeicosatetraenoic acid for up to 6 h. In contrast, levels of 12-hydroxyheptadecatrienoic acid (12-HHT) were markedly elevated. This metabolic profile was previously observed in vitro when recombinant cyclooxygenase (COX) enzymes were incubated with reduced glutathione (GSH). Analysis of peritoneal GSH levels revealed a fivefold elevation after urate crystal administration. The GSH synthesis inhibitor L-buthionine-[S,R]-sulfoximine partially reversed the urate crystal effect on both GSH elevation and PG synthesis. Moreover, addition of exogenous GSH to isolated peritoneal macrophages shifted AA metabolism from PGs to 12-HHT. Urate crystal administration reduced COX-1, but induced COX-2 expression in peritoneal cells. The reduction of COX-1 may contribute to the attenuation of PG synthesis after 1 and 2 h, but PG synthesis remained inhibited up to 6 h, when COX-2 levels were high. Overall, our results indicate that elevated GSH levels inhibit PG production in this model and provide in vivo evidence for the role of GSH in the regulation of PG biosynthesis.

Cyanate causes depletion of ascorbate in organisms

Ascorbate-dehydroascorbate redox cycle plays a key role in protecting organisms from an excess of oxidants. Recently, we found a novel reaction of dehydroascorbate with cyanate under the conditions of neutral pH and ordinary temperature. In this report, we demonstrated that through this irreversible reaction, cyanate causes the depletion of ascorbate in the matrix, where the ascorbate-dehydroascorbate redox cycle revolves. When the leaves of weed (Erigeron canadensis) were soaked in sodium cyanate solution generally used as a herbicide, the depletion of ascorbate as well as dehydroascorbate in them was observed, followed by the change in color from green to brown. These results suggest that a possible way of cyanate toxicity is to inflict oxidative stress on organisms.

Neonatal mouse assay for tumorigenicity: alternative to the chronic rodent bioassay

The chronic rodent bioassay for tumors has been utilized systematically for 25 years to identify chemicals with carcinogenic potential in man. In general, those chemicals exhibiting tumorigenicity at multiple sites in both mice and rats have been regarded as possessing strong carcinogenic potential in humans. In comparison, the value of data collected for those test chemicals exhibiting more sporadic tumorigenicity results (e.g., single species/single sex or dose-independent) has been questioned. As knowledge of the carcinogenic process has increased, several alternative test systems, usually faster and less expensive than the 2-year bioassay, have been suggested for identification of the strongly acting, transspecies carcinogens. The International Conference on Harmonization for Technical Requirements for the Registration of Pharmaceuticals for Human Use has proposed an international standard that allows for the use of one long-term rodent carcinogenicity study, plus one supplementary study to identify potential human pharmaceutical carcinogens. The neonatal mouse assay for tumorigenicity has been used since 1959; however, relative to other alternate tests, little has been written about this system. It is clear that this assay system successfully identifies transspecies carcinogens from numerous chemical classes, thus recommending itself as a strong candidate for a supplementary study to identify potential human carcinogens. In contrast, there are decidedly less data available from this assay in response to pharmaceuticals shown to exhibit weak and/or conflicting results in the 2-year bioassay, knowledge invaluable to the regulatory process. This paper reviews the historical development and our experience with the neonatal mouse assay and includes suggestions for a standardized protocol and strategies to document its response to "weak" and/or "nongenotoxic" carcinogens.

Protein folding coupled to disulphide bond formation

Protein folding that is coupled to disulphide bond formation has many experimental advantages. In particular, the kinetic roles and importance of all the disulphide intermediates can be determined, usually unambiguously. This contrasts with other types of protein folding, where the roles of any intermediates detected are usually not established. Nevertheless, there is considerable confusion in the literature about even the best-characterized disulphide folding pathways. This article attempts to set the record straight.

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.

Ascorbate metabolism and its regulation in animals

This article provides a comprehensive review on ascorbate metabolism in animal cells, especially in hepatocytes. The authors deal with the synthesis and the breakdown of ascorbate as a part of the antioxidant and carbohydrate metabolism. Hepatocellular and interorgan cycles with the participation of ascorbate are proposed, based on experiments with murine and human cells; reactions of hexuronic acid pathway, non-oxidative branch of the pentose phosphate cycle, glycolysis and gluconeogenesis are involved. Besides the well-known redox coupling between the two major water-soluble antioxidants (glutathione and ascorbate), their metabolic links have been also outlined. Glycogenolysis as a major source of UDP-glucuronic acid determines the rate of hexuronic acid pathway leading to ascorbate synthesis. Glycogenolysis is regulated by oxidized and reduced glutathione; therefore, glycogen, ascorbate and glutathione metabolism are related to each other. Hydrogen peroxide formation, due to the activity of gulonolactone oxidase catalyzing the last step of ascorbate synthesis, also affects the antioxidant status in hepatocytes. Based on new observations a complex metabolic regulation is supposed. Its element might be present also in humans who lost gulonolactone oxidase but they need and metabolize ascorbate. Finally, the obvious disadvantages and the possible advantages of the lost ascorbate synthesizing ability in humans are considered.

Antioxidant protection of NO-induced relaxations of the mouse anococcygeus against inhibition by superoxide anions, hydroquinone and carboxy-PTIO

1. The potential protective effect of several antioxidants [Cu/Zn superoxide dismutase (Cu/Zn SOD), ascorbate, reduced glutathione (GSH), and alpha-tocopherol (alpha-TOC)] on relaxations of the mouse anococcygeus muscle to nitric oxide (NO; 15 microM) and, where appropriate, nitrergic field stimulation (10 Hz; 10 s trains) was investigated. 2. The superoxide anion generating drug duroquinone (100 microM) reduced relaxations to exogenous NO by 54 +/- 6%; this inhibition was partially reversed by Cu/Zn SOD (250 u ml-1), and by ascorbate (500 microM). Following inhibition of endogenous Cu/Zn SOD activity with diethyldithiocarbamate (DETCA), duroquinone (50 microM) also reduced relaxations to nitrergic field stimulation (by 53 +/- 6%) and this effect was again reversed by Cu/Zn SOD and by ascorbate. Neither GSH (500 microM) nor alpha-TOC (400 microM) afforded any protection against duroquinone. 3. Xanthine (20 mu ml-1); xanthine oxidase (100 microM) inhibited NO-induced relaxations by 73 +/- 14%, but had no effect on those to nitrergic field stimulation, even after DETCA treatment. The inhibition of exogenous NO was reduced by Cu/Zn SOD (250 u ml-1) and ascorbate (400 microM), but was unaffected by GSH or alpha-TOC (both 400 microM). 4. Hydroquinone (100 microM) also inhibited relaxations to NO (by 52 +/- 10%), but not nitrergic stimulation. In this case, however, the inhibition was reversed by GSH (5-100 microM) and ascorbate (100-400 microM), although Cu/Zn SOD and alpha-TOC were ineffective. 5. 2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO, 50 microM) inhibited NO-induced relaxations by 50 +/- 4%, but had no effect on nitrergic responses; the inhibition was reduced by ascorbate (2-200 microM) and alpha-TOC (10-200 microM), but not by Cu/Zn SOD or GSH. 6. Hydroxocobalamin (5-100 microM) inhibited, equally, relaxations to both NO (-logIC40 3.14 +/- 0.33) and nitrergic stimulation (-logIC40 3.17 +/- 0.22). 7. Thus, a number of physiological antioxidants protected NO from superoxide anions, and from direct NO-scavengers. The possibility that the presence of these antioxidants within nitrergically-innervated tissues might explain the lack of effect of the NO inhibitors on nerve-induced relaxation, without the need to invoke a transmitter other than free radical NO, is discussed.

Retinoic acid modulation of glutathione and cysteine metabolism in chondrocytes

The major objective of this investigation was to determine the thiol status of chondrocytes and to relate changes in the level of glutathione and cysteine to maturation of the cells as they undergo terminal differentiation. Chondrocytes were isolated from the cephalic portion of chick embryo sterna and treated with all-trans retinoic acid for one week. We found that the addition of 100 nM retinoic acid to the cultures decreased the intracellular levels of glutathione and cysteine from 6.1 to 1.6 and 0.07 to 0.01 nmol/microgram DNA respectively; retinoic acid also caused a decrease in the extracellular concentration of cysteine. The decrease in chondrocyte thiols was dose and time dependent. To characterize other antioxidant systems of the sternal cell culture, the activities of catalase, glutathione reductase and superoxide dismutase were determined. Activities of all of those enzymes were high in the retinoic acid-treated cells; the conditioned medium also contained these enzymes and the cytosolic isoenzyme of superoxide dismutase. We probed the specificity of the thiol response by using immature caudal chondrocytes. Unlike the cephalic cells, retinoic acid did not change intracellular glutathione and extracellular cysteine levels, although the retinoid caused a reduction in the intracellular cysteine concentration. Finally, we explored the effect of medium components on chondrocyte thiol status. We noted that while ascorbate alone did not change cell thiol levels, it did cause a 4-fold decrease in the extracellular cysteine concentration. When retinoic acid and ascorbic acid were both present in the medium, there was a marked decrease in the level of glutathione. In contrast, the phosphate concentration of the culture medium served as a powerful modulator of both glutathione and cysteine. Results of the study clearly showed that there is a profound decrease in intracellular levels of both cysteine and glutathione and that thiol levels are responsive to ascorbic acid and the medium phosphate concentration. These findings point to a critical role for thiols in modulating events linked to chondrocyte maturation and cartilage matrix synthesis and mineralization.

Plasma methionine and cysteine kinetics in response to an intravenous glutathione infusion in adult humans

Glutathione (GSH), a tripeptide (gamma-glutamyl-cysteinyl-glycine), is thought to be both a storage and a transport form of cysteine (Cys). In a previous study (T. Hiramatsu, N.K. Fukagawa, J.S. Marchini, J. Cortiella, Y.-M. Yu, T.E. Chapman, and V.R. Young. Am. J. Clin. Nutr. 60: 525-533, 1994), the direct tracer-derived estimate of Cys flux was considerably higher than that predicted from estimates of protein turnover. To further examine the components of plasma Cys flux, seven normal-weight healthy adult men and women (26 +/- 2 yr) received stable isotope tracer infusions of L-[methyl-2H3;1-13C]methionine, L-[3,3-2H2]cysteine, and L-[methyl-2H3]leucine for 460 min. After a 3-h baseline period, GSH was administered at approximately 32 mumol.kg-1.h-1 until the end of the study. Expired breath and blood samples were obtained at timed intervals and analyzed for isotope enrichment using mass spectrometry. Leucine, alpha-ketoisocaproate, and methionine (carboxyl carbon, methyl moiety, remethylation, and transsulfuration) turnover were reduced during GSH administration (P < 0.01). In the final hour of GSH administration, Cys flux increased by 61% from 55.1 +/- 1.7 to 88.7 +/- 5.2 mumol.kg-1.h-1 (P < 0.01), which was essentially equivalent to the rate of exogenous GSH infusion. These data suggest that GSH breakdown accounts for approximately 50% of tracer-derived Cys flux basally and for all of the increase in measured Cys turnover during exogenous GSH infusion.

Ascorbic acid synthesis is stimulated by enhanced glycogenolysis in murine liver

Ascorbic acid synthesis was stimulated by glucagon, dibutyryl cyclic AMP, as well as phenylephrine vasopressin or okadaic acid, in hepatocytes prepared from fed mice. However, no such effect was observed in glycogen-depleted cells from starved animals, either in the presence or absence of glucose. The rate of ascorbate synthesis showed close correlation with the glucose release by hepatocytes. In mice the injection of glucagon increased plasma ascorbate concentration fifteenfold, and caused a sixfold elevation of the ascorbate content of the liver. These results show that hepatic ascorbate synthesis is dependent on glycogenolysis, and indicate a regulatory role of ascorbate released by the liver.

Effect of ascorbate or N-acetylcysteine treatment in a patient with hereditary glutathione synthetase deficiency

A 45-month-old girl with 5-oxoprolinuria (pyroglutamic aciduria), hemolysis, and marked glutathione depletion caused by deficiency of glutathione synthetase was followed before and during treatment with ascorbate or N-acetylcysteine. High doses of ascorbate (0.7 mmol/kg per day) or N-acetylcysteine (6 mmol/kg per day) were given for 1 to 2 weeks without any obvious deleterious side effects. Ascorbate markedly increased lymphocyte (4-fold) and plasma (8-fold) levels of glutathione. N-Acetylcysteine also increased lymphocyte (3.5-fold) and plasma (6-fold) levels of glutathione. After these treatments were discontinued, lymphocyte and plasma glutathione levels decreased rapidly to pretreatment levels. Ascorbate treatment was extended for 1 year, and lymphocyte (4-fold) and plasma (2- to 5-fold) glutathione levels remained elevated above baseline. In parallel, the hematocrit increased from 25.4% to 32.6%, and the reticulocyte count decreased from 11% to 4%. The results demonstrate that ascorbate and N-acetylcysteine can decrease erythrocyte turnover in patients with hereditary glutathione deficiency by increasing glutathione levels.

Glutathione ester delays the onset of scurvy in ascorbate-deficient guinea pigs

Previous studies showed that administration of ascorbate to glutathione (GSH)-deficient newborn rats and guinea pigs prevented toxicity and mortality and led to increased tissue and mitochondrial GSH levels; ascorbate thus spares GSH. In the present work, we tried to answer the converse question: Does administration of GSH spare ascorbate? Because administered GSH is not well transported into most cells, we gave GSH monoethyl ester (which is readily transported and converted into GSH intracellularly) to guinea pigs fed an ascorbate-deficient diet. We found that treatment with GSH ester significantly delays appearance of the signs of scurvy and that this treatment spares ascorbate; thus, the decrease of tissue levels of ascorbate was delayed. The findings support the conclusions that (i) GSH is essential for the physiological function of ascorbate because it is required in vivo for reduction of dehydroascorbate and (ii) there is metabolic redundancy and overlap of the functions of these antioxidants. The sparing effect of GSH in scurvy may be mediated through an increase in the reduction of dehydroascorbate (which would otherwise be degraded) and to antioxidant effects of GSH that are also produced by ascorbate. Other studies indicate that GSH deficiency in adult mice stimulates ascorbate synthesis in liver. During this work we found that administration of GSH itself is highly toxic to ascorbate-deficient guinea pigs when given in divided i.p. doses totaling 3.75 mmol/kg daily.