Synthesis and some major functions of vitamin C in animals

Inhibition of rabbit muscle isozymes by vitamin C

The ability of vitamins C, E and K to inhibit enzymes directly has been investigated. It was found that vitamin E and some analogs and menadione (vitamin K3) inhibited several enzymes irreversibility at concentrations below one millimolar. Ascorbate inhibits rabbit muscle 6-phosphofructokinase (MPFK-1; EC 2.7.1.11), muscle type LDH (EC 1.1.1.27), and muscle AK (EC 2.7.4.3) at low concentrations that do not inhibit equivalent liver isozymes. Ascorbate Ki values for muscle-type LDH and heart-type LDH isozymes are 0.007 and 3 mM, respectively. The ascorbate Ki value for rabbit skeletal muscle PFK-1 is 0.16 mM; liver PFK-I is not inhibited by ascorbate. Dehydroascorbate does not inhibit any enzyme at ascorbate concentrations normally found in cells. All ascorbate inhibitions are completely reactivated or nearly so by L-ascorbate oxidase, CYS, GSH, or DTT. We propose a hypothesis that ascorbate facilitates glycogen storage in muscle by inhibiting glycolysis. The relationship between ascorbate metabolism and diabetes is discussed.

Ascorbate regulation and its neuroprotective role in the brain

Ascorbic acid (vitamin C) occurs physiologically as the ascorbate anion: a water-soluble antioxidant that is found throughout the body. However, despite the high, homeostatically regulated levels of brain ascorbate, its specific functions in the CNS are only beginning to be elucidated. Certainly, it acts as part of the intracellular antioxidant network, and as such is normally neuroprotective. There is also evidence that it acts as a neuromodulator. A possibly unique role it might have is as an antioxidant in the brain extracellular microenvironment, where its concentration is modulated by glutamate-ascorbate heteroexchange at glutamate uptake sites. Ongoing studies of ascorbate and glutamate transporters should lead to rapid progress in understanding ascorbate regulation and function.

Ascorbic acid acts as an inhibitory transmitter in the hypothalamus to inhibit stimulated luteinizing hormone-releasing hormone release by scavenging nitric oxide

Because ascorbic acid (AA) is concentrated in synaptic vesicles containing glutamic acid, we hypothesized that AA might act as a neurotransmitter. Because AA is an antioxidant, it might therefore inhibit nitric oxidergic (NOergic) activation of luteinizing hormone-releasing hormone (LH-RH) release from medial basal hypothalamic explants by chemically reducing NO. Cell membrane depolarization induced by increased potassium concentration [K(+)] increased medium concentrations of both AA and LH-RH. An inhibitor of NO synthase (NOS), N(G)-monomethyl-l-arginine (NMMA), prevented the increase in medium concentrations of AA and LH-RH induced by high [K(+)], suggesting that NO mediates release of both AA and LH-RH. Calcium-free medium blocked not only the increase in AA in the medium but also the release of LH-RH. Sodium nitroprusside, which releases NO, stimulated LH-RH release and decreased the concentration of AA in the incubation medium, presumably because the NO released oxidized AA to dehydro-AA. AA (10(-5) to 10(-3) M) had no effect on basal LH-RH release but completely blocked high [K(+)]- and nitroprusside-induced LH-RH release. N-Methyl-d-aspartic acid (NMDA), which mimics the action of the excitatory amino acid neurotransmitter glutamic acid, releases LH-RH by releasing NO. AA (10(-5) to 10(-3) M) inhibited the LH-RH-releasing action of NMDA. AA may be an inhibitory neurotransmitter that blocks NOergic stimulation of LH-RH release by chemically reducing the NO released by the NOergic neurons.

Ascorbate and glutathione regulation in hibernating ground squirrels

Ground squirrels withstand up to 90% reductions in cerebral blood flow during hibernation as well as rapid reperfusion upon periodic arousals from torpor. Metabolic suppression likely plays a primary adaptive role which allows hibernating species to tolerate such phenomena. However, several other aspects of hibernation physiology are also consistent with tolerance to dramatic fluctuations in cerebral blood flow, suggesting that multiple neuroprotective adaptations may work in concert during hibernation. The purpose of the present work was to study the dynamics of the low molecular weight antioxidants, ascorbate and glutathione (GSH), during hibernation. Alterations in concentrations of ascorbate during hibernation and arousal in two species of hibernating ground squirrels suggest that it could play a protective role during hibernation or arousal. Samples were collected during the hibernation season from arctic ground squirrels (AGS; Spermophilus parryii) and 13-lined ground squirrels (TLS; S. tridecemlineatus) during prolonged torpor and in squirrels that did not hibernate or had not been hibernating for several weeks. We determined antioxidant levels in plasma, cerebrospinal fluid (CSF), and in frontal cortex, hippocampus and cerebellum using high-performance liquid chromatography (HPLC). Plasma ascorbate concentrations increased dramatically (3-4-fold) in both species during hibernation and rapidly returned to prehibernation levels upon arousal. By contrast, plasma GSH concentrations fell slightly or remained stable during hibernation. Ascorbate levels in the CSF doubled in hibernating AGS (not determined in TLS), while brain ascorbate content fell slightly (10-15%) in both species. Substantial increases in plasma and CSF ascorbate concentrations suggest that this antioxidant could play a protective role during hibernation and reperfusion upon arousal from hibernation.

Ascorbate compartmentalization in the CNS

Ascorbic acid, found physiologically as the ascorbate anion, is an abundant water-soluble antioxidant. It is concentrated in the intracellular compartment of all tissues in the body. The CNS has particularly high levels of ascorbate. Recent data from this laboratory indicate that ascorbate is distinctly compartmentalized between neurons and glia, with an average intracellular concentration of 10 mM in neurons and 1 mM in glial cells. These data can be contrasted with those for another important low molecular weight antioxidant, glutathione, which is somewhat more concentrated in glia than in neurons. The present review summarizes evidence for ascorbate compartmentalization between neurons and glia and considers these data in light of evidence for the roles of ascorbate as a neuroprotective antioxidant and as a neuromodulator in the CNS.

Ascorbic acid metabolism in rats and guinea pigs after the administration of ethanol

Ascorbic acid metabolism was studied in guinea pigs and rats after the administration of ethanol and a high dose of ascorbic acid (AA). Male guinea pigs were maintained for 30 days as follows: (1) controls (1 mg AA/100 g body wt.); (2) ethanol (1 mg AA/100 g body wt. + 900 mg ethanol/100 g body wt); (3) ascorbic acid (25 mg AA/100 g body wt.); (4) ascorbic acid + ethanol (25 mg AA/100 g body wt. + 900 mg ethanol/100 g body wt.). Rats were also grouped into four groups as in the case of guinea pigs, but the dose of AA was 200 mg/100 g body weight. Rats adjusted to ethanol intoxication by enhancing the biosynthesis of ascorbate as evidenced by elevated activity of L-gulono lactone oxidase (GLO). Hence ascorbate levels were not lowered in rats after administration of alcohol. However, alcohol administration lowered tissue levels of ascorbate in guinea pigs. But the supplementation of ascorbate along with alcohol raised the tissue level of this vitamin. Guinea pigs responded to the ascorbate deficiency during alcohol administration by lowering the degradation of ascorbate, as seen by the lower activity of the degrading enzyme gulono lactone hydrolase. It is concluded that on the administration of alcohol, guinea pigs are dependent upon additional exogenous supplies of ascorbic acid, whereas rats are not.

Hormone-regulated and glucose-sensitive transport of dehydroascorbic acid in immature rat granulosa cells

Ascorbic acid is concentrated in granulosa cells of the follicle, and ascorbate deficiency causes follicular atresia. Dehydroascorbic acid (DHAA), the oxidized form of ascorbic acid, serves as an important source for the recycling of ascorbate. As we previously demonstrated endocrine up-regulation of ascorbic acid transport by granulosa cells, we investigated DHAA as an alternate source of ascorbate in the follicle. Granulosa cells were cultured for 24 h, and DHAA uptake was initiated by the addition of 14C-labeled ascorbic acid (300 microM) in the presence of ascorbic acid oxidase (2 U/ml), which catalyzes DHAA production. Almost 90% of accumulated DHAA was present as ascorbic acid within 2 h. Preculture of cells for 24 h with FSH (50 ng/ml) and IGF-I (30 ng/ml) significantly stimulated DHAA uptake compared with the control (158 +/- 16 vs. 43 +/- 8 pmol/10(6) cells, respectively). DHAA uptake by granulosa cells was inhibited by D-glucose (ID50, approximately 2.5 mM) and by the glucose transport inhibitors phloretin (200 microM) and cytochalasin B (10 microM), which reduced uptake to 13 +/- 2% and 8 +/- 3% of the control, respectively. Northern and Western analysis of GLUT1 in granulosa cells following 24 h coincubation with FSH and IGF-I revealed up-regulation of GLUT1 at both the messenger RNA and protein levels (1.6- and 1.3-fold of control, respectively), suggesting that the stimulatory effects of FSH and IGF-I on DHAA transport are mediated by the induction of GLUT1. GLUT4 protein was not detectable by Western analysis. Endocrine-regulated DHAA transport may represent an important mechanism for maintaining adequate antioxidant tone within the developing follicle.

Vitamin C status of a campus population: college students get a C minus

Diet and the vitamin C status of two samples of college students were examined. Nonsmoking participants were recruited from a campus population during the fall and winter months. The prevalence of vitamin C deficiency (plasma vitamin C concentrations less than 11 mumol/L) ranged from 1% to 2% in the sampled campus populations. Marginal vitamin C status (plasma vitamin C concentrations from 11 to less than 28 mumol/L) was observed in 12% of the fall sample and 16% in the winter sample. Participants with marginal vitamin C status consumed significantly fewer servings of fruits and vegetables daily than participants with adequate vitamin C status. Marginal vitamin C status, which is even more pronounced in smokers, has been associated with fatigue and increased severity of respiratory tract infections. Because the vitamin C status of many college students, both smokers and nonsmokers, may be inadequate, health promotion or wellness programs for all students should emphasize the importance of adequate fruit and vegetable consumption.

Differential compartmentalization of brain ascorbate and glutathione between neurons and glia

Compartmentalization of brain ascorbate and glutathione between neurons and glia has been a source of controversy. To address this question, we determined the ascorbate and glutathione contents of brain tissue with defined, but varying, densities of neurons and glia. In developing rat cortex and hippocampus, glutathione content rose during gliogenesis, while ascorbate fell. By contrast, ascorbate, but not glutathione, increased markedly during granule cell proliferation and maturation in the developing cerebellum. Similarly, in tissue from adult cerebral cortex of species with distinct neuron densities, ascorbate content increased linearly with increasing neuron density in the order: human<rabbit<guinea-pig<rat<mouse, whereas glutathione was relatively constant. These data suggest that ascorbate predominates in neurons, whereas glutathione is slightly predominant in glia. Quantitative analysis of ascorbate and glutathione contents in these studies combined with appropriate intra- and extracellular volume fraction data permitted calculation of concentrations of ascorbate in neurons (10 mM) and glia (0.9 mM), and glutathione in neurons (2.5 mM) and glia (3.8 mM). The relative accuracy of these values was confirmed by their use in a model that reliably predicted changes in ascorbate and glutathione levels in rat cortex during the first three postnatal weeks and into adulthood. These findings not only provide new information about the intracellular composition of neurons and glia, but also have implications for understanding the roles of ascorbate and glutathione in normal brain function, as well as neuron and glia involvement in disease states linked to oxidative stress.

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.

Vitamin C depletion is associated with alterations in blood histamine and plasma free carnitine in adults

OBJECTIVE

The purpose of this study was to determine whether carnitine metabolism or histamine degradation would be useful parameters for investigating the optimal requirement for vitamin C.

METHODS

Twenty-two non-scorbutic subjects with subnormal vitamin C status (plasma vitamin C < 28 mumol/L) were placed on a metabolic diet low in vitamin C for 3 weeks and repleted with graded doses of vitamin C: 10, 30 and 60 mg vitamin C daily (group 1) or 10,125 and 250 mg vitamin C daily (group 2) for weeks 1, 2 and 3, respectively. Fasting blood samples were collected weekly and analyzed for plasma vitamin C, plasma free carnitine and blood histamine.

RESULTS

Group 1 subjects remained in a subnormal vitamin C state throughout the 3-week study, and blood histamine and plasma free carnitine were not impacted by the experimental treatment. Plasma vitamin C in group 2 subjects rose significantly during the study, and these subjects finished the study with an ample vitamin C status indicative of vitamin C intakes above the recommended dietary allowance. Both blood histamine and plasma free carnitine were inversely related to vitamin C status in group 2 subjects.

CONCLUSIONS

These data indicate that blood histamine and plasma free carnitine are altered in individuals with subnormal, non-scorbutic vitamin C status and provide evidence that metabolic changes independent of collagen metabolism occur prior to the manifestation of scurvy. Thus utilizing scurvy as an end-point to determine vitamin C requirements may not provide adequate vitamin C to promote optimal health and well-being.

Does diabetes mellitus increase the requirement for vitamin C?

This paper reviews the scientific evidence regarding the vitamin C status of people with diabetes mellitus and whether they might have increased dietary vitamin C requirements. English language articles published from 1935 to the present that either compare ascorbic acid concentrations of persons with and without diabetes mellitus or assess the impact of vitamin C supplementation on various health outcomes among persons with diabetes mellitus were examined. Most studies have found people with diabetes mellitus to have at least 30% lower circulating ascorbic acid concentrations than people without diabetes mellitus. Vitamin C supplementation had little impact on blood glucose concentrations, but was found to lower cellular sorbitol concentrations and to reduce capillary fragility. Much of the past research in this area has been methodologically weak. To further understand the relation of ascorbic acid and diabetes mellitus, randomized clinical trials of ascorbic acid supplementation should be a high priority for research.

Effect of dietary vitamin C on compression injury of the spinal cord in a rat mutant unable to synthesize ascorbic acid and its correlation with that of vitamin E

The roles of vitamin C on secondary pathological changes after spinal cord injury were investigated by evaluating the effects of dietary vitamin C on experimental spinal cord injury in a mutant strain of Wistar rats unable to synthesize ascorbic acid (ODS rats). Two groups of ODS rats were given vitamin C-deficient or vitamin C-supplemented diet for 1 week before injury. Motor disturbance induced by spinal cord injury was found to be greater in the vitamin C-deficient group. Histologically, the area of bleeding in the spinal cord was also greater in the vitamin C-deficient group. The levels of ascorbic acid and alpha-tocopherol in the spinal cord tissue and serum decreased during and after compression injury of the spinal cord. The decrease of alpha-tocopherol was similar in the two groups. However, the decrease of ascorbic acid was greater in the vitamin C-supplemented group. These results indicated that their protective effects against spinal cord injury are through scavenging water-soluble free radicals by vitamin C and lipid-soluble by vitamin E, and the effects of these vitamins were suggested to be independent.

Hyperglycemia-induced latent scurvy and atherosclerosis: the scorbutic-metaplasia hypothesis

Latent scurvy is characterized by a reversible atherosclerosis that closely resembles the clinical form of this disease. Acute scurvy is characterized by microvascular complications such as widespread capillary hemorrhaging. Vitamin C (ascorbate) is required for the synthesis of collagen, the protein most critical in the maintenance of vascular integrity. We suggest that in latent scurvy, large blood vessels use modified LDL--in particular lipoprotein(a)--in addition to collagen to maintain macrovascular integrity. By this mechanism, collagen is spared for the maintenance of capillaries, the sites of gas and nutrient exchange. The foam-cell phenotype of atherosclerosis is identified as a mesenchymal genetic program, regulated by the availability of ascorbate. When vitamin C is limited, foam cells develop and induce oxidative modification of LDL, thereby stabilizing large blood vessels via the deposition of LDL. The structural similarity between vitamin C and glucose suggests that hyperglycemia will inhibit cellular uptake of ascorbate, inducing local vitamin C deficiency.

Vitamin C redox reactions in blood of normal and malaria-infected mice studied with isoascorbate as a nonisotopic marker

It has been suggested that the host antimalarial response depends in part on phagocyte-derived oxidants and that the parasite itself exerts an oxidative stress on its erythrocytic environment. Intraerythrocytic malaria parasites are particularly susceptible to being damaged by oxidative drugs, several of which are under development as chemotherapeutic agents. Thus the antioxidant status and associated regulatory mechanisms of the blood during malaria infection are of great interest. The important antioxidant ascorbate (AH-) and isoascorbate (IAH-), an isomer that does not occur naturally in animals, were found to have similar redox properties. We therefore assessed the usefulness of IAH- as a marker for studies of AH- handling in vivo and in vitro under normal conditions and in murine malaria infection. DHIA added to whole blood from normal or Plasmodium vinckei-infected mice in vitro was rapidly taken up into blood cells and reduced to IAH-. Intracellular IAH- derived from the exogenous DHIA was released into the plasma by blood cells from malaria-infected mice but not those from normal mice. Uptake and reduction of DHIA had no effect on plasma or cellular levels of AH- under these conditions. IAH- injected i.v. into either normal or P. vinckei-infected mice was rapidly cleared in both cases and led to an increase in plasma levels of AH-; this suggested displacement of the latter from some intracellular site, presumably not associated with blood cells. DHIA administered as an intravascular bolus into either normal or malaria-infected mice was rapidly reduced. However, in contrast to the in vitro situation, the concentration of plasma IAH- derived from the injected DHIA was approximately the same in both the infected and control animals. The IAH- so formed disappeared quickly from the plasma. Intravenous injection of DHIA into malaria-infected mice caused a rapid, prolonged increase in the proportion of plasma vitamin C in the form of DHA, whereas in uninfected mice there was a transient decrease in plasma DHA followed by normalisation. The changes in plasma AH- and DHA following IV injection of a single dose of DHA closely paralleled those seen after DHIA administration. These observations indicate that: (i) blood cells from normal and malaria-infected mice take up and reduce DHIA in a similar fashion, but they have different ways of handling the resulting IAH-; (ii) cells other than blood cells are important in the reduction of plasma DHIA and DHA in vivo; (iii) malaria-infected mice are less capable of handling oxidative challenge than normal ones; (iv) in some circumstances IAH- and DHIA may be useful nonisotopic markers for studies of vitamin C handling in vitro and in vivo.

A vitamin as neuromodulator: ascorbate release into the extracellular fluid of the brain regulates dopaminergic and glutamatergic transmission

Ascorbate is an antioxidant vitamin that the brain accumulates from the blood supply and maintains at a relatively high concentration under widely varying conditions. Although neurons are known to use this vitamin in many different chemical and enzymatic reactions, only recently has sufficient evidence emerged to suggest a role for ascorbate in interneuronal communication. Ascorbate is released from glutamatergic neurons as part of the glutamate reuptake process, in which the high-affinity glutamate transporter exchanges ascorbate for glutamate. This heteroexchange process, which also may occur in glial cells, ensures a relatively high level of extracellular ascorbate in many forebrain regions. Ascorbate release is regulated, at least in part, by dopaminergic mechanisms, which appear to involve both the D1 and D2 family of dopamine receptors. Thus, amphetamine, GBR-12909, apomorphine, and the combined administration of D1 and D2 agonists all facilitate ascorbate release from glutamatergic terminals in the neostriatum, and this effect is blocked by dopamine receptor antagonists. Even though the neostriatum itself contains a high concentration of dopamine receptors, the critical site for dopamine-mediated ascorbate release in the neostriatum is the substantia nigra. Intranigral dopamine regulates the activity of nigrothalamic efferents, which in turn regulate thalamocortical fibers and eventually the glutamatergic corticoneostriatal pathway. In addition, neostriatonigral fibers project to nigrothalamic efferents, completing a complex multisynaptic loop that plays a major role in neostriatal ascorbate release. Although extracellular ascorbate appears to modulate the synaptic action of dopamine, the mechanisms underlying this effect are unclear. Evidence from receptor binding studies suggests that ascorbate alters dopamine receptors either as an allosteric inhibitor or as an inducer of iron-dependent lipid peroxidation. The applicability of these studies to dopamine receptor function, however, remains to be established in view of reports that ascorbate can protect against lipid peroxidation in vivo. Nevertheless, ample behavioral evidence supports an antidopaminergic action of ascorbate. Systemic, intraventricular, or intraneostriatal ascorbate administration, for example, attenuates the behavioral effects of amphetamine and potentiates the behavioral response to haloperidol. Some of these behavioral effects, however, may be dose-dependent in that treatment with relatively low doses of ascorbate has been reported to enhance dopamine-mediated behaviors. Ascorbate also appears to modulate glutamatergic transmission in the neostriatum. In fact, by facilitating glutamate release, ascorbate may indirectly oppose the action of dopamine, though the nature of the neostriatal dopaminergic-glutamatergic interaction is far from settled. Ascorbate also may alter the redox state of the NMDA glutamate receptor thus block NMDA-gated channel function.(ABSTRACT TRUNCATED AT 400 WORDS)

Transport of vitamin C in animal and human cells

The transport systems of animal and human tissues for vitamin C are reviewed with respect to their properties. It emerges that pure diffusion plays only a very minor role while a variety of more or less specific transporters is found on cellular membranes. Although most tissues prefer the reduced ascorbate over the oxidized dehydroascorbic acid and have high-affinity transporters for it, there are several examples for the reversed situation. Special attention is given to similarity or identity with glucose transporters, especially the GLUT-1 and the sodium-dependent intestinal and renal transporters, and to the very widespread dependence of ascorbate transport on sodium ions. The significance of ascorbate transport for vitamin C-requiring and nonrequiring species as well as alterations in states of disease can be seen from ample experimental evidence.

Effect of vanadium on L-ascorbic acid concentration in rat tissues

1. Two-month old Wistar rats of both sexes received, as sole drinking liquid, an aqueous solution of ammonium metavanadate (AMV) at a concentration of 0.01, 0.05, 0.15 and 0.30 mg V/ml (corresponding to 0.2, 1, 3 and 6 mM solution) over 4 weeks. 2. In the animal groups receiving AMV solution of 0.15 or 0.30 mg V/ml concentrations to drink, a statistically significant decrease of the uptake of food and AMV solution was observed, as compared with food and water taken up in the same time by the control group. 3. Moreover, a distinct decrease of the L-ascorbic acid level was noted in the liver, kidneys, spleen and adrenals. These differences proved statistically significant in single cases of animals receiving a solution of 0.05 and 0.15 mg V/ml concentration and in all animals given the solution at the highest vanadium concentration.

Ascorbate protects guinea pig tissues against lipid peroxidation

In recent years we and others have shown that ascorbic acid (AH2) is a potential scavenger of superoxide (O2.-) and peroxyl (LOO.) radicals, the species involved in lipid peroxidation (LPO) in animal tissues. In this paper we have demonstrated that AH2 protects guinea pig tissues from LPO both in vivo and in vitro. The extent of LPO has been determined by estimating malonaldehyde using the thiobarbituric acid test and HPLC and also by measuring the accumulation of fluorescent pigment and occurrence of protein changes in the microsomal membranes. In AH2-deficiency, LPO occurs progressively in guinea pig tissues, despite the presence of adequate levels of antioxidants like alpha-tocopherol, GSH, protein thiols, and scavenging enzymes, namely, superoxide dismutase, catalase, and glutathione peroxidase. In a model in vitro system, microsomal LPO initiated by O2.- is completely prevented by AH2 but not by alpha-tocopherol, GSH, uric acid, and catalase. AH2 is also the most effective antioxidant in preventing microsomal LPO mediated by tert-butylhydroperoxide or the chain propagating species LOO., generated from 2,2'-azobis (2-amidinopropane) hydrochloride. The results obtained with guinea pigs may be applicable to humans, because humans are also dependent on dietary AH2. Our data suggest that an adequate vitamin C nutrition may prevent common cellular degenerative diseases associated with LPO.

Redox metabolism of vitamin C in blood of normal and malaria-infected mice

As oxidative mechanisms have been suggested to be part of the host immune reaction against malarial parasites, we investigated the redox metabolism of the antioxidant vitamin C in the blood of control and malaria-infected mice. At the peak of infection (day 6) with the malaria parasite P. vinckei, plasma levels of ascorbate (AH-) were 10.8 +/- 0.9 micrograms/ml compared to 5.7 +/- 0.7 micrograms/ml in control mice, though no significant change was observed in the plasma concentration of dehydroascorbate (DHA). The plasma redox ratio of vitamin C, [AH-]:[DHA], was 7.4 in control mice and 18.5 in infected mice on day 6 post-inoculation. The increased AH- level in plasma of P. vinckei-infected mice was not due to differences in stabilities of either AH- or DHA in plasmas from control or P. vinckei-infected mice. DHA added to plasma was lost rapidly. In contrast, when added to whole blood. DHA was rapidly taken up and reduced to AH by blood cells from both normal mice and P. vinckei-infected mice. Most of the intracellular AH- derived from the exogenously added DHA was released into the plasma by blood cells from the infected but not normal mice. The observed release of AH- into the plasma by blood cells from infected mice was not caused by a plasma factor. Depletion of leukocytes from erythrocytes had no effect on the uptake and reduction of DHA by red blood cells, but the subsequent release of intracellular AH- occurred more rapidly.(ABSTRACT TRUNCATED AT 250 WORDS)

Testicular and plasma ascorbic acid levels in mice following dietary intake: a high-performance liquid chromatographic analysis

A modified buffer system is reported for the analysis of vitamin C in mouse plasma and testes, on a reversed-phase high-performance liquid chromatographic column with ultraviolet detection. The buffer, consisting of 0.1 M NaH2PO4 and 0.2 mM Na2EDTA adjusted to pH 3.1 with orthophosphoric acid, resolved the ascorbic acid (AA) peak allowing clear quantitation of the chemical. The method is also applicable to the assay of dehydroascorbic acid after its reduction to ascorbic acid, and overcomes problems of AA stability encountered in previously reported procedures. Using the present technique, variations in the vitamin levels of plasma and testes are studied from 3 to 29 days after the commencement of feeding a vitamin C-rich diet (1%, w/w) in mice. The plasma AA levels were elevated above the controls by a factor of 2.5 by day 8. Contrary to this, testicular AA levels increased marginally (1.2-fold) by day 12 and were maintained at levels close to the control values thereafter. It appears that the feedback inhibition mechanism which is effective in plasma is not operational in the testes. The findings are of clinical significance in that testicular AA levels do not change significantly as a result of dietary intake of vitamin C, whereas plasma AA levels do show an increase.

Evaluation of tissue ascorbic acid status in different hormonal states of female rat

The distribution pattern of ascorbic acid in the endocrine organs and/or tissues was investigated during various sexual states of female rats to demonstrate a correlation between ascorbic acid concentration and sex hormone activity. The concentration of ascorbic acid was estimated in the plasma, liver, pancreas, thyroid, ovary, adrenal and pituitary of contraceptive steroid treated, bilaterally ovariectomized, sham-operated and normal control animals at four different phases of estrous cycle to delineate a relationship between the level of ascorbic acid in different organs/tissues at various sex hormonal states. Statistically significant changes were observed in the concentration of ascorbic acid in different endocrine organs/tissues following contraceptive steroid treatment, ovariectomy and during the estrous phases. Our results suggest that the concentration of ascorbic acid in the endocrine organs and/or tissues varies during different sexual states of rat.

Glutathione deficiency increases hepatic ascorbic acid synthesis in adult mice

Glutathione deficiency, induced in adult mice by administering buthionine sulfoximine (an inhibitor of glutathione synthesis), led to a rapid and substantial increase in ascorbate in the liver. This effect was apparent 2-4 hr after giving the inhibitor; subsequently, the level of ascorbate decreased and that of dehydroascorbate increased markedly, supporting the conclusion that glutathione functions physiologically to keep ascorbate in its reduced form. In kidney and lung also, ascorbate levels decreased, and dehydroascorbate increased. Increased synthesis of ascorbate in glutathione-deficient adult mice seems to protect against tissue damage. In contrast, newborn rats, which (like guinea pigs and humans) apparently do not synthesize ascorbate, suffer severe damage to liver and other organs; previous studies showed that administration of ascorbate prevents such tissue damage. The findings support the view that the antioxidant actions of glutathione and ascorbate are closely linked and involve a mechanism in which decrease of the glutathione level, perhaps associated with an oxidative event, stimulates ascorbate synthesis.

Vitamin C--the primate fertility factor?

The loss of the ability of primates and man to synthesise ascorbic acid (vitamin C) is usually seen as an evolutionary accident, with no benefit to the species. This paper argues that the loss of this biosynthetic ability has allowed vitamin C to act as a 'fertility factor' in primate societies. It is argued that the requirement for vitamin C increases with age, and so in times of food shortages the older members of society suffer higher mortality than the younger. This reduces the median age of the population towards the younger and most fertile members, and so enables the population to regrow rapidly when food resources are restored.

Haematological effects of vanadium on living organisms

Although vanadium has been of great interest for many researchers over a number of years, its biochemical and physiological role is not yet fully clear. There are many papers describing the haematological consequences of its excess in living organisms and most of their data are quoted in this mini-review. The authors of these papers used various laboratory animals, different vanadium compounds, frequently different routes of administration and duration of intoxication. Hence a checklist and comparison of the results are rather difficult. Vanadium reduces the deformability of erythrocytes, and such cells are rather frequently retained in the reticuloendothelial system of the spleen and eliminated faster from the blood stream (Kogawa et al., 1976). Vanadium produces peroxidative changes in the erythrocyte membrane, this leading to haemolysis. Therefore, the depressed erythrocyte count in animals intoxicated with vanadium may be the consequence of both the haemolytic action of vanadium and the shortened time of survival of erythrocytes. Changes of the haem precursor level in blood serum and urine observed in humans exposed occupationally to vanadium suggest an influence of this element on haem synthesis. This problem requires, however, further studies and observations. Changes occurring under the influence of vanadium on the leukocyte system of animals suggest the influence of this element on the resistance of the organism, but the mechanism of the action of vanadium still requires elucidation.(ABSTRACT TRUNCATED AT 250 WORDS)

Vitamin C and the common cold

The effect of vitamin C on the common cold has been the subject of several studies. These studies do not support a considerable decrease in the incidence of the common cold with supplemental vitamin C. However, vitamin C has consistently decreased the duration of cold episodes and the severity of symptoms. The benefits that have been observed in different studies show a large variation and, therefore, the clinical significance may not be clearly inferred from them. The biochemical explanation for the benefits may be based on the antioxidant property of vitamin C. In an infection, phagocytic leucocytes become activated and they produce oxidizing compounds which are released from the cell. By reacting with these oxidants, vitamin C may decrease the inflammatory effects caused by them. Scurvy, which is caused by a deficiency in vitamin C, is mostly attributed to the decreased synthesis of collagen. However, vitamin C also participates in several other reactions, such as the destruction of oxidizing substances. The common cold studies indicate that the amounts of vitamin C which safely protect from scurvy may still be too low to provide an efficient rate for other reactions, possibly antioxidant in nature, in infected people.

Pentosidine: a molecular marker for the cumulative damage to proteins in diabetes, aging, and uremia

Collagen undergoes progressive browning with age and diabetes characterized by yellowing, fluorescence, and cross-linking. The present research was undertaken in order to investigate the nature of the collagen-linked fluorescence. Human collagen was exhaustively cleaved into peptides by enzymatic digestion. Upon purification, a highly fluorescent chromophore was identified and purified from old human collagen. Structure elucidation revealed the presence of an imidazo [4,5-b] pyridinium-type structure acting as a cross-link between arginine, lysine, and a pentose. This advanced glycosylation end-product and protein cross-link results from the reaction of pentoses with proteins and was named pentosidine. Further work indicated that long-term glycosylation of proteins with hexoses also leads to pentosidine formation through sugar fragmentation. The proposed mechanism of pentosidine formation involves the dehydration of the pentose-derived Amadori compound to form an intermediate which is attacked under base catalysis by the guanido group of arginine. The strict requirement for the Amadori rearrangement is uncertain. However, oxidation is definitely involved since pentosidine is not formed in the absence of oxygen. Five-carbon sugars contributing to pentosidine formation could be formed from larger sugars by oxidative fragmentation or from trioses, tetroses, and ketoses by condensation and/or reverse aldol reactions. Pentosidine increases exponentially in human skin at autopsy. Mean age-adjusted skin levels were significantly increased in subjects with uremia and especially in type 1 diabetics with uremia vs. controls. In skin biopsy, levels were significantly elevated in all diabetic (type 1) vs. control subjects. The highest degree of association was with the cumulative grade of diabetic complication (retinopathy, nephropathy, arterial stiffness, and joint stiffness). Pentosidine also forms in various proteins other than collagen, although to a much lesser extent. In blood, pentosidine is mainly associated with plasma proteins and is highly elevated during uremia. In the lens, it is associated with both water-soluble and -insoluble protein fractions and is especially elevated during brunescent cataract formation. The origin of pentosidine in vivo is uncertain. Evidence suggests that the pentoses are the most reactive sugars in pentosidine formation in vitro; however, the origin and importance of free pentoses in vivo, especially during the diabetic state, are not certain. Possible origins include hemolysis and/or a defect in the primary pentose metabolism.(ABSTRACT TRUNCATED AT 400 WORDS)

Administration of gulonolactone does not evoke ascorbic acid synthesis in teleost fish

Evidence is presented that for common carp (Cyprinus carpio L.), intraperitoneal injection of L-gulonolactone, a precursor of ascorbic acid synthesis in the D-glucuronic acid pathway, does not result in an increased concentration of ascorbate in tissue. Control fish injected with an equimolar amount of ascorbic acid have shown a significant increase in ascorbic acid concentration in the kidney, hepatopancreas, plasma and spleen. The ascorbate status in the carp body,i.e., the ascorbate nutritional history, produced significant differences in ascorbate withdrawal from circulation and probably in the catabolic rate. Acute fasting decreased ascorbate uptake into tissues as compared to fish fed a diet lacking ascorbate. Intraperitoneally injected ascorbate affects common carp being fed a diet containing 295 mg of total ascorbic acid kg(-1) by causing tissue to become saturated with vitamin C, similar to the tissues in the group undergoing acute fasting. There was no gulonolactone oxidase activity in the hepatopancreas of the common carp. These results suggest that the metabolic rate induced by feeding is the primary factor regulating ascorbate requirement.

Vitamin C and vitamin E status in the spontaneously diabetic BB rat before the onset of diabetes

Ascorbic acid (AA), dehydroascorbic acid (DHAA), and vitamin E were measured in tissues and plasma of 30 control and 30 spontaneously diabetic BioBreeding rats (BBdp) during development and before the onset of diabetes. At weaning, rats were fed an AIN-76 semisynthetic diet for 30, 64, or 113 days, after which plasma and tissues from 10 rats of each group were collected and analysed for AA, DHAA, and vitamin E. AA and DHAA levels were significantly increased in plasma and spleen of the diabetes-prone rats compared with those of the control group at 30 and 64 days, but the difference disappeared by 113 days. No differences were observed in liver, adrenals, thymus, and pancreas at any of the time periods. However, lower levels of vitamin E were observed in adrenal gland, thymus, and pancreas of the diabetes-prone rats. It is concluded that BBdp rats have an altered metabolism of AA, DHAA, and vitamin E, before the onset of diabetes. These changes could be due to genetic and physiological factors operating during development of this rat strain.

Regulation of ascorbic acid concentration in embryonic chick brain

The relationship between ascorbic acid concentration and cellular transport mechanisms was studied in chicken embryos (Gallus gallus domesticus). Unincubated (Day 0) fertile eggs did not contain detectable levels of ascorbic acid as assayed by high performance liquid chromatography with electrochemical detection. However, ascorbic acid concentration in brain increased to 5.6 nmol/mg tissue by Day 10 in ovo and then gradually declined 32% before birth. These levels were an order of magnitude greater than in skeletal muscle, where ascorbic acid concentration decreased sixfold between Days 8-20. Uptake of ascorbic acid was measured in brain cells that were either freshly isolated or grown in primary culture. Saturable, temperature- and Na(+)-dependent ascorbic acid transport was evident in freshly isolated cells as early as Day 6 and persisted throughout the period of ontogenic development. Primary cultures of embryonic chick brain cells were observed to take up ascorbic acid through a high-affinity (apparent Km = 37 microM, Vmax = 106 nmol ascorbic acid/g protein/min) mechanism. This transport system may maintain the high concentrations of ascorbic acid observed in the central nervous system during the ontogenic period when the levels of ascorbic acid in peripheral tissues change drastically.

Dose response of N-butyl-N-(4-hydroxybutyl)nitrosamine on urinary bladder carcinogenesis in mutant ODS rats lacking L-ascorbic acid synthesizing ability

With a daily intake of 250 ppm total ascorbic acid, ODS and F344 male rats were given 0.0125%, 0.025% or 0.05% N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) in the drinking water for 12 weeks, and additional groups received 0.05% BBN for 2, 4 or 8 weeks. The experiment was terminated after a total of 36 weeks. A greater response to urinary bladder carcinogenesis was observed in both strains with increasing dose of BBN or longer treatment period. However, the magnitude of urinary bladder carcinogenesis in ODS rats given the higher BBN concentrations and/or long periods of BBN treatment was less than in comparably treated F344 rats, but not with lower concentrations of BBN and/or shorter periods of BBN treatment.

Active transport of ascorbic acid into lens epithelium of the rat

The transport rates of radiolabeled ascorbic acid (AA) and dehydroascorbic acid, as well as 3-O-methyl-D-glucose and L-glucose from blood into aqueous humor, lens epithelium and lens 'cortex' compartments were studied in male Sprague-Dawley rats. In vivo pulse chase kinetic studies and modeling of transport from plasma and aqueous and on into idealized water compartments of lens epithelium and cortex allowed for the calculation of transport rate constants, Ki (min-1), in experiments utilizing L-glucose as a passive internal control. TLC chromatography was used to monitor intraocular labeled molecules deriving from labeled test molecules introduced via blood. Results indicate that AA enters aqueous humor at rates similar to L-glucose and likely via simple passive diffusion. In contrast, an active uptake of AA by lens epithelium was found with the calculated entry constant for ascorbate being more than 21 times faster than that of L-glucose. Concentrations in lens epithelium were found to be more than twice that of aqueous humor within only 7 min from the introduction of a [14C]AA bolus into blood. It was also found that very little AA continued on past the epithelium to the interior lens cortex compartment. Our data suggest no special uptake of AA by lens fiber cells. The non-metabolizable analog of D-glucose, 3-O-methyl-D-glucose, however, readily moves past the lens epithelium into fiber cells at much faster rates than the passive L-glucose marker and in a manner consistent with facilitated diffusion. The data suggest that even in a nocturnal species, such as a rat, which demonstrates relatively low circulating levels of ascorbic acid in plasma and aqueous humor, special mechanisms exist for moving ascorbic acid into intraocular tissues. More specifically, the lens epithelium actively takes up ascorbate for some, as yet unclear purpose while the interior fiber cells appear to have no special uptake mechanism for this molecule.

Monooxygenation of N-acetylhistamine mediated by L-ascorbate

In the presence of molecular oxygen and a catalytic amount of copper(II) ion, ascorbate almost completely degraded histamine (approx. 72%). The reaction was shown to occur at the imidazole group but not at the primary amino group in histamine. 4-[2-(Acetylamino)ethyl]-2,3-dihydroimidazol-2-one, a monooxygenated form of N-acetylhistamine, was first isolated as the primary product.

Effect of exogenous ascorbic acid intake on biosynthesis of ascorbic acid in mice

The effect of exogenous ascorbic acid intake on biosynthesis of ascorbic acid in mice has been studied. After the mice were on diets containing added ascorbic acid for two months, the activities of ascorbic acid synthesizing enzymes in the mouse liver homogenates were measured using L-gulono-gamma-lactone as a substrate. Exogenous ascorbic acid intake (0.5, 1 or 5% in the diet) was able to increase the concentration of ascorbic acid in the blood and to decrease the activities of ascorbic acid synthesizing enzymes in mouse liver. The results suggest that ascorbic acid synthesis was controlled by local regulatory mechanism or by the concentration of ascorbic acid in the hepatic portal blood. Ingestion of dietary erythorbic acid, a stereoisomer of ascorbic acid, had no effect on the activities of ascorbic acid synthesizing enzymes.

Effects of age and dietary vitamin C on the contents of ascorbic acid and acid-soluble thiol in lens and aqueous humour of guinea-pigs

Variations in ascorbate and thiol concentration in lens and aqueous humour, with age and vitamin C nutrition, are of potential biological importance. To study these relationships, Dunkin-Hartley guinea-pigs were maintained for periods of up to 1.3 yr on diets containing either high or low (marginal) vitamin C. Ascorbate contents of liver, spleen, adrenals, lens and aqueous humour, and acid-soluble thiol of lens and aqueous humour were measured. High vitamin C intake maintained ascorbate levels in the internal organs between five and 30 times the level attained by the low vitamin C intake and aqueous humour vitamin C was 10-20 times higher at high vitamin C intake. Lens ascorbate, however, was only about twice as high at high vitamin C intake than at low intake, and at both intake levels it declined steadily to about half its initial value, after 1.3 yr. Thus an animal aged 1.3 yr on the low intake had about one-quarter to one-fifth of the lens ascorbate level of a young animal receiving a generous intake. Acid-soluble thiol in the lens, in contrast to ascorbic acid, increased significantly with age but was not significantly affected by dietary vitamin C intake. Acid-soluble thiol in the aqueous humour was only 0.5-2% of the concentration found in lens, and unlike the lens thiol level, it declined with age. No sex differences were observed for ascorbate or thiol levels either in lens or in aqueous humour.

Uptake and release of ascorbic acid by rat Leydig cells in vitro

The mechanism by which ascorbic acid is transported into rat Leydig cells in vitro was investigated. Transport of [14C]ascorbic acid was temperature-dependent and inhibited in the absence of sodium in the incubation medium. The uptake of [14C]ascorbic acid was saturable (apparent Km approximately equal to 33 microM, Vmax approximately equal to 20 pmol/10(6) cells/min) and stereospecific. Uptake was not affected by removing glucose from the incubation medium, indicating that no significant uptake was occurring via the glucose transport system. Transport of [14C]ascorbic acid into Leydig cells was not affected by testosterone concentrations up to 7 microM or by LH at a concentration that causes maximum androgen secretion. Leydig cells preloaded with [14C]ascorbic acid rapidly released radioactivity into the incubation medium, a process that was unaffected by the presence of LH. The results suggest that ascorbic acid uptake by Leydig cells is by an electroneutral, sodium-dependent co-transport mechanism as in some, but not all, other types of cells.

Selected physical and biochemical parameters in the streptozotocin-treated guinea pig: insights into the diabetic guinea pig model

Since evidence suggests that ascorbic acid deficits may provoke certain diabetic complications, it becomes necessary to develop a diabetic animal model which, like man, is unable to synthesize this vitamin. To this end, the present study monitored the diabetogenic effects of streptozotocin (STZ, 150 mg/kg) in the male guinea pig, a species rarely used in diabetes research. Over a 3-week period, body weight and relative food intake were lower in the STZ group compared to controls. The mean daily water intake and urine volume of the STZ group after 1 week were 175 and 270% of their initial pretreatment values, respectively, while control values were unchanged. The STZ group also exhibited a persistent glycosuria throughout the study. At the end of 3 weeks, aldehyde fuchsin staining of pancreatic beta cell granules (an index of stored insulin) was 58% lower in the STZ group compared to controls. Plasma C-peptide (indicator of insulin secretion) was expressed in human equivalents (mean +/- SEM). C-peptide was reduced in the STZ group (103 +/- 65 pg/ml) compared to controls (549 +/- 96 pg/ml); however, no change in plasma glucose was observed. Plasma ascorbic acid levels also were lower for STZ animals (150 +/- 26 micrograms%) versus controls (410 +/- 28 micrograms%). This study 1) demonstrates a diabetic syndrome in the STZ-treated guinea pig based on a reduced growth rate, beta cell dysfunction, polydipsia, polyuria and glycosuria, and 2) suggests the usefulness of this diabetic model in studies of pathologic mechanisms influenced by ascorbic acid.

A highly sensitive high-performance liquid chromatography method for the estimation of ascorbic and dehydroascorbic acid in tissues, biological fluids, and foods

A highly sensitive procedure for determining ascorbic acid (AA) and dehydroascorbic acid (DHAA) by high-performance liquid chromatography with electrochemical detection in biological fluids, tissues, and foods is described. AA is separated in a C18 reverse-phase column after extraction from the sample with metaphosphoric acid. An aliquot of 20 microliter of diluted extract is injected into the column for the estimation of AA. DHAA is indirectly estimated by converting it to AA after reduction with DL-homocysteine at pH 7.0-7.2 for 30 min at 25 degrees C. After dilution, a 20-microliter aliquot is injected into the column to obtain total vitamin C (AA + DHAA). The concentration of DHAA is calculated by subtraction. AA can be reproducibly quantified at concentrations as low as 50 pg/20 microliter of sample extract. The method described here used a specially designed mobile phase, gave greater stability and a noiseless baseline, and increased substantially the sensitivity and precision. The procedure is rapid, analysis being completed within 10 min after sample preparation, and has been successfully applied to biological fluids, tissues, and foods.