Synthesis and some major functions of vitamin C in animals

Transport and stability of ascorbic acid in pituitary cultures

Ascorbic acid uptake in AtT-20 tumor cells and primary cultures of rat anterior and intermediate pituitary was sodium-dependent and showed half-maximal saturation between 9 and 18 microM ascorbate. When incubated in [14C]ascorbic acid at concentrations similar to those in serum (50 microM), all of the cells concentrated ascorbate 20- to 40-fold, producing intracellular ascorbate concentrations of 1-2 mM. HPLC analyses showed that over 90% of the intracellular label comigrated with authentic ascorbic acid. Although ascorbate was rapidly oxidized in culture medium in the absence of cells, incubation of ascorbate in culture medium in the presence of cells stabilized the ascorbate substantially. Unlike systems that transport dehydroascorbic acid, the ascorbate transport systems in all three preparations were not inhibited by glucose. Thus all three systems possess similar saturable, high-affinity, sodium-dependent active transport systems for ascorbic acid.

Alloxan toxicity in human and canine spermatozoa. Possible biochemical basis for a species difference in sensitivity

In view of the well known species differences in the sensitivity of pancreatic B-cells to the toxic glucose analogue alloxan, it was tested whether spermatozoa from two species with a different diabetogenic effect of alloxan displayed a similar difference in their sensitivity to this drug. In canine spermatozoa, less than 2 mM alloxan profoundly reduced the rate of glucose oxidation and cellular motility whereas more than 5 mM was required to significantly alter these parameters in human spermatozoa. Such species difference was not observed in spermatozoal sensitivity towards the inhibitory effects of tert-butyl hydroperoxide. The phenomenon is not attributable to a different rate of alloxan uptake since the drug is not incorporated by dog or human spermatozoa. The alloxan toxicity was counteracted by D-glucose and its 3-O-methyl analogue in both species, and was potentiated by ascorbic acid; however, only in man. The protective effect of D-glucose was much less marked in tert-butyl hydroperoxide-cytotoxicity. It is concluded that the observed species difference in spermatozoal alloxan sensitivity is not related to differences in alloxan uptake or in sensitivity to organic peroxides; differences in cellular scavenging of superoxide anion radicals and/or ascorbic acid metabolism may explain the lower sensitivity of human spermatozoa for alloxan.

Possible mechanisms responsible for the increased ascorbic acid content of Plasmodium vinckei-infected mouse erythrocytes

The possible mechanisms underlying the acquisition of an increased ascorbic acid content by mouse erythrocytes containing the malarial parasite Plasmodium vinckei were investigated. Ascorbic acid was taken up readily by parasitized red blood cells but not by controls, whilst its partly oxidized form, dehydroascorbic acid, entered both. The uptake of both ascorbic acid and dehydroascorbic acid into erythrocytes was increased as a result of malarial infection. Lysates prepared from parasitized red blood cells reduced exogenous dehydroascorbic acid to ascorbic acid at a higher rate than control red blood cell lysates; this difference was abolished following dialysis of the lysates, a process which removes endogenous reduced glutathione (GSH). The rates of chemical and enzymatic reduction of dehydroascorbic acid to ascorbic acid by GSH were of similar magnitude, thus calling into question the existence of a specific dehydroascorbate reductase in erythrocytes and parasites. These observations suggest that the increased uptake of dehydroascorbic acid into parasitized red blood cells may be a result of enhanced dehydroascorbate-reducing capacity, whilst the presence of the parasite induces a selective increase in the permeability of the erythrocyte plasma membrane to ascorbic acid. The endogenous ascorbic acid content of livers obtained from infected mice was 55% below the normal concentration and its relative rate of destruction during incubation in vitro was enhanced in comparison with that of control livers. Furthermore, the capacity of liver homogenates to synthesize ascorbic acid from glucuronic acid was greatly reduced in infected mice. Therefore it is unlikely that the increase in ascorbic acid content of parasitized red blood cells is a consequence of increased biosynthesis and release of ascorbic acid by the host liver. We have not been able to exclude the possibility that the malarial parasite itself may be capable of de novo synthesis of ascorbic acid.

The effects of maternal folic acid and vitamin C nutrition in early pregnancy on reproductive performance in the guinea-pig

1. The effect of different intakes of folic acid (FA) and vitamin C on pregnancy in the Dunkin-Hartley guinea-pig was examined. Female guinea-pigs were subjected to three graded intakes of FA and vitamin C ('deficient', 'intermediate' similar to recommended daily intakes (RDI), and 'supplemented') during early gestation and up to the time of neural tube closure (17th day of gestation), and then returned to the RDI of these vitamins. 2. Plasma and blood cell concentrations of these vitamins were measured once before and at the end of the dietary treatments. Reproductive performance was assessed in terms of the number of resorbed and aborted embryos and weight and size of the live fetuses on the 36th day of gestation. 3. The short-term deficiency of either of these two vitamins, insufficient to affect maternal health, had a dramatic effect on the reproductive performance. 4. The RDI of FA was significantly less effective than the supplemented intake in preventing embryonic deaths. The RDI of vitamin C produced lighter and smaller live fetuses than the supplemented intake. 5. The implications of these findings with regard to vitamin status in early pregnancy in man are discussed.

Fibrinogen coagulation without thrombin: reaction with vitamin C and copper(II)

We describe a novel method for inducing fibrinogen derived clots. The addition of vitamin C (0.1-1 mM) to a solution of fibrinogen (1 mg/ml) and Cu(II) (20-150 microM) results in protein coming out of solution. This phenomenon can be "read" by fibrometers as "clotting time". The reaction requires Cu(II) and can be prevented by a chelating agent, such as citrate, as well by a hydroxyl radical scavenger, such as mannitol. The insoluble protein, called "neofibe", is soluble in 4 M urea and 2% SDS. Isoelectric focusing and SDS-electrophoretic comparison of native fibrinogen with neofibe reveal molecular modifications of the starting protein. This reaction is an interesting example of a free radical mediated transformation of soluble protein into insoluble material. Some findings on the connection between hemostasis, vitamin C and Cu(II) are discussed.

Na+-independent dehydro-L-ascorbic acid uptake in renal brush-border membrane vesicles

A membrane preparation enriched in the brush-border component of the plasma membrane was isolated from rat renal superficial cortex by a divalent cation precipitation procedure. Uptake of dehydro-L-ascorbic acid, the oxidized form of L-ascorbic acid, by the brush-border membrane vesicles was studied. The uptake mechanism was found to be sodium-independent and insensitive to the trans-membrane electrical potential difference. Uptake was saturable and subject to cis-inhibition. Concentrative uptake was demonstrated only under conditions of trans-stimulation by structural analogs. The results suggest a mechanism of facilitated diffusion for the uptake of dehydro-L-ascorbic acid in renal brush-border membranes.

Ascorbic acid levels in aqueous and vitreous humors of the rabbit: effects of inflammation and ceruloplasmin

A HPLC method for determination of ascorbic acid and dehydroascorbic acid in plasma and aqueous and vitreous humors of rabbits is described. Values for total ascorbic acid concentration found in this study are in agreement with those of previous investigators. Endotoxin-induced ocular inflammation caused a decrease in the concentration of ascorbic acid in the aqueous humor and an increase in the vitreous humor. The additional lack of correlation between levels of ascorbic acid in normal aqueous and vitreous humors from the same uninflamed eye indicates that the aqueous humor is not the source of vitreal ascorbic acid. The copper concentration of the aqueous humor is increased during ocular inflammation, most likely due to the influx of ceruloplasmin from plasma when the blood-aqueous barrier is disrupted. Ceruloplasmin caused a decrease in the amount of ascorbic acid in the aqueous humor in vitro and the vitreous humor in vivo. The presence of ceruloplasmin in the aqueous humor during inflammation thus may contribute to the decreased concentration of ascorbic acid in this fluid.

Vitamins: an evolutionary perspective

Access to vitamins and the genetic endownment to utilize them maintain vitamin-dependent metabolic homeostasis in heterotrophs. Whereas the extent of adverse nutritional experiences has declined in modern human societies, phenocopies of deficiency diseases persist; accordingly, they have high heritability. The "vitamin-responsive hereditary metabolic diseases" identify DNA that specifies conserved apoenzyme domains interacting with coenzyme and the cellular processes providing access to coenzymes. Could heterozygosity at those loci also be a determinant of disease (or health) under certain circumstances?

Urate and ascorbate: their possible roles as antioxidants in determining longevity of mammalian species

Urate has been shown to be a major antioxidant in human serum and was postulated to have a biological role in protecting tissues against the toxic effects of oxygen radicals and in determining the longevity of primates. This possibility has been tested by determining if the maximum lifespan potentials of 22 primate and 17 non-primate mammalian species are positively correlated with the concentration of urate in serum and brain per specific metabolic rate. This analysis is based on the concept that the degree of protection a tissue has against oxygen radicals is proportional to antioxidant concentration per rate of oxygen metabolism of that tissue. Ascorbate, another potentially important antioxidant in determining longevity of mammalian species, was also investigated using this method. The results show a highly significant positive correlation of maximum lifespan potential with the concentration of urate in serum and brain per specific metabolic rate. No significant correlation was found for ascorbate. These results support the hypothesis that urate is biologically active as an antioxidant and is involved in determining the longevity of primate species, particularly for humans and the great apes. Ascorbate appears to have played little or no role as a longevity determinant in mammalian species.

Plasma levels of vitamin C components in normal and diabetic subjects

Recent reports of high dehydroascorbic acid levels in Asian subjects suffering from diabetes mellitus have led us to examine plasma dehydroascorbic acid levels in diabetic and non-diabetic Europeans. We can find no evidence that diabetic patients have higher plasma dehydroascorbic acid levels than nondiabetics, and consider some possible reasons for the discrepancy between this finding and those of earlier reports.

Nutritional need versus optimal intake

The lack of biochemical basis for "nutritional need" is discussed. The current use of "nutritional need" as the basis in nutritional recommendations suggests that the general approach of such recommendations should be re-evaluated. "Optimal intake" -approach is suggested as an alternative.

Plasma ascorbate concentrations and blood cell dehydroascorbate transport in patients with diabetes mellitus

Rates of dehydroascorbate uptake by blood granulocytes and mononuclear cells are slower, and plasma ascorbate concentrations are lower, among persons with diabetes mellitus than in nondiabetic subjects. These measurements do not correlate with one another or with simultaneously measured plasma glucose or glycosylated hemoglobin; they do not differ with type of diabetes or mode of treatment. In those diabetic granulocytes that exhibit slow dehydroascorbate uptake, maximal velocity (Vmax) transport rates for dehydroascorbate, 2-deoxyglucose, and 3-O-methylglucose are decreased, each to the same degree, while Km values for transport of these ligands are not different from those observed in nondiabetic cells. Since diffusion of these ligands is facilitated by a common transporter, these observations may reflect decreased numbers of glucose transporters in the plasma membranes of some diabetic leukocytes.

Ontogenetic development of L-gulonolactone oxidase activity in several vertebrates

Activity of L-gulonolactone oxidase (EC 1.1.3.8) in livers of fetal Rattus norvegicus and Mus musculus was detectable on the 18th day of gestation, increased rapidly to maxima at 15 and 5 days postpartum for the two species, respectively, and thereafter declined to adult levels. L-Gulonolactone oxidase was not detectable in liver or kidney of fetal guinea pigs at any stage of development. Near-term fetal snowshoe hares had higher activities of liver L-gulonolactone oxidase than observed in a large sample of adults. L-Gulonolactone oxidase was detectable in chicken (Gallus gallus) embryos by the sixth day of incubation, increased rapidly in the kidney with no discontinuity at hatching, reached a maximum at about the 35th day from the beginning of incubation, and then declined to adult levels. Barn swallow (Hirundo rustica) embryos appeared to synthesize little if any L-ascorbic acid; nestlings had considerably higher levels of L-gulonolactone oxidase than adults. Tadpoles of three species of frogs had appreciable levels of L-gulonolactone oxidase activity.

Loss of ascorbic acid from injured feline spinal cord

Feline spinal cord contains 0.97 mM ascorbic acid, as measured by the dinitrophenylhydrazine method. Greater than 90% is maintained in the reduced form. When functioning normally, the CNS conserves its ascorbic acid with a turnover rate of 2% per h. Following contusion injury severe enough to produce paraplegia, ascorbic acid is rapidly lost from injured spinal tissue. Thus, ascorbic acid is decreased 30% by 1 h and 50% by 3 h following injury. Oxidized ascorbic acid is increased at 1, but not 3, h following impact. As a consequence of its many functions in CNS, loss of ascorbic acid may contribute to derangements in spinal cord function following injury.

A spectrophotometric assay for dehydroascorbate reductase

A simple spectrophotometric assay for dehydroascorbate reductase based on the change in absorbance associated with the formation of ascorbic acid is described. Using a partially purified preparation from spinach leaves, the reaction was found to be linear with time and enzyme concentration. The reaction rate determined by this assay correlated well with that obtained by a high-performance liquid chromatography method. Possible advantages over currently available assays as well as potential applications are discussed.

Biosynthesis of ascorbate in yeast. Purification of L-galactono-1,4-lactone oxidase with properties different from mammalian L-gulonolactone oxidase

An enzyme from Saccharomyces cerevisiae which catalyzes the reaction: L-galactonolactone + O2 leads to L-ascorbate + H2O2 has been purified 466-fold from the mitochondrial fraction of a yeast homogenate. The enzyme has several properties that are different from the L-galactonolactone oxidase described by Nishikimi et al. [Arch. Biochem. Biophys. 191, 479-486 (1978)]. By gel filtration in the presence of sodium deoxycholate an apparent Mr of 70 000 was obtained for the active enzyme. Polyacrylamide-gradient gel electrophoresis in the presence of deoxycholate gave an Mr of 74 000, whereas sodium dodecylsulfate/polyacrylamide gel electrophoresis showed only one protein band corresponding to an Mr of 18 000. A tetrameric structure of the enzyme is thereby suggested. The substrate specificity is confined to the aldonoacid lactones L-galactono-, D-altrono-, L-fucono-, D-arabino- and D-threono-1,4-lactones. Competitive inhibition was demonstrated with L-gulono- and D-galactono-1,4-lactones. p-Chloromercuriphenyl sulfonate, iodoacetamide, N-ethylmaleimide, sulfite and sulfide were all inhibitory to the enzyme. No effect was seen when cyanide, azide, EDTA, alpha, alpha'-bipyridyl or bathocuproine disulfonate was added. An apparent Km of 0.3 mM with L-galactonolactone as a substrate was found. The Km for oxygen was 0.18 mM. The pH/activity curve exhibited a maximum around pH 8.9 and a shoulder at pH 6.5. Evidence of a covalently bound flavin coenzyme and involvement of an iron-sulfur cluster was obtained from difference spectra of oxidized minus substrate-reduced enzyme with peaks or shoulders of the oxidized enzyme at 475, 445, 410, 375 and 350 nm. In sodium dodecylsulfate/polyacrylamide gels the enzyme subunit(s) had a bright yellow fluorescence after fixation in 7% acetic acid or 5% formaldehyde. The galactonolactone oxidase is stable with 50% activity being lost in 6 months at + 5 degrees C.

Blood dehydroascorbic acid and diabetes mellitus in human beings

Ascorbic acid and dehydroascorbic acid were estimated in the blood of normal healthy subjects and diabetic patients. In normal subjects, blood contained only ascorbic acid while dehydroascorbic acid was practically absent. The ascorbic acid level was low in the blood of diabetic patients but the dehydroascorbic acid content was remarkably high, irrespective of age, sex, history of diabetes, or treatment. About 75% of blood dehydroascorbic acid was present in the erythrocytes: the rest was in plasma. High blood dehydroascorbic acid levels were also found in 90% of the non-diabetic offspring with both parents diabetic, in 24% of the non-diabetic offspring with one parent diabetic, and in 75% of the non-diabetic siblings of diabetic patients. It appears that, in persons having an hereditary predisposition to diabetes, high blood dehydroascorbic acid levels may be used as a marker for early detection of the disease.

Plasma ascorbic acid levels in sheep experimentally infected with Fasciola hepatica

Three groups of sheep, one control (A) and two infected with Fasciola hepatica metacercariae (B&C), were investigated. Each animal of group B was orally dosed with about 270 metacercariae and each sheep in group C received two such doses administered 14 days apart. In both infected groups, plasma ascorbic acid levels progressively decreased after exposure reaching a minimum by week 9, and then slightly increased during the subsequent five weeks. The decrease in ascorbic acid concentrations was mainly related to impaired synthesis as a result of the hepatic damage caused by the flukes.

Tissue concentrations and proliferative effects of massive doses of ascorbic acid in the mouse

The effect of ascorbic acid supplementation on CF1 mice fed ascorbic acid for approximately six months at dose levels of 1%, 5%, and 10% of diet was investigated by analysis of tissue ascorbic acid concentration in the liver, kidney, stomach, small intestine, and large bowel. The effect on epithelial cell proliferation was also examined in the small and large bowel but only at the 5% level. In the control animals, ascorbic acid concentration was lowest in the liver (0.406 +/- 0.07 mg/g) and highest in the small bowel (0.754 +/- 0.16 mg/g). Dietary intake of 5% and 10% ascorbic acid significantly elevated levels in the liver (0.741 +/- 0.13; p less than 0.05), and all doses of ascorbic acid significantly raised tissue concentrations in the kidney and colon. No difference was observed in the percentage of DNA-synthesizing cells in the jejunum of controls or animals fed 5% ascorbic acid at 1 or 24 hours after 3HTdR injection. However, at 1 hour a significantly decreased level of proliferation was observed in the distal colon of ascorbic-acid-treated mice compared with controls (labeling index [L.I.] = 7.3 +/- 0.28 vs. 10.1 +/- 1.15; p less than 0.05), and an even greater suppression of DNA synthesis was achieved by 24 hours (L.I. = 11.4 +/- 1.06 vs. 18.6 +/- 1.61; p less than 0.01). None of the doses of ascorbic acid employed was toxic to the experimental mice.

Increased ascorbic acid content in chronic lymphocytic leukemia B lymphocytes

Human lymphocyte extracts analyzed by high-performance liquid chromatography reveal a major UV-absorbing peak that was shown to be ascorbic acid by spectral, chemical, and enzymatic criteria. Because this peak appeared very prominent in the elution profile of chronic lymphocytic leukemia (CLL) lymphocyte extracts, we measured the ascorbic acid content in lymphocytes from the blood of normal subjects and untreated patients with chronic lymphocytic leukemia. A significantly higher concentration of 111 +/- 15.3 nmol per 10(8) cells (mean +/- SEM) was found in CLL lymphocytes than in normal blood lymphocytes, which contained 42.2 +/- 3.3 nmol per 10(8) cells. Selective enrichment with B and T cells showed that this difference was limited to the chronic lymphocytic leukemia B cell, which had a 5- to 15-fold higher content of ascorbic acid than normal B cells had. In contrast, the ascorbic acid level was similar in normal and CLL T cells. The very high ascorbic acid content provides the chronic lymphocytic leukemia B cell with a reducing substance that could react with oxidants or free radicals.

Human cell dehydroascorbate reductase. Kinetic and functional properties

Dehydroascorbate reductase (glutathione: dehydroascorbate oxidoreductase, EC 1.8.5.1) activity was examined in crude cytosol extracts of human cells. In blood neutrophil and lymphocyte extracts, the Km at pH 6.85 for dehydroascorbate was 1.3 mM, and for reduced glutathione 3.8 mM. Rates of dehydroascorbate uptake by intact human neutrophils, monocytes, lymphocytes and cultured fibroblasts were proportional to cytosol dehydroascorbate reductase activities. Dehydroascorbate reduction during dehydroascorbate uptake by these cells may be entirely enzymatic.

Plasma levels of folates, riboflavin, vitamin B6, and ascorbate in severely disturbed children

The recent upsurge in megavitamin therapy raises questions about the role of vitamin deficiencies and dependencies in mental health. With this in mind, the plasma levels of folic acid, ascorbic acid, pyridoxine, and riboflavin were studied in approximately 125 children admitted to a child psychiatric unit. There were no apparent decreased levels of vitamins in these children in terms of their age, race, or psychiatric diagnosis. It is postulated that vitamin deficiencies per se cannot be proposed as etiological factors in any of the psychiatric deficits represented. Megavitamin therapy, if successful, is not effective due to crrection of vitamin deficiencies as opposed to vitamin dependencies and may be due to the metabolic onus and consequent effects of such heavy doses of vitamins.

Ascorbic acid and diabetes mellitus

Dehydroascorbic acid, the oxidized form of vitamin C, causes diabetes when injected into animals and has been reported to be present in increased amounts in the blood of patients with diabetes mellitus and even in prediabetics. One of the earliest changes in diabetes mellitus is electron-microscopic evidence of damage to the inner endothelial lining of the blood vessels. Certain bioflavonoids, which are natural non-toxic food substances from plants, like rutin from buckwheat, prevent the oxidation of ascorbic acid and seem to protect the endothelium when given with vitamin C; it is therefore suggested that all vitamin C tablets should be combined with these flavonoids.