Characterization of dehydroascorbic acid solutions by liquid chromatography/mass spectrometry

Recent Advancements in Enhancing Antimicrobial Activity of Plant-Derived Polyphenols by Biochemical Means

Plants are a reservoir of phytochemicals, which are known to possess several beneficial health properties. Along with all the secondary metabolites, polyphenols have emerged as potential replacements for synthetic additives due to their lower toxicity and fewer side effects. However, controlling microbial growth using these preservatives requires very high doses of plant-derived compounds, which limits their use to only specific conditions. Their use at high concentrations leads to unavoidable changes in the organoleptic properties of foods. Therefore, the biochemical modification of natural preservatives can be a promising alternative to enhance the antimicrobial efficacy of plant-derived compounds/polyphenols. Amongst these modifications, low concentration of ascorbic acid (AA)–Cu (II), degradation products of ascorbic acid (DPAA), Maillard reaction products (MRPs), laccase–mediator (Lac–Med) and horse radish peroxidase (HRP)–H2O2 systems standout. This review reveals the importance of plant polyphenols, their role as antimicrobial agents, the mechanism of the biochemical methods and the ways these methods may be used in enhancing the antimicrobial potency of the plant polyphenols. Ultimately, this study may act as a base for the development of potent antimicrobial agents that may find their use in food applications.

Investigating protein thiol chemistry associated with dehydroascorbate, homocysteine and glutathione using mass spectrometry

RATIONALE

Oxidative stress is an imbalance between reactive free radical oxygen species and antioxidant defenses. Its consequences can lead to numerous pathologies. Regulating oxidative stress is the complex interplay between antioxidant recycling and thiol-containing regulatory proteins. Understanding these regulatory mechanisms is important for preventing onset of oxidative stress. The aim of this study was to investigae S-thiol protein chemistry associated with oxidized vitamin C (dehydroascorbate, DHA), homocysteine (HcySH) and glutathione (GSH) using mass spectrometry.

METHODS

Glutaredoxin-1 (Grx-1) was incubated with DHA, with and without GSH and HcySH. Disulfide formation was followed by electrospray ionization mass spectrometry (ESI-MS) of intact proteins and by LC/ESI-MS/MS of peptides from protein tryptic digestions. The mechanism of DHA-mediated S-thiolation was investigated using two synthetic peptides: AcFHACAAK and AcFHACE. Three proteins, i.e. human hemoglobin (HHb), recombinant peroxiredoxin 2 (Prdx2) and Grx-1, were S-homocysteinylated followed by S-transthiolyation with GSH and investigated by ESI-MS and ESI-MS/MS.

RESULTS

ESI-MS analysis reveals that DHA mediates disulfide formation and S-thiolation by HcySH as well as GSH of Grx-1. LC/ESI-MS/MS analysis allows identification of Grx-1 S-thiolated cysteine adducts. The mechanism by which DHA mediates S-thiolation of heptapeptide AcFHACAAK is shown to be via initial formation of a thiohemiketal adduct. In addition, ESI-MS of intact proteins shows that GSH can S-transthiolate S-homocysteinylated Grx-1_ HHb and Prdx2. The GS-S-protein adducts over time dominate the ESI-MS spectrum profile.

CONCLUSIONS

Mass spectrometry is a unique analytical technique for probing complex reaction mechanisms associated with oxidative stress. Using model proteins, ESI-MS reveals the mechanism of DHA-facilitated S-thiolation, which consists of thiohemiketal formation, disulfide formation or S-thiolation. Furthermore, protein S-thiolation by HcySH can be reversed by reversible GSH thiol exchange. The use of mass spectrometry with in vitro models of protein S-thiolation in oxidative stress may provide significant insight into possible mechanisms of action occurring in vivo.

Role of vitamin C in the function of the vascular endothelium

SIGNIFICANCE

Vitamin C, or ascorbic acid, has long been known to participate in several important functions in the vascular bed in support of endothelial cells. These functions include increasing the synthesis and deposition of type IV collagen in the basement membrane, stimulating endothelial proliferation, inhibiting apoptosis, scavenging radical species, and sparing endothelial cell-derived nitric oxide to help modulate blood flow. Although ascorbate may not be able to reverse inflammatory vascular diseases such as atherosclerosis, it may well play a role in preventing the endothelial dysfunction that is the earliest sign of many such diseases.

RECENT ADVANCES

Beyond simply preventing scurvy, evidence is mounting that ascorbate is required for optimal function of many dioxygenase enzymes in addition to those involved in collagen synthesis. Several of these enzymes regulate the transcription of proteins involved in endothelial function, proliferation, and survival, including hypoxia-inducible factor-1α and histone and DNA demethylases. More recently, ascorbate has been found to acutely tighten the endothelial permeability barrier and, thus, may modulate access of ascorbate and other molecules into tissues and organs.

CRITICAL ISSUES

The issue of the optimal cellular content of ascorbate remains unresolved, but it appears that low millimolar ascorbate concentrations are normal in most animal tissues, in human leukocytes, and probably in the endothelium. Although there may be little benefit of increasing near maximal cellular ascorbate concentrations in normal people, many diseases and conditions have either systemic or localized cellular ascorbate deficiency as a cause for endothelial dysfunction, including early atherosclerosis, sepsis, smoking, and diabetes.

FUTURE DIRECTIONS

A key focus for future studies of ascorbate and the vascular endothelium will likely be to determine the mechanisms and clinical relevance of ascorbate effects on endothelial function, permeability, and survival in diseases that cause endothelial dysfunction.

The SLC23 family of ascorbate transporters: ensuring that you get and keep your daily dose of vitamin C

The ascorbate transporters SVCT1 and SVCT2 are crucial for maintaining intracellular ascorbate concentrations in most cell types. Although the two transporter isoforms are highly homologous, they have different physiologic functions. The SVCT1 is located primarily in epithelial cells and has its greatest effect in reabsorbing ascorbate in the renal tubules. The SVCT2 is located in most non-epithelial tissues, with the highest expression in brain and neuroendocrine tissues. These transporters are hydrophobic membrane proteins that have a high affinity and are highly selective for ascorbate. Their ability to concentrate ascorbate inside cells is driven by the sodium gradient across the plasma membrane as generated by Na+/K+ ATPase. They can concentrate ascorbate 20 to 60-fold over plasma ascorbate concentrations. Ascorbate transport on these proteins is regulated at the transcriptional, translational and post-translational levels. Available studies show that transporter function is acutely regulated by protein kinases A and C, whereas transporter expression is increased by low intracellular ascorbate and associated oxidative stress. The knockout of the SVCT2 in mice is lethal on day 1 of life, and almost half of SVCT1 knockout mice do not survive to weaning. These findings confirm the importance both of cellular ascorbate and of the two transport proteins as key to maintaining intracellular ascorbate. LINKED ARTICLES This article is part of a themed section on Transporters. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2011.164.issue-7.

Ascorbic acid: a review of its chemistry and reactivity in relation to a wine environment

Extensive reviews of research are available on the use of ascorbic acid, and its consequent degradation pathways, in physiological conditions or food matrices. However, very little information can be found for wine-related systems. This review highlights the relevant chemistry and reactivity of ascorbic acid with a focus on its behavior and potential behavior in a wine environment. The review describes the use of ascorbic acid as a complementary antioxidant preservative to sulfur dioxide along with the metal-catalyzed and radical-dependent manner by which it achieves this role. The relevant degradation products of ascorbic acid in aerobic and anaerobic conditions are presented as well as the interaction of these degradation products with sulfur dioxide and other wine-relevant sulfur compounds. Limitations in existing knowledge, especially regarding the crossover between the antioxidant and pro-oxidant roles of ascorbic acid, are identified.

Vitamin C transport and its role in the central nervous system

Vitamin C, or ascorbic acid, is important as an antioxidant and participates in numerous cellular functions. Although it circulates in plasma in micromolar concentrations, it reaches millimolar concentrations in most tissues. These high ascorbate cellular concentrations are thought to be generated and maintained by the SVCT2 (Slc23a2), a specific transporter for ascorbate. The vitamin is also readily recycled from its oxidized forms inside cells. Neurons in the central nervous system (CNS) contain some of the highest ascorbic acid concentrations of mammalian tissues. Intracellular ascorbate serves several functions in the CNS, including antioxidant protection, peptide amidation, myelin formation, synaptic potentiation, and protection against glutamate toxicity. The importance of the SVCT2 for CNS function is supported by the finding that its targeted deletion in mice causes widespread cerebral hemorrhage and death on post-natal day 1. Neuronal ascorbate content as maintained by this protein also has relevance for human disease, since ascorbate supplements decrease infarct size in ischemia-reperfusion injury models of stroke, and since ascorbate may protect neurons from the oxidant damage associated with neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's. The aim of this review is to assess the role of the SVCT2 in regulating neuronal ascorbate homeostasis and the extent to which ascorbate affects brain function and antioxidant defenses in the CNS.

Hepatic expression of sodium-dependent vitamin C transporters: ontogeny, subtissular distribution and effect of chronic liver diseases

Ascorbic acid uptake is a key step in determining the overall bioactivity of this vitamin. Expression of Na-dependent vitamin C transporters (SVCT; SLC23A1 and SLC23A2) during long-term oxidative stress occurring in several chronic liver diseases may determine the antioxidant defence in this organ. In patients with hepatocellular cholestasis, primary biliary cirrhosis, haemochromatosis and non-alcoholic steatohepatitis, using real-time RT-PCR, an enhanced hepatic expression of both SLC23A1 and SLC23A2, but not other organic anions transporters, such as OATP1A2, OATP1B1 and OATP1B3, was found. To further investigate these findings, we used secondary biliary cirrhosis induced in rats by long-term biliary obstruction as a model of chronic liver disease accompanied by oxidative stress because of bile acid accumulation. In control rat liver, expression of Slc23a1 was low at birth, increased progressively up to adulthood and decreased in senescence, whereas expression of Slc23a2 did not change significantly after birth. In 8-week-old rats, immunohistochemistry and confocal microscopy studies revealed the expression in hepatocytes and bile duct cells of mainly Slc23a1, whereas both Slc23a1 and Slc23a2 were expressed in endothelial, stellate and Kupffer cells. In adult rats, when obstructive cholestasis was maintained for 8 weeks, a significant up-regulation of Slc23a2 accompanied by a down-regulation of Slc23a1 was found. In sum, there is a selective cell-type distribution of SVCT in the liver tissue, which, in addition to differential control in the expression of both isoforms, may play a role in the ability of different liver cell types to take up vitamin C under physiological and pathological conditions.

The SLC23 family of ascorbate transporters: ensuring that you get and keep your daily dose of vitamin C

The ascorbate transporters SVCT1 and SVCT2 are crucial for maintaining intracellular ascorbate concentrations in most cell types. Although the two transporter isoforms are highly homologous, they have different physiologic functions. The SVCT1 is located primarily in epithelial cells and has its greatest effect in reabsorbing ascorbate in the renal tubules. The SVCT2 is located in most non-epithelial tissues, with the highest expression in brain and neuroendocrine tissues. These transporters are hydrophobic membrane proteins that have a high affinity and are highly selective for ascorbate. Their ability to concentrate ascorbate inside cells is driven by the sodium gradient across the plasma membrane as generated by Na+/K+ ATPase. They can concentrate ascorbate 20 to 60-fold over plasma ascorbate concentrations. Ascorbate transport on these proteins is regulated at the transcriptional, translational and post-translational levels. Available studies show that transporter function is acutely regulated by protein kinases A and C, whereas transporter expression is increased by low intracellular ascorbate and associated oxidative stress. The knockout of the SVCT2 in mice is lethal on day 1 of life, and almost half of SVCT1 knockout mice do not survive to weaning. These findings confirm the importance both of cellular ascorbate and of the two transport proteins as key to maintaining intracellular ascorbate. LINKED ARTICLES This article is part of a themed section on Transporters. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2011.164.issue-7.

The co-application effects of fullerene and ascorbic acid on UV-B irradiated mouse skin

The role of fullerene as a pro-oxidant or anti-oxidant in Ultraviolet B ray (UV-B)-induced disorders in mouse skin was investigated. Fullerene gave no photo-toxic effect to UV-B-irradiated mouse skin. Since erythema was concentrated at the pore circumference in a UV-B irradiation experiment in mouse skin, the sebaceous gland pairs was strongly implicated as a site for the generation of reactive oxygen species (ROS). In a histological evaluation of the skin stained with CH(3)MDFDA (ROS index) and YO-Pro-1 (apoptosis index), the fluorescence intensity of a sebaceous gland significantly increased with UV-B irradiation. With the application of fullerene to UV-irradiated mouse skin, no toxicity was recognized in comparison with the control, and erythema, the ROS index, and the apoptosis index decrease with the application of fullerene. Ascorbyl radical (AA*) increased with the application of ascorbate (AA) to UV-B-irradiated mouse skin, and AA* decreased with the application of fullerene. The co-application of AA and fullerene, which suppressed AA* in vitro, significantly suppressed erythema, and also suppressed both the ROS index and apoptosis index in mouse skin after UV-B irradiation. In both mouse skin at 48 h after UV-B irradiation and in an attempt to reproduce this phenomenon artificially in vitro, a similar high AA* peak (AA*/H*>4) was observed in electron spin resonance (ESR) charts. The binding of fullerene with AA impairs the Fenton reaction between AA and Fe-protein based on the observation of ascorbate-specific UV absorption and a linear equation for the calibration curve. Therefore, fullerene may impair the intercalation of AA to a heme pocket by binding with AA. These results suggest that the co-application of AA and fullerene is effective against oxidative skin damage caused by UV-B irradiation, and the development of an AA* inhibitor such as fullerene should be useful for reducing organ damage associated with Fe-protein oxidation.

Vitamin C and the Role of Citrus Juices as Functional Food

The literature on the content and stability of vitamin C (ascorbic acid, AA) in citrus juices in relation to industrial practices is reviewed. The role of vitamin C from citrus juices in human diet is also reviewed. Citrus fruits and juices are rich in several types of bioactive compounds. Their antioxidant activity and related benefits derive not only from vitamin C but also from other phytochemicals, mainly flavonoids. During juice processing, temperature and oxygen are the main factors responsible for vitamin C losses. Nonthermal processed juices retain higher levels of vitamin C, but economic factors apparently delay the use of such methods in the citrus industry. Regarding packing material, vitamin C in fruit juice is quite stable when stored in metal or glass containers, whereas juice stored in plastic bottles has a much shorter shelf-life.

The limiting step for vitamin C absorption in humans is transcellular active transport across the intestinal wall where AA may be oxidized to dehydroascorbic acid (DHAA), which is easily transported across the cell membrane and immediately reduced back to AA by two major pathways. AA bioavailability in the presence of flavonoids has yielded controversial results. Whereas flavonoids seem to inhibit intestinal absorption of AA, some studies have shown that AA in citrus extract was more available than synthetic ascorbic acid alone. DHAA is reported to possess equivalent biological activity to AA, so recent studies often consider the vitamin C activity in the diet as the sum of AA plus DHAA. However, this claimed equivalence should be carefully reexamined.

Humans are one of the few species lacking the enzyme (L-gulonolactone oxidase, GLO) to convert glucose to vitamin C. It has been suggested that this is due to a mutation that provided a survival advantage to early primates, since GLO produces toxic H2O2. Furthermore, the high concentration of AA (and DHAA) in neural tissues could have been the key factor that caused primates (vertebrates with relative big brain) to lose the capacity to synthesize vitamin C.

Oxidative damage has many pathological implications in human health, and AA may play a central role in maintaining the metabolic antioxidant response. The abundance of citrus juices in the Mediterranean diet may provide the main dietary source for natural vitamin C.

Macrophage differentiation increases expression of the ascorbate transporter (SVCT2)

To determine whether macrophage differentiation involves increased uptake of vitamin C, or ascorbic acid, we assessed the expression and function of its transporter SVCT2 during phorbol ester-induced differentiation of human-derived THP-1 monocytes. Induction of THP-1 monocyte differentiation by phorbol 12-myristate 13-acetate (PMA) markedly increased SVCT2 mRNA, protein, and function. When ascorbate was present during PMA-induced differentiation, the increase in SVCT2 protein expression was inhibited, but differentiation was enhanced. PMA-induced SVCT2 protein expression was blocked by inhibitors of protein kinase C (PKC), with most of the affect due to the PKCbetaI and betaII isoforms. Activation of MEK/ERK was sustained up to 48 h after PMA treatment, and the inhibitors completely blocked PMA-stimulated SVCT2 protein expression, indicating an exclusive role for the classical MAP kinase pathway. However, inhibitors of NF-kappaB activation, NADPH oxidase inhibitors, and several antioxidants also partially prevented SVCT2 induction, suggesting diverse distal routes for control of SVCT2 transcription. Both known promoters for the SVCT2 were involved in these effects. In conclusion, PMA-induced monocyte-macrophage differentiation is enhanced by ascorbate and associated with increased expression and function of the SVCT2 protein through a pathway involving sustained activation of PKCbetaI/II, MAP kinase, NADPH oxidase, and NF-kappaB.

Vitamin C function in the brain: vital role of the ascorbate transporter SVCT2

Ascorbate (vitamin C) is a vital antioxidant molecule in the brain. However, it also has a number of other important functions, participating as a cofactor in several enzyme reactions, including catecholamine synthesis, collagen production, and regulation of HIF-1 alpha. Ascorbate is transported into the brain and neurons via the sodium-dependent vitamin C transporter 2 (SVCT2), which causes accumulation of ascorbate within cells against a concentration gradient. Dehydroascorbic acid, the oxidized form of ascorbate, is transported via glucose transporters of the GLUT family. Once in cells, it is rapidly reduced to ascorbate. The highest concentrations of ascorbate in the body are found in the brain and in neuroendocrine tissues such as adrenal, although the brain is the most difficult organ to deplete of ascorbate. Combined with regional asymmetry in ascorbate distribution within different brain areas, these facts suggest an important role for ascorbate in the brain. Ascorbate is proposed as a neuromodulator of glutamatergic, dopaminergic, cholinergic, and GABAergic transmission and related behaviors. Neurodegenerative diseases typically involve high levels of oxidative stress and thus ascorbate has been posited to have potential therapeutic roles against ischemic stroke, Alzheimer's disease, Parkinson's disease, and Huntington's disease.

Mitochondrial recycling of ascorbic acid as a mechanism for regenerating cellular ascorbate

Mitochondria are the major source of potentially damaging reactive oxygen species in most cells. Since ascorbic acid, or vitamin C, can protect against cellular oxidant stress, we studied the ability of mitochondria prepared from guinea pig skeletal muscle to recycle the vitamin from its oxidized forms. Although ascorbate concentrations in freshly prepared mitochondria were only about 0.2 mM, when provided with 6 mM succinate and 1 mM dehydroascorbate (the two-electron-oxidized form of the vitamin), mitochondria were able to generate and maintain concentrations as high as 4 mM, while releasing most of the ascorbate into the incubation medium. Mitochondrial reduction of dehydroascorbate was strongly inhibited by 1,3-bis(chloroethyl)-1-nitrosourea and by phenylarsine oxide. Despite existing evidence that mitochondrial ascorbate protects the organelle from oxidant damage, ascorbate failed to preserve mitochondrial alpha-tocopherol during prolonged incubation in oxygenated buffer. Nonetheless, the capacity for mitochondria to recycle ascorbate from its oxidized forms, measured as ascorbate-dependent ferricyanide reduction, was several-fold greater than total steady-state ascorbate concentrations. This, and the finding that more than half of the ascorbate recycled from dehydroascorbate escaped the mitochondrion, suggests that mitochondrial recycling of ascorbate might be an important mechanism for regenerating intracellular ascorbate.

Plasma Ascorbate Deficiency Is Associated With Impaired Reduction of Sulfamethoxazole-Nitroso in HIV Infection

OBJECTIVE

The objective of these studies was to determine the role of ascorbate deficiency in HIV infection in the defective detoxification of sulfamethoxazole-nitroso, the metabolite thought to mediate sulfonamide hypersensitivity reactions.

METHODS

Fifty-one HIV-infected patients and 26 healthy volunteers were evaluated. Vitamin supplementation histories were obtained, and blood samples were collected for determination of plasma ascorbate, dehydroascorbate, and cysteine concentrations, erythrocyte glutathione concentrations, and plasma reduction of sulfamethoxazole-nitroso in vitro.

RESULTS

Plasma ascorbate concentrations were significantly lower in HIV-positive patients not taking vitamin supplements (29.5 +/- 22.3 microM) than in healthy subjects (54.8 +/- 22.3 microM; P = 0.0005) and patients taking 500-1000 mg of ascorbate daily (82.5 +/- 26.3 microM; P < 0.0001). Plasma ascorbate deficiency was strongly correlated with impaired reduction of sulfamethoxazole-nitroso to its hydroxylamine (r = 0.60, P < 0.0001), and during in vitro reduction, the loss of plasma ascorbate was strongly associated with the amount of nitroso reduced (r = 0.70, P < 0.0001). Ascorbate added ex vivo normalized this reduction pathway. Erythrocyte glutathione concentrations were significantly lower in HIV-positive patients (0.98+/-0.32 mM) than in healthy subjects (1.45+/-0.49 mM; P = 0.001), but this finding was unrelated to ascorbate supplementation. There was trend toward lower plasma cysteine concentrations in patients (8.4+/-3.9 microM) than in controls (10.3+/-4.3 microM), but this trend was similarly unrelated to ascorbate supplementation. Dehydroascorbate concentrations were not significantly higher in HIV-positive patients (7.4+/-10.5%) than in healthy controls (4.0+/-6.2%), even in the subset of patients taking ascorbate (8.4+/-9.4%).

CONCLUSIONS

Ascorbate deficiency is common in HIV-positive patients and is associated with impaired detoxification of sulfamethoxazole-nitroso, the suspected proximate toxin in sulfonamide hypersensitivity. Patients taking daily ascorbate supplements (500-1000 mg) achieved high plasma ascorbate concentrations and did not show this detoxification defect. Ascorbate deficiency (or supplementation) was not associated with changes in glutathione or cysteine concentrations. These data suggest that ascorbate deficiency, independent of thiol status, may be an important determinant of impaired drug detoxification in HIV infection.