Spontaneous hydrolysis and dehydration of dehydroascorbic acid in aqueous solution

Kinetic Modeling Assisted Analysis of Vitamin C-Mediated Copper Redox Transformations in Aqueous Solutions

The kinetics of oxidation of micromolar concentrations of ascorbic acid (AA) catalyzed by Cu(II) in solutions representative of biological and environmental aqueous systems has been investigated in both the presence and absence of oxygen. The results reveal that the reaction between AA and Cu(II) is a relatively complex set of redox processes whereby Cu(II) initially oxidizes AA yielding the intermediate ascorbate radical (A•-) and Cu(I). The rate constant for this reaction was determined to have a lower limit of 2.2 × 104 M-1 s-1. Oxygen was found to play a critical role in mediating the Cu(II)/Cu(I) redox cycle and the oxidation reactions of AA and its oxidized forms. Among these processes, the oxidation of the ascorbate radical by molecular oxygen was identified to play a key role in the consumption of ascorbic acid, despite being a slow reaction. The rate constant for this reaction (A • - + O 2 → DHA + O 2 • - ) was determined for the first time with a calculated value of 54 ± 8 M-1 s-1. The kinetic model developed satisfactorily describes the Cu/AA/O2 system over a range of conditions including different concentrations of NaCl (0.2 and 0.7 M) and pH (7.4 and 8.1). Appropriate adjustments to the rate constant for the reaction between Cu(I) and O2 were found to account for the influence of the chloride ions and pH on the kinetics of the process. Additionally, the presence of Cu(III) as the primary oxidant resulting from the interaction between Cu(I) and H2O2 in the Cu(II)/AA system was confirmed, along with the coexistence of HO•, possibly due to an equilibrium established between Cu(III) and HO•.

Antioxidant Activity, Stability in Aqueous Medium and Molecular Docking/Dynamics Study of 6-Amino- and N-Methyl-6-amino-L-ascorbic Acid

The antioxidant activity and chemical stability of 6-amino-6-deoxy-L-ascorbic acid (D1) and N-methyl-6-amino-6-deoxy-L-ascorbic acid (D2) were examined with ABTS and DPPH assays and compared with the reference L-ascorbic acid (AA). In addition, the optimal storing conditions, as well as the pH at which the amino derivatives maintain stability, were determined using mass spectrometry. Comparable antioxidant activities were observed for NH-bioisosteres and AA. Moreover, D1 showed higher stability in an acidic medium than the parent AA. In addition, AA, D1, and D2 share the same docking profile, with wild-type human peroxiredoxin as a model system. Their docking scores are similar to those of dithiothreitol (DTT). This suggests a similar binding affinity to the human peroxiredoxin binding site.

Impact of High-Pressure Homogenization Parameters on Physicochemical Characteristics, Bioactive Compounds Content, and Antioxidant Capacity of Blackcurrant Juice

High-pressure homogenization (HPH) is one of the food-processing methods being tested for use in food preservation as an alternative to pasteurization. The effects of the HPH process on food can vary depending on the process parameters used and product characteristics. The study aimed to investigate the effect of pressure, the number of passes, and the inlet temperature of HPH processing on the quality of cloudy blackcurrant juice as an example of food rich in bioactive compounds. For this purpose, the HPH treatment (pressure of 50, 150, and 220 MPa; one, three, and five passes; inlet temperature at 4 and 20 °C) and the pasteurization of the juice were performed. Titratable acidity, pH, turbidity, anthocyanin, vitamin C, and total phenolics content, as well as colour, and antioxidant activity were measured. Heat treatment significantly decreased the quality of the juice. For processing of the juice, the best were the combinations of the following: one pass, the inlet temperature of 4 °C, any of the used pressures (50, 150, and 220 MPa); and one pass, the inlet temperature of 20 °C, and the pressure of 150 MPa. Vitamin C and anthocyanin degradation have been reported during the HPH. The multiple passes of the juice through the machine were only beneficial in increasing the antioxidant capacity but negatively affected the colour stability.

Dehydroascorbic Acid Affects the Stability of Catechins by Forming Conjunctions

Although tea catechins in green tea and green tea beverages must be stable to deliver good sensory quality and healthy benefits, they are always unstable during processing and storage. Ascorbic acid (AA) is often used to protect catechins in green tea beverages, and AA is easily oxidized to form dehydroascorbic acid (DHAA). However, the function of DHAA on the stability of catechins is not clear. The objective of this study was to determine the effects of DHAA on the stability of catechins and clarify the mechanism of effects by conducting a series of experiments that incubate DHAA with epigallocatechin gallate (EGCG) or catechins. Results showed that DHAA had a dual function on EGCG stability, protecting its stability by inhibiting hydrolysis and promoting EGCG consumption by forming ascorbyl adducts. DHAA also reacted with (−)-epicatechin (EC), (−)-epicatechin gallate (ECG), and (−)-epigallocatechin (EGC) to form ascorbyl adducts, which destabilized them. After 9 h of reaction with DHAA, the depletion rates of EGCG, ECG, EC, and EGC were 30.08%, 22.78%, 21.45%, and 13.55%, respectively. The ability of DHAA to promote catechins depletion went from high to low: EGCG, ECG, EGC, and EC. The results are important for the processing and storage of tea and tea beverages, as well as the general exploration of synergistic functions of AA and catechins.

Characterization of an l-Ascorbate Catabolic Pathway with Unprecedented Enzymatic Transformations

l-Ascorbate (vitamin C) is ubiquitous in both our diet and the environment. Here we report that Ralstonia eutropha H16 (Cupriavidus necator ATCC 17699) uses l-ascorbate as sole carbon source via a novel catabolic pathway. RNaseq identified eight candidate catabolic genes, sequence similarity networks, and genome neighborhood networks guided predictions for function of the encoded proteins, and the predictions were confirmed by in vitro assays and in vivo growth phenotypes of gene deletion mutants. l-Ascorbate, a lactone, is oxidized and ring-opened by enzymes in the cytochrome b₅₆₁ and gluconolactonase families, respectively, to form 2,3-diketo-l-gulonate. A protein predicted to have a WD40-like fold catalyzes an unprecedented benzilic acid rearrangement involving migration of a carboxylate group to form 2-carboxy-l-lyxonolactone; the lactone is hydrolyzed by a member of the amidohydrolase superfamily to yield 2-carboxy-l-lyxonate. A member of the PdxA family of oxidative decarboxylases catalyzes a novel decarboxylation that uses NAD⁺ catalytically. The product, l-lyxonate, is catabolized to α-ketoglutarate by a previously characterized pathway. The pathway is found in hundreds of bacteria, including the pathogens Pseudomonas aeruginosa and Acinetobacter baumannii.

3-O-ethyl-l-ascorbic acid: Characterisation and investigation of single solvent systems for delivery to the skin

l-ascorbic acid (AA), commonly known as vitamin C, has been widely used in topical formulations for many years as an antioxidant and anti-aging ingredient. However, the physicochemical properties of AA are not optimal for skin uptake and the molecule is also unstable, readily undergoing oxidation on exposure to air. The compound 3-o-ethyl-l-ascorbic acid (EA) has been developed as a stable vitamin C derivative and has been used in topical products. The aims of this work were to conduct a comprehensive characterisation of physicochemical properties of EA as well as to investigate the influence of various neat solvents on EA skin delivery. Nuclear magnetic resonance (NMR), mass spectroscopy, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to characterise the molecule. The pK_a of the compound and the partition coefficient logP_(o/w) were experimentally determined. A new HPLC method for analysis of the molecule was also developed and validated. A number of solvents for topical preparations were selected based on their wide use as excipients in topical formulations, their potential to act as skin penetration enhancers and their favourable safety profiles. The solubility and stability of EA was examined. Skin permeation of the molecule in full thickness porcine skin in vitro was investigated using Franz-type diffusion cells. The melting point, log P_(o/w) value and pK_a value of EA were determined to be 114.39 ± 0.5 °C, −1.07 ± 0.03 and 7.72 ± 0.01 respectively. Skin penetration of EA was evident for the following vehicles 1,2 hexanediol (HEX), glycerol (GLY), propylene glycol (PG), 1,2 pentanediol (1-2P), isopropyl alcohol (IPA), propylene glycol monolaurate (PGML) and propylene glycol monocaprylate (PGMC). Skin uptake but no permeation through the skin was observed for Transcutol® (TC) and dipropylene glycol (DiPG), while no penetration was observed for the solvents 1,5 pentanediol (1-5P) and tripropylene glycol (TriPG). The findings of the permeation experiments confirm the potential of simple formulations to deliver EA to the skin. Studies are ongoing to identify complex vehicles for synergistic enhancement of EA skin penetration. To our knowledge this is the first study to conduct a comprehensive characterization of EA and examine its skin uptake and permeation properties in porcine skin.

The oxidation of dehydroascorbic acid and 2,3-diketogulonate by distinct reactive oxygen species

l-Ascorbate, dehydro-l-ascorbic acid (DHA), and 2,3-diketo-l-gulonate (DKG) can all quench reactive oxygen species (ROS) in plants and animals. The vitamin C oxidation products thereby formed are investigated here. DHA and DKG were incubated aerobically at pH 4.7 with peroxide (H2O2), 'superoxide' (a ∼50 : 50 mixture of [Formula: see text] and [Formula: see text]), hydroxyl radicals (•OH, formed in Fenton mixtures), and illuminated riboflavin (generating singlet oxygen, 1O2). Products were monitored electrophoretically. DHA quenched H2O2 far more effectively than superoxide, but the main products in both cases were 4-O-oxalyl-l-threonate (4-OxT) and smaller amounts of 3-OxT and OxA + threonate. H2O2, but not superoxide, also yielded cyclic-OxT. Dilute Fenton mixture almost completely oxidised a 50-fold excess of DHA, indicating that it generated oxidant(s) greatly exceeding the theoretical •OH yield; it yielded oxalate, threonate, and OxT. 1O2 had no effect on DHA. DKG was oxidatively decarboxylated by H2O2, Fenton mixture, and 1O2, forming a newly characterised product, 2-oxo-l-threo-pentonate (OTP; '2-keto-l-xylonate'). Superoxide yielded negligible OTP. Prolonged H2O2 treatment oxidatively decarboxylated OTP to threonate. Oxidation of DKG by H2O2, Fenton mixture, or 1O2 also gave traces of 4-OxT but no detectable 3-OxT or cyclic-OxT. In conclusion, DHA and DKG yield different oxidation products when attacked by different ROS. DHA is more readily oxidised by H2O2 and superoxide; DKG more readily by 1O2 The diverse products are potential signals, enabling organisms to respond appropriately to diverse stresses. Also, the reaction-product 'fingerprints' are analytically useful, indicating which ROS are acting in vivo.

Ascorbic acid metabolism and functions: A comparison of plants and mammals

Ascorbic acid is synthesised by eukaryotes, the known exceptions being primates and some other animal groups which have lost functional gulonolactone oxidase. Prokaryotes do not synthesise ascorbate and do not need an ascorbate supply, so the functions that are essential for mammals and plants are not required or are substituted by other compounds. The ability of ascorbate to donate electrons enables it to act as a free radical scavenger and to reduce higher oxidation states of iron to Fe²⁺. These reactions are the basis of its biological activity along with the relative stability of the resulting resonance stabilised monodehydroascorbate radical. The importance of these properties is emphasised by the evolution of at least three biosynthetic pathways and production of an ascorbate analogue, erythroascorbate, by fungi. The iron reducing activity of ascorbate maintains the reactive centre Fe²⁺ of 2-oxoglutarate-dependent dioxygenases (2-ODDs) thus preventing inactivation. These enzymes have diverse functions and, recently, the possibility that ascorbate status in mammals could influence 2-ODDs involved in histone and DNA demethylation thereby influencing stem cell differentiation and cancer has been uncovered. Ascorbate is involved in iron uptake and transport in plants and animals. While the above biochemical functions are shared between mammals and plants, ascorbate peroxidase (APX) is an enzyme family limited to plants and photosynthetic protists. It provides these organisms with increased capacity to remove H₂O₂ produced by photosynthetic electron transport and photorespiration. The Fe reducing activity of ascorbate enables hydroxyl radical production (pro-oxidant effect) and the reactivity of dehydroascorbate (DHA) and reaction of its degradation products with proteins (dehydroascorbylation and glycation) is potentially damaging. Ascorbate status influences gene expression in plants and mammals but at present there is little evidence that it acts as a specific signalling molecule. It most likely acts indirectly by influencing the redox state of thiols and 2-ODD activity. However, the possibility that dehydroascorbylation is a regulatory post-translational protein modification could be explored.

Natural vitamin B12 and fucose supplementation of green smoothies with edible algae and related quality changes during their shelf life

BACKGROUND

Some algae are an excellent sources of vitamin B12, of special interest for vegetarian/vegan consumers, and of fucose to supplement fruit and vegetable beverages such as smoothies. Nevertheless, supplementation of smoothies with algae may lead to possible quality changes during smoothie shelf life that need to be studied. Therefore, the quality changes in fresh green smoothies supplemented (2.2%) with nine edible algae (sea lettuce, kombu, wakame, thongweed, dulse, Irish moss, nori, Spirulina and Chlorella) were studied throughout 24 days at 5 °C.

RESULTS

The initial vitamin C content - 238.7-326.0 mg kg^-1 fresh weight (FW) - of a 200 g portion of any of the smoothies ensured full coverage of its recommended daily intake, and still supplying 50-60% of the recommended intake after 7 days. Chlorella and Spirulina smoothies showed the highest vitamin B12 content (33.3 and 15.3 µg kg^-1 FW, respectively), while brown algae showed fucose content of 141.1-571.3 mg kg^-1 FW. These vitamin B12 and fucose contents were highly maintained during shelf life.

CONCLUSION

The Spirulina supplementation of a 200 g smoothie portion ensured full coverage of the recommended vitamin B12 intake, with lower vitamin C degradation, during a shelf life of 17 days. Furthermore, thongweed and kombu are also considered as excellent fucose sources with similar shelf life. © 2017 Society of Chemical Industry.

Magnetic resonance imaging with hyperpolarized agents: methods and applications

In the past decade, hyperpolarized (HP) contrast agents have been under active development for MRI applications to address the twin challenges of functional and quantitative imaging. Both HP helium (³He) and xenon (¹²⁹Xe) gases have reached the stage where they are under study in clinical research. HP ¹²⁹Xe, in particular, is poised for larger scale clinical research to investigate asthma, chronic obstructive pulmonary disease, and fibrotic lung diseases. With advances in polarizer technology and unique capabilities for imaging of ¹²⁹Xe gas exchange into lung tissue and blood, HP ¹²⁹Xe MRI is attracting new attention. In parallel, HP ¹³C and ¹⁵N MRI methods have steadily advanced in a wide range of pre-clinical research applications for imaging metabolism in various cancers and cardiac disease. The HP [1-¹³C] pyruvate MRI technique, in particular, has undergone phase I trials in prostate cancer and is poised for investigational new drug trials at multiple institutions in cancer and cardiac applications. This review treats the methodology behind both HP gases and HP ¹³C and ¹⁵N liquid state agents. Gas and liquid phase HP agents share similar technologies for achieving non-equilibrium polarization outside the field of the MRI scanner, strategies for image data acquisition, and translational challenges in moving from pre-clinical to clinical research. To cover the wide array of methods and applications, this review is organized by numerical section into (1) a brief introduction, (2) the physical and biological properties of the most common polarized agents with a brief summary of applications and methods of polarization, (3) methods for image acquisition and reconstruction specific to improving data acquisition efficiency for HP MRI, (4) the main physical properties that enable unique measures of physiology or metabolic pathways, followed by a more detailed review of the literature describing the use of HP agents to study: (5) metabolic pathways in cancer and cardiac disease and (6) lung function in both pre-clinical and clinical research studies, concluding with (7) some future directions and challenges, and (8) an overall summary.

Ascorbic acid intake and oxalate synthesis

In humans, approximately 60 mg of ascorbic acid (AA) breaks down in the body each day and has to be replaced by a dietary intake of 70 mg in women and 90 mg in men to maintain optimal health and AA homeostasis. The breakdown of AA is non-enzymatic and results in oxalate formation. The exact amount of oxalate formed has been difficult to ascertain primarily due to the limited availability of healthy human tissue for such research and the difficulty in measuring AA and its breakdown products. The breakdown of 60 mg of AA to oxalate could potentially result in the formation of up to 30 mg oxalate per day. This exceeds our estimates of the endogenous production of 10-25 mg oxalate per day, indicating that degradative pathways that do not form oxalate exist. In this review, we examine what is known about the pathways of AA metabolism and how oxalate forms. We further identify how gaps in our knowledge may be filled to more precisely determine the contribution of AA breakdown to oxalate production in humans. The use of stable isotopes of AA to directly assess the conversion of vitamin to oxalate should help fill this void.

Frontier orbitals and transition states in the oxidation and degradation of L-ascorbic acid: a DFT study

DFT calculations were carried out to investigate reaction paths of L-ascorbic acid (AAH2), hydroxyl radicals and water clusters. Frontier-orbital analyses were also performed to examine the regioselectivity of the OH˙ addition. Transition states of the electrolytic dissociation of AAH2 and intermediate carboxylic acids were found to have very small activation energies through proton transfers along hydrogen bonds. The ionized species (anions) are subject to the electrophilic attack of OH˙. The elementary processes of AAH2 → A˙(-) → dehydroascorbic acid → diketogulonic acid → threonic, oxalic, xylonic and lyxonic acids were investigated and discussed. The processes involved in the conversion of dehydroascorbic acid into a bicyclic hemiketal were also examined as a side-chain participating reaction. The oxidation and degradation of vitamin C up to threonic acid were described mainly as a donor (AAH2)-acceptor (OH˙) reaction.

Determination of ascorbic acid and its degradation products by high-performance liquid chromatography-triple quadrupole mass spectrometry

This study describes application of liquid chromatography coupled with triple quadrupole mass spectrometry (LC-MS) for evaluation of vitamin C stability, the objective being prediction of the degradation products. Detection was performed with an UV detector (UV-Vis) in sequence with a triple-quad mass spectrometer in the multiple reaction mode. The negative ion mode of ESI and MS-MRM transitions of m/z 175→115 (quantifier) and 175→89 (qualifier) for ascorbic acid was used. All the validation parameters were within the range of acceptance proposed by the Food and Drug Administration. The method was fully validated in terms of linearity, LOD, LOQ, accuracy, and interday precision. Validation experiments revealed good linearity with R(2) = 0.999 within the established concentration range, and excellent repeatability (9.3%). The LOD of the method was 0.1524 ng/mL whereas the LOQ was 0.4679 ng/mL. LC-MS methodology proves to be an improved, simple, and fast approach to determining the content of vitamin C and its degradation products with high sensitivity, selectivity, and resolving power within 6 minutes of analysis.

Effect of glucagon-like peptide-1 and ghrelin on liver metabolites in steers

Glucagon-like peptide 1 (GLP-1) and ghrelin have opposite regulatory effects on glucose metabolism in non-ruminants. However, mechanisms by which GLP-1 and ghrelin regulate nutrient partitioning, particularly in the liver, have been much less demonstrated in ruminants. A novel metabolomic method based on capillary electrophoresis time-of-flight mass spectrometry (CE-TOFMS) combined with multivariate statistical analysis was applied to address the GLP-1 and ghrelin-induced metabolic changes in the liver of steers. Three Holstein steers (400 ± 5.0 kg LW) fed a maintenance diet according to Japanese feeding standards were randomly assigned to three treatments (GLP-1, ghrelin and saline) in a 3 × 3 Latin square design with one week apart. Liver biopsies were taken 30 min after a single injection (1.0 μg/kg LW) of GLP-1 or ghrelin, and analysed for metabolites by Agilent CE-TOFMS system. Also, blood samples were collected for plasma hormones analysis. Results indicated that 20 and 10 liver metabolites were altered (P < 0.05) by GLP-1 and ghrelin, respectively. Pathway analysis showed that GLP-1 is involved in biochemical pathways related to glycolysis/gluconeogenesis, lipogenesis and lipid export from the liver, oxidative stress defence and protein turnover. Ghrelin was shown to be involved in pathways related to glycolysis, protein anabolism and phospholipid biosynthesis. However, plasma concentrations of insulin, growth hormone and glucagon did not differ between treatments. These results imply that GLP-1 and ghrelin are involved in multibiochemical pathways that go beyond simply regulating glucose metabolism. In addition, the effects of GLP-1 and ghrelin may potentially be independent of insulin and growth hormone, respectively.

Vitamin-C as anti-Helicobacter pylori agent: More prophylactic than curative- Critical review

Potential of nonantibiotic therapies for treatment of Helicobacter pylori-related acid peptic disease remains underexplored. Several clinical studies have shown that higher prevalence of H. pylori infection is associated with low Vitamin C (Vit C) level in serum and gastric juice. However, there is no consensus regarding the usefulness of Vit C supplementation in the management of H. pylori infection. Surveying the existing literature we conclude that high concentration of Vit C in gastric juice might inactivate H. pylori urease, the key enzyme for the pathogen's survival and colonization into acidic stomach. Once infection established, urease is not very important for its survival. The role of Vit-C as anti-H. pylori agent in peptic ulcer diseases appears to be preventive rather than curative. Rather than supplementing high dose of Vit C along with conventional triple therapy, it is preferable to complete the conventional therapy and thereafter start Vit C supplementation for extended period which would prevent reinfection in susceptible individuals, provided the patients are not achlorhydric. Further studies are required to prove the role of Vit C in susceptible population.

Oxidation of dehydroascorbic acid and 2,3-diketogulonate under plant apoplastic conditions

The rate of L-ascorbate catabolism in plants often correlates positively with the rate of cell expansion. The reason for this correlation is difficult to explore because of our incomplete knowledge of ascorbate catabolism pathways. These involve enzymic and/or non-enzymic oxidation to dehydroascorbic acid (DHA), which may then be hydrolysed to 2,3-diketogulonate (DKG). Both DHA and DKG were susceptible to further oxidation under conditions of pH and H₂O₂ concentration comparable with the plant apoplast. The kinetics of their oxidation and the identity of some of the products have been investigated here. DHA, whether added in pure form or generated in situ by ascorbate oxidation, was oxidised non-enzymically to yield, almost simultaneously, a monoanion (cyclic-oxalyl-threonate; cOxT) and a dianion (oxalyl-threonate; OxT). The monoanion was resistant to periodate oxidation, showing that it was not oxalic threonic anhydride. The OxT population was shown to be an interconverting mixture of 3-OxT and 4-OxT, differing in pK(a). The 3-OxT appeared to be formed earlier than 4-OxT, but the latter predominated at equilibrium. DKG was oxidised by H₂O₂ to two partially characterised products, one of which was itself further oxidised by H₂O₂ to yield threonate. The possible occurrence of these reactions in the apoplast in vivo and the biological roles of vitamin C catabolites are discussed.

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.

Alternative pathways of dehydroascorbic acid degradation in vitro and in plant cell cultures: novel insights into vitamin C catabolism

L-Ascorbate catabolism involves reversible oxidation to DHA (dehydroascorbic acid), then irreversible oxidation or hydrolysis. The precursor–product relationships and the identity of several major DHA breakdown products remained unclear. In the presence of added H2O2, DHA underwent little hydrolysis to DKG (2,3-dioxo-L-gulonate). Instead, it yielded OxT (oxalyl L-threonate), cOxT (cyclic oxalyl L-threonate) and free oxalate (~6:1:1), essentially simultaneously, suggesting that all three product classes independently arose from one reactive intermediate, proposed to be cyclic-2,3-O-oxalyl-L-threonolactone. Only with plant apoplastic esterases present were the esters significant precursors of free oxalate. Without added H2O2, DHA was slowly hydrolysed to DKG. Downstream of DKG was a singly ionized dicarboxy compound (suggested to be 2-carboxy-L-xylonolactone plus 2-carboxy-L-lyxonolactone), which reversibly de-lactonized to a dianionic carboxypentonate. Formation of these lactones and acid was minimized by the presence of residual unreacted ascorbate. In vivo, the putative 2-carboxy-L-pentonolactones were relatively stable. We propose that DHA is a branch-point in ascorbate catabolism, being either oxidized to oxalate and its esters or hydrolysed to DKG and downstream carboxypentonates. The oxidation/hydrolysis ratio is governed by reactive oxygen species status. In vivo, oxalyl esters are enzymatically hydrolysed, but the carboxypentonates are stable. The biological roles of these ascorbate metabolites invite future exploration.

Chemistry of ascorbic acid and sulfur dioxide as an antioxidant system relevant to white wine

The impact of the combined ascorbic acid and sulfur dioxide antioxidants on white wine oxidation processes was investigated using a range of analytical techniques, including flow injection analysis for free and total sulfur dioxide and two chromatographic methods for ascorbic acid, its oxidative degradation products and phenolic compounds. The combination of different analytical techniques provided a fast and simultaneous means for the monitoring of oxidation processes in a model wine system. In addition, the initial mole ratio of sulfur dioxide to ascorbic acid was varied and the model wine complexity was increased by the inclusion of metal ions (copper(II) and iron(II)). Sulfur dioxide was found not to be a significant binder of ascorbic acid oxidative degradation products and could not prevent the formation of certain phenolic pigment precursors. The results provide a detailed insight into the ascorbic acid/sulfur dioxide antioxidant system in wine conditions.

Hyperpolarized [1-13C]-ascorbic and dehydroascorbic acid: vitamin C as a probe for imaging redox status in vivo

Dynamic nuclear polarization (DNP) of (13)C-labeled metabolic substrates in vitro and their subsequent intravenous administration allow both the location of the hyperpolarized substrate and the dynamics of its subsequent conversion into other metabolic products to be detected in vivo. We report here the hyperpolarization of [1-(13)C]-ascorbic acid (AA) and [1-(13)C]-dehydroascorbic acid (DHA), the reduced and oxidized forms of vitamin C, respectively, and evaluate their performance as probes of tumor redox state. Solution-state polarization of 10.5 ± 1.3% was achieved for both forms at pH 3.2, whereas at pH 7.0, [1-(13)C]-AA retained polarization of 5.1 ± 0.6% and [1-(13)C]-DHA retained 8.2 ± 1.1%. The spin-lattice relaxation times (T(1)'s) for these labeled nuclei are long at 9.4 T: 15.9 ± 0.7 s for AA and 20.5 ± 0.9 s for DHA. Extracellular oxidation of [1-(13)C]-AA and intracellular reduction of [1-(13)C]-DHA were observed in suspensions of murine lymphoma cells. The spontaneous reaction of DHA with the cellular antioxidant glutathione was monitored in vitro and was approximately 100-fold lower than the rate observed in cell suspensions, indicating enzymatic involvement in the intracellular reduction. [1-(13)C]-DHA reduction was also detected in lymphoma tumors in vivo. In contrast, no detectable oxidation of [1-(13)C]-AA was measured in the same tumors, consistent with the notion that tumors maintain a reduced microenvironment. This study demonstrates that hyperpolarized (13)C-labeled vitamin C could be used as a noninvasive biomarker of redox status in vivo, which has the potential to translate to the clinic.

Tripartite ATP-independent periplasmic (TRAP) transporters in bacteria and archaea

The tripartite ATP-independent periplasmic (TRAP) transporters are the best-studied family of substrate-binding protein (SBP)-dependent secondary transporters and are ubiquitous in prokaryotes, but absent from eukaryotes. They are comprised of an SBP of the DctP or TAXI families and two integral membrane proteins of unequal sizes that form the DctQ and DctM protein families, respectively. The SBP component has a structure comprised of two domains connected by a hinge that closes upon substrate binding. In DctP-TRAP transporters, substrate binding is mediated through a conserved and specific arginine/carboxylate interaction in the SBP. While the SBP component has now been relatively well characterized, the membrane components of TRAP transporters are still poorly understood both in terms of their structure and function. We review the expanding repertoire of substrates and physiological roles for experimentally characterized TRAP transporters in bacteria and discuss mechanistic aspects of these transporters using data primarily from the sialic acid-specific TRAP transporter SiaPQM from Haemophilus influenzae, which suggest that TRAP transporters are high-affinity, Na(+)-dependent unidirectional secondary transporters.

Simultaneous chemical and photochemical protein crosslinking induced by irradiation of eye lens proteins in the presence of ascorbate: the photosensitizing role of an UVA-visible-absorbing decomposition product of vitamin C

Exposure to light has been implicated as a risk factor during aging of the eye lens and in cataract generation. In order to visualize the actual effect of UVA-visible light on this tissue, we incubated water-soluble eye lens proteins with ascorbate in the presence and absence of UVA-visible light for 3, 6 and 9 days at low oxygen concentration. The samples incubated in the presence of light were characterized by an initially small but continuous increase over time of the protein crosslinking. This was not the result of more extensive glycation because the decrease in amino group content of the proteins and the decomposition of ascorbate was the same in both irradiated and unirradiated samples. The augmented crosslinking capacity observed in the presence of UVA-visible light is due to the generation of a chromophore from the decomposition of ascorbate. This chromophore, obtained after 3, 6 and 9 days of incubation of solutions containing only ascorbate, induces both protein-crosslinking and oxidation after exposure to UVA-visible light in the presence of lens proteins. The extent of the crosslinking was proportional to the amount of the chromophore present in the solution. The presence of this chromophore was also determined when ascorbate was incubated with four-fold higher concentrations of N-α-acetyl lysine and N-α-acetyl arginine. When these samples were used as photosensitizers, the crosslinking degree was conditioned by the presence of this chromophore; nonetheless, the ascorbate-mediated advanced glycation end product (AGE) generation also made a contribution. The results of this work indicate that ascorbate oxidation, which generates the AGEs responsible for the chemical crosslinking of the lens proteins, also simultaneously produces a chromophore that can act as a photosensitizer, further increasing the protein crosslinking.

Polarographic determination of vitamin C after derivatization with o-phenylenediamine

A differential pulse polarographic (DPP) method has been developed for the determination of ascorbic acid (AA) and dehydroascorbic acid (DHA), the two main forms of Vitamin C. The method consists of the DPP analysis of a quinoxaline obtained by the derivatization of DHA with o-phenylenediamine. Results using the proposed method correlated well with those obtained by two reference methodologies: the common iodometric method and a published chromatographic methodology. It was also used in the study of Vitamin C degradation in fruit juices, showing that it involves an initial oxidation of AA to DHA, followed by hydrolytic degradation of the latter.

Radiation metabolomics. 3. Biomarker discovery in the urine of gamma-irradiated rats using a simplified metabolomics protocol of gas chromatography-mass spectrometry combined with random forests machine learning algorithm

Abstract Radiation metabolomics employing mass spectral technologies represents a plausible means of high-throughput minimally invasive radiation biodosimetry. A simplified metabolomics protocol is described that employs ubiquitous gas chromatography-mass spectrometry and open source software including random forests machine learning algorithm to uncover latent biomarkers of 3 Gy gamma radiation in rats. Urine was collected from six male Wistar rats and six sham-irradiated controls for 7 days, 4 prior to irradiation and 3 after irradiation. Water and food consumption, urine volume, body weight, and sodium, potassium, calcium, chloride, phosphate and urea excretion showed major effects from exposure to gamma radiation. The metabolomics protocol uncovered several urinary metabolites that were significantly up-regulated (glyoxylate, threonate, thymine, uracil, p-cresol) and down-regulated (citrate, 2-oxoglutarate, adipate, pimelate, suberate, azelaate) as a result of radiation exposure. Thymine and uracil were shown to derive largely from thymidine and 2'-deoxyuridine, which are known radiation biomarkers in the mouse. The radiation metabolomic phenotype in rats appeared to derive from oxidative stress and effects on kidney function. Gas chromatography-mass spectrometry is a promising platform on which to develop the field of radiation metabolomics further and to assist in the design of instrumentation for use in detecting biological consequences of environmental radiation release.

Modulation of gap-junction-dependent arterial relaxation by ascorbic acid

AIMS

To investigate whether ascorbic acid (AA) can influence endothelium-dependent relaxation by modulating the spread of endothelial hyperpolarization through the arterial wall via gap junctions.

METHODS

Force development and membrane potential were monitored by myography and sharp electrode techniques in isolated rabbit iliac arteries.

RESULTS

AA prevented the ability of the gap junction blocker 2-aminoethoxydiphenyl borate to inhibit endothelium-dependent relaxations and subintimal smooth muscle hyperpolarizations evoked by cyclopiazonic acid in the presence of nitric oxide (NO) synthase and cyclooxygenase blockade. AA also prevented the ability of a connexin-mimetic peptide targeted against Cx37 and Cx40 (37,40Gap 26) to attenuate the transmission of endothelial hyperpolarization to subintimal smooth muscle, and a peptide targeted against Cx43 (43Gap 26) to attenuate the spread of subintimal hyperpolarization to subadventitial smooth muscle and the associated mechanical relaxation. Parallel studies with endothelium-denuded preparations demonstrated that AA and cyclopiazonic acid both depressed relaxation evoked by the NO donor MAHMA NONOate.

CONCLUSIONS

The data suggest that AA can modulate arterial function through a previously unrecognized ability to preserve electrotonic signalling via myoendothelial and homocellular smooth muscle gap junctions under conditions where cell coupling is depressed. Underlying mechanisms do not involve amplification of 'residual' NO activity by AA.

Human Erythrocyte Membranes Contain a Cytochrome b561 That May Be Involved in Extracellular Ascorbate Recycling

Human erythrocytes contain an unidentified plasma membrane redox system that can reduce extracellular monodehydroascorbate by using intracellular ascorbate (Asc) as an electron donor. Here we show that human erythrocyte membranes contain a cytochrome b(561) (Cyt b(561)) and hypothesize that it may be responsible for this activity. Of three evolutionarily closely related Cyts b(561), immunoblots of human erythrocyte membranes showed only the duodenal cytochrome b(561) (DCytb) isoform. DCytb was also found in guinea pig erythrocyte membranes but not in erythrocyte membranes from the mouse or rat. Mouse erythrocytes lost a majority of the DCytb in the late erythroblast stage during erythropoiesis. Absorption spectroscopy showed that human erythrocyte membranes contain an Asc-reducible b-type Cyt having the same spectral characteristics as recombinant DCytb and biphasic reduction kinetics, similar to those of the chromaffin granule Cyt b(561). In contrast, mouse erythrocytes did not exhibit Asc-reducible b-type Cyt activity. Furthermore, in contrast to mouse erythrocytes, human erythrocytes much more effectively preserved extracellular Asc and transferred electrons from intracellular Asc to extracellular ferricyanide. These results suggest that the DCytb present in human erythrocytes may contribute to their ability to reduce extracellular monodehydroascorbate.

Novel ligands for the extracellular solute receptors of two bacterial TRAP transporters

Tripartite ATP-independent periplasmic (TRAP) transporters are relatively common prokaryotic secondary transporters which comprise an extracytoplasmic solute receptor (ESR) protein and two dissimilar membrane proteins or domains, yet the substrates and physiological functions of only a few of these systems are so far known. In this study, a biophysical approach was used to identify the ligands for the purified Rhodobacter capsulatus RRC01191 and Escherichia coli YiaO proteins, which are members of two phylogenetically distinct families of TRAP-ESRs found in diverse bacteria. In contrast to previous indirect evidence pointing to RRC01191 orthologues being involved in polyol uptake, it was shown that RRC01191 binds pyruvate, 2-oxobutyrate and a broad range of aliphatic monocarboxylic 2-oxoacid anions with varying affinities (K(d) values 0.08-3 muM), consistent with a predicted role in monocarboxylate transport related to branched-chain amino-acid biosynthesis. The E. coli YiaMNO TRAP transporter has previously been proposed to be an l-xylulose uptake system [Plantinga et al. (2004) Mol Membr Biol 21, 51-57], but purified YiaO did not bind l- or d-xylulose as judged by fluorescence spectroscopy, circular dichroism or mass spectrometry. Instead, these techniques showed that a breakdown product of l-ascorbate, 2,3-diketo-l-gulonate (2,3-DKG), binds by a simple one-step mechanism with sub-micromolar affinity. The data provide the first evidence for the existence of ESR-dependent transporters for 2-oxoacids and 2,3-DKG, homologues of which appear to be widespread amongst prokaryotes. The results also underline the utility of direct ESR ligand-binding studies for TRAP transporter characterization.

Association between hydrogen peroxide-dependent byproducts of ascorbic acid and increased hepatic acetyl-CoA carboxylase activity

BACKGROUND

Parenteral multivitamin preparation (MVP) induces fatty liver in neonatal guinea pig pups; this is prevented by photoprotection. Photo-excited riboflavin present in MVP generates H(2)O(2) and molecules with masses of 136 and 208. We hypothesized that H(2)O(2) initiates the peroxidation of ascorbic acid (AA), producing biologically active byproducts affecting hepatic lipid metabolism.

METHODS

Mass spectrometry (MS) documented the participation of H(2)O(2) and photo-excited riboflavin (Ribo) in the formation of AA byproducts. Sixteen 3-day-old guinea pig pups received an intravenous solution (50 g/L dextrose + 4.5 g/L NaCl + 1 kIU/L heparin) at 240 mL x kg(-1) x day(-1), enriched with control or test mixtures, for 4 days. The control mixture was photo-protected AA + Ribo (without byproducts or H(2)O(2)), and the test mixture was AA + Ribo treated to generate AA byproducts without H(2)O(2). Hepatic acetyl-CoA carboxylase (ACC) activity was determined after 4 days. Fourth-day urine samples were analyzed by MS. Data were treated by ANOVA (alpha = 0.05).

RESULTS

H(2)O(2) did not influence the classic degradation of AA, as the generation of 2,3-diketogulonic acid was not affected. In contrast, the formation of molecules with masses of 136 and 208 was H(2)O(2) and time dependent. ACC activity was higher (P <0.01) in animals receiving high concentration of these molecules; its hepatic activation correlated (P <0.01) with the urinary concentration of molecule-208.

CONCLUSIONS

H(2)O(2) at concentrations found in the clinical setting of total parenteral nutrition induce the transformation of dehydroascorbic acid into compounds that have the potential to affect lipid metabolism. These molecules have peroxide and aldehyde functions.

Ascorbate depletion: a critical step in nickel carcinogenesis?

Nickel compounds are known to cause respiratory cancer in humans and induce tumors in experimental animals. The underlying molecular mechanisms may involve genotoxic effects; however, the data from different research groups are not easy to reconcile. Here, we challenge the common premise that direct genotoxic effects are central to nickel carcinogenesis and probably to that of other metals. Instead, we propose that it is formation of metal complexes with proteins and other molecules that changes cellular homeostasis and provides conditions for selection of cells with transformed phenotype. This is concordant with the major requirement for nickel carcinogenicity, which is prolonged action on the target tissue. If DNA is not the main nickel target, is there another unique molecule that can be attacked with carcinogenic consequences? Our recent observations indicate that ascorbate may be such a molecule. Nickel depletes intracellular ascorbate, which leads to the inhibition of cellular hydroxylases, manifested by the loss of hypoxia-inducible factor (HIF)-1alpha and -2alpha hydroxylation and hypoxia-like stress. Proline hydroxylation is crucial for collagen and extracellular matrix assembly as well as for assembly of other protein molecules that have collagen-like domains, including surfactants and complement. Thus, the depletion of ascorbate by chronic exposure to nickel could be deleterious for lung cells and may lead to lung cancer. Key words: ascorbate, carcinogenesis, collagens, extracellular matrix, hypoxia-inducible transcription factor, metals, nickel, protein hydroxylation.

Depletion of intracellular ascorbate by the carcinogenic metals nickel and cobalt results in the induction of hypoxic stress

Exposure of cells to carcinogenic compounds of nickel(II) and cobalt(II) causes activation of the HIF-1 transcription factor and up-regulates a battery of hypoxia-inducible genes. However, the mechanism of HIF-1 activation by these metals is not known. It was shown recently that hydroxylation of prolines in the HIFalpha subunit of HIF-1 is required for its binding with the von Hippel-Lindau tumor suppressor protein and the subsequent proteasomal destruction. Here we show that responsible prolyl hydroxylases are targets for both nickel(II) and cobalt(II) because degradation of a reporter protein containing the oxygen-dependent degradation domain (Pro-402/564) of HIFalpha was abolished in a von Hippel-Lindau-dependent manner in cells exposed to nickel(II) or cobalt(II). The enzymatic activity of prolyl hydroxylases depends on iron as the activating metal, 2-oxoglutarate as a co-substrate, and ascorbic acid as a cofactor. Hydroxylase activity can be impaired by the depletion of any of these factors. We found that exposure of cells to nickel(II) or cobalt(II) did not affect the level of intracellular iron. Instead, nickel(II) or cobalt(II) exposure greatly depleted intracellular ascorbate. Co-exposure of cells to metals and ascorbate resulted in the increase of intracellular ascorbate and reversed both metal-induced stabilization of HIF-1alpha and HIF-1-dependent gene transcription. Because ascorbate is essential for maintaining iron in prolyl hydroxylases in the active iron(II) state, we suggest that the observed depletion of ascorbate by nickel(II) or cobalt(II) favors iron oxidation and thus inactivation of the enzyme.

Light-Induced Byproducts of Vitamin C in Multivitamin Solutions

Background: When solutions of multivitamin preparations (MVPs) are exposed to light, H2O2 as well as organic peroxides are generated and the concentration of vitamin C decreases. The aim of this study was to determine, using mass spectrometry, whether the generation of oxidative byproducts of vitamin C, such as dehydroascorbate (DHA) and 2,3-diketogulonic acid (DKG), accounted for the reported decrease in ascorbic acid in MVPs exposed to light.

Methods: Mass spectrometry was used to document the formation of byproducts of ascorbic acid in solutions containing a MVP, vitamin C + riboflavin, and vitamin C + H2O2 + Fe2+. The involvement of ascorbic acid and H2O2 in the formation of organic peroxides was tested by measuring peroxide concentrations in solutions containing H2O2 with or without ascorbic acid and with or without Fe2+ before and after addition of catalase.

Results: The loss of ascorbic acid in photo-exposed MVPs was associated with the concomitant generation of byproducts different from DHA and DKG. Among them, one mass fingerprint was particularly observed with solutions of vitamin C + riboflavin exposed to ambient light as well as with the solution of vitamin C + H2O2 + Fe2+, suggesting a Fenton-like reaction. This fingerprint was associated with the formation of catalase-resistant peroxides.

Conclusion: Exposure of MVPs to light leads to the rapid loss of ascorbic acid and generation of specific byproducts that differ from DHA and DKG. The conversion of vitamin C into byproducts could be of biological importance in accounting for the decrease in ascorbic acid concentrations and the generation of organic peroxides in light-exposed MVPs.

Monitoring of oxidation steps of ascorbic acid redox reaction by kinetics-sensitive voltcoulometry in unsupported and supported aqueous solutions and real samples

Aqueous solutions of ascorbic acid in unsupported and supported aqueous solutions and real samples were studied by the kinetics-sensitive double-step voltcoulommetric method with the aim to contribute to a better understanding of its behavior in biological systems. The data obtained from measurements made on analytes prepared in the laboratory, as well as those made on real samples (some commercial orange drinks, flash of the fresh fruits) point to the redox reaction of L-ascorbic acid (L-AH2) being very sensitive to both the presence of dissolved gaseous species (O2, CO2) and the ionic strenght in the analyte. Either the dissolved gaseous species, or the higher ionic strength caused by both the presence of supporting electrolyte and increased total concentration of ascorbic acid, respectively, give birth to the degradation of L-AH2. Naturally, the highest percentage of L-AH2 was spotted in fresh fruit.

The ascorbate-driven reduction of extracellular ascorbate free radical by the erythrocyte is an electrogenic process

Erythrocytes can reduce extracellular ascorbate free radicals by a plasma membrane redox system using intracellular ascorbate as an electron donor. In order to test whether the redox system has electrogenic properties, we studied the effect of ascorbate free radical reduction on the membrane potential of the cells using the fluorescent dye 3,3'-dipropylthiadicarbocyanine iodide. It was found that the erythrocyte membrane depolarized when ascorbate free radicals were reduced. Also, the activity of the redox system proved to be susceptible to changes in the membrane potential. Hyperpolarized cells could reduce ascorbate free radical at a higher rate than depolarized cells. These results show that the ascorbate-driven reduction of extracellular ascorbate free radicals is an electrogenic process, indicating that vectorial electron transport is involved in the reduction of extracellular ascorbate free radical.

Ascorbic acid-induced browning of (+)-catechin in a model wine system

The ability of ascorbic acid to induce browning of (+)-catechin in a model wine system has been studied. A significant increase in absorbance at 440 nm was observed over 14 days when ascorbic acid was incubated at 45 degrees C with (+)-catechin in a model wine base. The onset of browning was delayed for about 2 days, although the length of the lag period was dependent on the amount of molecular oxygen in the headspace of the reaction system. The lag period was not observed when a preoxidized solution of ascorbic acid was used, suggesting that a product of ascorbic acid oxidation is responsible for the onset of browning. Hydrogen peroxide, when added directly to (+)-catechin in the model system, was not capable of producing the same degree of browning as that generated by ascorbic acid. Liquid chromotography evidence is presented to show that different reaction products are produced by ascorbic acid and hydrogen peroxide.

Characterization of dehydroascorbic acid solutions by liquid chromatography/mass spectrometry

The composition of a commercial dehydroascorbic acid (DA) solution at pH 2 was investigated by liquid chromatography/mass spectrometry (LC/MS) and liquid chromatography/tandem mass spectrometry (LC/MS/MS) to establish the nature of its different forms and its decomposition products. In freshly prepared solutions, dimeric forms of DA and the hydrated bicyclic hemiketal of DA are the species mainly present in solution. In the presence of light, the initial dimeric species disappears over time to give other dehydrated dimers some of which decompose to the monomer. The comparison of these data with similar data obtained for ascorbic acid (AA) solutions under the same experimental conditions revealed that, in the presence of light, the aging of such AA solutions gives rise to only the hemiketal form of DA, and that no dimeric species of DA were formed. The presence of the hemiketal form of DA was not revealed by analysis of the same AA solutions using the conventional LC/UV technique. The natural form of DA from the oxidation of AA is the hydrated bicyclic form.

A single calibration graph for the direct determination of ascorbic and dehydroascorbic acids by electrogenerated luminescence based on Ru(bpy)(3)2+ in aqueous solution

Ascorbic (H2A) and dehydroascorbic (DA) acids were for the first time directly determined in a single chromatographic run by means of the tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)(3)2+) based electrogenerated chemiluminescence (ECL) detection. For the first time, it was demonstrated that DA, a nonelectroactive compound, is ECL active and is responsible for the ECL behavior of H2A. This fact, together with the lack of a DA standard, suggested the use of a calibration graph obtained for H2A, for determining both analytes. The proven ECL activity of DA, together with literature data relative to the standard redox potentials of the different species coming from H2A, led to a reconsideration of the proposed ECL reaction mechanism for H2A. The role of the OH- ion in the reaction mechanism of the two analytes appeared to be crucial. H2A and DA could be separated by a suitable C18-reversed-phase HPLC column using an aqueous 30 mM H3PO4 solution as the mobile phase. The optimal ECL response was achieved by polarizing the working electrode at 1.150 Vvs SCE (standard calomel electrode) (oxidation diffusion limiting potential for both H2A and Ru(bpy)(3)2+). The Ru(bpy)(3)2+ solution, at pH 10 for carbonate buffer, was mixed to the eluent solution in a postcolumn system, obtaining, still at pH 10, the final 0.25 mM Ru(bpy)(3)2+ concentration. The detection limit found for the two analytes was 1 x 10(-7) M. The method was successfully applied to the determination of the analytes in a commercially available orange fruit juice.

Dehydroascorbic acid

Dehydroascorbic acid (DHA) is an important, interesting but somewhat enigmatic compound in biological systems. DHA has many unique properties that set it apart from ascorbic acid (AA), and DHA has functions that may be very important beyond that in the AA:DHA cycle. Future studies should help to better clarify chemical activity of DHA and related products that form from DHA, as well as to highlight the role DHA plays in normal cellular homeostasis.

Oxygen-accepting antioxidants which arise during ascorbate oxidation

To identify potential antioxidant compounds derived from ascorbate, the hydrogen peroxide-induced oxidation of ascorbate and dehydroascorbate was studied by gas-chromatography electron impact mass spectrometry and liquid-chromatography electrospray mass spectrometry in real time. Significant differences in ascorbate and dehydroascorbate oxidation occurred at pH 3.3 compared to pH 7.4. Of note, the primary species present in dehydroascorbate (DHA)-containing solutions at pH 7.4 had a spectrum consistent with diketogulonate. Hydrogen peroxide exposure of DHA-containing solutions formed threonate more rapidly at pH 7.4 than at pH 3.3. In these solutions, a 5-carbon species with mass spectral characteristics of a 3,4,5-trihydroxy-2-ketopentanoate appeared to be an intermediate between diketogulonate and threonate, and was more labile than other species in the presence of hydrogen peroxide. These data suggest that a 3,4,5-trihydroxy-2-ketopentanoate is potentially a key antioxidant compound in the ascorbate degradation cascade and in ascorbate-containing solutions at physiologic pH.