Mechanism of ascorbic acid oxidation by cytochrome b(561)

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.

Proton-Coupled Electron Transport in Two Distinct CYBASC Paralogs of Arabidopsis thaliana: A Comparative Characterization of Highly Conserved Tyrosine and Lysine Residues

CYBASC proteins are ascorbate (AscH^-) reducible, diheme b-containing integral membrane cytochrome b₅₆₁ proteins (cytb₅₆₁), which are proposed to be involved in AscH^- recycling and facilitation of iron absorption. Two distinct CYBASC paralogs from the plant Arabidopsis thaliana, Atcytb₅₆₁-A (A-paralog) and Atcytb₅₆₁-B (B-paralog), have been found to differ in their visible-spectral characteristics and their interaction with AscH^- and ferric iron chelates. A previously determined crystal structure of the B-paralog provides the first insights into the structural organization of a CYBASC member and implies hydrogen bonding between the substrate AscH^- and the conserved lysine residues at positions 77 (B-K77) and 81 (B-K81). The function of the highly conserved tyrosine at position 70 (B-Y70) is not obvious in the crystal structure, but its localization indicates the possible involvement in proton-coupled electron transfer. Here we show that B-Y70 plays a major role in the modulation of the oxidation-reduction midpoint potential of the high-potential heme, E_M(b_H), as well as in AscH^- oxidation. Our results support the involvement of the functionally conserved B-K77 in the stabilization of the dianion Asc^2-. These findings are supported by the crystal structure of the B-paralog, but a comparative biochemical and biophysical characterization of the A- and B-paralogs implied distinct and more complex functions of the corresponding residues A-Y69 and A-K76 in the A-paralog. Our results emphasize the need for a high-resolution crystal structure of the A-paralog to illuminate the differences in functional organization between the two paralogs.

Kinetic Isotope Effects and Hydrogen Tunnelling in PCET Oxidations of Ascorbate: New Insights into Aqueous Chemistry?

Recent experimental studies of kinetic isotope effects (KIE-s) and hydrogen tunnelling comprising three proton-coupled electron transfer (PCET) oxidations of ascorbate monoanion, (a) in aqueous reaction solutions, (b) in the mixed water-organic cosolvent systems, (c) in aqueous solutions of various salts and (d) in fairly diluted aqueous solutions of the various partial hydrophobes are reviewed. A number of new insights into the wealth of the kinetic isotope phenomena in the PCET reactions have been obtained. The modulation of KIE-s and hydrogen tunnelling observed when partially hydrophobic solutes are added into water reaction solution, in the case of fairly diluted solutions is revealed as the strong linear correlation of the isotopic ratios of the Arrhenius prefactors Ah/Ad and the isotopic differences in activation energies ΔE_a (D,H). The observation has been proposed to be a signature of the involvement of the collective intermolecular excitonic vibrational dynamics of water in activation processes and aqueous chemistry.

Molecular Mechanisms of Oxygen Activation and Hydrogen Peroxide Formation in Lytic Polysaccharide Monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes for the degradation of recalcitrant polysaccharides such as chitin and cellulose. Unlike classical hydrolytic enzymes (cellulases), LPMOs catalyze the cleavage of the glycosidic bond via an oxidative mechanism using oxygen and a reductant. The full enzymatic molecular mechanisms, starting from the initial electron transfer from a reductant to oxygen activation and hydrogen peroxide formation, are not yet understood. Using quantum mechanics/molecular mechanics (QM/MM) metadynamics simulations, we have uncovered the oxygen activation mechanisms by LPMO in the presence of ascorbic acid, one of the most-used reductants in LPMOs assays. Our simulations capture the sequential formation of Cu(II)-O₂ ^- and Cu(II)-OOH^- intermediates via facile H atom abstraction from ascorbate. By investigating all the possible reaction pathways from the Cu(II)-OOH^- intermediate, we ruled out Cu(II)-O^• - formation via direct O-O cleavage of Cu(II)-OOH^-. Meanwhile, we identified a possible pathway in which the proximal O atom of Cu(II)-OOH^- abstracts a hydrogen atom from ascorbate, leading to Cu(I) and H₂O₂. The in-situ-generated H₂O₂ either converts to LPMO-Cu(II)-O^• - via a homolytic reaction, or diffuses into the bulk water in an uncoupled pathway. The competition of these two pathways is strongly dependent on the binding of the carbohydrate substrate, which plays a role in barricading the in-situ-generated H₂O₂ molecule, preventing its diffusion from the active site into the bulk water. Based on the present results, we propose a catalytic cycle of LPMOs that is consistent with the experimental information available. In particular, it explains the enigmatic substrate dependence of the reactivity of the LPMO with H₂O₂.

Chemical Transport Knockout for Oxidized Vitamin C, Dehydroascorbic Acid, Reveals Its Functions in vivo

Despite its transport by glucose transporters (GLUTs) in vitro, it is unknown whether dehydroascorbic acid (oxidized vitamin C, DHA) has any in vivo function. To investigate, we created a chemical transport knockout model using the vitamin C analog 6-bromo-ascorbate. This analog is transported on sodium-dependent vitamin C transporters but its oxidized form, 6-bromo-dehydroascorbic acid, is not transported by GLUTs. Mice (gulo^−/−) unable to synthesize ascorbate (vitamin C) were raised on 6-bromo-ascorbate. Despite normal survival, centrifugation of blood produced hemolysis secondary to near absence of red blood cell (RBC) ascorbate/6-bromo-ascorbate. Key findings with clinical implications were that RBCs in vitro transported dehydroascorbic acid but not bromo-dehydroascorbic acid; RBC ascorbate in vivo was obtained only via DHA transport; ascorbate via DHA transport in vivo was necessary for RBC structural integrity; and internal RBC ascorbate was essential to maintain ascorbate plasma concentrations in vitro/in vivo.

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Red cells in vivo obtain vitamin C (ascorbate) by dehydroascorbic acid transport.

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Red blood cell ascorbate is necessary to maintain red blood cell structural integrity.

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Red blood cell ascorbate maintains external plasma ascorbate concentrations in vivo by transmembrane electron transfer.

In animals and humans, it is unknown whether the oxidized form of vitamin C, termed dehydroascorbic acid, has a physiologic purpose. Using a mouse model and a custom-synthesized vitamin C analog, we show that red blood cells obtain their vitamin C by transport of dehydroascorbic acid, instead of vitamin C itself. The transported material is reduced inside and has at least two physiologic functions. One is to maintain structural integrity of red blood cells, and the other is to maintain vitamin C in the liquid part of blood, plasma.

Effects of reaction parameters on self-degradation of L-ascorbic acid and self-degradation kinetics

The degradation behavior of L-ascorbic acid (ASA) was investigated under different parameters of temperature, time, and pH. Higher temperatures and longer times accelerated the ASA degradation. Degradation product distributions changed with different pH values. As solution pH of 4.5 was beneficial for formation of uncolored intermediate products with an absorbance maximum at 294 nm. Formation of brown products was promoted at pH values from 5.8 to 6.8 with an absorbance maximum at 420 nm. Under different pH conditions, volatile products formation varied. Furfural and derivatives of furan were primary products due to the effects of pH. The non-enzymatic selfdegradation behavior of ASA was characteristic of first-order kinetics based on a classic dynamic model. Activation energy values varied under different pH values. An ASA degradation mechanism and pathway are proposed.

Reactive Oxygen Species Production Mediated by Humic-like Substances in Atmospheric Aerosols: Enhancement Effects by Pyridine, Imidazole, and Their Derivatives

Ambient particulate matter (PM) can cause adverse health effects via their ability to produce reactive oxygen species (ROS). Humic-like substances (HULIS), a complex mixture of amphiphilic organic compounds, have been demonstrated to contain the majority of redox activity in the water-extractable organic fraction of PM. Reduced organic nitrogen compounds, such as alkaloids resulting from biomass burning emissions, are among HULIS constituents. In this study, we examined the redox activities of pyridine, imidazole and their alkyl derivatives using a cell-free dithiothreitol (DTT) assay under simulated physiological conditions (37 °C, pH = 7.40). These compounds were found to have little redox activity on their own as measured by the DTT assay, but they enhanced ROS generation catalyzed by 1,4-naphthoquinone (as a model quinone compound) and HULIS isolated from multiple aerosol samples. The enhancement effect by the individual nitrogen-containing bases was determined to be proportional to their amount in the assay solutions. It is postulated that the underlying mechanism involves the unprotonated N atom acting as a H-bonding acceptor to facilitate hydrogen-atom transfer in the ROS generation cycle. The enhancement capability was found to increase with their basicity (i.e., pKa of their conjugated acids, BH(+)), consistent with the proposed mechanism for enhancement. Among the imidazole homologues, a linear relationship was observed between the enhancement factors (in log scale) of the unprotonated form of the imidazole compounds (B) and the pKa of their conjugated acids (BH(+)). This relationship predicts that the range of alkylimidazole homologues (C6-C13) observed in atmospheric HULIS would be 1.5-4.4 times more effective than imidazole in facilitating HULIS-mediated ROS generation. Our work reveals that the ability of atmospheric PM organics to catalyze generation of ROS in cells could be affected by coexisting redox inactive organic constituents and suggests further work deploying multiple assays be conducted to quantify redox capabilities and enhancement effects of the HULIS components.

Moving protons and electrons in biomimetic systems

An enormous variety of biological redox reactions are accompanied by changes in proton content at enzyme active sites, in their associated cofactors, in substrates and/or products, and between protein interfaces. Understanding this breadth of reactivity is an ongoing chemical challenge. A great many workers have developed and investigated biomimetic model complexes to build new ways of thinking about the mechanistic underpinnings of such complex biological proton-coupled electron transfer (PCET) reactions. Of particular importance are those model reactions that involve transfer of one proton (H(+)) and one electron (e(-)), which is equivalent to transfer of a hydrogen atom (H(•)). In this Current Topic, we review key concepts in PCET reactivity and describe important advances in biomimetic PCET chemistry, with a special emphasis on research that has enhanced efforts to understand biological PCET reactions.

Catalytic oxygen activation versus autoxidation for industrial applications: a physicochemical approach

The activation and use of oxygen for the oxidation and functionalization of organic substrates are among the most important reactions in a chemist's toolbox. Nevertheless, despite the vast literature on catalytic oxidation, the phenomenon of autoxidation, an ever-present background reaction that occurs in virtually every oxidation process, is often neglected. In contrast, autoxidation can affect the selectivity to a desired product, to those dictated by pure free-radical chain pathways, thus affecting the activity of any catalyst used to carry out a reaction. This critical review compares catalytic oxidation routes by transition metals versus autoxidation, particularly focusing on the industrial context, where highly selective and "green" processes are needed. Furthermore, the application of useful tests to discriminate between different oxygen activation routes, especially in the area of hydrocarbon oxidation, with the aim of an enhanced catalyst design, is described and discussed. In fact, one of the major targets of selective oxidation is the use of molecular oxygen as the ultimate oxidant, combined with the development of catalysts capable of performing the catalytic cycle in a real energy and cost effective manner on a large scale. To achieve this goal, insights from metallo-proteins that could find application in some areas of industrial catalysis are presented, as well as considering the physicochemical principles that are fundamental to oxidation and autoxidation processes.

Proton tunnelling and promoting vibrations during the oxidation of ascorbate by ferricyanide?

A combination of the temperature- and pressure-dependencies of the kinetic isotope effect on the proton coupled electron transfer during ascorbate oxidation by ferricyanide suggests that this reference reaction may exploit vibrationally assisted quantum tunnelling of the transferred proton.

Colloidal beading: sonication-induced stringing of selenium particles

We report the preparation of monodispersed Se colloidal aggregates (dimers and trimers) via sonication-induced aggregation of spherical monomers. Control over the size and morphology of the products was achieved by changing the aging and sonication times, respectively. The possible mechanisms for the formation of colloidal aggregates were discussed. This method can provide a simple and versatile approach to the production of colloidal molecules of particles composed of different materials, which will be useful for fundamental studies related to colloidal systems.

A convenient method for measuring blood ascorbate concentrations in patients receiving high-dose intravenous ascorbate

OBJECTIVE

A simple method of using fingerstick blood glucose (FSBG) monitors to estimate blood ascorbate values after high-dose intravenous (IV) ascorbate infusion is evaluated as a substitution for high-performance liquid chromatography (HPLC) measurement.

METHODS

In 33 participants, readings from FSBG monitors were taken before and after IV ascorbate infusions at various time points, with the postinfusion FSBG readings subtracted from the baseline glucose readings. The results of the subtractions (AAFSBG) were correlated with ascorbate concentrations detected by HPLC (AAHPLC).

RESULTS

A linear regression was found between ascorbate concentrations detected by the fingerstick method (AAFSBG) and by HPLC (AAHPLC). The linear correlations were identical in healthy subjects, diabetic subjects, and cancer patients. Analysis of variance obtained an AAFSBG/AAHPLC ratio of 0.90, with a 90% confidence interval of (0.69, 1.20). The corrections of AAFSBG improved similarity to AAHPLC but did not significantly differ from the uncorrected values.

CONCLUSION

The FSBG method can be used as an approximate estimation of high blood ascorbate concentration after IV ascorbate (>50 mg/dL, or 2.8 mM) without correction. However, this measurement is not accurate in detecting lower or baseline blood ascorbate. It is also important to highlight that in regard to glucose monitoring, FSBG readings will be erroneously elevated following IV ascorbate use and insulin should not be administered to patients based on these readings.

Redox regulation of calcium signaling in cancer cells by ascorbic Acid involving the mitochondrial electron transport chain

Previously, we have reported that ascorbic acid regulates calcium signaling in human larynx carcinoma HEp-2 cells. To evaluate the precise mechanism of Ca²⁺ release by ascorbic acid, the effects of specific inhibitors of the electron transport chain components on mitochondrial reactive oxygen species (ROS) production and Ca²⁺ mobilization in HEp-2 cells were investigated. It was revealed that the mitochondrial complex III inhibitor (antimycin A) amplifies ascorbate-induced Ca²⁺ release from intracellular stores. The mitochondrial complex I inhibitor (rotenone) decreases Ca²⁺ release from intracellular stores in HEp-2 cells caused by ascorbic acid and antimycin A. In the presence of rotenone, antimycin A stimulates ROS production by mitochondria. Ascorbate-induced Ca²⁺ release in HEp-2 cells is shown to be unaffected by catalase. The results obtained suggest that Ca²⁺ release in HEp-2 cells caused by ascorbic acid is associated with induced mitochondrial ROS production. The data obtained are in line with the concept of redox signaling that explains oxidant action by compartmentalization of ROS production and oxidant targets.

What are the differences between ascorbic acid and NADH as hydride and electron sources in vivo on thermodynamics, kinetics, and mechanism?

Ascorbic acid (AscH(2)) and dihydronicotinamide adenine dinucleotide (NADH) are two very important natural redox cofactors, which can be used as hydride, electron, and hydrogen atom sources to take part in many important bioreduction processes in vivo. The differences of the two natural reducing agents as hydride, hydrogen atom, and electron donors in thermodynamics, kinetics, and mechanisms were examined by using 5,6-isopropylidene ascorbate (iAscH(-)) and β-D-glucopyranosyl-1,4-dihydronicotinamide acetate (GluNAH) as their models, respectively. The results show that the hydride-donating ability of iAscH(-) is smaller than that of GluNAH by 6.0 kcal/mol, but the electron-donating ability and hydrogen-donating ability of iAscH(-) are larger than those of GluNAH by 20.8 and 8.4 kcal/mol, respectively, which indicates that iAscH(-) is a good electron donor and a good hydrogen atom donor, but GluNAH is a good hydride donor. The kinetic intrinsic barrier energy of iAscH(-) to release hydride anion in acetonitrile is larger than that of GluNAH to release hydride anion in acetonitrile by 6.9 kcal/mol. The mechanisms of hydride transfer from iAscH(-) and GluNAH to phenylxanthium perchlorate (PhXn(+)), a well-known hydride acceptor, were examined, and the results show that hydride transfer from GluNAH adopted a one-step mechanism, but the hydride transfer from iAscH(-) adopted a two-step mechanism (e-H(•)). The thermodynamic relation charts (TRC) of the iAscH(-) family (including iAscH(-), iAscH(•), iAsc(•-), and iAsc) and of the GluNAH family (including GluNAH, GluNAH(•+), GluNA(•), and GluNA(+)) in acetonitrile were constructed as Molecule ID Cards of iAscH(-) and of GluNAH in acetonitrile. By using the Molecule ID Cards of iAscH(-) and GluNAH, the character chemical properties not only of iAscH(-) and GluNAH but also of the various reaction intermediates of iAscH(-) and GluNAH all have been quantitatively diagnosed and compared. It is clear that these comparisons of the thermodynamics, kinetics, and mechanisms between iAscH(-) and GluNAH as hydride and electron donors in acetonitrile should be quite important and valuable to diagnose and understand the different roles and functions of ascorbic acid and NADH as hydride, hydrogen atom, and electron sources in vivo.

Heterologous production and characterisation of two distinct dihaem-containing membrane integral cytochrome b(561) enzymes from Arabidopsis thaliana in Pichia pastoris and Escherichia coli cells

Cytochrome (cyt) b(561) proteins are dihaem-containing membrane proteins, belonging to the CYBASC (cytochrome-b(561)-ascorbate-reducible) family, and are proposed to be involved in ascorbate recycling and/or the facilitation of iron absorption. Here, we present the heterologous production of two cyt b(561) paralogs from Arabidopsis thaliana (Acytb(561)-A, Acytb(561)-B) in Escherichia coli and Pichia pastoris, their purification, and initial characterisation. Spectra indicated that Acytb(561)-A resembles the best characterised member of the CYBASC family, the cytochrome b(561) from adrenomedullary chromaffin vesicles, and that Acytb(561)-B is atypical compared to other CYBASC proteins. Haem oxidation-reduction midpoint potential (E(M)) values were found to be fully consistent with ascorbate oxidation activities and Fe(3+)-chelates reductase activities. The ascorbate dependent reduction and protein stability of both paralogs were found to be sensitive to alkaline pH values as reported for the cytochrome b(561) from chromaffin vesicles. For both paralogs, ascorbate-dependent reduction was inhibited and the low-potential haem E(M) values were affected significantly by incubation with diethyl pyrocarbonate (DEPC) in the absence of ascorbate. Modification with DEPC in the presence of ascorbate left the haem E(M) values unaltered compared to the unmodified proteins. However, ascorbate reduction was inhibited. We concluded that the ascorbate-binding site is located near the low-potential haem with the Fe(3+)-chelates reduction-site close to the high-potential haem. Furthermore, inhibition of ascorbate oxidation by DEPC treatment occurs not only by lowering the haem E(M) values but also by an additional modification affecting ascorbate binding and/or electron transfer. Analytical gel filtration experiments suggest that both cyt b(561) paralogs exist as homodimers.

Proton-coupled electron transfer reactions at a heme-propionate in an iron-protoporphyrin-IX model compound

A heme model system has been developed in which the heme-propionate is the only proton donating/accepting site, using protoporphyrin IX-monomethyl esters (PPIX(MME)) and N-methylimidazole (MeIm). Proton-coupled electron transfer (PCET) reactions of these model compounds have been examined in acetonitrile solvent. (PPIX(MME))Fe(III)(MeIm)(2)-propionate (Fe(III)~CO(2)) is readily reduced by the ascorbate derivative 5,6-isopropylidine ascorbate to give (PPIX(MME))Fe(II)(MeIm)(2)-propionic acid (Fe(II)~CO(2)H). An excess of the hydroxylamine TEMPOH or of hydroquinone similarly reduces Fe(III)~CO(2), and TEMPO and benzoquinone oxidize Fe(II)~CO(2)H to return to Fe(III)~CO(2). The measured equilibrium constants, and the determined pK(a) and E(1/2) values, indicate that Fe(II)~CO(2)H has an effective bond dissociation free energy (BDFE) of 67.8 ± 0.6 kcal mol(-1). In these PPIX models, electron transfer occurs at the iron center and proton transfer occurs at the remote heme propionate. According to thermochemical and other arguments, the TEMPOH reaction occurs by concerted proton-electron transfer (CPET), and a similar pathway is indicated for the ascorbate derivative. Based on these results, heme propionates should be considered as potential key components of PCET/CPET active sites in heme proteins.

Tuning of the thermochemical and kinetic properties of ascorbate by its local environment: solution chemistry and biochemical implications

Ascorbate (vitamin C) is a ubiquitous biological cofactor. While its aqueous solution chemistry has long been studied, many in vivo reactions of ascorbate occur in enzyme active sites or at membrane interfaces, which have varying local environments. This report shows that the rate and driving force of oxidations of two ascorbate derivatives by the TEMPO radical (2,2',6,6'-tetramethylpiperidin-1-oxyl) in acetonitrile are very sensitive to the presence of various additives. These reactions proceed by the transfer of a proton and an electron (a hydrogen atom), as is typical of biological ascorbate reactions. The measured rate and equilibrium constants vary substantially with added water or other polar solutes in acetonitrile solutions, indicating large shifts in the reducing power of ascorbate. The correlation of rate and equilibrium constants indicates that this effect has a thermochemical origin rather than being a purely kinetic effect. This contrasts with previous examples of solvent effects on hydrogen atom transfer reactions. Potential biological implications of this apparently unique effect are discussed.

Importance of the conserved lysine 83 residue of Zea mays cytochrome b(561) for ascorbate-specific transmembrane electron transfer as revealed by site-directed mutagenesis studies

Cytochromes b(561), a novel class of transmembrane electron transport proteins residing in a large variety of eukaryotic cells, have a number of common structural features including six hydrophobic transmembrane alpha-helices and two heme ligation sites. We found that recombinant Zea mays cytochrome b(561) obtained by a heterologous expression system using yeast Pichia pastoris cells could utilize the ascorbate/mondehydroascorbate radical as a physiological electron donor/acceptor. We found further that a concerted proton/electron transfer mechanism might be operative in Z. mays cytochrome b(561) as well upon the electron acceptance from ascorbate to the cytosolic heme center. The well-conserved Lys(83) residue in a cytosolic loop was found to have a very important role(s) for the binding of ascorbate and the succeeding electron transfer via electrostatic interactions based on the analyses of three site-specific mutants, K83A, K83E, and K83D. Further, unusual behavior of the K83A mutant in pulse radiolysis experiments indicated that Lys(83) might also be responsible for the intramolecular electron transfer to the intravesicular heme. On the other hand, pulse radiolysis experiments on two site-specific mutants, S118A and W122A, for the well-conserved residues in the putative monodehydroascorbate radical binding site showed that their electron transfer activities to the monodehydroascorbate radical were very similar to those of the wild-type protein, indicating that Ser(118) and Trp(122) do not have major roles for the redox events on the intravesicular side.

Surprisingly long-lived ascorbyl radicals in acetonitrile: concerted proton-electron transfer reactions and thermochemistry

Proton-coupled electron transfer (PCET) reactions and thermochemistry of 5,6-isopropylidene ascorbate (iAscH-) have been examined in acetonitrile solvent. iAscH- is oxidized by 2,4,6-tBu3C6H2O. and by excess TEMPO. to give the corresponding 5,6-isopropylidene ascorbyl radical anion (iAsc.-), which persists for hours at 298 K in dry MeCN solution. The stability of iAsc.- is surprising in light of the transience of the ascorbyl radical in aqueous solutions and is due to the lack of the protons needed for radical disproportionation. A concerted proton-electron transfer (CPET) mechanism is indicated for the reactions of iAscH-. Redox potential, pKa and equilibrium measurements define the thermochemical landscape for 5,6-isopropylidene ascorbic acid and its derivatives in MeCN. These measurements give an O-H bond dissociation free energy (BDFE) for iAscH- of 65.4 +/- 1.5 kcal mol-1 in MeCN. Similar studies on underivatized ascorbate indicate a BDFE of 67.8 +/- 1.2 kcal mol-1. These values are much lower than the aqueous BDFE for ascorbate of 74.0 +/- 1.5 kcal mol-1 derived from reported data.

Molecular mechanisms of subtype-specific inhibition of neuronal T-type calcium channels by ascorbate

T-type Ca2+ channels (T-channels) are involved in the control of neuronal excitability and their gating can be modulated by a variety of redox agents. Ascorbate is an endogenous redox agent that can function as both an anti- and pro-oxidant. Here, we show that ascorbate selectively inhibits native Ca(v)3.2 T-channels in peripheral and central neurons, as well as recombinant Ca(v)3.2 channels heterologously expressed in human embryonic kidney 293 cells, by initiating the metal-catalyzed oxidation of a specific, metal-binding histidine residue in domain 1 of the channel. Our biophysical experiments indicate that ascorbate reduces the availability of Ca(v)3.2 channels over a wide range of membrane potentials, and inhibits Ca(v)3.2-dependent low-threshold-Ca2+ spikes as well as burst-firing in reticular thalamic neurons at physiologically relevant concentrations. This study represents the first mechanistic demonstration of ion channel modulation by ascorbate, and suggests that ascorbate may function as an endogenous modulator of neuronal excitability.

Heterologous expression and site-directed mutagenesis of an ascorbate-reducible cytochrome b561

Cytochromes b561 (Cyts b561) are ubiquitous membrane proteins catalyzing ascorbate-mediated trans-membrane electron transfer. A heterologous expression system in Saccharomyces cerevisiae was developed to study their structure-function relationship. Recombinant mouse chromaffin granule Cyt b561 (CGCytb) shows spectral characteristics, ascorbate reducibility, and redox potentials identical to that of the native bovine protein. Moreover, the reconstituted recombinant protein mediated trans-membrane electron transport with kinetic characteristics similar to that of bovine CGCytb. Site-directed mutant analysis supports the presence of two hemes coordinated by the highly conserved His pairs H52/H120 and H86/H159. Reduction of CGCytb by ascorbate showed biphasic kinetics (Kd1: 0.016 +/- 0.005 mM, Kd2: 1.24 +/- 0.19 mM). Mutation of a well-conserved Arg residue (R72) abolished high affinity CGCytb reduction by ascorbate, indicating that this residue may be critical for substrate binding. On the other hand, mutation of a Lys previously suggested to play a role in ascorbate binding (K83), did not affect the ascorbate-mediated reduction of the protein.

Ascorbate is particularly effective against LDL oxidation in the presence of iron(III) and homocysteine/cystine at acidic pH

Metal-catalyzed LDL oxidation is enhanced by the presence of homocysteine. In this study, the effectiveness of ascorbic acid against low-density lipoprotein (LDL) oxidation by iron(III) and copper(II) in the presence of homocysteine and the main plasma disulfide cystine was investigated. Relative to the degree of LDL oxidation reached in the absence of antioxidants, ascorbic acid was particularly effective against iron-catalyzed LDL oxidation at pH 6.0. This can be explained from its stability under acidic conditions and is likely to be important in ischemia, in inflammation and exhausting exercise. At pH 7.4, an ascorbic acid concentration at least as high as the concentration of homocysteine might be necessary to efficiently inhibit LDL oxidation by iron(III) and copper(II) in the presence of homocysteine and cystine. Histidine increased the efficiency of ascorbic acid as an antioxidant against copper-mediated oxidation in this system. The capacity of homocysteine to regenerate ascorbic acid from dehydroascorbic acid appeared to play a minor role in inhibition of ascorbic acid oxidation by copper as compared to copper chelation by homocysteine.

Imidazole facilitates electron transfer from organic reductants

In cyclic voltammetry studies at pH 8, imidazole facilitates oxidation of organic compounds that normally lose hydrogen atoms. High concentrations of imidazole shift the oxidizing wave of ascorbic acid, 2,3-dimethoxy-5-methyl-1,4-hydroquinone, and the vitamin E analogue Trolox toward lower potentials. By contrast, imidazole has no effect on the cyclic voltammogram of methyl viologen, which undergoes electron rather than hydrogen-atom transfer. The effect of imidazole is observed at pH 8.0 but only to a lesser extent at pH 5.5 indicating that imidazole must be unprotonated to facilitate oxidation. Digital simulation shows that these results are consistent with a mechanism in which imidazole acts as a proton acceptor permitting concerted proton/electron transfer by the organic reductant.

Thermodynamics and kinetics of proton-coupled electron transfer: stepwise vs. concerted pathways

Reactions that involve transfer of an electron and a proton can proceed by stepwise pathways involving initial electron transfer (ET) or initial proton transfer (PT), or by a concerted pathway without an intermediate. The concerted mechanism is termed proton-coupled electron transfer (PCET). Understanding such reactions requires knowledge of the thermodynamics of the possible ET, PT, and PCET steps. Many reactions have a large thermochemical bias favoring the PCET pathway. This bias is often sufficient to rule out stepwise mechanisms. The DeltaG degrees for ET, PT, or PCET has a strong influence on the rate of that step. Using the terminology of Marcus theory, PT and PCET reactions at C-H bonds have higher intrinsic barriers than such reactions at O-H or N-?H bonds. The intrinsic barriers to ET and PCET are often similar when there is a small intrinsic barrier to PT. Reactions with a thermochemical bias toward PCET and with similar intrinsic barriers for all the pathways are most likely to occur by concerted PCET.

Tissue-specific expression and developmental regulation of cytochrome b561 genes in Arabidopsis thaliana and Raphanus sativus

Ascorbate (Asc) is an essential molecule in many aspects of development and stress responses in plants and animals. Cytochromes b561 (cyts b561) are tightly coupled to Asc homeostasis. These proteins are found in mammalian tissues, where they are involved in the regeneration of Asc, serving the synthesis of catecholamine neurotransmitters, and in intestinal iron reduction. Plant genomes encode homologous membrane-associated, Asc-reducible cyts b561. The expression of these proteins in plants, however, has so far not been studied. We have now examined the expression of two Arabidopsis thaliana cyt b561-encoding genes-Artb561-1 and Artb561-2-using relative-quantitative RT-PCR and in situ hybridization (ISH) techniques. The genes show overlapping and distinct tissue- and organ-specific expression patterns. Transcripts of both genes are found in leaf epidermal cells, and expression seems to correlate with leaf maturation and cessation of cell elongation. Both genes are also expressed in the epidermal cell layer of stems and roots in the L1 layer of the shoot apex, in the vascular system of leaves, stems and roots, and in the root pericycle. In addition, Artb561-1 is expressed in the root cap, whereas Artb561-2 mRNA is found in the epidermis of lateral roots, in the root meristem, and in unfertilized ovules. These observations provide important information for the elucidation of the physiological function of cyts b561 in plants.

A phylogenetic study of cytochrome b561 proteins

BACKGROUND

As an antioxidant and cofactor to numerous metabolic enzymes, ascorbate has an essential role in plants and animals. Cytochromes b561 constitute a class of intrinsic membrane proteins involved in ascorbate regeneration. Despite their importance in ascorbate metabolism, no evolutionary analysis has been presented so far on this newly described protein family.

RESULTS

Cytochromes b561 have been identified in a large number of phylogenetically distant species, but are absent in fungi and prokaryotes. Most species contain three or four cytochrome b561 paralogous proteins, and the encoding genes usually have four or five exons. At the protein level, sequence similarities are rather low between cytochromes b561 within a single species (34-45% identity), and among phylogenetically distant species (around 30% identity). However, particular structural features characterizing this protein family are well conserved in members from all species investigated. These features comprise six transmembrane helices, four strictly conserved histidine residues, probably coordinating the two heme molecules, and putative ascorbate and monodehydro-ascorbate (MDHA) substrate-binding sites. Analysis of plant cytochromes b561 shows a separation between those from monocotyledonous and dicotyledonous species in a phylogenetic tree.

CONCLUSIONS

All cytochromes b561 have probably evolved from a common ancestral protein before the separation of plants and animals. Their phyletic distribution mirrors the use of ascorbate as primary antioxidant, indicating their role in ascorbate homeostasis and antioxidative defense. In plants, the differentiation into four cytochrome b561 isoforms probably occurred before the separation between monocots and dicots.

[Alloxan radical-induced generation of reactive oxygen species in the reaction system of alloxan with ascorbate]

The diabetogenic action of alloxan is thought to be initiated by generation of reactive oxygen species (ROS). Ascorbate can be an antioxidant in a predominantly aqueous environment, such as plasma and extracellular fluids. We have investigated the generation of ROS in the interaction of alloxan with ascorbate. Rapid oxygen consumption was observed in the reactin system of alloxan with ascorbate. The oxygen consumption was suppressed by superoxide dismutase and catalase, suggesting that superoxide and hydrogen peroxide could be generated in the reaction system. In addition, the generation of alloxan radical, an electron reductance of alloxan, and ascorbate free radical (AFR), an electron oxidant of ascorbate, was observed using electron spin resonance (ESR). Under anaerobic conditions, the ESR signal intensity of alloxan radical was significantly increased in comparison with that under aerobic conditions, whereas the intensity of AFR was significantly decreased. These results suggest that alloxan radical and AFR were generated in the reaction system of alloxan with ascorbate, and that the alloxan radical but not AFR reacted with molecular oxygen, resulting in the generation of ROS.