Functional coupling between enzymes of the chromaffin granule membrane

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.

Genetic diversity of ATP8 and ATP6 genes is associated with high-altitude adaptation in yak

ATP synthase 8 (ATP8) and ATPase synthase 6 (ATP6) play an important role in mitochondrial ATPase assembly. Mutations in either of these units could affect the ATP processing and the respiration chain in mitochondria. To find out if there were differences in gene diversity between Tibetan yaks and domestic cattle, we sequenced the ATP8 and ATP6 genes in 66 Tibetan yaks and 81 domestic cattle. We identified 20 SNPs in the ATP8 gene and 60 SNPs in the ATP6 gene. Ten SNPs detected in ATP8 were probably positively associated with high-altitude adaptation, of which SNPs m.8164 G > A, m.8210 G > A, m.8231 C > T and m. 8249 C > T resulted in amino acid changes. Similarly, SNPs m.8308A > G, m.8370A > C, m.8514G > A of ATP6 also appeared to be associated with high-altitude adaptability. Specifically, m.8308 A > G, located in the overlap region, might bring in a conserved region found in cytochrome b561 which play an important role in iron regulation, thus it might help the Tibetan yaks with this mutation to utilize rare oxygen efficiently. Considering all mutations, three of eight haplotypes identified in gene ATP8 were present only in Tibetan yaks, and six (H3 to H8) out of 21 haplotypes (H1 to H21) in gene ATP6 were restricted to Tibetan yaks. Haplotypes present only in Tibetan yaks could be positively associated with high-altitude adaptation.

A pulse radiolysis study of the dynamics of ascorbic acid free radicals within a liposomal environment

The dynamics of free-radical species in a model cellular system are examined by measuring the formation and decay of ascorbate radicals within a liposome with pulse radiolysis techniques. Upon pulse radiolysis of an N2O-saturated aqueous solution containing ascorbate-loaded liposome vesicles, ascorbate radicals are formed by the reaction of OH(·) radicals with ascorbate in unilamellar vesicles exclusively, irrespective of the presence of vesicle lipids. The radicals are found to decay rapidly compared with the decay kinetics in an aqueous solution. The distinct radical reaction kinetics in the vesicles and in bulk solution are characterized, and the kinetic data are analyzed.

Electron transfer reactions of candidate tumor suppressor 101F6 protein, a cytochrome b561 homologue, with ascorbate and monodehydroascorbate radical

The candidate tumor suppressor 101F6 protein is a homologue of adrenal chromaffin granule cytochrome b561, which is involved in the electron transfer from cytosolic ascorbate to intravesicular monodehydroascorbate radical. Since the tumor suppressor activity of 101F6 was enhanced in the presence of ascorbate, it was suggested that 101F6 might utilize a similar transmembrane electron transfer reaction. Detailed kinetic analyses were conducted on the detergent-solubilized recombinant human 101F6 for its electron transfer reactions with ascorbate and monodehydroascorbate radical by stopped-flow and pulse radiolysis techniques. The reduction of oxidized 101F6 with ascorbate was found to be independent of pH in contrast to those observed for chromaffin granule and Zea mays cytochromes b561 in which both cytochromes exhibited very slow rates at pH 5.0 but faster at pH 6.0 and 7.0. The absence of the inhibition for the electron acceptance from ascorbate upon the treatment with diethyl pyrocarbonate suggested that 101F6 might not utilize a "concerted proton/electron transfer mechanism". The second-order rate constant for the electron donation from the ascorbate-reduced 101F6 to the pulse-generated monodehydroascorbate radical was found to be 5.0 × 10(7) M(-1 )s(-1), about 2-fold faster than that of bovine chromaffin granule cytochrome b561 and about five times faster than that of Zea mays cytochrome b561, suggesting that human 101F6 is very effective for regenerating ascorbate from monodehydroascorbate radical in cells. Present observations suggest that 101F6 employs distinct electron transfer mechanisms on both sides of the membranes from those of other members of cytochrome b561 protein family.

Perturbation of dopamine metabolism by 3-amino-2-(4'-halophenyl)propenes leads to increased oxidative stress and apoptotic SH-SY5Y cell death

We have recently characterized a series of 3-amino-2-phenyl-propene (APP) derivatives as reversible inhibitors for the bovine adrenal chromaffin granule vesicular monoamine transporter (VMAT) that have been previously characterized as potent irreversible dopamine-beta-monooxygenase (DbetaM) and monoamine oxidase (MAO) inhibitors. Halogen substitution on the 4'-position of the aromatic ring gradually increases VMAT inhibition potency from 4'-F to 4'-I, parallel to the hydrophobicity of the halogen. We show that these derivatives are taken up into both neuronal and non-neuronal cells, and into resealed chromaffin granule ghosts efficiently through passive diffusion. Uptake rates increased according to the hydrophobicity of the 4'-substituent. More importantly, these derivatives are highly toxic to human neuroblastoma SH-SY5Y but not toxic to M-1, Hep G2, or human embryonic kidney 293 non-neuronal cells at similar concentrations. They drastically perturb dopamine (DA) uptake and metabolism in SH-SY5Y cells under sublethal conditions and are able to deplete both vesicular and cytosolic catecholamines in a manner similar to that of amphetamines. In addition, 4'-IAPP treatment significantly increases intracellular reactive oxygen species (ROS) and decreases glutathione (GSH) levels in SH-SY5Y cells, and cell death is significantly attenuated by the common antioxidants alpha-tocopherol, N-acetyl-l-cysteine and GSH, but not by the nonspecific caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone. DNA fragmentation analysis further supports that cell death is probably due to a caspase-independent ROS-mediated apoptotic pathway. Based on these and other findings, we propose that drastic perturbation of DA metabolism in SH-SY5Y cells by 4'-halo APP derivatives causes increased oxidative stress, leading to apoptotic cell death.

An Arabidopsis cytochrome b561 with trans-membrane ferrireductase capability

Ascorbate-reducible cytochromes b561 (Cyts-b561) are a class of intrinsic trans-membrane proteins. Tonoplast Cyt-b561 (TCytb), one of the four Cyt-b561 isoforms in Arabidopsis was localized to the tonoplast. We demonstrate here that the optical spectra, EPR spectra and redox potentials of recombinant TCytb are similar to those of the well characterized bovine chromaffin granule Cyt-b561. We provide evidence for the reduction of ferric-chelates by the reduced TCytb. It is also shown that TCytb is capable of trans-membrane electron transport from intracellular ascorbate to extracellular ferric-chelates in yeast cells.

Copper ions disrupt dopamine metabolism via inhibition of V-H+-ATPase: a possible contributing factor to neurotoxicity

The involvement of copper in the pathophysiology of neurodegeneration has been well documented but is not fully understood. Commonly, the effects are attributed to increased reactive oxygen species (ROS) production due to inherent redox properties of copper ions. Here we show copper can have physiological effects distinct from direct ROS production. First, we show that extragranular free copper inhibits the vesicular H(+)-ATPase of resealed chromaffin granule ghosts. Extragranular ascorbate potentiates this inhibition. The inhibition is mixed type with K(is) = 6.8 +/- 2.8 micromol/L and K(ii) = 3.8 +/- 0.6 micromol/L, with respect to ATP. Second, extracellular copper causes an inhibition of the generation of a pH-gradient and rapid dissipation of pre-generated pH and catecholamine gradients. Copper chelators and the ss-amyloid peptide 1-42 were found to effectively prevent the inhibition. The inhibition is reversible and time-independent suggesting the effects of extracellular copper on H(+)-ATPase is direct, and not due to ROS. The physiological significance of these observations was shown by the demonstration that extracellular copper causes a dramatic perturbation of dopamine metabolism in SH-SY5Y cells. Thus, we propose that the direct inhibition of the vesicular H(+)-ATPase may also contribute to the neurotoxic effects of copper.

Cytochrome b561 protein family: Expanding roles and versatile transmembrane electron transfer abilities as predicted by a new classification system and protein sequence motif analyses

Cytochrome b561 family was characterized by the presence of "b561 core domain" that forms a transmembrane four helix bundle containing four totally conserved His residues, which might coordinate two heme b groups. We conducted BLAST and PSI-BLAST searches to obtain insights on structure and functions of this protein family. Analyses with CLUSTAL W on b561 sequences from various organisms showed that the members could be classified into 7 subfamilies based on characteristic motifs; groups A (animals/neuroendocrine), B (plants), C (insects), D (fungi), E (animals/TSF), F (plants+DoH), and G (SDR2). In group A, both motif 1, {FN(X)HP(X)2M(X)2G(X)5G(X)ALLVYR}, and motif 2, {YSLHSW(X)G}, were identified. These two motifs were also conserved in group B. There was no significant features characteristic to groups C and D. A modified version of motif 1, {LFSWHP(X)2M(X)3F(X)3M(X)EAIL(X)SP(X)2SS}, was found in group E with a high degree of conservation. Both motif 3, {DP(X)WFY(L)H(X)3Q}, and motif 4, {K(X)R(X)YWN(X)YHH(X)2G(R/Y)} ,were found in group F at different regions from those of motifs 1 and 2. The "DoH" domain common to the NH2-terminal region of dopamine beta-hydroxylase was found to form fusion proteins with the b561 core domains in groups F and G. Based on these results, we proposed a hypothesis regarding structures and functions of the 7 subfamilies of cytochrome b561.

Selective Perturbation of the Intravesicular Heme Center of Cytochrome b561 by Cysteinyl Modification with 4,4′-Dithiodipyridine

Cytochrome b(561) from bovine adrenal chromaffin vesicles contains two hemes b with EPR signals at g(z) = 3.69 and 3.14 and participates in transmembrane electron transport from extravesicular ascorbate to an intravesicular monooxygenase, dopamine beta-hydroxylase. Treatment of purified cytochrome b(561) in an oxidized state with a sulfhydryl reagent, 4,4'-dithiodipyridine, caused the introduction of only one 4-thiopyridine group per b(561) molecule at either Cys57 or Cys125. About half of the heme centers of the modified cytochrome were reduced rapidly with ascorbate as found for the untreated sample, but the final reduction level decreased to approximately 65%. EPR spectra of the modified cytochrome showed that a part of the g(z) = 3.14 low-spin EPR species was converted to a new low-spin species with g(z) = 2.94, although a considerable part of the heme center was concomitantly converted to a high-spin g = 6 species. Addition of ascorbate to the modified cytochrome caused the disappearance or significant reduction of the EPR signals at g(z) = 3.69 and 3.14 of low-spin species and at g = 6.0 of the high-spin species, but not for the g(z) approximately 2.94 species. These results suggested that the bound 4-thiopyridone at either Cys57 or Cys125 affected the intravesicular heme center and converted it partially to a non-ascorbate-reducible form. The present observations suggested the importance of the two well-conserved Cys residues near the intravesicular heme center and implied their physiological roles during the electron donation to the monodehydroascorbate radical.

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.

Analysis of an Arabidopsis thaliana protein family, structurally related to cytochromes b561 and potentially involved in catecholamine biochemistry in plants

Cytochromes b561 (cyts b561) constitute a family of eukaryotic membrane proteins, catalysing ascorbate (Asc)-mediated trans-membrane electron transport, and hence likely involved in Asc regeneration. A class of proteins (DoH-CB) has been identified in plants and animals, containing the cyt b561 electron-transport domain (CB), combined with the catecholamine-binding regulatory domain of dopamine-beta-hydroxylase (DoH). A mammalian DoH-CB protein was previously reported to function as a cell-derived growth factor receptor (SDR2). We have performed an in silico analysis on DoH-CB proteins from Arabidopsis thaliana and demonstrate that structural features of both CB and DoH domains are well conserved. The combination of both domains may have evolved from a functional interaction between a cyt b561 and a DoH-containing protein, illustrating the so-called "Rosetta Stone" evolutionary principle, and this hypothesis is supported by sequence comparisons. DoH-CB proteins form a newly identified group of proteins, likely to play a key role in catecholamine action in plants. It is suggested that these proteins may function as trans-membrane electron shuttles, possibly regulated by catecholamines. The role and action of catecholamines in plants is poorly documented, but it is clear that they are involved in many aspects of growth and development. Whether the DoH-CB proteins functionally interact with Asc, as is the case for cyts b561, remains to be determined.

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.

Localization of an ascorbate-reducible cytochrome b561 in the plant tonoplast

As a free radical scavenger, and cofactor, ascorbate (ASC) is a key player in the regulation of cellular redox processes. It is involved in responses to biotic and abiotic stresses and in the control of enzyme activities and metabolic reactions. Cytochromes (Cyts) b561 catalyze ASC-driven trans-membrane electron transport and contribute to ASC-mediated redox reactions in subcellular compartments. Putative Cyts b561 have been identified in Arabidopsis (ecotype Columbia) on the basis of sequence similarity to their mammalian counterparts. However, little is known about the function or subcellular localization of this unique class of membrane proteins. We have expressed one of the putative Arabidopsis Cyt b561 genes (CYBASC1) in yeast and we demonstrate that this protein encodes an ASC-reducible b-type Cyt with absorbance characteristics similar to that of other members of this family. Several lines of independent evidence demonstrate that CYBASC1 is localized at the plant tonoplast (TO). Isoform-specific antibodies against CYBASC1 indicate that this protein cosediments with the TO marker on sucrose gradients. Moreover, CYBASC1 is strongly enriched in TO-enriched membrane fractions, and TO fractions contain an ASC-reducible b-type Cyt with alpha-band absorbance maximum near 561 nm. The TO ASC-reducible Cyt has a high specific activity, suggesting that it is a major constituent of this membrane. These results provide evidence for the presence of trans-membrane redox components in this membrane type, and they suggest the coupling of cytoplasmic and vacuolar metabolic reactions through ASC-mediated redox activity.

Kinetic evidence for channeling of dopamine between monoamine transporter and membranous dopamine-beta-monooxygenase in chromaffin granule ghosts

The nature of coupling between the uptake and dopamine-beta-monooxygenase (DbetaM) catalyzed hydroxylation of dopamine (DA) was studied in bovine chromaffin granule ghosts. Initial rate and transient kinetics of DA uptake and conversion were determined under a variety of conditions. The uptake kinetics of DA, norepinephrine (NE), and epinephrine demonstrate that DA is a better substrate than NE and epinephrine under optimal uptake conditions. The transient kinetics of DA accumulation and NE production under both optimal uptake and uptake and conversion conditions were zero-order with no detectable lag or burst periods. The mathematical analyses of the data show that a normal sequential uptake followed by the conversion process could not explain the observed kinetics, under any condition. On the other hand, all experimental data are in agreement with a mechanism in which DA is efficiently channeled from the vesicular monoamine transporter to membranous DbetaM for hydroxylation, prior to the release into the bulk medium of the ghost interior. The slow accumulation of DA under optimal conversion conditions appears to be caused by the slow leakage of DA from the channeling pathway to the ghost interior. Because DbetaM activity in intact granules is equally distributed between soluble and membranous forms of DbetaM, if an efficient channeling mechanism is operative in vivo, soluble DbetaM may not have access to the substrate, making the catalytic activity of soluble DbetaM physiologically insignificant, which is consistent with the increasing experimental evidence that membranous DbetaM may be the physiologically functional form.

Stromal cell-derived receptor 2 and cytochrome b561 are functional ferric reductases

Iron has a variety of functions in cellular organisms ranging from electron transport and DNA synthesis to adenosine triphosphate (ATP) and neurotransmitter synthesis. Failure to regulate the homeostasis of iron can lead to cognition and demyelination disorders when iron levels are deficient, and to neurodegenerative disorders when iron is in excess. In this study we show that three members of the b561 family of predicted ferric reductases, namely mouse cytochrome b561 and mouse and fly stromal cell-derived receptor 2 (SDR2), have ferric reductase activity. Given that a fourth member, duodenal cytochrome b (Dcytb), has previously been shown to be a ferric reductase, it is likely that all remaining members of this family also exhibit this activity. Furthermore, we show that the rat sdr2 message is predominantly expressed in the liver and kidney, with low expression in the duodenum. In hypotransferrinaemic (hpx) mice, sdr2 expression in the liver and kidney is reduced, suggesting that it may be regulated by iron. Moreover, we demonstrate the presence of mouse sdr2 in the choroid plexus and in the ependymal cells lining the four ventricles, through in situ hybridization analysis.

GSH is required to recycle ascorbic acid in cultured liver cell lines

Liver is the site of ascorbic acid synthesis in most mammals. As human liver cannot synthesize ascorbate de novo, it may differ from liver of other species in the capacity or mechanism for ascorbate recycling from its oxidized forms. Therefore, we compared the ability of cultured liver-derived cells from humans (HepG2 cells) and rats (H4IIE cells) to take up and reduce dehydroascorbic acid (DHA) to ascorbate. Neither cell type contained appreciable amounts of ascorbate in culture, but both rapidly took up and reduced DHA to ascorbate. Intracellular ascorbate accumulated to concentrations of 10-20 mM following loading with DHA. The capacity of HepG2 cells to take up and reduce DHA to ascorbate was more than twice that of H4IIE cells. In both cell types, DHA reduction lowered glutathione (GSH) concentrations and was inhibited by prior depletion of GSH with diethyl maleate, buthionine sulfoximine, and phenylarsine oxide. NADPH-dependent DHA reduction due to thioredoxin reductase occurred in overnight-dialyzed extracts of both cell types. These results show that cells derived from rat liver synthesize little ascorbate in culture, that cultured human-derived liver cells have a greater capacity for DHA reduction than do rat-derived liver cells, but that both cell types rely largely on GSH- or NADPH-dependent mechanisms for ascorbate recycling from DHA.

Chromaffin granule membranes contain at least three heme centers: direct evidence from EPR and absorption spectroscopy

Low-temperature electron paramagnetic resonance (EPR) spectroscopy, circular dichroism and two-component redox titration have previously provided evidence for two different ascorbate-reducible heme centers in cytochrome b(561) present in chromaffin granule membranes. These species have now been observed by room and liquid nitrogen temperature absorption spectroscopy. The visualization of these heme centers becomes possible as a consequence of utilizing chromaffin granule membranes prepared by a mild procedure. Additionally, a new redox center, not reducible by ascorbate, was discovered by both EPR and absorption spectroscopy. It constitutes about 15% of the heme absorbance of chromaffin membranes at 561 nm and has EPR characteristics of a well-organized highly axial low-spin heme center (thus making it unlikely that it is a denatured species). This species is either an alternative form of one of the hemes of cytochrome b(561) that has a very low redox potential or a b-type cytochrome distinct from b(561).

Higher-plant plasma membrane cytochrome b561: a protein in search of a function

During the past twenty years evidence has accumulated on the presence of a specific high-potential, ascorbate-reducible b-type cytochrome in the plasma membrane (PM) of higher plants. This cytochrome is named cytochrome b561 (cyt b561) according to the wavelength maximum of its alpha-band in the reduced form. More recent evidence suggests that this protein is homologous to a b-type cytochrome present in chromaffin granules of animal cells. The plant and animal cytochromes share a number of strikingly similar features, including the high redox potential, the ascorbate reducibility, and most importantly the capacity to transport electrons across the membrane they are located in. The PM cyt b561 is found in all plant species and in a variety of tissues tested so far. It thus appears to be a ubiquitous electron transport component of the PM. The cytochromes b561 probably constitute a novel class of transmembrane electron transport proteins present in a large variety of eukaryotic cells. Of particular interest is the recent discovery of a number of plant genes that show striking homologies to the genes coding for the mammalian cytochromes b561. A number of highly relevant structural features, including hydrophobic domains, heme ligation sites, and possible ascorbate and monodehydroascorbate binding sites are almost perfectly conserved in all these proteins. At the same time the plant gene products show interesting differences related to their specific location at the PM, such as potentially N-linked glycosylation sites. It is also clear that at least in several plants cyt b561 is represented by a multigene family. The current paper presents the first overview focusing exclusively on the plant PM cyt b561, compares it to the animal cyt b561, and discusses the possible physiological function of these proteins in plants.

The ascorbate: ascorbate free radical oxidoreductase from the erythrocyte membrane is not cytochrome b561

Erythrocytes contain a plasma membrane redox system that can reduce extracellular ascorbate radicals by using intracellular ascorbate as an electron donor. In this study, the hypothesis was tested that cytochrome b561 was a component of this system. Spectroscopic analysis of erythrocyte membrane preparations revealed the presence of cytochrome b5 and hemoglobin but also of a cytochrome with properties similar to cytochrome b561, reducible by ascorbate and insensitive to CO. The presence of cytochrome b561 was studied further by reverse transcriptase-PCR analysis of erythrocyte progenitor cells, reticulocytes. However, no cytochrome b561 mRNA could be found. These results were corroborated by Western blot analysis with an anti-cytochrome b561 serum. No cytochrome b561 protein could be detected in extracts of erythrocyte membranes. It is therefore concluded that erythrocytes do not contain cytochrome b561 in their membranes. The possible involvement of other b-cytochromes in ascorbate-ascorbate free radical oxidoreductase activity is discussed.

Erythrocytes reduce extracellular ascorbate free radicals using intracellular ascorbate as an electron donor

Ascorbate is readily oxidized in aqueous solution by ascorbate oxidase. Ascorbate radicals are formed, which disproportionate to ascorbate and dehydroascorbic acid. Addition of erythrocytes with increasing intracellular ascorbate concentrations decreased the oxidation of ascorbate in a concentration-dependent manner. Concurrently, it was found, utilizing electron spin resonance spectroscopy, that extracellular ascorbate radical levels were decreased. Control experiments showed that these results could not be explained by leakage of ascorbate from the cells, inactivation of ascorbate oxidase, or oxygen depletion. Thus, this means that intracellular ascorbate is directly responsible for the decreased oxidation of extracellular ascorbate. Exposure of ascorbate-loaded erythrocytes to higher levels of extracellular ascorbate radicals resulted in the detection of intracellular ascorbate radicals. Moreover, efflux of dehydroascorbic acid was observed under these conditions. These data confirm the view that intracellular ascorbate donates electrons to extracellular ascorbate free radical via a plasma membrane redox system. Such a redox system enables the cells to effectively counteract oxidative processes and thereby prevent depletion of extracellular ascorbate.

Extracellular reduction of the ascorbate free radical by human erythrocytes

We investigated the possibility that human erythrocytes can reduce extracellular ascorbate free radical (AFR). When the AFR was generated from ascorbate by ascorbate oxidase, intact cells slowed the loss of extracellular ascorbate, an effect that could not be explained by changes in enzyme activity or by release of ascorbate from the cells. If cells preserve extracellular ascorbate by regenerating it from the AFR, then they should decrease the steady-state concentration of the AFR. This was confirmed directly by electron paramagnetic resonance spectroscopy, in which the steady-state extracellular AFR signal varied inversely with the cell concentration and was a saturable function of the absolute AFR concentration. Treatment of cells N-ethylmaleimide (2 mM) impaired their ability both to preserve extracellular ascorbate, and to decrease the extracellular AFR concentration. These results suggest that erythrocytes spare extracellular ascorbate by enhancing recycling of the AFR, which could help to maintain extracellular concentrations of the vitamin.

Is ascorbic acid an antioxidant for the plasma membrane?

Ascorbic acid, or vitamin C, is a primary antioxidant in plasma and within cells, but it can also interact with the plasma membrane by donating electrons to the alpha-tocopheroxyl radical and a trans-plasma membrane oxidoreductase activity. Ascorbate-derived reducing capacity is thus transmitted both into and across the plasma membrane. Recycling of alpha-tocopherol by ascorbate helps to protect membrane lipids from peroxidation. However, neither the mechanism nor function of the ascorbate-dependent oxidoreductase activity is known. This activity has typically been studied using extracellular ferricyanide as an electron acceptor. Whereas an NADH:ferricyanide reductase activity is evident in open membranes, ascorbate is the preferred electron donor within cells. The oxidoreductase may be a single membrane-spanning protein or may only partially span the membrane as part of a trans-membrane electron transport chain composed of a cytochrome or even hydrophobic antioxidants such as alpha-tocopherol or ubiquinol-10. Further studies are needed to elucidate the structural components, mechanism, and physiological significance of this activity. Proposed functions for the oxidoreductase include stimulation of cell growth, reduction of the ascorbate free radical outside cells, recycling of alpha-tocopherol, reduction of lipid hydroperoxides, and reduction of ferric iron prior to iron uptake by a transferrin-independent pathway.

Mouse cytochrome b561: cDNA cloning and expression in rat brain, mouse embryos, and human glioma cell lines

A cDNA encoding cytochrome b561 has been isolated from a mouse brain cDNA library, which predicts a protein of 250 amino acids with a deduced Mr of 27,770. Northern blot analysis of different mouse and rat tissues revealed one major mRNA of 3300 bp, which is abundantly distributed in a number of neuroendocrine tissues. In addition, cytochrome mRNA levels in rat brain sections showed the highest distribution of cytochrome b561 in the hypothalamus, hippocampus, thalamus, and striatum, with a moderate level in the cerebral cortex, and the lowest levels in the olfactory bulb and cerebellum. Because non-neuronal cells in the central nervous system contained peptidyl alpha-amidating monooxygenase (PAM), to which cytochrome b561 donates its electrons, we used RT-PCR to document the coexpression of cytochrome b561 with PAM and dopamine beta-hydroxylase (DBH) in glioblastoma cells. Cytochrome b561 expression was detectable in the 11-day-old mouse embryo, and the level of its mRNA increased tenfold by 15 and 17 days of gestation.

Reduction of dopamine beta-monooxygenase. A unified model for apparent negative cooperativity and fumarate activation

The interactions of reductants with dopamine beta-monooxygenase (DbetaM) were examined using two novel classes of reductants. The steady-state kinetics of the previously characterized DbetaM reductant, N,N-dimethyl-1,4-p-phenylenediamine (DMPD), were parallel to the ascorbic acid-supported reaction with respect to pH dependence and fumarate activation. DMPD also displayed pH and fumarate-dependent apparent negative cooperativity demonstrating that the previously reported cooperative behavior of DbetaM toward the reductant is not unique to ascorbic acid. The 6-OH phenyl and alkylphenyl-substituted ascorbic acid derivatives were more efficient reductants for the enzyme than ascorbic acid. Kinetic studies suggested that these derivatives behave as pseudo bisubstrates with respect to ascorbic acid and the amine substrate. The lack of apparent cooperative behavior with these derivatives suggests that this behavior of DbetaM is not common for all the reductants. Based on these findings and additional kinetic evidence, the proposal that the apparent negative cooperativity in the interaction of ascorbic acid with DbetaM was due to the presence of a distinct allosteric regulatory site has been ruled out. In contrast to previous models, where fumarate was proposed to interact with a distinct anion binding site, the effect of fumarate on the steady-state kinetics of these novel reductants suggests that fumarate and the reductant may interact with the same site of the enzyme. In accordance with these observations and mathematical analysis of the experimental data, a unified model for the apparent negative cooperativity and fumarate activation of DbetaM in which both fumarate and the reductant interact with the same site of all forms of the enzyme with varying affinities under steady-state turnover conditions has been proposed.

The reduction of membrane-bound dopamine beta-monooxygenase in resealed chromaffin granule ghosts. Is intragranular ascorbic acid a mediator for extragranular reducing equivalents?

The role of internal and external reductants in the dopamine beta-monooxygenase (D beta M)-catalyzed conversion of dopamine to norepinephrine has been investigated in resealed chromaffin granule ghosts. The rate of norepinephrine production was not affected by the exclusion of internal ascorbate. The omission of ascorbate from the external medium drastically reduced the norepinephrine production without affecting the net rate of dopamine uptake. In the presence of the external reductant, the internal ascorbate levels were constant throughout the incubation period. The rate of norepinephrine production was not affected when ghosts were resealed to contain the D beta M reduction site inhibitor, imino-D-glucoascorbate. Ghosts incubated with external imino-D-glucoascorbate reduced the norepinephrine production. The weak D beta M reductant, 6-amino-L-ascorbic acid, was found to be a good external reductant for granule ghosts. The outcome of the above experiments was not altered when dopamine was replaced with the reductively inactive D beta M substrate, tyramine. These results and the known topology of membrane-bound D beta M disfavor the direct reduction of the enzyme by the external reductant. Our observations are consistent with the hypothesis that external ascorbate is the sole source of reducing equivalents for D beta M monooxygenation and that internal soluble ascorbate (or dopamine) may not directly reduce or mediate the reduction of membrane-bound D beta M in resealed granule ghosts.

The redox couple between glutathione and ascorbic acid: a chemical and physiological perspective

This article provides a comprehensive analysis of the redox reaction between glutathione/glutathione disulfide and ascorbic acid/dehydroascorbic acid. It includes an historical perspective of the progression of the experiments, first begun more than 60 years ago and continuing today with heightened importance. Indeed, the antioxidant capacity of glutathione and ascorbic acid, whether singly or in combination, linked via the redox couple, is a subject of intense interest for studies by bench scientists and clinicians, particularly because a growing body of evidence suggests that free radicals may be involved in a variety of diseases. The authors begin with a detailed summary of "test tube" experiments (the chemical perspective) that have revealed the conditions that regulate the rate of the redox coupling between glutathione and dehydroascorbic acid and that promote or inhibit the decomposition of dehydroascorbic acid in ordinary, buffered aqueous media; results obtained in the authors' laboratory are used for illustration purposes and uniformity of presentation. The authors then proceed to a critical examination of the extent to which the redox couple between glutathione and ascorbic acid operates in a cell, using the often published antioxidant cascade (See Fig. 1) as the model for the analysis (the physiological perspective). The evidence for and the evidence against the presence of the enzyme dehydroascorbate reductase in animal cells is outlined in a balanced way in an attempt to make sense of this continuing controversy. Next, the authors carefully document the many studies showing that exogenous dehydroascorbic acid is transported into cells where it is reduced to ascorbic acid by glutathione. Finally, they probe the functional significance and efficiency of the redox couple in monolayer cultures of human retinal pigment epithelial (RPE) cells, as a prototypical cellular model. The authors include the results of new experiments showing that incubation of RPE cells with a nitroxide, TEMPOL, leads to the selective oxidation of intracellular ascorbic acid. This approach is desirable because it dissects the cascade at a specific site and permits measurements of the levels of ascorbic acid and glutathione in the cells before, during, and after oxidation. The results show that only partial regeneration of ascorbic acid is obtained when control conditions are restored. However, if either ascorbic acid or dehydroascorbic acid is added to the media during the recovery period following treatment of cells with TEMPOL, then full recovery of ascorbic acid is observed.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of Mg-ATP in norepinephrine biosynthesis in intact chromaffin granules

Dopamine beta-monooxygenase converts dopamine to norepinephrine in intact chromaffin granules using intragranular ascorbic acid as a cosubstrate. Mg-ATP with external ascorbic acid is required for maximal norepinephrine biosynthesis. Mechanisms to explain these requirements were investigated specifically using intact granules. The effect of Mg-ATP was independent of membrane potential (delta psi) because norepinephrine biosynthesis was unchanged whether delta psi was positive or collapsed. Furthermore, the effect of Mg-ATP was independent of absolute intragranular and extragranular pH as well as the pH difference across the chromaffin granule membrane (delta pH). Nevertheless, norepinephrine biosynthesis was inhibited by N-ethylmaleimide, 4-chloro-7-nitrobenzofurazane, and N,N-dicyclohexylcarbodiimide, specific inhibitors of the secretory vesicle ATPase that may directly affect proton pumping. Biosynthesis occurred normally with other ATPase inhibitors that do not inhibit the ATPase in secretory vesicles. The data indicate that the effect of Mg-ATP with ascorbic acid is mediated by the granule membrane ATPase but independent of maintaining delta psi and delta pH. An explanation of these findings is that Mg-ATP, via the granule ATPase, may change the rate at which protons or dopamine are made available to dopamine beta-monooxygenase.

Ascorbic acid in the brain

Ascorbic acid is highly concentrated in the central nervous system. Measurement of the extracellular concentration of ascorbate in animals, mainly by the technique of voltammetry in vivo, has demonstrated fluctuation in release from neuropil, both spontaneously and in response to physical stimulation of the animal and to certain drugs. Although in the adrenal medulla ascorbate is co-released with catecholamines, release of ascorbate from brain cells is associated principally with the activity of glutamatergic neurones, mainly by glutamate-ascorbate heteroexchange across cell membranes of neurones or glia. This phenomenon is discussed in relation to a possible role of ascorbate as a neuromodulator or neuroprotective agent in the brain.

Electron transfer in chromaffin-vesicle ghosts containing peroxidase

In chromaffin vesicles, the enzyme dopamine beta-monooxygenase converts dopamine to norepinephrine. It is believed that reducing equivalents for this reaction are supplied by intravesicular ascorbic acid and that the ascorbate is regenerated by importing electrons from the cytosol with cytochrome b-561 functioning as the transmembrane electron carrier. If this is true, then the ascorbate-regenerating system should be capable of providing reducing equivalents to any ascorbate-requiring enzyme, not just dopamine beta-monooxygenase. This may be tested using chromaffin-vesicle ghosts in which an exogenous enzyme, horseradish peroxidase, has been trapped. If ascorbate and peroxidase are trapped together within chromaffin-vesicle ghosts, cytochrome b-561 in the vesicle membrane is found in the reduced form. Subsequent addition of H2O2 causes the cytochrome to become partially oxidized. H2O2 does not cause this oxidation if either peroxidase or ascorbate are absent. This argues that the cytochrome is oxidized by semidehydroascorbate, the oxidation product of ascorbate, rather than by H2O2 or peroxidase directly. The semidehydroascorbate must be internal because the ascorbate from which it is formed is sequestered and inaccessible to external ascorbate oxidase. This shows that cytochrome b-561 can transfer electrons to semidehydroascorbate within the vesicles and that the semidehydroascorbate may be generated by any enzyme, not just dopamine beta-monooxygenase.

Ascorbic acid: the concept of optimum requirements

Experiments with enzymes in situ that are dependent on ascorbic acid for maximum activity will provide critical information about ascorbic acid requirements. Our work with chromaffin tissue as a model system eventually will result in the determination of two dose-response curves for norepinephrine biosynthesis, representing cytosolic ascorbic acid and intragranular ascorbic acid (Fig. 11). These curves for norepinephrine biosynthesis can be combined with curves for other enzymatic events that are also dependent on ascorbic acid for maximal activity. These dose-response curves (Fig. 2) will allow determination of optimum ascorbic acid requirements based on specific product formation and minimum toxicity. These principles are adaptable to other vitamins as well as ascorbic acid, and could form the basis for a new approach to vitamin requirements.