Cytochrome b561 catalyzes transmembrane electron transfer

DCYTB is a predictor of outcome in breast cancer that functions via iron-independent mechanisms

BACKGROUND

Duodenal cytochrome b (DCYTB) is a ferrireductase that functions together with divalent metal transporter 1 (DMT1) to mediate dietary iron reduction and uptake in the duodenum. DCYTB is also a member of a 16-gene iron regulatory gene signature (IRGS) that predicts metastasis-free survival in breast cancer patients. To better understand the relationship between DCYTB and breast cancer, we explored in detail the prognostic significance and molecular function of DCYTB in breast cancer.

METHODS

The prognostic significance of DCYTB expression was evaluated using publicly available microarray data. Signaling Pathway Impact Analysis (SPIA) of microarray data was used to identify potential novel functions of DCYTB. The role of DCYTB was assessed using immunohistochemistry and measurements of iron uptake, iron metabolism, and FAK signaling.

RESULTS

High DCYTB expression was associated with prolonged survival in two large independent cohorts, together totaling 1610 patients (cohort #1, p = 1.6e-11, n = 741; cohort #2, p = 1.2e-05, n = 869; log-rank test) as well as in the Gene expression-based Outcome for Breast cancer Online (GOBO) cohort (p < 1.0e-05, n = 1379). High DCYTB expression was also associated with increased survival in homogeneously treated groups of patients who received either tamoxifen or chemotherapy. Immunohistochemistry revealed that DCYTB is localized on the plasma membrane of breast epithelial cells, and that expression is dramatically reduced in high-grade tumors. Surprisingly, neither overexpression nor knockdown of DCYTB affected levels of ferritin H, transferrin receptor, labile iron or total cellular iron in breast cancer cells. Because SPIA pathway analysis of patient microarray data revealed an association between DCYTB and the focal adhesion pathway, we examined the influence of DCYTB on FAK activation in breast cancer cells. These experiments reveal that DCYTB reduces adhesion and activation of focal adhesion kinase (FAK) and its adapter protein paxillin.

CONCLUSIONS

DCYTB is an important predictor of outcome and is associated with response to therapy in breast cancer patients. DCYTB does not affect intracellular iron in breast cancer cells. Instead, DCYTB may retard cancer progression by reducing activation of FAK, a kinase that plays a central role in tumor cell adhesion and metastasis.

Bringing bioelectricity to light

Any bilayer lipid membrane can support a membrane voltage. The combination of optical perturbation and optical readout of membrane voltage opens the door to studies of electrophysiology in a huge variety of systems previously inaccessible to electrode-based measurements. Yet, the application of optogenetic electrophysiology requires careful reconsideration of the fundamentals of bioelectricity. Rules of thumb appropriate for neuroscience and cardiology may not apply in systems with dramatically different sizes, lipid compositions, charge carriers, or protein machinery. Optogenetic tools are not electrodes; thus, optical and electrode-based measurements have different quirks. Here we review the fundamental aspects of bioelectricity with the aim of laying a conceptual framework for all-optical electrophysiology.

Human heart rate: heritability of resting and stress values in twin pairs, and influence of genetic variation in the adrenergic pathway at a microribonucleic acid (microrna) motif in the 3'-UTR of cytochrome b561 [corrected]

OBJECTIVES

The goal of this study was to understand the role of genetic variation in the catecholamine biosynthetic pathway for control of human heart rate (HR).

BACKGROUND

Human HR is an integrated cardiovascular trait predictive of morbidity and survival. Because the autonomic pathway exerts rapid control over the heart, we probed the role of heredity in the control of HR, focusing on a component of the autonomic sympathetic pathway already predictive of outflow responses: cytochrome b561 (CYB561), the electron shuttle in catecholamine vesicle membranes for transmitter biosynthesis.

METHODS

We studied hereditary control of HR with the twin pair design, at rest and during environmental (cold) stress. Single nucleotide polymorphism disruption of a microribonucleic acid (microRNA) recognition motif in the human CYB561 3'-UTR was identified computationally, and its differential effect on gene expression was demonstrated in a transfected luciferase reporter/3'-UTR variant. We exposed stem cell-derived human embryoid bodies to the microRNA mimic or antagomir oligonucleotides, and we observed the effects on contraction rate in proto-hearts.

RESULTS

Substantial heritability (h(2)) was demonstrated by using twin pair variance components for both basal/resting HR (h(2) 50.9 ± 6.4% of trait variation, p = 2.47 × 10(-10)) and stress-augmented HR (h(2) 55.1 ± 5.9%, p = 8.79 × 10(-13)), and the 2 HR traits shared genetic determination (genetic covariance ρG 0.747 ± 0.058, p = 2.85 × 10(-9)). CYB561 displayed 1 common genetic variant in the transcript region: A+1485G (rs3087776), in the 3'-UTR, 1485 bp downstream of the termination codon, in a conserved region, with the A-allele ancestral in primates. In a twin/sibling sample (n = 576), A+1485G influenced HR, both at rest (p = 0.010) and after environmental stress (p = 0.002), with the minor (A) allele displaying a recessive effect with lower HR. The effect of A+1485G on HR was extended by meta-analysis into 2 additional population samples (total n = 2,579), and the influence remained directionally consistent and significant (p = 0.007). A+1485G disrupted a microRNA (human microribonucleic acid-1294 [hsa-miR-1294]) recognition motif in the 3'-UTR, as demonstrated by a transfected luciferase reporter/human 3'-UTR variant system in 2 different neuronal/neuroendocrine cell types. The microRNA effect was further documented by cotransfection of an hsa-miR-1294 mimic, yielding an exaggerated decline in expression of the A-allele (better match) reporter (p = 4.3 × 10(-5)). Similar findings of differential 3'-UTR allelic susceptibility to hsa-miR-1294 were noted during expression of the full-length human CYB561 messenger ribonucleic acid with its cognate 3'-UTR. Finally, exposure of stem cell-derived human embryoid bodies to hsa-miR-1294 mimic or antagomir oligonucleotides yielded directionally opposite effects on contraction rate in proto-hearts.

CONCLUSIONS

HR is a substantially heritable trait, with genetic influence by variation in the adrenergic pathway, here shown for messenger ribonucleic acid translational control at the CYB561 step of transmitter formation. The results have implications for potentially modifiable autonomic pathways that influence this risk trait in the population.

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.

Functional characterization of the recombinant human tumour suppressor 101F6 protein, a cytochrome b(561) homologue

Candidate human tumour suppressor gene product, 101F6 protein, is a highly hydrophobic transmembrane protein and a member of cytochrome b(561) family. Purified 101F6 protein expressed in Pichia pastoris cells showed visible absorption spectra similar but distinct from those of cytochrome b(561). Haem content analysis indicated presence of two haems B per molecule. Midpoint potentials of the purified protein were found as +109 and +26 mV for two haems, slightly lower than those for bovine chromaffin granule or plant Zea mays cytochromes b(561). Electron paramagnetic resonance (EPR) spectra in oxidized state at 5 K showed only a highly anisotropic low-spin (HALS) signal at g(z) = 3.75. However, at 15 and 20 K, another HALS-type signal appeared at g(z) = 3.65 being overlapped with that of g(z) = 3.75. The rhombic EPR signal at g(z) = 3.16 previously seen in other cytochromes b(561) was not observed, suggesting distinct haem environments. Absence of the inhibition in the electron transfer from ascorbate by a treatment of 101F6 protein with diethylpyrocarbonate showed a remarkable contrast from those of other cytochromes b(561) where the 'concerted H(+)/e(-) transfer mechanism' at the cytosolic haem centre was blocked by specific Nε-carbethoxylation of haem-coordinating imidazole, suggesting that 101F6 protein might accept electrons via a mechanism distinct from other cytochromes b(561).

nemy encodes a cytochrome b561 that is required for Drosophila learning and memory

Although many genes have been shown to play essential roles in learning and memory, the precise molecular and cellular mechanisms underlying these processes remain to be fully elucidated. Here, we present the molecular and behavioral characterization of the Drosophila memory mutant nemy. We provide multiple lines of evidence to show that nemy arises from a mutation in a Drosophila homologue of cytochrome B561. nemy is predominantly expressed in neuroendocrine neurons in the larval brain, and in mushroom bodies and antennal lobes in the adult brain, where it is partially coexpressed with peptidyl alpha-hydroxylating monooxygenase (PHM), an enzyme required for peptide amidation. Cytochrome b561 was found to be a requisite cofactor for PHM activity and we found that the levels of amidated peptides were reduced in nemy mutants. Moreover, we found that knockdown of PHM gave rise to defects in memory retention. Altogether, the data are consistent with a model whereby cytochrome B561-mediated electron transport plays a role in memory formation by regulating intravesicular PHM activity and the formation of amidated neuropeptides.

Axial ligation and stoichiometry of heme centers in adrenal cytochrome b561

Cytochrome (cyt) b561 transports electrons across the membrane of chromaffin granules (CG) present in the adrenal medulla, supporting the biosynthesis of norepinephrine in the CG matrix. We have conducted a detailed characterization of cyt b561 using electron paramagnetic resonance (EPR) and optical spectroscopy on the wild-type and mutant forms of the cytochrome expressed in insect cells. The gz = 3.7 (low-potential heme) and gz = 3.1 (high-potential heme) signals were found to represent the only two authentic hemes of cyt b561; models that propose smaller or greater amounts of heme can be ruled out. We identified the axial ligands to hemes in cyt b561 by mutating four conserved histidines (His54 and His122 at the matrix-side heme center and His88 and His161 at the cytoplasmic-side heme center), thus confirming earlier structural models. Single mutations of any of these histidines produced a constellation of spectroscopic changes that involve not one but both heme centers. We hypothesize that the two hemes and their axial ligands in cyt b561 are integral parts of a structural unit that we term the "kernel". Histidine to glutamine substitutions in the cytoplasmic-side heme center but not in the matrix-side heme center led to the retention of a small fraction of the low-potential heme with gz = 3.7. We provisionally assign the low-potential heme to the matrix side of the membrane; this arrangement suggests that the membrane potential modulates electron transport across the CG membrane.

Three mammalian cytochromes b561 are ascorbate-dependent ferrireductases

Cytochromes b(561) are a family of transmembrane proteins found in most eukaryotic cells. Three evolutionarily closely related mammalian cytochromes b(561) (chromaffin granule cytochrome b, duodenal cytochrome b, and lysosomal cytochrome b) were expressed in a Saccharomyces cerevisiaeDeltafre1Deltafre2 mutant, which lacks almost all of its plasma membrane ferrireductase activity, to study their ability to reduce ferric iron (Fe(3+)). The expression of each of these cytochromes b(561) was able to rescue the growth defect of the Deltafre1Deltafre2 mutant cells in iron-deficient conditions, suggesting their involvement in iron metabolism. Plasma membrane ferrireductase activities were measured using intact yeast cells. Each cytochrome b(561) showed significant FeCN and Fe(3+)-EDTA reductase activities that were dependent on the presence of intracellular ascorbate. Site-directed mutagenesis of lysosomal cytochrome b was conducted to identify amino acids that are indispensable for its activity. Among more than 20 conserved or partially conserved amino acids that were investigated, mutations of four His residues (H47, H83, H117 and H156), one Tyr (Y66) and one Arg (R67) completely abrogated the FeCN reductase activity, whereas mutations of Arg (R149), Phe (F44), Ser (S115), Trp (W119), Glu (E196), and Gln (Q131) affected the ferrireductase activity to some degree. These mutations may affect the heme coordination, ascorbate binding, and/or ferric substrate binding. Possible roles of these residues in lysosomal cytochrome b are discussed. This study demonstrates the ascorbate-dependent transmembrane ferrireductase activities of members of the mammalian cytochrome b(561) family of proteins.

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.

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.

Molecular and functional roles of duodenal cytochrome B (Dcytb) in iron metabolism

Dcytb has been identified as the mammalian transplasma ferric reductase that catalyzes the reduction of ferric to ferrous iron in the process of iron absorption. Its mRNA and protein levels are up-regulated by several independent stimulators of iron absorption. Furthermore, its cDNA encodes putative binding sites for heme and ascorbic acid. Using Northern and Western blots, RT-PCR and confocal microscopy, we studied the expression and localisation of Dcytb in cell lines and tissues of CD1 mice. Dcytb expression and function were modulated by iron. Dcytb and DMT1, both predominantly localised in the apical region of the duodenum were up-regulated in iron deficiency. Dcytb, the iron regulated ferric reductase may also utilize cytoplasmic ascorbate as electron donor for transmembrane reduction of iron. Dcytb expression was found in other tissues apart from the duodenum and its regulation and functions at these other sites are of interest in iron metabolism.

Protein trafficking to chromaffin granules and proteolytic processing within regulated secretory vesicles of neuroendocrine chromaffin cells

Proteolytic processing within secretory vesicles is required for the production and secretion of biologically active peptide neurotransmitters and hormones, known collectively as neuropeptides. This chapter addresses several new aspects of proteolysis in secretory vesicles, chromaffin granules, with respect to sorting proneuropeptides or prohormones into such regulated secretory vesicles that use specific prohormone sorting signals. Concomitant with prohormone sorting, evidence for the role of chromogranin A in secretory granule biogenesis is presented. Secretory vesicle function involves endogenous serpin protease inhibitors for the regulation of proteolysis. The novel serpins endopin 1 and endopin 2 possess high homology to a(1)-antichymotrypsin, yet they possess distinct target protease specificities. The serpins PAI-1 and neuroserpin are also localized to chromaffin granules. In addition, regulation of secretory vesicle function involves cytochrome b561 that regulates reducing equivalents to maintain the intravesicular redox state. These studies demonstrate multiple components as regulatory factors in the control of secretory vesicle function for the biosynthesis and secretion of neuropeptides and catecholamines.

Mechanism of ascorbic acid oxidation by cytochrome b(561)

The 1 equiv reaction between ascorbic acid and cytochrome b(561) is a good model for redox reactions between metalloproteins (electron carriers) and specific organic substrates (hydrogen-atom carriers). Diethyl pyrocarbonate inhibits the reaction of cytochrome b(561) with ascorbate by modifying a histidine residue in the ascorbate-binding site. Ferri/ferrocyanide can mediate reduction of DEPC-treated cytochrome b(561) by ascorbic acid, indicating that DEPC-inhibited cytochrome b(561) cannot accept electrons from a hydrogen-atom donor like ascorbate but can still accept electrons from an electron donor like ferrocyanide. Ascorbic acid reduces cytochrome b(561) with a K(m) of 1.0 +/- 0.2 mM and a V(max) of 4.1 +/- 0.8 s(-1) at pH 7.0. V(max)/K(m) decreases at low pH but is approximately constant at pH >7. The rate constant for oxidation of cytochrome b(561) by semidehydroascorbate decreases at high pH but is approximately constant at pH <7. This suggests that the active site must be unprotonated to react with ascorbate and protonated to react with semidehydroascorbate. Molecular modeling calculations show that hydrogen bonding between the 2-hydroxyl of ascorbate and imidazole stabilizes the ascorbate radical relative to the monoanion. These results are consistent with the following mechanism for ascorbate oxidation. (1) The ascorbate monoanion binds to an unprotonated site (histidine) on cytochrome b(561). (2) This complex donates an electron to reduce the heme. (3) The semidehydroascorbate anion dissociates from the cytochrome, leaving a proton associated with the binding site. (4) The binding site is deprotonated to complete the cycle. In this mechanism, an essential role of the cytochrome is to bind the ascorbate monoanion, which does not react by outer-sphere electron transfer in solution, and complex it in such a way that the complex acts as an electron donor. Thermodynamic considerations show that no steps in this process involve large changes in free energy, so the mechanism is reversible and capable of fulfilling the cytochrome's function of equilibrating ascorbate and semidehydroascorbate.

b-type cytochromes in plasma membranes of Phaseolus vulgaris hypocotyls, Arabidopsis thaliana leaves, and Zea mays roots

The plasma membrane of higher plants contains more than one kind of b-type cytochromes. One of these has a high redox potential and can be fully reduced by ascorbate. This component, the cytochrome b561 (cyt b561), has its characteristic alpha-band absorbance close to 561 nm wavelength at room temperature. Cyt b561 was first isolated from etiolated bean hook plasma membranes by two consecutive anion exchange chromatography steps. During the first step performed at pH 8, cyt b561 did not bind to the anion exchange column, but other b-type cytochromes did. In the second step performed at pH 9.9, cyt b561 was bound to the column and was eluted from the column at an ionic strength of about 100 mM KCl. However, when the same protocol was applied to the solubilized plasma membrane proteins from Arabidopsis thaliana leaves and maize roots, the ascorbate-reducible cyt b561 bound already to the first anion exchange column at pH 8 and was eluted also at an ionic strength of about 100 mM KCl. Other b-type cytochromes than the ascorbate-reducible cyt b561 from the plasma membranes of Arabidopsis leaves and maize roots showed similar chromatographic characteristics to that of bean hypocotyls. These results demonstrate particular differences in the chromatographic behavior of cyt b561 from different sources.

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.

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.

Structural basis for the electron transfer across the chromaffin vesicle membranes catalyzed by cytochrome b561: analyses of cDNA nucleotide sequences and visible absorption spectra

We isolated cDNA clones for cytochromes b561 from sheep and porcine adrenal medullae using the RT-PCR technique. Comparison of the deduced amino acid sequences of various species showed that there are two fully-conserved regions in this cytochrome. In addition, one methionyl and six histidyl residues (potential heme ligands) are fully-conserved. Based on a plausible structural model in which a polypeptide spans the vesicle membranes six times and holds two heme B molecules, the first conserved sequence (69ALLVYRVFR77) is located on the extravesicular side of an alpha-helical segment and the second one (120SLHSW124) is located in an intravesicular loop connecting two alpha-helical segments, respectively. Consideration of the relative locations of the fully-conserved sequences, and the methionyl and histidyl residues in the model led to a proposal that the first and second conserved sequences are likely to form the binding sites for extravesicular ascorbic acid and intravesicular semidehydroascorbic acid, respectively. A mild alkaline-treatment of purified bovine cytochrome b561 in oxidized state led to a specific loss of an electron-accepting ability from ascorbic acid for a half of the heme center, suggesting a distinct role for each of the two hemes.

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.

Genomic structure and expression of the human gene encoding cytochrome b561, an integral protein of the chromaffin granule membrane

Cytochrome b561 is an electron transfer protein unique to neuroendocrine secretory vesicles. The Southern blot hybridization shows that it is a single copy gene highly conserved throughout phylogeny. The transcription unit spans approximately 11 kilobases, and heterologous transcription sites are located 404 bases 5' to the translation initiation codon. The sequence of the 5'-flanking region is GC-rich and lacks a typical TATA box at the usual position. However, it has a CAAT sequence at -132 and potential recognition sequences for several transcription factors including SP1, GR-PR-MMTV, AP4, gERE, JCV repeat, AP2, and NF-kappa B. Each of the five transmembrane segments are encoded by five consecutive exons. This corroborates the five-transmembrane model proposed for human, mouse, and Xenopus rather than six proposed for bovine. The cytochrome was found to be highly expressed in colon cancer cell lines, T cell lymphomas, and K-562 cell lines. However, in B-cell lymphomas such as Burkitt's and Daudi, the cytochrome b561 expression was completely shut down. The results in this report are the first to demonstrate the structural organization and regulatory sequences of the cytochrome b561 gene encoding an integral membrane protein of neuroendocrine storage vesicles of neurotransmitters and peptide hormones. Unexpected results on cytochrome b561 expression in cells of lymphocytic origin and its complex regulation in tumor cells provide new insights into cytochrome b561 gene regulation.

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)

Hormone- and growth factor-stimulated NADH oxidase

An NADH oxidase activity of animal and plant plasma membrane is described that is stimulated by hormones and growth factors. In plasma membranes of cancer cells and tissues, the activity appears to be constitutively activated and no longer hormone responsive. With drugs that inhibit the activity, cells are unable to grow although growth inhibition may be more related to a failure of the cells to enlarge than to a direct inhibition of mitosis. The hormone-stimulated activity in plasma membranes of plants and the constitutively activated NADH oxidase in tumor cell plasma membranes is inhibited by thiol reagents whereas the basal activity is not. These findings point to a thiol involvement in the action of the activated form of the oxidase. NADH oxidase oxidation by Golgi apparatus of rat liver is inhibited by brefeldin A plus GDP. Brefeldin A is a macrolide antibiotic inhibitor of membrane trafficking. A model is presented where the NADH oxidase functions as a thiol-disulfide oxidoreductase activity involved in the formation and breakage of disulfide bonds. The thiol-disulfide interchange is postulated as being associated with physical membrane displacement as encountered in cell enlargement or in vesicle budding. The model, although speculative, does provide a basis for further experimentation to probe a potential function for this enzyme system which, under certain conditions, exhibits a hormone- and growth factor-stimulated oxidation of NADH.

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.

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.

Several forms of chromaffin granule cytochrome b-561 revealed by EPR spectroscopy

Low-temperature EPR spectra of chromaffin granule membranes from bovine adrenal medulla reveal 3 different signals of the ferric cytochrome b-561. A typical gZ signal of a low-spin cytochrome observed at g approximately 3 is comprised of a high-potential component with gZ = 3.14 and a low-potential one with gZ = 3.11, the low-potential signal showing significantly faster relaxation. In addition, a highly temperature-sensitive heme signal at g = 3.7 is observed which is fully retained in the preparation of granule membranes with b-561 reduced by 50% but disappears upon full reduction of the cytochrome by ascorbate. The signal is strikingly similar to that of the mitochondrial low-potential cytochrome b heme (bL or b-566). The presence of several forms of b-561 in chromaffin granule membranes may provide a structural basis for the transmembrane electron transfer believe to be catalyzed by this hemoprotein.

Rat adrenal medulla: levels of chromogranins, enkephalins, dopamine beta-hydroxylase and of the amine transporter are changed by nervous activity and hypophysectomy

The levels of several constituents of chromaffin granules were determined in rat adrenal medulla after treatment with insulin or reserpine and after hypophysectomy. Insulin treatment induced increased levels of enkephalins, dopamine beta-hydroxylase, the amine carrier and of chromogranin B whereas catecholamines, cytochrome b-561 and chromagranin A remained slightly below control levels. Administration of reserpine led to similar changes with the exception of the enkephalins which increased much less. After hypophysectomy, chromogranin A was reduced to low levels; dopamine beta-hydroxylase, the amine carrier and catecholamines showed an intermediate reduction, whereas the other components remained at control levels. We can conclude that nervous stimulation of the adrenal medulla (by insulin and reserpine) induces the synthesis of enkephalins, dopamine beta-hydroxylase, the amine carrier and chromogranin B whereas the level of chromogranin A apparently depends on corticosteroids. Thus, major components of the secretory content and the membranes of chromaffin granules are regulated separately, which allows the biogenesis of chromaffin granules with a significantly altered composition.

The role of ascorbate in biomembrane energetics

The mechanism(s) whereby membrane translocations are energized are poorly understood. Our work has focused on transmembrane microsomal and plasma membrane redox constituents as a means to energize membranes via alternative mechanisms complementary to ATP-driven processes. One such component is NADH-ascorbate free radical (mono- or semidehydroascorbate) oxidoreductase. This activity is associated with the trans or exit face of the Golgi apparatus, transport vesicles that move between the Golgi apparatus and the plasma membrane, and with the plasma membrane itself. Various lines of evidence, mostly indirect, link this activity to membrane translocations. Included is an apparent activation of the reductase in membranes when coated with clathrin, a single large polypeptide chain involved in exocytosis and in receptor-mediated and absorptive endocytosis. The results are consistent with a role of the ascorbate free radical as an acceptor for electron transport-mediated transfer of electrons from NADH perhaps to oxygen by coated membranes as a part of a mechanism to drive membrane translocations via generation of a proton gradient or of a membrane potential. Additionally, plasma membrane redox may be important in the regulation of cell growth, but a strict dependence on ascorbate free radical for the latter seems less likely than with internal endomembranes, where redox function may strictly depend upon the restricted pool of regeneratable acceptor that the ascorbate free radical provides.

Secretory vesicle cytochrome b561: a transmembrane electron transporter

The major function of cytochrome b561 is now clear. This transmembrane protein transports electrons across a secretory vesicle bilayer to supply electrons to monooxygenases inside the secretory vesicle. Cytochrome b561 has been localized not only to adrenergic secretory vesicles, where it supplies electrons to dopamine beta-hydroxylase, but also to peptidergic secretory vesicles that contain peptidyl alpha-amidating monooxygenase. Thus, one would expect to find cytochrome b561 in the membranes of all neuroendocrine cells that contain amidated peptide secretory products. In addition, its wide occurrence as an integral membrane protein of secretory vesicles may make it useful for investigation of vesicle biogenesis and turnover. One of the most important potential roles of cytochrome b561 is that it can be used as a model protein to investigate long-range biological electron transport. This cytochrome is a single polypeptide, which can be purified easily and reconstituted into a functional assembly. It also catalyzes an experimentally unambiguous transmembrane transport of electrons. A full molecular characterization of the structure and function of this cytochrome may provide insights into biological electron transfer which would otherwise be difficult or impossible to obtain.

Mechanism of ascorbic acid regeneration mediated by cytochrome b561

In summary, ascorbic acid serves as a one-electron donor for dopamine beta-hydroxylase in chromaffin vesicles and probably for peptide amidating monooxygenase in neurohypophyseal secretory vesicles. It appears that the semidehydroascorbate that is produced is reduced by cytochrome b561 to regenerate intravesicular ascorbate. Cytochrome b561, a transmembrane protein, is reduced in turn by an extravesicular electron donor, probably cytosolic ascorbic acid. It will be interesting to see whether other ascorbate-requiring enzymes in other organelles use a similar ascorbate-regenerating system to provide an intravesicular supply of reducing equivalents.