Cytochrome b561 is identical with chromomembrin B, a major polypeptide of chromaffin granule membranes

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.

Spectral characterization of the recombinant mouse tumor suppressor 101F6 protein

Tumor suppressor protein 101F6, a gene product of the 3p21.3 (human) and 9F1 (mouse) chromosomal region, has recently been identified as a member of the cytochrome b561 (Cyt-b561) protein family by sequence homology. The His(6)-tagged recombinant mouse tumor suppressor Cyt-b561 protein (TSCytb) was recently expressed in yeast and purified, and the ascorbate reducibility was determined. TSCytb is auto-oxidizable and has two distinct heme b centers with redox potentials of approximately 40 and approximately 140 mV. Its split alpha-band in the dithionite-reduced spectrum at both 295 and 77 K is well resolved, and the separation between the two alpha-peaks is approximately 7 nm (approximately 222 cm(-1)). Singular value decomposition analysis of the split alpha-band in the ascorbate-reduced spectra revealed the presence of two major spectral components, each of them with split alpha-band but with different peak separations (6 and 8 nm). Similar minor differences in peak separation were obtained when the split alpha-bands in ascorbate-reduced difference spectra at low (<1 mM) and high (>10 mM) ascorbate concentrations were analysed. According to low-temperature electron paramagnetic resonance (EPR) spectroscopy, the two heme b centers are in the low-spin ferric state with maximum principal g values of 3.61 and 2.96, respectively. These values differ from the ones observed for other members of the Cyt-b561 family. According to resonance Raman spectroscopy, the porphyrin rings are in a relaxed state. The spectroscopic results are only partially in agreement with those obtained earlier for the native chromaffin granule Cyt-b561.

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.

An ascorbate-reducible cytochrome b561 is localized in macrophage lysosomes

Cytochromes b561 (Cyts b561) are a family of intrinsic membrane proteins involved in ascorbate-mediated transmembrane electron transport. The chromaffin granule Cyt b561 (CGCytb) is believed to transport electrons donated by extravesicular ascorbate (ASC) across the membrane to intravesicular monodehydroascorbate (MDA) supporting catecholamine synthesis in neuroendocrine tissues. Another isoform, the duodenal Cyt b561 (Dcytb), was reported to have ferric reductase activity, possibly facilitating intestinal iron uptake. Herein, a new Cyt b561 homologue, LCytb (for lysosomal Cytb561) was found expressed in the late endosomal-lysosomal membrane. LCytb shared high sequence similarity with CGCytb (45% identity) and Dcytb (42% identity). Moreover, four heme-coordinating His residues, and putative ASC and MDA binding sites were highly conserved. Recombinant LCytb exhibited an ASC-reducible b-type Cyt absorbance spectrum with alpha-band maximum at 561 nm in the spectrum of the reduced protein. Northern blots and Western blots revealed that LCytb was predominantly expressed in lung, spleen, thymus, testis and placenta. In situ hybridization and immunofluorescence studies further demonstrated that the protein was expressed in the alveolar macrophages of the lung, in the white pulp of the spleen, widespread in the thymus, and in the Sertoli cells of the testis. Sequence analysis indicated the presence of a (DE)XXXL(LI)-type signal in the C-terminal of the protein, predicting a late endosomal-lysosomal subcellular localization. This localization was confirmed by double labeling experiments in RAW264.7 and 293 cells, stably transfected with LCytb.

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.

Purification and characterization of bovine adrenal cytochrome b561 expressed in insect and yeast cell systems

Bovine adrenal chromaffin granule cytochrome (cyt) b561 is a transmembrane hemoprotein that plays a key role in transporting reducing equivalents from ascorbate to dopamine-beta-hydroxylase for catecholamine synthesis. We have developed procedures for expression and purification of functional bovine adrenal cyt b561 in insect and yeast cell systems. The bovine cyt b561 coding sequence, with or without a hexahistidine-tag sequence at the C-terminus, was cloned into the pVL1392 transfer vector under the control of the polyhedrin promoter to generate recombinant baculovirus for protein expression in Sf9 insect cells (approximately 0.5 mg detergent-solubilized cyt b561/L culture). For the yeast system, the cyt b561 cDNA was modified with a hexahistidine-tag sequence at the C-terminus, and inserted into the pPICZB vector under the control of the alcohol oxidase promoter. The recombinant plasmid was transformed into Pichia pastoris GS115 competent cells to give methanol-inducible cyt b561 expression (approximately 0.7 mg detergent-solubilized cyt b561/L culture). Recombinant His-tagged cyt b561 expressed in Sf9 or Pichia cells was readily solubilized from membrane fractions with dodecyl maltoside and purified to electrophoretic homogeneity by one-step chromatography on Ni-NTA affinity resin. The purified recombinant cytochrome from both systems had a heme to protein ratio close to two and was fully functional, as judged by comparison with the spectroscopic and kinetic parameters of the endogenous cytochrome from chromaffin granules. A novel procedure for isolation of chromaffin granule membranes was developed to utilize frozen adrenal glands instead of fresh tissue.

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.

Distinct roles of two heme centers for transmembrane electron transfer in cytochrome b561 from bovine adrenal chromaffin vesicles as revealed by pulse radiolysis

The reaction of monodehydroascorbate (MDA) radical with purified cytochrome b561 from bovine adrenal chromaffin vesicles was investigated by the technique of pulse radiolysis. Radiolytically generated MDA radical oxidized rapidly the reduced form of cytochrome b561 to yield the oxidized form. Subsequently the oxidized form of cytochrome b561 was re-reduced by ascorbate in the medium. The second-order rate constants of the reaction of MDA radical were increased with decreasing pH, whereas a maximum of the second-order rate constant for the reaction with ascorbate was obtained around pH 6.8. At excess MDA radical to cytochrome b561 concentration, only half of the heme in cytochrome b561 was oxidized, indicating that only one of the two heme centers can react with MDA radical. On the other hand, when the reactions were examined using cytochrome b561 pretreated in a mild alkaline condition in the oxidized state, the cytochrome b561 could not be oxidized with MDA radical, suggesting that the heme center specific for the electron donation to MDA radical is selectively modified upon the alkaline treatment. These results suggest that the two heme b centers have distinct roles for the electron donation to MDA radical and the electron acceptance from ascorbate, respectively.

Existence of two heme B centers in cytochrome b561 from bovine adrenal chromaffin vesicles as revealed by a new purification procedure and EPR spectroscopy

We have established a new purification procedure of cytochrome b561 from bovine adrenomedullary chromaffin vesicles. The heme content analysis of the purified sample indicated the presence of 1.7 molecules of heme B/cytochrome b561 molecule. EPR spectroscopy of the purified enzyme in oxidized state showed that there were three types of low spin heme species. Two of them showed usual EPR signals at gz = 3.14 and gz = 2.84 arising from the same heme and were interconvertible depending on pH. The other species showed a highly anisotropic low spin signal at gz = 3.70, with a lower redox potential than the others, and a temperature-sensitive character. These properties are very similar to low potential cytochrome b (bL or b566) of the mitochondrial complex III, indicating that the gz = 3.70 species is derived from a heme component different from the one that shows the usual low spin EPR signals. Based on our new structural model, these two heme B prosthetic groups are likely to be located on both sides of the membranes in close contact with the ascorbic acid- and semidehydroascorbic acid-binding sites, respectively, to facilitate the electron transfer across the membranes. This molecular architecture may provide a structural basis for the transmembrane electron transfer catalyzed by this hemoprotein.

Xenopus cytochrome b561: molecular confirmation of a general five transmembrane structure and developmental regulation at the gastrula stage

Cytochrome b561 transports electrons by a novel transmembrane vectoral pathway. Using the human cytochrome b561 cDNA probe, the Xenopus cytochrome b561 cDNA has been isolated and sequenced. A specific ATG was unambiguously identified because the Xenopus sequence has a stop codon 7 amino acids upstream from the initiation site ATG. The 741-bp open reading frame encodes a 247-amino-acid protein. The position of the initiating ATG site in Xenopus cytochrome b561 is consistent with the first ATG site in human and mouse proteins, and with the second of two ATG sites in the bovine protein. This result indicates that a five transmembrane domain model best represents the conformation of cytochrome b561 in membranes. Cytochrome b561 plays a fundamental role in the physiology of all neuroendocrine tissues, and the results presented here show that Xenopus cytochrome b561 mRNA is specific to neuroendocrine tissues and is developmentally regulated at the gastrula stage.

SDS-resistant aggregation of membrane proteins: application to the purification of the vesicular monoamine transporter

The vesicular monoamine transporter, which catalyses a H+/ monoamine antiport in monoaminergic vesicle membrane, is a very hydrophobic intrinsic membrane protein. After solubilization, this protein was found to have a high tendency to aggregate, as shown by SDS/PAGE, especially when samples were boiled in the classical Laemmli buffer before electrophoresis. This behavior was analysed in some detail. The aggregation was promoted by high temperatures, organic solvents and acidic pH, suggesting that it resulted from the unfolding of structure remaining in SDS. The aggregates were very stable and could be dissociated only by suspension in anhydrous trifluoroacetic acid. This SDS-resistant aggregation behaviour was shared by very few intrinsic proteins of the chromaffin granule membrane. Consequently, a purification procedure was based on this property. A detergent extract of chromaffin granule membranes enriched in monoamine transporter was heated and the aggregates were isolated by size-exclusion HPLC in SDS. The aggregates, containing the transporter, were dissociated in the presence of trifluoroacetic acid and analysed on the same HPLC column. This strategy might be of general interest for the purification of membrane proteins that exhibit SDS-resistant aggregation.

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.

Glycoprotein II from adrenal chromaffin granules is also present in kidney lysosomes

Glycoprotein II (GP II) is a protein found in the membranes of chromaffin granules from adrenal medulla. Immunoblotting (one- and two-dimensional) revealed that this antigen is also present in liver and in kidney. Subcellular fractionation of the latter organ indicated that GP II was present in lysosomes. This was confirmed by immunoelectron microscopy. The antiserum against GP II immunolabelled the membranes of organelles which could be identified as lysosomes by the labelling of their contents with an antiserum against cathepsin D. Thus GP II is an antigen common to secretory vesicles and lysosomes.

Identification and characterization of glycoproteins after extraction of bovine chromaffin-granule membranes with lithium di-iodosalicylate. Purification of glycoprotein II from the soluble fraction

Chromaffin-granule membranes were separated into insoluble and soluble fractions after extraction with lithium di-iodosalicylate (LDIS). These fractions were characterized by one- and two-dimensional gel electrophoresis, and glycoproteins were detected after electroblotting with peroxidase-labelled concanavalin A and wheat-germ agglutinin (WGA). The LDIS-insoluble fraction contained components identified as glycoproteins III, H, J and K (carboxypeptidase H). Microsequence analysis indicated that component J is an N-terminally extended form of glycoprotein K. A major glycoprotein, GpII (Mr 80,000-100,000), present in the LDIS-soluble fraction was purified by affinity chromatography on WGA-Sepharose. This was characterized by one- and two-dimensional gel electrophoresis with Coomassie Blue staining, by amino acid analysis and automated N-terminal sequence analysis. Extraction of chromaffin-granule membranes with LDIS is a simple and rapid procedure that facilitates studies concerned with the structure and function of membrane glycoproteins from these and other secretory granules.

Immunological characterization of a membrane glycoprotein of chromaffin granules: its presence in endocrine and exocrine tissues

A glycoprotein was isolated from detergent solubilized membranes of bovine chromaffin granules by high-performance liquid chromatography. Specific antisera raised against this glycoprotein reacted in one- and two-dimensional immunoblots with a heterogeneous component with a pI of 4.2-4.7 and Mr 100,000. The antiserum against bovine glycoprotein II cross-reacted with an analogous component in several species. The specific localization of glycoprotein II in chromaffin granules was established by density gradient centrifugation followed by immunoblotting. The antiserum, as shown by one- and two-dimensional immunoblotting, reacted with an analogous antigen in the posterior pituitary, in endocrine (anterior pituitary, parathyroid gland) and exocrine (parotid gland, pancreas) organs. In the pancreas the protein reacting with the antiserum was found in the membranes of zymogen granules. The results demonstrate for the first time that secretory vesicles of endocrine and exocrine tissues have at least one common antigen, i.e. the glycoprotein II. It seems likely that this protein is involved in a basic function common to all secretory vesicles.

Purified cytochrome b from human granulocyte plasma membrane is comprised of two polypeptides with relative molecular weights of 91,000 and 22,000

A new method has been developed for purification of cytochrome b from stimulated human granulocytes offering the advantage of high yields from practical quantities of whole blood. Polymorphonuclear leukocytes were treated with diisopropylfluorophosphate, degranulated and disrupted by nitrogen cavitation. Membranes enriched in cytochrome b were prepared by differential centrifugation. Complete solubilization of the cytochrome from the membranes was achieved in octylglucoside after a 1-M salt wash. Wheat germ agglutinin-conjugated Sepharose 4B specifically bound the solubilized cytochrome b and afforded a threefold purification. Eluate from the immobilized wheat germ agglutinin was further enriched by chromatography on immobilized heparin. The final 260-fold purification of the b-type cytochrome with a 20-30% yield was achieved by velocity sedimentation in sucrose density gradients. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of the purified preparation revealed two polypeptides of Mr 91,000 and Mr 22,000. Treatment of the 125I-labeled, purified preparation with peptide:N-glycosidase F, which removes N-linked sugars, decreased relative molecular weight of the larger species to approximately 50,000, whereas beta-elimination, which removes O-linked sugars, had little or no effect on the mobility of the Mr-91,000 polypeptide. Neither of the deglycosylation conditions had any effect on electrophoretic mobility of the Mr-22,000 polypeptide. Disuccinimidyl suberate cross-linked the two polypeptides to a new Mr of 120,000-135,000 by SDS-PAGE. Antibody raised to the purified preparation immunoprecipitated spectral activity and, on Western blots, bound to the Mr-22,000 polypeptide but not the Mr-91,000 polypeptide. Western blot analysis of granulocytes from patients with X-linked chronic granulomatous disease revealed a complete absence of the Mr-22,000 polypeptide. These results (a) suggest that the two polypeptides are in close association and are part of the cytochrome b, (b) provide explanation for the molecular weight discrepancies previously reported for the protein, and (c) further support the involvement of the cytochrome in superoxide production in human neutrophils.

Cytochrome b561 can be detected in many neuroendocrine tissues using a specific monoclonal antibody

CG7 is a monoclonal antibody that recognizes cytochrome b561, a major protein component of adrenal medullary chromaffin granules. Immunocytochemical studies using this antibody have demonstrated that cytochrome b561 is present in many neural and endocrine tissues and that its distribution is correlated with the presence of either catecholamines or amidated peptides in the tissue. These tissues include neuronal cell bodies and/or fibers in the gut, blood vessels, retina, and posterior pituitary, endocrine cells of the gut, anterior and intermediate lobes of the pituitary, heart muscle, and all adrenal medullary chromaffin cells. The discovery of cytochrome b561 in many neuropeptide-containing tissues regardless of the presence of catecholamines is consistent with a general role for cytochrome b561 as a secretory granule membrane electron carrier. Its expression may be linked to an ascorbic acid requirement by both catecholamine and neuropeptide biosynthetic enzymes.

Monoclonal antibodies to chromaffin cells can distinguish proteins specific to or specifically excluded from chromaffin granules

I have prepared a number of monoclonal antibodies to chromaffin cell membranes. One of these antibodies recognizes a number of antigenically related proteins that are present in all tissues examined. In the adrenal, these proteins are completely excluded from chromaffin granules but are present in other subcellular membrane fractions. This non-granule membrane-specific antibody has been designated NG3. A second antibody, CG7, binds to a single protein which segregates specifically into chromaffin granules. The protein recognized by CG7 is cytochrome b561, or chromomembrin B, one of the major protein components of chromaffin granule membranes. CG7 also labels a protein (the identical cytochrome b561) in bovine posterior pituitary neurosecretory vesicle membranes indicating that it functions in both peptidergic and catecholaminergic secretory granules. These two monoclonal antibodies provide useful probes of both granule and extra-granule membrane proteins for studies of membrane trafficking in chromaffin cells.

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.

Isolation and characterization of chromogranins A, B, and C from bovine chromaffin granules and a rat pheochromocytoma

Bovine chromaffin granules from adrenal medulla contain three acidic secretory proteins: chromogranins A, B, and C. For isolation of these proteins, methods based mainly on high performance liquid chromatography were developed. After removal of contaminating glycoproteins by lectin affinity chromatography, chromogranins were separated by high performance anion-exchange, gel-filtration, and reverse phase liquid chromatography. As a final purification step sodium dodecyl sulfate-gel electrophoresis was performed. Amino acid analysis of isolated bovine chromogranins revealed a similar composition of all three proteins, with glutamic acid being the most prominent amino acid. The methods developed for bovine proteins also proved suitable for isolating rat chromogranins A and B from a transplantable pheochromocytoma. Chromogranin C was not present in sufficient amounts to be isolated from this tissue. The chromogranins purified by these methods were used to raise specific antibodies in rabbits. The use of purified chromogranins together with specific antisera may be valuable in understanding the still undiscovered function of these proteins.

Fractionation of membrane proteins by temperature-induced phase separation in Triton X-114. Application to subcellular fractions of the adrenal medulla

After solubilization with the detergent Triton X-114, membrane proteins may be separated into three groups: if the membrane is sufficiently lipid-rich, one family of hydrophobic constituents separates spontaneously at low temperature; warming at 30 degrees C leads to separation of a detergent-rich phase and an aqueous phase. Using the chromaffin-granule membrane as a model, we found that many intrinsic membrane glycoproteins are found in the latter phase, probably maintained in solution by adherent detergent. They precipitate, however, when this is removed by dialysis, leaving in solution those truly hydrophilic proteins that were originally adhering to the membranes. We have used this method with mitochondria, and with Golgi- and rough-endoplasmic-reticulum-enriched microsomal fractions: it has proved to be a rapid and convenient method for effecting a partial separation of proteins from a variety of different membranes.

Isolation of ATPase I, the proton pump of chromaffin-granule membranes

Chromaffin-granule membranes contain two ATPases, which can be separated by (NH4)2SO4 fractionation after solubilization with detergents, or by phase segregation in Triton X-114. ATPase I (Mr 400000) is inhibited by trialkyltin, quercetin and alkylating agents, and hydrolyses both ATP and ITP. It contains up to five types of subunit, including a low-Mr hydrophobic polypeptide that reacts with dicyclohexylcarbodi-imide; these subunits are unrelated to those of mitochondrial F1F0-ATPase, as judged by size and reaction with antibodies. ATPase II (Mr 140000) is inhibited by vanadate, and is specific for ATP; it has not been extensively purified. Proton translocation by resealed chromaffin-granule 'ghosts', measured by uptake of methylamine or by quenching of the fluorescence of 9-amino-6-chloro-2-methoxyacridine, is supported by the hydrolysis of ATP or ITP, and inhibited by quercetin or alkylating agents, but not by vanadate. ATPase I must therefore be the proton translocator involved in the uptake of catecholamines and possibly of other components of the chromaffin-granule matrix, whereas ATPase II does not translocate protons.

Immunological studies on the distribution of chromogranin A and B in endocrine and nervous tissues

Bovine chromaffin granules contain two major families of acidic proteins, chromogranins A and B. The occurrence of these proteins in endocrine and nervous tissue was investigated by immunoblotting (one- and two-dimensional), and by immunohistochemistry. Immunoblotting revealed that in anterior hypophysis and in splenic nerve from ox, immunologically crossreacting proteins are present which in two-dimensional electrophoresis migrate to the same position as adrenal chromogranins A and B. Smaller proteins derived from chromogranin B by endogenous proteolysis were much less prominent in these tissues when compared with adrenal medulla. Immunohistochemistry performed in rat and bovine tissues established that chromogranin B is present in all cells of the adrenal medulla. It is also found in the anterior hypophysis, the endocrine pancreas, in enterochromaffin and in sympathetic ganglion cells, but e.g. is absent from posterior hypophysis and exocrine tissues. It is concluded that chromogranins A and B have a widespread distribution in endocrine and nervous tissue. Proteolytic processing of chromogranin B in the storage organelles of hypophysis and splenic nerves is apparently slower than that in chromaffin granules. The widespread distribution of the chromogranins resembling that of neuropeptides is a clear indication for some special, yet to be discovered, function.

Immunological characterization of secretory proteins of chromaffin granules: chromogranins A, chromogranins B, and enkephalin-containing peptides

The soluble proteins of bovine chromaffin granules can be resolved into about 40 proteins by two-dimensional electrophoresis. Use of several antisera enabled us to characterize most of these proteins with the immune replica technique. An antiserum against dopamine beta-hydroxylase reacted with one protein of Mr 75,000. Met-enkephalin antisera labeled eight proteins of Mr 23,000-14,000. A new method was developed to obtain highly purified chromogranin A for immunization. The antiserum reacted with chromogranin A and several smaller proteins of similar pI. This specific antiserum did not react with a second family of hitherto undescribed proteins, which we propose to call chromogranins B. An antiserum against these proteins was raised. It labeled several proteins ranging in Mr from 100,000 to 24,000 and focusing at pH 5.2. Subcellular fractionation established that chromogranins B are specifically localized in chromaffin granules of several species. They are secreted from the adrenal medulla during cholinergic stimulation. We conclude that apart from dopamine beta-hydroxylase chromaffin granules contain three families of immunologically unrelated proteins.

Rate of transmembrane electron transfer in chromaffin-vesicle ghosts

The chromaffin vesicle of the adrenal medulla contains a transmembrane electron carrier that may provide reducing equivalents for dopamine beta-hydroxylase in vivo. This electron-transfer system can be assayed by trapping ascorbic acid inside resealed membrane vesicles (ghosts), adding an external electron acceptor such as ferricytochrome c or ferricyanide, and following the reduction of these acceptors spectrophotometrically. Cytochrome c reduction is more rapid at high pH and is proportional to the amount of chromaffin-vesicle ghosts, at least at low ghost concentrations. At pH 7.0, ghosts loaded with 100 mM ascorbic acid reduce 60 microM cytochrome c at a rate of 0.035 +/- 0.010 mu equiv min-1 (mg of protein)-1 and 200 microM ferricyanide at a rate of 2.3 +/- 0.3 mu equiv min-1 (mg of protein)-1. The rate of cytochrome c reduction is accelerated to 0.105 +/- 0.021 mu equiv min-1 (mg of protein)-1 when cytochrome c is pretreated with equimolar ferrocyanide. Pretreatment of cytochrome c with ferricyanide also causes a rapid rate of reduction, but only after an initial delay. The ferrocyanide-stimulated rate of cytochrome c reduction is further accelerated by the protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), probably because FCCP dissipates the membrane potential generated by electron transfer. These rates of electron transfer are sufficient to account for electron transfer to dopamine beta-hydroxylase in vivo and are consistent with the mediation of electron transfer by cytochrome b-561.

An electron transfer dependent membrane potential in chromaffin-vesicle ghosts

Adrenal medullary chromaffin-vesicle membranes contain a transmembrane electron carrier that may provide reducing equivalents for intravesicular dopamine beta-hydroxylase in vivo. This electron transfer system can generate a membrane potential (inside positive) across resealed chromaffin-vesicle membranes (ghosts) by passing electrons from an internal electron donor to an external electron acceptor. Both ascorbic acid and isoascorbic acid are suitable electron donors. As an electron acceptor, ferricyanide elicits a transient increase in membrane potential at physiological temperatures. A stable membrane potential can be produced by coupling the chromaffin-vesicle electron-transfer system to cytochrome oxidase by using cytochrome c. The membrane potential is generated by transferring electrons from the internal electron donor to cytochrome c. Cytochrome c is then reoxidized by cytochrome oxidase. In this coupled system, the rate of electron transfer can be measured as the rate of oxygen consumption. The chromaffin-vesicle electron-transfer system reduces cytochrome c relatively slowly, but the rate is greatly accelerated by low concentrations of ferrocyanide. Accordingly, stable electron transfer dependent membrane potentials require cytochrome c, oxygen, and ferrocyanide. They are abolished by the cytochrome oxidase inhibitor cyanide. This membrane potential drives reserpine-sensitive norepinephrine transport, confirming the location of the electron-transfer system in the chromaffin-vesicle membrane. This also demonstrates the potential usefulness of the electron transfer driven membrane potential for studying energy-linked processes in this membrane.

Glycoproteins of the chromaffin granule membrane: separation by two-dimensional electrophoresis and identification by lectin binding

The proteins of highly purified chromaffin-granule membranes were separated by one- or two-dimensional electrophoresis, then transferred to nitrocellulose sheets; glycosylation was investigated by binding of several different radioiodinated lectins. Over 20 different glycosylated components were identified; comparison with mitochondrial and microsomal fractions suggested that most of the major glycoproteins are genuine components of the chromaffin granule membrane, rather than contaminants originating in other organelles. Two-dimensional electrophoresis revealed heterogeneity within several of the glycoproteins, and this is ascribed to differences in the state of glycosylation, on the basis of shifts in electrophoretic mobility produced by treatment with neuraminidase. Neuraminidase treatment of chromaffin granule membranes also enhances the binding of many lectins. The identities of the lectin-binding bands are discussed: neither cytochrome b561 nor the F1-like ATPase appears to be glycosylated. Chromogranin A, although a glycoprotein, does not bind any of the lectins tested, but a number of concanavalin-A binding proteins, as well as dopamine beta-hydroxylase, are present in the chromaffin granule lysate.

Isolation of a membrane protein by chromatofocusing: cytochrome b-561 of the adrenal chromaffin granule

Chromatofocusing, a form of ion-exchange chromatography in which proteins are separated on the basis of their differing isoelectric points, has been adapted for use with membrane proteins, solubilized by the non-ionic detergent Nonidet P-40. Using a two-step detergent extraction followed by chromatofocusing under high pressure, the highly hydrophobic protein cytochrome b-561 was isolated from chromaffin granule membranes and purified to near homogeneity in a functionally active form, in less than 5 h. Chromatofocusing conditions were optimized empirically since the behaviour of the chromaffin granule membrane proteins conformed less to the theory than that of soluble proteins, and the various factors affecting yield and resolution are discussed. The speed, high resolution and focusing effect could make this method particularly suitable for rapid isolation in a functionally active form of the many membrane proteins that are unstable in dilute solution and when removed from their lipid environment.

Isolation and immunological characterization of a glycoprotein from adrenal chromaffin granules

A glycoprotein (s-GP III) was isolated from the soluble lysate of chromaffin granules by chromatography with immunoaffinity and lectin columns. An identical protein (m-GP III) was shown to be present in the granule membranes. The apparent molecular weight of these glycoproteins as determined by the electrophoresis system of Laemmli (1970) was 43,000 under reducing conditions. In the absence of mercaptoethanol they aggregated to dimers. Antisera were raised against both the soluble and the membrane-bound forms of this glycoprotein. With these antisera GP III was further characterized: Immunoreplicas were obtained after two-dimensional electrophoresis of soluble and membrane-bound proteins of chromaffin granules. GP III was identified as a protein with a rather broad pI (4.6-5.3), indicating microheterogeneity. As shown by subcellular fractionation, m-GP III is specifically confined to chromaffin granules. GP III can therefore be used as a marker for the membranes of these organelles. The soluble form is secreted from adrenal medulla during stimulation with carbamylcholine chloride. An immunologically identical antigen was detected in adeno- and neurohypophysis. The physiological function of GP III is still unknown. It does not demonstrate any of the enzymatic activities so far known to occur in chromaffin granules.

Unusual redox behaviour of cytochrome b-561 from bovine chromaffin granule membranes

Redox titrations of cytochrome b-561 have been performed with the purified cytochrome and with intact and detergent-solubilized chromaffin-granule membranes. The midpoint redox potential of the cytochrome is 100-130 mV; this depends upon the composition of the buffer, but is independent of pH in the range 5.5-7.5; partial proteolysis of the cytochrome raises the midpoint potential to 160 mV. The Nernst plots of titration data have slopes of 75-115 mV, and are in some cases sigmoid in shape. This may be explained by negative cooperativity during redox transitions in oligomeric cytochrome b-561. Measurements of the haem and cytochrome content of chromaffin granule membrane suggest a haem content of 1 mol/mol protein. Chemical crosslinking of cytochrome b-561 suggests that it may exist as an oligomer of 4-6 polypeptide chains within the chromaffin granule membrane. Aggregation of purified cytochrome b-561 was shown by gel filtration studies and by immunological methods in SDS-polyacrylamide gels. Studies of the molecular weight of the aggregates suggest that the monomer has a molecular weight close to 22 000, but migrates anomalously slowly during electrophoresis.

The asymmetric orientation of cytochrome b561 in bovine chromaffin granule membranes

The topological arrangement of cytochrome b561 in the bovine adrenal medullary chromaffin granule membrane was investigated by radiolabeling and immunoprecipitation techniques using antibody raised against the purified cytochrome. The first labeling procedure involved a membrane-permeable amino group labeling reagent, ethyl acetimidate, and two membrane-nonpermeable amino group labeling reagents, isethionyl acetimidate and trinitrobenzenesulfonic acid. The second radiolabeling procedure involved lactoperoxidase-catalyzed iodination of the exposed tyrosines on the membrane-bound proteins. The labeled cytochrome b561 was isolated by immunoprecipitating detergent extracts of treated membranes, followed by electrophoresis of the precipitated cytochrome in polyacrylamide-dodecyl sulfate. From the analysis of both labeling techniques, cytochrome b561 appeared to be a transmembrane protein and a major portion of this protein was cytoplasmically exposed.

Biosynthetic relationship between the major matrix proteins of adrenal chromaffin granules

The matrix of the chromaffin granule contains a family of acidic proteins, collectively known as the chromogranins. It has been suggested that this family results from protease action on the major component, chromogranin A. Evidence for this has now been obtained from in vitro translation of adrenal medullary messenger RNA and immunoprecipitation of translation products using an antiserum directed against chromogranin A, but which also recognises other chromogranins.

Hydrophilic form of dopamine beta-hydroxylase from purified noradrenergic vesicles. Activities and comparisons

Bovine splenic nerve and adrenal medulla were used as homologous sources of dopamine beta-hydroxylase permitting the isolation of enzyme specific to a purified fraction of large dense cored noradrenergic vesicles and chromaffin granules, respectively. The hydrophilic (water soluble) form of the enzyme was purified to homogeneity on the bases of gel electrophoresis, isoelectric focusing, and double immunodiffusion tests from the physical lysates of the vesicles and granules. Amino acid analyses suggest that the hydrophilic dopamine beta-hydroxylase is the predominant form in the nerve vesicles. It has higher neutral and lower hydrophobic amino acid group residues when compared to the adrenomedullary enzyme prepared in this and most other laboratories. Among the neutral amino acids, this difference appears to reflect approximately 40% higher serine and glycine contents, and among the hydrophobic amino acids it may reflect in part approximately 25% lower leucine content. Although the terms hydrophilic and amphiphilic can be properly applied to certain chemical properties of the D beta H forms, it is not at all certain that these terms can be used quantitatively to describe the matrix and membrane associated forms of the enzyme, respectively.

Characterization of detergent-solubilized adenosine triphosphatase of chromaffin granule membranes

Purified bovine chromaffin granule membranes contain approximately 24 pmol/mg protein (16 copies per granule) of an F1-like adenosine 5'-triphosphatase, and 340 pmol/mg protein (200 copies per granule) of a low-molecular weight protein which reacts covalently with dicyclohexylcarbodiimide. These co-purify on electrofocusing and exclusion chromatography and are apparently components of a proton-translocating adenosine triphosphatase complex, that is involved in maintaining the high concentration of catecholamines in the granules. The membranes contain another adenosine 5'-triphosphatase, of lower molecular weight, which is sensitive to inhibition by vanadate but relatively insensitive to dicyclohexylcarbodiimide. The function of this enzyme is unknown.