Structure of bovine adrenal dopamine beta-monooxygenase, as deduced from cDNA and protein sequencing: evidence that the membrane-bound form of the enzyme is anchored by an uncleaved signal peptide

RNAi-mediated knock-down of the dopamine beta-hydroxylase gene changes growth of razor clams

Dopamine beta-hydroxylase (DβH) plays an essential role in the synthesis of catecholamines (CA) in neuroendocrine networks. In the razor clam, Sinonovacula constricta a novel gene for DβH (ScDβH-α) was identified that belongs to the copper type II ascorbate-dependent monooxygenase family. Expression analysis revealed ScDβH-α gene transcripts were abundant in the liver and expressed throughout development. Knock-down of ScDβH-α in adult clams using siRNA caused a reduction in the growth rate compared to control clams. Reduced growth was associated with strong down-regulation of gene transcripts for the growth-related factors, platelet derived growth factors A (PDGF-A) (P < 0.001) 24 h after ScDβH-α knock-down, vascular endothelial growth factor (VEGF1) (P < 0.001) and platelet derived growth factor B (PDGF-B-2) (P < 0.001) 24 h and 48 h after ScDβH-α knock-down and transforming growth factor beta (TGF-β1) (P < 0.001) 48 h and 72 h after ScDβH-α knock-down. Taken together the results suggest that the novel ScDβH-α gene through its role in CA synthesis is involved in growth regulation in the razor clam and possibly other bivalves.

Pro-apoptotic protein-protein interactions of the extended N-AChE terminus

The N-terminally extended “synaptic” acetylcholinesterase variant N-AChE-S operates to promote apoptosis; however, the protein partners involved in this function remain unknown. Here, we report that when microinjected to fertilized mouse oocytes, N-AChE-S caused embryonic death as early as the zygotic stage. To identify the putative protein partners involved, we first tried yeast two hybrid screening, but this approach failed, probably because of the N-AChE-S-induced lethality. In contrast, sequence analysis and a corresponding peptide array revealed possible partners, which were validated by co-immunoprecipitation. These include the kinases GSK3, Aurora and GAK, the membrane integrin receptors, and the death receptor FAS. Each of these could potentially modulate N-AChE-S-induced apoptosis with possible therapeutic value for the treatment of Alzheimer’s disease.

Catecholaminergic systems in stress: structural and molecular genetic approaches

Stressful stimuli evoke complex endocrine, autonomic, and behavioral responses that are extremely variable and specific depending on the type and nature of the stressors. We first provide a short overview of physiology, biochemistry, and molecular genetics of sympatho-adrenomedullary, sympatho-neural, and brain catecholaminergic systems. Important processes of catecholamine biosynthesis, storage, release, secretion, uptake, reuptake, degradation, and transporters in acutely or chronically stressed organisms are described. We emphasize the structural variability of catecholamine systems and the molecular genetics of enzymes involved in biosynthesis and degradation of catecholamines and transporters. Characterization of enzyme gene promoters, transcriptional and posttranscriptional mechanisms, transcription factors, gene expression and protein translation, as well as different phases of stress-activated transcription and quantitative determination of mRNA levels in stressed organisms are discussed. Data from catecholamine enzyme gene knockout mice are shown. Interaction of catecholaminergic systems with other neurotransmitter and hormonal systems are discussed. We describe the effects of homotypic and heterotypic stressors, adaptation and maladaptation of the organism, and the specificity of stressors (physical, emotional, metabolic, etc.) on activation of catecholaminergic systems at all levels from plasma catecholamines to gene expression of catecholamine enzymes. We also discuss cross-adaptation and the effect of novel heterotypic stressors on organisms adapted to long-term monotypic stressors. The extra-adrenal nonneuronal adrenergic system is described. Stress-related central neuronal regulatory circuits and central organization of responses to various stressors are presented with selected examples of regulatory molecular mechanisms. Data summarized here indicate that catecholaminergic systems are activated in different ways following exposure to distinct stressful stimuli.

Combinatorial Complexity of 5′ Alternative Acetylcholinesterase Transcripts and Protein Products

To explore the scope and significance of alternate promoter usage and its putative inter-relationship to alternative splicing, we searched expression sequence tags for the 5' region of acetylcholinesterase (ACHE) genes. Three and five novel first exons were identified in human and mouse ACHE genes, respectively. Reverse transcription-PCR and in situ hybridization validated most of the predicted transcripts, and sequence analyses of the corresponding genomic DNA regions suggest evolutionarily conserved promoters for each of the novel exons identified. Distinct tissue specificity and stress-related expression patterns of these exons predict combinatorial complexity with known 3' alternative AChE mRNA transcripts. Unexpectedly one of the 5' exons encodes an extended N terminus in-frame with the known AChE sequence, extending the increased complexity to the protein level. The resultant membrane variant(s), designated N-AChE, is developmentally regulated in human brain neurons and blood mononuclear cells. Alternative promoter usage combined with alternative splicing may thus lead to stress-dependent combinatorial complexity of AChE mRNA transcripts and their protein products.

Dopamine beta-monooxygenase signal/anchor sequence alters trafficking of peptidylglycine alpha-hydroxylating monooxygenase

Dopamine beta-monooxygenase (DBM) and peptidylglycine alpha-hydroxylating monooxygenase (PHM) are essential for the biosynthesis of catecholamines and amidated peptides, respectively. The enzymes share a conserved catalytic core. We studied the role of the DBM signal sequence by appending it to soluble PHM (PHMs) and expressing the DBMsignal/PHMs chimera in AtT-20 and Chinese hamster ovary cells. PHMs produced as part of DBMsignal/PHMs was active. In vitro translated and cellular DBMsignal/PHMs had similar masses, indicating that the DBM signal was not removed. DBMsignal/PHMs was membrane-associated and had the properties of an intrinsic membrane protein. After in vitro translation in the presence of microsomal membranes, trypsin treatment removed 2 kDa from DBMsignal/PHMs while PHMs was entirely protected. In addition, a Cys residue in DBMsignal/PHMs was accessible to Cys-directed biotinylation. Thus the chimera adopts the topology of a type II membrane protein. Pulse-chase experiments indicate that DBMsignal/PHMs turns over rapidly after exiting the trans-Golgi network. Although PHMs is efficiently localized to secretory granules, DBMsignal/PHMs is largely localized to the endoplasmic reticulum in AtT-20 cells. On the basis of stimulated secretion, the small amount of PHMs generated is stored in secretory granules. In contrast, the expression of DBMsignal/PHMs in PC12 cells yields protein that is localized to secretory granules.

Cell type-specific storage of dopamine beta-monooxygenase

Expression of dopamine beta-monooxygenase (DBM), the enzyme that converts dopamine into norepinephrine, is limited to adrenal chromaffin cells and a small population of neurons. We studied DBM trafficking to regulated granules by stably expressing rat DBM in AtT-20 corticotrope tumor cells, which contain regulated granules, and in Chinese hamster ovary (CHO) cells, which lack regulated granules. The behavior of exogenous DBM in both cell lines was compared with endogenous DBM in adrenal chromaffin cells. CHO cells secreted active DBM, indicating that production of active enzyme does not require features unique to neuroendocrine cells. Pulse-chase experiments indicated that early steps in DBM maturation followed a similar time course in AtT-20, CHO, and adrenal chromaffin cells. Use of a conformation-sensitive DBM antiserum indicated that acquisition of a folded structure occurred with a similar time course in all three cell types. Cell type-specific differences in DBM trafficking became apparent only when storage in granules was examined. As expected, DBM was stored in secretory granules in chromaffin cells; CHO cells failed to store DBM. Despite the fact that AtT-20 cells have regulated granules, exogenous DBM was not stored in these granules. Thus storage of DBM in secretory granules requires cell type specific factors.

Catecholamine biosynthesis and physiological regulation in neuroendocrine cells

The catecholamines are widely distributed in mammals and their levels and physiological functions are regulated at many sites. These include their release from neuroendocrine cells, the type and sensitivity of the multiple receptors in target cells, the efficacy of the reuptake system in the secretory cells, and the rates of catecholamine biosynthesis and degradation. In the present review the main focus will be on the more recent studies on the biosynthesis in neuroendocrine cells which involves a specific set of enzymes, with special reference to physiologically important regulatory mechanisms. Eight enzymes of the biosynthetic pathway have now been identified, cloned, expressed as recombinant proteins, characterized with respect to catalytic and regulatory properties, and some of them also crystallized. The identification of the tyrosine hydroxylase catalysed reaction as the rate-limiting step in the normal catecholamine biosynthesis has attracted most attention, both in terms of transcriptional and post-translational regulation. In certain human genetic disorders of catecholamine biosynthesis other enzymes in the pathway may become rate-limiting, notably those involved in the biosynthesis/regeneration of the natural co-factor tetrahydrobiopterin in the tyrosine hydroxylase reaction. The enzymes involved seem to be regulated by a variety of physiological factors, both on a long-term scale and a short-term basis, and include the relative rates of synthesis, degradation and state of activation of the biosynthetic enzymes, notably of tyrosine hydroxylase. Multiple surface receptors and signalling pathways are activated in response to extracellular stimuli and play an essential role in the regulation of catecholamine biosynthesis.

A PC12 variant lacking regulated secretory organelles: aberrant protein targeting and evidence for a factor inhibiting neuroendocrine gene expression

A variant of the PC12 pheochromocytoma cell line (termed A35C) has been isolated that lacks regulated secretory organelles and several constituent proteins. Northern and Southern blot analyses suggested a block at the transcriptional level. The proprotein-converting enzyme carboxypeptidase H was synthesised in the A35C cell line but was secreted by the constitutive pathway. Transient transfection of A35C cells with cDNAs encoding the regulated secretory proteins dopamine beta-hydroxylase and synaptotagmin I resulted in distinct patterns of mistargeting of these proteins. It is surprising that hybrid cells created by fusing normal PC12 cells with A35C cells exhibited the variant phenotype, suggesting that A35C cells express an inhibitory factor that represses neuroendocrine-specific gene expression.

Identification and cloning of a sequence homologue of dopamine beta-hydroxylase

We have identified and cloned a cDNA encoding a new member of the monooxygenase family of enzymes. This novel enzyme, which we call MOX (monooxygenase X; unknown substrate) is a clear sequence homologue of the enzyme dopamine beta-hydroxylase (DBH). MOX maintains many of the structural features of DBH, as evidenced by the retention of most of the disulfide linkages and all of the peptidyl ligands to the active site copper atoms. Unlike DBH, MOX lacks a signal peptide sequence and therefore is unlikely to be a secreted molecule. The steady-state mRNA levels of MOX are highest in the kidney, lung, and adrenal gland, indicating that the tissue distribution of MOX is broader than that of DBH. Antisera raised to a fusion protein of MOX identifies a single band of the expected mobility by Western blot analysis. MOX mRNA levels are elevated in some fibroblast cell strains at replicative senescence, through this regulation is not apparent in all primary cell strains. The gene for MOX resides on the q arm of chromosome 6 and the corresponding mouse homolog has been identified.

Regulation of the biosynthesis of large dense-core vesicles in chromaffin cells and neurons

1. The proteins of large dense-core vesicles (LDV) in neuroendocrine tissues are well characterized. Secretory components comprise chromogranins and neuropeptides. Intrinsic membrane proteins include cytochrome b-561, transporters, SV2, synaptotagmin, and synaptobrevin. 2. The effects of stimulation and of second messengers on the biosynthesis of LDV have been studied in detail. 3. Regulation of biosynthesis is complex. The cell can adapt to prolonged stimulation either by producing vesicles of normal size filled with a higher quantum of secretory peptides or by forming larger vesicles. In addition, some components, e.g., enzymes, can be upregulated specifically.

Membrane composition of adrenergic large and small dense cored vesicles and of synaptic vesicles: consequences for their biogenesis

The membrane proteins of adrenergic large dense cored vesicles, in particular those of chromaffin granules, have been characterized in detail. With the exception of the nucleotide carrier all major peptides have been cloned. There has been a controversy whether these vesicles contain antigens like synaptophysin, synaptotagmin and VAMP or synaptobrevin found in high concentration in synaptic vesicles. One can now conclude that large dense core vesicles also contain these peptides although in lower concentrations. The biosynthesis of large dense core vesicles is analogous to that of other peptide secreting vesicles of the regulated pathway. One cannot yet definitely define the biosynthesis of small dense core vesicles which apparently have a very similar membrane composition to that of large dense core vesicles. They may form directly from large dense core vesicles when their membranes have been retrieved after exocytosis. These membranes may become sorted in an endosomal compartment where peptides may be deleted or added. Such an addition could be derived from synaptophysin-rich vesicles present in adrenergic axons. However small dense core vesicle peptides may also be transported axonally independent of large dense core vesicles. For proving one of these possibilities some crucial experiments have been suggested.

A natural variant of bovine dopamine beta-monooxygenase with phenylalanine as residue 208: purification and characterization of the variant homo- and heterotetramers of (F208)4 and (F208)2(L208)2

Bovine dopamine beta-monooxygenase was purified from each of 18 individual adrenal glands by the method we have developed for the rapid purification of the enzyme from a single adrenal gland. Differential peptide mapping of the 18 enzyme preparations following fluorescence labeling of their cysteine residues revealed the presence of a novel variant with Phe as residue 208 in 14 adrenal glands; seven of them were homozygous for the variant allele and the remaining seven heterozygous. The variant enzyme was a tetramer and exhibited kinetic and structural properties similar to those of the wild-type tetramer (L208)4. These results indicate an allelic polymorphism and codominant expression of the two alleles of the enzyme gene.

Interpreting cDNA sequences: some insights from studies on translation

This review discusses some rules for assessing the completeness of a cDNA sequence and identifying the start site for translation. Features commonly invoked-such as an ATG codon in a favorable context for initiation, or the presence of an upstream in-frame terminator codon, or the prediction of a signal peptide-like sequence at the amino terminus-have some validity; but examples drawn from the literature illustrate limitations to each of these criteria. The best advice is to inspect a cDNA sequence not only for these positive features but also for the absence of certain negative indicators. Three specific warning signs are discussed and documented: (i) The presence of numerous ATG codons upstream from the presumptive start site for translation often indicates an aberration (sometimes a retained intron) at the 5' end of the cDNA. (ii) Even one strong, upstream, out-of-frame ATG codon poses a problem if the reading frame set by the upstream ATG overlaps the presumptive start of the major open reading frame. Many cDNAs that display this arrangement turn out to be incomplete; that is, the out-of-frame ATG codon is within, rather than upstream from, the protein coding domain. (iii) A very weak context at the putative start site for translation often means that the cDNA lacks the authentic initiator codon. In addition to presenting some criteria that may aid in recognizing incomplete cDNA sequences, the review includes some advice for using in vitro translation systems for the expression of cDNAs. Some unresolved questions about translational regulation are discussed by way of illustrating the importance of verifying mRNA structures before making deductions about translation.

Membrane-bound versus secreted forms of human asialoglycoprotein receptor subunits. Role of a juxtamembrane pentapeptide

The H2a alternatively spliced variant of the human asialoglycoprotein receptor H2 subunit differs from the H2b variant by an extra pentapeptide, EGHRG, present in the ectodomain next to the membrane-span. This difference causes retention and degradation in the endoplasmic reticulum (ER) of H2a when expressed without the H1 subunit in 3T3 cells. In contrast, a significant portion of singly expressed H2b is Golgi-processed and reaches the cell surface. Using a new specific anti-H2a antibody, we found that in HepG2 cells, H2a is rapidly cleaved to a 35-kDa fragment, comprising the entire ectodomain, most of which is secreted into the medium. The cleavage site for the secreted fragment was located at the lumenal end of the membrane span. No membrane-bound H2a exits the ER, indicating that the pentapeptide is a signal for ER retention and degradation of the membrane form but does not hinder secretion of the cleaved soluble form. H2a does not form a membrane receptor complex with H1 as H2b does. H2a is therefore not a subunit of the receptor but a precursor for a secreted form of the protein; signal peptidase is probably responsible for the cleavage to the soluble fragment. Therefore, the juxtamembrane sequence regulates the function of the transmembrane domain of a type II membrane protein as either a signal-anchor sequence (H2b) or as a cleaved signal sequence, which generates a secreted product (H2a).

Expression of human dopamine beta-hydroxylase in Drosophila Schneider 2 cells

Human dopamine beta-hydroxylase (DBH) has been expressed in transformed Drosophila Schneider 2 (S2) cells with yields of > 16 mg/l. Most of the activity was found in the culture fluid. Similarly, human neuroblastoma cells also secrete native DBH into the medium, but at a much lower level than recombinant Drosophila cells. We have purified native and recombinant human DBH by a modified purification procedure using SP-Sepharose, lentil lectin-Sepharose and gel-filtration chromatography and carried out studies to compare the two enzymes. Two variants of human DBH that differ by a single amino acid (either serine or alanine) at position 304 were expressed in Drosophila cells, purified, and found to have no significant difference in enzyme activity. The molecular mass of human DBH monomer has been determined from SDS/PAGE to be 73 kDa, but the recombinant DBH from Drosophila is smaller at 66 kDa. The difference may be due to glycosylation as deglycosylated enzymes from both sources are identical in size (61 kDa). The Km of tyramine for native and recombinant human enzymes are virtually the same but higher than bovine DBH by about 3-fold. Likewise, the inhibition of native and recombinant human DBH by fusaric acid and SKF102698 is not significantly different but IC50 values are 2-3-fold higher than that for the bovine enzyme. These results strongly support the conclusion that recombinant human DBH from Drosophila S2 cells can be used in place of human neuroblastoma-derived DBH for drug screening, characterization of the enzyme's physicochemical properties, and determination of structure-function relationships. The Drosophila expression system has thus provided a convenient source for large quantities of human DBH enzyme.

Neurosecretory vesicles can be hybrids of synaptic vesicles and secretory granules

We have investigated the relationship of the so-called small dense core vesicle (SDCV), the major catecholamine-containing neurosecretory vesicle of sympathetic neurons, to synaptic vesicles containing classic neurotransmitters and secretory granules containing neuropeptides. SDCVs contain membrane proteins characteristic of synaptic vesicles such as synaptophysin and synaptoporin. However, SDCVs also contain membrane proteins characteristic of certain secretory granules like the vesicular monoamine transporter and the membrane-bound form of dopamine beta-hydroxylase. In neurites of sympathetic neurons, synaptophysin and dopamine beta-hydroxylase are found in distinct vesicles, consistent with their transport from the trans-Golgi network to the site of SDCV formation in constitutive secretory vesicles and secretory granules, respectively. Hence, SDCVs constitute a distinct type of neurosecretory vesicle that is a hybrid of the synaptic vesicle and the secretory granule membranes and that originates from the contribution of both the constitutive and the regulated pathway of protein secretion.

Expression of human dopamine beta-hydroxylase in mammalian cells infected by recombinant vaccinia virus. Mechanisms for membrane attachment

Dopamine beta-hydroxylase (DBH) is found in neurosecretory vesicles in both membrane-bound and soluble forms. We expressed various human DBH cDNAs in two mammalian cell lines, using the vaccinia virus expression system. The expression of a full-length DBH cDNA (DBH-f) reproduced the native DBH electrophoretic pattern and led to the synthesis of an active enzyme composed of two subunits of 77 and 73 kDa. In contrast, a truncated cDNA lacking the first ATG (DBH-t) generated a single band of 73 kDa. Analysis of mutated recombinant clones demonstrates that the two polypeptides do not result from the use of an alternative translation initiator codon. These results, combined with deglycosylation experiments, allow us to attribute the double band pattern to an optional cleavage of the signal peptide. When the NH2-terminal extremity is shortened, cleavage becomes obligatory, underlining the role of the first 14 amino acids in the regulation of the cleavage of the signal peptide. Subcellular analysis of recombinant DBH-t and DBH-f proteins indicates that DBH is anchored to the membrane by two distinct mechanisms; one of them is due to the non-removal of the signal peptide, whereas the second one is independent of the presence of the signal sequence. Moreover, quantification of the fractionation experiments suggests that the two modes of membrane attachment are additive.

Oxygen-18 kinetic isotope effects in the dopamine beta-monooxygenase reaction: evidence for a new chemical mechanism in non-heme metallomonooxygenases

Previous studies of dopamine beta-monooxygenase (D beta M) have implicated the formation of a substrate-derived benzylic radical via a hydrogen atom abstraction mechanism [Miller & Klinman (1985) Biochemistry 24, 2114]. We now address the nature of the oxygen species catalyzing C-H bond cleavage through the measurement of oxygen-18 isotope effects as a function of substrate structure. Using deuterium isotope effects, together with experimental O-18 isotope effects with protonated and deuterated substrates, it has been possible to calculate intrinsic O-18 isotope effects. Since the D beta M mechanism includes many steps which may involve changes in bond order at dioxygen, e.g., the reversible binding of O2 to the active-site copper and its reductive activation to a copper-hydroperoxide species, the intrinsic O-18 isotope effect is expected to be the product of two terms: (1) an overall equilibrium O-18 isotope effect on steps leading from O2 binding to the formation of the intermediate which catalyzes C-H bond cleavage and (2) a kinetic O-18 isotope effect on the C-H bond cleavage step. Thus, the magnitude of a single O-18 isotope effect measurement cannot reveal the nature of the bonding at oxygen during substrate activation. In the present study we have measured the change in O-18 isotope effect as a function of substrate structure and reactivity, finding values of 18(V/K) which decrease from 1.0281 +/- 0.001 to 1.0216 +/- 0.0003 as the rate of the C-H bond cleavage step decreases from 680 to 2 s-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cAMP, glucocorticoids, and calcium on dopamine beta-hydroxylase gene expression in bovine chromaffin cells

To better understand the molecular mechanism underlying regulation of bovine dopamine beta-hydroxylase (DBH), the effects of elevated intracellular cAMP, glucocorticoids, and calcium were studied in primary cultured chromaffin cells. Elevation of intracellular cAMP by forskolin and treatment with its analog 8-bromo-cAMP caused an increase in the bovine DBH mRNA level by 3.5 +/- 0.5- and 7.8 +/- 0.9-fold, respectively, which was maximal at 6 h after the treatments. On the other hand, dexamethasone elicited no apparent change in DBH gene expression at various concentrations and time. The combined treatment with forskolin and dexamethasone resulted in the same degree of increase as that with forskolin alone. Increased intracellular calcium by the ionophore A23187 ranging from 50 to 500 nM caused DBH mRNA to decrease, which began to be observed after 6 h and was undetectable by 48 h. The results demonstrate the existence of coordinate and differential regulations among the enzymes involved in catecholamine biosynthesis in bovine adrenomedullary cells.

Biogenesis of constitutive secretory vesicles, secretory granules and synaptic vesicles

The formation of constitutive secretory vesicles and secretory granules from the trans-Golgi network is inhibited by brefeldin A, suggesting a role for ADP ribosylation factor and other coat proteins in the budding process, and is regulated by multiple heterotrimeric G proteins. Membrane proteins are sorted to secretory granules through signals in their cytoplasmic domains and, like secretory proteins, by aggregation of their lumenal domains. Synaptic vesicle membrane proteins undergo exocytotic-endocytotic recycling before synapse formation, consistent with the proposed biogenesis of synaptic vesicles from early endosomes.

Sequence of the gene coding for ammonia monooxygenase in Nitrosomonas europaea

Nitrosomonas europaea, a chemolithotrophic bacterium, was found to contain two copies of the gene coding for the presumed active site polypeptide of ammonia monooxygenase, the 32-kDa acetylene-binding polypeptide. One copy of this gene was cloned, and its complete nucleotide sequence is presented. Immediately downstream of this gene, in the same operon, is the gene for a 40-kDa polypeptide that copurifies with the ammonia monooxygenase acetylene-binding polypeptide. The sequence of the first 692 nucleotides of this structural gene, coding for about two-thirds of the protein, is presented. These sequences are the first sequences of protein-encoding genes from an ammonia-oxidizing autotrophic nitrifying bacterium. The two protein sequences are not homologous with the sequences of any other monooxygenase. From radioactive labelling of ammonia monooxygenase with [14C]acetylene it was determined that there are 23 nmol of ammonia monooxygenase per g of cells. The kcat of ammonia monooxygenase for NH3 in vivo was calculated to be 20 s-1.

The 5'-flanking region of the human dopamine beta-hydroxylase gene promotes neuron subtype-specific gene expression in the central nervous system of transgenic mice

Dopamine beta-hydroxylase (DBH, EC 1.14.17.1) catalyzes the conversion of dopamine to norepinephrine, the third step of catecholamine biosynthesis. We have previously created transgenic mice harboring a chimeric gene consisting of the 4-kb DNA fragment of the human DBH gene promoter and the human phenylethanolamine N-methyltransferase (PNMT, EC 2.1.1.28) cDNA, to express PNMT in norepinephrine- and epinephrine-producing cells in the brain, sympathetic ganglia, and adrenal medullary chromaffin cells (Kobayashi et al., Proc. Natl. Acad. Sci. U.S.A., 89 (1992) 1631-1635). In this paper, we produced for the first time the antibody that specifically detects human PNMT, but not mouse PNMT, with the synthetic oligopeptide characteristic of the human PNMT sequence, and used this antibody to investigate the cells expressing human PNMT in transgenic mice. Immunohistochemical analysis of transgenic mice showed typical expression of human PNMT immunoreactivity in norepinephrinergic and epinephrinergic neurons in brain, as well as norepinephrine- and epinephrine-producing cells in the adrenal gland, indicating that the 4-kb 5'-flanking region is essential for the tissue-specific expression of the DBH gene. We also detected the ectopic expression in some DBH-immunonegative cells in the olfactory bulb of transgenic mice.

Mouse dopamine beta-hydroxylase: primary structure deduced from the cDNA sequence and exon/intron organization of the gene

Genomic clones for mouse dopamine beta-hydroxylase (DBH) were isolated from two genomic libraries derived from DBA/2J and 129/SV mouse strains, by plaque hybridization with the human DBH cDNA probe. Subsequently, cDNA encoding mouse DBH was amplified with reverse transcription-polymerase chain reaction (RT-PCR) method using primers corresponding to 5'- and 3'-portions of the mouse DBH mRNA, subcloned into a plasmid vector, and subjected to nucleotide sequence analysis. The clone encoded a protein of 621 amino acids with a calculated molecular mass of 70,186 daltons. The predicted amino acid sequence of mouse DBH showed 87%, 80% and 79% identities with the rat, bovine and human enzymes, respectively. Several potential amino acid sequences that are involved in the posttranslational modification and catalytic function of DBH were identified in mouse DBH protein. Nucleotide sequence analysis of the overlapping genomic clones showed that the mouse DBH gene was composed of 12 exons about 17 kb in length. Typical TATA and CCAAT boxes were observed in the 5'-upstream region of the gene. Northern blot analysis of adrenal gland RNA detected a single size species of the mouse DBH mRNA.

Interaction of dopamine-beta-monooxygenase with chromaffin granule membrane lipids

The interaction between bovine adrenal medullary dopamine-beta-monooxygenase and liposomes from chromaffin granule membrane lipids as a function of pH, lipid and salt concentration was studied by ultracentrifugation. Efficient adsorption of dopamine-beta-monooxygenase to liposomes occurs in the pH range 5.0-6.5 and at low ionic strength. The adsorption was not detected in the case of apoenzyme. The membrane dopamine-beta-monooxygenase forms a complex with liposomes more effective than soluble does. The data obtained lead to certain conclusions about the specificity of complex between the enzyme and liposomes.

Biogenesis of secretory granules

The biogenesis of secretory granules in endocrine, neuroendocrine, and exocrine cells is thought to involve a selective aggregation of the regulated secretory proteins into a dense-cored structure. The dense-core is then enveloped by membrane in the trans-Golgi network and buds, forming an immature secretory granule. The immature secretory granule then undergoes a maturation process which gives rise to the mature secretory granule. The recent data on the processes of aggregation, budding and maturation are summarized here. In addition, the current knowledge about the mature secretory granule is reviewed with emphasis on the biogenesis of the membrane of this organelle.

H and T subunits of acetylcholinesterase from Torpedo, expressed in COS cells, generate all types of globular forms

We analyzed the production of Torpedo marmorata acetylcholinesterase (AChE) in transfected COS cells. We report that the presence of an aspartic acid at position 397, homologous to that observed in other cholinesterases and related enzymes (Krejci, E., N. Duval, A. Chatonnet, P. Vincens, and J. Massoulié. 1991. Proc. Natl. Acad. Sci. USA. 88:6647-6651), is necessary for catalytic activity. The presence of an asparagine in the previously reported cDNA sequence (Sikorav, J.L., E. Krejci, and J. Massoulié. 1987. EMBO (Eur. Mol. Biol. Organ.) J. 6:1865-1873) was most likely due to a cloning error (codon AAC instead of GAC). We expressed the T and H subunits of Torpedo AChE, which differ in their COOH-terminal region and correspond respectively to the collagen-tailed asymmetric forms and to glycophosphatidylinositol-anchored dimers of Torpedo electric organs, as well as a truncated T subunit (T delta), lacking most of the COOH-terminal peptide. The transfected cells synthesized similar amounts of AChE immunoreactive protein at 37 degrees and 27 degrees C. However AChE activity was only produced at 27 degrees C and, even at this temperature, only a small proportion of the protein was active. We analyzed the molecular forms of active AChE produced at 27 degrees C. The H polypeptides generated glycophosphatidylinositol-anchored dimers, resembling the corresponding natural AChE form. The cells also released non-amphiphilic dimers G2na. The T polypeptides generated a series of active forms which are not produced in Torpedo electric organs: G1a, G2a, G4a, and G4na cellular forms and G2a and G4na secreted forms. The amphiphilic forms appeared to correspond to type II forms (Bon, S., J. P. Toutant, K. Méflah, and J. Massoulié. 1988. J. Neurochem. 51:776-785; Bon, S., J. P. Toutant, K. Méflah, and J. Massoulié. 1988. J. Neurochem. 51:786-794), which are abundant in the nervous tissue and muscles of higher vertebrates (Bon, S., T. L. Rosenberry, and J. Massoulié. 1991. Cell. Mol. Neurobiol. 11:157-172). The H and T catalytic subunits are thus sufficient to account for all types of known AChE forms. The truncated T delta subunit yielded only non-amphiphilic monomers, demonstrating the importance of the T COOH-terminal peptide in the formation of oligomers, and in the hydrophobic character of type II forms.

Immunological characterization of the endoproteases PC1 and PC2 in adrenal chromaffin granules and in the pituitary gland

Specific antisera against synthetic fragment of the endoproteases, PC1 and PC2, were used to characterize these proteins. In one-dimensional immunoblots these antisera labelled components of 85 kDa for PC1 and of 70 kDa for PC2 in purified bovine chromaffin granules and anterior and posterior pituitary of ox and rat. In membranes of bovine chromaffin granules glycoprotein H was identified as the major PC2 immunoreactive spot. A major part of these endoproteases appeared membrane bound.

Multiple forms of human dopamine beta-hydroxylase in SH-SY5Y neuroblastoma cells

Dopamine beta-hydroxylase exists as three forms in human neuroblastoma (SH-SY5Y) cells. The membrane-bound form of the hydroxylase contains three different species with apparent relative molecular weights of 73,000, 77,000, and 82,000. The intracellular soluble form of dopamine beta-hydroxylase was present as a single species with an apparent molecular weight of 73,000. Pulse-chase experiments showed that membranous dopamine beta-hydroxylase contains two subunit forms of 73,000 and 77,000 after short chase times. The soluble hydroxylase was synthesized as a single species of 73,000 at approximately the same rate as the lower molecular weight species of the membranous enzyme. A constitutively secreted third form of the enzyme with an intermediate apparent molecular weight also incorporated [35S]sulfate, whereas no significant amount of [35S]sulfate was observed in the cellular forms of the enzyme. The [35S]sulfate was incorporated on N-linked oligosaccharides. Approximately 12% of the enzyme is released constitutively within 1 h. These results demonstrate that neuronal cells have the ability to constitutively secrete a specific form of dopamine beta-hydroxylase which may contribute to the levels of this enzyme found in plasma.

Amphiphilic and nonamphiphilic forms of bovine and human dopamine beta-hydroxylase

We show that human and bovine dopamine beta-hydroxylases (DBH) exist under three main molecular forms: a soluble nonamphiphilic form and two amphiphilic forms. Sedimentation in sucrose gradients and electrophoresis under nondenaturing conditions, by comparison with acetylcholinesterase (AChE), suggest that the three forms are tetramers of the DBH catalytic subunit and bind either no detergent, one detergent micelle, or two detergent micelles. By analogy with the Gna4 and Ga4 AChE forms, we propose to call the nonamphiphilic tetramer Dna4 and the amphiphilic tetramers Da4I and Da4II. In addition to the major tetrameric forms, DBH dimers occur as very minor species, both amphiphilic and nonamphiphilic. Reduction under nondenaturing conditions leads to a partial dissociation of tetramers into dimers, retaining their amphiphilic character. This suggests that the hydrophobic domain is not linked to the subunits through disulfide bonds. The two amphiphilic tetramers are insensitive to phosphatidylinositol phospholipase C, but may be converted into soluble DBH by proteolysis in a stepwise manner; Da4II----Da4I----Dna4. Incubation of soluble DBH with various phospholipids did not produce any amphiphilic form. Several bands corresponding to the catalytic subunits of bovine DBH were observed in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, but this multiplicity was not simply correlated with the amphiphilic character of the enzyme. In the case of human DBH, we observed two bands of 78 and 84 kDa. As previously reported by others, the presence of the heavy subunit characterizes the amphiphilic forms of the enzyme. We discuss the nature of the hydrophobic domain, which could be an uncleaved signal peptide, and the organization of the different amphiphilic and nonamphiphilic DBH forms. We present two models in which dimers may possess either one hydrophobic domain or two domains belonging to each subunit; in both cases, a single detergent micelle would be bound per dimer.

Genes for human catecholamine-synthesizing enzymes

Catecholamine neurotransmitters--dopamine, noradrenaline (norepinephrine), adrenaline (epinephrine)--are synthesized in catecholaminergic neurons from tyrosine, via dopa, dopamine and noradrenaline, to adrenaline. Four enzymes are involved in the biosynthesis of adrenaline: (1) tyrosine 3-mono-oxygenase (tyrosine hydroxylase, TH); (2) aromatic L-amino acid decarboxylase (AADC, or DOPA decarboxylase, DDC); (3) dopamine beta-mono-oxygenase (dopamine beta-hydroxylase, DBH); and (4) noradrenaline N-methyltransferase (phenylethanolamine N-methyltransferase, PNMT). We cloned full-length complementary DNAs (cDNAs) and genomic DNAs of human catecholamine-synthesizing enzymes (TH, AADC, DBH, PNMT) and determined the nucleotide sequences and the deduced amino acid sequences. We discovered multiple messenger RNAs (mRNAs) of human TH, human DBH, and human PNMT. Four types (types 1, 2, 3, and 4) of human TH mRNAs are produced by alternative mRNA splicing mechanism from a single gene. We found the multiple forms of TH in two species of monkeys, but only a single mRNA corresponding to human TH type 1 in Sunkus murinus and rat, suggesting that the multiplicity of TH mRNA is primate-specific. Total TH mRNA, especially the most abundant type 2 and type 1 mRNAs in the human brain, were found to be reduced during the process of aging. The multiple forms of human TH may give additional regulation to the human enzyme, probably through altered phosphorylation and activation. We have succeeded in producing transgenic mice carrying multiple copies of the human TH gene in brain and adrenal medulla. The level of human TH mRNA in brain was about 50-fold higher than that of endogenous mouse TH mRNA. In situ hybridization demonstrated an enormous region-specific expression of the transgene in substantia nigra and ventral tegmental area. TH immunoreactivity in these regions, Western blot analysis, and TH activity measurements proved definitely increased TH in transgenic mice, though not comparable to the increment of the mRNA. However, catecholamine levels in transgenics were not significantly different from those in non-transgenics. The results suggest complex regulatory mechanisms for human TH gene expression and for the catecholamine levels in transgenic mice. Kohsaka and Uchida in collaboration with us applied genetically engineered (human TH cDNA-transfected) non-neuronal cells to brain tissue transplantation in parkinsonian rat models. We isolated and sequenced a full-length cDNA encoding human AADC.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous solubilization of membrane-bound human testis angiotensin-converting enzyme expressed in Chinese hamster ovary cells

The testis isozyme of angiotensin-converting enzyme (ACE; EC 3.4.15.1) is a membrane-bound protein that, apart from the first 35 N-terminal residues, is identical to the C-terminal half of somatic ACE and contains the same putative C-terminal membrane anchor. Stable transfection of Chinese hamster ovary (CHO) cells with an expression vector containing the full-length human testis ACE cDNA results in the expression of two forms of recombinant human testis ACE (hTACE): membrane-bound ACE and, surprisingly, large quantities (up to 3 mg/liter) of soluble hTACE in the conditioned medium. Both forms are fully active and are physicochemically similar. However, by phase separation in Triton X-114, the soluble enzyme is hydrophilic, as is an anchor-minus mutant hTACE recovered from the medium of CHO cells transfected with a vector that contains a 3'-truncated testis ACE cDNA lacking the sequence encoding the membrane anchor. In contrast, the membrane-bound hTACE is amphipathic but is converted to a hydrophilic form on treatment with trypsin. The data establish that in ACE the hydrophobic sequence near the C terminus is necessary for membrane anchoring. Moreover, in CHO cells, membrane-bound hTACE is apparently solubilized by proteolytic cleavage of this anchor. A similar mechanism may account for the release of endothelial ACE in vivo to generate serum ACE and more generally for the constitutive processing and solubilization of analogously anchored proteins such as the amyloid precursor protein, among others. The release of membrane-bound ACE in CHO cells may, therefore, provide a useful system for the study of membrane-protein-solubilizing proteases.

The involvement of free radicals in the mechanisms of monooxygenases

The occurrence of free radicals in the mechanisms of monooxygenases reflects the chemistry of dioxygen and the inertness of typical substrates. Thus, oxidation of such substrates requires attack by reduced dioxygen-derived free radicals. Consequently, a molecule of NAD(P)H must be invested for each substrate molecule oxidized. Furthermore, since free radicals are difficult to control, deviations from the intended reaction course are frequent. These considerations are illustrated by examination of the generation and fate of enzyme- and substrate-derived free radicals at various stages in the catalytic cycles of two monooxygenases important in xenobiotic biotransformation, dopamine beta-hydroxylase and cytochrome P-450.

Antibodies raised against different oligopeptide segments of human dopamine-beta-hydroxylase

We raised antibodies against 3 oligopeptide segments of human dopamine-beta-hydroxylase (hDBH) corresponding to the N-terminal (hDBH-N), the intermediate (hDBH-I), and the C-terminal (hDBH-C) amino acid sequences (residues 26-43, 452-468, and 582-598), respectively. We characterized the antibodies in terms of specificity by means of Western blotting and immunohistochemistry. Anti-hDBH-N antiserum recognized DBH in the brain (noradrenergic neurons in the pons and medulla oblongata) and adrenal medulla, not only of human but also of mouse, rat and house shrew. In contrast, anti-hDBH-C antiserum recognized only human DBH. These observations suggest that the antibody raised against the hDBH-C terminal peptide may specifically recognize only human DBH.

Bovine dopamine beta-hydroxylase, primary structure determined by cDNA cloning and amino acid sequencing

A cDNA clone encoding bovine dopamine beta-hydroxylase (DBH) has been isolated from bovine adrenal glands. The clone hybridizes to two oligonucleotide probes, one based on a previously reported active site peptide [DeWolf, W. E., Jr., et al. (1988) Biochemistry 27, 9093-9101] and the other based on the human DBH sequence [Lamouroux, A., et al. (1987) EMBO J. 6, 3931-3937]. The clone contains a 1.9-kb open reading frame that codes for the soluble form of bovine DBH, with the exception of the first six amino acids. Direct confirmation of 93% of the cDNA-derived sequence was obtained from cleavage peptides by protein sequencing and mass spectrometry. Differences were found between these two sequences at only two positions. Of the four potential N-linked carbohydrate attachment sites, two, Asn-170 and Asn-552, were shown to be partially and fully glycosylated, respectively. Within the 69% of the protein sequence confirmed by mass spectrometry, no other covalent modifications were detected.

Synthesis of chromogranin A, dopamine beta-hydroxylase, and chromaffin vesicles

Primary cultures of bovine adrenal medullary cells synthesize chromogranin A (CgA) and dopamine beta-hydroxylase (DBH) and incorporate them into chromaffin vesicles. The incorporation of L-[35S]methionine into CgA, DBH, and total protein was approximately linear for 8 h at methionine concentrations of 12.5, 25, and 50 microM. Newly synthesized CgA and DBH were initially incorporated into vesicles of low buoyant density that matured over 24 h into vesicles having the greater buoyant density of chromaffin vesicles. Approximately 10% of the newly synthesized CgA is released constitutively within 4 h of formation, approximately 30-40% appears to be degraded, and the remainder is incorporated into chromaffin vesicles, which can secrete CgA in response to nicotinic stimulation. Newly synthesized DBH follows a similar course. Once incorporated into chromaffin vesicles, the newly synthesized CgA and DBH appear to be stable for 2-3 days and then decline with a half-time of 3-4 days. Primary cultures of bovine adrenal medullary cells are a good model system for studying factors regulating CgA and DBH synthesis and the formation of chromaffin vesicles.