Subunit exchange between membranous and soluble forms of bovine dopamine beta-hydroxylase

Determination of Dopamine-β-hydroxylase Activity in Human Serum Using UHPLC-PDA Detection

Dopamine-β-hydroxylase (DBH, EC 1.14.17.1) is an enzyme with implications in various neuropsychiatric and cardiovascular diseases and is a known drug target. There is a dearth of cost effective and fast method for estimation of activity of this enzyme. A sensitive UHPLC based method for the estimation of DBH activity in human sera samples based on separation of substrate tyramine from the product octopamine in 3 min is described here. In this newly developed protocol, a Solid Phase Extraction (SPE) sample purification step prior to LC separation, selectively removes interferences from the reaction cocktail with almost no additional burden on analyte recovery. The response was found to be linear with an r² = 0.999. The coefficient of variation for assay precision was < 10% and recovery > 90%. As a proof of concept, DBH activity in sera from healthy human volunteers (n = 60) and schizophrenia subjects (n = 60) were successfully determined using this method. There was a significant decrease in sera DBH activity in subjects affected by schizophrenia (p < 0.05) as compared to healthy volunteers. This novel assay employing SPE to separate octopamine and tyramine from the cocktail matrix may have implications for categorising subjects into various risk groups for Schizophrenia, Parkinson's disease as well as in high throughput screening of inhibitors.

Axonal transport of neuropeptides: Retrograde tracing study in live cell cultures of rat sympathetic cervical ganglia

Previous studies demonstrated that neuropeptides are transported with fast axonal transport. Considerable amounts (30-40%) of anterogradely transported peptides accumulated distal to a crush, apparently recycling to the cell bodies. In the present study, we used primary and compartmented cultures of sympathetic cervical ganglia (SCG) to address questions on the origin of the recycling peptides. In primary cultures, distinct labeling of neuropeptide Y (NPY) or secretoneurin (SN) immunoreactivities was detected in varicosities and in cell bodies, after administration of NPY or SN antibodies to the living cultures. Simultaneous addition to the medium with antibody against the N-terminal (lumen) domain of synaptotagmin, resulted in a partial overlapping between synaptotagmin and NPY/SN. In compartmented chamber cultures, in which cell body and proximal segments of the processes are restricted to the central chamber and the distal processes are present in peripheral compartments, antibody administration was performed in the peripheral compartment. KCl (60-120 mM) was added to the central chamber for 10 sec, followed by washing, and 30-60 min later clear labeling was detected in the cell bodies, suggesting that the antibodies were now present in structures that were transported from the distal segments in the peripheral compartment to the cell body. The results indicate 1) that peptide release from large dense cored vesicles is incomplete; 2) that the remaining peptides, together with the membrane, are retrogradely transported to cell bodies; and 3) that the recycling peptides accumulating distal to a crush of a peripheral nerve are most likely to be recycled from the nerve terminals.

Human phenotypes and animal knockout models of genetic autonomic disorders

Norepinephrine (NE) is a crucial neurotransmitter involved in autonomic regulation of blood pressure. Dopamine beta-hydroxylase (DBH), the norepinephrine transporter (NET), and the vesicular monoamine transporter subtype 2 catalyze intracellular NE biosynthesis, NE reuptake from the synapse, and vesicular transport, respectively. Genetic disorders in humans have been identified that render DBH, and the NET dysfunctional and result in cardiovascular and neurological abnormalities. Vesicular monoamine transporter subtype 2 (VMAT2) activity protects against neurotoxins, and reduced VMAT2 expression is implicated in drug addiction. Further investigation of the consequences of these genetic abnormalities has been achieved by the construction of mice strains deficient in the genes encoding DBH, NET, and VMAT2.

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.

Secretory granule content proteins and the luminal domains of granule membrane proteins aggregate in vitro at mildly acidic pH

A major unresolved issue in the field of secretory granule biogenesis is the extent to which the aggregation of granule content proteins is responsible for the sorting of regulated from constitutively secreted proteins. The aggregation process is postulated to take place in the trans-Golgi network and immature secretory granules as the proteins encounter mildly acidic pH and high calcium concentrations. We have developed in vitro assays that reconstitute the precipitation out of solution of secretory granule content proteins of anterior pituitary gland and adrenal medulla. In the assays, all of the major granule content polypeptides form a precipitate as the pH is titrated below 6.5, and this precipitate can be recovered in the pellet fraction after centrifugation. Addition of calcium is required for the aggregation of chromaffin granule content. In contrast to the proteins secreted by the regulated pathway, the constitutively secreted proteins IgG, albumin, and angiotensinogen, when added to the assays, remain predominantly in the supernatant. Among the individual proteins tested, prolactin is found to aggregate homophilically under these conditions and can drive the co-aggregation of other proteins, such as the chromogranins. Soluble forms of granule membrane proteins, including dopamine beta-hydroxylase and peptidyl glycine alpha-amidating enzyme also co-aggregated with granule content proteins. The results are consistent with the idea that spontaneous aggregation of proteins occurring under ionic conditions similar to those at the sites of granule formation is a property restricted to those proteins packaged in secretory granules. In addition, the association of luminal domains of membrane proteins with content proteins in vitro raises the possibility that analogous interactions between membrane-bound and content proteins also occur during granule formation in intact cells.

Comparative study of biochemical parameters and kinetic properties of dopamine-beta-hydroxylase activity from cat and rat adrenals

The specific adrenal dopamine-beta-hydroxylase activity measured in cat and rat was: (1) two times higher in cat than in rat; (2) significantly enhanced by ascorbate (up to 5 mM) that acts as a major activator; (3) differently affected by exogenous copper added to the incubation medium, leading to a decrease with large concentrations; and (4) similar, according to the kinetic parameters, thereby demonstrating a greater affinity for ascorbate than for tyramine.

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.

Dopamine beta-hydroxylase deficiency. A genetic disorder of cardiovascular regulation

Dopamine beta-hydroxylase (DBH) deficiency is a genetic disorder in which affected patients cannot synthesize norepinephrine, epinephrine, and octopamine in either the central nervous system or the peripheral autonomic neurons. Dopamine acts as a false neurotransmitter in their noradrenergic neurons. Neonates with DBH deficiency have had episodic hypothermia, hypoglycemia, and hypotension, but survivors sometimes cope relatively well until late childhood when overwhelming orthostatic hypotension profoundly limits their activities. The hypotension may be so severe that clonic seizures supervene. Most currently recognized patients are young or middle-aged adults. The diagnosis is established by the observation of severe orthostatic hypotension in a patient whose plasma norepinephrine/dopamine ratio is much less than one.

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

A full-length cDNA for dopamine beta-monooxygenase (D beta M) from bovine adrenal glands has been cloned and sequenced. The soluble and membrane-derived forms of D beta M have also been sequenced from their N-termini. While the observed sequences for the soluble protein correspond to those previously reported [Joh, T.H., & Hwang, O. (1986) Ann. N.Y. Acad. Sci. 493, 343-350], the heavy subunit of membrane-derived enzyme is found to contain a unique N-terminus. Alignment of this N-terminus with that deduced from cDNA cloning indicates identity at 22 (and possibly 26) out of 27 residues. This comparison leads us to conclude that the membranous form of bovine D beta M retains an uncleaved N-terminal signal peptide as the source of membrane anchoring.

Deglycosylated membranous and soluble dopamine beta-hydroxylase have identical apparent molecular weights

Dopamine beta-hydroxylase exists as soluble and membrane-bound forms in secretory vesicles. The soluble form of the enzyme contains identical subunits of 72 kDa and the membrane-bound form contains two non-identical subunits of 72 kDa and 75 kDa. The difference in the banding pattern on sodium dodecyl sulfate-polyacrylamide gel electrophoresis suggests the presence of either an extra peptide, or membrane-binding segment, or differential glycosylation of 75-kDa subunits of the membranous form. Soluble and membranous forms of the enzyme were deglycosylated with endoglycosidases to elucidate the contribution of the carbohydrate moieties to the banding pattern on a sodium dodecyl sulfate-polyacrylamide gel. The deglycosylated species of both forms appeared to be identical and showed a decrease in apparent molecular weights to 66 kDa. These results indicate that the banding pattern of soluble and membranous dopamine beta-hydroxylase on sodium dodecyl sulfate-polyacrylamide gel electrophoresis may not be due to a membrane-binding anchor but rather to carbohydrate moieties.

Microsequencing of dopamine beta-monooxygenase

Tryptic peptides and cyanogen bromide fragments of dopamine beta-monooxygenase (DBH) were prepared and separated on C-8 reverse phase columns by high pressure liquid chromatography. Absorbance profiles at both 220 nm and 280 nm were monitored so that peptides with aromatic residues could be isolated. These peptides were subjected to automated Edman degradation with a gas phase microsequencer.