Characterization of highly purified dopamine beta-hydroxylase

Suppression of substance P biosynthesis in sensory neurons of dorsal root ganglion by prodrug esters of potent peptidylglycine alpha-amidating monooxygenase inhibitors

Substance P as well as many other neuropeptides are synthesized as glycine-extended precursors and converted to the biologically active C-terminal amides by posttranslational modification. The final step of posttranslational processing is catalyzed by peptidylglycine alpha-amidating monooxygenase (PAM). In a previous study, N-substituted homocysteine analogs were found to be potent inhibitors of PAM partially purified from conditioned medium of cultured rat medullary thyroid carcinoma CA-77 cells. These compounds, however, were only modest inhibitors of substance P production in cultured dorsal root ganglion cells, possibly because of poor cell penetration. Several ester derivatives of hydrocinnamoyl-phenylalanyl-homocysteine, one of the most potent PAM inhibitors, were prepared to increase the intracellular accessibility of these compounds. Hydrocinnamoyl-phenylalanyl-(S-benzoyl-homocysteine) benzyl ester was identified as the most potent compound, inhibiting substance P biosynthesis in dorsal root ganglion cells with an IC50 of 2 microM. Inhibition of PAM resulted in a concomitant increase in the glycine-extended substance p (substance P-Gly) precursor peptide. In the presence of 3 microM benzyl ester derivative, the intracellular substance P-Gly level was 2.4-fold higher while the substance P level was 2.1-fold lower than the corresponding peptides in control cells. These results suggest that PAM inhibition represents an effective method for suppression of substance P biosynthesis and, therefore, may have therapeutic utility in conditions associated with elevated substance P levels. Furthermore, PAM inhibition may also prove useful in decreasing other amidated peptides.

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.

Interaction of non-conjugated olefinic substrate analogues with dopamine beta-monooxygenase: catalysis and mechanism-based inhibition

The reaction of dopamine beta-monooxygenase (DBM; EC 1.14.17.1) with the prototypical non-conjugated olefinic substrate, 2-(1-cyclohexenyl)ethylamine (CyHEA) [see Sirimanne and May (1988) J. Am. Chem. Soc. 110, 7560-7561], was characterized. CyHEA undergoes facile DBM-catalysed allylic hydroxylation to form (R)-2-amino-1-(1-cyclohexenyl)ethanol (CyHEA-OH) without detectable epoxidation or allylic hydroxylation to form (R)-2-amino-1-(1-cyclohexenyl)ethanol (CyHEA-OH) without detectable epoxidation or allylic rearrangement, and with stereochemistry consistent with that of DBM-catalysed benzylic hydroxylation and sulphoxidation. The kcat. of 90 s-1 for CyHEA oxygenation is about 75% of the kcat. for tyramine, the substrate commonly used in assays of DBM activity. DBM-catalysed oxygenation of CyHEA also results in mechanism-based inactivation of DBM, with the inactivation reaction yielding kinact. = 0.3 min-1 at pH 5.0 and 37 degrees C, and a partition ratio of 16,000. Although both CyHEA turnover and inactivation exhibit normal kinetics, CyHEA processing also results in gradual depletion of copper from DBM; however, mechanism-based irreversible DBM inactivation occurs independent of this copper depletion when sufficient copper is present in the assay solution. A likely mechanism for turnover-dependent DBM inactivation by CyHEA involves initial abstraction of an allylic hydrogen to form a resonance-stabilized allylic radical, which can then either partition to product or undergo attack by an active-site residue. Acyclic, non-conjugated olefinic analogues exhibit diminished substrate activity toward DBM. Thus, kcat. for oxygenation of cis-2-hexenylamine, which also produces only allylic alcohol product, is only 14% of that for CyHEA. Similarly, kinact./KI for turnover-dependent inactivation by the acyclic olefin 2-aminomethyl-1-pentene is more than an order of magnitude smaller than that for benzylic olefins. Our results establish that DBM catalyses allylic oxygenation of a number of non-conjugated olefinic substrate analogues with neither epoxidation nor allylic rearrangement occurring. The absence of epoxide products from non-conjugated olefinic substrates implies an inability of the activated copper-oxygen species of DBM to effect radical cation formation from a non-conjugated olefinic moiety. The striking contrast between DBM and cytochrome P-450, which carries out both epoxidation and allylic oxidation with non-conjugated olefinic substrates, is probably a reflection of the differences in redox potential of the activated oxygen species operative for these two enzymes.

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.

Characterization of a bifunctional peptidylglycine alpha-amidating enzyme expressed in Chinese hamster ovary cells

Peptidylglycine alpha-amidating enzyme (alpha-AE) catalyzes the conversion of glycine-extended prohormones to their biologically active alpha-amidated forms. We have derived a clonal Chinese hamster ovary cell line that secretes significant quantities of active alpha-AE. Enzyme production was increased by selection for methotrexate-resistant cells expressing a dicistronic message. Amplification of the alpha-AE gene was monitored by Southern blot analysis, enzyme activity, and immunoreactive protein throughout the selection process. The soluble enzyme is bifunctional as determined by the ability to convert either the glycine-extended substrate, dansyl-Tyr--Val--Gly, or the intermediate, dansyl-Tyr--Val--alpha-hydroxyglycine, to the dansyl-Tyr--Val--NH2 product. The recombinant alpha-AE was purified by a simple two-step chromatographic process. The purified enzyme is partially glycosylated and the glycosylated and nonglycosylated forms of the enzyme were separated on a Con A-Sepharose column. The kinetic constants for dansyl-Tyr--Val--Gly, dansyl-Tyr--Val--alpha-hydroxyglycine, ascorbate, and catechol were the same for both forms of alpha-AE. In addition, mimosine is competitive vs ascorbate with K(is) = 3.5 microM for the nonglycosylated alpha-AE and K(is) = 4.2 microM for the glycosylated alpha-AE. Therefore, the presence or absence of asparagine-linked oligosaccharide does not affect the catalytic efficiency of the enzyme. Overexpression of the recombinant enzyme in CHO cells greatly enhances expression of the endogenous gene, implicating a feedback mechanism on the alpha-AE gene.

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.

Enzymatic oxidation of xenobiotic chemicals

Studies with biomimetic models can yield considerable insight into mechanisms of enzymatic catalysis. The discussion above indicates how such information has been important in the cases of flavoproteins, hemoproteins, and, to a lesser extent, the copper protein dopamine beta-hydroxylase. Some of the moieties that we generally accept as intermediates (i.e., high-valent iron oxygen complex in cytochrome P-450 reactions) would be extremely hard to characterize were it not for biomimetic models and more stable analogs such as peroxidase Compound I complexes. Although biomimetic models can be useful, we do need to keep them in perspective. It is possible to alter ligands and aspects of the environment in a way that may not reflect the active site of the protein. Eventually, the model work needs to be carried back to the proteins. We have seen that diagnostic substrates can be of considerable use in understanding enzymes and examples of elucidation of mechanisms through the use of rearrangements, mechanism-based inactivation, isotope labeling, kinetic isotope effects, and free energy relationships have been given. The point should be made that a myriad of approaches need to be applied to the study of each enzyme, for there is potential for misleading information if total reliance is placed on a single approach. The point also needs to be made that in the future we need information concerning the structures of the active sites of enzymes in order to fully understand them. Of the enzymes considered here, only a bacterial form of cytochrome P-450 (P-450cam) has been crystallized. The challenge to determine the three-dimensional structures of these enzymes, particularly the intrinsic membrane proteins, is formidable, yet our further understanding of the mechanisms of enzyme catalysis will remain elusive as long as we have to speak of putative specific residues, domains, and distances in anecdotal terms. The point should be made that there is actually some commonality among many of the catalytic mechanisms of oxidation, even among proteins with different structures and prosthetic groups. Thus, we see that cytochrome P-450 has some elements of a peroxidase and vice versa; indeed, the chemistry at the prosthetic group is probably very similar and the overall chemistry seems to be induced by the protein structure. The copper protein dopamine beta-hydroxylase appears to proceed with chemistry similar to that of the hemoprotein cytochrome P-450 and, although not so thoroughly studied, the non-heme iron protein P. oleovarans omega-hydroxylase.(ABSTRACT TRUNCATED AT 400 WORDS)

Rapid freeze- and chemical-quench studies of dopamine beta-monooxygenase: comparison of pre-steady-state and steady-state parameters

The copper-containing enzyme dopamine beta-monooxygenase has been studied with regard to pre-steady-state kinetics of tyramine hydroxylation and reduction of enzyme-bound Cu2+ by chemical- and freeze-quench EPR techniques. The bulk of the enzyme-bound copper (approximately 70%) is reduced in a single-exponential process with a limiting rate constant of 250 s-1, Km = 0.9 mM, consistent with participation of both copper ions in the redox events of catalysis. The remaining copper is reduced much more slowly (k approximately 2 s-1) or not at all, attributed to a distribution of copper into inhibitory binding sites and the presence of some inactive enzyme. Knowledge of the Cu2+ reduction rate, together with rate constants calculated from steady-state isotope effects [Miller, S. M., & Klinman, J. P. (1985) Biochemistry 24, 2114-2127], has allowed prediction of pre-steady-state product formation transients. Measurement of these transients under conditions of excess ascorbate shows close agreement with prediction, supporting the validity of individual rate constants obtained from steady-state data. Kinetic modeling shows further that the predominant steady-state enzyme form is the enzyme-product complex (E-P), which is expected to show a correspondingly large (approximately 70% of total copper) EPR signal for bound Cu2+. Surprisingly, the steady state is characterized by a low (19% of total copper) EPR signal. This lack of correlation between the anticipated and observed steady-state EPR signal suggests either antiferromagnetic coupling in binuclear copper centers or reduction of Cu2+ in this enzyme form by ascorbic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

The reactivation of apodopamine beta-monooxygenase by vanadyl ions

Vanadyl ions may be used for reactivation of apodopamine beta-monooxygenase. Maximal activity of the enzyme was achieved at a 350-400-fold molar excess of vanadyl ions, whereas for maximal reconstitution with copper, an 8-10-fold molar excess of copper was necessary. AT higher concentrations of vanadyl as well as of copper, inhibition was observed.

On the copper transfer between dopamine beta-monooxygenase and Cu-thionein

Metallothionein saturated with copper is able to donate copper to apodopamine beta-monooxygenase. The complete recovery of dopamine beta-monooxygenase activity is observed at the molar ratio Cu-thionein/apoenzyme of 25. On the other hand, apothionein accepts copper easily from the holoenzyme.