Expression of rat tyrosine hydroxylase in insect tissue culture cells and purification and characterization of the cloned enzyme

Kinetic mechanism of phenylalanine hydroxylase: intrinsic binding and rate constants from single-turnover experiments

Phenylalanine hydroxylase (PheH) catalyzes the key step in the catabolism of dietary phenylalanine, its hydroxylation to tyrosine using tetrahydrobiopterin (BH(4)) and O(2). A complete kinetic mechanism for PheH was determined by global analysis of single-turnover data in the reaction of PheHΔ117, a truncated form of the enzyme lacking the N-terminal regulatory domain. Formation of the productive PheHΔ117-BH(4)-phenylalanine complex begins with the rapid binding of BH(4) (K(d) = 65 μM). Subsequent addition of phenylalanine to the binary complex to form the productive ternary complex (K(d) = 130 μM) is approximately 10-fold slower. Both substrates can also bind to the free enzyme to form inhibitory binary complexes. O(2) rapidly binds to the productive ternary complex; this is followed by formation of an unidentified intermediate, which can be detected as a decrease in absorbance at 340 nm, with a rate constant of 140 s(-1). Formation of the 4a-hydroxypterin and Fe(IV)O intermediates is 10-fold slower and is followed by the rapid hydroxylation of the amino acid. Product release is the rate-determining step and largely determines k(cat). Similar reactions using 6-methyltetrahydropterin indicate a preference for the physiological pterin during hydroxylation.

Identification by hydrogen/deuterium exchange of structural changes in tyrosine hydroxylase associated with regulation

The activity of tyrosine hydroxylase is regulated by reversible phosphorylation of serine residues in an N-terminal regulatory domain and catecholamine inhibition at the active site. Catecholamines such as dopamine bind very tightly to the resting enzyme; phosphorylation of Ser40 decreases the affinity for catecholamines by 3 orders of magnitude. The effects of dopamine binding and phosphorylation of Ser40 on the kinetics of deuterium incorporation into peptide bonds were examined by mass spectrometry. When dopamine is bound, three peptic peptides show significantly slower deuterium incorporation, 35-41 and 42-71 in the regulatory domain and 295-299 in the catalytic domain. In the phosphorylated enzyme, peptide 295-299 shows more rapid incorporation of deuterium, while 35-41 and 42-71 can not be detected. These results are consistent with tyrosine hydroxylase existing in two different conformations. In the closed conformation, the regulatory domain lies across the active site loop containing residues 295-298; this is stabilized when dopamine is bound in the active site. In the open conformation, the regulatory domain has moved out of the active site, allowing substrate access; this conformation is favored by phosphorylation of Ser40.

Characterization of metal ligand mutants of phenylalanine hydroxylase: Insights into the plasticity of a 2-histidine-1-carboxylate triad

The iron atom in the nonheme iron monooxygenase phenylalanine hydroxylase is bound on one face by His285, His290, and Glu330. This arrangement of metal ligands is conserved in the other aromatic amino acid hydroxylases, tyrosine hydroxylase and tryptophan hydroxylase. A similar 2-His-1-carboxylate facial triad of two histidines and an acidic residue are the ligands to the iron in other nonheme iron enzymes, including the alpha-ketoglutarate-dependent hydroxylases and the extradiol dioxygenases. Previous studies of the effects of conservative mutations of the iron ligands in tyrosine hydroxylase established that there is some plasticity in the nature of the ligands and that the three ligands differ in their sensitivity to mutagenesis. To determine the generality of this finding for enzymes containing a 2-His-1-carboxylate facial triad, the His285, His290, and Glu330 in rat phenylalanine hydroxylase were mutated to glutamine, glutamate, and histidine. All of the mutant proteins had low but measurable activities for tyrosine formation. In general, mutation of Glu330 had the greatest effect on activity and mutation of His290 the least. All of the mutations resulted in an excess of tetrahydropterin oxidized relative to tyrosine formation, with mutation of His285 having the greatest effect on the coupling of the two partial reactions. The H285Q enzyme had the highest activity as tetrahydropterin oxidase at 20% the wild-type value. All of the mutations greatly decreased the affinity for iron, with mutation of Glu330 the most deleterious. The results complement previous results with tyrosine hydroxylase in establishing the plasticity of the individual iron ligands in this enzyme family.

Oxygen dependence of tyrosine hydroxylase

The effects of dioxygen on tyrosine hydroxylase (TH) activity was studied, measuring the formation of DOPA from tyrosine, (3)H(2)O from 3,5-(3)H-tyrosine, or by direct oxygraphic determination of oxygen consumption. A high enzyme activity was observed during the initial 1-2 min of the reactions, followed by a decline in activity, possibly related to a turnover dependent substoichiometrical oxidation of enzyme bound Fe(II) to the inactive Fe(III) state. During the initial reaction phase, apparent K (m)-values of 29-45 microM for dioxygen were determined for all human TH isoforms, i.e. 2-40 times higher than previously reported for TH isolated from animal tissues. After 8 min incubation, the K (m) (O(2))-values had declined to an average of 20 +/- 4 microM. Thus, TH activity may be severely limited by oxygen availability even at moderate hypoxic conditions, and the enzyme is rapidly and turnover dependent inactivated at the experimental conditions commonly employed to measure in vitro activities.

Reduction and oxidation of the active site iron in tyrosine hydroxylase: kinetics and specificity

Tyrosine hydroxylase (TyrH) is a pterin-dependent enzyme that catalyzes the hydroxylation of tyrosine to form dihydroxyphenylalanine. The oxidation state of the active site iron atom plays a central role in the regulation of the enzyme. The kinetics of reduction of ferric TyrH by several reductants were determined by anaerobic stopped-flow spectroscopy. Anaerobic rapid freeze-quench EPR confirmed that the change in the near-UV absorbance of TyrH upon adding reductant corresponded to iron reduction. Tetrahydrobiopterin reduces wild-type TyrH following a simple second-order mechanism with a rate constant of 2.8 +/- 0.1 mM(-)(1) s(-)(1). 6-Methyltetrahydropterin reduces the ferric enzyme with a second-order rate constant of 6.1 +/- 0.1 mM(-)(1) s(-)(1) and exhibits saturation kinetics. No EPR signal for a radical intermediate was detected. Ascorbate, glutathione, and 1,4-benzoquinone all reduce ferric TyrH, but much more slowly than tetrahydrobiopterin, suggesting that the pterin is a physiological reductant. E332A TyrH, which has an elevated K(m) for tetrahydropterin in the catalytic reaction, is reduced by tetrahydropterins with the same kinetic parameters as those of the wild-type enzyme, suggesting that BH(4) does not bind in the catalytic conformation during the reduction. Oxidation of ferrous TyrH by molecular oxygen can be described as a single-step second-order reaction, with a rate constant of 210 mM(-)(1) s(-)(1). S40E TyrH, which mimics the phosphorylated state of the enzyme, has oxidation and reduction kinetics similar to those of the wild-type enzyme, suggesting that phosphorylation does not directly regulate the interconversion of the ferric and ferrous forms.

Biochemical conservation of recombinant Drosophila tyrosine hydroxylase with its mammalian cognates

Dopamine modulates several behavioral and developmental events; in the fruit fly Drosophila melanogaster, dopamine is a neurotransmitter, a neuromodulator, and a developmental signal. Studies in mammals suggest that these diverse roles for dopamine have been evolutionarily conserved. Fundamental regulation of dopamine occurs via tyrosine hydroxylase (TH), the first and rate-limiting enzyme in the catecholamine biosynthetic pathway. Mammalian TH is acutely regulated via phosphorylation-dephosphorylation mechanisms, which occur as a direct consequence of nerve stimulation. We have shown that the Drosophila homolog of TH, DTH, shares over 50% sequence identity with mammalian TH, and the serine residue corresponding to the major site of phosphorylation is conserved. We demonstrate using recombinant DTH protein generated in E. coli that its regulatory biochemical mechanisms closely parallel those from mammals. Drosophila thus provides a highly conserved and tractable model system in which to test the functional consequences of perturbing TH activity by acute regulatory mechanisms.

Mutation of regulatory serines of rat tyrosine hydroxylase to glutamate: effects on enzyme stability and activity

Tyrosine hydroxylase is phosphorylated at four serine residues in its amino-terminus by multiple kinases. Phosphorylation of serine 40 by cAMP-dependent protein kinase results in alleviation of dopamine inhibition [J. Biol. Chem. 267 (1992) 12639]. The other serines are at positions 8, 19, and 31. The effect of phosphorylation at these serines has been investigated using mutated forms of tyrosine hydroxylase containing glutamates at the positions of the serines. The S8E, S19E, and S31E tyrosine hydroxylase variants have similar steady-state kinetic parameters and similar binding affinity for catecholamines to wild-type enzyme. The S8E, S19E, S31E, and S40E variants differ in stability at elevated temperatures. The S40E variant is the least stable, while the others are all more stable than wild-type enzyme. The increased stability of S8E, S19E, and S31E tyrosine hydroxylases may be one of the physiological effects of phosphorylation. It may also have implications for the interpretation of activities of heterogeneous mixtures of tyrosine hydroxylase which have been phosphorylated.

Two functional but noncomplementing Drosophila tyrosine decarboxylase genes: distinct roles for neural tyramine and octopamine in female fertility

The trace biogenic amine tyramine is present in the nervous systems of animals ranging in complexity from nematodes to mammals. Tyramine is synthesized from tyrosine by the enzyme tyrosine decarboxylase (TDC), a member of the aromatic amino acid family, but this enzyme has not been identified in Drosophila or in higher animals. To further clarify the roles of tyramine and its metabolite octopamine, we have cloned two TDC genes from Drosophila melanogaster, dTdc1 and dTdc2. Although both gene products have TDC activity in vivo, dTdc1 is expressed nonneurally, whereas dTdc2 is expressed neurally. Flies with a mutation in dTdc2 lack neural tyramine and octopamine and are female sterile due to egg retention. Although other Drosophila mutants that lack octopamine retain eggs completely within the ovaries, dTdc2 mutants release eggs into the oviducts but are unable to deposit them. This specific sterility phenotype can be partially rescued by driving the expression of dTdc2 in a dTdc2-specific pattern, whereas driving the expression of dTdc1 in the same pattern results in a complete rescue. The disparity in rescue efficiencies between the ectopically expressed Tdc genes may reflect the differential activities of these gene products. The egg retention phenotype of the dTdc2 mutant and the phenotypes associated with ectopic dTdc expression contribute to a model in which octopamine and tyramine have distinct and separable neural activities.

Tyrosine hydroxylase phosphorylation: regulation and consequences

The rate-limiting enzyme in catecholamine synthesis is tyrosine hydroxylase. It is phosphorylated at serine (Ser) residues Ser8, Ser19, Ser31 and Ser40 in vitro, in situ and in vivo. A range of protein kinases and protein phosphatases are able to phosphorylate or dephosphorylate these sites in vitro. Some of these enzymes are able to regulate tyrosine hydroxylase phosphorylation in situ and in vivo but the identity of the kinases and phosphatases is incomplete, especially for physiologically relevant stimuli. The stoichiometry of tyrosine hydroxylase phosphorylation in situ and in vivo is low. The phosphorylation of tyrosine hydroxylase at Ser40 increases the enzyme's activity in vitro, in situ and in vivo. Phosphorylation at Ser31 also increases the activity but to a much lesser extent than for Ser40 phosphorylation. The phosphorylation of tyrosine hydroxylase at Ser19 or Ser8 has no direct effect on tyrosine hydroxylase activity. Hierarchical phosphorylation of tyrosine hydroxylase occurs both in vitro and in situ, whereby the phosphorylation at Ser19 increases the rate of Ser40 phosphorylation leading to an increase in enzyme activity. Hierarchical phosphorylation depends on the state of the substrate providing a novel form of control of tyrosine hydroxylase activation.

Stimulus-coupled interaction of tyrosine hydroxylase with 14-3-3 proteins

Tyrosine hydroxylase (TH) is phosphorylated by CaM kinase II and is activated in situ in response to a variety of stimuli that increase intracellular Ca(2+). We report here, using baculovirus-expressed TH, that the 14-3-3 protein binds and activates the expressed TH when the enzyme is phosphorylated at Ser-19, a site of CaM kinase II-dependent phosphorylation located in the regulatory domain of TH. Site-directed mutagenesis showed that a TH mutant in which Ser-19 was substituted by Ala retained enzymatic activity at the same level as the non-mutated enzyme, but was a poor substrate for CaM kinase II and did not bind the 14-3-3 protein. Likewise, a synthetic phosphopeptide (FRRAVpSELDA) corresponding to the part of the TH sequence, including phosphoSer-19, inhibited the interaction between the expressed TH and 14-3-3, while the phosphopeptide (GRRQpSLIED) corresponding to the site of cAMP-dependent phosphorylation (Ser-40) had little effect on complex formation. The complex was very stable with a dissociation constant of 3 nM. Furthermore, analysis of PC12nnr5 cells transfected with myc-tagged 14-3-3 showed that 14-3-3 formed a complex with endogenous TH when the cultured cells were exposed to a high K(+) concentration that increases intracellular Ca(2+) and phosphorylation of Ser-19 in TH. These findings suggest that the 14-3-3 protein participates in the stimulus-coupled regulation of catecholamine synthesis that occurs in response to depolarization-evoked, Ca(2+)-dependent phosphorylation of TH.

Differential regulation of Drosophila tyrosine hydroxylase isoforms by dopamine binding and cAMP-dependent phosphorylation

Tyrosine hydroxylase (TH) catalyzes the first step in dopamine biosynthesis in Drosophila as in vertebrates. We have previously reported that tissue-specific alternative splicing of the TH primary transcript generates two distinct TH isoforms in Drosophila, DTH I and DTH II (Birman, S., Morgan, B., Anzivino, M., and Hirsh, J. (1994) J. Biol. Chem. 269, 26559-26567). Expression of DTH I is restricted to the central nervous system, whereas DTH II is expressed in non-nervous tissues like the epidermis. The two enzymes present a single structural difference; DTH II specifically contains a very acidic segment of 71 amino acids inserted in the regulatory domain. We show here that the enzymatic and regulatory properties of vertebrate TH are generally conserved in insect TH and that the isoform DTH II presents unique characteristics. The two DTH isoforms were expressed as apoenzymes in Escherichia coli and purified by fast protein liquid chromatography. The recombinant DTH isoforms are enzymatically active in the presence of ferrous iron and a tetrahydropteridine co-substrate. However, the two enzymes differ in many of their properties. DTH II has a lower Km value for the co-substrate (6R)-tetrahydrobiopterin and requires a lower level of ferrous ion than DTH I to be activated. The two isoforms also have a different pH profile. As for mammalian TH, enzymatic activity of the Drosophila enzymes is decreased by dopamine binding, and this effect is dependent on ferrous iron levels. However, DTH II appears comparatively less sensitive than DTH I to dopamine inhibition. The central nervous system isoform DTH I is activated through phosphorylation by cAMP-dependent protein kinase (PKA) in the absence of dopamine. In contrast, activation of DTH II by PKA is only manifest in the presence of dopamine. Site-directed mutagenesis of Ser32, a serine residue occurring in a PKA site conserved in all known TH proteins, abolishes phosphorylation of both isoforms and activation by PKA. We propose that tissue-specific alternative splicing of TH has a functional role for differential regulation of dopamine biosynthesis in the nervous and non-nervous tissues of insects.

Human tyrosine hydroxylase isoforms. Inhibition by excess tetrahydropterin and unusual behavior of isoform 3 after camp-dependent protein kinase phosphorylation

Human tyrosine hydroxylase exists as four isoforms (hTH1-4), generated by alternative splicing of pre-mRNA, with tissue-specific distribution. Unphosphorylated hTH3 and hTH1 were produced in large amounts in Escherichia coli and purified to homogeneity. The phosphorylation sites were determined after labeling with [32P]phosphate in the presence of cAMP-dependent protein kinase (PKA) and calmodulin-dependent protein kinase II (CaM-PKII). Ser40 was phosphorylated by PKA, and both Ser19 and Ser40 were phosphorylated by CaM-PKII. The enzyme kinetics of hTH3 were determined in the presence of various concentrations of the natural co-substrate (6R)-tetrahydrobiopterin and compared with those of recombinant hTH1 (similar to rat TH). We show that, under initial velocity conditions, excess (6R)-tetrahydrobiopterin inhibits hTH3 and hTH1. The TH catalytic constants (kcat) were determined for each of the two isoenzymes: hTH3 is about five times more active than hTH1. Phosphorylation by CaM-PKII did not affect the kinetic parameters of hTH3. The classical activation of TH by PKA phosphorylation, demonstrated for hTH1, was not observed with hTH3. Furthermore, hTH3 escapes activity regulation by phosphorylation and is always more active than phosphorylated hTH1. The properties of the hTH3 enzyme may be relevant to diseases affecting dopaminergic cells.

Catecholamines, hypoxia and high altitude

Hypoxia is a potent activator of the sympathetic nervous system by stimulating arterial chemoreceptors. However, out of 15 laboratory studies on the effects of acute and prolonged hypoxia on catecholamines, 14 failed to show any changes in plasma or urinary noradrenaline and only four studies showed significant increases in plasma or urinary adrenaline. By contrast, six out of eight studies on MSNA showed increased sympathetic nerve activity to the leg. An increased clearance of plasma catecholamines during hypoxia may be a possible explanation. Furthermore, many of the studies had limitations in a number of subjects and catecholamine assays used. Emotional aspects of the study protocols, which could contribute to the increase in adrenaline, was only assessed by sham runs in one chamber study. However, 13 out of 14 reviewed field studies on subjects staying for more than 1 week at high altitude, reported increased plasma or urinary excretion of noradrenaline which may be compatible with increased sympathetic activity. Adrenaline changed to a lesser degree. Out of seven studies on more short-term (4 h to 3 days) exposure to high altitude, only one demonstrated significantly increased plasma noradrenaline. In this study, however, several subjects had been exposed to high altitude less than 1 week before the experiment. In a new study on 12 climbers reported in this paper, a temporary reduction in plasma catecholamines was found 2 days after arrival at 4200 m. There was a steady increase towards normal levels after 1 week. Plasma vasopressin (AVP) increased suggesting a compensatory mechanism. Both plasma noradrenaline and adrenaline were positively correlated with oxygen saturation in these subjects. Thus, in previously unacclimatized subjects, short-term exposure to high altitude does not increase plasma catecholamines, rather plasma levels decreased. In addition to increased clearance, there is some evidence of reduced synthesis of catecholamines during short-term hypoxia. The oxygen sensitivity of tyrosine hydroxylase (TH) activity, may be one possible mechanism.

Interaction of phosphorylated tryptophan hydroxylase with 14-3-3 proteins

Rabbit brain tryptophan hydroxylase (TPH) has been expressed in insect cells (Spodoptera frugiperda) as a histidine-tagged enzyme. The specific activity of the purified fusion enzyme is 80 nmol of 5-hydroxytryptophan/min/mg. Multifunctional regulatory 14-3-3 proteins were purified from fresh bovine brain. Phosphorylation and 14-3-3 proteins play important roles in the regulation of TPH activity. We have found that phosphorylation of TPH by cAMP-dependent protein kinase increased the activity of the hydroxylase by 25-30% and that 14-3-3 proteins increased the hydroxylase activity of phosphorylated TPH by approximately 45%. Under these conditions, the 14-3-3 proteins were not phosphorylated, and unphosphorylated TPH was not activated by 14-3-3 proteins. Surface plasmon resonance analysis demonstrated that 14-3-3 proteins bind to phosphorylated TPH with an affinity constant (Ka) of 4.5 x 10(7) M-1. Binding studies using affinity chromatography also showed that 14-3-3 proteins interact with phosphorylated TPH. The dephosphorylation of TPH by protein phosphatase-1 was inhibited by 14-3-3 proteins. Our results demonstrate that 14-3-3 proteins form a complex with phosphorylated brain TPH, thereby increasing its enzymatic activity and inhibiting its dephosphorylation.

Characterization of the active site iron in tyrosine hydroxylase. Redox states of the iron

Tyrosine hydroxylase is an iron-containing monooxygenase that uses a tetrahydropterin to catalyze the hydroxylation of tyrosine to dihydroxyphenylalanine in catecholamine biosynthesis. The role of the iron in this enzyme is not understood. Purification of recombinant rat tyrosine hydroxylase containing 0.5-0.7 iron atoms/subunit and lacking bound catecholamine has permitted studies of the redox states of the resting enzyme and the enzyme during catalysis. As isolated, the iron is in the ferric form. Dithionite or 6-methyltetrahydropterin can reduce the iron to the ferrous form. Reduction by 6-methyltetrahydropterin consumes 0.5 nmol/nmol of enzyme-bound iron, producing quinonoid 6-methyldihydropterin as the only detectable product. In the presence of oxygen, reoxidation to ferric iron occurs. During turnover the enzyme is in the ferrous form. However, a fraction is oxidized during turnover; this can be trapped by added catechol or by the dihydroxyphenylalanine formed during turnover.

Conformational properties and stability of tyrosine hydroxylase studied by infrared spectroscopy. Effect of iron/catecholamine binding and phosphorylation

The conformation and stability of recombinant tetrameric human tyrosine hydroxylase isoenzyme 1 (hTH1) was studied by infrared spectroscopy and by limited tryptic proteolysis. Its secondary structure was estimated to be 42% alpha-helix, 35% beta-extended structures (including beta-sheet), 14% beta-turns, and 10% nonstructured conformations. Addition of Fe(II) or Fe(II) plus dopamine to the apoenzyme did not significantly modify its secondary structure. However, an increased thermal stability and resistance to proteolysis, as well as a decreased cooperativity in the thermal denaturation transition, was observed for the ligand-bound forms. Thus, as compared with the apoenzyme, the ligand-bound subunits of hTH1 showed a more compact tertiary structure but weaker intersubunit contacts within the protein tetramer. Phosphorylation of the apoenzyme by cyclic AMP-dependent protein kinase did not change its overall conformation but allowed on iron binding a conformational change characterized by an increase (about 10%) in alpha-helix and protein stability. Our results suggest that the conformational events involved in TH inhibition by catecholamines are mainly related to modifications of tertiary and quaternary structural features. However, the combined effect of iron binding and phosphorylation, which activates the enzyme, also involves modifications of the protein secondary structure.

Identification of iron ligands in tyrosine hydroxylase by mutagenesis of conserved histidinyl residues

Tyrosine hydroxylase catalyzes the hydroxylation of tyrosine and other aromatic amino acids using a tetrahydropterin as the reducing substrate. The enzyme is a homotetramer; each monomer contains a single nonheme iron atom. Five histidine residues are conserved in all tyrosine hydroxylases that have been sequenced to date and in the related eukaryotic enzymes phenylalanine and tryptophan hydroxylase. Because histidine has been suggested as a ligand to the iron in these enzymes, mutant tyrosine hydroxylase proteins in which each of the conserved histidines had been mutated to glutamine or alanine were expressed in Escherichia coli. The H192Q, H247Q, and H317A mutant proteins contained iron in comparable amounts to the wild-type enzyme, about 0.6 atoms/sub-unit. In contrast, the H331 and H336 mutant proteins contained no iron. The first three mutant enzymes were active, with Vmax values 39, 68, and 7% that of the wild-type enzyme, and slightly altered V/Km values for both tyrosine and 6-methyltetrahydropterin. In contrast, the H331 and H336 mutant enzymes had no detectable activity. The EPR spectra of the H192Q and H247Q enzymes are indistinguishable from that of wild-type tyrosine hydroxylase, whereas that of the H317A enzyme indicated that the ligand field of the iron had been slightly perturbed. These results are consistent with H331 and H336 being ligands to the active site iron atom.

Catalytic core of rat tyrosine hydroxylase: terminal deletion analysis of bacterially expressed enzyme

Tyrosine hydroxylase (TH) catalyzes the rate-limiting step in catecholamine biosynthesis. This enzyme is hypothesized to consist of an amino-terminal regulatory domain and a carboxy-terminal catalytic domain. In the present studies, we have utilized recombinant DNA techniques to map the boundaries of the regulatory and catalytic domains of TH. We have isolated a full-length cDNA clone for rat pheochromocytoma TH and have expressed the enzyme in bacteria. Utilizing this bacterial expression system and polymerase chain reaction technology, we have constructed and subcloned genes for five amino-terminal deletion mutants (N delta 40, N delta 155, N delta 165, N delta 175 and N delta 200; N delta denotes amino-terminal deletion and the numerical value denotes the number of amino acids deleted) and two carboxy-terminal deletion mutants (C delta 19 and C delta 50). The catalytic core of rat tyrosine hydroxylase has been identified to include the region from amino acid #165 to amino acid #479. The amino-terminal deletion mutants, N delta 40, N delta 155 and N delta 165 are from 1.85 to 2.5-fold more active than unmodified recombinant TH, while the removal of 19 amino acids from the C-terminus (C delta 19) results in a 70% reduction in enzyme activity. Removal of additional sequences (ten more residues from the N-terminus [N delta 175]; or an additional 31 amino acids from the C-terminus [C delta 50]) results in protein that is totally without enzyme activity. As expected, removal of 40 (or more) N-terminal amino acids abolishes the ability of the catalytic subunit of the cAMP-dependent protein kinase to phosphorylate the recombinant enzyme; serine-40 is the phosphorylation site on TH for PKA. We conclude that the N-terminal boundary for the TH catalytic domain resides between residues 165 and 175 and that removal of this N-terminal domain (totally or partially) increases the activity of the enzyme. These findings confirm previous reports that proteolytic cleavage at amino acid #158 produces an active (and activated) catalytic fragment.

Deletion mutagenesis of rat PC12 tyrosine hydroxylase regulatory and catalytic domains

The functional organization of rat tyrosine hydroxylase was investigated by deletion mutagenesis of the regulatory and catalytic domains. A series of tyrosine hydroxylase cDNA deletion mutants were amplified by PCR, cloned into the pET3C prokaryotic expression vector, and the mutant proteins were partially purified from E. coli. The results show that the deletion of up to 157 N-terminal amino acids activated the enzyme, but further deletion to position 184 completely destroyed catalytic activity. On the carboxyl end, the removal of 43 amino acids decreased but did not eliminate activity, suggesting that this region may play a different role in the regulation of the enzyme. These findings place the amino end of the catalytic domain between residues 158 and 184 and the carboxyl end at or prior to position 455. Deletions within the first 157 amino acids in the N-terminus caused an increase in hydroxylating activity, a decrease in the apparent Km for tyrosine and phenylalanine substrates, and a substantial increase in the Ki for dopamine inhibition. The results define this region of the N-terminus as the regulatory domain of tyrosine hydroxylase, whose primary functions are to restrict the binding of amino acid substrates and to facilitate catecholamine inhibition. The results also suggest that the well-established role of the regulatory domain in restricting cofactor binding may be secondary to an increase in catecholamine binding, which in turn lowers the affinity for the cofactor. These findings provide new insight into the functional organization and mechanisms of regulation of tyrosine hydroxylase.

Multiple forms of tyrosine hydroxylase in human neuroblastoma cells: quantitation with isoform-specific antibodies

Human tyrosine hydroxylase (HTH) RNA undergoes alternative splicing, and four different forms of HTH mRNA have been previously identified. Rabbit antibodies were raised against octapeptides unique to each of the four isoforms of HTH predicted from these mRNAs. Blot immunolabeling of human adrenal medulla, pheochromocytoma, and several neuroblastoma cell lines with affinity-purified anti-HTH peptide antibodies demonstrated the presence of all four HTH isoforms in each of these tissues. Quantitative immunolabeling assays for HTH-1, -2, and -4 were established, and HTH isoform levels were determined in several human neuroblastoma cell lines. Whereas total HTH levels differed up to fourfold among the HTH-positive neuroblastoma cell lines studied [LA-N-1, LA-N-5, CHP-234, BE(2)-C, and BE(2)-M17], the relative abundances of HTH isoforms in each of the cell lines were similar. Immunocytochemical analyses demonstrated that HTH immunoreactivity was distributed unequally among the cells in each of these neuroblastoma lines, and morphological interconversion did not account for this heterogeneity. A direct relationship between the percentage of HTH-positive cells and overall HTH levels was also observed. This relationship, in the absence of an apparent clonal basis for the heterogeneity, suggests that HTH expression in neuroblastoma cells may be controlled in a relatively "all-or-none" (bimodal) fashion.

Baculovirus expression and characterization of catalytically active horseradish peroxidase

Studies of horseradish peroxidase (HRP), a prototypical enzyme, have provided much of the information that is available on the mechanisms and functions of hemoprotein peroxidases. HRP itself is widely used in biotechnological applications. Further progress in defining the structure and function of the enzyme, however, requires its expression in a heterologous system. We report here baculovirus-mediated, high yield expression of a synthetic gene for HRP in Spodoptera frugiperda cell culture. Expression of the soluble, glycosylated protein requires the 5'-leader sequence of the native gene. Recombinant horseradish peroxidase reacts with H2O2 to give compound I, II, and III spectra and a guaiacol oxidation activity, identical to those of the native enzyme. The integrity of the recombinant active site is confirmed by NMR spectroscopy and by catalytic reaction with ethylhydrazine to give a stabilized isoporphyrin that decays exclusively to delta-meso-ethylheme. Furthermore, thioanisoles are oxidized by recombinant and native HRP with the same enantiomeric specificity. HRP expressed in a baculovirus system, despite probable differences in glycosylation, is essentially identical to the native enzyme.

Expression of mouse tyrosine hydroxylase in Escherichia coli

Enzymatically active mouse tyrosine hydroxylase (TH) was successfully expressed at a high level in Escherichia coli using a T7 RNA polymerase directed expression system. The specific activity of mouse TH in E. coli cell lysate was 7.5 nmol/mg protein/min. Kinetic characteristics of recombinant TH were examined. Km for tyrosine and (6R)-tetrahydrobiopterin (6R-BH4) cofactor were determined to be 7.2 microM (420 microM 6R-BH4), 19 microM [( 6R-BH4] less than 55 microM, 20 microM tyrosine) and 54 microM [( 6R-BH4] greater than 55 microM, 20 microM tyrosine), respectively. These were in good agreement with previously reported values for this enzyme.

Recombinant human tyrosine hydroxylase isozymes. Reconstitution with iron and inhibitory effect of other metal ions

Human tyrosine 3-monooxygenase (tyrosine hydroxylase) exists as four different isozymes (TH1-TH4), generated by alternative splicing of pre-mRNA. Recombinant TH1, TH2 and TH4 were expressed in high yield in Escherichia coli. The purified isozymes revealed high catalytic activity [when reconstituted with Fe(II)] and stability at neutral pH. The isozymes as isolated contained 0.04-0.1 atom iron and 0.02-0.06 atom zinc/enzyme subunit. All three isozymes were rapidly activated (13-40-fold) by incubation with Fe(II) salts (concentration of iron at half-maximal activation = 6-14 microM), and were inhibited by other divalent metal ions, e.g. Zn(II), Co(II) and Ni(II). They all bind stoichiometric amounts of Fe(II) and Zn(II) with high affinity (Kd = 0.2-3 microM at pH 5.4-6.5). Similar time courses were observed for binding of Fe(II) and enzyme activation. In the absence of any free Fe(II) or Zn(II), the metal ions were released from the reconstituted isozymes. The dissociation was favoured by acidic pH, as well as by the presence of metal chelators and dithiothreitol. The potency of metal chelators to remove iron from the hydroxylase correlated with their ability to inhibit the enzyme activity. These studies show that tyrosine hydroxylase binds iron reversibly and that its catalytic activity is strictly dependent on the presence of this metal.