High-level expression of rat PC12 tyrosine hydroxylase cDNA in Escherichia coli: purification and characterization of the cloned enzyme

Expression and purification of recombinant human tyrosine hydroxylase as a fusion protein in Escherichia coli

Tyrosine hydroxylase is the rate-limiting step in the synthesis of dopamine and is tightly regulated. Previous studies have shown it to be covalently modified and potently inhibited by 3,4-dihydroxyphenylacetaldehyde (DOPAL), an endogenous neurotoxin via dopamine catabolism which is relevant to Parkinson's disease. In order to elucidate the mechanism of enzyme inhibition, a source of pure, active tyrosine hydroxylase was necessary. The cloning and novel purification of human recombinant TH from Escherichia coli is described here. This procedure led to the recovery of ~23 mg of pure, active and stable enzyme exhibiting a specific activity of ~17 nmol/min/mg. The enzyme produced with this procedure can be used to delineate the tyrosine hydroxylase inhibition by DOPAL and its relationship to Parkinson's disease. This procedure improves upon previous methods because the fusion protein gives rise to high expression and convenient affinity-capture, and the cleaved and highly purified hTH makes the product useful for a wider variety of applications.

A unique dual activity amino acid hydroxylase in Toxoplasma gondii

The genome of the protozoan parasite Toxoplasma gondii was found to contain two genes encoding tyrosine hydroxylase; that produces l-DOPA. The encoded enzymes metabolize phenylalanine as well as tyrosine with substrate preference for tyrosine. Thus the enzymes catabolize phenylalanine to tyrosine and tyrosine to l-DOPA. The catalytic domain descriptive of this class of enzymes is conserved with the parasite enzyme and exhibits similar kinetic properties to metazoan tyrosine hydroxylases, but contains a unique N-terminal extension with a signal sequence motif. One of the genes, TgAaaH1, is constitutively expressed while the other gene, TgAaaH2, is induced during formation of the bradyzoites of the cyst stages of the life cycle. This is the first description of an aromatic amino acid hydroxylase in an apicomplexan parasite. Extensive searching of apicomplexan genome sequences revealed an ortholog in Neospora caninum but not in Eimeria, Cryptosporidium, Theileria, or Plasmodium. Possible role(s) of these bi-functional enzymes during host infection are discussed.

Breast cancer-specific gene 1 interacts with the mitotic checkpoint kinase BubR1

The abnormal expression of breast cancer-specific gene 1 (BCSG1) in malignant mammary epithelial cells is highly associated with the development and progression of breast cancer. A series of in vitro and in vivo studies performed in our laboratory and others have demonstrated that BCSG1 expression significantly stimulates proliferation, invasion, and metastasis of breast cancer cells. However, currently little is known about how BCSG1 exerts its oncogenic functions. To elucidate the cellular mechanisms underlying the effects of BCSG1 in breast cancer cells, we used a yeast two-hybrid system to screen for proteins that could associate with BCSG1. Through this screening, we identified the mitotic checkpoint protein BubR1 as a novel binding partner of BCSG1. The specific association of BCSG1 with BubR1 in breast cancer cells was demonstrated by immunoprecipitation and GST pull-down assays. Intriguingly, experiments conducted in four different cell lines all showed that exogenous expressions of BCSG1 consistently reduce the cellular levels of the BubR1 protein without affecting BubR1 mRNA expression. The tendency of endogenous BCSG1 expression coinciding with lower BubR1 protein levels was also observed in seven out of eight breast cancer cell lines. We further showed that the reducing effect of BCSG1 on BubR1 protein expression could be prevented by treating BCSG1-transfected cells with MG-132, a selective 26S proteasome inhibitor, implying that the proteasome machinery may be involved in the BCSG1-induced reduction of the BubR1 protein. Accompanied with a reduction of BubR1 protein level, BCSG1 expression resulted in multinucleation of breast cancer cells upon treatment with spindle inhibitor nocodazole, indicating an impaired mitotic checkpoint. Taken together, our novel findings suggest that BCSG1 may accelerate the progression of breast cancer at least in part by compromising the mitotic checkpoint control through inactivation of BubR1.

Neuroblastoma directed therapy by a rational prodrug design of etoposide as a substrate for tyrosine hydroxylase

Tumor directed cytotoxic therapy is one of the major challenges for the success of chemotherapy. In order to accomplish this goal in neuroblastoma, we rationally designed a prodrug of etoposide as substrate for tyrosine hydroxylase, a well established neuroblastoma associated enzyme. Here, we report synthesis and characterization of a 3,4 dihydroxy-phenyl carbamate derivative of etoposide. In order to demonstrate activation by tyrosine hydroxylase, the coding sequence of murine tyrosine hydroxylase was generated by reverse transcriptase-polymerase chain reaction from NXS2 neuroblastoma cells and cloned into the pRSET-A bacterial expression vector. The enzyme was expressed in Escherichia coli, characterized by Western blot and enzymatic activity was demonstrated by conversion of tyrosine into DOPA in the presence of cofactors using reversed phase high-performance liquid chromatography. Under these enzymatic conditions, we demonstrate conversion of 3,4 dihydroxy-phenyl carbamate prodrug into free etoposide. This effect was clearly mediated by the enzyme since bacteria transformed with the empty vector were ineffective of prodrug activation. Furthermore, tyrosine hydroxylase positive cells exposed to the etoposide prodrug were effectively killed in contrast to tyrosine hydroxylase negative controls. These findings demonstrate that etoposide can be designed as a prodrug substrate for tyrosine hydroxylase and thereby establish proof of concept for neuroblastoma directed enzyme prodrug therapy.

Nitration and inactivation of tyrosine hydroxylase by peroxynitrite

Tyrosine hydroxylase (TH) is modified by nitration after exposure of mice to 1-methyl-4-phenyl-1,2,3,6-tetrahydrophenylpyridine. The temporal association of tyrosine nitration with inactivation of TH activity in vitro suggests that this covalent post-translational modification is responsible for the in vivo loss of TH function (Ara, J., Przedborski, S., Naini, A. B., Jackson-Lewis, V., Trifiletti, R. R., Horwitz, J., and Ischiropoulos, H. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 7659-7663). Recent data showed that cysteine oxidation rather than tyrosine nitration is responsible for TH inactivation after peroxynitrite exposure in vitro (Kuhn, D. M., Aretha, C. W., and Geddes, T. J. (1999) J. Neurosci. 19, 10289-10294). However, re-examination of the reaction of peroxynitrite with purified TH failed to produce cysteine oxidation but resulted in a concentration-dependent increase in tyrosine nitration and inactivation. Cysteine oxidation is only observed after partial unfolding of the protein. Tyrosine residue 423 and to lesser extent tyrosine residues 428 and 432 are modified by nitration. Mutation of Tyr(423) to Phe resulted in decreased nitration as compared with wild type protein without loss of activity. Stopped-flow experiments reveal a second order rate constant of (3.8 +/- 0.9) x 10(3) m(-1) s(-1) at pH 7.4 and 25 degrees C for the reaction of peroxynitrite with TH. Collectively, the data indicate that peroxynitrite reacts with the metal center of the protein and results primarily in the nitration of tyrosine residue 423, which is responsible for the inactivation of TH.

Dopamine inhibition of human tyrosine hydroxylase type 1 is controlled by the specific portion in the N-terminus of the enzyme

Tyrosine hydroxylase (TH), which converts L-tyrosine to L-DOPA, is a rate-limiting enzyme in the biosynthesis of catecholamines; its activity is regulated by feedback inhibition by catecholamine products including dopamine. To investigate the specific portion of the N-terminus of TH that determines the efficiency of dopamine inhibition, wild-type and N-terminal 35-, 38-, and 44-amino acid-deleted mutants (del-35, del-38, and del-44, respectively) of human TH type 1 were expressed as a maltose binding protein fusion in Escherichia coli and purified as a tetrameric form by affinity and size-exclusion chromatography. The fused-form wild-type enzyme possessed almost the same specific enzymatic activity as the previously reported recombinant nonfused form. Although maximum velocities of all N-terminus-deleted forms were about one-fourth of the wild-type value, there was no difference in Michaelis constants for L-tyrosine or (6R)-(L-erythro-1',2'-dihydroxypropyl)-2-amino-4-hydroxy-5,6,7,8-tetrahy dropteridine (6RBPH4) among the four enzymes. The iron contents incorporated into the three N-terminus-deleted mutants were significantly lower than that of wild type. However, there was no substantial difference in incorporated iron contents among the three mutants. The deletion of up to no less than 38 amino acid residues in the N-terminus made the enzyme more resistant to dopamine inhibition than the wild-type or del-35 TH form. Dopamine bound to the del-38 more than to the del-35 TH form. However, when incubation with dopamine was followed by further inhibition with the cofactor 6RBPH4 dopamine was expelled more readily from the del-38 than from the del-35 TH form. These observations suggest that the amino acid sequence Gly36-Arg37-Arg38 plays a key role in determining the competition between dopamine and 6RBPH4 and affects the efficiency of dopamine inhibition of the catalytic activity.

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.

Tyrosine hydroxylase: mechanisms of oxygen radical formation

A new mechanism of oxygen radical formation in dopaminergic neurons is proposed, based on the oxidative mechanism of tyrosine hydroxylase. The cofactor (6R,6S)-5,6,7,8-tetrahydrobiopterin can rearrange in solution which allows an autoxidation reaction producing O2.-, H2O2 and HO.. The combination of tyrosine hydroxylase and the cofactor produces more oxygen radicals than does the autoxidation of the cofactor. This production of oxygen radicals could be damaging to dopaminergic neurons. In the presence of tyrosine, the enzyme produces less radicals than it does in the absence of tyrosine. Mechanisms are proposed for the generation of reactive oxygen species during the autoxidation of the cofactor and during enzymatic catalysis. The generation, by tyrosine hydroxylase, of very small amounts of oxygen radicals over the period of 65 years could contribute to the oxidative stress that causes Parkinson's disease.

A chimeric tyrosine/tryptophan hydroxylase. The tyrosine hydroxylase regulatory domain serves to stabilize enzyme activity

The neurotransmitter biosynthetic enzymes, tyrosine hydroxylase (TH), and tryptophan hydroxylase (TPH) are each composed of an amino-terminal regulatory domain and a carboxyl-terminal catalytic domain. A chimeric hydroxylase was generated by coupling the regulatory domain of TH (TH-R) to the catalytic domain of TPH (TPH-C) and expressing the recombinant enzyme in bacteria. The chimeric junction was created at proline 165 in TH and proline 106 in TPH because this residue is within a conserved five amino-acid span (ValProTrpPhePro) that defines the beginning of the highly homologous catalytic domains of TH and TPH. Radioenzymatic activity assays demonstrated that the TH-R/TPH-C chimera hydroxylates tryptophan, but not tyrosine. Therefore, the regulatory domain does not confer substrate specificity. Although the TH-R/TPH-C enzyme did serve as a substrate for protein kinase (PKA), activation was not observed following phosphorylation. Phosphorylation studies in combination with kinetic data provided evidence that TH-R does not exert a dominant influence on TPH-C. Stability assays revealed that, whereas TH exhibited a t1/2 of 84 min at 37 degrees C, TPH was much less stable (t1/2 = 28.3 min). The stability profile of TH-R/TPH-C, however, was superimposable on that of TH. Removal of the regulatory domain (a deletion of 165 amino acids from the N-terminus) of TH rendered the catalytic domain highly unstable, as demonstrated by a t1/2 of 14 min. The authors conclude that the regulatory domain of TH functions as a stabilizer of enzyme activity. As a corollary, the well-characterized instability of TPH may be attributed to the inability of its regulatory domain to stabilize the catalytic domain.

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.

Spectroscopic and kinetic properties of unphosphorylated rat hepatic phenylalanine hydroxylase expressed in Escherichia coli. Comparison of resting and activated states

The non-heme iron-dependent metalloenzyme, rat hepatic phenylalanine hydroxylase (EC 1.14.16.1; phenylalanine 4-monooxygenase (PAH) was overexpressed in Escherichia coli and purified to homogeneity, allowing a detailed comparison of the kinetic, hydrodynamic, and spectroscopic properties of its allosteric states. The homotetrameric recombinant enzyme, which is highly active and contains 0.7-0.8 iron atoms per subunit, is identical to the native enzyme in several properties: Km, 6-methyltetrahydropterin = 61 microM and L-Phe = 170 microM; Vmax = 9 s-1 (compared to 45 microM, 180 microM, and 13 s-1 for the rat hepatic enzyme). L-Phe and lysolecithin treatment induce the rPAHT-->rPAHR (where r is recombinant) allosteric transformation necessary for rPAH activity. Characteristic changes in the fluorescence spectra, increased hydrophobicity, a large activation energy barrier, and a 10% volume increase of the tetrameric structure are consistent with a significant reorganization of the protein following allosteric activation. However, optical and EPR spectroscopic data suggest that only minor changes occur in the primary coordination sphere (carboxylate/histidine/water) of the catalytic iron center. Detailed steady state kinetic investigations, using 6-methyltetrahydropterin as cofactor and lysolecithin as activator, indicate rPAH follows a sequential mechanism. A catalytic Arrhenius Eact of 14.6 +/- 0.3 kcal/mol subunit was determined from temperature-dependent stopped-flow kinetics data. rPAH inactivates during L-Phe hydroxylation with a half-life of 4.3 min at 25 degrees C, corresponding to an Arrhenius Eact of 10 +/- 1 kcal/mol subunit for the inactivation process. Catechol binding (2.4 x 10(6) M-1) is shown to occur only at catalytically competent iron sites. Ferrous rPAH binds NO, giving rise to an ST = 3/2 spin system.

Structure and function of the aromatic amino acid hydroxylases

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A denaturant-insoluble form of tyrosine hydroxylase in PC12 pheochromocytoma cells

A 6M urea-insoluble form of tyrosine hydroxylase (THi) was detected in PC12 pheochromocytoma cells by western blotting immunodetection methods, and the characteristics and mechanisms of formation of this insoluble species were investigated. THi accounts for about 4% of the immunodetectable tyrosine hydroxylase in exponentially dividing pheochromocytoma cells. It is unlikely that a subpopulation of dead or dying cells is the source of THi since essentially no changes in THi levels were detected when cell death was intentionally increased. To measure the kinetics of formation of cellular THi, exponentially dividing cells were metabolically labeled first with [3H]leucine and then with [14C]leucine, and though both 3H and 14C were incorporated into soluble tyrosine hydroxylase, the near absence of 14C in THi demonstrated that a lag period of at least a day exists between biosynthesis of tyrosine hydroxylase and the accumulation of measurable THi. The cellular accumulation of THi can evidently be regulated by the cell, since upon nerve growth factor (NGF) treatment of cells the total content of tyrosine hydroxylase increased and the content of THi decreased to yield, overall, a fivefold lower proportion of THi after 4 days. A large increase in urea-insoluble enzyme was found upon sublethal exposure of cells to ferrous ion and hydrogen peroxide, indicating that oxidative damage via metal-ion-catalyzed formation of hydroxide free radical can yield an enzyme that is similar in its insolubility to THi.

High-level expression and deletion mutagenesis of human tryptophan hydroxylase

Human tryptophan hydroxylase has been expressed as a soluble and active form in Escherichia coli by fusion with an affinity tag, maltose-binding protein. The fusion protein has been purified to near homogeneity by affinity chromatography on crosslinked amylose resin. The purified fusion protein has a specific activity of 86 nmol of 5-hydroxytryptophan per min per mg of fusion protein. A series of truncation mutants have also been made to explore the domain organization of tryptophan hydroxylase. All deletion mutants were subject to affinity purification and kinetic characterization. While removal of the N-terminal 164 amino acids completely inactivates the enzyme, deletion of the first 91 residues results in a 7-fold reduction in specific activity. From the C terminus, deletion of 36, 55, or 112 amino acids abolishes the activity, whereas deletion of 19 residues decreases the specific activity by approximately 11-fold. These results are consistent with a model for tryptophan hydroxylase in which the enzyme consists of an N-terminal regulatory domain, a catalytic core, and a small C-terminal region of uncertain but important function.

Conformation and interaction of phenylalanine with the divalent cation at the active site of human recombinant tyrosine hydroxylase as determined by proton NMR

Recombinant human tyrosine hydroxylase has been purified as a metal-free apoenzyme (apo-hTH1) which tightly binds one Fe2+, Co2+, or Zn2+ per subunit with activation only by Fe2+ and competitive inhibition by the other cations. L-tyrosine and L-phenylalanine are alternative substrates for this enzyme, giving similar Vmax values, although the KM value for phenylalanine is about 8-fold greater than for tyrosine. Apo-hTH1 enhances the paramagnetic effects of Co2+ on 1/T1 and 1/T2 values of the protons of enzyme-bound phenylalanine both in the presence and in the absence of the oxidized form of the cofactor L-erythro-7,8-dihydrobiopterin (BH2), which was used as an inactive analog of the natural cofactor (6R)-1-erythro-tetrahydrobiopterin (BH4). No effects of hTH1-Zn2+ on 1/T1 or 1/T2 are found. From paramagnetic effects of hTH1-Co2+ on 1/T1 of phenylalanine protons at 250 and 600 MHz, in the presence of BH2, a correlation time (tau c) of 1.8 +/- 0.1 ps was found. Using this tau c value, and assuming that only one proton of the pairs H3,H5, and H2,H6 is experiencing the total paramagnetic effect (asymmetric limiting case), distances from enzyme-bound Co2+ to phenylalanine (+/- 1.2 A) of 6.1 A (H3 or H5), 6.3 A (H2 or H6), 7.0 A (H4), 7.3 A (H alpha), > or = 7.4 A (H beta-pro-S), and > or = 7.6 A (H beta-pro-R) were calculated. The distances to H3 or H5 and to H2 or H6 are slightly increased to 6.8 and 7.0 A, respectively, if each proton of both degenerate pairs equally experiences the paramagnetic effect of Co2+ (symmetric limiting case). These distances place the aromatic ring of phenylalanine in the second coordination sphere of the metal, which would permit an Fe-bound oxy or peroxy species to approach molecular contact with C3/C4, suggesting a direct role of Fe2+ in the hydroxylation reaction. The same correlation time and similar distances were found in the absence of BH2 with H4 of phenylalanine slightly closer to the metal. In the ternary hTH1-Zn(2+).BH2.phenylalanine complex, eight interproton distances in the enzyme-bound phenylalanine were determined by NOESY spectra at 600 MHz at 35-, 50-, and 75-ms mixing times. The conformation of enzyme-bound phenylalanine, consistent with the six Co(2+)-proton distances and the eight interproton distances, is partially extended with torsional angles chi 1 = 97 degrees +/- 3 degrees and chi 2 = -78 degrees +/- 2 degrees.

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.

Regulation of recombinant rat tyrosine hydroxylase by dopamine

Recombinant rat PC12 tyrosine hydroxylase, also called tyrosine 3-monooxygenase [L-tyrosine, tetrahydropteridine:oxygen oxidoreductase (3-hydroxylating), EC 1.14.16.2], purified from Escherichia coli is in an activated form with a low Km for the tetrahydrobiopterin cofactor and a pH optimum of 6.5. Pretreatment with low levels of the derived product, dopamine, inhibits catalytic activity, increases the Km for the cofactor, and shifts the pH curve towards a more acidic pH optimum. Labeled dopamine binds to tyrosine hydroxylase with high affinity (Kd = 1 microM) but low stoichiometry (r = 0.08 mol/mol of enzyme subunit). The binding of dopamine results in the appearance of a blue-green chromophore with lambda max at approximately 660 nm, which is consistent with the formation of a catecholamine-iron complex. In the absence of dopamine, the recombinant enzyme cannot be further activated by phosphorylation with cAMP-dependent protein kinase, although as much as 1 mol of phosphate is incorporated per mol of subunit. In contrast, the enzyme pretreated with dopamine is activated by phosphorylation in the same fashion and to the same extent as the native hydroxylase. The results suggest that the high-affinity binding of catecholamine products is a pivotal post-translational modification that determines the state of enzyme activation and the response to phosphorylation.

"7-tetrahydrobiopterin," a naturally occurring analogue of tetrahydrobiopterin, is a cofactor for and a potential inhibitor of the aromatic amino acid hydroxylases

The ability of 2-amino-4-hydroxy-7-[dihydroxylpropyl-(L-erythro)-5,6,7,8-tetrahyd ropterin] ("7-tetrahydrobiopterin" or 7-BH4) to substitute for the natural cofactor tetrahydrobiopterin (BH4) has been studied in vitro in the reactions of the three mammalian aromatic amino acid hydroxylases. With rat liver phenylalanine hydroxylase, the apparent Km for 7-BH4 is 160 microM, a value that is approximately 60-fold greater than that for the natural cofactor. In contrast, the hydroxylase reaction is severely inhibited by as little as 1 microM 7-BH4 when assayed in the presence of physiological concentrations of BH4. This inhibition can be overcome either by an increase in the concentration of BH4 or a decrease in the concentration of phenylalanine. With both rat brain tryptophan hydroxylase and rat pheochromocytoma tyrosine hydroxylase, the Km value for 7-BH4 is about one order of magnitude greater than the Km for BH4. Accordingly, 7-BH4 is a poor competitive inhibitor of both tryptophan and tyrosine hydroxylase. Thus, our results suggest that the observed hyperphenylalaninemia in patients who excrete 7-BH4 in their urine may arise directly from the inhibition of phenylalanine hydroxylase by low levels of this pterin. On the other hand, it is less likely that low levels of 7-BH4 would affect the activity of tyrosine or tryptophan hydroxylase in vivo.

Regulation of recombinant human tyrosine hydroxylase isozymes by catecholamine binding and phosphorylation. Structure/activity studies and mechanistic implications

Three isozymes of human tyrosine hydroxylase (hTH1, hTH2 and hTH4) were expressed in Escherichia coli and purified to homogeneity. Natural catecholamines and related synthetic compounds were found to be potent inhibitors, competitive to the tetrahydrobiopterin cofactor, of all the isozymes. Combining visible spectroscopy and equilibrium-binding studies, it was found that catecholamines bind to hTH1 and hTH2 with a stoichiometry of about 1.0 mol/mol enzyme subunit, interacting with the catalytic iron at the active site. All the isozymes tested were excellent substrates for cAMP-dependent protein kinase (Km = 5 microM, Vmax = 9.5 mumol.min-1.mg kinase-1). The incorporation of about 1.0 mol phosphate/subunit at Ser40 decreased the affinity of dopamine binding by a factor of 10. Conversely, the addition of stoichiometric amounts of Fe(II) and dopamine to the apoenzymes reduced both the affinity and stoichiometry of phosphorylation by cAMP-dependent protein kinase by 2-3-fold. These data provide evidence for a mutual interaction between the presumed regulatory and catalytic domains of hTH, and show that activation of the enzyme by phosphorylation and inactivation by binding of catecholamines are related events, which probably represent important mechanisms for the regulation of the enzyme activity in vivo.