Interaction of tyrosine hydroxylase with ribonucleic acid and purification with DNA-cellulose or poly(A)-sepharose affinity chromatography

AT-1 receptor and phospholipase C are involved in angiotensin III modulation of hypothalamic noradrenergic transmission

1. We previously reported that angiotensin III modulates noradrenergic neurotransmission in the hypothalamus of the rat. In the present work we studied the effects of angiotensin III on norepinephrine release and tyrosine hydroxylase activity. We also investigated the receptors and intracellular pathways involved in angiotensin III modulation of noradrenergic transmission. 2. In rat hypothalamic tissue labeled with [3H]norepinephrine 1, 10, and 100 nM and 1 microM losartan (AT1 receptor antagonist) had no effect on basal neuronal norepinephrine release, whereas 10 and 100 nM and 1 microM losartan partially diminished norepinephrine secretion evoked by 25 mM KCl. The AT2 receptor antagonist PD 123319 showed no effect either on basal or evoked norepinephrine release. The increase in both basal and evoked norepinephrine output induced by 1 microM angiotensin III was blocked by 1 microM losartan, but not by 1 microM PD 123319. 3. The phospholipase C inhibitor 5 microM neomicin inhibited the increase in basal and evoked norepinephrine release produced by 1 microM angiotensin III. 4. Tyrosine hydroxylase activity was increased by 1 microM angiotensin III and this effect was blocked by 1 microM LST and 5 microM neomicin, but not by PD 123319. On the other hand, 1 microM angiotensin III enhanced phosphatidyl inositol hydrolysis that was blocked by 1 microM losartan and 5 microM neomicin. PD 123319 (1 microM) did not affect ANG III-induced phosphatidyl inositol hydrolysis enhancement. 5. Our results confirm that angiotensin III acts as a modulator of noradrenergic transmission at the hypothalamic level through the AT1-phospholipase C pathway. This enhancement of hypothalamic noradrenergic activity suggests that angiotensin III may act as a central modulator of several biological processes regulated at this level by catecholamines, such as cardiovascular, endocrine, and autonomic functions as well as water and saline homeostasis.

Tetrahydropterin-dependent amino acid hydroxylases

Phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase constitute a small family of monooxygenases that utilize tetrahydropterins as substrates. When from eukaryotic sources, these enzymes are composed of a homologous catalytic domain to which are attached discrete N-terminal regulatory domains and short C-terminal tetramerization domains, whereas the bacterial enzymes lack the N-terminal and C-terminal domains. Each enzyme contains a single ferrous iron atom bound to two histidines and a glutamate. Recent mechanistic studies have begun to provide insights into the mechanisms of oxygen activation and hydroxylation. Although the hydroxylating intermediate in these enzymes has not been identified, the iron is likely to be involved. Reversible phosphorylation of serine residues in the regulatory domains affects the activities of all three enzymes. In addition, phenylalanine hydroxylase is allosterically regulated by its substrates, phenylalanine and tetrahydrobiopterin.

The aromatic amino acid hydroxylases

The enzymes phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase constitute the family of pterin-dependent aromatic amino acid hydroxylases. Each enzyme catalyzes the hydroxylation of the aromatic side chain of its respective amino acid substrate using molecular oxygen and a tetrahydropterin as substrates. Recent advances have provided insights into the structures, mechanisms, and regulation of these enzymes. The eukaryotic enzymes are homotetramers comprised of homologous catalytic domains and discrete regulatory domains. The ligands to the active site iron atom as well as residues involved in substrate binding have been identified from a combination of structural studies and site-directed mutagenesis. Mechanistic studies with nonphysiological and isotopically substituted substrates have provided details of the mechanism of hydroxylation. While the complex regulatory properties of phenylalanine and tyrosine hydroxylase are still not fully understood, effects of regulation on key kinetic parameters have been identified. Phenylalanine hydroxylase is regulated by an interaction between phosphorylation and allosteric regulation by substrates. Tyrosine hydroxylase is regulated by phosphorylation and feedback inhibition by catecholamines.

Effects of a diet deficient in tyrosine and queuine on germfree mice

A chemically-defined diet consisting of amino acids (including tyrosine), vitamins, trace elements, glucose, etc., known to support growth and reproduction through many generations when fed to germfree mice has been in use for many years in our laboratory. Classical nutritional studies showed that tyrosine was not a dietary requirement for higher mammals if an adequate amount of phenylalanine was present. Therefore, it was unexpected that when tyrosine was removed from this diet, the germfree mice developed ocular, neurological and other abnormalities which resulted in 100% fatalities usually within two weeks. Adding tyrosine back to the diet prevented the abnormalities from occurring. Conventional mice with a normal intestinal flora showed none of these symptoms when fed the same tyrosine-deficient diet. We added queuine to the tyrosine-deficient diet at a concentration of 0.1 microM. The germfree mice that were fed the diet supplemented with queuine were asymptomatic and remained alive until the termination of the experiments.

Inactivation of phosphorylated rat tyrosine hydroxylase by ascorbate in vitro

Tyrosine hydroxylase activity is reversibly controlled by the actions of several protein kinases. Previous studies showed that, following phosphorylation by protein kinase A, physiological concentrations of ascorbate irreversibly inactivate tyrosine hydroxylase. Several studies were performed to establish the mechanism of inactivation. We found that inactivation occurred under oxygen-free conditions. The results of this and other experiments suggest that oxygenated species such as superoxide or hydrogen peroxide were not required for inactivation by ascorbate. Inhibition of tyrosine hydroxylase by low concentrations of ascorbate raised the question concerning the mechanism for maintaining enzyme activity under physiological conditions. We report that tyrosine, N alpha-methyl tyrosine, 3-iodotyrosine, and phenylalanine protected the phosphorylated enzyme against ascorbate inactivation. Catecholamines (dopamine, norepinephrine, and some of their analogues) also protected the enzyme against ascorbate inactivation. We performed studies to assess conformational changes of tyrosine hydroxylase by measuring the extrinsic fluorescence using 8-anilino-1-naphthalenesulfonic acid as a reporter group. Phosphorylation of tyrosine hydroxylase by protein kinase A decreased the extrinsic fluorescence. Treatment of tyrosine hydroxylase with ascorbate produced a further decrease in fluorescence. These results provide evidence for conformational changes following these treatments. In contrast to extrinsic fluorescence, the circular dichroic spectrum of tyrosine hydroxylase failed to change following phosphorylation by protein kinase A or inhibition by ascorbate. The spectrum was consistent with a secondary structure of tyrosine hydroxylase with 55% alpha helix, 20% beta sheet, 2% beta turn, and 23% random coil.

Tyrosine hydroxylase activity and extrinsic fluorescence changes produced by polyanions

The activity of tyrosine hydroxylase in vitro is affected by many factors, including pH, phosphorylation by several protein kinases, and polyanions. We investigated the activation of tyrosine hydroxylase by RNA or DNA (polyanions), using purified rat PC12 cell enzyme. RNA and DNA each increased tyrosine hydroxylase activity in the presence of subsaturating (125 microM) tetrahydrobiopterin at pH 6. RNA increased enzyme activity up to 6-fold with an EC50 of 3 micrograms/ml. RNA and DNA each increased tyrosine hydroxylase activity by decreasing the Km of the enzyme for tetrahydrobiopterin from 3 mM to 295 microM in the presence of 100 micrograms/ml RNA or 171 microM in the presence of 100 micrograms/ml DNA. We used the apolar fluorescent probe 8-anilino-1-naphthalenesulphonic acid (1,8-ANS) as a reporter group to provide the first evidence for changes in conformation related to changes in activity. At pH 6.0, 1,8-ANS bound to tyrosine hydroxylase and exhibited a characteristic fluorescence spectrum. At pH 7.2, both enzyme activity and fluorescence decreased. DNA or heparin (another polyanion) activated tyrosine hydroxylase and decreased fluorescence of the reporter group 30% at pH 6.0. This decrease suggests that these polyanions altered the conformation of tyrosine hydroxylase. The activating effects of polyanions were diminished at physiological pH (6.8-7.2) or in the presence of bivalent-cation salts (10 mM) or univalentcation salts (100 mM). These results suggest that polyanions play a minimal role, if any, in the physiological regulation of tyrosine hydroxylase activity.

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

A rat cDNA containing the complete coding sequence for rat tyrosine hydroxylase (tyrosine 3-monooxygenase, EC 1.14.16.2) was isolated from a rat PC12 cDNA library and subcloned in a bacterial expression plasmid, and large amounts of functional enzyme were produced in Escherichia coli. The recombinant enzyme was purified approximately 20-fold to a final specific activity of 1.8 mumol/min per mg of protein, with a yield of 30%. As much as 1 mg of pure protein could be obtained from 1 g of wet bacterial cells. The purified hydroxylase was shown to be homogeneous by denaturing polyacrylamide electrophoresis and isoelectric focusing. Amino acid analysis of the N terminus (25 residues) revealed 100% identity with rat PC12 tyrosine hydroxylase, as deduced from its cDNA sequence. Several of the kinetic properties of the recombinant enzyme resembled those of the native PC12 hydroxylase. However, in contrast to the native enzyme, the purified recombinant hydroxylase was shown to be in an activated form. Phosphorylation with cAMP-dependent protein kinase resulted in stoichiometric incorporation of phosphate, but the kinetic profile of the recombinant enzyme was unaffected. Several clues to these differences are considered that may provide insight into the structural features important to the regulation of tyrosine hydroxylase.

Reversible denaturation of the gene V protein of bacteriophage f1

The guanidine hydrochloride (GuHCl)-induced denaturation of the gene V protein of bacteriophage f1 has been studied, using the chemical reactivity of a cysteine residue that is buried in the folded protein and the circular dichroism (CD) at 211 and 229 nm as measures of the fraction of polypeptide chains in the folded form. It is found that this dimeric protein unfolds in a single cooperative transition from a folded dimer to two unfolded monomers. A folded, monomeric form of the gene V protein was not detected at equilibrium. The kinetics of unfolding of the gene V protein in 3 M GuHCl and the refolding in 2 M GuHCl are also consistent with a transition between a folded dimer and two unfolded monomers. The GuHCl concentration dependence of the rates of folding and unfolding suggests that the transition state for folding is near the folded conformation.

A new form of baker's yeast transketolase. An enzyme-RNA complex

Using an immunosorbent, a new form of transketolase, namely, an enzyme-RNA complex, was isolated from a baker's yeast extract. Spontaneous fission of RNA (or its enzymic hydrolysis by RNase) is accompanied by a sharp increase in the catalytic activity of transketolase, which may be directly related to the enzyme's regulation mechanism.

Enhancement of dopamine release in vivo from the rat striatum by dialytic perfusion of 6R-L-erythro-5,6,7,8-tetrahydrobiopterin

We have previously reported that intracerebroventricular administration of 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (6R-BH4), a cofactor for tyrosine hydroxylase, enhances biosynthesis of 3,4-dihydroxyphenylethylamine (dopamine) in the rat brain. In the present study, we have more precisely examined the effects of 6R-BH4 on dopamine release in vivo from the rat striatum using brain microdialysis. The amount of dopamine collected in striatal dialysates was determined using HPLC with electrochemical detection after purification with an alumina batch method. When the striatum was dialyzed with Ringer solution containing various concentrations of 6R-BH4 (0.25, 0.5, and 1.0 mM), dopamine levels in striatal dialysates increased in a concentration-dependent manner. Biopterin had little effect on dopamine levels in dialysates. The 6R-BH4-induced increase in dopamine levels in dialysates was abolished after pretreatment with tetrodotoxin (50 microM) added to the perfusion fluid, but after pretreatment with nomifensine (100 mg/kg, intraperitoneal injection), an inhibitor of dopamine uptake mechanism, a larger increase was observed. After inhibition of tyrosine hydroxylase by pretreatment with alpha-methyl-p-tyrosine (250 mg/kg, intraperitoneal injection), most of the increase persisted. These results suggest that 6R-BH4 has a dopamine-releasing action, which is not dependent on biosynthesis of dopamine.

The control of 5-hydroxytryptamine and dopamine synthesis in the brain: a theoretical approach

The transport of the eight amino acids (phenylalanine, tyrosine, tryptophan, valine, leucine, isoleucine, histidine and methionine) using the large neutral amino acid transporter of the blood-brain barrier (BBB) has been calculated using published kinetic data. The fate of the amino acids has been followed from blood to interstitial space, to cell and through metabolism which included, for tyrosine and tryptophan, the hydroxylases. The system was analysed in terms of flux control coefficients. Since the summation theorem did not hold, the system clearly behaved as a non-homogeneous system. At physiological levels of these eight amino acids, the largest contribution to the control of the flux of tyrosine is given by the hydroxylase step, followed by the diffusional component of the transport across the BBB. For tryptophan it is the hydroxylase step, followed by the carrier-mediated transport across the BBB. For the other amino acids it is the metabolism, followed by the diffusional component of the BBB transport. These parameters for tyrosine and tryptophan were determined at increased levels of blood phenylalanine, tyrosine or histidine. The flux through tryptophan hydroxylase can be affected by high blood levels of tyrosine and histidine to values also observed in hyperphenylalaninaemia. Since hypertyrosinaemia (type II) and hyperhistidinaemia are not associated with mental retardation, it is concluded that interference with transport across the BBB of tyrosine and tryptophan, as well as the flux through tryptophan hydroxylase leading to the synthesis of 5-hydroxytryptamine, do not contribute to the cause of permanent brain dysfunction in hyperphenylalaninaemia. It can be calculated that addition of tyrosine to the diet to raise the blood tyrosine level in phenylketonuria patients may have a beneficial effect for the synthesis of neurotransmitters derived from tyrosine.

In vitro activation of bovine adrenal tyrosine hydroxylase by rabbit skeletal muscle actin: evidence for a possible role of cytoskeletal elements as an activator for cytoplasmic enzymes

Tyrosine hydroxylase prepared from the soluble fraction of bovine adrenal medulla was markedly activated by rabbit skeletal muscle G-actin, and this activation was accompanied by a decrease in the apparent Km of the enzyme for the pterin cofactor. The activating effect of G-actin on the soluble enzyme was still observed in the medium containing a high concentration of salt or excess amounts of proteinase inhibitors. Furthermore, this effect was not affected by either cytochalasin B or DNase I. These results therefore suggest that G-actin interacts with the enzyme molecule at the binding site(s) different from that involved in actin polymerization, and that it causes the activation of the soluble enzyme as a result of an allosteric alteration in the enzyme structure, thus giving rise to the possibility that cytoskeletal elements play an important role in the regulation of catecholamine synthesis as a factor modulating the activity of cytoplasmic tyrosine hydroxylase.