Purification of GTP cyclohydrolase I from human liver and production of specific monoclonal antibodies

Comprehensive analysis of GTP cyclohydrolase I activity in Mycobacterium tuberculosis H37 Rv via in silico studies

GTP cyclohydrolase I enzyme (GTPCH-I) is a rate limiting enzyme in the biosynthesis pathway of tetrahydrobiopterin (BH4) and tetrahydrofolate (THF) compounds; latter being are an essential compounds involved in many biological functions. This enzyme has been evaluated structurally and functionally in many organisms to understand its putative role in cell processes, kinetics, regulations, drug targeting in infectious diseases, pain sensitivity in humans, and so on. In Mycobacterium tuberculosis (a human pathogen causing tuberculosis), this GTPCH-I activity has been predicted to be present in Rv3609c gene (folE) of H₃₇ Rv strain, which till date has not been studied in detail. In order to understand in depth, the structure and function of folE protein in M. tuberculosis H₃₇ Rv, in silico study was designed by using many different bioinformatics tools. Comparative and structural analysis predicts that Rv3609c gene is similar to folE protein ortholog of Listeria monocytogenes (cause food born disease), and uses zinc ion as a cofactor for its catalysis. Result shows that mutation of folE protein at 52th residue from tyrosine to glycine or variation in pH and temperature can lead to high destability in protein structure. Studies here have also predicted about the functional regions and interacting partners involved with folE protein. This study has provided clues to carry out experimentally the analysis of folE protein in mycobacteria and if found suitable will be used for drug targeting.

A typical N-terminal extensions confer novel regulatory properties on GTP cyclohydrolase isoforms in Drosophila melanogaster

The cofactor tetrahydrobiopterin plays critical roles in the modulation of the signaling molecules dopamine, serotonin, and nitric oxide. Deficits in cofactor synthesis have been associated with several human hereditary diseases. Responsibility for the regulation of cofactor pools resides with the first enzyme in its biosynthetic pathway, GTP cyclohydrolase I. Because organisms must be able to rapidly respond to environmental and developmental cues to adjust output of these signaling molecules, complex regulatory mechanisms are vital for signal modulation. Mammalian GTP cyclohydrolase is subject to end-product inhibition via an associated regulatory protein and to positive regulation via phosphorylation, although target residues are unknown. GTP cyclohydrolase is composed of a highly conserved homodecameric catalytic core and non-conserved N-terminal domains proposed to be regulatory sites. We demonstrate for the first time in any organism that the N-terminal arms of the protein serve regulatory functions. We identify two different modes of regulation of the enzyme mediated through the N-terminal domains. The first is end-product feedback inhibition, catalytically similar to that of the mammalian enzyme, except that feedback inhibition by the cofactor requires sequences in the N-terminal arms rather than a separate regulatory protein. The second is a novel inhibitory interaction between the N-terminal arms and the active sites, which can be alleviated through the phosphorylation of serine residues within the N termini. Both mechanisms allow for acute and highly responsive regulation of cofactor production as required by downstream signaling pathways.

Biosynthesis of pteridines. Reaction mechanism of GTP cyclohydrolase I

GTP cyclohydrolase I catalyses the hydrolytic release of formate from GTP followed by cyclization to dihydroneopterin triphosphate. The enzymes from bacteria and animals are homodecamers containing one zinc ion per subunit. Replacement of Cys110, Cys181, His112 or His113 of the enzyme from Escherichia coli by serine affords catalytically inactive mutant proteins with reduced capacity to bind zinc. These mutant proteins are unable to convert GTP or the committed reaction intermediate, 2-amino-5-formylamino-6-(beta-ribosylamino)-4(3H)-pyrimidinone 5'-triphosphate, to dihydroneopterin triphosphate. The crystal structures of GTP complexes of the His113Ser, His112Ser and Cys181Ser mutant proteins determined at resolutions of 2.5A, 2.8A and 3.2A, respectively, revealed the conformation of substrate GTP in the active site cavity. The carboxylic group of the highly conserved residue Glu152 anchors the substrate GTP, by hydrogen bonding to N-3 and to the position 2 amino group. Several basic amino acid residues interact with the triphosphate moiety of the substrate. The structure of the His112Ser mutant in complex with an undefined mixture of nucleotides determined at a resolution of 2.1A afforded additional details of the peptide folding. Comparison between the wild-type and mutant enzyme structures indicates that the catalytically active zinc ion is directly coordinated to Cys110, Cys181 and His113. Moreover, the zinc ion is complexed to a water molecule, which is in close hydrogen bond contact to His112. In close analogy to zinc proteases, the zinc-coordinated water molecule is suggested to attack C-8 of the substrate affording a zinc-bound 8R hydrate of GTP. Opening of the hydrated imidazole ring affords a formamide derivative, which remains coordinated to zinc. The subsequent hydrolysis of the formamide motif has an absolute requirement for zinc ion catalysis. The hydrolysis of the formamide bond shows close mechanistic similarity with peptide hydrolysis by zinc proteases.

GTP cyclohydrolase I from Tetrahymena pyriformis: cloning of cDNA and expression

A full-length cDNA clone for GTP cyclohydrolase I (EC 3.5.4.16) was isolated from a Tetrahymena pyriformis cDNA library by plaque hybridization. The nucleotide sequence determination revealed that the length of the cDNA insert was 1516 bp. The coding region encoded a protein of 223 amino acid residues with a calculated molecular mass of 25 416 Da. The deduced amino acid sequence of Tetrahrymena GTP cyclohydrolase I showed sequence identity with that of Escherichia coli (55%). The identity of T. pyriformis GTP cyclohydrolase I with sequences of Dictyostelium discoideum, Saccharomyces cerevisiae, Drosophila melanogaster, mouse, rat, and human enzymes was less marked and was 30, 30, 25, 28, 28, and 27%, respectively. RNA blot analysis showed a single mRNA species of 2.1 kb in this protozoan. The mRNA level of GTP cyclohydrolase I increased during synchronous cell division induced by intermittent heat treatment. The results suggest that the mRNA expression is associated with the cell cycle of T. pyriformis.

Enzymes related to catecholamine biosynthesis in Tetrahymena pyriformis. Presence of GTP cyclohydrolase I

We first identified GTP cyclohydrolase I activity (EC 3.5.4.16) in the ciliated protozoa, Tetrahymena pyriformis. The Vmax value of the enzyme in the cellular extract of T. pyriformis was 255 pmol mg-1 protein h-1. Michaelis-Menten kinetics indicated a positive cooperative binding of GTP to the enzyme. The GTP concentration producing half-maximal velocity was 0.8 mM. By high-performance liquid chromatography (HPLC) with fluorescence detection, a major peak corresponding to D-monapterin (2-amino-4-hydroxy-6-[(1'R,2'R)-1',2',3'-trihydroxypropyl]pteridin e, D-threo-neopterin) and minor peaks of D-erythro-neopterin and L-erythro-biopterin were found to be present in the cellular extract of Tetrahymena. Thus, it is strongly suggested that Tetrahymena converts GTP into unconjugated pteridine derivatives. In this study, dopamine was detected as the major catecholamine, while neither epinephrine nor norepinephrine was identified. Indeed, this protozoa was shown to possess the activity of a dopamine synthesizing enzyme, aromatic L-amino acid decarboxylase. On the other hand, activities of tyrosine hydroxylase or tyrosinase which converts tyrosine into dopa, the substrate of aromatic L-amino acid decarboxylase, could not be detected in this protozoa. Furthermore, neither dopamine beta-hydroxylase activity nor phenylethanolamine N-methyltransferase activity could be identified by the HPLC methods.

Rat GTP cyclohydrolase I is a homodecameric protein complex containing high-affinity calcium-binding sites

Recombinant rat liver GTP cyclohydrolase I has been prepared by heterologous gene expression in Escherichia coli and characterized by biochemical and biophysical methods. Correlation averaged electron micrograph images of preferentially oriented enzyme particles revealed a fivefold rotational symmetry of the doughnut-shaped views with an average particle diameter of 10 nm. Analytical ultracentrifugation and quantitative scanning transmission electron microscopy yielded average molecular masses of 270 kDa and 275 kDa, respectively. Like the Escherichia coli homolog, these findings suggest that the active enzyme forms a homodecameric protein complex consisting of two fivefold symmetric pentameric rings associated face-to-face. Examination of the amino acid sequence combined with calcium-binding experiments and mutational analysis revealed a high-affinity, EF-hand-like calcium-binding loop motif in eukaryotic enzyme species, which is absent in bacteria. Intrinsic fluorescence measurements yielded an approximate dissociation constant of 10 nM for calcium and no significant binding of magnesium. Interestingly, a loss of calcium-binding capacity observed for two rationally designed mutations within the presumed calcium-binding loop of the rat GTP cyclohydrolase I yielded a 45% decrease in enzyme activity. This finding suggests that failure of calcium binding may be the consequence of a mutation recently identified in the causative GTP cyclohydrolase I gene of patients suffering from dopa responsive dystonia.

GTP-cyclohydrolase-I like immunoreactivity in rat brain

GTPCH-I immunoreactive structures in the rat brain were studied using a polyclonal antibody raised in the chick. General mapping was made using the avidin-biotin-peroxidase technique and compared with the distribution of tyrosine hydroxylase and serotonin immunoreactivities. Double immunofluorescence was performed in order to establish real intracellular colocalization. GTPCH-I immunoreactivity was generally found to be low. Immunostained neurons were present in all the serotonin cell groups. In catecholaminergic neurons, although tyrosine hydroxylase immunoreactivity was always very high, GTPCH-I immunoreactivity was extremely variable, from relatively strong (substantia nigra, ventral tegmental area) to low (locus coeruleus, caudal part of the hypothalamus), extremely low (rostral hypothalamus, ventral brainstem) or almost absent (dorsal brainstem, some hypothalamic nuclei). When feasible, double immunolabeling revealed that all the serotonin cells and most of the tyrosine hydroxylase cells were also expressing GTPCH-I. Our results argue in favor of a regulation of tyrosine hydroxylase activity by the intracellular synthesis of BH4.

Atomic structure of GTP cyclohydrolase I

BACKGROUND

Tetrahydrobiopterin serves as the cofactor for enzymes involved in neurotransmitter biosynthesis and as regulatory factor in immune cell proliferation and the biosynthesis of melanin. The biosynthetic pathway to tetrahydrobiopterin consists of three steps starting from GTP. The initial reaction is catalyzed by GTP cyclohdrolase I (GTP-CH-I) and involves the chemically complex transformation of the purine into the pterin ring system.

RESULTS

The crystal structure of the Escherichia coli GTP-CH-I was solved by single isomorphous replacement and molecular averaging at 3.0 A resolution. The functional enzyme is a homodecameric complex with D5 symmetry, forming a torus with dimensions 65 A x 100 A. The pentameric subunits are constructed via an unprecedented cyclic arrangement of the four-stranded antiparallel beta-sheets of the five monomers to form a 20-stranded antiparallel beta-barrel of 35 A diameter. Two pentamers are tightly associated by intercalation of two antiparallel helix pairs positioned close to the subunit N termini. The C-terminal domain of the GTP-CH-I monomer is topologically identical to a subunit of the homohexameric 6-pyruvoyl tetrahydropterin synthase, the enzyme catalyzing the second step in tetrahydrobiopterin biosynthesis.

CONCLUSIONS

The active site of GTP-CH-I is located at the interface of three subunits. It represents a novel GTP-binding site, distinct from the one found in G proteins, with a catalytic apparatus that suggest involvement of histidines and, possibly, a cystine in the unusual reaction mechanism. Despite the lack of significant sequence homology between GTP-CH-I and 6-pyruvoyl tetrahydropterin synthase, the two proteins, which catalyze consecutive steps in tetrahydrobiopterin biosynthesis, share a common subunit fold and oligomerization mode. In addition, the active centres have an identical acceptor site for the 2-amino-4-oxo pyrimidine moiety of their substrates which suggests an evolutionarily conserved protein fold designed for pterin biosynthesis.

Tetrahydrobiopterin cofactor biosynthesis: GTP cyclohydrolase I mRNA expression in rat brain and superior cervical ganglia

GTP cyclohydrolase I (GTPCH) is the rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin, the reduced pteridine cofactor required for catecholamine (CA), indoleamine, and nitric oxide biosynthesis. We have used the reverse transcription-polymerase chain reaction technique, based on the published cDNA sequence for rat liver GTPCH, to clone a portion of the GTPCH transcript from rat adrenal gland mRNA and have used this clone for the analysis of GTPCH mRNA in brain and other tissues of the rat by northern blot, nuclease protection assay, and in situ hybridization. Two GTPCH mRNA transcripts of 1.2 and 3.8 kb in length were detected by northern blot, with the 1.2-kb form predominating in the liver and the 3.8-kb form in the pineal gland, adrenal gland, brainstem, and hypothalamic neurons maintained in culture. In situ hybridization studies localized GTPCH mRNA to CA-containing perikarya in the locus ceruleus, ventral tegmental area, and substantia nigra, pars compacta. Levels of GTPCH mRNA in central and peripheral catecholamine neurons determined by nuclease protection assay were increased twofold 24 h after a single injection of the CA-depleting drug reserpine; both the 1.2- and 3.8-kb transcripts were increased in the adrenal gland. Low levels of GTPCH mRNA were also detected by nuclease protection assay in the striatum, hippocampus, and cerebellum, brain regions that do not contain monoaminergic perikarya.

Induction of GTP-cyclohydrolase I mRNA expression by lectin activation and interferon-gamma treatment in human cells associated with the immune response

The development of tetrahydrobiopterin synthesis during lectin stimulation of resting human T lymphocytes (Kerler et al. [1989] FEBS Lett., 250:622-624), the interferon-gamma induced neopterin production by human monocytes/macrophages (Huber et al. [1984] J. Exp. Med., 160:310-316), and the control of tetrahydrobiopterin synthesis in activated T cells by the synergistic action of interferon-gamma and interleukin 2 (Ziegler et al. [1990] J. Biol. Chem. 265:17026-17030) were previously explained by modulation of the apparent GTP-cyclohydrolase I activation. In this study we demonstrate that increases in GTP-cyclohydrolase I activity which occur after lectin induction and after cytokine treatment correlate with increased steady state mRNA levels specific for this enzyme. The enhancement of interferon-gamma induced enzyme activity in primed T cells by interleukin 2 also corresponds to further increases in mRNA expression. The steady state GTP-cyclohydrolase I mRNA levels in primed T cells, however, do not correlate with the steep decline which follows the culmination of enzyme activity 44 hours after treatment. This indicates that the down-regulation of apparent GTP-cyclohydrolase I activity is caused by posttranslational modification of the protein.

Regulation of the L-arginine-dependent and tetrahydrobiopterin-dependent biosynthesis of nitric oxide in murine macrophages

Nitric oxide is a recently discovered biomolecule with a broad range of actions. The present study investigated the regulation of nitric oxide synthase activity by dexamethasone and the cofactor tetrahydrobiopterin in murine macrophages. The influence of the tetrahydrobiopterin biosynthesis inhibitors 2,4-diamino-6-hydroxypyrimidine, an inhibitor of GTP cyclohydrolase I, and phenprocoumon, an inhibitor of sepiapterin reductase, on the synthesis of nitric oxide was investigated. Dexamethasone decreased the nitric oxide production due to direct inhibition of the induction of nitric oxide synthase and of GTP cyclohydrolase. Substitution of tetrahydrobiopterin via sepiapterin could not overcome the dexamethasone-mediated inhibition. 2,4-Diamino-6-hydroxypyrimidine abolished nitric oxide synthesis and synergized with dexamethasone, completely eliminating nitric oxide production. Phenprocoumon inhibited production of nitric oxide via interference with later steps of tetrahydrobiopterin biosynthesis. An exogenous supply of tetrahydrobiopterin through sepiapterin led to a further increase of nitric oxide production, even in fully activated macrophages. The amount of nitric oxide produced by murine macrophages is therefore limited by the amount of tetrahydrobiopterin present in the cells. Inhibitors of tetrahydrobiopterin biosynthesis could provide a novel approach for therapy of pathological conditions mediated by nitric oxide, such as septic shock.

In vitro immunization with antigen directly blotted from SDS-polyacrylamide gels to polyvinylidene difluoride membranes

A new immunization method has been developed for the production of monoclonal antibodies. This technique uses small amounts of partially purified and weak immunogenic antigen, bound to membranes after blotting from SDS-PAGE. For this purpose two different membranes have been tested. Immobilon-P polyvinylidene difluoride (PVDF) membranes were less mitogenic than nitrocellulose membranes, and were therefore selected for the in vitro immunization using 6-pyruvoyl tetrahydropterin synthase as antigen. The in vitro immunization method was then used for the production of monoclonal antibodies against 6-pyruvoyl tetrahydropterin synthase, one of the key enzymes on the biosynthetic pathway of tetrahydrobiopterin, the natural cofactor of the mammalian aromatic amino acid hydroxylases. The antibodies obtained were mainly of the IgM type.

Differential metabolism of tetrahydrobiopterin in monoamine neurons: a hypothesis based upon clinical and basic research

This chapter has attempted to describe and integrate some of the clinical and basic research that support our hypothesis that the metabolism of BH4 is normally heterogeneous across different populations of monoamine-containing neurons. Based upon this hypothesis, there may now be reason to support the idea that certain neuropsychiatric illnesses, which are though to be the result (at least in part) of altered monoamine metabolism, might find their roots in an abnormal metabolism of BH4 within specific monoaminergic cell groups. Such a specific dysfunction might not be apparent in the rest of the brain or peripheral nervous system, thereby being difficult to detect. Perhaps the application of molecular biological techniques to studies of BH4 metabolism in man will shed new light on these problems.

Species and tissue specificity of mammalian GTP cyclohydrolase I messenger RNA

Northern blot analysis of rat RNA from cell lines and isolated organs with a specific rat cDNA probe detected two GTP cyclohydrolase I mRNA species of approx. 1.4 and 3.6 kb. The ratio between these two species varies between 0.6 and 2.4 in different rat organs. Using primers derived from highly conserved regions in the rat and Escherichia coli cDNA sequences a human GTP cyclohydrolase I probe was obtained by means of reverse transcription and PCR (polymerase chain reaction). The human PCR product consisting of 555 bp was cloned and sequenced. It shows a 92% identity with the published sequence of the rat gene. The analysis of various human cell lines with this specific probe shows only one species of GTP cyclohydrolase I mRNA with an approximate size of 3.6 kb.

Allosteric characteristics of GTP cyclohydrolase I from Escherichia coli

The kinetic and regulatory properties of GTP cyclohydrolase I were investigated using an improved enzyme assay and direct determination of the product, dihydroneopterin triphosphate. The enzyme was purified from Escherichia coli to absolute homogeneity as demonstrated by N-terminal sequencing of up to 50 amino acid residues. A 30-residue internal fragment showed 42% similarity with rat liver GTP cyclohydrolase I. The enzyme did not obey Michaelis-Menten kinetics or show a sigmoid reaction curve. The substrate saturation kinetics were found to be slow with low response to minor changes in GTP concentrations. GTP cyclohydrolase I has a relatively high apparent Km. The values are slightly different for enzyme purified by GTP-agarose (100 microM) and UTP-agarose (110 microM). Low turnover numbers of 12/min and 19/min were calculated for the respective enzyme preparations. GTP-cyclohydrolase-I activity was modulated in Vmax by K+, divalent cations, UTP and tetrahydrobiopterin. Divalent cations, such as Mg2+, had an activating effect with an optimum at 8 mM Mg2+. A different catalytic function and formation of a new, unidentified product by GTP cyclohydrolase I was observed in the presence of Ca2+. In the presence of 1 mM EDTA and Mg2+, GTP-cyclohydrolase-I activity was strongly inhibited by chelate complexes. UTP proved not to be a competitive inhibitor, but a positive modulator. The inhibition by chelate complexes was totally abolished by UTP. Tetrahydrobiopterin showed an inhibitory effect, with 50% inhibition at 100 microM tetrahydrobiopterin. UTP was able to reduce the inhibition by tetrahydrobiopterin. Using monoclonal antibody 1F11 (related to the GTP-binding site), and monoclonal antibody NS7 (mimicking tetrahydrobiopterin), different binding sites were demonstrated for GTP and tetrahydrobiopterin on each enzyme subunit. Western-blot competition analysis revealed a UTP-binding site different from the binding sites of GTP and tetrahydrobiopterin. Based on the kinetic behaviour and the kind of modulations observed we defined GTP cyclohydrolase I as an M-class allosteric enzyme.

Multiple mRNA forms of human GTP cyclohydrolase I

To isolate full length cDNA clones encoding human GTP cyclohydrolase I, the first and rate-limiting enzyme in tetrahydrobiopterin biosynthesis, a cDNA library generated from human liver was screened by plaque hybridization. Analysis of the clones, hybridized with rat cDNA fragment, by restriction mapping and partial sequencing showed the existence of three kinds of cDNAs. All three cDNAs were identical in their central and 5' regions. They were, however, found to diverge at 3' ends. Furthermore, the three species of mRNAs corresponding to the three cDNAs were detected in human liver by reverse transcription-polymerase chain reaction (RT-PCR) analysis. These results indicate that, in humans, GTP cyclohydrolase I molecules are encoded by at least three distinct mRNAs.

Control of cell-cycle-associated tetrahydrobiopterin synthesis in rat thymocytes

The cell-cycle progression of rat thymocytes from G0 through G1 to DNA synthesis is associated with a transient synthesis of H4biopterin, the concentration of which reaches a maximum at the time of S-phase entry and then decreases. This synthesis of H4biopterin is controlled by the specific activity of GTP cyclohydrolase I, which peaks in G1/S cells. In contrast, the catalytic activity of sepiapterin reductase remains constant throughout the cell-cycle. At G0 the steady state mRNA levels specific for GTP cyclohydrolase I and sepiapterin reductase, respectively, are below the limits of detection. Both accumulate as the thymocytes progress through the cell-cycle but lack cyclic down regulation. The data indicate that the variations in H4biopterin synthesis during the cell-cycle are caused by growth regulated increase in GTP cyclohydrolase I mRNA expression, with subsequent post-translational inactivation. This latter is likely due to the degree of enzyme phosphorylation.

The mtrAB operon of Bacillus subtilis encodes GTP cyclohydrolase I (MtrA), an enzyme involved in folic acid biosynthesis, and MtrB, a regulator of tryptophan biosynthesis

mtrA of Bacillus subtilis was shown to be the structural gene for GTP cyclohydrolase I, an enzyme essential for folic acid biosynthesis. mtrA is the first gene in a bicistronic operon that includes mtrB, a gene involved in transcriptional attenuation control of the trp genes. mtrA of B. subtilis encodes a 20-kDa polypeptide that is 50% identical to rat GTP cyclohydrolase I. Increased GTP cyclohydrolase I activity was readily detected in crude extracts of B. subtilis and Escherichia coli in which MtrA was overproduced. Biochemical evidence indicating that MtrA catalyzes dihydroneopterin triphosphate and formic acid formation from guanosine triphosphate is presented. It was also shown that mtrB of B. subtilis encodes a 6-kDa polypeptide. Expression of mtrB is sufficient for transcriptional attenuation control of the B. subtilis trp gene cluster in Escherichia coli. Known interrelationships between genes involved in folic acid and aromatic amino acid biosynthesis in B. subtilis are described.

Production of monoclonal antibodies against human 6-pyruvoyl tetrahydropterin synthase and immunocytochemical localization of the enzyme

Monoclonal antibodies were produced against human pituitary gland 6-pyruvoyl tetrahydropterin synthase, one of the key enzymes in the biosynthesis of tetrahydrobiopterin, by in vitro immunization with the antigen directly blotted from SDS-PAGE to polyvinylidene difluoride membranes. The antibodies produced show crossreactivity in the enzyme linked immunosorbent assay, not only with the human 6-pyruvoyl tetrahydropterin synthase but some also with the same enzyme isolated from salmon liver. 6-Pyruvoyl tetrahydropterin synthase was localized immuno-enzymatically in peripheral blood smears and in skin fibroblasts by the use of these monoclonal antibodies and the alkaline phosphatase monoclonal anti-alkaline phosphatase labeling technique.

Human liver 6-pyruvoyl tetrahydropterin reductase is biochemically and immunologically indistinguishable from aldose reductase

6-Pyruvoyl tetrahydropterin reductase has been implicated in the biosynthesis of tetrahydrobiopterin. Using immunochemical and biochemical techniques the purified human liver enzyme was shown to be identical to aldose reductase. This suggests that 6-pyruvoyl tetrahydropterin reductase may play an additional role in the reduction of aldehydes derived from the biogenic amine neuro-transmitters and corticosteroid hormones as well as in the pathogenesis of diabetic complications, as has been postulated for aldose reductase.