Isolation and characterization of GTP cyclohydrolase I from mouse liver. Comparison of normal and the hph-1 mutant

Biochemical characterization of the tetrahydrobiopterin synthesis pathway in the oleaginous fungus Mortierella alpina

We characterized the de novo biosynthetic pathway of tetrahydrobiopterin (BH₄) in the lipid-producing fungus Mortierella alpina. The BH₄ cofactor is essential for various cell processes, and is probably present in every cell or tissue of higher organisms. Genes encoding two copies of GTP cyclohydrolase I (GTPCH-1 and GTPCH-2) for the conversion of GTP to dihydroneopterin triphosphate (H₂-NTP), 6-pyruvoyltetrahydropterin synthase (PTPS) for the conversion of H₂-NTP to 6-pyruvoyltetrahydropterin (PPH₄), and sepiapterin reductase (SR) for the conversion of PPH₄ to BH₄, were expressed heterologously in Escherichia coli. The recombinant enzymes were produced as His-tagged fusion proteins and were purified to homogeneity to investigate their enzymic activities. Enzyme products were analysed by HPLC and electrospray ionization-MS. Kinetic parameters and other properties of GTPCH, PTPS and SR were investigated. Physiological roles of BH₄ in M. alpina are discussed, and comparative analyses between GTPCH, PTPS and SR proteins and other homologous proteins were performed. The presence of two functional GTPCH enzymes has, as far as we are aware, not been reported previously, reflecting the unique ability of this fungus to synthesize both BH₄ and folate, using the GTPCH product as a common substrate. To our knowledge, this study is the first to report the comprehensive characterization of a BH₄ biosynthesis pathway in a fungus.

Folate biofortification of lettuce by expression of a codon optimized chicken GTP cyclohydrolase I gene

Folates are essential coenzymes involved in one-carbon metabolism. Folate deficiency is associated with a higher risk of newborns with neural tube defects, spina bifida, and anencephaly, and an increased risk of cardiovascular diseases, cancer, and impaired cognitive function in adults. In plants folates are synthesized in mitochondria from pterin precursors, which are synthesized from guanosine-5'-triphosphate (GTP) in the cytosol (pterin branch), and p-aminobenzoate (PABA), derived from chorismate in plastids (PABA branch). We generated transgenic lettuce lines expressing a synthetic codon-optimized GTP-cyclohydrolase I gene (gchI) based on native Gallus gallus gene. Immunoblotting analyses confirmed the presence of the gchI in transgenic lines. Twenty-nine transgenic lines were generated and 19 exhibited significant increase in the folate content, ranging from 2.1 to 8.5-fold higher when compared to non-transgenic lines. The folate content in enriched lettuce would provide 26% of the Dietary Reference Intakes for an adult, in a regular serving. Although the lettuce lines generated here exhibited high folate enhancement over the control, better folate enrichment could be further achieved by engineering simultaneously both PABA and pterin pathways.

Cloning, expression, purification, and characterization of Nocardia sp. GTP cyclohydrolase I

The sequence of the gene from Nocardia sp. NRRL 5646 encoding GTP cyclohydrolase I (GCH), gch, and its adjacent regions was determined. The open reading frame of Nocardia gch contains 684 nucleotides, and the deduced amino acid sequence represents a protein of 227 amino acid residues with a calculated molecular mass of 24,563Da. The uncommon start codon TTG was identified by matching the N-terminal amino acid sequence of purified Nocardia GCH with the deduced amino acid sequence. A likely ribosomal binding site was identified 9bp upstream of the translational start site. The 3' end flank region encodes a peptide that shares high homology with dihydropteroate synthases. Nocardia GCH has 73 and 60% identity to the proteins encoded by the putative gch of Mycobacterium tuberculosis and Streptomyces coelicolor, respectively. Nocardia GCH was highly expressed in Escherichia coli cells carrying a pHAT10 based expression vector, and moderately expressed in Mycobacterium smegmatis cells carrying a pSMT3 based expression vector. Enterokinase digestion of recombinant Nocardia GCH, and in-gel digestion of Nocardia GCH and recombinant GCH followed by MALDI-TOF-MS analysis, confirmed that the actual subunit size of the enzyme was 24.5kDa. Thus, we conclude that the active form of native Nocardia GCH is a decamer. Our earlier incorrect conclusion was that the native enzyme was an octamer derived from the anomalous SDS-PAGE migration of the subunit.

GTP cyclohydrolase I: purification, characterization, and effects of inhibition on nitric oxide synthase in nocardia species

GTP cyclohydrolase I (GTPCH) catalyzes the first step in pteridine biosynthesis in Nocardia sp. strain NRRL 5646. This enzyme is important in the biosynthesis of tetrahydrobiopterin (BH4), a reducing cofactor required for nitric oxide synthase (NOS) and other enzyme systems in this organism. GTPCH was purified more than 5,000-fold to apparent homogeneity by a combination of ammonium sulfate fractionation, GTP-agarose, DEAE Sepharose, and Ultragel AcA 34 chromatography. The purified enzyme gave a single band for a protein estimated to be 32 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular mass of the native enzyme was estimated to be 253 kDa by gel filtration, indicating that the active enzyme is a homo-octamer. The enzyme follows Michaelis-Menten kinetics, with a Km for GTP of 6.5 micromoles. Nocardia GTPCH possessed a unique N-terminal amino acid sequence. The pH and temperature optima for the enzyme were 7.8 and 56 degrees C, respectively. The enzyme was heat stable and slightly activated by potassium ion but was inhibited by calcium, copper, zinc, and mercury, but not magnesium. BH4 inhibited enzyme activity by 25% at a concentration of 100 micromoles. 2,4-Diamino-6-hydroxypyrimidine (DAHP) appeared to competitively inhibit the enzyme, with a Ki of 0.23 mM. With Nocardia cultures, DAHP decreased medium levels of NO2- plus NO3-. Results suggest that in Nocardia cells, NOS synthesis of nitric oxide is indirectly decreased by reducing the biosynthesis of an essential reducing cofactor, BH4.

Genetic engineering of Escherichia coli for production of tetrahydrobiopterin

Tetrahydrobiopterin (BH4) is an essential cofactor for various enzymes in mammals. In vivo, it is synthesized from GTP via the three-step pathway of GTP cyclohydrolase I (GCHI), 6-pyruvoyl-tetrahydropterin synthase (PTPS) and sepiapterin reductase (SPR). BH4 is a medicine used to treat atypical hyperphenylalaninemia. It is currently synthesized by chemical means, which consists of many steps, and requires costly materials and complicated procedures. To explore an alternative microbial method for BH4 production, we utilized recombinant DNA technology to construct recombinant Escherichia coli (E. coli) strains carrying genes expressing GCHI, PTPS and SPR enzymes. These strains successfully produced BH4, which was detected as dihydrobiopterin and biopterin, oxidation products of BH4. In order to increase BH4 productivity we made further improvements. First, to increase the de novo GTP supply, an 8-azaguanine resistant mutant was isolated and an additional guaBA operon was introduced. Second, to augment the activity of GCHI, the folE gene from E. coli was replaced by the mtrA gene from Bacillus subtilis. These modifications provided us with a strain showing significantly higher productivity, up to 4.0 g of biopterin/L of culture broth. The results suggest the possibility of commercial BH4 production by our method.

Studies on the genotype-phenotype relation in the hph-1 mouse mutant deficient in guanosine triphosphate (GTP) cyclohydrolase I activity

The guanosine triphosphate (GTP) cyclohydrolase I (GTP-CHI) catalyses the rate-limiting step in the de novo synthesis of tetrahydrobiopterin, a cofactor of three aromatic amino acid hydroxylases, one of which is phenylalanine hydroxylase. The hph-1 mouse mutant deficient in GTP-CHI activity exhibits hyperphenylalaninemia which peculiarly disappears at 3 weeks of age, thus corresponding to the increase in liver GTP-CHI activity. The present gas chromatographic-mass spectrometric analysis of the phenylalanine and catecholamine metabolisms demonstrated the former metabolism to remain disturbed even in adult hph-1, which demonstrated a metabolic basis for sensitivity to the phenylalanine challenge in adult hph-1. A Northern blot analysis showed the hepatic GTP-CHI RNA expression in hph-1 at 2, 3 and 4 weeks of age to parallel the peculiar time course of the enzyme activity previously reported. No mutation was detected in either the coding region or the 5' flanking region (nt.-1 to -746) of the GTP-CHI gene of the hph-1. Further molecular genetic analyses are therefore required to elucidate the mechanism of the peculiar phenotype of hph-1.

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.

Purification and characterization of GTP cyclohydrolase I from Streptomyces tubercidicus, a producer of tubercidin

GTP cyclohydrolase I catalyzing the first reaction in the biosynthesis of pterin moiety of folic acid in bacteria, was purified from Streptomyces tubercidicus by at least 203-fold with a yield of 32% to apparent homogeneity, using ammonium sulfate fractionation, DEAE-cellulose, Sepharose CL-6B, and hydroxylapatite column chromatography. The molecular weight of the native enzyme was estimated to be 230,000 daltons by gel permeation chromatography. The purified enzyme gave a single band on sodium dodesyl sulfate-polyacrylamide gel electrophoresis and its molecular weight was apparently 58,000 daltons. These results indicate that the enzyme consists of four subunits with the same molecular weight. The K(m) and Vmax values for GTP of the purified enzyme were determined to be 80 microM and 90 nmol/min (mg protein), respectively. The optimum pH and temperature for the enzyme reaction were pH 7.5-8.5 and 40-42 degrees C, respectively. Coenzyme or metal ion was not required for the enzyme activity. The enzyme activity was inhibited by most divalent cations, while it was slightly activated by potassium ion. In case of nucleotides, CTP, GMP, GDP, and UTP inhibited enzyme activity, among which GDP exhibited the strongest inhibitory effect.

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.

Purification, cloning, and functional expression of dihydroneopterin triphosphate 2'-epimerase from Escherichia coli

Dihydroneopterin triphosphate (H2NTP) 2'-epimerase from Escherichia coli catalyzes the epimerization of H2NTP to dihydromonapterin triphosphate (H2MTP). The enzyme was purified 954-fold to apparent homogeneity by a combination of ammonium sulfate fractionation and column chromatography of Cibacron blue 3GA dye ligand, phenyl-Sepharose CL-4B, methotrexate-agarose, and Superdex 200 HR 10/30 FPLC column. The molecular mass of the epimerase determined on a Superdex column was 82.6 kDa, while the subunit molecular mass determined on SDS-polyacrylamide gel electrophoresis was 13.7 kDa. This implies that the epimerase most probably exists as homohexamer. The 20-amino acid sequence from the N terminus was determined (AQPAAIIRIKNLRLRTFIGI). Based on this sequence, the gene encoding the epimerase was cloned using a simple polymerase chain reaction approach. Translation of the nucleotide sequence of the cloned gene revealed the presence of an open reading frame containing 120 amino acids with a predicted molecular mass of 13,993 Da. The epimerase gene located in a 2.3-kilobase BamHI-EcoRI fragment from Kohara's clone 406 was overexpressed 300-fold, which was confirmed by the prominent increase in the 14-kDa protein band on SDS-polyacrylamide electrophoresis gels. It showed no homology with the sequences of isomerases or other enzymes in GenBank/EMBL data bases.

A missense mutation in a patient with guanosine triphosphate cyclohydrolase I deficiency missed in the newborn screening program

A patient with guanosine triphosphate cyclohydrolase I deficiency passed the newborn phenylketonuria screening program. The characteristic clinical phenotype developed in a 5-month-old patient; elevated plasma phenylalanine, undetectable urinary pterins, and absence of the enzyme activity in a liver biopsy were present. A point mutation that results in an amino acid substitution from methionine to isoleucine at position 211 was proposed to be the cause for this new phenotypic expression of guanosine triphosphate cyclohydrolase I deficiency.

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.

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.

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.