Inhibition of GTP cyclohydrolase I by pterins

Tetrahydrobiopterin in cardiovascular health and disease

Tetrahydrobiopterin (BH4) functions as a cofactor for several important enzyme systems, and considerable evidence implicates BH4 as a key regulator of endothelial nitric oxide synthase (eNOS) in the setting of cardiovascular health and disease. BH4 bioavailability is determined by a balance of enzymatic de novo synthesis and recycling, versus degradation in the setting of oxidative stress. Augmenting vascular BH4 levels by pharmacological supplementation has been shown in experimental studies to enhance NO bioavailability. However, it has become more apparent that the role of BH4 in other enzymatic pathways, including other NOS isoforms and the aromatic amino acid hydroxylases, may have a bearing on important aspects of vascular homeostasis, inflammation, and cardiac function. This article reviews the role of BH4 in cardiovascular development and homeostasis, as well as in pathophysiological processes such as endothelial and vascular dysfunction, atherosclerosis, inflammation, and cardiac hypertrophy. We discuss the therapeutic potential of BH4 in cardiovascular disease states and attempt to address how this modulator of intracellular NO-redox balance may ultimately provide a powerful new treatment for many cardiovascular diseases.

Mitochondria oxidative stress, connexin43 remodeling, and sudden arrhythmic death

BACKGROUND

Previously, we showed that a mouse model (ACE8/8) of cardiac renin-angiotensin system activation has a high rate of spontaneous ventricular tachycardia and sudden cardiac death secondary to a reduction in connexin43 level. Angiotensin-II activation increases reactive oxygen species (ROS) production, and ACE8/8 mice show increased cardiac ROS. We sought to determine the source of ROS and whether ROS played a role in the arrhythmogenesis.

METHODS AND RESULTS

Wild-type and ACE8/8 mice with and without 2 weeks of treatment with L-NIO (NO synthase inhibitor), sepiapterin (precursor of tetrahydrobiopterin), MitoTEMPO (mitochondria-targeted antioxidant), TEMPOL (a general antioxidant), apocynin (nicotinamide adenine dinucleotide phosphate oxidase inhibitor), allopurinol (xanthine oxidase inhibitor), and ACE8/8 crossed with P67 dominant negative mice to inhibit the nicotinamide adenine dinucleotide phosphate oxidase were studied. Western blotting, detection of mitochondrial ROS by MitoSOX Red, electron microscopy, immunohistochemistry, fluorescent dye diffusion technique for functional assessment of connexin43, telemetry monitoring, and in vivo electrophysiology studies were performed. Treatment with MitoTEMPO reduced sudden cardiac death in ACE8/8 mice (from 74% to 18%; P<0.005), decreased spontaneous ventricular premature beats, decreased ventricular tachycardia inducibility (from 90% to 17%; P<0.05), diminished elevated mitochondrial ROS to the control level, prevented structural damage to mitochondria, resulted in 2.6-fold increase in connexin43 level at the gap junctions, and corrected gap junction conduction. None of the other antioxidant therapies prevented ventricular tachycardia and sudden cardiac death in ACE8/8 mice.

CONCLUSIONS

Mitochondrial oxidative stress plays a central role in angiotensin II-induced gap junction remodeling and arrhythmia. Mitochondria-targeted antioxidants may be effective antiarrhythmic drugs in cases of renin-angiotensin system activation.

Sepiapterin improves angiogenesis of pulmonary artery endothelial cells with in utero pulmonary hypertension by recoupling endothelial nitric oxide synthase

Persistent pulmonary hypertension of the newborn (PPHN) is associated with decreased blood vessel density that contributes to increased pulmonary vascular resistance. Previous studies showed that uncoupled endothelial nitric oxide (NO) synthase (eNOS) activity and increased NADPH oxidase activity resulted in marked decreases in NO bioavailability and impaired angiogenesis in PPHN. In the present study, we hypothesize that loss of tetrahydrobiopterin (BH4), a critical cofactor for eNOS, induces uncoupled eNOS activity and impairs angiogenesis in PPHN. Pulmonary artery endothelial cells (PAEC) isolated from fetal lambs with PPHN (HTFL-PAEC) or control lambs (NFL-PAEC) were used to investigate the cellular mechanisms impairing angiogenesis in PPHN. Cellular mechanisms were examined with respect to BH4 levels, GTP-cyclohydrolase-1 (GCH-1) expression, eNOS dimer formation, and eNOS-heat shock protein 90 (hsp90) interactions under basal conditions and after sepiapterin (Sep) supplementation. Cellular levels of BH4, GCH-1 expression, and eNOS dimer formation were decreased in HTFL-PAEC compared with NFL-PAEC. Sep supplementation decreased apoptosis and increased in vitro angiogenesis in HTFL-PAEC and ex vivo pulmonary artery sprouting angiogenesis. Sep also increased cellular BH4 content, NO production, eNOS dimer formation, and eNOS-hsp90 association and decreased the superoxide formation in HTFL-PAEC. These data demonstrate that Sep improves NO production and angiogenic potential of HTFL-PAEC by recoupling eNOS activity. Increasing BH4 levels via Sep supplementation may be an important therapy for improving eNOS function and restoring angiogenesis in PPHN.

Mechanisms of H2O2-induced oxidative stress in endothelial cells

Hydrogen peroxide, produced by inflammatory and vascular cells, induces oxidative stress that may contribute to endothelial dysfunction. In smooth muscle cells, H(2)O(2) induces production of O(2)*(-) by activating NADPH oxidase. However, the mechanisms whereby H(2)O(2) induces oxidative stress in endothelial cells are poorly understood. We examined the effects of H(2)O(2) on O(2)*(-) levels on porcine aortic endothelial cells (PAEC). Treatment with 60 micromol/L H(2)O(2) markedly increased intracellular O(2)*(-) levels (determined by conversion of dihydroethidium to hydroxyethidium) and produced cytotoxicity (determined by propidium iodide staining) in PAEC. Overexpression of human manganese superoxide dismutase in PAEC reduced O(2)*(-) levels and attenuated cytotoxicity resulting from treatment with H(2)O(2). L-NAME, an inhibitor of nitric oxide synthase (NOS), and apocynin, an inhibitor of NADPH oxidase, reduced O(2)*(-) levels in PAEC treated with H(2)O(2), suggesting that both NOS and NADPH oxidase contribute to H(2)O(2)-induced O(2)*(-) in PAEC. Inhibition of NADPH oxidase using apocynin and NOS rescue with L-sepiapterin together reduced O(2)*(-) levels in PAEC treated with H(2)O(2) to control levels. This suggests interaction-distinct NOS and NADPH oxidase pathways to superoxide. We conclude that H(2)O(2) produces oxidative stress in endothelial cells by increasing intracellular O(2)*(-) levels through NOS and NADPH oxidase. These findings suggest a complex interaction between H(2)O(2) and oxidant-generating enzymes that may contribute to endothelial dysfunction.

Genome-scale reconstruction of the metabolic network in Staphylococcus aureus N315: an initial draft to the two-dimensional annotation

Background

Several strains of bacteria have sequenced and annotated genomes, which have been used in conjunction with biochemical and physiological data to reconstruct genome-scale metabolic networks. Such reconstruction amounts to a two-dimensional annotation of the genome. These networks have been analyzed with a constraint-based formalism and a variety of biologically meaningful results have emerged. Staphylococcus aureus is a pathogenic bacterium that has evolved resistance to many antibiotics, representing a significant health care concern. We present the first manually curated elementally and charge balanced genome-scale reconstruction and model of S. aureus' metabolic networks and compute some of its properties.

Results

We reconstructed a genome-scale metabolic network of S. aureus strain N315. This reconstruction, termed iSB619, consists of 619 genes that catalyze 640 metabolic reactions. For 91% of the reactions, open reading frames are explicitly linked to proteins and to the reaction. All but three of the metabolic reactions are both charge and elementally balanced. The reaction list is the most complete to date for this pathogen. When the capabilities of the reconstructed network were analyzed in the context of maximal growth, we formed hypotheses regarding growth requirements, the efficiency of growth on different carbon sources, and potential drug targets. These hypotheses can be tested experimentally and the data gathered can be used to improve subsequent versions of the reconstruction.

Conclusion

iSB619 represents comprehensive biochemically and genetically structured information about the metabolism of S. aureus to date. The reconstructed metabolic network can be used to predict cellular phenotypes and thus advance our understanding of a troublesome pathogen.

Interleukin-1beta-induced nitric oxide production and inhibition of insulin secretion in rat islets of langerhans is dependent upon the nitric oxide synthase cofactor tetrahidrobiopterin

Interleukin (IL)-1 beta-induced inhibition of glucose-stimulated insulin secretion in rat islets of Langerhans is mediated in part by nitric oxide (NO). The NO synthase cofactor 5,6,7,8-tetrahydrobiopterin (BH(4)) supports NO synthesis in many cell types and IL-1 beta-induced NO generation and inhibition of insulin secretion have been previously correlated with intracellular BH(4 )levels in rat insulinoma cells. Using rat islets and the beta cell line BRIN-BD11, we have investigated whether synthesis of BH(4) limits IL-1beta-induced NO generation and inhibition of glucose-induced insulin secretion. IL-1 beta-induced NO generation by BRIN cells and islets was reduced by 2,4-diamino-6-hydroxypyrimidine (DAHP), an inhibitor of de novo BH(4) synthesis. Sepiapterin, the substrate for salvage pathway BH(4) synthesis, reversed this inhibitory effect of DAHP in islets but not BRIN cells. DAHP reversed IL-1 beta-induced inhibition of islet insulin secretion, an effect prevented by sepiapterin. We conclude that BH(4) generation is necessary for IL-1 beta-induced NO generation in rat islets and BRIN cells. While a contribution of non-NO mediators cannot be excluded, our results support the proposal that IL-1 beta-induced, NO-mediated inhibition of insulin secretion in rat islets is dependent on the NOS cofactor BH(4).

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.

Protein kinase C phosphorylates and activates GTP cyclohydrolase I in rat renal mesangial cells

GTP cyclohydrolase I is the rate-limiting enzyme in the de novo synthesis pathway of tetrahydrobiopterin which is an essential cofactor for all NO synthase isoforms. The expression of GTP cyclohydrolase I is regulated on a transcriptional level by a variety of cytokines like interleukin 1beta or tumor necrosis factor alpha. The present paper reports that stimulation of protein kinase C by angiotensin II, platelet-derived growth factor BB or the phorbol ester 12-O-tetradecanoylphorbol-13-acetate triggers the phosphorylation and activation of GTP cyclohydrolase I. These data establish that in addition to transcriptional regulation, there is a prominent post-transcriptional modulation of enzyme activity.

GTP cyclohydrolase I inhibition by the prototypic inhibitor 2, 4-diamino-6-hydroxypyrimidine. Mechanisms and unanticipated role of GTP cyclohydrolase I feedback regulatory protein

2,4-Diamino-6-hydroxypyrimidine (DAHP) is considered to be a selective and direct-acting inhibitor of GTP cyclohydrolase I (GTPCH), the first and rate-limiting enzyme in the pathway for synthesis of tetrahydrobiopterin (BH4). Accordingly, DAHP has been widely employed to distinguish whether de novo BH4 synthesis is required in a given biological system. Although it has been assumed that DAHP inhibits GTPCH by direct competition with substrate GTP, this has never been formally demonstrated. In view of apparent structural homology between DAHP and BH4, we questioned whether DAHP may mimic BH4 in its inhibition of GTPCH by an indirect mechanism, involving interaction with a recently cloned 9.5-kDa protein termed GTPCH Feedback Regulatory Protein (GFRP). We show by reverse transcription-polymerase chain reaction that GFRP mRNA is constitutively expressed in rat aortic smooth muscle cells and further induced by treatment with immunostimulants. Moreover, functional GFRP is expressed and immunostimulant-induced BH4 accumulates in sufficient quantity to trigger feedback inhibition of GTPCH. Studies with DAHP reveal that GFRP is also essential to achieve potent inhibition of GTPCH. Indeed, DAHP inhibits GTPCH by dual mechanisms. At a relatively low concentration, DAHP emulates BH4 and engages the GFRP-dependent feedback inhibitory system; at higher concentrations, DAHP competes directly for binding with GTP substrate. This knowledge predicts that DAHP would preferably target GTPCH in tissues with abundant GFRP.

Role of tetrahydrobiopterin availability in the regulation of nitric-oxide synthase expression in human mesangial cells

Human mesangial cells express an inducible form of nitric-oxide synthase (iNOS) after treatment with cytokines. Tetrahydrobiopterin (BH4), an essential cofactor for NOS, is required for cytokine-induced NO generation. We report here that BH4 is necessary not only for the activity but also for the expression of iNOS in human mesangial cells. Inhibition of de novo BH4 synthesis with 2,4-diamino-6-hydroxypyrimidine (DAHP) significantly attenuated iNOS activity as well as mRNA and protein expression in response to interleukin 1beta plus tumor necrosis factor alpha (IL-1beta/TNF-alpha). In contrast, sepiapterin, which provides BH4 through the pterin salvage pathway, strongly potentiated IL-1beta/TNF-alpha-induced iNOS expression and abrogated the inhibitory effect of DAHP. Inhibition of the pterin salvage pathway with methotrexate abolished sepiapterin potentiation of iNOS induction but did not alter the effect of IL-1beta/TNF-alpha. Determination of intracellular pteridines confirmed that sepiapterin markedly raised BH4 content, an effect that was blocked by methotrexate. These results suggest that BH4 availability plays an important role in the regulation of iNOS expression. The effect of BH4 appears to be mediated, at least in part, by an increase in mRNA stability, as indicated by the observation that DAHP shortened, whereas sepiapterin prolonged the half-life of IL-1beta/TNF-alpha-induced iNOS mRNA. Taken together, our results suggest that the biosynthesis of BH4 contributes to cytokine induction of iNOS expression in human mesangial cells through the stabilization of iNOS mRNA.

Biosynthesis of pteridines in Neurospora crassa, Phycomyces blakesleeanus and Euglena gracilis: detection and characterization of biosynthetic enzymes

Occurrence, biosynthesis and some functions of tetrahydrobiopterin (H4biopterin) in animals are well known. The biochemistry of H4biopterin in other organisms than animals was hitherto not widely investigated. Recently H4biopterin was found in the phytoflagellate Euglena gracilis, in the zygomycete Phycomyces blakesleeanus and in the ascomycete Neurospora crassa. In Euglena, Neurospora and Phycomyces the enzymatic activities of GTP cyclohydrolase I, 6-pyruvoyl tetrahydropterin synthase and sepiapterin reductase are detectable and the biosynthesis follows the same steps as were shown for animals. The biosynthetic enzymes, however, show a much lower sensitivity to those inhibitors that act on vertebrate enzymes. 2,4-Diamino-6-hydroxypyrimidine as inhibitor of GTP cyclohydrolase I and N-acetylserotonin or N-methoxyacetylserotonin as inhibitors of sepiapterin reductase can decrease pteridine biosynthesis significantly, in vitro and in vivo. The apparent Km values are in general higher when compared with the respective animal enzymes. In Neurospora, the conversion of GTP to dihydroneopterin triphosphate was closely associated with subsequent production of 6-hydroxymethyl-7,8-dihydropterin due to the high activity of dihydroneopterin aldolase, different from all other tested organisms. Investigations involving inhibition of pteridine synthesis might be a useful tool for evaluating the hypothesis that pteridines in fungi and plants are co-chromophores of various blue light-dependent, flavin-containing photoreceptors.

Induction of pterin synthesis is not required for cytokine-stimulated tryptophan metabolism

Activation of the immune system which occurs in inflammatory disease leads to parallel increases in pterin synthesis and increased production of neuroactive L-tryptophan metabolites. Several model systems were studied to determine whether pterins, which are cofactors for hydroxylation reactions, could be required in the oxidative kynurenine pathway of L-tryptophan degradation. Treatment of mice with interferon-gamma increased L-tryptophan metabolism without any corresponding change in tissue biopterin concentrations. Cytokine-treated human fibroblasts, macrophages and glioblastoma cells all showed increases in kynurenine production, which were completely independent of pterin synthesis. When pterin synthesis de novo was blocked, either by an inhibitor of GTP cyclohydrolase or because of a genetic deficiency of one of the enzymes of the pathway of pterin biosynthesis, cytokine-stimulated increases in tryptophan metabolism were unaffected. Furthermore, increasing intracellular tetrahydrobiopterin concentrations by treating cells with sepia-pterin also had no effect on markers of tryptophan metabolism. Therefore, both normal and cytokine-stimulated L-tryptophan metabolism appears to be completely independent of pterin biosynthesis.

Feedback regulation mechanisms for the control of GTP cyclohydrolase I activity

Guanosine triphosphate (GTP) cyclohydrolase I, the rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin (BH4), is subject to feedback inhibition by BH4, a cofactor for phenylalanine hydroxylase. Inhibition was found to depend specifically on BH4 and the presence of another protein (p35). The inhibition occurred through BH4-dependent complex formation between p35 protein and GTP cyclohydrolase I. Furthermore, the inhibition was specifically reversed by phenylalanine, and, in conjunction with p35, phenylalanine reduced the cooperativity of GTP cyclohydrolase I. These findings also provide a molecular basis for high plasma BH4 concentrations observed in patients with hyperphenylalaninemia caused by phenylalanine hydroxylase deficiency.

Tetrahydrobiopterin synthesis is induced by LPS in vascular smooth muscle and is rate-limiting for nitric oxide production

GTPCH1 mRNA and BH4 synthesis is increased by LPS in vascular smooth muscle. Our data suggest that induction of GTPCH1 and NOS represent two arms of a common pathway required for immunostimulant-evoked NO synthesis. This conclusion is consistent with the view that the major function of immunostimulant-evoked BH4 is to support NOS. Moreover, GTPCH1 and other enzymes of the de novo BH4 synthetic pathway may prove to be important targets for therapy of clinical conditions arising from NO overproduction. As we begin to reveal the molecular events governing the induction and expression of GTPCH1 and NOS, additional therapeutic approaches for treating NO overproduction are certain to be revealed.

Pterins inhibit nitric oxide synthase activity in rat alveolar macrophages

1. The synthesis of nitrite and citrulline from L-arginine by immune-stimulated rat alveolar macrophages and the modulation of this synthesis were studied. 2,4-Diamino-6-hydroxypyrimidine (DAHP), 6R-5,6,7,8-tetrahydro-L-biopterin (BH4) and L-sepiapterin were potent inhibitors of the recombinant interferon-gamma induced production of nitrogen oxides in intact cultured cells with I50 values for BH4 and L-sepiapterin of approximately 10 microM. They were equally effective in inhibiting the induced production of citrulline. This inhibitory effect was concentration-dependent for all three modulators investigated. 2. The inhibitory effects were not dependent on incubation times of either 24 or 48 h, on the immune-stimulus used (lipopolysaccharide, interferon-gamma), or whether these stimuli were added during or after the induction period. 3. Pterin-6-carboxylic acid (PCA), which cannot be converted into BH4, and methotrexate (MTX), which inhibits dihydrofolatereductase but not de novo biosynthesis of BH4, did not change the production of nitrite. 4. The data indicate that DAHP, an inhibitor of the de novo biosynthesis of the co-factor BH4, blocks the nitric oxide synthase activity in intact cells. Since the pterins BH4 and L-sepiapterin blocked the L-arginine dependent production of nitrite and citrulline, the activity of nitric oxide synthase in phagocytic cells may be regulated by metabolic endproducts of the de novo biosynthesis of BH4.

Tetrahydrobiopterin-dependent formation of nitrite and nitrate in murine fibroblasts

The present study demonstrates that murine dermal fibroblasts produce nitrite (NO2-) and nitrate (NO3-) upon treatment with interferon gamma (IFN-gamma). This formation is dependent on L-arginine and can be inhibited by the L-arginine analogue NG-monomethyl-L-arginine. The effect of IFN-gamma is drastically increased by cotreatment with tumor necrosis factor alpha (TNF-alpha), interleukin 1 (IL-1), or lipopolysaccharide (LPS). The tested cytokines also induce formation of tetrahydrobiopterin in murine fibroblasts. Inhibition of guanosine triphosphate-cyclohydrolase I, the key enzyme of tetrahydrobiopterin de novo synthesis with 2,4-diamino-6-hydroxy-pyrimidine, leads to decreased formation of NO2- and NO3-. This effect can be reversed by addition of sepiapterin, which provides tetrahydrobiopterin via a salvage pathway. Methotrexate, which inhibits the salvage pathway, blocks the restoration of NO2- and NO3- production by sepiapterin. The cytotoxic effect of combinations of IFN-alpha with TNF-gamma, IL-1, or LPS is attenuated by inhibition of tetrahydrobiopterin synthesis. These results show that intracellular concentrations of tetrahydrobiopterin control the amount of NO2- and NO3- produced in situ and suggest that the role of cytokine-induced tetrahydrobiopterin synthesis is to provide cells with the active cofactor for production of nitrogen oxides.

Cerebrospinal fluid neopterin in human immunodeficiency virus type 1 infection

We evaluated cerebrospinal fluid (CSF) concentrations of neopterin, a putative marker of activated macrophages, in 97 subjects infected with human immunodeficiency virus type 1 who had a spectrum of neurological complications. The highest CSF neopterin concentrations occurred in those with neurological opportunistic infections, primary central nervous systems lymphoma, and acquired immunodeficiency syndrome (AIDS) dementia complex. In general, the CSF concentration of neopterin was independent of CSF cell count and blood-brain barrier disruption to albumin. In the patients with AIDS dementia complex, CSF neopterin concentrations correlated with severity of disease and decreased in conjunction with clinical improvement following treatment with zidovudine. These results suggest that CSF neopterin, although not disease-specific, may be useful as a surrogate marker for the presence of AIDS dementia complex and its response to antiviral therapy.

Parallel induction of tetrahydrobiopterin biosynthesis and indoleamine 2,3-dioxygenase activity in human cells and cell lines by interferon-gamma

In all of eight tested human cells and cell lines with inducible indoleamine 2,3-dioxygenase (EC 1.13.11.17) tetrahydrobiopterin biosynthesis was activated by interferon-gamma. This was demonstrated by GTP cyclohydrolase I (EC 3.5.4.16) activities and intracellular neopterin and biopterin concentrations. Pteridine synthesis was influenced by extracellular tryptophan. In T 24-cell extracts, submillimolar concentrations of tetrahydrobiopterin stimulated the indoleamine 2,3-dioxygenase reaction.

Effects of sepiapterin and 6-acetyldihydrohomopterin on the guanosine triphosphate cyclohydrolase I of mouse, rat and the fruit-fly Drosophila

The regulation of GTP cyclohydrolase I would lead to the regulation of tetrahydrobiopterin, an important cofactor for synthesis of neurotransmitters. In an attempt to extend a previous finding [Bellahsene, Dhondt, & Farriaux (1984) Biochem. J. 217, 59-65] that GTP cyclohydrolase I of rat liver is inhibited by subnanomolar concentrations of reduced biopterin and sepiapterin, we found that this could not be verified with the enzyme from mouse liver, fruit-fly (Drosophila) heads or, indeed, from rat liver. It was shown, however, that 12 microM-sepiapterin inhibited mouse liver GTP cyclohydrolase I. Another compound, namely 6-acetyldihydrohomopterin, was also employed in the present study to explore its effect on enzymes that lead to its synthesis in Drosophila and for effects on mammalian systems; at 2-5 microM this compound was shown to stimulate one form of mouse liver GTP cyclohydrolase I and then to inhibit at higher concentrations (40 microM). Neither sepiapterin nor 6-acetyldihydrohomopterin caused any effect on the Drosophila head enzyme. On the other hand, the sigmoid GTP concentration curve for the Drosophila enzyme may indicate a regulatory characteristic of this enzyme. Another report, on the lower level of GTP cyclohydrolase I in mutant mouse liver [McDonald, Cotton, Jennings, Ledley, Woo & Bode (1988) J. Neurochem. 50, 655-657], was confirmed and extended. Instead of having 10% activity, we find that the hph-1 mouse mutant has less than 2% activity in the liver. These studies demonstrate that micromolar levels of reduced pterins may have regulatory effects on GTP cyclohydrolase I and that a mouse mutant is available that has low enough activity to be considered as a model for human atypical phenylketonuria.

Human liver GTP cyclohydrolase I: purification and some properties

Human liver guanosine triphosphate (GTP) cyclohydrolase I has been purified more than 1,700-fold to what appears to be homogeneity. The active enzyme complex has an estimated molecular weight of 453,000 +/- 11,500 by gel filtration chromatography. It consists of a polypeptide of 149,000 +/- 4,000 mol wt by SDS-polyacrylamide gel electrophoresis. The activity of the enzyme is heat stable and is inhibited by di- and trivalent cations. The enzyme has an optimum pH of 7.7 in sodium phosphate buffer. It uses GTP as a sole substrate, with a Km of 116 microM.

Regulation of GTP cyclohydrolase I and dihydropteridine reductase in rat pheochromocytoma PC 12 cells

The addition of 8-bromo cyclic AMP, forskolin, theophylline, and 3-isobutyl-1-methylxanthine to the medium of PC 12 cells resulted in an increase in GTP cyclohydrolase I activity, but had no effect on dihydropteridine reductase activity, except theophylline which caused a decrease in dihydropteridine reductase activity at 96 h. GTP cyclohydrolase I activity peaked at 24 h and returned to normal 96 h after drug treatment. Cycloheximide decreased GTP cyclohydrolase I activity at 48 and 96 h, but had little effect on dihydropteridine reductase activity. The addition of reserpine selectively increased only GTP cyclohydrolase I activity. The addition of tetrahydrobiopterin and sepiapterin, however, coordinately inhibited both GTP cyclohydrolase I and dihydropteridine reductase activities. It appears that GTP cyclohydrolase I activity in PC 12 cells is regulated by cyclic AMP stimulation and by end-product inhibition, whereas dihydropteridine reductase activity is only subject to pterin inhibition.