Immunological studies on the participation of 6-pyruvoyl tetrahydropterin (2'-oxo) reductase, an aldose reductase, in tetrahydrobiopterin biosynthesis

Biosynthesis of Pteridines in Insects: A Review

Pteridines are important cofactors for many biological functions of all living organisms, and they were first discovered as pigments of insects, mainly in butterfly wings and the eye and body colors of insects. Most of the information on their structures and biosynthesis has been obtained from studies with the model insects Drosophila melanogaster and the silkworm Bombyx mori. This review discusses, and integrates into one metabolic pathway, the different branches which lead to the synthesis of the red pigments "drosopterins", the yellow pigments sepiapterin and sepialumazine, the orange pigment erythropterin and its related yellow metabolites (xanthopterin and 7-methyl-xanthopterin), the colorless compounds with violet fluorescence (isoxanthopterin and isoxantholumazine), and the branch leading to tetrahydrobiopterin, the essential cofactor for the synthesis of aromatic amino acids and biogenic amines.

Expression of BmDHFR is up-regulated to trigger an increase in the BH4/BH2 ratio when the de novo synthesis of BH4 is blocked in silkworm, Bombyx mori

Tetrahydrobiopterin (BH4) is a vital coenzyme for several enzymes involved in diverse enzymatic reactions in animals. BH4 deficiency can lead to metabolic and neurological disorders due to dysfunction in its metabolism. Sepiapterin reductase (SPR) and dihydrofolate reductase (DHFR) are crucial enzymes in the BH4 de novo synthesis pathway and salvage pathway, respectively. Dihydrobiopterin (BH2) is an oxidized product of BH4 metabolism. The ratio of BH4/BH2 is a key indicator of the stability of BH4 levels. The de novo pathway of BH4 synthesis is well-defined; however, little is known about the mechanisms of the salvage pathway in insects. Herein, we used the natural BmSPR mutant silkworm (lem) as a resource material. Our results reveal that the BmDHFR expression and the BH4/BH2 ratio were remarkably higher in lem as compared to the wild-type silkworm. In BmN cells, knockdown of BmSpr showed increased BmDHFR expression, while the BH4/BH2 ratio decreased after BmDhfr knockdown by RNAi. Furthermore, simultaneous RNAi of BmSpr and BmDhfr showed a further decrease in the BH4/BH2 ratio. These manifest that the expression of BmDHFR is up-regulated to trigger an increase in the BH4/BH2 ratio when the de novo synthesis of BH4 is blocked in silkworm. Additionally, the knockdown of BmSpr in wild-type silkworms also showed an increased BmDHFR level and BH4/BH2 ratio. Taken together, when the silkworm BH4 de novo synthesis pathway is blocked, the salvage pathway is activated, and BmDHFR plays an important role in maintaining the metabolic balance of silkworm BH4. This study enriches our understanding of the molecular mechanism of the BH4 salvage pathway and lays a good foundation for further studies on BH4 using the silkworm as a model insect.

Evaluation of the silkworm lemon mutant as an invertebrate animal model for human sepiapterin reductase deficiency

Human sepiapterin reductase (SR) deficiency is an inherited disease caused by SPR gene mutations and is a monoamine neurotransmitter disorder. Here, we investigated whether the silkworm lemon mutant could serve as a model of SR deficiency. A point mutation in the BmSPR gene led to a five amino acid deletion at the carboxyl terminus in the lemon mutant. In addition, classical phenotypes seen in SR deficient patients were observed in the lemon mutant, including a normal phenylalanine level, a decreased dopamine and serotonin content, and an increased neopterin level. A recovery test showed that the replenishment of l-dopa significantly increased the dopamine level in the lemon mutant. The silkworm lemon mutant also showed negative behavioural abilities. These results suggest that the silkworm lemon mutant has an appropriate genetic basis and meets the biochemical requirements to be a model of SR deficiency. Thus, the silkworm lemon mutant can serve as a candidate animal model of SR deficiency, which may be helpful in facilitating accurate diagnosis and effective treatment options of SR deficiency.

Analysis of Catecholamines and Pterins in Inborn Errors of Monoamine Neurotransmitter Metabolism-From Past to Future

Inborn errors of monoamine neurotransmitter biosynthesis and degradation belong to the rare inborn errors of metabolism. They are caused by monogenic variants in the genes encoding the proteins involved in (1) neurotransmitter biosynthesis (like tyrosine hydroxylase (TH) and aromatic amino acid decarboxylase (AADC)), (2) in tetrahydrobiopterin (BH₄) cofactor biosynthesis (GTP cyclohydrolase 1 (GTPCH), 6-pyruvoyl-tetrahydropterin synthase (PTPS), sepiapterin reductase (SPR)) and recycling (pterin-4a-carbinolamine dehydratase (PCD), dihydropteridine reductase (DHPR)), or (3) in co-chaperones (DNAJC12). Clinically, they present early during childhood with a lack of monoamine neurotransmitters, especially dopamine and its products norepinephrine and epinephrine. Classical symptoms include autonomous dysregulations, hypotonia, movement disorders, and developmental delay. Therapy is predominantly based on supplementation of missing cofactors or neurotransmitter precursors. However, diagnosis is difficult and is predominantly based on quantitative detection of neurotransmitters, cofactors, and precursors in cerebrospinal fluid (CSF), urine, and blood. This review aims at summarizing the diverse analytical tools routinely used for diagnosis to determine quantitatively the amounts of neurotransmitters and cofactors in the different types of samples used to identify patients suffering from these rare diseases.

Combining Human and Rodent Genetics to Identify New Analgesics

Most attempts at rational development of new analgesics have failed, in part because chronic pain involves multiple processes that remain poorly understood. To improve translational success, one strategy is to select novel targets for which there is proof of clinical relevance, either genetically through heritable traits, or pharmacologically. Such an approach by definition yields targets with high clinical validity. The biology of these targets can be elucidated in animal models before returning to the patients with a refined therapeutic. For optimal treatment, having biomarkers of drug action available is also a plus. Here we describe a case study in rational drug design: the use of controlled inhibition of peripheral tetrahydrobiopterin (BH4) synthesis to reduce abnormal chronic pain states without altering nociceptive-protective pain. Initially identified in a population of patients with low back pain, the association between BH4 production and chronic pain has been confirmed in more than 12 independent cohorts, through a common haplotype (present in 25% of Caucasians) of the rate-limiting enzyme for BH4 synthesis, GTP cyclohydrolase 1 (GCH1). Genetic tools in mice have demonstrated that both injured sensory neurons and activated macrophages engage increased BH4 synthesis to cause chronic pain. GCH1 is an obligate enzyme for de novo BH4 production. Therefore, inhibiting GCH1 activity eliminates all BH4 production, affecting the synthesis of multiple neurotransmitters and signaling molecules and interfering with physiological function. In contrast, targeting the last enzyme of the BH4 synthesis pathway, sepiapterin reductase (SPR), allows reduction of pathological BH4 production without completely blocking physiological BH4 synthesis. Systemic SPR inhibition in mice has not revealed any safety concerns to date, and available genetic and pharmacologic data suggest similar responses in humans. Finally, because it is present in vivo only when SPR is inhibited, sepiapterin serves as a reliable biomarker of target engagement, allowing potential quantification of drug efficacy. The emerging development of therapeutics that target BH4 synthesis to treat chronic pain illustrates the power of combining human and mouse genetics: human genetic studies for clinical selection of relevant targets, coupled with causality studies in mice, allowing the rational engineering of new analgesics.

Investigation of a potential role for aldose reductase AlrA in tetrahydropteridine synthesis in Dictyostelium discoideum Ax2

Dictyostelium discoideum Ax2 is well-known for the synthesis of d-threo-tetrahydrobiopterin (DH4) with a smaller amount of l-erythro-tetrahydrobiopterin (BH4). DH4 synthesis from 6-pyruvoyltetrahydropterin (PPH4) is catalyzed by aldose reductase (AR)-like protein and sepiapterin reductase (SR) via an intermediate 1′-oxo-2′-d-hydroxypropyl tetrahydropterin, which is non-enzymatically oxidized to d-sepiapterin in the absence of SR. However, l-sepiapterin was a dominant product in the reaction of a cellular extract of spr− disrupted in the SR gene. In order to investigate its potential role in tetrahydropteridine synthesis, the enzyme catalyzing l-sepiapterin synthesis from PPH4 was purified from spr −. Via mass spectrometry, the protein was identified to be encoded by alrA. AlrA consists of 297 amino acid residues sharing a high sequence identity with human AR. However, in the co-incubation assay, DH4 synthesis was not detected and, furthermore, the recombinant AlrA was observed to suppress BH4 synthesis by SR, which was known to prefer 1′-oxo-2′-d-hydroxypropyl tetrahydropterin to PPH4. Although intracellular DH4 level in alrA − was decreased to 60% of the wild type, it is presumed to result from the antioxidant function of DH4. Therefore, despite the structural and catalytic identities with human AR, AlrA seems to be involved in neither BH4, nor DH4 synthesis under normal physiological conditions.

Aldose reductase deficiency leads to oxidative stress-induced dopaminergic neuronal loss and autophagic abnormality in an animal model of Parkinson's disease

Fungicide exposure causes degeneration of dopaminergic neurons and contributes to Parkinson's disease (PD). Benomyl inhibits enzymes responsible for detoxifying the reactive dopamine metabolite 3,4-dihydroxyphenylacetaldehyde. Aldose reductase (AR) is known as tetrahydrobiopterin (BH₄) reductase that generates BH₄, a cofactor for tyrosine hydroxylase (TH) involved in dopamine synthesis. AR also acts as an aldehyde reductase involved in detoxifying 3,4-dihydroxyphenylacetaldehyde. In PD patients, the level of AR is significantly lower in the cerebellum. To determine if AR deficiency contributes to PD, AR wild-type (AR^+/+) and knockout (AR^-/-) mice were administrated with 1-methyl-4-phenyl -1,2,3,6- tetrahydropyridine (MPTP). The MPTP-treated AR^-/- mice showed more severe behavioral deficits and brain damage than that of AR^+/+ mice. Contrary to expectation, under normal or MPTP-treated condition, AR^-/- mice showed a significant elevation of BH₄ and dopamine in the midbrain, suggesting that either AR does not contribute to BH₄ production, or other BH₄ synthetic pathways are induced. The AR^-/- brain showed upregulation of peroxynitrite, inducible nitric oxide synthase and downregulation of antioxidant enzymes, Cu/Zn superoxide dismutase (SOD) and peroxiredoxin 2 (Prx2), which indicate an increase in oxidative stress. In line with the animal data, pretreating the SH-SY5Y cells with AR inhibitors (Fidarestat or Epalrestat) before MPP⁺ treatment, increased severe cell death and mitochondrial fragmentation with downregulation of SOD were observed when compared to the MPP⁺ treatment alone. Cycloxygenase 2 (COX2), which can lead to the oxidation of dopamine, was upregulated in AR^-/- brains. Autophagic proteins, beclin-1 and LC3B were also downregulated. The loss of dopaminergic neurons was associated with activation of p-ERK1/2. These findings suggest that AR plays an important role in protecting dopaminergic neuron against neurotoxic metabolites in PD.

Functional analysis of sepiapterin reductase in Drosophila melanogaster

Sepiapterin reductase (SR) is a key enzyme involved in the biosynthesis of tetrahydrobiopterin (BH4), an essential cofactor for the synthesis of important biogenic amines, including catecholamines and serotonin. BH4 deficiencies have been implicated in several neurological disorders. Here, we characterized sepiapterin reductase (SR) loss-of-function mutants in Drosophila melanogaster and demonstrated that SR mutations are responsible for hyposensitivity to oxidative stress. Biochemical analysis further revealed that SR activity and BH4 levels in SR mutants were significantly reduced. Furthermore, we showed that the levels of phosphorylated Akt and total Akt protein were increased in SR mutants. Our findings indicate that SR plays an important role in the Akt pathway and that SR mutants will be a valuable tool for investigating the physiological functions of BH4.

Biochemical and structural studies of 6-carboxy-5,6,7,8-tetrahydropterin synthase reveal the molecular basis of catalytic promiscuity within the tunnel-fold superfamily

6-Pyruvoyltetrahydropterin synthase (PTPS) homologs in both mammals and bacteria catalyze distinct reactions using the same 7,8-dihydroneopterin triphosphate substrate. The mammalian enzyme converts 7,8-dihydroneopterin triphosphate to 6-pyruvoyltetrahydropterin, whereas the bacterial enzyme catalyzes the formation of 6-carboxy-5,6,7,8-tetrahydropterin. To understand the basis for the differential activities we determined the crystal structure of a bacterial PTPS homolog in the presence and absence of various ligands. Comparison to mammalian structures revealed that although the active sites are nearly structurally identical, the bacterial enzyme houses a His/Asp dyad that is absent from the mammalian protein. Steady state and time-resolved kinetic analysis of the reaction catalyzed by the bacterial homolog revealed that these residues are responsible for the catalytic divergence. This study demonstrates how small variations in the active site can lead to the emergence of new functions in existing protein folds.

Tetrahydrobiopterin: biochemistry and pathophysiology

BH4 (6R-L-erythro-5,6,7,8-tetrahydrobiopterin) is an essential cofactor of a set of enzymes that are of central metabolic importance, including four aromatic amino acid hydroxylases, alkylglycerol mono-oxygenase and three NOS (NO synthase) isoenzymes. Consequently, BH4 is present in probably every cell or tissue of higher organisms and plays a key role in a number of biological processes and pathological states associated with monoamine neurotransmitter formation, cardiovascular and endothelial dysfunction, the immune response and pain sensitivity. BH4 is formed de novo from GTP via a sequence of three enzymatic steps carried out by GTP cyclohydrolase I, 6-pyruvoyltetrahydropterin synthase and sepiapterin reductase. An alternative or salvage pathway involves dihydrofolate reductase and may play an essential role in peripheral tissues. Cofactor regeneration requires pterin-4a-carbinolamine dehydratase and dihydropteridine reductase, except for NOSs, in which the BH4 cofactor undergoes a one-electron redox cycle without the need for additional regeneration enzymes. With regard to the regulation of cofactor biosynthesis, the major controlling point is GTP cyclohydrolase I. BH4 biosynthesis is controlled in mammals by hormones and cytokines. BH4 deficiency due to autosomal recessive mutations in all enzymes, except for sepiapterin reductase, has been described as a cause of hyperphenylalaninaemia. A major contributor to vascular dysfunction associated with hypertension, ischaemic reperfusion injury, diabetes and others, appears to be an effect of oxidized BH4, which leads to an increased formation of oxygen-derived radicals instead of NO by decoupled NOS. Furthermore, several neurological diseases have been suggested to be a consequence of restricted cofactor availability, and oral cofactor replacement therapy to stabilize mutant phenylalanine hydroxylase in the BH4-responsive type of hyperphenylalaninaemia has an advantageous effect on pathological phenylalanine levels in patients.

Candidate genes for alcohol preference identified by expression profiling in alcohol-preferring and -nonpreferring reciprocal congenic rats

BACKGROUND

Selectively bred alcohol-preferring (P) and alcohol-nonpreferring (NP) rats differ greatly in alcohol preference, in part due to a highly significant quantitative trait locus (QTL) on chromosome 4. Alcohol consumption scores of reciprocal chromosome 4 congenic strains NP.P and P.NP correlated with the introgressed interval. The goal of this study was to identify candidate genes that may influence alcohol consumption by comparing gene expression in five brain regions of alcohol-naïve inbred alcohol-preferring and P.NP congenic rats: amygdala, nucleus accumbens, hippocampus, caudate putamen, and frontal cortex.

RESULTS

Within the QTL region, 104 cis-regulated probe sets were differentially expressed in more than one region, and an additional 53 were differentially expressed in a single region. Fewer trans-regulated probe sets were detected, and most differed in only one region. Analysis of the average expression values across the 5 brain regions yielded 141 differentially expressed cis-regulated probe sets and 206 trans-regulated probe sets. Comparing the present results from inbred alcohol-preferring vs. congenic P.NP rats to earlier results from the reciprocal congenic NP.P vs. inbred alcohol-nonpreferring rats demonstrated that 74 cis-regulated probe sets were differentially expressed in the same direction and with a consistent magnitude of difference in at least one brain region.

CONCLUSIONS

Cis-regulated candidate genes for alcohol consumption that lie within the chromosome 4 QTL were identified and confirmed by consistent results in two independent experiments with reciprocal congenic rats. These genes are strong candidates for affecting alcohol preference in the inbred alcohol-preferring and inbred alcohol-nonpreferring rats.

Tetrahydrobiopterin, superoxide, and vascular dysfunction

(6R)-5,6,7,8-Tetrahydrobiopterin (BH(4)) is an endogenously produced pterin that is found widely distributed in mammalian tissues. BH(4) works as a cofactor of aromatic amino acid hydroxylases and nitric oxide synthases. In the vasculature a deficit of BH(4) is implicated in the mechanisms of several diseases including atherosclerosis, hypertension, diabetic vascular disease, and vascular complications from cigarette smoking and environmental pollution. These ill-effects are connected to the ability of BH(4) to regulate reactive oxygen species levels in the endothelium. The possibility of using BH(4) as a therapeutical agent in cardiovascular medicine is becoming more compelling and many biochemical and physiological aspects involved in this application are currently under investigation. This review summarizes our current understanding of BH(4) reactivity and some aspects of cellular production and regulation.

Expression analysis of the aldo-keto reductases involved in the novel biosynthetic pathway of tetrahydrobiopterin in human and mouse tissues

Tetrahydrobiopterin (BH(4)) acts as a cofactor of the aromatic amino-acid hydroxylases, and its deficiency may result in hyperphenylalaninemia (HPA) and decreased production of the neurotransmitters. BH(4) is synthesized by sepiapterin reductase (SPR) from 6-pyruvoyl-tetrahydropterin (PPH(4)). A patient with SPR deficiency shows no HPA; however, an SPR knockout mouse exhibits HPA. We have reported on the SPR-unrelated novel biosynthetic pathway from PPH(4) to BH(4) (salvage pathway II) in which 3alpha-hydroxysteroid dehydrogenase type 2 and aldose reductase work in concert. In this study, we performed the expression analysis of both proteins in humans and wild-type mice. The results of expression analysis indicated that salvage pathway II worked in human liver; however, it did not act in human brain or in mouse liver and brain. For this reason, a patient with SPR deficiency may show progressive neurological deterioration without HPA, and SPR knockout mice may exhibit HPA and abnormal locomotion activity.

A murine model for human sepiapterin-reductase deficiency

Tetrahydrobiopterin (BH(4)) is an essential cofactor for several enzymes, including all three forms of nitric oxide synthases, the three aromatic hydroxylases, and glyceryl-ether mono-oxygenase. A proper level of BH(4) is, therefore, necessary for the metabolism of phenylalanine and the production of nitric oxide, catecholamines, and serotonin. BH(4) deficiency has been shown to be closely associated with diverse neurological psychiatric disorders. Sepiapterin reductase (SPR) is an enzyme that catalyzes the final step of BH(4) biosynthesis. Whereas the number of cases of neuropsychological disorders resulting from deficiencies of other catalytic enzymes involved in BH(4) biosynthesis and metabolism has been increasing, only a handful of cases of SPR deficiency have been reported, and the role of SPR in BH(4) biosynthesis in vivo has been poorly understood. Here, we report that mice deficient in the Spr gene (Spr(-/-)) display disturbed pterin profiles and greatly diminished levels of dopamine, norepinephrine, and serotonin, indicating that SPR is essential for homeostasis of BH(4) and for the normal functions of BH(4)-dependent enzymes. The Spr(-/-) mice exhibit phenylketonuria, dwarfism, and impaired body movement. Oral supplementation of BH(4) and neurotransmitter precursors completely rescued dwarfism and phenylalanine metabolism. The biochemical and behavioral characteristics of Spr(-/-) mice share striking similarities with the symptoms observed in SPR-deficient patients. This Spr mutant strain of mice will be an invaluable resource to elucidate many important issues regarding SPR and BH(4) deficiencies.

Structure of Chlorobium tepidum sepiapterin reductase complex reveals the novel substrate binding mode for stereospecific production of L-threo-tetrahydrobiopterin

Sepiapterin reductase (SR) is involved in the last step of tetrahydrobiopterin (BH(4)) biosynthesis by reducing the di-keto group of 6-pyruvoyl tetrahydropterin. Chlorobium tepidum SR (cSR) generates a distinct BH(4) product, L-threo-BH(4) (6R-(1'S,2'S)-5,6,7,8-BH(4)), whereas animal enzymes produce L-erythro-BH(4) (6R-(1'R,2'S)-5,6,7,8-BH(4)) although it has high amino acid sequence similarities to the other animal enzymes. To elucidate the structural basis for the different reaction stereospecificities, we have determined the three-dimensional structures of cSR alone and complexed with NADP and sepiapterin at 2.1 and 1.7 A resolution, respectively. The overall folding of the cSR, the binding site for the cofactor NADP(H), and the positions of active site residues were quite similar to the mouse and the human SR. However, significant differences were found in the substrate binding region of the cSR. In comparison to the mouse SR complex, the sepiapterin in the cSR is rotated about 180 degrees around the active site and bound between two aromatic side chains of Trp-196 and Phe-99 so that its pterin ring is shifted to the opposite side, but its side chain position is not changed. The swiveled sepiapterin binding results in the conversion of the side chain configuration, exposing the opposite face for hydride transfer from NADPH. The different sepiapterin binding mode within the conserved catalytic architecture presents a novel strategy of switching the reaction stereospecificities in the same protein fold.

D-threo-tetrahydrobiopterin is synthesized via 1'-oxo-2'-D-hydroxypropyl-tetrahydropterin in Dictyostelium discoideum Ax2

The biosynthesis of D-threo-tetrahydrobiopterin (DH4, tetrahydrodictyopterin) in Dictyostelium discoideum Ax2 was investigated through the mutant disrupted in the gene encoding sepiapterin reductase (SR) by insertional inactivation. The mutant cells, being completely devoid of SR protein, showed 18.1% of L-erythro-tetrahydrobiopterin (BH4) and 0.6% of DH4 productions in the wild type cells. The mutant cells were also identified to excrete D- and L-sepiapterin, which were presumed to originate from intracellular 1'-oxo-2'-D-hydroxypropyl- and 1'-oxo-2'-L-hydroxypropyl-tetrahydropterin (H4-pterin), respectively. Furthermore, in a coupled assay with Dictyostelium SR, the mutant cell extract exhibited a novel enzyme activity converting 6-pyruvoyltetrahydropterin to 1'-oxo-2'-D-hydroxypropyl-H4-pterin. These results are clear demonstration of the in vivo synthesis of DH4 via 1'-oxo-2'-D-hydroxypropyl-H4-pterin as well as an alternative synthesis of BH4 and DH4 in the complete absence of SR.

Tetrahydrobiopterin is synthesized from 6-pyruvoyl-tetrahydropterin by the human aldo-keto reductase AKR1 family members

Tetrahydrobiopterin (BH(4)) is a cofactor for aromatic amino acid hydroxylases and nitric oxide synthase. The biosynthesis includes two reduction steps catalyzed by sepiapterin reductase. An intermediate, 6-pyruvoyltetrahydropterin (PPH(4)) is reduced to 1(')-oxo-2(')-hydroxypropyl-tetrahydropterin (1(')-OXPH(4)) or 1(')-hydroxy-2(')-oxopropyl-tetrahydropterin (2(')-OXPH(4)), which is further converted to BH(4). However, patients with sepiapterin reductase deficiency show normal urinary excretion of pterins without hyperphenylalaninemia, suggesting that other enzymes catalyze the two reduction steps. In this study, the reductase activities for the tetrahydropterin intermediates were examined using several human recombinant enzymes belonging to the aldo-keto reductase (AKR) family and short-chain dehydrogenase/reductase (SDR) family. In the reduction of PPH(4) by AKR family enzymes, 2(')-OXPH(4) was formed by 3 alpha-hydroxysteroid dehydrogenase type 2, whereas 1(')-OXPH(4) was produced by aldose reductase, aldehyde reductase, and 20 alpha-hydroxysteroid dehydrogenase, and both 1(')-OXPH(4) and 2(')-OXPH(4) were detected as the major and minor products by 3 alpha-hydroxysteroid dehydrogenases (types 1 and 3). The activities of aldose reductase and 3 alpha-hydroxysteroid dehydrogenase type 2 (106 and 35 nmol/mg/min, respectively) were higher than those of the other enzymes (0.2-4.0 nmol/mg/min). Among the SDR family enzymes, monomeric carbonyl reductase exhibited low 1(')-OXPH(4)-forming activity of 5.0 nmol/mg/min, but L-xylulose reductase and peroxisomal tetrameric carbonyl reductase did not form any reduced product from PPH(4). Aldose reductase reduced 2(')-OXPH(4) to BH(4), but the other enzymes were inactive towards both 2(')-OXPH(4) and 1(')-OXPH(4). These results indicate that the tetrahydropterin intermediates are natural substrates of the human AKR family enzymes and suggest a novel alternative pathway from PPH(4) to BH(4), in which 3 alpha-hydroxysteroid dehydrogenase type 2 and aldose reductase work in concert.

Escherichia coli 6-pyruvoyltetrahydropterin synthase ortholog encoded by ygcM has a new catalytic activity for conversion of sepiapterin to 7,8-dihydropterin

The putative gene (ygcM) of Escherichia coli was verified in vitro to encode the ortholog of 6-pyruvoyltetrahydropterin synthase (PTPS). Unexpectedly, the enzyme was found to convert sepiapterin to 7,8-dihydropterin without any cofactors. The enzymatic product 7,8-dihydropterin was identified by HPLC and mass spectrometry analyses, suggesting a novel activity of the enzyme to cleave the C6 side chain of sepiapterin. The optimal activity occurred at pH 6.5-7.0. The reaction rate increased up to 3.2-fold at 60-80 degrees C, reflecting the thermal stability of the enzyme. The reaction required no metal ion and was activated slightly by low concentrations (1-5 mM) of EDTA. The apparent K(m) value for sepiapterin was determined as 0.92 mM and the V(max) value was 151.3 nmol/min/mg. The new catalytic function of E. coli PTPS does not imply any physiological role, because sepiapterin is not an endogenous substrate of the organism. The same activity, however, was also detected in a PTPS ortholog of Synechocystis sp. PCC 6803 but not significant in Drosophila and human enzymes, suggesting that the activity may be prevalent in bacterial PTPS orthologs.

Mutations in the sepiapterin reductase gene cause a novel tetrahydrobiopterin-dependent monoamine-neurotransmitter deficiency without hyperphenylalaninemia

Classic tetrahydrobiopterin (BH(4)) deficiencies are characterized by hyperphenylalaninemia and deficiency of monoamine neurotransmitters. In this article, we report two patients with progressive psychomotor retardation, dystonia, severe dopamine and serotonin deficiencies (low levels of 5-hydroxyindoleacetic and homovanillic acids), and abnormal pterin pattern (high levels of biopterin and dihydrobiopterin) in cerebrospinal fluid. Furthermore, they presented with normal urinary pterins and without hyperphenylalaninemia. Investigation of skin fibroblasts revealed inactive sepiapterin reductase (SR), the enzyme catalyzing the final two-step reaction in the biosynthesis of BH(4). Mutations in the SPR gene were detected in both patients and their family members. One patient was homozygous for a TC-->CT dinucleotide exchange, predicting a truncated SR (Q119X). The other patient was a compound heterozygote for a genomic 5-bp deletion (1397-1401delAGAAC) resulting in abolished SPR-gene expression and an A-->G transition leading to an R150G amino acid substitution and to inactive SR as confirmed by recombinant expression. The absence of hyperphenylalaninemia and the presence of normal urinary pterin metabolites and of normal SR-like activity in red blood cells may be explained by alternative pathways for the final two-step reaction of BH(4) biosynthesis in peripheral and neuronal tissues. We propose that, for the biosynthesis of BH(4) in peripheral tissues, SR activity may be substituted by aldose reductase (AR), carbonyl reductase (CR), and dihydrofolate reductase, whereas, in the brain, only AR and CR are fully present. Thus, autosomal recessive SR deficiency leads to BH(4) and to neurotransmitter deficiencies without hyperphenylalaninemia and may not be detected by neonatal screening for phenylketonuria.

The enzyme that synthesizes tetrahydrobiopterin from 6-pyruvoyl-tetrahydropterin in the lemon mutant silkworm consists of two carbonyl reductases

Tetrahydrobiopterin plays an important role in the biosynthesis of certain neurotransmitters. Using DEAE-Sepharose FF column chromatography, we separated the enzyme that synthesizes tetrahydrobiopterin from 6-pyruvoyl-tetrahydropterin [which is different from sepiapterin reductase (EC 1.1.1.153)] in the lemon mutant of the silkworm Bombyx mori into two fractions, which were named carbonyl reductase I (CR I) and carbonyl reductase II (CR II). The CR I enzyme converted 6-pyruvoyl-tetrahydropterin to 6-lactoyl-tetrahydropterin, while CR II converted 6-pyruvoyl-tetrahydropterin to 1'-hydroxy-2'-oxopropyl-tetrahydropterin, both reactions occurring only in the presence of NADPH. Neither of the two carbonyl reductases alone was able to catalyze the conversion of 6-pyruvoyl-tetrahydropterin to tetrahydrobiopterin in the presence of NADPH. However, when CR I was mixed with CR II in the reaction mixture, 6-pyruvoyl-tetrahydropterin was reduced to tetrahydrobiopterin in the presence of NADPH. Moreover, CR I catalyzed the formation of tetrahydrobiopterin from 1'-hydroxy-2'-oxopropyl-tetrahydropterin, while CR II converted 6-lactoyl-tetrahydropterin to tetrahydrobiopterin, both reactions occurring only in the presence of NADPH. Our results suggest that there are two potential routes for formation of tetrahydrobiopterin from 6-pyruvoyl-tetrahydropterin in the lemon mutant silkworm. In the first route, 1'-hydroxy-2'-oxopropyl-tetrahydropterin is formed from 6-pyruvoyl-tetrahydropterin by CR II and then reduced to tetrahydrobiopterin by CR I, both reactions occurring only in the presence of NADPH. In the other route, 6-pyruvoyl-tetrahydropterin is reduced to 6-lactoyl-tetrahydropterin by CR I and then converted to tetrahydrobiopterin by CR II, both reactions occurring only in the presence of NADPH.

Cloning of mouse sepiapterin reductase gene and characterization of its promoter region

We have isolated and characterized approximately 5 kb mouse sepiapterin reductase gene (Spr) and a highly homologous pseudogene (Sprp). The authentic Spr gene is present as a single copy in the mouse genome and is composed of three exons containing the entire coding region. The primer extension experiment located the transcription initiation site in a putative pyrimidine-rich Inr element. The promoter region of the Spr gene is embedded within a CpG island. It was shown that the promoter region is devoid of distinctive TATA and CAAT boxes. Transient transfection of a series of 5' deletion derivatives of the Spr promoter showed the sequence between -83 and -51 to be essential for promoter activity. The pseudogene Sprp lacks promoter region and exon 3.

Correlation of rat hepatic phenylalanine hydroxylase, with tetrahydrobiopterin and GTP concentrations

Hepatic phenylalanine hydroxylase is reported to be more abundant in experimentally-diabetic rats; whereas livers of animals fed a high protein diet, where gluconeogenesis also prevails, have normal amounts of this enzyme. In this study, in addition to seeking an explanation for this effect of experimental diabetes, we also examined the effects of providing alternative dietary gluconeogenic substrates. In rats fed a diet composed of 40% (w/w) glycerol, the specific activities of hepatic phenylalanine hydroxylase are decreased to about 60% of control values. There is no effect on the apparent state of phosphorylation of the enzyme. However, studies on the incorporation of radiolabelled leucine into liver phenylalanine hydroxylase suggested that there was a decreased rate of synthesis. Similarly, animals fed a diet containing 85% (w/w) fructose also have diminished phenylalanine hydroxylase activities. Under all of the above circumstances and also in streptozotocin-induced diabetic animals, alterations in the concentrations of the hydroxylase cofactor, tetrahydrobiopterin and of GTP closely correlate with the effects on the enzyme activities. They are elevated in livers of diabetic animals and significantly diminished in livers of rats fed diets rich in glycerol or fructose. These observations suggest that in adult rat both liver tetrahydrobiopterin concentrations and the expression of hepatic phenylalanine hydroxylase are regulated by GTP [210].

Inhibition of NADPH supply by 6-aminonicotinamide: effect on glutathione, nitric oxide and superoxide in J774 cells

We have examined the integrity of J774 cell nitric oxide (NO) production and glutathione maintenance, whilst NADPH supply was compromised by inhibition of the pentose pathway with 6-aminonicotinamide. In resting cells 6-phosphogluconate accumulation began after 4 h and glutathione depletion after 24 h of 6-aminonicotinamide treatment. Cellular activation by lipopolysaccharide/interferon-lambda decreased glutathione by approximately 50% and synchronous 6-aminonicotinamide treatment exacerbated this to 31.2% of control (P < 0.05). In activated cells NO2- production was inhibited by 60% with 6-aminonicotinamide (P < 0.01), and superoxide production by 50% (P < 0.01) in zymosan-activated cells. NADPH production via the pentose pathway is therefore important to sustain macrophage NO production whilst maintaining protective levels of glutathione.

Discovery of a new tetrahydrobiopterin-synthesizing enzyme in the lemon mutant of the silkworm Bombyx mori

A new tetrahydrobiopterin-synthesizing enzyme, which is different from sepiapterin reductase (EC 1.1.1.153), was discovered in the integument of the lemon mutant of the silkworm Bombyx mori. This enzyme converted 6-pyruvoyltetrahydropterin to tetrahydrobiopterin, an essential cofactor in the hydroxylation of aromatic amino acids, in the presence of NADPH. The reaction proceeded via 6-lactoyltetrahydropterin and 1'-hydroxy-2'-oxopropyltetrahydropterin as intermediates. The molecular mass of this enzyme was estimated to be 40 kDa. N-Acetylserotonin, a potent inhibitor of sepiapterin reductase, slightly inhibited the enzymatic reaction. In the presence of 0.5 mM N-acetylserotonin, the formation of tetrahydrobiopterin by sepiapterin reductase purified from the normal strain silkworm was completely inhibited. However, the formation of tetrahydrobiopterin by the enzyme purified from the lemon mutant was inhibited by only about 50%. These results suggest an alternative biosynthetic pathway to tetrahydrobiopterin.

Sepiapterin reductase and the biosynthesis of tetrahydrobiopterin in Drosophila melanogaster

Ammonium sulfate fractionation and standard column chromatography techniques have been used to purify the enzyme sepiapterin reductase to electrophoretic homogeneity from pupae of Drosophila melanogaster. This purification constitutes a 1000-fold increase in the specific activity of the enzyme. The native molecular weight of the enzyme was determined to be ca 67,000 Da, while the subunit molecular weight is estimated to be 36,000-39,000 Da. The apparent Km for 6-lactoyltetrahydropterin (lactoyl-H4pterin) is 50 microns. The Drosophila enzyme is sensitive to inhibition by the biogenic amine, N-acetyl serotonin, and (to a lesser extent) melatonin, but its activity is not affected by serotonin, epinephrine or norepinephrine. The enzyme was shown to be an integral component of the Drosophila enzyme system which functions in catalyzing the conversion of dihydroneopterin triphosphate (H2NTP) to (6R)-5,6,7,8-tetrahydrobiopterin (H4biopterin). It appears that although purified Drosophila sepiapterin reductase can catalyze low levels of conversion of 6-pyruvoyltetrahydropterin (pyruvoyl-H4pterin) to H4 biopterin in the presence of NADPH, the efficient conversion of pyruvoyl-H4pterin to H4biopterin requires the presence of both sepiapterin reductase and pyruvoyl-H4pterin reductase.

Cloning and sequencing of cDNA encoding human sepiapterin reductase--an enzyme involved in tetrahydrobiopterin biosynthesis

A full-length cDNA clone for sepiapterin reductase, an enzyme involved in tetrahydrobiopterin biosynthesis, was isolated from a human liver cDNA library by plaque hybridization. The nucleotide sequence of hSPR 8-25, which contained an entire coding region of the enzyme, was determined. The clone encoded a protein of 261 amino acids with a calculated molecular mass of 28,047 daltons. The predicted amino acid sequence of human sepiapterin reductase showed a 74% identity with the rat enzyme. We further found a striking homology between human SPR and carbonyl reductase, estradiol 17 beta-dehydrogenase, and 3 beta-hydroxy-5-ene steroid dehydrogenase, especially in their N-terminal region.

Control of pteridine biosynthesis in the natural killer-like cell line YT

The natural killer-like cell line YT constitutively expresses GTP-cyclohydrolase activity whereas 6-pyruvoyltetrahydropterin synthase and sepiapterin reductase are absent. The product, dihydroneopterin triphosphate, is dephosphorylated and oxidized causing neopterin to accumulate in the cells. The activities of the H4biopterin synthesizing enzymes are not controlled by IFN-gamma or the synergistic action of both IFN-gamma and IL-2 as has been shown for monocytes/macrophages (Huber C. et al. (1984) J. Exp. Med. 160, 310) and CD4+ T cells, respectively (Ziegler I. et al. (1990) J. Biol. Chem. 265, 17026). Sepiapterin reductase specifically is induced by incubation of the cells with sepiapterin, leaving GTP-cyclohydrolase, 6-pyruvoyltetrahydropterin synthase and other enzymes related to pteridine metabolism (dihydropteridine reductase, dihydrofolate reductase) unaffected. The data indicate that H4biopterin synthesis is individually regulated in the diverse cellular components of the immune system.

Isolation and expression of rat liver sepiapterin reductase cDNA

Sepiapterin reductase (7,8-dihydrobiopterin: NADP+ oxidoreductase, EC 1.1.1.153) catalyzes the terminal step in the biosynthetic pathway for tetrahydrobiopterin, the cofactor necessary for aromatic amino acid hydroxylation. We report here the isolation of a cDNA clone for rat liver sepiapterin reductase. The cDNA has been excised from a lambda vector and the DNA sequence was determined. The insert contains the coding sequence for at least 95% of the rat enzyme and is fused to the Escherichia coli beta-galactosidase N-terminal segment and the lac promoter. The N-terminal region of the clone contains an extraordinarily high G + C content. The amino acid sequence deduced from the clone is in agreement with the size and composition of the enzyme and was matched to several tryptic peptide sequences. The enzyme encoded by the cDNA insert was shown to have sepiapterin reductase activity after expression in E. coli. Structural similarities were identified between this protein and several enzymes that should contain similar nucleotide and pteridine binding sites.