High-performance liquid chromatography with column switching for the analysis of biogenic amine metabolites and pterins

Tetrahydrobioterin (BH4) Pathway: From Metabolism to Neuropsychiatry

Tetrahydrobipterin (BH4) is a pivotal enzymatic cofactor required for the synthesis of serotonin, dopamine and nitric oxide. BH4 is essential for numerous physiological processes at periphery and central levels, such as vascularization, inflammation, glucose homeostasis, regulation of oxidative stress and neurotransmission. BH4 de novo synthesis involves the sequential activation of three enzymes, the major controlling point being GTP cyclohydrolase I (GCH1). Complementary salvage and recycling pathways ensure that BH4 levels are tightly kept within a physiological range in the body. Even if the way of transport of BH4 and its ability to enter the brain after peripheral administration is still controversial, data showed increased levels in the brain after BH4 treatment. Available evidence shows that GCH1 expression and BH4 synthesis are stimulated by immunological factors, notably pro-inflammatory cytokines. Once produced, BH4 can act as an anti- inflammatory molecule and scavenger of free radicals protecting against oxidative stress. At the same time, BH4 is prone to autoxidation, leading to the release of superoxide radicals contributing to inflammatory processes, and to the production of BH2, an inactive form of BH4, reducing its bioavailability. Alterations in BH4 levels have been documented in many pathological situations, including Alzheimer's disease, Parkinson's disease and depression, in which increased oxidative stress, inflammation and alterations in monoaminergic function are described. This review aims at providing an update of the knowledge about metabolism and the role of BH4 in brain function, from preclinical to clinical studies, addressing some therapeutic implications.

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.

Serum Biopterin and Neopterin Levels as Predictors of Empty Follicles

OBJECTIVE

This study measured serum and follicular fluid (FF) levels of biopterin, neopterin, vascular endothelial growth factor (VEGF), and macrophage colony-stimulating factor (M-CSF) in patients receiving mild ovarian stimulation for oocyte retrieval.

PATIENTS AND METHODS

Infertile patients who underwent ovarian stimulation were divided into the following: Group 1, no oocyte retrieval (n = 12), and Group 2, retrieval of more than four oocytes (n = 13). Median total gonadotropin dose in both groups was 150 IU. Biopterin and neopterin levels were measured using high-performance liquid chromatography. VEGF and M-CSF levels were measured by enzyme-linked immunosorbent assay.

RESULTS

Compared to Group 2, serum and FF levels of neopterin and VEGF and serum levels of M-CSF were significantly increased, and serum and FF levels of biopterin were significantly decreased in Group 1 (P < 0.05 each).

CONCLUSION

Biopterin and neopterin levels showed similar differences in FF and serum of patients with empty follicles. Decreased biopterin and increased neopterin in serum could predict poor oocyte retrieval.

Determination of six pterins in urine by LC-MS/MS

BACKGROUND

The present work describes an analytical method for urinary pterins by LC-MS/MS, with emphasis on the separation of 6- and 7-positional isomers of bio- and neopterins.

RESULTS

Urine sample preparation consisted of oxidation by MnO(2), filtration and direct dilution in the mobile phase. The method was validated in urine spiked at five concentration levels with true triplicates of each level. Separation of the pterins, including the positional isomers, was achieved by employing a LUNA amino column. Six pterins were quantified (pterin, isoxanthopterin, 6-biopterin, 7-biopterin, 6-neopterin, 7-neopterin) and a linear behavior was observed; LOD varied from 7 to 360 pg/ml and correlation coefficients above 0.98 were obtained for all pterins. In addition, pterin levels were evaluated in 41 urine samples of healthy subjects, in ten urine samples of patients with classical phenylketonuria (PKU) and in one with atypical PKU.

CONCLUSION

The proposed method allowed to identify, separate and quantify six pterins in urine, using a simple and rapid sample preparation. The atypical PKU was unequivocally differentiated from the classical form, demonstrating that this method could be very useful for characterization and follow-up of diseases.

Technical and biochemical factors affecting cerebrospinal fluid 5-MTHF, biopterin and neopterin concentrations

BACKGROUND

The diagnosis of pediatric neurologic disorders with a deficiency in the biosynthesis of either the neurotransmitters serotonin and dopamine, or the co-factor tetrahydrobiopterin or a cerebral 5-methyltetrahydrofolate (5-MTHF) deficiency, strongly relies on a robust analysis of neurotransmitter metabolites, pterins and 5-MTHF in the cerebrospinal fluid (CSF). The aim of this study was to investigate which technical and biochemical factors affect the CSF concentration of 5-MTHF, neopterin and biopterin in a pediatric population.

METHODS

We studied effects of the ventriculo-spinal gradient, total protein concentration, pretreatment with ascorbic acid (in case of 5-MTHF analysis), pretreatment of CSF with trichloro acetic acid (TCA)/dithiotreitol (DTE) and oxidation with either iodine or manganese oxide (in case of pterin analysis), storage time and age of the patients. We included CSF samples from children until the age of 18 years and analysed 5-MTHF, neopterin, biopterin, homovanillic acid (HVA), 5-hydroxy-indoleacetic acid (5-HIAA) and total protein.

RESULTS

The major findings of our study are: (1) CSF 5-MTHF, neopterin and biopterin concentrations are not affected by the ventriculo-spinal gradient; (2) pretreatment of CSF with ascorbic acid has negligible effects on 5-MTHF concentrations; (3) pretreatment of CSF with TCA/DTE and oxidation with iodine results in the most accurate determination of neopterin and biopterin; (4) when adjusted for age and total protein, CSF 5-MTHF correlated with 5-HIAA, but not with HVA; (5) the reference value of 5-MTHF in CSF in childhood is age-dependent (r=-0.634; p0.001); (6) we did not observe an age-dependency for neopterin and biopterin in CSF.

CONCLUSION

5-MTHF, neopterin and biopterin can be analysed in any volume of CSF that is collected. For correct analysis of pterins, CSF will have to be pretreated to stabilize the concentrations and stored properly, whereas such pretreatment is not necessary for 5-MTHF.

Correction of murine PKU following AAV-mediated intramuscular expression of a complete phenylalanine hydroxylating system

Phenylketonuria (PKU) caused by phenylalanine hydroxylase (PAH) deficiency leads to toxic accumulation of phenylalanine (Phe). PAH is predominantly expressed in liver and its activity requires a supply of tetrahydrobiopterin (BH(4)) cofactor, but we propose that expression of a complete Phe hydroxylating system (PAH plus BH(4) synthetic enzymes) in skeletal muscle will lead to therapeutic reduction of blood Phe levels in Pah(enu2) mice, a model of human PKU. In order to test this hypothesis, we first developed transgenic Pah(enu2) mice that lack liver PAH activity but coexpress, in their skeletal muscle, PAH and guanosine triphosphate cyclohydrolase I (GTPCH). The latter is responsible for the committing enzymatic step in BH(4) biosynthesis. Despite sufficient muscle enzyme expression, these mice remained hyperphenylalaninemic, thereby suggesting that expression of additional BH(4) synthetic enzymes would be necessary. A recombinant triple-cistronic adeno-associated virus-2 (AAV2) pseudotype 1 vector expressing PAH along with GTPCH and 6-pyruvoyltetrahydrobiopterin synthase (PTPS), the next step in BH(4) synthesis, was generated. Injection of this vector into the gastrocnemius muscles of Pah(enu2) mice led to stable and long-term reduction of blood Phe and reversal of PKU-associated coat hypopigmentation. We propose that muscle-directed gene therapy will be a viable alternative treatment approach to PKU and other inborn errors of metabolism.

Long-term outcome and neuroradiological findings of 31 patients with 6-pyruvoyltetrahydropterin synthase deficiency

Tetrahydrobiopterin (BH(4)) deficiency is an autosomal recessive disorder caused by enzyme defects in the biosynthesis or recycling of BH(4). Patients with BH(4) deficiency present with severe neurological signs and symptoms and require a different treatment from classical phenylketonuria. During the last 12 years, 31 cases of BH(4) deficiency were identified in our department. They were all classified as 6-pyruvoyl-tetrahydropterin synthase (PTPS) deficiency. They were diagnosed at the ages of 2.5-48 months and treated with BH(4), L-dopa and 5-hydroxytryptophan immediately after diagnosis. The average development quotients (DQ) at diagnosis and after treatment for more than 3 years were 53+/- 16, and 78+/- 15, respectively. A significant negative correlation was observed between the level of the DQ and the age at which treatment was commenced (r = -0.751, p = 0.002). Developmental profiles were uneven. Language, adaptability and at later age mathematics were particularly weak areas. Only two patients achieved a good performance in mathematics. Eleven patients who were treated with drugs from ages of 2.9-48 months had neuroradiological scanning. Computed tomography disclosed calcification in lentiform nuclei in one patient and magnetic resonance imaging disclosed delayed myelination and abnormal high intensity signal in cerebral white matter in all of them. Even though most of abnormalities were reversible, small patchy or spotted areas were still present on these regions after treatment for 10-46 months. In summary, our study supports the substantial efficacy of the current therapeutic approach in PTPS deficiency of normalizing amine neurotransmitters with three drugs as early as possible. For the first time, calcifications could be detected in patients with PTPS deficiency. Abnormalities in white matter on magnetic resonance imaging were not related to clinical manifestations and most were reversible.

Tetrahydrobiopterin-deficient hyperphenylalaninemia in the Chinese

BACKGROUND

Hyperphenylalaninemia (HPA) may be caused by either a deficiency in phenylalanine-4-hydroxylase or in tetrahydrobiopterin (BH4), the essential cofactor required for the hydroxylation of aromatic amino acids. The most common forms of BH4 deficiency are 6-pyruvoyl-tetrahydropterin synthase (PTPS) deficiency (MIM 261640) and dihydropteridine reductase (DHPR) deficiency (MIM 261630), which require a different treatment from classical HPA.

RESULTS

Approximately 86% of BH4-deficient HPA in the Chinese population was found to be caused by PTPS deficiency. Eleven missense (73C-->G, 120T-->G, 155A-->G, 166G-->A, 200C-->T, 209T-->A, 226C-->T, 259C-->T, 286G-->A, 317C-->T, 430G-->C), one splicing (IVS3+1G-->A) and two deletion mutations (116-119delTGTT, 169-171delGTG) were identified in 37 unrelated PTPS-deficient Chinese families. Among these, 155A-->G, 259C-->T and 286G-->A mutation accounted for about 80% of the mutant alleles. The 155A-->G and 286G-->A mutations were found to be the common mutation in southern and northern Chinese, respectively. Only two Chinese DHPR-deficient families were detected among about 300 Chinese hyperphenylalaninemia cases. A single base transition 508G-->A on the DHPR cDNA was identified in two consanguineous DHPR-deficient siblings. A reduced level of DHPR mRNA expression was found in the other DHPR-deficient patient, which suggested that the mutation might lie in the regulatory region of the DHPR gene.

CONCLUSIONS

The BH4-deficient HPA was estimated to make up around 30% of the Chinese population in Taiwan suffering from HPA, which is much higher than in Caucasian populations (1.5-2% of HPA).

Neopterin: a review

Neopterin was discovered in bee larvae, in worker bees and in royal jelly. The compound was termed "neopterin" to denote that it might start a new (Greek, neo) epoch in pteridine research. Increased concentrations of neopterin were reported in patients with viral infections, suggesting that increased neopterin may originate from the immune response of patients to the infections. In vitro studies revealed that human monocytes/macrophages produce neopterin when stimulated by interferon-gamma. Neopterin can easily be detected in serum and urine. The most important clinical applications for the determination of neopterin are prognostic indicator of malignant diseases, follow-up control of chronic infections, monitoring of immune-stimulatory therapy, differential diagnosis of acute viral and bacterial infections, prognostic indicator in HIV infections and early indications of complications in allograft recipients. In recent years new physiological functions of neopterin have been discovered such as inducing or enhancing cytotoxicity, inducing apoptosis and the role of a chain breaking antioxidant. This review will focus on the immunological and physiological properties of neopterin.

Dihydropteridine reductase deficiency in a large consanguineous Tunisian family: clinical, biochemical, and neuropathologic findings

We report the case of a large consanguineous Tunisian family of seven siblings suffering from dihydropteridine reductase deficiency with either typical clinical, biochemical, or autopsy findings. Two cousins also were reported to have the same symptoms. This metabolic disorder is characterized by severe microcephaly, psychomotor regression, and progressive basal ganglia calcifications. Dihydropteridine reductase assay on samples collected from the two brothers still alive did not show measurable activity. The sister and four brothers died between the ages of 3 years and 7 years. A neuropathology study done on the sister showed diffuse demyelination throughout the white matter and spongy vacuolation in the subthalamic nuclei, the superior cerebellar peduncles and the tegmentum tracts of the brain stem. The anterointernal part of the putamen was completely necrotic with nearly total nerve cell loss. Abnormal vascular proliferation and calcification of the walls of small, medium, and large arteries and veins, as well as diffusely scattered pericapillary and isolated calcospherites, were seen in this necrotic region. We think that folate deficiency may be involved in the pathogenesis of the basal ganglia calcification.

Tetrahydrobiopterin-dependent stabilization of neuronal nitric oxide synthase dimer reduces susceptibility to phosphorylation by protein kinase C in vitro

Binding of (6R)-5,6,7,8-tetrahydro-L-biopterin (H4B) stabilizes the homodimeric structure of neuronal nitric oxide synthase (nNOS). In the present study, low-temperature sodium dodecylsulfate-polyacrylamide gel electrophoresis revealed differential susceptibility of stabilized and non-stabilized dimers to in vitro phosphorylation by protein kinase C. Protein kinase C preferentially phosphorylated the non-stabilized dimer. Although a low extent of phosphorylation was detected in the stabilized dimer, most of it was estimated to be due to phosphorylation of the dimer before its stabilization. Phosphorylation did not affect the stabilizing effect of H4B. These results indicate that H4B-dependent dimer stabilization prevents nNOS from protein kinase C-dependent phosphorylation in vitro.

Hyperprolactinemia, a tool in treatment control of tetrahydrobiopterin deficiency: endocrine studies in an affected girl

Severe tetrahydrobiopterin (BH4) deficiency is a naturally occurring model of cerebral catecholamine and serotonin shortage. Examination of the stimulated release and physiologic secretion pattern of several hormones in affected individuals permits certain conclusions concerning the involvement of these neurotransmitters in hormone regulation. Treatment, moreover, permits the ranking of the quality of the therapeutic regimens in use according to the degree of hormonal alteration. The 24-h secretion pattern of prolactin, GH, cortisol, and melatonin and the stimulated release of prolactin, GH, TSH, and gonadotropins were studied in an affected girl. Severe hyperprolactinemia with disruption of the pulsatile and circadian secretion pattern was the prevailing feature. The GH physiologic secretion pattern was not affected, but its stimulation was impaired. Melatonin displayed a normal circadian secretion pattern; the rhythm, however, was advanced by several hours. Conventional treatment of BH4 deficiency, i.e. BH4, 5-hydroxytryptophan, and L-DOPA/carbidopa (the last named given in three doses per day), suppresses prolactin levels merely for a few hours. L-DOPA/carbidopa given at shorter intervals or, even better, as a slow release preparation, is more effective in suppressing prolactin levels. Our data indicate immense hyperprolactinemia but few other hormonal disturbances in severe BH4 deficiency. Prolactin secretion may serve as an extremely sensitive marker for the hypothalamic dopamine content under different therapeutic regimens. Treatment with an L-DOPA/carbidopa slow release preparation produces virtually normal prolactin levels.

Effects of activating and deactivating cytokines on the functionally linked tetrahydrobiopterin. No pathways in vascular smooth muscle cells

The functional relationship of nitric oxide (NO) production and synthesis of tetrahydrobiopterin (BH4), the requisite cofactor for NO synthase, was investigated in rat aortic smooth muscles cells (SMC). Inflammatory cytokines induced BH4 and NO synthesis in different ratios, IL-1 beta induced mainly NO synthesis with concomitant but limiting amounts of BH4 for maximal NO production. TNF alpha did not induce NO synthesis but induced BH4 synthesis. IFN gamma was ineffective on both the induction of NO and BH4 synthesis. TGF beta downregulated NO production but did not affect BH4 biosynthesis. IL-4 and IL-10 had no effect on both BH4 and NO synthesis. Activating cytokines strongly synergized in induction of NO production, whereas endogenous BH4 production became insufficient for maximal NO synthesis. Exogenous cofactor in the form of sepiapterin or authentic BH4, but not the natural isomer 7-BH4, enhanced NO production twofold. Inhibition of BH4 synthesis with dicumarol abolished NO production that could be restored in the presence of BH4.

High-performance liquid chromatographic methods for the quantification of tetrahydrobiopterin biosynthetic enzymes

Tetrahydrobiopterin is a cofactor in hydroxylation reactions, including phenylalanine 4-monooxygenase, tyrosine 3-monooxygenase, tryptophan 5-monooxygenase, alkyl glycol ether monooxygenase and nitric oxide synthase. Determination of its biosynthesis is carried out to diagnose inherited diseases leading to partial defects in tetrahydrobiopterin synthesis. In addition, tetrahydrobiopterin synthesis is induced by proinflammatory cytokines, and intracellular levels of tetrahydro-biopterin in many cases limit the activity of tetrahydrobiopterin-dependent reactions, such as nitric oxide synthase in intact cells. Biosynthesis of tetrahydrobiopterin from guanosine 5'-triphosphate (GTP) requires the action of three enzymes, GTP-cyclohydrolase I (E.C. 3.5.4.16), 6-pyruvoyl tetrahydropterin synthase (EC, 4.6.1.10) and sepiapterin reductase (E.C. 1.1.1.153). Methods for quantification of biopterin and related pteridines in biological matrices by HPLC and application of these for determining the activity of the three tetrahydrobiopterin biosynthetic enzymes are reviewed in this article.

Retrovirus-mediated gene transfer of 6-pyruvoyl-tetrahydropterin synthase corrects tetrahydrobiopterin deficiency in fibroblasts from hyperphenylalaninemic patients

Tetrahydrobiopterin (BH4) deficiency, a variant form of hyperphenylalaninemia with progressive neurological dysfunction, is primarily caused by autosomal recessive mutations in the gene encoding the 6-pyruvoyl-tetrahydropterin synthase (PTPS). PTPS is a biosynthetic enzyme for the BH4 co-factor, and its deficiency is associated with a malfunction of the phenylalanine catabolism in the liver and a lack of biogenic amine neurotransmitters dopamine and serotonin in the brain. We have previously isolated the wild-type PTPS cDNA and identified several mutations responsible for a decreased enzyme in patients. This study reports the in vitro correction of BH4 deficiency by using retrovirus mediated transfer of the PTPS cDNA into primary fibroblast cultures established from different patients. The Bing packaging cell line was used for amphotropic virus production. Following PTPS gene transfer, stimulation with cytokines restored biosynthesis of BH4 in originally defective cells to values comparable to those of heterozygous fibroblasts from clinically healthy subjects. These results not only provide a direct proof that the mutations in PTPS were causative for the mutant phenotype, but they are also the first step toward gene therapy as a potential alternative approach to treat BH4 deficiency.

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.

Antenatal diagnosis of tetrahydrobiopterin deficiency by quantification of pterins in amniotic fluid and enzyme activity in fetal and extrafetal tissue

Prenatal diagnosis of tetrahydrobiopterin (BH4) deficiency was undertaken by evaluating the pterin patterns in amniotic fluid and the specific enzyme activities in fetal or extrafetal tissues. This allowed the prenatal diagnosis in 19 pregnancies at risk. In 8 families with a child already affected by dihydropteridine reductase deficiency 4 fetuses were diagnosed as homozygotes and 4 as heterozygotes for the defect. In 11 families with a child affected by 6-pyruvoyl tetrahydropterin synthase deficiency 4 fetuses were homozygous, 4 heterozygous and 3 normal. This study also advanced our knowledge of tetrahydrobiopterin metabolism during fetal development. The key enzymes involved in the biosynthesis of BH4 are expressed early and allow the fetus to be autotrophous for its cofactor requirement. In a twin pregnancy, both fetuses were diagnosed to be heterozygotes for dihydropteridine reductase deficiency and primapterin (7-biopterin) in amniotic fluid was increased. This indicates that pterin-4 alpha-carbinolamine dehydratase activity seems to be differently expressed during fetal life. As a consequence, pterins detected in amniotic fluid are of fetal origin and 6- and 7-substituted pterins can be present in amniotic fluid in higher proportions when compared with other body fluids.

Analysis of 6R- and 6S-tetrahydrobiopterin and other pterins by reversed-phase ion-pair liquid-chromatography with fluorimetric detection by post-column sodium nitrite oxidation

A rapid and sensitive reversed-phase ion-pair liquid chromatographic system with fluorimetric detection by post-column sodium nitrite oxidation was established for measuring six pterin compounds (6R-5,6,7,8-tetrahydrobiopterin, 6S,5,6,7,8-tetrahydrobiopterin, 7,8-dihydrobiopterin, biopterin, pterin and D-neopterin). The coefficients of variation for these pterins were 0.705-3.714%, and the minimum detectable amount was ca. 10-20 pg at a signal-to-noise ratio of 3. A linear detector response was also verified. The concentrations of the pterin compounds in rat tissues were measured by the described method. Furthermore, by means of brain microdialysis, the output of pterin compounds from rat striatum was detected. Therefore, these results demonstrate that this system can be applied to analyses not only of various rat tissues but also of dialysates collected in vivo.

Differential diagnosis of hyperphenylalaninaemia by a combined phenylalanine-tetrahydrobiopterin loading test

We describe a new fully reliable method for the differential diagnosis of tetrahydrobiopterin-dependent hyperphenylalaninaemia (HPA). The method comprises the combined phenylalanine (Phe) plus tetrahydrobiopterin (BH4) oral loading test and enables the selective screening of BH4 deficiency when pterin analysis is not available or when a clear diagnosis has not been previously made. It should be performed together with the measurement of dihydropteridine reductase (DHPR) activity in blood. The new combined loading test was performed in nine patients with primary HPA, three with classical phenylketonuria (PKU), three with DHPR deficiency, and three with 6-pyruvoyl tetrahydropterin synthase (PTPS) deficiency. Three hours after oral Phe loading (100 mg/kg body weight), synthetic BH4 was administered orally at doses of either 7.5 or 20 mg/kg body weight. Amino acid (Phe and tyrosine) and pterin (neopterin and biopterin) metabolism and kinetics were analysed. By exploiting the decrease in serum Phe 4 and 8 h after administration, a clear response was obtained with the higher BH4 dose (20 mg/kg body weight), allowing detection of all cases of BH4 deficiency, as well as differentiation of BH4 synthesis from regeneration defects. Since DHPR deficient patients who were previously shown to be non-responsive to the simple BH4 loading test gave a positive response, the combined Phe plus BH4 loading test can be used as a more reliable tool for the differential diagnosis of HPA in these patients. Moreover, it takes advantage of being performed while patients are on a Phe-restricted diet.

Hyperphenylalaninemia and pterin metabolism in serum and erythrocytes

The relationship between blood phenylalanine concentrations and serum and erythrocyte biopterin and neopterin concentrations was investigated in 20 phenylketonuric patients with different dietary compliance. At serum phenylalanine concentrations ranging from 43 to 1004 mumol/l, a good correlation was found with serum biopterin (r = 0.76, P < 0.001) and with red blood cell biopterin (r = 0.62, P < 0.001). A similar correlation was found between serum neopterin and phenylalanine (r = 0.60, P < 0.001). The correlation between red blood cell neopterin and serum phenylalanine was less evident, however (r = 0.47, P < 0.005). After oral loading with phenylalanine (100 mg/kg body weight), serum and red blood cell biopterin concentrations increased in patients with classical phenylketonuria as well as in one patient with dihydropteridine reductase deficiency in response to the induced acute hyperphenylalaninemia. One patient suffering from 6-pyruvoyl tetrahydropterin synthase deficiency was loaded orally with tetrahydrobiopterin (20 mg/kg body weight). The kinetics of administered cofactor confirmed its rapid absorption, with early increase of serum concentrations followed by its transport into the red blood cells. The half-life of biopterin was approximately 7 h in serum and 15 h in red blood cells. Because both values are less than the half-life of phenylalanine (20-30 h) in serum, biopterin measurement offers no advantage in monitoring dietary control in hyperphenylalaninemic patients.

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

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

Tetrahydrobiopterin and Parkinson's disease

Two patients with Parkinson's disease were treated with 1 g tetrahydrobiopterin (BH4) for 5 days. Clinical improvement was not observed. In the cerebrospinal fluid (CSF) a 4-8 fold increase in the concentration of homovanillic acid (HVA), and a 3-fold increase in the concentration of 5-hydroxyindole acetic acid (5-HIAA) was measured. However, the concentration of HVA reached, was only approximately half as high, as that of patients treated with madopar (DOPA + benserazid). In urine, the excretion of HVA increased 13-37 fold, when the patients were treated with madopar, whereas no increase in the HVA excretion was measured after the BH4 administration. Additionally, 2 patients with Parkinson's disease were treated with 1 g BH4 in combination with 15 g tyrosine for 3 days, and 1 parkinsonian patient was treated with 15 g tyrosine daily for 7 weeks. No increase in the CSF concentrations of HVA or 5-HIAA was observed. The results suggest, the BH4 in the dosage used, is not effective in the treatment of Parkinson's disease.

Rapid and sensitive method for high-performance liquid chromatographic analysis of pterins in biological fluids

A rapid and sensitive high-performance liquid chromatographic (HPLC) method for the analysis of the most important urinary pterins is described. The method involves a preliminary sample oxidation to stabilize and convert pterins into their fluorescent forms and a purification by anion-exchange chromatography, followed by a short reversed-phase HPLC separation with fluorometric detection and quantitation of the different pterins. A complete HPLC analysis is accomplished in as little as 15 min. The sensitivity of the method allows the detection of as little as 20 pg of each pterin with a mean recovery greater than 99% for all pterins analysed. Reference values were obtained from 50 normal babies aged between 1 and 120 days. A significant correlation was found between urinary biopterin levels and the age of the babies (r = 0.445), while neopterin did not show any significant correlation with age. The "biopterin neopterin creatinine ratio" (BNCR index) was also significantly correlated with the age of the babies (r = 0.428). This rapid and sensitive method for pterin determination in biological fluids may be useful in the differential diagnosis of the various hyperphenylalaninemic conditions identified by neonatal mass screening programmes.

Analysis and clinical significance of pterins

This review briefly describes the biochemistry of pterins, their involvement in pathological processes and the use of pterin measurement in diagnosis and monitoring of disease. Chromatographic and other methods of pterin analysis are detailed with particular emphasis being placed on the need for correct sample collection and handling.

Two mutations of dihydropteridine reductase deficiency

Two patients with dihydropteridine reductase (DHPR) deficiency, in one case due to the absence of any enzyme protein (DHPR- cross reactive material (CRM)-) and in the other case due to the production of a mutant type devoid of catalytic activity (DHPR- CRM+) were examined. This latter form of malignant phenylketonuria, whose relative frequency seems to be higher in the Italian population, possibly has a worse prognosis. The earlier onset and the greater severity of clinical symptoms are associated with a more pronounced hydroxylation defect, as shown by higher degree of neonatal hyperphenylalaninaemia, unresponsiveness to an oral tetrahydrobiopterin load, lower concentrations of neurotransmitter metabolites, and reduced tyrosine production after an oral phenylalanine load.

Tetrahydrobiopterin deficiency: assay for 6-pyruvoyl-tetrahydropterin synthase activity in erythrocytes, and detection of patients and heterozygous carriers

6-Pyruvoyl-tetrahydropterin synthase (PTS), a key enzyme in the synthesis of tetrahydrobiopterin in man, is defective in the most frequent variant of tetrahydrobiopterin-deficient hyperphenylalaninaemia (atypical phenylketonuria). An assay for PTS activity in erythrocytes was developed. It is based on the PTS-catalysed formation of tetrahydrobiopterin from dihydroneopterin triphosphate in the presence of magnesium, sepiapterin reductase, NADPH, dihydropteridine reductase, and NADH, and fluorimetric measurement of the product as biopterin by high performance liquid chromatography (HPLC) after oxidation with iodine. The PTS activity was higher in younger erythrocytes, including reticulocytes, than in older ones. Fetal erythrocytes showed approx. four times higher activities than those of adults. Using a more purified human liver sepiapterin reductase fraction which gave a lower yield than a crude preparation, adult controls (n = 8) showed a mean erythrocyte PTS activity of 17.6 (range 11.0-29.5) microU/g Hb. Nine of 11 patients with typical PTS deficiency showed activities between 0% and 8% of the mean of controls, and two of 11 showed 14% and 20%, respectively. The obligate heterozygotes (n = 16) had activities of 19% (range 8%-31%) of the mean of controls, i.e., significantly less than the expected 50%. Four patients with the "peripheral" type of the disease showed 7%-10% of the mean of controls. Thus, the assay did not distinguish between patients and heterozygotes in every family. The assay is well suited to the identification of heterozygotes of PTS deficiency in family studies.

Hyperphenylalaninemia due to deficiency of 6-pyruvoyl tetrahydropterin synthase. Unusual gene dosage effect in heterozygotes

We have identified deficient biopterin synthesis in four probands and one sib with persistent postnatal hyperphenylalaninemia. The metabolic findings were associated with a benign clinical presentation and normal biopterin level in cerebrospinal fluid in the newborn period, indicating the peripheral (hepatic) form of this autosomal recessive phenotype. Impaired development was apparent at 3 months in one proband not treated early. Treatment with oral tetrahydropterin restored adequate phenylalanine hydroxylase activity; it also maintained or improved CNS function. The deficient enzyme in these subjects is 6-pyruvoyl tetrahydropterin synthase (PTS). Erythrocyte activity of PTS in homozygotes (or compound heterozygotes) is less than 10% of normal. Heterozygotes have 20%-50% of normal PTS activity (enzyme phenotype), a finding compatible with a range of gene dosage effects, some abnormal. The metabolic phenotype in heterozygotes (urine biopterin excretion) did not correlate with erythrocyte PTS activity. The complex relationship between erythrocyte PTS activity, and biopterin synthesis in these families indicates genetic heterogeneity at the PTS locus.

Biosynthesis and metabolism of pterins in peripheral blood mononuclear cells and leukemia lines of man and mouse

The cellular origin and the control of neopterin release associated with immune stimulation was studied in cell cultures. Using purified human mononuclear cells, the intracellular change in concentrations of GTP and pterins was measured under various kinds of stimulation. Three enzymes involved in tetrahydrobiopterin biosynthesis, i.e. GTP cyclohydrolase I, 6-pyruvoyl tetrahydropterin synthase and sepiapterin reductase, were also determined. Human macrophages stimulated with culture supernatant from activated T-lymphocytes were the main producers of neopterin. In these cells, GTP cyclohydrolase I activity was elevated due to high GTP levels and therefore neopterin accumulated. Human macrophages lack 6-pyruvoyl tetrahydropterin synthase activity. Exogenous tetrahydrobiopterin added to the culture medium of stimulated T cells and macrophages suppressed the elevation of GTP cyclohydrolase I activity and neopterin concentration, but not the elevation of intracellular GTP. Stimulation of macrophages with recombinant human interferon-gamma and neutralization of the effect of T cell supernatants by addition of a monoclonal antibody specific for human interferon-gamma showed that immune interferon induced the alterations in GTP cyclohydrolase I activity and neopterin concentration. In the human macrophage line U-937 and in the leukemia line HL-60, no GTP cyclohydrolase I activity or intracellular pterins were detected, but high levels of GTP. In mouse mononuclear cells, no neopterin was detected, but biopterin and pterin. After stimulation, biopterin was elevated in the same way as neopterin in human mononuclear cells. This is explained by the different regulation of the rate-limiting steps of tetrahydrobiopterin biosynthesis in man and in mouse. These results suggest that neopterin is an unspecific marker for the activation of the cellular immune system.

"Peripheral" tetrahydrobiopterin deficiency with hyperphenylalaninaemia due to incomplete 6-pyruvoyl tetrahydropterin synthase deficiency or heterozygosity

Four patients in three families with "peripheral" tetrahydrobiopterin deficiency were investigated. They were characterized biochemically by a tetrahydrobiopterin-responsive hyperphenylalaninaemia, a high neopterin/biopterin ratio in urine and plasma, and normal or elevated concentrations of biopterin, homovanillic acid, and 5-hydroxyindole acetic acid in cerebrospinal fluid. From measurements of the activity of erythrocyte 6-pyruvoyl tetrahydropterin synthase (PTS, formerly called phosphate-eliminating enzyme) and phenylalanine loading tests in the patients and their parents, one patient was demonstrated to be heterozygous for PTS deficiency. The others were obviously genetic compounds (allelism) with incomplete PTS deficiency. Three of the children developed normally, two of them under treatment with tetrahydrobiopterin. In the latter two patients, significantly lower concentrations of biopterin, homovanillic acid, and 5-hydroxyindole acetic acid in cerebrospinal fluid were noted at age 7 months (when treatment was interrupted) than those observed at 3 and 5 weeks, respectively. The infant who is heterozygous for PTS deficiency was born small for gestational age and showed a moderately delayed psychomotor development. It is concluded that "peripheral" tetrahydrobiopterin deficiency is caused by a partial PTS deficiency with sufficient activity to cover the tetrahydrobiopterin requirement of tyrosine 3-hydroxylase and trytophan 5-hydroxylase in brain but not enough for phenylalanine 4-hydroxylase in liver. For therapy, tetrahydrobiopterin, 2-5 mg/kg in a single oral dose per day, is recommended to keep plasma phenylalanine normal. A careful observation of the mental development is indicated.

Biosynthesis of tetrahydrobiopterin. Purification and characterization of 6-pyruvoyl-tetrahydropterin synthase from human liver

6-Pyruvoyl-tetrahydropterin synthase, which catalyzes the first step in the conversion of 7,8-dihydroneopterin triphosphate to tetrahydrobiopterin, was purified approximately 140,000-fold to apparent homogeneity from human liver. The molecular mass of the enzyme is estimated to be 83 kDa. 7,8-Dihydroneopterin triphosphate was a substrate of the enzyme in the presence of Mg2+, and the pH optimum of the reaction was 7.5 in Tris HCl buffer. The Km value for 7,8-dihydroneopterin triphosphate was 10 microM. The product of this enzymatic reaction was the presumed intermediate 6-pyruvoyl-tetrahydropterin. This latter compound was converted to tetrahydrobiopterin in the presence of NADPH and partially purified sepiapterin reductase from human liver. The conditions and the effect of N-acetylserotonin on this reaction, and on the formation of the intermediates 6-(1'-hydroxy-2'-oxopropyl)-tetrahydropterin and 6-(1' oxo-2'-hydroxypropyl)-tetrahydropterin have been studied.

Interferon-gamma enhances biosynthesis of pterins in peripheral blood mononuclear cells by induction of GTP-cyclohydrolase I activity

In a recent publication, evidence was presented that cellular immune responses are associated with increased in vivo and in vitro excretion of neopterin. Our study aimed at investigating the biosynthesis of unconjugated pterins in highly purified human macrophages and T lymphocytes before and during stimulation with supernatants of activated T cells or with recombinant human interferon-gamma (IFN-gamma) by monitoring the following parameters: substrate concentration (GTP, guanosine triphosphate), activity of the enzyme initiating the biosynthesis of pterins (GTP-cyclohydrolase I) and product concentrations of total neopterin, biopterin, and pterin. In contrast to T cells and other tissues, macrophages were unable to produce tetrahydrobiopterin. This was indicated by our failure to detect biopterin and pterin. Instead, products of the first biosynthetic step accumulated, which were measured as total neopterin. We concluded that in macrophages the other enzymes required for biosynthesis of tetrahydrobiopterin are limiting. GTP concentration correlated with GTP cyclohydrolase I activity. An increase in both was induced by IFN-gamma and suppressed by neutralization of T-cell supernatants with monoclonal antibodies having specificity for IFN-gamma. Addition of tetrahydrobiopterin to the culture medium only led to a suppressed increase in GTP cyclohydrolase I activity and neopterin, but not in GTP concentration. Thus, it appears that IFN-gamma selectively stimulates the early steps of pterin biosynthesis in macrophages, thereby leading to accumulation and excretion of dihydroneopterin and neopterin. Although the physiological role of this phenomenon remains obscure, the fact that it seems to reflect endogenous release of IFN-gamma deserves particular attention.

Estimation of tetrahydrobiopterin and other pterins in cerebrospinal fluid using reversed-phase high-performance liquid chromatography with electrochemical and fluorescence detection

We describe an isocratic, reversed-phase high-performance liquid chromatographic method for the simultaneous measurement of fully oxidised, dihydro- and tetrahydropterins in cerebrospinal fluid. Tetrahydrobiopterin is detected electrochemically using an ESA Coulochem detector in the redox mode. Dihydropterins are detected by fluorescence following post-column electrochemical oxidation, and fully oxidised pterins by their natural fluorescence. Apart from addition of antioxidants, no sample preparation is required. Comparison is made with methods requiring chemical oxidation for detection of tetrahydrobiopterin. Some results on children with neurological disease are presented.

Automated high-performance liquid chromatographic method with column switching for the determination of neurotransmitters and related compounds, ascorbic acid and uric acid in tissue extracts

An automated high-performance liquid chromatographic method with electrochemical and fluorimetric detection and on-line data evaluation is described for the simultaneous measurement of indoleaminergic and catecholaminergic neurotransmitters, some of their metabolites and precursors and ascorbic and uric acids. Deproteinized tissue extracts from the central nervous system or peripheral organs are injected without prior purification (recovery greater than 90%). A switching system enables the compounds to be passed as necessary through one, two or three reversed-phase columns, which are then eluted simultaneously (analysis time 25 min). Fifty samples per day can be analysed with a precision of 95% for neurotransmitters and about 90% for ascorbic and uric acids.