Atypical phenylketonuria with defective biopterin metabolism. Monotherapy with tetrahydrobiopterin or sepiapterin, screening and study of biosynthesis in man

The impact of tetrahydrobiopterin administration on endothelial function before and after smoking cessation in chronic smokers

Cardiovascular disease mortality is reduced following smoking cessation but the reversibility of specific atherogenic risk factors such as endothelial dysfunction is less established. We assessed brachial artery flow-mediated dilation (FMD) in 57 chronic smokers and 15 healthy controls, alone and after oral tetrahydrobiopterin (BH4) administration, to assess the extent to which reduced bioactivity of BH4, a cofactor for the endothelial nitric oxide synthase enzyme (eNOS), contributes to smoking-associated reductions in FMD. Thirty-four smokers then ceased cigarette and nicotine use for 1 week, after which FMD (±BH4 administration) was repeated. Brachial artery FMD was calculated as the peak dilatory response observed relative to baseline (%FMD). Endothelium-independent dilation was assessed by measuring the dilatory response to sublingual nitroglycerin (%NTG). Chronic smokers exhibited reduced %FMD relative to controls: (5.6±3.0% vs. 8.1±3.7%; P<0.01) and %NTG was not different between groups (P=0.22). BH4 administration improved FMD in both groups (P=0.03) independent of smoking status (P=0.78) such that FMD was still lower in smokers relative to controls (6.6±3.3% vs. 9.8±3.2%; P<0.01). With smoking cessation, FMD increased significantly (from 5.0±2.9 to 7.8±3.2%;P<0.01); %NTG was not different (P=0.57) and BH4 administration did not further improve FMD (P=0.33). These findings suggest that the blunted FMD observed in chronic smokers, likely due at least in part to reduced BH4 bioactivity and eNOS uncoupling, can be restored with smoking cessation. Post-cessation BH4 administration does not further improve endothelial function in chronic smokers, unlike the effect observed in nonsmokers, indicating a longer-term impact of chronic smoking on vascular function that is not acutely reversible.

The role of tetrahydrobiopterin and catecholamines in the developmental regulation of tyrosine hydroxylase level in the brain

Tyrosine hydroxylase (TH) is a rate-limiting enzyme for dopamine synthesis and requires tetrahydrobiopterin (BH4) as an essential cofactor. BH4 deficiency leads to the loss of TH protein in the brain, although the underlying mechanism is poorly understood. To give insight into the role of BH4 in the developmental regulation of TH protein level, in this study, we investigated the effects of acute and subchronic administrations of BH4 or dopa on the TH protein content in BH4-deficient mice lacking sepiapterin reductase. We found that BH4 administration persistently elevated the BH4 and dopamine levels in the brain and fully restored the loss of TH protein caused by the BH4 deficiency in infants. On the other hand, dopa administration less persistently increased the dopamine content and only partially but significantly restored the TH protein level in infant BH4-deficient mice. We also found that the effects of BH4 or dopa administration on the TH protein content were attenuated in young adulthood. Our data demonstrate that BH4 and catecholamines are required for the post-natal augmentation of TH protein in the brain, and suggest that BH4 availability in early post-natal period is critical for the developmental regulation of TH protein level.

Tetrahydrobiopterin, its mode of action on phenylalanine hydroxylase, and importance of genotypes for pharmacological therapy of phenylketonuria

In about 20%-30% of phenylketonuria (PKU) patients (all phenotypes of PAH deficiency), Phe levels may be controlled through phenylalanine hydroxylase cofactor tetrahydrobiopterin therapy. These patients can be diagnosed by an oral tetrahydrobiopterin challenge and are characterized by mutations coding for proteins with substantial residual PAH activity. They can be treated with a commercially available synthetic form of tetrahydrobiopterin, either as a monotherapy or as adjunct to the diet. This review article summarizes molecular and metabolic bases of PKU and the importance of the tetrahydrobiopterin loading test used for PKU patients. On the basis of in vitro residual PAH activity, more than 1,200 genotypes from patients challenged with tetrahydrobiopterin were categorized as predictive for tetrahydrobiopterin responsiveness or non-responsiveness and correlated with the loading test, phenotype, and residual in vitro PAH activity. The coexpression of two distinct PAH mutant alleles revealed possible dominance effects (positive or negative) by one of the mutations on residual activity as result of interallelic complementation. The treatment of the transfected cells with tetrahydrobiopterin showed an increase in residual PAH activity with several mutations coexpressed.

Rapid clearance of supplemented tetrahydrobiopterin is driven by high-capacity transporters in the kidney

Tetrahydrobiopterin (BH(4)) is an essential cofactor of aromatic amino acid hydroxylases and NO synthase. Supplementation of BH(4) potentially targets cardiovascular dysfunction as well as inherited BH(4) deficiencies and BH(4)-responsive phenylketonuria. However, the high cost/effect ratio of the recommended daily dose of BH(4) supplementation acts against further popularization of this therapy. The aim of this study was to attenuate urinary excretion with the intention of improving efficacy of BH(4) supplementation. The rapid excretion of BH(4) in the urine was confirmed to be the major route of supplemented BH(4) loss. In addition to glomerular filtration into the urine, a dominant rapid exclusion by renal secretion was observed in rats (T((1/2))=16 min) when the plasma BH(4) was higher than about 1 nmol/mL (more than 10 times higher than normal), due to BH(4) supplementation. The rapidity of the process was slowed by prior administration of cyclosporin A, a representative anti-excretory drug, and the excretion decelerated to a moderate rate (T((1/2))=53 min). By the combined administration of BH(4) plus cyclosporin A, the blood BH(4) levels were dramatically elevated. It was hypothesized that the drug interfered with kidney excretion of BH(4) rather than by attenuating organ tissue distribution by inhibiting biopterin uptake from the plasma. Consistent with this hypothesis, biopterin levels after BH(4) administration were elevated in major organs in the presence of anti-excretory drugs without notable change in their BH(4) fraction which was consistently 95% or higher regardless of combined administration with the drugs. Targeting these putative transporters would be a promising approach for improving the efficiency of BH(4) supplementation therapy.

Up to date knowledge on different treatment strategies for phenylketonuria

Dietary management for phenylketonuria was established over half a century ago, and has rendered an immense success in the prevention of the severe mental retardation associated with the accumulation of phenylalanine. However, the strict low-phenylalanine diet has several shortcomings, not the least of which is the burden it imposes on the patients and their families consequently frequent dietary non-compliance. Imperfect neurological outcome of patients in comparison to non-PKU individuals and nutritional deficiencies associated to the PKU diet are other important reasons to seek alternative therapies. In the last decade there has been an impressive effort in the investigation of other ways to treat PKU that might improve the outcome and quality of life of these patients. These studies have lead to the commercialization of sapropterin dihydrochloride, but there are still many questions regarding which patients to challenge with sapropterin what is the best challenge protocol and what could be the implications of this treatment in the long-term. Current human trials of PEGylated phenylalanine ammonia lyase are underway, which might render an alternative to diet for those patients non-responsive to sapropterin dihydrochloride. Preclinical investigation of gene and cell therapies for PKU is ongoing. In this manuscript, we will review the current knowledge on novel pharmacologic approaches to the treatment of phenylketonuria.

Detection of tetrahydrobiopterin by LC-MS/MS in plasma from multiple species

BACKGROUND

Tetrahydrobiopterin (BH4) is a naturally occurring pteridine and cofactor for a variety of enzymes, including phenylalanine-4-hydroxylase, nitric oxide synthetase and glyceryl ether monooxygenase. BH4 is readily oxidized to dihydrobiopterin and biopterin (B), however only BH4 can provide proper cofactor functions. BH4 is the active ingredient in Kuvan™ for the treatment of phenylketonuria. In order to measure BH4 in plasma from nonclinical and clinical samples with good accuracy, precision, sensitivity and robustness, an LC-MS/MS method was validated. To overcome the oxidation of BH4 in postcollection plasma, the approach was to measure the concentration of BH4 indirectly by measuring B concentration and applying an oxidation conversion ratio. Different endogenous levels of BH4 are determined in human, monkey, dog, rabbit, rat and mouse plasma. Furthermore, the conversion ratio of BH4 to B for each species is different and determined empirically. Plasma is transferred into cryogenic vials containing 0.1% dithioerythritol to prevent oxidation of BH4. The samples are then extracted and oxidized under basic conditions. B is measured with LC-MS/MS using negative ion mode.

RESULTS

The method is accurate, and precise to within 15%. The lower limit of quantitation in matrix is 5, 50 or 100 ng/ml, depending on the species endogenous levels of BH4. The pharmacokinetics of a single oral dose at three concentrations of BH4 administered to C57BL/6 mice is presented. In this mouse study, the T(1/2) of BH4 in plasma was approximately 1.2 h.

CONCLUSION

The validated LC-MS/MS method to determine plasma BH4 concentration described herein has been used to support many nonclinical and clinical toxicokinetic and pharmacokinetic studies. BH4 is sensitive to oxidation and has a complicated biology. The method successfully supported the approval of Kuvan for the treatment of phenylketonuria.

Tetrahydrobiopterin in intestinal lumen: its absorption and secretion in the small intestine and the elimination in the large intestine

In treating hereditary deficiency of tetrahydrobiopterin (BH(4)), supplementation with BH(4) might be the ultimate choice of therapy. Oral administration of BH(4) has been believed to be inefficient owing to poor absorption of BH(4) in the intestine. In this study, we found a considerable amount of BH(4) as well as its oxidized pterins in the ingredients of intestinal lumen of mice when they were served food that did not contain significant amounts of biopterin. Ligation of the biliary duct led to significant decrease in luminal biopterin. Supplementation of BH(4) either by intraperitoneal administration of sepiapterin or of 6RBH(4) ((6R)-L-erythro-5,6,7,8-tetrahydrobiopterin) increased the BH(4) content in the intestinal lumen with a slight delay after the rise of blood BH(4). In these mice, biopterin appeared in the large intestine, caecum and colon, 2 h after the administration. The appearance of BH(4) in the large intestine was accompanied by a large amount of pterin (2-amino-4-hydroxypteridine). The amounts of biopterin + pterin that appeared in the large intestine after intraperitoneal administration of BH(4) were not greater than those found after oral administration at the same dose. When the mice were treated with a large dose of antibiotics prior to the BH(4) administration, the amount of biopterin increased in the caecum but the amount of pterin decreased greatly. These results suggested that a large proportion of BH(4) administered moved to the large intestine, where most biopterin was decomposed presumably by enteric bacteria. Nonetheless, most of the orally administered biopterin was taken up by the small intestine and the amount of biopterin reaching the large intestine was almost the same as that which appeared after direct injection of 6RBH(4) into the peritoneal cavity.

Cellular accumulation of tetrahydrobiopterin following its administration is mediated by two different processes; direct uptake and indirect uptake mediated by a methotrexate-sensitive process

In our previous study on tetrahydrobiopterin (BH4) accumulation in organs of mice administered with 6RBH4, it was demonstrated that the intestinal mucosa was able to take up BH4 directly but that the liver could accomplish this only indirectly via a pathway involving the dihydrofolate reductase reaction. This observation was largely based on the fact that BH4 deposition in the liver was completely inhibited by prior treatment with methotrexate whereas deposition in the intestinal mucosa was only partially inhibited. To investigate the distinctive features of BH4 uptake in these organs, Caco-2 of intestinal epithelial origin and isolated hepatocytes were analyzed for cellular BH4 uptake in vitro. Both cell types exhibited a similar profile of BH4 accumulation but their response to methotrexate differed; the accumulation of BH4 in the hepatocytes was almost completely inhibited by methotrexate, whereas no inhibition was observed in Caco-2 cells, suggesting that the process of BH4 accumulation in Caco-2 cells, unlike hepatocytes, did not involve enzymic reduction by dihydrofolate reductase. Furthermore, 6SBH4, a synthetic diastereomer of BH4, was loaded into Caco-2 cells and the accumulated BH4 was identified as 6SBH4. These results provided strong evidence that BH4 had directly accumulated in Caco-2 cells. The distinctive features of BH4 deposition in the intestinal mucosa and liver reflected the means by which Caco-2 cells or hepatocytes, both representative cells of these tissues, took up extracellular BH4, i.e., in a direct or indirect manner, respectively.

Accumulated BH4 in mouse liver caused by administration of either 6R- or 6SBH4 consisted solely of the 6R-diastereomer: evidence of oxidation to BH2 and enzymic reduction

Mice were given (i.p.) L-erythro-(6S)-tetrahydrobiopterin (6SBH4) or 6RBH4 and the increase in liver BH4 in both groups was almost the same. The C6-chirality of liver BH4 was determined by HPLC. After administration of 6SBH4, the liver BH4 consisted mainly of 6RBH4 (>95%). These findings show that the exogenous BH4 was oxidized to 7,8BH2 which was then taken up and enzymically reduced back to BH4 in the liver.

Tetrahydrobiopterin augments endothelium-dependent dilatation in sedentary but not in habitually exercising older adults

Endothelium-dependent dilatation (EDD) is impaired with ageing in sedentary, but not in regularly exercising adults. We tested the hypotheses that differences in tetrahydrobiopterin (BH(4)) bioactivity are key mechanisms explaining the impairment in EDD with sedentary ageing, and the maintenance of EDD with ageing in regularly exercising adults. Brachial artery flow-mediated dilatation (FMD), normalized for local shear stress, was measured after acute oral placebo or BH(4) in young sedentary (YS) (n = 10; 22 +/- 1 years, mean +/- s.e.m.), older sedentary (OS) (n = 9; 62 +/- 2), and older habitually aerobically trained (OT) (n = 12; 66 +/- 1) healthy men. At baseline, FMD was approximately 50% lower in OS versus YS (1.12 +/- 0.09 versus 0.57 +/- 0.09 (Deltamm (dyn cm(-2))) x 10(-2), P < 0.001; 1 dyn = 10(-5) N), but was preserved in OT (0.93 +/- 0.08 (Deltamm (dyn cm(-2))) x 10(-2)). BH4 administration improved FMD by approximately 45% in OS (1.00 +/- 0.10 (Deltamm (dyn cm(-2))) x 10(-2), P < 0.01 versus baseline), but did not affect FMD in YS or OT. Endothelium-independent dilatation neither differed between groups at baseline nor changed with BH4 administration. These results suggest that BH4 bioactivity may be a key mechanism involved in the impairment of conduit artery EDD with sedentary ageing, and the EDD-preserving effect of habitual exercise.

Tetrahydrobiopterin uptake in supplemental administration: elevation of tissue tetrahydrobiopterin in mice following uptake of the exogenously oxidized product 7,8-dihydrobiopterin and subsequent reduction by an anti-folate-sensitive process

In order to increase the tissue level of tetrahydrobiopterin (BH4), supplementation with 6R-tetrahydrobiopterin (6RBH4) has been widely employed. In this work, the effectiveness of 6RBH4 was compared with 7,8-dihydrobiopterin (7,8BH2) and sepiapterin by administration to mice. Administration of 6RBH4 was the least effective in elevating tissue BH4 levels in mice while sepiapterin was the best. In all three cases, a dihydrobiopterin surge appeared in the blood. The appearance of the dihydrobiopterin surge after BH4 treatment suggested that systemic oxidation of the administered BH4 had occurred before accumulation of BH4 in the tissues. This idea was supported by the following evidences: 1) An increase in tissue BH4 was effectively inhibited by methotrexate, an inhibitor of dihydrofolate reductase which reduces 7,8BH2 to BH4. 2) When the unnatural diastereomer 6SBH4 was administered to mice, a large proportion of the recovered BH4 was in the form of the 6R-diastereomer, suggesting that this BH4 was the product of a dihydrofolate reductase process by which 7,8BH2 converts to 6RBH4. These results indicated that the exogenous BH4 was oxidized and the resultant 7,8BH2 circulated through the tissues, and then it was incorporated by various other tissues and organs through a pathway shared by the exogenous sepiapterin and 7,8BH2 in their uptake. It was demonstrated that maintaining endogenous tetrahydrobiopterin in tissues under ordinary conditions was also largely dependent on an methotrexate-sensitive process, suggesting that cellular tetrahydrobiopterin was maintained both by de novo synthesis and by salvage of extracellular dihydrobiopterin.

Dihydropteridine reductase deficiency in man: from biology to treatment

In 1975, dihydropteridine reductase (DHPR) deficiency was first recognized as a cause of tetrahydrobiopterin (BH(4)) deficiency, leading to hyperphenylalaninemia (HPA) and impaired biogenic amine deficiency. So far, more than 150 patients scattered worldwide have been reported and major progresses have been made in the understanding of physiopathology, screening, diagnosis, treatment, and molecular genetics of this inherited disease. Present knowledge on different aspects of DHPR deficiency, largely derived from authors' personal experience, is traced in this article.

High frequency of tetrahydrobiopterin-responsiveness among hyperphenylalaninemias: a study of 1,919 patients observed from 1988 to 2002

Tetrahydrobiopterin (BH(4))-responsive hyperphenylalaninemia (HPA) is a recently described variant of phenylalanine hydroxylase deficiency. In contrast to patients with classical phenylketonuria, these patients respond to BH(4) loading tests (20mg/kg) with decrease of plasma phenylalanine levels 4 and 8 h after administration and they can be treated with BH(4) monotherapy. We retrospectively evaluated 1,919 loading tests from 33 different countries performed in our laboratory between 1988 and 2002 of which 278 loading tests were performed with 6R-BH(4), which is about 33% more active than the formerly used 6R,S-BH(4). The loading tests were performed between the ages of one week and 4.6 years, using 2.6-30.0 mg 6R,S- or 6R-BH(4)/kg. Plasma phenylalanine levels before the test ranged from 121 to 4,705 micromol/L. We calculated the phenylalanine "hydroxylation rate" 4 and 8 h after BH(4) administration and plotted the slope of the hydroxylation rate against the phenylalanine levels at time 0. The slope was greater than 3.75 in 65, 74, 33, 17, 0, and 10% of patients with basal phenylalanine levels of 120-400, 400-800, 800-1,200, 1,200-1,600, 1,600-2,200, and >2,200 micromol/L, respectively, when loaded with 20 mg 6R-BH(4)/kg (p>0.0001). This is 5-20 times higher compared with tests using 6R,S-BH(4) or lower doses of BH(4). More than 70% of patients with mild HPA (<800 micromol/L) are found to be BH(4) responders. Therapy with BH(4) (approximately 10mg/kg/day) was initiated in several patients instead of a low-phenylalanine diet, resulting in much better treatment compliance. Our data further demonstrate that BH(4) loading tests can only distinguish between BH(4) responders and non-responders. To differentiate between BH(4) and phenylalanine hydroxylase deficiencies additional tests are essential.

Tetrahydrobiopterin deficiency increases neuronal vulnerability to hypoxia

Tetrahydrobiopterin (BH4) is an essential co-factor for nitric oxide synthases (NOS). The aim of the present work was to study whether BH4 deficiency affects the vulnerability of neurones in primary culture to hypoxia. Intracellular BH4 levels were depleted by pre-incubating neurones with 5 mm 2,4-diamino-6-hydroxypyrimidine (DAHP) for 18 h, after which cells were exposed for 1 h to normoxic or hypoxic conditions. Our results showed that whereas neurones were resistant to hypoxia-induced cellular damage, BH4 deficiency in neurones led to oxidative stress, mitochondrial depolarization, ATP depletion and necrosis after 1 h of hypoxia. Indeed, hypoxia specifically inhibited mitochondrial complex IV activity in BH4-deficient neurones. All these effects were counteracted when neuronal BH4 levels were restored by incubating cells with exogenous BH4 during the hypoxic period. Moreover, hypoxia-induced damage in BH4-deficient neurones was prevented when Nomega-nitro-l-arginine monomethyl ester (NAME), haemoglobin or superoxide dismutase plus catalase were present during the hypoxic period, suggesting that peroxynitrite might be involved in the process. In fact, BH4 deficiency elicited neuronal NO dysfunction, resulting in an increase in peroxynitrite generation by cells, as shown by the enhancement in tyrosine nitration; this was prevented by supplements of BH4, NAME, haemoglobin or superoxide dismutase plus catalase during hypoxia. Our results suggest that BH4 deficiency converts neuronal NOS into an efficient peroxynitrite synthase, which is responsible for the increase in neuronal vulnerability to hypoxia-induced mitochondrial damage and necrosis.

Stimulation of the brain NO/cyclic GMP pathway by peripheral administration of tetrahydrobiopterin in the hph-1 mouse

Mutations in GTP-cyclohydrolase I (GTP-CH) have been identified as causing a range of inborn errors of metabolism, including dopa-responsive dystonia. GTP-CH catalyses the first step in the biosynthesis of tetrahydrobiopterin (BH4), a cofactor necessary for the synthesis of catecholamines and serotonin. Current therapy based on monoamine neurotransmitter replacement may be only partially successful in correcting the neurological deficits. The reason might be that BH4 is also a cofactor for nitric oxide synthase. Using a strain of mutant GTP-CH-deficient (hph-1) mice, we demonstrate that in addition to impaired monoamine metabolism, BH4 deficiency is also associated with diminished nitric oxide synthesis in the brain (as evaluated by measuring the levels of cyclic GMP), when compared with wild-type animals. We have found a decline in the levels of BH4 with age in all animals, but no gender-related differences. We found a strong association between the levels of BH4 and cyclic GMP in hph-1 mice but not in wild-type animals. We also demonstrate that acute peripheral administration of BH4 (100 micromol/kg s.c.) in hph-1 mice significantly elevated the brain BH4 concentration and subsequently cyclic GMP levels in cerebellum, with peaks at 2 and 3 h, respectively. We suggest that BH4 administration should be considered in BH4 deficiency states in addition to monoamine replacement therapy.

Human phenylalanine hydroxylase gene expression in kidney and other nonhepatic tissues

Phenylalanine hydroxylase (PAH) is the key enzyme in phenylalanine metabolism. PAH deficiency results in hyperphenylalaninemia, leading to severe mental retardation in the classical form of the disease, phenylketonuria (PKU). Previously the expression of PAH could only unambiguously be demonstrated in human liver, whereas in rodents PAH expression has been established in kidney and liver. Reports concerning PAH activity in other human or rodent tissues were severely questioned by subsequent investigations such that they did not gain general recognition. Conducting Northern blot analyses, we detected the PAH transcript in RNA isolated from human liver, kidney, pancreas, and brain. PAH gene expression in human kidney was subsequently investigated by RNase protection assay analyses, RNA in situ hybridization, immunohistochemistry, enzyme assay, and cDNA isolation. These experiments allowed the conclusive verification of a functional PAH enzyme in human kidney. The primary structure of the kidney transcript corresponded to the structure of the liver transcript. Human kidney PAH may play a significant role in phenylalanine homeostasis of the organism, as impaired phenylalanine hydroxylation has been observed in renal failure and differences in the regulation of the kidney versus the liver enzyme have been indicated. These results provide new aspects to research into the basis for the heterogeneity of hyperphenylalaninemia phenotypes and establish that the expression of the human PAH gene is not limited to the liver.

Issues for consideration in dihydropteridine reductase (DHPR) deficiency: a variant form of hyperphenylalaninaemia

An adult male with intellectual disabilities demonstrated deterioration in many skills over a number of years and an increase in his temper outbursts was also reported. At 18 years of age, he had been diagnosed as having dihydropteridine reductase (DHPR) deficiency, but treatment had proved unsuccessful in the short term and was discontinued. Dihydropteridine reductase deficiency is a recessively inherited disorder of the amino acid metabolism resulting in a deficiency of tetrahydrobiopterin, an essential cofactor for phenylalanine, tyrosine and tryptophan metabolism. This causes a severe deficiency of neurotransmitters in the brain. Following further neurological examinations, treatment for the subject was recommenced at the age of 30 years. Few reports of late-diagnosis DHPR have been documented. This paper outlines one case report of DHPR, highlighting the importance of diagnosis, medical treatment and nursing care.

RFLP haplotyping and mutation analysis of the phenylalanine hydroxylase gene in Dutch phenylketonuria families

Restriction fragment length polymorphism haplotyping of mutated and normal phenylalanine hydroxylase (PAH) alleles in 49 Dutch phenylketonuria (PKU) families was performed. All mutant PAH chromosomes identified by haplotyping (n = 98) were screened for eight of the most predominant mutations. Compound heterozygosity was proven in 40 kindreds. Homozygosity was found for the IVS/2nt1 mutation in 5 families, and for the R158Q and IVS10nt546 mutations in one family each. All patients from these families suffer from severe PKU, providing additional proof that these mutations are deleterious for the PAH gene. Genotypical heterogeneity was evident for mutant haplotype 1 (n = 27) carrying the mutations R261Q (n = 12), E280K (n = 4, P281L (n = 1) and unknown (n = 10), and likewise for mutant haplotype 4 (n = 30) carrying the mutations R158Q (n = 13), Y414C (n = 1) and unknown (n = 16). Mutant haplotype 3 (n = 20), in tight association with mutation IVS12nt1, appeared to be in strong linkage disequilibrium (LDE) with its normal counterpart allele (n = 4). Mutant haplotype 6 (n = 4), in tight association with the IVS10nt546 mutation, showed moderate LDE with its counterpart allele (n = 1). The distribution of the mutant PAH haplotypes 1, 3 and 4 among the Dutch PKU population resembles that in other Northern and Western European countries, but it is striking that mutant haplotype 2 and its associated mutation R408W is nearly absent in The Netherlands, in strong contrast to its neighbouring countries.

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.

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.

Simple column liquid chromatographic assay for serum neopterin

A simple high-performance liquid chromatographic assay for serum neopterin with highly sensitive fluorimetric detection (limit 1 nM) is proposed. Comparison with results obtained by radioimmunoassay revealed a good correlation between the two techniques. The potential use of this method for the follow-up of liver transplant patients is discussed.

A missense mutation, S349P, completely inactivates phenylalanine hydroxylase in north African Jews with phenylketonuria

The majority of hyperphenylalaninemias (HPAs) result from mutations at the gene for phenylalanine hydroxylase (PAH). The broad phenotypic variability of these conditions, ranging from phenylketonuria (PKU) to mild benign HPA, is underlain by a wide spectrum of mutations giving rise to various genotypic combinations. Mutant PAH alleles, labeled by specific polymorphic haplotypes and mutations, are becoming useful markers in human population genetics. We report here a mutant PAH allele found in Jews from Morocco and Tunisia, marked by haplotype 4 and a missense mutation, TCASer-->CCAPro, at codon 349 in exon 10 of the gene. In vitro expression of the mutation showed normal levels of mRNA with virtually no enzymatic activity or protein immunoreactivity, pointing to a highly unstable protein. A homozygote for this mutation showed the most severe ("classical") type of PKU, while compound heterozygotes showed two other types of HPA--"atypical" PKU and "high benign" HPA--illustrating the interplay between different mutations that gives rise to various HPAs.

Abnormalities of biogenic amine metabolism

The term biogenic amine is an umbrella term that encompasses all amines with an origin in biological processes. This review will be restricted to the biogenic amine abnormalities that affect the metabolism of serotonin and the catecholamines. The synthesis and catabolism of these neurotransmitters are outlined, and a summary is given of the neurological details, biochemical features, and treatment of the inborn errors that primarily affect their metabolism. An idea is also developed that proposes that abnormalities of biogenic amine metabolism are far more common than is currently considered, and that the search for these problems may be appropriate in any neonate or infant who presents with neurological problems of unknown origin.

Enhancement of dopamine release in vivo from the rat striatum by dialytic perfusion of 6R-L-erythro-5,6,7,8-tetrahydrobiopterin

We have previously reported that intracerebroventricular administration of 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (6R-BH4), a cofactor for tyrosine hydroxylase, enhances biosynthesis of 3,4-dihydroxyphenylethylamine (dopamine) in the rat brain. In the present study, we have more precisely examined the effects of 6R-BH4 on dopamine release in vivo from the rat striatum using brain microdialysis. The amount of dopamine collected in striatal dialysates was determined using HPLC with electrochemical detection after purification with an alumina batch method. When the striatum was dialyzed with Ringer solution containing various concentrations of 6R-BH4 (0.25, 0.5, and 1.0 mM), dopamine levels in striatal dialysates increased in a concentration-dependent manner. Biopterin had little effect on dopamine levels in dialysates. The 6R-BH4-induced increase in dopamine levels in dialysates was abolished after pretreatment with tetrodotoxin (50 microM) added to the perfusion fluid, but after pretreatment with nomifensine (100 mg/kg, intraperitoneal injection), an inhibitor of dopamine uptake mechanism, a larger increase was observed. After inhibition of tyrosine hydroxylase by pretreatment with alpha-methyl-p-tyrosine (250 mg/kg, intraperitoneal injection), most of the increase persisted. These results suggest that 6R-BH4 has a dopamine-releasing action, which is not dependent on biosynthesis of dopamine.

Tetrahydrobiopterin and biogenic amine metabolism in neuropsychiatry, immunology, and aging

Tetrahydrobiopterin (BH4) is essential for biogenic amine synthesis, and alterations in its metabolism occur at birth (atypical PKU), in neuropsychiatric illnesses, and in aging. BH4 therapy has been attempted in atypical PKU and in neuropsychiatric illness with some success and may become more viable as more is learned about BH4 metabolism and ways are discovered to elevate brain BH4 levels. It is intriguing to consider that a genetic defect in BH4 biosynthesis occurring at birth might go unrecognized and contribute to altered biogenic amine metabolism that occurs in neuropsychiatric illness. Since there seems to be a sensitivity of BH4 metabolism to genetic alterations, it is possible that altered BH4 metabolism is involved in some of deleterious effects associated with the aging process. A link between genetic alterations in BH4 metabolism at birth and adult neuropsychiatric illness and aging remains to be established, although this seems plausible. The presence of BH4 and other pterins in cells of the immune system as well as the pineal gland and other neuroendocrine tissues suggests the potential for other functions of pterins. Hopefully, future research will uncover the full potential for the therapeutic use of BH4 in a variety of diseases as well as elucidating other potential roles for pterin molecules which are present in many different systems.

Localization of GTP cyclohydrolase I in human peripheral blood smears using a specific monoclonal antibody and an immune-alkaline phosphatase labeling technique

GTP cyclohydrolase I, the enzyme catalyzing the first step in the cofactor biosynthesis for the aromatic amino acid hydroxylases, has been localized in situ. By the use of a monoclonal antibody specific to human GTP cyclohydrolase I, the enzyme has been visualized immuno-enzymatically by alkaline phosphatase monoclonal anti-alkaline phosphatase labeling. In routine blood smears lymphocytes, monocytes/macrophages, and granulocytes show strong intraplasmatic staining. Premature erythrocytes show clear staining of the reticulated cytoplasmatic structure, while mature erythrocytes are completely negative. Neither is there any staining for GTP cyclohydrolase I in the blast cells of a case of T-cell acute lymphoblastic leukemia. These results closely confirm the prior finding that mature erythrocytes as well as most malignant mononuclear cells lack GTP cyclohydrolase I activity, and they indicate that in these cells the enzyme protein may be absent.

Direct analysis of tetrahydrobiopterin in cerebrospinal fluid by high-performance liquid chromatography with redox electrochemistry: prevention of autoxidation during storage and analysis

Measurement of tetrahydrobiopterin in cerebrospinal fluid requires careful handling of samples during storage and analysis. Addition of dithioerythritol and deoxygenation of the mobile phase with helium prevents breakdown of tetrahydrobiopterin during chromatography. Tetrahydrobiopterin in cerebrospinal fluid is unstable at room temperature, 100% being lost within 3.5 h, this breakdown does not generate equivalent quantities of dihydrobiopterin and biopterin. Addition of dithioerythritol and diethylenetriaminepenta-acetic acid to cerebrospinal fluid prevents breakdown of tetrahydrobiopterin for 6 mth at -70 degrees C and for up to 5 h at 4 degrees C. At room temperature less than 5% of tetrahydrobiopterin was lost after 2 h.

"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.

Biopterin in Parkinson's disease

Tetrahydrobiopterin is an essential co-factor in the natural synthesis of dopamine. Oral tetrahydrobiopterin was given in small doses to four patients with early Parkinson's disease but had no discernible effect.

A model for hyperphenylalaninaemia due to tetrahydrobiopterin deficiency

A model for tetrahydrobiopterin deficiency in mice is described. Elevated levels of phenylalanine produced in the model were shown to be dramatically reduced after injection of tetrahydrobiopterin. A comparison of several reduced pterins for their efficacy in the system is described. The unnatural S isomer of tetrahydrobiopterin was shown to be active in the system.

Clinical role of pteridine therapy in tetrahydrobiopterin deficiency

In most patients with deficiency of tetrahydrobiopterin (BH4) continuous administration of BH4 or of a synthetic analogue such as 6-methyltetrahydropterin (6-MPH4) lowers plasma phenylalanine concentrations to the therapeutic range. The effective dose of BH4 varies from 1 to 2 mg kg-1 daily in patients with defective biopterin synthesis, to 5 mg kg-1 or more in patients with dihydropteridine reductase (DHPR) deficiency. The cost of 2 mg kg-1 day-1 of BH4 is comparable to the cost of a low phenylalanine diet. Higher doses of pterins given orally (20 mg kg-1) raise the levels of tetrahydropterin in cerebrospinal fluid (CSF) to normal in patients with defective biopterin synthesis in whom initial concentration of biopterin species are low. In some, but not all, such patients pterin therapy also raises CSF amine metabolite concentrations and ameliorates symptoms. High dose therapy does not appear to be effective in raising CSF pterin levels in patients with DHPR deficiency who already accumulate dihydrobiopterin (BH2) in CSF. Central folate deficiency is an additional cause of neurological deterioration in patients with DHPR deficiency who require supplementation with folate as folinic acid. It is suggested that the accumulation of BH2 in such patients competitively interferes with folate metabolism.

Hyperphenylalaninaemia due to impaired dihydrobiopterin biosynthesis: leukocyte function and effect of tetrahydrobiopterin therapy

We have described the clinical and biochemical status of two patients with tetrahydrobiopterin (BH4) deficiency due to impaired dihydrobiopterin biosynthesis. BH4 administration appeared to improve the mental and psychological status more than did neurotransmitter replacement therapy alone. This enhancement of activities of daily life was seen with a dose of BH4 as low as 1.25 mg kg-1 day-1. Granulocyte adherence capacity was below normal and recovered after BH4 therapy in both patients. B-cell differentiation capacity was altered either before or after therapy.

Differential diagnosis of tetrahydrobiopterin deficiency

Six hundred and seventy-three children (483 newborns and 190 older selected children) were screened for tetrahydrobiopterin (BH4) deficiency by HPLC of urine pterins and BH4 load test. One patient with GTP cyclohydrolase I deficiency, 36 patients with dihydrobiopterin synthetase (DHBS) deficiency (of which six were in the newborn and 30 in the older children) and 14 with dihydropteridine reductase deficiency (DHPR) were found. All 37 patients with defective BH4 biosynthesis responded to a BH4 load by lowering of the elevated serum phenylalanine concentration but four of 14 patients with DHPR deficiency did not. Measurement of DHPR activity in blood spots on Guthrie cards is recommended. Since subvariants of patients with BH4 deficiency exist, homovanillic acid, 5-hydroxyindole acetic acid, pterins, phenylalanine, and tyrosine in cerebrospinal fluid should be measured for diagnosis and the control of therapy. The activity of the phosphate-eliminating enzyme (a key enzyme in BH4 biosynthesis and part of "DHBS") was measured in human liver and activities of approx. 1 n U (mg protein)-1 were found. In the liver biopsy of a patient with DHBS deficiency no activity (less than 3% of controls) was demonstrated.

Transient hyperphenylalaninaemia with a high neopterin to biopterin ratio in urine

A case of transient hyperphenylalaninaemia with a maturational delay of dihydropteridine synthesis is described. With the Guthrie test, the patient showed a blood phenylalanine level of 38 mg dl-1, which had fallen to a normal value without a phenylalanine restricted diet by 3 months of age. The neopterin level and the neopterin to biopterin ratio in the patient's urine were very high at 19 days of age. The blood phenylalanine level did not decrease when tetrahydrobiopterin (2.5 mg kg-1) was administered at 19 days of age, while administration of tetrahydrobiopterin (7.5 mg kg-1) at 20 days of age had decreased the blood phenylalanine level to 50% of the preloading level after 24 h. The oral phenylalanine loading test showed the pattern of classic phenylketonuria (PKU) at 15 days of age, but it showed the normal pattern at 1 year 8 months of age.

Combined tetrahydrobiopterin-phenylalanine loading test in the detection of partially defective biopterin synthesis

Deficiency in the synthesis of biopterin causes neonatal hyperphenylalaninemia. We report a 10-year-old girl of normal appearance with a partial defect in biopterin synthesis, normal intelligence and normal serum phenylalanine levels (95 mumol/l) (1.6 mg/dl). During her 1st year of life serum phenylalanine levels were 250 mumol/l (4 mg/dl) and phenylalanine loading performed at 6 months and 1 year of age was not followed by an increase in serum tyrosine. At 9 years of age she had developed a severely abnormal EEG with focal spike activity but no observable clinical abnormalities. Determination of urinary pterins showed abnormal low levels of biopterin and high levels of neopterin. Phenylalanine loading combined with oral administration of tetrahydrobiopterin (BH4) was followed by a normal increase in serum tyrosine and a normal decrease in serum phenylalanine. Considering the importance of BH4 for the synthesis of dopamine, catecholamines, and serotonin we suggest that these cases should be followed carefully. If neurological symptoms appear, e.g., epilepsy, it may be worthwhile to consider treatment with BH4 and neurotransmitter precursors.

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

An automatic high-performance liquid chromatographic system with facilities for column switching is described which involves minimal pre-treatment of biological samples, separates complex mixtures of compounds in a short period of time and uses fluorimetric or amperometric detection. The system has been applied to the analysis of oxidized pterins in urine and reduced pterins in cerebrospinal fluid and rat brain fractions (R- and S-enantiomers of tetrahydrobiopterin resolved). The system can also be used for the analysis of most of the dopamine and serotonin metabolites in cerebrospinal fluid and brain fractions from norepinephrine to serotonin.

Tetrahydrobiopterin deficiencies: preliminary analysis from an international survey

Tetrahydrobiopterin deficiency is a rare cause of hyperphenylalaninemic syndromes. The natural history of the disease is characterized by progressive neurologic illness unresponsive to a phenylalanine-restricted diet. Fifty patients have been reported. From the documented cases, the following statements can be made: (1) An incidence of 2% among hyperphenylalaninemic babies can be reasonably estimated. (2) Most patients have high neonatal blood phenylalanine concentrations, but some have only mild elevations. (3) Among the available diagnostic tests, measurement of urine pteridines should be proposed in all hyperphenylalaninemic babies, (4) The tolerance to dietary phenylalanine is generally high. (5) The results of neurotransmitter replacement therapy are encouraging, but treatment should be started within the first month and requires a strict follow-up protocol. Consequently, in every newborn infant with positive Guthrie test results, a rapid investigation of BH4 metabolism should be accomplished in order to differentiate between phenylalanine-hydroxylase deficiencies (phenylketonuria, mild hyperphenylalaninemia, transient hyperphenylalaninemia) and BH4 deficiencies.

GTP cyclohydrolase I deficiency, a new enzyme defect causing hyperphenylalaninemia with neopterin, biopterin, dopamine, and serotonin deficiencies and muscular hypotonia

A 4-year-old patient is described with hyperphenylalaninemia, severe retardation in development, severe muscular hypotonia of the trunk and hypertonia of the extremities, convulsions, and frequent episodes of hyperthermia without infections. Urinary excretion of neopterin, biopterin, pterin, isoxanthopterin, dopamine, and serotonin was very low, although the relative proportions of pterins were normal. In lumbar cerebrospinal fluid, homovanillic acid, 5-hydroxyindoleacetic acid, neopterin and biopterin were low. Oral administration of L-erythro tetrahydrobiopterin normalized the elevated serum phenylalanine within 4 h, serum tyrosine was increased briefly and serum alanine and glutamic acid for a longer time. Urinary dopamine and serotonin excretion were also increased. Administration of an equivalent dose of D-erythro tetrahydroneopterin was ineffective and demonstrated that this compound is not a cofactor in vivo and cannot be transformed into an active cofactor. GTP cyclohydrolase I activity was not detectable in liver biopsies from the patient. The presence of an endogenous inhibitor in the patient's liver was excluded. This is the first case of a new variant of hyperphenylalaninemia in which the formation of dihydroneopterin triphosphate and its pterin metabolites in liver is markedly diminished. Normal activities of xanthine oxidase and sulfite oxidase were apparent since uric acid levels were normal and no increase in hypoxanthine, xanthine, and S-sulfocysteine concentrations could be observed in urine. It is concluded that the molybdenum cofactor of these enzymes may not be derived from dihydroneopterin triphosphate in man. Also, since no gross abnormalities in the patient's immune system could be found, it seems unlikely that dihydroneopterin triphosphate metabolites, such as neopterin, participate actively in immunological processes, as postulated by others. See Note added in proof.

Biopterin synthesis defect. Treatment with L-dopa and 5-hydroxytryptophan compared with therapy with a tetrahydropterin

We have identified a generalized deficiency of monoamine neurotransmitters in a patient with a defect in biopterin synthesis. Neurotransmitter precursors (L-3,4-dihydroxyphenylalanine [L-dopa]; 5-hydroxytryptophan [5-HTP] and a tetrahydropterin [6-methyltetrahydropterin (6MPH4)] were investigated for their ability to normalize monoamine neurotransmitter metabolism. Before treatment, the concentrations of dopamine (DA), norepinephrine, epinephrine, and six monoamine metabolites were very low or undetectable in plasma, cerebrospinal fluid, or urine. L-Dopa and 5-HTP replacement was begun at age 7 mo. This therapy generally corrected the deficiency of monoamines and their metabolites, and improved neurological development until the age of 25 mo. Despite these benefits, the intermittent administration of L-dopa could not produce a stable improvement of acute neurological function or DA metabolism. In the 3 h after L-dopa administration, plasma DA and the motor activity and alertness of the patient rose and fell in parallel. Doses of L-dopa that were clinically optimal produced normal plasma levels of norepinephrine and epinephrine, but excessive concentrations of DA and its metabolites. Furthermore, the clinical and biochemical effects of L-dopa were inhibited by phenylalanine and 5-HTP, respectively, demonstrating that these amino acids have antagonistic pharmacological effects. Physiological correction of the monoamine deficit and the hyperphenylalaninemia of this disorder was attempted at age 35 mo using high doses (8-38 mg/kg per d) of 6MPH4. 6MPH4, a synthetic analogue of tetrahydrobiopterin, controlled the hyperphenylalaninemia. Significant concentrations of 6MPH4 were obtained in the cerebrospinal fluid; no neurological improvement or stimulation of monoamine synthesis in the central nervous system was detected. These findings indicate the complexity in replacement therapy with L-dopa and 5-HTP, but suggest that this treatment may be partially effective in biopterin-deficient patients who are unresponsive to high doses of tetrahydropterins.

Dihydropteridine reductase deficiency: non-response to oral tetrahydrobiopterin load test

In a child with dihydropteridine reductase deficiency (McKusick 26163) none of four separate oral tetrahydrobiopterin loading tests resulted in a decrease of the serum phenylalanine concentration. This test cannot be recommended for screening hyperphenylalaninaemic patients for dihydropteridine reductase deficiency.