Pterin-4a-carbinolamine dehydratase from Pseudomonas aeruginosa: characterization, catalytic mechanism and comparison to the human enzyme

The three-dimensional structure of pterin-4a-carbinolamine dehydratase (PCD) from Pseudomonas aeruginosa has been solved. Based on this we have investigated the roles of putative active center residues through functional replacement by site-directed mutagenesis. Three histidines, His73, His74 and His91, appear to be involved in dehydration catalysis. The three-dimensional positions of these residues match those of corresponding histidines at the active center of human PCD. Based on the coincidence of catalytic parameters, and on the similar effects induced by the mutations, it is concluded that the substrate binding mode and the reaction mechanisms of bacterial and human PCD are basically identical.

Location of the active site and proposed catalytic mechanism of pterin-4a-carbinolamine dehydratase

Based on the recently solved three-dimensional structure of pterin-4a-carbinolamine dehydratase from rat/human liver the involvement of the proposed active-site residues Glu57, Asp60, His61, His62, Tyr69, His79, Arg87 and Asp88 was examined by site-directed mutagenesis. Most of the mutants showed reduced activity, and only the Glu57-->Ala mutant and the His61-->Ala, His62-->Ala double mutant were fully devoid of activity. The dissociation constants of quinonoid 6,6-dimethyl-7,8-dihydropterin were significantly increased for binding to the Glu57-->Ala, His61-->Ala, His62-->Ala single mutants and the His61-->Ala, His62-->Ala double mutant, confirming that His61 and His62 are essential for substrate binding and catalysis. The mechanism of dehydration is proposed to involve base catalysis at the N(5)-H group of the substrate by His61.

Human pterin-4 alpha-carbinolamine dehydratase/dimerization cofactor of hepatocyte nuclear factor-1 alpha. Characterization and kinetic analysis of wild-type and mutant enzymes

Pterin-4a-carbinolamine dehydratase/dimerization cofactor for hepatocyte nuclear factor-1 alpha is a protein with two different functions. We have overexpressed and purified the human wild-type protein, and its Cys81Ser and Cys81Arg mutants. The Cys81Arg mutant has been proposed to be causative in a hyperphenylalaninaemic patient [Citron, B. A., Kaufman, S., Milstien, S., Naylor, E. W., Greene, C. L. & Davis, M. D. (1993) Am. J. Hum. Genet. 53, 768-774]. The dehydratase behaves as a tetramer on gel filtration, while cross-linking experiments showed mono-, di-, tri-, and tetrameric forms, irrespective of the presence of the single Cys81. Sulfhydryl-modifying reagents did not affect the activity, but rather showed that Cys81 is exposed. Various pterins bind and quench the tryptophan fluorescence suggesting the presence of a specific binding site. The fluorescence is destroyed upon light irradiation. Wild-type and the Cys81Ser protein enhance the rate of the phenylalanine hydroxylase assay approximately 10-fold, a value similar to that of native dehydratase from rat liver; the Cys81Arg mutant, in contrast, has significantly lower activity. This is compatible with the hypothesis that the dehydratase is a rate-limiting factor for the in vivo phenylalanine hydroxylase reaction. The three proteins enhance the spontaneous dehydration of the synthetic substrate 6,6-dimethyl-7,8-dihydropterin-4a-carbinolamine approximately 50-70-fold at 4 degrees C and pH 8.5. The results are discussed in view of the recently solved three-dimensional structure of the enzyme [Ficner, R., Sauer, U. W., Stier, G. & Suck, D. (1995) EMBO J. 14, 2032-2042].

Effect of high-protein meal plus aspartame ingestion on plasma phenylalanine concentrations in obligate heterozygotes for phenylketonuria

The effect of a protein-rich meal alone or in combination with 85 mumol/kg body weight aspartame (APM) on plasma phenylalanine and large neutral amino acids (LNAA) was evaluated in obligate heterozygotes for phenylketonuria (PKU) and normal subjects (controls). Thirteen PKU heterozygotes (seven women, six men) and 13 controls (five women, eight men) ingested a 12-noon meal providing approximately 303 mumol/kg Phe. In addition, 10 PKU heterozygotes (five women, five men) and 10 controls (five women, five men) ingested the same meal with 85 mumol/kg APM (providing 75 mumol/kg Phe). Plasma amino acids were analyzed at baseline (-4 and 0 hours) and at 1, 3, and 20 hours after the meal or meal plus APM. Compared with the meal alone, ingestion of the meal plus APM significantly increased plasma Phe concentrations in both controls and PKU heterozygotes. Mean plasma Phe values were higher for controls at 1 hour (95 +/- 7 mumol/L) and for PKU heterozygotes at 3 hours (153 +/- 21 mumol/L). After the addition of APM to the meal, the highest mean plasma Phe concentration was only slightly greater than the usual postprandial range for both controls and PKU heterozygotes. Ingestion of the meal did not increase the plasma Phe/LNAA ratio in either controls or PKU heterozygotes. Compared with baseline, the plasma Phe/LNAA ratio increased significantly 1 hour after combined ingestion of the meal plus APM in both groups (P = .020 and P = .008, respectively); however, the ratios were well below the range of Phe/LNAA values in individuals with mild hyperphenylalaninemia, who are clinically normal and do not require a Phe-restricted diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Phenylalanine hydroxylase-stimulating protein/pterin-4 alpha-carbinolamine dehydratase from rat and human liver. Purification, characterization, and complete amino acid sequence

Phenylalanine hydroxylase-stimulating protein, also known as pterin-4 alpha-carbinolamine dehydratase (PHS/PCD), was purified from rat and, for the first time, from human liver. We obtained their complete protein primary sequence using a combination of liquid secondary ionization mass spectrometry/tandem quadrupole mass spectrometry, electrospray ionization mass spectrometry, and Edman microsequence analysis. The amino acid sequences of human and rat PHS/PCD were found to be identical. Surprisingly, the primary structure of PHS/PCD is also essentially identical to a protein of the cell nucleus, named dimerization cofactor of hepatocyte nuclear factor 1 alpha, recently reported to be involved in transcription (Mendel, D. M., Khavari, P. A., Conley, P. B., Graves, M. K., Hansen, L. P., Admon, A., and Crabtree, G. R. (1991) Science 254, 1762-1767).

7-substituted pterins in humans with suspected pterin-4a-carbinolamine dehydratase deficiency. Mechanism of formation via non-enzymatic transformation from 6-substituted pterins

A recently described new form of hyperphenylalaninemia is characterized by the excretion of 7-substituted isomers of biopterin and neopterin and 7-oxo-biopterin in the urine of patients. It has been shown that the 7-substituted isomers of biopterin and neopterin derive from L-tetrahydrobiopterin and D-tetrahydroneopterin and are formed during hydroxylation of phenylalanine to tyrosine with rat liver dehydratase-free phenylalanine hydroxylase. We have now obtained identical results using human phenylalanine hydroxylase. The identity of the pterin formed in vitro and derived from L-tetrahydrobiopterin as 7-(1',2'-dihydroxypropyl)pterin was proven by gas-chromatography mass spectrometry. Tetrahydroneopterin and 6-hydroxymethyltetrahydropterin also are converted to their corresponding 7-substituted isomers and serve as cofactors in the phenylalanine hydroxylase reaction. Dihydroneopterin is converted by dihydrofolate reductase to the tetrahydro form which is biologically active as a cofactor for the aromatic amino acid monooxygenases. The 6-substituted pterin to 7-substituted pterin conversion occurs in the absence of pterin-4a-carbinolamine dehydratase and is shown to be a nonenzymatic process. 7-Tetrahydrobiopterin is both a substrate (cofactor) and a competitive inhibitor with 6-tetrahydrobiopterin (Ki approximately 8 microM) in the phenylalanine hydroxylase reaction. For the first time, the formation of 7-substituted pterins from their 6-substituted isomers has been demonstrated with tyrosine hydroxylase, another important mammalian enzyme which functions in the hydroxylation of phenylalanine and tyrosine.

7-Substituted pterins: formation during phenylalanine hydroxylation in the absence of dehydratase

Previously we described a new form of human hyperphenylalaninemia characterized by the formation of 7-substituted pterins. We present evidence strongly suggesting that the 7-substituted pterins are formed by rearrangement of 6-substituted pterins. This rearrangement occurs during the phenylalanine hydroxylase reaction cycle which normally involves the enzymes phenylalanine hydroxylase, pterin-4a-OH-dehydratase, and q-dihydropterin reductase, specifically in the absence of dehydratase activity. We conclude that formation of 7-substituted pterins in humans is a consequence of an absence of dehydratase activity, which might result from a genetic defect. A chemical mechanism for this rearrangement is presented. Our results also suggest that tetrahydroneopterin can be a cofactor for the phenylalanine hydroxylase system in vivo.

7-Substituted pterins. A new class of mammalian pteridines

Three novel pteridines have been isolated from the urine of patients with a new variant of 6-(L-erythro-1',2'-dihydroxypropyl)-5,6,7,8-tetrahydropterin (tetrahydrobiopterin) deficiency, showing hyperphenylalaninemia. From the results of high performance liquid chromatography, oxidative degradation, and gas chromatography-electron impact mass spectrometry, their structures were identified as 7-(D-erythro-1',2',3'-trihydroxypropyl)-pterin (7-neopterin), 7-(L-erythro-1',2'-dihydroxypropyl)-pterin (7-biopterin), and 6-oxo-7-(L-erythro-1',2'-dihydroxypropyl)-pterin (6-oxo-7-biopterin). The ratio of biopterin to 7-biopterin in the patients' urines was 1:1, and after oral loading with tetrahydrobiopterin, 7-biopterin excretion rose parallel to biopterin. This finding suggests that 7-substituted pterins may be formed endogenously by a yet unknown isomerization reaction. The cause of hyperphenylalaninemia is still unclear. The activities of the enzymes involved in tetrahydrobiopterin biosynthesis and regeneration were found to be normal in the patients, and no effect of 7-biopterin on these enzymes was observed in vitro. However, compared with the normal cofactor, tetrahydrobiopterin, the Km values of tetrahydro-7-biopterin for phenylalanine hydroxylase and dihydropteridine reductase are 20 and 5 times higher, respectively.

1H-NMR and mass spectrometric studies of tetrahydropterins. Evidence for the structure of 6-pyruvoyl tetrahydropterin, an intermediate in the biosynthesis of tetrahydrobiopterin

The conversion of dihydroneopterin triphosphate in the presence of 6-pyruvoyl tetrahydropterin synthase was followed by 1H-NMR spectroscopy. The interpretation of the spectra of the product is unequivocal: they show formation of a tetrahydropterin system carrying a stereospecifically oriented substituent at the asymmetric C(6) atom. The spectra are compatible with formation of a (3')-CH3 function, and with complete removal of the 1' and 2' hydrogens of dihydroneopterin triphosphate. The fast-atom-bombardment/mass spectrometry study of the same product yields a [M + H]+ ion at m/z 238 compatible with the structure of 6-pyruvoyl tetrahydropterin. The data support the proposed structure of 6-pyruvoyl tetrahydropterin as a key intermediate in the biosynthesis of tetrahydrobiopterin.

Human liver 6-pyruvoyl tetrahydropterin reductase is biochemically and immunologically indistinguishable from aldose reductase

6-Pyruvoyl tetrahydropterin reductase has been implicated in the biosynthesis of tetrahydrobiopterin. Using immunochemical and biochemical techniques the purified human liver enzyme was shown to be identical to aldose reductase. This suggests that 6-pyruvoyl tetrahydropterin reductase may play an additional role in the reduction of aldehydes derived from the biogenic amine neuro-transmitters and corticosteroid hormones as well as in the pathogenesis of diabetic complications, as has been postulated for aldose reductase.

Purification and characterization of 6-pyruvoyl tetrahydropterin synthase from salmon liver

Salmon liver was chosen for the isolation of 6-pyruvoyl tetrahydropterin synthase, one of the enzymes involved in tetrahydrobiopterin biosynthesis. A 9500-fold purification was obtained and the purified enzyme showed two single bands of 16 and 17 kDa on SDS/PAGE. The native enzyme (68 kDa) consists of four subunits and needs free thiol groups for enzymatic activity as was shown by reacting the enzyme with the fluorescent thiol reagent N-(7-dimethylamino-4-methylcoumarinyl)-maleimide. The enzyme is heat-stable up to 80 degrees C, has an isoelectric point of 6.0-6.3, and a pH optimum at 7.5. The enzyme is Mg2+ -dependent and has a Michaelis constant for its substrate dihydroneopterin triphosphate of 2.2 microM. The turnover number of the purified salmon liver enzyme is about 50 times as high as that of the enzyme purified from human liver. It does not bind to the lectin concanavalin A, indicating that it is free of mannose and glucose residues. Polyclonal antibodies raised against the purified enzyme in Balb/c mice were able to immunoprecipitate enzyme activity. The same polyclonal serum was not able to immunoprecipitate enzyme activity of human liver 6-pyruvoyl tetrahydropterin synthase, nor was any cross-reaction in ELISA tests seen.

Purification of GTP cyclohydrolase I from human liver and production of specific monoclonal antibodies

GTP cyclohydrolase I, the first enzyme in the de novo biosynthesis of tetrahydrobiopterin, was enriched more than 13,000-fold from human liver by preparative isoelectric focusing using Sephadex G-200 SF gels. The pI of the active enzyme was determined as 5.6 by analytical isoelectric focusing in the same matrix. The native enzyme has an apparent molecular mass of 440 kDa and appears to be composed of eight 50-kDa subunits as estimated from SDS/PAGE. The enriched enzyme preparation was used to produce specific monoclonal antibodies. From 11 monoclonal antibodies obtained, one was extensively characterized for further applications. This monoclonal antibody belongs to the IgM class and shows immunoreactivity with GTP cyclohydrolase I both from man and from Escherichia coli. It is capable of highly sensitive detection of GTP cyclohydrolase I by ELISA and by Western blot analysis. The monoclonal antibody was used for the immunoenzymatic localisation of GTP cyclohydrolase I in human peripheral blood mononuclear cells. Furthermore, it was possible to demonstrate the absence of immunoreactivity in cells with GTP cyclohydrolase I deficiency. The antibody's use as a tool either for differential diagnosis of atypical phenylketonuria due to GTP cyclohydrolase I deficiency or prenatal diagnosis of this severe inherited metabolic disease is now under investigation.

Primapterin, anapterin, and 6-oxo-primapterin, three new 7-substituted pterins identified in a patient with hyperphenylalaninemia

Three unknown compounds present in the urine of a patient with mild hyperphenylalaninemia were identified to be L-erythro-7-iso-biopterin, D-erythro-7-iso-neopterin, and L-erythro-6-oxo-7-iso-biopterin. The newly identified pterins were named primapterin, anapterin, and 6-oxo-primapterin, respectively. Primapterin and anapterin are present in very low concentrations in every human urine, as well as in the liver of man and mouse, whereas 6-oxo-primapterin was detected in the patient's urine only. Substantial amounts of primapterin were excreted in the patient described. The metabolic origin of primapterin and anapterin is still obscure.

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.

Identification of new steroids in patients with 17 alpha-hydroxylase deficiency by capillary gas chromatography/mass spectrometry

Urines of two children with 17 alpha-hydroxylase deficiency contained a number of 5-pregnane- and pregnenediols, -triols and -tetrols with a hydroxy or oxo group in position 11 of the steroid ring. They are formed mainly from progesterone via 11-hydroxyprogesterone, pregnanolone and corticosterone, respectively, or from pregnenolone. Three metabolites not previously described, 16-hydroxypregnenolone, 6,21-dihydroxypregnanediol and 6-hydroxytetrahydrocorticosterone, were identified.

Entrance of tetrahydropterin derivatives in brain after peripheral administration: effect on biogenic amine metabolism

The ability of various structural analogs of tetrahydrobiopterin to enter the rat brain and remain in the reduced form after i.p. administration was tested because tetrahydrobiopterin itself enters poorly. The total content of various pterins in different brain areas was measured by high-pressure liquid chromatography-fluorescence detection, whereas high-pressure liquid chromatography-electrochemical detection was used to measure the content of reduced pterins. The effect of injection of certain tetrahydropterins on brain content of biogenic amines and their major metabolites was also monitored. In general, when the position 6 side-chain of tetrahydrobiopterin was made shorter and smaller in size, entrance into the brain from the periphery was markedly enhanced. Increasing the lipophilicity or size of the position 6 side-chain did not allow for better entry into brain and, in some cases, hindered entry. Under the conditions tested, none of the tetrahydropterins influenced the brain content of biogenic amines or their major metabolites; higher brain concentrations of tetrahydropterins are probably necessary to modify central nervous system biogenic amine metabolism. Other types of structural modifications or experimental approaches may be necessary to achieve higher brain concentrations of active cofactors, which may be required for the successful treatment of certain human diseases with tetrahydropterins.

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.

Tetrahydrobiopterin biosynthetic pathway and deficiency

It has been proven that the most common defect in the tetrahydrobiopterin biosynthesis is caused by 6-pyruvoyl tetrahydropterin synthase deficiency. The enzyme 6-pyruvoyl tetrahydropterin synthase consists of four identical subunits which convert dihydroneopterin triphosphate to 6-pyruvoyl tetrahydropterin in the presence of magnesium. UV, NMR, and MS data prove that the enzyme catalyzes the elimination of triphosphate as well as the intramolecular rearrangement. The 6-pyruvoyl tetrahydropterin synthase activity was measured in fetal erythrocytes and together with the neopterin and biopterin measurements in amniotic fluid this enabled performing prenatal diagnosis of 6-pyruvoyl tetrahydropterin synthase deficiency. Peripheral tetrahydrobiopterin deficiency was shown to be due to an incomplete 6-pyruvoyl tetrahydropterin synthase deficiency or heterozygosity.

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.

Prenatal diagnosis of "dihydrobiopterin synthetase" deficiency, a variant form of phenylketonuria

Amniocentesis was performed at 19 weeks gestation in a mother who had previously delivered a boy with "dihydrobiopterin synthetase" (DHBS) deficiency. The amniotic fluid contained neopterin in high (136 nmol/l) and biopterin in very low concentrations (1.8 nmol/l). The activity of the phosphate-eliminating enzyme (PEE, also called 6-pyruvoyl tetrahydropterin synthase, substrate: 7,8-dihydroneopterin triphosphate) which is present in liver and erythrocytes and defective in DHBS deficiency, was measured in the erythrocytes of the family members. The fetal sample showed only 2% of the activity of healthy adult controls and was comparable with that of the affected sibling. Obligate heterozygotes had activities around 20% of the controls. Two fetal control samples showed even higher activities than adult erythrocytes, Sepiapterin reductase activities wer normal in all cases. At autopsy, PEE deficiency was confirmed in the liver of the fetus. We concluded that DHBS deficiency (and most probably also GTP cyclohydrolase I deficiency) can be diagnosed by metabolite measurements in amniotic fluid. PEE activity is measurable in erythrocytes, although the assay needs to be improved. Since maternal tetrahydrobiopterin does not cross the placenta, treatment of a tetrahydrobiopterin-deficient fetus with tetrahydrobiopterin in utero is not possible.

Purification of 6-pyruvoyl-tetrahydropterin synthase from human liver

The enzyme which catalyzes the first step in the conversion of dihydroneopterin triphosphate to tetrahydrobiopterin has been purified approx. 40,000-fold from human liver to apparent homogeneity. The enzyme has a native molecular weight of approximately 83,000 and consists of four identical subunits, each of which has a molecular weight of approximately 19,000. It contains carbohydrates and is remarkably stable to heat treatment. In the presence of purified sepiapterin reductase, Mg2+, and NADPH, this enzyme catalyzes efficiently the formation of tetrahydrobiopterin from dihydroneopterin triphosphate. This indicates that these two proteins are sufficient for the overall conversion.

Tetrahydrobiopterin biosynthesis. Studies with specifically labeled (2H)NAD(P)H and 2H2O and of the enzymes involved

The biosynthesis of tetrahydrobiopterin from either dihydroneopterin triphosphate, sepiapterin, dihydrosepiapterin or dihydrobiopterin was investigated using extracts from human liver, dihydrofolate reductase and purified sepiapterin reductase from human liver and rat erythrocytes. The incorporation of hydrogen in tetrahydrobiopterin was studied in either 2H2O or in H2O using unlabeled NAD(P)H or (R)-(4-2H)NAD(P)H or (S)-(4-2H)NAD(P)H. Dihydrofolate reductase catalyzed the transfer of the pro-R hydrogen of NAD(P)H during the reduction of 7,8-dihydrobiopterin to tetrahydrobiopterin. Sepiapterin reductase catalyzed the transfer of the pro-S hydrogen of NADPH during the reduction of sepiapterin to 7,8-dihydrobiopterin. In the presence of partially purified human liver extracts one hydrogen from the solvent is introduced at position C(6) and the 4-pro-S hydrogen from NADPH is incorporated at each of the C(1') and C(2') position of BH4. Label from the solvent is also introduced into position C(3'). These results suggest that dihydrofolate reductase is not involved in the biosynthesis of tetrahydrobiopterin from dihydroneopterin triphosphate. They are consistent with the assumption of the occurrence of a 6-pyruvoyl-tetrahydropterin intermediate, which is proposed to be formed upon triphosphate elimination from dihyroneopterin triphosphate, and via an intramolecular redox reaction. Our results suggest that the reduction of 6-pyruvoyl-tetrahydropterin might be catalyzed by sepiapterin reductase.

Atypical phenylketonuria with "dihydrobiopterin synthetase" deficiency: absence of phosphate-eliminating enzyme activity demonstrated in liver

An assay for the phosphate-eliminating enzyme (PEE) activity in liver was developed which required only 5-10 mg tissue. PEE catalyses the elimination of inorganic triphosphate from dihydroneopterin triphosphate, which is the second and irreversible step in the biosynthesis of tetrahydrobiopterin (BH4). In the presence of substrate, magnesium, NADPH, and a sepiapterin reductase fraction from human liver, PEE catalysed the formation of BH4 which was measured by HPLC and electrochemical detection. In adult human liver, a PEE activity of 1.02 +/- 0.134 microU/mg protein (mean +/- 1 SD; n = 5) was observed. In liver needle biopsy material from five patients with defective biopterin biosynthesis, no PEE activity was found (less than 2% and 6% of the control values, respectively). The presence of an endogenous inhibitor was excluded. In a patient who died without definite diagnosis and in a patient with beta-thalassaemia liver PEE activity was increased. Sepiapterin reductase activity was present in all cases. Results indicate that in "dihydrobiopterin synthetase" deficiency, the most frequent of the rare BH4-deficient variants of hyperphenylalaninaemia, the molecular defect consists in a defect of PEE.

Purification and properties of the phosphate eliminating enzyme involved in the biosynthesis of BH4 in man

An enzyme catalyzing the elimination of triphosphate from 7,8-dihydroneopterin triphosphate in the presence of Mg2+ has been purified approx. 3000 fold from human liver. It has a molecular weight of approx. 63'000, a pI value of 4.4 - 4.6 and is stable at 80 degrees C for 5 min. This enzyme catalyzes the formation of tetrahydrobiopterin in the presence of sepiapterin reductase, Mg2+ and NADPH. It is thus possible, that it also catalyzes the internal oxidoreduction leading to formation of the intermediate 6-pyruvoyl-tetrahydropterin, suggesting that no further enzyme is obligatory for biosynthesis of tetrahydrobiopterin.

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.

Biosynthesis of tetrahydrobiopterin in man

The biosynthesis of tetrahydrobiopterin (BH4) from dihydroneopterin triphosphate (NH2P3) was studied in human liver extract. The phosphate-eliminating enzyme (PEE) was purified approximately 750-fold. The conversion of NH2P3 to BH4 was catalyzed by this enzyme in the presence of partially purified sepiapterin reductase. Mg2+ and NADPH. The PEE is heat stable when heated at 80 degrees C for 5 min. It has a molecular weight of 63 000 daltons. One possible intermediate 6-(1'-hydroxy-2'-oxopropyl)5,6,7,8-tetrahydropterin(2'-oxo-tetrahydropte rin) was formed upon incubation of BH4 in the presence of sepiapterin reductase and NADP+ at pH 9.0. Reduction of this compound with NaBD4 yielded monodeutero threo and erythro-BH4, the deuterium was incorporated at the 2' position. This and the UV spectra were consistent with a 2'-oxo-tetrahydropterin structure. Dihydrofolate reductase (DHFR) catalyzed the reduction of BH2 to BH4 and was found to be specific for the pro-R-NADPH side. The sepiapterin reductase catalyzed the transfer of the pro-S hydrogen of NADPH during the reduction of sepiapterin to BH2. In the presence of crude liver extracts the conversion of NH2P3 to BH4 requires NADPH. Two deuterium atoms were incorporated from (4S-2H)NADHP in the 1' and 2' position of the BH4 side chain. Incorporation of one hydrogen from the solvent was found at position C(6). These results are consistent with the occurrence of an intramolecular redox exchange between the pteridine nucleus and the side chain and formation of 6-pyruvoyl-5,6,7,8-tetrahydropterin(tetrahydro-1'-2'-dioxopterin) as intermediate.