The enzymatic conversion of dihydroneopterin triphosphate to tripolyphosphate and 6-pyruvoyl-tetrahydropterin, an intermediate in the biosynthesis of other pterins in Drosophila melanogaster

Biosynthesis of Pteridines in Insects: A Review

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

Evolution of the GST omega gene family in 12 Drosophila species

Gene families provide a unique system to study the evolutionary relationships between related genes both within and between organisms. We can ascertain whether members of a gene family in different species are orthologs or paralogs. We may also search for evidence that may explain why duplicate genes are present. The availability of genome sequences for 12 Drosophila species allows us to address these questions with respect to the evolution of one gene family, the glutathione S transferase (GST) omega class. This gene family is of particular interest because of its relationship with human disease and its presence in a wide range of species.

Identification and characteristics of the structural gene for the Drosophila eye colour mutant sepia, encoding PDA synthase, a member of the omega class glutathione S-transferases

The eye colour mutant sepia (se1) is defective in PDA {6-acetyl-2-amino-3,7,8,9-tetrahydro-4H-pyrimido[4,5-b]-[1,4]diazepin-4-one or pyrimidodiazepine} synthase involved in the conversion of 6-PTP (2-amino-4-oxo-6-pyruvoyl-5,6,7,8-tetrahydropteridine; also known as 6-pyruvoyltetrahydropterin) into PDA, a key intermediate in drosopterin biosynthesis. However, the identity of the gene encoding this enzyme, as well as its molecular properties, have not yet been established. Here, we identify and characterize the gene encoding PDA synthase and show that it is the structural gene for sepia. Based on previously reported information [Wiederrecht, Paton and Brown (1984) J. Biol. Chem. 259, 2195-2200; Wiederrecht and Brown (1984) J. Biol. Chem. 259, 14121-14127; Andres (1945) Drosoph. Inf. Serv. 19, 45; Ingham, Pinchin, Howard and Ish-Horowicz (1985) Genetics 111, 463-486; Howard, Ingham and Rushlow (1988) Genes Dev. 2, 1037-1046], we isolated five candidate genes predicted to encode GSTs (glutathione S-transferases) from the presumed sepia locus (region 66D5 on chromosome 3L). All cloned and expressed candidates exhibited relatively high thiol transferase and dehydroascorbate reductase activities and low activity towards 1-chloro-2,4-dinitrobenzene, characteristic of Omega class GSTs, whereas only CG6781 catalysed the synthesis of PDA in vitro. The molecular mass of recombinant CG6781 was estimated to be 28 kDa by SDS/PAGE and 56 kDa by gel filtration, indicating that it is a homodimer under native conditions. Sequencing of the genomic region spanning CG6781 revealed that the se1 allele has a frameshift mutation from 'AAGAA' to 'GTG' at nt 190-194, and that this generates a premature stop codon. Expression of the CG6781 open reading frame in an se1 background rescued the eye colour defect as well as PDA synthase activity and drosopterins content. The extent of rescue was dependent on the dosage of transgenic CG6781. In conclusion, we have discovered a new catalytic activity for an Omega class GST and that CG6781 is the structural gene for sepia which encodes PDA synthase.

Localization of sepiapterin reductase in pigment cells of Oryzias latipes

Body colors of poikilothermal vertebrates are derived from three distinct types of pigment cells, melanophores, erythro/xanthophores and irido/leucophores. It is well known that melanin in melanophores is synthesized by tyrosinase within a specific organelle termed the melanosome. Although sepiapterin reductase (SPR) is an important enzyme involved in metabolizing biopterin and sepiapterin (a conspicuous pteridine as a coloring pigment in xanthophores) the distribution of SPR has not been shown in pigment cells. An antibody raised in rabbits against rat SPR was used to demonstrate the presence of SPR in pigment cells of Oryzias latipes. This study, which used immunohistochemistry with fluorescence or peroxidase/diaminobenzidine as markers, revealed that SPR could be detected readily in xanthophores, but only faintly in melanophores. These results suggest that sepiapterin is metabolized within xanthophores. Moreover, these experiments show that a protein sharing immunological cross-reactivity with rat SPR is located in teleost O. latipes xanthophores, which is significant considering the relationship of pteridine metabolism between poikilothermal vertebrates and mammals. Further progress in investigations of the roles of pteridines in vertebrates will be promoted by using these fish which can be bred in mass rather easily in the laboratory.

The pteridine pathway in zebrafish: regulation and specification during the determination of neural crest cell-fate

This review describes pteridine biosynthesis and its relation to the differentiation of neural crest derivatives in zebrafish. During the embryonic development of these fish, neural crest precursor cells segregate into neural elements, ectomesenchymal cells and pigment cells; the latter then diversifying into melanophores, iridophores and xanthophores. The differentiation of neural cells, melanophores, and xanthophores is coupled closely with the onset of pteridine synthesis which starts from GTP and is regulated through the control of GTP cyclohydrolase I activity. De novo pteridine synthesis in embryos of this species increases during the first 72-h postfertilization, producing H4biopterin, which serves as a cofactor for neurotransmitter synthesis in neural cells and for tyrosine production in melanophores. Thereafter, sepiapterin (6-lactoyl-7,8-dihydropterin) accumulates as yellow pigment in xanthophores, together with 7-oxobiopterin, isoxanthopterin and 2,4,7-trioxopteridine. Sepiapterin is the key intermediate in the formation of 7-oxopteridines, which depends on the availability of enzymes belonging to the xanthine oxidoreductase family. Expression of the GTP cyclohydrolase I gene (gch) is found in neural cells, in melanoblasts and in early xanthophores (xanthoblasts) of early zebrafish embryos but steeply declines in xanthophores by 42-h postfertilization. The mechanism(s) whereby sepiapterin branches off from the GTP-H4biopterin pathway is currently unknown and will require further study. The surge of interest in zebrafish as a model for vertebrate development and its amenability to genetic manipulation provide powerful tools for analysing the functional commitment of neural crest-derived cells and the regulation of pteridine synthesis in mammals.

Development of the pteridine pathway in the zebrafish, Danio rerio

In the zebrafish, the peripheral neurons and the pigment cells are derived from the neural crest and share the pteridine pathway, which leads either to the cofactor tetrahydrobiopterin or to xanthophore pigments. The components of the pteridine pattern were identified as tetrahydrobiopterin, sepiapterin, 7-oxobiopterin, isoxanthopterin, and 2,4,7-trioxopteridine. The expression of GTP cyclohydrolase I activity during the first 24-h postfertilization, followed by 6-pyruvoyl-5,6,7,8-tetrahydropterin synthase and sepiapterin reductase, suggest an early supply of tetrahydrobiopterin for neurotransmitter synthesis in the neurons and for tyrosine supply in the melanophores. At 48-h postfertilization, sepiapterin formation branches off the de novo pathway of tetrahydrobiopterin synthesis. Sepiapterin, via 7,8-dihydrobiopterin and biopterin, serves as a precursor for the formation of 7-oxobiopterin, which may be further catabolized to isoxanthopterin and 2,4,7-trioxopteridine. Neither 7, 8-dihydrobiopterin nor biopterin is a substrate for xanthine oxidoreductase. In contrast, both of these compounds are oxidized at C-7 by a xanthine oxidase variant form, which is inactivated by KCN, but is insensitive to allopurinol. The oxidase and the dehydrogenase form of xanthine oxidoreductase as well as the xanthine oxidase variant have specific developmental patterns. It follows that GTP cyclohydrolase I, the formation of sepiapterin, and the xanthine oxidoreductase family control the pteridine pathway in the zebrafish.

Biosynthesis of pteridines. NMR studies on the reaction mechanisms of GTP cyclohydrolase I, pyruvoyltetrahydropterin synthase, and sepiapterin reductase

GTP cyclohydrolase I catalyzes a ring expansion affording dihydroneopterin triphosphate from GTP. [1',2',3',4',5'-13C5, 2'-2H1]GTP was prepared enzymatically from [U-13C6]glucose for use as enzyme substrate. Multinuclear NMR experiments showed that the reaction catalyzed by GTP cyclohydrolase I involves the release of a proton from C-2' of GTP that is exchanged with the bulk solvent. Subsequently, a proton is reintroduced stereospecifically from the bulk solvent. This is in line with an Amadori rearrangement mechanism. The proton introduced from solvent occupies the pro-7R position in the enzyme product. The data also confirm that the reaction catalyzed by pyruvoyltetrahydropterin synthase results in the incorporation of solvent protons into positions C-6 and C-3' of the enzyme product. On the other hand, the reaction catalyzed by sepiapterin reductase does not involve any detectable incorporation of solvent protons into tetrahydrobiopterin.

The role of pterins in depression and the effects of antidepressive therapy

Urinary excretion of neopterins (N) and biopterins (B) was measured in 48 patients with depression before and after treatment with placebo, antidepressants, or electroconvulsive therapy (ECT), and in 26 healthy control subjects. Patients prior to and after treatment had a significantly greater neopterin/biopterin (N:B) ratio than control subjects. There was a significant correlation between N:B ratios and the severity of depression and plasma cortisol. As a raised N:B ratio implies failure to convert neopterin to biopterin, it is possible that reduced availability of tetrahydrobiopterin, the essential cofactor for the formation of noradrenaline, serotonin and dopamine, may exert rate-limiting control over the synthesis of monoamines implicated in the pathogenesis of depressive illness.

Sepiapterin reductase and the biosynthesis of tetrahydrobiopterin in Drosophila melanogaster

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

Pterin metabolism in depression: an extension of the amine hypothesis and possible marker of response to ECT

Urinary excretion of neopterins and biopterins was measured in 23 patients with severe depression before and after receiving electroconvulsive therapy (ECT) and 26 healthy control subjects. Patients with psychotic depression and those responding to ECT had neopterin:biopterin (N:B) ratio significantly higher than controls before commencing ECT and positive therapeutic response was associated with reduction of N:B ratio towards control values. As a raised N:B ratio implies failure to convert neopterin to biopterin it is possible that reduced availability of tetrahydrobiopterin, the essential cofactor for the formation of noradrenaline, serotonin and dopamine, may exert rate limiting control over the synthesis of monoamines implicated in the pathogenesis of depressive disorders. The N:B ratio may be a marker for certain depressive subtypes and response to ECT.

Computer studies on the stereostructure and quantum chemical properties of 6-pyruvoyl tetrahydropterin, the key intermediate of tetrahydrobiopterin biosynthesis

The optimized geometry of the conformation of atoms constituting the 6-pyruvoyl tetrahydropterin molecule, the labile key intermediate of tetrahydrobiopterin biosynthesis, was obtained by molecular orbital calculations within the MINDO/3 framework. The stereostructure of the molecule showing the preferred mode for binding to sepiapterin reductase or pyruvoyl tetrahydropterin reductase was drawn in perspective. The resulting structure with the equatorial staggered configuration of the 6-1',2'-dioxopropyl (pyruvoyl) side chain indicated that O(1') and H(6) were located in the trans position around the C(6)-C(1') bond and that the two vicinal carbonyls in the side chain were fixed in the incomplete trans form. The calculation of atomic charges and LUMO coefficients of these carbonyls suggests that the C2'-carbonyl may be more reactive toward NADPH than the C1'-carbonyl in the enzymatic reaction.

Human carbonyl and aldose reductases: new catalytic functions in tetrahydrobiopterin biosynthesis

New catalytic functions of human carbonyl- and aldose reductase in tetrahydrobiopterin biosynthesis are proposed. 6-Pyruvoyl tetrahydropterin, an intermediate in the biosynthesis of tetrahydrobiopterin, was converted to 6-lactoyl tetrahydropterin and 1'-hydroxy-2'-oxopropyl tetrahydropterin by carbonyl reductase under anaerobic condition. 1'-Hydroxy-2'-oxopropyl tetrahydropterin was subsequently metabolized to tetrahydrobiopterin by aldose reductase. Based on these results alternative pathways for the synthesis of tetrahydrobiopterin in patients with genetic defects of sepiapterin reductase are suggested.

Purification and characterization of 6-pyruvoyl-tetrahydropterin synthase from Drosophila melanogaster

The enzyme 6-pyruvoyl-tetrahydropterin synthase (PTP synthase), which catalyzes the conversion of 7,8-dihydroneopterin triphosphate to 6-pyruvoyl tetrahydropterin, has been purified approx. 230-fold to apparent homogeneity from head extracts of Drosophila melanogaster. A partially purified 6-pyruvoyl-tetrahydropterin reductase (PTP reductase) was also prepared and in its presence, along with Mg2+ and NADPH, the purified PTP synthase converted 7,8-dihydroneopterin triphosphate to metastable 6-lactoyltetrahydropterin, which was autoxidized to sepiapterin under aerobic conditions. Purified PTP synthase had a specific activity of 3792 units per mg protein and migrated as a single protein band on both nondenaturing polyacrylamide gel electrophoresis (PAGE) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The purified active enzyme consisted of at least two identical subunits which had a molecular mass of 37.5 kDa on SDS-PAGE and NH2-Asx-Pro- as N-terminal amino acids. The native enzyme in crude extract was shown to be more complex, existing as higher multimeric forms.

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.

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.

Depression and tetrahydrobiopterin: the folate connection

Total biopterin, neopterin and creatinine were measured in spot urine samples from affective disorder patients on lithium therapy and control subjects. Folic acid was also measured in plasma in a sample of the patients. The mean neopterin: biopterin ratio was significantly higher in the 76 patients (3.2 +/- 0.5) than in the 61 controls (1.8 +/- 0.1). In female patients biopterin levels were significantly lower than in controls. In the control groups there was a significant correlation between the molar concentration of neopterin and biopterin. No such correlation was found in the patients. These data indicate that tetrahydrobiopterin (BH4) biosynthesis is reduced in this group. A significant positive correlation was found between plasma folate and urinary biopterin. It is suggested that folate deficiency may impair the synthesis of BH4, a cofactor essential for the synthesis of 5-HT and other monoamines that are involved in the pathogenesis of affective disorders.

Biosynthesis of tetrahydrobiopterin: a sensitive assay of 6-pyruvoyltetrahydropterin synthase using [2'-3H]dihydroneopterin 3'-triphosphate as substrate

Pyruvoyltetrahydropterin synthase catalyzes the release of tritiated water from [2'-3H]dihydroneopterin 3'-triphosphate. The tritiated water formed during the enzymatic reaction is separated from substrate by adsorption of the latter to activated charcoal. The sensitivity and specificity of the assay allows the determination of the enzyme in crude cell extract.

Biosynthesis of biopterin. Studies on the mechanism of 6-pyruvoyltetrahydropteridine synthase

[1'-3H]- and [2'-3H]dihydroneopterin triphosphate (NH2TP) were prepared enzymatically from [4-3H]- and [5-3H]glucose and converted to tetrahydrobiopterin (BH4) by an extract from bovine adrenal medulla. The formation of BH4 from both [1'-3H]- and [2'-3H]-NH2TP proceeds with virtually complete loss of the respective tritium label. The breaking of the CH-bond at C-1' is characterized by a kinetic isotope effect of 2.6 +/- 0.5. A smaller kinetic isotope effect of 1.5 +/- 0.2 was found for the breaking of the CH-bond at C-2'.

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.

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.

On the role of sepiapterin reductase in the biosynthesis of tetrahydrobiopterin

Rat erythrocyte sepiapterin reductase can catalyze the NADPH-dependent reduction of tetrahydropterin substrates with relative velocities of sepiapterin greater than lactoyltetrahydropterin greater than or equal to pyruvoyltetrahydropterin greater than 1'-hydroxy-2'-oxopropyltetrahydropterin; L-erythrotetrahydrobiopterin is the product of the reduction of all three tetrahydropterins. The 1' position of the 1',2'-diketone, pyruvoyltetrahydropterin, is reduced first; the product of this first reduction is 1'-hydroxy-2'-oxopropyltetrahydropterin. Both steps are inhibited by N-acetylserotonin. An antibody to sepiapterin reductase purified from rat erythrocytes was produced in rabbits, and the purified antibody is highly specific for sepiapterin reductase. This antibody is an inhibitor of both sepiapterin reductase activity and tetrahydrobiopterin biosynthesis in crude extracts of rat adrenal and brain. The antibody inhibits the production of both the biosynthetic intermediate, 1'-hydroxy-2'-oxopropyltetrahydropterin, and tetrahydrobiopterin. The results indicate that sepiapterin reductase is on the biosynthetic pathway to tetrahydrobiopterin, and catalyzes the complete reduction of pyruvoyltetrahydropterin to tetrahydrobiopterin. In contrast, homogenates of whole rat adrenal also produce large quantities of lactoyltetrahydropterin which suggests that in some tissues this compound may also be an intermediate in tetrahydrobiopterin biosynthesis. The synthesis of lactoyltetrahydropterin is not inhibited by the antibody to sepiapterin reductase and therefore does not appear to be catalyzed by sepiapterin reductase. However, sepiapterin reductase is responsible for the conversion of lactoyltetrahydropterin to tetrahydrobiopterin. The source of sepiapterin in biosynthetic reactions was found to be oxidative decomposition of lactoyltetrahydropterin.

"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.

Pigment cell differentiation: the relationship between pterin content, allopurinol treatment, and the melanoid gene in axolotls

The effects of allopurinol (an inhibitor of the enzyme xanthine dehydrogenase (XDH] and the melanoid gene on pigment cell differentiation in the axolotl were examined by analyzing pigment components of the xanthophore (pterins). Pterin contents of skin extracts (70% ethanol) from wild type, allopurinol-treated and melanoid axolotls were determined by thin layer chromatography (TLC) and fluorometric scanning of TLC plates. Heights of peaks produced were used as a quantitative measure for pterin content. Results reveal that melanoid animals contain significantly reduced amounts of all seven pterins examined as compared with wild type animals. Allopurinol-treated animals have reduced levels of four pterins (xanthopterin, isoxanthopterin, biopterin and sepiapterin) as compared with the wild type. These findings suggest that the alterations in pterin biosynthetic pathways, either by drug-induced inhibition of XDH activity or by the melanoid gene, produce similar dramatic changes in pigment phenotype which are manifested by alterations in pigment cell differentiation.

Pigment patterns in mutants affecting the biosynthesis of pteridines and xanthommatin in Drosophila melanogaster

Eye-color mutants of Drosophila melanogaster have been analyzed for their pigment content and related metabolites. Xanthommatin and dihydroxanthommatin (pigments causing brown eye color) were measured after selective extraction in acidified butanol. Pteridines (pigments causing red eye color) were quantitated after separation of 28 spots by thin-layer chromatography, most of which are pteridines and a few of which are fluorescent metabolites from the xanthommatin pathway. Pigment patterns have been studied in 45 loci. The pteridine pathway ramifies into two double branches giving rise to isoxanthopterin, "drosopterins," and biopterin as final products. The regulatory relationship among the branches and the metabolic blockage of the mutants are discussed. The Hn locus is proposed to regulate pteridine synthesis in a step between pyruvoyltetrahydropterin and dihydropterin. The results also indicate that the synthesis and accumulation of xanthommatin in the eyes might be related to the synthesis of pteridines.

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.