Pteridine cofactors

The role of phenylalanine hydroxylase in lipogenesis in the oleaginous fungus Mortierella alpina

Phenylalanine hydroxylase (PAH) catalyses the irreversible hydroxylation of phenylalanine to tyrosine, which is the rate-limiting reaction in phenylalanine metabolism in animals. A variety of polyunsaturated fatty acids can be synthesized by the lipid-producing fungus Mortierella alpina, which has a wide range of industrial applications in the production of arachidonic acid. In this study, RNA interference (RNAi) with the gene PAH was used to explore the role of phenylalanine hydroxylation in lipid biosynthesis in M. alpina. Our results indicated that PAH knockdown decreased the PAH transcript level by approximately 55% and attenuated cellular fatty acid biosynthesis. Furthermore, the level of NADPH, which is a critical reducing agent and the limiting factor in lipogenesis, was decreased in response to PAH RNAi, in addition to the downregulated transcription of other genes involved in NADPH production. Our study indicates that PAH is part of an overall enzymatic and regulatory mechanism supplying NADPH required for lipogenesis in M. alpina.

Tetrahydrobiopterin Plays a Functionally Significant Role in Lipogenesis in the Oleaginous Fungus Mortierella alpina

Tetrahydrobiopterin (BH₄) is well-known as a cofactor of phenylalanine hydroxylase (PAH) and nitric oxide synthase (NOS), but its exact role in lipogenesis is unclear. In this study, the GTP cyclohydrolase I (GTPCH) gene was overexpressed to investigate the role of BH₄ in lipogenesis in oleaginous fungus Mortierella alpina. Transcriptome data analysis reveal that GTPCH expression was upregulated when nitrogen was exhausted, resulting in lipid accumulation. Significant changes were also found in the fatty acid profile of M. alpina grown on medium that contained a GTPCH inhibitor relative to that of M. alpina grown on medium that lacked the inhibitor. GTPCH overexpression in M. alpina (the MA-GTPCH strain) led to a sevenfold increase in BH₄ levels and enhanced cell fatty acid synthesis and poly-unsaturation. Increased levels of nicotinamide adenine dinucleotide phosphate (NADPH) and upregulated expression of NADPH-producing genes in response to enhanced BH₄ levels were also observed, which indicate a novel aspect of the NADPH regulatory mechanism. Increased BH₄ levels also enhanced phenylalanine hydroxylation and nitric oxide synthesis, and the addition of an NOS or a PAH inhibitor in the MA-GTPCH and control strain cultures decreased fatty acid accumulation, NADPH production, and the transcript levels of NADPH-producing genes. Our research suggests an important role of BH₄ in lipogenesis and that the phenylalanine catabolism and arginine-nitric oxide pathways play an integrating role in translating the effects of BH₄ on lipogenesis by regulating the cellular NADPH pool. Thus, our findings provide novel insights into the mechanisms of efficient lipid biosynthesis regulation in oleaginous microorganisms and lay a foundation for the genetic engineering of these organisms to optimize their dietary fat yield.

Functional Characterization of a Condensation Domain That Links Nonribosomal Peptide and Pteridine Biosynthetic Machineries in Photorhabdus luminescens

Nonribosomal peptide synthetases (NRPSs) produce a wide variety of biologically important small molecules. NRPSs can interface with other enzymes to form hybrid biosynthetic systems that expand the structural and functional diversity of their products. The pepteridines are metabolites encoded by an unprecedented pteridine-NRPS-type hybrid biosynthetic gene cluster in Photorhabdus luminescens, but how the distinct enzymatic systems interface to produce these molecules has not been examined at the biochemical level. By an unknown mechanism, the genetic locus can also affect the regulation of other enzymes involved in autoinducer and secondary metabolite biosynthesis. Here, through in vitro protein biochemical assays, we demonstrate that an atypical NRPS condensation (C) domain present in the pathway condenses acyl units derived from α-keto acids onto a free 5,6,7,8-tetrahydropterin core, producing the tertiary cis-amide-containing pepteridines. Solution studies of the chemically synthesized molecules led to the same amide regiochemistries that were observed in the natural products. The biochemical transformations mediated by the C domain destroy the radical scavenging activity of its redox active tetrahydropterin substrate. Secondary metabolite analyses revealed that the pepteridine locus affects select metabolic pathways associated with quorum sensing, antibiosis, and symbiosis. Taken together, the results suggest that the pathway likely regulates cellular redox and specialized metabolic pathways through engagement with the citric acid cycle.

Role of dihydrofolate reductase in tetrahydrobiopterin biosynthesis and lipid metabolism in the oleaginous fungus Mortierella alpina

Mortierella alpina is a well-known polyunsaturated fatty acid-producing oleaginous fungus. Analysis of the Mort. alpina genome suggests that there is a putative dihydrofolate reductase (DHFR) gene playing a role in the salvage pathway of tetrahydrobiopterin (BH4), which has never been explored in fungi before. DHFR is the sole source of tetrahydrofolate and plays a key role in maintaining BH4 levels. Transcriptome data analysis revealed that DHFR was up-regulated by nitrogen exhaustion, when Mort. alpina starts to accumulate lipids. Significant changes were found in the fatty acid profile in Mort. alpina grown on medium containing DHFR inhibitors compared to Mort. alpina grown on medium without inhibitors. To explore the role of DHFR in folate/BH4 metabolism and its relationship to lipid biosynthesis, we expressed heterologously the gene encoding DHFR from Mort. alpina in Escherichia coli and we purified the recombinant enzyme to homogeneity. The enzymatic activity was investigated by liquid chromatography and MS and VIS-UV spectroscopy. The kinetic parameters and the effects of temperature, pH, metal ions and inhibitors on the activity of DHFR were also investigated. The transcript level of cytosolic NADPH-producing gene involved in folate metabolism is down-regulated by DHFR inhibitors, which highlights the functional significance of DHFR in lipid biosynthesis. The relationship between DHFR and lipid metabolism is thus of major importance, and folate metabolism may be an alternative NADPH source in fatty acid synthesis. To our knowledge, this study is the first to report the comprehensive characterization of a BH4salvage pathway in a fungus.

Role of the phenylalanine-hydroxylating system in aromatic substance degradation and lipid metabolism in the oleaginous fungus Mortierella alpina

Mortierella alpina is a filamentous fungus commonly found in soil that is able to produce lipids in the form of triacylglycerols that account for up to 50% of its dry weight. Analysis of the M. alpina genome suggests that there is a phenylalanine-hydroxylating system for the catabolism of phenylalanine, which has never been found in fungi before. We characterized the phenylalanine-hydroxylating system in M. alpina to explore its role in phenylalanine metabolism and its relationship to lipid biosynthesis. Significant changes were found in the profile of fatty acids in M. alpina grown on medium containing an inhibitor of the phenylalanine-hydroxylating system compared to M. alpina grown on medium without inhibitor. Genes encoding enzymes involved in the phenylalanine-hydroxylating system (phenylalanine hydroxylase [PAH], pterin-4α-carbinolamine dehydratase, and dihydropteridine reductase) were expressed heterologously in Escherichia coli, and the resulting proteins were purified to homogeneity. Their enzymatic activity was investigated by high-performance liquid chromatography (HPLC) or visible (Vis)-UV spectroscopy. Two functional PAH enzymes were observed, encoded by distinct gene copies. A novel role for tetrahydrobiopterin in fungi as a cofactor for PAH, which is similar to its function in higher life forms, is suggested. This study establishes a novel scheme for the fungal degradation of an aromatic substance (phenylalanine) and suggests that the phenylalanine-hydroxylating system is functionally significant in lipid metabolism.

Neurotoxin-induced impairment of biopterin synthesis and function: Initial stage of a Parkinson-like dopamine deficiency syndrome

Disorders of the function of the tyrosine hydroxylase play an important role in the occurrence of the Parkinson syndrome. The enzyme that catalyses the first, rate-limiting step in the biosynthesis to dopamine requires the cofactor tetrahydrobiopterin. This compound supplies the reduction equivalent for activation of molecular oxygen. Binding of the cofactor to the enzyme is affected by phosphorylation or dephosphorylation of the enzyme protein and, thereby, influences the activity. Nerve and chromaffin cells that synthesize dopamine, noradrenaline and serotonin are able to synthesize the cofactor tetrahydrobiopterin de novo from guanosine-triphosphate as a precursor. In patients suffering from Parkinson's disease a remarkable decrease in biopterin content was found in the brain. The function of the dopaminergic system was studied with an experimental Parkinson model. The antimetabolite 6-aminonicotinamide induces a dopamine deficit in the striatum with a significant slowdown in the utilization of this transmitter. The abolition of the 6-aminonicotinamide-induced muscular rigidity by l-DOPA and dopamine agonists implies that the antimetabolite produces a Parkinson-like syndrome in rats. There are reports on the molecular basis of this effect which are also important for understanding possible disturbances of the synthesis of biopterins. The effector 6-aminonicotinamide-adenine-dinucleotide-phosphate (6-ANADP), which blocks the pentose phosphate pathway, is formed by an enzymatic neurotoxic synthesis. The clonal cell line PC-12 was used to study the molecular basis of the disturbances occurring in the dopaminergic system. These cells contain all the enzymes for catecholamine synthesis, including those for the synthesis of the cofactor tetrahydrobiopterin. Addition of 6-aminonicotinamide to the culture medium resulted in the synthesis of the neurotoxic agent, 6-ANADP, by a glycohydrolase localized in the endoplasmic reticulum. The synthesis of biopterin was depressed after application of 6-aminonicotinamide. The decrease of intracellular tetrahydrobiopterin and total biopterin resulted in reduced DOPA production. The decreased content of biopterin cofactor synthesis was compensated for by the addition of the precursor sepiapterin, indicating that the NADPH-dependent reductases in biopterin synthesis were not inhibited by the antimetabolic nucleotide 6-ANADP. DOPA production was not fully normalized by sepiapterin. Addition of NADH to the medium resulted in a further increase of DOPA production, probably by activation of the recycling pathway. The first step in the synthesis of biopterin from GTP to 7,8-neopterin-triphosphate seems to be particularly sensitive to the action of exogenous neurotoxins. A further sensitive site of action in synthesis to the cofactor BH(4) concerns the function of the dihydropteridin-reductase, which recycles qBH(2) to BH(4). Neurotoxin-induced impairment of biopterin synthesis is probably a pathogenetically important disorder at the initial stage of Parkinson's disease.

Biochemical characterization of the tetrahydrobiopterin synthesis pathway in the oleaginous fungus Mortierella alpina

We characterized the de novo biosynthetic pathway of tetrahydrobiopterin (BH₄) in the lipid-producing fungus Mortierella alpina. The BH₄ cofactor is essential for various cell processes, and is probably present in every cell or tissue of higher organisms. Genes encoding two copies of GTP cyclohydrolase I (GTPCH-1 and GTPCH-2) for the conversion of GTP to dihydroneopterin triphosphate (H₂-NTP), 6-pyruvoyltetrahydropterin synthase (PTPS) for the conversion of H₂-NTP to 6-pyruvoyltetrahydropterin (PPH₄), and sepiapterin reductase (SR) for the conversion of PPH₄ to BH₄, were expressed heterologously in Escherichia coli. The recombinant enzymes were produced as His-tagged fusion proteins and were purified to homogeneity to investigate their enzymic activities. Enzyme products were analysed by HPLC and electrospray ionization-MS. Kinetic parameters and other properties of GTPCH, PTPS and SR were investigated. Physiological roles of BH₄ in M. alpina are discussed, and comparative analyses between GTPCH, PTPS and SR proteins and other homologous proteins were performed. The presence of two functional GTPCH enzymes has, as far as we are aware, not been reported previously, reflecting the unique ability of this fungus to synthesize both BH₄ and folate, using the GTPCH product as a common substrate. To our knowledge, this study is the first to report the comprehensive characterization of a BH₄ biosynthesis pathway in a fungus.

Pteridines as inhibitors of xanthine oxidase: structural requirements

Different pteridine derivatives were investigated for their inhibitory action on xanthine oxidase. From 27 investigated compounds, 13 showed concentration-dependent inhibition of the enzyme. Concentrations necessary for 50% inhibition ranged from <0.1 up to >100 microM. Different types of inhibition were found concerning xanthine and pterin as substrates: competitive, noncompetitive and mixed type. Out of 18 aromatic compounds tested, 12 were inhibitors. Only one out of nine reduced derivatives served as inhibitor. A simple regression model was used to specify the structural requirements for a pteridine to be an inhibitor. The most characteristic features of an inhibitor are aromaticity and no substitution at position 7 of the pteridine ring.

GTP cyclohydrolase I mRNA induction and tetrahydrobiopterin synthesis in human endothelial cells

The key role of tetrahydrobiopterin (BH4) in the synthesis of nitric oxide by human umbilical vein endothelial cells (HUVEC) has been demonstrated. We characterized the induction of BH4 synthesis in a cell line (ECV) derived from HUVEC and primary HUVEC. A significant induction of guanosine triphosphate cyclohydrolase I (GTPCH) mRNA was observed in response to TNF, IL-1beta, and IFNgamma in ECV and HUVEC. The induction of GTPCH mRNA was abolished by actinomycin D. The cytokines led to an increased accumulation of BH4 in ECV. This effect was prevented by 2,4-diamino-6-hydroxypyrimidine, a selective inhibitor of GTPCH, as well as by actinomycin D and by cycloheximide. Results provide evidence for an increase in GTPCH activity and in BH4 levels in response to immunostimulants in human endothelial cells.

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.

Biosynthesis of tetrahydrofolate. Sequence of GTP cyclohydrolase I from Escherichia coli

The sequence of the gene coding for GTP cyclohydrolase I of Escherichia coli and of the adjacent regions was determined. The open reading frame contains 669 nucleotides. The deduced amino-acid sequence represents a protein consisting of 223 amino-acid residues with a molecular mass of 24,873 Da. Partial amino-acid sequences of the N-terminal region and of 5 peptides obtained by trypsin and BrCN cleavage were determined by Edman degradation and were in full agreement with the sequence deduced from the nucleotide sequence. The starting methionine is removed by posttranslational modification. The protein shows extensive homology to the recently reported GTP cyclohydrolase from rats.

Inhibition of monocyte luminol-dependent chemiluminescence by tetrahydrobiopterin, and the free radical oxidation of tetrahydrobiopterin, dihydrobiopterin and dihydroneopterin

Luminol-dependent chemiluminescence of normal human monocytes activated by zymosan is demonstrated to be inhibited by tetrahydrobiopterin in a concentration-dependent manner. The reduced pterins tetrahydrobiopterin, dihydrobiopterin, and dihydroneopterin are all shown to be readily oxidized by the hydroxyl radical. The susceptibility of reduced pterins to free radical attack may explain the inhibition of chemiluminescence observed and an additional role of reduced pterins as free radical scavengers in tissues is considered.

Interferon-gamma for the treatment of metastatic renal cancer: dose-dependent stimulation and downregulation of beta-2 microglobulin and neopterin responses

Considering rIFN-gamma as a potent biological response modifier (BRM), we started an open phase II trial with rIFN-gamma in patients with advanced renal cell carcinoma (RCC). For optimization of the dose and schedule of rIFN-gamma, two biochemical serum markers, neopterin and beta-2 microglobulin, were chosen to monitor the biological response. In order to test the magnitude and kinetics of rIFN-gamma-induced neopterin and beta-2 microglobulin release in the serum, rIFN-gamma was administered thrice at three different dose levels in a randomly assigned order (0.01; 0.1; 0.5 mg). Neopterin and beta-2 microglobulin were assessed by means of commercially available radioimmunoassays. The results revealed: 1) strong, reproducible and dose-dependent increments of both markers after the first injection 2) downregulation of the magnitude of neopterin responses with repeated injections at each of the three dose levels tested, and 3) a dose-dependent downregulation of the magnitude of beta-2 microglobulin responses and of serum baseline values at the highest dose level tested. From these data, we conclude that both the dose and schedule might be of importance for optimization of biological responses to exogenously applied rIFN-gamma.

Association between the behavioral and neurochemical effects of amphetamine: hemispheric asymmetry study

Effects of amphetamine on concentrations of dopamine, norepinephrine and serotonin in several monoamine-containing cell body and terminal regions were examined left and right separately in rats. Results suggest that amphetamine reduced the L-R asymmetry of most of these measures, and this effect is more significant in the cell body than in the terminal regions. Behaviorally, amphetamine also decreased L-R asymmetry of the spontaneous turning behavior in rats and this latter effect is most closely associated with the reductions of dopamine and norepinephrine asymmetries in the substantia nigra and reduction of norepinephrine asymmetry in the locus coeruleus.

Dihydropteridine reductase in schizophrenic patients

The activity of the enzyme dihydropteridine reductase (DHPR) has been recently found to be one of the factors controlling the rate of synthesis of dopamine, norepinephrine, and serotonin, thought to be involved in the etiology of schizophrenia. Several lines of evidence suggest that peripheral and brain DHPR enzymes may be identical. In addition, peripheral DHPR activity has been hypothesized to be important in determining the level of phenylethylamine, a putative psychotogen that is produced peripherally and crosses the blood-brain barrier. Since DHPR activity has never been investigated in schizophrenic patients, we measured the whole blood activity in 20 schizophrenic patients and 20 matched controls. There was no difference between the groups in DHPR activity.

Application of gas chromatography-mass spectrometry to the study of biopterin metabolism in man. Detection of biolumazine and 2'-deoxysepialumazine

The separation characteristics of the trimethylsilyl ether derivatives of various naturally occurring and synthetic pteridines on a apolar glass capillary column, together with their mass spectra, permit their identification and quantitation in biological samples. Examples are given of the determination of the ratio of monapterin to neopterin in urine, of monitoring excreted pterin metabolites after a loading test with 6-methyltetrahydropterin in urine and of structure elucidation of lumazines , previously unknown in man. 6- Methylisoxanthopterin was shown to be the main metabolite in urine after administration of 6-methyl-5,6,7,8-tetrahydropterin. Biolumazine and 2'- deoxysepialumazine were found in human faeces after administration of ( 6R ,S)-5,6,7,8-tetrahydro-L-erythro-biopterin.

Measurement of urinary neopterin in normal pregnant and non-pregnant women and in women with benign and malignant genital tract neoplasms

Urinary neopterin was measured in healthy women (n = 209) and men (n = 208), in patients with benign gynecological tumors (n = 53), in women with precancerous lesions of the cervix and the endometrium (n = 24) and in women with cancer of the genital tract (n = 108). In addition urinary neopterin measurements were made in 109 pregnant women and 20 women in the puerperium. No significant difference was found between mean neopterin values in patients with benign gynecological tumors, in women with precancerous lesions and in healthy women. Patients with cancer had significantly higher mean urinary neopterin levels than the control group. Raised neopterin levels were found in 56% of patients with genital tract cancer, the figures varying between 93% for ovarian cancer and 47% for cancer of the cervix. Some of the cancer patients had serial urinary neopterin measurements and in about 80% there was some relation between urinary neopterin values and clinical progress as judged clinically and radiologically, the best agreement existing in patients with ovarian cancer. Significantly higher mean neopterin values were found during normal pregnancy and in the early puerperium than in non-pregnant healthy controls. Raised urinary neopterin excretion may be due to enhanced cell proliferation and alloantigenic activation of T-lymphocytes.

Urinary neopterine excretion in patient with genital cancer

Urinary neopterine values in 96 healthy women were compared with those in a group of 63 patients with cancer of the genitals who had not yet undergone treatment. Significantly higher median neopterine levels were found in the patients with carcinoma (p less than 0.001). In the 63 cancer patients who had not yet been treated, the diagnosis was made correctly in 65% of the cases by monitoring neopterine levels. In the follow-up of 72 patients with cancer of the genitals in 74% the neopterine values corresponded with the clinico-roentgenological findings. In 22%, false positive neopterine values (no recurrence of the tumor), and in 4% false negative values (recurrence, or progression of the carcinoma) were observed. These results confirm previous reports concerning the significance of pteridine excretion in patients with cancer. However, more investigations have to be performed to establish the value of neopterine sampling in monitoring patients with malignant tumors.

Biopterin cofactor biosynthesis: independent regulation of GTP cyclohydrolase in adrenal medulla and cortex

Guanosine triphosphate cyclohydrolase, the enzyme that is apparently rate-limiting in biopterin biosynthesis, is increased in adrenal cortex and medulla of rats treated with insulin or reserpine. Denervation and hypophysectomy block the increase in medullary and cortical enzyme activity, respectively, whereas cycloheximide presents the increase in both tissues. These results provide evidence for induction and regulation of guanosine triphosphate cyclohydrolase.

Evolution of the coenzymes

The biosyntheses of the coenzymes exhibit various characteristics consistent with the idea that coenzymes evolved from very simple beginning through a succession of symbiotic unions.

Partial purification and some properties of biopterin synthase and dihydropterin oxidase from Drosophila melanogaster

An enzyme which has been named "biopterin synthase" has been discovered in Drosophila melanogaster. This enzyme, which has been purified 200-fold from extracts of Drosophila, catalyzes the conversion of sepiapterin to dihydrobiopterin, or oxidized sepiapterin to biopterin. The Km values for the two substrates are 63 microM for sepiapterin and 10 microM for oxidized sepiapterin. NADPH is required in this enzymatic reaction. An analysis of enzyme activity during development in Drosophila indicates a correlation between enzyme activity and biopterin content at various development stages. Another enzyme, called "dihyropterin oxidase," was also discovered and partially purified. This enzyme catalyzes the oxidation of dihydropterin compounds to the corresponding pterin compounds. For example, sepiapterin (a dihydroterin) is oxidized to oxidized sepiapterin in the presence of this enzyme. The only dihydropterin that has been tested that is not a substrate for this enzyme is dihydroneopterin triphosphate, the compound thought to be a precursor for all naturally occurring pterins and dihydropterins. Since the action of dihydropterin oxidase is reduced significantly when the concentration of oxygen is very low, it is likely that this enzyme uses molecular oxygen as the oxidizing agent during the oxidation of dihydropterins. Neither NAD+ or NADP+ is required. In the presence of the two enzymes dihydropterin oxidase and biopterin synthase, sepiapterin is converted to biopterin. However, in the presence of biopterin synthase alone, sepiapterin is converted to dihydrobiopterin.

Purification and properties of the enzymes from Drosophila melanogaster that catalyze the synthesis of sepiapterin from dihydroneopterin triphosphate

Sepiapterin synthase, the enzyme system responsible for the synthesis of sepiapterin from dihydroneopterin triphosphate, has been partially purified from extracts of the heads of young adult fruit flies (Drosophila melanogaster). The sepiapterin synthase system consists of two components, termed "enzyme A" (MW 82,000) and "enzyme B" (MW 36,000). Some of the properties of the enzyme system are as follows: NADPH and a divalent cation, supplied most effectively as MgCl2, are required for activity; optimal activity occurs are pH 7.4 and 30 C; the Km for dihydroneopterin triphosphate is 10 microM; and a number of unconjugated pterins, including biopterin and sepiapterin, are inhibitory. Dihydroneopterin cannot be used as substrate in place of dihydroneopterin triphosphate. Evidence is presented in support of a proposed reaction mechanism for the enzymatic conversion of dihydroneopterin triphosphate to sepiapterin in which enzyme A catalyzes the production of a labile intermediate by nonhydrolytic elimination of the phosphates of dihydroneopterin triphosphate, and enzyme B catalyzes the conversion of this intermediate, in the presence of NADPH, to sepiapterin. An analysis of the activity of sepiapterin synthase during development in Drosophila revealed the presence of a small amount of activity in eggs and young larvae and a much larger amount in late pupae and young adults. Sepiapterin synthase activity during development corresponds with the appearance of sepiapterin in the flies. Of a variety of eye color mutants of Drosophila melanogaster tested for sepiapterin synthase activity, only purple (pr) flies contained activity that was significantly lower than that found in the wild-type flies (22% of the wild-type activity). Further studies indicated that the amount of enzyme A activity is low in purple flies, whereas the amount of enzyme B activity is equal to that present in wild-type flies.

Enhanced urinary excretion of 7,8-dihydro-6-hydroxylumazine by mice bearing the Ehrlich ascites tumour

A fluorescent urinary metabolite, excreted by mice carrying the Ehrlich ascites tumour (EAT), has been identified as 7,8-dihydro-6-hydroxylumazine. This finding agrees with other data which show that a disorder in pteridine metabolism is commonly associated with the presence of malignant growth.

Isolation and characterization of dihydropteridine reductase from Pseudomonas species

Dihydropteridine reductase isolated from the bacterium Pseudomonas species (ATCC 11299a) has been purified approximately 450-fold byammonium sulfate precipitation and diethylaminoethyl-cellulose chromatographic procedures. The preparation is at least 80% pure as judged by polyacrylamide gels. Its molecular weight was determined to be about 44,000. Both dihydropteridine reductase and phenylalanine hydroxylase activities were found to be higher in cells adapted to a medium containing L-phenylalanine or L-tyrosine as the sole carbon source than in those grown in L-asparagine. The substrate of the reductase is quinonoid dihydropteridine, and the product is tentatively identified as a tetrahydropteridine through its ability to serve as a cofactor for phenylalanine hydroxylase. The enzyme shows no marked specificity for the pteridine cofactor that occurs naturally in this organism, L-threo-neopterin. The pH optimum for the reductase is 7.2, and nicotinamide adenine dinucleotide, reduced form, is the preferred cosubstrate. Inhibition of the reduced and untreated enzyme by several sulfhydryl reagents was observed. A metal requirement for the reductase could not be demonstrated. Dihydropteridine reductase was found to be inhibited by aminopterin in a competitive manner with respect to the quinonoid dihydro form of 2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetrahydropteridine.

Quantitative determination of unconjugated pterins in urine by gas chromatography/mass fragmentography

A gas chromatographic/mass fragmentographic method is described which permits the determination of unconjugated pterins in urine. After the addition of 6,7-dimethylpterin as an internal standard, the acidified urine samples are purified by liquid chromatography on Dowex-50 and Dowex-1 columns. The pterins are then converted to their corresponding trimethylsilyl derivatives and the base peaks of biopterin (m/e 409), neopterin (m/e 409) and 6,7-dimethylpterin (m/e 320) are determined. The method is sensitive and specific and permits the processing of large numbers of samples. By means of this method, the urinary excretion of biopterin and neopterin from 9 healthy subjects has been determined.

Phenylketonuria due to a deficiency of dihydropteridine reductase

The onset of neurologic symptoms in a child who had markedly elevated blood phenylalanine levels during the first two weeks of life and who was promptly treated with a low phenylalanine diet, with excellent control of serum phenylalanine levels, suggested that this child had an unusual form of phenylketonuria. In assays of the components of the phenylalanine hydroxylating system (open liver biopsy at 14 months), the activity of phenylalanine hydroxylase was 20 per cent of the average normal adult value. By contrast, no dihydropteridine reductase activity was detected in the patient's liver, brain or cultured skin fibroblasts. Since dihydropteridine reductase is also essential for the biosynthesis of dopamine, norepinephrine, and serotonin, disturbed neurotransmitter function may be responsible for the patient's neurologic deterioration. On the basis of these results, assay of reductase in cultured skin fibroblasts may be advisable in the initial diagnosis of phenylketonuria.