Synthesis, utilization, and structure of the tetrahydropterin intermediates in the bovine adrenal medullary de novo biosynthesis of tetrahydrobiopterin

Genetic engineering of Escherichia coli for production of tetrahydrobiopterin

Tetrahydrobiopterin (BH4) is an essential cofactor for various enzymes in mammals. In vivo, it is synthesized from GTP via the three-step pathway of GTP cyclohydrolase I (GCHI), 6-pyruvoyl-tetrahydropterin synthase (PTPS) and sepiapterin reductase (SPR). BH4 is a medicine used to treat atypical hyperphenylalaninemia. It is currently synthesized by chemical means, which consists of many steps, and requires costly materials and complicated procedures. To explore an alternative microbial method for BH4 production, we utilized recombinant DNA technology to construct recombinant Escherichia coli (E. coli) strains carrying genes expressing GCHI, PTPS and SPR enzymes. These strains successfully produced BH4, which was detected as dihydrobiopterin and biopterin, oxidation products of BH4. In order to increase BH4 productivity we made further improvements. First, to increase the de novo GTP supply, an 8-azaguanine resistant mutant was isolated and an additional guaBA operon was introduced. Second, to augment the activity of GCHI, the folE gene from E. coli was replaced by the mtrA gene from Bacillus subtilis. These modifications provided us with a strain showing significantly higher productivity, up to 4.0 g of biopterin/L of culture broth. The results suggest the possibility of commercial BH4 production by our method.

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.

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.

2,4-Diamino-6-hydroxypyrimidine, an inhibitor of GTP cyclohydrolase I, suppresses nitric oxide production by chicken macrophages

Biosynthesis of nitric oxide (.NO) from L-arginine by nitric oxide synthase (NOS) represents a major cytotoxic effector function of macrophages. It has been shown that most mammalian NOS requires tetrahydrobiopterin (BH4) as a cofactor and that inhibition of BH4 synthesis results in suppressed .NO production. Chicken L-arginine metabolism differs from that of mammals in that chickens cannot synthesize L-arginine de novo. Therefore, it is important to examine whether chicken macrophage .NO synthesis is also BH4-dependent. 2,4-diamino-6-hydroxypyrimidine (DAHP), a specific inhibitor for GTP cyclohydrolase I (GTP-CH; EC 3.5.4.16), the rate-limiting enzyme in de novo pterin synthesis, was used to block synthesis of BH4. Both chicken peritoneal macrophages (PECs) and the avian MC29 virus-transformed macrophage cell line, HD11, exhibited a dose-dependent reduction in .NO production (measured as nitrite accumulation) relative to DAHP concentration. Authentic BH4 and a substrate for pterin salvage pathway of BH4 synthesis, sepiapterin, were both capable of restoring the production of .NO in DAHP-treated PECs and HD11 macrophages. These results suggest that chicken macrophages require active synthesis of BH4 to produce .NO and that chemicals interfering with BH4 synthesis may result in suppressed .NO production and, hence, .NO-mediated immune function.

Tetrahydrobiopterin turnover in cultured rat sympathetic neurons: developmental profile, pharmacologic sensitivity, and relationship to norepinephrine synthesis

We have examined the turnover of 5,6,7,8-tetrahydrobiopterin (BH4) and the effect of decreasing BH4 levels on in situ tyrosine hydroxylase (TH) activity and norepinephrine (NE) content in a homogeneous population of NE-containing neurons derived from the superior cervical ganglion (SCG) of the neonatal rat and maintained in tissue culture. Initial studies indicated that the level of BH4 within SCG cultures increased fourfold between 5 and 37 days in vitro (DIV). This increase in BH4 levels was determined to result from an increase in the rate of BH4 biosynthesis without a change in the rate of degradation. Regardless of culture age, the BH4 content of SCG neurons was observed to turn over with a half-life of approximately 2.5 h. BH4 synthesis by SCG neurons was found to be five times more sensitive to inhibition by 2,4-diamino-6-hydroxypyrimidine (DAHP) and 25 times less sensitive to inhibition by N-acetylserotonin than was previously reported for CNS neurons in culture. Under basal conditions, the rates of in situ TH activity and BH4 biosynthesis were similar. In response to inhibition of BH4 biosynthesis by DAHP and a 90-95% decrease in BH4 levels, in situ TH activity declined by 75%. NE levels declined by 30% following a 24-h period of inhibition of BH4 synthesis. After 2 days of BH4 synthesis inhibition, the level of NE was decreased by 47%. On treatment days 3 and 4, the decline in NE content plateaued at 24% of control levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of cell-cycle-associated tetrahydrobiopterin synthesis in rat thymocytes

The cell-cycle progression of rat thymocytes from G0 through G1 to DNA synthesis is associated with a transient synthesis of H4biopterin, the concentration of which reaches a maximum at the time of S-phase entry and then decreases. This synthesis of H4biopterin is controlled by the specific activity of GTP cyclohydrolase I, which peaks in G1/S cells. In contrast, the catalytic activity of sepiapterin reductase remains constant throughout the cell-cycle. At G0 the steady state mRNA levels specific for GTP cyclohydrolase I and sepiapterin reductase, respectively, are below the limits of detection. Both accumulate as the thymocytes progress through the cell-cycle but lack cyclic down regulation. The data indicate that the variations in H4biopterin synthesis during the cell-cycle are caused by growth regulated increase in GTP cyclohydrolase I mRNA expression, with subsequent post-translational inactivation. This latter is likely due to the degree of enzyme phosphorylation.

Production of monoclonal antibodies against human 6-pyruvoyl tetrahydropterin synthase and immunocytochemical localization of the enzyme

Monoclonal antibodies were produced against human pituitary gland 6-pyruvoyl tetrahydropterin synthase, one of the key enzymes in the biosynthesis of tetrahydrobiopterin, by in vitro immunization with the antigen directly blotted from SDS-PAGE to polyvinylidene difluoride membranes. The antibodies produced show crossreactivity in the enzyme linked immunosorbent assay, not only with the human 6-pyruvoyl tetrahydropterin synthase but some also with the same enzyme isolated from salmon liver. 6-Pyruvoyl tetrahydropterin synthase was localized immuno-enzymatically in peripheral blood smears and in skin fibroblasts by the use of these monoclonal antibodies and the alkaline phosphatase monoclonal anti-alkaline phosphatase labeling technique.

6-Pyruvoyl tetrahydropterin synthase assay in extracts of cultured human cells using high-performance liquid chromatography with fluorescence detection of biopterin

An assay for 6-pyruvoyl tetrahydropterin synthase, the second enzyme in the conversion of guanosine triphosphate into tetrahydrobiopterin, has been developed. Cell extracts were incubated with enzymatically prepared dihydroneopterin triphosphate (80 microM) in the presence of Mg2+ (12 mM), excess sepiapterin reductase (EC 1.1.1.153) (2 nmol/min) and NADPH (2 mM). 6-Pyruvoyl tetrahydropterin, the product of the reaction, was thus converted into tetrahydrobiopterin. After oxidation of the reduced biopterin derivatives in acidic iodine solution, biopterin was enriched and separated from the abundant neopterin phosphates by solid-phase extraction on a strong cation exchanger. Biopterin was then directly eluted on a reversed-phase liquid chromatographic column and detected fluorimetrically using excitation at 353 nm and emission at 438 nm. The biopterin concentrations formed by the coupled enzyme reaction increased linearly with incubation times up to 90 min. The assay allows the quantification of 6-pyruvoyl tetrahydropterin synthase in cultured human cells.

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.

Immune activation and the anaemia associated with chronic inflammatory disorders

Chronic inflammatory disorders are associated with an increased risk of patients developing anaemia. There is some evidence that cytokines released during cell-mediated immune responses are capable of inhibiting bone marrow haematopoiesis. In vitro, interferon gamma and tumour-necrosis factor alpha inhibit growth of erythroid precursor cells. The mode of action of these cytokines is probably associated with their antiproliferative capacity. Decrease of serum iron and increase of storage iron in patients appears to be a consequence of the defense strategy of macrophages during long-lasting inflammatory disorders. Decreased serum iron correlates to decreased haemoglobin concentrations. In view of this, the development of anaemia seems likely to result from the altered iron metabolism induced by stimulated macrophages. Low haemoglobin levels and associated hypoxia up-regulate the release of erythropoietin, which can explain why increased circulating erythropoietin is usually found in patients with anaemia.

Synthesis of tetrahydrobiopterin in Friend erythroleukemia cells and its modulator effect on cell proliferation

The induction of the enzymes in the tetrahydrobiopterin pathway by dimethyl sulfoxide (DMSO) was investigated in subclones F4N and B8/3 of the proerythroblastoid Friend erythroleukemia cell line (MEL). GTP-cyclohydrolase, the initial enzyme in the biosynthetic pathway, is virtually absent in both clones, but expression increases during 3 days of DMSO treatment. The final enzyme levels show 12-fold (subclone B8/3) and 40-fold (subclone F4N) increases compared to initial values. Enhancement of 6-pyruvoyl tetrahydropterin synthase activity is detectable 6 h after exposure to DMSO and continues to increase in the 3-day time period to 2.4-fold and 1.8-fold levels in subclones B8/3 and F4N, respectively. Sepiapterin reductase is present in unstimulated F4N cells and absent in B8/3 cells. The enzyme activity is not affected by DMSO treatment in either cell line. This explains why DMSO treatment causes accumulation of tetrahydrobiopterin in the MEL subclone F4N, but not in subclone B8/3. MEL cells are devoid of phenylalanine hydroxylase for which tetrahydrobiopterin serves as cofactor. In F4N, but not in B8/3, tetrahydrobiopterin modulates the rate of [3H]thymidine incorporation, thus being functionally linked with cell proliferation rather than with differentiation. In contrast to T lymphocytes, periods of tetrahydrobiopterin synthesis and of modulator function are uncoupled in MEL cells.

Tetrahydrobiopterin synthesis rate and turnover time in neuronal cultures from embryonic rat mesencephalon and hypothalamus

6-(R)-(L-erythro-1',2'-Dihydroxypropyl)-2-amino- 4-hydroxy-5,6,7,8-tetrahydropteridine (tetrahydrobiopterin, BH4) synthesis rate and turnover time were estimated in cultures derived from the embryonic rat mesencephalon (MES) and hypothalamus (HYP) by following the decline in BH4 levels after blockade of BH4 biosynthesis by N-acetylserotonin (NAS) or 2,4-diamino-6-hydroxypyrimidine (DAHP). BH4 content of both culture systems decreased by 75% following an 8-h incubation with maximally effective concentrations of NAS (200 microM) or DAHP (10 mM). Parameters describing BH4 metabolism were calculated from steady-state levels of BH4 and first-order rate constants determined by a nonlinear regression analysis of the exponential BH4 decline. These parameters were confirmed using an alternative procedure that examined the first-order rate of recovery of BH4 following termination of BH4 synthesis inhibition. Steady-state levels of BH4 in HYP cultures (70.3 +/- 9.4 pg/culture) were significantly greater than that for MES (46.5 +/- 2.8 pg/culture). The average fractional rate constants of BH4 loss for MES (0.153 +/- 0.015/h) and HYP (0.159 +/- 0.014/h) were equivalent. The calculated rate of BH4 synthesis was significantly greater for HYP (11.29 +/- 2.13 pg/culture/h) than for MES (7.11 +/- 0.85 pg/culture/h), owing to the greater steady-state concentration of BH4. BH4 turnover time for MES (6.68 +/- 0.67 h) and HYP (6.40 +/- 0.62 h) and half-life for MES (4.63 +/- 0.46 h) and HYP (4.44 +/- 0.43 h) did not differ. The turnover of the cofactor is thus rapid enough that alterations in its synthesis or degradation could acutely modify the rate of monoamine biosynthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Analysis of the tetrahydrobiopterin synthesizing system during maturation of murine reticulocytes

The enzymes of tetrahydrobiopterin synthesis have been studied in murine bone marrow, in spleen, in erythrocytes, and in reticulocytes. Mice with chemically induced and with genetically conditioned reticulocytosis as found in the lactate dehydrogenase deficient strain (Ldh-1c/Ldh-1c) were used for analysis of reticulocytic enzyme activities. The activity of the biopterin synthesizing system is highest in bone marrow even though it amounts to only about 10% as compared with liver. The first enzyme of the biosynthetic pathway, GTP-cyclohydrolase, virtually disappears during the final maturation step of reticulocytes. In contrast, the activities of 6-pyruvoyltetrahydropterin synthase and of sepiapterin reductase of erythrocytes are only reduced to about one half of the reticulocyte level. The absence of biopterin in erythrocytes is therefore caused by the loss of the enzyme that initiates the pterin biosynthetic pathway.

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.

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.

Repetitive recycling of guanosine triphosphate cyclohydrolase I for synthesis of dihydroneopterin triphosphate

A procedure for enzymatic production of dihydroneopterin triphosphate is described that allows GTP cyclohydrolase I to be reused repetitively. The reaction takes place in an ultrafiltration cell, and the product is collected in the filtrate, whereas the enzyme remains in the cell to be reused with additional substrate. This is repeated until the enzyme activity drops below a desirable level. The purity of the dihydroneopterin triphosphate is satisfactory for utilization of this compound for studies on enzymes involved in the synthesis of tetrahydrobiopterin and drosopterin. A procedure for purification of dihydroneopterin triphosphate is described that uses C18-silica and silica cartridges.

Identification of 5,6,7,8-tetrahydropterin and 5,6,7,8-tetrahydrobiopterin in Drosophila melanogaster

Using reversed-phase high-performance liquid chromatography with electrochemical detection we have demonstrated the occurrence of 5,6,7,8-tetrahydropterin and 5,6,7,8-tetrahydrobiopterin in Drosophila melanogaster. The former is the first time that has been detected in vivo. The identification has been based on the retention times, hydrodinamic voltagrams and the differential concentration in three strains of Drosophila melanogaster. Compared to the wild type, the Punch2 mutant has diminished levels of both pteridines, whereas Henna-recessive3 lacks completely tetrahydropterin and has increased levels of tetrahydrobiopterin, as expected according to their biochemical lesions.

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.

On the substrate specificity of bovine liver dihydrofolate reductase: new unconjugated dihydropterin substrates

The substrate specificity of dihydrofolate reductase from cells of different origin has been thought to be quite narrow, and unconjugated dihydropterins such as 6-methyl-dihydropterin are known to be very poor substrates. We have reinvestigated the substrate specificity of several dihydropterins and, in addition, have observed that in a new series of unconjugated dihydropterins of the general structure 6-CH2O(CH2)nCH3 several compounds are excellent substrates for the bovine liver enzyme, but none of them bind as well as dihydrofolate. The substrate activity (apparent Vmax) of these compounds increases from 17 to 110% that of the natural substrate, dihydrofolate, as n is increased from 0 to 3. In contrast, these unconjugated dihydropterins are very poor substrates for the Escherichia coli enzyme.