Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:1.5.1.3 (dihydrofolate reductase)
 5,819 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The ability of the enzyme dihydrofolate reductase to catalyze the formation of tetrahydrobiopterin from dihydrobiopterin was used to develop a method for measuring the activity of this enzyme in vivo. This method can be used to determine the activity of the enzyme in tissues as well as the extent and duration of inhibition of the enzyme by antifolates. Sepiapterin, which is converted to dihydrobiopterin by the enzyme sepiapterin reductase, was as effective a precursor as dihydrobiopterin and has been used in these studies because of its greater stability relative to dihydrobiopterin. Assay conditions must be established for each tissue and enzyme activity can be determined either by measuring the rate of disappearance of dihydrobiopterin or the rate of formation of tetrahydrobiopterin.
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PMID:In vivo measurement of dihydrofolate reductase and its inhibition by antifolates. 318 62

Mammalian cells and tissues were found to have two pathways for the biosynthesis of tetrahydrobiopterin (BH4): (i) the conversion of GTP to BH4 by a methotrexate-insensitive de novo pathway, and (ii) the conversion of sepiapterin to BH4 by a pterin salvage pathway dependent on dihydrofolate reductase (5,6,7,8-tetrahydrofolate: NADP+ oxidoreductase, EC 1.5.1.3) activity. In a Chinese hamster ovary cell mutant lacking dihydrofolate reductase (DUKX-B11), endogenous formation of BH4 proceeds normally but, unlike the parent cells, these cells or extracts of them do not convert sepiapterin or 7,8-dihydrobiopterin to BH4. KB cells, which do not contain detectable levels of GTP cyclohydrolase or BH4 but do contain dihydrofolate reductase, readily convert sepiapterin to BH4 and this conversion is completely prevented by methotrexate. In supernatant fractions of bovine adrenal medulla, the conversion of sepiapterin to BH4 is completely inhibited by methotrexate. Similarly, this conversion in rat brain in vivo is methotrexate-sensitive. Sepiapterin and 7,8-dihydrobiopterin apparently do not enter the de novo pathway of BH4 biosynthesis and may be derived from labile intermediates which have not yet been characterized.
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PMID:Biosynthesis of tetrahydrobiopterin by de novo and salvage pathways in adrenal medulla extracts, mammalian cell cultures, and rat brain in vivo. 657 16

Using Escherichia coli guanosine triphosphate cyclohydrolase, dihydroneopterin triphosphate was synthesized from guanosine triphosphate and was compared with sepiapterin as a substrate for tetrahydrobiopterin formation in bovine adrenal medulla extracts. The dihydrofolate reductase inhibitor, methotrexate, blocks the formation of tetrahydrobiopterin from sepiapterin but not from dihydroneopterin triphosphate. Reduced nicotinamide adenine dinucleotide phosphate and a divalent metal ion are required in partially purified preparations (gel filtration of 40-60% ammonium sulfate fraction on Ultrogel ACA-34) for the biosynthesis of tetrahydrobiopterin from dihydroneopterin triphosphate. Sepiapterin was converted only to dihydrobiopterin in the same fractions since dihydrofolate reductase was removed. The evidence indicates that both dihydroneopterin triphosphate and sepiapterin are good precursors of tetrahydrobiopterin but they are not on the same pathway, contrary to previous proposals.
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PMID:Tetrahydrobiopterin is synthesized by separate pathways from dihydroneopterin triphosphate and from sepiapterin in adrenal medulla preparations. 663 80

The synthesis of nitric oxide by proximal tubule-inducible nitric oxide synthase requires tetrahydrobiopterin as a cofactor. To determine whether tetrahydrobiopterin synthesis is required for nitric oxide production, nitrite release by mouse proximal tubule cells treated with 2,4-diamino-6-hydroxypyrimidine, an inhibitor of the rate-limiting enzyme in the de novo synthesis of tetrahydrobiopterin from guanosine triphosphate, guanosine triphosphate cyclohydrolase I, was measured. Treatment with lipopolysaccharide (0.1 micrograms/mL) and interferon-gamma (100 U/mL) for 12 h increased nitrite production from 2.7 +/- 0.2 to 25.4 +/- 1.3 nmol/mg of protein (P < 0.001; N = 9). 2,4-Diamino-6-hydroxypyrimidine (6 mM) reduced lipopolysaccharide/interferon-gamma-induced nitrite production by 53.1 +/- 3.4%. Sepiapterin, a substrate for tetrahydrobiopterin synthesis via the dihydrofolate reductase-dependent pterin salvage pathway, prevented the inhibition by 2,4-diamino-6-hydroxypyrimidine, an effect that was blocked by methotrexate. In conclusion, guanosine triphosphate cyclohydrolase I activity is required for cytokine-induced nitric oxide production by proximal tubular epithelium. The inhibition of guanosine triphosphate cyclohydrolase I could prove useful in the treatment of nitric oxide-mediated renal disorders.
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PMID:Guanosine triphosphate cyclohydrolase I regulates nitric oxide synthesis in renal proximal tubules. 775 97

(6R)-5,6,7,8-Tetrahydrobiopterin (BH4), which is synthesized intracellularly from GTP, caused a concentration-dependent increase in rat pheochromocytoma (PC12) cell proliferation when added exogenously. Incubation with sepiapterin, which is converted enzymatically to BH4 within cells, also increased PC12 cell proliferation and BH4 levels concomitantly. These sepiapterin effects were mediated by BH4 as inhibition of sepiapterin conversion to BH4 by a sepiapterin reductase inhibitor, N-acetyl-serotonin, blocked the increase in proliferation and the elevation of BH4 levels. 7,8-Dihydrobiopterin (BH2) also increased BH4 levels and PC12 cell proliferation, both of which were reversed by methotrexate, which blocks the conversion of BH2 to BH4 by dihydrofolate reductase. The BH4-induced increase in PC12 cell proliferation was not related to elevated catecholamine or nitric oxide synthesis as inhibitors of tyrosine hydroxylase or nitric oxide synthase did not reduce the BH4 effect. BH4 and its precursors did not alter intracellular cAMP levels, suggesting that this second messenger is not involved in the enhancement of PC12 cell proliferation by BH4. Sepiapterin and BH4 also enhanced the proliferation of SV40-transformed human fibroblasts and rat C6 glioma cells, indicating that the stimulatory effect of BH4 on cell proliferation is not restricted to PC12 cells.
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PMID:Mitogenic effects of tetrahydrobiopterin in PC12 cells. 856

The authors evaluated whether supplementation of tetrahydrobiopterin (BH4) or the BH4 precursor in vascular smooth muscle cells (VSMC) that are deficient in de novo BH4 production affected the ability of these cells to synthesize nitric oxide (NO). GTP cyclohydrolase I (GTPCH) is the rate-limiting enzyme for the de novo synthetic pathway of BH4. The selective GTPCH inhibitor 2,4-diamino-6-hydroxypyrimidine (DAHP) restricts the de novo synthesis of BH4 in VSMC. Thus, treatment with DAHP and cytokines should lead to the intracellular accumulation of inactive inducible nitric oxide synthase (iNOS) apoenzyme (relative to BH4), which can be reactivated upon the repletion of BH4. Using such VSMC deficient in de novo BH4, the authors evaluated their ability to synthesize NO by supplementing with BH4 or the BH4 precursor sepiapterin or dihydrobiopterin. Adding BH4 to VSMC increased NO production in a concentration-dependent manner. Sepiapterin (SEP) or 7,8-dihydrobiopterin (BH2) also dose-dependently induced NO generation. Nitric oxide was produced in the order SEP >BH2>> BH4 at half-maximal concentrations for stimulation of 0.05, 0.1, and 1 micromol/L, respectively. Addition of BH4 or SEP substantially induced iNOS enzyme activity, which was quantified as the formation of [3H]L-citrulline from [3H]L-arginine. SEP or BH2 in the presence of methotrexate, a selective inhibitor of dihydrofolate reductase, no longer induced NO production. In contrast, although the amount of NO induced by supplemented BH4 was substantially depressed, higher concentrations of BH4, but not SEP or BH2, significantly caused NO production even in the presence of methotrexate. Thus, BH4 produced from BH2 is largely responsible for NO production in VSMC that are deficient in de novo BH4. NOS would preferentially use BH4 that is regenerated from BH2 via dihydrofolate reductase, rather than BH4 or BH2 obtained from the cytosolic milieu.
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PMID:Exogenous biopterins requirement for iNOS function in vascular smooth muscle cells. 1288 22