Biopterin derivatives in normal and phenylketonuric patients after oral loads of L-phenylalanine, L-tyrosine, and L-tryptophan.

Leeming, R. J.; Blair, J. Anthony; Green, A; Raine, D. N.

doi:10.1136/adc.51.10.771

Cited by 75 publications

(48 citation statements)

References 16 publications

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“…However, unlike 2 other patients who were apparently affected by the same phenotype but in whom biopterin derivatives were not depressed, this child had a very low biopterin level in his plasma and urine with only a small increase after an oral phenylalanine load. Indeed in normal subjects and in heterozygotes for PKU, as well as in classical PKU or its variants, a 4-to 5-fold increase in biopterin concentration is observed after a rise in phenylalanine plasma level, the biopterin/phenylalanine ratio being above 15 when the phenylalanine level is in the normal range, and at about S in untreated PKU or in phenylalaninechallenged subjects.3 6 The increase in biopterin concentration is even greater in dihydropteridine reductase deficiency and in such cases large amounts of dihydrobiopterin and dihydroxanthopterin are found in urine.515…”

Section: Discussionmentioning

confidence: 99%

Biopterin defect in a normal-appearing child affected by a transient phenylketonuria.

Rey¹,

Leeming²,

Blair³

et al. 1980

Archives of Disease in Childhood

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Section: Discussionmentioning

confidence: 99%

Biopterin defect in a normal-appearing child affected by a transient phenylketonuria.

Rey¹,

Leeming²,

Blair³

et al. 1980

Archives of Disease in Childhood

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“…For the fitting, we calculated the molar extinction coefficient and partial specific volume from the composition of recombinant GFRP using the residue values of Perkins (19); these were 11,237 M Ϫ1 cm Ϫ1 and 0.739 cm 3 /g, respectively. The sedimentation equilibrium experiment utilized three double sector cells and was carried out at three protein concentrations (8,16, and 24 M initially), a rotor speed of 16,000 rpm, and 20°C. In each cell, 100 l of protein solution and 110 l of buffer (20 mM Hepes-KOH, 0.1 M KCl, and 1 mM EDTA (pH 7.6)) were loaded.…”

Section: Methodsmentioning

confidence: 99%

GTP Cyclohydrolase I Feedback Regulatory Protein Is a Pentamer of Identical Subunits

Yoneyama

Brewer

Hatakeyama

1997

Journal of Biological Chemistry

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GTP cyclohydrolase I feedback regulatory protein (GFRP) mediates feedback inhibition of GTP cyclohydrolase I activity by tetrahydrobiopterin and also mediates the stimulatory effect of phenylalanine on the enzyme activity. To characterize the molecular structure of GFRP, we have purified it from rat liver using an efficient step of affinity chromatography and isolated cDNA clones, based on partial amino acid sequences of peptides derived from purified GFRP. Comparison between the amino acid sequence deduced from the cDNA and the N-terminal amino acid sequence of purified GFRP showed that the mature form of GFRP consists of 83 amino acid residues with a calculated M r of 9,542. The isolated GFRP cDNA was expressed in Escherichia coli as a fusion protein with six consecutive histidine residues at its N terminus. The fusion protein was affinitypurified and digested with thrombin to remove the histidine tag. The resulting recombinant GFRP showed kinetic properties similar to those of GFRP purified from rat liver. Cross-linking experiments using dimethyl suberimidate indicated that GFRP was a pentamer of 52 kDa. Sedimentation equilibrium measurements confirmed the pentameric structure of GFRP by giving an average M r of 49,734, which is 5 times the calculated molecular weight of the recombinant GFRP polypeptide. Based on the pentameric structure of GFRP, we have proposed a model for the quaternary structure of GFRP and GTP cyclohydrolase I complexes. 6,7, 1 is an essential cofactor for aromatic amino-acid hydroxylases and nitric-oxide synthases (1, 2). The intracellular level of BH 4 is subsaturating for these enzyme reactions and thus is thought to be an important regulator of the activities of these enzymes. BH 4 is synthesized from GTP by a pathway composed of four enzyme reactions (1). Most important for regulation of BH 4 biosynthesis is GTP cyclohydrolase I (EC 3.5.4.16), which catalyzes the first and rate-limiting step of the conversion of GTP to dihydroneopterin triphosphate. GTP cyclohydrolase I is present in many organisms, ranging from bacteria to animal. In bacteria, this enzyme functions as the first enzyme in the biosynthesis of folic acid and not of BH 4 (1). Rat GTP cyclohydrolase I is composed of multiple identical subunits and shows positive cooperativity against GTP (3, 4).We have recently reported the identification of a new regulatory protein (GFRP) which mediates the feedback inhibition of GTP cyclohydrolase I by the end product of this pathway, BH 4 (5). GFRP and BH 4 inhibit the enzyme activity of GTP cyclohydrolase I by decreasing its maximum velocity while having little effect on the affinity of GTP, indicating noncompetitive inhibition. GFRP exerts its inhibitory effect on GTP cyclohydrolase I by forming a complex with GTP cyclohydrolase I in the presence of BH 4 and GTP. Formation of the complex between the two proteins is reversible, depending on the presence of BH 4 and GTP. Furthermore, the inhibition of GTP cyclohydrolase I by GFRP and BH 4 is specifically reversed by L-phenylalanine. ...

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“…In this regard, nature appears to have evolved a mechanism that ensures rapid detoxification of Phe. The observation that oral administration of Phe elicits an increase in plasma BH4 in normal individuals (20) implies that modulation of GTPCH activity by GFRP and Phe is operative in vivo. Specific reversal of GTPCH inhibition by Phe provides a simple in vitro diagnostic test of whether GFRP participates in the action of a given inhibitor of GTPCH.…”

Section: Phenylalanine and Gfrp Modulate The Inhibition By Dahp Of Immentioning

confidence: 99%

GTP Cyclohydrolase I Inhibition by the Prototypic Inhibitor 2,4-Diamino-6-Hydroxypyrimidine

Xie¹,

Smith²,

Gross³

1998

Journal of Biological Chemistry

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2,4-Diamino-6-hydroxypyrimidine (DAHP) is considered to be a selective and direct-acting inhibitor of GTP cyclohydrolase I (GTPCH), the first and rate-limiting enzyme in the pathway for synthesis of tetrahydrobiopterin (BH4). Accordingly, DAHP has been widely employed to distinguish whether de novo BH4 synthesis is required in a given biological system. Although it has been assumed that DAHP inhibits GTPCH by direct competition with substrate GTP, this has never been formally demonstrated. In view of apparent structural homology between DAHP and BH4, we questioned whether DAHP may mimic BH4 in its inhibition of GT-PCH by an indirect mechanism, involving interaction with a recently cloned 9.5-kDa protein termed GTPCH Feedback Regulatory Protein (GFRP). We show by reverse transcription-polymerase chain reaction that GFRP mRNA is constitutively expressed in rat aortic smooth muscle cells and further induced by treatment with immunostimulants. Moreover, functional GFRP is expressed and immunostimulant-induced BH4 accumulates in sufficient quantity to trigger feedback inhibition of GTPCH. Studies with DAHP reveal that GFRP is also essential to achieve potent inhibition of GTPCH. Indeed, DAHP inhibits GTPCH by dual mechanisms. At a relatively low concentration, DAHP emulates BH4 and engages the GFRP-dependent feedback inhibitory system; at higher concentrations, DAHP competes directly for binding with GTP substrate. This knowledge predicts that DAHP would preferably target GTPCH in tissues with abundant GFRP.

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Biopterin derivatives in normal and phenylketonuric patients after oral loads of L-phenylalanine, L-tyrosine, and L-tryptophan.

Cited by 75 publications

References 16 publications

Biopterin defect in a normal-appearing child affected by a transient phenylketonuria.

Biopterin defect in a normal-appearing child affected by a transient phenylketonuria.

GTP Cyclohydrolase I Feedback Regulatory Protein Is a Pentamer of Identical Subunits

GTP Cyclohydrolase I Inhibition by the Prototypic Inhibitor 2,4-Diamino-6-Hydroxypyrimidine

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