Tetrahydrobiopterin (BH4) - (6R-L-erythro-5,6,7,8-tetrahydrobiopterin)
- BH4 is present in probably every cell or tissue of higher organisms
- Plays a key role in a number of biological processes and pathological states associated with
- Monoamine neurotransmitter formation
- Cardiovascular and endothelial dysfunction
- Immune response
- Pain sensitivity
- Natural cofactor of the hydroxylases of aromatic aminoacids
Essential for the activity of various enzymes, including:
- 4 aromatic amino acid hydroxylases
- phenylalanine (Phe) hydroxylase
- Alkylglycerol mono-oxygenase
- Three NOS (NO synthase) isoenzymes
Tetrahydrobiopterin is the cofactor for the hydroxylation of:
Essential for the production of.
- Monoamine neurotransmitters.
Phenylketonuria (PKU) patients
- Chronically exposed to high Phe levels
- High urinary excretion of BH4 metabolites neopterin and biopterin is observed
BH4 levels are maintained in vivo
- De novo biosynthesis from its precursor guanosine triphosphate
- Via dihydroneopterin triphosphate
- Salvage of quinonoid dihydrobiopterin
- Formed from BH4 during its cofactor role
- By dihydropteridine reductase
- Central role in neurotransmitter biosynthesis
- Disturbances in BH4 metabolism can have severe neurological consequences
- Exemplified by malignant hyperphenylalaninaemia
- Genetic disorder of biopterin metabolism
- BH4 is much reduced
- CSF biopterin levels
- Decrease with age suggesting that BH4 metabolism is altered in the CNS in normal aging
- SDAT BH4 levels in serum and CSF
- BH4 biosynthesis in the temporal lobe
- Diminished cofactor availability resulting in reduced noradrenalin levels in the CNS
- May be a contributing factor in the pathogenesis of SDAT
- Develp changes of Alzheimer's disease
- X noradrenergic system has been suggested in Down's syndrome
- Altered brain biopterin metabolism in. aging, SDAT and Down's syndrome
- Biosynthetic precusor of tetrahydrobiopterin
- Appear in urine
- A pterin derivative, is a NO synthase cofactor.
- Not increase tetrahydrobiopterin biosynthesis in the rat as previously thought
- Reduce liver biopterin levels
- Increas urinary biopterin levels in the rat
- Reduced brain pterin levels but had no influence on liver pterin
BH4 is formed de novo
- From GTP
- Via a sequence of three enzymatic steps carried out by
- GTP cyclohydrolase I
- Major controlling point
- 6-pyruvoyltetrahydropterin synthase
- Sepiapterin reductase
- Converts sepiapterin to biopterin
An alternative or salvage pathway involves
- Dihydrofolate reductase
- May play an essential role in peripheral tissues
Cofactor regeneration requires
- BH4 is the principal active cofactor, and a recycling pathway converts BH2 to BH4
- Pterin-4a-carbinolamine dehydratase
- Dihydropteridine reductase
- Except for NOSs
- BH4 cofactor undergoes a one-electron redox cycle without the need for additional regeneration enzymes
- BH4 biosynthesis is controlled in mammals by hormones and cytokines.
- Due to autosomal recessive mutations in all enzymes
- Except for sepiapterin reductase
- Described as a cause of hyperphenylalaninaemia
- Major contributor to vascular dysfunction associated with hypertension, ischaemic reperfusion injury, diabetes and others
- Appears to be an effect of oxidized BH4
- Leads to an increased formation of oxygen-derived radicals instead of NO by decoupled NOS
- Furthermore, several neurological diseases have been suggested to be a consequence of restricted cofactor availability
- Oral cofactor replacement therapy to stabilize mutant phenylalanine hydroxylase in the BH4-responsive type of hyperphenylalaninaemia
- Has an advantageous effect on pathological phenylalanine levels in patients
Tetrahydrobiopterin (BH4) functions
- As a H-donor cofactor of aromatic amino acid hydroxylases (Nagatsu et al., 1972; Kaufman and Fisher, 1974)
- Rate-limiting enzymes for producing monoamine neurotransmitters in the sympathetic nervous system
- Becomes converted to quinonoid dihydrobioptrin (q-BH2)
- Through 4alpha-carbinolamine in the process
- Regeneration of BH4 from q-BH2 by dihydropteridine reductase (DHPR; NADH: quinonoid dihydropteridine oxidoreductase [EC 1. 6. 99. 7]) with NADH
- Allows this cofactor to function catalytically
- Biosynthesis of BH4 from GTP by three enzymes (Katoh and Akino, 1986)
- GTP cyclohydrolase I (Hatakeyama et al., 1989)
- 6-pyruvoyltetrahydropterin synthase (Inoue et al., 1991)
- Sepiapterin reductase (SPR) (Sueoka and Katoh, 1982)
- The BH4-recy-cling by DHPR
- Is important to control the concentration of the cofactor in the cell
- DHPR reduces with NADH various quinonoid dihydropterins derived from BH4 and other tetrahydropterins such as
- 6-methyl tetrahydropterin (6M-PH4)
- Transient activation of DHPR by
- M-calpain, a widely distributed Ca2+-activated protease, in vitro.
- Reaction: 4a-hydroxytetrahydrobiopterin = 6,7-dihydrobiopterin + H2O
- 4a-hydroxytetrahydrobiopterin = 6-[(1R,2S)-1,2-dihydroxypropyl]-5,6,7,8-tetrahydro-4a-hydroxypterin
- 6,7-dihydrobiopterin = 6-[(1R,2S)-1,2-dihydroxypropyl]-6,7-dihydropterin
- Other name(s): 4alpha-hydroxy-tetrahydropterin dehydratase; 4a-carbinolamine dehydratase; pterin-4alpha-carbinolamine dehydratase; 4a-hydroxytetrahydrobiopterin hydro-lyase
- Systematic name: a-hydroxytetrahydrobiopterin hydro-lyase (6,7-dihydrobiopterin-forming)
- In concert with EC 18.104.22.168, 6,7-dihydropteridine reductase, the enzyme recycles 4a-hydroxytetrahydrobiopterin back to tetrahydrobiopterin, a cosubstrate for several enzymes, including aromatic amino acid hydroxylases.
- The enzyme is bifunctional
- Also acts as a dimerization cofactor of hepatocyte nuclear factor-1alpha (HNF-1).
- Synthetic analogue
- Lowers plasma phenylalanine concentrations to the therapeutic range
- Effective dose of BH4 varies
- From 1 to 2 mg kg-1 daily in patients with defective biopterin synthesis
- To 5 mg kg-1 or more in patients with dihydropteridine reductase (DHPR) deficiency
- The cost of 2 mg kg-1 day-1 of BH4 is comparable to the cost of a low phenylalanine diet.
- Higher doses of pterins given orally (20 mg kg-1)
- Raise the levels of tetrahydropterin in cerebrospinal fluid (CSF) to normal
- In patients with defective biopterin synthesis in whom initial concentration of biopterin species are low
- In some, but not all, such patients pterin therapy also raises CSF amine metabolite concentrations and ameliorates symptoms.
- High dose therapy does not appear to be effective in raising CSF pterin levels in patients with DHPR deficiency who already accumulate dihydrobiopterin (BH2) in CSF.
- Central folate deficiency
- Is an additional cause of neurological deterioration in patients with DHPR deficiency who require supplementation with folate as folinic acid.
- It is suggested that the accumulation of BH2 in such patients competitively interferes with folate metabolism.
- Reaction: 7,8-dihydroneopterin 3'-triphosphate = 6-pyruvoyl-5,6,7,8-tetrahydropterin + triphosphate
- 7,8-dihydroneopterin 3'-triphosphate = 6-[(1S,2R)-1,2-dihydroxy-3-triphosphooxypropyl]-7,8-dihydropterin
- Other name(s): 2-amino-4-oxo-6-[(1S,2R)-1,2-dihydroxy-3-triphosphooxypropyl]-7,8-dihydroxypteridine triphosphate lyase; 6-[(1S,2R)-1,2-dihydroxy-3-triphosphooxypropyl]-7,8-dihydroxypteridin triphosphate-lyase (6-pyruvoyl-5,6,7,8-tetrahydropterin-forming)
- Systematic name: 7,8-dihydroneopterin 3'-triphosphate triphosphate-lyase (6-pyruvoyl-5,6,7,8-tetrahydropterin-forming)
- Catalyses triphosphate elimination and an intramolecular redox reaction in the presence of Mg2+.
- It has been identified in human liver.
- This enzyme is involved in the de novo synthesis of tetrahydrobiopterin from GTP, with the other enzymes involved being EC 22.214.171.124 (sepiapterin reductase) and EC 126.96.36.199 (GTP cyclohydrolase I)
- Reaction: a 5,6,7,8-tetrahydropteridine + NAD(P)+ = a 6,7-dihydropteridine + NAD(P)H + H+
- Other name(s): 6,7-dihydropteridine:NAD(P)H oxidoreductase; DHPR; NAD(P)H2:6,7-dihydropteridine oxidoreductase; NADH-dihydropteridine reductase; NADPH-dihydropteridine reductase; NADPH-specific dihydropteridine reductase; dihydropteridine (reduced nicotinamide adenine dinucleotide) reductase; dihydropteridine reductase; dihydropteridine reductase (NADH); 5,6,7,8-tetrahydropteridine:NAD(P)H+ oxidoreductase
- Systematic name: 5,6,7,8-tetrahydropteridine:NAD(P)+ oxidoreductase
- Substrate is the quinonoid form of dihydropteridine
- Not identical with EC 188.8.131.52 dihydrofolate reductase
- Infarct size was significantly smaller in the rats (50 ± 2%) and pigs (54 ± 5%) given l-arginine + BH4 in comparison with the vehicle groups (rats 65 ± 3% and pigs 86 ± 5%, P < 0.05).
- Neither l-arginine nor BH4 alone significantly reduced infarct size.
- Myocardial BH4 levels were 3.5- to 5-fold higher in pigs given l-arginine + BH4 and BH4 alone.
- The generation of superoxide in the ischemic-reperfused myocardium was reduced in pigs treated with intracoronary l-arginine + BH4 versus the vehicle group (P < 0.05).
- Administration of l-arginine + BH4 before reperfusion protects the heart from ischemia–reperfusion injury.
- The cardioprotective effect is mediated via NOS-dependent pathway resulting in diminished superoxide generation.
- Since its first discovery in 1958 found in various kinds of cyanobacteria, namely
- Anacystis nidulans (14),
- Oscillatoria sp. (15),
- Spirulina platensis (16),
- Synechococcus sp. (17)
Biopterin - 2-Amino-6-(1,2-dihydroxypropyl)-4(1H)-pteridinone
Downregulation of CR6 interacting factor 1 (CRIF1)
- Induce mitochondrial dysfunction
- Resulting in reduced activity of endothelial nitric oxide synthase (eNOS) and NO production in endothelial cells
- Tetrahydrobiopterin (BH4) is an important cofactor in regulating the balance between NO (eNOS coupling) and superoxide production (eNOS uncoupling)
- CRIF1 deficiency
- Increased eNOS uncoupling
- Depleted levels of total biopterin and BH4
- By reducing the enzymes of BH4 biosynthesis (GCH-1, PTS, SPR, and DHFR) in vivo and vitro
- Supplementation of CRIF1-deficient cells with BH4
- Significantly increased the recovery of Akt and eNOS phosphorylation and NO synthesis
- Scavenging ROS with MitoTEMPO treatment
- Replenished BH4 levels by elevating levels of GCH-1, PTS, and SPR
- But with no effect on the level of DHFR
- Downregulation of DHFR synthesis regulators p16 or p21 in CRIF1-deficient cells
- Partially recovered the DHFR expression
- CR6 interacting factor 1 (CRIF1)
- One of the largest mitoribosomal subunits
- Essential for the synthesis and insertion of oxidative phosphorylation polypeptides (OXPHOS) in the mitochondrial membrane
- Lack of CRIF1
- Major factor underlying misfolded mitochondrial OXPOS subunits
- Production of excessive mitochondrial ROS in vascular endothelial cells
- Stimulates endothelial dysfunction via the inactivation of eNOS and decreased NO production
- mitochondrial ROS that has been linked to mitochondrial dysfunction also mediates the initiation of eNOS uncoupling
- CRIF1 deficiency limited the common substrate L-arginine to NO synthesis and resulted in eNOS uncoupling.
- Reduced BH4 levels, which resulted in eNOS uncoupling and mediated the development of hypertension
GTP cyclohydrolase I
- Dominant form of GTP cyclohydrolase I deficiency results in biopterin deficiency/insufficiency only in the brain
6-pyruvoyl tetrahydropterin synthase
- Can be identified in newborns by an elevated phenylalanine
Dihydropteridine reductase (DHPR)
- Can be identified in newborns by an elevated phenylalanine
- Also require folinic acid supplements
- Relatively benign
- Associated with transient hyperphenylalaninaemia
Diagnosis relies on:
- Pterin metabolites in urine
- Dihydropteridine reductase in blood spots
- Neurotransmitters and pterins in the CSF
- Demonstration of reduced enzyme activity (red blood cells or fibroblasts)
- Causative mutations in the relative genes
BH4 deficiency is no longer malignant if therapy:
- Is promptly initiated to reduce plasma phenylalanine levels
- Replace missing neurotransmitters
- Special diet and/or BH4 supplements
- Administration of l-dopa, carbidopa, 5-hydroxytryptophan
- In certain cases MAO-B inhibitor
Single-gene defects affecting the gene GCH1
- Block the first step in biopterin synthesis lead to
Dopamine-responsive dystonia - Segawa's syndrome
- Due to the role of BH4 in synthesising neurotransmitters
- Including Dopamine
- Treated with supplementation with levodopa
- Does not require BH4 for conversion to dopamine
- GCH1 defects
- Autosomal dominant
- One defective gene copy is required for the condition to occur
- BH4 depletion in P. berghei infection may compromise both nitric oxide synthesis and phenylalanine metabolism
Methotrexate - DHFR inhibition
- Knockdown of DHFR expression both result in significantly reduced levels of BH4 relative to BH2
- DHFR inhibition does not alter total cellular biopterin concentrations
- Reduction in the ratio of BH4 to BH2 is sufficient to increase superoxide production and inhibit L-arginine to L-citrulline conversion, a measure of coupled NOS activity
phenylalanine 4-monooxygenase - phenylalaninase; phenylalanine 4-hydroxylase; phenylalanine hydroxylase - PAH
- L-phenylalanine,tetrahydropteridine:oxygen oxidoreductase (4-hydroxylating)
- Reaction: L-phenylalanine + a 5,6,7,8-tetrahydropteridine + O2 = L-tyrosine + a 4a-hydroxy-5,6,7,8-tetrahydropteridine
- The active centre contains mononuclear iron(II).
- The reaction involves an arene oxide that rearranges to give the phenolic hydroxy group.
- This results in the hydrogen at C-4 migrating to C-3 and in part being retained.
- This process is known as the NIH-shift.
- The 4a-hydroxytetrahydropteridine formed can dehydrate to 6,7-dihydropteridine, both spontaneously and by the action of EC 184.108.40.206, 4a-hydroxytetrahydrobiopterin dehydratase.
- The 6,7-dihydropteridine must be enzymically reduced back to tetrahydropteridine,
- By EC 220.127.116.11, 6,7-dihydropteridine reductase, before it slowly rearranges into the more stable but inactive compound 7,8-dihydropteridine.
- L-arginine (LA) and tetrahydrobiopterin (BH4) are main substrates in the production of NO, which mediates pulmonary vasodilation. Administration of either LA or BH4 decrease pulmonary artery pressure (PAP).
- A combined administration of both may have synergistic effects in the therapy of PAH.
- Administration of LA alone
- Restore vascular endothelial NO production
- Decrease PAP in rodents and in humans
- ENOS activity highly depends on the intracellular presence of tetrahydrobiopterin (BH4)
- Binding of BH4 to eNOS
- Increases stability of the active eNOS dimer
- Significantly increases enzymatic turnover of LA
- BH4 deficiency leads to
- Uncoupling of the dimeric eNOS molecule
- Enhances generation of superoxide anion
- Reacts with and neutralizes NO before it is able to accomplish its vasodilatory effects.
Asymmetric dimethylarginine (ADMA)
- Is a methylated LA
- Independent risk factor for cardiovascular mortality
- Inhibits eNOS function and mediates vasoconstriction
- Patients with high ADMA levels LA supplementation was necessary to enhance statin-mediated eNOS function
- Human cohort of PAH patients
- ADMA levels were reduced after oral administration of a phosphodiesterase-5 inhibitor
- On neopterin and biopterin concentrations in blood plasma of common carp
- [Petr Maršálek, Ivana Mikuliková, Helena Modrá, Zdeňka Svobodová]
- Neopterin and biopterin are often used as markers of cell mediated immunity.
- Prochloraz is a widely used imidazole fungicide in horticulture and agriculture
- 60 juvenile common carp were divided into four groups
- 15 fish and exposed to prochloraz at concentrations of 0, 50, 150 and 380 mcg·l-1 28 days
- Concentrations of neopterin (25 ± 7.6 nmol·l-1) and biopterin (190 ± 29 nmol·l-1) in plasma
- Untreated common carp were comparable with those in mammals
- Neopterin concentrations significantly (P < 0.01) increased after exposure to prochloraz
- In comparison to non-exposed fish
- Biopterin concentrations were not influenced by exposure to prochloraz
Sapropterin Dihydrochloride - 6R-BH4
Klinická studie na endotelovou dysfunkci
- Sapropterin Dihydrochloride 5 mg/kg twice a day administered as whole tablets orally within 1 hour after morning and evening meals for 13 days and the last dose within 1 hour after a morning meal on Day 14 and Day 28.
Sepiapterin reductase (L-erythro-7,8-dihydrobiopterin forming) - L-erythro-7,8-dihydrobiopterin:NADP+ oxidoreductase
- Reaction: (1) L-erythro-7,8-dihydrobiopterin + NADP+ = sepiapterin + NADPH + H+ (2) L-erythro-tetrahydrobiopterin + 2 NADP+ = 6-pyruvoyl-5,6,7,8-tetrahydropterin + 2 NADPH + 2 H+
- Sepiapterin = 2-amino-6-lactoyl-7,8-dihydropteridin-4(3H)-one
- Tetrahydrobiopterin = 5,6,7,8-tetrahydrobiopterin = 2-amino-6-(1,2-dihydroxypropyl)-5,6,7,8-tetrahydropteridin-4(3H)-one
- Enzyme catalyses the final step in the de novo synthesis of tetrahydrobiopterin from GTP
- Found in higher animals and some fungi and bacteria
- Produces the erythro form of tetrahydrobiopterin
- EC 18.104.22.1685, sepiapterin reductase (L-threo-7,8-dihydrobiopterin forming).
- Tetrahydrobiopterin is biosynthesized from guanosine triphosphate (GTP)
- By three chemical reactions mediated by the enzymes
- GTP cyclohydrolase I (GTPCH),
- 6-pyruvoyltetrahydropterin synthase (PTPS),
- Sepiapterin reductase (SR)
- BH4 can be oxidized by one or two electron reactions, to generate BH4 or BH3 radical and BH2
- ascorbic acid - vitamin C can reduce BH3 radical into BH4
- Preventing the BH3 radical from reacting with other free radicals (superoxide and peroxynitrite)
- Ascorbic acid is oxidized to dehydroascorbic acid during this process, can be recycled back
- Folic acid and its metabolites
- Seem to be particularly important in the recycling of BH4 and NOS coupling
Sapropterin dihydrochloride (BH4*2HCL)
- Tablet - approved for use in the United States as a tablet in December 2007
- Powder in December 2013. It was approved for
- Use in the European Union in December 2008
- Canada in April 2010
- Japan in July 2008
Kuvan and Biopten
- Typical cost of treating a patient with Kuvan is US$100,000 per year
- BioMarin holds the patent for Kuvan until at least 2024
- Par Pharmaceutical has a right to produce a generic version by 2020
- Sapropterin is indicated in tetrahydrobiopterin deficiency caused by:
- GTP cyclohydrolase I (GTPCH) deficiency,
- 6-pyruvoyltetrahydropterin synthase (PTPS) deficiency
- BH4*2HCL is FDA approved for use in
- Phenylketonuria (PKU), along with dietary measures
- Most people with PKU have little or no benefit from BH4*2HCL
- Oxidized form of biopterin is excreted much more rapidly than BH4 (Hoshiga et al., 1993).
- Gamma-interferon and other cytokines
- Increase biopterin production by inducing GTP cyclohydrolase 1 (Thöny et al., 2000)
- Prevent cytokine-mediated induction of GTP cyclohydrolase 1 (Simmons et al., 1996), apparently via a cyclic adenosine monophosphate (cAMP)–-mediated signaling cascade (Ohtsuki et al., 2002)
- Food deprivation of animals
- Increases biopterin production and concentration in the blood (Koller et al., 1990).
- Inflammation and infection
- Tend to depress availability of biopterin in tissues, such as
- Endothelia of small blood vessels
- Due to rapid oxidation of the reduced form (McNeill and Channon, 2012).
- BH4 is converted to the quinoid form of BH2 (qBH2)
- And must be converted back to BH4 via a salvage pathway
- Sepiapterin reductase deficiency
- Increased CSF biopterin concentrations because of increased degradation of BH4.