"7-tetrahydrobiopterin," a naturally occurring analogue of tetrahydrobiopterin, is a cofactor for and a potential inhibitor of the aromatic amino acid hydroxylases

Insights into Molecular Structure of Pterins Suitable for Biomedical Applications

Pterins are an inseparable part of living organisms. Pterins participate in metabolic reactions mostly as tetrahydropterins. Dihydropterins are usually intermediates of these reactions, whereas oxidized pterins can be biomarkers of diseases. In this review, we analyze the available data on the quantum chemistry of unconjugated pterins as well as their photonics. This gives a comprehensive overview about the electronic structure of pterins and offers some benefits for biomedicine applications: (1) one can affect the enzymatic reactions of aromatic amino acid hydroxylases, NO synthases, and alkylglycerol monooxygenase through UV irradiation of H₄pterins since UV provokes electron donor reactions of H₄pterins; (2) the emission properties of H₂pterins and oxidized pterins can be used in fluorescence diagnostics; (3) two-photon absorption (TPA) should be used in such pterin-related infrared therapy because single-photon absorption in the UV range is inefficient and scatters in vivo; (4) one can affect pathogen organisms through TPA excitation of H₄pterin cofactors, such as the molybdenum cofactor, leading to its detachment from proteins and subsequent oxidation; (5) metal nanostructures can be used for the UV-vis, fluorescence, and Raman spectroscopy detection of pterin biomarkers. Therefore, we investigated both the biochemistry and physical chemistry of pterins and suggested some potential prospects for pterin-related biomedicine.

Autoxidation and photooxidation of tetrahydrobiopterin: a theoretical study

Pterins are naturally occurring pigments and enzyme cofactors widespread in living organisms. Tetrahydrobiopterin (H4Bip) is a coenzyme of aromatic amino acid hydroxylases, NO-synthases, and alkylglycerol monooxygenases. This coenzyme is prone to oxidation in the presence of molecular oxygen, a so-called autoxidation. The reactions participating in H4Bip autoxidation are well known. However, our study is an attempt to evaluate theoretically the feasibility of reactions participating in autoxidation. To do so, we have calculated the Gibbs free energy of elementary reactions between H4Bip, its derivatives, molecular oxygen, and reactive oxygen species (ROS). In the last few years, we have established the photosensitized oxidation of H4Bip experimentally. Thus, we have also evaluated the feasibility of H4Bip photooxidation reactions, which may occur according to both type-I and type-II photosensitized oxidation. We calculated Fukui indices for H4Bip and found particular atoms in the molecule that interact with nucleophiles (for example, singlet oxygen 1O2) and radicals (in particular, molecular oxygen 3O2). Therefore, we evaluated the probability of H4Bip autoxidation reactions, photooxidation reactions, and the reactivity of particular atoms in H4Bip molecule using the theoretical methods of quantum chemistry.

Expanding the Spectrum of Dopa-Responsive Dystonia (DRD) and Proposal for New Definition: DRD, DRD-plus, and DRD Look-alike

Previously, we defined DRD as a syndrome of selective nigrostriatal dopamine deficiency caused by genetic defects in the dopamine synthetic pathway without nigral cell loss. DRD-plus also has the same etiologic background with DRD, but DRD-plus patients have more severe features that are not seen in DRD because of the severity of the genetic defect. However, there have been many reports of dystonia responsive to dopaminergic drugs that do not fit into DRD or DRD-plus (genetic defects in the dopamine synthetic pathway without nigral cell loss). We reframed the concept of DRD/DRD-plus and proposed the concept of DRD look-alike to include the additional cases described above. Examples of dystonia that is responsive to dopaminergic drugs include the following: transportopathies (dopamine transporter deficiency; vesicular monoamine transporter 2 deficiency); SOX6 mutation resulting in a developmentally decreased number of nigral cells; degenerative disorders with progressive loss of nigral cells (juvenile Parkinson's disease; pallidopyramidal syndrome; spinocerebellar ataxia type 3), and disorders that are not known to affect the nigrostriatal dopaminergic system (DYT1; GLUT1 deficiency; myoclonus-dystonia; ataxia telangiectasia). This classification will help with an etiologic diagnosis as well as planning the work up and guiding the therapy.

Photooxidation of tetrahydrobiopterin under UV irradiation: possible pathways and mechanisms

Tetrahydrobiopterin (H4 Bip) is a cofactor for several key enzymes, including NO synthases and aromatic amino acid hydroxylases (AAHs). Normal functioning of the H4 Bip regeneration cycle is extremely important for the work of AAHs. Oxidized pterins may accumulate if the H4 Bip regeneration cycle is disrupted or if H4 Bip autoxidation occurs. These oxidized pterins can photosensitize the production of singlet molecular oxygen (1)O2 and thus cause oxidative stress. In this context, we studied the photooxidation of H4 Bip in phosphate buffer at pH 7.2. We found that UV irradiation of H4 Bip affected its oxidation rate (quantum yield Φ300 = (2.7 ± 0.4) × 10(-3)). The effect of UV irradiation at λ = 350 nm on H4 Bip oxidation was stronger, especially in the presence of biopterin (Bip) (Φ350 = (9.7 ± 1.5) × 10(-3)). We showed that the rate of H4 Bip oxidation linearly depends on Bip concentration. Experiments with KI, a selective quencher of triplet pterins at micromolar concentrations, demonstrated that the oxidation is sensitized by the triplet state biopterin (3) Bip. Apparently, electron transfer sensitization (Type-I mechanism) is dominant. Energy transfer (Type-II mechanism) and singlet oxygen generation play only a secondary role. The mechanisms of H4 Bip photooxidation and their biological meaning are discussed.

Dopa-responsive dystonia

Clinical characteristics and pahophysiologies of dopa-responsive dystonia are discussed by reviewing autosomal-dominant GTP cyclohydrolase-I deficiency (AD GCHI D), recessive deficiencies of enzymes of pteridine metabolism, and recessive tyrosine hydroxylase (TH). Pteridine and TH metabolism involve TH activities in the terminals of the nigrostriatal dopamine neuron which show high in early childhood and decrease exponentially with age, attaining stational low levels by the early 20s. In these disorders, TH in the terminals follows this course with low levels and develops particular symptoms with functional maturation of the downstream structures of the basal ganglia; postural dystonia through the direct pathway and descending output matured earlier in early childhood and parkinsonism in TH deficiency in teens through the D2 indirect pathway ascending output matured later. In action-type AD GCHI D, deficiency of TH in the terminal on the subthalamic nucleus develops action dystonia through the descending output in childhood, focal and segmental dystonia and parkinsonism in adolescence and adulthood through the ascending pathway maturing later. Dysfunction of dopamine in the terminals does not cause degenerative changes or higher cortical dysfunction. In recessive disorders, hypofunction of serotonin and noradrenaline induces hypofunction of the dopamine in the perikaryon and shows cortical dysfunction.

Specific interaction of the diastereomers 7(R)- and 7(S)-tetrahydrobiopterin with phenylalanine hydroxylase: implications for understanding primapterinuria and vitiligo

Pterin-4a-carbinolamine dehydratase (PCD) is an essential component of the phenylalanine hydroxylase (PAH) system, catalyzing the regeneration of the essential cofactor 6(R)-L-erythro-5,6,7,8-tetrahydrobiopterin [6(R)BH4]. Mutations in PCD or its deactivation by hydrogen peroxide result in the generation of 7(R,S)BH4, which is a potent inhibitor of PAH that has been implicated in primapterinuria, a variant form of phenylketonuria, and in the skin depigmentation disorder vitiligo. We have synthesized and separated the 7(R) and 7(S) diastereomers confirming their structure by NMR. Both 7(R)- and 7(S)BH4 function as poor cofactors for PAH, whereas only 7(S)BH4 acts as a potent competitive inhibitor vs. 6(R)BH4 (Ki=2.3-4.9 microM). Kinetic and binding studies, as well as characterization of the pterin-enzyme complexes by fluorescence spectroscopy, revealed that the inhibitory effects of 7(R,S)BH4 on PAH are in fact specifically based on 7(S)BH4 binding. The molecular dynamics simulated structures of the pterin-PAH complexes indicate that 7(S)BH4 inhibition is due to its interaction with the polar region at the pterin binding site close to Ser-251, whereas its low efficiency as cofactor is related to a suboptimal positioning toward the catalytic iron. 7(S)BH4 is not an inhibitor for tyrosine hydroxylase (TH) in the physiological range, presumably due to the replacement of Ser-251 by the corresponding Ala297. Taken together, our results identified structural determinants for the specific regulation of PAH and TH by 7(S)BH4, which in turn aid in the understanding of primapterinuria and acute vitiligo.

Decreased phenylalanine uptake and turnover in patients with vitiligo

The human epidermis has the full machinery for autocrine L-phenylalanine turnover to L-tyrosine in keratinocytes and melanocytes. Phenylalanine hydroxylase (PAH) activities increase linearly with inherited skin colour (skin phototype I-VI, Fitzpatrick classification) yielding eightfold more activities in black skin compared to white skin. Moreover, UVB irradiation (1 MED) significantly increases epidermal PAH activities 24 h after exposure. Importantly, L-phenylalanine uptake and turnover in the pigment forming melanocytes is vital for initiation of melanogenesis. In this context it was shown that the uptake of this amino acid is regulated by calcium. The depigmentation disorder vitiligo provides a unique model to follow impaired L-phenylalanine turnover in the skin as well as in serum because affected individuals hold an impaired epidermal 6BH4 de novo synthesis/recycling and regulation including low epidermal PAH activities. After overnight fasting and oral loading with L-phenylalanine (100 mg/kg body weight), 29.6% of 970 patients tested (n=287/970) yielded serum phenylalanine/tyrosine ratios >or=4 and 35.3% (n=342/970) had mild to moderate hyperphenylalaninaemia (HPA), while 9.3% (n=90/970) had both serum L-phenylalanine levels >or=2.0 mg/dl and phe/tyr ratios >or=4.0. Isolated HPA was found in 26% (n=252/970), whereas 20.3% had only increased ratios (n=197/970). None of the patients had phenylketonuria and the family history for this metabolic disease was negative. The IQ followed normal Gaussian distribution. In vitro L-phenylalanine uptake/turnover studies on primary epidermal melanocytes originating from these patients demonstrated a significantly decreased calcium dependent L-phenylalanine uptake and turnover compared to healthy control cells. Based on our observation, we would like to propose that phenylalanine uptake/turnover is under tight control by calcium which in turn could offer an additional novel mechanism in the aetiology of HPA.

Vitiligo

O vitiligo é doença de pele de causa desconhecida que acomete cerca de 1% da população, comprometendo de modo semelhante homens e mulheres, preferencialmente entre 10 e 30 anos de idade. Alguns fatores precipitantes para essa doença são: estresse físico e emocional, traumas mecânicos e substâncias químicas, como derivados do fenol. Doenças auto-imunes, principalmente as tireoidianas, podem estar associadas ao vitiligo. Novas terapias têm sido propostas, como o uso de imunomoduladores tópicos, aliadas àquelas já consolidadas, como os psoralenos e os corticosteróides; o sucesso terapêutico, entretanto, está estritamente relacionado à qualidade da relação médico/paciente.

Activation/deactivation of acetylcholinesterase by H2O2: more evidence for oxidative stress in vitiligo

Previously it has been demonstrated that the human epidermis synthesises and degrades acetylcholine and expresses both muscarinic and nicotinic receptors. These cholinergic systems have been implicated in the development of the epidermal calcium gradient and differentiation in normal healthy skin. In vitiligo severe oxidative stress occurs in the epidermis of these patients with accumulation of H2O2 in the 10(-3)M range together with a decrease in catalase expression/activity due to deactivation of the enzyme active site. It was also shown that the entire recycling of the essential cofactor (6R)-l-erythro-5,6,7,8-tetrahydrobiopterin via pterin-4a-carbinolamine dehydratase (PCD) and dihydropteridine reductase (DHPR) is affected by H2O2 oxidation of Trp/Met residues in the enzyme structure leading to deactivation of these proteins. Using fluorescence immunohistochemistry we now show that epidermal H2O2 in vitiligo patients yields also almost absent epidermal acetylcholinesterase (AchE). A kinetic analysis using pure recombinant human AchE revealed that low concentrations of H2O2 (10(-6)M) activate this enzyme by increasing the Vmax>2-fold, meanwhile high concentrations of H2O2 (10(-3)M) inhibit the enzyme with a significant decrease in Vmax. This result was confirmed by fluorescence excitation spectroscopy following the Trp fluorescence at lambdamax 280nm. Molecular modelling based on the established 3D structure of human AchE supported that H2O2-mediated oxidation of Trp(432), Trp(435), and Met(436) moves and disorients the active site His(440) of the enzyme, leading to deactivation of the protein. To our knowledge these results identified for the first time H2O2 regulation of AchE. Moreover, it was shown that H2O2-mediated oxidation of AchE contributes significantly to the well-established oxidative stress in vitiligo.

Perturbed 6-tetrahydrobiopterin recycling via decreased dihydropteridine reductase in vitiligo: more evidence for H2O2 stress

To date there is ample evidence that patients with vitiligo accumulate millimolar concentrations of hydrogen peroxide (H2O2) in their epidermis as well as in their blood lymphocytes/monocytes. Several enzymes are affected by this H2O2 including catalase, glutathione peroxidase, and 4 alpha-carbinolamine dehydratase. The latter enzyme disrupts the recycling of the essential cofactor (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (6BH4) for the aromatic amino acid hydroxylases as well as the nitric oxide synthases. In this report we have elucidated the influence of H2O2 on dihydropteridine reductase (DHPR), the last enzyme in the 6BH4-recycling process. Here we show for the first time that concentrations of less than 30 microM H2O2 increase DHPR activities, whereas levels greater than 30 microM H2O2 deactivate the enzyme based on the oxidation of Met146 and Met151 in the sequence, consequently leading to disruption of the NADH-dependent enzyme active site. This oxidation was confirmed by Fourier transform-Raman spectroscopy yielding the expected SO band at 1025 cm-1 characteristic of methionine sulfoxide. Hence these results unmasked a novel regulatory mechanism for DHPR enzyme activity. Moreover, we also demonstrated that DHPR activities in whole blood of patients with vitiligo are significantly decreased in untreated patients, whereas activities are normalized after removal of epidermal H2O2 with a topical pseudocatalase (PC-KUS). Taken together, these new data add more evidence to a systemic involvement of H2O2 in the pathomechanism of vitiligo.

Autosomal dominant guanosine triphosphate cyclohydrolase I deficiency (Segawa disease)

Autosomal dominant guanosine triphosphate cyclohydrolase I (GCH-I) deficiency (Segawa disease) is a dopa-responsive dystonia caused by mutation of the GCH-I gene located on 14q22.1-q22.2. Neurohistochemical examination revealed a decrease of the tyrosine hydroxylase protein as well as its activity in the striatum and decrease of dopamine content, particularly in its ventral portion rich in D1 receptors (striatal direct pathways). Neuroimaging, clinical neurophysiological, and biochemical studies showed preservation of the structure and function of the terminal of the nigrostriatal DA neuron. Clinical neurophysiological studies showed no progressive decrement of DA activities. As the enzymatic activity of pteridine metabolism is highest in the early developmental course, it may modulate dopamine receptors maturing early in the developmental course. Its product, tetrahydrobiopterin, has higher affinity to tyrosine hydroxylase among hydroxylases. Thus, partial deficiency of tetrahydrobiopterin caused by heterozygous mutation of the GCH-I gene decreases dopamine activity rather selectively. This affects the DA receptors that mature early and demonstrates characteristic symptoms age-dependently along with the developmental decrement of the tyrosine hydroxylase activities at the terminals and the maturational processes of the projecting neurons of the basal ganglia. A difference in the ratio of mutant/wild-type GCH-I mRNA that depends on the locus of mutation may explain intrafamilial and interfamilial variation of phenotype.

In situ and in vitro evidence for DCoH/HNF-1 alpha transcription of tyrosinase in human skin melanocytes

Human epidermal melanocytes hold the full capacity for autocrine de novo synthesis/regulation/recycling of the essential cofactor 6-tetrahydrobiopterin (6BH(4)) for conversion of L-phenylalanine via phenylalanine hydroxylase to L-tyrosine and for production of L-Dopa via tyrosine hydroxylase to initiate both pigmentation and catecholamine synthesis in these neural crest-derived cells. Earlier we have demonstrated pterin-4a-carbinolamine dehydratase (PCD) mRNA and enzyme activities in epidermal melanocytes and keratinocytes. This protein dimerises also the transcription factor hepatocyte nuclear factor 1 (HNF-1), leading to activation of multiple genes. This study demonstrates for the first time DCoH/HNF-1 alpha expression and transcriptional activity in human epidermal melanocytes in vitro and in situ and identified tyrosinase, the key enzyme for pigmentation, as a new transcriptional target. Specific binding of DCoH/HNF-1 complex to the human tyrosinase promoter was confirmed by gel shift analysis. These results provide a novel mechanism in the regulation of skin pigmentation.

6R-Tetrahydrobiopterin induces dopamine synthesis in a human neuroblastoma cell line, LA-N-1. A cellular model of DOPA-responsive dystonia

Dopa-responsive dystonia (DRD) is an extrapyramidal disorder caused by deficit of 5,6,7,8-tetrahydrobiopterin (BH4), cofactor for tyrosine hydroxylase (TH). In these patients the nigrostriatal dopaminergic neurons normally express TH and the cellular machinery for the dopamine uptake. LA-N-1 is a human neuroblastoma cell line expressing tyrosine hydroxylase. Here we show that LA-N-1 cells are able to take up exogenous dopamine (DA) by an high-affinity mechanism; significant amounts of serotonin and its metabolite 5HIAA, but neither DA nor its metabolites, DOPAC and HVA, could be measured in the cell culture homogenate. 5,6,7,8-Tetrahydrobiopterin, cofactor for both tyrosine and tryptophan hydroxylases, is able to activate dopamine synthesis and also decreases the content of 5HIAA by 50%, indicating that LA-N-1 might be a useful model for studying dopamine-serotonin interaction in cultured cells and the neuronal mechanism of DRD.

Epidermal H(2)O(2) accumulation alters tetrahydrobiopterin (6BH4) recycling in vitiligo: identification of a general mechanism in regulation of all 6BH4-dependent processes?

It has been shown in vivo that patients with the depigmentation disorder vitiligo accumulate hydrogen peroxide (H(2)O(2)) accompanied by low catalase levels and high concentrations of 6- and 7-biopterin in their epidermis. Earlier it was demonstrated that epidermal 4a-OH-tetrahydrobiopterin dehydratase, an important enzyme in the recycling process of 6(R)-L-erythro 5,6,7,8 tetrahydrobiopterin (6BH(4)), has extremely low activities in these patients concomitant with a build-up of the abiogenic 7-isomer (7BH(4)), leading to competitive inhibition of epidermal phenylalanine hydroxylase. A topical substitution for the impaired epidermal catalase with a pseudocatalase effectively removes epidermal H(2)O(2), yielding a recovery of epidermal 4a-OH-tetrahydrobiopterin dehydratase activities and physiologic 7BH(4) levels in association with successful repigmentation demonstrating recovery of the 6BH(4) recycling process. Examination of recombinant enzyme activities, together with 4a-OH-tetrahydrobiopterin dehydratase expression in the epidermis of untreated patients, identifies H(2)O(2)-induced inactivation of this enzyme. These results are in agreement with analysis of genomic DNA from these patients yielding only wild-type sequences for 4a-OH-tetrahydrobiopterin dehydratase and therefore ruling out the previously suspected involvement of this gene. Furthermore, our data show for the first time direct H(2)O(2) inactivation of the important 6BH(4) recycling process. Based on this observation, we suggest that H(2)O(2) derived from various sources could be a general mechanism in the regulation of all 6BH(4)-dependent processes.

Cloning and expression of recombinant human pineal tryptophan hydroxylase in Escherichia coli: purification and characterization of the cloned enzyme

The first step in the biosynthesis of melatonin in the pineal gland is the hydroxylation of tryptophan to 5-hydroxytryptophan. A cDNA of human tryptophan hydroxylase (TPH) was cloned from a library of human pineal gland and expressed in Escherichia coli. This cDNA sequence is identical to the cDNA sequence published from the human carcinoid tissue [1]. This human pineal hydroxylase gene encodes a protein of 444 amino acids and a molecular mass of 51 kDa estimated for the purified enzyme. Tryptophan hydroxylase from human brainstem exhibits high sequence homology (93% identity) with the human pineal hydroxylase. The recombinant tryptophan hydroxylase exists in solution as tetramers. The expressed human pineal tryptophan hydroxylase has a specific activity of 600 nmol/min/mg when measured in the presence of tetrahydrobiopterin and L-tryptophan. The enzyme catalyzes the hydroxylation of tryptophan and phenylalanine at comparable rates. Phosphorylation of the hydroxylase by protein kinase A or calmodulin-dependent kinase II results in the incorporation of 1 mol of phosphate/mol of subunit, but this degree of phosphorylation leads to only a modest (30%) increase in BH(4)-dependent activity when assayed in the presence of 14-3-3. Rapid scanning ultraviolet spectroscopy has revealed the formation of the transient intermediate compound, 4alpha-hydroxytetrahydrobiopterin, during the hydroxylation of either tryptophan or phenylalanine catalyzed by the recombinant pineal TPH.

Dopa-responsive dystonia: recent advances and remaining issues to be addressed

It is evident from this review that there is much that we know and much that we still do not know about DRD. In terms of diagnosis and clinical management, there is general agreement that patients with childhood-onset dystonic symptoms of unknown etiology should be treated initially with levodopa with the later addition, if necessary, of other medications (for example, BH4, 5-hydroxytryptophan). Although the results of molecular genetic and CSF studies are, at this time, unlikely to significantly alter clinical management of the patient, these analyses could be useful in providing information on prognosis (that is, DRD versus progressive neurodegenerative disorders or more severe metabolic disorders). It is also clear that notwithstanding the discovery of GCH1 and hTH mutations responsible for DRD, there remain many important unresolved issues regarding this disorder, including questions of female predominance, phenotypic heterogeneity, and presence of childhood-onset dystonia versus the expected parkinsonism resulting from a striatal DA deficit. We are confident that answers to these interesting questions on DRD will, in addition to providing clarification of the mechanisms of this disorder, provide exciting information relating to the pathogenesis of other types of dystonia as well as PD and to long-standing issues regarding a role of DA and serotonin in normal human brain development.

Characterization of expression of the gene for human pterin carbinolamine dehydratase/dimerization cofactor of HNF1

Pterin carbinolamine dehydratase/dimerization cofactor of HNF1 (PCD/DCoH) is a dual-function protein. In the cytoplasm it acts as a dehydratase in the regeneration of tetrahydrobiopterin, the cofactor for aromatic amino acid hydroxylases. In the nucleus, it functions as a dimerization cofactor of HNF1 and increases the transcriptional activity of HNF1. To deepen our understanding of this protein, we characterized its expression in human tissues and cells. Human PCD/DCoH was present predominantly in liver and kidney, with significant amounts in testis and ovary, trace amounts in lung, and undetectable levels in whole brain, heart, and spleen. It was expressed in all of the cells that were examined. Importantly, it was also present in the nucleus of HeLa cells, which lack HNF1, and in the cytoplasm of fibroblasts that have little or no tetrahydrobiopterin. The expression of human PCD/DCoH in the liver and nonhepatic cells was compared at both the mRNA and protein levels. Although the mRNA level in liver was only fourfold higher than that in keratinocytes and fibroblasts, the hepatic PCD/DCoH protein level was 20-fold higher than that in normal human epidermal keratinocytes and dermal fibroblasts. Cloning of the 5' and 3' untranslated region (UTR) of human keratinocyte PCD/DCoH revealed that it has 53 bp more of GC-rich 5' untranslated sequence than the published liver PCD/DCoH. In vitro transcription and translation analysis showed that the longer 5' UTR resulted in about a 35% decrease in translation efficiency. These data show that human PCD/DCoH is not only present in cells where tetrahydrobiopterin is synthesized or HNF1 is present but is a widely distributed protein. Its differential expression in different tissues and cells is regulated not only at the transcriptional level but also at the translational level.

Identification of hepatic nuclear factor 1 binding sites in the 5' flanking region of the human phenylalanine hydroxylase gene: implication of a dual function of phenylalanine hydroxylase stimulator in the phenylalanine hydroxylation system

Phenylalanine hydroxylase stimulator (PHS) is a component of the phenylalanine hydroxylation system that is involved in the regeneration of the cofactor tetrahydrobiopterin. It is also identical to the dimerization cofactor of hepatocyte nuclear factor 1 (HNF1) (DCoH) that is able to enhance the transcriptional activity of HNF1. Moreover, it has the structural potential for binding macromolecules such as proteins and nucleic acids, consistent with its involvement in gene expression. We investigated whether PHS/DCoH could enhance the expression of phenylalanine hydroxylase (PAH). Cotransfection assays showed that DCoH itself could not transactivate the 9-kb human PAH 5' flanking fragment. However, this 9-kb fragment was transactivated by HNF1 in a dose-dependent manner with a maximum of nearly 8-fold activation; DCoH potentiated this transactivation by another 1.6-fold. The HNF1 binding sites were located at -3.5 kb in a region that is 77.5% identical to the mouse liver-specific hormone-inducible PAH gene enhancer. This study suggests a possible dual function of PHS in vivo in the human phenylalanine hydroxylation system: it is involved in the regeneration of the cofactor tetrahydrobiopterin and can also enhance the expression of the human PAH gene.

Studies on the enzymatic and transcriptional activity of the dimerization cofactor for hepatocyte nuclear factor 1

The relationship between the enzymatic and the transcriptional activity of the bifunctional protein pterin-4a-carbinolamine dehydratase/dimerization cofactor for hepatocyte nuclear factor 1 (DCoH) has been elucidated by site-directed mutagenesis. DCoH dimers harbor a binding site for hepatocyte nuclear factor 1 (HNF1), two active centers that bind pterins, and a saddle-shaped surface that resembles nucleic acid binding domains. Two domains of the protein have been selectively targeted to determine if a change in one activity affects the other. No strong correlation has been found, supporting the idea that carbinolamine dehydratase activity is not required for HNF1 binding in vitro or transcriptional coactivation in vivo. Double mutations in the active center, however, influence the in vivo transcriptional activity but not HNF1 binding. This finding suggests that some active center residues also are used during transcription, possibly for binding of another (macro)molecule. Several mutations in the saddle led to a surprising increase in transcription, therefore linking this domain to transcriptional regulation as well. The transcriptional function of DCoH therefore is composed of two parts, HNF1 binding and another contributing effect that involves the active site and, indirectly, the saddle.

Expression of 4alpha-carbinolamine dehydratase in human epidermal keratinocytes

4alpha-Carbinolamine dehydratase is a bifunctional protein involved in the regeneration of tetrahydrobiopterin during the hydroxylation of the aromatic amino acids. It is also a dimerization cofactor of HNF1 and therefore is believed to function as part of the hepatic gene transcription system. In view of the recent discoveries that the distribution and developmental pattern of the dehydratase do not correlate strictly with those of the aromatic amino acid hydroxylases and HNF1, the hypothesis that the dehydratase may have other unknown functions has been put forward. In the present paper, we demonstrate unambiguously that human epidermal keratinocytes express detectable levels of this protein as indicated by enzyme assay, immunoprecipitation, Western blot, and RT-PCR. Its complete coding sequence has been cloned and was found to be identical with the human liver counterpart. The possible function of the dehydratase in skin is discussed.

Pteridines in the control of pigmentation

The influence of UVB irradiation on the metabolic pathway for the production of L-tyrosine from L-phenylalanine in the human epidermis has been examined in 12 healthy volunteers with photo skin types I-VI (Fitzpatrick classification). This metabolic pathway involves the induction of GTP-cyclohydrolase 1 (GTP-CH-1), the rate-limiting enzyme for de novo synthesis of (6R)L-erythro-5,6,7,8-tetrahydrobiopterin (6-BH4). This essential cofactor controls the production of L-tyrosine from L-phenylalanine via phenylalanine hydroxylase (PAH). The de novo synthesis of 6-BH4 depends on the induction of GTP-CH-1, e.g., by tumor necrosis factor-alpha (TNF alpha). Epidermal suction blister tissues were taken before (0 h) and after (24 and 72 h) UVB exposure with a standardized dosage [1 minimal erythema dose (MED)]. In all cases, there was a significant increase in TNF alpha release, GTP-CH-1 activity, total 6-biopterin level, and PAH activity, indicative of enhanced L-tyrosine production. The response of this metabolic cascade over baseline activities was pronounced in fair photo skin types (I-III) compared to dark skin (IV-VI). Taken together, our results suggest that UVB can control the direct supply of L-tyrosine in the epidermis, and this process may represent an important factor in de novo melanogenesis.

High-resolution structures of the bifunctional enzyme and transcriptional coactivator DCoH and its complex with a product analogue

DCoH, the dimerization cofactor of hepatocyte nuclear factor 1 (HNF-1), functions as both a transcriptional coactivator and a pterin dehydratase. To probe the relationship between these two functions, the X-ray crystal structures of the free enzyme and its complex with the product analogue 7,8-dihydrobiopterin were refined at 2.3 A resolution. The ligand binds at four sites per tetrameric enzyme, with little apparent conformational change in the protein. Each active-site cleft is located in a subunit interface, adjacent to a prominent saddle motif that has structural similarities to the TATA binding protein. The pterin binds within an arch of aromatic residues that extends across one dimer interface. The bound ligand makes contacts to three conserved histidines, and this arrangement restricts proposals for the enzymatic mechanism of dehydration. The dihedral symmetry of DCoH suggests that binding to the dimerization domain of HNF-1 likely involves the superposition of two-fold rotation axes of the two proteins.

Characterization of the wild-type form of 4a-carbinolamine dehydratase and two naturally occurring mutants associated with hyperphenylalaninemia

The characterization of 4a-carbinolamine dehydratase with the enzymatically synthesized natural substrate revealed non-Michaelis-Menten kinetics. A Hill coefficient of 1.8 indicates that the dehydratase exists as a multisubunit enzyme that shows cooperativity. A mild form of hyperphenylalaninemia with high 7-biopterin levels has been linked to mutations in the human 4a-carbinolamine dehydratase gene. We have now cloned and expressed two mutant forms of the protein based on a patient's DNA sequences. The kinetic parameters of the mutant C82R reveal a 60% decrease in Vmax but no change in Km (approximately 5 microM), suggesting that the cysteine residue is not involved in substrate binding. Its replacement by arginine possibly causes a conformational change in the active center. Like the wild-type enzyme, this mutant is heat stable and forms a tetramer. The susceptibility to proteolysis of C82R, however, is markedly increased in vitro compared with the wild-type protein. We have also observed a decrease in the expression levels of C82R protein in transfected mammalian cells, which could be due to proteolytic instability. The 18-amino acid-truncated mutant GLu-87--> termination could not be completely purified and characterized due to minute levels of expression and its extremely low solubility as a fusion protein. No dehydratase activity was detected in crude extracts from transformed bacteria or transfected mammalian cells. Considering the decrease in specific activity and stability of the mutants, we conclude that the patient probably has less than 10% residual dehydratase activity, which could be responsible for the mild hyperphenylalaninemia and the high 7-biopterin levels.

Structure and function of the aromatic amino acid hydroxylases

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Regulation of melanin biosynthesis in the human epidermis by tetrahydrobiopterin

The participation of (6R) 5,6,7,8-tetrahydrobiopterin (6-BH4) in regulating the tyrosine supply for melanin biosynthesis was investigated by the examination of human keratinocytes, melanocytes, and epidermal suction blisters from normal human skin and from patients with the depigmentation disorder vitiligo. Cells, as well as total epidermis, contained high phenylalanine hydroxylase activities and also displayed the capacity to synthesize and recycle 6-BH4, the essential cofactor for this enzyme. In vitiligo, 4a-hydroxy-BH4 dehydratase activity was extremely low or absent, yielding an accumulation of the nonenzymatic by-product 7-tetrahydrobiopterin (7-BH4) at concentrations up to 8 x 10(-6) M in the epidermis. This by-product is a potent competitive inhibitor in the phenylalanine hydroxylase reaction with an inhibition constant of 10(-6) M. Thus, 6-BH4 seems to control melanin biosynthesis in the human epidermis, whereas 7-BH4 may initiate depigmentation in patients with vitiligo.

Pseudomonas aeruginosa possesses homologues of mammalian phenylalanine hydroxylase and 4 alpha-carbinolamine dehydratase/DCoH as part of a three-component gene cluster

Pseudomonas aeruginosa possesses a multigene operon that includes phenylalanine hydroxylase (PhhA; phenylalanine 4-monooxygenase, EC 1.14.16.1). phhA encodes PhhA (M(r) = 30,288), phhB (M(r) = 13,333) encodes a homologue of mammalian 4 alpha-carbinolamine dehydratase/homeodomain protein transregulator, and phhC encodes an aromatic aminotransferase (M(r) = 43,237). The reading frames specifying phhB and phhC overlap by 2 bases. The P. aeruginosa PhhA appears to contain iron and is pterin dependent. Unlike the multimeric mammalian hydroxylase, the native P. aeruginosa enzyme is a monomer. The P. aeruginosa PhhA is homologous with mammalian PhhA, tryptophan hydroxylase, and tyrosine hydroxylase. Expression of PhhA from its native promoter required phhB. This may suggest a positive regulatory role for phhB, consistent with the dual catalytic and regulatory roles of the corresponding mammalian homologue.

Mutation in the 4a-carbinolamine dehydratase gene leads to mild hyperphenylalaninemia with defective cofactor metabolism

Hyperphenylalaninemias represent a major class of inherited metabolic disorders. They are most often caused by mutations in the phenylalanine hydroxylase gene and, less frequently but with usually more serious consequences, in genes necessary for the synthesis and regeneration of the cofactor, tetrahydrobiopterin. This cofactor is absolutely required for all aromatic amino acid hydroxylations, and, recently, nitric oxide production from L-arginine has also been found to be dependent on tetrahydrobiopterin. Phenylalanine hydroxylase catalyzes a coupled reaction in which phenylalanine is converted to tyrosine and in which tetrahydrobiopterin is converted to the unstable carbinolamine, 4a-hydroxytetrahydrobiopterin. The enzyme, carbinolamine dehydratase, catalyzes the dehydration of the carbinolamine to quinonoid dihydropterin. A decreased rate of dehydration of this compound has been hypothesized to be responsible for the production of 7-biopterin found in certain mildly hyperphenylalaninemic individuals. We have now identified nonsense and missense mutations in the 4a-carbinolamine dehydratase gene in a hyperphenylalaninemic child who excretes large amounts of 7-biopterin. This finding is consistent with the role of the carbinolamine dehydratase in the phenylalanine hydroxylation reaction. Together with previously identified inherited disorders in phenylalanine hydroxylase and dihydropteridine reductase, there are now identified mutations in the three enzymes involved in the phenylalanine hydroxylation system. In addition, the genetics of this system may have broader implications, since the product of the dehydratase gene has previously been shown to play an additional role (as dimerization cofactor for hepatocyte nuclear factor-1 alpha) in the regulation of transcription, through interaction with hepatocyte nuclear factor-1 alpha.