Tetrahydrobiopterin is synthesized from 6-pyruvoyl-tetrahydropterin by the human aldo-keto reductase AKR1 family members

Peripheralized sepiapterin reductase inhibition as a safe analgesic therapy

The development of novel analgesics for chronic pain in the last 2 decades has proven virtually intractable, typically failing due to lack of efficacy and dose-limiting side effects. Identified through unbiased gene expression profiling experiments in rats and confirmed by human genome-wide association studies, the role of excessive tetrahydrobiopterin (BH4) in chronic pain has been validated by numerous clinical and preclinical studies. BH4 is an essential cofactor for aromatic amino acid hydroxylases, nitric oxide synthases, and alkylglycerol monooxygenase so a lack of BH4 leads to a range of symptoms in the periphery and central nervous system (CNS). An ideal therapeutic goal therefore would be to block excessive BH4 production, while preventing potential BH4 rundown. In this review, we make the case that sepiapterin reductase (SPR) inhibition restricted to the periphery (i.e., excluded from the spinal cord and brain), is an efficacious and safe target to alleviate chronic pain. First, we describe how different cell types that engage in BH4 overproduction and contribute to pain hypersensitivity, are themselves restricted to peripheral tissues and show their blockade is sufficient to alleviate pain. We discuss the likely safety profile of peripherally restricted SPR inhibition based on human genetic data, the biochemical alternate routes of BH4 production in various tissues and species, and the potential pitfalls to predictive translation when using rodents. Finally, we propose and discuss possible formulation and molecular strategies to achieve peripherally restricted, potent SPR inhibition to treat not only chronic pain but other conditions where excessive BH4 has been demonstrated to be pathological.

Simultaneous assay of urine sepiapterin and creatinine in patients with sepiapterin reductase deficiency

OBJECTIVES

Sepiapterin reductase deficiency (SRD) causes central nervous system symptoms due to dopamine and serotonin depletion because sepiapterin reductase plays an important role in tetrahydrobiopterin biosynthesis. SRD cannot be detected by newborn screening because of the absent hyperphenylalaninemia. To diagnose SRD biochemically, confirmation of reduced monoamine metabolites and elevated sepiapterin in the cerebrospinal fluid (CSF) has been considered necessary, because a past study showed no elevation of urine sepiapterin. Recently, however, the elevation of urine sepiapterin in SRD was reported.

METHODS

We developed a fast method to measure sepiapterin and creatinine simultaneously using high-performance liquid chromatography with fluorescence and ultraviolet detection. Urine sepiapterin and creatinine were measured in three SRD patients, two SRD carriers, four SRD siblings, and 103 non-SRD patients.

RESULTS

In the three SRD cases, concentrations of urine sepiapterin were 1086, 914, and 575 µmol/mol creatinine (upper limit: 101.7 µmol/mol creatinine), and were markedly higher than those in other groups. CSF sepiapterin concentration was also measured in one SRD case and it was 4.1 nmol/L (upper limit: 0.5 nmol/L).

CONCLUSIONS

The simultaneous determination of urine sepiapterin and creatinine appears helpful for the diagnosis of SRD. This assay system can also be used to measure sepiapterin in the CSF.

Molecular and metabolic bases of tetrahydrobiopterin (BH4) deficiencies

Tetrahydrobiopterin (BH4) deficiency is caused by genetic variants in the three genes involved in de novo cofactor biosynthesis, GTP cyclohydrolase I (GTPCH/GCH1), 6-pyruvoyl-tetrahydropterin synthase (PTPS/PTS), sepiapterin reductase (SR/SPR), and the two genes involved in cofactor recycling, carbinolamine-4α-dehydratase (PCD/PCBD1) and dihydropteridine reductase (DHPR/QDPR). Dysfunction in BH4 metabolism leads to reduced cofactor levels and may result in systemic hyperphenylalaninemia and/or neurological sequelae due to secondary deficiency in monoamine neurotransmitters in the central nervous system. More than 1100 patients with BH4 deficiency and 800 different allelic variants distributed throughout the individual genes are tabulated in database of pediatric neurotransmitter disorders PNDdb. Here we provide an update on the molecular-genetic analysis and structural considerations of these variants, including the clinical courses of the genotypes. From a total of 324 alleles, 11 are associated with the autosomal recessive form of GTPCH deficiency presenting with hyperphenylalaninemia (HPA) and neurotransmitter deficiency, 295 GCH1 variant alleles are detected in the dominant form of L-dopa-responsive dystonia (DRD or Segawa disease) while phenotypes of 18 alleles remained undefined. Autosomal recessive variants observed in the PTS (199 variants), PCBD1 (32 variants), and QDPR (141 variants) genes lead to HPA concomitant with central monoamine neurotransmitter deficiency, while SPR deficiency (104 variants) presents without hyperphenylalaninemia. The clinical impact of reported variants is essential for genetic counseling and important for development of precision medicine.

Protective Effect of Aldo-keto Reductase 1B1 Against Neuronal Cell Damage Elicited by 4'-Fluoro-α-pyrrolidinononanophenone

Chronic exposure to cathinone derivatives increases the risk of severe health hazards, whereas little is known about the detailed pathogenic mechanisms triggered by the derivatives. We have recently shown that treatment with α-pyrrolidinononanophenone (α-PNP, a highly lipophilic cathinone derivative possessing a long hydrocarbon main chain) provokes neuronal cell apoptosis and its 4'-fluorinated analog (F-α-PNP) potently augments the apoptotic effect. In this study, we found that neuronal SK-N-SH cell damage elicited by F-α-PNP treatment is augmented most potently by pre-incubation with an AKR1B1 inhibitor tolrestat, among specific inhibitors of four aldo-keto reductase (AKR) family members (1B1, 1C1, 1C2, and 1C3) expressed in the neuronal cells. In addition, forced overexpression of AKR1B1 remarkably lowered the cell sensitivity to F-α-PNP toxicity, clearly indicating that AKR1B1 protects from neurotoxicity of the derivative. Treatment of SK-N-SH cells with F-α-PNP resulted in a dose-dependent up-regulation of AKR1B1 expression and activation of its transcription factor NF-E2-related factor 2. Metabolic analyses using liquid chromatography/mass spectrometry/mass spectrometry revealed that AKR1B1 is hardly involved in the F-α-PNP metabolism. The F-α-PNP treatment resulted in production of reactive oxygen species and lipid peroxidation byproduct 4-hydroxy-2-nonenal (HNE) in the cells. The enhanced HNE level was reduced by overexpression of AKR1B1, which also lessened the cell damage elicited by HNE. These results suggest that the AKR1B1-mediated neuronal cell protection is due to detoxification of HNE formed by F-α-PNP treatment, but not to metabolism of the derivative.

Sepiapterin reductase: Characteristics and role in diseases

Sepiapterin reductase, a homodimer composed of two subunits, plays an important role in the biosynthesis of tetrahydrobiopterin. Furthermore, sepiapterin reductase exhibits a wide distribution in different tissues and is associated with many diseases, including brain dysfunction, chronic pain, cardiovascular disease and cancer. With regard to drugs targeting sepiapterin reductase, many compounds have been identified and provide potential methods to treat various diseases. However, the underlying mechanism of sepiapterin reductase in many biological processes is unclear. Therefore, this article summarized the structure, distribution and function of sepiapterin reductase, as well as the relationship between sepiapterin reductase and different diseases, with the aim of finding evidence to guide further studies on the molecular mechanisms and the potential clinical value of sepiapterin reductase. In particular, the different effects induced by the depletion of sepiapterin reductase or the inhibition of the enzyme suggest that the non-enzymatic activity of sepiapterin reductase could function in certain biological processes, which also provides a possible direction for sepiapterin reductase research.

Dimeric dihydrodiol dehydrogenase is an efficient primate 1,5-anhydro-D-fructose reductase

1,5-Anhydro-D-fructose (AF), a metabolite of the anhydrofructose pathway of glycogen metabolism, has recently been shown to react with intracellular proteins and form advanced glycation end-products. The reactive AF is metabolized to non-reactive 1,5-anhydro-D-glucitol by AF reductase in animal tissues and human cells. Pig and mouse AF reductases were characterized, but primate AF reductase remains unknown. Here, we examined the AF-reducing activity of eleven primate NADPH-dependent reductases with broad substrate specificity for carbonyl compounds. AF was reduced by monkey dimeric dihydrodiol dehydrogenase (DHDH), human aldehyde reductase (AKR1A1) and human dicarbonyl/L-xylulose reductase (DCXR). DHDH showed the lowest K_M (21 μM) for AF, and its k_cat/K_M value (1208 s^-1mM^-1) was much higher than those of AKR1A1 (1.3 s^-1mM^-1), DCXR (1.1 s^-1mM^-1) and the pig and mouse AF reductases. AF is a novel substrate with higher affinity and catalytic efficiency than known substrates of DHDH. Docking simulation study suggested that Lys156 in the substrate-binding site of DHDH contributes to the high affinity for AF. Gene database searches identified DHDH homologues (with >95% amino acid sequence identity) in humans and apes. Thus, DHDH acts as an efficient AF reductase in primates.

Analysis of Catecholamines and Pterins in Inborn Errors of Monoamine Neurotransmitter Metabolism-From Past to Future

Inborn errors of monoamine neurotransmitter biosynthesis and degradation belong to the rare inborn errors of metabolism. They are caused by monogenic variants in the genes encoding the proteins involved in (1) neurotransmitter biosynthesis (like tyrosine hydroxylase (TH) and aromatic amino acid decarboxylase (AADC)), (2) in tetrahydrobiopterin (BH₄) cofactor biosynthesis (GTP cyclohydrolase 1 (GTPCH), 6-pyruvoyl-tetrahydropterin synthase (PTPS), sepiapterin reductase (SPR)) and recycling (pterin-4a-carbinolamine dehydratase (PCD), dihydropteridine reductase (DHPR)), or (3) in co-chaperones (DNAJC12). Clinically, they present early during childhood with a lack of monoamine neurotransmitters, especially dopamine and its products norepinephrine and epinephrine. Classical symptoms include autonomous dysregulations, hypotonia, movement disorders, and developmental delay. Therapy is predominantly based on supplementation of missing cofactors or neurotransmitter precursors. However, diagnosis is difficult and is predominantly based on quantitative detection of neurotransmitters, cofactors, and precursors in cerebrospinal fluid (CSF), urine, and blood. This review aims at summarizing the diverse analytical tools routinely used for diagnosis to determine quantitatively the amounts of neurotransmitters and cofactors in the different types of samples used to identify patients suffering from these rare diseases.

Synthesis of Potent and Selective Inhibitors of Aldo-Keto Reductase 1B10 and Their Efficacy against Proliferation, Metastasis, and Cisplatin Resistance of Lung Cancer Cells

Aldo-keto reductase 1B10 (AKR1B10) is overexpressed in several extraintestinal cancers, particularly in non-small-cell lung cancer, where AKR1B10 is a potential diagnostic marker and therapeutic target. Selective AKR1B10 inhibitors are required because compounds should not inhibit the highly related aldose reductase that is involved in monosaccharide and prostaglandin metabolism. Currently, 7-hydroxy-2-(4-methoxyphenylimino)-2H-chromene-3-carboxylic acid benzylamide (HMPC) is known to be the most potent competitive inhibitor of AKR1B10, but it is nonselective. In this study, derivatives of HMPC were synthesized by removing the 4-methoxyphenylimino moiety and replacing the benzylamide with phenylpropylamide. Among them, 4c and 4e showed higher AKR1B10 inhibitory potency (IC₅₀ 4.2 and 3.5 nM, respectively) and selectivity than HMPC. The treatments with the two compounds significantly suppressed not only migration, proliferation, and metastasis of lung cancer A549 cells but also metastatic and invasive potentials of cisplatin-resistant A549 cells.

Investigation of a potential role for aldose reductase AlrA in tetrahydropteridine synthesis in Dictyostelium discoideum Ax2

Dictyostelium discoideum Ax2 is well-known for the synthesis of d-threo-tetrahydrobiopterin (DH4) with a smaller amount of l-erythro-tetrahydrobiopterin (BH4). DH4 synthesis from 6-pyruvoyltetrahydropterin (PPH4) is catalyzed by aldose reductase (AR)-like protein and sepiapterin reductase (SR) via an intermediate 1′-oxo-2′-d-hydroxypropyl tetrahydropterin, which is non-enzymatically oxidized to d-sepiapterin in the absence of SR. However, l-sepiapterin was a dominant product in the reaction of a cellular extract of spr− disrupted in the SR gene. In order to investigate its potential role in tetrahydropteridine synthesis, the enzyme catalyzing l-sepiapterin synthesis from PPH4 was purified from spr −. Via mass spectrometry, the protein was identified to be encoded by alrA. AlrA consists of 297 amino acid residues sharing a high sequence identity with human AR. However, in the co-incubation assay, DH4 synthesis was not detected and, furthermore, the recombinant AlrA was observed to suppress BH4 synthesis by SR, which was known to prefer 1′-oxo-2′-d-hydroxypropyl tetrahydropterin to PPH4. Although intracellular DH4 level in alrA − was decreased to 60% of the wild type, it is presumed to result from the antioxidant function of DH4. Therefore, despite the structural and catalytic identities with human AR, AlrA seems to be involved in neither BH4, nor DH4 synthesis under normal physiological conditions.

Inhibition of aldo-keto reductase family 1 member B10 by unsaturated fatty acids

A human member of the aldo-keto reductase (AKR) superfamily, AKR1B10, is a cytosolic NADPH-dependent reductase toward various carbonyl compounds including reactive aldehydes, and is normally expressed in intestines. The enzyme is overexpressed in several extraintestinal cancers, and suggested as a potential target for cancer treatment. We found that saturated and cis-unsaturated fatty acids inhibit AKR1B10. Among the saturated fatty acids, myristic acid was the most potent, showing the IC₅₀ value of 4.2 μM cis-Unsaturated fatty acids inhibited AKR1B10 more potently, and linoleic, arachidonic, and docosahexaenoic acids showed the lowest IC₅₀ values of 1.1 μM. The inhibition by these fatty acids was reversible and kinetically competitive with respect to the substrate, showing the K_i values of 0.24-1.1 μM. These fatty acids, except for α-linoleic acid, were much less inhibitory to structurally similar aldose reductase. Site-directed mutagenesis study suggested that the fatty acids interact with several active site residues of AKR1B10, of which Gln114, Val301 and Gln303 are responsible for the inhibitory selectivity. Linoleic and arachidonic acids also effectively inhibited AKR1B10-mediated 4-oxo-2-nonenal metabolism in HCT-15 cells. Thus, the cis-unsaturated fatty acids may be used as an adjuvant therapy for treatment of cancers that up-regulate AKR1B10.

Urine sepiapterin excretion as a new diagnostic marker for sepiapterin reductase deficiency

Sepiapterin reductase deficiency (SRD) causes depletion of biogenic amines in the brain, early onset motor disorder, and intellectual disability. The diagnostic marker for this rare disease is increased sepiapterin and biopterin in CSF. Through a new analytic methodology we demonstrated accumulation of sepiapterin in urine of four SRD patients several times greater than that found in healthy controls and carriers, regardless of age or treatment. Our findings suggest a new interpretation of current theories of peripheral pterin metabolism and provide a new noninvasive diagnostic tool for children with early onset cryptogenetic developmental delay and/or movement disorder.

Cuticle integrity and biogenic amine synthesis in Caenorhabditis elegans require the cofactor tetrahydrobiopterin (BH4)

Tetrahydrobiopterin (BH4) is the natural cofactor of several enzymes widely distributed among eukaryotes, including aromatic amino acid hydroxylases (AAAHs), nitric oxide synthases (NOSs), and alkylglycerol monooxygenase (AGMO). We show here that the nematode Caenorhabditis elegans, which has three AAAH genes and one AGMO gene, contains BH4 and has genes that function in BH4 synthesis and regeneration. Knockout mutants for putative BH4 synthetic enzyme genes lack the predicted enzymatic activities, synthesize no BH4, and have indistinguishable behavioral and neurotransmitter phenotypes, including serotonin and dopamine deficiency. The BH4 regeneration enzymes are not required for steady-state levels of biogenic amines, but become rate limiting in conditions of reduced BH4 synthesis. BH4-deficient mutants also have a fragile cuticle and are generally hypersensitive to exogenous agents, a phenotype that is not due to AAAH deficiency, but rather to dysfunction in the lipid metabolic enzyme AGMO, which is expressed in the epidermis. Loss of AGMO or BH4 synthesis also specifically alters the sensitivity of C. elegans to bacterial pathogens, revealing a cuticular function for AGMO-dependent lipid metabolism in host-pathogen interactions.

Synthesis of 8-hydroxy-2-iminochromene derivatives as selective and potent inhibitors of human carbonyl reductase 1

Human carbonyl reductase 1 (CBR1), a member of the short-chain dehydrogenase/reductase superfamily, reduces anthracycline anticancer drugs to their less potent anticancer C-13 hydroxy metabolites, which are linked with pathogenesis of cardiotoxicity, a side effect of the drugs.

Nitric oxide confers cisplatin resistance in human lung cancer cells through upregulation of aldo-keto reductase 1B10 and proteasome

In this study, we show that exposure of human lung cancer A549 cells to cisplatin (cis-diamminedichloroplatinum, CDDP) promotes production of nitric oxide (NO) through generation of reactive oxygen species (ROS) and resulting upregulation of inducible NO synthase (iNOS). The incubation of the cells with a NO donor, diethylenetriamine NONOate, not only reduced the CDDP-induced cell death and apoptotic alterations (induction of CCAAT-enhancer-binding protein homologous protein and caspase-3 activation), but also elevated proteolytic activity of 26S proteasome, suggesting that the activation of proteasome function contributes to the reduction of CDDP sensitivity by NO. Monitoring expression levels of six aldo-keto reductases (AKRs) (1A1, 1B1, 1B10, 1C1, 1C2, and 1C3) during the treatment with the NO donor and subsequent CDDP sensitivity test using the specific inhibitors also proposed that upregulation of AKR1B10 by NO is a key process for acquiring the CDDP resistance in A549 cells. Treatment with CDDP and NO increased amounts of nitrotyrosine protein adducts, indicative of peroxynitrite formation, and promoted the induction of AKR1B10, inferring a relationship between peroxynitrite formation and the enzyme upregulation in the cells. The treatment with CDDP or a ROS-related lipid aldehyde, 4-hydroxy-2-nonenal, facilitated the iNOS upregulation, which was restored by increasing the AKR1B10 expression. In contrast, the facilitation of NO production by CDDP treatment was hardly observed in AKR1B10-overexpressing A549 cells and established CDDP-resistant cancer cells (A549, LoVo, and PC3). Collectively, these results suggest the NO functions as a key regulator controlling AKR1B10 expression and 26S proteasome function leading to gain of the CDDP resistance.

Biochemical and structural studies of 6-carboxy-5,6,7,8-tetrahydropterin synthase reveal the molecular basis of catalytic promiscuity within the tunnel-fold superfamily

6-Pyruvoyltetrahydropterin synthase (PTPS) homologs in both mammals and bacteria catalyze distinct reactions using the same 7,8-dihydroneopterin triphosphate substrate. The mammalian enzyme converts 7,8-dihydroneopterin triphosphate to 6-pyruvoyltetrahydropterin, whereas the bacterial enzyme catalyzes the formation of 6-carboxy-5,6,7,8-tetrahydropterin. To understand the basis for the differential activities we determined the crystal structure of a bacterial PTPS homolog in the presence and absence of various ligands. Comparison to mammalian structures revealed that although the active sites are nearly structurally identical, the bacterial enzyme houses a His/Asp dyad that is absent from the mammalian protein. Steady state and time-resolved kinetic analysis of the reaction catalyzed by the bacterial homolog revealed that these residues are responsible for the catalytic divergence. This study demonstrates how small variations in the active site can lead to the emergence of new functions in existing protein folds.

Synthesis and structure-activity relationship of 2-phenyliminochromene derivatives as inhibitors for aldo-keto reductase (AKR) 1B10

Inhibitors of a human member (AKR1B10) of the aldo-keto reductase superfamily are regarded as promising therapeutics for the treatment of cancer. Recently, we have discovered (Z)-2-(4-methoxyphenylimino)-7-hydroxy-N-(pyridin-2-yl)-2H-chromene-3-carboxamide (1) as the potent competitive inhibitor using the virtual screening approach, and proposed its 4-methoxy group on the 2-phenylimino moiety as an essential structural prerequisite for the inhibition. In this study, 18 derivatives of 1 were synthesized and their inhibitory potency against AKR1B10 evaluated. Among them, 7-hydroxy-2-(4-methoxyphenylimino)-2H-chromene-3-carboxylic acid benzylamide (5n) was the most potent inhibitor showing a Ki value of 1.3nM. The structure-activity relationship of the derivatives indicated that the 7-hydroxyl group on the chromene ring, but not the 4-methoxy group, was absolutely required for inhibitory activity, The molecular docking of 5n in AKR1B10 and site-directed mutagenesis of the enzyme residues suggested that the hydrogen-bond interactions between the 7-hydroxyl group of 5n and the catalytic residues (Tyr49 and His111) of the enzyme, together with a π-stacking interaction of the benzylamide moiety of 5n with Trp220, are important for the potent inhibition.

Pathophysiological roles of aldo-keto reductases (AKR1C1 and AKR1C3) in development of cisplatin resistance in human colon cancers

Cisplatin (cis-diamminedichloroplatinum, CDDP) is widely used for treatment of patients with solid tumors formed in various organs including the lung, prostate and cervix, but is much less sensitive in colon and breast cancers. One major factor implicated in the ineffectiveness has been suggested to be acquisition of the CDDP resistance. Here, we established the CDDP-resistant phenotypes of human colon HCT15 cells by continuously exposing them to incremental concentrations of the drug, and monitored expressions of aldo-keto reductases (AKRs) 1A1, 1B1, 1B10, 1C1, 1C2 and 1C3. Among the six AKRs, AKR1C1 and AKR1C3 are highly induced with the CDDP resistance. The resistance lowered the sensitivity toward cellular damages evoked by oxidative stress-derived aldehydes, 4-hydroxy-2-nonenal and 4-oxo-2-nonenal that are detoxified by AKR1C1 and AKR1C3. Overexpression of AKR1C1 or AKR1C3 in the parental HCT15 cells mitigated the cytotoxicity of the aldehydes and CDDP. Knockdown of both AKR1C1 and AKR1C3 in the resistant cells or treatment of the cells with specific inhibitors of the AKRs increased the sensitivity to CDDP toxicity. Thus, the two AKRs participate in the mechanism underlying the CDDP resistance probably via detoxification of the aldehydes resulting from enhanced oxidative stress. The resistant cells also showed an enhancement in proteolytic activity of proteasome accompanied by overexpression of its catalytic subunits (PSMβ9 and PSMβ10). Pretreatment of the resistant cells with a potent proteasome inhibitor Z-Leu-Leu-Leu-al augmented the CDDP sensitization elicited by the AKR inhibitors. Additionally, the treatment of the cells with Z-Leu-Leu-Leu-al and the AKR inhibitors induced the expressions of the two AKRs and proteasome subunits. Collectively, these results suggest the involvement of up-regulated AKR1C1, AKR1C3 and proteasome in CDDP resistance of colon cancers and support a chemotherapeutic role for their inhibitors.

Reduction of cytotoxic p-quinone metabolites of tert-butylhydroquinone by human aldo-keto reductase (AKR) 1B10

2-tert-Butylhydroquinone (BHQ), an antioxidant used as a food additive, exhibits an anticancer effect, whereas it is carcinogenic in rodents at high doses. BHQ is metabolized into cytotoxic tert-butylquinone (BQ), which is further converted to 6-tert-butyl-2,3-epoxy-4-hydroxy-5-cyclohexen-1-one (TBEH) through 6-tert-butyl-2,3-epoxy-4-benzoquinone (TBE), which induces chromosomal aberration. The reductases for BQ and TBE may be protective against the toxicity of the two p-quinones, but the responsible human enzymes remain unidentified. In this study, we compared the ability of 12 human recombinant enzymes in the aldo-keto reductase (AKR) and short-chain dehydrogenase/reductase superfamilies to reduce BQ and TBE. Among them, AKR1B10 was the most efficient catalyst of the stoichiometric two-electron reduction of BQ and TBE into BHQ and TBEH, respectively. BQ and TBE are more cytotoxic towards endothelial cells than BHQ and TBEH, and their cytotoxicity was decreased by the overexpression of AKR1B10 in the cells. Additionally, AKR1B10 gene expression in human HCT116 cells was up-regulated by treatments with BHQ, BQ and TBE. These results suggest a role for the enzyme in protection at least against the toxicity of the two p-quinone metabolites of BHQ.

Functional promiscuity of the COG0720 family

The biosynthesis of GTP derived metabolites such as tetrahydrofolate (THF), biopterin (BH(4)), and the modified tRNA nucleosides queuosine (Q) and archaeosine (G(+)) relies on several enzymes of the Tunnel-fold superfamily. A subset of these proteins includes the 6-pyruvoyltetrahydropterin (PTPS-II), PTPS-III, and PTPS-I homologues, all members of the COG0720 family that have been previously shown to transform 7,8-dihydroneopterin triphosphate (H(2)NTP) into different products. PTPS-II catalyzes the formation of 6-pyruvoyltetrahydropterin in the BH(4) pathway, PTPS-III catalyzes the formation of 6-hydroxylmethyl-7,8-dihydropterin in the THF pathway, and PTPS-I catalyzes the formation of 6-carboxy-5,6,7,8-tetrahydropterin in the Q pathway. Genes of these three enzyme families are often misannotated as they are difficult to differentiate by sequence similarity alone. Using a combination of physical clustering, signature motif, phylogenetic codistribution analyses, in vivo complementation studies, and in vitro enzymatic assays, a complete reannotation of the COG0720 family was performed in prokaryotes. Notably, this work identified and experimentally validated dual function PTPS-I/III enzymes involved in both THF and Q biosynthesis. Both in vivo and in vitro analyses showed that the PTPS-I family could tolerate a translation of the active site cysteine and was inherently promiscuous, catalyzing different reactions on the same substrate or the same reaction on different substrates. Finally, the analysis and experimental validation of several archaeal COG0720 members confirmed the role of PTPS-I in archaeosine biosynthesis and resulted in the identification of PTPS-III enzymes with variant signature sequences in Sulfolobus species. This study reveals an expanded versatility of the COG0720 family members and illustrates that for certain protein families extensive comparative genomic analysis beyond homology is required to correctly predict function.

Design, synthesis and evaluation of caffeic acid phenethyl ester-based inhibitors targeting a selectivity pocket in the active site of human aldo-keto reductase 1B10

Inhibitors of a human aldo-keto reductase, AKR1B10, are regarded as promising therapeutics for the treatment of cancer, but those with both high potency and selectivity compared to the structurally similar aldose reductase (AKR1B1) have not been reported. In this study, we have found that, among honeybee propolis products, caffeic acid phenethyl ester (CAPE) inhibited AKR1B10 (IC(50) = 80 nM) with 7-fold selectivity over AKR1B1. Based on a model of docked CAPE in AKR1B10, its derivatives were designed, synthesized and evaluated for inhibitory potency. Among them, 3-(4-hydroxy-2-methoxyphenyl)acrylic acid 3-(3-hydroxyphenyl)propyl ester (10c) was the most potent competitive inhibitor (K(i) = 2.6 nM) with 790-fold selectivity for AKR1B10 over AKR1B1. Molecular docking of 10c and site-directed mutagenesis of AKR1B10 residues suggested that the interactions between the 2-methoxy and 3-hydroxy groups of 10c and the enzyme's Val301 and Gln114, respectively, are important for the inhibitor's selectivity. Additionally, the sub-μM concentration of 10c significantly suppressed the farnesal metabolism and cellular proliferation in AKR1B10-overexpressing cells.

Tetrahydrobiopterin: biochemistry and pathophysiology

BH4 (6R-L-erythro-5,6,7,8-tetrahydrobiopterin) is an essential cofactor of a set of enzymes that are of central metabolic importance, including four aromatic amino acid hydroxylases, alkylglycerol mono-oxygenase and three NOS (NO synthase) isoenzymes. Consequently, BH4 is present in probably every cell or tissue of higher organisms and plays a key role in a number of biological processes and pathological states associated with monoamine neurotransmitter formation, cardiovascular and endothelial dysfunction, the immune response and pain sensitivity. BH4 is formed de novo from GTP via a sequence of three enzymatic steps carried out by GTP cyclohydrolase I, 6-pyruvoyltetrahydropterin synthase and sepiapterin reductase. An alternative or salvage pathway involves dihydrofolate reductase and may play an essential role in peripheral tissues. Cofactor regeneration requires pterin-4a-carbinolamine dehydratase and dihydropteridine reductase, except for NOSs, in which the BH4 cofactor undergoes a one-electron redox cycle without the need for additional regeneration enzymes. With regard to the regulation of cofactor biosynthesis, the major controlling point is GTP cyclohydrolase I. BH4 biosynthesis is controlled in mammals by hormones and cytokines. BH4 deficiency due to autosomal recessive mutations in all enzymes, except for sepiapterin reductase, has been described as a cause of hyperphenylalaninaemia. A major contributor to vascular dysfunction associated with hypertension, ischaemic reperfusion injury, diabetes and others, appears to be an effect of oxidized BH4, which 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, and 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.

Selective inhibition of the tumor marker aldo-keto reductase family member 1B10 by oleanolic acid

A human member of the aldo-keto reductase (AKR) superfamily, AKR1B10, was recently suggested as a therapeutic target in the treatment of several types of cancer. Due to its high sequence identity with human aldose reductase (AKR1B1), selective inhibition of AKR1B10 compared with AKR1B1 is required for the development of anticancer agents. In this study, we have examined AKR1B10 inhibition by seven pentacyclic triterpenes (1-7) that show potential anticancer properties. Among them, oleanolic acid (1) was found to be the most potent competitive inhibitor (inhibition constant, 72 nM) with the highest AKR1B10/AKR1B1 selectivity ratio of 1370. Molecular docking of 1 with AKR1B10 and AKR1B1 and site-directed mutagenesis studies suggested that the nonconserved residues Val301 and Gln303 in AKR1B10 are important for determining its inhibitory potency and selectivity. Oleanolic acid (1) also inhibited the cellular metabolism by AKR1B10 (IC(50), 4 μM) and decreased mitomycin C tolerance of colon cancer HT29 cells. Thus, the selective and potent inhibition of AKR1B10 by 1 may be related to a possible cancer inhibitory role.

Queuosine deficiency in eukaryotes compromises tyrosine production through increased tetrahydrobiopterin oxidation

Queuosine is a modified pyrrolopyrimidine nucleoside found in the anticodon loop of transfer RNA acceptors for the amino acids tyrosine, asparagine, aspartic acid, and histidine. Because it is exclusively synthesized by bacteria, higher eukaryotes must salvage queuosine or its nucleobase queuine from food and the gut microflora. Previously, animals made deficient in queuine died within 18 days of withdrawing tyrosine, a nonessential amino acid, from the diet (Marks, T., and Farkas, W. R. (1997) Biochem. Biophys. Res. Commun. 230, 233-237). Here, we show that human HepG2 cells deficient in queuine and mice made deficient in queuosine-modified transfer RNA, by disruption of the tRNA guanine transglycosylase enzyme, are compromised in their ability to produce tyrosine from phenylalanine. This has similarities to the disease phenylketonuria, which arises from mutation in the enzyme phenylalanine hydroxylase or from a decrease in the supply of its cofactor tetrahydrobiopterin (BH4). Immunoblot and kinetic analysis of liver from tRNA guanine transglycosylase-deficient animals indicates normal expression and activity of phenylalanine hydroxylase. By contrast, BH4 levels are significantly decreased in the plasma, and both plasma and urine show a clear elevation in dihydrobiopterin, an oxidation product of BH4, despite normal activity of the salvage enzyme dihydrofolate reductase. Our data suggest that queuosine modification limits BH4 oxidation in vivo and thereby potentially impacts on numerous physiological processes in eukaryotes.

Partial biopterin deficiency disturbs postnatal development of the dopaminergic system in the brain

Postnatal development of dopaminergic system is closely related to the development of psychomotor function. Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the biosynthesis of dopamine and requires tetrahydrobiopterin (BH4) as a cofactor. To clarify the effect of partial BH4 deficiency on postnatal development of the dopaminergic system, we examined two lines of mutant mice lacking a BH4-biosynthesizing enzyme, including sepiapterin reductase knock-out (Spr(-/-)) mice and genetically rescued 6-pyruvoyltetrahydropterin synthase knock-out (DPS-Pts(-/-)) mice. We found that biopterin contents in the brains of these knock-out mice were moderately decreased from postnatal day 0 (P0) and remained constant up to P21. In contrast, the effects of BH4 deficiency on dopamine and TH protein levels were more manifested during the postnatal development. Both of dopamine and TH protein levels were greatly increased from P0 to P21 in wild-type mice but not in those mutant mice. Serotonin levels in those mutant mice were also severely suppressed after P7. Moreover, striatal TH immunoreactivity in Spr(-/-) mice showed a drop in the late developmental stage, when those mice exhibited hind-limb clasping behavior, a type of motor dysfunction. Our results demonstrate a critical role of biopterin in the augmentation of TH protein in the postnatal period. The developmental manifestation of psychomotor symptoms in BH4 deficiency might be attributable at least partially to high dependence of dopaminergic development on BH4 availability.

Selective inhibition of the tumor marker AKR1B10 by antiinflammatory N-phenylanthranilic acids and glycyrrhetic acid

A human aldose reductase-like protein, AKR1B10 in the aldo-keto reductase (AKR) superfamily, was recently identified as a tumor marker of several types of cancer. Tolrestat, an aldose reductase inhibitor (ARI), is known to be the most potent inhibitor of the enzyme. In this study, we compared the inhibitory effects of other ARIs including flavonoids on AKR1B10 and aldose reductase to evaluate their specificity. However, ARIs showed lower inhibitory potency for AKR1B10 than for aldose reductase. In the search for potent and selective inhibitors of AKR1B10 from other drugs used clinically, we found that non-steroidal antiinflammatory N-phenylanthranilic acids, diclofenac and glycyrrhetic acid competitively inhibited AKR1B10, showing K(i) values of 0.35-2.9 microM and high selectivity to this enzyme (43-57 fold versus aldose reductase). Molecular docking studies of mefenamic acid and glycyrrhetic acid in the AKR1B10-nicotinamide adenine dinucleotide phosphate (NADP(+)) complex and site-directed mutagenesis of the putative binding residues suggest that the side chain of Val301 and a hydrogen-bonding network among residues Val301, Gln114 and Ser304 are important for determining the inhibitory potency and selectivity of the non-steroidal antiinflammatory drugs. Thus, the potent and selective inhibition may be related to the cancer chemopreventive roles of the drugs, and their structural features may facilitate the design of new anti-cancer agents targeting AKR1B10.

Roles of rat and human aldo-keto reductases in metabolism of farnesol and geranylgeraniol

Farnesol (FOH) and geranylgeraniol (GGOH) with multiple biological actions are produced from the mevalonate pathway, and catabolized into farnesoic acid and geranylgeranoic acid, respectively, via the aldehyde intermediates (farnesal and geranylgeranial). We investigated the intracellular distribution, sequences and properties of the oxidoreductases responsible for the metabolic steps in rat tissues. The oxidation of FOH and GGOH into their aldehyde intermediates were mainly mediated by alcohol dehydrogenases 1 (in the liver and colon) and 7 (in the stomach and lung), and the subsequent step into the carboxylic acids was catalyzed by a microsomal aldehyde dehydrogenase. In addition, high reductase activity catalyzing the aldehyde intermediates into FOH (or GGOH) was detected in the cytosols of the extra-hepatic tissues, where the major reductase was identified as aldo-keto reductase (AKR) 1C15. Human reductases with similar specificity were identified as AKR1B10 and AKR1C3, which most efficiently reduced farnesal and geranylgeranial among seven enzymes in the AKR1A-1C subfamilies. The overall metabolism from FOH to farnesoic acid in cultured cells was significantly decreased by overexpression of AKR1C15, and increased by addition of AKR1C3 inhibitors, tolfenamic acid and R-flurbiprofen. Thus, AKRs (1C15 in rats, and 1B10 and 1C3 in humans) may play an important role in controlling the bioavailability of FOH and GGOH.

Candidate genes for alcohol preference identified by expression profiling in alcohol-preferring and -nonpreferring reciprocal congenic rats

BACKGROUND

Selectively bred alcohol-preferring (P) and alcohol-nonpreferring (NP) rats differ greatly in alcohol preference, in part due to a highly significant quantitative trait locus (QTL) on chromosome 4. Alcohol consumption scores of reciprocal chromosome 4 congenic strains NP.P and P.NP correlated with the introgressed interval. The goal of this study was to identify candidate genes that may influence alcohol consumption by comparing gene expression in five brain regions of alcohol-naïve inbred alcohol-preferring and P.NP congenic rats: amygdala, nucleus accumbens, hippocampus, caudate putamen, and frontal cortex.

RESULTS

Within the QTL region, 104 cis-regulated probe sets were differentially expressed in more than one region, and an additional 53 were differentially expressed in a single region. Fewer trans-regulated probe sets were detected, and most differed in only one region. Analysis of the average expression values across the 5 brain regions yielded 141 differentially expressed cis-regulated probe sets and 206 trans-regulated probe sets. Comparing the present results from inbred alcohol-preferring vs. congenic P.NP rats to earlier results from the reciprocal congenic NP.P vs. inbred alcohol-nonpreferring rats demonstrated that 74 cis-regulated probe sets were differentially expressed in the same direction and with a consistent magnitude of difference in at least one brain region.

CONCLUSIONS

Cis-regulated candidate genes for alcohol consumption that lie within the chromosome 4 QTL were identified and confirmed by consistent results in two independent experiments with reciprocal congenic rats. These genes are strong candidates for affecting alcohol preference in the inbred alcohol-preferring and inbred alcohol-nonpreferring rats.

Potent and selective inhibition of the tumor marker AKR1B10 by bisdemethoxycurcumin: probing the active site of the enzyme with molecular modeling and site-directed mutagenesis

A human member of the aldo-keto reductase (AKR) superfamily, AKR1B10, shares high sequence identity with aldose reductase (AR), and was recently identified as a therapeutic target in the treatment of several types of cancer. We have compared the inhibitory effects of plant components on recombinant AKR1B10 and AR. AKR1B10 was inhibited by curcuminoids, magnolol, honokiol and resveratrol, with IC(50) values of 0.06-5 microM, which were lower than their values for AR. Among them, bisdemethoxycurcumin was the most potent competitive inhibitor (K(i)=22 nM) with the highest selectivity (85-fold versus AR), and acted as an effective inhibitor in cellular level. In contrast, demethoxycurcumin and curcumin showed >3-fold less potency and selectivity. Molecular docking studies of the curcuminoids in the AKR1B10-NADP(+) complex and site-directed mutagenesis of the putative binding residues suggest that Gln114, Val301 and Gln303 are important for determining the inhibitory potency and selectivity of the curcuminoids.

Expression analysis of the aldo-keto reductases involved in the novel biosynthetic pathway of tetrahydrobiopterin in human and mouse tissues

Tetrahydrobiopterin (BH(4)) acts as a cofactor of the aromatic amino-acid hydroxylases, and its deficiency may result in hyperphenylalaninemia (HPA) and decreased production of the neurotransmitters. BH(4) is synthesized by sepiapterin reductase (SPR) from 6-pyruvoyl-tetrahydropterin (PPH(4)). A patient with SPR deficiency shows no HPA; however, an SPR knockout mouse exhibits HPA. We have reported on the SPR-unrelated novel biosynthetic pathway from PPH(4) to BH(4) (salvage pathway II) in which 3alpha-hydroxysteroid dehydrogenase type 2 and aldose reductase work in concert. In this study, we performed the expression analysis of both proteins in humans and wild-type mice. The results of expression analysis indicated that salvage pathway II worked in human liver; however, it did not act in human brain or in mouse liver and brain. For this reason, a patient with SPR deficiency may show progressive neurological deterioration without HPA, and SPR knockout mice may exhibit HPA and abnormal locomotion activity.

Kinetic studies of AKR1B10, human aldose reductase-like protein: endogenous substrates and inhibition by steroids

A human member of the aldo-keto reductase (AKR) superfamily, AKR1B10, was identified as a biomarker of lung cancer, exhibiting high sequence identity with human aldose reductase (AKR1B1). Using recombinant AKR1B10 and AKR1B1, we compared their substrate specificity for biogenic compounds and inhibition by endogenous compounds and found the following unique features of AKR1B10. AKR1B10 efficiently reduced long-chain aliphatic aldehydes including farnesal and geranylgeranial, which are generated from degradation of prenylated proteins and metabolism of farnesol and geranylgeraniol derived from the mevalonate pathway. The enzyme oxidized aliphatic and aromatic alcohols including 20alpha-hydroxysteroids. In addition, AKR1B10 was inhibited by steroid hormones, bile acids and their metabolites, showing IC(50) values of 0.03-25 microM. Kinetic analyses of the alcohol oxidation and inhibition by the steroids and tolrestat, together with the docked model of AKR1B10-inhibitor complex, suggest that the inhibitory steroids and tolrestat bind to overlapping sites within the active site of the enzyme-coenzyme complex. Thus, we propose a novel role of AKR1B10 in controlling isoprenoid homeostasis that is important in cholesterol synthesis and cell proliferation through salvaging isoprenoid alcohols, as well as its metabolic regulation by endogenous steroids.

Involvement of an aldo-keto reductase (AKR1C3) in redox cycling of 9,10-phenanthrenequinone leading to apoptosis in human endothelial cells

9,10-Phenanthrenequinone (9,10-PQ), a major quinone found in diesel exhaust particles, is considered to generate reactive oxygen species (ROS) through its redox cycling. Here, we show that 9,10-PQ evokes apoptosis in human aortic endothelial cells (HAECs) and its apoptotic signaling includes ROS generation and caspase activation. The 9,10-PQ-induced cytotoxicity was inhibited by ROS scavengers, indicating that intracellular ROS generation is responsible for the 9,10-PQ-induced apoptosis. Comparison of mRNA expression levels and kinetic constants in the 9,10-PQ reduction among 10 human reductases suggests that aldo-keto reductase 1C3 (AKR1C3) is a 9,10-PQ reductase in HAECs. In in vitro 9,10-PQ reduction by AKR1C3, the reduced product 9,10-dihydroxyphenanthrene and superoxide anions were formed, suggesting the enzymatic two-electron reduction of 9,10-PQ that thereby causes oxidative stress through its redox cycling. In addition, the participation of AKR1C3 in 9,10-PQ-redox cycling was confirmed by the data that AKR1C3 overexpression in endothelial cells augmented the ROS generation and cytotoxicity by 9,10-PQ, and the ROS scavengers inhibited the toxic effects. Pretreatment of the overexpressing cells with AKR1C3 inhibitors, flufenamic acid and indomethacin, suppressed the 9,10-PQ-induced GSH depletion. These results suggest that AKR1C3 is a key enzyme in the initial step of 9,10-PQ-induced cytotoxicity in HAECs.

Carbonyl Reductases and Pluripotent Hydroxysteroid Dehydrogenases of the Short-chain Dehydrogenase/reductase Superfamily

Carbonyl reduction of aldehydes, ketones, and quinones to their corresponding hydroxy derivatives plays an important role in the phase I metabolism of many endogenous (biogenic aldehydes, steroids, prostaglandins, reactive lipid peroxidation products) and xenobiotic (pharmacologic drugs, carcinogens, toxicants) compounds. Carbonyl-reducing enzymes are grouped into two large protein superfamilies: the aldo-keto reductases (AKR) and the short-chain dehydrogenases/reductases (SDR). Whereas aldehyde reductase and aldose reductase are AKRs, several forms of carbonyl reductase belong to the SDRs. In addition, there exist a variety of pluripotent hydroxysteroid dehydrogenases (HSDs) of both superfamilies that specifically catalyze the oxidoreduction at different positions of the steroid nucleus and also catalyze, rather nonspecifically, the reductive metabolism of a great number of nonsteroidal carbonyl compounds. The present review summarizes recent findings on carbonyl reductases and pluripotent HSDs of the SDR protein superfamily.

Two Greek siblings with sepiapterin reductase deficiency

BACKGROUND

Sepiapterin reductase (SR) deficiency is a rare inherited disorder of neurotransmitter metabolism; less than 25 cases have been described in the literature so far.

METHODS

We describe the clinical history and extensive cerebrospinal fluid (CSF) and urine examination of two Greek siblings with the diagnosis of SR deficiency. The diagnosis was confirmed by enzyme activity measurement in cultured fibroblasts and by mutation analysis.

RESULTS

Both patients suffered from a progressive and complex L-dopa responsive movement disorder. Very low concentrations of the neurotransmitter metabolites homovanillic acid (HVA), 5-hydroxyindolacetic acid (5-HIAA) and 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) were observed in CSF. CSF neopterin and biopterin concentrations were abnormal in one case only, whereas in both cases sepiapterin concentrations were abnormally high and 5-hydroxytryptophan was undetectable. Urine concentrations of HVA, 5-HIAA and vanillyl mandelic acid (VMA) were decreased in both cases. Both patients had no detectable SR enzyme activity in primary dermal fibroblasts, and upon analysis of genomic DNA revealed the same homozygous point mutation introducing a premature stop codon into the reading frame of the SPR gene (mutant allele K251X).

CONCLUSIONS

Our cases illustrate that, apart from HVA and 5-HIAA analysis, the specific quantification of sepiapterin in CSF, rather than neopterin and biopterin alone, is crucial to the final diagnosis of SR deficiency. In addition, urinary concentrations of neurotransmitter metabolites may be abnormal in SR deficiency and may provide an initial indication of SR deficiency before CSF analysis is performed. The known, impressive beneficial response of SR deficient patients to treatment with L-dopa, is illustrated again in our cases.

A brain-specific decrease of the tyrosine hydroxylase protein in sepiapterin reductase-null mice--as a mouse model for Parkinson's disease

Sepiapterin reductase (SPR) is an enzyme that acts in the third and final step of tetrahydrobiopterin (BH4) biosynthesis. The human Spr gene locates within the region of 2.5MB mapped to PARK3, an autosomal dominant form of familial Parkinson's diseases. In order to explore the role of SPR in the metabolism of BH4, we produced and analyzed Spr-deficient mice. Most of Spr-null mice survived beyond two weeks. Whereas the BH4 contents in the homozygous mutant mice were greatly decreased than those in wild-type and heterozygous mice, the substantial amounts of BH4 were remained even 17 days after delivery. Spr-null mice exhibited severe monoamine deficiencies and a tremor-like phenotype after weaning. The amount of TH protein in the brain of Spr-null mice was less than 10% of wild-type, while TH protein in the adrenal, phenylalanine hydroxylase protein in the liver, and nNOS in the brain were not altered. These data suggest an essential role of SPR in the biosynthesis of BH4, and that the SPR gene could be a candidate gene for PARK3.

Folate salvage in plants: pterin aldehyde reduction is mediated by multiple non-specific aldehyde reductases

Folates undergo oxidative cleavage in vivo, releasing a pterin aldehyde fragment that can be re-used in folate synthesis if the aldehyde group is reduced. High levels of NADPH-dependent reductase activity against pterin-6-aldehyde and its dihydro form were detected in Arabidopsis, pea and other plants; modeling predicted that the activity would maintain in vivo pterin aldehyde pools at extremely low levels (<0.2 pmol g(-1) FW). Subcellular fractionation showed that the pea leaf activity is mainly cytosolic, and anion exchange chromatography revealed multiple isoforms, all of which catalyzed reduction of other aldehydes. Arabidopsis seed activity likewise comprised various isoforms. An Arabidopsis gene (At1g10310) encoding a pterin aldehyde reductase was identified by searching the short-chain dehydrogenase/reductase family for proteins predicted to be NADPH-linked, and sharing conserved residues with reductases that mediate analogous reactions. The recombinant protein behaved as a dimer in size exclusion chromatography. In addition to pterin aldehydes, it catalyzed the reduction of diverse aromatic and aliphatic aldehydes: Vmax values varied <5-fold, but Km values ranged from 3.6 microm to 1.7 mm, those for pterin-6-aldehyde and dihydropterin-6-aldehyde being 36 and 56 microm, respectively. Activity with dihydropterin-6-aldehyde was unusually high at 0 degrees C. The At1g10310 transcript was most abundant in seeds, but, as expected for multiple isoforms, inactivating the At1g10310 gene caused only a minor change in seed pterin aldehyde reductase activity. We conclude that pterin aldehyde salvage in plants involves multiple, generalist NADPH-linked reductases, and that the At1g10310 enzyme is typical of these and hence suitable for use in engineering studies of folate turnover.

Sepiapterin reductase expression is increased in Parkinson's disease brain tissue

The PARK3 locus on chromosome 2p13 has shown linkage to both the development and age of onset of Parkinson's disease (PD). One candidate gene at this locus is sepiapterin reductase (SPR). Sepiapterin reductase catalyzes the final step in the biosynthetic pathway of tetrahydrobiopterin (BH(4)), an essential cofactor for aromatic amino acid hydrolases including tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis. The expression of SPR was assayed using semiquantitative real-time RT-PCR in human post-mortem cerebellar tissue from neuropathologically confirmed PD cases and neurologically normal controls. The expression of other enzymes involved in BH(4) biosynthesis, including aldose reductase (AKR1B1), carbonyl reductase (CBR1 and CBR3), GTP-cyclohydrolase I (GCH1), and 6-pyruvoyltetrahydrobiopterin (PTS), was also examined. Single-nucleotide polymorphisms around the SPR gene that have been previously reported to show association to PD affection and onset age were genotyped in these samples. Expression of SPR showed a significant 4-fold increase in PD cases relative to controls, while the expression of AKR1B1 and PTS was significantly decreased in PD cases. No difference in expression was detected for CBR1, CBR3, and GCH1. Genetic variants did not show a significant effect on SPR expression, however, this is likely due to the low frequency of rare genotypes in the sample. While the association of SPR to PD is not strong enough to support that this is the PARK3 gene, this study further implicates a role for SPR in idiopathic PD.

Mutations in the BH4-metabolizing genes GTP cyclohydrolase I, 6-pyruvoyl-tetrahydropterin synthase, sepiapterin reductase, carbinolamine-4a-dehydratase, and dihydropteridine reductase

Tetrahydrobiopterin (BH(4)) deficiencies are a highly heterogeneous group of disorders with several hundred patients, and so far a total of 193 different mutant alleles or molecular lesions identified in the GTP cyclohydrolase I (GTPCH), 6-pyruvoyl-tetrahydropterin synthase (PTPS), sepiapterin reductase (SR), carbinolamine-4a-dehydratase (PCD), or dihydropteridine reductase (DHPR) genes. The spectrum of mutations causing a reduction in one of the three biosynthetic (GTPCH, PTPS, and SR) or the two regenerating enzymes (PCD and DHPR) is tabulated and reviewed. Furthermore, current genomic variations or SNPs are also compiled. Mutations in GCH1 are scattered over the entire gene, and only 5 out of 104 mutant alleles, present in a homozygous state, are reported to cause the autosomal recessive form of inheritable hyperphenylalaninemia (HPA) associated with monoamine neurotransmitter deficiency. Almost all other 99 different mutant alleles in GCH1 are observed together with a wild-type allele and cause Dopa-responsive dystonia (DRD, Segawa disease) in a dominant fashion with reduced penetrance. Compound heterozygous or homozygous mutations are spread over the entire genes for PTS with 44 mutant alleles, for PCBD with nine mutant alleles, and for QDPR with 29 mutant alleles. These mutations cause an autosomal recessive inherited form of HPA, mostly accompanied by a deficiency of the neurotransmitters dopamine and serotonin. Lack of sepiapterin reductase activity, an autosomal recessive variant of BH(4) deficiency presenting without HPA, was diagnosed in patients with seven different mutant alleles in the SPR gene in exons 2 or 3 or in intron 2. Details on all mutations presented here are constantly updated in the BIOMDB database (www.bh4.org).