Setsuko Katoh

Direct inhibition of brain sepiapterin reductase by a catecholamine and an indoleamine

Summary Rat brain sepiapterin reductase, which is involved in the biosynthesis of tetrahydrobiopterin, was directly inhibited by a catecholamine and an indoleamine which require tetrahydrobiopterin as a cofactor in the biosyntheses of themselves. A competitive inhibition appeared for sepiapterin reductase with respect to pterin substrate by 1-norepinephrine (catecholamine) and serotonin, N-acetyl-serotonin and melatonin (indoleamines): Ki values of them were estimated as 3.4 mM, 2.3 mM, 0.20 μM and 30 μM, respectively with partially purified sepiapterin reductase from whole rat brain. Some possible co-regulatory mechanisms to control the formation of catecholamines or indoleamines and of tetrahydrobiopterin are discussed.

Cloning and sequencing of cDNA encoding human sepiapterin reductase: An enzyme involved in tetrahydrobiopterin biosynthesis

A full-length cDNA clone for sepiapterin reductase, an enzyme involved in tetrahydrobiopterin biosynthesis, was isolated from a human liver cDNA library by plaque hybridization. The nucleotide sequence of hSPR 8-25, which contained an entire coding region of the enzyme, was determined. The clone encoded a protein of 261 amino acids with a calculated molecular mass of 28,047 daltons. The predicted amino acid sequence of human sepiapterin reductase showed a 74% identity with the rat enzyme. We further found a striking homology between human SPR and carbonyl reductase, estradiol 17 beta-dehydrogenase, and 3 beta-hydroxy-5-ene steroid dehydrogenase, especially in their N-terminal region.

Carbonyl reductase activity of sepiapterin reductase from rat erythrocytes

A homogeneous preparation of sepiapterin reductase, an enzyme involved in the biosynthesis of tetrahydrobiopterin, from rat erythrocytes was found to be responsible for the reduction with NADPH of various carbonyl compounds of non-pteridine derivatives including some vicinal dicarbonyl compounds which were reported in the previous paper (Katoh, S. and Sueoka, T. (1984) Biochem, Biophys. Res. Commun. 118, 859-866) in addition to the general substrate, sepiapterin (2-amino-4-hydroxy-6-lactoyl-7,8-dihydropteridine). The compounds sensitive as substrates of the enzyme were quinones, e.g., p-quinone and menadione; other vicinal dicarbonyls, e.g., methylglyoxal and phenylglyoxal; monoaldehydes, e.g., p-nitrobenzaldehyde; and monoketones, e.g., acetophenone, acetoin, propiophenone and benzylacetone. Rutin, dicoumarol, indomethacin, and ethacrynic acid inhibited the enzyme activity toward either a carbonyl compound of a non-pteridine derivative or sepiapterin as substrate. Sepiapterin reductase is quite similar to general aldo-keto reductases, especially to carbonyl reductase.

Biosynthesis of biopterin by rat brain.

Abstract: A method for the determination of [14C]biopterin biosynthesis from [14C]guanosine‐5′‐triphosphate by a desalted preparation from rat striatum, based on sequential reverse‐phase and cation‐exchange high performance liquid chromatography, is described. Synthesis of reduced forms of biopterin by this striatal extract was found to be dependent on enzymatic activity, guanosine‐5′‐triphosphate, magnesium ions, and a reduced pyridine nucleotide. As demonstrated by the technique of isotope dilution, isotope trapping, 6‐lactyl‐7,8‐dihydropterin (sepiapterin) was found to be an intermediate in biopterin biosynthesis that is catalyzed by the striatal extract. Rat brain was also shown to synthesize biopterin in vivo from intraventricularly administered [14C]guanosine or sepiapterin. Intraventricular injection of sepiapterin increased dihydro‐ and 5,6,7,8‐tetrahydrobiopterin levels in rat brain by more than eightfold. The temporal relationship between the appearance of dihydro‐ and 5,6,7,8‐tetrahydrobiopterin following intraventricular injection of sepiapterin suggests that dihydrobiopterin is the immediate product of sepiapterin reduction which is then reduced further to the functional cofactor 5,6,7,8‐tetra‐hydrobiopterin. Therefore, in contrast to previous reports, the biosynthesis of biopterin by rat brain does not appear to differ from that occurring in other, nonneural tissues.

Dyspropterin, an intermediate formed from dihydroneopterin triphosphate in the biosynthetic pathway of tetrahydrobiopterin

The structure of dyspropterin, a new name given to an intermediate which is formed from dihydroneopterin triphosphate in the biosynthetic pathway of tetrahydrobiopterin, has been studied. Sepiapterin reductase (EC 1.1.1.153) was found to reduce dyspropterin to tetrahydrobiopterin in the presence of NADPH. Several lines of evidence showing the formation of tetrahydrobiopterin have been presented. Stoichiometric analysis revealed that there is a 1:2 relationship between the production of biopterin and the oxidation of NADPH during the reductase-catalyzed reduction of dyspropterin. The tetrahydrobiopterin production from dyspropterin was enhanced by dihydropteridine reductase (EC 1.6.99.7). Dyspropterin could also serve as a cofactor in phenylalanine hydroxylase (EC 1.14.16.1) system. These results are consistent with the view that dyspropterin is 6-(1,2-dioxopropyl)-5,6,7,8-tetrahydropterin. Based on our findings, the biosynthetic pathway of tetrahydrobiopterin from dihydroneopterin triphosphate has been discussed.

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 Parkinsons 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.

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.

Phosphorylation by Ca2+/calmodulin-dependent protein kinase II and protein kinase C of sepiapterin reductase, the terminal enzyme in the biosynthetic pathway of tetrahydrobiopterin

Sepiapterin reductase, the terminal enzyme in the biosynthetic pathway of tetrahydrobiopterin, was stoichiometrically phosphorylated by Ca2+/calmodulin‐dependent protein kinase II and protein kinase C (Ca2+/phospholipid‐dependent protein kinase) in vitro. Maximal incorporation of phosphate into the enzyme subunit by these was 3.05 ± 0.05 (n = 4) and 0.74 ± 0.03 (n = 5) 32P mol per mol enzyme subunit, respectively. The enzyme was not phosphorylated by cyclic nucleotide‐dependent protein kinase of either the cAMP‐dependent or cGMP‐dependent type in this study. Dihydropteridine reductase, another enzyme working in direct supply of tetrahydrobiopterin, was also a good substrate for Ca2+/calmodulin‐dependent protein kinase II. Phosphorylation of sepiapterin reductase by these protein kinases modified the kinetic properties of the enzyme. It is likely that these multifunctional Ca2+‐activated protein kinases may play a role in the regulation of the physiological function of the BH4‐generating enzymes in vivo, as was previously found in the case of BH4‐requiring enzymes.

The complete amino acid sequence of the mature form of rat sepiapterin reductase

The partial amino acid sequence of rat sepiapterin reductase was determined using peptides generated by cleavage of the S-carboxyamidomethylated protein with Achromobacter protease I, cyanogen bromide, chymotrypsin or BNPS-skatole. The protein began with N-acetyl methionyl residue at the N-terminus and ended with isoleucyl residue at the C-terminus. The present results essentially coincided with the amino acid sequence predicted from the nucleotide sequence of the cDNA recently reported by Citron et al. (Proc. Natl. Acad. Sci. USA 87, 6436-6440 (1990)), clarified the processing event during the biosynthesis and provided the complete amino acid sequence of the mature form of the enzyme.

Biopterin and neopterin in human saliva

Presence of biopterin and neopterin in human saliva was investigated by HPLC after iodine oxidation in acidic medium. Concentrations of biopterin and neopterin (M +/- SEM) were 1.271 +/- 0.254 and 0.358 +/- 0.075 ng per ml, respectively, in saliva of apparently healthy young male adults, ages 20 to 22 years (n = 9). Nearly identical value of the neopterin/biopterin ratio (0.29 +/- 0.07) was obtained for each of these specimens. Monapterin, the L-threo-isomer of neopterin (0.084 +/- 0.022 ng per ml saliva), and other unconjugated pterins such as xanthopterin, 6-hydroxymethylpterin and pterin were also found in the saliva. These pterins were all detectable in saliva of young female adults with similar levels to those of male saliva. Another fluorescent compound which was identical with 7-iso biopterin in retention time on HPLC was observed in all specimens of normal saliva examined.