Effects of Systemic Administration of 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine to Mice on Tyrosine Hydroxylase, l-3,4-Dihydroxyphenylalanine Decarboxylase, Dopamine β-Hydroxylase, and Monoamine Oxidase Activities in the Striatum and Hypothalamus

The effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (30 mg/kg subcutaneously per day for 8 days) to C57BL/6N mice were studied on tyrosine hydroxylase (TH), L-3,4-dihydroxyphenylalanine decarboxylase (DDC), and monoamine oxidase (MAO) activities in the striatum, and TH, DDC, dopamine-beta-hydroxylase (DBH), and MAO activities in the hypothalamus. Treatment with MPTP led to a large decrease in TH activity and a parallel decrease in DDC activity in the striatum, as compared with the saline controls. In contrast, MPTP administration did not cause a decrease of the activities of TH, DDC, and DBH in the hypothalamus. There was also no reduction in MAO activities of striatum and hypothalamus. These data indicate that MPTP administration to mice results in specific degeneration of the dopaminergic nigrostriatal pathway and that DDC in the mouse striatum may mainly be localized in the dopaminergic neurons with TH.     

Effects of Apomorphine on In Vivo Release of Dopamine and Its Metabolites in the Prefrontal Cortex and the Striatum, Studied by a Microdialysis Method

The effects of apomorphine (0.1-2.5 mg/kg) on release of endogenous dopamine and extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the prefrontal cortex and the striatum were examined in vivo by a microdialysis method. Apomorphine significantly reduced release of dopamine and the extracellular levels of dopamine metabolites, DOPAC and HVA, not only in the striatum, but also in the prefrontal cortex. These findings indicate that dopamine autoreceptors modulate in vivo release of dopamine in the prefrontal cortex.     

Functionally important residues tyrosine-171 and serine-158 in sepiapterin reductase.

The active site of sepiapterin reductase (SPR), which is a member of the NADP(H)-preferring short-chain dehydrogenase/reductase (SDR) family and acts as the terminal enzyme in the biosynthetic pathway of tetrahydrobiopterin cofactor (BH4), was investigated by truncation and site-directed mutagenesis. The truncation mutants showed that N-terminal and C-terminal residues contribute to bind coenzyme and substrate, respectively. The mutant rSPRA29V showed decreased activity; however, the A-X-L-L-S sequence, which has been reported as a putative pterin binding site, was estimated to preferably work as a component in the region for binding coenzyme rather than substrate. Site-directed mutants of rSPRS158D, rSPRY171V, and rSPRK175I showed low, but significant, activity having similar Km values and kcat/Km values less than 25%, for both sepiapterin and NADPH. Both amino acids Tyr-171 and Ser-158 are located within a similar distance to the carbonyl group of the substrate in the crystal structure of mouse SPR, and the double point mutant rSPRY171V+S158D was indicated to be inactive. These results showed that Ser-158, Tyr-171, and Lys-175 contributed to the catalytic activity of SPR, and both Tyr-171 and Ser-158 are simultaneously necessary on proton transfer to the carbonyl functional groups of substrate.     

Demonstration of aromatic l-amino acid decarboxylase activity in human brain with l-dopa and l-5-hydroxytryptophan as substrates by high-performance liquid chromatography with electrochemical detection

The enzymatic decarboxylations of l-DOPA and l-5-hydroxytryptophan (l-5-HTP) by aromatic l-amino acid decarboxylase (AADC) were measured with homogenates from human brain regions, caduate nucleus and hypothalamus, using our new and highly sensitive methods for l-DOPA decarboxylase and l-5-HTP decarboxylase by high-performance liquid chromatography with electrochemical detection (HPLC-ED). Dopamine formed from l-DOPA as substrate was measured for DOPA decarboxylase activity using d-DOPA for the blank. For 5-HTP decarboxylase activity, serotonin (5-HT) formed from l-5-HTP was measured, and the blank value in presence of NSD-1055 was subtracted. NSD-1055 inhibited 5-HTP decarboxylase activity completely at a concentration of 0.2 mM. In this study, the properties of l-5-HTP decarboxylase activity in human caudate nucleus were first examined. AADC activities in human brains were found to be widely variable for both l-DOPA and l-5-HTP as substrates. The ratio of the activities for l-DOPA and l-5-HTP were found to be significantly higher in hypothalamus than in caudate nucleus. AADC activity for l-DOPA in the brain was found to be linear up to 40 min of incubation, while that for l-5-HTP was found to be linear up to 240 min of incubation. The optimum pyridoxal phosphate concentration was found to be similar for both substrates and was between 0.01 and 0.1 mM. The optimum pH values were found to be 7.2 and 8.2 for l-DOPA decarboxylase and l-5-HTP decarboxylase, respectively. K_m and V_max values for a human caudate nucleus l-DOPA decarboxylase were found to be 414 μM and 482 pmol/min/g wet weight, respectively, while those for l-5-HTP decarboxylase were found to be 90 μM and 71 pmol/min/g wet weight, respectively.     

Distribution of post-proline cleaving enzyme in human brain and the peripheral tissues

Summary We have studied the distribution of post-propline cleaving enzyme activity in the various tissues in humans using 7-(succinyl-Gly-Pro)-4-methylcoumarinamide as substrate. The post-propline cleaving enzyme activity was high in muscle, testes, kidney and submandibular gland, but was low in the heart, mesenterium and aorta. In the brain, relatively high post-propline cleaving enzyme activity was observed in the cerebral cortex, but other brain regions showed a very low enzyme activity.On Sephadex G-100 column chromatography, enzyme activity in human kidney showed a major peak and a minor peak. The major peak coincided with the enzyme in human cerebral cortex, but was different from human serum enzyme. Diisopropylfluorophosphate, a serine protease inhibitor, strongly inhibited the enzyme activity of each active fraction. The enzyme in the cerebral cortex and kidney was inhibited by heavy metals and thiol blocking agents. However, inhibition of enzyme activity in the serum was not observed with such inhibitors. Therefore, we suppose that post-proline cleaving enzyme activity in the brain is similar, if not identical, to that in the kidney.     

Change of tyrosine hydroxylase in the parkinsonian brain and in the brain of MPTP-treated mice as revealed by homospecific activity

Changes in homospecific activity (unit of enzyme activity per unit of enzyme protein; Rush, Kindler and Udenfriend, 1974. Biochem. Biophys. Res. Commun., 61, 38) of tyrosine hydroxylase (TH) in the striatum of the brain were examined in MPTP-treated mice and parkinsonian patients. After a single injection of MPTP to mice, TH activity was acutely inhibited onlyin situ without changes in in vitro TH activity (Vmax) and TH protein; TH homospecific activity (TH Vmax/TH protein) did not change. After repeated injection of MPTP to mice for 8 days, in situ TH activity, in vitro TH Vmax, and TH protein were decreased in parallel, and TH homospecific activity did not change The result indicates that the decreases in in situ TH activity and in TH Vmax are due to the decrease in TH protein by nerve degeneration of dopaminergic neurons in MPTP treated mice. However, when MPP⁺ was infused in the striatum of rats for 3 hours, in vitro TH activity (Vmax) was decreased without changes in TH protein. Thus, TH homospecific activity was decreased. The results indicate that MPP⁺ inactivates TH protein in the striatum after continued infusion. In contrast, the homospecific activity of TH in post-mortem parkinsonian striatum was increased 3-fold. The increase in homospecific activity of residual TH in parkinsonian brain suggests such molecular changes in TH molecules as result in a compensatory increase in TH activity.Special issue dedicated to Dr. Sidney Udenfriend.     

Molecular cloning and characterization of a cDNA encoding a novel apoplastic protein preferentially expressed in a shikonin-producing callus strain of Lithospermum erythrorhizon

A cDNA (LEPS-2) encoding a novel cell wall protein was cloned from shikonin-producing callus tissues of Lithospermum erythrorhizon by differential display between a shikonin-producing culture strain and a non-producing strain. The LEPS-2 cDNA encoded a polypeptide of 184 amino acids. The deduced amino acid sequence exhibited no significant homology with known proteins. Expression of LEPS-2 gene as well as accumulation of LEPS-2 protein was highly correlated with shikonin production in L. erythrorhizon cells in culture. In the intact plant, expression of LEPS-2 was detected only in the roots where shikonin pigments accumulated. Cell fractionation experiments and immunocytochemical analysis showed that the protein was localized in the apoplast fraction of the cell walls. The shikonin pigments were also stored on the cell walls as oil droplets. These results indicate that expression of the LEPS-2 is closely linked with shikonin biosynthesis and the LEPS-2 protein may be involved in the intra-cell wall trapping of shikonin pigments.     

Enzymes related to catecholamine biosynthesis in Tetrahymena pyriformis. Presence of GTP cyclohydrolase I.

We first identified GTP cyclohydrolase I activity (EC 3.5.4.16) in the ciliated protozoa, Tetrahymena pyriformis. The V_max value of the enzyme in the cellular extract of T. pyriformis was 255 pmol mg⁻¹ protein h⁻¹. Michaelis–Menten kinetics indicated a positive cooperative binding of GTP to the enzyme. The GTP concentration producing half-maximal velocity was 0.8 mM. By high-performance liquid chromatography (HPLC) with fluorescence detection, a major peak corresponding to -monapterin (2-amino-4-hydroxy-6-[(1′R,2′R)-1′,2′,3′-trihydroxypropyl]pteridine, -threo-neopterin) and minor peaks of -erythro-neopterin and -erythro-biopterin were found to be present in the cellular extract of Tetrahymena. Thus, it is strongly suggested that Tetrahymena converts GTP into unconjugated pteridine derivatives. In this study, dopamine was detected as the major catecholamine, while neither epinephrine nor norepinephrine was identified. Indeed, this protozoa was shown to possess the activity of a dopamine synthesizing enzyme, aromatic -amino acid decarboxylase. On the other hand, activities of tyrosine hydroxylase or tyrosinase which converts tyrosine into dopa, the substrate of aromatic -amino acid decarboxylase, could not be detected in this protozoa. Furthermore, neither dopamine β-hydroxylase activity nor phenylethanolamine N-methyltransferase activity could be identified by the HPLC methods.     

Limited proteolysis of tyrosine hydroxylase by Ca(2+)-activated neutral protease (calpain).

Limited proteolysis of tyrosine hydroxylase (TH) by calpain, Ca(2+)-activated neutral protease, was studied. Cleavage of TH with calpain in vitro produced molecules having Mrs of approximately 57,000 and 56,000 in SDS-polyacrylamide gel electrophoresis. The N-terminal amino acid sequence, Ser-Pro-Arg-Phe-Val, of the 56-kDa species indicated that calpain cleaved off the N-terminal region (residues 1-30) encoded by the first exon including Ser-8 and Ser-19, the phosphorylation sites by proline-directed protein kinase (PDPK) and by Ca2+/calmodulin-dependent protein kinase II (kinase II), respectively, from the native TH. The removal of the N-terminal region from the native molecule induced a slight but significant activation of TH at pH 7.0. The native TH behaved as the tetramer with an Mr of 240,000. In contrast, calpain-cleaved TH showed the monomeric Mr by gel permeation chromatography and increased Ki for catecholamine which inhibits the native TH in competition to the coenzyme, DL-6-methyl-5,6,7,8-tetrahydropterin (6MPH4). These results imply that calpain cleavage would effectively release TH from the feedback inhibition by removal of the N-terminal region resulting in disruption of the quaternary structure.     

New chromogenic substrates for X-prolyl dipeptidyl-aminopeptidase

We have synthesized several new chromogenic substrates, p-nitroanilides of the dipeptides, Gly-Pro, Ala-Pro, Lys-Pro, Arg-Pro, Glu-Pro, and Asp-Pro, for X-prolyl dipeptidyl-aminopeptidase. These have permitted the development of a simple assay of the enzyme in which p-nitroaniline liberated directly or after the Bratton-Marshall reaction is measured spectrophotometrically. The enzyme activity was measured in human serum or in homogeneous enzyme purified from human submaxillary gland. The homogeneous enzyme hydrolyzed each substrate to produce X-Pro and p-nitroaniline. The optimum pH was at 8.7, except with Arg-Pro p-nitroanilide (8.0). Serum enzyme hydrolyzed Gly-Pro p-nitroanilide to p-nitroaniline and Gly-Pro, which was further hydrolyzed to Gly and Pro by an imidodipeptidase in serum. Gly-Pro β-naphthylamide or Gly-Pro-Leu was a competitive inhibitor with each X-Pro p-nitroanilide as substrate. Gly-Pro p-nitroanilide had the highest activity among the substrates at pH 8.7, followed by p-nitroanilides of Ala, Lys, Arg, Glu, and Asp in a decreasing order of activity.