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Modulation of phenylalanine and tyrosine concentrations by ischemia and guanosine in neuronal PC12 cells

  • Bettina Thauerer , Simon Geisler , Dietmar Fuchs and Gabriele Baier-Bitterlich EMAIL logo
Published/Copyright: December 19, 2013
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Pteridines
From the journal Pteridines Volume 24 Issue 3-4

Abstract

Survival of neurons is dependent on the presence of trophic and non-trophic factors. We have previously observed the protective capacity of purine nucleosides in hypoxic neuronal cell cultures. Guanosine appeared especially interesting with respect to its remarkable neurite-stimulating aptitude. Here we report the effect of ischemic stress and guanosine on the concentration of the essential amino acids phenylalanine and tyrosine, in culture supernatants of PC12 cells. In ischemic neuronal cultures, a substantial rise of phenylalanine and tyrosine levels was observed, indicating inhibition of cellular metabolism due to neuronal stress. Elevated phenylalanine and tyrosine concentrations were reduced by the addition of guanosine. Guanosine even down-regulated the ratio of phenylalanine to tyrosine. Results may support current efforts to propagate guanosine as a neuroprotective and neuroregenerative substance.

Keywords: guanosine; ischemia; phenylalanine; phenylalanine-4-hydroxylase; PC12 cells; purine nucleosides; tyrosine

Introduction

Multiple signaling pathways regulate the critical balance between cell death and survival in ischemia reperfusion. Following ischemia, dying, injured, and hypoxic cells release soluble nucleotide and nucleoside pools [1], which are then further metabolized to other purine derivatives [2] that remain elevated for days after the insult [3]. We have previously studied neuronal signaling after chemical and physiological hypoxia and observed the protective capacity of purine nucleosides in both rat PC12 cells [4–6] and primary cerebellar granule neurons [7, 8] (see our review, reference [9]). Among purine nucleosides, guanosine gained our special attention due to its outstanding neurite-stimulating capacity, and we have recently reported on its neuroprotection of serum-stressed PC12 cells [10]. Guanosine derives from guanosine triphosphate (GTP) by stepwise dephosphorylation to guanosine diphosphate and guanosine monophosphate by ecto-nucleotidases [11–14]. However, GTP is also a source for the biochemical conversion by the interferon-γ -inducible enzyme GTP cyclohydrolase I [15] to 7,8-dihydroneopterin triphosphate, which is further metabolized by dephosphorylation and oxidation to neopterin, 7,8-dihydroneopterin, and biopterin derivatives [16–18] (see Figure 3). Interestingly, the protective effects clearly distinguish guanosine from its relative, 7,8-dihydroneopterin. Earlier data [19, 20] showed that increased concentrations of 7,8-dihydroneopterin might lead to enhanced apoptosis and disturbance of the redox balance of human leukemic Jurkat T cells. Raised neopterin concentrations were also detected in cerebrospinal fluids from HIV-infected patients with neurological/ psychiatric symptoms [21]. Alternatively, 7,8-dihydroneopterin triphosphate is further metabolized by 6-pyruvoyl tetrahydropterin synthase to 6-pyruvoyl-tetrahydropterin, which is converted by two NADPH-dependent reductions to 5,6,7,8-tetrahydrobiopterin (BH4), the essential cofactor of phenylalanine 4-hydroxylase (PAH), which converts phenylalanine to tyrosine, and of all aromatic amino acid monoxygenases [22, 23]. Tyrosine is then further hydroxylated, in dependence of BH4 as a cofactor, to l-3, 4-dihydroxyphenylalanine, the precursor of the neurotransmitters dopamine, norepinephrine, and epinephrine [24, 25]. Increased blood levels of phenylalanine were reported in patients with various pathologies linked to inflammation and immune activation [26–29], and phenylalanine concentrations were associated with increased concentrations of the immune activation marker neopterin [30–35].

Phenylalanine is enzymatically converted by PAH to tyrosine, whereby an increase of the phenylalanine-to-tyrosine ratio may serve as an estimate of PAH activity [36]. Increased levels of phenylalanine but also higher phenylalanine-to-tyrosine ratios have been described in patients after burns with reduced survival [27, 37] (Figure 3). Based on this coherence, we were inclined to study the effect of guanosine on phenylalanine and tyrosine concentrations in normoxic and in ischemic neuronal PC12 cell cultures.

Materials and methods

Cell culture

PC12 cells (LGC Promochem ATCC, Manassas, VA, USA) were grown in Dulbecco′s Modified Eagle’s Medium (DMEM) high-glucose medium supplemented with 1% Pen/Strep, 1% l-glutamine, and 10% fetal calf serum (all from PAA Laboratories, Vienna, Austria) at 37°C with 5% CO2 and 21% O2 on collagen-S type I-coated culture dishes (Becton Dickinson, Canaan, CT, USA). Cells were subcultured at a density of about 80% or 2 days before onset of an experiment.

Induction of ischemic conditions

In PC12 cells, ischemic stress was induced by simultaneous deprivation of oxygen and glucose (OGD). OGD was achieved using an incubator designed for maintaining hypoxia (HERAcell 240, Thermo Electron Corporation, Vienna, Austria) that was set to continuous conditions of 1% O2 balanced with N2 and controlled with O2 and CO2 sensors and additional exchange of DMEM high-glucose to glucose-free DMEM.

Cell viability assay

PC12 cells (5×104) were cultured for 1 day on collagen-S type I-coated chamber slides (NUNC, Rochester, NY, USA). After this time, PC12 cells were stimulated under normoxic (21% O2, DMEM high-glucose) or ischemic (1% O2, DMEM glucose-free) conditions. Cells were additionally treated with guanosine (500 μmol/L; Sigma-Aldrich, Vienna, Austria). After 24 h, cell viability was measured by staining the cells with fluorescent dyes (Hoechst 33342; 10 μg/mL; Life Technologies, Vienna, Austria) for 10 min and with propidium iodide (PI; 5 μg/mL; Sigma-Aldrich, Vienna, Austria) for an additional 5 min at 37°C. The Hoechst dye is membrane permeable and stains the nuclei of both living and dead cells (total cell number). However, PI is membrane impermeable and stains only the DNA of cells with disrupted plasma membranes. Cells were visualized on a fluorescence microscope (Zeiss Axioplan2, Zeiss, Vienna, Austria) equipped with a spot camera (RT-slider 2.3.1, Visitron Systems, Puchheim, Germany), using filters for Hoechst (excitation: 400 nm; emission: 420 nm) and PI (excitation: 570 nm; emission: 590 nm). Pictures of both fluorescence excitations were merged in Adobe Photoshop 6.01 (Adobe Systems Inc., San Jose, USA), and double-stained cells (pink) represent dead and blue viable ones.

Phenylalanine and tyrosine analysis

PC12 cells (1×107) were cultured on collagen-S type I-coated dishes and stimulated under normoxic (21% O2, DMEM high-glucose) or ischemic (1% O2, DMEM glucose-free) conditions. Cells were additionally treated with guanosine (500 μmol/L) for 24 h. After this time point, supernatants were collected and measurements of phenylalanine and tyrosine concentrations were performed as previously described [38].

In brief, 100 μL of supernatant, 100 μL of internal standard, 500 μmol/L of 3-nitro-l-tyrosine, and 25 μL of 2 mol/L of trichloroacetic acid were vortexed and centrifuged to precipitate proteins. Supernatants were diluted 1:25 with 15 mmol/L of potassium dihydrogen phosphate. An albumin-based mixture with 100 μmol/L of phenylalanine and tyrosine was treated in the same way as supernatants and served as an external calibrator. Separation of analytes was achieved on reversed-phase high-performance liquid chromatography using 15 mmol/L of aqueous potassium dihydrogen phosphate as an elution buffer. Phenylalanine and tyrosine were detected by their natural fluorescence at an excitation wavelength of 210 nm and an emission wavelength of 302 nm [38], employing a fluorescence detection device (ProStar 360, Varian, Palo Alto, CA, USA), and the phenylalanine-to-tyrosine ratio was calculated.

Statistical analysis

All data represent the means±standard error of the mean of the indicated number of independent experiments. Statistical analysis was performed using SPSS software (version 19.0.0; IBM, Chicago, USA). With an unpaired one-tailed t-test, the significance of differences between two experimental groups was assessed. The acceptance level of statistical significance was p<0.05.

Results and discussion

Following cultivation of neuronal PC12 cells in normoxic and ischemic conditions in a hypoxia incubator, ischemic culture supernatants showed a substantial rise in phenylalanine and also, to a minor extent, in tyrosine concentrations (Figure 1A and B).

Figure 1 Phenylalanine and tyrosine concentrations in normoxic and ischemic PC12 cell cultures. Neuronal PC12 cells were cultivated in normoxic and ischemic conditions in a hypoxia incubator. Ischemic culture supernatants showed a substantial rise in phenylalanine and also, to a minor extent, in tyrosine concentrations (A and B), but phenylalanine-to-tyrosine ratios showed no significant difference in normoxic as compared to ischemic samples (C). Cell death was studied by double fluorescence analysis of the Hoechst dye (total) and propidium iodide (dead cells) in normoxic and ischemic cell cultures (D). Values represent the mean±SEM, n=3. Differences were analyzed using unpaired one-tailed t-test: *p<0.05, **p<0.01.
Figure 1

Phenylalanine and tyrosine concentrations in normoxic and ischemic PC12 cell cultures. Neuronal PC12 cells were cultivated in normoxic and ischemic conditions in a hypoxia incubator. Ischemic culture supernatants showed a substantial rise in phenylalanine and also, to a minor extent, in tyrosine concentrations (A and B), but phenylalanine-to-tyrosine ratios showed no significant difference in normoxic as compared to ischemic samples (C). Cell death was studied by double fluorescence analysis of the Hoechst dye (total) and propidium iodide (dead cells) in normoxic and ischemic cell cultures (D). Values represent the mean±SEM, n=3. Differences were analyzed using unpaired one-tailed t-test: *p<0.05, **p<0.01.

Increased blood levels of phenylalanine were reported in patients with various pathologies linked to inflammation and immune activation [26–29], whereby an increase of the essential amino acid phenylalanine is likely to result from an impaired conversion of phenylalanine to tyrosine by PAH. Disturbances of BH4 levels are, for example, critical in individuals suffering from neurodegenerative diseases [33, 35]. An impaired PAH activity could be due to insufficient supply of the cofactor BH4, since BH4 was shown to be very sensitive to oxidative stress [39]. Its depletion may lead to a suppression of BH4-dependent enzymes like PAH or tyrosine hydroxylase [40]. This fact may explain why tyrosine concentrations were also increased in our supernatants from ischemic cultures as compared to control values. Hence, phenylalanine-to-tyrosine ratios, which allow an estimate of the activity of PAH [25, 36, 41], showed no significant difference in normoxic as compared to hypoxic samples and potentially reflect an inhibition of cellular metabolism due to ischemic starvation rather than to a specific effect on enzymes related to phenylalanine metabolism (Figure 1C).

In addition, cell death was studied by double fluorescence analysis of the Hoechst dye and propidium iodide. In agreement with earlier studies by our group [4, 42, 43], ischemia led to a substantial increase in cell death (Figure 1D) and thus support the hypothesis that up-regulated phenylalanine and tyrosine concentrations are a direct consequence of ischemic starvation.

The addition of guanosine significantly rescued PC12 cell death induced by ischemia (Figure 2). The data presented here are in line with previous reports on the positive effects of purine nucleosides on hypoxic neuronal cells (for a review, see reference [9]) and support current efforts to propagate purine nucleosides as neuroprotective and neuroregenerative substances [9, 44–50]. In support of these data, results obtained from the analysis of phenylalanine and tyrosine concentrations showed that, upon addition of guanosine, phenylalanine as well as tyrosine levels were diminished (Figure 2A and B). Interestingly, the phenylalanine-to-tyrosine ratio was also reduced by guanosine. However, the difference was only significant for phenylalanine under normoxic conditions (Figure 2A).

Figure 2 Effect of guanosine on phenylalanine and tyrosine concentrations in normoxic and ischemic PC12 cell cultures. Neuronal PC12 cells were cultivated in normoxic and ischemic conditions in a hypoxia incubator with and without the addition of purine nucleoside guanosine. Guanosine-treated cell culture supernatants showed a substantial decrease in phenylalanine and also, to a minor extent, in tyrosine concentrations (A and B), accompanied by decreased phenylalanine-to-tyrosine ratios (C). Cell death was studied by double fluorescence analysis of the Hoechst dye (total) and propidium iodide (dead cells) in normoxic and ischemic cell cultures treated with and without guanosine and reflects its neuroprotective effect (D). Values represent the mean±SEM, n=3. Differences were analyzed using unpaired one-tailed t-test: **p<0.01.
Figure 2

Effect of guanosine on phenylalanine and tyrosine concentrations in normoxic and ischemic PC12 cell cultures. Neuronal PC12 cells were cultivated in normoxic and ischemic conditions in a hypoxia incubator with and without the addition of purine nucleoside guanosine. Guanosine-treated cell culture supernatants showed a substantial decrease in phenylalanine and also, to a minor extent, in tyrosine concentrations (A and B), accompanied by decreased phenylalanine-to-tyrosine ratios (C). Cell death was studied by double fluorescence analysis of the Hoechst dye (total) and propidium iodide (dead cells) in normoxic and ischemic cell cultures treated with and without guanosine and reflects its neuroprotective effect (D). Values represent the mean±SEM, n=3. Differences were analyzed using unpaired one-tailed t-test: **p<0.01.

Analysis of cell viability showed that guanosine may efficiently protect PC12 cells from ischemic insult (Figure 2D). The data are in line with previous findings of guanosine-mediated protection in serum-starved [10] and in hypoxic neuronal cell cultures [42] and support current efforts analyzing guanosine-mediated neuroprotection [9].

This result might imply either a direct specific guanosine-mediated stimulating effect on PAH activity – although alterations of phenylalanine-to-tyrosine ratios were not significant – or a guanosine-mediated elevation of the GTP pool, which potentially leads to elevated levels of BH4 [51] (Figure 3). Based on our data, we assume that the GTP-derivative guanosine mediates the protection of cells in stress situations by regulation of different mechanisms. As discussed earlier, guanosine was shown to regulate specific intrinsic signaling mechanisms [9, 42], and these mechanisms are likely supported by the regulation of the levels of essential amino acids such as phenylalanine and tyrosine (Figure 3). The exact mechanism, however, will be subject to future investigations.

Figure 3 Schematic model of the sources of guanosine and other products derived from GTP. The purine nucleoside guanosine is the product of the stepwise dephosphorylation of guanosine triphosphate (GTP) to guanosine diphosphate (GDP) and guanosine monophosphate (GMP) by ectonucleotide diphosphohydrolases (E-NTPDases, 1) and by ecto-5′-nucleotidase (2). In addition, GTP can be enzymatically cleaved by GTP-cyclohydrolase I (GCH-I, 3) to 7,8-dihydroneopterin triphosphate. Neopterin and 7,8-dihydroneopterin are then metabolized by dephosphorylation and oxidation (4). Alternatively, 7,8-dihydroneopterin triphosphate is further metabolized by 6-pyruvoyl tetrahydropterin synthase to 6-pyruvoyl-tetrahydropterin (5), which is converted by two NADPH-dependent reductions (6) to tetrahydrobiopterin (BH4). BH4 is the essential cofactor of phenylalanine 4-hydroxylase (PAH, 7), converting phenylalanine to tyrosine, and of other monoxygenases (8). Guanosine-mediated downregulation of phenylalanine-to-tyrosine conversion potentially contributes to its neuroprotective effects either by directly controlling PAH activity or by feedback-regulation of GTP and BH4 concentrations.
Figure 3

Schematic model of the sources of guanosine and other products derived from GTP. The purine nucleoside guanosine is the product of the stepwise dephosphorylation of guanosine triphosphate (GTP) to guanosine diphosphate (GDP) and guanosine monophosphate (GMP) by ectonucleotide diphosphohydrolases (E-NTPDases, 1) and by ecto-5′-nucleotidase (2). In addition, GTP can be enzymatically cleaved by GTP-cyclohydrolase I (GCH-I, 3) to 7,8-dihydroneopterin triphosphate. Neopterin and 7,8-dihydroneopterin are then metabolized by dephosphorylation and oxidation (4). Alternatively, 7,8-dihydroneopterin triphosphate is further metabolized by 6-pyruvoyl tetrahydropterin synthase to 6-pyruvoyl-tetrahydropterin (5), which is converted by two NADPH-dependent reductions (6) to tetrahydrobiopterin (BH4). BH4 is the essential cofactor of phenylalanine 4-hydroxylase (PAH, 7), converting phenylalanine to tyrosine, and of other monoxygenases (8). Guanosine-mediated downregulation of phenylalanine-to-tyrosine conversion potentially contributes to its neuroprotective effects either by directly controlling PAH activity or by feedback-regulation of GTP and BH4 concentrations.

Corresponding author: Gabriele Baier-Bitterlich, Division of Neurobiochemistry, Medical University of Innsbruck, Biocenter, Innrain 80-82, 6020 Innsbruck, Austria, E-mail:

References

1. Neary JT, Rathbone MP, Cattabeni F, Abbracchio MP, Burnstock G. Trophic actions of extracellular nucleotides and nucleosides on glial and neuronal cells. Trends Neurosci 1996;19:13–8.10.1016/0166-2236(96)81861-3Search in Google Scholar

2. Zetterstrom T, Vernet L, Ungerstedt U, Tossman U, Jonzon B, Fredholm BB. Purine levels in the intact rat brain. Studies with an implanted perfused hollow fibre. Neurosci Lett 1982;29:111–5.10.1016/0304-3940(82)90338-XSearch in Google Scholar

3. Uemura Y, Miller JM, Matson WR, Beal MF. Neurochemical analysis of focal ischemia in rats. Stroke 1991;22:1548–53.10.1161/01.STR.22.12.1548Search in Google Scholar

4. Tomaselli B, zur Nedden S, Podhraski V, Baier-Bitterlich G. p42/44 MAPK is an essential effector for purine nucleoside-mediated neuroprotection of hypoxic PC12 cells and primary cerebellar granule neurons. Mol Cell Neurosci 2008;38:559–68.10.1016/j.mcn.2008.05.004Search in Google Scholar

5. Tomaselli B, Podhraski V, Böck G, Baier-Bitterlich G. Early cellular responses of purine nucleoside-mediated protection of hypoxia-induced injuries of neuronal PC12 cells. Am J Biochem Biotechnol 2005;2:161–7.Search in Google Scholar

6. Tomaselli B, Podhraski V, Heftberger V, Bock G, Baier-Bitterlich G. Purine nucleoside-mediated protection of chemical hypoxia-induced neuronal injuries involves p42/44 MAPK activation. Neurochem Int 2005;46:513–21.10.1016/j.neuint.2005.02.003Search in Google Scholar

7. Bocklinger K, Tomaselli B, Heftberger V, Podhraski V, Bandtlow C, Baier-Bitterlich G. Purine nucleosides support the neurite outgrowth of primary rat cerebellar granule cells after hypoxia. Eur J Cell Biol 2004;83:51–4.10.1078/0171-9335-00362Search in Google Scholar

8. Heftberger V, Tomaselli B, Podhraski V, Baier-Bitterlich G. Purine nucleoside mediated protection of primary cerebellar granule cells after hypoxic insult. In: Focus on Neurochemistry Research. New York: Nova Science Publishers, 2005.Search in Google Scholar

9. Thauerer B, Zur Nedden S, Baier-Bitterlich G. Purine nucleosides: endogenous neuroprotectants in hypoxic brain. J Neurochem 2012;121:329–42.10.1111/j.1471-4159.2012.07692.xSearch in Google Scholar

10. Thauerer B, Baeumer EB, Fellner L, Kuzdas D, Ramberger M, Baier-Bitterlich G. Guanosine protects neuronal PC12 cells from serum deprivation-induced cell death. Pteridines 2012;23:27–32.10.1515/pteridines.2012.23.1.27Search in Google Scholar

11. Ciccarelli R, Di Iorio P, D′Alimonte I, Giuliani P, Florio T, Caciagli F, et al. Cultured astrocyte proliferation induced by extracellular guanosine involves endogenous adenosine and is raised by the co-presence of microglia. Glia 2000;29:202–11.10.1002/(SICI)1098-1136(20000201)29:3<202::AID-GLIA2>3.0.CO;2-CSearch in Google Scholar

12. Robson SC, Sevigny J, Zimmermann H. The E-NTPDase family of ectonucleotidases: structure function relationships and pathophysiological significance. Purinergic Signal 2006;2:409–30.10.1007/s11302-006-9003-5Search in Google Scholar

13. Schmidt AP, Lara DR, Souza DO. Proposal of a guanine-based purinergic system in the mammalian central nervous system. Pharmacol Ther 2007;116:401–16.10.1016/j.pharmthera.2007.07.004Search in Google Scholar

14. Zimmermann H. Biochemistry, localization and functional roles of ecto-nucleotidases in the nervous system. Prog Neurobiol 1996;49:589–618.10.1016/0301-0082(96)00026-3Search in Google Scholar

15. Werner ER, Werner-Felmayer G, Fuchs D, Hausen A, Reibnegger G, Yim JJ, et al. Tetrahydrobiopterin biosynthetic activities in human macrophages, fibroblasts, THP-1, and T 24 cells. GTP-cyclohydrolase I is stimulated by interferon-gamma, and 6-pyruvoyl tetrahydropterin synthase and sepiapterin reductase are constitutively present. J Biol Chem 1990;265:3189–92.10.1016/S0021-9258(19)39752-2Search in Google Scholar

16. Hausen A, Bichler A, Fuchs D, Hetzel H, Reibnegger G, Wachter H. Neopterin, a biochemical indicator of cellular immune reactions, in the detection and control of patients with neoplastic diseases. Cancer Detect Prev 1985;8:121–8.Search in Google Scholar

17. Huber C, Batchelor JR, Fuchs D, Hausen A, Lang A, Niederwieser D, et al. Immune response-associated production of neopterin. Release from macrophages primarily under control of interferon-gamma. J Exp Med 1984;160:310–6.10.1084/jem.160.1.310Search in Google Scholar

18. Wachter H, Grassmayr K, Hausen A. Enhanced urinary excretion of 7,8-dihydro-6-hydroxylumazine by mice bearing the Ehrlich ascites tumour. Cancer Lett 1979;6:61–6.10.1016/S0304-3835(79)80001-4Search in Google Scholar

19. Baier-Bitterlich G, Baier G, Fuchs D, Bock G, Hausen A, Utermann G, et al. Role of 7,8-dihydroneopterin in T-cell apoptosis and HTLV-1 transcription in vitro. Oncogene 1996;13:2281–5.Search in Google Scholar

20. Baier-Bitterlich G, Wachter H, Fuchs D. Oxidative stress and apoptosis in HIV infection. Science 1996;271:582–3.10.1126/science.271.5249.582Search in Google Scholar PubMed

21. Fuchs D, Moller AA, Reibnegger G, Stockle E, Werner ER, Wachter E. Decreased serum tryptophan in patients with HIV-1 infection correlates with increased serum neopterin and with neurologic/psychiatric symptoms. J Acquir Immune Defic Syndr 1990;3:873–6.Search in Google Scholar

22. Heintz C, Cotton RG, Blau N. Tetrahydrobiopterin, its mode of action on phenylalanine hydroxylase, and importance of genotypes for pharmacological therapy of phenylketonuria. Hum Mutat 2013;34:927–36.10.1002/humu.22320Search in Google Scholar PubMed

23. Werner-Felmayer G, Golderer G, Werner ER. Tetrahydrobiopterin biosynthesis, utilization and pharmacological effects. Curr Drug Metabol 2002;3:159–73.10.2174/1389200024605073Search in Google Scholar PubMed

24. Neurauter G, Schrocksnadel K, Scholl-Burgi S, Sperner-Unterweger B, Schubert C, Ledochowski M, et al. Chronic immune stimulation correlates with reduced phenylalanine turnover. Curr Drug Metabol 2008;9:622–7.10.2174/138920008785821738Search in Google Scholar PubMed

25. Shintaku H. Disorders of tetrahydrobiopterin metabolism and their treatment. Curr Drug Metabol 2002;3:123–31.10.2174/1389200024605145Search in Google Scholar PubMed

26. Watanabe A, Higashi T, Sakata T, Nagashima H. Serum amino acid levels in patients with hepatocellular carcinoma. Cancer 1984;54:1875–82.10.1002/1097-0142(19841101)54:9<1875::AID-CNCR2820540918>3.0.CO;2-OSearch in Google Scholar

27. Rath T, Roth E, Keidl R, Meissl G. Phenylalanine: total amino acid ratio in 45 burn patients. Scand J Plastic Reconstr Surg Hand Surg 1987;21:297–300.10.3109/02844318709086465Search in Google Scholar

28. Roth E, Zoch G, Schulz F, Karner J, Muhlbacher F, Hamilton G, et al. Amino acid concentrations in plasma and skeletal muscle of patients with acute hemorrhagic necrotizing pancreatitis. Clin Chem 1985;31:1305–9.10.1093/clinchem/31.8.1305Search in Google Scholar

29. Ollenschlager G, Jansen S, Schindler J, Rasokat H, Schrappe-Bacher M, Roth E. Plasma amino acid pattern of patients with HIV infection. Clin Chem 1988;34:1787–9.10.1093/clinchem/34.9.1781Search in Google Scholar

30. Fahey JL, Taylor JM, Detels R, Hofmann B, Melmed R, Nishanian P, et al. The prognostic value of cellular and serologic markers in infection with human immunodeficiency virus type 1. N Engl J Med 1990;322:166–72.10.1056/NEJM199001183220305Search in Google Scholar

31. Melichar B, Solichova D, Freedman RS. Neopterin as an indicator of immune activation and prognosis in patients with gynecological malignancies. Int J Gynecol Cancer 2006;16:240–52.10.1111/j.1525-1438.2006.00294.xSearch in Google Scholar

32. Murr C, Widner B, Wirleitner B, Fuchs D. Neopterin as a marker for immune system activation. Curr Drug Metabol 2002;3:175–87.10.2174/1389200024605082Search in Google Scholar

33. Ploder M, Neurauter G, Spittler A, Schroecksnadel K, Roth E, Fuchs D. Serum phenylalanine in patients post trauma and with sepsis correlate to neopterin concentrations. Amino Acids 2008;35:303–7.10.1007/s00726-007-0625-xSearch in Google Scholar

34. Strohmaier W, Redl H, Schlag G, Inthorn D. Elevated d-erythro-neopterin levels in intensive care patients with septic complications. Prog Clin Biol Res 1987;236B:59–66.Search in Google Scholar

35. Wissmann P, Geisler S, Leblhuber F, Fuchs D. Immune activation in patients with Alzheimer′s disease is associated with high serum phenylalanine concentrations. J Neurol Sci 2013;329: 29–33.10.1016/j.jns.2013.03.007Search in Google Scholar

36. Anderson DN, Wilkinson AM, Abou-Saleh MT, Blair JA. Recovery from depression after electroconvulsive therapy is accompanied by evidence of increased tetrahydrobiopterin-dependent hydroxylation. Acta Psychiatr Scand 1994;90:10–3.10.1111/j.1600-0447.1994.tb01547.xSearch in Google Scholar

37. Chang XJ, Yang CC, Hsu WS, Xu WZ, Shih TS. Serum and erythrocyte amino acid pattern: studies on major burn cases. Burns Incl Therm Inj 1983;9:240–8.10.1016/0305-4179(83)90053-0Search in Google Scholar

38. Neurauter G, Grahmann AV, Klieber M, Zeimet A, Ledochowski M, Sperner-Unterweger B, et al. Serum phenylalanine concentrations in patients with ovarian carcinoma correlate with concentrations of immune activation markers and of isoprostane-8. Cancer Lett 2008;272:141–7.10.1016/j.canlet.2008.07.002Search in Google Scholar

39. Connor MJ, Pheasant AE, Blair JA. The identification of p-acetamidobenzoate as a folate degradation product in rat urine. Biochem J 1979;178:795–7.10.1042/bj1780795Search in Google Scholar

40. Widner B, Fuchs D, Leblhuber F, Sperner-Unterweger B. Does disturbed homocysteine and folate metabolism in depression result from enhanced oxidative stress? J Neurol Neurosurg Psychiatr 2001;70:419.10.1136/jnnp.70.3.419Search in Google Scholar

41. Ponzone A, Guardamagna O, Spada M, Ponzone R, Sartore M, Kierat L, et al. Hyperphenylalaninemia and pterin metabolism in serum and erythrocytes. Clin Chim Acta 1993;216:63–71.10.1016/0009-8981(93)90139-USearch in Google Scholar

42. Thauerer B, zur Nedden S, Baier-Bitterlich G. Vital role of protein kinase C-related kinase in the formation and stability of neurites during hypoxia. J Neurochem 2010;113:432–46.10.1111/j.1471-4159.2010.06624.xSearch in Google Scholar PubMed PubMed Central

43. zur Nedden S, Tomaselli B, Baier-Bitterlich G. HIF-1 alpha is an essential effector for purine nucleoside-mediated neuroprotection against hypoxia in PC12 cells and primary cerebellar granule neurons. J Neurochem 2008;105:1901–14.10.1111/j.1471-4159.2008.05275.xSearch in Google Scholar PubMed PubMed Central

44. Abbracchio MP, Burnstock G, Verkhratsky A, Zimmermann H. Purinergic signalling in the nervous system: an overview. Trends Neurosci 2009;32:19–29.10.1016/j.tins.2008.10.001Search in Google Scholar PubMed

45. Benowitz LI, Carmichael ST. Promoting axonal rewiring to improve outcome after stroke. Neurobiol Dis 2010;37:259–66.10.1016/j.nbd.2009.11.009Search in Google Scholar PubMed PubMed Central

46. Burnstock G. Purinergic signalling and disorders of the central nervous system. Nat Rev Drug Discov 2008;7:575–90.10.1038/nrd2605Search in Google Scholar PubMed

47. Ciccarelli R, Di Iorio P, Giuliani P, D′Alimonte I, Ballerini P, Caciagli F, et al. Rat cultured astrocytes release guanine-based purines in basal conditions and after hypoxia/hypoglycemia. Glia 1999;25:93–8.10.1002/(SICI)1098-1136(19990101)25:1<93::AID-GLIA9>3.0.CO;2-NSearch in Google Scholar

48. Dunwiddie TV, Masino SA. The role and regulation of adenosine in the central nervous system. Annu Rev Neurosci 2001;24:31–55.10.1146/annurev.neuro.24.1.31Search in Google Scholar

49. Fredholm BB, Norden B. Fuels for transportation. Ambio 2010;39(Suppl 1):31–5.10.1007/s13280-010-0062-zSearch in Google Scholar

50. Rathbone MP, Saleh TM, Connell BJ, Chang R, Su C, Worley B, et al. Systemic administration of guanosine promotes functional and histological improvement following an ischemic stroke in rats. Brain Res 2011;1407:79–89.10.1016/j.brainres.2011.06.027Search in Google Scholar

51. Hatakeyama K, Harada T, Kagamiyama H. IMP dehydrogenase inhibitors reduce intracellular tetrahydrobiopterin levels through reduction of intracellular GTP levels. Indications of the regulation of GTP cyclohydrolase I activity by restriction of GTP availability in the cells. J Biol Chem 1992;267:20734–9.10.1016/S0021-9258(19)36747-XSearch in Google Scholar

Received: 2013-10-22
Accepted: 2013-10-30
Published Online: 2013-12-19
Published in Print: 2013-12-01

©2013 by Walter de Gruyter Berlin Boston

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  7. Homocysteine metabolism in different human cells
  8. Extraction of ursolic acid from Ocimum sanctum and synthesis of its novel derivatives: effects on extracellular homocysteine, dihydrofolate reductase activity and proliferation of HepG2 human hepatoma cells
  9. Risk factors of atherosclerosis in patients with history of breast cancer
  10. Systemic immune response and peripheral blood cell count in patients with a history of breast cancer
  11. Evaluation of dihydropteridine reductase activities in patients with kidney failure
  12. Tetrahydrobiopterin compounds modulate intracellular signaling and reactive oxygen species levels in an in vitro model of ischemia-reperfusion injury
  13. Neopterin suppresses the activity of tryptophan-degrading enzyme indoleamine 2,3-dioxygenase in human peripheral blood mononuclear cells
  14. Modulation of phenylalanine and tyrosine concentrations by ischemia and guanosine in neuronal PC12 cells
  15. Identification of four common α-thalassemia gene deletions among a group with hemoglobinopathies in Sétif population, Algeria
  16. Demographic factors and associated anomalies in fetuses with neural tube defects
https://doi.org/10.1515/pterid-2013-0036
Keywords for this article
guanosine; ischemia; phenylalanine; phenylalanine-4-hydroxylase; PC12 cells; purine nucleosides; tyrosine
Creative Commons
BY-NC-ND 3.0

Articles in the same Issue

  1. Masthead
  2. Masthead
  3. Reviews
  4. Antiviral activity of interferon-γ involved in impaired immune function in infectious diseases
  5. Is folic acid supplementation to food benefit or risk for human health?
  6. Original articles
  7. Homocysteine metabolism in different human cells
  8. Extraction of ursolic acid from Ocimum sanctum and synthesis of its novel derivatives: effects on extracellular homocysteine, dihydrofolate reductase activity and proliferation of HepG2 human hepatoma cells
  9. Risk factors of atherosclerosis in patients with history of breast cancer
  10. Systemic immune response and peripheral blood cell count in patients with a history of breast cancer
  11. Evaluation of dihydropteridine reductase activities in patients with kidney failure
  12. Tetrahydrobiopterin compounds modulate intracellular signaling and reactive oxygen species levels in an in vitro model of ischemia-reperfusion injury
  13. Neopterin suppresses the activity of tryptophan-degrading enzyme indoleamine 2,3-dioxygenase in human peripheral blood mononuclear cells
  14. Modulation of phenylalanine and tyrosine concentrations by ischemia and guanosine in neuronal PC12 cells
  15. Identification of four common α-thalassemia gene deletions among a group with hemoglobinopathies in Sétif population, Algeria
  16. Demographic factors and associated anomalies in fetuses with neural tube defects
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