TAILIEUCHUNG - Báo cáo khoa học: Xanthosine and xanthine Substrate properties with purine nucleoside phosphorylases, and relevance to other enzyme systems

Substrate properties of xanthine (Xan) and xanthosine (Xao) for purine nucleoside phosphorylases (PNP) of mammalian origin have been reported previously, but only at a single arbitrarily selected pH and with no kinetic con-stants. Additionally, studies have not taken into account the fact that, at physiological pH, Xao (pKa¼) is a mono-anion, while Xan (pKa¼) is an equilibrium mixture of the neutral and monoanionic forms. | Eur. J. Biochem. 269 4048-4057 2002 FEBS 2002 doi Xanthosine and xanthine Substrate properties with purine nucleoside phosphorylases and relevance to other enzyme systems Gerasim Stoychev1 Borys Kierdaszuk1 and David Shugar1 2 1 Department of Biophysics Institute of Experimental Physics University of Warsaw Poland 2Institute of Biochemistry and Biophysics Polish Academy of Sciences Warsaw Poland Substrate properties of xanthine Xan and xanthosine Xao for purine nucleoside phosphorylases PNP of mammalian origin have been reported previously but only at a single arbitrarily selected pH and with no kinetic constants. Additionally studies have not taken into account the fact that at physiological pH Xao pKa is a monoanion while Xan pKa is an equilibrium mixture of the neutral and monoanionic forms. Furthermore the monoanionic forms unlike those of guanosine Guo and inosine Ino and guanine Gua and hypoxanthine Hx are still 6-oxopurines. The optimum pH for PNP from human erythrocytes and calf spleen with both Xao and Xan is in the range 5-6 whereas those with Guo and Gua and Ino and Hx are in the range 7-8. The pH-dependence of substrate properties of Xao and Xan points to both neutral and anionic forms as substrates with a marked preference for the neutral species. Both neutral and anionic forms of 6-thioxanthine pKa but not of 2-thioxan-thine pKa are weaker substrates. Phosphorolysis of Xao to Xan by calf spleen PNP at pH levels off at 83 conversion due to equilibrium with the reverse synthetic pathway equilibrium constant and not by product inhibition. Replacement of Pi by arsenate led to complete arsenolysis of Xao. Kinetic parameters are reported for the phosphorolytic and reverse synthetic pathways at several selected pH values. Phosphorolysis of 200 M Xao by the human enzyme at pH is inhibited by Guo IC50 10 2 jm Hx iC50 7 1 jiMkindGui IC50 jm . With Gua inhibition was shown to be .

• Báo cáo khoa học
• Report medicine
• traditional medicine
• scientific reports
• molecular Biology
• tumor cells
• Antimicrobial
• medical knowledge
• medical research
• analysis of cytoplasmic
• oxidation
• Crystal structure
• bacteria
• hemoglobin
• medicine
• cell proliferation
• breast cancer
• chemical reaction
• gastric cancer
• hormone medicine
• biochemical studies
• environmental medicine