ToxSci Advance Access originally published online on August 9, 2007
Toxicological Sciences 2007 100(1):36-43; doi:10.1093/toxsci/kfm211
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Glutathione-Dependent Reduction of Arsenate by Glycogen Phosphorylase—A Reaction Coupled to Glycogenolysis
Department of Pharmacology and Pharmacotherapy, Toxicology Section, University of Pécs, Medical School, Szigeti út 12, H-7624 Pécs, Hungary
1 To whom correspondence should be addressed. Fax: +36-72-536-218. E-mail: zoltan.gregus{at}aok.pte.hu.
Received June 13, 2007; accepted August 2, 2007
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Abstract |
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Arsenate (As(V)) is reduced in the body to the more toxic arsenite (As(III)). We have shown that two enzymes catalyzing phosphorolytic cleavage of their substrates, namely purine nucleoside phosphorylase and glyceraldehyde-3-phosphate dehydrogenase, can reduce As(V) in presence of an appropriate thiol and their substrates. Another phosphorolytic enzyme that may also reduce As(V) is glycogen phosphorylase (GP). With inorganic phosphate (Pi), GP catalyzes the breakdown of glycogen to glucose-1-phosphate; however, it also accepts As(V). Testing the hypothesis that GP can reduce As(V), we incubated As(V) with the phosphorylated GPa or the dephosphorylated GPb purified from rabbit muscle and quantified the As(III) formed from As(V) by high-performance liquid chromatography–hydride generation–atomic fluorescence spectrometry. In the presence of adenosine monophosphate (AMP), glycogen, and glutathione (GSH), both GP forms reduced As(V) at rates increasing with enzyme and As(V) concentrations. The As(V) reductase activity of GPa was 10-fold higher than that of GPb. However, incubating GPb with GP kinase and ATP (that converts GPb to GPa) increased As(V) reduction by phosphorylase up to the rate produced by GPa incubated under the same conditions. High concentration of inorganic sulfate, which activates GPb like phosphorylation, also promoted reduction of As(V) by GPb. As(V) reduction by GPa (like As(V) reduction in rats) required GSH. It also required glycogen (substrate for GP) and was stimulated by AMP (allosteric activator of GP) even at low micromolar concentrations. Pi, substrate for GP competing with As(V), inhibited As(III) formation moderately at physiological concentrations. Glucose-1-phosphate, the product of GP-catalyzed glycogenolysis, also decreased As(V) reduction. Summarizing, GP is the third phosphorolytic enzyme identified capable of reducing As(V) in vitro. For reducing As(V) by GP, GSH and glycogen are indispensable, suggesting that the reduction is linked to glycogenolysis. While its in vivo significance remains to be tested, further characterization of the GP-catalyzed As(V) reduction is presented in the adjoining paper.
Key Words: arsenate; reduction; glycogen phosphorylase; glutathione; glycogen breakdown; arsenolysis.