Advance Access publication August 9, 2007 Glutathione-Dependent Reduction of Arsenate by Glycogen Phosphorylase—A Reaction Coupled to Glycogenolysis

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 nucle-oside phosphorylase and glyceraldehyde-3-phosphate dehydroge-nase, 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 phos-phorylated 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 mono-phosphate (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 o