Periodate inactivates muscle glycogen phosphorylase by modifying the enzyme active site

Incubation of skeletal muscle glycogen phosphorylase with sodium periodate's results into irreversible loss of enzyme activity. The rate of inactivation is influenced by the ionic strength of the medium and by the presence of caffeine, but not by nucleotides. During the reaction, cysteine residues slowly reactive towards DTNB are modified and the coenzyme is released. These results suggest the presence of cysteine residues at the protein site involved in the binding of the phosphate group of pyridoxal phosphate.