Reversible inhibition of skeletal muscle phosphoprotein phosphatase by ATP, phosphate and fluoride.

A phosphoprotein phosphatase preparation which showed activity towards glycogen synthase, phosphorylase, phosphorylase kinase, and phosphohistones was reversibly inhibited (70–90%) by preincubation with free ATP (apparent K_i about 0.3 mM). Other nucleotides (ADP2 (apparent K_a 3μM) prior to assay. Other divalent metals (Co⁺⁺ > Zn⁺⁺ > Mg⁺⁺) were partially effective in reversing the inhibition. It is concluded that ATP by virtue of its special structure and metal binding capacity possibly removes a catalytically important metal ion from the enzyme.