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 (ADP<AMP-P(NH)P<GTP<ATP) and EDTA were only partially effective in inhibiting phosphatase activity. Inhibition was also achieved by preincubation with 50 mM KF or by 16 hr. dialysis against 5 mM potassium phosphate buffer. The inhibition by all these ligands was reversed by a second preincubation with MnCl₂ (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.