A Comparison between Heat-Stable Phosphatase Inhibitors and Activators from Rabbit Skeletal Muscle and Liver and Their Effects upon Different Preparations of Phosphoprotein Phosphatase

Abstract

The separation and properties of heat-stable phosphatase effectors from muscle and liver were studied using phosphoprotein phosphatases with molecular weights of 250000, 65000 and 30000. Muscle inhibitors that were only active when phosphorylated were eluted from DEAE-cellulose by 20 mM NaCl (inhibitor 1) and 65 mM NaCl (inhibitor 1 ′). Available evidence suggests that these are different forms of the same protein and that inhibitor 1′ more closely resembles the native inhibitor. Liver extracts contained only the form eluted by 20 mM NaCl. These inhibitors did not affect the activity of the smallest enzyme. Both tissues also contained a non-phosphorylatable inhibitor of phosphorylase phosphatase activity (inhibitor 2) and an activator of phosphohistone phosphatase activity that affected all three enzymes. There was no evidence for the presence of effectors that were specific for only one of the phosphatases employed. The ratio of phosphorylatable to non-phosphorylatable inhibitor was much lower in liver than in muscle. Corresponding effectors from muscle and liver had similar physical properties and similar effects upon the activities of the three enzymes. Phosphorylase phosphatase activity was not affected by the activator. The 250000-Mr enzyme was relatively less sensitive than the 65000-Mr enzyme to inhibitor 2, and more sensitive to inhibitor 1′. With phosphohistone as substrate, only the largest enzyme was inhibited by inhibitor 1 and no enzyme was inhibited by inhibitor 1′ or inhibitor 2. There was no apparent competition between effectors. The presence of one effector did not modify the action of any of the others.