1. The activity and the kinetic properties of purified hepatic phosphorylases a and b from rabbit and rat have been investigated in the glycogenolytic direction with a radiochemical assay. 2. In contrast with the a form, phosphorylase b has an absolute requirement for both AMP and a lyotropic salt. When the latter effectors are included, the b/a-form activity ratio remains low (0.03-0.15) at the hepatic concentration of Pi, because the b form has an exceedingly low affinity for this substrate. 3. Only phosphorylase b is significantly inhibited by glucose, glucose 6-phosphate and MgATP2-. Assays in the presence of substrastes, stimulators and inhibitors in the physiological concentration range indicate that glycogenolysis in the liver depends strictly on the conversion of phosphorylase b into a. Even at 1 mM-AMP the b/a-form activity ratio does not exceed 0.01. 4. Current spectrophotometric procedures for the glycogenolytic assay of phosphorylase in crude liver preparations are highly specific for the a form; the measurement of total phosphorylase (a + b) would require impractical modifications, and is better performed in the direction of glycogen synthesis.

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