[Delayed activation of muscle glycogen phosphorylase b on incubation of the enzyme with adenosine-5'-monophosphate].

When studying the enzyme activity of glycogen phosphorylase b from rabbit skeletal muscles by the turbidimetric method or by the method based on the determination of inorganic phosphate (the product of the enzymatic reaction) in the direction of glycogen synthesis we observed that 10-15 min preincubation of the enzyme with the allosteric activator (AMP) results in an increase in the initial rate of the enzymatic reaction (nu) in relation to the corresponding value of nu measured by the initiation of the enzymatic reaction by the addition of the mixture of glucose 1-phosphate and AMP (0.02 M Hepes, pH 6.8; 37 degrees C). Glycogen with molecular mass of (264-276).10(6) dalton was used in the kinetic experiments. For 1 mM AMP the time-dependent process of phosphorylase b activation under the action of AMP follows the exponential law with the apparent rate constant of the first order equal to 0.43 min-1 (turbidimetric method of measurement of enzyme activity). When AMP concentration increases, the degree of activation of phosphorylase b reaches a limiting value equal to 1.75 (6 mM glucose 1-phosphate, 0.2 mg/ml glycogen). The activation effect decreases with increasing glycogen concentration and disappears at saturating concentrations of high-molecular weight substrate. Incubation of phosphorylase b with AMP causes the lowering of Michaelis constant for glucose 1-phosphate. It is assumed that enhancement of the rate of the enzymatic reaction catalyzed by phosphorylase b during incubation with AMP is due to association of the enzyme molecules adsorbed to a glycogen particle resulting in an increase in the affinity of the enzyme for glycogen.