SLOW ACTIVATION OF MUSCLE GLYCOGEN-PHOSPHORYLASE-B DURING INCUBATION WITH ADENOSINE 5'-MONOPHOSPHATE

When studying the enzymatic activity of glycogen phosphorylase b from rabbit skeletal muscles in the direction of glycogen synthesis, both t urbidimetrically and by determination of inorganic phosphate (the prod uct of the enzymatic reaction), we observed that preincubation of the enzyme with the allosteric activator AMP for 10-15 min increased the i nitial rate of the enzymatic reaction (nu) as compared with the corres ponding value measured after initiating the enzymatic reaction by the addition of a 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) d altons was used in the kinetic experiments. With 1 mM AMP the time-dep endent activation of phosphorylase b is exponential with an apparent f irst-order rate constant of 0.43 min(-1) (by the turbidimetric method) . As AMP concentration is increased, the activation of phosphorylase b reaches a limiting value of 1.75 (6 mM glucose 1-phosphate, 0.2 mg/ml glycogen). The activating effect decreases with increasing glycogen c oncentration and disappears at saturating concentrations of the high-m olecular-weight substrate. Incubation of phosphorylase b with AMP lowe rs the Michaelis constant for glucose 1-phosphate. It is assumed that the enhancement of the rate of the enzymatic reaction catalyzed by pho sphorylase b during incubation with AMP is due to self-association of enzyme molecules adsorbed to a glycogen particle resulting in an incre ase in the affinity of the enzyme for glycogen.