Effect of metabolites and phosphorylase on the D to I conversion of glycogen synthase from human polymorphonuclear leukocytes

Rate and duration of net glycogen synthesis following glucose administration to fasted human leukocytes

When glucose was added to fasted human leukocytes in a final concentration of 0.5-5 mM there was a phase of glycogen synthesis followed by a phase of glycogen breakdown. The duration of the phase of net glycogen synthesis increased with increasing concentrations of glucose applied, but the net rate of glycogen synthesis was inversely related to this figure and decreased from approx. 7 nmol/10(7) cells per min at 0.5 mM glucose to an average of 4 nmol/10(7) cells per min at 5 mM glucose.

The inhibitory effect of phosphorylase a on the activation of glycogen synthase depends on the type of synthase phosphatase

The activity of glycogen synthase phosphatase in rat liver stems from the co-operation of two proteins, a cytosolic S-component and a glycogen-bound G-component. It is shown that both components possess synthase phosphatase activity. The G-component was partially purified from the enzyme-glycogen complex. Dissociative treatments, which increase the activity of phosphorylase phosphatase manyfold, substantially decrease the synthase phosphatase activity of the purified G-component. The specific inhibition of glycogen synthase phosphatase by phosphorylase a, originally observed in crude liver extracts, was investigated with purified liver synthase b and purified phosphorylase a. Synthase phosphatase is strongly inhibited, whether present in a dilute liver extract, in an isolated enzyme-glycogen complex, or as G-component purified therefrom. In contrast, the cytosolic S-component is insensitive to phosphorylase a. The activation of glycogen synthase in crude extracts of skeletal muscle is not affected by phosphorylase a from muscle or liver. Consequently we have studied the dephosphorylation of purified muscle glycogen synthase, previously phosphorylated with any of three protein kinases. Phosphorylase a strongly inhibits the dephosphorylation by the hepatic G-component, but not by the hepatic S-component or by a muscle extract. These observations show that the inhibitory effect of phosphorylase a on the activation of glycogen synthase depends on the type of synthase phosphatase.

A continuous assay method for glycogen synthesis from UDPglucose

A light scattering assay was developed for continuous monitoring of glycogen synthesis from UDPglucose. A close linear correlation between light-scattering signal and the amount of radioactively labelled glucose incorporated into glycogen suggests that the scattering signal is determined by the average molecular weight of the glycogen molecules. The method has the advantage of continuity, and it is suitable to follow the time course of the reaction at different conditions and to determine the initial rate of glycogen synthesis in a relatively simple and fast way. The possibility of measurement of glycogen synthase activity in muscle extracts was also tested.

The hysteretic properties of glycogen synthase I

Glycogen-free synthase I from human polymorphonuclear leukocytes is activated by its own substrate, glycogen, in a slow, time-dependent process (hysteretic activation). This lag in response to addition of glycogen depends on the concentration of glycogen, pH and temperature. At pH 7.4 and at a temperature of 30 degrees C, the half-time of activation t 1/2 decreases from 89 min at 0.004 mg/ml glycogen to 6 min at 25 mg/ml. The activation is accelerated by increasing temperature and pH, but is not influenced by enzyme concentration, glucose 6-phosphate, UDP, high ionic strength, EDTA, mercaptoethanol, glucose, sucrose or amylase limit dextrin. The Km for UDP-glucose (0.024 mM) and the activity ratio were unchanged during the activation process. The activation can be described by vt = vf + (vo - vf) e-kt where vt, vf and vo are velocities at times t, O and infinity and k is a complex rate constant. Evidence from ultracentrifugation and kinetic studies is presented to substantiate the hypothesis that the underlying mechanism is a simple biolecular process: enzyme + glycogen in equilibrium enzyme-glycogen complex, with the dissociation constant Ks = 0.003 mg/ml. The hysteretic activation may become rate-limiting during experiments in vitro with synthase. The possibility of a physiological role in glycogen metabolism, perhaps in the form of a concerted hysteresis with H+ is discussed.