The Effect of Glucose and of a Treatment by Glucocorticoids on the Activation in vitro of Liver Glycogen Synthetase

Abstract

The removal of glucose, AMP and other small molecules from mouse liver extract by filtration through Sephadex 6-25 has allowed us t o demonstrate that the activation in vitro of glycogen synthetase is much more rapidly attained in the presence of glucose and also when the animals have received prednholone 3 h before sacrifice. These effects are the result of a shortening of the lag period that precedes the activation of the synthetase; they are conveniently studied in filtrates that have been enriched with sulfate or phosphate ions. Like glucose, caffeine and nicotinamide shorten the lag period and are without action when this period is over; a-particulate glycogen has the opposite effect. No lag period is observed in the presence of AMP, particularly when associated with magnesium acetate. When a liver Sephadex filtrate is incubated in the absence of salts, glycogen synthetase b is converted into a partially active form that is rapidly transformed into synthetase a upon the addition of salt.These results support the hypothesis that glycogen synthetase phosphatase is initially present in the liver filtrate as a b enzyme, active only in the presence of AMP and magnesium; this b form is converted into its active, a, homologue through the action of a synthetase phosphatase activating enzyme, which is more active after a treatment with glucocorticoids, is stimulated by glucose, caffeine and nicotinamide and inhibited by glycogen, AMP and fluoride. I n the liver of an untreated animal, the enzyme i s predominantly in the b form [2,3]; it is activated within a few min after an intravenous load of glucose [4,6] or within 2 to 3 h after the administration of glucocorticoids [5-71. Once activated, the enzyme is rapidly reconverted into b after the injection of glucagon [5,8] epinephrine or cyclic AMP [5].The studies in Witro (9-111 support the hypothesis that the activation and the inactivation of the synthetase in liver occurs, as in muscle [12], by dephosphorylation and rephosphorylation, with a phosphatase and b a s e , respectively. The latter enzyme is stimulated by cyclic AMP and this effect adequately explains the inactivation of the synthetase under the action of glucagon or epinephrine. Up to now, it had not been possible to show unequivocally what part of the system was mod5ed by glucose and by glucocorticoids.Enzymes. Glycogen synthetase or UDPG: a-l,4-gluoan a-4-glucosyltransferase (EC 2.4.1.11) ; glycogen phosphorylaae or a-1,4-glucan: orthophosphate glucosyltransferese (EC 2.4.1.1) ; phosphorylase phosphatase or phosphorylase phosphohydrolaae (EC 3.1.3.17).

Overexpression of glutamine: fructose-6-phosphate amidotransferase in the liver of transgenic mice results in enhanced glycogen storage, hyperlipidemia, obesity, and impaired glucose tolerance.

“…Other factors such as the nature and size of the glycogen particles (31), the state and amount of glycogenin (32), the subcellular partitioning of synthase (33), and the partitioning and cycling of its substrates (34) have also been implicated in the determination of net glycogen storage. Decreased synthase activity in the fed state may be due to excess glycogen accumulation (35,36) and thus be only indirectly related to the expression of the trans- gene. Studies using hepatocytes cultured under controlled conditions are under way to further explore these questions.…”

Thyroid Metabolism in the Recessive Sex-Linked Dwarf Female Chicken

“…Since an increase in liver glycogen was shown to be associated with hypothyroidic condition in chickens (Snedecor and King, 1964;Snedecor, 1968), such an increase in dwarf chickens may be indicative of defective thyroid function or a failure to utilize thyroid hormones (Grandhi et al, 1975b). DeWulf et al (1970) demonstrated that liver glycogen synthetase phosphatase activating enzyme formation is induced by glucocorticoids and its activity stimulated by the presence of glucose in liver. It has also been shown that normally chicks have an in vivo glucose absorption rate 3 to 4 times greater than rats and this will be further enhanced by administration of thyroid hormones (Hazelwood, 1965).…”

Dual function and common identity of proteins in glycogen metabolism: Reply to a hypothesis

“…Ryman and Whelan [I] have revived our previous hypothesis [4] that synthetase phosphatase exists in two forms. This enzyme is indeed inactive in a fresh liver Sephadex filtrate and becomes active within about 20 min upon incubation of the filtrate at 20" [4] .…”

“…Ryman and Whelan [I] have revived our previous hypothesis [4] that synthetase phosphatase exists in two forms. This enzyme is indeed inactive in a fresh liver Sephadex filtrate and becomes active within about 20 min upon incubation of the filtrate at 20" [4] . This latency had first been interpreted as the time required for the conversion of an inactive into an active form by a 'synthetase phosphatase activating enzyme' [4] ; this hypothetical enzyme and phosphorylase phosphatase had so many properties in common that we suggested their identity [5] .…”

“…Their hypothesis was partly based on results obtained in our laboratory and recently reviewed in this journal [2] However, other facts included in the same review [2] were neglected, presumably because they were described in a very concise way; in the meantime, a more complete report of our work has been published [3].Ryman and Whelan [I] have revived our previous hypothesis [4] that synthetase phosphatase exists in two forms. This enzyme is indeed inactive in a fresh liver Sephadex filtrate and becomes active within about 20 min upon incubation of the filtrate at 20" [4] . This latency had first been interpreted as the time required for the conversion of an inactive into an active form by a 'synthetase phosphatase activating enzyme ' [4] ; this hypothetical enzyme and phosphorylase phosphatase had so many properties in common that we suggested their identity [5] .…”