Glucose Phosphorylating Activities in the Islets of Langerhans of the Rat. Effect of Fasting

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Abstract
In the present study, the characterization of glucose phosphorylating activities in islet extracts and the effect of fasting on these activities have been studied. 4 mM glucose 6-phosphate strongly inhibits hexokinase activity, while glucokinase increased its activity. N-acetyl-glucosamine inhibits hexokinase activity (40% inhibition), while glucokinase increased its activity (19% increment). Both phosphorylating activities reaches a maximum when the Mg/ATP ratio is 1; increments in Mg/ATP ratio inhibits glucokinase activity while hexokinase activity is less dependent of Mg/ATP ratio. Fasting induces a progressive decrease of both phosphorylating activities in islet extracts. After 48 h fasting both activities are decreased 50%. After 96 h fasting, glucokinase activity disappeared completely, while hexokinase was 80% reduced. Our data suggest that hexokinase is less affected by fasting than glucokinase. In addition, other authors have reported that other glycolytic enzymes are little altered by fasting. Hence the fasting-induced-adaptation of glucokinase could account for the reduced rate of glycolysis and subsequently reduced insulin secretory response towards glucose.

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