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Glucose phosphorylating activities in the islets of Langerhans of the rat. Effect of fasting.

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.