The high Km glucose phosphorylation enzyme glucokinase is believed to be the α-cell glucose sensor, i.e., the site in glucose metabolism that determines the sensitivity and specificity of glucose-induced insulin secretion. We investigated the regulation of this enzyme by measuring glucokinase Vmax and protein levels in isolated islets from hyperinsulinemic rats. Rats were infused for 48 h with 2 ml/h of 20% glucose, 50% glucose, or 0.45% NaCl (control rats). At the end of the infusion, 20% glucose-infused rats were normoglycemic and hyperinsulinemic (2.3-fold rise in basal plasma insulin level). Their islets had a 2.3-fold increase in insulin secretion at 8.3 mM glucose (51 ± 10% of capacity vs. 22 ± 5% in NaCl rats, P < 0.03), a 75% increase in glucokinase Vmax and little if any increase in glucokinase protein level (111 ± 3% of control). The rats infused with 50% glucose had marked hyperglycemia and higher basal plasma insulin levels. Their islets were maximally stimulated by 8.3 mM glucose in combination with a 270% increase in glucokinase Vmax and a 69 ± 11% increase in glucokinase protein level. Hexokinase Vmax was also doubled. Thus, compensatory increases in β-cell glucose phosphorylation are a key mechanism for adaptive hyperinsulinemia. Our results show two types of regulation for the β-cell high Km phosphorylation enzyme, glucokinase. The content of glucokinase protein is controlled by the plasma glucose level. Variable catalytic activity of this protein was also observed in this study.
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Original Articles| May 01 1994
Increased Catalytic Activity of Glucokinase in Isolated Islets From Hyperinsulinemic Rats
Address correspondence and reprint requests to Dr. Jack Leahy, New England Medical Center no. 268, 750 Washington Street, Boston, MA 02111.
Chuan Chen, Lisa Bumbalo, Jack L Leahy; Increased Catalytic Activity of Glucokinase in Isolated Islets From Hyperinsulinemic Rats. Diabetes 1 May 1994; 43 (5): 684–689. https://doi.org/10.2337/diab.43.5.684
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