Glucokinase in pancreatic B-cells and its inhibition by alloxan

Abstract

Characterization of glucokinase in pancreatic B-cells from ob/ob mice and from rat liver revealed identical characteristics. A narrow substrate specificity; high Km values for the two substrates, D\ x=r eq-\ glucose and D-mannose, in the range of 10 and 20 mmol/l, respectively; higher Vmax values for D-glucose than for D-mannose; inhibition of glucokinase activities by D-mannoheptulose and by a specific glucokinase antibody. These characteristics distinguish glucokinase in soluble cytoplasmic fractions of pancreatic B-cells and liver from low Km hexokinases. Alloxan is a pancreatic B-cell cytotoxic agent, which has been widely used as a tool for the elucidation of the mechanisms of insulin secretion, because its inhibitory action on insulin secretion has been presumed to be intimately related to the mechanism of glucose-induced insulin secretion. Alloxan inhibited glucokinase but not hexokinase activity in cytoplasmic fractions of pancreatic B-cells and liver. The half maximal inhibitory concentration of alloxan was 5 \g=m\mol/l. Glucokinase activity was protected from alloxan toxicity only by D-glucose and D-mannose; the \g=a\ anomer of D-glucose provided significantly greater protection than the \ g=b\ anomer. The non-metabolizable sugar 3-0-methyl-D-glucose did not provide protection of glucokinase activity against inhibition by alloxan. Thus, inhibition of pancreatic B-cell glucokinase may contribute to the inhibition of glucose-induced insulin secretion by alloxan. These results support the contention that glucokinase regulates the metabolic flux rate through the glycolytic chain in the pancreatic B-cell and thereby generates the signal for glucose-induced

Physiological and Molecular Characteristics of Rat Hypothalamic Ventromedial Nucleus Glucosensing Neurons

“…Neurons not meeting standard latencies and/or the minimal AUC criteria for GE or GI neurons were classified as NG. Subsets of neurons were also categorized by their response to alloxan (Table 1), a GK inhibitor (47,48) that alters glucose-evoked Ca 2ϩ response in both GE and GI neurons with an effective concentration of ϳ4 mmol/l, which is not toxic to neurons (10). sc-RT-PCR.…”

The mechanisms of alloxan- and streptozotocin-induced diabetes

“…Selective inhibition of glucose-induced insulin secretion is the major pathophysiological effect of the thiol group reactivity of alloxan [37][38][39]. Alloxan has a central 5-carbonyl group that reacts very avidly with thiol groups.…”

“…Initially, insulin biosynthesis, glucose-induced insulin secretion and glucose metabolism (both glucose oxidation and oxygen consumption) are all affected [93][94][95]. On the other hand, streptozotocin has no immediate, direct inhibitory effect upon glucose transport [77] or upon glucose phosphorylation by glucokinase [39]. However, at later stages of functional beta cell impairment, deficiencies in terms of gene expression and protein production lead to the deterioration of both glucose transport and metabolism [96].…”

Chronic Exposure to Excess Nutrients Left-shifts the Concentration Dependence of Glucose-stimulated Insulin Secretion in Pancreatic β-Cells

“…Glucokinase Measurement-The method used to measure glucokinase (GK) activity from cell extracts has been described previously (34). Briefly, cells were sonicated and centrifuged at 4,500 ϫ g for 5 min.…”