Regulation of Glucose Metabolism in Pancreatic Islets

Abstract

We evaluated the possible role of islet glucokinase in controlling the rate of islet glucose metabolism, and thereby the rate of glucose-induced insulin release. The activities of glucokinase, hexokinase, P-fructokinase, and glyceraldehyde-P dehydrogenase were quantitated in sonicated or isotonically homogenized islet preparations using pyridine nucleotide-dependent fluorometric assays. In sonicates, about 1/4 of the islet glucose phosphorylating activity was due to an enzyme with kinetic properties similar to glucokinase; 3/4 of the activity was due to hexokinase. The procedure for determining islet glucokinase activity was improved by centrifuging isotonic islet homogenates at 12,000 x g. The supernatant fraction was enriched for glucokinase. About 1/2 of the glucose phosphorylating activity in this fraction was due to glucokinase and 1/2 was due to hexokinase. The glucokinase activity in islet homogenates was !23 of the activity of hexokinase, 1/40 of the activity of P-fructokinase, and 1/400 of the activity of glyceraldehyde-P dehydrogenase. Detailed concentration dependency curves of glucose and mannose utilization were also obtained with intact isolated pancreatic rat islets. Glucose and mannose usage in islets was governed by two superimposed hyperbolic systems differing in Km and Vmax. A high Km system (Km for glucose 11 mM and for mannose 21 mM) predominated. A low Km system (Km for glucose 215 and for mannose 530 microM) contributed about 15% to the total activity. The available data with intact islets could be rationalized by the existence of two distinct hexose phosphorylating enzymes with differing capacities and kinetic properties. These enzymes, tentatively identified as glucokinase and hexokinase, could coexist in the same cell or could be distributed among different cell types. The possible physiologic significance of these results is discussed, emphasizing the idea of dual control of glycolysis and insulin release by glucokinase and hexokinase. An earlier proposal that glucokinase serves as glucoreceptor of beta-cells [J. Biol. Chem. 243:2730 (1968)] is greatly strengthened by the present studies.

Glucose Oscillations Can Activate an Endogenous Oscillator in Pancreatic Islets

“…Efflux is through the mitochondrial pyruvate dehydrogenase (PDH) complex, which decarboxylates the final glycolytic product pyruvate to supply acetyl CoA to the citric acid cycle. The glycolytic model is a system of differential equations for the rates of concentration change of G i and PFK1 substrate, fructose 6-phosphate (F6P), and product, fructose 1,6-bisphosphate (FBP): derived by assuming the glucose 6-phosphate isomerase reaction upstream to PFK1 and the series of reactions downstream to PFK1 are at equilibrium [23, 24]. The glycolytic subsystem is coupled through ATP to a conductance-based model of electrical activity on the plasma membrane: a rise in glycolytic flux causes ATP production that reduces ATP-sensitive K + (K(ATP)) membrane ion channel conductance.…”

Co‐localization of glucokinase with actin filaments

“…Enzyme activities were assayed fluorometrically by a modification of the method of Trus et al [13]. COS‐7 cell transfectants were harvested with trypsin–EDTA, washed twice with phosphate‐buffered saline (PBS) to remove glucose, and homogenized with seven strokes in a homogenizing buffer (20 mM K 2 HPO 4 , 110 mM KCl, 1 mM MgCl 2 , 1 mM EDTA, and 1 mM dithiothreitol, pH 7.4) using a Potter‐Elvehjem glass–Teflon homogenizer.…”

Pancreatic beta cell line MIN6 exhibits characteristics of glucose metabolism and glucose-stimulated insulin secretion similar to those of normal islets

“…Considerable evidence has been collected that the glucose signal for insulin secretion is generated by glucose metabolism in beta cells [17,18]. Recent studies have shown that glucose transport and phosphorylation steps are important for glucose-stimulated insulin secretion in pancreatic beta cells [18][19][20].…”

“…Considerable evidence has been collected that the glucose signal for insulin secretion is generated by glucose metabolism in beta cells [17,18]. Recent studies have shown that glucose transport and phosphorylation steps are important for glucose-stimulated insulin secretion in pancreatic beta cells [18][19][20]. The present study was designed to characterize the properties of glucose transport, phosphorylation, utilization and glucose-stimulated insulin secretion in MIN6 cells and to determine whether MIN6 cells represent an appropriate model for investigat-ing the mechanism of glucose-stimulated insulin secretion.…”