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 ¼ of the islet glucose phosphorylating activity was due to an enzyme with kinetic properties similar to glucokinase; % of the activity was due to hexokinase. The procedure for determining islet glucokinase activity was improved by centrifuging isotonic islet homogenates at 12,000 × g. The supernatant fraction was enriched for glucokinase. About ½ of the glucose phosphorylating activity in this fraction was due to glucokinase and ½ was due to hexokinase. The glucokinase activity in islet homogenates was ⅓ of the activity of hexokinase, V40 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 μM) 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 β-cells [J. Biol. Chem. 243:2730 (1968] is greatly strengthened by the present studies.