Discrimination of glucose anomers by glucokinase from liver and transplantable insulinoma.
Phosphorylation of alpha- and beta-D-glucose by glucokinase from rat liver or a radiation-induced, transplantable insulinoma was investigated. Glucokinase partially purified by ion exchange chromatography on DEAE-Cibacron blue F3GA agarose was incubated for brief periods (1 or 3 min) with glucose anomers. Glucokinase from both liver and insulinoma tissue had a higher affinity for alpha-D-glucose (S0.5 = 6-7 mM) than beta-D-glucose (S0.5 = 12-14 mM). The maximum velocity was 15-20% lower for alpha-D-glucose than beta-D-glucose. Cooperative rate dependence with respect to glucose concentration was observed with both anomers (nH = 1.4). These kinetic data imply that both anomers of glucose are phosphorylated by glucokinase, however, at the physiological range of glucose concentrations below 15 mM, the higher affinity of alpha-D-glucose results in higher rates than with beta-D-glucose. At clearly pathological glucose concentrations exceeding 20 mM, the observed velocities are slightly higher with beta- than alpha-D-glucose. Glucokinase is thought to be the glucose sensor of pancreatic beta cells. The present data indicating a preferential phosphorylation of alpha-D-glucose compared to beta-D-glucose by glucokinase, supports the glucokinase-glucose sensor hypothesis, because it parallels the well established greater potency of alpha-D-glucose as a stimulant of insulin release.
D007516:Adenoma, Islet Cell / Q000201:enzymology*; D000818:Animals; D005941:Glucokinase / Q000378:metabolism*; D005947:Glucose; D007340:Insulinoma / Q000201:enzymology*; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D009374:Neoplasms, Experimental / Q000201:enzymology; D009381:Neoplasms, Radiation-Induced / Q000201:enzymology*; D010190:Pancreatic Neoplasms / Q000201:enzymology*; D051381:Rats; D013237:Stereoisomerism; D013379:Substrate Specificity
D005941:Glucokinase; D005947:Glucose