Anomeric specificity of the stimulatory effect of D-glucose on D-fructose phosphorylation by human liver glucokinase

D-Glucose was recently reported to stimulate D-fructose phosphorylation by human B-cell glucokinase. The present study aims at investigating the anomeric specificity of such a positive cooperativity. The a-anomer of D-glucose was found to increase much more markedly than beta-D-glucose the phosphorylation of D-fructose by human liver glucokinase. Such an anomeric preference diminished at high concentrations of the D-glucose anomers, i.e. when the effect of the aldohexose upon D-fructose phosphorylation became progressively less marked. A comparison between the effects of the two anomers of D-glucose and those of equilibrated D-glucose upon D-fructose phosphorylation by human liver glucokinase indicated that the results obtained with the equilibrated aldohexose were not significantly different from those expected from the combined effects of each anomers of D-glucose. In isolated rat islets incubated for 60 min at 4 degreesC, alpha-D-glucose (5.6 mM), but not beta-D-glucose (also 5.6 mm), augmented significantly the conversion of D-[U-C-14]fructose (5.0 mM) to acidic radioactive metabolites. Likewise, in islets prelabeled with Ca-45 and perifused at 37 degreesC, D-fructose (20.0 mM) augmented Ca-45 efflux and provoked a biphasic stimulation of insulin release from islets exposed to alpha-D-glucose (5.6 mm), while inhibiting Ca-45 efflux and causing only a sluggish and modest increase in insulin output from islets exposed to beta-D-glucose (also 5.6 mm). The enhancing action of D-glucose upon D-fructose phosphorylation by glucokinase thus displays an obvious anomeric preference for alpha-D-glucose, and such an anomeric specificity remains operative in intact pancreatic islets.