Abstract
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 alpha-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 degrees C, alpha-D-glucose (5.6 mm), but not beta-D-glucose (also 5.6 mm), augmented significantly the conversion of D-[U-(14)C]fructose (5.0 mm) to acidic radioactive metabolites. Likewise, in islets prelabeled with (45)Ca and perifused at 37 degrees C, D-fructose (20.0 mm) augmented (45)Ca efflux and provoked a biphasic stimulation of insulin release from islets exposed to alpha-D-glucose (5.6 mm), while inhibiting (45)Ca 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.
Highlights
Creatic islets prepared from either Goto-Kakizaki rats or adult rats that had been injected with streptozotocin during the neonatal period, i.e. in two animal models of non-insulin-dependent diabetes mellitus [4, 5]
The experimental conditions used in this study to assess the anomeric specificity of the effect of D-glucose upon the phosphorylation, metabolism, and insulinotropic action of D-fructose were selected to minimize the interconversion of the D-glucose anomers [6]
The metabolic data collected from islets incubated for 60 min at 4 °C are compatible with the view that the anomeric specificity of the enhancing action of D-glucose upon D-fructose phosphorylation, as documented in the experiments conducted in the presence of human liver glucokinase, is operative in intact pancreatic islets
Summary
Creatic islets prepared from either Goto-Kakizaki rats or adult rats that had been injected with streptozotocin during the neonatal period, i.e. in two animal models of non-insulin-dependent diabetes mellitus [4, 5]. The effects of the two anomers of D-glucose upon D-[U-14C]fructose conversion to 14CO2 and 14C-labeled acidic metabolites and upon the cationic and insulin secretory responses to D-fructose were examined in isolated rat pancreatic islets. The experiments were conducted over 10 min of incubation at 25 °C (D-fructose phosphorylation), 60 min of incubation at 4 °C (D-fructose metabolism in islets) or with D-glucose anomers maintained for 90 min or less at 4 °C (perifused islets) to minimize the interconversion of the glucose anomers [6] Under these conditions, the fraction of ␣-D-glucose converted to -D-glucose, expressed relative to the equilibrium value, is close to 5.4 and 9.0% after 60 and 90 min of incubation at 4 °C and to 17.8% after 10 min incubation at 25 °C [6]. The mean value for the fractional conversion of each anomer during the incubation period used for the measurement of biological variables is close to only half of these percentages
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