Hai-Xia Zhang

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 α-anomer ofd-glucose was found to increase much more markedly than β-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 upond-fructose phosphorylation became progressively less marked. A comparison between the effects of the two anomers ofd-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 °C, α-d-glucose (5.6 mm), but not β-d-glucose (also 5.6 mm), augmented significantly the conversion ofd-[U-14C]fructose (5.0 mm) to acidic radioactive metabolites. Likewise, in islets prelabeled with45Ca and perifused at 37 °C, d-fructose (20.0 mm) augmented 45Ca efflux and provoked a biphasic stimulation of insulin release from islets exposed to α-d-glucose (5.6 mm), while inhibiting45Ca efflux and causing only a sluggish and modest increase in insulin output from islets exposed to β-d-glucose (also 5.6 mm). The enhancing action ofd-glucose upon d-fructose phosphorylation by glucokinase thus displays an obvious anomeric preference for α-d-glucose, and such an anomeric specificity remains operative in intact pancreatic islets.

Labelling of lipids by D-[1-14C]glucose, D-[6-14C] glucose and D-[3-3H]glucose in pancreatic islets from normal and GK rats.

In pancreatic islets prepared from either normal or GK rats and incubated at either low (2.8 mM) or high (16.7 mM) D-glucose concentration, the labelling of both lipids and their glycerol moiety is higher in the presence of D-[1-14C]glucose than D-[6-14C]glucose. The rise in D-glucose concentration augments the labelling of lipids, the paired 14C/3H ratio found in islets exposed to both D-[1-14C]glucose or D-[6-14C]glucose and D-[3-3H]glucose being even slightly higher at 16.7 mM D-glucose than that found, under otherwise identical conditions, at 2.8 mM D-glucose. Such a paired ratio exceeds unity in islets exposed to D-[1-14C]glucose. The labelling of islet lipids by D-[6-14C]glucose is about 30 times lower than the generation of acidic metabolites from the same tracer. These findings indicate (i) that the labelling of islet lipids accounts for only a minor fraction of D-glucose catabolism in pancreatic islets, (ii) a greater escape to L-glycerol-3-phosphate of glycerone-3-phosphate generated from the C1-C2-C3 moiety of D-glucose than D-glyceraldehyde-3-phosphate produced from the C4-C5-C6 moiety of the hexose, (iii) that only a limited amount of [3-3H]glycerone 3-phosphate generated from D-[3-3H]glucose is detritiated at the triose phosphate isomerase level before being converted to L-glycerol-3-phosphate, and (iv) that a rise in D-glucose concentration results in an increased labelling of islet lipids, this phenomenon being somewhat more pronounced in the case of D-[1-14C]glucose or D-[6-14C]glucose rather than D-[3-3H]glucose.