Kinetics and specificity of human B-cell glucokinase: relevance to hexose-induced insulin release

Solution NMR Spectroscopy for the Study of Enzyme Allostery

Allostery is a ubiquitous biological regulatory process in which distant binding sites within a protein or enzyme are functionally and thermodynamically coupled. Allosteric interactions play essential roles in many enzymological mechanisms, often facilitating formation of enzyme-substrate complexes and/or product release. Thus, elucidating the forces that drive allostery is critical to understanding the complex transformations of biomolecules. Currently, a number of models exist to describe allosteric behavior, taking into account energetics as well as conformational rearrangements and fluctuations. In the following Review, we discuss the use of solution NMR techniques designed to probe allosteric mechanisms in enzymes. NMR spectroscopy is unequaled in its ability to detect structural and dynamical changes in biomolecules, and the case studies presented herein demonstrate the range of insights to be gained from this valuable method. We also provide a detailed technical discussion of several specialized NMR experiments that are ideally suited for the study of enzymatic allostery.

Glucose-dependent modulation of insulin secretion and intracellular calcium ions by GKA50, a glucokinase activator

Because glucokinase is a metabolic sensor involved in the regulated release of insulin, we have investigated the acute actions of novel glucokinase activator compound 50 (GKA50) on islet function. Insulin secretion was determined by enzyme-linked immunosorbent assay, and microfluorimetry with fura-2 was used to examine intracellular Ca(2+) homeostasis ([Ca(2+)](i)) in isolated mouse, rat, and human islets of Langerhans and in the MIN6 insulin-secreting mouse cell line. In rodent islets and MIN6 cells, 1 micromol/l GKA50 was found to stimulate insulin secretion and raise [Ca(2+)](i) in the presence of glucose (2-10 mmol/l). Similar effects on insulin release were also seen in isolated human islets. GKA50 (1 micromol/l) caused a leftward shift in the glucose-concentration response profiles, and the half-maximal effective concentration (EC(50)) values for glucose were shifted by 3 mmol/l in rat islets and approximately 10 mmol/l in MIN6 cells. There was no significant effect of GKA50 on the maximal rates of glucose-stimulated insulin secretion. In the absence of glucose, GKA50 failed to elevate [Ca(2+)](i) (1 micromol/l GKA50) or to stimulate insulin release (30 nmol/l-10 micromol/l GKA50). At 5 mmol/l glucose, the EC(50) for GKA50 in MIN6 cells was approximately 0.3 micromol/l. Inhibition of glucokinase with mannoheptulose or 5-thioglucose selectively inhibited the action of GKA50 on insulin release but not the effects of tolbutamide. Similarly, 3-methoxyglucose prevented GKA50-induced rises in [Ca(2+)](i) but not the actions of tolbutamide. Finally, the ATP-sensitive K(+) channel agonist diazoxide (200 micromol/l) inhibited GKA50-induced insulin release and its elevation of [Ca(2+)](i.) We show that GKA50 is a glucose-like activator of beta-cell metabolism in rodent and human islets and a Ca(2+)-dependent modulator of insulin secretion.

Opposite effects of D-fructose on total versus cytosolic ATP/ADP ratio in pancreatic islet cells

D-fructose (10 mM) augments, in rat pancreatic islets, insulin release evoked by 10 mM D-glucose. Even in the absence of D-glucose, D-fructose (100 mM) displays a positive insulinotropic action. It was now examined whether the insulinotropic action of D-fructose could be attributed to an increase in the ATP content of islet cells. After 30-60 min incubation in the presence of D-glucose and/or D-fructose, the ATP and ADP content was measured by bioluminescence in either rat isolated pancreatic islets (total ATP and ADP) or the supernatant of dispersed rat pancreatic islet cells exposed for 30 s to digitonine (cytosolic ATP and ADP). D-fructose (10 and 100 mM) was found to cause a concentration-related decrease in the total ATP and ADP content and ATP/ADP ratio below the basal values found in islets deprived of exogenous nutrient. Moreover, in the presence of 10 mM D-glucose, which augmented both the total ATP content and ATP/ADP ratio above basal value, D-fructose (10 mM) also lowered these two parameters. The cytosolic ATP/ADP ratio, however, was increased in the presence of D-glucose and/or D-fructose. Under the present experimental conditions, a sigmoidal relationship was found between such a cytosolic ATP/ADP ratio and either (86)Rb net uptake by dispersed islet cells or insulin release from isolated islets. These data provide, to our knowledge, the first example of a dramatic dissociation between changes in total ATP content or ATP/ADP ratio and insulin release in pancreatic islets exposed to a nutrient secretagogue. Nevertheless, the cationic and insulinotropic actions of d-glucose and/or d-fructose were tightly related to the cytosolic ATP/ADP ratio.

Adenine nucleotide pattern in rat pancreatic islets exposed to nutrient secretagogues

The effects of D-glucose, D-mannose, D-galactose, Dglyceraldehyde, pyruvate, L-lactate, 2-ketoisocaproate, L-leucine, and/or L-glutamine on the ATP and ADP content of rat isolated pancreatic islets were reevaluated in order to compare changes evoked by these nutrient secretagogues in the islet ATP content and ATP/ADP ratio to their effects upon insulin release. Although being compatible with the fuel concept for nutrient-stimulated insulin secretion, the results of this study also argue against the monolithic view that the adenine nucleotide pattern in islet cells represents the sole coupling factor between metabolic and more distal events in the process of nutrient-stimulated insulin release.

Immediate and delayed effects of D-fructose upon insulin, somatostatin, and glucagon release by the perfused rat pancreas

Novel information was recently provided concerning the reciprocal effects of D-glucose and D-fructose upon their respective metabolism in rat pancreatic islets. In the light of such findings, this study aims at comparing the effects of D-glucose and D-fructose on insulin, somatostatin, and glucagon release from the isolated perfused rat pancreas. A rise in D-glucose concentration from 3.3 to 5.0 or 7.3 mM or the administration of D-fructose (17 and 40 mM) in the presence of 3.3 mM D-glucose stimulated insulin release in a concentration-related manner, but failed to affect somatostatin output. The secretion of glucagon was decreased in all cases. The secretory response to L-arginine (5 mM), 25 min after restoring the basal concentration of D-glucose, was more markedly affected, in terms of potentiation of insulin and somatostatin release and reduction of glucagon output, after prior administration of D-fructose than after a prior increase in D-glucose concentration. These findings argue against any major role for a paracrine regulation of hormonal release and, instead, are consistent with a causal link between metabolic and secretory events in the islet cells. Nevertheless, the present results emphasize differences in the response of distinct pancreatic endocrine cell types to the same or distinct hexoses.

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 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.

Interference of D-mannoheptulose with D-glucose phosphorylation, metabolism and functional effects: comparison between liver, parotid cells and pancreatic islets

D-mannoheptulose is currently used as a tool to inhibit, in a competitive manner, D-glucose phosphorylation, metabolism and functional effects in the pancreatic islet B-cell. In order to better understand the mode of action of the heptose, we have explored its effect upon D-glucose phosphorylation in liver, parotid cells and islet homogenates, this allowing to characterize the interference of the heptose with glucokinase and/or hexokinase. The effect of D-mannoheptulose upon the metabolism of D-glucose was also examined in both intact parotid cells and pancreatic islets. Last, the effect of D-mannoheptulose upon glucose-stimulated insulin release was reinvestigated over large concentration ranges of both the heptose and hexose. The experimental data revealed a mixed type of D-mannoheptulose inhibitory action upon D-glucose phosphorylation, predominantly of the non-competitive and competitive type, in liver and parotid homogenates, respectively. Despite efficient inhibition of hexose phosphorylation in both parotid cell and islet homogenates, the heptose suppressed the metabolic and functional responses to D-glucose only in pancreatic islets, whilst failing to affect adversely D-glucose catabolism in parotid cells. These findings suggest that factors such as the intracellular transport and availability of the heptose may interfere with the expression of its antagonistic action upon D-glucose metabolism.

Energy-dependent intracellular translocation of glucokinase in rat pancreatic islets

It was recently reported that hyperglycemia provokes a rapid and sustained translocation of glucokinase in rat pancreatic B-cells, and it was speculated that this may be associated with enhancement of its catalytic activity, as possibly attributable to the mitochondrial binding of the enzyme. In the present work, the activities of both hexokinase and glucokinase were measured in particulated and cytosolic subcellular fractions prepared from islets first incubated for 60 min either in the absence of exogenous nutrient or in the presence of D-glucose, tested at both low (2.8 mmol/L) and high (16.7 mmol/L) concentrations. The relative contribution of the cytosolic domain to the total activity of glucokinase recovered in the two subcellular fractions was higher in islets deprived of exogenous nutrient than in islets first exposed to 2.8 or 16.7 mmol/L D-glucose, the results obtained at each of the latter two hexose concentrations being comparable to one another. The subcellular distribution of hexokinase, however, was not significantly different in islets deprived of D-glucose or exposed to the hexose. These findings are interpreted as indicative of an energy-dependent translocation of glucokinase in the B-cell, distinct from the redistribution of the enzyme occurring in response to a rise in D-glucose concentration above its physiological value.

Glucose-induced positive cooperativity of fructose phosphorylation by human B-cell glucokinase

Human B-cell glucokinase displays sigmoidal kinetics towards D-glucose or D-mannose, but fails to do so towards D-fructose. Yet, D-glucose, D-mannose and 2-deoxy-D-glucose confer to the enzyme positive cooperativity towards D-fructose. For instance, in the presence of 5 mM D-[U-14C]fructose, its rate of phosphorylation is increased to 214.3 +/- 11.0%, 134.0 +/- 4.3% and 116.5 +/- 3.0% of paired control value by D-glucose, D-mannose and 2-deoxy-D-glucose (each 6 mM), respectively. D-glucose and, to a lesser extent, D-mannose also display reciprocal kinetic cooperativity. D-fructose, however, fails to affect D-glucose or D-mannose phosphorylation under conditions in which positive cooperativity is otherwise observed. These findings are relevant to the reciprocal effects of distinct hexoses upon their phosphorylation by B-cell glucokinase and, as such, to the metabolic and functional response evoked in pancreatic islet B-cells by these sugars, when tested either separately or in combination.

Regulation, perturbation, and correction of metabolic events in pancreatic islets

The physiological regulation of nutrient catabolism in islet cells, its perturbation in non-insulin-dependent diabetes mellitus, and the tools available to compensate for such a perturbation are reviewed. In terms of physiology, emphasis is placed on the relevance of glucokinase to hexose-induced insulin release, protein-to-protein interaction and enzyme-to-enzyme channelling, and the preferential stimulation of mitochondrial oxidative events in glucose-stimulated B-cells. In terms of pathology, attention is drawn to the deficiency of FAD-linked mitochondrial glycerophosphate dehydrogenase. Last, as far as therapeutic aspects are concerned, the potential usefulness of hypoglycemic sulfonylureas and meglitinide analogs, adenosine analogs, non-glucidic nutrients, and GLP-1 is underlined.