Discrimination of glucose anomers by glucokinase from liver and transplantable insulinoma

Mitoenergetic Dysfunction Triggers a Rapid Compensatory Increase in Steady-State Glucose Flux

ATP can be produced in the cytosol by glycolytic conversion of glucose (GLC) into pyruvate. The latter can be metabolized into lactate, which is released by the cell, or taken up by mitochondria to fuel ATP production by the tricarboxylic acid cycle and oxidative phosphorylation (OXPHOS) system. Altering the balance between glycolytic and mitochondrial ATP generation is crucial for cell survival during mitoenergetic dysfunction, which is observed in a large variety of human disorders including cancer. To gain insight into the kinetic properties of this adaptive mechanism we determined here how acute (30 min) inhibition of OXPHOS affected cytosolic GLC homeostasis. GLC dynamics were analyzed in single living C2C12 myoblasts expressing the fluorescent biosensor FLII(12)Pglu-700μδ6 (FLII). Following in situ FLII calibration, the kinetic properties of GLC uptake (V1) and GLC consumption (V2) were determined independently and used to construct a minimal mathematical model of cytosolic GLC dynamics. After validating the model, it was applied to quantitatively predict V1 and V2 at steady-state (i.e., when V1 = V2 = Vsteady-state) in the absence and presence of OXPHOS inhibitors. Integrating model predictions with experimental data on lactate production, cell volume, and O2 consumption revealed that glycolysis and mitochondria equally contribute to cellular ATP production in control myoblasts. Inhibition of OXPHOS induced a twofold increase in Vsteady-state and glycolytic ATP production flux. Both in the absence and presence of OXPHOS inhibitors, GLC was consumed at near maximal rates, meaning that GLC consumption is rate-limiting under steady-state conditions. Taken together, we demonstrate here that OXPHOS inhibition increases steady-state GLC uptake and consumption in C2C12 myoblasts. This activation fully compensates for the reduction in mitochondrial ATP production, thereby maintaining the balance between cellular ATP supply and demand.

Anomeric specificity of human liver and B-cell glucokinase: modulation by the glucokinase regulatory protein

The anomeric specificity of the wild-type recombinant forms of human liver and B-cell glucokinase was investigated using radioactive anomers of d-glucose as tracers. With d-glucose at anomeric equilibrium and at 30 degrees C, the maximal velocity, Hill number, and K(s) amounted, respectively, to 16 micromol min(-1) mg(-1), 1.8 and 6.9 mM in the case of liver glucokinase, and 7.3 micromol min(-1) mg(-1), 2.0 and 7.1 mM in the case of B-cell glucokinase. Whether at 20-22 or 30 degrees C, the maximal velocity, Hill number, and K(m) were significantly lower with alpha-d-glucose than with beta-d-glucose in both liver and B-cell glucokinase. As a result of these differences, the reaction velocity was higher with alpha-d-glucose at low hexose concentrations, while the opposite situation prevailed at high hexose concentrations. In the presence of 0.2 mM d-fructose 6-phosphate, the glucokinase regulatory protein caused a concentration-related inhibition of d-glucose phosphorylation, such an effect fading out at high concentrations of either d-glucose or glucokinase relative to that of its regulatory protein. The phosphorylation of alpha-d-glucose by liver glucokinase appeared more resistant than that of beta-d-glucose to the inhibitory action of d-fructose 6-phosphate, as mediated by the glucokinase regulatory protein. Such a phenomenon failed to achieve statistical significance in the case of the B-cell glucokinase. It is proposed that this information, especially the novel findings concerning the anomeric difference in both Hill number and sensitivity to the glucokinase regulatory protein, should be taken into account when considering the respective contributions of alpha- and beta-d-glucose to the overall phosphorylation of equilibrated d-glucose by glucokinase.

Alteration of anomeric preference of glucose-induced insulin secretion by glyceraldehyde

Glucokinase activity in pancreatic islets was dose-dependently inactivated by D-glyceraldehyde, whereas islet hexokinase activity was not altered. In untreated islets, alpha-D-glucose stimulated insulin secretion more efficiently than beta-D-glucose at a glucose concentration of 10 mM. However, glyceraldehyde highly attenuated the insulin-secretory response of pancreatic islets to alpha-D-glucose compared with that to beta-D-glucose. Thus, there was apparently no anomeric preference of glucose-induced insulin secretion in glyceraldehyde-treated islets. Glyceraldehyde affected neither the alpha-preference of glucose phosphorylation by glucokinase nor the beta-preference of glucose phosphorylation by hexokinase. Our study suggests that defective discrimination of glucose anomers by glyceraldehyde-treated islets may be caused by inactivation of glucokinase.

Alpha- and beta-anomeric preference of glucose-induced insulin secretion at physiological and higher glucose concentrations, respectively

We determined the anomeric preference of glucose phosphorylation by islet glucokinase, glucose utilization by pancreatic islets, and insulin secretion induced by glucose over a wide range of glucose concentrations. alpha-D-Glucose was phosphorylated faster than beta-D-glucose by islet glucokinase at lower glucose concentrations (5 and 10 mM), whereas the opposite anomeric preference was observed at higher glucose concentrations (40 and 60 mM). At 20 mM, there was no significant difference in phosphorylation rate between the two anomers. Similar patterns of anomeric preference were observed both in islet glucose utilization and in glucose-induced insulin secretion. The present study affords strong evidence that glucokinase is responsible for the anomeric preference of glucose-stimulated insulin secretion through anomeric discrimination in islet glucose utilization.

Hexokinase and 'glucokinase' in liver metabolism

Rat liver contains four hexokinase isoenzymes, one of which, despite often being called 'glucokinase', is no more specific for glucose than the others. However, it does differ from them in displaying a sigmoid kinetic response to glucose, requiring much higher glucose concentrations for activity, and being insensitive to physiological concentrations of glucose 6-phosphate.

A computer model of pancreatic islet glycolysis

We have modeled an experiment with perifused pancreatic islet cells using our BIOSSIM language. The experiment and the resulting model are concerned with glucose uptake and glycolysis by the beta-cells of pancreatic islets. Although glycolysis appears to be involved in insulin release, we do not have enough information to represent insulin release in detail. The rapid entry of glucose into the beta-cell is promoted by a carrier having a very high tissue capacity. Phosphorylation of glucose by the low affinity enzyme glucokinase appears to be limiting for glycolysis. The effects of several hexose diphosphate activators of phosphofructokinase are modeled. Model behavior is described. The kinetic parameters of the enzyme submodels are given. Because of the difficulties of preparing large amounts of experimental material, information on pancreatic islet metabolism is limited. This model is a plausible explanation of the experimental results. Recent work on the genetically engineered glucose transporter and glucokinase is discussed.

Phosphorylation by liver glucokinase of D-glucose anomers at anomeric equilibrium

The relative contribution of each anomer of D-glucose to the overall phosphorylation rate of the hexose tested at anomeric equilibrium was examined in rat liver postmicrosomal supernatants under conditions aimed at characterizing the activity of glucokinase, with negligible interference of either hexokinase, N-acetyl-D-glucosamine kinase or glucose-6-phosphatase (acting as a phosphotransferase). Both at 10 degrees and 30 degrees C, the relative contribution of each anomer was unaffected by the concentration of D-glucose. At both temperatures, the alpha/beta ratio for the contribution of each anomer was slightly, but significantly, lower than the alpha/beta ratio of anomer concentrations. These findings, which are consistent with the anomeric specificity of glucokinase in terms of affinity, cooperativity and maximal velocity, reveal that the preferred alpha-anomeric substrate for both glycogen synthesis and glycolysis is generated by glucokinase at a lower rate than is beta-D-glucose-6-phosphate.

Anomeric preference of glucose utilization in human erythrocytes loaded with glucokinase

Human erythrocytes were loaded with homogeneous rat liver glucokinase by an encapsulation method based on hypotonic hemolysis and isotonic resealing. As assayed at 10 mM glucose, glucokinase and hexokinase activities in glucokinase-loaded erythrocytes were 218 and 384 nmol/min/gHb, respectively; whereas hexokinase activity in both intact and unloaded red cells, which contain no glucokinase activity, was about 400 nmol/min/gHb. No difference in the rate of lactate production from glucose anomers between intact and unloaded erythrocytes suggested that the encapsulation procedure itself did not affect glucose utilization in red cells. Alpha-anomeric preference in lactate production from glucose was observed in glucokinase-loaded erythrocytes, whereas the beta anomer of glucose was more rapidly utilized than the alpha anomer in intact and unloaded erythrocytes. The results indicate that the step of glucose phosphorylation determines the anomeric preference in glucose utilization by human erythrocytes, since glucokinase and hexokinase are alpha- and beta-preferential, respectively, in glucose phosphorylation.

Anomeric specificity and kinetics of glucokinase: theoretical unsuitability of the Hill equation

The kinetics of the low-Km hexokinase isoenzymes, which obey the Michaelis-Menten equation, can be established from the Km (Michaelis constant) and Vmax (maximal velocity) values for either equilibrated D-glucose or its alpha- and beta-anomers. In the case of the high-Km glucokinase isoenzyme, however, the sigmoidal substrate dependency and the competition between the two anomers of D-glucose do not allow, theoretically, to assign any meaningful value to either the Km, Vmax or n (Hill number) constants for equilibrated D-glucose. Thus, with equilibrated D-glucose, the concentration dependency fails to display a rectilinear relationship in the Hill plot. These observations illustrate the shortcomings of current biochemical studies in which the anomeric heterogeneity of D-glucose is ignored.

Pancreatic islet discrimination of hexose anomers. I. Steady-state computer simulation

Pancreatic islets detect glucose level by phosphorylating it and converting the glycolytic rate to a signal to secrete insulin. Insulin secretion is greater from the alpha- than from the beta-anomer when the D-glucose level is below 22 mM. D-mannose behaves similarly but at nearly twofold higher concentrations. Two explanations have been proposed: 1) glucokinase, which has the same anomeric preference, is the principal hexose phosphorylating enzyme and limits glycolytic rate. 2) Phosphofructokinase limits glycolysis and hexokinase is the principal enzyme phosphorylating hexose; hexosediphosphate activators of phosphofructokinase are more readily synthesized from alpha-anomers of hexose phosphates. We have simulated both alternatives with a detailed anomerically specific model of the hexose-metabolizing glycolytic enzymes. The pathway preference for alpha-anomer of both hexoses was adequately reproduced with anomerically active limiting glucokinase. The other mechanism did not reproduce the observed pathway preference.

Anomer specificity of glucose-6-phosphatase and glucokinase

The anomeric form of glucose produced by glucose-6-phosphatase was studied using an apparatus that specifically measures beta-D-glucose. The time course of beta-D-glucose formation from glucose-6-P by glucose-6-phosphatase is essentially linear. In the presence of mutarotase, this rate is reduced to 70% of that obtained in the absence of mutarotase. When detergent treated microsomes were used, the rate of beta-D-glucose formation is unaffected by mutarotase. These results suggest that only beta-anomer of glucose is produced by microsomal glucose-6-phosphatase and this specificity is determined by translocase for glucose-6-P or glucose. It was also demonstrated that alpha-D-glucose is the substrate for glucokinase.

Anomeric specificity of D-glucose metabolism in rat adipocytes

The anomeric specificity of D-glucose metabolism was investigated in rat adipocytes exposed for 60 min at 8 degrees C to pure alpha- or beta-D-glucose or to equilibrated D-glucose. The rate of D-[5-3H]glucose utilization was higher with alpha- than beta-D-glucose. However, as judged from the oxidation of D-[1-14C]glucose and D-[6-14C]glucose anomers, the fraction of D-glucose catabolism occurring via the pentose cycle was higher with beta- than alpha-D-glucose. In the presence of equilibrated D-glucose, the utilization of alpha-D-[5-3H]glucose and the oxidation of both alpha-D-[1-14C]glucose and alpha-D-[6-14C]glucose were higher, relative to the anomer concentration, than the corresponding values for beta-D-glucose. It is concluded that the anomeric specificity of D-glucose metabolism is operative in adipocytes, even when they are exposed to equilibrated D-glucose.

Anomeric specificity of mammalian hexokinase

The anomeric specificity of hexokinase was examined in crude homogenates of rat parotid gland, erythrocytes and pancreatic islets. At 8 degrees C, the alpha/beta ratio in maximal velocity averaged 0.73, 0.66 and 0.75 in the parotid, erythrocytes and pancreatic islets, respectively. Hexokinase displayed a greater affinity for alpha- than beta-D-glucose as judged from three criteria: the Km value, the reaction velocity measured with mixtures of the two anomers and their effect upon the phosphorylation of D-[U-14C] glucose in anomeric equilibrium. The latter procedure yielded an alpha/beta ratio in Km close to 0.51, 0.49 and 0.39 in parotid, erythrocytes and pancreatic islets, respectively. Within the limits of this study, the anomeric specificity of mammalian hexokinase would appear to be a mirror image of that of yeast hexokinase.

Temperature dependency of the anomeric specificity of yeast and bovine hexokinases

The phosphorylation of alpha- and beta-D-glucose by either yeast hexokinase or beef heart hexokinase was measured at both 10 and 30 degrees C. At 30 degrees C, the anomeric specificity of yeast hexokinase represented a mirror image of that of bovine hexokinase, in terms of both maximal velocity and affinity. A decrease in temperature apparently accentuated the anomeric difference in both maximal velocity and affinity of bovine hexokinase. Such a difference consisted in a higher maximal velocity with beta- than alpha-D-glucose, but a greater affinity for the alpha- than beta-anomer. In yeast hexokinase, however, the decrease in temperature suppressed the anomeric difference in maximal velocity and inversed the anomeric difference in affinity. In the case of both enzymes, the fall in temperature decreased more the maximal velocity recorded with alpha-D-glucose than that measured with beta-D-glucose, and severely lowered the Km for alpha-D-glucose, whilst failing to affect significantly the Km for beta-D-glucose. These findings, which allow to reconcile prior apparently conflicting data, reveal that the anomeric behaviour of hexokinase is affected by the ambient temperature. Our data also support the view that hexokinase underwent a phylogenic evolution in terms of its anomeric specificity.

Reciprocal influence of glucose anomers upon their respective phosphorylation by hexokinase

The phosphorylation of D-glucose (1.0mM) was measured in homogenates of tumoral islet cells incubated at 7 degrees C in the presence of labelled alpha- and/or beta-D-glucose, with or without exogenous glucose 6-phosphate. The close-to-maximal reaction velocity of hexokinase was higher with beta- than alpha-D-glucose. The latter anomer inhibited beta-D-glucose phosphorylation more than the beta-anomer decreased the phosphorylation of alpha-D-glucose. This behaviour was accounted for by the higher affinity of hexokinase for alpha- than for beta-D-glucose. These direct measurements of the relative contribution of each anomer to the overall rate of glucose phosphorylation in the presence of mixed populations of alpha- and beta-D-glucose validate the concept that the phosphorylation of D-glucose displays anomeric specificity even when the hexose is used at anomeric equilibrium. Glucose 6-phosphate inhibited the phosphorylation of the two anomers more severely when alpha-D-glucose rather than beta-D-glucose was the most abundant anomer.

Glucose metabolism in insulin-producing tumoral cells

Homogenates of insulin-producing tumoral cells catalyzed the phosphorylation of glucose, mannose, and fructose. The kinetics of phosphorylation at increasing glucose concentrations, the inhibitory effect of glucose 6-phosphate, and the comparison of results obtained with distinct hexoses indicated the presence of both low-Km hexokinase-like and high-Km enzymatic activities, the results being grossly comparable to those collected in normal pancreatic islets. Relative to protein content, the glucose-phosphorylating enzymatic activity was higher in tumoral than normal islet cells. The activity of other enzymes was either lower (glutamate dehydrogenase), moderately higher (phosphoglucomutase, lactate dehydrogenase) or considerably greater (ornithine decarboxylase) in tumoral than in normal islet cells. In intact tumoral cells, incubated under increasing glucose concentrations, the oxidation of D-[U-14C]glucose and the output of lactic and pyruvic acids reached a close-to-maximal value at 2.8 mM glucose. The ratios for glucose oxidation/utilization and lactate/pyruvate output were much lower in tumoral than in normal islet cells. Although glucose caused a modest increase in insulin output from the tumoral cells, this effect was saturated at a low glucose concentration (2.8 mM) and less marked than that of other secretagogues (e.g., L-leucine, L-ornithine, or forskolin). Thus, despite a close-to-normal enzymatic equipment for glucose phosphorylation, the tumoral cells displayed severe abnormalities in the metabolism and secretory response to this hexose. These findings point to regulatory mechanisms distal to glucose phosphorylation in the control of glucose metabolism in insulin-producing cells.

Glucokinase is not the pancreatic B-cell glucoreceptor

A series of recent experimental findings are reviewed to indicate that glucokinase does not represent the pancreatic B-cell glucoreceptor. Whether in liver, pancreatic islet or insulin-producing tumoral cell homogenates, glucokinase fails to yield a higher reaction velocity with alpha-than beta-D-glucose. At a high glucose concentration (40 mmol/l), when the phosphorylation of glucose by glucokinase is indeed higher with beta- than alpha-D-glucose, no preference for beta-D-glucose is observed in intact islets, as judged from the utilization of D-[5-3H]glucose, production of lactic acid, oxidation of D-[U-14C]glucose, net uptake of 45Ca or release of insulin. The glucose 6-phosphate content of intact islets is higher in the presence of beta- than alpha-D-glucose. At a low glucose concentration (3.3 mmol/l), when the participation of glucokinase to hexose phosphorylation is minimal, alpha-D-glucose is still better metabolized and stimulates both 45Ca net uptake and insulin release more efficiently than beta-D-glucose, despite the fact that hexokinase yields a higher reaction velocity with beta- than alpha-D-glucose. In intact islets, beta-D-glucose is used preferentially to alpha-D-glucose in the pentose cycle pathway as judged from the oxidation of alpha- or beta-D-[1-14C]glucose relative to that of alpha- or beta-D-[6-14C]glucose. In islets removed from fasted rats, the rate of glycolysis is more severely decreased than expected from the repression of glucokinase. The metabolism of glucose in tumoral insulin-producing cells differs, in several respects, from that in normal pancreatic islets, although the pattern of hexokinase and glucokinase activities is similar in these two types of cells. All these observations point to the participation of regulatory sites distal to glucose phosphorylation in the control of glucose metabolism in islet cells.

Radiometric oil well assay for glucokinase in microscopic structures

Glucokinase (ATP:D-glucose 6-phosphotransferase, EC 2.7.1.1) plays a pivotal role in hepatic glucose metabolism and serves as the glucose sensor in pancreatic islet beta-cells. Biochemical studies of this enzyme are complicated by the cellular heterogeneity of the liver and the pancreas and because the presence of hexokinases (ATP:D-hexose 6-phosphotransferases, EC 2.7.1.1) seriously interferes with currently available analytical procedures. A radiometric assay was designed to deal with these problems. It is based on the liberation of 3H2O from D-[2-3H(N)]glucose 6-phosphate, the product of the glucokinase reaction, using exogenous phosphoglucose isomerase (D-glucose-6-phosphate ketol-isomerase, EC 5.3.1.9). Interference by hexokinases was largely eliminated by using glucose 6-phosphate as inhibitor and the sensitivity of the assay was greatly increased by using small volumes with the oil well procedure. The assay was sufficiently sensitive to detect about 1 pg of glucokinase. It thus allowed the application of quantitative histochemical procedures to the study of intralobular hepatic glucokinase profiles and the pancreatic beta-cell glucose sensor. The quantitative histochemical procedures were sufficiently sensitive and reliable for measuring important kinetic constants of glucokinase (i.e., the Km and the Hill number) in microscopic samples of tissue.

New perspectives on pancreatic islet glucokinase

Control of blood sugar involves the complex interaction of the pancreatic glucose-sensing beta-cells with the liver, which serves as the primary site of glucose disposal after a meal. Glucokinase occupies an important role in controlling glucose phosphorylation and metabolism both in the liver and in pancreatic islets. In the beta-cells, glucokinase functions as pacemaker of glycolysis at physiological glucose levels. It determines the unique characteristics of islet hexose usage, that is, the rate, affinity, cooperativity, and anomeric discrimination of glucose metabolism. Because glycolysis controls hexose-induced insulin release, glucokinase is considered the best-qualified candidate for the elusive glucose sensor of beta-cells. A deficiency of glucokinase would disturb glucose homeostasis. Decreased islet glucokinase would diminish islet glycolysis and would result in a higher set point of beta-cells for glucose-induced insulin release. Decreased liver glucokinase would cause less efficient hepatic glucose disposal. Human maturity-onset diabetes (type II diabetes) has these characteristics. It is thus conceivable that certain forms of type II diabetes are due to a glucokinase deficiency.