Underestimation of D-glucose phosphorylation as measured by 3H2O production from D-[2-3H]glucose

Enzyme-to-enzyme channelling in the early steps of glycolysis in rat pancreatic islets

The metabolism of D-glucose displays anomeric specificity in rat pancreatic islets. The aim of the present report is to investigate whether such a situation implies enzyme-to-enzyme tunnelling of metabolites in the early steps of glycolysis. For such a purpose, the modelling of alpha- and beta-D-glucose catabolism, itself based on available information concerning both the utilisation of these two anomers and the intrinsic properties of phosphoglucoisomerase, was first examined. According to a theoretical model with enzyme-to-enzyme channelling, the generation of 3HOH from D-[2-3H]glucose should be higher in islets exposed to beta-D-glucose rather than alpha-D-glucose, whilst the opposite situation should prevail in the case of D-[5-3H]glucose conversion to 3HOH. Experimental data collected in rat islets incubated for 60 min at 4 degrees C in the presence of either alpha- or beta-D-glucose mixed with tracer amounts of either alpha- or beta-D-[2- 3H]glucose and alpha- or beta-D-[5-3H]glucose indicate that the beta/alpha ratio for D-[2-3H]glucose conversion to 3HOH is indeed higher than the beta/alpha ratio for D-[5-3H]glucose conversion to 3HOH. These findings are consistent with the postulated enzyme-to-enzyme tunnelling of glycolytic intermediates between hexokinase isoenzyme(s), phosphoglucoisomerase and, possibly, phosphofructokinase.

Metabolism of tritiated D-glucose anomers in rat erythrocytes

It was recently proposed that alpha-D-glucose 6-phosphate may undergo enzyme-to-enzyme channelling between glucokinase and phosphoglucoisomerase in rat pancreatic islets. The present study aims at exploring whether a different situation prevails in cells deprived of glucokinase, namely in erythrocytes. At anomeric equilibrium, the ratio between D-[2-3H]glucose and D-[5-3H]glucose conversion to 3HOH was lower in rat erythrocytes incubated for 60 min at 4 degrees C in the presence of 2.8 mM, rather than 8.3 mM, D-glucose. This coincided with both a greater relative increase in beta-D-[5-3H]glucose, as compared to alpha-D-[5-3H]glucose, conversion to 3HOH and an increase in the beta/alpha ratio for 3HOH generation from D-[5-3H]glucose in response to an increase in the anomeric concentration from 2.8 to 8.3 mM, the suppression of the difference between the beta/alpha ratios for 3HOH generation from D-[2-3H]glucose and D-[5-3H]glucose in the erythrocytes incubated at 8.3 mM, as distinct from 2.8 mM, alpha- and beta-D-glucose, and a [2-3H]/[5-3H] ratio for 3HOH generation lower than unity in erythrocytes exposed to alpha-D-glucose but not significantly different from unity in the presence of beta-D-glucose. These findings emphasize the relevance of alpha-D-glucose 6-phosphate channelling between hexokinase and phosphoglucoisomerase as a determinant of the difference between D-[2-3H]glucose and D-[5-3H]glucose conversion to 3HOH, and reveal that the regulation of such a tunnelling process by the concentration of the D-glucose represents, in rat erythrocytes, a mirror image of that observed in rat pancreatic islets. The regulation of this process thus tightly depends on the identity of the hexokinase enzyme mainly responsible for the phosphorylation of D-glucose in distinct cell types.

D-glucose metabolism in normal dispersed islet cells and tumoral INS-1 cells

The metabolism of D-glucose was characterized in both normal dispersed rat islet cells and the 2-mercaptoethanol-dependent insulin-secreting cells of the INS-1 line. The normal and tumoral islet cells differed from one another by the relative magnitude, concentration dependency and hierarchy of the increase in the production of 3HOH from D-[5-(3)H]glucose and 14C-labelled CO2, acidic metabolites and amino acids from D-[U-14C]glucose at increasing concentrations of the hexose. For instance, whilst the paired ratio between D-[U-14C]glucose oxidation and D-[5-(3)H]glucose utilization augmented in a typical sigmoidal manner in normal islet cells exposed to increasing concentrations of D-glucose, it progressively decreased under the same experimental conditions in INS-1 cells. Nevertheless, the absolute values and concentration-response relationship for the increase in ATP generation rate attributable to the catabolism of D-glucose were virtually identical in normal and tumoral cells. These findings indicate that the analogy in the secretory response to D-glucose of normal and INS-1 islet cells, although coinciding with a comparable response to the hexose in terms of ATP generation, contrasts with a vastly different pattern of D-glucose metabolism in these two cell types.

Coupled glucose transport and metabolism in cultured neuronal cells: determination of the rate-limiting step

In brain and nerves the phosphorylation of glucose, rather than its transport, is generally considered the major rate-limiting step in metabolism. Since little is known regarding the kinetic coupling between these processes in neuronal tissues, we investigated the transport and phosphorylation of [2-3H]glucose in two neuronal cell models: a stable neuroblastoma cell line (NCB20), and a primary culture of isolated rat dorsal root ganglia cells. When transport and phosphorylation were measured in series, phosphorylation was the limiting step, because intracellular glucose concentrations were the same as those outside of cells, and because the apparent Km for glucose utilization was lower than expected for the transport step. However, the apparent Km was still severalfold higher than the Km of hexokinase I. When [2-3H]glucose efflux and phosphorylation were measured from the same intracellular glucose pool in a parallel assay, rates of glucose efflux were three- to-fivefold greater than rates of phosphorylation. With the parallel assay, we observed that activation of glucose utilization by the sodium channel blocker veratridine caused a selective increase in glucose phosphorylation and was without effect on glucose transport. In contrast to results with glucose, both cell types accumulated 2-deoxy-D-[14C]glucose to concentrations severalfold greater than extracellular concentrations. We conclude from these studies that glucose utilization in neuronal cells is phosphorylation-limited, and that the coupling between transport and phosphorylation depends on the type of hexose used.

Control of glucose metabolism in newborn pig enterocytes: evidence for the role of hexokinase

The objective of the present work was to identify the regulatory step(s) in the post-natal development of a high glycolytic capacity previously evidenced in newborn pig enterocytes (Darcy-Vrillon et al. (1994) Pediat. Res., 36, 175-181. Glucose entry via the Na+/glucose cotransporter, estimated by the uptake of the non-metabolizable analogue methyl alpha-D-[U-14C]glucopyranoside, slightly decreased between birth and 2 days of sucking. The flux of glucose metabolized into the pentose cycle pathway slightly increased but could not account for the 3-fold increase observed in the glycolytic capacity. Whereas the maximal activity of 6-phosphofructo-1-kinase did not change between stages, there was a significant increase in hexokinase activity as well as in the flux of glucose phosphorylated. These findings suggest that the stimulation of glucose phosphorylation through hexokinase is the key event leading to an increased glycolytic capacity of small intestinal cells at the onset of sucking.

Enzyme-to-enzyme tunnelling between phosphoglucoisomerase and phosphofructokinase

1. Cross-linked and permeabilized rat erythrocytes were incubated for 2-5 min at 37 degrees C in the presence of ATP and either D-[U-14C]glucose 6-phosphate (3 mM) mixed with unlabelled D-fructose 6-phosphate (1 mM) or D-[U-14C]fructose 6-phosphate (1 mM) mixed with unlabelled D-glucose 6-phosphate (3 mM). 2. The contribution of molecules derived from the radioactive ketohexose ester relative to the total amount of newly formed D-fructose 1,6-bisphosphate was lower than the time-related average value for such a relative contribution in the pool of D-fructose 6-phosphate. 3. From such a difference, it was calculated that, under the present experimental conditions, 13.1 +/- 2.0% of the molecules of D-fructose 1,6-bisphosphate formed during incubation are directly derived from D-glucose 6-phosphate by a process of enzyme-to-enzyme channelling between phosphoglucoisomerase and phosphofructokinase, rather than originating from the free pool of D-fructose 6-phosphate. 4. A comparable value of 13.2 +/- 3.2% was reached when the process of enzyme-to-enzyme tunnelling was judged from the 3H/14C ratio in D-fructose 1,6-bisphosphate formed by permeabilized erythrocytes exposed for 5-15 min to D-glucose 6-phosphate (3 or 5 mM) mixed with tracer amounts of both D-[1-14C]glucose 6-phosphate and D-[2-3H]glucose 6-phosphate.

Menadione- and cyclic AMP-induced alteration of the ratio between D-[2-3H]glucose detritiation and phosphorylation in rat erythrocytes

When rat erythrocytes are exposed to both dibutyryl-cyclic AMP and isobutylmethylxanthine, the rates of D-glucose phosphorylation and glycolysis are both markedly increased. On the contrary, menadione by increasing the flow rate through the pentose phosphate pathway, decreases the generation rate of triose phosphates from exogenous D-glucose. Yet, both procedures lead to a decrease in the production of 3HOH from D-[2-3H]glucose relative to its phosphorylation rate. It is proposed, therefore, that such a radioactive measurement should no more be considered as a reliable index of D-glucose phosphorylation, at variance with current practice.

Dissociated effects of 2-deoxy-D-glucose on D-[2-3H]glucose and D-[5-3H]glucose conversion into 3HOH in rat erythrocytes

When rat erythrocytes were preincubated with 2-deoxy-D-glucose, the generation of both 3H-labelled acidic metabolites and 3HOH from D-[5-3H]glucose, the total production of L-lactate, and the generation of 14CO2, 14C-labelled acidic metabolites and 14C-labelled lactate from D-[1-14C]glucose or D-[U-14C]glucose were all lower than in erythrocytes preincubated in the absence of a hexose or in the presence of 3-O-methyl-D-glucose. However, preincubation with 2-deoxy-D-glucose failed to decrease the generation of 3H-labelled acidic metabolites and L-[3-3H]lactate from D-[2-3H]glucose, while decreasing the production of 3HOH more severely from D-[2-3H]glucose than from D-[5-3H]glucose. This may be attributable not solely to inhibition of D-glucose phosphorylation by 2-deoxy-D-glucose and 2-deoxy-D-glucose 6-phosphate, but also to inhibition by 2-deoxy-D-glucose 6-phosphate of hexose 6-phosphate interconversion in the reaction catalysed by phosphoglucoisomerase, as also observed with the purified enzyme. The generation of 3HOH from D-[2-3H]glucose should therefore be considered as a tool to assess the efficiency of interconversion of hexose 6-phosphates in the reaction catalysed by phosphoglucoisomerase, rather than to estimate D-glucose phosphorylation rate.

Metabolism of tritiated D-glucose in rat erythrocytes

The metabolism of D-[U-14C]glucose, D-[1-14C]glucose, D-[6-14C]glucose, D-[1-3H]glucose, D-[2-3H]glucose, D-[3-3H]glucose, D-[3,4-3H]glucose, D-[5-3H]glucose, and D-[6-3H]glucose was examined in rat erythrocytes. There was a fair agreement between the rate of 3HOH production from either D-[3-3H]glucose and D-[5-3H]glucose, the decrease in the 2,3-diphosphoglycerate pool, its fractional turnover rate, the production of 14C-labeled lactate from D-[U-14C]glucose, and the total lactate output. The generation of both 3HOH and tritiated acidic metabolites from D-[3,4-3H]glucose indicated incomplete detritiation of the C4 during interconversion of fructose-1,6-bisphosphate and triose phosphates. Erythrocytes unexpectedly generated 3HOH from D-[6-3H]glucose, a phenomenon possibly attributable to the detritiation of [3-3H]pyruvate in the reaction catalyzed by glutamate pyruvate transaminase. The production of 3HOH from D-[2-3H]glucose was lower than that from D-[5-3H]glucose, suggesting enzyme-to-enzyme tunneling of glycolytic intermediates in the hexokinase/phosphoglucoisomerase/phosphofructokinase sequence. The production of 3HOH from D-[1-3H]glucose largely exceeded that of 14CO2 from D-[1-14C]glucose, a situation tentatively ascribed to the generation of 3HOH in the phosphomannoisomerase reaction. It is further speculated that the adjustment in specific radioactivity of D-[1-3H]glucose-6-phosphate cannot simultaneously match the vastly different degrees of isotopic discrimination in velocity at the levels of the reactions catalyzed by either glucose-6-phosphate dehydrogenase or phosphoglucoisomerase. The interpretation of the present findings thus raises a number of questions, which are proposed as a scope for further investigations.

Phosphoglucoisomerase-catalyzed interconversion of hexose phosphates. A model for D-[2-3H]glucose metabolism in human erythrocytes

When D-[2-3H]glucose 6-phosphate mixed with the unlabeled ester is converted to D-[1-3H]fructose 6-phosphate and 3HOH in the phosphoglucoisomerase reaction and then to D-[1-3H]fructose 1,6-bisphosphate in the phosphofructokinase reaction, the specific radioactivity of the latter metabolite and the production of 3HOH relative to the total generation of tritiated end products are both inversely related to the concentration of phosphofructokinase. In human erythrocytes, the modeling of D-[2-3H]glucose metabolism, based on the activity of phosphoglucoisomerase in cell homogenates and on the steady-state content of D-glucose 6-phosphate and D-fructose 6-phosphate in intact cells, indicates that the back-and-forth interconversion of these esters is about five-times higher than the net glycolytic flux. Yet, the production of 3HOH from D-[2-3H]glucose is about 20% lower than the net glycolytic flux, as judged from the production of 3HOH from D-[5-3H]glucose. Thus, an incomplete detriation of D-[2-3H]glucose is not incompatible with an extensive interconversion of hexose 6-phosphates in the reaction catalyzed by phosphoglucoisomerase.

Hexose metabolism in pancreatic islets: enzyme-to-enzyme tunnelling of hexose 6-phosphates

The fate of unlabelled D-glucose and D-[2-3H]glucose in pancreatic islets was simulated taking into account experimental values for glycolytic flux, intracellular concentration of D-glucose 6-phosphate and phosphoglucoisomerase activity. The model, which also takes into account the isotopic discrimination in velocity and intramolecular transfer of tritium between D-[2-3H]glucose 6-phosphate and D-[1-3H]fructose 6-phosphate in the reaction catalyzed by phosphoglucoisomerase, revealed that the predicted generation of 3HOH from D-[2-3H]glucose was much higher than the true experimental value. Such a discrepancy is reinforced by the consideration that the generation of 3HOH from D-[2-3H]glucose in islet cells is not solely attributable to the phosphoglucoisomerase-catalyzed detritiation of hexose 6-phosphates metabolized in the glycolytic pathway. In order to reconcile experimental and theoretical values for 3HOH production, it was found necessary to postulate enzyme-to-enzyme tunnelling of hexose 6-phosphates in the hexokinase/phosphoglucoisomerase/phosphofructokinase sequence. It is proposed that such a tunnelling may favour the anomeric specificity of D-glucose metabolism in islet cells, by restricting the anomerization of hexose 6-phosphates.

Phosphoglucoisomerase-catalyzed interconversion of hexose phosphates: a model for the interconversion of D-[2-3H]glucose 6-phosphate and D-[1-3H]fructose 6-phosphate

Based on experimental data, a model is proposed for the interconversion of either unlabelled hexose phosphates or D-[2-3H]glucose 6-phosphate and D-[1-3H]fructose 6-phosphate in the reaction catalyzed by phosphoglucoisomerase. This model takes into account the known differences in maximal velocity and affinity for each substrate, the intramolecular transfer of tritium between C1 and C2, and the isotopic discrimination between unlabelled and tritiated esters. This model reveals that, in a close system characterized by the progressive detritiation of hexose phosphates, the concentration ratio of D-glucose 6-phosphate to D-fructose 6-phosphate is much higher with the tritiated than unlabelled esters, a paradoxical increase in the specific radioactivity of D-glucose 6-phosphate above its initial value being even observed during the initial period of exposure of D-[2-3H]glucose 6-phosphate to phosphoglucoisomerase. The extension of this model to an open system may be essential for the correct interpretation of radioactive data collected in intact cells exposed to D-[2-3H]glucose.

Perturbation of pancreatic islet function in glucose-infused rats

The secretory behavior of insulin- and glucagon-producing cells was found to be perturbed in isolated perfused pancreases removed from rats infused with hypertonic solutions of glucose for 48 hours. The anomalies included a high basal release of insulin and a paradoxical increase in insulin output and decrease in glucagon release in response to a fall in D-glucose concentration. Likewise, in isolated islets prepared from the glucose-infused rats, L-arginine or theophylline stimulated insulin release at a low ambient concentration of D-glucose, at variance with the situation found in islets removed from normal rats. These secretory perturbations could not be attributed to any obvious defect in either the transport of D-glucose into islet cells or its further utilization and oxidation, but coincided with the abnormal accumulation of glycogen in the B-cell. It is proposed that the latter anomaly may play a role in the altered dynamics of insulin release found in animals or patients with long-term hyperglycemia.

Phosphoglucoisomerase-catalyzed interconversion of hexose phosphates; diastereotopic specificity, isotopic discrimination and intramolecular hydrogen transfer

When D-[1-3H]fructose 6-phosphate generated from D-[2-3H]glucose 6-phosphate is converted, in a monodirectional manner to D-glucose 6-phosphate and then 6-phospho-D-gluconate, about 42% of the radioactivity is transferred from the C1 of the ketohexose ester to the C2 of the aldohexose phosphate, whereas the remaining 58% are produced as 3H2O. The velocity of the reaction catalyzed by phosphoglucoisomerase represents, in the case of the tritiated substrate, only 43% of that recorded with D-[U-14C]fructose 6-phosphate, such an isotopic discrimination being attributable mainly to a difference in maximal velocity rather than affinity. The phenomena of both intramolecular hydrogen transfer and isotopic discrimination were less pronounced than when D-[2-3H]glucose 6-phosphate is converted, in a monodirectional manner, to D-fructose 6-phosphate and then D-fructose 1,6-bisphosphate. In contrast, when either D-[1-3H]glucose 6-phosphate or D-[1-3H]fructose 6-phosphate prepared from D-[1-3H]glucose were tested, no 3H2O was formed, all radioactivity being recovered, respectively, in tritiated D-fructose 1,6-bisphosphate or NADP3H. Nevertheless, phosphoglucoisomerase was also found to discriminate between D-[U-14C]glucose 6-phosphate and D-[1-3H]glucose 6-phosphate or between D-[U-14C]fructose 6-phosphate and D-[1-3H]fructose 6-phosphate prepared from D-[1-3H]glucose. The reaction velocity with the tritiated esters averaged 78-83% of those recorded with the 14C-labelled esters. Such an isotopic discrimination was again attributable mainly to a difference in maximal velocity rather than affinity. These findings indicate that the mode of preparation of D-[1-3H]fructose cannot be ignored in considering the fate of this tritiated hexose, as ruled by the intrinsic properties, and especially the diastereotopic specificity of phosphoglucoisomerase.