Fructose metabolism via the pentose cycle in tumoral islet cells

Metabolism of glycerol-1,2,3-trimethylsuccinate in rat hepatocytes

The metabolism of glycerol-1,2,3-trimethylsuccinate ester was investigated in rat hepatocytes. The ester displayed a greater nutritional value than D-glucose, as a precursor of either CO2 or glycogen. In terms of 14CO2 production, the value calculated from experiments conducted in the presence of 1.9 mM [U-14C] glycerol-1,2,3-trimethylsuccinate, glycerol-1,2,3-trimethyl[1,4-14C] succinate and glycerol- 1,2,3-trimethyl[2,3-14C] succinate represented about 50 times that found in cells incubated with 1.0 mM D-[U-14C] glucose. For glycogen synthesis, the results found with the ester were approximately 7-8 times higher than those found with the hexose. A further advantage of the ester over D-glucose consisted in the fact that, at increasing concentrations of these nutrients, a maximal metabolic response may be reached at lower levels of glycerol- 1,2,3-trimethylsuccinate than D-glucose. By comparison with previous data obtained in the same experimental model, glycerol-1,2,3-trimethylsuccinate was also found to display a higher nutritional value than the dimethyl ester of succinic acid. It is proposed, therefore, that glycerol-1,2,3-trimethylsuccinate could be used to support ATP generation in cells endangered by an imbalance between the rate of synthesis and hydrolysis of this adenine nucleotide.

Improved viability and metabolic behavior of hepatocytes after liver storage in the presence of a succinic acid ester

BACKGROUND

Selected esters of succinic acid were recently proposed as novel nutrients to support ATP generation in cells endangered by an imbalance between the formation and breakdown of this adenine nucleotide. In the present study, a new ester, glycerol-1,2,3-trimethylsuccinate, was examined for its potential beneficial effect in the procedures preceding liver transplantation.

METHODS

The viability and metabolic behavior of hepatocytes were examined after perfusion and storage of rat livers for 20 hr at 4 degrees C with a Belzer UW-CSS solution in the absence or presence or 2 mM glycerol-1,2,3-trimethylsuccinate.

RESULTS

Although it failed to affect significantly the release of cellular enzymes during storage and the protein or glycogen content of the liver, and was unable to prevent the storage-induced decrease in both biosynthetic activity and D-[U-14C]glucose incorporation into glycogen in isolated hepatocytes, the ester restored to a close-to-normal value the viability of the hepatocytes and opposed the starvation-like effects of liver storage upon both the conversion of D-[U-14C]glucose to 14CO2 and radioactive amino acids and the de novo generation of 14C-labeled D-glucose from [2-14C]pyruvate.

CONCLUSIONS

Because succinic acid esters are efficiently metabolized in several cell types, the present results suggest that such esters may have a wide field of application in transplantation procedures.

The effect of fructose metabolism on the accumulation of inositol phosphates in rat pancreatic islets

The mechanism by which glucose recognition of B cells results in the release of inositol 1,4,5-trisphosphate is not known at present. In pancreatic islets, fructose shares a common metabolic pathway with glucose from the second step of glycolysis and can augment insulin secretion at stimulatory glucose levels. To evaluate the impact of glycolysis on the release of inositol 1,4,5-trisphosphate, we studied the effect of glucose and fructose metabolism on insulin secretion and the activation of inositol-specific phospholipase C, using collagenase digested rat pancreatic islets incorporated with 3H-labelled myo-inositol. Inositol phosphates, generated by the cleavage of phosphatidyl inositol by inositol phospholipase C, were analyzed using fast protein liquid chromatography. The islets were exposed to 3.3, 5.5 and 12 mmol 1(-1) glucose for 45 min in the absence or presence of 10, 20 or 30 mmol 1(-1) fructose, and the amount of insulin released into the medium was measured. Intracellular inositol phosphate accumulation was measured under the same glucose concentrations with 0, 10 and 30 mmol 1(-1) fructose. As expected, fructose alone had no insulinotropic effect, but potentiated the glucose-induced (5.5 and 12 mmol 1(-1)) insulin secretion at concentrations of 10-30 mmol 1(-1). Glucose (12 vs. 3.3 mmol 1(-1)) significantly increased both intracellular content of inositol 1,4,5-trisphosphate, as well as its metabolite inositol 1,3,4-trisphosphate. Fructose, however, had no potentiating effects on the accumulation of inositol phosphates. It is therefore supposed that glucose does not activate inositol-specific phospholipase C via the glycolysis. Further, since fructose did not activate inositol-specific phospholipase C, this stimulation is likely to be induced by glucose as such.

NADP-malate dehydrogenase activity in rat erythrocytes. Comparison with pyruvate kinase in relation to coupling with lactate dehydrogenase

The present study explores the possible channelling of pyruvate generated by either pyruvate kinase or NADP-malate dehydrogenase to lactate dehydrogenase in cross-linked and permeabilized erythrocytes. The generation of both unlabelled and 14C-labelled pyruvate and lactate was measured in rat erythrocytes, which were prepared for cross-linking with dimethyl suberimidate and permeabilization by digitonin and then exposed to unlabelled or 14C-labelled malate and/or phospho-enol-pyruvate. Rat erythrocytes were found to display NADP-malate dehydrogenase activity. Under conditions in which the generation rates of pyruvate from either phospho-enol-pyruvate (15 microM) or malate (0.5 mM) were not vastly different from one another, a greater fraction of the 2-keto acid was converted to lactate when produced from phospho-enol-[1-14C]pyruvate rather than [U-14C]malate. This difference was most obvious when the availability of exogenous NADH was close to or somewhat below that theoretically required to ensure full conversion of endogenously formed pyruvate to lactate. These findings are compatible with the view that pyruvate generated at the pyruvate kinase level is converted to lactate more efficiently than pyruvate produced in the reaction catalysed by NADP-malate dehydrogenase.

Metabolism of succinic acid methyl esters in myocytes

The metabolism of the dimethyl ester of succinic acid (SAD) was examined in myocytes. When expressed in terms of CO(2) output, the oxidation of SAD (10 mM) only represented 30-40% of that of an equimolar concentration of D-glucose. Except for a modest decrease in D-[5-(3)H]glucose utilization, SAD failed to affect the catabolism of exogenous D-glucose. SAD also failed, like D-glucose, to augment O(2) consumption by the myocytes. These findings indicate that SAD is less efficiently metabolized in myocytes than in hepatocytes or pancreatic islets. It is nevertheless argued that the methyl esters of succinic acid could be efficiently used as nutrients by myocytes in situations of ATP depletion.

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.

Hexose metabolism in pancreatic islets: unequal oxidation of the two carbons of glucose-derived acetyl residues

The fate of the C1 and C2 of glucose-derived acetyl residues was examined in rat pancreatic islets. The production of 14CO2 from D-[2-14C]glucose exceeded that from D-[6-14C]glucose, in the same manner as the oxidation of [1-14C]acetate exceeded that of [2-14C]acetate. The difference in 14CO2 output from D-[2-14C]glucose and D-[6-14C]glucose was matched by complementary differences in the generation of 14C-labeled acidic metabolites and amino acids. Even the production of 14C-labeled L-lactate was somewhat higher in the case of D-[6-14C]glucose than D-[2-14C]glucose. The ratio between D-[2-14C]glucose and D-[6-14C]glucose oxidation progressively decreased at increasing concentrations of the hexose (2.8, 7.0, and 16.7 mM), was higher after 30 than 120 min incubation, and was decreased in the presence of a nonmetabolized analogue of L-leucine. These findings are consistent with the view that the difference between D-[6-14C]glucose and D-[2-14C]glucose oxidation is mainly attributable to the inflow into the Krebs cycle of unlabeled metabolites generated from endogenous nutrients, this being compensated by the exit of partially labeled metabolites from the same cycle. The present results also indicate that the oxidation of glucose-derived acetyl residues relative to their generation in the reaction catalyzed by pyruvate dehydrogenase is higher than that estimated from the ratio between D-[6-14C]glucose and D-[3,4-14C]glucose conversion to 14CO2.

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.

Generation of 3HOH from D-[6-3H]glucose by erythrocytes: role of pyruvate alanine interconversion

Human and rat erythrocytes were found to generate 3HOH from D-[6(N)-3H]glucose. The rate of 3HOH production represented 7-10% of the glycolytic flux. The generation of 3HOH appeared attributable, in part at least, to the detritiation of [3-3H]pyruvate during the interconversion of the 2-keto acid and L-alanine in the reaction catalyzed by glutamate-pyruvate transaminase. Indeed, purified pig heart glutamate-pyruvate transaminase, as well as homogenates prepared from rat erythrocytes or pancreatic islets, catalyzed the generation of 3HOH from L-[3-3H]alanine. When the production of tritiated pyruvate from L-[3-3H]alanine was coupled to the conversion of the 2-keto acid to L-lactate, the production of 3HOH accounted for one-third of the reaction velocity, the latter failing to display isotopic discrimination. In these experiments, the production of 3HOH was abolished by amino-oxyacetate. Likewise, in intact rat erythrocytes, aminooxyacetate inhibited the generation of 3HOH and tritiated L-alanine from D-[6-3H]glucose (or D-[1-3H]glucose), as well as the generation of 3HOH from L-[3-3H]alanine. In pancreatic islets, however, aminooxyacetate failed to affect significantly the generation of 3HOH from D-[6-3H]glucose. These findings indicate that the generation of 3HOH from D-[6-3H]glucose is mainly attributable to an intermolecular tritium transfer in transaminase reaction, at least in cells devoid of mitochondria.

An investigation of fructose utilization in Acanthocheilonema viteae

The capacity of Acanthocheilonema viteae to metabolize fructose was investigated in vitro. In common with other filarial species A. viteae oxidized fructose to lactate but its rate of consumption was only 40% of the glucose-containing control value. Fructose was not incorporated into glycogen. Release of 14CO2 from [U-14C]fructose was not detected in the presence of glucose and was about 40% of the glucose-containing value under conditions where fructose was the sole hexose substrate. Fructose consumption and lactate excretion increased in proportion to the external concentration of fructose. However, worm viability was not maintained in fructose over a 120 h in vitro incubation. In the presence of fructose, protein synthesis (measured incorporation of [35S]methionine into acid-insoluble material) was reduced compared to the glucose-containing control group; but was significantly greater than the value obtained under glucose-free conditions.

Impairment of the mitochondrial oxidative response to D-glucose in pancreatic islets from adult rats injected with streptozotocin during the neonatal period

Pancreatic islets removed from adult rats injected with streptozotocin during the neonatal period display an impaired secretory response to D-glucose and, to a lesser extent, to L-leucine. Despite normal to elevated hexokinase and glucokinase activities in the islets of these glucose-intolerant animals and despite normal mitochondrial binding of the hexokinase isoenzymes, the metabolic response to a high concentration of D-glucose is severely affected, especially in terms of D-[6-14C]glucose oxidation. Thus, the ratio in D-[6-14C]glucose oxidation/D-[5-3H]glucose utilization is much less markedly increased in response to a rise in hexose concentration and, at a high concentration of D-glucose (16.7 mmol/l), less markedly decreased by the absence of Ca2+ and presence of cycloheximide in diabetic than control rats. This metabolic defect contrasts with (1) a close-to-normal or even increased capacity of the islets of diabetic rats to oxidize D-[6-14C]glucose, [2-14C]pyruvate, L-[U-14C]glutamine and L-[U-14C]leucine at low, non-insulinotropic, concentrations of these substrates; (2) a lesser impairment of the oxidation of L-[U-14C]leucine tested in high concentration (20 mmol/l), the effect of Ca2+ deprivation upon the latter variable being comparable in diabetic and control rats; (3) an unaltered transamination of either [2-14C]pyruvate or L-[U-14C]leucine; and (4) a modest perturbation of glycolysis. The most obvious alteration in glycolysis consists in a lesser increase of the glycolytic flux in response to a rise of D-glucose concentration in diabetic than control rats, this coinciding with an apparent decrease in affinity of glucokinase for the hexose.(ABSTRACT TRUNCATED AT 250 WORDS)

D-glucose and L-leucine metabolism in neonatal and adult cultured rat pancreatic islets

Neonatal and adult rat islets, cultured for 7-9 days in the presence of 10.5 mM D-glucose, were incubated for 120 min with either D-glucose (2.8 and 16.7 mM) or L-leucine (1.0 and 20.0 mM). The total and anaerobic rates of glycolysis, as judged respectively through the generation of 3H2O from D-[5-3H]glucose and 14C-labelled lactate from D-[3,4-14C]glucose or D-[6-14C]glucose were higher in neonatal than adult islets, but increased to a lesser relative extent in neonatal than adult islets in response to a rise in hexose concentration. The flow through the pentose phosphate pathway, as judged from the difference between D-[1-14C]glucose and D-[6-14C]glucose oxidation was higher in neonatal than adult islets. The flow through the reaction catalyzed by pyruvate dehydrogenase, as judged from the oxidation of D-[3,4-14C]glucose, was lower in neonatal than adult islets incubated in the presence of 16.7 mM (but not 2.8 mM) D-glucose. The oxidation of acetyl residues relative to their generation rate, as judged from the ratio of D-[6-14C]glucose to D-[3,4-14C]glucose oxidation, was not affected by the hexose concentration whether in neonatal or adult islets, but was about twice higher in the latter than former islets. The rate of D-[6-14C]glucose oxidation was also higher in adult than neonatal islets, especially at the high concentration of D-glucose. In both neonatal and adult islets, a rise in hexose concentration stimulated preferentially the oxidation of D[3,4-14C]glucose or D-[6-14C]glucose relative to the utilization of D-[5-3H]glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Hexose metabolism in pancreatic islets. Feedback control of D-glucose oxidation by functional events

A rise in extracellular D-glucose concentration in pancreatic islet cells causes a greater relative increase in the oxidation of pyruvate and acetyl residues than in glycolysis. A possible explanation for such an unusual situation was sought in the present study. The preferential stimulation of mitochondrial oxidative events was found to display a sigmoidal dependency on hexose concentration, and an exponential time course during prolonged exposure of the islets to a high concentration of D-glucose. The preferential stimulation of mitochondrial oxidative events was abolished in islets incubated in the presence of cycloheximide and absence of Ca2+, in which case the oxidation of D-[6-14C]glucose was more severely inhibited than that of D-[3,4-14C]glucose. Likewise, the inhibitor of protein biosynthesis and the absence of Ca2+ affected the oxidation of L-[U-14C]leucine preferentially, relative to that of L-[1-14C]leucine, in islets exposed to a high, but not a low, concentration of the amino acid. These results demonstrate that in pancreatic islets it is possible to dissociate both glycolysis from mitochondrial oxidative events and the oxidation of acetyl residues from their generation rate. Moreover, the experimental data suggest that nutrient-responsive and ATP-requiring functional processes exert a feedback control on mitochondrial respiration in this fuel-sensor organ.

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

In rat pancreatic islets, tumoral islet cells (RINm5F line), parotid gland, and in human erythrocytes, but not in rat hepatocytes, the production of 3H2O from D-[2-3H]glucose is 20-30% lower than from D-[5-3H]glucose. This coincides with the production of tritiated lactic acid from D-[2-3H]glucose and may be attributable to an intramolecular hydrogen transfer in the phosphoglucoisomerase reaction. It is concluded that the production of 3H2O from D-[2-3H]glucose is not a reliable tool to assess the total rate of hexose phosphorylation.

Hexose metabolism in pancreatic islets stimulation by D-glucose of [2-3H]glycerol detritiation

1. In pancreatic islets, a rise in glucose concentration is known to increase the ratio between D-[6-14C]glucose oxidation and D-[5-3H]glucose utilization. The opposite situation was found to prevail in parotid cells. 2. In rat pancreatic islets, D-glucose caused a concentration-related stimulation of 3H2O production from [2-3H]glycerol, but failed to affect 3H2O production from [1(3)-3H]glycerol or 14CO2 production from [U-14C]glycerol. At the low concentration used in most of these experiments (i.e. 1.0 mM), glycerol failed to affect D-[U-14C]glucose oxidation. 3. These findings suggest that the preferential stimulation by D-glucose of mitochondrial oxidative events in pancreatic islets represents an unusual situation in secretory cells and involves an accelerated circulation in the glycerol phosphate shuttle.