Lactate-stimulated ethanol oxidation in isolated hepatocytes

1. Hepatocytes isolated from starved rats and incubated without other substrates oxidized ethanol at a rate of 0.8-0.9mumol/min per g wet wt. of cells. Addition of 10mm-lactate increased this rate 2-fold. 2. Quinolinate (5mm) or tryptophan (1mm) decreased the rate of gluconeogenesis with 10mm-lactate and 8mm-ethanol from 0.39 to 0.04-0.08mumol/min per g wet wt. of cells, but rates of ethanol oxidation were not decreased. From these results it appears that acceleration of ethanol oxidation by lactate is not dependent upon the stimulation of gluconeogenesis and the consequent increased demand for ATP. 3. As another test of the relationship between ethanol oxidation and gluconeogenesis, the initial lactate concentration was varied from 0.5mm to 10mm and pyruvate was added to give an initial [lactate]/[pyruvate] ratio of 10. This substrate combination gave a large stimulation of ethanol oxidation (from 0.8 to 2.6mumol/min per g wet wt. of cells) at low lactate concentrations (0.5-2.0mm), but rates remained nearly constant (2.6-3.0mumol/min per g wet wt. of cells) at higher lactate concentrations (2.0-10mm). 4. In contrast, owing to the presence of ethanol, the rate of glucose synthesis was only slightly increased (from 0.08 to 0.12mumol/min per g wet wt. of cells) between 0.5mm- and 2.0mm-lactate and continued to increase (from 0.12 to 0.65mumol/min per g wet wt. of cells) with lactate concentrations between 2 and 10mm. 5. In the presence of ethanol, O(2) uptake increased with increasing substrate concentration over the entire range. 6. Changes in concentrations of glutamate and 2-oxoglutarate closely paralleled changes in the rate of ethanol oxidation. 7. In isolated hepatocytes, rates of ethanol oxidation are lower than those in vivo apparently because of depletion of malate-aspartate shuttle intermediates during cell preparation. Rates are returned to those observed in vivo by substrates that increase the intracellular concentration of shuttle metabolites.     

Phosphorus-31 nuclear magnetic resonance studies of wild-type and glycolytic pathway mutants of Saccharomyces cerevisiae.

High-resolution phosphorus-31 nuclear magnetic resonance (31P NMR) spectra of wild-type and mutant strains of Saccharomyces cerevisiae were observed at a frequency of 145.7 MHz. Levels of various phosphorus metabolites were investigated upon addition of glucose under both aerobic and anaerobic conditions. Three mutant strains were isolated and their biochemical defects characterized: pfk lacked phosphofructokinase activity; pgi lacked phosphoglucose isomerase activity; and cif had no glucose catabolite repression of the fructose bisphosphatase activity. Each mutant strain was found to accumulate characteristic sugar phosphates when glucose was added to the cell suspension. In the case of the phosphofructokinase deficient mutant, the appearance of a pentose shunt metabolite was observed. 31P NMR peak assignments were made by a pH titration of the acid extract of the cells. Separate signals for terminal, penultimate, and central phosphorus atoms in intracellular polyphosphates allowed the estimation of their average molecular weight. Signals for glycero(3)phosphochline, glycero(3)phosphoserine, and glycero(3) phosphoethanolamine as well as three types of nucleotide diphosphate sugars could be observed. The intracellular pH in resting and anaerobic cells was in the range 6.5--6.8 and the level of adenosine 5'-triphosphate (ATP) low. Upon introduction of oxygen, the ATP level increased considerably and the intracellular pH reached a value of pH 7.2--7.3, irrespective of the external medium pH, indicating active proton transport in these cells. A new peak representing the inorganic phosphate of one of the cellular organelles, whose pH differed from the cytoplasmic pH, could be detected under appropriate conditions.     

Substrate-induced alterations of high energy phosphate metabolism and contractile function in the perfused heart

The bioenergetic basis by which the Krebs cycle substrate pyruvate increased cardiac contractile function over that observed with the Embden-Meyerhof substrate glucose was investigated in the isovolumic guinea pig heart. Alterations in the content of the high energy phosphate metabolites and the rate of high energy phosphate turnover were measured by 31P NMR. These were correlated to the changes in contractile function and rates of myocardial oxygen consumption. Maximum left ventricular developed pressure (LVDP) and high energy phosphates were observed with 16 mM glucose or 10 mM pyruvate. In hearts perfused with 16 mM glucose, the intracellular phosphocreatine (PCr) concentration was 15.2 +/- 0.6 mM with a PCr/Pi ratio of 10.3 +/- 0.9. The O2 consumption was 5.35 mumol/g wet weight/min, and these hearts exhibited a LVDP of 97 +/- 3.7 mm Hg at a constant paced rate of 200 beats/min. In contrast, when hearts were switched to 10 mM pyruvate, the PCr concentration was 18.3 +/- 0.4 mM, the PCr/Pi ratio was 30.4 +/- 2.2, the O2 consumption was 6.67 mumol/g wet weight/min, and the LDVP increased to 125 +/- 3.3 mm Hg. From NMR saturation transfer experiments, the steady-state flux of ATP synthesis from PCr was 4.9 mumol/s/g of cell water during glucose perfusion and 6.67 mumol/s/g of cell water during pyruvate perfusion. The flux of ATP synthesis from ADP was measured to be 0.99 mumol/s/g of cell water with glucose and calculated to be 1.33 mumol/s/g of cell water with pyruvate. These results suggest that pyruvate quite favorably alters myocardial metabolism in concert with the increased contractile performance. Thus, as a mechanism to augment myocardial performance, pyruvate appears to be unique.     

The liver is an organ site for the release of inosine metabolized by non-glycolytic pig red cells

The metabolic energy source used by the pig red cell, which is unable to metabolize blood-borne glucose, was examined. Potential physiological substrates include adenosine, inosine, ribose, deoxyribose, dihydroxyacetone and glyceraldehyde, of which inosine was previously implicated. A net ATP synthesis by red cells occurs during in situ perfusion through the adult miniature pig liver. HPLC analysis of the perfusate revealed the presence primarily of inosine and hypoxanthine. Inosine production by the liver was 0.015 mumol/g per min. Moreover, red cells maintain ATP when suspended in a balanced salt medium during a 6 h incubation at 38 degrees C, in which inosine is continuously infused to give an external concentration of no more than 3 mumol/l, mimicking its plasma level. Inosine consumption under these infusion conditions was 56 nmol/ml cell per h, which is two orders of magnitude lower than when inosine is present in millimolar concentration. The total red cell inosine consumption of 9.63 mumol/h is much less than the total liver inosine production of 212 mumol/h. These findings suggest that the liver is an organ site elaborating inosine, and that maintenance of a 3 mumol/l inosine in plasma is sufficient to meet the energy requirements of the pig red cells.     

Kinetic study of a change in intracellular ATP level associated with aerobic catabolism of ethanol by Streptococcus mutans.

Streptococcus mutans, a group of lactic acid bacteria and a normal inhabitant of the human oral cavity, generates ATP by substrate-level phosphorylation coupled to oxidation of ethanol (an end product of fermentation of sugars) into acetate in the presence of oxygen (K. Fukui, K. Kato, Kodama, H. Ohta, T. Shima moto, and T. Shimono, Proc. Jpn. Acad. 64B:13-16, 1988). Kinetic measurements were made of the cellular responses of S. mutans FA-1 to ethanol in comparison with those to glucose. In contrast to oxygen-independent acid production from glucose, oxygen was absolutely required for acid production from ethanol. Ethanol elicited a marked increase in the intracellular ATP concentration (ATPi) from a starved level to a steady level which was held constant as long as oxygen was present in the medium. Once oxygen was exhausted, ATPi returned to the starved level without delay. On the contrary, ATPi changes induced by glucose, which were independent of oxygen, followed a rather complicated time course before a steady level was established. Both the steady ATPi and the rate of accompanying oxygen consumption were functions of the ethanol concentration. These two parameters were linearly correlated, indicating that the unimolecular ATP turnover rate, which is independent of the rate of ATP generation in the steady state, can be calculated for cells energized by ethanol. The estimated turnover rate was 1.5 s-1 at 37 degrees C, which is comparable to that for other bacteria energized by glucose under nongrowing conditions.     

Absence of glucose-induced cAMP signaling in the Saccharomyces cerevisiae mutants cat1 and cat3 which are deficient in derepression of glucose-repressible proteins

Addition of glucose to derepressed cells of the yeast Saccharomyces cerevisiae induces a transient, specific cAMP signal. Intracellular acidification in these cells, as caused by addition of protonophores like 2,4-dinitrophenol (DNP) causes a large, lasting increase in the cAMP level. The effect of glucose and DNP was investigated in glucose-repressed wild type cells and in cells of two mutants which are deficient in derepression of glucose-repressible proteins, cat1 and cat3. Addition of glucose to cells of the cat3 mutant caused a transient increase in the cAMP level whereas cells of the cat1 mutant and in most cases also repressed wild type cells did not respond to glucose addition with a cAMP increase. The glucose-induced cAMP increase in cat3 cells and the cAMP increase occasionally present in repressed wild type cells however could be prevented completely by addition of a very low level of glucose in advance. In derepressed wild type cells this does not prevent the specific glucose-induced cAMP signal at all. These results indicate that repressed cells do not show a true glucose-induced cAMP signal. When DNP was added to glucose-repressed wild type cells or to cells of the cat1 and cat3 mutants no cAMP increase was observed. Addition of a very low level of glucose before the DNP restored the cAMP increase which points to lack of ATP as the cause for the absence of the DNP effect. These data show that intracellular acidification is able to enhance the cAMP level in repressed cells. The glucose-induced artefactual increase occasionally observed in repressed cells is probably caused by the fact that their low intracellular pH is only restored after the ATP level has increased to such an extent that it is no longer limiting for cAMP synthesis. It is unclear why the artefactual increases are not always observed. Measurement of glucose- and DNP-induced activation of trehalase confirmed the physiological validity of the changes observed in the cAMP level. Our results are consistent with the idea that the glucose-induced signaling pathway contains a glucose-repressible protein and that the protein is located before the point where intracellular acidification triggers activation of the pathway.Abbreviations CCCP carbonyl cyanide m-chlorophenylhydrazone - DNP 2,4-dinitrophenol - Mes 4-morpholineethanesulfonic acid     

Cellobiose versus glucose utilization by the ruminal bacterium Ruminococcus albus.

Cellulose degradation and metabolism in the rumen can be adversely affected by the presence of soluble sugars, but relatively little information is available on substrate preferences of cellulolytic bacteria. When the ruminal bacterium Ruminococcus albus was incubated with a combination of cellobiose and glucose, the organism preferentially utilized the disaccharide. This preference appeared to be related to repression of glucose uptake systems in cellobiose-grown cells. Glucose transport kinetics exhibited low- and high-affinity uptake, and high-affinity transport was apparently driven by ATP hydrolysis. Bacterial yield in continuous culture was as much as 38% greater when the organism was grown on cellobiose versus glucose, and the increased yield could be partially attributed to constitutive cellobiose phosphorylase activity. The maintenance coefficient of glucose-grown cells was significantly greater than that of cells provided with cellobiose (0.225 g of glucose per g of protein per h versus 0.042 g of cellobiose per g of protein per h), and this result suggested that more energy was devoted to glucose uptake. Substrate affinities were examined in carbon-excess continuous culture, and affinities for glucose and cellobiose were relatively low (0.97 and 3.16 mM, respectively). Although R. albus maintained a proton motive force of approximately 60 mV from pH 6.7 to 5.5, growth ceased below pH 6.0, and this inhibition of growth may have been caused by a depletion of ATP at low pH.     

Glucose-induced alterations of intracellular ionized magnesium in human lymphocytes

The intracellular ionic content of human erythrocytes may be altered by hyperglycaemia. Despite this, very little is known about the cellular mechanisms linking glucose and cellular magnesium homeostasis. We measured intracellular ionized magnesium in human lymphocytes, by means of a fluorimetric technique, total intracellular magnesium by means of atomic absorption spectrophotometry and intracellular ATP by means of HPLC. The incubation of lymphocytes with D-glucose in the absence of insulin was followed by a significant decrease in intracellular ionized magnesium; this effect did not occur when the cells were incubated with L-glucose. The effect of glucose on intracellular ionized magnesium was blocked by amphotericin B and the EC(50) of the effect of glucose on intracellular ionized magnesium was about 5 mmol/l of glucose. The increase of intracellular ionized magnesium in cells incubated in the absence of glucose was followed by a decrease in intracellular ATP. In a Na(+)-free medium the decrease of intracellular ionized magnesium in the presence of glucose was still present and the incubation of lymphocytes with glucose did not modify total intralymphocyte magnesium. By selective permeabilization of cell membranes, we established that glucose could not increase compartmentalized intracellular ionized magnesium. Our data supports the hypothesis that glucose per se induces a substantial decrease in intracellular ionized magnesium, which is probably due to an augmented binding of intracellular ionized magnesium to cellular ATP.     

Glucose metabolism in perfused skeletal muscle. Interaction of insulin and exercise on glucose uptake.

1. The interaction of insulin and isometric exercise on glucose uptake by skeletal muscle was studied in the isolated perfused rat hindquarter. 2. Insulin, 10 m-i.u./ml, added to the perfusate, increased glucose uptake more than 10-fold, from 0.3-0.5 to 5.2-5.4 mumol/min per 30g of muscle in hindquarters of fed and 48h-starved rats respectively. In contrast, it did not stimulate glucose uptake in hindquarters from rats in diabetic ketoacidosis. 3. In the absence of added insulin, isometric exercise, induced by sciatic-nerve stimulation, increased glucose uptake to 4 and 3.4 mumol/min per 30g of muscle in fed and starved rats respectively. It had a similar effect in rats with moderately severe diabetes, but it did not increase glucose uptake in rats with diabetic ketoacidosis or in hindquarters of fed rats that had been "washed out" with an insulin-free perfusate. Insulin, at concentrations which did not stimulate glucose uptake in resting muscle, restored the stimulatory effect of exercise in these situations. 4. The stimulation of glucose uptake by exercise was independent of blood flow and the degree of tissue hypoxia; also it could not be reproduced by perfusing resting muscle with a medium previously used in an exercise experiment. 5. At rest glucose was not detectable in muscle cell water of fed and starved rats even when perfused with insulin. In the presence of insulin, a small accumulation of glucose, 0.25 mM, was noted in the muscle of ketoacidotic diabetic rats, suggesting inhibition of glucose phosphorylation, as well as of transport. 6. During exercise, the calculated intracellular concentration of glucose in the contracting muscle increased to 1.1-1.6mM in the fed, starved and moderately diabetic groups. Insulin significantly increased the already high rates of glucose uptake by the hindquarters of these animals but it did not alter the elevated intracellular concentration of glucose. 7. In severely diabetic rats, exercise did not cause glucose to accumulate in the cell in the absence of insulin. In the presence of insulin, it increased glucose uptake to 6.1 mumol/min per 30g of muscle and intracellular glucose to 0.72 mM. 8. The data indicate that the stimulatory effect of exercise on glucose uptake requires the presence of insulin. They suggest that in the absence of insulin, glucose uptake is not enhanced by exercise owing to inhibition of glucose transport into the cell.     

Changes of ATP, polyphosphate and K+ contents in Saccharomyces carlsbergensis during uptake of Mn2+ and glucose

The process of prolonged Mn2+ uptake by the yeast Saccharomyces carlsbergensis in the presence of 100 mM glucose and in the absence of phosphate can be divided into two steps. The first step (0-20 min) of Mn2+ uptake (4.3 mumol/g of wet cells) is characterized by an intense K+ efflux (23.8 mumol/g), synthesis of high molecular weight polyphosphate (HPP) (8.1 mumol/g) and decrease of ATP content (0.06 mumol/g). Simultaneously about 0.6 mumol of glucose is taken up and the level of low molecular weight polyphosphate (LPP) remains practically unchanged. The second step (20-120 min) of Mn2+ uptake (15.6 mumol/g) is characterized by a drop in HPP (16.6 mumol/g) and the synthesis of LPP (19.0 mumol/g). The ATP content decreases by 0.87 mumol/g as compared to the control, while that of K+ increases (5.7 mumol/g). During the first step of Mn2+ uptake the energy of the K+ concentration gradient may be used both for Mn2+ influx (2K+: 1Mn2+) and synthesis of HPP (1P:1.9K+). During the second step the Mn2+ accumulation is apparently driven by HPP conversion into LPP (1:1) and by ATPases serving the Mn2+/H+ exchange.     

Influence of transport energization on the growth yield of Escherichia coli.

The growth yields of Escherichia coli on glucose, lactose, galactose, maltose, maltotriose, and maltohexaose were estimated under anaerobic conditions in the absence of electron acceptors. The yields on these substrates exhibited significant differences when measured in carbon-limited chemostats at similar growth rates and compared in terms of grams (dry weight) of cells produced per mole of hexose utilized. Maltohexaose was the most efficiently utilized substrate, and galactose was the least efficiently utilized under these conditions. All these sugars were known to be metabolized to glucose 6-phosphate and produced the same pattern of fermentation products. The differences in growth yields were ascribed to differences in energy costs for transport and phosphorylation of these sugars. A formalized treatment of these factors in determining growth yields was established and used to obtain values for the cost of transport and hence the energy-coupling stoichiometries for the transport of substrates via proton symport and binding-protein-dependent mechanisms in vivo. By this approach, the proton-lactose stoichiometry was found to be 1.1 to 1.8 H+ per lactose, equivalent to approximately 0.5 ATP used per lactose transported. The cost of transporting maltose via a binding-protein-dependent mechanism was considerably higher, being over 1 to 1.2 ATP per maltose or maltodextrin transported. The formalized treatment also permitted estimation of the net ATP yield from the metabolism of these sugars; it was calculated that the growth yield data were consistent with the production of 2.8 to 3.2 ATP in the metabolism of glucose 6-phosphate to fermentation products.     

Effect of sugars on D-arabitol production and glucose metabolism in Saccharomyces rouxii.

The effect of sugars on the production of d-arabitol and on the glucose catabolic pathways was investigated in the osmotrophic yeast Saccharomyces rouxii. The activity of d-arabitol dehydrogenase, which served as a measure of total d-arabitol production, increased when cells were grown in the presence of increasing glucose concentrations. Growth in sucrose had no effect on the enzyme activity. A high intracellular concentration of d-arabitol could be demonstrated when the cells were grown in a 60% glucose medium and could be eliminated by anaerobic growth or growth in the presence of 4 mg of chloramphenicol per ml. A mutant was isolated that would not grow in 60% glucose; although the regulation of d-arabitol dehydrogenase was altered in this strain, the production of d-arabitol was not eliminated. The activity of d-arabitol dehydrogenase followed the growth phases of the parent strain when the cells were preadapted to 30% glucose. If the cells were adapting from 1 to 30% glucose, a large increase in enzyme activity was detected before growth occurred. Protein synthesis was found to be involved in this increase in activity. There was an increased participation of the pentose phosphate pathway when the cells were grown in the presence of increasing glucose concentrations. The mutant strain had only an 11% pentose phosphate pathway participation compared with 20% for the parent strain in glucose. The results suggest that the active pentose phosphate pathway is involved in glucose tolerance by providing a plentiful supply of reduced nicotinamide adenine dinucleotide phosphate which is necessary for cell survival.     

Inhibition of peptide chain initiation in lysates from ATP-depleted cells.I. Stages in the evolution of the lesion and its reversal by thiol compounds, cyclic AMP or purine derivatives and phosphorylated sugars.

Anaerobic incubation of rabbit reticulocytes at 37 degrees C in Krebs-Ringer solution supplemented with hemin but devoid of glucose resulted at the end of 1-2h in a drastic decline of their ATP content and an attendant arrest of protein synthesis. Subsequent provision of glucose and reoxygenation of the cells was followed by a rapid replenishment of the ATP pool, while resumption of protein synthesis was markedly delayed. This lag period could be considerably reduced by addition of 5-10 mM adenine or 2,6-diaminopurine to the incubation medium. Lysates prepared from ATP-depleted cells exhibited disaggregation of the polysomes and an inhibition of the nedogenously coded protein synthesis, when tested in a cell-free system supplied with an adequate ATP generator. Both alterations increased in severity with the progressive decay of the intracellular ATP pool. The early phase of partial inhibition following a 40-70% decrease of the cellular ATP level was fully reversible by fortifying the cell-free preparation with dithiothreitol or a suitable NADPH-generating system. Aternative, the inhibition could be also overcome by millimolar amounts of adenine, 2,6-diaminopurine and a variety of other purine derivatives or cyclic AMP. The effect of these compounds was unrelated to the endogenous cyclic AMP pool. Joint addition of both dithiothreitol and cyclic AMP or adenine was necessary for relieving the initiation block in lysates derived from cells depleted of 80-90% of their ATP content. On further aggravating the conditions of energy starvation, an additional requirement for phosphorylated sugars, e.g. glucose 6-phosphate or fructose 1,6-diphosphate, became apparent. ATP depletion brought about by exposing the cells to Antimycin A or 2,4-dinitrophenol resulted in a lesion which was indistinguishable from that induced by anaerobic incubation. On the other hand, energy deprivation in cell-free lysates from untreated reticulocytes, preincubated in the absence of an ATP-generating system failed to duplicate the deleterious effect of intracellular ATP depletion. Some aspects bearing on the biochemical mechanism of the lesion and its reversal are discussed in the light of the available data.     

Adenosine triphosphate pool levels and endogenous metabolism in Arthrobacter crystallopoietes during growth and starvation

The adenosine triphosphate (ATP) content of Arthrobactery crystallopoietes was measured during growth, starvation and recovery from starvation. During exponential growth of the cells as spheres in a glucose salts medium, the level of ATP per cell remained constant at 8.0×10^-10 g/cell. Morphogenesis to rodshaped cells and an increased growth rate following addition of casein hydrolysate was accompanied by an almost two-fold increase in the ATP level. As division of the rod-shaped cells proceeded, the level of ATP declined. After growing as rods for 12–14 h the cells underwent fragmentation to spheres during which time the ATP level again increased to the original value of 8.0×10^-10 g/cell. As the spherical cells resumed growth on the residual glucose, their ATP content declined for a short period and then remained relatively constant. During starvation of sphere or rod-shaped cells for one week, the ATP level declined by approximately 70% during the first 40–50 h and then remained constant. The endogenous metabolism rate of spherical cells declined during the first 10–20 h of starvation and then remained constant at approximately 0.02% of the cell carbon being utilized per h. Addition of glucose to spherical cells which had been starved for one week increased both the ATP content per cell and their rate of endogenous metabolism. The ATP content fluctuated and then remained at a level higher than maintained during starvation while endogenous metabolism quickly declined.Non-Standard Abbreviations ATP adenosine triphosphate - GS glucose mineral salts - HC casein hydrolysate - PVP polyvinylpyrrolidone - DMSO dimethylsulfoxide - MOPS morpholinopropane sulfonic acid - EDTA ethylene diaminetetraacetic acid     

The effect of gamma radiation and neocarzinostatin of NAD and ATP levels in mouse leukaemia cells

When mouse leukaemia cells are treated with γ-radiation or neocarzinostatin the intracellular NAD and ATP levels fall rapidly. We have shown that the ATP response is a consequence of the decreased NAD level. We suggest that this low NAD level results in decreased glycolytic activity and that there is a subsequent accumulation of phosphorylated sugars associated with the fall in ATP. Under these extreme conditions, therefore, the NAD level probably regulates the rate of glycolysis in cells which are utilising a rapidly metabolisable sugar as their energy source.     

Carbon-13 and phosphorus-31 NMR study of hepatic metabolism in the perfused rat liver

Phosphorus-31 nuclear magnetic resonance (NMR) has been used to determine non-invasively absolute concentrations of phosphorylated metabolites in the perfused rat liver. It has been shown that the NMR method does detect cytoplasmic ATP and ADP (ATP:ADP ratio of 15 +/- 3) with no contribution from mitochondrial adenine nucleotides. The concentration of ATP was 7.2 +/- 0.3 mM in the cytosol of well-oxygenated liver, after two hours of perfusion with a Krebs-Ringer buffer. Other phosphorylated metabolites were detected, mainly inorganic phosphate (1.1 mumol/g liver wet weight), phosphorylcholine (1.0 mumol/g wet weight), glycerophosphorylethanolamine (0.34 mumol/g wet weight) and glycerophosphorylcholine (0.30 mumol/g wet weight). The intracellular pH measured from the position of the Pi resonance has a value of 7.2 +/- 0.1. It is likely that the detectable Pi originates from the cytosolic compartment since a pH value of 7.4-7.6 would be expected for the mitochondrial matrix. Natural abundance carbon-13 NMR has also been used to follow the glycogen breakdown in situ by measuring the intensity of the glycogen C-1 resonance in the perfused liver spectrum as a function of the perfusion time. The glycogenolytic process has been studied as a function of the glucose content of the perfusate. Rate of glycogenolysis from 2.7 to 0.16 muEq glycosyl units g wet weight-1 min-1 were found when glucose concentration in the perfusate was varied from 0 to 50 mM. The fate of 90% enriched [2-13C] acetate has been studied in the perfused rat liver by 13C-NMR in order to investigate the mitochondrial metabolism and the interrelations between cytosolic and mitochondrial pools of metabolites. Some compounds of the intermediary metabolism where found to be extensively labelled, e.g. glutamate, glutamine, acetoacetate and beta-hydroxybutyrate. Under our experimental conditions, labelling of glutamate reached a steady-state within 30 min after the onset of perfusion of 20 mM acetate. In addition, the observed incorporation of carbon-13 isotope into glutamine can be linked to the operation of the glutamate-glutamine antiporter and to the high activity of the cytosolic glutamate synthetase. The finding of both active glutaminase and glutamine synthetase activity in the same liver cells is an evidence of the existence of an active glutamine-glutamate futile cycle.     

D-xylulose-induced depletion of ATP and Pi in isolated rat hepatocytes

Xylitol is known to cause hepatic ATP catabolism by inducing the trapping of Pi in the form of glycerol 3-P as a consequence of an increase in the NADH:NAD+ ratio, resulting from the oxidation of xylitol to D-xylulose. The question was therefore raised whether D-xylulose also depletes hepatic ATP. In isolated rat hepatocytes, 5 mM D-xylulose decreased ATP by 80% within 5 min compared to 40% with 5 mM xylitol. Intracellular Pi decreased by 70% within the same time interval with both compounds, but was restored three-fold faster with D-xylulose. The rate of utilization of D-xylulose reached 5 mumol.min-1.g-1 of cells, as compared with 1.5 for xylitol, indicating that reduction of xylitol into D-xylulose is a rate-limiting step in the metabolism of the polyol. D-Xylulose barely modified the concentration of glycerol 3-P but increased xylulose 5-P from 0.02 to 0.5 mumol/g within 5 min. The main cause of the ATP- and Pi-depleting effects of D-xylulose was found to be an accumulation of sedoheptulose 7-P from a basal value of 0.1 to 5 mumol/g of cells after 10 min. Ribose 5-P increased from 0.03 to 0.5 mumol/g at 5 min. Ribose 1-P also accumulated, albeit outside of the cells. This extracellular accumulation can be explained by the release of intracellular purine nucleoside phosphorylase from damaged hepatocytes acting on inosine that had diffused out of the cells. Smaller increases in the concentrations of sedoheptulose 7-P and pentose phosphates were recorded after incubations of the cells with xylitol.     

Real-time analysis of intracellular glucose and calcium in pancreatic beta cells by fluorescence microscopy

Glucose is the physiological stimulus for insulin secretion in pancreatic beta cells. The uptake and phosphorylation of glucose initiate and control downstream pathways, resulting in insulin secretion. However, the temporal coordination of these events in beta cells is not fully understood. The recent development of the FLII(12)Pglu-700μ-δ6 glucose nanosensor facilitates real-time analysis of intracellular glucose within a broad concentration range. Using this fluorescence-based technique, we show the shift in intracellular glucose concentration upon external supply and removal in primary mouse beta cells with high resolution. Glucose influx, efflux, and metabolism rates were calculated from the time-dependent plots. Comparison of insulin-producing cells with different expression levels of glucose transporters and phosphorylating enzymes showed that a high glucose influx rate correlated with GLUT2 expression, but was largely also sustainable by high GLUT1 expression. In contrast, in cells not expressing the glucose sensor enzyme glucokinase glucose metabolism was slow. We found no evidence of oscillations of the intracellular glucose concentration in beta cells. Concomitant real-time analysis of glucose and calcium dynamics using FLII(12)Pglu-700μ-δ6 and fura-2-acetoxymethyl-ester determined a glucose threshold of 4mM for the [Ca(2+)](i) increase in beta cells. Indeed, a glucose concentration of 7mM had to be reached to evoke large amplitude [Ca(2+)](i) oscillations. The K(ATP) channel closing agent glibenclamide was not able to induce large amplitude [Ca(2+)](i) oscillations in the absence of glucose. Our findings suggest that glucose has to reach a threshold to evoke the [Ca(2+)](i) increase and subsequently initiate [Ca(2+)](i) oscillations in a K(ATP) channel independent manner.     

Mechanism of control of adenylate cyclase activity in yeast by fermentable sugars and carbonyl cyanide m-chlorophenylhydrazone

The phosphorylation of fructose-1,6-bisphosphatase is preceded by a transient increase in the intracellular level of cyclic AMP which activates a cyclic AMP-dependent protein kinase (Pohlig, G., and Holzer, H. (1985) J. Biol. Chem. 260, 13818-13823). Possible mechanisms by which sugars or ionophores might activate adenylate cyclase and thereby lead to an increase in cyclic AMP concentrations were studied. Studies with permeabilized yeast cells demonstrated that neither sugar intermediates nor carbonyl cyanide m-chlorophenylhydrazone are able to increase adenylate cyclase activity. In the light of striking differences of the effects of fermentable sugars and of carbonyl cyanide m-chlorophenylhydrazone on parameters characterizing the membrane potential, it seems not reasonable that the activity of adenylate is under control of the membrane potential. Rapid quenching of 9-aminoacridine fluorescence after addition of fermentable sugars to starved yeast cells indicated an intracellular acidification. The 31P NMR technique showed a fast drop of the intracellular pH from 6.9 to 6.55 or 6.4 immediately after addition of glucose or carbonyl cyanide m-chlorophenylhydrazone. The time course of the decrease of the cytosolic pH coincides with the transient increase of cyclic AMP concentration and the 50% inactivation of fructose-1,6-bisphosphatase under the conditions of the NMR experiments. Kinetic studies of adenylate cyclase activity showed an approximately 2-fold increase of activity when the pH was decreased from 7.0 to 6.5, which is the result of a decrease in the apparent Km for ATP with no change in Vmax. These studies suggest that activation of adenylate cyclase by decrease in the cytosolic pH starts a chain of events leading to accumulation of cyclic AMP and phosphorylation of fructose-1,6-bisphosphatase.     

The influence of adenosine on intermediary metabolism of isolated hepatocytes.

The effect of adenosine was tested on the energetic metabolism of fed rat liver cells after isolation. The cells were incubated in a buffered saline medium with glucose (5 mM) and adenosine (1 mM) for 30 minutes at 37 degrees C. This increased the concentration of the adenylic nucleotides ATP (+57 per cent, ADP (+39 per cent). Cyclic AMP was increased (+50 per cent) and the intracellular inorganic phosphate decreased (-22 per cent). These changes were accompaned by a decrease of glycogenolysis, glucose consumption and lactate production. Measurement of glycolytic intermediates showed decreased concentrations of fructose 1,6-bis-phosphate and 3-phosphoglycerate proportional to the increase in ATP concentration. The near-equilibrium of the glyceraldehyde 3-phosphate dehydrogenase-phosphoglycerate kinase system was not modified by adenosine. The decrease of the NAD+/NADH ratio along with the increase of the ATP/ADP X PO4 ratio explains the decrease of 3-phosphoglycerate. The decrease in glucose consumption can be explained by the cross over at the phosphofructokinase stage with the decrease of fructose 1,6-bisphosphate. The major part of adenosine was deaminated as indicated by an increase in the production of ammonia and urea. The effects of inosine, or adenosine along with an inhibitor of adenosine deaminase (pentostatin) suggest that adenosine acts on the glucose consumption through adenylic nucleotides. However the increase of the adenylic nucleotide level cannot totally explain the other metabolic changes: decrease of the NAD+/NADH cytoplasmic ratio, constancy of this ratio in mitochondria, decrease of gluconeogenesis from lactate. A direct action of adenosine can therefore be expected.