Rates of utilization and fates of glucose, glutamine, pyruvate, fatty acids and ketone bodies by mouse macrophages.

The concentrations of ATP and the ATP/AMP concentration ratios were maintained in thioglycollate-elicited mouse peritoneal macrophages incubated in vitro for 90 min in the presence or absence of added substrate: rates of glycolysis, lactate formation and glutamine utilization were approximately linear with time for at least 60 min of incubation. The rate of oxygen consumption by macrophages was only increased above the basal rate (i.e. that in the absence of added substrate) by addition of succinate or pyruvate, or by addition of the uncoupling agent carboxyl cyanide m-chlorophenylhydrazone ('CCCP'); it was decreased by 75% by the addition of KCN. These findings suggest that metabolism of endogenous substrate can provide most, if not all, of the energy requirement of these cells, at least for a short period. The rates of glucose and glutamine utilization by incubated macrophages were approx. 300 and 100 nmol/min per mg of protein respectively. A large proportion of the glutamine that is utilized is converted into glutamate and aspartate, and very little (perhaps less than 10%) is oxidized. Similarly almost all of the glucose that is utilized is converted into lactate and very little is oxidized. This characteristic is similar to that of resting lymphocytes and rapidly dividing cells; in non-proliferating macrophages it may be a mechanism to provide precision in control of the rate of biosynthetic processes that utilize intermediates of these pathways, e.g. purines and pyrimidines for mRNA for the synthesis of secretory proteins and glycerol 3-phosphate for phospholipid synthesis for membrane recycling. No utilization of acetoacetate or 3-hydroxybutyrate by macrophages was detected. In contrast, both butyrate and oleate were oxidized. The rate of [14C]oleate conversion into 14CO2 (1.3 nmol/h per mg of protein) could account for most of the oxygen consumption by incubated macrophages, suggesting that long-chain fatty acids might provide an important fuel in situ. This may be one explanation for the secretion of lipoprotein lipase by these cells, to provide fatty acids for oxidation from the degradation of local triacylglycerol.     

Metabolism of radiolabelled energy-yielding substrates by rat Sertoli cells

The rates of metabolism in vitro of 3H- or 14C-labelled glucose, pyruvate, glutamine and leucine by Sertoli cells from immature rats were estimated. The overall rate of glucose utilization exceeded by far the rates of oxidation of pyruvate (derived from glucose) via the citric acid cycle and glucose metabolism via the oxidative branch of the pentose phosphate pathway. This pattern of glucose metabolism was not markedly altered after stimulation of glucose metabolism by FSH. The rate of oxidation of exogenous pyruvate indicated that the energy yield from glucose metabolism by Sertoli cells could be dependent on the extracellular concentrations of pyruvate and lactate. There is no evidence that a high rate of aerobic glycolysis is of vital importance for Sertoli cells. In medium containing glucose and all amino acids, 14C-labelled glutamine and leucine were converted to 14CO2 at considerable rates. It was calculated that the oxidation of glutamine and leucine in addition to glucose and fatty acids can yield much of the required energy of Sertoli cells.     

Metabolism of isolated kidney tubules. Interactions between lactate, glutamine and oleate metabolism.

Kidney-cortex tubule suspensions were prepared by collagenase treatment of kidney cortex from fed and starved rats. This preparation, consisting mainly of proximal convoluted tubules was incubated with three major renal substrates, L-lactate, glutamine and oleate to study the dose dependence of substrate uptake rates from medium substrate combinations. All three substances, when added at near physiological concentrations, modified the uptake rate and fate of the other substrates. In accordance with previous observations, oleate inhibited lactate uptake, and lactate decreased glutamine metabolism. Glutamine on the other hand led to a marked increase in lactate uptake. Both, glutamine and lactate increased oleate metabolism. Glucose was the main product of lactate and glutamine metabolism, lactate being preferentially taken up for this process. Oleate led to a net synthesis of triglycerides in the tubules, which was stimulated by the addition of lactate and glutamine. More than 75% of the oleate taken up was recovered as triglycerides. In the absence of fatty acids, triglyceride content of tubules decreased. The results indicate that oleate is taken up in preference to lactate and glutamine when all three substrates are offered to the tubule. Glucose and triglycerides are the main metabolic products of tubular substrate metabolism. Whereas glucose is released into the medium, triglycerides are stored in the tubule cell.     

Glutamine metabolism in isolated incubated adipocytes of the rat.

1. Phosphate-dependent glutaminase activity in the epididymal fat-pad was 15.1 nmol/min per mg of protein. Glutaminase activity demonstrated differences with respect to adipose-tissue sites. Considerable variation was found in different sites of adipose tissue from lean control and Zucker obese animals. 2. Adipocytes incubated in the presence of 2 mM-glutamine utilized glutamine at a rate of 1.8 mumol/h per g dry wt., and glutamate, ammonia, lactate and alanine were produced. Addition of glucose plus insulin increased the rates of glutamine utilization and glutamate, ammonia, lactate and alanine production. Isoprenaline alone or plus glucose further stimulated the rate of glutamine utilization and formation of end products. 3. The rate of incorporation of 14C from glutamine into CO2 was similar to that of glucose, but the rate of incorporation into triacylglycerol was much less. Addition of unlabelled glucose or glucose plus insulin stimulated the rate of incorporation of [14C]glutamine into triacylglycerol, but had no effect on that of 14CO2 formation. Isoprenaline plus glucose increased the rate of incorporation of [14C]glutamine into CO2, but decreased the rate of incorporation into triacylglycerol. 4. In the absence of insulin, the rate of [14C]glutamine incorporation into triacylglycerol was related to the glucose concentration (0-10 mM). However, in the presence of insulin, the rate of incorporation of [14C]glutamine was maximal at 1 mM-glucose.     

Glucose and glutamine metabolism in rat thymocytes.

The metabolism of glucose and glutamine in freshly prepared resting and concanavalin A-stimulated rat thymocytes was studied. Concanavalin A addition enhanced uptake of both glucose and glutamine and led to an increase in oxidative degradation of both substrates to CO2. With variously labelled [14C]glucose, it was shown that the pathways of glucose dissimilation were equally stimulated by the mitogen. A disproportionately large percentage of the extra glucose taken up was converted into lactate, but concanavalin A also caused an increase in the oxidation of pyruvate as judged by the enhanced release of 14CO2 from [2-14C]-, [3,4-14C]- and [6-14C]-glucose. Addition of glutamine did not affect glucose metabolism. The major end products of glutamine metabolism by resting and mitogen-stimulated rat thymocytes were glutamate, aspartate, CO2 and NH3. Virtually no lactate was formed from glutamine. Concanavalin A enhanced the formation of all end products except glutamate, indicating that more glutamine was metabolized beyond the stage of glutamate in the mitogen-activated cells. Addition of glucose caused a significant decrease in the rates of glutamine utilization and conversion into aspartate and CO2 in the absence and in the presence of concanavalin A. In the presence of glucose, almost all nitrogen of the metabolized glutamine was accounted for as NH3 released via the glutaminase and/or glutamate dehydrogenase reactions. In the absence of glucose, part (18%) of the glutamine nitrogen was metabolized by the resting and to a larger extent (38%) by the mitogen-stimulated thymocytes via a transaminase or amidotransferase reaction.     

Metabolism of pyruvate by isolated rat mesenteric lymphocytes, lymphocyte mitochondria and isolated mouse macrophages.

1. The activities of pyruvate dehydrogenase in rat lymphocytes and mouse macrophages are much lower than those of the key enzymes of glycolysis and glutaminolysis. However, the rates of utilization of pyruvate (at 2 mM), from the incubation medium, are not markedly lower than the rate of utilization of glucose by incubated lymphocytes or that of glutamine by incubated macrophages. This suggests that the low rate of oxidation of pyruvate produced from either glucose or glutamine in these cells is due to the high capacity of lactate dehydrogenase, which competes with pyruvate dehydrogenase for pyruvate. 2. Incubation of either macrophages or lymphocytes with dichloroacetate had no effect on the activity of subsequently isolated pyruvate dehydrogenase; incubation of mitochondria isolated from lymphocytes with dichloroacetate had no effect on the rate of conversion of [1-14C]pyruvate into 14CO2, and the double-reciprocal plot of [1-14C]pyruvate concentration against rate of 14CO2 production was linear. In contrast, ADP or an uncoupling agent increased the rate of 14CO2 production from [1-14C]pyruvate by isolated lymphocyte mitochondria. These data suggest either that pyruvate dehydrogenase is primarily in the a form or that pyruvate dehydrogenase in these cells is not controlled by an interconversion cycle, but by end-product inhibition by NADH and/or acetyl-CoA. 3. The rate of conversion of [3-14C]pyruvate into CO2 was about 15% of that from [1-14C]pyruvate in isolated lymphocytes, but was only 1% in isolated lymphocyte mitochondria. The inhibitor of mitochondrial pyruvate transport, alpha-cyano-4-hydroxycinnamate, inhibited both [1-14C]- and [3-14C]-pyruvate conversion into 14CO2 to the same extent, and by more than 80%. 4. Incubations of rat lymphocytes with concanavalin A had no effect on the rate of conversion of [1-14C]pyruvate into 14CO2, but increased the rate of conversion of [3-14C]pyruvate into 14CO2 by about 50%. This suggests that this mitogen causes a stimulation of the activity of pyruvate carboxylase.     

End products of glucose and glutamine metabolism by cultured cell lines

Rates of CO2 production from glucose and glutamine, intracellular metabolite levels, and release of metabolic end products into the culture medium were determined for 13 cultured cell lines, including a glycolysis-defective mutant. All the non-mutant lines synthesized pyruvate, lactate, alanine, proline, aspartate, and citrate, so that the metabolism of glucose and glutamine resulted mainly in the production of these compounds and only to a lesser extent in complete oxidation to CO2. These data and the pattern of metabolites produced by the mutant line were consistent with a model characterized by incomplete glutamine oxidation leading to end product accumulation. Multiple linear regression analysis identified the metabolite levels most highly correlated with the intracellular citrate level and with the amount of citrate released into the medium. The analysis also showed that the rates of CO2 production from glucose and glutamine were themselves positively correlated, suggesting that the oxidation of the two substrates is coordinately controlled under normal culture conditions.     

The influence of insulin on glucose and fatty acid metabolism in the isolated perfused rat hind quarter.

Glucose and fatty acid metabolism of resting skeletal muscle were studied by perfusion of the isolated rat hind leg with a hemoglobin-free medium. Tissue integrity was demonstrated by normal ATP, ADP and creatine phosphate levels, by a sufficient oxygen supply, and by a normal appearance of perfused muscle specimens under the electron microscope. The rates of glucose and fatty acid uptake, and of lactate, alanine, glycerol and fatty acid release were constant over a perfusion period of 60 min. Insulin (1 unit/l) caused a more than threefold increase in glucose uptake, a stimulation of lactate production, and a 20% increase in the muscular glycogen levels. Fatty acids and alanine release were significantly diminished by insulin, but glycerol release did not change. The uptake of oleate by the rat hind leg was dependent on the medium concentration in a range of 0.7-1.9mM oleate, and was stimulated by insulin. Glucose uptake was not influenced by oleate, whether sodium was present or not. When the leg was perfused with [1-14C]oleate, 75% of the incorporated fatty acids were found in muscle lipids, 10% were oxidized to CO2, and 5% were recovered in bone lipids. The absolute amount of oleate oxidation was not altered by insulin. In all experiments with and without glucose in the medium, 70-80% of the 14C label incorporated into muscle lipids was found in the triglyceride fraction. In the presence of glucose, insulin significantly increased the incorporation of [1-14C]oleate into muscle triglycerides, whereas no insulin effect, either on fatty acid uptake or on triglyceride formation, could be observed when glucose was omitted from the perfusate. The present results indicate that a "glucose-fatty acid cycle" as found in rat heart muscle does not operate in resting peripheral skeletal muscle tissue. They also demonstrate that the stimulating effect of insulin on muscular fatty acid uptake and triglyceride synthesis is dependent on glucose supply. This finding can be intrepreted as a stimulation of fatty acid esterification by sn-glycerol 3-phosphate derived from an increased glucose turnover, which is in turn due to insulin.     

Glucose and amino acid metabolism in an established line of skeletal muscle cells

The suitability of an established myogenic line (L₆) for the study of skeletal muscle intermediary metabolism was investigated. Myoblasts were grown in tissue culture for ten days at which time they had differentiated into multinucleated myotubes. Myotube preparations were then incubated for up to 96 hours in 10 ml of Dulbecco's modified Eagle medium containing 10% fetal calf serum. Glucose was utilized at a nearly linear rate, 3.0 nmol/min/mg protein. Intracellular glucose was detectable throughout the incubation, even when medium glucose was as low as 16 mg%. During the initial 28 hours of incubation, when net lactate production was observed, only 35% of the glucose utilized was converted to lactate. Alanine was produced in parallel to lactate at an average rate of 0.6 nmol/min/mg protein. In concert with active glutamine utilization, high rates of ammoniagenesis were observed as medium glutamine decreased from 3.3 mM to 0.49 mM and medium ammonia increased from 2.3 mM to 6.2 mM, between zero time and 96 hours of incubation, respectively. The cells maintained stable ATP and citrate levels, and physiologic intracellular lactate/pyruvate ratios (10–24) throughout 96 hours of incubation. These results suggest (1) glucose utilization by skeletal muscle in tissue culture is limited by phosphorylation, not transport; (2) as much as 50% of glucose-derived pyruvate enters mitochondrial pathways; (3) glutamine carbon may be utilized simultaneously with glucose consumption and this process accounts for high rates of ammoniagenesis.     

Utilization of pyruvate and pyruvate precursors by normal and carcinogen-altered rat tracheal epithelial cells in culture

The metabolism of [14C]pyruvate, [14C]glucose, [14C]glutamine and [14C]alanine was compared between normal rat tracheal epithelial cells and carcinogen-altered cells derived from dimethylbenz(a)anthracene-exposed tracheal implants. Normal primary cultures (NPC) of tracheal cells are distinguished by their need for pyruvate-supplemented medium for growth and survival. The altered cells were selected out by their survival in the unsupplemented medium. Compared to the selected primary cultures (SPC), the NPC showed a three- to four-fold higher incorporation of radioactivity from [2-14C]pyruvate in all the macromolecular fractions, as well as in all the metabolites isolated from the acid soluble fraction and from lactic acid isolated from the medium. [U-14C]glucose was also incorporated at higher levels into lactic acid isolated from the acid soluble fraction and the medium of NPC. These data indicate a higher rate of glycolysis in the normal tracheal cells. This was supported by the findings of a two-fold greater glucose consumption and two-fold higher production of lactic acid isolated from the NPC medium. Lactate dehydrogenase activity was also two-fold higher in NPC. Thus, despite the apparently higher level of pyruvate production in the NPC, exogenous pyruvate is necessary to satisfy the metabolic needs of NPC. The utilization of [U-14C]glutamine or [U-14C]alanine was not markedly different between NPC and SPC. Furthermore, radioactivity from both of the amino acids was recovered in lactic acid in the medium, indicating that both cell types can derive pyruvic acid from either glutamine or alanine. SPC apparently do not use these routes to supply higher levels of pyruvic acid for survival in culture. The oxidation of none of the radioactive metabolites into CO2 was distinctly different between NPC and SPC except for the 1.7-fold higher utilization of [1-14C]glucose along the oxidative arm of the pentose cycle in the normal cells.     

Indole-3-acetic acid increases glutamine utilization by high peroxidase activity-presenting leukocytes

Indole-3-acetic acid (IAA) is toxic for human tumor cells and in association with horseradish peroxidase (HRP) can be used as a new prodrug/enzyme combination for targeted cancer therapy. The toxic effect of IAA on neutrophils, macrophages and lymphocytes is associated with cell peroxidase activity, which is high in neutrophils and low in lymphocytes. The effect of IAA on glucose and glutamine metabolism in leukocytes presenting different peroxidase activities: neutrophils, thioglycollate-elicited macrophages and lymphocytes was investigated. A time-course effect (from 6 to 48 h in culture) of IAA on glucose and glutamine metabolism of neutrophils, thioglycollate-elicited macrophages, and lymphocytes was then carried out. Addition of IAA (0.25 mM) did not have a marked effect on glucose utilization and lactate formation by the three cell types but it raised glutamine consumption and glutamate production by neutrophils and macrophages. IAA had no effect on glutamine consumption and glutamate production by lymphocytes. A strong relationship was found between glutamine utilization (0.999) and glutamate production (0.999) and peroxidase activity. IAA did not change the activities of hexokinase, glucose-6-phosphate dehydrogenase, citrate synthase, lactate dehydrogenase, and phosphate-dependent glutaminase of 24 h cultured neutrophils and lymphocytes. The effect of IAA (1 mM) on glucose and glutamine metabolism was also investigated by 1 h incubated leukocytes in PBS. IAA did not affect glucose and glutamine metabolism of lymphocytes but enhanced glucose and glutamine metabolism by 1 h incubated neutrophils and thioglycollate-elicited macrophages. IAA caused a marked increase on oxygen consumption by neutrophils, which was more pronounced in the presence of the glutamine as compared to glucose. The stimulation of oxygen consumption leads to a reduction in NADH/NAD+ ratio that activates the flux of substrates through the Krebs cycle. Since glutamine is mainly metabolized through the left hand side of the Krebs cycle, a reduction in the redox state of the cells may accelerate the flux of substrates through glutaminolysis. The toxic results presented here show that the affect of IAA in association with peroxidase involves activation of glutamine metabolism.     

Amino acid sequence of the trypsin-generated C3d fragment from human complement factor C3.

Energy metabolism in proliferating cultured rat thymocytes was compared with that of freshly prepared non-proliferating resting cells. Cultured rat thymocytes enter a proliferative cycle after stimulation by concanavalin A and Lymphocult T (interleukin-2), with maximal rates of DNA synthesis at 60 h. Compared with incubated resting thymocytes, glucose metabolism by incubated proliferating thymocytes was 53-fold increased; 90% of the amount of glucose utilized was converted into lactate, whereas resting cells metabolized only 56% to lactate. However, the latter oxidized 27% of glucose to CO2, as opposed to 1.1% by the proliferating cells. Activities of hexokinase, 6-phosphofructokinase, pyruvate kinase and aldolase in proliferating thymocytes were increased 12-, 17-, 30- and 24-fold respectively, whereas the rate of pyruvate oxidation was enhanced only 3-fold. The relatively low capacity of pyruvate degradation in proliferating thymocytes might be the reason for almost complete conversion of glucose into lactate by these cells. Glutamine utilization by rat thymocytes was 8-fold increased during proliferation. The major end products of glutamine metabolism are glutamate, aspartate, CO2 and ammonia. A complete recovery of glutamine carbon and nitrogen in the products was obtained. The amount of glutamate formed by phosphate-dependent glutaminase which entered the citric acid cycle was enhanced 5-fold in the proliferating cells: 76% was converted into 2-oxoglutarate by aspartate aminotransferase, present in high activity, and the remaining 24% by glutamate dehydrogenase. With resting cells the same percentages were obtained (75 and 25). Maximal activities of glutaminase, glutamate dehydrogenase and aspartate aminotransferase were increased 3-, 12- and 6-fold respectively in proliferating cells; 32% of the glutamate metabolized in the citric acid cycle was recovered in CO2 and 61% in aspartate. In resting cells this proportion was 41% and 59% and in mitogen-stimulated cells 39% and 65% respectively. Addition of glucose (4 mM) or malate (2 mM) strongly decreased the rates of glutamine utilization and glutamate conversion into 2-oxoglutarate by proliferating thymocytes and also affected the pathways of further glutamate metabolism. Addition of 2 mM-pyruvate did not alter the rate of glutamine utilization by proliferating thymocytes, but decreased the rate of metabolism beyond the stage of glutamate significantly. Formation of acetyl-CoA in the presence of pyruvate might explain the relatively enhanced oxidation of glutamate to CO2 (56%) by proliferating thymocytes.     

End products of glucose and glutamine metabolism by L929 cells

Products of glucose and glutamine metabolism by L929 cells were detected and quantitated by gas chromatography and mass spectrometry of the oxime-trimethylsilyl derivatives. This method allowed detection and identification of all major carboxylic and amino acids produced in the system. Although lactic acid was expected to be the major product, alanine, citric, glutamic, aspartic, and pyruvic acids were also released into the culture medium at significant rates. Incorporation of labeled carbon from D-[U-13C]glucose showed that the alanine, lactic, and pyruvic acids were derived from glucose as was one-third of the citric acid carbon. The rate of glucose utilization for production of these end products was 29-fold greater than the rate of glucose oxidation to CO2, and calculated ATP production from alanine and pyruvate synthesis exceeded that from lactate synthesis by nearly 2-fold. Utilization of glutamine for synthesis of aspartic, glutamic, and citric acids also exceeded the rate of glutamine oxidation, thereby making end-product synthesis from glucose and glutamine the dominant cellular metabolic activity. In the absence of glucose, synthesis and intracellular levels of aspartic and glutamic acids increased, whereas synthesis and cell content of the other acids decreased markedly. This response is consistent with the metabolic pattern proposed by Moreadith and Lehninger (Moreadith, R.W., and Lehninger, A.L. (1984) J. Biol. Chem. 259, 6215-6221) in which much of the glutamine used by these cells is converted to aspartate in the absence of a pyruvate source and to aspartate or citrate in the presence of pyruvate.     

Growth and metabolism of marine fish Chinook salmon embryo cells: response to lack of glucose and glutamine

A peculiar phenomenon, differing from the response of mammalian cells, occurred when Chinook salmon embryo (CHSE) cells were passaged in the medium lacking of both glucose and glutamine. To elucidate metabolic mechanism of CHSE cells, the metabolism parameters, key metabolic enzymes, and ATP levels were measured at different glucose and glutamine concentrations. In the glutamine-free culture, hexokinase activity kept constant, and lactate dehydrogenase (LDH) activity decreased. This indicated that lack of glutamine did not expedite glucose consumption but made it shift to lower lactate production and more efficient energy metabolism. The results coincided with the experimental results of unaltered specific glucose consumption rate and decreased yield coefficients of lactate to glucose. In the glucose-free culture, simultaneous increase of glutaminase activity and of specific ammonia production rate suggested an increased flux into the glutaminolysis pathway, and increases of both glutamate dehydrogenase activity and yield coefficient of ammonia to glutamine showed an increased flux into deamination pathway. However, when glucose and glutamine were both lacking, the specific consumption rates of most of amino acids increased markedly, together with decrease of LDH activity, indicating that pyruvate derived from amino acids, away from lactate production, remedied energy deficiency. When both glucose and glutamine were absent, intracellular ATP contents and the energy charge remained virtually unaltered.Revisions requested 16 December 2004; Revisions received 24 January 2005     

Stoichiometry,Kinetics, and Regulation of Glucose and Amino Acid Metabolism of a Recombinant BHK Cell Line in Batch and Continuous Cultures

Batch and continuous cultures were carried out to study the stoichiometry, kinetics, and regulation of glucose and amino acid metabolism of a recombinant BHK cell line, with particular attention to the metabolism at low levels of glucose and glutamine. The apparent yields of cells on glucose and glutamine, lactate on glucose, and ammonium on glutamine were all found to change significantly at low residual concentrations of glucose (<5 mmol/L) and glutamine (<1 mmol/L) . The uptake rates of glucose and glutamine were markedly reduced at low concentrations, leading to a more effective utilization of these nutrients for energy metabolism and biosynthesis and reduced formation rates of lactate and ammonium. However, the consumption of other amino acids, especially the essential amino acids leucine, isoleucine, and valine and the nonessential amino acids serine and glutamate, was strongly enhanced at low glutamine concentration. Quantitatively, it was shown that the cellular yields and rates associated with glucose metabolism were primarily determined by the residual glucose concentration, while those associated with glutamine metabolism depended mainly on the residual glutamine. Both experimental results and analysis of the kinetic data with models showed that the glucose metabolism of BHK cells is not affected by glutamine except for a slight influence under glucose limitation and glutaminolysis not by glucose, at least not significantly under the experimental conditions. Compared to hybridoma and other cultured animal cells, the recombinant BHK cell line showed remarkable differences in terms of nutrient sensitivity, stoichiometry, and amino acid metabolism at low levels of nutrients. These cell-line-specific stoichiometry and nutrient needs should be considered when designing an optimal medium and/or feeding strategy for achieving high cell density and high productivity of BHK cells. In this work, a cell density of 1.1 × 10⁷ cells/mL was achieved in a conventional continuous culture by using a proper feed medium.     

Comparative analysis of glucose and glutamine metabolism in transformed mammalian cell lines,insect and primary liver cells

Glucose and glutamine metabolism in several cultured mammalian cell lines (BHK, CHO, and hybridoma cell lines) were investigated by correlating specific utilization and formation rates with specific maximum activities of regulatory enzymes involved in glycolysis and glutaminolysis. Results were compared with data from two insect cell lines and primary liver cells. Flux distribution was measured in a representative mammalian (BHK) and an insect (Spodoptera frugiperda) cell line using radioactive substrates. A high degree of similarity in many aspects of glucose and glutamine metabolism was observed among the cultured mammalian cell lines examined. Specific glucose utilization rates were always close to specific hexokinase activities, indicating that formation of glucose-6-phosphate from glucose (catalyzed by hexokinase) is the rate limiting step of glycolysis. No activity of the key enzymes connecting glycolysis with the tricarboxylic acid cycle, such as pyruvate dehydrogenase, pyruvate carboxylase, and phosphoenolpyruvate carboxykinase, could be detected. Flux distribution in BHK cells showed glycolytic rates very similar to lactate formation rates. No glucose- or pyruvate-derived carbon entered the tricarboxylic acid cycle, indicating that glucose is mainly metabolized via glycolysis and lactate formation. About 8% of utilized glucose was metabolized via the pentose phosphate shunt, while 20 to 30% of utilized glucose followed pathways other than glycolysis, the tricarboxylic acid cycle, or the pentose phosphate shunt. About 18% of utilized glutamine was oxidized, consistent with the notion that glutamine is the major energy source for mammalian cell lines. Mammalian cells cultured in serum-free low-protein medium showed higher utilization rates, flux rates, and enzyme activities than the same cells cultured in serum-supplemented medium. Insect cells oxidized glucose and pyruvate in addition to glutamine. Furthermore, insect cells produced little or no lactate and were able to channel glycolytic intermediates into the tricarboxylic acid cycle. Metabolic profiles of the type presented here for a variety of cell lines may eventually enable one to interfere with the metabolic patterns of cells relevant to biotechnology, with the hope of improving growth rate and/or productivity. © 1996 Wiley-Liss, Inc.     

Influence of different ammonium,lactate and glutamine concentrations on CCO cell growth

In this study the effects of ammonium and lactate on a culture of channel catfish ovary (CCO) cells were examined. We also made investigation on the influence of glutamine, since our previous research revealed that this amino acid stimulated CCO cell growth more than glucose in a concentration-dependent manner. The effect of ammonium in cell culture included the considerable decrease in cell growth rate with eventual growth arrest as well as the retardation of glucose consumption. At ammonium concentrations above 2.5 mM, the cells displayed specific morphological changes. The effect of lactate was different to that of ammonium since the cell growth rate was progressively decreasing with the increase of lactate concentration, whereas the glucose consumption rate remained almost unchanged. Besides that, it was found that lactate was steadily eliminated from the culture medium when its initial concentration was relatively high. The influence of glutamine on CCO cell propagation showed that nutrient requirements of this cell line were mainly dependent on glutamine rather than glucose. The increase in glutamine concentration led to the increase in cell growth rate and consequent ammonia accumulation while the glucose utilization and lactate production were reduced. Without glutamine in culture medium cell growth was arrested. However, the lack of glucose reversed the stimulating effect of glutamine by decreasing cell growth rate and affecting amino acid utilization.     

Reciprocal regulation of glucose and glutamine utilization by cultured human diploid fibroblasts

Human diploid fibroblasts utilize both glucose and glutamine as energy sources. The utilization of glutamine by fibroblasts is regulated by glucose, and vice versa. This conclusion is supported by the following observations: (1) essentially identical growth rates were observed in Eagle's minimum essential medium (MEM)3 in which the glucose concentration was either 5.5 mM or was maintained between 25 and 40 micrometer, (2) the total glutamine utilization by fibroblasts increase at least 30% in medium with 25 micrometer to 70 micrometer glucose compared to medium with 5.5 mM glucose, while the rate of glutamine-1 or 5-14C oxidation to CO2 increased 5-fold as the glucose concentration was decreased to zero, (3) 2 mM glutamine inhibited glucose-6-14C oxidation by 88% and stimulated glucose-1-14C by 77% in log phase cells and (4) glutamine oxidation in normal medium contributed approximately 30% of the energy requirement of human diploid fibroblasts.     

The pentose cycle and insulin release in isolated mouse pancreatic islets during starvation.

When islets from mice were incubated with 16.7 mM-glucose, previous starvation for 48 h decreased the rate of insulin release by approx. 50% and glucose utilization was decreased by approx. 35%. The maximally extractable activity of glucose 6-phosphate dehydrogenase was diminished by 28% after starvation. The formation of 14CO2 from both [1-14C]glucose was, however, higher than the rate of oxidation of [6-14C]-glucose in islets from both fed and starved mice. The fraction of glucose utilized that was oxidized (specific 14CO2 yield) ranged from one-fifth to one-third and was higher in islets from starved mice with both [1-14C]glucose and [6-14C]glucose as substrate. The contribution of pentose-cycle oxidation to total glucose metabolism was small (3% in the fed state and 4% in the starved state). The absolute rates of glucose carbon metabolism via the pentose-cycle oxidation to total glucose metabolism was small (3% in the fed state and 4% in the starved state). The absolute rates of glucose carbon metabolism via the pentose cycle and the turnover of NADPH in this pathway were identical in islets from fed and starved animals. After incubation at 16.7 mM-glucose for 30 min the contents of glucose (6-phosphate and 6-phosphogluconate were both unchanged by starvation. It is concluded that there is no correlation between the decreased sensitivity of the insulin secretory mechanism during starvation and the metabolism of glucose via the pentose cycle, the islet content of glucose 6-phosphate or 6-phosphogluconate.     

Contribution of pH-sensitive metabolic processes to pH homeostasis in isolated rat kidney tubules

The metabolism of isolated rat kidney tubules suspended in calcium-free physiological saline buffered with phosphate was found to be sensitive to changes in the pH of the suspending medium. Lowering the pH from 7.8 to 6.4 brought about increases in the rates of oxidation of added succinate, glutamate or glutamine as well as in the production of glucose from lactate, glutamine, succinate and fructose. The cellular ATP level was also higher in tubules incubated at pH 6.4. In contrast, the utilization of added glucose was greater at pH 7.8 than at pH 6.4, a substantial amount of lactate being produced at the higher pH. When glucose and either lactate or glutamine were provided as co-substrates glucose was the preferred fuel at pH 7.8 but the alternative substrate was the more readily utilized at pH 6.4. As a consequence of the metabolic activities of the tubules the pH of the suspending medium changed, utilization of lactate, glutamate or glutamine causing a rise in pH while conversion of glucose to lactate caused a fall in pH. In cases where two substrates were metabolized concurrently over a period of 3 h the extracellular pH tended towards a plateau level of approximately pH 7.4. It is proposed that pH-sensitive metabolism in isolated kidney tubules contributes to pH homeostasis in the cellular environment.