Peroxynitrite formation from macrophage-derived nitric oxide.

Peroxynitrite formation by rat alveolar macrophages activated with phorbol 12-myristate 13-acetate was assayed by the Cu,Zn superoxide dismutase-catalyzed nitration of 4-hydroxyphenylacetate. The inhibitor of nitric oxide synthesis N-methyl-L-arginine prevented the Cu,Zn superoxide dismutase-catalyzed nitration of 4-hydroxyphenylacetate by stimulated macrophages, while Cu-depleted Zn superoxide dismutase did not catalyze the formation of 3-nitro-4-hydroxyphenylacetate either in vitro or in the presence of activated macrophages. The rate of phenolic nitration by activated macrophages was 9 +/- 2 pmol x 10(6) cells-1 x min-1 (mean +/- STD). Only 8% of synthetic peroxynitrite was trapped by superoxide dismutase, which suggested that the rate of peroxynitrite formation may have been as high as 0.11 nmol x 10(6) cells-1 x min-1. This upper estimate was consistent with N-methyl-L-arginine increasing the amount of superoxide detected with cytochrome c by 0.12 nmol x 10(6) cells-1 x min-1. The rate of nitrite and nitrate accumulation was 0.10 +/- 0.001 nmol x 10(6) cells-1 x min-1, suggesting that the majority of nitric oxide produced by activated macrophages may have been converted to peroxynitrite. The formation of a relatively long lived, strong oxidant from the reaction of nitric oxide and superoxide in activated macrophages may contribute to inflammatory cell-mediated tissue injury.     

The fuels of respiration of the isolated perfused rat kidney

The oxidation of ten substrates: monosaccharides, fatty acids and amino acids, was studied in the isolated perfused rat kidney. Glucose, when offered 3.75 mM, contributed to tissue respiration by a rate equivalent to 18% of the total O₂-consumption of the preparation. The corresponding data for the other nine substrates, each offered in the presence of 3.75 mM glucose, were as follows: pyruvate: 66 %, lactate: 45 %, acetate: 34 %, palmitate: 30 %, glutamate: 25 %, fructose: 18 %, propionate: 12 %, alanine: 10 %, and tyrosine: < 1 %. Under the conditions used less than 2.2 % of the metabolized glucose, pyruvate, lactate and acetate respectively were recovered in the lipid fraction of the kidney, indicating direct oxidation of the respiratory fuels offered and a rather low turnover rate of the endogeneous lipid pool.     

Fatty acids are not an important fuel for coronary microvascular endothelial cells

Summary The metabolism by coronary microvascular endothelial cells (CMEC) of the heart typical substrates palmitate and lactate was compared to that of glucose and glutamine. Confluent cultures of CMEC were used. Palmitate oxidation was saturable and independent of the exogenous albumin concentration. Palmitate, 300 M, lactate, 1 mM, and glutamine, 0.5 mM, were oxidized to 35, 46, and 56 nmol CO₂/h × mg protein. These oxidation rates were decreased by 80, 66, and 48% in presence of 5 mM glucose. The largest energy yield was obtained by glycolytic breakdown of glucose. Glucose, 5 mM, was degraded to lactate by 99%, and oxidized in the Krebs cycle by only 0.04%. 1% was catabolized via the hexose monophosphate pathway. The rate of glucose oxidation in the Krebs cycle could be 30-fold increased by the uncoupler 2,4-dinitrophenol, 30 µM. At concentrations lower than 1 mM the amount of glucose oxidized in the Krebs cycle also grew, indicating existence of the Crabtree effect. The energy demand of CMEC seems to be of the same order as that of the arrested heart.     

Glucose and lactate oxidation rates in the fetal lamb

Both glucose and lactate are nutrients of the ovine fetus. Each may be used by the fetus as a fuel for oxidation or as a source of carbon for energy storage and net tissue accretion. The present report describes the oxidation rates of glucose and lactate in vivo for the fetal lamb over a relatively short time period. The fraction of fetal glucose or lactate oxidized was defined as the ratio of 14CO2 excretion across the umbilical circulation to the net entry of [14C]glucose or [14C]lactate into fetal tissues. The fraction of glucose oxidized over a 3-hr study averaged 61.2%, accounting for 2.55 mg X min-1 X kg-1 of glucose oxidized and for 28% of the simultaneous net oxygen uptake. The fraction of lactate oxidized averaged 71.5%, accounting for 4.12 mg X min-1 X kg-1 of lactate oxidized. Oxidation fractions and rates for both glucose and lactate increased with their concentrations in fetal blood suggesting sparing of other fuels for oxidation at higher glucose and lactate concentrations.     

Support of Xenopus laevis spermatogenesis in vitro by different energy substrates

The support of Xenopus laevis spermatogenesis in vitro by different energy-yielding substrates has been investigated. Isolated spermatogenic cells maintained their levels of adenosine-triphosphate for 24 h in serum-free medium containing only amino acids as energy substrates. DL-Aminocarnitine, an inhibitor of carnitine palmitoyltransferase, reduced cell viability 87% during a 15-h culture in the same medium, indicating that beta oxidation of endogenous fatty acids is a significant source of energy when exogenous substrates are unavailable. Isolated spermatocytes developed into spermatids for 7 days in medium supplemented with either pyruvate, oxaloacetate, or lactate, with maximal survival and development at 0.5 mM pyruvate, 2.0 mM oxaloacetate, and 4.0 mM lactate. Few spermatocytes survived more than 3 days in serum-free medium supplemented with only glucose and amino acids as energy substrates. In contrast, glucose-supplemented medium supported spermatocyte differentiation for 14 days in testis fragment culture and 7 days in spermatocyte-Sertoli cell cocultures due to the excretion of lactate and pyruvate by Xenopus Sertoli cells during culture in glucose-supplemented medium. Glucose also enhanced spermatocyte development in medium containing dialyzed, heat-inactivated fetal calf serum. Spermatogenic cells oxidized glucose to CO2 with C1 oxidized 6- to 7-fold more than C6, suggesting that glucose may be metabolized in the hexose monophosphate shunt. The results are discussed in comparison to energy metabolism in mammalian testes and spermatogenic cells.     

Metabolism of Lactate in the Rat Brain During the Early Neonatal Period

The metabolism of lactate in isolated cells from early neonatal rat brain has been studied. In these circumstances, lactate was mainly oxidized to CO2, although a significant portion was incorporated into lipids (78% sterols, 4% phosphatidylcholine, 2% phosphatidylethanolamine, and 1% phosphatidylserine). The rate of lactate incorporation into CO2 and lipids was higher than those found for glucose and 3-hydroxybutyrate. Lactate strongly inhibited glucose oxidation through the pyruvate dehydrogenase-catalyzed reaction and the tricarboxylic acid cycle while scarcely affecting glucose utilization by the pentose phosphate pathway. Lipogenesis from glucose was strongly inhibited by lactate without relevant changes in the rate of glycerol phosphate synthesis. These results suggest that lactate inhibits glucose utilization at the level of the pyruvate dehydrogenase-catalyzed reaction, which may be a mechanism to spare glucose for glycerol and NADPH synthesis. The effect of 3-hydroxybutyrate inhibiting lactate utilization only at high concentrations of 3-hydroxybutyrate suggests that before ketogenesis becomes active, lactate may be the major fuel for the neonatal brain. (-)-Hydroxycitrate and aminooxyacetate markedly inhibited lipogenesis from lactate, suggesting that the transfer of lactate carbons through the mitochondrial membrane is accomplished by the translocation of both citrate and N-acetylaspartate.     

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.     

Glucose and glutamine utilization by rat lymphocytes, monocytes and neutrophils in culture: a comparative study

Glucose and glutamine utilization and production of glutamate and lactate were determined for up to 48 h in lymphocytes, monocytes and neutrophils cultured in medium rich in metabolites and vitamins. Glucose was utilized by the three cell types in culture in the following order: neutrophils > monocytes > lymphocytes, whereas lactate was produced in the order: monocytes > neutrophils > lymphocytes. The consumption of glucose followed the activity of glucose-6-phosphate dehydrogenase but it was not related to hexokinase activity. Glutamine was consumed by the three leukocyte types in culture as follows: neutrophils > lymphocytes > or = monocytes. The consumption of glutamine was not fully related to the activity of phosphate-dependent glutaminase. The production of glutamate was not remarkably different among the three cell types. For comparison, glutamine and glucose utilization and glutamate and lactate production were also evaluated using 1-h incubated leukocytes. Under this condition, only glucose or glutamine was added to the medium. Glucose was utilized as follows: neutrophils > monocytes > lymphocytes, whereas lactate was produced in the following order: monocytes > or = neutrophils > lymphocytes. Glutamine was consumed as follows: neutrophils > lymphocytes > monocytes, whereas glutamate was produced as follows: neutrophils > or = monocytes = lymphocytes. The ratio of the amount of glucose/glutamine consumed by 1-h incubated cells was 0.5 for neutrophils, 1.5 for monocytes, and 0.3 for lymphocytes. However, the three cell types cultured for 48 h utilized glucose to a much higher degree than glutamine. The ratio of the amount of glucose/glutamine utilized by the cultured cells was 8.9 for neutrophils, 16.4 for monocytes, and 6.7 for lymphocytes. These observations support the proposition that glutamine is required in much higher amounts than glucose to accomplish the total metabolic requirement of leukocytes. Under conditions closer to physiological when the availability of a variety of metabolites and vitamins is not restricted, glucose is the preferred substrate for lymphocytes, monocytes and neutrophils.     

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.     

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.     

Microcalorimetric determination of the kinetics of substrate utilisation by non-growing suspensions of Neisseria sicca

The metabolism of various substrates by non-growing suspensions of Neisseria sicca was investigated by a flow-microcalorimetric technique. Substrate utilisation showed Michaelis kinetics allowing determination of saturation constants (Km) and maximum specific rates of substrate utilisation (Vmax). Pyruvate, lactate, a number of tricarboxylic acid cycle intermediates, and amino acids (aspartate, glutamate and proline) were rapidly metabolised [Vmax 5-35 mumol (g dry wt cells)-1 min-1]; Km values were between 4 and 20 microM. Glucose, glycerol, acetate and the other amino acids investigated gave only a slight or no increase in power. The pattern of substrate utilisation is discussed in relation to the role of carbonic anhydrase in N. sicca.     

Lactate Formation in Primary and Metastatic Colon Cancer Cells at Hypoxia and Normoxia

High glucose consumption and lactate synthesis in aerobic glycolysis are a hallmark of cancer cells. They can form lactate also in glutaminolysis, but it is not clear how oxygen availability affects this process. We studied lactate synthesis at various oxygen levels in human primary (SW480) and metastatic (SW620) colon cancer cells cultured with L‐Ser and/or L‐Asp. Glucose and lactate levels were determined colorimetrically, amino acids by HPLC, expression of AST1‐mRNA and AST2‐mRNA by RT‐PCR. In both lines glucose consumption and lactate synthesis were higher at 10% than at 1% oxygen, and lactate/glucose ratio was increased above 2.0 by L‐Asp. AST1‐mRNA expression was independent on oxygen and cell line, but AST2‐mRNA was lower at hypoxia in SW480. We conclude that, in both cell lines at 1% hypoxia, lactate is formed mainly from glucose but at 10% normoxia also from L‐Asp. At 10% normoxia, lactate synthesis is more pronounced in primary than metastatic colon cancer cells.     

Influx and incorporation into protein of L-phenylalanine in the perfused rat pancreas: effects of amino acid deprivation and carbachol.

The rate of protein synthesis in the isolated perfused rat pancreas was measured from the rate of incorporation of L-[3H]phenylalanine into total protein, and was compared with the transport of this amino acid into the epithelium. Unidirectional (15 s) and net (15-30 min) uptake of L-[3H]phenylalanine was measured relative to D-[14C]mannitol (extracellular marker) using a cell loading technique. The fractional rate of protein synthesis in the pancreas was also measured in vivo using a flooding dose technique and found to be 118 +/- 10% day-1 (corresponding to an absolute rate of incorporation of L-Phe into protein of 36.1 +/- 3 nmol min-1 g-1) in overnight fasted rats. Compared with the in vivo rate, the perfused pancreas exhibited a markedly lower rate of protein synthesis which increased significantly when amino acids were added to the perfusate (15.6 +/- 1.9 vs. 22.5 +/- 0.9% day-1 or 4.7 +/- 0.6 vs. 6.9 +/- 0.3 nmol L-Phe min-1 g-1). Carbachol (3 x 10(-7) M) stimulated protein synthesis provided amino acids were also supplied in the perfusate. Protein synthesis rates measured under all conditions in vivo and in vitro were at least an order of magnitude lower than the unidirectional influx (121 +/- 14 nmol min-1 g-1) of L-phenylalanine into the pancreatic epithelium. These results demonstrate that amino acid transport across the basolateral membrane of the epithelium is not rate-limiting for pancreatic protein synthesis.     

Fuel homeostasis in the harbor seal during submerged swimming

1. The turnover rates and oxidation rates of plasma glucose, lactate, and free fatty acids (FFA) were measured in three harbor seals (average mass = 40 kg) at rest or during voluntary submerged swimming in a water flume at 35% (1.3 m.s-1) and 50% (2 m.s-1) of maximum oxygen consumption (MO2max). 2. For seals resting in water, the total turnover rates for glucose, lactate, and FFA were 23.2, 26.2, and 7.5 mumols.min-1.kg-1, respectively. Direct oxidation of these metabolites accounted for approximately 7%, 27%, and 33% of their turnover and 3%, 7%, and 18% of the total ATP production, respectively. 3. For swimming seals, MO2max was achieved at a drag load equivalent to a speed of 3 m.s-1 and averaged 1.85 mmol O2.min-1.kg-1, which is 9-fold greater than resting metabolism in water at 18 degrees C. 4. At 35% and 50% MO2max, glucose turnover and oxidation rates did not change from resting levels. Glucose oxidation contributed about 1% of the total ATP production during swimming. 5. At 50% MO2max, lactate turnover and anaerobic ATP production doubled, but the steady state plasma lactate concentration remained low at 1.1 mM. Lactate oxidation increased 63% but still contributed only 4% of the total ATP production. Anaerobic metabolism contributed about 1% of the total ATP production at rest and during swimming. 6. The plasma FFA concentration and turnover rate increased only 24% and 37% over resting levels, respectively, at 50% MO2max. However, the oxidation rate increased almost 3.5-fold and accounted for 85% of the turnover.(ABSTRACT TRUNCATED AT 250 WORDS)     

Glutamine and glucose metabolism in bovine blood lymphocytes.

1. Glutamine and glucose metabolism was studied in bovine blood lymphocytes incubated at 37 degrees C in the presence of Krebs-Ringer bicarbonate buffer (pH 7.4) containing 1 mM [U-14C]glutamine and 5 mM [U-14C]glucose, respectively. 2. The major metabolic products from glutamine were ammonia, glutamate, and to a lesser extent, aspartate and CO2. Glucose was metabolized mainly to lactate and, to a lesser extent, pyruvate and CO2. These findings indicate incomplete oxidation of glutamine and glucose carbons in bovine blood lymphocytes. 3. Glucose provided three-fold greater amounts of energy to bovine blood lymphocytes than did glutamine on the basis of their measured end-products. Glycolysis accounted for 50% of glucose-derived ATP production. 4. Our findings suggest similar metabolic patterns of glutamine and glucose in lymphocytes between ruminants and non-ruminant species (e.g. rats). However, in contrast to rat peripheral lymphocytes, glucose, rather than glutamine, was a major energy substrate for bovine blood lymphocytes.     

Energy sources for glutamate neurotransmission in the retina: absence of the aspartate/glutamate carrier produces reliance on glycolysis in glia

The mitochondrial transporter, the aspartate/glutamate carrier (AGC), is a necessary component of the malate/aspartate cycle, which promotes the transfer into mitochondria of reducing equivalents generated in the cytosol during glycolysis. Without transfer of cytosolic reducing equivalents into mitochondria, neither glucose nor lactate can be completely oxidized. In the present study, immunohistochemistry was used to demonstrate the absence of AGC from retinal glia (Müller cells), but its presence in neurons and photoreceptor cells. To determine the influence of the absence of AGC on sources of ATP for glutamate neurotransmission, neurotransmission was estimated in both light- and dark-adapted retinas by measuring flux through the glutamate/glutamine cycle and the effect of light on ATP-generating reactions. Neurotransmission was 80% faster in the dark as expected, because photoreceptors become depolarized in the dark and this depolarization induces release of excitatory glutamate neurotransmitter. Oxidation of [U-14C]glucose, [1-14C]lactate, and [1-14C]pyruvate in light- and dark-adapted excised retinas was estimated by collecting 14CO2. Neither glucose nor lactate oxidation that require participation of the malate/aspartate shuttle increased in the dark, but pyruvate oxidation that does not require the malate/aspartate shuttle increased to 36% in the dark. Aerobic glycolysis was estimated by measuring the rate of lactate appearance. Glycolysis was 37% faster in the dark. It appears that in the retina, ATP consumed during glutamatergic neurotransmission is replenished by ATP generated glycolytically within the retinal Müller cells and that oxidation of glucose within the Müller cells does not occur or occurs only slowly.     

Halothane-Induced Alterations of Glucose and Pyruvate Metabolism in Rat Cerebra Synaptosomes

Synaptosomes isolated from rat cerebra were used to study the effects of the inhalational anesthetic, halothane, on cholinergic processes. To identify possible mechanisms responsible for the depression of acetylcholine synthesis, we examined the effects of halothane on precursor metabolite metabolism involved with supplying the cytosol with acetyl-CoA for acetylcholine synthesis. Three percent halothane/air (vol/vol) depressed 14CO2 evolution from labeled pyruvate and glucose. Steady-state 14CO2 evolution from [1-14C]glucose was depressed 84% by halothane, while 14CO2 evolution from [6-14C]glucose and [3,4-14C]glucose was decreased 67 and 52%, respectively, when compared with control conditions. Halothane inhibited the activities of both pyruvate dehydrogenase (14% depression) and ATP-citrate lyase (32% depression). Total synaptosomal acetyl-CoA concentrations were unaffected by halothane. Three percent halothane/air (vol/vol) caused a 77% increase in medium glucose depletion rate from 1.38 nmol (mg protein)-1 min-1 to 2.44 nmol (mg protein)-1 min-1. Production of lactate by the synaptosomes in the presence of halothane increased by 231% from a control rate of 1.44 nmol (mg protein)-1 min-1 to 4.77 nmol (mg protein)-1 min-1. Lactate production rate from pyruvate was also enhanced by 56% in the presence of halothane. These data lend support to the concept that the NAD+/NADH potential may be involved in the halothane-induced depression of acetylcholine synthesis.     

Comparison of the oxidation of glutamine,glucose, ketone bodies and fatty acids by human diploid fibroblasts

The contribution of glutamine, glucose, ketone bodies and fatty acids to the oxidative energy metabolism of human diploid fibroblasts was studied. The rate of glutamine oxidation by fibroblasts was 98 nmol/h per mg cell protein compared to 2 nmol/h per mg cell protein or less for glucose, acetoacetate, d-3-hydroxybutyrate, octanoic acid and palmitic acid. Glucose inhibited glutamine oxidation by 85%, while the other substrates had no effect. Therefore, these cells meet their energy requirement almost solely by anaerobic glycolysis and glutamine oxidation.     

Ketone body utilization for energy production and lipid synthesis in isolated rat brain capillaries

Isolated brain capillaries from 2-month-old rats were incubated for 2 h in the presence of [3-14C]acetoacetate, D-3-hydroxy[3-14C]butyrate, [U-14C]glucose, [1-14C]acetate or [1-14C]butyrate. Labelled CO2 was collected as an index of oxidative metabolism and incorporation of label precursors into lipids was determined. The rate of CO2 production from glucose was slightly higher than from the other substrates. Interestingly, acetoacetate was oxidized at nearly the same rate as glucose. This shows that ketone bodies could be used as a source of energy by brain capillaries. Radiolabelled substrates were also used for the synthesis of lipids, which was suppressed by the addition of albumin. The incorporation of [U-14C]glucose in total lipids was 10-times higher than that from other precursors. However, glucose labelled almost exclusively the glycerol backbone of phospholipids, especially of phosphatidylcholine. Ketone bodies as well as glucose were incorporated mainly into phospholipids, whereas acetate and butyrate were mainly incorporated into neutral lipids. The contribution to fatty acid synthesis of various substrates was in the following order: butyrate greater than or equal to acetate greater than ketone bodies greater than or equal to glucose. All precursors except glucose were used for sterol synthesis. Glucose produced almost exclusively the glycerol backbone of phospholipids.     

AMINO ACID METABOLISM IN GLIAL CELLS: HOMEOSTATIC REGULATION OF INTRA- and EXTRACELLULAR MILIEU BY C-6 GLIOMA CELLS

Abstract— The amino acid and carbohydrate metabolism of confluent cultures of C-6 glioma cells has been investigated. It was observed that the presence of glutamine in the incubation fluid was essential to maintain high glutamine levels in the cells during a 2 h incubation. When cells were incubated in a cerebrospinal fluid-like medium glutamate, glutamine, aspartate and γ-aminobutyrate (GABA) levels were comparable to those occurring in whole forebrain of adult rat in vivo. Glucose uptake was high, approx 1 μmol/mg protein/2 h, 50% of which was accounted for by lactate production. Of the remaining glucose uptake a substantial proportion was unaccounted for by known oxygen-coupled citric acid cycle flux, or glycogen or amino acid synthesis. Interestingly, the cells released into the medium significant amounts of the neuroinhibitory amino acids, GABA and glycine, and rapidly cleared the medium of the neuroexcitatory amino acids glutamate and aspartate. Metabolism of [2-¹⁴C]glucose and [³H]acetate by the cells indicated rapid labelling of the glutamate and aspartate pools of the cells by glucose in 1 h, but the relative specific activities of glutamine and GABA were much lower. The metabolism of tracer concentrations of [³H]acetate to glutamate by the cells indicated greater dilution of this isotope compared to that of labelled glucose. However, the ratio of ³H to ¹⁴C radioactivity in glutamate and other amino acids was similar to that in the mixture of glucose and acetate added to the medium. Therefore, some active route of acetate metabolism which communicates metabolically with the route of glucose metabolism to glutamate appears to exist in the cells. Significant acetate activation and fatty acid turnover would explain the present results. Some of the amino acid labelling patterns observed in these studies are not consistent with these glial-like cells behaving as models for the small compartment of amino acid metabolism in brain. Enzyme measurements corroborated the metabolic studies. Glutamate decarboxylase activity was 3–10% of the level found in whole brain. GABA transaminase was also low compared to brain as was glutamine synthetase. Glutamate dehydrogenase was present at levels equal to or higher than those of whole brain.