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PubMed1970

Carbohydrate metabolism in Spirochaeta stenostrepta.

The pathways of carbohydrate metabolism in Spirochaeta stenostrepta, a free-living, strictly anaerobic spirochete, were studied. The organism fermented glucose to ethyl alcohol, acetate, lactate, CO(2), and H(2). Assays of enzymatic activities in cell extracts, and determinations of radioactivity distribution in products formed from (14)C-labeled glucose indicated that S. stenostrepta degraded glucose via the Embden-Meyerhof pathway. The spirochete utilized a clostridial-type clastic reaction to metabolize pyruvate to acetyl-coenzyme A, CO(2), and H(2), without production of formate. Acetyl-coenzyme A was converted to ethyl alcohol by nicotinamide adenine dinucleotide-dependent acetaldehyde and alcohol dehydrogenase activities. Phosphotransacetylase and acetate kinase catalyzed the formation of acetate from acetyl-coenzyme A. Hydrogenase and lactate dehydrogenase activities were detected in cell extracts. A rubredoxin was isolated from cell extracts of S. stenostrepta. Preparations of this rubredoxin stimulated acetyl phosphate formation from pyruvate by diethylaminoethyl cellulose-treated extracts of S. stenostrepta, an indication that rubredoxin may participate in pyruvate cleavage by this spirochete. Nutritional studies showed that S. stenostrepta fermented a variety of carbohydrates, but did not ferment amino acids or other organic acids. An unidentified growth factor present in yeast extract was required by the organism. Exogenous supplements of biotin, riboflavin, and vitamin B(12) were either stimulatory or required for growth.

Acetates↗
PubMed1975

Nonequilibration of acetyl units from fatty acids and glucose in mammary glands of lactating mice.

Lipid synthesis by lactating mouse mammary gland slices from [1-carbon14] propionate, [1-carbon14] valerate, and [1-carbon-14] heptanoate, in the presence of glucose and from [uniformly labeled carbon-14] glucose in the presence of propionate, valerate, and heptanoate was studied. Regardless of the labeled substrate, almost all of the radioactivity incorporated into the total lipid fraction was in triglycerides. In experiments with labeled glucose and either the propionic. the valeric, or the heptylic acid, the carbon-14 fatty acids in triglycerides were almost exclusively composed of even-chain fatty acids. When the substrate was either [1-carbon-14] propionate, [1-carbon-14] valerate, or [1-carbon-14] heptanoate, in the presence of glucose, the pattern changed to one in which a larger proportion of the radioactivity was in the odd-chain fatty acids. These findings suggest that in mouse mammary gland, acetyl units produced via beta-oxidation of fatty acids may not equilibrate with those formed from oxidation of pyruvate which is derived from glucose.

Acetates↗
PubMed1978

Fructose, a substrate for energy-yielding metabolism in the heart?

Effects of fructose and glucose on cardiac function and 14CO2 production from radioactive hexoses were studied in isolated working guinea pig hearts perfused at constant filling pressure via the left atrium. When hearts were perfused without exogenous substrates external work declined, indicating exhaustion of endogenous energy sources. It was possible to restore left ventricular function with concentrations of glucose in the physiological range; in order to achieve a similar restoration with fructose as the only exogenous substrate concentrations higher than 40mM were necessary. 14CO2 necessary. 14CO2 production from [U-14C]glucose was half-maximal at about 2mM glucose. On the other hand, 14CO2 production from [U-14C]fructose did not show saturation kinetics and, at concentrations around 2mM, was less than 5% of the rates observed with [U-14C]glucose. Moreover, fructose oxidation was suppressed in the presence of 5mM glucose. The data suggest that fructose cannot serve as a major substrate in the heart under in vivo conditions.

Animals↗
PubMed1980

Electrically induced release of amino acids formed from (U14C) glucose in rat brain cortex slices, studied by a simplified dansylation procedure.

The nonessential amino acids glutamate, aspartate, glutamine, gamma-aminobutyrate (GABA), alanine, glycine, and proline present in rat thin brain cortex slices were labeled by in vitro incubation of these with [U-14C]glucose, and the efflux of such endogenous radioactive amino acids and of lactate was studied in a superfused system, under control conditions or when the slices were depolarized by varous procedures. When electrical stimuli known to induce selective neurotransmitter release (1 or 1.5 volt, sine wave 60 Hz) were applied for 10 sec to the slices, no significant increase in amino acid efflux was found. When more intense stimuli (4 volt, 60 Hz) were applied for 60 sec, or extracellular potassium was raised to 56 mM, both conditions being known to induce nonselective substance release, the efflux of essentially all amino acids and of lactate was markedly increased. Increases in efflux were proportionately larger for glutamate, aspartate, and gamma-aminobutyrate, and this could be accounted for by their greater intracellular chemical (or electrochemical) potentials, but not because of a selective release mechanism for them. Amino acids were analyzed as their 1-dimethylaminoaphthalene-5-sulfonyl (dansyl) derivatives, by a modification of existing procedures in which the dansyl (DNS) derivatives were efficiently extracted from acidified incubation fluid into an organic phase. This rapidly desalted the derivatives and allowed their concentration and chromatographic separation on thin-layer silica gel sheets with little loss.

Amino Acids↗
PubMed1978

The effect of metabolic state on incorportion of [14C] pantothenate into CoA in rat liver and heart.

The effect of metabolic or hormonal status on CoA biosynthesis was studied by comparing the rates of incorporation of [14C]-panthothenate into CoA in fasted and glucose-fed rats. Rat hearts and livers were freeze-clamped 1.5 hours after intravenous injection of [14C] pantothenate. CoA, pantothenate, and other pathway intermediates were separated by chromatography of tissue extracts on DEAE cellulose paper. Compared to the fasted rats, rats forced-fed glucose 0.5 hours before pantothenate injection 69% lower incorporation of radioactivity into CoA in liver, a 69% lower specific radioactivity of liver CoA, a 63% lower specific radioactivity of liver mitochondrial CoA, and a 44% lower incorporation of radioactivity in CoA in heart. The accumulation of labeled pathway intermediates was negligible. The cysteine content of liver was equal for the two conditions. There was no difference in level of unacylated CoASH for fasted and glucose-fed rats, suggesting that hormonal effects on degree of CoA acylation are not involved in this regulator mechanism. The specific radioactivities and concentrations of pantothenate in heart or liver were also nearly equal for the two conditions, so the regulatory mechanism does not involve hormonal effects on pantothenate uptake into tissues. Thus, the effect of glucose-feeding (and related changes of metabolite and insulin-glucagon levels) on the incorporation o[14C] pantothenate into CoA is exerted at a locus on the biosynthetic and/or degradative pathway between pantothenate and CoA.

Animals↗
PubMed1970

Molecular weights and metabolism of rat brain proteins.

1. Rats were injected with [U-(14)C]glucose and after various intervals extracts of whole brain proteins (and in some cases proteins from liver, blood and heart) were prepared by high-speed centrifugation of homogenates in 0.9% sodium chloride or 0.5% sodium deoxycholate. 2. The extracts were subjected to gel filtration on columns of Sephadex G-200 equilibrated with 0.9% sodium chloride or 0.5% sodium deoxycholate. 3. Extracts prepared with both solvents displayed on gel filtration a continuous range of proteins of approximate molecular weights ranging from less than 2x10(4) to more than 8x10(5). 4. The relative amount of the large proteins (mol.wt.>8x10(5)) was conspicuously higher in brain and liver than in blood. 5. At 15min after the injection of [U-(14)C]glucose the smaller protein molecules (mol.wt.<2x10(4)) were significantly radioactive, whereas no (14)C could be detected in the larger (mol.wt.>2x10(4)) protein molecules. The labelling of all protein samples was similar within 4h after injection of [U-(14)C]glucose. Fractionation of brain proteins into distinctly different groups by the methods used in the present work yielded protein samples with a specific radioactivity comparable with that of total brain protein. 6. No evidence could be obtained by the methods used in the present and previous work to indicate the presence of a significant amount of ;metabolically inert protein' in the brain. 7. It is concluded that: (a) most or all of the brain proteins are in a dynamic state of equilibrium between continuous catabolism and anabolism; (b) the continuous conversion of glucose into protein is an important part of the maintenance of this equilibrium and of the homoeostasis of brain proteins in vivo.

Animals↗
PubMed1988

Measuring brain glucose phosphorylation with labeled glucose.

This study tested whether glucose labeled at the C-6 position generates metabolites that leave brain so rapidly that C-6-labeled glucose cannot be used to measure brain glucose phosphorylation (CMRGlc). In pentobarbital-anesthetized rats, the parietal cortex uptake of [14C]glucose labeled in the C-6 position was followed for times ranging from 10 s to 60 min. We subtracted the observed radioactivity from the radioactivity expected with no loss of labeled metabolites from brain by extrapolation of glucose uptake in an initial period when loss was negligible. The observed radioactivity was a monoexponentially declining function of the total radioactivity expected in the absence of metabolite loss. The constant of decline was 0.0077.min-1 for parietal cortex. Metabolites were lost from the beginning of the experiment. However, with correction for the loss of labeled metabolites, it was possible to determine an average CMRGlc between 4 and 60 min of circulation of 64 +/- 4 (SE; n = 49) mumol.hg-1.min-1.

Animals↗
PubMed1993

Effects of intestinal insulin-like peptide on glucose catabolism in mealworm larval fat body in vitro: dependence on extracellular Ca2+ for its stimulatory action.

In vitro hormonally induced variations of glucose catabolism in mealworm fat body tissue were examined by a microradiorespirometric method. An insulin-like peptide (ILP) extracted from the midgut of last larval instar mealworm larvae significantly modified glucose catabolism and was dependent on energy metabolism and on the Ca2+ concentration in the culture medium. Using two different labelled substrate molecules, the stimulatory effects of ILP (compared with those of mammalian insulin) on the relative use of the pentose cycle as opposed to the glycolytic-citric acid cycle by the mealworm fat body were measured in vitro. Metabolic variations were evaluated using either [1-14C]glucose or [6-14C]glucose as substrates. Time course and dose-response curves of ILP and the hormonally induced variations in total CO2 and 14CO2 kinetics were determined. Modification in the specific radioactivity kinetics of 14CO2 derived from [1-14C] glucose and [6-14C]glucose molecules under hormonal effects were observed. As demonstrated in in vivo studies, ILP stimulated the relative utilization of the pentose cycle. However, this effect was observed much more rapidly, but for a shorter time, with fat body in vitro. Mammalian insulin produced similar, but not identical effects. Variations in transmembranous Ca2+ cellular exchanges, induced by either EGTA, nifedipine, or calcium ionophore ionomycin included in the culture medium, indicated that the stimulatory effects of ILP depends on this cation.

Animals↗
PubMed1995

Metabolism of D-[1-3H]glucose, D-[2-3H]glucose, D-[5-3H]glucose, D-[6-3H]glucose and D-[U-14C]glucose by rat and human erythrocytes incubated in the presence of H2O or D2O.

The present study investigates whether heavy water affects the efficiency of 3HOH production from D-[1-3H]glucose, D-[2-3H]glucose, D-[5-3H]glucose and D-[6-3H]glucose relative to the total generation of tritiated metabolites produced by either rat or human erythrocytes. The relative 3HOH yield was close to 95% with D-[5-3H]glucose, 72% with D-[2-3H]glucose, 22-32% with D-[1-3H]glucose, and only 12% with D-[6-3H]glucose. In the latter case, the comparison of the specific radioactivity of intracellular and extracellular acidic metabolites, expressed relative to that of 14C-labelled metabolites produced from D-[U-14C]glucose, indicated that the generation of 3HOH from D-[6-3H]glucose occurs at distal metabolic steps, such as the partial reversion of the pyruvate kinase reaction or the interconversion of pyruvate and L-alanine in the reaction catalysed by glutamate-pyruvate transaminase. As a rule, the substitution of H2O by D2O only caused minor to negligible changes in the relative 3HOH yield. This implies that the unexpectedly high deuteration of 13C-labelled D-glucose metabolites recently documented in erythrocytes exposed to D2O cannot be attributed to any major interference of heavy water with factors regulating both the deuteration and detritiation efficiency, such as the enzyme-to-enzyme tunnelling of specific glycolytic intermediates.

Animals↗
PubMed1995

Role of glycine 1 and lysine 2 in the glycation of bovine gamma B-crystallin. Site-directed mutagenesis of lysine to threonine.

To determine the role of Gly-1 and Lys-2 of bovine gamma B-crystallin in glycation and cross-linking, Lys-2 was changed to Thr by site-directed mutagenesis. A polymerase chain reaction was used to perform site-directed mutagenesis on the third codon (AAG-->ACG) of bovine gamma B-crystallin cDNA. The wild type and mutant cDNAs were cloned into pMON5743 and expressed in JM101 Escherichia coli cells, and the identity of gamma B-crystallin was confirmed by Western blotting after purification by cation exchange high performance liquid chromatography. Glycation of gamma B-crystallin by [14C]glucose was reduced significantly due to the mutation of Lys-2, supporting the view that Lys-2 is a major glycation site. Peptide mapping showed the presence of two major labeled peptides containing N-terminal sequences, and in the mutant these peptides had longer retention times and reduced radioactivity. Amino acid analysis, after glycation with [14C]glucose, revealed N-terminal glycine as the most predominant glycation site. Lys-2 was glycated slower than Gly-1 but faster than Lys-163. Glycation with DL-glyceraldehyde showed an important role for both Gly-1 and Lys-2 in the glycation-mediated gamma B-crystallin cross-linking.

Amino Acid Sequence↗
PubMed1975

Placental transfer of glutamate and its metabolites in the primate.

When radioactive glutamate was infused into pregnant rhesus monkeys, 69 to 88 per cent of radioactivity in the maternal plasma remained in association with glutamate while 10 to 22 per cent was converted to glucose. In the fetal plasma, glucose and lactate accounted for more than 80 per cent of radioactivity, with less than 2 per cent of the label found in glutamate. Maternal glutamate infusions resulting in a ten- to twenty-fold increase in maternal plasma glutamate levels (60 to 100 mumoles per 100 ml.) had no effect upon fetal glutamate levels. Infusions producing maternal glutamate levels 70 times normal (280 mumoles per 100 ml.) did result in some transfer of glutamate to the fetal circulation. Labeled glutamate administered to the fetus at 1.5 to 2.4 Gm. per kilogram of fetal weight did not result in glutamate transfer to the maternal circulation. Infusion of glutamate to the fetus at 5 Gm. per kilogram of fetal weight increased fetal plasma glutamate levels to 2, 000 mumoles per 100 ml. and resulted in some transfer of glutamate to maternal circulation. Glutamate metabolites (lactate and glucose) were readily transferred across the placenta in either direction. These studies indicate that the primate placenta is virtually impermeable to glutamate unless extreme elevations of plasma glutamate are induced.

Amino Acids↗
PubMed1985

The deoxyglucose method adapted for studies of glucose metabolism in the early chick embryo.

14C-2-deoxyglucose (DG), currently employed in in vivo studies of brain glucose metabolism, has been used for determination of glucose consumption in the in vitro developing chick embryo. DG, presented in traces, accumulates in the embryo in proportion with incubation time. Analysis of tissue homogenates shows that the accumulated radioactivity is due to both phosphorylated (DGP) and nonphosphorylated DG. As it is only the radioactivity originating from the DGP that is proportional to glucose utilization, the nonphosphorylated DG must be washed out. The washout shows two distinct kinetics: a fast one corresponding to DG that has entered the cells but has not yet been phosphorylated and a slow one that is probably due to a dephosphorylated DGP coming from a different cellular compartment. On the basis of these results the optimal experimental conditions have been defined, allowing quantitative studies of glucose metabolism during the first day of development of the chicken embryo. From 18 to 24 hr of incubation (end of gastrulation), total glucose consumption increases from 50 nmol X h-1 at stage 3-4 to 90 nmol X h-1 at stage 6-7. This increase mainly reflects the growth of the blastodisc. Comparison with the values of O2 uptake measured at the same period of development suggests that only a fraction of the glucose consumed is oxidized, the major part being converted aerobically to lactate.

Animals↗
PubMed1986

Photoaffinity labeling of the human erythrocyte glucose transporter with 8-azidoadenosine.

8-Azidoadenosine was employed as a possible covalent probe of the erythrocyte nucleoside transporter. 8-Azidoadenosine was shown to enter human erythrocytes by a saturable mechanism (apparent Km for influx 80 microM) that was inhibited by nitrobenzylthioinosine (NBMPR), a potent inhibitor of nucleoside transport, and competitively inhibit uridine influx and NBMPR binding. Irradiation with UV light of human erythrocyte membranes or a partially purified preparation of the nucleoside transporter in the presence of [3H]8-azidoadenosine and dithiothreitol (as a free radical scavenger) resulted in selective covalent incorporation into the band 4.5 region of sodium dodecyl sulfate-polyacrylamide gels (Mr 66,000-45,000). Covalent labeling of band 4.5 was inhibited by adenosine, uridine, and inosine, but NBMPR had no effect. Surprisingly, D-glucose and cytochalasin B, but not L-glucose and cytochalasin E, blocked covalent attachment of the ligand. No incorporation of radioactivity into membranes from rabbit and pig erythrocytes was observed, cells which transport nucleosides rapidly, but have little or no functional glucose carrier. Limited treatment with trypsin of unsealed human erythrocyte membranes photolabeled with [3H]8-azidoadenosine yielded a single radioactive fragment of Mr 19,000, a pattern identical to that obtained with [3H]cytochalasin B-labeled membranes. These results suggest that, despite 8-azidoadenosine being a permeant for the nucleoside transporter, under photoactivation 8-azidoadenosine preferentially labeled the glucose carrier.

Adenosine↗
PubMed1994

Uridine Diphosphate Glucose Metabolism and Callose Synthesis in Cultured Pollen Tubes of Nicotiana alata Link et Otto.

Membrane preparations from cultured pollen tubes of Nicotiana alata Link et Otto contain a Ca2+ -independent (1-3)-[beta]-D-glucan (callose) synthase activity that has a low affinity for UDP-glucose, even when activated by treatment with trypsin (H. Schlupmann, A. Basic, S.M. Read [1993] Planta 191: 470-481). Therefore, we investigated whether UDP-glucose was a likely substrate for callose synthesis in actively growing pollen tubes. Deposition of (1-3)-[beta]-glucan occurred at a constant rate, 1.4 to 1.7 nmol glucose min-1, in tubes from 1 mg of pollen from 3 h after germination; however, the rate of incorporation of radioactivity from exogenous [14C]-sucrose into wall polymers was not constant, but increased until at least 8 h after germination, probably due to decreasing use of internal reserves. UDP-glucose was a prominent ultraviolet-absorbing metabolite in pollen-tube extracts, with 1.6 nmol present in tubes from 1 mg of pollen, giving a calculated cytoplasmic concentration of approximately 3.5 mM. Radioactivity from [14C]-sucrose was rapidly incorporated into sugar monophosphates and UDP-glucose by the growing tubes, consistent with a turnover time for UDP-glucose of less than 1 min; the specific radioactivity of extracted UDP-[14C]glucose was equal to that calculated from the rate of incorporation of [14C]sucrose into wall glucans. Large amounts of less metabolically active neutral sugars were also present. The rate of synthesis of (1-3)-[beta]-glucan by nontrypsin-treated pollen-tube membrane preparations incubated with 3.5 mM UDP-glucose and a [beta]-glucoside activator was slightly greater than the rate of deposition of (1-3)-[beta]-glucan by intact pollen tubes. These data are used to assess the physiological significance of proteolytic activation of pollen-tube callose synthase.

Journal Article↗
PubMed1967

Mechanism of fumaric acid accumulation in Rhizopus nigricans.

It is doubtful that the glyoxylate bypass plays a significant role in the accumulation of fumaric acid by fungi, as has been postulated. In high glucose media, which favor fumarate production, isocitrate lyase (threo-D(s) isocitrate glyoxylate lyase), which is the key enzyme of the glyoxylate bypass, is strongly repressed. The specific activity of this enzyme remains low as long as glucose is present in the medium, even though fumarate formation proceeds at a high level. In addition, the activity of isocitrate lyase is inhibited by phosphoenolpyruvate, which would be formed from glucose. Alternatively, evidence is presented that bulk accumulation of fumaric acid under aerobic conditions in high glucose media takes place through a C(3) plus C(1) carbon dioxide fixation. CO(2) fixation was measured by the direct incorporation of NaHC(14)O(3) into fumaric acid, and by demonstrating that the specific radioactivity of fumaric acid formed from uniformly labeled C(14)-glucose was decreased in the presence of nonradioactive carbonate. The extent of decrease in specific radioactivity is in accord with a C(3) plus C(1) CO(2) fixation mechanism.

Carbon Isotopes↗
PubMed1980

Selective release of non-electrolytes from liposomes upon perturbation of bilayers by temperature change or polyene antibiotics.

A new system for assaying the permeability characteristics of liposomes was established using Amicon cells equipped with a membrane filter (pore size, 0.3 micrometer). In this system, damage of liposomes during the assay procedure was negligible. Changes in permeability to non-electrolytes, such as glucose (Mr 180), sucrose (Mr 342), inulin (Mr 5000) and dextran (Mr 75000), induced by perturbation of the bilayers were examined with this system. The following results were obtained on the barrier properties of multilamellar liposomes modified by various treatments. 1. Amphotericin B and nystatin did not cause any change in permeability to glucose of egg yolk phosphatidylcholine liposomes prepared in physiological saline and containing trace amounts of radioactive markers in their aqueous compartments. Both antibiotics, however, induced nonspecific release of glucose, sucrose, inulin and dextran from liposomes that contained 0.3 M glucose in their aqueous compartments. These antibiotics first seem to form pores through which small ions can permeate; Na+ and Cl- can enter the liposomes through these pores, whereas glucose in the liposomes cannot pass out. As a result, the liposomes become swollen with consequent severe disruption of their membranes. 2. Filipin and digitonin disrupted the membrane structures, resulting in release of large molecules such as dextran even in the absence of an osmotic mechanism. 3. Perturbation of the phase equilibrium by temperature change resulted in formation of 'pores'. The penetration of cations and anions through these 'pores' was apparently much faster than that of glucose, since when liposomes swollen in 0.3 M glucose were incubated in salt solution they were disrupted by an osmotic mechanism releasing not only glucose but also dextran. Most of the 'pores' were not large enough to allow passage of large non-electrolytes, such as inulin and dextran, since no appreciable amounts of these markers were released from liposomes under conditions where there should be no osmotic gradient. 4. At a temperature well above the phase transition temperature, egg yolk phosphatidylcholine liposomes exhibited specific release of glucose. This process did not involve an osmotic gradient, indicating that it was mainly due to diffusion of the solutes through the bilayers.

Amphotericin B↗
PubMed1972

Pentose cycle and reducing equivalents in rat mammary-gland slices.

1. Slices of mammary gland of lactating rats were incubated with glucose labelled uniformly with (14)C and in positions 1, 2, 3 and 6, and with (3)H in all six positions. Glucose carbon atoms are incorporated into CO(2), fatty acids, lipid glycerol, the glucose and galactose moieties of lactose, lactate, soluble amino acids and proteins. C-3 of glucose appears in fatty acids. The incorporation of (3)H into fatty acids is greatest from [3-(3)H]glucose. (3)H from [5-(3)H]glucose appears, apart from in lactose, nearly all in water. 2. The specific radioactivity of the galactose moiety of lactose from [1-(14)C]- and [6-(14)C]-glucose was less, and that from [2-(14)C]- and [3-(14)C]-glucose more, than that of the glucose moiety. There was no randomization of carbon atoms in the glucose moiety, but it was extensive in galactose. 3. The pentose cycle was calculated from (14)C yields in CO(2) and fatty acids, and from the degradation of galactose from [2-(14)C]glucose. A method for the quantitative determination of the contribution of the pentose cycle, from incorporation into fatty acids from [3-(14)C]glucose, is derived. The rate of the reaction catalysed by hexose 6-phosphate isomerase was calculated from the randomization pattern in galactose. 4. Of the utilized glucose, 10-20% is converted into lactose, 20-30% is metabolized via the pentose cycle and the rest is metabolized via the Embden-Meyerhof pathway. About 10-15% of the triose phosphates and pyruvate is derived via the pentose cycle. 5. The pentose cycle is sufficient to provide 80-100% of the NADPH requirement for fatty acid synthesis. 6. The formation of reducing equivalents in the cytoplasm exceeds that required for reductive biosynthesis. About half of the cytoplasmic reducing equivalents are probably transferred into mitochondria. 7. In the Appendix a concise derivation of the randomization of C-1, C-2 and C-3 as a function of the pentose cycle is described.

Animals↗