Glucose metabolism during osmotic stress in isolated axons of Eriocheir sinensis

Isolated axons of $\text{Eriocheir sinensis}$ show high ratios of ¹⁴CO₂ production from glucose-1-¹⁴C to ¹⁴CO₂ production from glucose-6-¹⁴C (ratio C₁/C₆). During osmotic stresses, there is a modification in ¹⁴CO production from glucose-6-¹⁴C as well as in the ratio C₁/C₆ while ¹⁴CO₂ production from glucose-1-¹⁴C does not change significantly. These results are interpreted in terms of activity of oxidative and non-oxidative pathways of glucose metabolism.     

No contribution of the pentose phosphate pathway in dormancy-breaking of cocklebur seeds

The role of the oxidative pentose phosphate (PP) pathway in the dormancy-breaking of cocklebur (Xanthium pennsylvanicum Wallr.) seeds was investigated. D-[1-¹⁴C]-glucose or D-[6-¹⁴C]-glucose was fed to dormant and non-dormant lower seeds or to their axial or cotyledonary segments which were imbibed for different durations, and C₆/C₁ ratios of respired ¹⁴CO₂ as an index of the PP pathway activity were calculated. Contrary to expectation, there was no significant difference in the C₆/C₁ ratios between the dormant and non-dormant seeds or segments during a water imbition period of 24 h, although the PP pathway actually operated already in an early stage of water imbibition. Also concerning the activities of G6PDH and 6PGDH, the key enzymes of this pathway, no difference between the dormant and non-dormant seeds was found. It was thus concluded that, unlike other seeds, there is no contribution of the PP pathway to the regulation of dormancy of the cocklebur seed.     

Activity of the pentose phosphate pathway during lignification

Summary We did this work to see if there is a correlation between lignin synthesis and the activity of the pentose phosphate pathway. Excision of the third internode of the stem of Coleus blumei Benth. followed by incubation on sucrose and indoleacetic acid led to extensive formation of tracheids. During this lignification we determined the activities of glucose-6-phosphate dehydrogenase and fructose-1,6-diphosphate aldolase, and the extent to which [1-¹⁴C]-,[3,4-¹⁴C]-, and [6-¹⁴C]glucose labelled CO₂ and the major cellular components. The results indicate that the pentose phosphate pathway was active during lignification, and that the activity of this pathway relative to glycolysis increased at the onset of lignification. Explants of storage tissue of Helianthus tuberosus L. were cultured under conditions which caused extensive lignification. ¹⁴CO₂ production from [1-¹⁴C]-, [3,4-¹⁴C]-, and [6-¹⁴C]glucose indicated activity of the pentose phosphate pathway during tracheid formation. We suggest that lignification is accompanied by appreciable activity of the pentose phosphate pathway and that this could provide the reducing power for lignin synthesis.Abbreviations NADP nicotinamide-adenine dinucleotide phosphate - IAA indoleacetic acid     

The Metabolism of Carbon-14 Specifically Labelled Glucose in Leaves Showing Tobacco Mosaic Virus Induced Local Necrotic Lesions

Glucose metabolism of healthy and tobacco mosaic virus-infected leaf-discs of Nicotiana tabocum L. var. Xanthi showing local-necrotic lesions was investigated using glucose-¹⁴C. Local lesion formation following inoculation with tobacco mosaic virus resulted in enhanced glucose metabolism reflected by an increased rate of release of ¹⁴CO₂ from glucose-U-¹⁴C and greater incorporation of ¹⁴C into all cell fractions. When specifically labelled glucose was fed to healthy and tobacco mosaic virus infected leaves, the C₆/C₁ ratio (rate of release of ¹⁴CO₂ from glucose-6-¹⁴C/rate of release of ¹⁴CO₂ from glucose-l-¹⁴C) was similar for healthy and virus-infected leaves. The C₆/C₁ ratios recorded from 0.30 to 0.50 indicate that both the glycolytic and pentose phosphate pathways participate in glucose catobolism in healthy and virus-infected leaves. Although the C₆/C₁ ratio was the same as that of the healthy leaf the rate of release of ¹⁴CO₂ from glucose-6-¹⁴C and glucose-1-¹⁴C was greatly increased in the virus-infected leaf. The increased glucose catabolism occurs by both glycolytic and pentose phosphate pathways in the virus-infected leaf.     

Randomization of carbon atoms in the glucose molecule and changes of specific radioactivity of14CO2 liberated by the callus tissue ofDaucus carota L. from glucose-6- and -1-14C

On incubation of the callus tissue ofDaucus carota L. in solutions of glucose-6-¹⁴C and -1-¹⁴C the distribution of radioactivity in the molecule of endogenous glucose will change and the ratio of activities of liberated¹⁴CO₂ (C₆/C₁) will rise The limits of possible changes of specific activity of¹⁴CO₂ and of the C₆/C₁ ratio were calculated with respect to the observed randomization and it was shown that the mutual exchange of carbon atoms in the molecules is not the decisive cause of the rise of the ratio. The specific radioactivity of¹⁴C in CO₂ is as much as 12 times higher than that of endogenous glucose and fructose and about twice as high as the theoretical maximum. This might indicate that in addition to the cytoplasmic fraction of glucose the callus cells contain a fraction of low metabolic activity, most likely in the vacuoles, that could account for some of the increase of the C₆/C₁ value. The main reason for the changes in the C₆/C₁ ratio is envisaged in the establishment of isotopic equilibrium between the pentose cycle and glycolysis and other metabolic systems, in particular via triose phosphates, the radioactivity of which can greatly affect the C₆/C₁ ratio, as was shown in a model experiment.     

Pathways of carbohydrate oxidation in leaves of Pisum sativum and Triticum aestivum

The aim of this work was to establish the pathways of carbohydrate oxidation used in the dark by leaves of Pisum sativum and Triticum aestivum. Segments of young and mature leaves of pea released the carbons of glucose-[¹⁴C] as ¹⁴CO₂ in the order 3,4 > 1 > 2 > 6 whereas in segments of young and mature leaves of wheat the order was 3,4 > 1 > 6 > 2. The detailed labelling of the constituents of mature leaves of wheat by glucose-[1-¹⁴C], -[2-¹⁴C], -[3,4-¹⁴C], and -[6-¹⁴C] was determined and showed that the high yield of CO₂ from C-6 relative to that from C-2 was due to release of C-6 during pentan synthesis. Estimates were made of the maximum catalytic activities of phosphofructokinase and glucose-6-phosphate dehydrogenase in pea and wheat leaves of three ages. The results of all the above investigations strongly indicate that both pea and wheat leaves in the dark oxidize carbohydrate via glycolysis and the pentose phosphate pathway with the latter accounting for no more than a third of the total. No evidence was obtained of any major change in the relative activities of the two pathways during the development of either type of leaf.     

Alterations in carbohydrate metabolism of γ-irradiated cavendish banana

γ-Irradiation of preclimacteric banana resulted in a gradual increase in fructose content, which reached a maximum in 6 days. Although the catabolism of glucose-U-¹⁴C was less in irradiated banana, incorporation of label into fructose was high. Initial fructose accumulation in irradiated banana may be due to a shift in glucose utilization from the glycolytic to the pentose phosphate pathway. The ratio of resporatory CO₂ from glucose-6-¹⁴C and glucose-1-¹⁴C was halved in irradiated bananas indicating predominance of the pentose phosphate pathway. The radioactivity of fructose derived from glucose-6-¹⁴C was almost twice that from glucose-1-¹⁴C in irradiated bananas, whilst in control both fruit the labelled precursors yielded equal amounts. Studies on individual enzymes in these two pathways showed an increase in phosphorylase, phosphoglucomutase, glucose-6-phosphate dehydrogenase and fructose-6-phosphatase and a decrease in hexokinase in irradiated banana.     

Isotopic studies of carbohydrate metabolism during basidiospore germination in Schizophyllum commune

Summary The metabolism and fate of specifically labeled glucose-¹⁴C were compared to mannitol-l-¹⁴C and arabitol-l-¹⁴C during basidiospore germination of Schizophyllum commune on glucose-asparagine minimal broth. Glucose-l-¹⁴C metabolism led to more ¹⁴CO₂ evolution than glucose-6-¹⁴C in spores and the former activity increased upon germination. Liberation of ¹⁴CO₂ from glucose-3,4-¹⁴C increased at 8 h to 12 h of germination and exceeded the amount of radioactive ¹⁴CO₂ released from glucose-1-¹⁴C. The ¹⁴CO₂ released from glucose-2-¹⁴C increased continually during germination while only minor changes in ¹⁴CO₂ evolution occurred with glucose-6-¹⁴C. Unlabeled ethanol (0.25 M) inhibited ¹⁴CO₂ evolution with glucose-3,4-¹⁴C and ungerminated spores and this inhibition disappeared upon germination.More ¹⁴CO₂ was evolved from labeled glucose during germination and less radioactivity became associated with cellular material. Of the latter, alcohol-soluble extracts of spores or germlings contained mainly radioactive trehalose, less mannitol and little or no labeled arabitol, and this decreased upon germination. Germlings also converted more radioactive glucose-¹⁴C into KOH-insoluble material and KOH-soluble components. Spores or germlings converted arabitol-1-¹⁴C primarily into trehalose and this was not the case for mannitol-1-¹⁴C.     

THE HEXOSE MONOPHOSPHATE PATHWAY IN ETHYLNITROSOUREA INDUCED TUMORS OF THE NERVOUS SYSTEM

Respiration studies in vitro, in which tissue slices were incubated with [1-¹⁴C]glucose or [6-¹⁴C]glucose and ¹⁴CO₂ collected, resulted in C-1/C-6 ¹⁴CO₂ ratios that were higher in slices of tumor and newborn brain than in slices of adult brain. In adult brain, the C-1/C-6 ¹⁴CO₂ ratio averaged close to unity. In slices of tumor and newborn brain however, the mean C-1/C-6 ratio was greater than three. Addition of phenazine methosulfate (PMS) increased conversion of [1-¹⁴C]glucose to ¹⁴CO₂ in slices of normal adult brain 5-fold, and in slices of newborn brain and tumor, approx 12-fold. Injection of animals with 6-aminonicotinamide (6-AN) decreased conversion of [1-¹⁴C]glucose in slices of normal brain 30% but decreased conversion in tumor slices by 80%. Evidence supports the presence of an active hexose monophosphate pathway (HMP) in tumors of the nervous system regulated in part by available NADP⁺ levels. Inhibition by 6-AN was more effective in tumors than in normal adult brain.     

Alteration by dinitrocresol of pathways for glucose oxidation in eggs of arbacia punctulata

1. The C¹⁴O₂ production by Arbacia eggs and embryos from glucose-1-C¹⁴, glucose-2-C¹⁴, and glucose-6-C¹⁴ has been measured without and with dinitrocresol in the incubation medium. In the absence of the dinitrocresol, the C¹⁴O₂ production from glucose-1-C¹⁴ is more rapid than from glucose-2-C¹⁴ and much more rapid than from glucose-6-C¹⁴; this, together with previous findings, indicates that glucose is utilized in Arbacia eggs predominantly via the TPN shunt rather than via the aldolase step of the glycolytic pathway. In the presence of the dinitrocresol, C¹⁴O₂ from glucose-6-C¹⁴ approaches that from glucose-1-C¹⁴, indicating that, in the presence of this reagent, glucose utilization is diverted from the shunt to the glycolytic pathway. 2. Incorporation of C¹⁴ from glucose labelled in the 1-, 2-, or 6- positions into other metabolic products of the eggs and embryos is also inhibited by dinitrocresol, particularly incorporation into the acid-insoluble fraction containing nucleoproteins.     

Reductant for glutamate synthase in generated by the oxidative pentose phosphate pathway in non-photosynthetic root plastids

Purified pea root plastids were supplied with glutamine, 2-oxoglutarate and phosphorylated sugars. Formation of glutamate was linear for 75 min and dependent upon the intactness of the organelle. Glucose-6-phosphate and ribose-5-phosphate were the most effective substrates in supporting glutamate synthesis. Flux through the oxidative pentose phosphate pathway during glutamate synthesis in purified plastids was followed by monitoring the release of ¹⁴CO₂ from [1-¹⁴C]glucose-6-phosphate. ¹⁴CO₂ evolution from C-1 was dependent upon the presence of both glutamine and 2-oxoglutarate and could be inhibited by the application of azaserine. The data are discussed in view of the role of the oxidative pentose phosphate pathway in non-photosynthetic plastids.     

Hexose monophosphate pathway in synapses

Abstract— Synaptosomes isolated from rat cerebral cortex converted [l-¹⁴C]glucose more rapidly than [6-²⁴C]glucose to ^,14CO₂. The ratio of C-l: C-6 in ¹⁴CO₂ was 3-9, thus suggesting that the hexose monophosphate shunt (HMP) pathway was functional in synapses in vitro. When changes in the ratio of C-l: C-6 in ¹⁴CO₂ were used as an index of shunt activity, glucose oxidation by this route was stimulated by electron acceptors as well as by neurohormones, including norepinephrine, acetylcholine and serotonin. Brain mince also exhibited a C-l: C-6 ratio of 3-2 when short (15 min) incubations were employed. Negative results previously reported are attributable to prolonged incubation during which depletion of NADP or randomization of the labelled carbons in radioactive glucose could have occurred. Our experiments excluded the incorporation of glucose into macromolecules as a specific role for the hexose monophosphate pathway. The generation of NADPH for numerous metabolic reactions including the maintenance of membrane SH groups and the oxidation and hydroxylation reactions may represent the functions of the hexose monophosphate in synaptosomes and account for its stimulation by neurohormones.     

Female dimorphism in honey-bee larvae with respect to glucose metabolism

The rate of conversion of glucose-1-¹⁴C and glucose-6-¹⁴C to ¹⁴CO₂ and lipid was monitored in queen and worker larvae between 48 and 96 hr of age. A definite dimorphism in glucose metabolism between castes was established. Worker larvae 72 hr of age have a much greater $\text{C}{}_6\text{C}{}_1$ ratio than do queen larvae of the same age. The ratios were of the same order of magnitude for both castes of larvae younger or older than 72 hr. Queen larvae were shown to have a greater rate of lipid synthesis than worker larvae.     

Dynamic aspects of radiorespirometry during the cell cycle of Candida utilis: Some observations in phased culture under carbon-, nitrogen-, and phosphorus-limited growth

Many changes that occur in a cell during the cell cycle can be demonstrated in synchronous cultures and can reveal dimensions of cell metabolism not attainable by the study of balanced growth of asynchronous populations in batch cultures or the steady state in chemostat cultures. The release of ¹⁴CO₂ from specifically labeled glucose by phased (continuously synchronized) cultures follows a characteristic pattern (profile) that depends upon the stage in the cell cycle and the period of labeling used. Successive profiles throughout a cycle showed differences that were altered under different nutrient-limiting growth conditions. Profiles obtained with glucose-1-¹⁴C, glucose-2-¹⁴C, glucose-3,4-¹⁴C, and glucose-6-¹⁴C and phased cells of Candida utilis under N-, P-, and C-limited growth demonstrated the variable character of the metabolic activity that occurred in the cells while contour changes within the profiles across the cycle indicated possible correlations with activities of the hexose monophosphate, Embden-Meyerhof-Parnas, and tricarboxylic acid cycle pathways during the cell cycle. The basis of these changes and their use as elementary parameters for study of problems of physiological changes in vivo are considered.     

METABOLITES AND ENZYMES OF THE PENTOSE PHOSPHATE PATHWAY IN ISOLATED NERVE ENDINGS1

Abstract— The activities of each enzyme associated with the pentose phosphate pathway as well as the non-enzymatic intermediates in this pathway were measured in synaptosomes isolated from rat cerebral cortex. The specific activities of transketolase (EC 2.2.1.1) and transaldolase (EC 2.2.1.2) were significantly lower in synaptosomes than cerebral cortex; however, the specific activities of glucose-6-phosphate dehydrogenase (EC 1.1.1.49), 6-phosphogluconate dehydrogenase (EC 1.1.1.44), ribosephosphate isomerase (EC 5.3.1.6) and ribulosephosphate epimerase (EC 5.1.3.1.) were comparable in homogenates of synaptosomal fractions and cerebral cortex. Concentrations of most intermediates of the pentose pathway were also similar in extracts of synaptosomes and brain homogenates. Six hours after treatment of rats with the nicotinamide analog, 6-aminonicotinamide (6-AN), 6-phosphogluconate levels in synaptosomes were increased 5-fold; however, glucose-6-phosphate levels remained unchanged. During a 30 min in uitro incubation 6-phosphogluconate levels increased approx 2-fold in synaptosomes obtained from 6-AN treated rats but did not change in synaptosomes from untreated rats. During the same period glucose-6-phosphate levels decreased in synaptosomes from both control and 6-AN treated rats. The conversion of both [1-¹⁴C]glucose and [6-¹⁴C]glucose to ¹⁴CO₂ was depressed in synaptosomes from 6-AN treated rats; however, the ratio of the two isotopes converted to ¹⁴CO₂ was essentially the same. It is concluded that the pentose phosphate pathway is active in nerve endings both in vivo and in vitro.     

Photosynthetic carbon metabolism during leaf ontogeny in Zea mays L.: Enzyme studies

The activities of several enzymes, including ribulose-1,5-diphosphate (RuDP) carboxylase (EC 4.1.1.39) and phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) were measured as a function of leaf age in Z. mays. Mature leaf tissue had a RuDP-carboxylase activity of 296.7 mol CO₂ g^-1 fresh weight h^-1 and a PEP-carboxylase activity of 660.6 mol CO₂ g^-1 fresh weight h^-1. In young corn leaves the activity of the two enzymes was 11 and 29%, respectively, of the mature leaves. In senescent leaf tissue, RuDP carboxylase activity declined more rapidly than that of any of the other enzymes assayed. On a relative basis the activities of NADP malic enzyme (EC 1.1.1.40), aspartate (EC 2.6.1.1) and alanine aminotransferase (EC 2.6.1.2), and NAD malate dehydrogenase (EC 1.1.1.37) exceeded those of both PEP and RuDP carboxylase in young and senescent leaf tissue. Pulse-chase labeling experiments with mature and senescent leaf tissue show that the predominant C₄ acid differs between the two leaf ages. Labeling of alanine in senescent tissue never exceeded 4% of the total ¹⁴C remaining during the chase period, while in mature leaf tissue alanine accounted for 20% of the total after 60 s in ¹²CO₂. The activity of RuDP carboxylase during leaf ontogeny in Z. mays parallels the development of the activity of this enzyme in C₃ plants.Abbreviations RuDP ribulose-1,5-diphosphate - PEP phosphoenol pyruvate - PGA 3-phosphoglycerate     

Respiration of 14CO2 by intact animals of various species given l-[1-14C]fucose or d-[1-14C]arabinose

The production of ¹⁴CO₂ from l-[1-¹⁴C]fucose and d-[1-¹⁴C]arabinose has been studied in five mammalian species.Cats, guinea pigs, mice, and rabbits respired about 22% of the label of l[1-¹⁴C]fucose or of d-[1-¹⁴C]arabinose within 6 h after intraperitoneal injection of the sugar. Rats respired only 1.5% of the l-fucose label and 5% of the d-arabinose label in the same time period.Liver homogenates from cat, guinea pig, and rabbit produced significantly more ¹⁴CO₂ from l-[1-¹⁴C]fucose or d-[1-¹⁴C]arabinose than mouse or rat liver homogenates. Unlike those of the other species, guinea pig liver homogenates had very low l-fucose dehydrogenase activity.The results suggest that substantial catabolism of l-fucose and d-arabinose occurs in the tissues of some animal species. Investigators wishing to employ l-fucose as a tracer of glycoprotein metabolism must, therefore, ensure that the species that they employ does not metabolize l-fucose to products interfering with their studies.     

Regulation of pathways of glucose metabolism in kidney. The effect of experimental diabetes on the activity of the pentose phosphate pathway and the glucuronate-xylulose pathway.

Measurements have been made of the activities of enzymes of the pentose phosphate pathway, the glucuronate-xylulose pathway, hexokinase and phosphofructokinase in kidney of diabetic and normal rats. The activities of these enzymes keep pace with kidney growth, remaining constant per gram tissue but showing a marked increase on the basis of total activity per 100 g body wt. The formation of ¹⁴CO₂ from [1-¹⁴C]glucose and [6-¹⁴C]glucose by kidney slices from diabetic rats was decreased to approximately half the control value; evidence was obtained for an equivalent dilution of the glucose 6-phosphate pool. Correction of the ¹⁴CO₂ yields for the change in specific activity of glucose 6-phosphate yielded values consistent with the enzyme profile. Calculations from specific yields of ¹⁴CO₂ provided evidence for an increased flux of glucose via the pentose phosphate pathway in the kidney in diabetes. The results are discussed in relation to kidney hypertrophy in diabetes and the requirement for ribose 5-phosphate and NADPH for biosynthetic reactions and in relation to the thickening of the basement membrane in diabetes. These results are in accord with the concept of glucose overutilization by non-insulin-requiring tissues.     

Relationship between the pentose-phosphate pathway and the de novo synthesis of fatty acids and cholesterol in oligodendrocyte-enriched glial cultures

This study focuses on the activity of the pentose-phosphate pathway and its relationship to de novo synthesis of fatty acids and cholesterol in oligodendrocyte-enriched glial cell cultures derived from 1-week old rat brain. The proportion of glucose that was metabolized along the pentose-phosphate pathway was estimated by measuring ¹⁴CO₂ production from [1-¹⁴C]-, [2-¹⁴C]- and [6-¹⁴C]glucose, the utilization of glucose and the production of lactate. Incorporation of ¹⁴C from [¹⁴C]glucose and from [3-¹⁴C]acetoacetate into lipids was analysed. The pentose- phosphate pathway produced much more CO₂ from glucose than the Krebs cycle, although it accounted for only a small part of the consumption of glucose (< 3%). The higher ¹⁴CO₂ production from [2-¹⁴C]glucose than from [6-¹⁴C]glucose indicated that recycling of the products of the pentose-phosphate pathway takes place in these cells.Gradual inhibition of the pathway with increasing concentrations of 6-aminonicotinamide resulted in a parallel inhibition of the conversion of acetoacetate and of glucose into fatty acids and into cholesterol. Glycolysis was also strongly inhibited in the presence of 6-aminonicotinamide whereas the activity of the Krebs cycle was not affected.These results suggest that de novo synthesis of fatty acids and cholesterol by oligodendrocytes of neonatal rats is closely geared to the activity of the pentose-phosphate pathway in these cells.