Glucose Precursors Research Articles

Regulation of heme oxygenase activity in Cyanidium caldarium by light, glucose, and phycobilin precursors

Cyanobacteria, red algae, and cryptophytes contain phycobiliproteins which function as photosynthetic light-harvesting pigments. The chromophores of phycobiliproteins are phycobilins, open-chain tetrapyrroles that are synthesized from protoheme. The first step of phycobilin formation is the conversion of protoheme to biliverdin IX alpha in a reaction that is catalyzed by heme oxygenase. In the unicellular red alga, Cyanidium caldarium, light is required for the accumulation of phycobiliproteins. It has been reported previously that the synthesis of the apoprotein components of allophycocyanin and phycocyanin is induced by light in C. caldarium, that the phycobilin precursors, delta-aminolevulinic acid (ALA), protoporphyrin IX, and protoheme can substitute for light, and that the regulation is exerted at the level of mRNA synthesis. We have determined that a key enzyme of phycobilin formation is induced by light in C. caldarium. Extractable heme oxygenase activity is low in dark-grown cells, and it increases approximately 6-fold during the first 24 h after the cells are illuminated. After 24 h, the activity decreases to a level approximately equal to the initial activity. Heme oxygenase is induced in unilluminated cells by administration of ALA. D-Glucose, which is known to inhibit phycocyanin accumulation in C. caldarium, inhibits the induction of heme oxygenase by light or ALA. Induction of heme oxygenase by light or ALA is blocked by cycloheximide, an inhibitor of cytoplasmic protein synthesis, but not by chloramphenicol, an inhibitor of chloroplast protein synthesis. Rifampicin, an inhibitor of algal chloroplast RNA synthesis, and gabaculine, a competitive inhibitor of ALA biosynthesis, block the induction of heme oxygenase by light but not by ALA. These results indicate that heme oxygenase in C. caldarium is induced by phycobilin precursors. The induction by light and the repression of the induction by D-glucose are probably indirect effects mediated by the effects of light and D-glucose on phycobilin precursor formation. The results also indicate that heme oxygenase is encoded by a nuclear gene and is synthesized on cytoplasmic ribosomes.

Differing patterns of carbohydrate metabolism in liver and muscle

The effect of a period of starvation followed by refeeding on skeletal muscle glycogen was investigated by the use of double-labelled radioactive glucose precursors in rats. Skeletal muscle glycogen, which is not depleted to anything like the extent of liver glycogen, shows a remarkable stability with respect to its overall molecular size distribution during starvation and subsequent refeeding. The experiments also indicate that there is a control mechanism in muscle tissue enabling the synthesis of lysosomal glycogen to be switched off during the initial part of the refeeding process. The results emphasise the inadequacy of the Cori cycle and a modified version is proposed.

Effect of phenylephrine on the compartmentation of inorganic phosphate in perfused rat liver during gluconeogenesis and urea synthesis: a 31P-n.m.r.-spectroscopic study.

The transport of Pi between the cytosol and the mitochondria was investigated in perfused rat liver stimulated with phenylephrine and metabolic precursors of glucose and urea: pyruvate, lactate, NH4+ and ornithine. The relative concentrations of phosphorus metabolites in the liver were measured by 31P-n.m.r. spectroscopy. When added simultaneously, phenylephrine and the precursors induced a decrease in the Pi level which in 4-5 min reached a new steady state at 73% of the control level. After 5 min or more of stimulation the ATP level had also decreased. When the stimulation ended, Pi and ATP returned to their initial levels within 15 min. In mitochondria isolated after 5 min of stimulation, Pi was increased more than 2-fold as compared with control mitochondria and, in addition, an accumulation of Pi from the perfusion buffer into the liver was observed. Phenylephrine by itself did not cause any significant changes in the ATP or Pi levels, whereas the glucose and urea precursors in the absence of phenylephrine induced a 9% decrease in Pi, while ATP remained constant. The Pi content of mitochondria isolated under these conditions was not significantly increased as compared with control mitochondria. These results showed that Pi accumulated into the mitochondria by a mechanism possibly involving exchange for malate, and that a major part of the intramitochondrial Pi was invisible by n.m.r.

MATERNAL DIABETES DOES NOT DECREASE GLUCOSE AND CLYCEROL PRODUCTION IN NORMOCLYCAEMIC NEWDOUNS

Neonatal hypoglycemia is frequently observed following pregnancies complicated by diabetes. This might be a result of decreased glycogenolysis/gluconeogenesis caused by neonatal hyperinsulinemia. The aim of the study was to investigate the capacity for production of glucose and glycerol, a glucose precursor, in infants of diabetic and healthy mothers. Subjects and methods: Eight normoglycaemic, fasting, term infants from diabetes pregnancies (IDDM/GDM) and two control infants were studied at a postnatal age of 4-10 h. The glucose (GPR) and glycerol production rates (GlycPR) were studied by use of 6,62H2-glucose and 2-13C-glycerol. The isotopic enrichments and concentrations of glucose (P-gluc) and glycerol (P-glyc) in plasma were analyzed Isotopic enrichments were measured by gas chromatography/mass spectrometry during periods of steady slate and were used for calculation of production rates. Conclusion: Normoglycaemic infants of diabetic mothers have capacity for glucose and glycerol production, at rates similar to those in control infants.

Regulation of mannitol biosynthesis and degradation by Cryptococcus neoformans.

Cryptococcus neoformans, an encapsulated yeast that is an opportunistic pathogen of AIDS patients, produced and secreted mannitol when incubated with an appropriate carbon source. Glucose, fructose, and mannose were good growth substrates and were converted to mannitol. Maltose and xylose were good growth substrates but were not converted to mannitol. Cells of C. neoformans that were grown on a non-mannitol-generating carbon source, such as peptone or xylose, were able to convert glucose to mannitol only after a prolonged lag period in the presence of glucose. It was concluded that the enzymes of the mannitol biosynthetic pathway were not constitutively expressed but were induced in response to glucose or to a glucose metabolite. Enzymes required to catabolize mannitol, however, were constitutively expressed. The production of mannitol was inhibited by anaerobiosis, by the respiratory poison rotenone, and by polyethylenesulfonate, a specific inhibitor of fungal NADP-dependent dehydrogenases. When cells were incubated with deuterated glucose, the deuterium content of the mannitol produced was much lower than that of the glucose precursor, indicating that the glucose was diluted by an intracellular pool of an intermediate. We had previously shown that C. neoformans contains a large intracellular pool of glucose 6-phosphate, and we now conclude that this pool of glucose 6-phosphate is metabolically active.

Quantitative animal nutrition and metabolism: a general review

Examples of increasing productivity of livestock enterprises over the last three to four decades, and the contribution to such advances, of scientific knowledge including that of the nutrition of farm livestock are referred to. Space limitations neccessitate restricting the review to consideration of ruminant livestock, with particular emphasis on ruminant digestive processes, on milk synthesis and on certain aspects of energy metabolism. Factors affecting the supply of amino acids to the host animal are referred to. The importance of synchronising N and energy supply to the rumen microorganisms to maximize microbial protein synthesis is emphasised and the need for knowledge of the extent to which particular feed proteins escape fermentation within the rumen. Concerning milk synthesis, the importance of an adequate supply of glucose or glucose precursors is mentioned as are the causes of the low milk fat syndrome. Limitations to existing knowledge of amino acid supply and milk protein synthesis are noted. Finally, aspects of ruminant energy metabolism studies are considered; particular stress is given to the importance of energy transactions in the intestinal wall as a major contributor to overall heat increment.

Triiodo-L-thyronine stimulates glycogen synthesis in rat hepatocyte cultures.

The hyperthyroid state is associated with low hepatic glycogen levels, but paradoxically with a high activity of glycogen synthase and low activity of glycogen phosphorylase. We determined the effects of triiodo-L-thyronine (T3) on glycogen synthesis and glycogen synthase activity in rat hepatocytes in vitro. Culture of rat hepatocytes with T3 (100 nM-1 microM) for 16 h-40 h increases glycogen synthesis from glucose and gluconeogenic precursors. The stimulation of glycogen synthesis by T3 was associated with an increase in the activity of glycogen synthase and was additive with the long-term effects of insulin but not with the short-term stimulation of glycogen synthesis by insulin. Culture of hepatocytes with T3 (at concentrations up to 1 microM) did not affect the responsiveness of glycogen synthesis to short-term stimulation by insulin but culture with 10 microM-T3 decreased the responsiveness to insulin without affecting the basal rate. It is suggested that the high activity of glycogen synthase in the hyperthyroid state is due to a direct effect of T3 on the hepatocyte, but the low hepatic glycogen content is probably due to either secondary metabolite and/or endocrine changes or to impaired responsiveness to insulin. T3 may have an anabolic role in the control of hepatic glycogen storage in the euthyroid postprandial state.

Regulation of glucose transporters by connective tissue activating peptide-III isoforms.

Connective tissue activating peptide-III (CTAP-III) is a component of platelet alpha-granules which elicits a series of responses in connective tissue cells referred to as activation, including increased glucose consumption and mitogenesis and increased secretion of hyaluronic acid and glycosaminoglycans. As anticipated by a requirement for glucose or glucose precursors in the activation process, an early event following CTAP-III activation of connective tissue cells is an increase in glucose transport. The present study investigates the molecular basis for this increase in glucose transport. Murine 3T3-F442A fibroblasts were found to respond to CTAP-III in a manner similar to human connective tissue cells (synovial cells, chondrocytes, skin fibroblasts). CTAP-III increases the rate of glucose transport to similar extents at 4 and 24 h, and at physiologic (micrograms/ml) concentrations of CTAP-III. A proteolytic cleavage product of recombinant CTAP-III (rCTAP-III-Leu-21 (des-1-15)), also known as neutrophil-activating peptide-2 (NAP-2), was found to be equally effective as CTAP-III, whereas NAP-1/interleukin-8, another member of the CTAP-III super-family, was ineffective in stimulating glucose transport. This contrasts with neutrophil chemotaxis, in which CTAP-III (des-1-15)/NAP-2 acts similarly to NAP-1/interleukin 8 while CTAP-III is ineffective. CTAP-III appears to elicit a different type of glucose transport response than many other growth factors in that its response is sustained (greater than or equal to 24 h) rather than transient (peak approximately 4 h) in confluent as well as in subconfluent cells. Western blot analysis using antibodies to the GLUT-1 glucose transporter revealed an increased level of GLUT-1 protein in response to CTAP-III isoforms that corresponded in magnitude (on a percentage basis) to the increased level of glucose transport. The increased levels of GLUT-1 protein in response to CTAP-III and rCTAP-III-Leu-21 (des-1-15)/NAP-2 were accompanied by an increase in levels of GLUT-1 mRNA of a magnitude sufficient to account for observed increased levels of GLUT-1. These results are consistent with CTAP-III isoforms stimulating glucose transport in connective tissue cells by increasing levels of GLUT-1 mRNA and is one of the few known instances in which increases in levels of GLUT-1 mRNA and protein are sufficient to account for observed increases in glucose transport. They also provide further evidence that CTAP-III (des-1-15)/NAP-2 binds to more than one type of receptor and that CTAP-III acts in a manner different than other well characterized growth factors (e.g. platelet-derived growth factor, transforming growth factor-beta) in that it causes a sustained (greater than or equal to 24 h) elevation in glucose transport in confluent as well as subconfluent cells.

Pathways for oxidative fuel provision to working muscles: ecological consequences of maximal supply limitations.

The study of metabolic fuel provision and its regulation has reached an exciting stage where specific molecular events can be correlated with parameters of the organism's ecology. This paper examines substrate supply pathways from storage sites to locomotory muscle mitochondria and discusses ecological implications of the limits for maximal flux through these pathways. The relative importance of the different oxidative fuels is shown to depend on aerobic capacity. Very aerobic, endurance-adapted animals such as long distance migrants favor the use of lipids and intramuscular fuels over carbohydrates and circulatory fuels. The hypothesis of functional co-adaptation between oxygen and metabolic fuel supply systems allows us to predict that the capacity of several biochemical processes should be scaled with maximal oxygen consumption. Key enzymes, transmembrane transporter proteins, glucose precursor supply and soluble fatty acid transport proteins must all be geared to support higher maximal glucose and fatty acid fluxes in aerobic than in sedentary species.

Therapy of Diseases of Ruminant Intermediary Metabolism

Diseases of intermediary metabolism include ketosis and fatty liver of dairy cattle and pregnancy toxemia of ewes. These conditions occur when there is a failure of the homeostatic mechanisms regulating the mobilization of fats and the conservation of carbohydrates. The therapeutic approach is to reestablish the normal homeostatic patterns of fuel utilization. Suppression of excessive ketogenesis is the most important factor in reestablishing homeostasis. Ketogenesis can be suppressed by a number of therapeutic agents that act either by suppressing the mobilization of fatty acids or by inhibiting the transport of fatty acids into the hepatic mitochondria, the site at which fatty acids are converted to ketone bodies. Useful therapies include bolus glucose infusions, glucose precursors, and glucocorticoids.

Correlated physiological responses to selection forcarcass lean content in sheep

Correlated responses in physiological traits in lines of Texel-Oxford sheep selected for high or lowcarcass lean content were examined. Serum samples were taken at the end of performance test, 20 weeks of age, from 66 rams when fed ad libitum and every 24 h when fasted for 56 h. The lean line had higher serum concentrations of β-hydroxybutyrate (BHB), non-esterified fatty acids (NEFA) and insulin-like growth factor- 1 (IGF-1) with lower concentrations of triglyceride (TRIG), creatinine (CREA) and UREA, but only the differences in UREA were statistically significant. There were substantial changes in serum concentrations of physiological traits in response to fasting as glucose and IGF-1 decreased while BHB, NEFA, TRIG, CREA and UREA increased. The correlated responses suggested that the lean line preferentially synthesises protein rather than deposits fat during normal feeding. When the animals are fasted, there may be relatively greater use of fat as an energy source in the lean line, rather than using products from protein catabolism as glucose precursors. The accuracy of selection was examined when physiological traits were incorporated into a selectionindex, which included the performance test traits: liveweight, ultrasonic backfat and muscle depths, to predict genetic merit for carcass lean content. UREA may be a useful predictor of genetic merit for lean meat production as it was correlated with estimated carcass lean content and there were substantial differences between the selection lines.

Intermediary Metabolism in Pregnancy: First Theme of the Freinkel Era

During the first half of gestation in the rat, maternal net body weight increases rapidly, whereas in the second half of gestation, the mass of maternal structures declines, coincident with the rate of maternal fat accumulation. Enhanced maternal food intake, extrahepatic tissue lipoprotein lipase (LPL) activity, and adipose tissue lipogenesis are responsible for the progressive accumulation of maternal fat. However, during late gestation, decreased fat synthesis in maternal adipose tissue, enhanced lipolytic activity, and decreased LPL activity deplete maternal fat depots. These changes, plus enhanced endogenous production of triglyceride-rich lipoproteins, are also responsible for maternal hypertriglyceridemia. This condition benefits the offspring in two ways: 1) enhanced LPL activity in maternal liver when fasting increases triglyceride consumption for ketone body synthesis, giving the basis for accelerated starvation; and 2) induction of LPL activity in the mammary gland before parturition diverts maternal circulating triglycerides to milk synthesis in preparation for lactation. The magnitude of the maternal-fetal glucose transfer was higher than that of any of the other substrates studied, including alanine, and despite actions to spare glucose, this transfer causes maternal hypoglycemia, which is especially intense in the fasting condition. This increases sympathoadrenal activity in the mother, which may contribute to her active gluconeogenesis. Glycerol was a more efficient glucose precursor than alanine and pyruvate, and whereas glycerol placental transfer is very small, it is proposed that the fetus benefits from this product of adipose tissue lipolysis when it is previously converted into glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycero-3-phosphate is a precursor of glucose but not of glycero-3-phosphocholine in rat liver

The previous claims by others of a novel choline transfer reaction involving CDP-choline and leading to the conversion of [U- 14C]glycero-3-phosphate to glycero-3-phosphocholine in liver sub-cellular fractions could not be confirmed. Apparent labelling of glycero-3-phosphocholine, isolated by paper chromatography, was, in fact, explained by contamination by other radioactive metabolites, especially d-glucose. A synergistic effect on gluconeogenesis from [U- 14C]glycero-3-phosphate was obtained by incubating well-washed mitochondria with washed microsomes.

Assessment of 18F Gaseous Releases During the Production of 18F-fluorodeoxyglucose

Fluorodeoxyglucose labeled with 18F (18F-FDG) is the most commonly used radiopharmaceutical in positron emission tomography (PET). Fluorine-18-labeled FDG is used as a diagnostic tool in PET studies to monitor the physiology of the brain, diagnose heart function and disease, and to image cancerous tumors. At the University of California, Los Angeles (UCLA), three cyclotrons produce [18F]-fluoride ion using 18O-enriched water targets. Fluorine-18, which has a half-life of 109.8 min, is produced using an 18O(p.n.)18F reaction and is chemically processed to yield 18F-FDG. This study presents data which demonstrate that during the radiochemical processes involved in the production of 18F-FDG, gaseous effluent containing 18F is released. Forty cyclotron production runs with average end of cyclotron bombardment activities of 15.9 +/- 1.88 GBq (430 +/- 50.8 mCi) and end of radiochemical synthesis activities of 5.40 +/- 1.27 GBq (146 +/- 34.3 mCi) yield 18F gaseous effluent releases ranging from 0 to 2560 MBq (0 to 69.2 mCi) with a mean of 437 MBq (11.8 mCi). Temporal correlation of the 18F gaseous releases during 18F-FDG radiochemical production has tied the 18F release to the addition of the glucose precursor (mannotriflate) and ethyl ether in the radiochemical processing. The results are presented in terms of activities released and dilution factors required from the release stack point to maintain controlled (occupational) and uncontrolled (public) area limits in accordance with the recommendations of the International Commission on Radiological Protection and the regulatory requirements of the federal government.

Effects of epinephrine on the net hepatic uptake of lactate, pyruvate, and glycerol in sheep

Epinephrine caused hyperglycemia in part by increasing gluconeogenesis. However, the mechanism of its gluconeogenic effects has not been studied in ruminants. This study was undertaken to examine the effect of epinephrine on the net hepatic uptake of selected glucose precursors in sheep. The major abdominal blood vessels of the sheep were catheterized in normal and alloxan diabetic sheep. Glucose production, metabolic clearance of glucose, and the hepatic removal of certain glucose precursors were determined before, during, and after epinephrine infusion. Epinephrine increased the hepatic glucose output, the concentrations of lactate and glycerol in plasma, and the net hepatic uptake and fractional hepatic extraction of lactate and glycerol. These effects were independent of changes in the concentrations of insulin and glucagon in plasma. These results show that epinephrine directly stimulates hepatic gluconeogenesis in sheep.

Stereoselective synthesis of L‐[4‐13C]carnitine

AbstractThe stereoselective synthesis of L‐[4‐13C]carnitine was achieved in 5 steps. The label was introduced from K13CN into an easily separated diastereomeric pair of 3‐deoxy‐D‐[1‐13C]aldohexoses. Reductive amination of the labeled aldohexose yielded the corresponding D‐1‐(dimethylamino)[1‐13C]alditol which was oxidized in two steps and alkylated with iodomethane to yield L‐[4‐13C]carnitine. The stereochemical integrity at C‐2 of the 3‐deoxy‐D‐[1‐13C]glucose precursor was maintained throughout the synthesis of L‐[4‐13C]carnitine.

Glucose and Gluconeogenic Substrate Exchange by the Forearm Skeletal Muscle in Hyperglycemic and Insulin-Treated Type II Diabetic Patients

To determine the contribution of skeletal muscle to fasting hyperglycemia in noninsulin dependent type II diabetes (NIDDM), the forearm balance of glucose, lactate, and alanine was quantified in 25 control subjects, 21 hyperglycemic (blood glucose: 11.6 mmol/L), and 19 insulin-treated patients with NIDDM (blood glucose: 5.8 mmol/L). Forearm glucose uptake was similar in controls (4.6 +/- 0.6 mumol L-1 min-1) and in hyperglycemic diabetic patients (4.5 +/- 0.9 mumol L-1 min-1). In spite of this, in the diabetic patients lactate (5.1 +/- 0.8 mumol L-1 min-1) and alanine (2.6 +/- 0.4) release by the forearm was 3- and 2-fold higher than in the control group (lactate: 1.7 +/- 0.8, P less than 0.005; and alanine: 1.3 +/- 0.2, P less than 0.05, respectively). The ratio of lactate release to glucose uptake was 57% and 18% in diabetic and control subjects, respectively. Insulin administration did not affect either glucose uptake or the release of gluconeogenic substrates by the forearm. It is concluded that: 1) in fasting patients with NIDDM, glucose is taken up by the skeletal muscle in normal amounts but preferentially used nonoxidatively with lactate formation. This suggests that, although the muscle does not contribute directly to fasting hyperglycemia, it may play an indirect role through an increased delivery of glucose precursors; and 2) insulin-induced normoglycemia is maintained by mechanisms that do not involve the exchange of glucose and gluconeogenic substrates by the skeletal muscle.

Effect of insulin on the utilization of propionate in gluconeogenesis in sheep

The effects of insulin on the utilization of propionate in glucose synthesis were studied in fed and fasted sheep. Insulin was infused at 0.40 microU/h into the mesenteric vein. Glucose was infused to prevent hypoglycaemia. The rate of incorporation of [2-14C]propionate into glucose was determined before and during insulin infusion. After 150 min of insulin infusion endogenous glucose synthesis was about 70% of control values, whereas the incorporation of [14C]propionate into plasma glucose was 94% of control values. In contrast, the incorporation of other glucose precursors into glucose was decreased 30-50% by insulin. Therefore, insulin does not appear to decrease the utilization of propionate in gluconeogenesis. These results are consistent with the proposition that insulin differentially affects the rate of incorporation of glucose precursors into glucose in ruminant animals.

Regulation of glycogen synthesis from glucose and gluconeogenic precursors by insulin in periportal and perivenous rat hepatocytes

Regulation of glycogen synthesis from glucose and gluconeogenic precursors by insulin in periportal and perivenous rat hepatocytes

Regulation of glycogen synthesis from glucose and gluconeogenic precursors by insulin in periportal and perivenous rat hepatocytes

Glycogen synthesis in hepatocyte cultures is dependent on: (1) the nutritional state of the donor rat, (2) the acinar origin of the hepatocytes, (3) the concentrations of glucose and gluconeogenic precursors, and (4) insulin. High concentrations of glucose (15-25 mM) and gluconeogenic precursors (10 mM-lactate and 1 mM-pyruvate) had a synergistic effect on glycogen deposition in both periportal and perivenous hepatocytes. When hepatocytes were challenged with glucose, lactate and pyruvate in the absence of insulin, glycogen was deposited at a linear rate for 2 h and then reached a plateau. However, in the presence of insulin, the initial rate of glycogen deposition was increased (20-40%) and glycogen deposition continued for more than 4 h. Consequently, insulin had a more marked effect on the glycogen accumulated in the cell after 4 h (100-200% increase) than on the initial rate of glycogen deposition. Glycogen accumulation in hepatocyte cultures prepared from rats that were fasted for 24 h and then re-fed for 3 h before liver perfusion was 2-fold higher than in hepatocytes from rats fed ad libitum and 4-fold higher than in hepatocytes from fasted rats. The incorporation of [14C]lactate into glycogen was 2-4-fold higher in periportal than in perivenous hepatocytes in both the absence and the presence of insulin, whereas the incorporation of [14C]glucose into glycogen was similar in periportal and perivenous hepatocytes in the absence of insulin, but higher in perivenous hepatocytes in the presence of insulin. Rates of glycogen deposition in the combined presence of glucose and gluconeogenic precursors were similar in periportal and perivenous hepatocytes, whereas in the presence of glucose alone, rates of glycogen deposition paralleled the incorporation of [14C]glucose into glycogen and were higher in perivenous hepatocytes in the presence of insulin. It is concluded that periportal and perivenous hepatocytes utilize different substrates for glycogen synthesis, but differences between the two cell populations in the relative utilization of glucose and gluconeogenic precursors are dependent on the presence of insulin and on the nutritional state of the rat.