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At least 1,423 records · Page 79Linked to original sources
PubMed1985

Hexose monophosphate shunt and cholesterol synthesis in the diabetic and fasting states.

The livers of streptozotocin-induced diabetic and fasted rats showed a decreased cholesterol synthesis measured by in vitro incorporation of [2-14C]acetate. A significant decrease of glucose-6-phosphate dehydrogenase (G-6-PD), 6-phosphogluconate dehydrogenase (6-PGD), and pyruvate kinase (PK) was also observed 7 days after administration of streptozotocin. These enzymatic activities were also low in livers of 72 hr fasted animals. An increase of glucose-6-phosphatase (G-6-Pase) was observed consistently in diabetic as well as in fasted rats. Suitable amounts of insulin and refeeding normalized the alterated enzymatic activities in diabetic and in fasted animals, respectively.

Animals↗
PubMed1983

Hexose transport in microvascular endothelial cells cultured from bovine retina.

The nature of glucose transport at the microvascular blood-retinal barrier was studied using primary cultures of microvascular endothelial cells from bovine retina. Uptake of 3-O-methyl-D-glucose (3MG), a non-metabolizable glucose analogue, was rapid and equilibrative. 3MG uptake could be inhibited by traditional glucose transport inhibitors such as phloretin, phlorizin and cytochalasin B but not by agents that deplete intracellular ATP (2,4-dinitrophenol) or that abolish the sodium gradient (ouabain). Uptake of 3MG by the cells could be stimulated by preloading with 50 mM-glucose, a phenomenon known as counter-transport. Insulin had no effect on 3MG uptake in the cells even after prolonged insulin deprivation. These results demonstrate the existence of a facilitative-diffusion type of glucose transport system in endothelial cells of the retinal microvasculature. Studies with 2-deoxy-D-glucose demonstrated that transport was not rate limiting for metabolism and that the endothelial cells contain free sugar when exposed to an extracellular sugar concentration in the physiologic range. The presence of this free sugar within the cell is necessary for efficient transendothelial transfer of glucose from blood to retina but it could also provide the basis for capillary damage in diabetes mellitus.

3-O-Methylglucose↗
PubMed1983

Non-oxidative synthesis of pentose 5-phosphate from hexose 6-phosphate and triose phosphate by the L-type pentose pathway.

1. Ribose 5-phosphate was non-oxidatively synthesized from glucose 6-phosphate and triose phosphate by an enzyme extract prepared from rat liver (RLEP). Analysis of the intermediates by GLC, ion-exchange chromatography and specific enzymatic analysis, revealed the presence of the following intermediates of the L-type pentose pathway: altro-heptulose 1,7-bisphosphate, arabinose 5-phosphate and D-glycero D-ido octulose 8-phosphate. 2. With either [1-14C] or [2-14C]glucose 6-phosphate as diagnostic substrates, the distribution of 14C in ribose 5-phosphate was determined. At early time intervals (0.5-8 hr), [1-14C]glucose 6-phosphate introduced 14C into C-1, C-3 and C-5 of ribose 5-phosphate, at 17 hr 14C was confined to C-1. With [2-14C]glucose 6-phosphate as substrate, 14C was confined to C-2, C-3 and C-5 of ribose 5-phosphate during early times (0.5-8 hr), while at 17 hr 14C was located in C-2. 3. The transketolase exchange reaction, [14C]ribose 5-phosphate + altro-heptulose 7-phosphate in equilibrium ribose 5-phosphate + [14C]altro-heptulose 7-phosphate, was demonstrated for the first time using purified transketolase, its activity was measured and it is proposed to play a major role in the relocation of 14C into C-3 and C-5 or ribose 5-phosphate during the prediction labelling experiments. 4. The coupled transketolase-transaldolase reactions, 2 fructose 6-phosphate in equilibrium altro-heptulose 7-phosphate + xylulose 5-phosphate and 2 altro-heptulose 7-phosphate in equilibrium fructose 6-phosphate + D-glycero D-altro octulose 8-phosphate were demonstrated with purified enzymes, but are concluded to play a minor role in the non-oxidative synthesis of pentose 5-phosphate and octulose phosphate by (RLEP). 5. The formation of gem diol and dimers of erythrose 4-phosphate is proposed to account in part for the failure to detect monomeric erythrose 4-phosphate in the carbon balance studies. 6. The equilibrium value for the pentose pathway acting by the reverse mode in vitro was measured and contrasted with the value for the pathway acting in the forward direction. The initial specific rates of the pentose pathway reactions in vitro for the reverse and forward directions are measured. 7. The study which includes carbon balance, time course changes and 14C prediction labelling experiments reports a comprehensive investigation of the mechanism of the pentose pathway acting reversibly.

Animals↗
PubMed1986

Variations of fructose-2,6-diphosphate levels in cultured HT29 human colon cancer cells: influence of hexoses and lactate concentrations.

Under the standard conditions of culture, Fru-2,6-P2 level in HT29 cells is transitorily increased as a consequence of medium change; the peak value occurs after 2 hr, followed by a gradual return to a basal value (40 pmol/mg protein) which is maintained as long as medium glucose concentration stands above 2 mM. A 20 hr glucose deprivation lowers Fru-2,6-P2 level to trace value, but, when glucose is reintroduced, the peak value is much higher; large Fru-2,6-P2 accumulation is correlated with higher rates of glucose uptake and lactate release, which suggests an activation of glycolysis at the level of phosphofructokinase-1. Fru-2,6-P2 level depends on the glucose concentration within the range of 0 to 5 mM. At this concentration and above, maximal effect is reached. Previous glucose deprivation renders the Fru-2,6-P2 forming system more sensitive to glucose. When given instead of glucose, fructose enters the glycolytic pathway and produces same effect as glucose on the Fru-2,6-P2 level. Galactose turns it to almost zero which coincides with low glycolytic rate. Acidity of the culture medium favorishes the Fru-2,6-P2 formation; however, change in pH cannot explain the variations of Fru-2,6-P2 level observed under the standard culture conditions. Lactate concentrations over 10 mM in the medium are found to significantly inhibit the Fru-2,6-P2 producing system. Therefore, lactate accumulation in the medium could be an important factor controlling Fru-2,6-P2 level during standard cell culture.

Adenocarcinoma↗