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At least 1,009 records · Page 56Linked to original sources
PubMed1991

Modeling the dependence of hexose distribution volumes in brain on plasma glucose concentration: implications for estimation of the local 2-deoxyglucose lumped constant.

The steady-state distribution volumes of glucose, 3-O-methylglucose, and 2-deoxyglucose (2DG) are known to change as the concentration of glucose in plasma ranges from hypo- to hyperglycemic values. Model estimates of the three distribution volumes were compared with distribution volume values experimentally measured in the brains of conscious rats as the concentration of glucose in plasma was varied from 2 to 28 mM. The dependence on plasma glucose concentration of the 2DG lumped constant, the factor that relates the phosphorylation rate of 2DG to the net rate of glucose utilization at unit specific radioactivity in the plasma, had been determined previously in separate series of experiments. The model was extended to incorporate this dependence of the lumped constant. In the model both the transport and the phosphorylation barriers were assumed to be single and saturable. The values of their respective half-saturation concentrations and the ratio of the two maximum velocities for glucose were assumed to be invariant over the entire range of plasma glucose concentration. Good agreement between measured and estimated values for the distribution volumes and the lumped constant was attained over the full range of plasma glucose concentration. The model estimates reflected the progressive transport limitation of the brain glucose content as plasma glucose levels were reduced to hypoglycemic values. The results also indicated that these changes should be evident in the time course of 2DG in brain following administration by bolus or continuous infusion, and thus that indexes of local lumped constant change could be derived from the time course data.

3-O-Methylglucose↗
PubMed1966

Formation of glucose from hexoses, pentoses, polyols and related substances in kidney cortex.

1. Slices of rat kidney cortex, on incubation in a saline medium, formed d-glucose from the following substances: d-fructose, d-galactose, d-mannose, l-sorbose, l-arabinose, d-xylose, glycerol, myo-inositol, l-iditol, sorbitol, xylitol, ribitol, methylglyoxal, dihydroxyacetone, l-glyceraldehyde, d-glyceraldehyde, dl-glyceraldehyde, dl-glycerate. Values for the rates of glucose formation from these precursors are given. 2. No glucose was formed from l-rhamnose, d-arabitol, d-arabinose, d-ribose, l-fucose, d-lyxose, mannitol, dulcitol, d-glucuronate, propane-1,2-diol and propan-2-ol. 3. The pathways of glucose formation from the various precursors are discussed (Scheme 1). 4. l-Glyceraldehyde inhibited the formation of glucose from d-glyceraldehyde.

Alcohols↗
PubMed1967

Studies on glycogen synthesis in pigeon liver homogenates. Incorporation of hexose into glycogen.

Liver homogenates of avian species, but not of mammals, form glycogen from glucose, mannose, fructose and galactose. Incorporation of labelled glucose, fructose and mannose, but not of labelled galactose, into glycogen is diluted isotopically by unlabelled glucose. Except for fructose, glycogen formation from other substrates by pigeon liver homogenates compares favourably with that from the same substrates in pigeon liver slices. Optimum conditions for glycogen synthesis from glucose by pigeon liver homogenate are: medium of incubation, 0.175m-sucrose-45mm-potassium chloride-15mm-glycylglycine buffer, pH7.5; concentration of substrate, 15mm; concentration of tissue, less than 120mg./ml.; temperature of incubation, 37-43 degrees ; atmosphere, oxygen. Uncouplers of oxidative phosphorylation, Ca(2+), EDTA, PP(i), 2-deoxyglucose 6-phosphate and microsomal fraction of rat liver are inhibitory to glycogen synthesis from glucose. Starvation of pigeons for 24 and 48hr. leads to a slight stimulation of glycogen synthesis in their liver homogenates as compared with fed controls. Pigeon liver homogenates can be separated into subcellular fractions that on reconstitution can synthesize glycogen. All the enzymes of the glycogen pathway except soluble high-K(m) glucokinase are present in pigeon liver.

Animals↗
PubMed1976

Phosphonomethyl analogues of hexose phosphates.

The analogue of fructose 1,6-bisphosphate in which the phosphate group, -O-PO3H2, on C-6 is replaced by the phosphonomethyl group, -CH2-PO3H2, was made enzymically from the corresponding analogue of 3-phosphoglycerate. It was a substrate for aldolase, which was used to form it, but not for fructose 1,6-bisphosphatase. It was hydrolysed chemically to yield the corresponding analogue of fructose 6-phosphate [i.e. 6-deoxy-6-(phosphonomethyl)-D-fructose, or, more strictly, 6,7-dideoxy-7-phosphono-D-arabino-2-heptulose]. This proved to be a substrate for the sequential actions of glucose 6-phosphate isomerase, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. Thus seven out of the nine enzymes of the glycolytic and pentose phosphate pathways so far tested catalyse the reactions of the phosphonomethyl isosteres of their substrates.

Fructose-Bisphosphatase↗
PubMed1984

Hexose metabolism in pancreatic islets. Inhibition of hexokinase.

In islet homogenates, hexokinase-like activity (Km 0.05 mM; Vmax. 1.5 pmol/min per islet) accounts for the major fraction of glucose phosphorylation. Yet the rate of glycolysis in intact islets incubated at low glucose concentrations (e.g. 1.7 mM) sufficient to saturate hexokinase only represents a minor fraction of the glycolytic rate observed at higher glucose concentrations. This apparent discrepancy between enzymic and metabolic data may be attributable, in part at least, to inhibition of hexokinase in intact islets. Hexokinase, which is present in both islet and purified B-cell homogenates, is indeed inhibited by glucose 6-phosphate (Ki 0.13 mM) and glucose 1,6-bisphosphate (Ki approx. 0.2 mM), but not by fructose 2,6-bisphosphate. In intact islets, the steady-state content of glucose 6-phosphate (0.26-0.79 pmol/islet) and glucose 1,6-bisphosphate (5-48 fmol/islet) increases, in a biphasic manner, at increasing concentrations of extracellular glucose (up to 27.8 mM). From these measurements and the intracellular space of the islets, it was estimated that the rate of glucose phosphorylation as catalysed by hexokinase represents, in intact islets, no more than 12-24% of its value in islet homogenates.

Animals↗
PubMed1991

Hexose metabolism in pancreatic islets. Glucose-induced and Ca(2+)-dependent activation of FAD-glycerophosphate dehydrogenase.

A rise in extracellular D-glucose concentration causes in pancreatic islets a preferential stimulation of aerobic, relative to total, glycolysis. The possible participation in such a phenomenon of a glucose-induced and Ca(2+)-dependent activation of FAD-glycerophosphate dehydrogenase was investigated. In islet homogenates, the activity of the mitochondrial and Ca(2+)-responsive FAD-glycerophosphate dehydrogenase was about two orders of magnitude lower than that of the cytosolic and Ca(2+)-insensitive NAD-glycerophosphate dehydrogenase. In islet mitochondria, Ca2+ increased the affinity of the FAD-glycerophosphate dehydrogenase for L-glycerol 3-phosphate, but did not affect the maximal reaction velocity. In the presence of 0.1 mM-L-glycerol 3-phosphate, the Ka for Ca2+ was close to 0.1 microM. When intact islets were preincubated in the presence of both D-glucose and Ca2+, the activity of FAD-glycerophosphate dehydrogenase measured in intact mitochondria incubated in the presence of 1.2 microM-Ca2+ was higher than that recorded under the same conditions in islets preincubated in the absence of D-glucose and/or Ca2+. These findings support the view that, in islets exposed to a high concentration of D-glucose, a Ca(2+)-induced activation of mitochondrial FAD-glycerophosphate dehydrogenase favours the transfer of reducing equivalents by the glycerol phosphate shuttle, and hence accounts, in part at least, for the preferential stimulation of aerobic glycolysis.

Animals↗
PubMed1999

The detoxification of cumene hydroperoxide by the glutathione system of cultured astroglial cells hinges on hexose availability for the regeneration of NADPH.

The ability of astroglia-rich primary cultures derived from the brains of newborn rats to detoxify exogenously applied cumene hydroperoxide (CHP) was analyzed as a model to study glutathione-mediated peroxide detoxification by astrocytes. Under the conditions used, 200 microM CHP disappeared from the incubation buffer with a half-time of approximately 10 min. The half-time of CHP in the incubation buffer was found strongly elevated (a) in cultures depleted of glutathione by a preincubation with buthionine sulfoximine, an inhibitor of glutathione synthesis, (b) in the presence of mercaptosuccinate, an inhibitor of glutathione peroxidase, and (c) in the absence of glucose, a precursor for the regeneration of NADPH. The involvement of glutathione peroxidase in the clearance of CHP was confirmed by the rapid increase in the level of GSSG after application of CHP. The restoration of the initial high ratio of GSH to GSSG depended on the presence of glucose during the incubation. The high capacity of astroglial cells to clear CHP and to restore the initial ratio of GSH to GSSG was fully maintained when glucose was replaced by mannose. In addition, fructose and galactose at least partially substituted for glucose, whereas exogenous isocitrate and malate were at best marginally able to replace glucose during peroxide detoxification and regeneration of GSH. These results demonstrate that CHP is detoxified rapidly by astroglial cells via the glutathione system. This metabolic process strongly depends on the availability of glucose or mannose as hydride donors for the regeneration of the NADPH that is required for the reduction of GSSG by glutathione reductase.

Animals↗