scholarly journals Monosaccharide transport into lactating-rat mammary acini

1982 ◽  
Vol 204 (2) ◽  
pp. 493-501 ◽  
Author(s):  
L C Threadgold ◽  
H G Coore ◽  
N J Kuhn
Keyword(s):  
Plasma Membrane ◽  
Cytochalasin B ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Monosaccharide Transport ◽  
Rat Mammary Gland ◽  
The Mean ◽  
Rate Limiting ◽  
Rate Of Consumption ◽  
Uptake And Release

The uptake and release of 3-O-methyl-D-[3H]glucose at 37 degrees C by acini, prepared from lactating-rat mammary gland with collagenase, was inhibited by glucose, phloretin, cytochalasin B, HgCl2 and low temperature. Uptake and phosphorylation of 2-deoxy-D-[3H]glucose, studied in greater detail, could be ascribed to a specific, saturable, inhibitable, process of apparent Km 16 mM and Vmax. approx. 56 nmol/min per mg of protein, plus a non-specific, non-inhibitable process that was monitored with [14C]fructose. The mean rate of uptake of 5 mM-2-deoxyglucose (16 nmol/min per mg of protein) was similar to the rate of consumption of 5 mM-glucose, suggesting that transport was a rate-limiting step in the overall metabolism of glucose. This accords with evidence for a glucose gradient across the plasma membrane.

scholarly journals Effect of ATP depletion and temperature on the transferrin-mediated uptake and release of iron by BeWo choriocarcinoma cells

1989 ◽  
Vol 259 (3) ◽  
pp. 685-692 ◽  
Author(s):  
A van der Ende ◽  
A du Maine ◽  
A L Schwartz ◽  
G J Strous
Keyword(s):  
Iron Uptake ◽  
Iron Chelator ◽  
Atp Depletion ◽  
Iron Release ◽  
Extracellular Medium ◽  
Bewo Cells ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting ◽  
Uptake And Release

We have recently described the transferrin-mediated uptake and release of iron by BeWo cells [van der Ende, du Maine, Simmons, Schwartz & Strous (1987) J. Biol. Chem. 262, 8910-8916]. We now extend our studies of the mechanisms responsible for uptake and release of iron by these cells. Following preloading, 59Fe release was maximal (about 12%) after about 4 h. Replacement of the extracellular medium with an equal volume of fresh medium either prior to or following the time at which equilibrium was reached further stimulated 59Fe release. Both the rate and maximum amount of iron release decreased if longer loading times were used. Preincubation of BeWo cells for 15 min with 10 mM-sodium cyanide and 50 mM-2-deoxyglucose prior to the determination of 59Fe release did not alter the amount released into medium (which did not contain a high-affinity iron chelator). However, under these conditions, the uptake of 59Fe was dramatically inhibited as a result of prolongation of the transferrin-transferrin-receptor complex recycling time. These results demonstrate that the release of iron from BeWo cells is independent of cellular ATP levels, whereas iron uptake is ATP-dependent. Rates of both 59Fe release and 59Fe uptake were temperature-dependent. Analysis of these data via an Arrhenius plot suggests a single rate-limiting step for the release and uptake processes between 0 and 37 degrees C. The apparent energies of activation of these processes are very similar (approx. 59.0 kJ/mol for iron release and 50.6 kJ/mol for iron uptake), which raises the possibility that the release and uptake of iron share a common thermodynamically rate-limiting step. Possible mechanisms involved in iron release out of the cell and out of the endosome are discussed.

scholarly journals Use of a genetic variant to study the hexose transport properties of human skin fibroblasts

1990 ◽  
Vol 265 (3) ◽  
pp. 823-829 ◽  
Author(s):  
O T Mesmer ◽  
B A Gordon ◽  
C A Rupar ◽  
T C Y Lo
Keyword(s):  
Human Skin ◽  
Transport System ◽  
Cytochalasin B ◽  
Transport Systems ◽  
Hexose Transport ◽  
Glucose Starvation ◽  
Human Skin Fibroblasts ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting

Human skin fibroblasts from ‘normal’ subjects were found to possess at least two hexose transport systems. One system was responsible for the uptake of 2-deoxy-D-glucose (dGlc), D-glucose and D-galactose, whereas the other was responsible primarily for the uptake of 3-O-methyl-D-glucose (MeGlc). The transport of dGlc was the rate-limiting step in the uptake process; over 97% of the internalized dGlc was phosphorylated and the specific activity of hexokinase was several times higher than that for dGlc transport. The dGlc transport system was activated by glucose starvation, and was very sensitive to inhibition by cytochalasin B and energy uncouplers. Fibroblasts isolated from a patient with symptoms of hypoglycaemia were found to differ from their normal counterparts in the dGlc transport system. They exhibited a much higher transport affinity for dGlc, D-glucose and D-galactose, with no change in the respective transport capacity. Transport was not the rate-limiting step in dGlc uptake by these cells. Moreover, the patient's dGlc transport system was no longer sensitive to inhibition by cytochalasin B and energy uncouplers. This suggested that the intrinsic properties of the patient's dGlc transport system were altered. It should be noted that the patient's dGlc transport system could still be activated by glucose starvation. Despite the changes in the dGlc transport system, the MeGlc transport system in the patient's fibroblasts remained unaltered. The observed difference in the properties of the two hexose transport systems in the ‘normal’ and the patient's fibroblasts strongly suggests that the two transport systems may be coded or regulated by different genes. The present finding provides the first genetic evidence from naturally occurring fibroblasts indicating the presence of two different hexose transport systems.

scholarly journals Passage rate and total clearance rate from the rumen of cows fed on grass silages differing in cell-wall content

1995 ◽  
Vol 73 (1) ◽  
pp. 41-49 ◽  
Author(s):  
Marlou W. Bosch ◽  
Marianne Bruining
Keyword(s):  
Organic Matter ◽  
Crude Protein ◽  
Growth Stages ◽  
Passage Rate ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
The Mean ◽  
Voluntary Intake ◽  
Rate Limiting ◽  
Different Growth Stages

Four non-lactating, rumen-fistulated cows were fed ad lib. on two grass silages (first cut (FC) and second cut (SC)) harvested at different growth stages, resulting in different crude-protein (CP) and neutral-detergent-fibre (NDF) contents (FC, 152 g CP/kg, 515g NDF/kg and SC, 210 g CP/kg, 442 g NDF/kg). Voluntary intake and rumen contents, total as well as organic matter were higher for silage FC. Fractional passage rate from the rumen, calculated from the logarithmic decline in Cr-NDF rumen pool, was higher for silage FC (0·0395/h and 0·0446/h for silages SC and FC respectively). When fractional passage rates from the rumen were calculated by dividing the intake of indigestible organic matter by the mean rumen poo! of this fraction, the same differences between silages were found, although the actual levels were much lower (0·0258$sol;h and 0·0300/h for silages SC and FC respectively). The results from the present experiment suggest that disappearance rate from the rumen of particles with a size between 1·25 and 0·071 mm is the rate-limiting step in the control of rumen fill.

Transport of fluorescein in trichomes of Lycopersicon esculentum

1982 ◽  
Vol 60 (4) ◽  
pp. 397-402 ◽  
Author(s):  
Gregor F. Barclay ◽  
Carol A. Peterson ◽  
Melvin T. Tyree
Keyword(s):  
Lycopersicon Esculentum ◽  
Cytoplasmic Streaming ◽  
Cytochalasin B ◽  
The Other ◽  
Square Root ◽  
Inhibiting Effect ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Other Hand ◽  
Rate Limiting

Translocation of the dye disodium fluorescein (uranin) in trichomes of Lycopersicon esculentum (tomato) was nonpolar and proportional to the square root of time. Inhibition of cytoplasmic streaming by cytochalasin B had no effect on the rate of dye movement. On the other hand, disruption of plasmodesmatal connections between adjacent cells by plasmolysis strongly diminished the rate of fluorescein translocation. Subsequent deplasmolysis of the cells did not remove the inhibiting effect of plasmolysis. The data are consistent with the interpretation that dye movement proceeds by diffusion, the rate-limiting step being transport through plasmodesmatal connections.

Product Release is Not the Rate-Limiting Step during Cytochalasin B-Induced ATPase Activity of Monomeric Actin

1991 ◽  
Vol 46 (1-2) ◽  
pp. 139-144 ◽  
Author(s):  
Peter Dancker ◽  
Lore Hess ◽  
Karl Ritter
Keyword(s):  
Steady State ◽  
Atpase Activity ◽  
Rate Constant ◽  
Cytochalasin B ◽  
Single Turnover ◽  
Rate Limiting Step ◽  
Hydrolysis Products ◽  
Limiting Step ◽  
Rate Limiting ◽  
Hydrolysis Of

Abstract Under conditions where cytochalasin B induces ATPase activity of monomeric actin (0.3 mᴍ MgCl2, 1 mᴍ EGTA , 30 (μᴍ cytochalasin B, 1 mᴍ ATP) the rate constant of the ex­change of actin-bound ε-ATP for free ATP is about 4 -6 times faster than steady state ATPase activity. When a stoichiometric ATP -actin complex is extracted with PCA (single turnover ex­periment) the apparent rate constant of Pi generation is not faster than steady state ATPase activity. -The experiments suggest that the hydrolysis of actin-bound ATP and not the subse­quent release of hydrolysis products is rate-limiting during cytochalasin-induced ATPase activi­ty of actin.

Ornithine Decarboxylase Activity in Cultured Endothelial Cells Stimulated by Serum, Thrombin and Serotonin

1978 ◽  
Vol 39 (02) ◽  
pp. 496-503 ◽  
Author(s):  
P A D’Amore ◽  
H B Hechtman ◽  
D Shepro
Keyword(s):  
Endothelial Cells ◽  
Ornithine Decarboxylase ◽  
Decarboxylase Activity ◽  
Aortic Endothelial Cells ◽  
Ornithine Decarboxylase Activity ◽  
Rate Limiting Step ◽  
Bovine Aortic Endothelial Cells ◽  
Limiting Step ◽  
Rate Limiting ◽  
Serum Serotonin

SummaryOrnithine decarboxylase (ODC) activity, the rate-limiting step in the synthesis of polyamines, can be demonstrated in cultured, bovine, aortic endothelial cells (EC). Serum, serotonin and thrombin produce a rise in ODC activity. The serotonin-induced ODC activity is significantly blocked by imipramine (10-5 M) or Lilly 11 0140 (10-6M). Preincubation of EC with these blockers together almost completely depresses the 5-HT-stimulated ODC activity. These observations suggest a manner by which platelets may maintain EC structural and metabolic soundness.

Dynamics of hepatic and peripheral insulin effects suggest common rate-limiting step in vivo

Diabetes ◽  
1993 ◽  
Vol 42 (2) ◽  
pp. 296-306 ◽  
Author(s):  
D. C. Bradley ◽  
R. A. Poulin ◽  
R. N. Bergman
Keyword(s):  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting
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