Concentrations Of UDPG Research Articles

Mechanisms of blood glucose homeostasis

The mechanisms by which glycogen metabolism, glycolysis and gluconeogenesis are controlled in the liver both by hormones and by the concentration of glucose are reviewed. The control of glycogen metabolism occurs by phosphorylation and dephosphorylation of both glycogen phosphorylase and glycogen synthase catalysed by various protein kinases and protein phosphatases. The hormonal effect is to stimulate glycogenolysis by the intermediary of cyclic AMP, which activates directly or indirectly the protein kinases. The glucose effect is to activate the protein phosphatase system; this occurs by the direct binding of glucose to glycogen phosphorylase which is then a better substrate for phosphorylase phosphatase and is inactivated. Since phosphorylase a is a strong inhibitor of synthase phosphatase, its disappearance allows the activation of glycogen synthase and the initiation of glycogen synthesis. When glycogen synthesis is intense, the concentrations of UDPG and of glucose 6-phosphate in the liver decrease, allowing a net glucose uptake by the liver. Glucose uptake is indeed the difference between the activities of glucokinase and glucose 6-phosphatase. Since the Km of the latter enzyme is far above the physiological concentration of its substrate, the decrease in glucose 6-phosphate concentration proportionally reduces its activity. The control of glycolysis and of gluconeogenesis occurs mostly at the level of the interconversion of fructose 6-phosphate and fructose 1,6-bisphosphate under the action of phosphofructokinase 1 and fructose 1,6-bisphosphatase. Fructose 2,6-bisphosphate is a potent stimulator of the first of these two enzymes and an inhibitor of the second. It is formed from fructose 6-phosphate and ATP by phosphofructokinase 2 and hydrolysed by a fructose 2,6-bisphosphatase. These two enzymes are part of a single bifunctional protein which is a substrate for cyclic AMP-dependent protein kinase. Its phosphorylation causes the inactivation of phosphofructokinase 2 and the activation of fructose 2,6-bisphosphatase, resulting in the disappearance of fructose 2,6-bisphosphate. The other major effector of these two enzymes is fructose 6-phosphate, which is the substrate of phosphofructokinase 2 and a potent inhibitor of fructose 2,6-bisphosphatase; these properties allow the formation of fructose 2,6-bisphosphate when the level of glycaemia and secondarily that of fructose 6-phosphate is high.

Anhydrobiosis in nematodes: Control of the synthesis of trehalose during induction

AbstractControl of the synthesis of trehalose during the induction of anhydrobiosis was examined. Kinetic parameters for TPS did not change during the induction, indicating that there was no de novo synthesis of an isozyme. TPS activity doubled in the presence of a mixture of Na+, K+, Mg++, and Ca++ at concentrations normally found in the worms during induction. The concentration of glucose‐6‐phosphate increased from about 2 mM in wet worms to about 12 mM at the end of induction, and the concentration of UDPG increased from about 1 mM in wet worms to about 9 mM at the end of induction. The concentration of UDPG was found to be rate limiting. Trehalase activity decreased by half during the induction period. The significance of these data as related to the control of the synthesis of trehalose was discussed.

PRODUCTION AND UTILIZATION OF GLUCOSE 1-PHOSPHATE IN THE EGGS OF THE SEA URCHIN ANTHOCIDARIS CRASSISPINA.

Concentrations of G1P, G6P, UDPG, UTP and PPi were measured in the eggs of the sea urchin, Anthocidaris crassispina. Activities of phosphorylase a (EC 2.4.1.1), phosphoglucomutase (EC 2.7.5.1), UDPG pyrophosphorylase (EC 2.7.7.9) and pyrophosphatase (EC 3.6.1.1) were also estimated. Levels of G1P and G6P increase following fertilization, but concentrations of UDPG and UTP in unfertilized eggs are very similar to those in fertilized eggs. PPi is undetectable. In unfertilized and fertilized eggs, the G1P level is very low as compared with the G6P level and is far less than that expected from the equilibrium constant in a reaction catalyzed by phosphoglucomutase. Since the phosphoglucomutase activity is higher by about 20 times than the phosphorylase a activity, G1P is probably produced in the reverse reaction, catalyzed by phosphoglucomutase, rather than in the reaction catalyzed by phosphorylase. The G1P thus produced seems to be utilized thoroughly in the reaction catalyzed by UDPG pyrophosphorylase. The reaction seems to be irreversible and tends to go to UDPG production in sea urchin eggs, since the PPi level is negligible due to high pyrophosphatase activity. The utilization of G1P in the reaction catalyzed by UDPG pyrophosphorylase seems to keep the G1P level low.

Hormonal regulation of glycogen metabolism in human fetal liver. II. Regulation of glycogen synthase activity.

During the first two thirds of gestation, the concentrations of UDPG, ATP, ADP, and Mg++ in human fetal liver remain constant, whereas the concentration of Pi decreases twofold and the G-6-P and AMP concentrations increase. Incubation of human fetal liver explants with glucagon or insulin did not alter the concentrations of any of these intermediates. ATP, ADP, and Pi are inhibitors of human fetal liver glycogen synthase D-form activity, while G-6-P and AMP and Mg++ are stimulators. Ca++ at concentrations of less than 0.1 mM was found to stimulate glycogen synthase D activity. This effect of Ca++ was also observed in "physiologic" mixtures containing UDPG, G-6-P, ATP, ADP, AMP, Pi, and Mg++ at concentrations found either in liver in utero or in explants. 45Ca++ efflux from perifused (rat) fetal liver explants was stimulated by glucagon. These data provide a picture of the metabolite regulation of human fetal liver glycogen synthase activity in which the D-form may largely control glycogen synthesis in utero and hormonal effects on glycogen synthase may be induced by effects of Ca++ on the D-form.

β-Glucan synthesis by cell-free extracts from Lolium multiflorum endosperm

1. 1. Particulate fractions from mid-log phase suspension cultures of Lolium multiflorum endosperm incorporate [ 14C]glucose from UDP[U- 14C]glucose into polymeric products and into unidentified compounds soluble in chloroform-methanol and 66% aqueous ethanol. 2. 2. The fraction from the cell homogenate sedimenting at 10 000–250 000 × g was most active in polysaccharide synthesis. 3. 3. Glucose was the only radioactive sugar detected in the high molecular weight product. 4. 4. The product was shown to be a mixture of β-glucans by fractionation on the basis of solubility in water and 1 M NaOH and from the pattern of products released by three β-glucan hydrolases with different substrate specificities for β-1, 3-, β-1, 4- and β-1, 3; 1,4-glucans. 5. 5. The proportion of 1,3- and 1,4-linkages in the total products was shown to vary with the concentration of UDPG. At 1 μM UDPG 90% of the residues in the total product were 1,4-linked, whereas the proportion fell to 49% at 1 mM UDPG. 6. 6. The water soluble polsaccharide which is the predominant product at 100 μM UDPG was examined by enzymic hydrolysis and shown to be a β-1,3;1,4-glucan similar to the glucan present in the Lolium endosperm cell walls. 7. 7. The UDPG concentration in the cells during growth has been measured. It reaches a maximum of 30 μM at mid-log phase.

The glycogen phosphorylase of Tetrahymena pyriformis: II. Inhibition and inactivation by EDTA and ATP and other kinetic properties

Some kinetic properties of an AMP-independent glycogen phosphorylase (EC 2.4.1.1) of Tetrahymena pyriformis have been examined. The K m value for glucose-1-phosphate was about 2.4 m m and was unaffected by AMP. Glucose-6-phosphate did not affect the activity of the enzyme, but UDPG and ATP inhibited, the former being more potent than the latter. When added to reaction mixtures, EDTA inhibited the activity of the enzyme very rapidly. This inhibition could not be reversed by the addition of divalent ions or by the removal of EDTA by dialysis or gel filtration. Preincubation of the enzyme with EDTA or ATP caused an irreversible inactivation. The rate of inactivation was slower than the rate of inhibition mentioned above. ATP was much less effective than EDTA in inactivating the enzyme. The inactivation caused by ATP was proportional to concentration and to time of exposure to ATP but the kinetics of EDTA inactivation were more complex. AMP and structurally related nucleoside monophosphates neither stimulated nor inhibited the activity of Tetrahymena phosphorylase but they could, however, protect the enzyme against the inhibiting effects of ATP and EDTA. There was a good correlation between the ability of nucleoside monophosphates to protect against ATP inactivation and against EDTA inhibition. During storage, however, the sensitivity of the enzyme to EDTA inactivation changed differently from its sensitivity to ATP inactivation, indicating that EDTA and ATP probably inactivated the enzyme by different mechanisms. The relative concentrations of UDPG, ATP, and AMP may play an important regulatory role in vivo. An additional regulatory mechanism, depending on the growth conditions of the organism, is possible since it was shown that phosphorylase isolated from glucose-grown cells was much more resistant to heat inactivation and to inhibition by EDTA and by ATP than the enzyme isolated from cells grown in the absence of glucose.

Incorporation of glucose into an insoluble polyglycoside during oscillatory controlled glycolysis in yeast cells.

In the first few minutes of oscillatory controlled glycolysis yeast cells convert 50% of all glucose taken up into ethanol and glycerol, and incorporate 80–90% of the rest into some in-soluble polyglycoside. Glucose uptake in glycolysing yeast cells is a pulsatory process which is in phase with fruc-tose-1,6-diphosphate synthesis. The concentration of UDPG, an intermediate in polyglucoside synthesis, is oscillating in phase with glucose-6-phosphate. The cells use 80–90% of all ATP synthesized in glycolysis for uptake and storage of glucose.

The stimulation of glycogen synthesis andof glycogen synthetase in the liver by glucocorticoids.

In the N.M.R.I. mouse which has been treated by prednisolone the blood glucose level is only slightly increased and its turnover rate is not changed; the conversion of glucose to liver glycogen is greatly stimulated and the activity of glycogen synthetase in the liver is increased in a similar proportion, at least when it is measured in a concentrated liver homogenate. This activation of the glycogen synthetase also accounts for a simultaneous decrease in the concentration of UDPG and of glucose 6‐phosphate in the liver.

Improved method for measuring galactose-1-phosphate uridyl transferase activity of erythrocytes

Previously described UDPG consumption assays have been modified by increasing the concentration of UDPG, introducing a correction factor for non-linearity, using saline instead of distilled water to produce an optically clear extract of blood (thus obviating turbidity corrections) and by correcting for non-specific reduction of DPN by contaminants in UDPG dehydrogenase, when present. The enzyme has been found to be remarkably stable in red blood cells even when stored at room temperature. A modification of the procedure will yield reliable results on blood samples of as small as 100 μl. Good differentiation has been obtained between heterozygotes for galactosemia, heterozygotes for the Duarte Variant, and normal subjects. Examination of samples from subjects with various hematologic disorders indicates that abnormally high values are obtained in the presence of a reticulocytosis and when red cells are hypochromic.