On the number of steady states in a multiple futile cycle

The multisite phosphorylation-dephosphorylation cycle is a motif repeatedly used in cell signaling. This motif itself can generate a variety of dynamic behaviors like bistability and ultrasensitivity without direct positive feedbacks. In this paper, we study the number of positive steady states of a general multisite phosphorylation–dephosphorylation cycle, and how the number of positive steady states varies by changing the biological parameters. We show analytically that (1) for some parameter ranges, there are at least n + 1 (if n is even) or n (if n is odd) steady states; (2) there never are more than 2n − 1 steady states (in particular, this implies that for n = 2, including single levels of MAPK cascades, there are at most three steady states); (3) for parameters near the standard Michaelis–Menten quasi-steady state conditions, there are at most n + 1 steady states; and (4) for parameters far from the standard Michaelis–Menten quasi-steady state conditions, there is at most one steady state.      

The use of 6-labeled glucose to assess futile cycling in Escherichia coli

To assess the "futile cycle" fructose-6-P leads to fructose-1,6-P2 leads to fructose-6-P in Escherichia coli we have grown the cells on [6-14C]glucose and determined label in the 1-position of glucose obtained from glycogen. In a variety of strains, including a wild type and a mutant without fructose diphosphatase, 1-position labeling was negligible. But there was little label in the 1-position of fructose-1,6-P2 either, which shows that hexose diphosphate and triose-P are not in equilibrium in this organism. Therefore, the lack of 1-position labeling in glycogen does not necessarily indicate lack of futile cycling. One strain, however, a temperature-sensitive glyceraldehyde-3-P dehydrogenase mutant grown at permissive temperature, gave substantial labeling of the 1-position of fructose-1,6-P2. In this strain 1-position labeling in glycogen was low, indicating minimal futile cycling.     

微量元素对糖尿病大鼠糖脂代谢的影响

目的观察微量元素铬对糖尿病大鼠糖脂代谢的影响。方法选糖尿病大鼠经灌胃给予有机铬水溶液治疗12周后,分别观察口服有机铬200μg/d及400μg/d的糖尿病大鼠空腹血糖及血脂水平(血清总胆固醇、甘油三酯、低密度脂蛋白和高密度脂蛋白)。实验分为4组:1组为正常对照组;2组为铬200μg/d组;3组为铬400μg/d组;4组为糖尿病对照组。结果有机铬具有明显降低血糖、血清总胆固醇、低密度脂蛋白和甘油三酯及升高高密度脂蛋白的作用(P0.05~P0.01)。结论有机铬能明显改善糖尿病大鼠的糖脂代谢。     

Stimulation of glucose catabolism in Escherichia coli by a potential futile cycle.

Fifteen-fold overexpression of phosphoenolpyruvate synthase (Pps) (EC 2.7.9.2) in Escherichia coli stimulated oxygen consumption in glucose minimal medium. A further increase in Pps overexpression to 30-fold stimulated glucose consumption by approximately 2-fold and resulted in an increased excretion of pyruvate and acetate. Insertion of two codons at the PvuII site in the pps gene abolished the enzymatic activity and eliminated the above-described effects. Both the active and the inactive proteins were detected at the predicted molecular weight by polyacrylamide gel electrophoresis. Therefore, the observed physiological changes were due to the activity of Pps. The higher specific rates of consumption of oxygen and glucose indicate a potential futile cycle between phosphoenolpyruvate (PEP) and pyruvate. A model for the stimulation of glucose uptake is presented; it involves an increased PEP/pyruvate ratio caused by the overexpressed Pps activity, leading to a stimulation of the PEP:sugar phosphotransferase system.     

Hexose metabolism in pancreatic islets. Insignificance of D-glucose futile cycling in rat islets.

When rat pancreatic islets were incubated in the presence of unlabelled D-glucose (16.7 mM) and 3HOH, the production of 3H-labelled material susceptible to be phosphorylated by yeast hexokinase and then detritiated by yeast phosphoglucoisomerase did not exceed 2.66 +/- 0.21 pmol/islet per 180 min, i.e. about 1% of the rate of exogenous D-[5-3H]glucose utilization. Such a material accounted for 43 +/- 4% of the total radioactivity, associated with tritiated hexose(s). It is proposed, therefore, that the futile cycling of D-glucose in the reactions catalyzed in the islet cells by the hexokinase isoenzymes and glucose-6-phosphatase represents a negligible fraction of the total rate of D-glucose phosphorylation.     

Effects of training on glucose metabolism of gluconeogenesis-inhibited short-term-fasted rats

To evaluate the effects of endurance training on gluconeogenesis and blood glucose homeostasis, trained as well as untrained short-term-fasted rats were injected with mercaptopicolinic acid (MPA), a gluconeogenic inhibitor, or the injection vehicle. Glucose kinetics were assessed by primed-continuous venous infusion of [U-14C]- and [6-3H]glucose at rest and during submaximal exercise at 13.4 m/min on level grade. Arterial blood was sampled for the determination of blood glucose and lactate concentrations and specific activities. In resting untrained sham-injected rats, blood glucose and lactate were 7.6 +/- 0.2 and 1.3 +/- 0.1 mM, respectively; glucose rate of appearance (Ra) was 71.1 +/- 12.1 mumol.kg-1.min-1. MPA treatment lowered blood glucose, raised lactate, and decreased glucose Ra. Trained animals had significantly higher glucose Ra at rest and during exercise. At rest, trained MPA-treated rats had lower blood glucose, higher blood lactate, and similar glucose Ra and disappearance rates (Rd) than trained sham-injected animals. Exercising sham-injected untrained animals had increased blood glucose and glucose Ra compared with rest. Exercising trained sham-injected rats had increased blood glucose and glucose Ra and Rd but no change in blood lactate compared with untrained sham-injected animals. In the trained animals during exercise, MPA treatment increased blood lactate and decreased blood glucose and glucose Ra and Rd. There was no measurable glucose recycling in trained or untrained MPA-treated animals either at rest or during submaximal exercise. There was no difference in running time to exhaustion between trained and untrained MPA-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of deoxycorticosterone acetate on glucose metabolism in nondiabetic and streptozotocin-diabetic rats.

A previous study in our laboratory showed that streptozotocin (STZ) induced diabetic, deoxycorticosterone acetate (DOCA) induced hypertensive rats exhibited significantly lower levels of plasma glucose than did normotensive diabetic animals. The present experiments further investigate the effects of DOCA treatment on fasting levels of plasma glucose and insulin and on their changes after oral glucose challenge in nondiabetic and STZ-diabetic rats. It was found that, in nondiabetic rats, DOCA-induced hypertension was associated with normal glucose levels and glucose tolerance but with significantly lower levels of plasma insulin. DOCA-treated diabetic animals showed significantly lower levels of plasma glucose, but their plasma insulin concentrations were not significantly different from those of the DOCA vehicle treated diabetic rats. DOCA-treated diabetic rats also had significantly higher plasma levels of cholesterol and triglycerides. It is suggested that DOCA may have a direct or indirect action on the assimilation, production, or utilization of glucose, perhaps leading to an improvement in insulin sensitivity and subsequently a decrease in insulin secretion.     

Effects of parenteral palatinose on glucose metabolism in normal and streptozotocin diabetic rats

The present experiment was carried out to investigate the metabolism of palatinose (6-O-alpha-D-glucopyranosyl-D-fructose) in the rat. The bolus injection of palatinose (0.5 g/kg) in the tail vein of normal and streptozotocin (STZ) diabetic rats caused significant increments in glucose and insulin concentrations. However, in severe STZ diabetic rats (greater than 300 mg/dl of fasting plasma glucose) no significant change in glucose and insulin concentrations was observed. In liver perfusion, the gradual decrease in glucose output from the normal and mild STZ diabetic rat livers perfused with 20 mM Krebs-Ringer-Tris buffer pH 7.4 was prevented by the addition of 5.5 mM palatinose in the perfusate and fructose was detected in the effluent during the palatinose infusion. The results indicate that palatinose is metabolized to glucose and fructose in both normal and diabetic rat tissues, and this causes the increase in blood glucose concentration. On the other hand, the direct stimulatory effect of insulin release from pancreatic B-cell was not observed when the palatinose was infused into the isolated perfused rat pancreas. The study suggest that palatinose administered parenterally is metabolized by tissues and expected to be used as a source of fluid and energy supply.     

Effects of high-fat diet and exercise training on intracellular glucose metabolism in rats

We examined the effects of high-fat diet (HFD) and exercise training on insulin-stimulated whole body glucose fluxes and several key steps of glucose metabolism in skeletal muscle. Rats were maintained for 3 wk on either low-fat (LFD) or high-fat diet with or without exercise training (swimming for 3 h per day). After the 3-wk diet/exercise treatments, animals underwent hyperinsulinemic euglycemic clamp experiments for measurements of insulin-stimulated whole body glucose fluxes. In addition, muscle samples were taken at the end of the clamps for measurements of glucose 6-phosphate (G-6-P) and GLUT-4 protein contents, hexokinase, and glycogen synthase (GS) activities. Insulin-stimulated glucose uptake was decreased by HFD and increased by exercise training (P < 0.01 for both). The opposite effects of HFD and exercise training on insulin-stimulated glucose uptake were associated with similar increases in muscle G-6-P levels (P < 0.05 for both). However, the increase in G-6-P level was accompanied by decreased GS activity without changes in GLUT-4 protein content and hexokinase activities in the HFD group. In contrast, the increase in G-6-P level in the exercise-trained group was accompanied by increased GLUT-4 protein content and hexokinase II (cytosolic) and GS activities. These results suggest that HFD and exercise training affect insulin sensitivity by acting predominantly on different steps of intracellular glucose metabolism. High-fat feeding appears to induce insulin resistance by affecting predominantly steps distal to G-6-P (e.g., glycolysis and glycogen synthesis). Exercise training affected multiple steps of glucose metabolism both proximal and distal to G-6-P. However, increased muscle G-6-P levels in the face of increased glucose metabolic fluxes suggest that the effect of exercise training is quantitatively more prominent on the steps proximal to G-6-P (i.e., glucose transport and phosphorylation).     

Effects of dietary oyster extract on lipid metabolism, blood pressure, and blood glucose in SD rats, hypertensive rats, and diabetic rats

Oyster extract was prepared by hydrolysis of oyster protein with proteases, Aloase (a protease from Bacillus subtilis), and Pancitase (a protease from Aspergillus oryzae). Rats were fed a diet containing 20% casein (the control diet) or 15% casein and 5% oyster extract (the oyster extract diet) as the protein source. The oyster extract diet exerted a significant reduction in serum cholesterol and liver triglyceride concentrations as compared with the control diet in Sprague-Dawley (SD) rats fed cholesterol-supplemented diets for 4 weeks. The activities of cytosolic fatty acid synthase and glucose-6-phosphate dehydrogenase were significantly lower in the oyster extract group than in the control group in the liver of SD rats. Hepatic cholesterol and triglyceride concentrations were significantly lower in spontaneously hypertensive (SH) rats and Otsuka Long-Evans Tokushima Fatty (OLETF) rats, type 2 diabetic rats, fed the oyster extract diet, for 4 weeks and 4 months respectively, than in those fed the control diet in the cholesterol-free diet. Blood pressure was significantly lower in the oyster extract group than in the control group at the 2nd and 4th weeks after the beginning of feeding experimental diets in SH rats. These results suggest that oyster extract prepared by hydrolysis of oyster induces triglyceride-lowering activity in the liver through a decrease in hepatic lipogenesis in SD rats, and that it exerts the antihypertensive effect in SH rats.     

Potential intercellular futile cycling of carbohydrates in diabetes.

A simple and rapid method for the isolation of a large quantity of cytochrome c oxidase from bovine heart mitochondria was developed, based on selective solubilization of mitochondrial protein with first Triton and then lauryl maltoside. Gel filtration shows that the lauryl maltoside-solubilized oxidase preparation is in a hydrodynamically homogeneous state with a Stokes radius of 7.5 +/- 0.2 nm. It contains 8.0 mumol of haem (with an a/a3 ratio of 1)/g of protein. The catalytic constant (maximum turnover number) with respect to cytochrome c approaches 600 S-1. After further purification of the solubilized enzyme on a sucrose-gradient centrifugation, the purified enzyme has a haem content of 10.3 mumol/g of protein and eight major polypeptide bands shown on SDS/polyacrylamide-gel electrophoresis.     

Effects of oral administration of benzylamine on glucose tolerance and lipid metabolism in rats

Repeated administration of benzylamine plus vanadate have been reported to exhibit anti-hyperglycemic effects in different models of diabetic rats. Likewise oral treatment withMoringa oleifera extracts which contain the alkaloïd moringine, identical to benzylamine, has also been shown to prevent hyperglycemia in alloxan-induced diabetic rats. With these observations we tested whether prolonged oral administration of benzylamine could interact with glucose and/or lipid metabolism. Seven week old male Wistar rats were treated for seven weeks with benzylamine 2.9 g/l in drinking water and were submitted to glucose tolerance tests. A slight decrease in water consumption was observed in benzylamine-treated animals while there was no change in body and adipose tissue weights at the end of treatment. Blood glucose and plasma insulin, triacylglycerol or cholesterol levels were not modified. However, benzylamine treatment resulted in a decrease in plasma free fatty acids in both fed and fasted conditions. Benzylamine treatment improved glucose tolerance as shown by the reduction of hyperglycemic response to intra-peritoneal glucose load. Oral benzylamine treatment did not alter the response of adipocytes to insulin nor to insulin-like actions of benzylamine plus vanadate, viain vitro activation of glucose transport or inhibition of lipolysis. This work demonstrates for the first time that oral administration of benzylamine alone influences glucose and lipid metabolism. However, these results obtained in normoglycemic rats require to be confirmed in diabetic models.     

Anomeric specificity of glucose metabolism in the pentose cycle

The production of 3H2O from alpha- and beta-D-[5-3H]glucose and that of 14CO2 from either alpha- and beta-D-[1-14C] or alpha- and beta-D-[6-14C]glucose were measured in rat pancreatic islets and tumoral insulin-producing cells incubated at 7 degrees C. The ratio in 14CO2 output from D-[1-14C]glucose/D-[6-14C]glucose, the fraction of glucose metabolism occurring through the pentose cycle, and the flow rate through such a cycle were always higher in the presence of beta- than alpha-D-glucose. This indicates that the anomeric specificity of glucose-6-phosphate dehydrogenase is operative in intact islet cells.     

Effects of thyrotoxicosis and selective hepatic autonomic denervation on hepatic glucose metabolism in rats

Thyrotoxicosis is known to induce a broad range of changes in carbohydrate metabolism. Recent studies have identified the sympathetic and parasympathetic nervous system as major regulators of hepatic glucose metabolism. The present study aimed to investigate the pathogenesis of altered endogenous glucose production (EGP) in rats with mild thyrotoxicosis. Rats were treated with methimazole in drinking water and l-thyroxine (T(4)) from osmotic minipumps to either reinstate euthyroidism or induce thyrotoxicosis. Euthyroid and thyrotoxic rats underwent either a sham operation, a selective hepatic sympathetic denervation (Sx), or a parasympathetic denervation (Px). After 10 days of T(4) administration, all animals were submitted to a hyperinsulinemic euglycemic clamp combined with stable isotope dilution to measure EGP. Plasma triiodothyronine (T(3)) showed a fourfold increase in thyrotoxic compared with euthyroid animals. EGP was increased by 45% in thyrotoxic compared with euthyroid rats and correlated significantly with plasma T(3). In thyrotoxic rats, hepatic PEPCK mRNA expression was increased 3.5-fold. Relative suppression of EGP during hyperinsulinemia was 34% less in thyrotoxic than in euthyroid rats, indicating hepatic insulin resistance. During thyrotoxicosis, Sx attenuated the increase in EGP, whereas Px resulted in increased plasma insulin with unaltered EGP compared with intact animals, compatible with a further decrease in hepatic insulin sensitivity. We conclude that chronic, mild thyrotoxicosis in rats increases EGP, whereas it decreases hepatic insulin sensitivity. Sympathetic hepatic innervation contributes only to a limited extent to increased EGP during thyrotoxicosis, whereas parasympathetic hepatic innervation may function to restrain EGP in this condition.     

Hysteresis, alternative stationary states and auto-oscillations in an open futile cycle one of the reactions of which is substrate inhibited]

In connection with the problem of regulation of futile (energy-dissipating) cycles in cell metabolism, a kinetic model has been investigated of an open cycle S1 (see article) S2, in which one of the enzymes (E-) is inhibited by the excess of its substrate S2. The quasi-stationary net velocity of the utilization of substrate S1 in the cycle as a function of its concentration is shown to be of a hysteretic character. Owing to this the alternative stationary states and self-oscillations may occur in the cycle. Under certain conditions the transition from one alternative state to another may reverse the direction of the net flux of conversion from S1 to S2 or vice versa. The self-oscillations are associated with a periodical change in the net flux direction. It is suggested the participation of glycogen (starch) in the self-oscillatory mechanism of the futile cycle formed by the phosphofructokinase and fructose bisphosphatase reactions may give rise to oscillations with the period of 10(3)-10(4) min, which may serve as the basis for the cell clock.