The role of futile cycles in the regulation of carbohydrate metabolism in the liver

Impaired chloroplast positioning affects photosynthetic capacity and regulation of the central carbohydrate metabolism during cold acclimation

Photosynthesis and carbohydrate metabolism of higher plants need to be tightly regulated to prevent tissue damage during environmental changes. The intracellular position of chloroplasts changes due to a changing light regime. Chloroplast avoidance and accumulation response under high and low light, respectively, are well known phenomena, and deficiency of chloroplast movement has been shown to result in photodamage and reduced biomass accumulation. Yet, effects of chloroplast positioning on underlying metabolic regulation are less well understood. Here, we analysed photosynthesis together with metabolites and enzyme activities of the central carbohydrate metabolism during cold acclimation of the chloroplast unusual positioning 1 (chup1) mutant of Arabidopsis thaliana. We compared cold acclimation under ambient and low light and found that maximum quantum yield of PSII was significantly lower in chup1 than in Col-0 under both conditions. Our findings indicated that net CO₂ assimilation in chup1 is rather limited by biochemistry than by photochemistry. Further, cold-induced dynamics of sucrose phosphate synthase differed significantly between both genotypes. Together with a reduced rate of sucrose cycling derived from kinetic model simulations our study provides evidence for a central role of chloroplast positioning for photosynthetic and metabolic acclimation to low temperature.

Real-time detection of hepatic gluconeogenic and glycogenolytic states using hyperpolarized [2-13C]dihydroxyacetone

Glycogenolysis and gluconeogenesis are sensitive to nutritional state, and the net direction of flux is controlled by multiple enzymatic steps. This delicate balance in the liver is disrupted by a variety of pathological states including cancer and diabetes mellitus. Hyperpolarized carbon-13 magnetic resonance is a new metabolic imaging technique that can probe intermediary metabolism nondestructively. There are currently no methods to rapidly distinguish livers in a gluconeogenic from glycogenolytic state. Here we use the gluconeogenic precursor dihydroxyacetone (DHA) to deliver hyperpolarized carbon-13 to the perfused mouse liver. DHA enters gluconeogenesis at the level of the trioses. Perfusion conditions were designed to establish either a gluconeogenic or a glycogenolytic state. Unexpectedly, we found that [2-(13)C]DHA was metabolized within a few seconds to the common intermediates and end products of both glycolysis and gluconeogenesis under both conditions, including [2,5-(13)C]glucose, [2-(13)C]glycerol 3-phosphate, [2-(13)C]phosphoenolpyruvate (PEP), [2-(13)C]pyruvate, [2-(13)C]alanine, and [2-(13)C]lactate. [2-(13)C]Phosphoenolpyruvate, a key branch point in gluconeogenesis and glycolysis, was monitored in functioning tissue for the first time. Observation of [2-(13)C]PEP was not anticipated as the free energy difference between PEP and pyruvate is large. Pyruvate kinase is the only regulatory step of the common glycolytic-gluconeogenic pathway that appears to exert significant control over the kinetics of any metabolites of DHA. A ratio of glycolytic to gluconeogenic products distinguished the gluconeogenic from glycogenolytic state in these functioning livers.

Muscle glycogen inharmoniously regulates glycogen synthase activity, glucose uptake, and proximal insulin signaling

Insulin-stimulated glucose uptake and incorporation of glucose into skeletal muscle glycogen contribute to physiological regulation of blood glucose concentration. In the present study, glucose handling and insulin signaling in isolated rat muscles with low glycogen (LG, 24-h fasting) and high glycogen (HG, refed for 24 h) content were compared with muscles with normal glycogen (NG, rats kept on their normal diet). In LG, basal and insulin-stimulated glycogen synthesis and glycogen synthase activation were higher and glycogen synthase phosphorylation (Ser(645), Ser(649), Ser(653), Ser(657)) lower than in NG. GLUT4 expression, insulin-stimulated glucose uptake, and PKB phosphorylation were higher in LG than in NG, whereas insulin receptor tyrosyl phosphorylation, insulin receptor substrate-1-associated phosphatidylinositol 3-kinase activity, and GSK-3 phosphorylation were unchanged. Muscles with HG showed lower insulin-stimulated glycogen synthesis and glycogen synthase activation than NG despite similar dephosphorylation. Insulin signaling, glucose uptake, and GLUT4 expression were similar in HG and NG. This discordant regulation of glucose uptake and glycogen synthesis in HG resulted in higher insulin-stimulated glucose 6-phosphate concentration, higher glycolytic flux, and intracellular accumulation of nonphosphorylated 2-deoxyglucose. In conclusion, elevated glycogen synthase activation, glucose uptake, and GLUT4 expression enhance glycogen resynthesis in muscles with low glycogen. High glycogen concentration per se does not impair proximal insulin signaling or glucose uptake. "Insulin resistance" is observed at the level of glycogen synthase, and the reduced glycogen synthesis leads to increased levels of glucose 6-phosphate, glycolytic flux, and accumulation of nonphosphorylated 2-deoxyglucose.

Compartmentalization of transport and phosphorylation of glucose in a hepatoma cell line

The first steps of glucose metabolism are carried out by members of the families of GLUTs (glucose transporters) and HKs (hexokinases). Previous experiments using the inhibitor of glucose transport, CB (cytochalasin B), revealed that compartmentalization of GLUTs and HKs is a major factor in the control of glucose uptake in L6 myotubes [Whitesell, Ardehali, Printz, Beechem, Knobel, Piston, Granner, Van Der Meer, Perriott and May (2003) Biochem. J. 370, 47-56]. In the present paper, we evaluate compartmentalization of GLUTs and HKs in a hepatoma cell line, H4IIE, which is characterized by excess GLUT activity, HKI in a particulate and a cytosolic fraction, and insignificant G6Pase (glucose-6-phosphatase) activity. The measured activity of glucose transport exceeded the rate of phosphorylation approx. 30-fold. Treatment with 25 microM CB (K(i) approximately 3 microM in H4IIE cells) paradoxically increased the excess of GLUTs over phosphorylation (GLUTs are inhibited 80%, while phosphorylation is inhibited 98%). The global relationships of the data could be reconciled most simply by a two-compartment model. In this model, phosphorylation of glucose is carried out by a subset of HK molecules supplied by a subset of GLUTs that are more sensitive to CB than the other GLUTs. The agent, DCC (dicyclohexylcarbodi-imide) caused HKI to translocate from the particulate compartment to the cytosolic compartment and potently inhibited glucose phosphorylation. The particulate compartment may represent the mitochondria, to which the more CB-sensitive GLUTs may control the transport of glucose.

Dual role of phosphofructokinase-2/fructose bisphosphatase-2 in regulating the compartmentation and expression of glucokinase in hepatocytes

Hepatic glucokinase is regulated by a 68-kDa regulatory protein (GKRP) that is both an inhibitor and nuclear receptor for glucokinase. We tested the role of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2) in regulating glucokinase compartmentation in hepatocytes. PFK2 catalyzes formation or degradation of the regulator of glycolysis fructose 2,6-bisphosphate (fructose 2,6-P2), depending on its phosphorylation state (ser-32), and is also a glucokinase-binding protein. Incubation of hepatocytes at 25 mmol/l glucose causes translocation of glucokinase from the nucleus to the cytoplasm and an increase in fructose 2,6-P2. Glucagon caused phosphorylation of PFK2-ser-32, lowered the fructose 2,6-P2 concentration, and inhibited glucose-induced translocation of glucokinase. These effects of glucagon were reversed by expression of a kinase-active PFK2 mutant (S32A/H258A) that overrides the suppression of fructose 2,6-P2 but not by overexpression of wild-type PFK2. Overexpression of PFK2 potentiated glucokinase expression in hepatocytes transduced with an adenoviral vector-encoding glucokinase by a mechanism that does not involve stabilization of glucokinase protein from degradation. It is concluded that PFK2 has a dual role in regulating glucokinase in hepatocytes: it potentiates glucokinase protein expression by posttranscriptional mechanisms and favors its cytoplasmic compartmentation. Thus, it acts in a complementary mechanism to GKRP, which also regulates glucokinase protein expression and compartmentation.

Dynamic profiling of the glucose metabolic network in fasted rat hepatocytes using [1,2-13C2]glucose

Recent studies in metabolic profiling have underscored the importance of the concept of a metabolic network of pathways with special functional characteristics that differ from those of simple reaction sequences. The characterization of metabolic functions requires the simultaneous measurement of substrate fluxes of interconnecting pathways. Here we present a novel stable isotope method by which the forward and reverse fluxes of the futile cycles of the hepatic glucose metabolic network are simultaneously determined. Unlike previous radio-isotope methods, a single tracer [1,2-13C2]D-glucose and mass isotopomer analysis is used. Changes in fluxes of substrate cycles, in response to several gluconeogenic substrates, in isolated fasted hepatocytes from male Wistar rats were measured simultaneously. Incubation with these substrates resulted in a change in glucose-6-phosphatase/glucokinase and glycolytic/gluconeogenic flux ratios. Different net redistributions of intermediates in the glucose network were observed, resulting in distinct metabolic phenotypes of the fasted hepatocytes in response to each substrate condition. Our experimental observations show that the constraints of concentrations of shared intermediates, and enzyme kinetics of intersecting pathways of the metabolic network determine substrate redistribution throughout the network when it is perturbed. These results support the systems-biology notion that network analysis provides an integrated view of the physiological state. Interaction between metabolic intermediates and glycolytic/gluconeogenic pathways is a basic element of cross-talk in hepatocytes, and may explain some of the difficulties in genotype and phenotype correlation.

Preferential oxidation of glycogen in isolated working rat heart

We tested the hypothesis that glycogen is preferentially oxidized in isolated working rat heart. This was accomplished by measuring the proportion of glycolytic flux (oxidation plus lactate production) specifically from glycogen which is metabolized to lactate, and comparing it to the same proportion determined concurrently from exogenous glucose during stimulation with epinephrine. After prelabeling of glycogen with either 14C or 3H, a dual isotope technique was used to simultaneously trace the disposition of glycogen and exogenous glucose between oxidative and non-oxidative pathways. Immediately after the addition of epinephrine (1 microM), 40-50% of flux from glucose was directed towards lactate. Glycogen, however, did not contribute to lactate, being almost entirely oxidized. Further, glycogen utilization responded promptly to the abrupt increase in contractile performance with epinephrine, during the lag in stimulation of utilization of exogenous glucose, suggesting that glycogen serves as substrate reservoir to buffer rapid increases in demand. Preferential oxidation of glycogen may serve to ensure efficient generation of ATP from a limited supply of endogenous substrate, or as a mechanism to limit lactate accumulation during rapid glycogenolysis.

Mammalian glucokinase and its gene

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Dissociated effects of 2-deoxy-D-glucose on D-[2-3H]glucose and D-[5-3H]glucose conversion into 3HOH in rat erythrocytes

When rat erythrocytes were preincubated with 2-deoxy-D-glucose, the generation of both 3H-labelled acidic metabolites and 3HOH from D-[5-3H]glucose, the total production of L-lactate, and the generation of 14CO2, 14C-labelled acidic metabolites and 14C-labelled lactate from D-[1-14C]glucose or D-[U-14C]glucose were all lower than in erythrocytes preincubated in the absence of a hexose or in the presence of 3-O-methyl-D-glucose. However, preincubation with 2-deoxy-D-glucose failed to decrease the generation of 3H-labelled acidic metabolites and L-[3-3H]lactate from D-[2-3H]glucose, while decreasing the production of 3HOH more severely from D-[2-3H]glucose than from D-[5-3H]glucose. This may be attributable not solely to inhibition of D-glucose phosphorylation by 2-deoxy-D-glucose and 2-deoxy-D-glucose 6-phosphate, but also to inhibition by 2-deoxy-D-glucose 6-phosphate of hexose 6-phosphate interconversion in the reaction catalysed by phosphoglucoisomerase, as also observed with the purified enzyme. The generation of 3HOH from D-[2-3H]glucose should therefore be considered as a tool to assess the efficiency of interconversion of hexose 6-phosphates in the reaction catalysed by phosphoglucoisomerase, rather than to estimate D-glucose phosphorylation rate.

Effect of treatment in vivo of rats with bacterial endotoxin on fructose 2,6-bisphosphate metabolism and L-pyruvate kinase activity and flux in isolated liver cells

The effect of treatment of rats with bacterial endotoxin on fructose 2,6-bisphosphate (Fru-2,6-P2) metabolism was investigated in isolated liver cells prepared from 18 h-starved animals. The results obtained support the hypothesis that a stimulation of 6-phosphofructo-1-kinase (PFK-1) activity and an inhibition of fructose-1,6-bisphosphatase (Fru-1,6-P2ase) may be one mechanism underlying the inhibition of gluconeogenesis from lactate and pyruvate by endotoxin. We suggest that the stimulation of PFK-1 and inhibition of Fru-1,6-P2ase activity is the result of a 2-3-fold increase in Fru-2,6-P2. The latter is not due to changes in the total activity or phosphorylation state of the bifunctional 6-phosphofructo-2-kinase (PFK-2)/fructose-2,6-bisphosphatase, but appears to be the result of a decrease in the cytosolic concentration of phosphoenolpyruvate (PEP), an inhibitor of PFK-2 activity. The effect of endotoxin is resistant to the presence of glucagon, which has comparable effects in cells prepared from both control and endotoxin-treated animals. The mechanism by which endotoxin treatment of the rat decreases PEP and gluconeogenesis remains to be established. However, it does not involve alterations in either the total activity or the phosphorylation state of pyruvate kinase, nor does it involve increased flux through this enzyme in the intact cell, which is in fact decreased in this model of septic shock. It is suggested that the decreased flux may result from a lower rate of formation of PEP, suggesting that the prime lesion in sepsis is an inhibition of one or more of the steps leading to PEP formation.

Inhibition of glycolysis by 5-amino-4-imidazolecarboxamide riboside in isolated rat hepatocytes

5-Amino-4-imidazolecarboxamide riboside (AICAriboside; Z-riboside), the nucleotide corresponding to AICAribotide (AICAR or ZMP), an intermediate of the 'de novo' pathway of purine nucleotide biosynthesis, has been shown to inhibit gluconeogenesis in isolated rat hepatocytes [Vincent, Marangos, Gruber & Van den Berghe (1991) Diabetes 40, 1259-1266]. We now report that glycosis is also inhibited and even more sensitive to AICAriboside in these cells. In hepatocyte suspensions from fasted rats, production of lactate from 15 mM-glucose was half-maximally inhibited by 25-50 microM-AICAriboside. AICAriboside influenced two regulatory steps of glycolysis: (1) it decreased the release of 3H2O from [2-3H]glucose and the concentrations of both glucose 6-phosphate and fructose 6-phosphate, indicating that it diminished the phosphorylation of glucose by glucokinase; (2) it decreased the concentration of fructose 2,6-bisphosphate (Fru-2,6-P2), the main physiological stimulator of liver 6-phosphofructo-1-kinase. Further studies showed that AICAriboside induced an inactivation of 6-phosphofructo-2-kinase, the enzyme that produces Fru-2,6-P2, without affecting the concentration of cyclic AMP. Similarly to the inhibiton of gluconeogenesis by AICAriboside, the inhibition of glycolysis became apparent after an approx. 10 min latency and persisted when the cells were washed after addition of AICAriboside, strongly suggesting that the effects were also exerted by the Z-nucleotides, which accumulate after addition of AICAriboside to hepatocytes. An increased uptake of lactate was evident when 50-200 microM-AICAriboside was added 15 min after addition of glucose. This can be explained by the higher sensitivity of glycolysis, as compared with gluconeogenesis, to inhibition by AICAriboside, and reveals the simultaneous operation of both processes.

Control of glycolysis in cultured chick embryo hepatocytes. Fructose 2,6-bisphosphate content and phosphofructokinase-1 activity are stimulated by insulin and epidermal growth factor

Chick embryo hepatocytes were maintained in monolayer culture in a serum-free chemically defined medium for periods of up to 2 days. Over this time period, insulin provoked selective increases (up to 5-fold) in factors relevant to the control of glycolysis: the activities of phosphofructokinase-1 (PFK-1), phosphofructokinase-2 (PFK-2) and hexokinase isoenzymes and the content of fructose 2,6-bisphosphate (F26BP). Half-maximal effects of insulin on pFK-1 activity were in the physiological range (0.1 nM). Changes in enzyme activities and F26BP content in response to insulin were correlated with stimulation of glycolytic flux as estimated by radioisotopic flux. These data are discussed in relation to known changes which occur in hepatic glycolytic activity and PFK-1 activity in the intact chick around hatching. The effects of insulin on F26BP content, PFK-1 activity and glycolytic flux were mimicked by epidermal growth factor (EGF). In contrast, phorbol esters produced minimal actions on any of the above parameters. Our data indicate that protein kinase C is not involved in the actions of insulin or EGF in control of F26BP content or PFK-1 activity. This work indicates that the related tyrosyl kinase receptors of insulin and EGF may provoke identical responses within hepatocytes, but through the utilization of different transduction systems which merge to common control points.

Quantitative analysis of intermediary metabolism in rat hepatocytes incubated in the presence and absence of ethanol with a substrate mixture including ketoleucine

Hepatocytes isolated from livers of fed rats were incubated with a mixture of glucose (10 mM), ribose (1.0 mM), acetate (1.25 mM), alanine (3.5 mM), glutamate (2.0 mM), aspartate (2.0 mM), 4-methyl-2-oxovaleric acid (ketoleucine) (3.0 mM), and, in paired flasks, 10 mM-ethanol. One substrate was 14C-radiolabelled in any given incubation. Incorporation of 14C into glucose, glycogen, CO2, lactate, alanine, aspartate, glutamate, acetate, urea, lipid glycerol, fatty acids and the 1- and 2,3,4-positions of ketone bodies was measured after 20 and 40 min of incubation under quasi-steady-state conditions. Data were analysed with the aid of a realistic structural metabolic model. In each of the four conditions examined, there were approx. 77 label incorporation measurements and several measurements of changes in metabolite concentrations. The considerable excess of measurements over the 37 independent flux parameters allowed for a stringent test of the model. A satisfactory fit to these data was obtained for each condition. There were large bidirectional fluxes along the gluconeogenic/glycolytic pathways, with net gluconeogenesis. Rates of ureagenesis, oxygen consumption and ketogenesis were high under all four conditions studied. Oxygen utilization was accurately predicted by three of the four models. There was complete equilibration between mitochondrial and cytosolic pools of acetate and of CO2, but for several of the metabolic conditions, two incompletely equilibrated pools of mitochondrial acetyl-CoA and oxaloacetate were required. Ketoleucine was utilized at a rate comparable to that reported by others in perfused liver and entered the mitochondrial pool of acetyl-CoA directly associated with ketone body formation. Ethanol, which was metabolized at rates comparable to those in vivo, caused relatively few changes in overall flux patterns. Several effects related to the increased NADH/NAD+ ratio were observed. Pyruvate dehydrogenase was completely inhibited and the ratio of acetoacetate to 3-hydroxybutyrate was decreased; flux through glutamate dehydrogenase, the citric acid cycle, and ketoleucine dehydrogenase were, however, only slightly inhibited. Net production of ATP occurred in all conditions studied and was increased by ethanol. Futile cycling was quantified at the glucose/glucose 6-phosphate, glycogen/glucose 6-phosphate, fructose 6-phosphate/fructose 1,6-bis-phosphate, and phosphoenolpyruvate/pyruvate/oxaloacetate substrate cycles. Cycling at these four loci consumed about 22% of cellular ATP production in control hepatocytes and 14% in ethanol-treated cells.

Mild type II diabetes markedly increases glucose cycling in the postabsorptive state and during glucose infusion irrespective of obesity

Glucose cycling (GC; G in equilibrium G6P) equals 14% of glucose production in postabsorptive man. Our aim was to determine glucose cycling in six lean and six overweight mild type II diabetics (fasting glycemia: 139 +/- 10 and 152 +/- 7 mg/dl), in postabsorptive state (PA) and during glucose infusion (2 mg/kg per min). 14 control subjects were weight and age matched. GC is a function of the enzyme that catalyzes the reaction opposite the net flux and is the difference between hepatic total glucose output (HTGO) (2-[3H]glucose) and hepatic glucose production (HGP) (6-[3H]-glucose). Postabsorptively, GC is a function of glucokinase. With glucose infusion the flux is reversed (net glucose uptake), and GC is a function of glucose 6-phosphatase. In PA, GC was increased by 100% in lean (from 0.25 +/- 0.07 to 0.43 +/- .08 mg/kg per min) and obese (from 0.22 +/- 0.05 to 0.50 +/- 0.07) diabetics. HGP and HTGO increased in lean and obese diabetics by 41 and 33%. Glucose infusion suppressed apparent phosphatase activity and gluconeogenesis much less in diabetics than controls, resulting in marked enhancement (400%) in HTGO and HGP, GC remained increased by 100%. Although the absolute responses of C-peptide and insulin were comparable to those of control subjects, they were inappropriate for hyperglycemia. Peripheral insulin resistance relates to decreased metabolic glucose clearance (MCR) and inadequate increase of uptake during glucose infusion. We conclude that increases in HGP and HTGO and a decrease of MCR are characteristic features of mild type II diabetes and are more pronounced during glucose infusion. There is also an increase in hepatic GC, a stopgap that controls changes from glucose production to uptake. Postabsorptively, this limits the increase of HGP and glycemia. In contrast, during glucose infusion, increased GC decreases hepatic glucose uptake and thus contributes to hyperglycemia. Obesity per se did not affect GC. An increase in glucose cycling and turnover indicate hepatic insulin resistance that is observed in addition to peripheral resistance. It is hypothesized that in pathogenesis of type II diabetes, augmented activity of glucose-6-phosphatase and kinase may be of importance.

Hormonal control of substrate cycling in humans

Recent studies have established the existence of substrate cycles in humans, but factors regulating the rate of cycling have not been identified. We have therefore investigated the acute response of glucose/glucose-6P-glucose (glucose) and triglyceride/fatty acid (TG/FA) substrate cycling to the infusion of epinephrine (0.03 microgram/kg.min) and glucagon. The response to a high dose glucagon infusion (2 micrograms/kg.min) was tested, as well as the response to a low dose infusion (5 ng/kg.min), with and without the simultaneous infusion of somatostatin (0.1 microgram/kg.min) and insulin (0.1 mU/kg.min). Additionally, the response to chronic prednisone (50 mg/d) was evaluated, both alone and during glucagon (low dose) and epinephrine infusion. Finally, the response to hyperglycemia, with insulin and glucagon held constant by somatostatin infusion and constant replacement of glucagon and insulin at basal rates, was investigated. Glucose cycling was calculated as the difference between the rate of appearance (Ra) of glucose as determined using 2-d1- and 6,6-d2-glucose as tracers. TG/FA cycling was calculated by first determining the Ra glycerol with d5-glycerol and the Ra FFA with [1-13C]palmitate, then subtracting Ra FFA from three times Ra glycerol. The results indicate that glucagon stimulates glucose cycling, and this stimulatory effect is augmented when the insulin response to glucagon infusion is blocked. Glucagon had minimal effect on TG/FA cycling. In contrast, epinephrine stimulated TG/FA cycling, but affected glucose cycling minimally. Prednisone had no direct effect on either glucose or TG/FA cycling, but blunted the stimulatory effect of glucagon on glucose cycling. Hyperglycemia, per se, had no direct effect on glucose or TG/FA cycling. Calculations revealed that stimulation of TG/FA cycling theoretically amplified the sensitivity of control of fatty acid flux, but no such amplification was evident as a result of the stimulation of glucose cycling by glucagon.

Palmitate inhibits liver glycolysis. Involvement of fructose 2,6-bisphosphate in the glucose/fatty acid cycle

In hepatocytes from overnight-fasted rats incubated with glucose, palmitate decreased the production of lactate, the detritiation of [2-3H]- and [3-3H]-glucose, and the concentration of fructose 2,6-bisphosphate. Similarly, perfusion of hearts from fed rats with beta-hydroxybutyrate resulted in an inhibition of the detritiation of [3-3H]glucose and a fall in fructose 2,6-bisphosphate concentration. This fall could result from an increase in citrate (hepatocytes and heart) and sn-glycerol 3-bisphosphate concentration. It is suggested that a fall in fructose 2,6-bisphosphate concentration participates in the inhibition of glycolysis by fatty acids and ketone bodies.

Stimulation of glycogen synthesis and lipogenesis by glutamine in isolated rat hepatocytes

Glutamine stimulated glycogen synthesis and lactate production in hepatocytes from overnight-fasted normal and diabetic rats. The effect, which was half-maximal with about 3 mM-glutamine, depended on glucose concentration and was maximal below 10 mM-glucose. beta-2-Aminobicyclo[2.2.1.]heptane-2-carboxylic acid, an analogue of leucine, stimulated glutaminase flux, but inhibited the stimulation of glycogen synthesis by glutamine. Various purine analogues and inhibitors of purine synthesis were found to inhibit glycogen synthesis from glucose, but they did not abolish the stimulatory effect of glutamine on glycogen synthesis. The correlation between the rate of glycogen synthesis and synthase activity suggested that the stimulation of glycogen synthesis by glutamine depended solely on the activation of glycogen synthase. This activation of synthase was not due to a change in total synthase, nor was it caused by a faster inactivation of glycogen phosphorylase, as was the case after glucose. It could, however, result from a stimulation of synthase phosphatase, since, after the addition of 1 nM-glucagon or 10 nM-vasopressin, glutamine did not interfere with the inactivation of synthase, but did promote its subsequent re-activation. Glutamine was also found to inhibit ketone-body production and to stimulate lipogenesis.

The characteristics and site of inhibition of gluconeogenesis in rat liver cells by bacterial endotoxin. Stimulation of phosphofructokinase-1

The characteristics and site of inhibition of gluconeogenesis by endotoxin were investigated in liver cells isolated from control and endotoxin-treated rats. Endotoxin treatment was associated with inhibition (40-50%) of gluconeogenesis from lactate plus pyruvate over a range of concentrations of substrate and of oleate and with or without glucose or glucagon. Similar inhibition was observed with asparagine, proline, glutamine, alanine and a substrate mixture, but not with glycerol, glyceraldehyde, dihydroxyacetone or endogenous substrates. There was no change in cellular ATP content or in the rates of ketogenesis or ureogenesis from asparagine, proline or glutamine. Other effects on isotopic fluxes, metabolite contents, enzyme activities and control coefficients were consistent with the suggestion that the effects of endotoxin on gluconeogenesis are exerted at the level of phosphofructokinase-1, and not at phosphoenolpyruvate carboxykinase, pyruvate kinase, pyruvate carboxylase or glucokinase.

Inhibition of gluconeogenesis by hypoglycin in the rat. Evidence for inhibition of glucose-6-phosphatase in vivo

Treatment of rats with hypoglycaemic doses of hypoglycin has been shown to abolish the relative detritiation of [2-3H,U-14C]glucose [Osmundsen, Billington, Taylor & Sherratt (1978) Biochem. J. 170, 337-342], indicating that both the Cori and the glucose/glucose 6-phosphate cycles were inhibited in vivo. This inhibition was confirmed and, in addition, it was shown that the conversion in vivo of both [14C]lactate and [14C]fructose into glucose was decreased after hypoglycin treatment. These results suggest that hypoglycin poisoning results in the inhibition in vivo of glucose-6-phosphatase activity, which participates in the overall inhibition of gluconeogenesis and hypoglycaemia. Clofibrate feeding apparently protected the rats against the inhibition of the fructose-to-glucose conversion by hypoglycin. However, in isolated hepatocytes prepared from hypoglycin-treated rats, the conversion of [14C]fructose into glucose and the recycling of [2-3H,U-14C]glucose were not different from that in control hepatocytes. This suggests that the inhibition was lost during preparation of the hepatocytes. The direct measurement of glucose-6-phosphatase activity showed that it was inhibited when measured in concentrated, but not dilute, homogenates prepared from hypoglycin-treated rats.

Assessment of insulin action in insulin-dependent diabetes mellitus using [6(14)C]glucose, [3(3)H]glucose, and [2(3)H]glucose. Differences in the apparent pattern of insulin resistance depending on the isotope used

To determine whether [2(3)H], [3(3)H], and [6(14)C]glucose provide an equivalent assessment of glucose turnover in insulin-dependent diabetes mellitus (IDDM) and nondiabetic man, glucose utilization rates were measured using a simultaneous infusion of these isotopes before and during hyperinsulinemic euglycemic clamps. In the nondiabetic subjects, glucose turnover rates determined with [6(14)C]glucose during insulin infusion were lower (P less than 0.02) than those determined with [2(3)H]glucose and higher (P less than 0.01) than those determined with [3(3)H]glucose. In IDDM, glucose turnover rates measured with [6(14)C]glucose during insulin infusion were lower (P less than 0.05) than those determined with [2(3)H]glucose, but were not different from those determined with [3(3)H]glucose. All three isotopes indicated the presence of insulin resistance. However, using [3(3)H]glucose led to the erroneous conclusion that glucose utilization was not significantly decreased at high insulin concentrations in the diabetic patients. [6(14)C] and [3(3)H]glucose but not [2(3)H]glucose indicated impairment in insulin-induced suppression of glucose production. These results indicate that tritiated isotopes do not necessarily equally reflect the pattern of glucose metabolism in diabetic and nondiabetic man.

Increased glucose turnover and glucose cycling in acromegalic patients with normal glucose tolerance

To characterize the diabetogenic effects of growth hormone, we simultaneously measured glucose turnover with 2-3H- and 6-3H-glucose in six acromegalic patients with normal fasting blood glucose and oral glucose tolerance tests. Eight healthy volunteers served as controls. All subjects were studied under both basal conditions and during glucose infusion (2 mg X kg-1 X min-1). We determined true glucose production and irreversible glucose uptake using 6-3H-glucose and glucose cycling (difference between 2-3H- and 6-3H-glucose). After an overnight fast, glucose production was higher than normal in the acromegalic patients (2.18 +/- 0.15 vs 1.85 +/- 0.03 mg X kg-1 X min-1, p less than 0.05) despite hyperinsulinaemia. The metabolic clearance rate was normal. During the glucose infusion, glucose production was not suppressed as effectively in the acromegalic patients as in controls nor was glucose uptake augmented, while metabolic clearance rate was decreased. In acromegaly, basal glucose cycling was increased (0.44 +/- 0.08 vs 0.25 +/- 0.07 mg X kg-1 X min-1, p less than 0.05). Furthermore cycling of endogenous glucose measured during glucose infusion was also augmented (0.41 +/- 0.05 vs 0.24 +/- 0.05 mg X kg-1 X min-1, p less than 0.05). Hence the increase of glucose cycling (70%) was much more pronounced than that of glucose production (17%). In conclusion, small defects in glucose metabolism in acromegaly can be detected with sensitive tracer methods. These derangements are confined to the liver under fasting conditions, but are of both hepatic and extrahepatic origin during glucose loading.

Antagonistic regulation of the glucose/glucose 6-phosphate cycle by insulin and glucagon in cultured hepatocytes

Flux through the glucose/glucose 6-phosphate cycle in cultured hepatocytes was measured with radiochemical techniques. Utilization of [2-3H]glucose was taken as a measure of glucokinase flux. Liberation of [14C]glucose from [U-14C]glycogen and from [U-14C]lactate, as well as the difference between the utilization of [2-3H]glucose and of [U-14C]glucose, were taken as measures of glucose-6-phosphatase flux. At constant 5 mM-glucose and 2 mM-lactate concentrations insulin increased glucokinase flux by 35%; it decreased glucose-6-phosphatase flux from glycogen by 50%, from lactate by 15% and reverse flux from external glucose by 65%, i.e. overall by 40%. Glucagon had essentially no effect on glucokinase flux; it enhanced glucose-6-phosphatase flux from glycogen by 700%, from lactate by 45% and reverse flux from external glucose by 20%, i.e. overall by 110%. At constant glucose concentrations cellular glucose 6-phosphate concentrations were essentially not altered by insulin, but were increased by glucagon by 230%. In conclusion, under basic conditions without added hormones the glucose/glucose 6-phosphate cycle showed only a minor net glucose uptake, of 0.03 mumol/min per g of hepatocytes; this flux was increased by insulin to a net glucose uptake of 0.21 mumol/min per g and reversed by glucagon to a net glucose release of 0.22 mumol/min per g. Since the glucose 6-phosphate concentrations after hormone treatment did not correlate with the glucose-6-phosphatase flux, it is suggested that the hormones influenced the enzyme activity directly.

Control of gluconeogenesis in rat liver cells. Flux control coefficients of the enzymes in the gluconeogenic pathway in the absence and presence of glucagon

We have used control analysis to quantify the distribution of control in the gluconeogenic pathway in liver cells from starved rats. Lactate and pyruvate were used as gluconeogenic substrates. The flux control coefficients of the various enzymes in the gluconeogenic pathway were calculated from the elasticity coefficients of the enzymes towards their substrates and products and the fluxes through the different branches in the pathway. The elasticity coefficients were either calculated from gamma/Keq. ratios (where gamma is the mass-action ratio and Keq. is the equilibrium constant) and enzyme-kinetic data or measured experimentally. It is concluded that the gluconeogenic enzyme pyruvate carboxylase and the glycolytic enzyme pyruvate kinase play a central role in control of gluconeogenesis. If pyruvate kinase is inactive, gluconeogenic flux from lactate is largely controlled by pyruvate carboxylase. The low elasticity coefficient of pyruvate carboxylase towards its product oxaloacetate minimizes control by steps in the gluconeogenic pathway located after pyruvate carboxylase. This situation occurs when maximal gluconeogenic flux is required, i.e. in the presence of glucagon. In the absence of the hormone, when pyruvate kinase is active, control of gluconeogenesis is distributed among many steps, including pyruvate carboxylase, pyruvate kinase, fructose-1,6-bisphosphatase and also steps outside the classic gluconeogenic pathway such as the adenine-nucleotide translocator.

Effect of glutamine on fructose 2,6-bisphosphate and on glucose metabolism in HeLa cells and in chick-embryo fibroblasts

Glutamine caused a dose-dependent decrease in fructose 2,6-bisphosphate concentration in both HeLa cells and chick-embryo fibroblasts. The effect was complete within 15 min in HeLa cells, but required more than 9 h in the fibroblasts. Half-maximal effects were obtained with 0.1-0.3 mM-glutamine. In chick-embryo fibroblasts, but not in HeLa cells, glutamine induced a time-dependent decrease in the activity of phosphofructokinase-2, which correlated with the decrease in fructose 2,6-bisphosphate. Glutamine decreased the glycolytic flux by about 25% only in chick-embryo fibroblasts. The difference in glycolytic response between the two types of cells might correspond to a difference in the sensitivity of phosphofructokinase-1 for fructose 2,6-bisphosphate. In HeLa cells, glutamine caused a 2-3-fold stimulation of the synthesis of glycogen, a 50% decrease in the concentration of fructose 1,6-bisphosphate and a more than 80% decrease in the concentration of 5-phosphoribosyl pyrophosphate; the concentrations of hexose 6-phosphates and ATP were not affected.

Substrate cycling between gluconeogenesis and glycolysis in euthyroid, hypothyroid, and hyperthyroid man

Substrate, or futile cycles, have been hypothesized to be under hormonal control, and important in metabolic regulation and thermogenesis. To define the role of thyroid hormones in the regulation of substrate cycling in glycolysis and gluconeogenesis, we measured rates of cycling in normal (n = 4), hypothyroid (n = 5), and hyperthyroid (n = 5) subjects employing a stable isotope turnover technique. Glucose labeled with deuterium at different positions (2-D1-, 3-D1-, and 6,6-D2-glucose) was given as a primed-constant infusion in tracer doses, and arterialized plasma samples were obtained and analyzed by gas-chromatography mass-spectrometry for the steady state enrichment of glucose that was labeled at the various positions. The rate of appearance (Ra) was then calculated for each isotopic tracer. The difference between the Ra determined by 2-D1-glucose (Ra2) and the Ra determined by 3-D1-glucose (Ra3) represents the substrate cycling rate (SCR) between glucose and glucose-6-phosphate. The difference between the Ra determined by 3-D1-glucose (Ra3) and the Ra determined by 6,6-D2-glucose (Ra6) represents the SCR between fructose-6-phosphate and fructose-1,6-diphosphate. The difference between Ra2 and Ra6 represents the combined SCR of both cycles. In normal subjects (serum thyroxine [T4] = 8.4 +/- 1.2 microgram/dl (all expressions, mean +/- SD), n = 4), the rates of appearance for Ra2, Ra3, and Ra6 were 3.23 +/- 0.56, 2.64 +/- 0.50, and 2.00 +/- 0.27 mg/kg X min, respectively, whereas those in the hypothyroid subjects (T4 = 1.0 +/- 0.8 microgram/dl; n = 5) were 1.77 +/- 0.56 (P less than 0.01), 1.52, 1.57 +/- 0.31 (P less than 0.05) mg/kg X min, respectively. Conversely, the rates of appearance for Ra2 and Ra6 in the hyperthyroid subjects (T4 = 23.9 +/- 3.6 micrograms/dl) were 3.94 +/- 0.43 (P less than 0.05) and 2.54 +/- 0.22 (P less than 0.02), respectively, compared with the normal subjects. On the basis of these data, we noted that the normal subjects had a combined SCR of 1.23 +/- 0.35 mg/kg X min. In contrast, the hypothyroid patients had a significantly decreased combined SCR, 0.20 +/- 0.54 mg/kg X min (P less than 0.02). The hyperthyroid patients had a combined SCR of 1.39 +/- 0.23 mg/kg X min (P less than NS). To determine whether these cycles responded to thyroid hormone treatment, these same hypothyroid subjects were acutely treated for 1 wk with parenteral 50 micrograms/d sodium L-triiodothyronine and chronically with 100-150 micrograms/d L-thyroxine. After 7 d, their mean oxygen consumption rate and carbon dioxide production rate increased significantly from 102+/-13 micromol/kg.min, to 147+/-34 micromol/kg.min (P<0.05), and from 76+/-13 micromol/kg.min to 111+/-19 micromol/kg.min (P<0.05), respectively. The combined SCR (Ra(2)--Ra(6) remained unchanged at 0.07+/-0.37 mg/kg.min. However, after 6 mo of oral L-thyroxine therapy (T(4)=9.5+/-1.4 microgram/kl) the treated hypothyroid patients had increased their combined SCR (Ra(2)--Ra(6)) to 0.86 +/-0.23 mg/kg.min (P<0.02), a value not significantly different from the combined SCR of normal subjects. We conclude that substrate cycling between glucose and glucose-6-phosphate and between fructose-6-phosphate and fructose-1,6-diphosphate occurs in man and is affected by thyroid hormone. Substrate cycles may represent a mechanism by which thyroid hormone alters the sensitivity of certain reactions to metabolic signals.

Quantitative analysis of intermediary metabolism in hepatocytes incubated in the presence and absence of glucagon with a substrate mixture containing glucose, ribose, fructose, alanine and acetate

Hepatocytes were isolated from the livers of fed rats and incubated, in the presence and absence of 100 nM-glucagon, with a substrate mixture containing glucose (10 mM), fructose (4 mM), alanine (3.5 mM), acetate (1.25 mM), and ribose (1 mM). In any given incubation one substrate was labelled with 14C. Incorporation of 14C into glucose, glycogen, CO2, lactate, alanine, glutamate, lipid glycerol and fatty acids was measured after 20 and 40 min of incubation under quasi-steady-state conditions [Borowitz, Stein & Blum (1977) J. Biol. Chem. 252, 1589-1605]. These data and the measured O2 consumption were analysed with the aid of a structural metabolic model incorporating all reactions of the glycolytic, gluconeogenic, and pentose phosphate pathways, and associated mitochondrial and cytosolic reactions. A considerable excess of experimental measurements over independent flux parameters and a number of independent measurements of changes in metabolite concentrations allowed for a stringent test of the model. A satisfactory fit to the data was obtained for each condition. Significant findings included: control cells were glycogenic and glucagon-treated cells glycogenolytic during the second interval; an ordered (last in, first out) model of glycogen degradation [Devos & Hers (1979) Eur. J. Biochem. 99, 161-167] was required in order to fit the experimental data; the pentose shunt contributed approx. 15% of the carbon for gluconeogenesis in both control and glucagon-treated cells; net flux through the lower Embden-Meyerhof pathway was in the glycolytic direction except during the 20-40 min interval in glucagon-treated cells; the increased gluconeogenesis in response to glucagon was correlated with a decreased pyruvate kinase flux and lactate output; fluxes through pyruvate kinase, pyruvate carboxylase, and phosphoenolpyruvate carboxykinase were not coordinately controlled; Krebs cycle activity did not change with glucagon treatment; flux through the malic enzyme was towards pyruvate formation except for control cells during interval II; and 'futile' cycling at each of the five substrate cycles examined (including a previously undescribed cycle at acetate/acetyl-CoA) consumed about 26% of cellular ATP production in control hepatocytes and 21% in glucagon-treated cells.

Regulation of fructose 2,6-bisphosphate concentration in white adipose tissue

Injection of insulin to fed rats diminished the concentration of fructose 2,6-bisphosphate in white adipose tissue. Incubation of epididymal fat-pads or adipocytes with insulin stimulated lactate release and sugar detritiation and also decreased fructose 2,6-bisphosphate concentration. Such a decrease was, however, not observed in fat-pads from starved or alloxan-diabetic rats. Incubation of adipocytes from fed rats with various concentrations of glucose or fructose led to a dose-dependent rise in fructose 2,6-bisphosphate which correlated with lactate output and detritiation of 3-3H-labelled sugar. In adipocytes from fed rats, palmitate stimulated the detritiation of [3-3H]glucose without affecting lactate production and fructose 2,6-bisphosphate concentration. Incubation of epididymal fat-pads from fed rats in the presence of antimycin stimulated lactate output but decreased fructose 2,6-bisphosphate concentration. Changes in lipolytic rates brought about by noradrenaline, insulin, adenosine and corticotropin in adipocytes from fed rats were not related to changes in fructose 2,6-bisphosphate or to rates of lactate output. In fed rats, the activity of 6-phosphofructo-2-kinase was not changed after treatment of adipocytes with insulin, noradrenaline or adenosine. It is suggested that the decrease in fructose 2,6-bisphosphate concentration observed after insulin treatment can be explained by the increase in sn-glycerol 3-phosphate, an inhibitor of 6-phosphofructo-2-kinase.

Difference in glucose sensitivity of liver glycolysis and glycogen synthesis. Relationship between lactate production and fructose 2,6-bisphosphate concentration

Incubation of isolated rat hepatocytes from fasted rats with 0-6 mM-glucose caused an increase in [fructose 2,6-bisphosphate] (0.2 to about 5 nmol/g) without net lactate production. A release of 3H2O from [3-3H]glucose was, however, detectable, indicating that phosphofructokinase was active and that cycling occurred between fructose 6-phosphate and fructose 1,6-bisphosphate. A relationship between [fructose 2,6-bisphosphate] and lactate production was observed when hepatocytes were incubated with [glucose] greater than 6 mM. Incubation with glucose caused a dose-dependent increase in [hexose 6-phosphates]. The maximal capacity of liver cytosolic proteins to bind fructose 2,6-bisphosphate was 15 nmol/g, with affinity constants of 5 X 10(6) and 0.5 X 10(6) M-1. One can calculate that, at 5 microM, more than 90% of fructose 2,6-bisphosphate is bound to cytosolic proteins. In livers of non-anaesthetized fasted mice, the activation of glycogen synthase was more sensitive to glucose injection than was the increase in [fructose 2,6-bisphosphate], whereas the opposite situation was observed in livers of fed mice. Glucose injection caused no change in the activity of liver phosphofructokinase-2 and decreased the [hexose 6-phosphates] in livers of fed mice.

Measurement of the rate of substrate cycling between fructose 6-phosphate and fructose 1,6-bisphosphate in skeletal muscle by using a single-isotope technique

The effects of several agents on the rates of the fructose 6-phosphate/fructose 1,6-bisphosphate substrate cycle were measured in incubated epitrochlearis muscles of the rat by monitoring the transfer of radiolabel from [6-14C]glucose to the 1-position of glucose residues in glycogen. The cycling rates observed were almost identical with those previously obtained by using the well-established dual-isotope technique. In particular, it was found that the beta-adrenoceptor agonist isoprenaline increased the cycling rate about 12-fold.

Abnormal meal carbohydrate disposition in insulin-dependent diabetes. Relative contributions of endogenous glucose production and initial splanchnic uptake and effect of intensive insulin therapy

Postprandial hyperglycemia in insulin-deficient, insulin-dependent diabetic subjects may result from impaired suppression of endogenous glucose production and/or abnormal disposition of meal-derived glucose. To investigate the relative contributions of these processes and to determine whether 2 wk of near normoglycemia achieved by using intensive insulin therapy could restore the pattern of glucose disposal to normal, meal-related and endogenous rates of glucose appearance were measured isotopically after ingestion of a mixed meal that contained deuterated glucose in seven lean insulin-dependent and five lean nondiabetic subjects. Diabetic subjects were studied once when insulin deficient and again during intensive insulin therapy after 2 wk of near normoglycemia. Total glucose production was determined by using tritiated glucose and the contribution of meal-related glucose was determined by using the plasma enrichment of deuterated glucose. The elevated basal and peak postprandial plasma glucose concentrations (252 +/- 33 and 452 +/- 31 mg/dl) of diabetic subjects when insulin deficient were decreased by intensive insulin therapy to values (82 +/- 6 and 193 +/- 10 mg/dl, P less than 0.01) that approximated those of nondiabetic subjects (93 +/- 3 and 140 +/- 15 mg/dl, respectively). Total and endogenous rates of glucose appearance (3,091 +/- 523 and 1,814 +/- 474 mg/kg per 8 h) in the diabetic subjects were significantly (P less than 0.02) greater than those in non-diabetic subjects (1,718 +/- 34 and 620 +/- 98 mg/kg per 8 h, respectively), whereas meal-derived rates of glucose appearance did not differ. Intensive insulin therapy decreased (P less than 0.01) both total (1,581 +/- 98 mg/kg per 8 h) and endogenous (478 +/- 67 mg/kg per 8 h) glucose appearance to rates that approximated those observed in the nondiabetic subjects, but did not alter meal-related glucose appearance. Thus, excessive entry of glucose into the peripheral circulation in insulin-deficient diabetic patients after ingestion of a mixed meal resulted from a lack of appropriate suppression of endogenous glucose production rather than impairment of initial splanchnic glucose uptake. Intensive insulin therapy restored postprandial suppression of endogenous glucose production to rates observed in nondiabetic subjects.

Role of fructose 2,6-bisphosphate in the control by glucagon of gluconeogenesis from various precursors in isolated rat hepatocytes

Hepatocytes from overnight-starved rats were incubated with 1-20 mM-fructose, -dihydroxyacetone, -glycerol, -alanine or -lactate and -pyruvate with or without 0.1 microM-glucagon. The production of glucose and lactate was measured, as was the content of fructose 2,6-bisphosphate. The concentrations of fructose (below 5 mM) and dihydroxyacetone (above 1 mM) that gave rise to an increase in fructose 2,6-bisphosphate were those at which a glucagon effect on the production of glucose and lactate could be observed. Glycerol had no effect on fructose 2,6-bisphosphate content or on production of lactate, and glucagon did not stimulate the production of glucose from this precursor. With alanine or lactate/pyruvate as substrates, glucagon stimulated glucose production whether the concentration of fructose 2,6-bisphosphate was increased or not. The extent of inactivation of pyruvate kinase by glucagon was not affected by the presence of the various gluconeogenic precursors. The role of fructose 2,6-bisphosphate in the effect of glucagon on gluconeogenesis from precursors entering the pathway at the level of triose phosphates or pyruvate is discussed.

Quantitative analysis of flux along the gluconeogenic, glycolytic and pentose phosphate pathways under reducing conditions in hepatocytes isolated from fed rats

Hepatocytes were isolated from the livers of fed rats and incubated with a mixture of glucose (10 mM), ribose (1 mM), mannose (4 mM), glycerol (3 mM), acetate (1.25 mM), and ethanol (5 mM) with one substrate labelled with 14C in any given incubation. Incorporation of label into CO2, glucose, glycogen, lipid glycerol and fatty acids, acetate and C-1 of glucose was measured at 20 and 40 min after the start of the incubation. The data (about 48 measurements for each interval) were used in conjunction with a single-compartment model of the reactions of the gluconeogenic, glycolytic and pentose phosphate pathways and a simplified model of the relevant mitochondrial reactions. An improved method of computer analysis of the equations describing the flow of label through each carbon atom of each metabolite under steady-state conditions was used to compute values for the 34 independent flux parameters in this model. A good fit to the data was obtained, thereby permitting good estimates of most of the fluxes in the pathways under consideration. The data show that: net flux above the level of the triose phosphates is gluconeogenic; label in the hexose phosphates is fully equilibrated by the second 20 min interval; the triose phosphate isomerase step does not equilibrate label between the triose phosphates; substrate cycles are operating at the glucose-glucose 6-phosphate, fructose 6-phosphate-fructose 1,6-bisphosphate and phosphoenolpyruvate-pyruvate-oxaloacetate cycles; and, although net flux through the enzymes catalysing the non-oxidative steps of the pentose phosphate pathway is small, bidirectional fluxes are large.

Stimulation of glycolysis and accumulation of a stimulator of phosphofructokinase in hepatocytes incubated with vasopressin

Vasopressin stimulates glycolysis in hepatocytes prepared from fed rats, or from starved rats when incubated with glucose. It causes the stimulation of phosphofructokinase activity and the accumulation of a stimulator of phosphofructokinase, which is probably fructose 2,6-bisphosphate, the recently discovered stimulatory of phosphofructokinase.