Cyclic AMP-dependent phosphorylation of rat liver 6-phosphofructo 2-kinase, fructose 2,6-bisphosphatase

Short- and Long-Term Adaptation to Altered Levels of Glucose: Fifty Years of Scientific Adventure

My graduate and postdoctoral training in metabolism and enzymology eventually led me to study the short- and long-term regulation of glucose and lipid metabolism. In the early phase of my career, my trainees and I identified, purified, and characterized a variety of phosphofructokinase enzymes from mammalian tissues. These studies led us to discover fructose 2,6-P₂, the most potent activator of phosphofructokinase and glycolysis. The discovery of fructose 2,6-P₂ led to the identification and characterization of the tissue-specific bifunctional enzyme 6-phosphofructo-2-kinase:fructose 2,6-bisphosphatase. We discovered a glucose signaling mechanism by which the liver maintains glucose homeostasis by regulating the activities of this bifunctional enzyme. With a rise in glucose, a signaling metabolite, xylulose 5-phosphate, triggers rapid activation of a specific protein phosphatase (PP2ABδC), which dephosphorylates the bifunctional enzyme, thereby increasing fructose 2,6-P₂ levels and upregulating glycolysis. These endeavors paved the way for us to initiate the later phase of my career in which we discovered a new transcription factor termed the carbohydrate response element binding protein (ChREBP). Now ChREBP is recognized as the masterregulator controlling conversion of excess carbohydrates to storage of fat in the liver. ChREBP functions as a central metabolic coordinator that responds to nutrients independently of insulin. The ChREBP transcription factor facilitates metabolic adaptation to excess glucose, leading to obesity and its associated diseases.

Dimer interface rearrangement of the 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase rat liver isoenzyme by cAMP-dependent Ser-32 phosphorylation

The bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) is a key regulator of carbohydrate metabolism in liver. The goal of this study was to elucidate the regulatory role of Ser-32 phosphorylation on the kinase domain mediated dimerization of PFK-2/FBPase-2. Fluorescence-based mammalian two-hybrid and sensitized emission fluorescence resonance energy transfer analyses in cells revealed preferential binding within homodimers in contrast to heterodimers. Using isolated proteins a close proximity of two PFK-2/FBPase-2 monomers was only detectable in the phosphorylated enzyme dimer. Thus, a flexible kinase interaction mode exists, suggesting dimer conformation mediated coupling of hormonal and posttranslational enzyme regulation to the metabolic response in liver.

N- and C-termini modulate the effects of pH and phosphorylation on hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

Liver and skeletal muscle isoforms of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF2K/Fru-2,6-P(2)ase) isoenzymes are products of alternatively spliced first exons of the same gene, with common kinase and bisphosphatase domains. The muscle-specific exon-1 encodes nine unique amino acids, that lack the cAMP-dependent protein kinase (PK-A) phosphorylation site, and differ in sequence from those encoded by the liver-specific exon-1 (32 amino acids), contributing to its much lower affinity for fructose 6-phosphate (Fru-6-P). PK-A phosphorylation of the liver isoform at Ser(32) reduces the affinity of the kinase for Fru-6-P, and stimulates the bisphosphatase V(max). In the present study, we have defined the locus of interaction of the N-terminal residues with the N-terminal kinase and C-terminal domains by successive N- and C-terminal deletions. This study shows that: (1) residues Gly(5)-Glu(6)-Leu(7) of the liver isoform are responsible for increasing the affinity of 6PF2K for Fru-6-P, maintaining the inhibition of Fru-2,6-P(2)ase activity, and mediating the effects of PK-A phosphorylation on the two activities; (2) the loss of Fru-6-P inhibition of the bisphosphatase and the enhancement of its V(max), rather than the inhibition of the kinase, may be responsible for the behaviour of the muscle isoform primarily as a bisphosphatase; (3) the composition of residues 24-32 of the liver form appears to confer the enhanced kinase catalytic rate of this form over that of the muscle isoform. It is concluded that specific regions of the N-terminus of liver and skeletal muscle 6PF2K/Fru-2,6-P(2)ase have a role in adapting the two activities to work in the physiological range of pH and substrate concentrations found in each particular tissue.

Enzymatic and fluorescence studies of four single-tryptophan mutants of rat testis fructose 6-phosphate,2-kinase:fructose 2,6-bisphosphatase

In order to determine environments around four tryptophan residues, located in the N-terminus, in the kinase and in the phosphatase domains of rat testis Fru 6-P,2-kinase:Fru 2,6-bisphosphatase, mutant enzymes containing a single tryptophan were constructed by site-directed mutagenesis. The kinetic constants of these mutant enzymes were similar to those of the wild-type enzyme. The sum of the fluorescence intensities of the enzymes was 1.5 x that of the wild-type enzyme, and Trp 299, Trp 64, Trp 15, and Trp 320 contributed 38%, 28%, 17%, and 17%, respectively. The fluorescence polarization of the wild-type enzyme was significantly lower than any of the mutant enzymes, suggesting proximity of two tryptophan residues in the wild-type enzyme. The polarization in the presence of Fru 6-P affected only Trp 15, which suggested that it is located near the Fru 6-P binding site, but Trp 64 is not. Inactivation of both enzyme activities and unfolding of these enzymes in guanidine were monitored by activity assays and fluorescence intensities and maxima. Both Fru 6-P,2-kinase and Fru 2,6-bisphosphatase activities of all these enzymes were inactivated between 0.7 and 1 M guanidine. Enzymes containing Trp 64 or Trp 15 showed biphasic fractional unfolding curves, but those of Trp 299 or Trp 320 showed gradual steady changes. Fluorescence quenching by iodide indicated that Trp 64 was not accessible and that other Trp residues were only slightly accessible to solvent. These results suggest that all the Trp residues are in heterogeneous environments and that none are exposed on the protein surface.

Covalent control of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: insights into autoregulation of a bifunctional enzyme

The hepatic bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF-2-K/Fru-2,6-P2ase), E.C. 2.7-1-105/E.C. 3-1-3-46, is one member of a family of unique bifunctional proteins that catalyze the synthesis and degradation of the regulatory metabolite fructose-2,6-bisphosphate (Fru-2,6-P2). Fru-2,6-P2 is a potent activator of the glycolytic enzyme 6-phosphofructo-1-kinase and an inhibitor of the gluconeogenic enzyme fructose-1,6-bisphosphatase, and provides a switching mechanism between these two opposing pathways of hepatic carbohydrate metabolism. The activities of the hepatic 6PF-2-K/Fru-2,6-P2ase isoform are reciprocally regulated by a cyclic AMP-dependent protein kinase (cAPK)-catalyzed phosphorylation at a single NH2-terminal residue, Ser-32. Phosphorylation at Ser-32 inhibits the kinase and activates the bisphosphatase, in part through an electrostatic mechanism. Substitution of Asp for Ser-32 mimics the effects of cAPK-catalyzed phosphorylation. In the dephosphorylated homodimer, the NH2- and COOH-terminal tail regions also have an interaction with their respective active sites on the same subunit to produce an autoregulatory inhibition of the bisphosphatase and activation of the kinase. In support of this hypothesis, deletion of either the NH2- or COOH-terminal tail region, or both regions, leads to a disruption of these interactions with a maximal activation of the bisphosphatase. Inhibition of the kinase is observed with the NH2-truncated forms, in which there is also a diminution of cAPK phosphorylation to decrease the Km for Fru-6-P. Phosphorylation of the bifunctional enzyme by cAPK disrupts these autoregulatory interactions, resulting in inhibition of the kinase and activation of the bisphosphatase. Therefore, effects of cyclic AMP-dependent phosphorylation are mediated by a combination of electrostatic and autoregulatory control mechanisms.

Tolbutamide inhibits glucagon-induced phosphorylation of 6PF-2-K/Fru-2,6-P2ase in rat hepatocytes

In previous studies, we demonstrated that tolbutamide inhibits a phosphorylation of hepatic 6-phosphofructo-2-kinase (6PF-2-K)/fructose-2,6-bisphosphatase (Fru-2,6-P2ase) catalyzed by the adenosine 3',5'-cyclic monophosphate-dependent protein kinase in a reconstruction system using the purified enzyme from the rat liver. In the current study, to assess a role of tolbutamide on hepatic 6PF-2-K/Fru-2,6-P2ase physiologically, we used intact rat hepatocytes and examined effects of tolbutamide on a phosphorylation of the bifunctional enzyme in the presence of glucagon. Glucagon induced a rapid phosphorylation of hepatic 6PF-2-K/Fru-2,6-P2ase accompanied by an inhibition of 6PF-2-K activity and a stimulation of Fru-2,6-P2ase activity in a dose-dependent manner. Tolbutamide inhibited glucagon-induced phosphorylation of the bifunctional enzyme protein in a dose-dependent manner. By adding 2 mM tolbutamide, reduced activity of 6PF-2-K and increased activity of Fru-2,6-P2ase in the presence of 10(-9) M glucagon were partially restored. The present results suggest the possibility that tolbutamide modulates the activity of hepatic 6PF-2-K/Fru-2,6-P2ase through inhibiting a phosphorylation of the enzyme protein. The counterregulatory influence of tolbutamide on the effect of glucagon suggests a possible mechanism for the extrapancreatic effect of sulfonylurea drugs.

CS-045, a new oral antidiabetic agent, stimulates fructose-2,6-bisphosphate production in rat hepatocytes

Fructose-2,6-bisphosphate is a potent activator of 6-phosphofructo-1-kinase, a key enzyme in glycolysis. We previously revealed that sulfonylureas stimulate fructose-2,6-bisphosphate production in the rat liver by activating 6-phosphofructo-2-kinase. In the present study, we show that CS-045, a new antidiabetic agent, activated 6-phosphofructo-2-kinase and raised fructose-2,6-bisphosphate levels in dispersed rat hepatocytes. This action was time- and dose-dependent. Ten micromolar CS-045 raised the fructose-2,6-bisphosphate content linearly to the submaximal level in 20 min. Dose dependency was observed in the range of 1-30 microM. Thirty micromolar CS-045 completely reversed the inhibitory effect of 0.1 nM glucagon on fructose-2,6-bisphosphate production. CS-045 activated 6-phosphofructo-2-kinase by decreasing the Km value for the substrate (fructose-6-phosphate) without affecting the Vmax. The combination of suboptimal doses of CS-045 and tolbutamide increased fructose-2,6-bisphosphate content more than that induced by each agent alone. These results indicate that CS-045 may reduce plasma glucose by facilitating glycolysis in the liver.

Effects of tolbutamide on fructose-2,6-bisphosphate formation and ketogenesis in hepatocytes from diabetic rats

To assess the extrapancreatic action of sulfonylurea directly in the diabetic, effects of tolbutamide on hepatocyte fructose-2,6-bisphosphate (F-2,6-P2) formation and ketone production were investigated using isolated hepatocytes from streptozotocin (STZ)-induced diabetic rats. The basal level of hepatocyte F-2,6-P2 was significantly higher in diabetic rats within 2 weeks after STZ (40 mg/kg body weight) injection compared with that in the nondiabetic control group. Ultimately, a marked decrease in the F-2,6-P2 level was observed at 4 weeks after STZ administration (10% of the control). Although the addition of tolbutamide further increased the hepatocyte F-2,6-P2 level during the first week after STZ injection, no significant effect was observed after the second week and on from the initial STZ. Treatment of diabetes with insulin restored the stimulatory effect of tolbutamide on the hepatocyte F-2,6-P2 formation. Tolbutamide, independently of insulin treatment, lowered the ketone production of hepatocytes from diabetic rats. The present results indicate that insulin is necessary, in advance, for sulfonylurea to stimulate the liver F-2,6-P2 formation, while tolbutamide inhibition of hepatocyte ketone production is independent of insulin. These results provide further support for the role of sulfonylurea in regulating hepatic energy metabolism in the diabetic.

Phosphorylation of rat tyrosine hydroxylase and its model peptides in vitro by cyclic AMP-dependent protein kinase

The enzyme tyrosine hydroxylase catalyzes the first step in the biosynthesis of dopamine, norepinephrine, and epinephrine. Tyrosine hydroxylase is a substrate for cyclic AMP-dependent protein kinase as well as other protein kinases. We determined the Km and Vmax of rat pheochromocytoma tyrosine hydroxylase for cyclic AMP-dependent protein kinase and obtained values of 136 microM and 7.1 mumol/min/mg of catalytic subunit, respectively. These values were not appreciably affected by the substrates for tyrosine hydroxylase (tyrosine and tetrahydrobiopterin) or by feedback inhibitors (dopamine and norepinephrine). The high Km of tyrosine hydroxylase correlates with the high content of tyrosine hydroxylase in catecholaminergic cells. We also determined the kinetic constants for peptides modeled after actual or potential tyrosine hydroxylase phosphorylation sites. We found that the best substrates for cyclic AMP-dependent protein kinase were those peptides corresponding to serine 40. Tyrosine hydroxylase (36-46), for example, exhibited a Km of 108 microM and a Vmax of 6.93 mumol/min/mg of catalytic subunit. The next best substrate was the peptide corresponding to serine 153. The peptide containing the sequence conforming to serine 19 was a very poor substrate, and that conforming to serine 172 was not phosphorylated to any significant extent. The primary structure of the actual or potential phosphorylation sites is sufficient to explain the substrate behavior of the native enzyme.

Occurrence and characterization of fructose 6-phosphate, 2-kinase and fructose 2,6-bisphosphatase in Euglena gracilis

1. Fructose 6-phosphate, 2-kinase and fructose 2,6-bisphosphatase occurred in Euglena gracilis SM-ZK, and is located in cytosol. 2. Fructose 6-phosphate, 2-kinase and fructose 2,6-bisphosphatase were partially purified, and both enzyme activities were not separated during the partial purification. 3. The pH optimum for fructose 6-phosphate, 2-kinase activity was 7.0. The saturation curve of the enzyme activity for ATP concentration was hyperbolic, and the Km value for the substrate was 0.88 mM. On the other hand, the saturation curve of the enzyme activity for fructose 6-phosphate concentration was sigmoidal, and the K0.5 value for the substrate was 70 microM. 4. The pH optimum for fructose 2,6-bisphosphatase activity was 6.5. The saturation curve for fructose 2,6-bisphosphate concentration was sigmoidal, and the K0.5 value for the substrate was 1.29 microM. Fructose 2,6-bisphosphate showed a substrate inhibition at high concentration over 5 microM, and the enzyme activity was completely inhibited by 20 microM of fructose 2,6-bisphosphate.

Expression of the bisphosphatase domain of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase in Escherichia coli

The fructose-2,6-bisphosphatase domain of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (EC 2.7.105/EC 3.1.3.46) was expressed in Escherichia coli by using an expression system based on bacteriophage T7 RNA polymerase. The protein was efficiently expressed (i) as a fusion protein that starts at the T7 major capsid protein initiation site in a pET expression vector and (ii) as a protein that starts within the bisphosphatase sequence by translation reinitiation. Both proteins have similar properties. The protein was purified to homogeneity by anion-exchange chromatography and gel filtration. The purified fructose-2,6-bisphosphatase domain was active and no 6-phosphofructo-2-kinase activity was found associated with it. In contrast to the dimeric bifunctional enzyme, the fructose-2,6-bisphosphatase domain behaved as a monomer of 30 kDa. The turnover number and kinetic properties of the separate bisphosphatase domain were similar to those of the bisphosphatase of the bifunctional enzyme, including the ability to form a phosphoenzyme intermediate. These results support the hypothesis that the rat liver enzyme consists of two independent domains and is a member of a class of enzymes formed by gene fusion.

Electrophoretic determination of fructose 6-phosphate,2-kinase

An electrophoretic determination of fructose 6-phosphate,2-kinase activity has been devised. The enzymes partially purified from bovine liver and heart were subjected to polyacrylamide gel electrophoresis and the production of fructose 2,6-bisphosphate, coupled to the activation of potato PPi:phosphofructokinase, was detected as the dark band due to the disappearance of the fluorescence evoked by NADH consumption. The enzyme from bovine heart showed slower electrophoretic mobility than that from bovine liver, strongly suggesting the possibility that they may be distinct enzyme forms. Rat liver enzyme also gave a mobility different from that of both bovine liver and heart enzymes. Our present procedure will provide a new tool for understanding fructose-6-phosphate,2-kinase/fructose 2,6-bisphosphatase system in various tissues.

ATP-dependent activation of a new form of spinach leaf 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase

A novel form of 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase that possesses little 2-kinase or bisphosphatase activity as isolated has been partially purified from spinach (Spinacia oleracea L.) leaves. However, the new form can be activated by pretreatment with Mg X ATP at room temperature. After ATP activation, the fructose 2,6-bisphosphatase activity has a Michaelis constant for fructose 2,6-bisphosphate of about 1 mM, and is inhibited by high substrate concentrations (greater than 2 mM) and both end products. The kinase/phosphatase activity ratio of the new form was dependent on pH and varied from 0.3 at pH 7.0 to 5.0 at pH 8.2. In contrast, the previously characterized form of the enzyme (which is isolated in an active form and is unaffected by preincubation with Mg X ATP) had an activity ratio of about 2 that was insensitive to pH over the range tested. The ATP-dependent activation of the new enzyme form was stimulated by fructose 6-phosphate and inhibited by glucose 6-phosphate. These results explain why activation is not observed during assay of this enzyme, and indicate that the activation process may be regulated by metabolites. Collectively, these data provide further evidence for the existence, in spinach leaves, of two molecular forms of the enzyme which exhibit different kinetic properties.

Hormonal regulation of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: kinetic models

A graphical method to reveal the so-called 'critical fragments' in schemes of biochemical systems is considered. These fragments produce multiple steady states or self-oscillations in systems. As an example, the bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, regulated by glucagon through enzyme phosphorylation, is discussed. It is shown that this enzyme may act as a metabolic switching mechanism in discontinuous or oscillatory regimes, depending on the specific structure of its kinetic scheme. The boundaries of concentrational and parameter domains for these critical phenomena are also predicted.

Effects of 5-hydroxytryptamine, cyclic AMP, AMP, and fructose 2,6-bisphosphate on phosphofructokinase activity in Hymenolepis diminuta

1. 5-HT (10(-4) M) had no effect on the activity of phosphofructokinase in Hymenolepis diminuta. Concentrations of ATP above 33 microM inhibited PFK activity; AMP and cyclic AMP relieved this inhibition. 2. Local levels of cyclic AMP may be indirectly modulated by NaF, guanylyl imidophosphate, or 5-HT in the presence of GTP, which stimulates adenylyl cyclase activity x2 in H. diminuta homogenates. 3. Fructose 2,6-bisphosphate (F2BP), a physiological regulator of PFK activity in rat liver, also relieved ATP-induced inhibition of PFK. F2BP was present in supernatants from the worms at about 20 mumol/g wet wt. 4. 5-HT may cause an increase in the rate of glycolysis in H. diminuta by elevating either cyclic AMP and/or AMP levels; these nucleotides can in turn increase PFK activity.

Bisphosphorylated metabolites of glycerate, glucose, and fructose: functions, metabolism and molecular pathology

2,3-Bisphosphoglycerate, glucose 1,6-P2 and fructose 2,6-P2 have been recognized as regulatory signals implicated in the control of metabolism, oxygen affinity of red cells and other cellular functions. The alterations of their metabolism constitute a novel area in molecular pathology. The concentration of 2,3-bisphosphoglycerate in erythrocytes changes in a number of pathological conditions. An inherited deficiency of the multifunctional enzyme involved in the synthesis and breakdown of 2,3-bisphosphoglycerate in erythrocytes has been reported. The levels of glucose 1,6-P2 are reduced in the liver and in the muscle of rats with experimentally induced diabetes. In muscle of genetically dystrophic mice a decrease in the levels of glucose 1,6-P2 has been found, probably resulting from enhancement of glucose 1,6-P2 phosphatase activity. Fructose 2,6-P2 levels are decreased in the liver of experimental diabetic mice and rats, and elevated in the liver of genetically obese animals.

Hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: phosphate dependence and effects of other oxyanions

The effects of various oxyanions on the activities of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (EC 2.7.1.105/3.1.3.46) were examined. No evidence could be found for an absolute dependence of the kinase activity on inorganic phosphate as was recently reported by M. Laloux, E. Van Schaftingen, and H.-G. Hers ((1985) Eur. J. Biochem. 148, 155-159). Rather, phosphate and arsenate activated the kinase by decreasing the enzyme's Km for fructose 6-phosphate without affecting its Km for ATP or Vmax. The Km of the kinase for fructose 6-phosphate in the presence of inorganic phosphate was found to be significantly lower (6 microM) than previously reported (30 microM) when the hydrolysis of fructose 2,6-bisphosphate by the concomitant bisphosphatase activity at low Fru 6-P concentrations was taken into account. The KA's for phosphate and arsenate activation of the kinase were 0.2 and 0.3 mM, respectively. A number of other oxyanions, including pyrophosphate, sulfate, tungstate, selenate, and molybdate all inhibited the kinase by increasing the Km for fructose 6-phosphate. The apparent Ki's for inhibition of the kinase were in the 0.5-1 mM range. In contrast, all of these oxyanions activated the bisphosphatase, with half-maximal effects requiring millimolar concentrations. Inorganic phosphate was the most potent activator with a KA of 1 mM. In contrast to the other oxyanions, vanadate and meta-periodate inhibited the kinase but had no effect on the bisphosphatase. Vanadate appeared to be a noncompetitive inhibitor since its effects were not overcome by Pi, ATP, or fructose 6-phosphate, and the species responsible was shown to be decavanadate. Like vanadate, meta-periodate had no effect on the bisphosphatase, though it was a potent inhibitor (I0.5 = 30 microM) of the kinase. Its effects were shown to be time-dependent and reversed by dithiothreitol, suggesting that it acted by an oxidative mechanism. These results augment the mounting body of evidence that the enzyme's two reactions are catalyzed at discrete active sites.

Tolbutamide and insulin stimulation of fructose-2,6-bisphosphate formation in hepatocytes differ

The effects of tolbutamide and pancreatic hormones on liver fructose-2,6-bisphosphate (F-2,6-P2) formation were examined using isolated rat hepatocytes. Glucagon decreased the F-2,6-P2 level in a dose-dependent manner. Insulin (greater than 10(-9) M) increased the F-2,6-P2 level reduced by glucagon (less than 10(-9) M), but did not show a stimulatory effect on this activator formation in the absence of glucagon. On the other hand, tolbutamide increased the F-2,6-P2 level in hepatocytes regardless of the presence or absence of glucagon. Tolbutamide (2 mM) stimulation on liver F-2,6-P2 formation was enhanced by the concomitant addition of insulin (10(-8) M) in the presence of glucagon (3 X 10(-11) M). These observations suggest that the regulatory effect of tolbutamide on liver F-2,6-P2 level is independent of that of insulin.

Effect of tolbutamide on fructose-6-phosphate,2-kinase and fructose-2,6-bisphosphatase in rat liver

The effects of tolbutamide on the activities of fructose-6-phosphate,2-kinase and fructose-2,6-bisphosphatase were examined using rat hepatocytes. Tolbutamide stimulated fructose-6-phosphate,2-kinase activity and inhibited fructose-2,6-bisphosphatase activity, resulting in an increase of fructose-2,6-bisphosphate level. Changes in the activities of the enzyme by tolbutamide were due to variation in the Km value, but not dependent on alteration of Vmax. Glucagon inhibition of fructose-2,6-bisphosphate formation resulting from an inactivation of fructose-6-phosphate,2-kinase and an activation of fructose-2,6-bisphosphatase was released by tolbutamide. Tolbutamide stimulation of fructose-2,6-bisphosphate formation through regulation of fructose-6-phosphate,2-kinase/fructose-2,6-bisphosphatase may produce enhancement of glycolysis and inhibition of gluconeogenesis in the liver.

Reaction of phosphofructokinase 2/fructose 2,6-bisphosphatase with monoclonal antibodies. A proof of the bifunctionality of the enzyme

Monoclonal antibodies were derived from mice immunized against homogeneous chicken liver phosphofructokinase 2/fructose 2,6-bisphosphatase. Of 112 clones, 30 were found to secrete antibodies that specifically reacted with the antigen in enzyme-linked immunoabsorbant assay (ELISA) while 17, which were ELISA-negative, produced antibodies that affected the enzymic activity of the antigen. Four clones were subcloned and used for an extensive investigation of the reaction of the corresponding antibodies with the supposedly bifunctional enzyme. A definite proof of the bifunctionality of the enzyme was obtained from the two following observations. First, the two activities were similarly retained by the four antibodies that had been coupled to Sepharose. Second, one of the antibodies inhibited both activities with the same efficiency. Furthermore, the antigen-antibody reaction led to the formation of aggregates with an apparent molecular mass of several megadaltons, showing that the two subunits of the antigen reacted with the same antibody and were therefore identical. The four monoclonal antibodies affected the activity of phosphofructokinase 2. This effect was seen as an up to 17-fold activation as well as an up to 85% inhibition. Only one of the four antibodies (antibody 10) had inhibitory effects on fructose 2,6-bisphosphatase, an effect which was in part explained by a decrease in the rate of formation of the intermediary phosphoenzyme. All the effects described above were obtained on both the chicken liver and the pigeon muscle enzymes but with lower doses of antibody in the case of the former enzyme. Antibody 10 was also shown to react with mouse liver phosphofructokinase 2/fructose 2,6-bisphosphatase, and with phosphofructokinase 2 from chicken brain, heart and testis and from frog skeletal muscle and liver. None of the four antibodies cross-reacted with phosphofructokinase 2 from Saccharomyces cerevisiae or from spinach leaves.

Purification and properties of phosphofructokinase 2/fructose 2,6-bisphosphatase from chicken liver and from pigeon muscle

Phosphofructokinase 2 and fructose 2,6-bisphosphatase extracted from either chicken liver or pigeon muscle co-purified up to homogeneity. The two homogeneous proteins were found to be dimers of relative molecular mass (Mr) close to 110,000 with subunits of Mr 54,000 for the chicken liver enzyme and 53,000 for the pigeon muscle enzyme. The latter also contained a minor constituent of Mr 54,000. Incubation of the chicken liver enzyme with the catalytic subunit of cyclic-AMP-dependent protein kinase in the presence of [gamma-32P]ATP resulted in the incorporation of about 0.8 mol phosphate/mol enzyme. Under similar conditions, the pigeon muscle enzyme was phosphorylated to an extent of only 0.05 mol phosphate/mol enzyme and all the incorporated phosphate was found in the minor 54,000-Mr constituent. The maximal activity of the native avian liver phosphofructokinase 2 was little affected by changes of pH between 6 and 10. Its phosphorylation by cyclic-AMP-dependent protein kinase resulted in a more than 90% inactivation at pH values below 7.5 and in no or little change in activity at pH 10. Intermediary values of inactivation were observed at pH values between 8 and 10. Muscle phosphofructokinase 2 had little activity at pH below 7 and was maximally active at pH 10. Its partial phosphorylation resulted in a further 25% decrease of its already low activity measured at pH 7.1 and in a negligible inactivation at pH 8.5. Phosphoenolpyruvate and citrate inhibited phosphofructokinase 2 from both origins non-competitively. The muscle enzyme and the phosphorylated liver enzyme displayed much more affinity for these inhibitors than the native liver enzyme. Fructose 2,6-bisphosphatase from both sources had about the same specific activity but only the chicken liver enzyme was activated about twofold upon incubation with ATP and cyclic-AMP-dependent protein kinase. All enzyme forms were inhibited by fructose 6-phosphate and this inhibition was released by inorganic phosphate and by glycerol 3-phosphate. Both liver and muscle fructose 2,6-bisphosphatases formed a 32P-labeled enzyme intermediate when incubated in the presence of fructose 2,6-[2-32P]bisphosphate.

Tissue distribution, immunoreactivity, and physical properties of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

6-Phosphofructo-2-kinase (EC 2.7.1.105) and fructose-2,6-bisphosphatase (EC 3.1.3.46) activities were determined in various rat tissues, the latter by using a method based on the formation of a phosphorylated enzyme intermediate during the course of catalysis. Both activities from liver, skeletal muscle, lung, kidney, and testis copurified during polyethylene glycol fractionation, anion-exchange and blue Sepharose chromatography, and gel filtration. The Stokes radius of these enzymes and of the liver bifunctional enzyme was 45 A. Extrahepatic tissues had only 10% or less of the kinase activity found in liver. The results indicate that a liver-type bifunctional enzyme is present in most extrahepatic tissues but that it is minimally expressed. However, the ratio of kinase to bisphosphatase activity in most extrahepatic tissues was 4- to 6-fold higher than in liver, whereas heart 6-phosphofructo-2-kinase had no associated bisphosphatase activity, although its Stokes radius was also 45 A. The heart enzyme was not precipitated by an antiserum to the liver enzyme, whereas only a fraction of the kidney and testis activities was precipitated by this antiserum. The data support the existence of a distinct form of extrahepatic 6-phosphofructo-2-kinase, most readily demonstrated in heart, which may not be bifunctional.

Adipose-tissue phosphofructokinase. Rapid purification and regulation by phosphorylation in vitro

A new procedure for the purification of phosphofructokinase using Blue Dextran-Sepharose is described. This allowed an approx. 1000-fold purification of phosphofructokinase from rat white and brown adipose tissue to be achieved in essentially a single step. The purified enzymes from both tissues were found to exhibit hyperbolic kinetics with fructose 6-phosphate, to be inhibited by ATP and citrate, and to be activated by 5'-AMP, phosphate and fructose 2,6-bisphosphate. The enzymes were phosphorylated by the catalytic subunit of cyclic AMP-dependent protein kinase, and phosphorylation was found to be associated with increases in activity when the enzymes were assayed under appropriate sub-optimal conditions. In particular, the phosphorylated enzymes exhibited less inhibition by ATP and the white-adipose-tissue enzyme was more sensitive to activation by fructose 2,6-bisphosphate. It is suggested that an increase in the cytoplasmic concentration of cyclic AMP in tissues other than liver may result in an increase in glycolysis through the phosphorylation of phosphofructokinase by cyclic AMP-dependent protein kinase.

Studies of the structure of fructose-6-phosphate 2-kinase:fructose-2,6-bisphosphatase

Some physicochemical properties of a homogeneous preparation of a bifunctional enzyme, fructose-6-phosphate 2-kinase:fructose-2,6-bisphosphatase, were determined. The molecular weight of the enzyme is 101 000 as determined by high-speed sedimentation equilibrium. The molecular weight of dissociated enzyme is 55 000 in 6 M guanidinium chloride by sedimentation equilibrium and in sodium dodecyl sulfate by polyacrylamide gel electrophoresis. A value of 4.7 was observed for the isoelectric point. Tryptic peptide maps and high-performance liquid chromatography of the trypsin-digested enzyme revealed approximately 60 peptides. Amino acid analysis of the enzyme shows that it contains 27 lysine and 36 arginine residues per 55 000 daltons. No free N-terminal amino acid residue was detectable, suggesting that it is blocked. Hydrolysis of the enzyme by carboxypeptidases A and B releases tyrosine followed by histidine and arginine, indicating that the amino acid sequence at the carboxyl terminus is probably -Arg-His-Tyr. Tryptic digestion of [32P]phosphofructose-6-phosphate 2-kinase:fructose-2,6-bisphosphatase yields a 32P-labeled peptide detected by tryptic peptide mapping and high-performance liquid chromatography. Thermolysin digestion of CNBr-cleaved 32P-enzyme also yields a single 32P-peptide. These results indicate that fructose-6-phosphate 2-kinase:fructose-2,6-bisphosphatase is a dimer of 55 000 daltons and the subunits are very similar, if not identical.

Isolation and characterization of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from bovine liver

6-Phosphofructo-2-kinase and fructose-2,6-bisphosphatase activities were copurified to homogeneity from bovine liver. The purification scheme consisted of polyethylene glycol precipitation, anion-exchange and Blue-Sepharose chromatography, substrate elution from phosphocellulose, and gel filtration. The bifunctional enzyme had an apparent molecular weight of 102,000 and consisted of two subunits (Mr 49,000). The kinase had a Km for ATP of 12 microM and a S0.5 for fructose 6-phosphate of 150 microM while the bisphosphatase had a Km for fructose 2,6-bisphosphate of 7 microM. Both activities were subject to modulation by various effectors. Inorganic phosphate stimulated both activities, while alpha-glycerolphosphate inhibited the kinase and stimulated the bisphosphatase. The pH optimum for the 6-phosphofructo-2-kinase activity was 8.5, while the fructose-2,6-bisphosphatase reaction was maximal at pH 6.5. Incubation of the purified enzyme with [gamma-32P]ATP and the catalytic subunit of the cAMP-dependent protein kinase resulted in 32P incorporation to the extent of 0.7 mol/mol enzyme subunit with concomitant inhibition of the kinase activity and activation of the bisphosphatase activity. The mediation of the bisphosphatase reaction by a phosphoenzyme intermediate was suggested by the isolation of a stable labeled phosphoenzyme when the enzyme was incubated with fructose 2,6-[2-32P]bisphosphate. The pH dependence of hydrolysis of the phospho group suggested that it was linked to the N3 of a histidyl residue. The 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from bovine liver has properties essentially identical to those of the rat liver enzyme, suggesting that hepatic fructose 2,6-bisphosphate metabolism is under the same control in both species.

Phosphate dependency of phosphofructokinase 2

Experiments performed at micromolar concentrations of inorganic phosphate support the conclusion that liver phosphofructokinase 2 would be completely inactive in the absence of inorganic phosphate or arsenate. The concentration of inorganic phosphate that allowed half-maximal activity decreased with increasing pH, being approximately 0.11 mM at pH 6.5 and 0.05 mM at pH 8. The effect of phosphate was to increase V and to decrease Km for fructose 6-phosphate, without affecting Km for ATP. Citrate and P-enolpyruvate inhibited the enzyme non-competitively with fructose 6-phosphate and independently of the concentration of inorganic phosphate. Phosphorylation of the enzyme by the catalytic subunit of cyclic-AMP-dependent protein kinase did not markedly modify the phosphate requirement and its effect of inactivating phosphofructokinase 2 could not be counteracted by excess phosphate. A nearly complete phosphate dependency was also observed with phosphofructokinase 2 purified from Saccharomyces cerevisiae or from spinach leaves. By contrast, the fructose 2,6-bisphosphatase activity of the liver bifunctional enzyme was not dependent on the presence of inorganic phosphate. Phosphate increased this activity about threefold when measured in the absence of added fructose 6-phosphate and a half-maximal effect was reached at approximately 0.5 mM phosphate. Like glycerol phosphate, phosphate counteracted the inhibition of fructose 2,6-bisphosphatase by fructose 6-phosphate, but a much higher concentration of phosphate than of glycerol phosphate was required to reach this effect.

Sulfonylurea stimulates liver fructose-2,6-bisphosphate formation in proportion to its hypoglycemic action

The effects of 5 sulfonylureas on fructose-2,6-bisphosphate (F-2,6-P2) formation using isolated perfused rat liver were examined. All sulfonylureas examined stimulated dose-dependent formation of the activator in a limited range of the concentration. A maximum effect on F-2,6-P2 formation was observed at the concentration of 10(-3) M of tolbutamide or chlorpropamide, at 10(-4) M of gliclazide or acetohexamide and at 10(-6) M of glibenclamide. These concentrations of sulfonylurea correspond with those in blood when therapeutical doses of the drug are administered orally. Sulfonamide and biguanide did not show any stimulatory effect on F-2,6-P2 level. The results demonstrate that stimulation of liver F-2,6-P2 formation is a common characteristic of sulfonylureas and suggest strongly that one of the extrapancreatic actions of sulfonylurea is stimulation of F-2,6-P2 formation followed by enhancement of glycolysis and inhibition of gluconeogenesis in the liver.

Contribution of different protein phosphatases to the dephosphorylation of 6-phosphofructo-1-kinase and 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase in rat liver

The nature of rat liver protein phosphatases involved in the dephosphorylation of the glycolytic key enzyme 6-phosphofructo-1-kinase and the regulatory enzyme 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase was investigated. In terms of the classification system proposed by Ingebritsen & Cohen [(1983) Eur. J. Biochem. 132, 255-261], only the type-2 protein phosphatases 2A (which can be separated into 2A1 and 2A2) and 2C act on these substrates. Fractionation of rat liver extracts by anion-exchange chromatography and gel filtration revealed that protein phosphatase 2A is responsible for most of the 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase phosphatase activity (activity ratio 2A/2C = 4:1). On the other hand, 6-phosphofructo-1-kinase phosphatase activity is equally distributed between protein phosphatases 2A (2A1 plus 2A2) and 2C. In addition, the possible role of low-Mr compounds for the control of purified protein phosphatase 2C was examined. At near-physiological concentrations, none of the metabolites studied significantly affected the rate of dephosphorylation of 6-phosphofructo-1-kinase, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, pyruvate kinase or fructose-1,6-bisphosphatase.

The mechanism by which ethanol decreases the concentration of fructose 2,6-bisphosphate in the liver

The intragastric administration of ethanol to fed rats caused in their liver, within about 1 h, a 20-fold decrease in the concentration of fructose 2,6-bisphosphate, an activation of fructose 2,6-bisphosphatase, an inactivation of phosphofructo-2-kinase but no change in the concentration of cyclic AMP. Incubation of isolated hepatocytes in the presence of ethanol caused a rapid increase in the concentration of sn-glycerol 3-phosphate and a slower and continuous decrease in the concentration of fructose 2,6-bisphosphate with no change in that of hexose 6-phosphates. There was also a relatively slow activation of fructose 2,6-bisphosphatase and inactivation of phosphofructo-2-kinase. Glycerol and acetaldehyde had effects similar to those of ethanol on the concentration of phosphoric esters in the isolated liver cells. 4-Methylpyrazole cancelled the effect of ethanol but reinforced those of acetaldehyde. High concentrations of glucose or of dihydroxyacetone caused an increase in the concentration of hexose 6-phosphates and counteracted the effect of ethanol to decrease the concentration of fructose 2,6-bisphosphate. As a rule, hexose 6-phosphates had a positive effect and sn-glycerol 3-phosphate had a negative effect on the concentration of fructose 2,6-bisphosphate in the liver, so that, at a given concentration of hexose 6-phosphates, there was an inverse relationship between the concentration of fructose 2,6-bisphosphate and that of sn-glycerol 3-phosphate. These effects could be explained by the ability of sn-glycerol 3-phosphate to inhibit phosphofructo-2-kinase and to counteract the inhibition of fructose 2,6-bisphosphatase by fructose 6-phosphate. sn-Glycerol 3-phosphate had also the property to accelerate the inactivation of phosphofructo-2-kinase by cyclic AMP-dependent protein kinase whereas fructose 2,6-bisphosphate had the opposite effect. The changes in the activity of phosphofructo-2-kinase and fructose 2,6-bisphosphatase appear therefore to be the result rather than the cause of the decrease in the concentration of fructose 2,6-bisphosphate.