6-Phosphofructo-2-kinase/fructose-2,6-biphosphatase 4 is essential for p53-null cancer cells

The bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase-4 (PFKFB4) controls metabolic flux through allosteric regulation of glycolysis. Here we show that p53 regulates the expression of PFKFB4 and that p53-deficient cancer cells are highly dependent on the function of this enzyme. We found that p53 downregulates PFKFB4 expression by binding to its promoter and mediating transcriptional repression via histone deacetylases. Depletion of PFKFB4 from p53-deficient cancer cells increased levels of the allosteric regulator fructose-2,6-bisphosphate, leading to increased glycolytic activity but decreased routing of metabolites through the oxidative arm of the pentose-phosphate pathway. PFKFB4 was also required to support the synthesis and regeneration of nicotinamide adenine dinucleotide phosphate (NADPH) in p53-deficient cancer cells. Moreover, depletion of PFKFB4-attenuated cellular biosynthetic activity and resulted in the accumulation of reactive oxygen species and cell death in the absence of p53. Finally, silencing of PFKFB4-induced apoptosis in p53-deficient cancer cells in vivo and interfered with tumour growth. These results demonstrate that PFKFB4 is essential to support anabolic metabolism in p53-deficient cancer cells and suggest that inhibition of PFKFB4 could be an effective strategy for cancer treatment.

Direct and indirect activation of eukaryotic elongation factor 2 kinase by AMP-activated protein kinase

BACKGROUND

Eukaryotic elongation factor 2 (eEF2) kinase (eEF2K) is a key regulator of protein synthesis in mammalian cells. It phosphorylates and inhibits eEF2, the translation factor necessary for peptide translocation during the elongation phase of protein synthesis. When cellular energy demand outweighs energy supply, AMP-activated protein kinase (AMPK) and eEF2K become activated, leading to eEF2 phosphorylation, which reduces the rate of protein synthesis, a process that consumes a large proportion of cellular energy under optimal conditions.

AIM

The goal of the present study was to elucidate the mechanisms by which AMPK activation leads to increased eEF2 phosphorylation to decrease protein synthesis.

METHODS

Using genetically modified mouse embryo fibroblasts (MEFs), effects of treatments with commonly used AMPK activators to increase eEF2 phosphorylation were compared with that of the novel compound 991. Bacterially expressed recombinant eEF2K was phosphorylated in vitro by recombinant activated AMPK for phosphorylation site-identification by mass spectrometry followed by site-directed mutagenesis of the identified sites to alanine residues to study effects on the kinetic properties of eEF2K. Wild-type eEF2K and a Ser491/Ser492 mutant were retrovirally re-introduced in eEF2K-deficient MEFs and effects of 991 treatment on eEF2 phosphorylation and protein synthesis rates were studied in these cells.

RESULTS & CONCLUSIONS

AMPK activation leads to increased eEF2 phosphorylation in MEFs mainly by direct activation of eEF2K and partly by inhibition of mammalian target of rapamycin complex 1 (mTORC1) signaling. Treatment of MEFs with AMPK activators can also lead to eEF2K activation independently of AMPK probably via a rise in intracellular Ca²⁺. AMPK activates eEF2K by multi-site phosphorylation and the newly identified Ser491/Ser492 is important for activation, leading to mTOR-independent inhibition of protein synthesis. Our study provides new insights into the control of eEF2K by AMPK, with implications for linking metabolic stress to decreased protein synthesis to conserve energy reserves, a pathway that is of major importance in cancer cell survival.

AMPK antagonizes hepatic glucagon-stimulated cyclic AMP signalling via phosphorylation-induced activation of cyclic nucleotide phosphodiesterase 4B

Biguanides such as metformin have previously been shown to antagonize hepatic glucagon-stimulated cyclic AMP (cAMP) signalling independently of AMP-activated protein kinase (AMPK) via direct inhibition of adenylate cyclase by AMP. Here we show that incubation of hepatocytes with the small-molecule AMPK activator 991 decreases glucagon-stimulated cAMP accumulation, cAMP-dependent protein kinase (PKA) activity and downstream PKA target phosphorylation. Moreover, incubation of hepatocytes with 991 increases the V_max of cyclic nucleotide phosphodiesterase 4B (PDE4B) without affecting intracellular adenine nucleotide concentrations. The effects of 991 to decrease glucagon-stimulated cAMP concentrations and activate PDE4B are lost in hepatocytes deleted for both catalytic subunits of AMPK. PDE4B is phosphorylated by AMPK at three sites, and by site-directed mutagenesis, Ser304 phosphorylation is important for activation. In conclusion, we provide a new mechanism by which AMPK antagonizes hepatic glucagon signalling via phosphorylation-induced PDE4B activation.

The diabetes drug Metformin decreases hepatic glucose production and activates AMP-activated protein kinase (AMPK). Here the authors provide evidence that AMPK activation antagonizes glucagon signalling by activating PDE4B, lowering cAMP levels and decreasing PKA activation.

Identification of phosphorylation sites on human deoxycytidine kinase after overexpression in eucaryotic cells

Compelling evidence suggests that deoxycytidine kinase (dCK), a key enzyme in the salvage of deoxyribonucleosides and in the activation of clinically relevant nucleoside analogues, can be regulated by reversible phosphorylation. In this study, we show that dCK overexpressed in HEK-293T cells was labelled after incubation of the cells with [32P]orthophosphate. Tandem mass spectrometry allowed the identification of 4 in vivo phosphorylation sites, Thr3, Ser11, Ser15, and Ser74. These results provide the first evidence that dCK is constitutively multiphosphorylated in intact cells. In addition, site-directed mutagenesis demonstrated that phosphorylation of Ser74, the major in vivo phosphorylation site, is crucial for dCK activity.

Regulation of oxidative enzyme activity and eukaryotic elongation factor 2 in human skeletal muscle: influence of gender and exercise

AIM

To investigate gender-related differences in the responses of oxidative enzymes and eukaryotic elongation factor-2 (eEF2) to exercise.

METHODS

The influence of exercise (90 min, 60%VO(2peak)) on citrate synthase (CS) and beta-hydroxyacyl-CoA dehydrogenase (HAD) activity and mRNA content, together with eEF2 expression and phosphorylation at rest, were assessed in skeletal muscle of untrained (UT) and endurance trained (ET) females and males.

RESULTS

Citrate synthase and HAD mRNA were higher in females than in males (27% and 48%, respectively, P < 0.05) whereas CS and HAD activity did not differ between females and males (NS). In females only, CS activity was enhanced (P < 0.05) by 90 min exercise. Resting CS mRNA content did not differ between UT and ET but, nevertheless, CS activity was 56% higher in ET than in UT volunteers (P < 0.001). HAD mRNA and activity were not influenced by training status (NS). In UT, CS mRNA was enhanced 37% (P < 0.05) by exercise whereas exercise did not change CS mRNA in ET (NS). eEF2 expression was 31% higher (P < 0.05) and eEF2 Thr56 phosphorylation (which leads to translation inhibition) was 24% lower (P < 0.05) in females than in males. eEF2 expression and phosphorylation were unaffected by training status (NS).

CONCLUSION

Basal transcriptional, translational, and/or post-translational control of CS and HAD seems to be gender-dependent. Also, gender differences in translation and/or post-translational protein modification of CS occur during exercise. Accordingly, the potential for peptide-chain elongation, based on eEF2 expression and phosphorylation, appears to be higher in females than in males.

New targets of AMP-activated protein kinase

The discovery of the AMP-activated protein kinase (AMPK) more than a decade ago has shed much light on the cellular response to stresses characterized by a fall in the concentration of ATP and an increase in the AMP/ATP ratio. All conditions known to increase this ratio activate AMPK, whose major role is to act as an emergency signal to conserve ATP. It does so by inhibiting anabolic processes and by activating pathways producing ATP. In recent years, our laboratory has discovered new targets of AMPK. The purpose of this short review is to summarize our contribution to this field.

Insulin signal transduction in rat small intestine: role of MAP kinases in expression of mucosal hydrolases

The postreceptor events regulating the signal of insulin downstream in rat intestinal cells have not yet been analyzed. Our objectives were to identify the nature of receptor substrates and phosphorylated proteins involved in the signaling of insulin and to investigate the mechanism(s) by which insulin enhances intestinal hydrolases. In response to insulin, the following proteins were rapidly phosphorylated on tyrosine residues: 1) insulin receptor substrates-1 (IRS-1), -2, and -4; 2) phospholipase C-isoenzyme-gamma; 3) the Ras-GTPase-activating protein (GAP) associated with Rho GAP and p62(Src); 4) the insulin receptor beta-subunit; 5) the p85 subunits of phosphatidylinositol 3-kinase (PI 3-kinase); 6) the Src homology 2 alpha-collagen protein; 7) protein kinase B; 8) mitogen-activated protein (MAP) kinase-1 and -2; and 9) growth receptor-bound protein-2. Compared with controls, insulin enhanced the intestinal activity of MAP kinase-2 and protein kinase B by two- and fivefold, respectively, but did not enhance p70/S6 ribosomal kinase. Administration of an antireceptor antibody or MAP-kinase inhibitor PD-98059 but not a PI 3-kinase inhibitor (wortmannin) to sucklings inhibited the effects of insulin on mucosal mass and enzyme expression. We conclude that normal rat enterocytes express all of the receptor substrates and mediators involved in different insulin signaling pathways and that receptor binding initiates a signal enhancing brush-border membrane hydrolase, which appears to be regulated by the cascade of MAP kinases but not by PI 3-kinase.

Phosphorylation and activation of heart PFK-2 by AMPK has a role in the stimulation of glycolysis during ischaemia

BACKGROUND

The role of protein phosphorylation in the Pasteur effect--the phenomenon whereby anaerobic conditions stimulate glycolysis--has not been addressed. The AMP-activated protein kinase (AMPK) is activated when the oxygen supply is restricted. AMPK acts as an energy-state sensor and inhibits key biosynthetic pathways, thus conserving ATP. Here, we studied whether AMPK is involved in the Pasteur effect in the heart by phosphorylating and activating 6-phosphofructo-2-kinase (PFK-2), the enzyme responsible for the synthesis of fructose 2,6-bisphosphate, a potent stimulator of glycolysis.

RESULTS

Heart PFK-2 was phosphorylated on Ser466 and activated by AMPK in vitro. In perfused rat hearts, anaerobic conditions or inhibitors of oxidative phosphorylation (oligomycin and antimycin) induced AMPK activation, which correlated with PFK-2 activation and with an increase in fructose 2,6-bisphosphate concentration. Moreover, in cultured cells transfected with heart PFK-2, oligomycin treatment resulted in a parallel activation of endogenous AMPK and PFK-2. In these cells, the activation of PFK-2 was due to the phosphorylation of Ser466. A dominant-negative construct of AMPK abolished the activation of endogenous and cotransfected AMPK, and prevented both the activation and phosphorylation of transfected PFK-2 by oligomycin.

CONCLUSIONS

AMPK phosphorylates and activates heart PFK-2 in vitro and in intact cells. AMPK-mediated PFK-2 activation is likely to be involved in the stimulation of heart glycolysis during ischaemia.

A family of highly conserved glycosomal 2-hydroxyacid dehydrogenases from Phytomonas sp

Phytomonas sp. contains two malate dehydrogenase isoforms, a mitochondrial isoenzyme with a high specificity for oxaloacetate and a glycosomal isozyme that acts on a broad range of substrates (Uttaro, A. D., and Opperdoes, F.R. (1997) Mol. Biochem. Parasitol. 89, 51-59). Here, we show that the low specificity of the latter isoenzyme is the result of a number of recent gene duplications that gave rise to a family of glycosomal 2-hydroxyacid dehydrogenase genes. Two of these genes were cloned, sequenced, and overexpressed in Escherichia coli. Although both gene products have 322 amino acids, share 90.4% identical residues, and have a similar hydrophobicity profile and net charge, their kinetic properties were strikingly different. One isoform behaved as a real malate dehydrogenase with a high specificity for oxaloacetate, whereas the other showed no activity with oxaloacetate but was able to reduce other oxoacids, such as phenyl pyruvate, 2-oxoisocaproate, 2-oxovalerate, 2-oxobutyrate, 2-oxo-4-methiolbutyrate, and pyruvate.

Identification, cloning, and heterologous expression of a mammalian fructosamine-3-kinase

Fructosamines are thought to play an important role in the development of diabetic complications. Little is known about reactions that could metabolize these compounds in mammalian tissues, except for recent indications that they can be converted to fructosamine 3-phosphates. The purpose of the present work was to identify and characterize the enzyme responsible for this conversion. Erythrocyte extracts were found to catalyze the ATP-dependent phosphorylation of 1-deoxy-1-morpholinofructose (DMF), a synthetic fructosamine. The enzyme responsible for this conversion was purified approximately 2,500-fold by chromatography on Blue Sepharose, Q Sepharose, and Sephacryl S-200 and shown to copurify with a 35,000-M(r) protein. Partial sequences of tryptic peptides were derived from the protein by nanoelectrospray-ionization mass spectrometry, which allowed for the identification of the corresponding human and mouse cDNAs. Both cDNAs encode proteins of 309 amino acids, showing 89% identity with each other and homologous to proteins of unknown function predicted from the sequences of several bacterial genomes. Both proteins were expressed in Escherichia coli and purified. They were shown to catalyze the phosphorylation of DMF, fructoselysine, fructoseglycine, and fructose in order of decreasing affinity. They also phosphorylated glycated lysozyme, though not unmodified lysozyme. Nuclear magnetic resonance analysis of phosphorylated DMF and phosphorylated fructoseglycine showed that the phosphate was bound to the third carbon of the 1-deoxyfructose moiety. The physiological function of fructosamine-3-kinase may be to initiate a process leading to the deglycation of fructoselysine and of glycated proteins.

Partial purification and characterization of a wortmannin-sensitive and insulin-stimulated protein kinase that activates heart 6-phosphofructo-2-kinase

A wortmannin-sensitive and insulin-stimulated protein kinase (WISK), which phosphorylates and activates cardiac 6-phosphofructo-2-kinase (PFK-2), was partially purified from perfused rat hearts. Immunoblotting showed that WISK was devoid of protein kinase B (PKB), serum- and glucocorticoid-regulated protein kinase and protein kinase Czeta (PKCzeta). Comparison of the inhibition of WISK, PKCalpha and PKCzeta by different protein kinase inhibitors suggested that WISK was not a member of the PKC family. In addition, WISK contained no detectable phosphoinositide-dependent protein kinase-1 (PDK1) activity. WISK phosphorylated recombinant heart PFK-2 in a time-dependent manner to the extent of 0.4 mol of phosphate incorporated/mol of enzyme subunit, and increased the V(max) of PFK-2 twofold, without affecting the K(m) for fructose 6-phosphate. WISK phosphorylated Ser-466 to a greater extent than Ser-483 in recombinant heart PFK-2, and both sites were demonstrated to be phosphorylated to the same extent by PKB. Gel filtration and in-gel kinase analysis indicated that WISK was a monomer with a M(r) of 56500. Treatment of WISK with protein phosphatase 2A (PP2A) catalytic subunits reversed the effect of insulin, suggesting the involvement of an upstream activating kinase. Indeed, PDK1 was able to partially reactivate the PP2A-treated WISK and this reactivation was not enhanced by PtdIns(3,4,5)P(3)-containing vesicles. Moreover, a single 57000-M(r) band was labelled on incubation of the dephosphorylated WISK preparation with PDK1 and [gamma-(32)P]ATP. These findings provide evidence for the existence of a new protein kinase in the insulin signalling pathway, probably downstream of PDK1.

Heart 6-phosphofructo-2-kinase activation by insulin results from Ser-466 and Ser-483 phosphorylation and requires 3-phosphoinositide-dependent kinase-1, but not protein kinase B

Previous studies have shown that (i) the insulin-induced activation of heart 6-phosphofructo-2-kinase (PFK-2) is wortmannin-sensitive, but is insensitive to rapamycin, suggesting the involvement of phosphatidylinositol 3-kinase; and (ii) protein kinase B (PKB) activates PFK-2 in vitro by phosphorylating Ser-466 and Ser-483. In this work, we have studied the effects of phosphorylation of these residues on PFK-2 activity by replacing each or both residues with glutamate. Mutation of Ser-466 increased the V(max) of PFK-2, whereas mutation of Ser-483 decreased citrate inhibition. Mutation of both residues was required to decrease the K(m) for fructose 6-phosphate. We also studied the insulin-induced activation of heart PFK-2 in transfection experiments performed in human embryonic kidney 293 cells. Insulin activated transfected PFK-2 by phosphorylating Ser-466 and Ser-483. Kinase-dead (KD) PKB and KD 3-phosphoinositide-dependent kinase-1 (PDK-1) cotransfectants acted as dominant negatives because both prevented the insulin-induced activation of PKB as well as the inactivation of glycogen-synthase kinase-3, an established substrate of PKB. However, the insulin-induced activation of PFK-2 was prevented only by KD PDK-1, but not by KD PKB. These results indicate that the insulin-induced activation of heart PFK-2 is mediated by a PDK-1-activated protein kinase other than PKB.

Expression and regulation of 6-phosphofructo-2-kinase/fructose- 2,6-bisphosphatase isozymes in white adipose tissue

The aim of this work was to identify the 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) isozyme(s) present in white adipose tissue. Ion-exchange chromatography of PFK-2 from rat epididymal fat pads yielded an elution pattern compatible with the presence of both the L (liver) and M (muscle) isozymes. This was consistent with a study of the phosphorylation of the purified adipose tissue enzyme by cAMP-dependent protein kinase, by specific labelling of the preparation with [2-32P]fructose 2,6-bisphosphate and by reaction with antibodies. Characterization of the PFK-2/FBPase-2 mRNAs showed that mature adipocytes express the mRNA that codes for the L isozyme and the two mRNAs that code for the M isozyme. Preadipocytes expressed mRNA that codes for the M isozyme. Incubation of rat epididymal fat pads with adrenaline stimulated glycolysis but decreased fructose 2,6-bisphosphate concentrations without significant inactivation of PFK-2. These results support previous findings showing that fructose 2,6-bisphosphate is not involved in the adrenaline-induced stimulation of glycolysis in white adipose tissue.

Mechanisms of control of heart glycolysis

This review focuses on the mechanisms of control of heart glycolysis under conditions of normal and reduced oxygen supply. The kinetic properties and the biochemical characteristics of control steps (glucose transporters, hexokinase, glycogen phosphorylase and phosphofructokinases) in the heart are reviewed in the light of recent findings and are considered together to explain the control of glycolysis by substrate supply and availability, energy demand, oxygen deprivation and hormones. The role of fructose 2,6-bisphosphate in the control of glycolysis is analysed in detail. This regulator participates in the stimulation of heart glycolysis in response to glucose, workload, insulin and adrenaline, and it decreases the glycolytic flux when alternative fuels are oxidized. Fructose 2,6-bisphosphate integrates information from various metabolic and signalling pathways and acts as a glycolytic signal. Moreover, a hierarchy in the control of glycolysis occurs and is evidenced in the presence of adrenaline or cyclic AMP, which relieve the inhibition of glycolysis by alternative fuels and stimulate fatty acid oxidation. Insulin and glucose also stimulate glycolysis, but inhibit fatty acid oxidation. The mechanisms of control underlying this fuel selection are discussed. Finally, the study of the metabolic adaptation of glucose metabolism to oxygen deprivation revealed the implication of nitric oxide and cyclic GMP in the control of heart glucose metabolism.

Mutagenesis of the fructose-6-phosphate-binding site in the 2-kinase domain of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

Multiple alignment of several isozyme sequences of the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase revealed conserved residues in the 2-kinase domain. Among these residues, three asparagine residues (Asn76, Asn97 and Asn133; numbering refers to the liver isozyme sequence) and three threonine residues (Thr132, Thr134 and Thr135) are located near the fructose 6-phosphate-binding site in the crystal structure of the bifunctional enzyme. The role of these residues in substrate binding and catalysis in the 6-phosphofructo-2-kinase domain has been studied by mutagenesis to alanine. Since the crystal structure of 6-phosphofructo-2-kinase does not contain fructose 6-phosphate, this substrate was docked into the putative binding site by computer modelling, and its interactions with the protein were predicted. Analysis of the mutagenesis-induced changes in kinetic properties and of the substrate-docking model revealed that all these residues are directly or indirectly involved in fructose-6-phosphate binding. All the mutants displayed an increased Km for fructose 6-phosphate (10-200-fold). We propose that Asn133 stabilises Arg138, which itself makes a direct electrostatic bond with the 6-phosphate group of fructose 6-phosphate, that Asn76 interacts with the C3 hydroxyl group of fructose 6-phosphate, that Thr132 makes a hydrogen bond with the C6 oxygen of this substrate, and that Thr134 interacts with two residues involved in fructose-6-phosphate binding, Thr132 and Tyr199. On the other hand, Asn97 and Thr135 play structural roles, by maintaining the structure of the fructose-6-phosphate-binding pocket.

The glycosomal ATP-dependent phosphofructokinase of Trypanosoma brucei must have evolved from an ancestral pyrophosphate-dependent enzyme

Trypanosoma brucei contains an ATP-dependent phosphofructokinase (PFK), located in its glycosomes, which are peroxisome-like organelles sequestering the majority of its glycolytic enzymes. In this paper, we report the cloning and sequencing of the single-copy gene encoding this enzyme. Its amino-acid sequence is more similar to pyrophosphate (PPi)-dependent PFKs than to other ATP-dependent PFKs. A phylogenetic analysis suggests that the enzyme must have been derived from a PPi-dependent ancestral PFK, which changed its phospho-donor specificity during evolution. The enzyme is no longer capable of using PPi as phospho substrate, nor can it catalyze the reverse reaction as PPi-PFKs generally can. Moreover, the presence of a high pyrophosphatase activity in the cell renders it unlikely that PPi can function as free-energy source in present-day trypanosomes. It remains to be determined which mutations were responsible for the change in phospho-substrate specificity of the trypanosomatid PFK. As a result of its particular evolutionary history, the T. brucei PFK shows many structural differences, even at the active site, when compared with other ATP-dependent PFKs. These differences offer great potential for the structure-based design of trypanocidal drugs.

Identification of a novel Ca2+-stimulated S6-kinase in rat liver

Extracellular calcium addition transiently stimulated two S6 peptide kinase activities in isolated rat hepatocytes. Mono Q chromatography revealed that the activities eluting at 0.15 M NaCl and 0.18 M NaCl were stimulated 4-fold and 2-fold, respectively. The kinase stimulated by calcium was a 40000-Mr S6 peptide kinase, as demonstrated by partial purification from whole liver. The protein kinase did not crossreact with antibodies directed against the N- or C-terminal part of p70 ribosomal S6 kinase (p70(S6K)) and the C-terminal part of p90 ribosomal S6 kinase (p90(rsk)). Following digestion of 40000-Mr S6 peptide kinase with trypsin, six peptides were sequenced. There was no similarity with the sequences of p70(S6K) and p90(rsk). Moreover, the obtained sequences could not be identified in the SwissProt or EMBL-genebank databases, suggesting that 40000-Mr S6 peptide kinase probably represents a novel protein kinase.

Phosphorylation and activation of heart 6-phosphofructo-2-kinase by protein kinase B and other protein kinases of the insulin signaling cascades

To understand the insulin-induced activation of 6-phosphofructo-2-kinase (PFK-2) of the bifunctional enzyme PFK-2/fructose-2,6-bisphosphatase in heart, the effect of phosphorylation by protein kinases of the insulin signaling pathways on PFK-2 activity was studied. Purified PFK-2/fructose-2, 6-bisphosphatase from bovine heart is a mixture of two isoforms (Mr 58,000 and 54,000 on SDS-polyacrylamide gels). The Mr 54,000 protein is an alternatively spliced form, lacking phosphorylation sites for protein kinases. Recombinant enzymes corresponding to the Mr 58,000 (BH1) and Mr 54,000 (BH3) forms were expressed and used as substrates for phosphorylation. The recombinant BH1 isoform was phosphorylated by p70 ribosomal S6 kinase (p70(s6k)), mitogen-activated protein kinase-activated protein kinase-1, and protein kinase B (PKB), whereas the recombinant BH3 isoform was a poor substrate for these protein kinases. Treatment with all protein kinases activated PFK-2 in the recombinant BH1 preparation. Phosphorylation of the recombinant BH1 isoform correlated with PFK-2 activation and was reversed by treatment with protein phosphatase 2A. All the protein kinases phosphorylated Ser-466 and Ser-483 in the BH1 isoform, but to different extents: p70(s6k) preferentially phosphorylated Ser-466, whereas mitogen-activated protein kinase-activated protein kinase-1 and PKB phosphorylated Ser-466 and Ser-483 to a similar extent. We propose that PKB is part of the insulin signaling cascade for PFK-2 activation in heart.

Human L-3-phosphoserine phosphatase: sequence, expression and evidence for a phosphoenzyme intermediate

We report the sequence of the cDNA encoding human L-3-phosphoserine phosphatase. The encoded polypeptide contains 225 residues and shows 30% sequence identity with the Escherichia coli enzyme. The human protein was expressed in a bacterial expression system and purified. Similar to known L-3-phosphoserine phosphatases, it catalyzed the Mg2(+)-dependent hydrolysis of L-phosphoserine and an exchange reaction between L-serine and L-phosphoserine. In addition we found that the enzyme was phosphorylated upon incubation with L-[32P]phosphoserine, which indicates that the reaction mechanism proceeds via the formation of a phosphoryl-enzyme intermediate. The sensitivity of the phosphoryl-enzyme to alkali and to hydroxylamine suggests that an aspartyl- or a glutamyl-phosphate was formed. The nucleotide sequence of the cDNA described in this article has been deposited in the EMBL data base under accession number Y10275.

Cloning, sequencing and expression of rat liver 3-phosphoglycerate dehydrogenase

Rat liver d-3-phosphoglycerate dehydrogenase was purified to homogeneity and digested with trypsin, and the sequences of two peptides were determined. This sequence information was used to screen a rat hepatoma cDNA library. Among 11 positive clones, two covered the whole coding sequence. The deduced amino acid sequence (533 residues; Mr 56493) shared closer similarity with Bacillus subtilis 3-phosphoglycerate dehydrogenase than with the enzymes from Escherichia coli, Haemophilus influenzae and Saccharomyces cerevisiae. In all cases the similarity was most apparent in the substrate- and NAD+-binding domains, and low or insignificant in the C-terminal domain. A corresponding 2.1 kb mRNA was present in rat tissues including kidney, brain and testis, whatever the dietary status, and also in livers of animals fed a protein-free, carbohydrate-rich diet, but not in livers of control rats, suggesting transcriptional regulation. The full-length rat 3-phosphoglycerate dehydrogenase was expressed in E. coli and purified. The recombinant enzyme and the protein purified from liver displayed hyperbolic kinetics with respect to 3-phosphoglycerate, NAD+ and NADH, but substrate inhibition by 3-phosphohydroxypyruvate was observed; this inhibition was antagonized by salts. Similar properties were observed with a truncated form of 3-phosphoglycerate dehydrogenase lacking the C-terminal domain, indicating that the latter is not implicated in substrate inhibition or in salt effects. By contrast with the bacterial enzyme, rat 3-phosphoglycerate dehydrogenase did not catalyse the reduction of 2-oxoglutarate, indicating that this enzyme is not involved in human D- or L-hydroxyglutaric aciduria.

Site-directed mutagenesis of Lys-174, Asp-179 and Asp-191 in the 2-kinase domain of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

In a structural model of the 2-kinase domain of the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase based on the analogy with adenylate kinase, Lys-174, Asp-179 and Asp-191 residues are located in the putative active site. Asp-179 and Asp-191 are conserved in all known 6-phosphofructo-2-kinase sequences. In contrast, Lys-174 is conserved except in a yeast isoenzyme, fbp26, where it is replaced by glycine. Yeast fbp26 possesses fructose-2,6-bisphosphatase activity, but is devoid of 6-phosphofructo-2-kinase activity. Mutation of Asp-179 and Asp-191 of the rat liver isoenzyme to alanine increased the Km of 6-phosphofructo-2-kinase for fructose 6-phosphate 2000- and 1000-fold respectively, whereas mutation of Lys-174 to glycine decreased the Vmax of 6-phosphofructo-2-kinase more than 4000-fold. In contrast, none of the mutations affected the kinetic parameters of fructose-2,6-bisphosphatase. CD and fluorescence measurements indicated that the mutations had no effect on the structure and stability of the recombinant proteins. The results show that Asp-179 and Asp-191 participate in fructose 6-phosphate binding, whereas Lys-174 is important for catalysis. Therefore the natural mutation of Lys-174 to glycine in the fbp26 yeast isoenzyme could explain the lack of 6-phosphofructo-2-kinase activity. These results support a novel 6-phosphofructo-2-kinase structure model based on adenylate kinase.

Modelling the 2-kinase domain of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase on adenylate kinase

Simultaneous multiple alignment of available sequences of the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase revealed several segments of conserved residues in the 2-kinase domain. The sequence of the kinase domain was also compared with proteins of known three-dimensional structure. No similarity was found between the kinase domain of 6-phosphofructo-2-kinase and 6-phosphofructo-1-kinase. This questions the modelling of the 2-kinase domain on bacterial 6-phosphofructo-1-kinase that has previously been proposed [Bazan, Fletterick and Pilkis (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 9642-9646]. However, sequence similarities were found between the 2-kinase domain and several nucleotide-binding proteins, the most similar being adenylate kinase. A structural model of the 2-kinase domain based on adenylate kinase is proposed. It accommodates all the results of site-directed mutagenesis studies carried out to date on residues in the 2-kinase domain. It also allows residues potentially involved in catalysis and/or substrate binding to be predicted.

Mutagenesis of charged residues in a conserved sequence in the 2-kinase domain of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

Arg-136, Glu-137, Arg-138 and Arg-139 are conserved in all sequences of the 2-kinase domain of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. Their role was studied by site-directed mutagenesis. All the mutations had little, if any, effect on fructose-2,6-bisphosphatase activity. Mutations of Arg-136 and Glu-137 into Ala caused only minor modifications of phosphofructo-2-kinase activity. In contrast, mutation of Arg138 into Ala increased 280-fold the Km for fructose 6-phosphate of phosphofructo-2-kinase. Mutation of Arg-139 into Ala resulted in decreases in phosphofructo-2-kinase Vmax/Km for MgATP and fructose 6-phosphate 600-fold and 5000-fold respectively. Mutation of Arg-139 into Lys and Gln increased the Km of phosphofructo-2-kinase for MgATP (20-fold and 25-fold respectively) and for fructose 6-phosphate (8-fold and 13-fold), and the IC50 for MgADP (30-fold and 50-fold) and for magnesium citrate (7-fold and 25-fold). However, these two mutations did not affect nucleotide binding, as measured by quenching of intrinsic fluorescence. The changes in kinetic properties induced by mutations could not be attributed to structural changes. It is proposed that Arg-138 is involved in fructose 6-phosphate binding and that Arg-139 is probably involved in the stabilization of the transition state and so participates in catalysis.

Myelin protein Po as a potential autoantigen in autoimmune inner ear disease

We have previously shown that a 30,000 Mr protein extracted from guinea pig inner ear tissue is recognized by autoantibodies present in the serum of patients suffering from autoimmune inner ear disease. This protein was localized in the modiolus and in the organ of Corti. We have now identified this protein by a combination of microsequencing and matrix-assisted laser desorption ionization time-of-flight mass spectrometry of its tryptic peptides. A partial sequence of the protein was thereby determined. These data and 2-dimensional gel electrophoresis followed by immunoblotting experiments showed that the 30,000 Mr inner ear antigen is the major peripheral myelin protein Po. This suggests that protein Po may be an important autoantigen in autoimmune inner ear disease.

Signaling pathway involved in the activation of heart 6-phosphofructo-2-kinase by insulin

Incubation of isolated rat cardiomyocytes with insulin increased 2-deoxyglucose uptake, glycogen synthesis, and fructose 2, 6-bisphosphate content. Half-maximal effects were obtained with 1-2 nM insulin. The insulin-induced increase in fructose 2,6-bisphosphate content was preceded by a 2-3-fold activation of 6-phosphofructo-2-kinase, which was independent of glucose transport. Insulin activated phosphatidylinositol 3-kinase and p70 ribosomal S6 kinase (p70 S6 kinase), but had no significant effect on mitogen-activated protein kinase, although phorbol 12-myristate 13-acetate activated the latter. The effect of insulin on fructose 2, 6-bisphosphate, 6-phosphofructo-2-kinase, and phosphatidylinositol 3-kinase was blocked by wortmannin. However, rapamycin, which inhibited p70 S6 kinase activation, and PD 98059, an inhibitor of the mitogen-activated protein kinase pathway, had no effect on the insulin-induced activation of 6-phosphofructo-2-kinase. Heart 6-phosphofructo-2-kinase can therefore be regarded as a glycolytic target of insulin. Its activation by insulin might be mediated by phosphatidylinositol 3-kinase.

The ATP-binding site in the 2-kinase domain of liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. Study of the role of Lys-54 and Thr-55 by site-directed mutagenesis

All known 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase isozymes contain a sequence (GX4GK(S/T)) in the 6-phosphofructo-2-kinase domain corresponding to the so-called nucleotide binding fold signature or Walker A motif. Mutagenesis and crystal structure data from several nucleotide binding proteins, which also contain this sequence, showed the importance of the lysine and serine/threonine residues in nucleotide binding. We have studied the role of Lys-54 and Thr-55 in MgATP binding in the 6-phosphofructo-2-kinase domain of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase by site-directed mutagenesis. Lys-54 was mutated to methionine, whereas Thr-55 was mutated to valine, serine, and cysteine. Three mutants, Lys-54 to Met and Thr-55 to Cys or Val, displayed more than a 5000-fold decrease in 6-phosphofructo-2-kinase activity compared with the wild type. The mutations had no effect on fructose-2, 6-bisphosphatase activity and did not affect the activation of fructose-2,6-bisphosphatase after phosphorylation by cyclic 3', 5'-AMP-dependent protein kinase. Binding experiments with ATP, ADP, and their analogs (3'-N-methylanthraniloyl derivatives) showed that these two residues do not play the same role. Lys-54 is involved in ATP binding, whereas Thr-55 is important for catalysis.

Protein kinase signaling pathway triggered by cell swelling and involved in the activation of glycogen synthase and acetyl-CoA carboxylase in isolated rat hepatocytes

Incubation of isolated hepatocytes with glutamine or proline or in hypotonic media is known to activate glycogen synthase and acetyl-CoA carboxylase as a result of cell swelling. We report here that the same experimental conditions caused an activation of phosphatidylinositol 3-kinase and p70 ribosomal protein S6 kinase (p70 S6 kinase) but did not modify the activity of p42 mitogen-activated protein kinase. In addition, rapamycin, an inhibitor of p70 S6 kinase activation, prevented the amino acid- and hypotonicity-induced activation of p70 S6 kinase but did not block the activation of glycogen synthase and acetyl-CoA carboxylase, thus ruling out p70 S6 kinase as a necessary component in the activation pathway. By contrast, wortmannin or LY294002, inhibitors of phosphatidylinositol 3-kinase, completely blocked the activation of phosphatidylinositol 3-kinase and p70 S6 kinase and partly blocked the activation of glycogen synthase and acetyl-CoA carboxylase. Therefore, phosphatidylinositol 3-kinase might be a component of the signaling pathway that is triggered by cell swelling and is responsible, at least in part, for the activation of glycogen synthase and acetyl-CoA carboxylase. Incubation of hepatocytes with 0.1 microM epidermal growth factor doubled the activity of p42 mitogen-activated protein kinase without activating glycogen synthase.

Study of the roles of Arg-104 and Arg-225 in the 2-kinase domain of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase by site-directed mutagenesis

The roles of Arg-104 and Arg-225 located in the 2-kinase domain of the bifunctional enzyme 6-phosphofructo-2-kinase (PFK-2)/fructose-2,6-bisphosphatase (FBPase-2) have been studied by site-directed mutagenesis. In recombinant rat liver PFK-2/FBPase-2, mutation of Arg-225 to Ser increased the Km of PFK-2 for fructose-6-phosphate (Fru-6-P) 7-fold at pH 6 and decreased PFK-2 activity at suboptimal substrate concentrations between pH 6 and 9.5. The mutation had no effect on the Vmax of PFK-2 or on the Km of PFK-2 for MgATP. The mutation also increased the Vmax. of FBPase-2 4-fold without changing the Km for Fru-2,6-P2 or IC50 of Fru-6-P. These findings are in agreement with a previous study [Rider and Hue (1992) Eur. J. Biochem. 207, 967-972] on the protection by Fru-6-P of the labelling of Arg-225 by phenylglyoxal, and suggest that Arg-225 participates in Fru-6-P binding. In recombinant rat muscle PFK-2/FBPase-2, mutation of Arg-104 to Ser increased the Km for Fru-6-P 60-fold, increased the IC50 of citrate, increased the Vmax. 1.5-3-fold at pH 8.5 and altered the pH profile of PFK-2 activity. It did not affect the Km of PFK-2 for MgATP. The mutation also decreased the Vmax. of FBPase-2 3-fold, increased the Km for Fru-2,6-P2 70-fold and increased the IC50 of Fru-6-P at least 300-fold. Although the dimeric structure was maintained in the mutant, its PFK-2 activity was more sensitive towards inactivation by guanidinium chloride than the wild-type enzyme activity. The findings indicate that Arg-104 is involved in Fru-6-P binding in the PFK-2 domain and that it might also bind citrate. Structural changes resulting from the mutation might be responsible for the changes in kinetic properties of FBPase-2.

An agarose-based gel-concentration system for microsequence and mass spectrometric characterization of proteins previously purified in polyacrylamide gels starting at low picomole levels

An agarose-based concentration gel system is described for eluting and concentrating proteins previously purified either in one-dimensional or two-dimensional gels. Using the technique, proteins can be concentrated from about 1 ml into volumes as small as 10 microliters. After the proteins have been melted out of the agarose gels, they can be digested with proteases, producing peptide patterns similar to those observed with in-solution digestions. The overall peptide recovery, calculated from the amount of protein loaded on the primary separating gel to the collection of fragments after HPLC, is at least 70% of the peptide yields obtained with digests of the same amount of protein in free solution. These results are routinely obtained with 50 pmol amounts (referring to amounts of protein initially loaded on the primary gel). Proteins can also be analysed by a combination of microsequencing and on-line electrospray mass spectrometry, allowing their identification by peptide mass fingerprinting.

Site-directed mutagenesis of rat muscle 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: role of Asp-130 in the 2-kinase domain

Asp-130 of the recombinant skeletal-muscle 6-phosphofructo-2-kinase (PFK-2)/fructose-2,6-bisphosphatase was mutated into Ala in order to study its role in catalysis and/or substrate binding. The D130A mutant displayed a 30- to 140-fold decreased 2-kinase Vmax, depending on the pH, and a 30- and 60-fold increase in Km for MgATP and Fru-6-P respectively at pH 8.5 compared with the wild-type. Mutagenesis of Asp-130 to Ala had no effect on the 2-phosphatase activity, and fluorescence measurements indicated that the changes in kinetic properties of PFK-2 in the D130A mutant were not due to instability. The role of Asp-130 in the 2-kinase reaction is discussed and compared with that of Asp-103 of 6-phosphofructo-1-kinase from Escherichia coli, which binds Mg2+.

Cloning and expression of novel isoforms of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from bovine heart

Distinct 6-phosphofructo-2-kinase (PFK-2)/fructose 2,6-bisphosphatase (FBPase-2) cDNAs were cloned from bovine heart, showing that PFK-2/FBPase-2 gene B, which contains 16 exons, codes for at least five mRNAs. Three of them (B1, B2, B4) could encode the 58,000-M(r) isozyme. In B2 mRNA, exon 15 encodes four more residues than in B1. In B4 mRNA, exon 15 encodes six more residues than in B1, but exon 16 (20 residues) is missing. B3 mRNA corresponds to the 54,000-M(r) isozyme. It lacks exon 15 and also differs from the other mRNAs in the 5' noncoding region. B5 mRNA encodes a truncated form. When expressed in E. coli, the recombinant isoforms corresponding to all these mRNAs except B5 exhibited PFK-2 activity.

Rat muscle 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. Study of the kinase domain by site-directed mutagenesis

Sequence alignment and modeling of the 2-kinase domain of the liver bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2, 6-bisphosphatase, on 6-phosphofructo-1-kinase from Bacillus stearothermophilus and Escherichia coli (Bazan, J. F., Fletterick, R. J., and Pilkis, S. J. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 9642-9646) suggested that Cys-160 of the 2-kinase would correspond to Asp-127 of the 1-kinase, which acts as a general base catalyst. We have studied the validity of this alignment by site-directed mutagenesis of residues in the 2-kinase domain of skeletal muscle 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. Cys-160 was mutated to Asp or Ser. Two adjacent residues, Glu-157 and Asp-162, either of which could act as a general base catalyst, were mutated to Ala. Asp-162 corresponds to Asp-129 in the bacterial 1-kinase, which is also essential for catalysis and might bind Mg2+. None of these mutations significantly decreased the Vmax of the 2-kinase, suggesting that the mutated amino acids are not essential for catalysis and therefore do not play the same role as Asp-127 and Asp-129 in the bacterial 1-kinase. Mutation of Glu-157 and Asp-162 to alanine had no effect on the kinetic parameters of the bifunctional enzyme, indicating that these two negatively charged residues are not involved in catalysis and substrate binding.

Role of fructose 2,6-bisphosphate in the control of heart glycolysis

The aim of this work was to study whether changes in fructose 2,6-bisphosphate concentration are correlated with variations of the glycolytic flux in the isolated working rat heart. Glycolysis was stimulated to different extents by increasing the concentration of glucose, increasing the workload, or by the addition of insulin. The glycolytic flux was measured by the rate of detritiation of [2-3H]- and [3-3H]glucose. Under all the conditions tested, an increase in fructose 2,6-bisphosphate content was observed. The glucose- or insulin-induced increase in fructose 2,6-bisphosphate content was related to an increase in the concentration of fructose 6-phosphate, the substrate of 6-phosphofructo-2-kinase. An increase in the workload correlated with a 50% decrease in the Km of 6-phosphofructo-2-kinase for fructose 6-phosphate. Similar changes in Km have been observed when purified heart 6-phosphofructo-2-kinase was phosphorylated in vitro by the cyclic AMP-dependent protein kinase or by the calcium/calmodulin-dependent protein kinase. Since the concentration of cyclic AMP was not affected by increasing the workload, it is possible that the change in Km of 6-phosphofructo-2-kinase, which was found in hearts submitted to a high load, resulted from phosphorylation by calcium/calmodulin protein kinase; other possibilities are not excluded. Anoxia decreased the external work developed by the heart, stimulated glycolysis and glycogenolysis, but did not increase fructose 2,6-bisphosphate.

6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from frog skeletal muscle: purification, kinetics and immunological properties

Fructose 2,6-bisphosphate is the most potent activator of 6-phosphofructo-1-kinase, a key regulatory enzyme of glycolysis in animal tissues. This study was prompted by the finding that the content of fructose 2,6-bisphosphate in frog skeletal muscle was dramatically increased at the initiation of exercise and was closely correlated with the glycolytic flux during exercise. 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, the enzyme system catalyzing the synthesis and degradation of fructose 2,6-bisphosphate, was purified from frog (Rana esculenta) skeletal muscle and its properties were compared with those of the rat muscle type enzyme expressed in Escherichia coli using recombinant DNA techniques. 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from frog muscle was purified 5600-fold. 6-Phosphofructo-2-kinase and fructose-2,6-bisphosphatase activities could not be separated, indicating that the frog muscle enzyme is bifunctional. The enzyme preparation from frog muscle showed two bands on sodium dodecylsulphate polyacrylamide gel electrophoresis. The minor band had a relative molecular mass of 55,800 and was identified as a liver (L-type) isoenzyme. It was recognized by an antiserum raised against a specific amino-terminal amino acid sequence of the L-type isoenzyme and was phosphorylated by the cyclic AMP-dependent protein kinase. The major band in the preparations from frog muscle (relative molecular mass = 53,900) was slightly larger than the recombinant rat muscle (M-type) isoenzyme (relative molecular mass = 53,300). The pH profiles of the frog muscle enzyme were similar to those of the rat M-type isoenzyme, 6-phosphofructo-2-kinase activity was optimal at pH 9.3, whereas fructose-2,6-bisphosphatase activity was optimal at pH 5.5. However, the 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from frog muscle differed from other M-type isoenzymes in that, at physiological pH, the maximum activity of 6-phosphofructo-2-kinase exceeded that of fructose-2,6-bisphosphatase, the activity ratio being 1.7 (at pH 7.2) compared to 0.2 in the rat M-type isoenzyme. 6-Phosphofructo-2-kinase activity from the frog and rat muscle enzymes was strongly inhibited by citrate and by phosphoenolpyruvate whereas glycerol 3-phosphate had no effect. Fructose-2,6-bisphosphatase activity from frog muscle was very sensitive to the non-competitive inhibitor fructose 6-phosphate (inhibitor concentration causing 50% decrease in activity = 2 mumol.l-1).(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular forms of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase expressed in rat skeletal muscle

The rat cDNA for the muscle-type (M) isozyme of 6-phosphofructo-2-kinase (PFK-2)/fructose-2,6-bisphosphatase (FBPase-2) contains two putative translation initiation sites. To determine whether the M isozyme expressed in rat skeletal muscle corresponds to the short (PFK2M-sf) or the long (PFK2M-lf) isoform, we have expressed them in Escherichia coli. A third construction was also expressed in which the second ATG codon was deleted (PFK2M-lf delta ATG) to ensure that initiation started at the first ATG. The properties of these recombinant proteins were compared with those of the PFK-2/FBPase-2 present in rat skeletal muscle and liver. The recombinant proteins displayed PFK-2 and FBPase-2 activities and the M(r) values of the subunits measured by SDS-polyacrylamide gel electrophoresis were compatible with the calculated ones. The purified recombinant lf form contained not only the expected lf band (54,500 M(r)) but also the sf band (52,000 M(r)), indicating that the expression system could synthesize the long and the short isoforms from the same mRNA. The kinetic properties of the recombinant sf form were not different from those of the rat muscle enzyme. By contrast, lf delta ATG PFK-2 displayed a higher Km for its substrates and a lower Vmax. Immunoblotting with an antibody directed against the long isoform revealed a 54,500 M(r) band both in the lf and the lf delta ATG recombinant, but no band in rat skeletal muscle extracts. In these extracts, one band of 52,000 and a minor one of 54,500 M(r) were detected by an anti PFK-2/FBPase-2 antibody. The 54,500 M(r) band was recognized by an antibody directed against the L isozyme, suggesting that a small amount of the latter is expressed in skeletal muscle. Thus, the M isozyme differs from the L isozyme by replacement of the first 32 amino acids of the L isozyme by an unrelated nonapeptide.

Evidence for new phosphorylation sites for protein kinase C and cyclic AMP-dependent protein kinase in bovine heart 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

Bovine heart 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) was phosphorylated by incubation with [gamma-32P]MgATP and cyclic AMP-dependent protein kinase (PKA) or protein kinase C (PKC). After digestion with chymotrypsin, the phosphorylation sites for the two protein kinases were identified by peptide mapping, and microsequencing. Evidence for new phosphorylation sites for PKA (Ser-483) and PKC (Ser-84 and Ser-466) was obtained.

Inactivation of liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase by phenylglyoxal. Evidence for essential arginine residues

Treatment of liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase with the arginine-specific reagent, phenylglyoxal, irreversibly inactivated both 6-phosphofructo-2-kinase and fructose-6-bisphosphatase in a time-dependent and dose-dependent manner. Fructose 6-phosphate protected against 2,6-phosphofructo-2-kinase inactivation, whereas MgGTP protected against fructose-2,6-bisphosphatase inactivation. Semi-logarithmic plots of the time course of inactivation by different phenylglyoxal concentrations were non-linear, suggesting that more than one arginine residue was modified. The stoichiometry of phenylglyoxal incorporation indicated that at least 2 mol/mol enzyme subunit were incorporated. Enzyme which had been phosphorylated by cyclic-AMP-dependent protein kinase was inactivated to a lesser degree by phenylglyoxal, suggesting that the serine residue (Ser32) phosphorylated by cyclic-AMP-dependent protein kinase interacts with a modified arginine residue. Chymotryptic cleavage of the modified protein and microsequencing showed that Arg225, in the 6-phosphofructo-2-kinase domain, was one of the residues modified by phenylglyoxal. The protection by fructose 6-phosphate against the labelling of chymotryptic fragments containing Arg225, suggests that this residue is involved in fructose 6-phosphate binding in the 6-phosphofructo-2-kinase domain of the bifunctional enzyme.

The two forms of bovine heart 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase result from alternative splicing

Purified bovine heart 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) showed two bands with subunit M(r) of 58,000 and 54,000 when analysed by SDS/PAGE. Both the 58,000- and 54,000-M(r) forms were phosphorylated by cyclic AMP-dependent protein kinase (PKA) and by protein kinase C (PKC) in vitro. Phosphorylation by PKA decreased the apparent Km of PFK-2 for one of its substrates, fructose 6-phosphate, while phosphorylation by PKC did not correlate with any change in PFK-2 activity. The differences between the 58,000- and 54,000-M(r) forms were studied by electroblotting, peptide mapping and microsequencing. Residues 451-510, which correspond to exon 15 in the rat and contain phosphorylation sites for PKA (Ser-466) and PKC (Thr-475), were absent from the 54,000-M(r) form. Peptide mapping after phosphorylation by [gamma-32P]MgATP and PKC showed a phosphorylated peptide containing Thr-475, which was present in the 58,000-M(r) form but not in the 54,000-M(r) form. The fact that the latter form was phosphorylated by PKC and PKA suggests that other phosphorylation sites for PKA and PKC are located outside the region encoded by exon 15. Finally, analysis of RNA from bovine heart showed that the tissue contains two PFK-2/FBPase-2 mRNAs, only one of which was recognized by a probe specific to the region coding for Ser-466 and Thr-475. Taken together, these findings demonstrate that the 58,000- and 54,000-M(r) forms of bovine heart PFK-2/FBPase-2 result from alternative splicing of the same primary transcript.

Inhibition of 6-phosphofructo-2-kinase activity by mercaptopurines

The activity of 6-phosphofructo-2-kinase (PFK-2), the enzyme that catalyses the synthesis of fructose 2,6-bisphosphate (Fru-2,6-P2), was inhibited by mercaptopurines in vitro. Inhibition was observed with the purified enzyme from rat liver and bovine heart, and in extracts from rat lymphocytes and hepatoma cells, chick embryo fibroblasts, and human HeLa and lymphoblastoid cells. Half-maximal effect was obtained with 0.1-0.2 mM mercaptopurine and maximal inhibition ranged between 50 and 90% depending on the enzyme preparation. The inhibition resulted from a decrease in Vmax with no change in Km for ATP. The inhibition was relieved by treatment of the enzyme with thiol reducing agents, suggesting that it involves the formation of a mixed disulfide between mercaptopurine and thiol group(s) essential for enzyme activity. Incubation of intact lymphocytes or lymphoblastoid cells with 2- or 6-mercaptopurine resulted in a decrease in Fru-2,6-P2 content and lactate release. A decrease in Fru-2,6-P2 content but no change in lactate release was observed in HeLa cells and fibroblasts treated with 6-mercaptopurine but not with 2-mercaptopurine. Treatment of HeLa cells with 6-mercaptopurine resulted in a decreased PFK-2 activity which could be restored by treatment of the cell extract with dithiothreitol. In isolated rat hepatocytes and perfused rat hearts mercaptopurines had little or no effect on the Fru-2,6-P2 content and lactate release. These results suggest that the effect of 6-mercaptopurine of arresting growth in lymphoid cells might involve the inhibition of glycolysis in addition to the known inhibition of de novo purine nucleotide synthesis.

Vanadate inhibits liver fructose-2,6-bisphosphatase

Vanadate was found to be a reversible non-competitive inhibitor of chicken liver fructose-2,6-bisphosphatase. The inhibition was best observed in the presence of glycerol 2- or 3-phosphate and half-maximal effect was obtained with about 0.15 mM vanadate. Vanadate decreased the extent of phosphorylation of the enzyme (E-P) by fructose 2,6-[2-32P]bisphosphate. This did not result from an increased rate of E-P breakdown, as is the case with phosphoglycerate mutase, an enzyme which shares structural and functional similarity to fructose-2,6-bisphosphate. The data were consistent with the formation of a dead-end transition state analogue of phosphate in the active site. Inhibition of fructose-2,6-bisphosphatase by vanadate offers a likely explanation for the increase in fructose 2,6-bisphosphate concentration brought about by vanadate in isolated rat hepatocytes.

Inactivation of liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase by o-phthalaldehyde

The two activities of chicken liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase were inactivated by o-phthalaldehyde. Absorbance and fluorescence spectra of the modified enzyme were consistent with the formation of an isoindole derivative (1 mol/mol of enzyme subunit). The inactivation of 6-phosphofructo-2-kinase by o-phthalaldehyde was faster than the inactivation of fructose-2,6-bisphosphatase, which was concomitant with the increase in fluorescence. The substrates of 6-phosphofructo-2-kinase did not protect the kinase against inactivation, whereas fructose-2,6-bisphosphate fully protected against o-phthalaldehyde-induced inactivation of the bisphosphatase. Addition of dithiothreitol prevented both the increase in fluorescence and the inactivation of fructose-2,6-bisphosphatase, but not that of 6-phosphofructo-2-kinase. It is proposed that o-phthalaldehyde forms two different inhibitory adducts: a non-fluorescent adduct in the kinase domain and a fluorescent isoindole derivative in the bisphosphatase domain. A lysine and a cysteine residue could be involved in fructose-2,6-bisphosphate binding in the bisphosphatase domain of the protein.

Activation of rat liver plasma-membrane diacylglycerol kinase by vasopressin and phenylephrine

Plasma-membrane fractions were prepared from the livers of rats injected with 0.15 M-NaCl (controls) or vasopressin (1 nmol/kg body wt.). When assayed in the presence of deoxycholate, vasopressin increased the Vmax. of plasma-membrane diacylglycerol kinase 2-4-fold, and the apparent Km of the enzyme for 1,2-dioleoyl-sn-glycerol was doubled. The effect of vasopressin on the Vmax. of plasma-membrane diacylglycerol kinase was twice as great between pH 7 and 8.5 than at pH 6 or 6.5. Vasopressin doubled the activity of diacylglycerol kinase in the plasma-membrane fraction when the enzyme was assayed with phosphatidylserine rather than deoxycholate as stimulator, and when either 1-stearoyl-2-arachidonoyl-sn-glycerol or 1,2-dioleoyl-sn-glycerol was the substrate. In perfused livers vasopressin (10 nM) increased the Vmax. of plasma-membrane diacylglycerol kinase 2-fold, and phenylephrine (3 microM) gave a similar effect. Vasopressin doubled diacylglycerol kinase activity in hepatocytes that had been preincubated for 55 min, but not in cells that had only been preincubated for 15 min.

Rat hepatoma (HTC) cell 6-phosphofructo-2-kinase differs from that in liver and can be separated from fructose-2,6-bisphosphatase

6-Phosphofructo-2-kinase was purified from rat liver and hepatoma (HTC) cells. The HTC cell enzyme had kinetic properties different from those of the liver enzyme (more sensitive to inhibition by citrate and not inhibited by sn-glycerol 3-phosphate) and was not a substrate of the cyclic-AMP-dependent protein kinase. Unlike the liver enzyme, which is bifunctional and phosphorylated by fructose 2,6-[2-32P]bisphosphate, the HTC cell enzyme contained no detectable fructose-2,6-bisphosphatase activity and phosphorylation by fructose 2,6-[2-32P]-bisphosphate could not be detected. HTC cell fructose-2,6-bisphosphatase could be separated from 6-phosphofructo-2-kinase activity by purification. Antibodies raised against liver 6-phosphofructo-2-kinase did not precipitate HTC cell fructose-2,6-bisphosphatase whose kinetic properties were completely different from those of the liver enzyme.

Fructose 2,6-bisphosphate and its phosphorothioate analogue. Comparison of their hydrolysis and action on glycolytic and gluconeogenic enzymes

Purified chicken liver 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase was phosphorylated either from fructose 2,6-bis[2-32P]phosphate or fructose 2-phosphoro[35S]thioate 6-phosphate. The turnover of the thiophosphorylated enzyme intermediate as well as the overall phosphatase reaction was four times faster than with authentic fructose 2,6-bisphosphate. Fructose 2-phosphorothioate 6-phosphate was 10-100-fold less potent than authentic fructose 2,6-bisphosphate in stimulating 6-phosphofructo-1-kinase and pyrophosphate:fructose 6-phosphate phosphotransferase, but about 10 times more potent in inhibiting fructose 1,6-bisphosphatase. The analogue was twice as effective as authentic fructose 2,6-bisphosphate in stimulating pyruvate kinase from trypanosomes.

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.

Complete nucleotide sequence coding for rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase derived from a cDNA clone

cDNA clones for 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase were isolated from rat liver expression libraries in lambda gt11 by antibody, oligonucleotide, and cDNA screening. One 1860 bp long clone contained a full-length nucleotide sequence coding for the 470 amino acids of each of the two identical subunits of the bifunctional enzyme. This clone also contained untranslated sequences, one 173 bp long upstream from the ATG start codon and one 271 bp long downstream from the TGA stop codon. The clone was terminated by a poly(A) tail of 29 nucleotides.

Fructose 2,6-bisphosphate in rat erythrocytes. Inhibition of fructose 2,6-bisphosphate synthesis and measurement by glycerate 2,3-bisphosphate

The concentration of fructose 2,6-bisphosphate found in freshly isolated erythrocytes was below the limit of detection (20 pmol/ml of packed cells). However, it increased to about 250 pmol/ml of cells when erythrocytes were incubated with glucose at pH 6.9, but not at pH 7.4 or 8.2. This could be explained by variations in the content of glycerate 2,3-bisphosphate, which was found to inhibit 6-phosphofructo-2-kinase, the enzyme responsible for fructose 2,6-bisphosphate synthesis. Glycerate 2,3-bisphosphate was also found to inhibit the potato enzyme (pyrophosphate:fructose-6-phosphate 1-phosphotransferase) used for the measurement of fructose 2,6-bisphosphate.