Phosphofructokinase from Ascaris suum. Purification and properties

Molecular and biochemical characterisation of abomasal nematode parasites Teladorsagia circumcincta and Haemonchus contortus phosphofructokinases and their recognition by the immune host

Full length cDNAs encoding phosphofructokinase (PFK) were cloned from Teladorsagia circumcincta (TcPFK) and Haemonchus contortus (HcPFK). TcPFK (2361 bp) and HcPFK (2367 bp) cDNA encoded 787 and 789 amino acid proteins respectively. The predicted amino acid sequences showed 98% similarity with each other and 70% with a Caenorhabditis elegans PFK. Substrate binding sites were completely conserved in both proteins. Soluble N-terminal His-tagged PFK proteins were expressed in Escherichia coli strain BL21, purified and characterised. The recombinant TcPFK and HcPFK had very similar kinetic properties: the pH optima were pH 7.0, Km for fructose 6-phosphate was 0.50 ± 0.01 and 0.55 ± 0.01 mM respectively when higher (inhibiting concentration, 0.3 mM) ATP concentration was used and the curve was sigmoidal. The Vmax for TcPFK and HcPFK were 1110 ± 16 and 910 ± 10 nM min(-1 )mg(-1) protein respectively. Lower ATP concentration (non-inhibiting, 0.01 mM) did not change the Vmax for TcPFK and HcPFK (890 ± 10 and 860 ± 12 nM min(-1 )mg(-1) protein) but the substrate affinity doubled and Km for fructose 6-phosphate were 0.20 ± 0.05 and 0.25 ± 0.01 mM respectively. Recognition of TcPFK and HcPFK by mucosal and serum antibodies in nematode exposed animals demonstrates antigenicity and suggests involvement in the host response to nematode infection.

The kinetics and regulation of phosphofructokinase from Teladorsagia circumcincta

Phosphofructokinase (PFK-1) activity was examined in L(3) and adult Teladorsagia circumcincta, both of which exhibit oxygen consumption. Although activities were higher in the adult stage, the kinetic properties of the enzyme were similar in both life cycle stages. T. circumcincta PFK-1 was subject to allosteric inhibition by high ATP concentration, which increased both the Hill coefficient (from 1.4±0.2 to 1.7±0.2 in L(3)s and 2.0±0.3 to 2.4±0.4 in adults) and the K(½) for fructose 6 phosphate (from 0.35±0.02 to 0.75±0.05mM in L(3)s and 0.40±0.03 to 0.65±0.05mM in adults). The inhibitory effects of high ATP concentration could be reversed by fructose 2,6 bisphosphate and AMP, but glucose 1,6 bisphosphate had no effect on activity. Similarly, phosphoenolpyruvate had no effect on activity, while citrate, isocitrate and malate exerted mild inhibitory effects, but only at concentrations exceeding 2mM. The observed kinetic properties for T. circumcincta PFK-1 were very similar to those reported for purified Ascaris suum PFK-1, though slight differences in sensitivity to ATP concentration suggests there may be subtle variations at the active site. These results are consistent with the conservation of properties of PFK-1 amongst nematode species, despite between species variation in the ability to utilise oxygen.

Forty years of helminth biochemistry

This review describes some of the developments in helminth biochemistry that have taken place over the last 40 years. Since the early 1970s the main anabolic and catabolic pathways in parasitic helminths have been worked out. The mode of action of the majority of anthelmintics is now known, but in many cases the mechanisms of resistance remain elusive. Developments in helminth biochemistry have depended heavily on developments in other areas. High throughput methods such as proteomics, transcriptomics and genome sequencing are now generating vast amounts of new data. The challenge for the future is to interpret and understand the biological relevance of this new information.

Optimum activity of the phosphofructokinase from Ascaris suum requires more than one metal ion

Phosphofructokinase (PFK) catalyzes the phosphorylation of fructose 6-phosphate (F6P) to give fructose 1,6-bisphosphate (FBP) using MgATP as the phosphoryl donor. As the concentration of Mg(2+) increases above the concentration needed to generate the MgATP chelate complex, a 15-fold increase in the initial rate was observed at low MgATP. The effect of Mg(2+) is limited to V/K(MgATP), and initial rate studies indicate an equilibrium-ordered addition of Mg(2+) before MgATP. Isotope partitioning of the dPFK:MgATP complex indicates a random addition of MgATP and F6P at low Mg(2+), with the rate of release of MgATP from the central E:MgATP:F6P complex 4-fold faster than the net rate constant for catalysis. This can be contrasted with the ordered addition of MgATP prior to F6P at high Mg(2+). The addition of fructose 2,6-bisphosphate (F26P(2)) has no effect on the mechanism at low Mg(2+), with the exception of a 4-fold increase in the affinity of the enzyme for F6P. At high Mg(2+), F26P(2) causes the kinetic mechanism to become random with respect to MgATP and F6P and with MgATP released from the central complex half as fast as the net rate constant for catalysis. The latter is in agreement with previous studies [Gibson, G. E., Harris, B. G., and Cook, P. F. (1996) Biochemistry 35, 5451-5457]. The overall effect of Mg(2+) is a decrease in the rate of release of MgATP from the E:MgATP:F6P complex, independent of the concentration of F26P(2).

Phosphofructokinase from muscle of the marine giant barnacle Austromegabalanus psittacus: kinetic characterization and effect of in vitro phosphorylation

The kinetic properties of phosphofructokinase from muscle of the giant cirripede Austromegabalanus psittacus were characterized, after partial purification by ion exchange chromatography on DEAE-cellulose. This enzyme showed differences regarding PFKs from other marine invertebrates: the affinity for fructose 6-phosphate (Fru 6-P) was very low, with an S(0.5) of 22.6+/-1.4 mM (mean+/-S.D., n=3), and a high cooperativity (n(H) of 2.90+/-0.21; mean+/-S.D., n=3). The barnacle PFK showed hyperbolic saturation kinetics for ATP (apparent K(m ATP)=70 microM, at 5 mM Fru 6-P, in the presence of 2 mM ammonium sulfate). ATP concentrations higher than 1 mM inhibited the enzyme. Ammonium sulfate activated the PFK several folds, increasing the affinity of the enzyme for Fru 6-P and V(max). 5'-AMP (0.2 mM) increased the affinity for Fru 6-P (S(0.5) of 6.2 mM). Fructose 2,6-bisphosphate activated the PFK, with a maximal activation at concentrations higher than 2 microM. Citrate reverted the activation of PFK produced by 0.2 mM 5'-AMP (IC(50 citrate)=2.0 mM), producing a higher inhibition than that exerted on other invertebrate PFKs. Barnacle muscular PFK was activated in vitro after exposure to exogenous cyclic-AMP (0.1 mM) as well as by phosphatidylserine (50 microg/ml), indicating a possible control by protein kinase A and a phospholipid dependent protein kinase (PKC). The results suggest a highly regulated enzyme in vivo, by allosteric mechanisms and also by protein phosphorylation.

CLONING AND NUCLEOTIDE SEQUENCE OF A FULL-LENGTH cDNA ENCODING ASCARIS SUUM PHOSPHOFRUCTOKINASE

The nucleotide sequence of a full-length cDNA encoding phosphofructokinase (PFK) enzyme from the parasitic nematode Ascaris suum was determined. The entire sequence of 2,653 bases comprises a single open reading frame of 2,452 bases and a noncoding region of 201 bases after the stop codon. The mature protein contains 812 amino acids and has a molecular mass of 90,900 Da. The amino acid sequences of several peptides derived from the purified protein show excellent correspondence with the translated nucleotide sequence. Comparison of the amino acid sequence of the protein with those of 3 other worms as well as those of human, rabbit, and bacterial enzymes reveals highly conserved regions interrupted with stretches of lesser sequence similarity. Analyses of the subunit primary structure reveal, as in other eukaryotic PFKs, that the amino-terminal half is homologous to the carboxy-terminal half, supporting the hypothesis that the PFK gene evolved by duplication of the prokaryotic gene and that the allosteric sites arose by mutations at the catalytic site. The location of the phosphorylation site is unique and different compared with other PFKs and plays a key role in regulation of the enzyme activity. Structural motifs such as the putative substrate and effector binding domains and also the key amino acids involved therein are clearly identified by alignment of all the PFK protein sequences.

Minor participation of cAMP on the protein kinase phosphorylation of mitochondrial and cytosolic fractions from Ascaris suum: a comparative study with porcine heart muscle

In contrast to porcine heart muscle in which cAMP effectively activated the phosphorylation of cytosolic proteins, cAMP exerted a minor effect on the phosphorylation of proteins from the soluble fraction of Ascaris suum muscle. Similarly, cAMP did not enhance the kinase activity in the mitochondrial membranes from porcine heart and A. suum, although major differences in protein phosphorylation were observed between both fractions. However, cAMP-dependent protein kinases (PKA) were evidenced in the parasitic soluble mitochondrial fraction, since the phosphorylation of histone IIA and kemptide was augmented in this fraction, in the presence of cAMP. An increase in the phosphorylation of exogenously added A. suum phosphofructokinase was also obtained when cAMP was added to the parasite soluble mitochondrial fraction. The phosphorylation of phosphofructokinase by this fraction was inhibited when kemptide and cAMP were included in the reaction mixture, suggesting substrate competition for the same PKA. Although PKI (6-22), a reported inhibitor of the catalytic subunit of mammalian cAMP-dependent PKAs, did not affect the endogenous phosphorylation of proteins in the various A. suum fractions, an inhibition on the phosphorylation of exogenously added kemptide and phosphofructokinase was observed when PKI (6-22) was incubated with the parasite mitochondrial soluble fraction.

Kinetic characterization of a T-state of Ascaris suum phosphofructokinase with heterotropic negative cooperativity by ATP eliminated

The affinity analogue, 2',3'-dialdehyde ATP has been used to chemically modify the ATP-inhibitory site of Ascaris suum phosphofructokinase, thereby locking the enzyme into a less active T-state. This enzyme form has a maximum velocity that is 10% that of the native enzyme in the direction of fructose 6-phosphate (F6P) phosphorylation. The enzyme displays sigmoid saturation for the substrate fructose 6-phosphate (S0.5 (F6P) = 19 mM and nH = 2.2) at pH 6.8 and a hyperbolic saturation curve for MgATP with a Km identical to that for the native enzyme. The allosteric effectors, fructose 2,6-bisphosphate and AMP, do not affect the S0.5 for F6P but produce a slight (1.5- and 2-fold, respectively) V-type activation with Ka values (effector concentration required for half-maximal activation) of 0.40 and 0.24 mM, respectively. Their activating effects are additive and not synergistic. The kinetic mechanism for the modified enzyme is steady-state-ordered with MgATP as the first substrate and MgADP as the last product to be released from the enzyme surface. The decrease in V and V/K values for the reactants likely results from a decrease in the equilibrium constant for the isomerization of the E:MgATP binary complex, thus favoring an unisomerized form. The V and V/KF6P are pH dependent with similar pK values of about 7 on the acid side and 9.8 on the basic side. The microenvironment of the active site appears to be affected minimally as evidenced by the similarity of the pK values for the groups involved in the binding site for F6P in the modified and native enzymes.

Isotope partitioning with Ascaris suum phosphofructokinase is consistent with an ordered kinetic mechanism

Isotope partitioning and initial velocity studies have been used to study the kinetic mechanism of Ascaris suum phosphofructokinase (PFK) at pH 8.0 for the native enzyme (nPFK), and at pH 6.8 for a form of enzyme desensitized (dPFK) to hysteresis in the reaction time course, to ATP allosteric inhibition, and to F6P homotropic cooperativity. Complete trapping (P*max approximately equal to 100%) of the E:MgATP* complex as fructose (1-32P)-1, 6-bisophosphate for both enzyme forms is consistent with the previously proposed steady-state ordered mechanism [Rao, G.S.J., Harris, B.G., & Cook, P.F. (1987) J.Biol. Chem. 262, 14074-14079] with MgATP binding before fructose 6-phosphate (F6P). K'F6P values for trapping of MgATP of 0.54 +/- 0.09 mM for nPFK and 0.85 +/- 0.15 mM for dPFK were obtained. Saturating amounts of the heterotropic activator fructose 2, 6-bisphosphate (F26P2) gives no change in the trapping parameters for nPFK with a P*max of 100% and a K'F6P of 0.40 +/- 0.06 mM. For dPFK, however, F26P2 causes a decrease in both parameters, giving a P*max of 54% and a K'F6P of 0.26 +/- 0.07 mM. The partial trapping of E:MgATP* in the presence of F26P2 for dPFK suggests that the activator changes the kinetic mechanism from an ordered to a random binding of substrates. Initial velocity studies confirm the change in mechanism. Uncompetitive inhibition by arabinose 5-phosphate (Ara5P), a dead-end inhibitory analog of F6P, versus MgATP for nPFK in the absence and presence of F26P2 is consistent with an ordered mechanism with MgATP adding to enzyme prior to F6P. An uncompetitive pattern is also obtained with dPFK for Ara5P versus MgATP in the absence of F26P2, but the pattern becomes noncompetitive in the presence of F26P2, consistent with a change to a random mechanism. No trapping of the E:[14C]F6P complex could be detected, indicating either that the E:[14C]F6P complex does not form in a significant amount under the conditions used or that the off-rate for F6P from enzyme is much faster than the net rate constant for formation of the first product, FBP. The data are consistent with a predominantly ordered mechanism with MgATP binding prior to F6P. The minor pathway with MgATP dissociating from the E:F6P:MgATP ternary complex becomes apparent for the dPFK in the presence of F26P2.

Acid-base catalytic mechanism and pH dependence of fructose 2,6-bisphosphate activation of the Ascaris suum phosphofructokinase

A form of phosphofructokinase (PFK) from Ascaris suum desensitized to hysteresis in the reaction time course and ATP allosteric inhibition has been used to study the activation by fructose 2,6-bisphosphate (F26P2) at varied pH in both reaction directions. In the direction of phosphorylation of F6P, V and V/KMgATP are constant over the pH range 6-9, while V/KF6P decreases at low pH, giving a pK value of 7.0, and at high pH, giving a pK of 8.9. V and V/KMgATP are insensitive to the presence of F26P2, but V/KF6P is increased by a constant amount in the presence of saturating F26P2 over the entire pH range studied. The concentration of F26P2 that gives half the change in V/KF6P, Kact, increases as the pH decreases, giving a pK of 7.4, reflecting an enzyme group that must be unprotonated for optimum binding of F26P2. In the direction of phosphorylation of MgADP, V and V/KMgADP are pH-independent, and both are insensitive to the presence of F26P2. V/KFBP decreases at high pH, giving a pK of about 7.3, and is increased by a constant amount in the presence of F26P2 over the entire pH range studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and properties of phosphofructokinase from Dictyostelium discoideum

Phosphofructokinase (PFruK) from the slime mold Dictyostelium discoideum has been purified to homogeneity over 15,000-fold with a 29% yield. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis of the final preparation revealed a single band of 95 kDa. The native molecular mass was determined by gel filtration to be 382 kDa, indicating that the enzyme is a homotetramer. An antibody raised in rabbits against the 95-kDa band immunoprecipitated PFruK activity while it did not react with the enzyme from yeast and mammalian cells. The apparent pI was 6.8 and the pH optimum was 7.6. The enzyme had an activation energy (Ea) of 29.1 kJ/mol. The amino acid composition was distinctive in having high Ser, Gly and Glx and low Ala, Val and Tyr compared with other eukaryotic PFruKs. Enzyme activity did not have a sigmoidal saturation curve for fructose 6-phosphate, was only mildly inhibited by MgATP at acidic pH values, was not affected by enzyme concentration and was insensitive to any of the typical allosteric effectors of PFruKs from other sources. However, the enzyme binds fructose 2,6-bisphosphate as indicated by protection against thermal denaturation. Treatment with cAMP-dependent protein kinase led to phosphorylation of the enzyme without change in activity. The metabolic significance of these properties and their relationship to structure/function are discussed.

Histochemical localization of key glycolytic and related enzymes in adult Onchocerca fasciata

The activities of some key enzymes of the glycolytic and pentose phosphate pathways were investigated histochemically in adult female Onchocerca fasciata (Nematoda: Filarioidea). The distribution patterns of phosphofructokinase (PFK), aldolase (ALD), glyceraldehyde 3-phosphate dehydrogenase (G3PDH) and glucose 6-phosphate dehydrogenase (G6PDH) in different tissues of the worm were determined by employing NitroBlue Tetrazolium (NBT). The glycolytic enzymes PFK, ALD, and G3PDH were distributed throughout the hypodermal tissue, somatic muscles and reproductive organs. These enzyme activities were predominantly expressed in the hypodermal and reproductive tissues, both of which appeared to be metabolically more active than adjacent tissues. The high activities of the enzymes studied in the hypodermal tissue when compared with the minimal or low activity in the intestinal epithelium support the assumption that the worm's intestine, in contrast to the body wall, plays no significant role in the nutrient acquisition process. The results emphasize that both the glycolytic and hexose monophosphate pathways of carbohydrate metabolism are active components in energy production and biosynthetic processes in the various tissues of the worm. The functional significance of these glucose-metabolizing enzymes has been discussed with regard to their location in the tissues concerned.

Phosphofructokinase from white muscle of the rainbow trout, Oncorhynchus mykiss: purification and properties

Phosphofructokinase was purified and characterized from the white skeletal muscle of rainbow trout Oncorhynchus mykiss. Purification involved three steps: ion-exchange chromatography on hydroxyapatite and affinity chromatography on phosphocellulose and ATP-agarose. A final specific activity of 75 units per mg of protein at 22 degrees C and pH 7.2 with 40% recovery was obtained. The purified enzyme gave a single band on SDS-PAGE with a subunit molecular mass of 76.5 +/- 0.6 kDa. Based on gel filtration analysis, the active form of the enzyme was found to be composed of six identical subunits. A high isoelectric point (7.1) was found for this enzyme. Arrhenius plots of the enzyme activity showed a sharp transition at 15-16 degrees C. The pH optimum of the enzyme was 8.0-8.5 at physiological level of ATP and positive modulators shifted the optimum to lower pH values. Amino-acid analysis revealed a lower content of the aromatic residues Phe, Tyr and Trp and higher level of Ser residue than in the rabbit muscle enzyme.

Modification of the ATP inhibitory site of the Ascaris suum phosphofructokinase results in the stabilization of an inactive T state

Treatment of the Ascaris suum phosphofructokinase (PFK) with 2',3'-dialdehyde ATP (oATP) results in an enzyme form that is inactive. The conformational integrity of the active site, however, is preserved, suggesting that oATP modification locks the PFK into an inactive T state that cannot be activated. A rapid, irreversible first-order inactivation of the PFK is observed in the presence of oATP. The rate of inactivation is saturable and gives a KoATP of 1.07 +/- 0.27 mM. Complete protection against inactivation is afforded by high concentrations of ATP, and the dependence of the inactivation rate on the concentration of ATP gives a Ki of 326 +/- 26 microM for ATP which is 22-fold higher than the Km for ATP at the catalytic site but close to the binding constant for ATP to the inhibitory site. Fructose 6-phosphate, fructose 2,6-bisphosphate, and AMP provide only partial protection against modification. The pH dependence of the inactivation rate gives a pKa of 8.4 +/- 0.1. Approximately 2 mol of [3H]oATP is incorporated into a subunit of PFK concomitant with 90% loss of activity, and ATP prevents the derivatization of 1 mol/subunit. The oATP-modified enzyme is not activated by AMP or fructose 2,6-bisphosphate. oATP has no effect on the activity of a desensitized form of PFK in which the ATP inhibitory site is modified with diethyl pyrocarbonate but with the active site intact [Rao, G.S.J., Wariso, B.A., Cook, P.F., Hofer, H.W., & Harris, B.G. (1987) J. Biol. Chem. 262, 14068-14073].(ABSTRACT TRUNCATED AT 250 WORDS)

Some enzymes of carbohydrate metabolism in Mesocestoides corti and Heterakis spumosa

The activities of selected enzymes of carbohydrate metabolism were measured in tetrathyridia of Mesocestoides corti and in adult females and males of Heterakis spumosa. When the species were compared, only lactate dehydrogenase and phosphoenolpyruvate carboxykinase activities were considerably higher in M. corti. Activities of other enzymes were higher in H. spumosa, with malate dehydrogenase activity being considerably so. In H. spumosa, enzyme activity was higher, and succinate dehydrogenase markedly so in males, when compared with females. Tetrathyridia aged 170 and 210 days show relatively stable malate and lactate dehydrogenase activities, and mice of ICR and BALB/c strains are suitable for the maintenance of tetrathyridia.

Phosphofructokinase from mollusc muscle is activated by phosphorylation

Phosphofructokinase was purified from muscle tissue of two different molluscs, edible snails, Helix pomatia (gastropoda), and mussels, Mytilus edulis (bivalvia). Under denaturing conditions, both enzymes had a molecular mass of 82 kDa. In the presence of ATP-Mg2+, the enzymes were rapidly phosphorylated in vitro by the catalytic subunit of cyclic AMP (cAMP)-dependent protein kinase purified from snail muscle and also by the C subunit of protein kinase from bovine heart. The extent of phosphorylation was 0.6 and 0.5 phosphate residues per subunit for the snail and the mussel phosphofructokinase, respectively. Phosphorylation of both phosphofructokinases effected a decrease in ATP inhibition at neutral or slightly acidic pH values and increased the affinity for fructose 6-phosphate. The resulting activation in the presence of suboptimum fructose 6-phosphate concentrations was more distinct for the snail enzyme. In addition, phosphorylated phosphofructokinase from mussels exhibited a marked increase in Vmax when activated by either 5'-AMP or fructose 2,6-bisphosphate.

Trypsin modification of phosphofructokinase from Ascaris suum

Phosphofructokinase from Ascaris suum is a tetramer with subunits of 90 kDa. Treatment of the native enzyme with trypsin (10%, w/w) followed by SDS-gel electrophoresis was shown to immediately generate a 40-kDa fragment followed by a gradual formation of two other fragments of 37 and 32 kDa. The loss of catalytic activity during the digestion was less than 50%. Gel filtration of the digested enzyme under non-denaturing conditions showed a Mr almost that of the native enzyme. Digestion of the phosphorylated enzyme resulted in an 80% release of the phosphorylated peptide over the period of 1 h. The digested enzyme was inhibited less by ATP than the native enzyme, but it was still positively affected by the effectors, fructose 2,6-bisphosphate and AMP. The results are interpreted to suggest that the structure of the ascarid phosphofructokinase is similar to that of the mammalian enzyme.

Glycogen synthesis in the obliquely striated muscle of Ascaris suum

A glycogen synthase, designated GS II, which occurs in a protein/carbohydrate complex has been purified from Ascaris suum muscle. The purified GS-II complex which is eluted from concanavalin-A--Sepharose contains proteins with Mr 140,000 and 66,000 and a glycoprotein with a carbohydrate/protein mass ratio of 3:1. GS II activity was totally dependent on glucose 6-phosphate, but exogenous glycogen was not required for polysaccharide synthesis. The GS-II complex was not phosphorylated by cyclic-AMP-dependent protein kinase, and antibodies to the protein and carbohydrate components of GS II did not cross react with the purified cyclic-AMP-regulated glycogen synthase (GS I) from A. suum muscle. Polysaccharide which was synthesized de novo by the complex was added to the large-molecular-mass glycoprotein in GS II. The glycogen-like character of the newly synthesized polysaccharide was confirmed by the observation that glycogen phosphorylase utilized the polymer as substrate in both the synthesis and degradation reactions. A model is discussed in which a core glycoprotein serves as the substrate for a glycogen synthase which is distinctly different from GS I.

Purification of phosphofructokinase using transition-state analogue affinity chromatography

A novel purification of phosphofructokinase has been achieved in a two step process using ion-exchange affinity chromatography and a transition-state analogue affinity column matrix. The procedure can be performed in one day, and gives a 25% yield of the starting material. The transition-state analogue chromatography is carried out using an ADP-agarose column in the presence of fructose 6-phosphate, magnesium ions and nitrate ions. In the presence of nitrate ion plus substrate, phosphofructokinase binds immobilized ADP while other proteins pass through the column. Previous studies with creatine kinase have shown that the nitrate ion mimics the planar phosphate in the transition state resulting in a complex which is stable under the relatively high ionic strength of the column buffer. This permits the elution of phosphofructokinase in a single peak of high specific activity. This column typically results in a 20-30 fold increase in specific activity with only a small loss of activity.

Phosphofructokinase from Fasciola hepatica: activation by phosphorylation and other regulatory properties distinct from the mammalian enzyme

Phosphofructokinase from the liver fluke, Fasciola hepatica, was phosphorylated by the catalytic subunit of cyclic AMP-dependent protein kinase isolated from this organism. Phosphorylated fluke phosphofructokinase had a sevenfold lower apparent Km for its substrate, Fru-6-P, and an eightfold higher 0.5 Vopt for ATP, the enzyme's primary inhibitor, than native phosphofructokinase. Activation of fluke phosphofructokinase following phorphorylation by a mammalian protein kinase catalytic subunit was previously reported (E. S. Kamemoto and T. E. Mansour (1986) J. Biol. Chem. 261, 4346-4351). The catalytic subunit of protein kinase isolated from the liver fluke phosphorylated sites on fluke phosphofructokinase similar to those phosphorylated by the mammalian enzyme. Maximal phosphate incorporation was 0.3 mol P/mol of protomer. The native enzyme was found to contain 1.3 mol P/mol of protomer. In contrast to fluke phosphofructokinase, activity of the mammalian heart enzyme was slightly decreased following phosphorylation. The dependence of allosteric interaction on an acidic pH observed with the mammalian phosphofructokinase was not observed with the fluke enzyme. Unlike mammalian phosphofructokinase, allosteric kinetics of the fluke enzyme was observed at alkaline pH (8.0). Fluke phosphofructokinase was found to be relatively insensitive to inhibition by citrate, a known potent inhibitor of the mammalian enzyme. Fru-2,6-P2, a potent modifier of phosphofructokinase from a variety of sources, was found to activate both native and phosphorylated fluke phosphofructokinase. The most potent activators of fluke phosphofructokinase were found to be Fru-2,6-P2, AMP, and phosphorylation. The endogenous level of Fru-2,6-P2 in the flukes was determined to be 29 +/- 1.3 nmol/g wet wt, a level that may well modulate enzyme activity. Fru-6-P,2-kinase, the enzyme responsible for synthesis of Fru-2,6-P2, was found to be present in the flukes. Our results suggest physiological roles for phosphorylation and Fru-2,6-P2 in regulation of fluke phosphofructokinase.

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.

Reversible activation and inactivation of phosphofructokinase from Ascaris suum by the action of tissue-homologous protein phosphorylating and dephosphorylating enzymes

In the presence of ATP-Mg2+, purified phosphofructokinase from Ascaris suum muscle was effectively phosphorylated and activated in vitro by a protein kinase purified from the same tissue. Both effects were reversed by the action of a purified protein phosphatase from the same tissue. The findings suggest the presence of a highly potent interconversion mechanism for phosphofructokinase in the muscle of the parasitic nematode.

Progress in molecular parasitology

Substantial progress has been made in the last ten years in understanding the structural and functional organization of parasitic protozoa and helminths and the complex physiological relationships that exist between these organisms and their hosts. By employing the new powerful techniques of biochemistry, molecular biology and immunology the genomic organization in parasites, the molecular basis of parasite's variation in surface antigens and the biosynthesis, processing, transport and membrane anchoring of these and other surface proteins were extensively investigated. Significant advances have also been made in our knowledge of the specific and often peculiar strategies of intermediary metabolism, cell compartmentation, the role of oxygen for parasites and the mechanisms of antiparasitic drug action. Further major fields of interest are currently the complex processes which enables parasites to evade the host's immune defense system and other mechanisms which have resulted in the specific adaptations which enabled parasites to survive within their host environments. Various approaches in molecular and biochemical parasitology and in immunoparasitology have been proven to be of high potential for serodiagnosis, immunoprophylaxis and drug design.

Allosteric regulatory properties of muscle phosphofructokinase

We have reviewed the allosteric regulatory properties of skeletal muscle phosphofructokinase and recent results on the phosphorylation of this enzyme. The number and affinities of various ligand binding sites are described, and a simple three state model is presented to explain the kinetic and ligand-binding properties of the enzyme. Data describing a lack of fit to a concerted transition model are presented. The widespread occurrence of partial phosphorylation of phosphofructokinase at a specific site near the carboxyl terminus is documented, as well as the lack of significant kinetic consequences of such phosphorylation.