ATP-dependent Phosphofructokinase Research Articles

Sucrose is metabolised by sucrose synthase and glycolysis within the phloem complex of Ricinus communis L. seedlings

Metabolites and enzyme activities were measured in the phloem sap exuding from a cut hypocotyl of germinating castor-bean (Ricinus communis L.) seedlings. The sap contained considerable quantities of adenine nucleotides, uridine nucleotides, uridine diphosphoglucose (UDPGlc), all the major phosphorylated metabolites required for glycolysis, fructose-2,6-bisphosphate and pyrophosphate. Supplying 200 mM glucose instead of sucrose to the cotyledons resulted in high concentrations of glucose in the sap, but did not modify the metabolite levels. In contrast, when 200 mM fructose was supplied we found only a low level of fructose but a raised sucrose concentration in the sap, which was accompanied by a three-to fourfold decrease of UDPGlc, and an increase of pyrophosphate, UDP and UTP. The measured levels of metabolites are used to estimate the molar mass action ratios and in-vivo free-energy change associated with the various reactions of sucrose breakdown and glycolysis in the phloem. It is concluded that the reactions catalysed by ATP-dependent phosphofructokinase and pyruvate kinase are removed from equilibrium in the phloem, whereas the reactions catalysed by sucrose synthase, UDPGlc-pyrophosphorylase, phosphoglucose mutase, phosphoglucose isomerase, aldolase, triose-phosphate isomerase, phosphoglycerate mutase and enolase are close to equilibrium within the conducting elements of the phloem. Since the exuded sap contained negligible or undetectable activities of the enzymes, it is concluded, that the responsible proteins are bound, or sequesterd behind plasmodesmata, possibly in the companion cells. It is argued that sucrose mobilisation via a reversible reaction catalysed by sucrose synthase is particularily well suited to allow the rate of sucrose breakdown in the phloem to respond to changes in the metabolic requirement for ATP, and for UDPGlc during callose production. It is also calculated that the transport of nucleotides in the phloem sap implies that there must be a very considerable uptake or de-novo biosynthesis of these cofactors in the phloem.

Pyrophosphate: fructose-6-phosphate 1-phosphotransferase and biosynthetic capacity during differentiation of hypocotyls of Vigna seedlings

The relationship between the activity of pyrophosphate: fructose-6-phosphate 1-phosphotransferase (PFP) and the capacity for biosynthesis of macromolecules was examined in segments from different parts of hypocotyls of etiolated seedlings of Vigna mungo and V. radiata. The relative ratio of the maximum activity of PFP to that of ATP-dependent phosphofructokinase (PFK) (PFP/PFK ratio) was high in young tissues and decreased with differentiation and ageing of the tissues. The highest level of fructose-2,6-bisphosphate was observed in the youngest part of hypocotyls of V. mungo. The level was markedly decreased with ageing of tissues. The levels of PPi and ATP were also higher in younger parts than in older parts of the hypocotyls, but the ratio of the level of PPi to that of ATP was almost constant in all parts of the hypocotyl. A good correlation was found between the PFP/PFK ratio and the biosynthetic capacity, as estimated from the rate of incorporation of [U- 14C]sucrose into ethanol-insoluble macromolecules.

Evidence for an interaction between cytosolic aldolase and the ATP- and pyrophosphate-dependent phosphofructokinases in carrot storage roots

Immunoaffinity chromatography was employed to identify potential plant cytosolic aldolase (ALD c) binding proteins. A clarified homogenate of carrot storage root was chromatographed on a column of protein-A—Sepharose that had been covalently coupled to anti-(carrot root ALD,) immunoglobulin G. The column was washed with phosphate-buffered saline (PBS), followed by step-wise elution with increasing concentrations of NaCl in PBS. Several proteins were eluted following application of the salt gradient. Western blotting identified the major eluting proteins to be the PP i-dependent phosphofructokinase (PFP) and the cytosolic form of the ATP-dependent phosphofructokinase (PFK c), enzymes that are metabolically sequential to ALD c. The results suggest that ALD c may specifically interact with PFP and PFK c in carrots.

Substrate Specificity of Pyrophosphate:Fructose 6-Phosphate 1-Phosphotransferase from Potato Tuber

The aim of this work was to establish the precise ionic form of the reactants used by pyrophosphate:fructose-6-phosphate phosphotransferase. The enzyme was purified to near-homogeneity from potato (Solanum tuberosum L.) tubers. Changes in enzyme activity when the pH of the assay and the concentration of fructose 6-phosphate, pyrophosphate, and magnesium are varied independently indicate that fructose 6-phosphate(2-) and MgP(2)O(7) (2-) are the reacting species in the glycolytic direction. Analogous experiments with fructose 1,6-bisphosphate, inorganic phosphate, and magnesium demonstrate that the enzyme uses fructose 1,6-bisphosphate(4-), HPO(4) (2-), and Mg(2+) in the gluconeogenic direction. The ionic species used in the glycolytic direction are comparable with those required by bacterial ATP-dependent phosphofructokinase. This is consistent with the proposal that the active site of pyrophosphate:fructose-6-phosphate phosphotransferase in plants is equivalent to that of the bacterial phosphofructokinase (SM Carlisle et al. [1990] J Biol Chem 265: 18366-18371).

Enzymes Catalyzing the Reversible Conversion of Fructose-6-Phosphate and Fructose-1,6-Bisphosphate in Maize (Zea mays L.) Kernels

The significance of the glycolytic and gluconeogenic conversion of fructose-6-phosphate and fructose-1,6-bisphosphate on sugar metabolism was investigated in maize (Zea mays L.) kernels. Maximum extractable activities of the pyrophosphate (PPi) dependent phosphofructokinase, fructose-1,6-bisphosphatase, and the ATP-dependent phosphofructokinase were measured in normal and four maize genotypes, which accumulate relatively more sugars and less starch, to determine how these enzymes are affected by the genetic lesions. Normal endosperm accumulated more dry matter than the high sugar/low starch genotypes, but protein contents did not differ greatly among the genotypes. Mutation of several starch biosynthetic enzymes had little impact on the activities of PPi-dependent phosphofructokinase, fructose-1,6-bisphosphatase, and ATP-dependent phosphofructokinase, despite the altered capacity of the cell to synthesize starch. The PPi-dependent phosphofructokinase appeared to be more active toward glycolysis in all genotypes studied. Activity of the PPi-dependent phosphofructokinase in shrunken (low sucrose synthase genotype) did not differ from the activity in other genotypes, suggesting that the gluconeogenic production of PPi may not be the primary role of the enzyme. As expected, shrunken kernels contained more sugars and less starch than normal kernels throughout kernel development except at the very early stages. Developmental profiles of normal kernels also showed marked changes in the PPi-dependent phosphofructokinase activity, whereas the level of ATP-dependent phosphofructokinase activity remained relatively steady during kernel development. In addition, the ATP-dependent phosphofructokinase, and not the PPi-dependent phosphofructokinase, appeared to correlate more closely with respiration rate. These findings suggest that glycolysis catalyzed by the ATP-dependent phosphofructokinase may serve primarily to support energy production, and glycolysis catalyzed by the PPi-dependent phosphofructokinase may contribute mainly to generation of biosynthetic intermediates.

Properties of ATP-dependent Phosphofructokinase from Endosperm of Developing Wheat (Triticum aestivum L.) Grains

ATP-dependent phosphofructokinase (ATP:D-fructose S-phosphate, 1-phosphotransferase, EC 2.7.1.11, PFK) from endosperm of developing wheat grains was purified to apparent homogeneity with about 45% recovery using ammonium sulfate fractionation, ion-exchange chromatography on DEAE-cellulose and gel filtration through Sepharose CL-SB and Sephadex G-200. The purified enzyme with a molecular weight of about 182 kD, was a heterotetramer with subunit molecular weights ranging between 20 and 80 kD. The enzyme exhibited maximum activity at pH 7.9 and was highly specific for its substrates. The enzyme had absolute requirement for Mg2+. At pH 7.9, the Km values as determined by Lineweaver-Burk plots were 1.43 and 0.70 mM, respectively for fru-S-P and ATP. Fru-2, S-P2 had no effect on the activity of the enzyme. The enzyme was inhibited strongly by citrate, ADP, 3-PGA and PEP with Ki values of 2.40, 1.75, 2.10 and 0.80 mM, respectively. Citrate and PEP inhibited the enzyme competitively with respect to both fru-S-P and ATP. ADP and 3-PGA inhibited the enzyme non-competitively and competitively, respectively with respect to fru-S-P and in a mixed manner with respect to ATP. Hill plot values indicated co-operative interaction of citrate, 3-PGA and PEP with the enzyme.

Cloning, sequencing, and expression of pyrophosphate-dependent phosphofructokinase from Propionibacterium freudenreichii.

Pyrophosphate-dependent 6-phosphofructo-1-kinase (PPi-PFK) from Propionibacterium freudenreichii is a non-allosteric enzyme with properties dissimilar to those of other described phosphofructokinases. The enzyme was cloned into pBluescript, sequenced, and expressed in Escherichia coli at levels 15 times higher than those observed in Propionibacterium. The gene consists of 1215 bases which code for a protein of 404 amino acids and a mass of 43,243 daltons. High G + C in the codon usage (66%) of the gene is consistent with the classification of Propionibacterium in the High-G + C subdivision of the Gram-positive bacteria. While showing no sequence identity to the non-allosteric ATP-dependent phosphofructokinase of E. coli, alignments of the amino acid sequence with other PFKs reveal degrees of identities among the amino halves of the proteins, from 26% between the Propionibacterium and potato PPi-PFKs, and 29% between Propionibacterium and E. coli ATP-PFKs. These levels of identities indicate that the amino halves of these proteins are homologous. Identities between the carboxyl half of Propionibacterium PFK and carboxyl halves of other sequences are below 20%, suggesting that the carboxyl half is not homologous. Despite the poor conservation, most of the residues that take part in the binding of fructose-6-P or Mg-PPi could be readily identified by analogy to the structure of the E. coli PFK. Both the fructose-6-P and ATP-binding sites are conserved, indicating that PPi binds to the homologous site of the E. coli ATP-binding site.

Contrasting roles for pyrophosphate:fructose-6-phosphate phosphotransferase during aging of tissue slices from potato tubers and carrot storage tissues

The role of pyrophosphate:fructose-6-phosphate phosphotransferase (PFP) during wounding-induced respiration in tissue slices from a starch-storing tissue (potato tubers) and a sucrose-storing tissue (carrot root) has been compared. (a) In both tissues, there was a rapid decrease of glycerate-3-phosphate, a gradual increase of triose-phosphate and fructose-1,6-bisphosphate, no change or an increase of hexose phosphates, and a rapid increase of fructose-2,6-bisphosphate. Inorganic pyrophosphate responded differently in the two tissues, decreasing in potato and increasing in carrot. (b) Comparison of key enzyme activities in extracts showed differences between the two tissues. Potato tubers contained high pyruvate kinase, relative to ATP-dependent phosphofructokinase, and also contained high PFP. Carrot roots contained comparable capacities of pyruvate kinase and ATP-dependent phosphofructokinase, and two-fold lower PFP. (c) It is argued that PFP probably has a glycolytic function in potato tubers, supplementing the rather low ATP-dependent phosphofructokinase activity. In carrot, ATP-dependent phosphofructokinase is relatively high, allowing PFP to operate in the reverse direction, and generate inorganic pyrophosphate which is needed for sucrose mobilisation.

Enzyme Sets of Glycolysis, Gluconeogenesis, and Oxidative Pentose Phosphate Pathway Are Not Complete in Nongreen Highly Purified Amyloplasts of Sycamore (Acer pseudoplatanus L.) Cell Suspension Cultures

Differential centrifugation and Percoll-gradient centrifugation of protoplast lysates of suspension-cultured cells of sycamore (Acer pseudoplatanus L.) yielded pure amyloplasts. Contamination of the final amyloplast preparation by foreign compartments was assessed by measuring marker enzyme activities. The activity of alkaline pyrophosphatase was taken as a 100% plastid marker; relative to this marker, mitochondria (cytochrome c oxidase) averaged 0.34%, microbodies (catalase) 0.61%, and cytosol (alcohol dehydrogenase) 0.09%. Enzymatic activities of the glycolytic, gluconeogenic, pentose phosphate and the starch degradation pathways were found to be present in these amyloplast extracts in appreciable amounts. But the pyrophosphate-dependent phosphofructokinase and phosphoglyceromutase were judged to be essentially absent from amyloplasts because the activities of these enzymes were not enriched above the level of contaminating enzymatic activities in the amyloplast fractions. Additionally, the in vitro activities of starch phosphorylase, ATP dependent phosphofructokinase, NAD dependent glyceraldehyde-3 phosphate dehydrogenase, and glucose-6 phosphate dehydrogenase did not seem to support carbon fluxes from starch to triose phosphates as calculated from the rate of starch disappearance during carbon starvation of the cells. These results provide additional, indirect evidence for the recently emerged view that, in addition to the well known phosphate-triosephosphate translocator, another hexose phosphate and possibly also an ATP/ADP translocating system play major roles in nongreen plastids.

Pyrophosphate-dependent phosphofructokinase. Conservation of protein sequence between the alpha- and beta-subunits and with the ATP-dependent phosphofructokinase.

Full-length cDNA clones for the alpha- and beta-subunits of pyrophosphate-fructose 6-phosphate 1-phosphotransferase have been isolated from a cDNA expression library derived from potato tuber poly(A)+ RNA. The nucleotide sequences indicate that the alpha- and beta-subunits are related with about 40% of amino acid residues being identical. A comparison of the deduced amino acid sequences of both subunits of this enzyme with that of the major ATP-dependent fructose 6-phosphate 1-phosphotransferase from Escherichia coli (Shirakihara, Y., and Evans, P. R. (1988) J. Mol. Biol. 204, 973-994) showed little homology between the proteins except for regions involved in the binding of fructose 6-phosphate/fructose, 1,6-bisphosphate and possibly between regions binding pyrophosphate and the beta- and gamma-phosphates of ADP/ATP. A comparison of the derived secondary structures of the two subunits of the PPi-dependent enzyme with the known secondary structure of the E. coli ATP-dependent enzyme indicated that the overall structure of these enzymes is similar. These data suggest that catalytic activity resides on the beta-subunit of the pyrophosphate-dependent enzyme.

Effect of sink isolation on sugar uptake and starch synthesis by potato-tuber storage parenchyma

Import into potato (Solarium tuberosum L. cv. Record) tubers was terminated by removing the sink at its connection with the stolon. The ability of discs of storage tissue from the excised tubers to take up exogenous sugars and convert them to starch was compared with that of discs from untreated tubers from the same plant population. In rapidly-growing control tubers, glucose and fructose were taken up to a greater extent than sucrose, 77% of the glucose being converted to starch within 3 h (compared with 64% and 27% for fructose and sucrose, respectively). These values fell as the tubers aged but the ranking (glucose > fructose > sucrose) was maintained, emphasising a severe rate-limiting step following the import of sucrose into the growing tuber. Sink isolation had little effect on the ability of the storage cells to take up exogenous sucrose across the plasmalemma for up to 7 d after sink isolation. However, the ability of the same cells to convert the sucrose to starch was severely inhibited within 24 h, as was the sensitivity of starch synthesis to turgor. In the case of glucose, sink isolation inhibited both the uptake and the conversion to starch, the latter being inhibited to a greater degree. A detailed metabolic study of tubers 7 d after excision showed that, with sucrose as substrate, 94% of the radioactivity in the soluble sugar pool was recovered in sucrose following sink isolation (92% in control tubers). However, with glucose as substrate, 80% of the radioactivity was recovered as sucrose following tuber excision (28% in control tubers), providing evidence that sucrose synthesis acts as a major alternative carbon sink when starch synthesis is inhibited. In the same tubers, sucrose-synthase activity decreased by 70% following sink isolation, compared with a 45% reduction in ADP-glucose pyrophosphorylase. Activities of UDP-glucose pyrophosphorylase, starch phosphorylase, starch synthase nd both PPi- and ATP-dependent phosphofructokinases remained unchanged. Acid-invertase activity increased fivefold.

Induction of Pyrophosphate:Fructose 6-Phosphate 1-Phosphotransferase by Anoxia in Rice Seedlings

Rice (Oryza sativa) seeds were imbibed for 3 days and the seedlings were further incubated for 8 days in the presence of either air or nitrogen. In aerobiosis, the specific activity of pyrophosphate:fructose 6-phosphate 1-phosphotransferase and that of the ATP-dependent phosphofructokinase increased about fourfold. In anaerobiosis, the specific activity of ATP-dependent phosphofructokinase remained stable, whereas that of pyrophosphate:fructose 6-phosphate 1-phosphotransferase increased as much as in the presence of oxygen and there was also a fourfold increase in the concentration of fructose 2,6-bisphosphate, a potent stimulator of that enzyme. These data suggest a preferential involvement of pyrophosphate:fructose 6-phosphate 1-phosphotransferase rather than of ATP-dependent phosphofructokinase in glycolysis during anaerobiosis.

Regulation of Carbon Partitioning to Respiration during Dark Ammonium Assimilation by the Green Alga Selenastrum minutum

The assimilation of NH(4) (+) causes a rapid increase in respiration to provided carbon skeletons for amino acid synthesis. In this study we propose a model for the regulation of carbon partitioning from starch to respiration and N assimilation in the green alga Selenastrum minutum. We provide evidence for both a cytosolic and plastidic fructose-1,6-bisphosphatase. The cytosolic form is inhibited by AMP and fructose-1,6-bisphosphate and the plastidic form is inhibited by phosphate. There is only one ATP dependent phosphofructokinase which, based on immunological cross reactivity, has been identified as being localized in the plastid. It is inhibited by phosphoenolpyruvate and activated by phosphate. No pyrophosphate dependent phosphofructokinase was found. The initiation of dark ammonium assimilation resulted in a transient increase in ADP which releases pyruvate kinase from adenylate control. This activation of pyruvate kinase causes a rapid 80% drop in phosphoenolpyruvate and a 2.7-fold increase in pyruvate. The pyruvate kinase mediated decrease in phosphoenolpyruvate correlates with the activation of the ATP dependent phosphofructokinase increasing carbon flow through the upper half of glycolysis. This increased the concentration of triosephosphate and provided substrate for pyruvate kinase. It is suggested that this increase in triosephosphate coupled with the glutamine synthetase mediated decline in glutamate, serves to maintain pyruvate kinase activation once ADP levels recover. The initiation of NH(4) (+) assimilation causes a transient 60% increase in fructose-2,6-bisphosphate. Given the sensitivity of the cytosolic fructose-1,6-bisphosphatase to this regulator, its increase would serve to inhibit cytosolic gluconeogenesis and direct the triosephosphate exported from the plastid down glycolysis to amino acid biosynthesis.

Cytosolic Phosphofructokinase from Spinach Leaves

Cytosolic ATP-dependent phosphofructokinase (PFK) from spinach leaves (Spinacia oleracea L.) was enriched 2600-fold by (NH(4))(2)SO(4) fractionation, DEAE anion exchange chromatography, Blue Sepharose CL-6B, and ATP agarose type 3-affinity chromatography. The final preparation had a specific activity of 417 nkat per milligram protein and exhibited four bands between 50 and 70 kilodaltons following denaturing electrophoresis. Only one band of ATP- and fructose 6-phosphate (F-6-P)-dependent, Pistimulated activity was detected following isoelectric focusing PAGE and nondenaturing discontinuous PAGE of the final preparation. Crude extracts contained, in addition to the band observed in the final preparation, a second band that was inhibited by Pi. The latter band is presumably chloroplastic PFK. PFK was stimulated by the anions Pi(2-), Cl(-), SO(4) (2-), NO(3) (-), HAsO(4) (2-), and HCO(3) (-) but was not affected by NH(4) (+). Pi and Mg(2+) changed the response of PFK toward pH and affected the saturation kinetics of F-6-P. In general, activity was highest when Pi was high and (or) Mg(2+) was low. Phosphoenolpyruvate (PEP), 2-PGA, and PPi, but not 3-PGA, inhibited PFK. Although the inhibition by PEP and 2-PGA was reduced or relieved by Pi, the inhibition by PPi was not affected by Pi. F-2, 6-P(2) had no effect upon the activity of PFK. It is proposed that, in the cytosol of spinach leaves, PFK is likely to be more active during the dark, when cytosolic Pi levels are high, than in the light.

Metabolism of Mollicutes: the Embden--Meyerhof--Parnas Pathway and the Hexose Monophosphate Shunt

Cell-free extracts of eighteen fermentative and nonfermentative Mollicutes were examined for enzyme activities associated with the hexose monophosphate shunt (HMS) and Embden---Meyerhof---Parnas (EMP) pathway. All Acholeplasma spp. had glucose-6-phosphate (G6P) dehydrogenase (EC 1.1.1.49), 6-phosphogluconate (6PG) dehydrogenase (EC 1.1.1.44) and hexokinase (EC 2.7.1.1) activity. Of these three enzyme activities, hexokinase was also detected in Mycoplasma sp. Let. 1 but in no other fermentative or nonfermentative Mycoplasma spp. The Acholeplasma and fermentative Mycoplasma spp. possessed all other HMS and EMP activities examined. All Acholeplasma spp. possessed a pyrophosphate (PPi)-dependent phosphofructokinase (PFK) (EC 2.7.1.90) while fermentative Mycoplasma spp. possessed an ATP-dependent PFK (EC 2.7.1.11). Transaldolase (EC 2.2.1.2) activity was detected in some, but not all Acholeplasma and fermentative Mycoplasma spp. 2-Deoxyribose-5-phosphate aldolase (EC 4.1.2.4) activity was present in all mollicute extracts tested except for Mycoplasma gallisepticum and Mycoplasma sp. Let. 1. The two nonfermentative Mycoplasma spp. lacked all enzyme activities of the HMS pathway except for ribulose-5-phosphate epimerase activity, and of the EMP pathway only phosphoglucose isomerase and the enzymes converting glyceraldehyde 3-phosphate (G3P) to phosphoenolpyruvate (PEP) were detected. We believe that the three major observations of this study are: (1) all Mycoplasma spp. lack G6P and 6PG dehydrogenase activities, suggesting a reduction in their NADPH pool, which may relate to the lipid growth requirement of this genus; (2) the fermentative Mycoplasma spp. have an ATP-dependent PFK activity, while the fermentative Acholeplasma spp. have a PPi-dependent PFK activity; and (3) the nonfermentative Mycoplasma spp. lack ATP and PPi-dependent PFK and fructose-1,6-bisphosphate aldolase activities but, like the fermentative Mollicutes, can convert three-carbon compounds, G3P to PEP through the three-carbon arm of the EMP pathway.

Inorganic pyrophosphate: Fructose-6-phosphate 1-phosphotransferase of the potato tuber is related to the major ATP-dependent phosphofructokinase of [formula omitted

A procedure was developed for the purification of inorganic pyrophosphate: fructose-6-phosphate 1-phospho-transferase (PPi-PFK) from potato tubers. The enzyme has the structure α 4β 4 with a subunit of 68 kDa and a β subunit of 60 kDa. The structural relationship of this enzyme to other PFKs and to fructose bisphosphatase was examined by immunoprecipitation and immunoblotting. Antibodies to the plant enzyme did not react with E.coli PFK. No cross-reaction was seen among the following enzymes or their antibodies: yeast fructose bisphosphatase; rabbit PFKs A,B, or the enzyme from brain; and the two subunits of the potato PPi-PFK. On the other hand, antibody to E.coli PFK-1 strongly cross-reacts with the 60 kDa polypeptide but not 68 kDa peptide.

Kinetic Properties of the ATP-Dependent Phosphofructokinase Isoenzymes from Cucumber Seeds

Kinetic Properties of the ATP-Dependent Phosphofructokinase Isoenzymes from Cucumber Seeds

Banana Ripening: Implications of Changes in Internal Ethylene and CO2 Concentrations, Pulp Fructose 2,6-Bisphosphate Concentration, and Activity of Some Glycolytic Enzymes

In ripening banana (Musa acuminata L. [AAA group, Cavandish subgroup] cv. Valery) fruit, the steady state concentration of the glycolytic regulator fructose 2,6-bisphosphate (Fru 2,6-P(2)) underwent a transient increase 2 to 3 hours before the respiratory rise, but coincident with the increase in ethylene synthesis. Fru 2,6-P(2) concentration subsequently decreased, but increased again approximately one day after initiation of the respiratory climacteric. This second rise in Fru 2,6-P(2) continued as ripening proceeded, reaching approximately five times preclimacteric concentration. Pyrophosphate-dependent phosphofructokinase glycolytic activity exhibited a transitory rise during the early stages of the respiratory climacteric, then declined slightly with further ripening. Cytosolic fructose 1,6-bisphosphatase activity did not change appreciably during ripening. The activity of ATP-dependent phosphofructokinase increased approximately 1.6-fold concurrent with the respiratory rise. A balance in the simultaneous glycolytic and gluconeogenic carbon flow in ripening banana fruit appears to be maintained through changes in substrate levels, relative activities of glycolytic enzymes and steady state levels of Fru 2,6-P(2).

Cytosolic ATP-Dependent Phosphofructokinase from Spinach

ATP-dependent 6-phosphofructokinase (PFK) activity is present in both chloroplastic and in nonchloroplastic fractions isolated from spinach protoplasts. The activity in the extra-chloroplastic fraction was stimulated 2- to 3.5-fold by 25 mm inorganic phosphate (Pi), the chloroplast-associated activity was inhibited 2- to 5-fold. The Pi stimulated activity was ATP-dependent and was not an artifact due to the presence of fructose 6-P, Pi, pyrophosphatase, and pyrophosphate fructose 6-P 1-phosphotransferase (PFP). PFK activities, which expressed characteristics similar to those separated from protoplasts, could be separated following ammonium sulfate fractionation of crude extracts; the ammonium sulfate treatment also separated both PFK activities from PFP. It is concluded that spinach leaves contain a cytosolic PFK. This activity is relatively stable, is stimulated by Pi over a wide pH range, is not a result of the transformation of another enzyme activity, and has an activity that is similar to, or slightly less than, that of the cytosolic PFP.

Pyrophosphate- and ATP-dependent phosphofructokinases in pollen of Typha latifolia.

Pyrophosphate- and ATP-dependent phosphofructokinases in pollen of Typha latifolia.