Inhibition of UDP-N-acetylglucosamine pyrophosphorylase by uridine

UDP-N-acetylglucosamine pyrophosphorylases (UTP: 2-acetamido-2-deoxy-alpha-D-glucose-1-phosphate uridylyltransferase, EC 2.7.7.23) from baker's yeast and Neurospora crassa IFO 6178 were inhibited by uridine which is the nucleoside moiety of UDP-GlcNAc. The inhibition was shown in both directions of pyrophosphorolysis and of synthesis of UDP-GlcNAc. Kinetic analysis revealed that uridine demonstrated a noncompetitive type of inhibition with UDP-GlcNAc and competitive inhibition with PPi. The Ki values for the baker's yeast enzyme were 1.8 mM for UDP-GlcNAc and 0.16 mM for PPi, and the values for the Neurospora enzyme were 1.1 mM for UDP-GlcNAc and 0.15 mM for PPi, respectively. Uridine did not bind irreversibly to the enzyme, as the activity was restored with dialysis. No other nucleosides caused inhibition of the enzyme activity except uridine. Some uridine derivatives, such as 5-hydroxyuridine, 5,6-dihydrouridine and pseudouridine, also inhibited the enzyme activity. But doexyuridine showed only slight inhibition, and 5'-UMP and orotidine caused no inhibition of the enzyme activity.     

Properties of Pyrophosphate:Fructose-6-Phosphate Phosphotransferase from Endosperm of Developing Wheat (Triticum aestivum L.) Grains

Pyrophosphate:fructose-6-phosphate phosphotransferase (PFP, EC 2.7.1.90) from endosperm of developing wheat (Triticum aestivum L.) grains was purified to apparent homogeneity with about 52% recovery using ammonium sulfate fractionation, ion exchange chromatography on DEAE-cellulose and gel filtration through Sepharose-CL-6B. The purified enzyme, having a molecular weight of about 170,000, was a dimer with subunit molecular weights of 90,000 and 80,000, respectively. The enzyme exhibited maximum activity at pH 7.5 and was highly specific for pyrophosphate (PPi). None of the nucleoside mono-, di- or triphosphate could replace PPi as a source of energy and inorganic phosphate (Pi). Similarly, the enzyme was highly specific for fructose-6-phosphate. It had a requirement for Mg²⁺ and exhibited hyperbolic kinetics with all substrates including Mg²⁺. K_m values as determined by Lineweaver-Burk plots were 322, 31, 139, and 129 micromolar, respectively, for fructose-6-phosphate, PPi, fructose-1,6-bisphosphate and Pi. Kinetic constants were determined in the presence of fructose-2,6-bisphosphate, which stimulated activity about 20-fold and increased the affinity of the enzyme for its substrates. Initial velocity studies indicated kinetic mechanism to be sequential. At saturating concentrations of fructose-2,6-bisphosphate (1 micromolar), Pi strongly inhibited PFP; the inhibition being mixed with respect to both fructose-6-phosphate and PPi, with K_i values of 0.78 and 1.2 millimolar, respectively. The inhibition pattern further confirmed the mechanism to be sequential with random binding of the substrates. Probable role of PFP in endosperm of developing wheat grains (sink tissues) is discussed.     

6-phosphofructokinase (pyrophosphate). Properties of the enzyme from Entamoeba histolytica and its reaction mechanism.

The inorganic pyrophosphate-requiring 6-phosphofructokinase of Entamoeba histolytica has been further investigated. The molecular weight of the enzyme is approximately 83,000 and its isoelectric point occurs at pH 5.8 to 6.0. The divalent cation requirement for reaction was explored. In the direction of fructose 6-phosphate formation half-maximal rate required 500 muM magnesium ion; in the direction of fructose bisphosphate formation 8 muM magnesium ion sufficed. ATP, PPi, polyphosphate, acetyl phosphate, or carbamyl phosphate cannot replace PPi as phosphate donor for the conversion of fructose 6-phosphate to fructose bisphosphate. In the direction of fructose 6-phosphate formation arsenate can replace orthophosphate. Isotope exchange studies indicate that little or no exchange occurs between Pi and PPi or between fructose 6-phosphate and fructose bisphosphate in the absence of a third substrate. These findings appear to rule out phosphoenzyme formation and a ping-pong reaction mechanism. PPi, Pi, and fructose bisphosphate are competitive inhibitors of fructose bisphosphate, PPi, and fructose 6-phosphate, respectively. This argues against an ordered mechanism and suggests a random mechanism. Fructose 6-phosphate and Pi were noncompetitive with respect to each other indicating the formation of a dead end complex. These product inhibition relationships are in accord with a Random Bi Bi mechanism.     

Regulation of UDPG pyrophosphorylase in Acetabularia mediterranea

The kinetic properties of UDPG pyrophosphorylase (glucosyl-1-phosphate uridyl transferase, EC 2.7.7.9) suggest that it may play a key role in the regulation of metabolism in Acetabularia mediterranea. The enzyme-catalyzed reaction is readily reversible in vitro, and has been assayed in both directions. The enzyme shows substrate inhibition by UDPG and UTP at substrate concentrations in excess of 2 mM. The kinetic behavior of the enzyme is consistent with the hypothesis that it catalyzes an ordered bisubstrate biproduct reaction in which G-1-P is the leading substrate, and UTP is the leading product. A plot of initial velocity vs. PPi concentration is sigmoid, indicating a cooperative homotropic effect. PGAL inhibits the reaction in the direction: UTP + G-1-P leads to UDPG + PPi It has no effect on the reverse reaction. The responses of the enzyme may serve to regulate the allocation of G-1-P between anabolic and catabolic pathways.     

Fluoride-Induced Inhibition of Starch Biosynthesis in Developing Potato, Solanum tuberosum L., Tubers Is Associated with Pyrophosphate Accumulation

Pretreatment of discs excised from developing tubers of potato (Solanum tuberosum L.) with 10 millimolar sodium fluoride induced a transient increase in 3-phosphoglycerate content. This was followed by increases in triose-phosphate, fructose 1,6-bisphosphate and hexose-phosphate (glucose 6-phosphate + fructose 6-phosphate + glucose 1-phosphate). The effect of fluoride is attributed to an inhibition of glycolysis and a stimulation of triose-phosphate recycling (the latter confirmed by the pattern of ¹³C-labeling [NMR] in sucrose when tissue was supplied with [2-¹³C]glucose). Fluoride inhibited the incorporation of [U-¹⁴C] glucose, [U-¹⁴C]sucrose, [U-¹⁴C]glucose 1-phosphate, and [U-¹⁴C] glycerol into starch. The incorporation of [U-¹⁴C]ADPglucose was unaffected. Inhibition of starch biosynthesis was accompanied by an almost proportional increase in the incorporation of ¹⁴C into sucrose. The inhibition of starch synthesis was accompanied by a 10-fold increase in tissue pyrophosphate (PPi) content. Although the subcellular localization of PPi was not determined, a hypothesis is presented that argues that the PPi accumulates in the amyloplast due to inhibition of alkaline inorganic pyrophosphatase by fluoride ions.     

Evidence for a novel metabolic pathway of (ADP-ribose)n: pyrophosphorolysis of ADP-ribose in HeLa S3 cell nuclei

ADP-ribose liberated from (ADP-ribose)n by the action of (ADP-ribose)n glycohydrolase was converted to ATP and ribose 5-phosphate (ribose 5-P) in the presence of pyrophosphate (PPi) in HeLa S3 cell nuclei. This reaction was reversible and dependent on the simultaneous presence of ADP-ribose, PPi, Mg2+, and nuclei. These results suggest the presence of a novel enzyme in the nuclei, designated as ADP-ribose pyrophosphorylase, which catalyzes the reaction shown in Equation 1. ADP-ribose + PPi in equilibrium ATP + Ribose 5-P (1) This reaction could represent a pathway for the biosynthesis of ATP from (ADP-ribose)n in eukaryotic cell nuclei.     

菠萝叶片依赖焦磷酸的磷酸果糖激酶的纯化及其特性

经硫酸铵分部,DEAE—纤维素、羟基磷灰石、Sephadex G—200及磷酸纤维素柱层析,从菠萝叶片分离得到电泳均一的依赖焦磷酸的磷酸果糖激酶(PFP)。SDS电泳图谱表明有一条分子量为62kD的主带和一条57 kD的弱带。Fru—2,6—P_2对酶的正反应活性有促进作用。动力学研究表明,Fru—2,6—P_2增加V_(max)及酶对底物Fru—6—P和Mg~(2+)的亲和性。     

Inorganic pyrophosphatase activity in cell-free extracts of Ureaplasma urealyticum

Cell-free extracts of Ureaplasma urealyticum strains Pi and T960 (CX8) (serovars 6 and 8, respectively) metabolized inorganic pyrophosphate (PPi). The inorganic pyrophosphatase (PPase) activity was greatest with Mg2+ as cofactor, but Mn2+ acted as a poor substitute. The PPases of the two serovars differed electrophoretically. Although the highest PPase activity was obtained using PPi as substrate, the enzyme could also utilize to a lesser degree both tripolyphosphate and trimetaphosphate. No activity was observed against beta-glycerophosphate, naphthyl phosphates, glucose 6-phosphate, fructose 6-phosphate, fructose 1,6-bisphosphate, thiamin pyrophosphate, phosphoribosylpyrophosphate, ADP or ATP. Acid- and alkaline-phosphatase activities were observed with naphthyl phosphates as substrates, but they did not have the same electrophoretic mobility on gels as the PPase activity. U. urealyticum PPase was inhibited by oxidized glutathione, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, phenylglyoxal, p-chloromercuribenzoic acid, Mn2+, Zn2+ and Ca2+. Neither reduced glutathione, L-cysteine nor Co2+ enhanced activity. PPi can act as a substrate or regulator of certain metabolic reactions, and PPi metabolism can function in bacterial bioenergetics; its role in ureaplasmas is presently unclear.     

Isolation and Characterization of Pyrophosphate:D-Fructose-6-phosphate 1-Phosphotransferase from Cucumber Seeds

Pyrophosphate:D-fructose-6-phosphate 1-phosphotransferase waspurified over 700-fold from germinating cucumber (Cucumis sativuscv. Fletcher) seeds. The purified enzyme has a specific activityof 5.2 µmol.min^–1.mg protein^–1 in the presenceof 1 µM fru-2,6-P₂. The pH optima is similar for boththe forward and reverse reactions (pH 7.5–7.8). Magnesium,manganese and cobalt activate the enzyme, with the highest affinitybeing for magnesium. The enzyme exhibits normal Michaelis-Mentenkinetics in both the presence and absence of fru-2,6-P₂. Half-maximumactivation of the enzyme was obtained with 35 nM fru-2,6-P2.Fru-2,6-P₂ stimulates activity by increasing V_max and increasingthe affinity for fru-6-P, fru-1,6-P₂ and PPi. Phosphate causesnoncompetitive inhibition with respect to both fru-6-P and PPi.On the basis of the steadystate substrate interaction and Piinhibition data a sequential ternary complex mechanism is proposed. (Received April 28, 1986; Accepted July 9, 1986)     

Guanosine monophosphate synthetase from Ehrlich ascites cells. Multiple inhibition by pyrophosphate and nucleosides.

GMP synthetase (xanthosine-5'-phosphate: ammonia ligase (AMP-forming), EC 6.3.4.1) from Ehrlich ascites cells was found to be subject to multiple inhibition by its reaction product, PPi, and some analogs of adenosine. PPi and the nucleoside (N) inhibitors were also capable of individually inhibiting this enzyme. Under no conditions did the inhibition appear to be irreversible or "pseudoinactivating" in nature. The individual inhibition by PPi was competitive with respect to ATP (KI = 0.42 mM). Conversely, in the absence of PPi, the binding of N was noncompetitive with ATP, but shifted to a competitive pattern when PPi was present. Furthermore, with the inhibitors in concert, there was an apparent lowering of the KI values for both inhibitors. This data was consistent with either PPi functioning to tighten the binding of N at a noncatalytic site (positive cooperativity) or with PPi actually opening a second binding site for N in addition to the non-catalytic site. Although this study did not distinguish which of these events was occurring, it did reveal that the intensity of the effect of PPi appeared to be constant. That is, for various N inhibitors with a range of independently determined KI values from 26 to 1650 muM, the ratio of their KI values determined in the absence of PPi to the values determined in the presence of PPi was always 38 +/- 1.     

Probing the pyrophosphate-binding site in potato tuber UDP-glucose pyrophosphorylase with pyridoxal diphosphate.

Potato tuber UDP-glucose pyrophosphorylase (EC 2.7.7.9) catalyzes the reversible uridylyl transfer from UDP-glucose to MgPPi forming glucose 1-phosphate and MgUTP, according to an ordered bi-bi mechanism in which UDP-glucose and MgPPi bind in this order. To probe the active site of this enzyme, we have applied pyridoxal 5'-diphosphate, a reactive PPi analogue. The enzyme was rapidly inactivated when incubated with the reagent in the presence of Mg2+ followed by sodium borohydride reduction. The degree of the inactivation was decreased by MgUTP, MgPPi, and glucose 1-phosphate, but enhanced by UDP-glucose. The enhancement was prevented by co-addition of Pi, the competitive inhibitor with respect to PPi. The complete inactivation corresponded to the incorporation of 0.9-1.1 mol of reagent/mol of enzyme monomer. In the presence of UDP-glucose, labels were almost exclusively incorporated into Lys-329. Thus, this residue may be located near the bound MgPPi and its modification is promoted, probably through conformational changes, by the binding of UDP-glucose to the enzyme. The results of the modification by the same reagent of the mutant enzymes in which Lys-329 and Lys-263 are individually replaced by Gln suggest the roles of these lysyl residues in the binding of MgPPi and in the UDP-glucose-induced conformational changes, respectively.     

Determination of the inorganic pyrophosphate level and its subcellular localization in Chara corallina

In order to determine the concentration of pyrophosphate (PPi) and its subcellular distribution in Chara corallina, a new method to concentrate PPi from cell extracts was developed. PPi was extracted and concentrated as Ca2P2O7 under alkaline conditions. The amount of PPi in the precipitate was measured using an enzyme system containing pyrophosphate:fructose-6-phosphate 1-phosphotransferase (EC 2.7.1.90) coupled to NADH oxidation in the presence of [ethylene-bis(oxyethylenenitrilo)]tetraacetic acid. The subcellular localization of PPi and inorganic phosphate (Pi) was studied using the intracellular perfusion technique. The relative volumes of the cytoplasm (6.4%) and the vacuole (93.6%) were determined by perfusing Lucifer Yellow CH into the vacuole and by assuming that the Lucifer Yellow CH dead space represented the cytoplasmic volume. The volume of the chloroplast layer was determined microscopically, and it was found that it occupied 10% of the Chara cytoplasm. PPi was present predominantly in the cytosol at a level of 193 microM, while it existed in the vacuole at a level of only 2.20 microM and less than 1 microM in chloroplasts. By contrast, Pi was distributed almost equally in the cytosol (12.0 mM), chloroplasts (16.2 mM), and the vacuole (6.70 mM). The electrochemical potential gradient across the tonoplast for H+ (delta mu H+ = -11.6 to -18.0 KJ/mol) was nearly equal to the free energy release from the hydrolysis of PPi in cytoplasm (delta Gpp = -18.9 KJ/mol), indicating that the H+-translocating inorganic pyrophosphatase can work as a H+ pump in C. corallina.     

Uridine diphosphate glucose synthase from calf liver: determinants of enzyme activity in vitro.

The reaction catalyzed by calf liver uridine diphosphate glucose synthase (pyrophosphorylase) (EC 2.7.7.9; UTP + glucose 1-phosphate = UDP-glucose + PPi) is an example of an enzymic reaction in which a nucleoside triphosphate other than ATP is the immediate source of metabolic energy. Kinetic properties of the enzyme, acting in the direction of UCP-glucose formation were investigated in vitro. The reaction was inhibited by UDP-glucose (0.072), Pi (11), UDP (1.6), UDP-xylose (0.87), UDP-glucuronate (1.3), and UDP-galacturonate (0.95). The numbers in parentheses indicate the concentration (mM) required for half-maximal inhibition under the conditions used. Other compounds tested, including ATP, ADP, and AMP, had no effect. Over a range of concentrations of UTP (0.04-0.8 MM) and UDP-glucose (0.05-0.03 mM), the reaction rate was more dependent on the concentration ratio [UDP-glucose]/[UTP] than on the absolute concentration of either compound. Comparison of the kinetic properties in vitro with estimates of metabolite levels in vivo suggests that (1) the enzyme operates in a range far from its maximal rate, and (2) the concentrations of glucose 1-phosphate and Pi and the ratio [UDP-glucose]/[UTP] may be the most important determinants of UDP-glucose synthase activity.     

Effects of 2,4-dinitrophenol and anoxia on the inorganic-pyrophosphate content of the spadix of Arum maculatum and the root apices of Pisum sativum

This work was done to determine whether the inorganic-pyrophosphate (PPi) content of plant tissues changes when the rate of glycolysis is altered. Treatment of excised clubs of the spadix of Arum maculatum L. and root apices of Pisum sativum L. with 2,4-dinitrophenol increased the rates of respiration but had no detectable effects on PPi contents. When the two tissues were subjected to up to 60 min anoxia, no changes in PPi were detected. Anoxia was shown to lead to a fall in ATP and concomitant rises in ADP and AMP in pea roots. It is argued (i) that variation in the rate of glycolysis was not accompanied by detectable changes in PPi content, (ii) that this observation does not favour the view that pyrophosphate fructose 6-phosphate 1-phosphotransferase mediates appreciable entry into glycolysis, and (iii) that PPi content can be maintained when respiratory-chain phosphorylation is inhibited.Abbreviations FW fresh weight - PFK(PPi) pyrophosphate fructose 6-phosphate 1-phosphotransferase - PPi inorganic pyrophosphate     

Effect of inorganic pyrophosphate and its diphosphonate analogs on glucose-6-phosphatase dehydrogenase activity of mouse organs

The glucose-6-phosphatase dehydrogenase (EC 1.1.1.49) reaction of mouse organs was studied as affected by PPi and its diphosphonate analogs. It is shown that in vitro and hydroxy-1-ethane-1,1-diphosphonic acid) inhibit the mentioned enzyme of the mouse spleen and liver. The effect of hydroxyl-1-ethane-1,1-diphosphonic acid was used as an example to show that inhibition of glucose-6-phosphate dehydrogeanse by diphosphonates belongs to the mixed type characterized by changes in the Km and Vmax values. For the spleen enzyme Km equals 0.064 mM, Vmax - 4.7 Mg of NADPH per 1 mg of protein-1. h-1. Administration of methylene diphosphonic acid causes an inhibition in vivo of the glucose-6-phosphatase dehydrogenate activity of the liver but not of the spleen and thymus. Basing on the isoenzymic composition of the enzyme for the mentioned organs, it is possible to suppose that the difference in the methylene diphosphonic acid effect in the liver and lymphoid organs may depend on the differences in its isoenzymic spectrum. The fact that in vivo methylene diphosphonic acid in a dose having an immuno-depressive action has no influence on the activity of glucose-6-phosphatase dehydrogenase in the lymphoid organs, may evidence for the absence of the indirect immunodepressive effect of diphosphonate by affecting this enzyme.     

Potential Role of Pyrophosphate:Fructose 6-Phosphate Phosphotransferase in Carbohydrate Metabolism of Cold Stored Tubers of Solanum tuberosum cv Bintje

To gain a better understanding of the mechanism of cold induced sweetening, sugar accumulation in potato, Solanum tuberosum cv Bintje, was compared to the maximum activity of inorganic pyrophosphate (PPi):fructose 6-phosphate 1-phosphotransferase (EC 2.7.1.90) and the concentration of two regulatory metabolites. Mature tubers accumulated reducing sugars and sucrose at an almost linear rate of 13.4 and 5.2 micromole per day per gram dry weight at 2°C and 4.5 and 1.3 micromole per day per gram dry weight, respectively, at 4°C. During storage at 8°C sugar accumulation was nil. Sugar accumulation was preceded by a lag phase of about 4 days. The accumulation of reducing sugars persisted for at least 4 weeks, whereas sucrose accumulation declined after 2 weeks of storage. The ratio of glucose:fructose changed concomitantly with sugar increase from 65:35 to equimolarity. The maximum activity of PPi:fructose 6-phosphate 1-phosphotransferase was 2.51 and 2.25 units per gram dry weight during storage at 2 and 8°C, respectively. The temperature coefficient of this enzyme from potatoes kept at 2 or 8°C was 2.12 and 2.48, respectively. The endogenous concentration of fructose 2,6-biphosphate increased from 0.15 to 1 nanomole per gram dry weight during storage at 2 and 4°C but remained the same throughout storage at 8°C. After exposure to 2°C an initial increase in the concentration of PPi was observed from 4.0 to 5.6 nanomoles per gram dry weight. Pyrophosphate concentration did not change during storage at 4°C but decreased slightly at 8°C. All observed changes became annulled after transfer of cold stored tubers to 18°C. These data strongly indicate that PPi:fructose 6-phosphate 1-phosphotransferase can be fully operational in cold stored potato tubers and the lack of increase in PPi concentration supports the functioning of this enzyme during sugar accumulation.     

Kinetic and structural properties of inorganic pyrophosphatase from the pathogenic bacterium Helicobacter pylori

Inorganic pyrophosphatase (PPase) catalyzes the hydrolysis of pyrophosphate (PPi) to orthophosphate (Pi) and controls the level of PPi in cells. PPase plays an essential role in energy conservation and provides the energy for many biosynthetic pathways. The Helicobacter pylori pyrophosphatase (HpPPase) gene was cloned, expressed, purified, and found to have a molecular weight of 20 kDa. The K(m) and V (max) of HpPPase were determined as 214.4 microM and 594 micromol Pi min(-1) mg(-1), respectively. PPi binds Mg(2+) to form a true substrate that activates the enzyme. However, free PPi could be a potent inhibitor for HpPPase. The effects of the inhibitors NaF, ATP, iminodiphosphate, and N-ethylmaleimide on HpPPase activity were evaluated. NaF showed the highest inhibition of the enzyme. Crystal structures of HpPPase and the PPi-HpPPase complex were determined. HpPPase comprises three alpha-helices and nine beta-strands and folds as a barrel structure. HpPPase forms a hexamer in both the solution and crystal states, and each monomer has its own PPi-binding site. The PPi binding does not cause a significant conformational change in the PPi-HpPPase complex, which might represent an inhibition state for HpPPase in the absence of a divalent metal ion.     

PPi-dependent phosphofructotransferase (phosphofructokinase) activity in the mollicutes (mycoplasma) Acholeplasma laidlawii.

A PPi-dependent phosphofructotransferase (PPi-fructose 6-phosphate 1-phosphotransferase, EC 2.7.1.90) which catalyzes the conversion of fructose 6 phosphate (F-6-P) to fructose 1,6-bisphosphate (F-1, 6-P2) was isolated from a cytoplasmic fraction of Acholeplasma laidlawii B-PG9 and partially purified (430-fold). PPi was required as the phosphate donor. ATP, dATP, CTP, dCTP, GTP, dGTP, UTP, dUTP, ITP, TTP, ADP, or Pi could not substitute for PPi. The PPi-dependent reaction (2.0 mM PPi) was not altered in the presence of any of these nucleotides (2.0 mM) or in the presence of smaller (less than or equal to 300 microM) amounts of fructose 2,6-bisphosphate, (NH4)2SO4, AMP, citrate, GDP, or phosphoenolpyruvate. Mg2+ and a pH of 7.4 were required for maximum activity. The partially purified enzyme in sucrose density gradient experiments had an approximate molecular weight of 74,000 and a sedimentation coefficient of 6.7. A second form of the enzyme (molecular weight, 37,000) was detected, although in relatively smaller amounts, by using Blue Sepharose matrix when performing electrophoresis experiments. The back reaction, F-1, 6-P2 to F-6-P, required Pi; arsenate could substitute for Pi, but not PPi or any other nucleotide tested. The computer-derived kinetic constants (+/- standard deviation) for the reaction in the PPi-driven direction of F-1, 6-P2 were as follows: v, 38.9 +/- 0.48 mM min-1; Ka(PPi), 0.11 +/- 0.04 mM; Kb(F-6-P), 0.65 +/- 0.15 mM; and Kia(PPi), 0.39 +/- 0.11 mM. A. laidlawii B-PG9 required PPi not only for the PPi-phosphofructotransferase reaction which we describe but also for purine nucleoside kinase activity. a dependency unknown in any other organism. In A. laidlawii B-PG9, the PPi requirement may be met by reactions in this organism already known to synthesize PPi (e.g., dUTPase and purine nucleobase phosphoribosyltransferases). In almost all other cells, the conversion of F-6-P to F-1,6-P2 is ATP dependent, and the reaction is generally considered to be the rate-limiting step of glycolysis. The ability of A. laidlawii B-PG9 and one other acholeplasma to use PPi instead of ATP as an energy source may offer these cytochrome-deficient organisms some metabolic advantage and may represent a conserved metabolic remnant of an earlier evolutionary process.     

A source of apparent pyrophosphate:fructose 6-phosphate phosphotransferase activity in rabbit muscle phosphofructokinase

In the presence of UDPglucose, rabbit muscle phosphofructokinase appeared to use PPi as a phosphoryl donor, as reported previously (Biochem. Biophys. Res. Commun. 121, 842-847). This apparent activity was due to conversion of UDPglucose and PPi to glucose 1-phosphate and UTP, the latter being metabolized by phosphofructokinase. Auxiliary enzymes used in the assays were contaminated by UDPglucose pyrophosphorylase. This contamination was sufficient to account for, and had similar properties to, the apparent PPi-dependent activity. Without auxiliary enzymes phosphofructokinase could not use PPi. These findings indicate that the apparent interconversion of phosphofructokinase and PPi:fructose 6-phosphate phosphotransferase must be re-assessed.     

The role of inorganic phosphate in the regulation of C4 photosynthesis

Inorganic phosphate participates in many fundamental processes within the plant cell. Its broad influence on plant metabolism is related to such key operations as metabolite transport, enzyme regulation and carbohydrate metabolism in general. This review discusses these topics with special emphasis on the role assigned to this ubiquitous anion within the C₄ pathway of photosynthesis.Abbreviations DHAP dihydroxyacetone phosphate - Ga3P glyceraldehyde-3-phosphate - NAD(P)-ME-NAD(P) dependent malic enzyme - PEP phosphoenolpyruvate - 3-PGA 3-phosphoglycerate - PFK and PFP-ATP- and PPi dependent fructose-6-phosphate 1-phosphotransferase - PPDK pyruvate:orthophosphate dikinase - RPPC reductive pentose-phosphate cycle - RuBisCO ribulose bisphosphate carboxylase-oxygenase - SPS sucrose-6-phosphate synthase