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PubMed1992

Influence of fructose 2,6-bisphosphate and MgATP on rat liver phosphofructokinase at pH 7: evidence for a complex interdependence.

The relationship between fructose 2,6-bisphosphate (Fru-2,6-BP) activation and MgATP inhibition of rat liver phosphofructokinase has been comprehensively evaluated at pH 7. When either ligand is varied at a fixed concentration of the other, its influence on the concentration of fructose 6-phosphate (Fru-6-P) required to produce half-maximal velocity, Ka, is usually well described by the same simple, single-modifier linkage expression that described the actions of these ligands at pH 9. However, the effects of both ligands together cannot be described by the same overall linkage relationship that described their actions at pH 9. Specifically, despite an overall antagonistic relationship between the binding of MgATP and that of Fru-2,6-BP, very low concentrations of Fru-2,6-BP appear to facilitate the binding of MgATP to an appreciable degree. Also, MgATP at high concentration appears to inhibit the binding of Fru-2,6-BP to a significantly greater extent than its actions at lower concentration would predict. These additional features of MgATP-Fru-2,6-BP interaction have been incorporated into an overall linkage expression describing the actions of both MgATP and Fru-2,6-BP on Ka for Fru-6-P. The best fit parameters predict the data to within an average standard error of +/- 21%.

Adenosine Triphosphate↗
PubMed1983

Heterofermentative glucose metabolism by glucose transport-impaired mutants of oral streptococcal bacteria during growth in batch culture.

Spontaneous mutants defective in a membrane component of the phosphoenolpyruvate-glucose phosphotransferase system were isolated by plating cells of Streptococcus sanguis 10556, Streptococcus mutans GS5-2 and NCTC 10449 on agar containing lactose and 2-deoxyglucose. Toluenized cells of these mutants were defective in their ability to catalyse the phosphoenolpyruvate-dependent phosphorylation of 2-deoxyglucose. The parental strains were mainly homofermentative when grown in batch culture in the presence of various sugars. Nevertheless, the mutants produced acetate, formate and ethanol when cultured in the presence of glucose but were homofermentative when grown in the presence of lactose or maltose. Analysis of one mutant isolated from Strep. sanguis (mutant GS26) revealed normal levels of glucokinase, glucose-6-phosphate dehydrogenase, puruvate kinase and lactate dehydrogenase. This last enzyme was dependent on fructose 1,6-diphosphate for catalytic activity. The determination of the intracellular level of fructose 1,6-diphosphate (FDP) during growth of the cells in batch culture showed that the mutant strains contained 2 to 15 times less FDP than the parental strains. Growth experiments performed at pH 6.0 and 7.0 with Strep. sanguis and its PTS-negative mutant GS26 suggested that the regulation of pyruvate metabolism in this bacterium include the intracellular level of FDP and the initial hydrogen concentration of the growth medium. The results also suggested that, in these bacteria, an active PTS is required to maintain the intracellular concentration of FDP high enough to keep the cell homofermentative during growth in batch culture.

Biological Transport↗
PubMed1992

Pilocarpine-induced increases in the activity of 6-phosphofructo-2-kinase and the fructose-2,6-bisphosphate content of rat salivary glands.

The activity of 6-phosphofructo-1-kinase (PFK-1), an important regulatory enzyme of glycolysis, was determined after injection of the sialagogue pilocarpine. The fructose-2,6-bisphosphate content of the glands and 6-phosphofructo-2-kinase (PFK-2) activity were also measured. The increase in PFK-1 activity after pilocarpine treatment was likely to be due to the increase in the content of its potent modulator, fructose-2,6-bisphosphate. This in turn was assumed to be due to the increase in the activity of the active form of PFK-2.

Animals↗
PubMed1980

Phosphocretine production coupled to the glycolytic reactions in the cytosol of cardiac cells.

Phosphocreatine production ctalyzed by a cytosolic fraction from cardiac muscle containing all glycolytic enzymes and creatine kinase in a soluble form has been studied in the presence of creatine, adenine nucleotides and different glycolytic intermedites as substrates. Glycolytic depletion of glucose, fructose 1,6bis(phosphate) and phosphoenolpyruvate to lactte was coupled to efficient phosphocreatine production. The molar ratio of phosphocreatine to lactate produced was close to 2.0 when fructose 1,6bis(phosphate) was used as substrate and 1.0 with phosphoenolpyruvate. In these processes the creatine kinase reaction was not the rate-limiting step: themass action ratio of the creatine kinase reaction was very close to its equilibrium value and the maximal rate of the forward creatine kinase reaction exceeded that of glycolytic flux by about 6-fold when fructose 1,6-bis(phosphate) was used as a substrate. Therefore, the creatine kinase raction was continuously in the state of quasi-equilibrium and the efficient syntheses of phosphocreatine observed is a result of constant removal of ADP by the glycolytic system at an almost unchanged level of ATP ([ATP]>>[ADP]), this leading to a continuous shift of the creatine kinase equilibrium position. When phosphocreatine was added initially at concentrations of 5---15 mM the rate of the coupled creatine kinase and glycolytic reactions was very significantly inhibited due to a sharp decrease in the steady-state concentration of ADP. Therefore, under conditions of effective phosphocreatine production in heart mitochondria, which maintain a high phosphocreatine: creatine ratio in the myoplasm in vivo, the glycolytic flux may be suppressed due to limited availability of ADP restricted by the creatine kinase system. The possible physiological role of the control of the glycolytic flux by the creatine kinase system is discussed.

Creatine Kinase↗
PubMed1978

The genetic system of the L-type pyruvate kinase forms in man. Subunit structure, interrelation and kinetic characteristics of the pyruvate kinase enzymes from erythrocytes and liver.

Pyruvate kinase (ATP: pyruvate 2-O-phosphotransferase, EC 2.7.1.40) from human liver and red cells has been purified to homogeneity; its subunit structure and some of its kinetic characteristics have been studied. The influence of a partial proteolysis by trypsin on the subunit structure, the isozymic pattern and the kinetic characteristics of red cell and liver enzyme have been investigated. From the results of this study we may conclude that: 1. Liver (L-type) pyruvate kinase is composed of 4 identical L subunits while the major form of erythrocyte enzyme (PK-R2) is a heterotetramer designated as L2L2', the molecular weight of L' being slightly higher than that of L subunits (63 000 and 58 000 respectively). Pyruvate kinase PK-R1, predominant in the erythroblasts and the young red cells, is composed of four identical L' subunits. 2. A mild tryptic attack is able to transform PK-R1 into PK-R2, then PK-R2 into pyruvate kinase L (PK-L). The same proteolytic treatment transforms the L' subunits into L ones. 3. Consequently L-type pyruvate kinase seems to be initially synthesized in the erythroid precursors as an L4' enzyme secondarily partially proteolysed into L2L2'. In liver a very active proteolytic system would be responsible for the total transformation into L4 pyruvate kinase. 4. L4' enzyme exhibits Michaelis-Menten kinetic behaviour with an apparent Michaelis constant of 3.8 mM whereas L4 enzyme shows both positive and negative homotropic interactions towards phosphoenolpyruvate and has [S] 0.5 of 1.2 mM. The characteristics of L2L2' are roughly intermediate between those of L4' and of L4. Fructose 1,6-biphosphate decreases [S]0.5 for these three pyruvate kinase forms without suppressing the differences in the apparent affinity for phosphoenolpyruvate of these enzymes. 5. L4 pyruvate kinase is more inhibited by Mg-ATP than L4', with L2L2' in the intermediate range. 6. Tryptic treatment of each enzyme form studied transforms its kinetic behaviour into that observed for L4.

Erythrocytes↗
PubMed1979

Functional changes associated with the sequential transformation of L'4 into L4 pyruvate kinase.

The functional changes, associated with the sequential transformation of L'4 into L4 pyruvate kinase (ATP:pyruvate 2-O-phosphotransferase, EC 2.7.1.40) were studied. L'4 enzyme from human erythrocytes shows strong hysteretic behaviour: the initial rate of the enzyme preincubated with an unsaturating concentration of phosphoenolpyruvate is much higher than of the enzyme preincubated with ADP, at the same phosphoenolpyruvate concentration, although the "final activity" (the activity of the linear part of the reaction progress curve) was the same in both cases. This phenomenon was observed both in the presence and absence of fructose 1,6-diphosphate. High concentrations of both Mg2+free and MgATP2- diminish the difference in initial rate, between the ADP and phosphoenolpyruvate preincubated enzymes: Mg2+free by stabilizing the phosphoenolpyruvate-induced form; ATPMg2- by stabilizing the ADP-induced form. The magnitude of the difference in initial rates of the ADP-or phosphoenolpyruvate-preincubated enzyme is a function of both substrates. L4 pyruvate kinase (either from human liver or trypsin treated L'4 enzyme) does not, or to a very slight extent, show such behaviour. L'2L2 pyruvate kinase shows behaviour intermediate between L'4 and L4 enzymes. A model is proposed to describe the kinetic behaviour of L'4 and L4 enzymes.

Adenosine Diphosphate↗
PubMed1980

Studies on the regulation of chloroplast fructose-1,6-bisphosphatase. Activation by fructose 1,6-bisphosphate.

Chloroplast fructose-1,6-bisphosphatase (D-fructose 1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) isolated from spinach leaves, was activated by preincubation with fructose 1,6-bisphosphate. The rate of activation was slower than the rate of catalysis, and dependent upon the temperature and the concentration of fructose 1,6-bisphosphate. The addition of other sugar diphosphates, sugar monophosphates or intermediates of the reductive pentose phosphate cycle neither replaced fructose 1,6-bisphosphate nor modified the activation process. Upon activation with the effector the enzyme was less sensitive to trypsin digestion and insensitive to mercurials. The activity of chloroplast fructose-1,6-bisphosphatase, preincubated with fructose 1,6-bisphosphate, returned to its basal activity after the concentration of the effector was lowered in the preincubation mixture. The results provide evidence that fructose-1,6-bisphosphatase resembles other regulatory enzymes involved in photosynthetic CO2 assimilation in its activation by chloroplast metabolites.

Chloroplasts↗
PubMed1980

Purification and properties of adenylosuccinate synthetase from Yoshida sarcoma ascites tumor cells.

Adenylosuccinate synthetase (IMP:L-aspartate ligase (GDP-forming), EC 6.3.4.4) was purified about 750-fold to a homogeneous state from Yoshida sarcoma ascites tumor cells. A yield of 38% purified enzyme was achieved by a procedure including affinity chromatography on hadacidin-Sepharose 4B. Ultracentrifugal analyses showed that the molecular weight of the native enzyme was 102 000 with an s20,w value of 4.5 and that the molecular weight in 6 M guanidine-HCl was 47 000. These values indicate that the native enzyme is composed of two subunits. The isoelectric point was determined to be 5.9 by isoelectric focusing. The optimum pH for activity was 6.8-7.0. The Km values for IMP, aspartate and GTP were calculated to be 4.1, 9.8 and 0.7 . 10(-4) M, respectively. The antibiotic, hadacidin was strongly inhibitory, causing competitive inhibition with respect to aspartate with a Ki value of 2.5 . 10(-6) M. Nucleoside mono- and diphosphate also inhibited the enzyme activity, but their inhibitions were not apparently specific. The purified enzyme showed full activity in the presence of Mg2+, and Mg2+ could be partially replaced by Mn2+, Co2+, Ca2+ or Cu2+. Divalent metal ions, such as Cd2+, Pb2+, Zn2+, Cu2+ and Mn2+, interfered with the activity by antagonizing Mg2+. Hg2+ or PCMB inactivated the enzyme, suggesting that an SH-group may be important for activity.

Adenylosuccinate Synthase↗
PubMed1980

Comparison of the mechanisms of two distinct aldolases from Escherichia coli grown on gluconeogenic substrates.

Escherichia coli grown on gluconeogenic compounds as carbon sources produced two chemically and physically distinct types of fructose-1,6-biphosphate aldolases (D-fructose-1,6-bisphosphate D-glyceraldehyde-3-phosphatelyase, EC 4.1.2.13), while these bacteria produced only a single enzyme when grown on glucose or fructose. We have investigated this enzyme in several strains of Escherichia coli (Crookes, K-12, and B) grown on glucose, fructose lactate, pyruvate, alanine and glycerol by comparing chemical properties and mechanisms of action. Comparison of these mechanisms was accomplished by following the fate of 18O in the keto position of fructose 1,6-bisphosphate during the aldolase catalyzed cleavage reaction. The results show that the two enzymes have different mechanisms of action and are consistent with a Schiff-base mechanism for the one which was induced by gluconeogenic substrates and metal-chelate mechanism for the constitutive enzyme.

Alanine↗
PubMed1981

Reversible microsomal binding of hepatic aldolase.

Fructose-1,6-bisphosphate aldolase (D-fructose-1,6-bisphosphate D-glyceraldehyde-3-phosphate lyase, EC 4.1.2.13) partitions between the microsomes and the cytosol when a rat liver homogenate is fractionated by differential centrifugation. Gel electrophoresis and immunodiffusion indicate that the one isozyme present in the liver of the young adult rat is found in both fractions. The association of the aldolase with membranes is differentially sensitive to a variety of metabolites and inorganic salts. In the absence of cellular salts, 1 mM fructose 1,6-bisphosphate or glucose 1,6-bisphosphate elutes 50% of the enzyme from the microsomes. About 9 mM Pi or citrate is necessary to produce the same effect. With other metabolites or inorganic salts higher concentrations are required. The fraction of total enzyme which partitions with the microsomes when a homogenate is submitted to high speed centrifugation, correlates inversely with the level of fructose 1,6-bisphosphate in the supernatant solution and this concentration is higher when the tissue concentration in the homogenate is greater. The Km for fructose 1,6-bisphosphate of 3 . 10(-4) for aldolase bound to microsomes is decreased to 6 . 10(-6) M when the enzyme is dissociated from the membranes with salt. These observations appear relevant to the ongoing discussion regarding the physiological relevance of the subcellular localization of glycolytic enzymes.

Animals↗
PubMed1980

The effect of hemin, adenosine 3',5'-monophosphate and phosphorylated sugars on Met-tRNAfMet deacylase activity in rabbit reticulocyte lysates.

The effect of hemin, phosphorylated sugars, adenosine 3',5'-monophosphate (cyclic AMP) and a number of purines on a specific initiator tRNA deacylase activity in rabbit reticulocytes has been investigated. In the concentration range established to be optimal for maximal stimulation of translation (5.5-30.0 microM), hemin produces a 20-82% inhibition of Met-tRNAfMet deacylation. In contrast, all phosphorylated sugars tested, with the exception of fructose 1,6-diphosphate, are without effect. High concentrations of cyclic AMP (2-4 mM) also significantly inhibit the deacylase activity. The role of hemin and Met-tRNAfMet deacylase in the control of peptide initiation are discussed.

Acyltransferases↗
PubMed1978

Proteolytic modification of pig and rat liver pyruvate kinase type L including phosphorylatable site.

The phosphorylated or phosphate-accepting site of pyruvate kinase from pig and rat liver was removed without inactivation by incubation with subtilisin. At different time intervals the subtilisin was inactivated with phenylmethylsulfonyl fluoride and the amount of remaining phosphorylatable or phosphorylated sites of pyruvate kinase estimated by incubation with an excess of [32P]-ATP and protein kinase. It was found that to get the same rate of modification the subtilisin concentration required to modify unphosphorylated pyruvate kinase was approximately ten times higher than that used for removal of the phosphorylated site of phosphorylated site of phosphorylated enzyme. It was shown that the proteolytically-modified pyruvate kinase had an increased apparent Km for phosphoenolpyruvate without a change in V, when compared to unmodified unphosphorylated and phosphorylated pyruvate kinase. The removal of the phosphorylated site was not associated with loss of the allosteric sites for ATP and Fru-1,6-P2. The possibility that phosphorylation of the pyruvate kinase increases its degradation rate in vivo is briefly discussed.

Adenosine Triphosphate↗