Km Values For Mg2 Research Articles

Phosphoenolpyruvate carboxylase (PEPC): Mutational analysis of a flexible loop and a putative binding site for an allosteric activator, glucose 6-phosphate (G6P)

The crystal structure of E.coli PEPC and Zea mays C4-form PEPC have been resolved recently, and we found an untraceable loop structure from K702 to G708 in E.coli numbering (from K762 to G768 in Zea mays numbering). This loop structure has flexibility and mobilty even in crystal state and participate in forming the active site (Kai et al. (1999) Proc. Natl. Acad. Sci. USA, 96, 823-828). Five mutants of this loop in Zea mays PEPC have been constructed. In the double mutation P765R/G766R, Vmax increased and extent of activation by an allosteric activator, G6P, increased. However, the Km value for Mg2+ increased. The two mutations P765R and P765G/G766R decreased the extent of activation by G6P. P765G mutant decreased the activity of the bicarbonate dependent PEP hydrolysis reaction and increased the extent of activation by G6P. In the insertional mutation of Arg residue between P765 and G766, Vmax increased and the extent of activation by G6P increased. These results indicated that this loop has multiple functions. From the studies of three-dimentional structure of Zea mays PEPC, we found the putative G6P binding site (R183, R184, R231 and R372). We prepared four mutations (R183Q, R183Q, R183Q/R184Q, and R231Q) and found that R183 and R184 are essential for G6P activation. During the course of these experiments, we obtained the data suggesting the presence of two affinity states for HCO3-in wild type PEPC.

Lysine 194 Is Functional in Isocitrate Lyase from Escherichia coli

Lysine 194 in conserved stretch 1 of tetrameric isocitrate lyase from Escherichia coli has been replaced by using the restriction-enzyme-site elimination method of directed mutagenesis. Expression of subunits of each variant and of wild-type (wt) enzyme was equivalent and all variants assembled into tetrameric proteins. The variants K194R and K194H had kcat values relative to that of wt enzyme taken as 100 of 11 and 7, respectively. Km values for Mg2+-Ds-isocitrate (in mM units) were: 0.13 for wt-enzyme; 0.12 for the K194R variant; and 0.55 for the K194H variant. Substitution at position 194 of Leu or Glu resulted in zero catalytic activity. These results establish that Lys 194 is another functional residue in conserved stretch one of isocitrate lyase from E. coli besides H184, K193, C195, and H197. Because K194 can be specifically replaced by the basic residues His and Arg with resultant lowered activity and by His with an increased Km value, it may contribute to a cation center and facilitate substrate binding as well as orientation of the developing transition state.

Convenient Isolation and Kinetic Mechanism of Glutathionylspermidine Synthetase from Crithidia fasciculata

Trypanothione, the essential metabolite in the oxidant defense system of trypanosomatids, is synthesized by two distinct proteins, glutathionylspermidine synthetase and trypanothione synthetase. Glutathionylspermidine synthetase was purified to homogeneity from the trypanosomatid Crithidia fasciculata by aqueous two-phase systems and chromatography. The enzyme showed a specific activity of 38 micromol of glutathionylspermidine formed per min per mg of protein. Its molecular mass was 78 kDa in SDS-polyacrylamide gel electrophoresis, and it appeared predominantly monomeric in native polyacrylamide gel electrophoresis and gel filtration. The isoelectric point was at pH 4.6, and the pH optimum was near 7.6. Partial amino acid sequencing revealed homology with, but low similarity to, the glutathionylspermidine synthetase/amidase of Escherichia coli, and amidase activity was not detected in glutathionylspermidine synthetase of C. fasciculata. The kinetics of trypanosomatid glutathionylspermidine synthetase revealed a rapid equilibrium random mechanism with limiting Km values for Mg2+-ATP, GSH, and spermidine of 0.25 +/- 0.02, 2.51 +/- 0.33, and 0.47 +/- 0. 09 mM, respectively, and a kcat of 415 +/- 78 min-1. Partial reactions at restricted cosubstrate supply were not detected by 31P NMR, supporting the necessity of a quarternary complex formation for catalysis. ADP inhibited competitively with respect to ATP (Ki = 0. 08 mM) and trypanothione exerted a feedback inhibition competitive with GSH (Ki = 0.48 mM).

Evidence for two catalytically different magnesium-binding sites in acetohydroxy acid isomeroreductase by site-directed mutagenesis

Alignment of procaryotic and plant acetohydroxy acid isomeroreductase (EC 1.1.1.86) reveals five conserved regions designated domains I, II, III, IV, and V. Domain I has been previously proposed to correspond to the NADPH-binding site [Dumas et al. (1991) Biochem. J. 277, 469-475] and domain III to a putative magnesium-binding site [Sista & Bowman (1992) Gene 120, 115-118]. The binding and the function of this cation are of particular importance. First, Mg2+ is essential for the two-step reaction catalyzed by this enzyme: an isomerization followed by an NADPH-dependent reduction. Second, the plant acetohydroxy acid isomeroreductase exhibits Kd and Km values for Mg2+ of 5 microM and 6 microM, respectively. Such values correspond to the strongest affinity known between an enzyme and the metal ion. To determine if domain III of acetohydroxy acid isomeroreductase is effectively involved in magnesium binding, and with the goal to assign a function to the other conserved domains, site-directed mutagenesis was performed on each charged or polar conserved amino acids of domains II-V of the spinach acetohydroxy acid isomeroreductase. The results demonstrate that mutation of each of these amino acids leads to a partial or complete inactivation of enzyme activity. Steady-state kinetic analysis and equilibrium binding experiments show that both domains III and IV are directly involved in the binding of magnesium. Also, they suggest that magnesium bound to domain III plays a role in the reductive half-reaction, whereas, magnesium bound to domain IV is involved in the isomerization half-reaction.

Solubilization and Reconstitution of the Mg2+/2H+ Antiporter of the Lutoid Tonoplast from Hevea brasiliensis Latex.

The Mg2+/2H+ antiporter recently described on lutoid membrane (Z. Amalou, R. Gibrat, C. Brugidou, P. Trouslot, J.d'Auzac [1992] Plant Physiol 100: 255-260) was solubilized by octylglucoside and reconstituted into soybean liposomes using the detergent dilution method. Magnesium efflux or influx experiments were used to generate a H+ influx or efflux, respectively, monitored with the fluorescent probe 9-amino-6-chloro-2-methoxyacridine. Both experiments gave saturable H+ fluxes as a function of internal or external Mg2+ concentrations with similar kinetic parameters Km and Vmax. The Km value for Mg2+ (about 2 mM) was identical to that previously found in lyophilized-resuspended lutoid (reference therein), whereas the Vmax value was 14-fold higher. Since only 10% of the initial proteins were recovered in proteoliposomes, and electrophoretic patterns of the two kinds of vesicles differed significantly, it was inferred that the increase in Vmax was due essentially to an enrichment of the protein antiporter in the reconstituted fraction, owing to a selective effect of octylglucoside at both solubilization and reconstitution steps. None of the various divalent cations used could dissipate the pH gradient of control liposomes of soybean lipids, unless the divalent/H+ exchanger A23187 was added, whereas a rapid dissipation of the pH gradient was observed with reconstituted proteoliposomes from lutoid proteins, with the cation selectivity sequence Zn2+ > Cd2+ > Mg2+ in the millimolar concentration range. The divalent ions Ca2+, Ba2+, and Mn2+ were incapable of generating a H+ efflux in reconstituted proteoliposomes, whereas both Mg2+/H+ and Ca2+/H+ exchanges were observed in lyophilized-resuspended lutoids. Therefore, the lutoid membrane seems to contain separate Mg2+/H+ and Ca2+/H transport systems, the latter being eliminated during the solubilization/reconstitution of lutoid membrane proteins.

An improved assay method for UDP-glucuronyltransferase activity towards 5-hydroxytryptamine and the characteristic properties of the enzyme.

A simple and reproducible assay method for UDP-glucuronyltransferase (GT) towards 5-hydroxytryptamine (5-HT) was developed. It consists of the removal of unconjugated 5-HT by 0.6 N NH4OH-saturated n-amyl alcohol and the colorimetric estimation of 5-HT glucuronide. Using this assay method, some properties of the enzyme activity in rat liver microsomes were studied. Simple Michaelis-Menten kinetics was followed with respect to 5-HT and the apparent Km value for 5-HT was 0.1 mM. However, the deviation from this kinetics was observed with respect to UDP-glucuronic acid (UDPGA). The apparent Km values for UDPGA were 0.6 mM and 5 mM. The enzyme activity was stimulated by divalent cations. For Mg2+, the enzyme did not obey this kinetics, and the apparent Km values for Mg2+ were 1 mM and 10 mM. In the presence of Mg2+, the apparent Km value for 5-HT did not change but the Vmax value increased. On the other hand, the addition of a low concentration of Mg2+ decreased the apparent Km value for UDPGA and increased the Vmax value. The addition of a high concentration of Mg2+ did not change the apparent Km value for UDPGA but increased the Vmax value. These results indicate that the enzyme activity is stimulated by the formation of Mg(2+)-UDPGA complex which showed higher affinity for the enzyme than UDPGA at the low concentration of Mg2+ and further stimulated by the formation of Mg(2+)-enzyme complex at the higher concentration of Mg2+.

Purification and properties of a kinase from Escherichia coli K-12 that phosphorylates two periplasmic transport proteins.

The phosphorylation in vivo and in vitro of the arginine-ornithine and the lysine-arginine-ornithine (LAO) periplasmic transport proteins of Escherichia coli K-12 was previously reported (Celis, R. T. F. (1984) Eur. J. Biochem. 145, 403-411). The phosphorylative reaction required ATP (as a direct energy donor), Mg2+, and a kinase that can be released by osmotic shock treatment of the cells. The enzyme was purified to electrophoretic homogeneity. The enzyme exhibited an ATPase activity and a kinase activity. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate gave an apparent molecular weight of 43,000 for the enzyme. The native protein showed the same molecular weight, suggesting that the protein is a monomer. The protein showed an apparent isoelectric point of 4.8 on isoelectric focusing. The two enzymatic reactions required a divalent cation and the apparent Km value for Mg2+ for the kinase activity was 0.5 mM. Mn2+ and Co2+ served as well as Mg2+, whereas Zn2+ and Ca2+ did not support activity. The ATPase activity of the enzyme yielded an apparent Km value for ATP of 50 microM. A similar value, Km of 100 microM, was calculated for the kinase activity with different concentrations of ATP. The enzyme showed a pH optimum of 7.3.

Properties of phosphofructokinase from carp, eel and rainbow trout liver.

Phosphofructokinase (EC 2.7.1.11) from rainbow trout liver, eelliver, and carp hepatopancreas was purified by ammonium sulfate fractionation, followed by ethanol fractionation, DEAF-cel-lulose chromatography, and Sephadex G-200 gel filtration. The rainbow trout and eel enzymes were obtained as a homogeneous peak by gel filtration. In the carp liver, there was a considerable loss of enzyme activity during dialysis. The molecular weight of the enzyme was estimated to be about 340, 000 by gel filtration, and the pH optimum of the activity was found to be around 8. The Km values for F6P of the rainbow trout, eel and carp enzymes were 19, 24, and 71μM, respectively, and that for ATP were 15, 19, and 22μM, respectively. The reaction mechanism by which the carp enzyme differed from that of the other two fish enzymes. Fish phosphofructokinase required Mg2+ for activation. The Km values for Mg2+ of the three fish enzymes ranged from O.83 to 1.1mM. The enzyme showed maximum activity around i mM ATP in the presence of 2mM F6P beyond which the activity decreased with increasing ATP concentration. ADP inhibited the enzyme activity, depending of F6P and ATP concentrations. AMP was found to activate the enzyme in the presence of ATP at inhibitory concentrations. The enzyme also utilized ITP, UTP, and CTP as a phosphoryl donor.

Stimulation of guanylate cyclase activity and reduction of adenylate cyclase activity by granulocyte-macrophage colony-stimulating factor in human blood neutrophils.

Human neutrophils were incubated with granulocyte-macrophage (GM)-CSF and examined for changes in second messenger systems. Twofold increases in cGMP but not cAMP were measured after 5 to 20 min with 100 U/ml GM-CSF. Guanylate cyclase activities in membrane and cytosol fractions were increased to the same extent whether measured in the presence of Mg2+ or Mn2+, or in the cytosol with Mg2+ + N-methyl-N'-nitro-N-nitroso-guanidine. Kinetic studies of the cytosol enzyme showed no changes in the Km values for Mg2+ and Mn2+dependent guanylate cyclase activities (0.91 and 0.022 mM, respectively), whereas Vm values were increased after treating intact cells with GM-CSF. Two peaks of guanylate cyclase activity were observed, one at 10 and another at 60 min after adding 100 U/ml GM-CSF, whereas only one peak at 5 min occurred with 1 U/ml. Adenylate cyclase activity was reduced by nearly 50% after adding 100 U/ml GM-CSF for 10 to 30 min. These effects were also seen in the presence of several hormonal and nonhormonal adenylate cyclase stimulators. In contrast, small increases in adenylate cyclase activity occurred after adding 1 U/ml GM-CSF. In experiments to examine the pathway of guanylate cyclase activation by GM-CSF, we observed no changes in inositol phosphates, intracellular calcium ion, or cytosolic protein kinase C. The augmentation of chemotactic peptide-induced superoxide production by GM-CSF concentrations, may be related to the effects of the higher levels of GM-CSF to stimulate late increases in guanylate cyclase or decreases in adenylate cyclase.

Properties of ATP-sulfurylase from marine alga Porphyra yezoensis.

ATP-sulfurylase purified from Porphyra yezoensis showed maximal activity at pH 8.2, and 37°C. It was shown that the actual substrate was Mg2+-ATP complex, and that ree-ATP was an inhibitor of the forward reaction. APS was a potent product inhibitor, competitive with respect to Mg2+-ATP and a mixed type inhibitor with respect to molybdate. The Km values for Mg2+-ATP, molyb-date, PPi, and APS were 0.67mM, 6.7mM, 33μM, and <1μM, respectively. Initial velocity studies showed that the mechanism of the molybdolysis reaction was a sequantial type. The molybdolysis reaction was inhibited by several nucleotides and anions of a sulfate analog, but was not affected by p-chloromercuribenzoate.

The roles of magnesium ions in the reaction catalysed by phosphofructokinase from Trypanosoma brucei.

The involvement of Mg2+ ions in the reaction catalysed by phosphofructokinase from Trypanosoma brucei was studied. The true substrate for the enzyme was shown to be the MgATP2-complex, and free Mg2+ ions are also required for enzyme activity. At concentrations of MgATP2- of 2.92 mM and greater, and a fructose 6-phosphate concentration of 1 mM and in the presence of EDTA as a Mg2+ buffer, the Km value for Mg2+ was determined to be 294 +/- 18 microM. Neither MgATP nor free ATP is an inhibitor of the enzyme, although apparent inhibition by the latter can be observed as a consequence of the decrease in free Mg2+ by chelation.

Pyruvate decarboxylase from Zymomonas mobilis. Isolation and partial characterization

Pyruvate decarboxylase (EC 4.1.1.1) from the ethanol producing bacterium Zymomonas mobilis was purified to homogeneity. This enzyme is an acidic protein with an isoelectric point of 4.87 and has an apparent molecular weight of 200,000±10,000. The enzyme showed a single band in sodium dodecylsulfate gel electrophoresis with a molecular weight of 56,500±4,000 which indicated that the enzyme consists of four probably identical subunits. The dissociation of the cofactors Mg2+ and thiamine pyrophosphate at pH 8.9 resulted in a total loss of enzyme activity which could be restored to 99.5% at pH 6.0 in the presence of both cofactors. For the apoenzyme the apparent Km values for Mg2+ and thiamine pyrophosphate were determined to be 24 μM and 1.28 μM. The apparent Km value for the substrate pyruvate was 0.4 mM. Antiserum prepared against this purified pyruvate decarboxylase failed to crossreact with cell extracts of the reportedly pyruvate decarboxylase positive bacteria Sarcina ventriculi, Erwinia amylovora, or Gluconobacter oxydans, or with cell extracts of Saccharomyces cerevisiae.

Purification of peroxisomal malate synthase from alkane-grown Candida tropicalis and some properties of the purified enzyme.

Malate synthase, one of the key enzymes in the glyoxylate cycle, was purified from peroxisomes of alkane-grown yeast, Candida tropicalis. The enzyme was mainly localized in the matrix of peroxisomes, judging from subcellular fractionation followed by exposure of the organelles to hypotonic conditions. The molecular mass of this peroxisomal malate synthase was determined to be 250,000 daltons by gel filtration on a Sepharose 6B column as well as by ultracentrifugation. On sodium dodecylsulfate/polyacrylamide slab-gel electrophoresis, the molecular mass of the subunit of the enzyme was demonstrated to be 61,000 daltons. These results revealed that the native form of this enzyme was homo-tetrameric. Peroxisomal malate synthase showed the optimal activity pH at 8.0 and absolutely required Mg2+ for enzymatic activity. The Km values for Mg2+, acetyl-CoA and glyoxylate were 4.7 mM, 80 microM and 1.0 mM, respectively.

Isolation and characterization of acetylcholine receptor membrane-associated (nonreceptor v2-protein) and soluble electrocyte creatine kinases.

Creatine kinase has been identified as a most prominent component of Torpedo electric organ and a minority constituent of the acetylcholine receptor (AChR) membranes obtained therefrom. Purification by low temperature ethanol extraction, precipitation of the Mg2+-enzyme complex, and mercurial-agarose chromatography yield preparations of soluble kinase with specific activities greater than 550 units/mg protein. Retention times in ion-exchange high performance liquid chromatography, electrophoretic behavior, immunochemical properties, tryptic mapping, and amino acid composition enable the comparison of creatine kinase isoenzymes. The denatured subunits of the predominant species have pI values of 6.3-6.8 and Mr = 40,000-42,000 characteristic of the so-called v2 proteins and show cross-reactivity with antibodies against the BB ("brain" type) creatine kinase. The MM ("muscle" type) antigens could be detected in the total electrocyte, but not in the AChR membranes; they have a slightly lower molecular weight and higher pI. The in situ membrane association of the BB isoenzyme is confirmed by immunocytochemistry. The apparent Km values for the substrate creatine phosphate are 2.2 mM for the AChR membrane-associated enzyme and 2.5 mM for the muscle form. The apparent Km values for Mg2+-ADP are 0.54 and 0.22 mM, respectively. Thus, a 2-fold higher affinity in the binding of ADP to the binary enzyme-creatine-P complex results from membrane association.

Purification and properties of H+-translocating, Mg2+-adenosine triphosphatase from vacuolar membranes of Saccharomyces cerevisiae.

H+-translocating, Mg2+-ATPase was solubilized from vacuolar membranes of Saccharomyces cerevisiae with the zwitterionic detergent N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate and purified by glycerol density gradient centrifugation. Partially purified vacuolar membrane H+-ATPase, which had a specific activity of 18 units/mg of protein, was separated almost completely from acid phosphatase and alkaline phosphatase. The purified enzyme required phospholipids for maximal activity and hydrolyzed ATP, GTP, UTP, and CTP, with this order of preference. Its Km value for Mg2+-ATP was determined to be 0.21 mM and its optimal pH was 6.9. ADP inhibited the enzyme activity competitively, with a Ki value of 0.31 mM. The activity of purified ATPase was strongly inhibited by N,N'-dicyclohexylcarbodiimide, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, tributyltin, 7-chloro-4-nitrobenzoxazole, diethylstilbestrol, and quercetin, but was not affected by oligomycin, sodium azide, sodium vanadate, or miconazole. It was not inhibited at all by antiserum against mitochondrial F1-ATPase or mitochondrial F1-ATPase inhibitor protein. These results indicated that vacuolar membrane H+-ATPase is different from either yeast plasma membrane H+-ATPase or mitochondrial F1-ATPase. The vacuolar membrane H+-ATPase was found to be composed of two major polypeptides a and b of Mr = 89,000 and 64,000, respectively, and a N,N'-dicyclohexylcarbodiimide binding polypeptide c of Mr = 19,500, whose polypeptide composition was also different from those of either plasma membrane H+-ATPase or mitochondrial F1-ATPase of S. cerevisiae.

Characteristics of the reverse reaction of NADP+‐isocitrate dehydrogenase from castor bean seeds

The reductive carboxylation of α‐ketoglutarate by purified NADP+‐isocitrate dehydrogenase (EC 1.1.1.42) from maturing castor bean seeds (Ricinus communis L.) has been characterized. The optimum pH for the reaction was 6.5, whereas pH 8.5 was optimum for oxidation of isocitrate (forward reaction). The enzyme utilized NADH as well as NADPH as the reducing agent in the reverse reaction, but only NADP+ in the forward reaction. The Km values for NADPH and NADH were 0.044 and 2.8 mM respectively, and for α‐ketoglutarate and HCO3 4.1 and 3.7 mM. The enzyme was activated by various cations including Mg2+, Mn2+, Co2+, Zn2+, Ni2+ and Co2+. Km values for Mg2+ Mn2+, Co2+ and Zn2+ were 12, 34, 37 and 49μM respectively.

Calorimetric studies of ligand-induced modulation of calcium adenosine 5'-triphosphatase from sarcoplasmic reticulum.

The enthalpy change (delta H degrees ') associated with the binding of Mg2+ to the sarcoplasmic reticulum calcium adenosine 5'-triphosphatase [(Ca2+)ATPase] is -76 kcal/mol. The affinity constant for Mg2+ obtained from calorimetric measurements agrees with the Km value for Mg2+ in the phosphorylation of the enzyme by inorganic phosphate (Pi). The delta H degrees ' of binding of Pi to the enzyme is -23.5 kcal/mol, and the affinity constant for Pi obtained from the calorimetry also agrees with the Km value for Pi in the phosphorylation reaction. delta H degrees ' of Mg2+ binding is reduced to -35 kcal/mol in the presence of either 20 mM Pi or 1.2 mM Ca2+ without a significant change in the affinity of the enzyme for Mg2+. delta H degrees ' of Pi binding to the enzyme drops to -8.5 kcal/mol in the presence of 10 mM Mg2+ without a significant change in the affinity of the enzyme for Pi. On the other hand, the presence of Ca2+ does not affect the delta H degrees ' for the binding of the substrate analogue 5'-adenylyl beta,gamma-imidodiphosphate [App(NH)p], and the presence of this analogue does not affect the delta H degrees ' for Ca2+ binding. The results suggest a model in which a conformational change, largely controlled by Mg2+ binding to the enzyme, leads to the formation of the covalent phosphoprotein intermediate.

Responses of the transmembrane potential coupled to the ATP-evoked catecholamine release in isolated chromaffin granules.

Using suspensions of chromaffin granules isolated from bovine adreno-medullae, the effects of Mg2+.ATP on membrane potential and catecholamine release across membranes of granules were investigated. The release of catecholamine was monitored by the measurement of changes in endogenous fluorescence of the amines, while the fluorometric method with 3,3'-dipropylthiadicarbocyanine iodide (diS-C3-(5)) was applied to the measurement of membrane potential changes. The catecholamine release brought about by Mg2+.ATP was found to obey Michaelis-Menten kinetics with Km-value for Mg2+ of about 0.15 mM and to be inhibited by dicyclohexylcarbodiimide (DCCD), an inhibitor of membrane-bound H+-ATPase. Concomitantly with such an amine release, depolarization of the granule membrane was observed to occur. The magnitude of this depolarization closely paralleled with the rate of the amine release and followed a saturation kinetics of Michaelis-Menten type with Km for Mg2+ of 0.21 mM and for ATP of 0.22 mM. Moreover, such a depolarization was competitively suppressed by DCCD. Based on these results, the role of the transmembrane potential change of granules in the catecholamine release was discussed.

Effects of polyamines on in vitro dna synthesis by DNA polymerases from calf thymus.

DNA synthesizing reactions catalyzed both large and small species of calf thymus DNA polymerase (DNA polymerase-alpha and -beta) [EC 2.7.7.7] were stimulated to comparable extents by the presence of spermidine or spermine, and it was not possible to differentiate these two species in terms of their sensitivities to polyamines. Optimal concentrations for stimulation were 0.5-1.0 mM for spermidine and 2-10 mjM for spermine. Excess polyamines strongly inhibited the reactions. The modes of stimulation were as follows: 1) Stimulation was observed with templates bearing long single-stranded sections when either natural DNA or synthetic homopolymer-oligomer duplex was used. 2) The nautral DNA-dependent reaction was stimulated by polyamines at suboptimal concentrations of Mg2+; the apparent Km value for Mg2+ was lowered on adding polyamines, while the Vmax value was unchanged. When synthetic homopolymer-oligomer duplex was used as a template, the reaction was stimulated spermidine even at the optimal concentration of Mn2+. 3) Polyamines markedly influenced the salt requirements of the reactions of DNA polymerase. Spermidine could replace salts such as KC1 or NaC1 at concentrations less than 1/100 of those of salts.

Control of rabbit liver fructose-1, 6-diphosphatase activity by magnesium ions.

EDTA at a concentration of 1 muM produced a threshold effect in the activation of purified rabbit liver fructose-1, 6-diphosphatase [EC 3.1.3.11] in the presence of 5 mM Mg2+ at pH 7.2. Without EDTA, biphasic activation curves were produced by Mg2+. A double-reciprocal plot of the data gave the Km values corresponding to the two linear regions. They were 0.19 and 0.83 mM at pH 7.5, and 0.055 and 0.83 mM at pH 9.1. In the presence of 5muM EDTA a sigmoidal curve was obtained for Mg2+ activation in the range of noninhibitory Mg2+ concentrations at pH 7.2. The apparent Km value for Mg2+ was 0.15 mM, and the Hill coefficient was 2.0. At pH 9.1 cooperativity among the Mg2+ sites disappeared, and the apparent Km value for Mg2+ was 0.055 mM. These Km values at pH 7.2 or 9.1 corresponded to the smaller of the biphasic Km values obtained without EDTA. In the absence of EDTA, no inhibition by Mg2+ was observed in the Mg2+ concentration range below 10 mM. In the presence of EDTA, the enzyme was inhibited markedly by Mg2+ at concentrations above 0.5 mM at pH 7.2, and was more sensitive to inhibition at pH 9.1. The effects of pH on the Km value for Mg2+ activation and on the Mg2+ inhibition contributed to an apparent shift of the pH optimum for activity induced by EDTA. Cooperative interaction among fructose-1, 6-diphosphate sites was observed for the enzyme in the presence of EDTA. The Hill coefficient was approximatley 1.8, and the apparent Km value for the substrate was 0.74 muM. EDTA appears to make liver fructose-1, 6-diphosphatase very sensitive to various effectors. It is suggested that Mg2+ serves as a regulator for the enzyme activity.