Purification of the 56-kDa component of the bacteriophage T7 primase/helicase and characterization of its nucleoside 5'-triphosphatase activity

Bacteriophage T7 gene 4 protein, purified from phage-infected cells, consists of a mixture of a 56- and a 63-kDa species that provides primase and helicase activities for T7 DNA replication. The 56-kDa species has been purified 1800-fold from Escherichia coli cells containing a plasmid that encodes this gene 4 protein. The purified 56-kDa protein is homogeneous, as determined by denaturing gel electrophoresis, and is monomeric in its native form, as indicated by gel filtration. The binding of the 56-kDa protein to single-stranded DNA is stimulated by nucleoside 5'-triphosphates, as is the case for a mixture of the two molecular weight species. In the presence of DNA, the 56-kDa protein preferentially hydrolyzes dTTP (Bernstein, J. A., and Richardson, C. C. (1988) Proc. Natl. Acad. Sci. U. S. A. 85, 396-400). Since nucleoside 5'-triphosphatase activity is necessary for both helicase activity and for translocation of gene 4 protein to primase recognition sites, we have characterized this activity using the 56-kDa protein alone. In the DNA-dependent hydrolysis reaction, the enzyme displays a Km of 10 mM for dTTP, and a Vmax of 2.9 x 10(-5) M/min/mg of protein (at 2.5 micrograms/ml). There is little cooperativity with respect to dTTP binding (Hill coefficient = 1.1) except in the presence of ribonucleoside 5'-triphosphate, an inhibitor of dTTP hydrolysis (Hill coefficient greater than 1.5). The apparent KD for single-stranded circular DNA is 0.2 microM. The active species in dTTP hydrolysis is an oligomer of at least two subunits, as indicated by the effect of enzyme concentration upon the rate of DNA-dependent hydrolysis. The 56-kDa protein also catalyzes DNA-independent hydrolysis of dTTP with a Km of 0.11 mM and a Vmax of 1.3 x 10(-7) M/min/mg of protein (at 8 micrograms/ml). The active species in DNA-independent dTTP hydrolysis is also an oligomer.

Rapid purification of inositol monophosphate phosphatase from beef brain

A procedure is described for preparation of homogeneous inositol monophosphate phosphatase (EC 3.1.3.25) from beef brain in less than 2 days with an overall recovery of 15-25%. This enzyme, an essential part of the inositol phospholipid cycle in brain, is a proposed site of action of lithium ions in manic-depressive disorders. The major purification steps are: a) removal of most interfering protein by heat denaturation at 75 degrees C for 1 h, b) separation by anion exchange at a pH (6.0) near the enzyme's pI (4.9), and c) adsorption of most remaining impurities on a Procion Red affinity column.

Properties of inositol polyphosphate 1-phosphatase

We recently described inositol polyphosphate 1-phosphatase, an enzyme which cleaves the 1-phosphate from inositol 1,4-bisphosphate (Ins(1,4)P2) and inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) (Inhorn, R. C., and Majerus, P. W. (1987) J. Biol. Chem. 262, 15946-15952). We have now purified the enzyme to homogeneity from calf brain. The enzyme hydrolyzes 50.3 mumol of Ins(1,4)P2/min/mg protein. The enzyme has an apparent mass of 44,000 daltons as determined both by gel filtration chromatography and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, suggesting that it is monomeric. Lithium ions inhibit Ins(1,3,4)P3 hydrolysis uncompetitively with an apparent Ki of approximately 0.3 mM LiCl. Calcium inhibits hydrolysis of Ins(1,4)P2 and Ins(1,3,4)P3 equally, with approximately 40% inhibition occurring at 1 microM free Ca2+. Rabbit polyclonal antiserum against purified inositol polyphosphate 1-phosphatase was prepared which immunoprecipitates approximately 0.3 milliunits of activity/microliter serum (1 unit = 1 mumol of Ins(1,4)P2 hydrolyzed per min). This antiserum was used to determine the enzyme content in several bovine tissues, all of which had a similar intrinsic specific activity (i.e. approximately 0.3 milliunits/microliter antiserum). Tissues studied included brain, heart, kidney, liver, lung, parotid, spleen, testis, and thymus. Approximately 10-15% of the total inositol polyphosphate 1-phosphatase activity in calf brain homogenates remains in a particulate fraction; antiserum also binds 0.3 milliunits of membrane-associated activity/microliter antiserum. Thus, a single enzyme can account for Ins(1,4)P2 hydrolytic activity in the bovine tissues. Ins(1,3,4)P3 metabolism was also investigated in bovine tissue homogenates. Inositol polyphosphate 1-phosphatase accounts for greater than 80% of the hydrolytic activity in all tissues studied except brain, where inositol polyphosphate 4-phosphatase is the major enzyme that hydrolyzes Ins(1,3,4)P3. The apparent Km of inositol polyphosphate 1-phosphatase for Ins(1,3,4)P3 varies approximately 3-4-fold among the bovine tissues.

Dephosphorylation of 1D-myo-inositol 1,4-bisphosphate in rat liver

Dephosphorylation of 1D-myo-inositol 1,4-bisphosphate [Ins(1,4)P2] in rat liver is catalysed by a cytosolic phosphatase that removes the 1-phosphate group. The Km for Ins(1,4)P2 is approx. 17 microM. Li+ (100 mM) causes 50% inhibition of Ins(1,4)P2 phosphatase activity when activity is measured at the very low substrate concentration of 10 nM, but on raising the substrate concentration to 100 microM there is a greater than 10-fold increase in sensitivity to Li+, suggesting that Li+ acts mainly, but not entirely, as an uncompetitive inhibitor of Ins(1,4)P2 phosphatase. In addition, rat liver cytosol shows Li+-sensitive phosphatase activity against 1D-myo-inositol 1-,3- and 4-monophosphates. The Ins(1,4)P2 1-phosphatase and inositol monophosphatase activities all share an apparent Mr of 47 x 10(3), as determined by gel-filtration chromatography. However, the Ins(1,4)P2 1-phosphatase is more sensitive to inactivation by heat, and can be separated from inositol monophosphatase activity by anion-exchange chromatography. We conclude that rat liver cytosol contains an Ins(1,4)P2 1-phosphatase that is distinct from, but in many ways similar to, inositol monophosphatase.

Regulation of bile acid synthesis. Isolation and characterization of microsomal phosphatases

In preliminary experiments, we have shown that rat liver microsomes possess phosphatase activity which was inhibited in the presence of sodium fluoride. We have now separated six microsomal phosphatase fractions appearing to be isoenzymes. They all possess different kinetic constants and are not equally inhibited by tartrate and fluoride ions, inhibitors of phosphatase activity. One phosphatase fraction, in fact, is almost completely unaffected by fluoride ion. More pertinent to our interest, these isoenzymes exhibit differing abilities to modulate the activities of hydroxymethylglutaryl CoA reductase, acyl-CoA:cholesterol O-acetyltransferase, and cholesterol 7 alpha-hydroxylase. Interaction of four of the fractions with rat liver microsomes resulted in a decrease in cholesterol 7 alpha-hydroxylase activity; two were without effect.

Horse brain acylphosphatase: purification and characterization

Two structurally different acylphosphatases found in horse brain were purified; they were not immunologically related. The molecular masses were almost identical and the kinetic parameters were rather similar. The data reported indicate that one of the purified brain acylphosphatases and an enzyme, previously isolated from horse muscle, are the same protein. The presence of this acylphosphatase form in the brain has not been reported before. The other acylphosphatase seemed to be the same as the enzyme which had been purified from calf brain and partially characterized by Diederich and Grisolia [(1969) J. Biol. Chem. 244, 2412-2417]. Furthermore, this enzyme seems to be identical to the acylphosphatase recently purified in our laboratory from human erythrocytes.

Purification and properties of inositol-1,4-bisphosphatase from bovine brain

Inositol-1,4-bisphosphatase has been purified 13,000-fold from bovine brain supernatant. The enzyme is monomeric, with an apparent subunit Mr of 40,000. Maximal hydrolytic rates were observed in Tris buffer, pH 7.8, in the presence of 9 mM-Mg2+. The enzyme acted as a 1-phosphatase, hydrolysing both inositol 1,4-bisphosphate [Ins(1,4)P2] (Km 0.04 mM) and inositol 1,3,4-trisphosphate [Ins(1,3,4)P3] (Km 0.5 mM) to inositol 4-phosphate and inositol 3,4-bisphosphate respectively. Li+ inhibited the hydrolysis of both substrates in an uncompetitive manner, with apparent Ki values of 9.63 mM and 0.46 mM for Ins(1,4)P2 and Ins(1,3,4)P3 respectively.

Rat hepatoma (HTC) cell 6-phosphofructo-2-kinase differs from that in liver and can be separated from fructose-2,6-bisphosphatase

6-Phosphofructo-2-kinase was purified from rat liver and hepatoma (HTC) cells. The HTC cell enzyme had kinetic properties different from those of the liver enzyme (more sensitive to inhibition by citrate and not inhibited by sn-glycerol 3-phosphate) and was not a substrate of the cyclic-AMP-dependent protein kinase. Unlike the liver enzyme, which is bifunctional and phosphorylated by fructose 2,6-[2-32P]bisphosphate, the HTC cell enzyme contained no detectable fructose-2,6-bisphosphatase activity and phosphorylation by fructose 2,6-[2-32P]-bisphosphate could not be detected. HTC cell fructose-2,6-bisphosphatase could be separated from 6-phosphofructo-2-kinase activity by purification. Antibodies raised against liver 6-phosphofructo-2-kinase did not precipitate HTC cell fructose-2,6-bisphosphatase whose kinetic properties were completely different from those of the liver enzyme.

Purification and properties of myo-inositol-1-phosphatase from bovine brain

myo-Inositol-1-phosphatase from bovine brain was purified over 2000-fold. The native enzyme has a Mr of 59,000, and on SDS/polyacrylamide-gel electrophoresis the subunit Mr was 31,000. Thus the native enzyme is a dimer of two apparently identical subunits. The enzyme, purified to a specific activity of more than 300 units/mg of protein (1 unit of enzyme activity corresponds to the release of 1 mumol of Pi/h at 37 degrees C), catalysed the hydrolysis of a variety of phosphorylated compounds, the best one, in terms of V/Km, being D-myo-inositol 1-phosphate. Kinetic constants of compounds tested, including both isomers of glycerophosphate and two deoxy forms of beta-glycerophosphate, were measured. They show the importance of the two hydroxyl groups which are adjacent to the phosphate in myo-inositol 1-phosphate. With a wide variety of substrates Li+ was found to be an uncompetitive inhibitor whose Ki varied with substrate structure.

Purification and characterization of myocardial fructose-6-phosphate,2-kinase and fructose-2,6-bisphosphatase

Fructose-6-P,2-kinase:fructose-2,6-bisphosphatase has been purified to homogeneity from beef heart. The enzyme was bifunctional and the specific activities of the kinase and the phosphatase of the pure enzyme were 60 and 30 milliunits/mg, respectively. The molecular weight of the enzyme was 118,000, consisting of two subunits of 58,000. In some preparations of the enzyme a minor protein with a subunit Mr of 54,000 was present. This minor protein (54,000) was also bifunctional and showed the same immunoreactivity as the major protein. The specific activity of fructose-6-P,2-kinase of the minor component was three times higher than that of the major enzyme (58,000), but fructose-2,6-bisphosphatase activity was the same. These two forms have been separated by phosphocellulose chromatography. The tryptic peptide maps of these enzymes were very similar. The 58,000 enzyme was phosphorylated by cAMP-dependent protein kinase but the 54,000 enzyme was not. These results indicated that the minor 54,000 protein might be a proteolytically digested form of the 58,000 enzyme. The Km of the kinase for fructose-6-P and ATP was 70 microM and 260 microM, respectively for both the 58,000 and the 54,000 enzymes. Km for fructose-2,6-P2 and Ki for fructose-6-P of the phosphatase was approximately 40 and 11 microM, respectively. The enzyme was phosphorylated by fructose-2,6-P2 but the stoichiometry of the phosphate incorporation was 0.05 mol/mol subunit, while 0.4 mol/mol was incorporated in rat liver enzyme under the same conditions.

A unique deoxyguanosine triphosphatase is responsible for the optA1 phenotype of Escherichia coli

Escherichia coli optA1, a mutant unable to support the growth of T7 phage containing mutations in gene 1.2, contains reduced amounts of dGTP. Extracts of E. coli optA1 catalyze the hydrolysis of dGTP at a rate 50-fold greater than do extracts of E. coli optA+. The dGTPase responsible for the increased hydrolysis has been purified to apparent homogeneity. Purification of the protein is facilitated by its high affinity for single-stranded DNA. By using this purification scheme an identical dGTPase has been purified from E. coli optA+. The purified proteins catalyze the hydrolysis of dGTP to yield deoxyguanosine and tripolyphosphate. The products of hydrolysis, chromatographic properties, denatured molecular mass of 56 kDa, N-terminal amino acid sequence, substrate specificity, and heat inactivation indicate that the proteins purified from optA1 and from optA+ cells are identical and identify the enzyme as the deoxyguanosine 5'-triphosphate triphosphohydrolase purified to homogeneity from wild-type E. coli [Seto, D., Bhatnagar, S. K. & Bessman, M. J. (1988) J. Biol. Chem. 263, 1494-1499]. OptA1 cells contain approximately equal to 50-fold more active molecules of the 56-kDa dGTPase than do E. coli optA+ cells.

Multiple forms of sheep serum A-esterase activity associated with the high-density lipoprotein

Five lipoproteins of sheep serum expressing A-esterase activity, but with differing activities towards four organophosphate substrates, were separated by a combination of gel filtration and ion-exchange chromatography. Each had an Mr of approx. 360,000 and contained a major peptide of Mr 28,000-30,000 that appeared to be present as several isoforms on urea/agarose isoelectric focusing. In every case this peptide split into a number of bands on urea/agarose isoelectric focusing. The bands appear to represent isoforms of the peptide, and four lipoproteins yielded characteristic patterns of bands. This peptide resembles the apolipoprotein A-I of human serum, and available evidence suggests that this is the protein that expresses A-esterase activity. Evidence is presented for the existence of different species of high-density lipoprotein HDL2 particles containing different complements of peptide isoforms and expressing contrasting substrate specificities towards organophosphates.

The purification and properties of myo-inositol monophosphatase from bovine brain

1. An inositol monophosphatase was purified to homogeneity from bovine brain. 2. The enzyme is a dimer of subunit Mr 29,000. 3. The enzyme hydrolyses both enantiomers of myo-inositol 1-phosphate and both enantiomers of myo-inositol 4-phosphate, but has no activity towards inositol bisphosphates, inositol trisphosphates or inositol 1,3,4,5-tetrakisphosphate. 4. Several non-inositol-containing monophosphates are also substrates. 5. The enzyme requires Mg2+ for activity, and Zn2+ supports activity to a small extent. 6. Other bivalent cations (including Zn2+) are inhibitors, competitive with Mg2+. 7. Phosphate, but not inositol, is an inhibitor competitive with substrate. 8. Li+ inhibits hydrolysis of inositol 1-phosphate and inositol 4-phosphate uncompetitively with different apparent Ki values (1.0 mM and 0.26 mM respectively).

Type B nucleoside-diphosphatase of rat brain. Purification and properties of an enzyme with high thiamin pyrophosphatase activity

Type B nucleoside-diphosphatase was purified from membranes of rat brain by solubilization with a non-ionic detergent and successive column chromatographies on DEAE-cellulose DE-52, concanavalin-A-Sepharose, Bio-Gel HT, blue-Sepharose CL-6B, chelating Sepharose 6B, Ultrogel AcA44 and TSK gel G3000 SW. The purified enzyme gave a single protein band on SDS/polyacrylamide gel electrophoresis and its molecular mass was estimated to be 75 kDa. It hydrolyzed thiamin diphosphate as well as GDP, IDP and UDP. Thiamin diphosphate (TPP) was hydrolyzed twice as efficiently as nucleoside diphosphates in the presence of Mn2+ at pH 7.4. The Km values for TPP, GDP, IDP and UDP were 0.66, 0.40, 0.54 and 1.06 mM respectively. ATP, ADP and pyridoxal 5'-phosphate inhibited thiamin-pyrophosphatase activity competitively and their Ki values were 2.3 mM, 1.0 mM and 0.59 mM respectively. The optimum pH of thiamin-pyrophosphatase activity was 7.4 in the presence of Mn2+ and that of GDP-hydrolytic activity was 6.5 in the presence of Mg2+.

Biochemical properties of purified transverse tubules isolated from skeletal muscle triads

Transverse tubules (t-tubules) were prepared from muscle by dissociation of intact triads during centrifugation in ion-free sucrose gradients. They were further purified by the removal of contaminating sarcoplasmic reticulum after loading with calcium phosphate. Purification was accompanied by enrichment in markers specific for t-tubules, e.g., nitrendipine binding sites. According to gel electrophoresis the purified t-tubules contained three major protein bands of 104, 70, and 30 kDa. When solubilized with detergents there was a two- to threefold increase in Mg2+-ATPase activity, and a corresponding increase in the 30-kDa protein band. The 104-kDa protein was shown to be a (Na+ + K+)-ATPase because of its phosphorylation by [gamma-32P]ATP in the presence of sodium ions. The orientation of the t-tubule membrane was predominantly inside-out.

Zn2+-dependent acid p-nitrophenylphosphatase from chicken liver. Purification, characterization and subcellular localization

1. Zn2+-dependent acid p-nitrophenylphosphatase from chicken liver was purified to homogeneity. 2. The purified enzyme moves as a single electrophoretic band at pH 8.3 in 7.5% acrylamide and was coincident with the enzyme activity. 3. Gel filtration on Sephadex G-200 gave an apparent molecular weight of 110,000 with two apparent identical subunits of 54,000-56,000 as determined by sodium dodecyl sulphate gel electrophoresis. 4. The maximum of enzyme activity was obtained in the presence of 3-5 mM ZnCl2 at pH 6-6.2, however, higher concentrations of metal are inhibitory. The enzyme hydrolyses p-nitrophenylphosphate, o-carboxyphenylphosphate and phenylphosphate, was insensitive to NaF and was inhibited by phosphate and ATP. The Km for p-nitrophenylphosphate was 0.28 x 10(-3)M at pH 6 in 50 mM sodium acetate/100 mM NaCl. 5. Phosphate is a competitive inhibitor (Ki = 0.5 x 10(-3)M) whereas ATP seems to be a non-competitive inhibitor (Ki = 0.35 x 10(-3)M). The isoelectric point determined by isoelectric focusing on polyacrylamide gel is 7.5. 6. Cell fractionation studies indicate that the Zn2+-dependent acid p-nitrophenylphosphatase of chicken liver is a soluble enzyme form.

Purification and characterization of two types of soluble inositol phosphate 5-phosphomonoesterases from rat brain

Two soluble forms of inositol phosphate 5-phosphomonoesterase have been partially purified and characterized from rat brain and are referred to as type 1 and type 2 according to their order of elution from DEAE-Sepharose. Together, these enzymes represent 26 +/- 3% (mean +/- S.E., n = 4) of the total inositol 1,4,5-triphosphate (Ins(1,4,5)P3) phosphatase activity assayed in crude brain homogenate and are present in approximately equal total activities in a 100,000 x g supernatant, with the remainder being membrane-bound. Both soluble enzymes require Mg2+ for activity, are moderately inhibited by Ca2+ in the micromolar range, and can be inhibited by millimolar concentrations of a variety of phosphorylated compounds. The type 1 enzyme has been purified to a specific activity of 1.06 mumol/min/mg protein. It elutes as a 60-kDa protein on Sephacryl S-200. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the type 1 enzyme correlates with a pair of protein bands of 66 and 60 kDa. It has apparent Km values of 3 and 0.8 microM for Ins(1,4,5)P3 and inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4), respectively, and hydrolyses Ins(1,4,5)P3 approximately 12 times faster than Ins(1,3,4,5)P4. The type 2 enzyme has been purified to a specific activity of 15.2 mumol/min/mg protein, elutes as a protein of 160 kDa on Sephacryl S-300, and migrates as a similarly sized subunit on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It has an apparent Km for Ins(1,4,5)P3 of 18 microM. Its apparent Km for Ins(1,3,4,5)P4, however, is greater than 150 microM, suggesting that this enzyme is primarily an Ins(1,4,5)P3 5-phosphomonoesterase. The relationship of these two enzymes to the inositol tris/tetrakisphosphate pathway is discussed.

Purification and properties of porcine testis acylphosphatase

Acylphosphatase has been purified from porcine testis and its properties were compared with those of porcine skeletal muscle acylphosphatase. The molecular weight of the testis enzyme was found to be 10,600, similar to that of porcine skeletal muscle acylphosphatase, on sedimentation equilibrium analysis. The specific activity of the testis enzyme was 10,800 mumol/min/mg at 25 degrees C with benzoyl phosphate as substrate, i.e., higher than that of the muscle enzyme, 7,200 mumol/min/mg, under the same conditions. The pI of the testis enzyme was 8.3, i.e., lower than that of the muscle enzyme, 10.6. There were marked differences in the amino acid compositions of the two enzymes. In particular two histidine residues were present in the testis enzyme but none were present in the muscle enzyme, and no cysteine residue was present in the testis enzyme but one was present in the muscle enzyme. The carboxyl terminal amino acid of the testis enzyme seemed to be lysine, while that of the muscle enzyme is tyrosine. The peptide maps of the testis and muscle enzymes indicated considerable differences in the amino acid sequences of the two enzymes. Differences in the antigenic structures of the two enzymes were demonstrated on enzyme linked immunoassaying and double immunodiffusion. These results indicate that the porcine testis acylphosphatase is an isozyme different from the porcine skeletal muscle acylphosphatase.

Inositol polyphosphate 1-phosphatase from calf brain. Purification and inhibition by Li+, Ca2+, and Mn2+

We recently identified an enzyme which we have designated inositol polyphosphate 1-phosphatase that hydrolyzes both inositol 1,3,4-trisphosphate (Ins-1,3,4-P3) and inositol 1,4-bisphosphate (Ins-1,4-P2), yielding inositol 3,4-bisphosphate and inositol 4-phosphate, respectively, as products (Inhorn, R. C., Bansal, V.S., and Majerus, P.W. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 2170-2174). We have now purified the inositol polyphosphate 1-phosphatase 3600-fold from calf brain supernatant. The purified enzyme has an apparent molecular mass of 44,000 daltons as determined by gel filtration and is free of other inositol phosphate phosphatase activities. The enzyme hydrolyzes Ins-1,4-P2 with an apparent Km of approximately 4-5 microM, while it degrades Ins-1,3,4-P3 with an apparent Km of approximately 20 microM. The enzyme hydrolyzes these substrates at approximately the same maximal velocity. Inositol polyphosphate 1-phosphatase shows a sigmoidal dependence upon magnesium ion, with 0.3 mM Mg2+ causing half-maximal stimulation. A Hill plot of the data is linear with a value of n = 1.9, suggesting that the enzyme binds magnesium cooperatively. Calcium and manganese inhibit enzyme activity, with 50% inhibition at approximately 6 microM. Lithium inhibits Ins-1,4-P2 hydrolysis uncompetitively with a Ki of approximately 6 mM. This mechanism of lithium inhibition is similar to that observed for the inositol monophosphate phosphatase (originally designated myo-inositol-1-phosphatase; Hallcher, L.M., and Sherman, W.R. (1980) J. Biol. Chem. 255, 10896-10901), suggesting that these two enzymes are related. Lithium also inhibits Ins-1,3,4-P3 hydrolysis with an estimated Ki of 0.5-1 mM.

[High molecular weight forms of GTP-binding regulatory proteins from the bovine cerebellum]

A new form of a low Km GTPase belonging to the family of regulatory GTP-binding G-proteins has been identified in bovine cerebellum. The molecular weight of this G-protein is several times as high as that of other G-proteins known to be alpha beta gamma heterotrimers: i. e., Gs, Gi, Go, transducin and a new G-protein which had recently been isolated in our laboratory from bovine cerebellum. The high molecular weight G-protein is stable against dissociation; its molecular mass does not change after treatment with DTT, colchicine and NaF. Using antibodies against the alpha-subunit of the formerly isolated cerebellar G-protein and the transducin beta-subunit, it was demonstrated that the both immunoreactive subunits are present in the high molecular weight G-protein. The two forms of the cerebellar G-proteins, i. e., "high" and "low molecular weight" ones, differ drastically in terms of the Mg2+ effect on their GTPase activity. Whereas at submicromolar concentrations of Mg2+ the GTPase activity of the former is virtually absent, the GTPase activity of the latter is more elevated in the presence of EDTA than in the presence of Mg2+.

Characterization of a phosphoenzyme intermediate in the reaction of phosphoglycolate phosphatase

When 32P-glycolate and phosphoglycolate phosphatase from spinach are mixed, 32P is incorporated into acid precipitated protein. Properties that relate the phosphorylation of the enzyme to the phosphatase are: the Km value for P-glycolate is similar for protein phosphorylation and substrate hydrolysis; the 32P in the phosphoenzyme is diluted by unlabeled P-glycolate or the specific alternative substrate, ethyl-P; the activator Cl- enhances the effectiveness of ethyl-P as a substrate and as an inhibitor of the formation of 32P-enzyme; and 32P is lost from the enzyme when 32P-glycolate is consumed. The phosphorylated protein has a molecular weight of 34,000, which is half that of the native protein and is similar in size to the labeled band that is seen on sodium dodecyl sulfate-polyacrylamide gels. The enzyme-bound phosphoryl group appears to be an acylphosphate from its pH stability, being quite stable at pH 1, less stable at pH 5, and very unstable above pH 5. The bond is readily hydrolyzed in acid molybdate and it is sensitive to cleavage by hydroxylamine at pH 6.8. The demonstration of enzyme phosphorylation by 32P-glycolate resolves the dilemma presented by initial rate studies in which alternative substrates appeared to have different mechanisms (Rose, Z. B., Grove, D. S., and Seal, S. N. (1986) J. Biol. Chem. 261, 10996-11002).

Fructose-2,6-bisphosphatase and 6-phosphofructo-2-kinase are separable in yeast

Fructose-2,6-bisphosphatase was purified from yeast and separated from 6-phosphofructo-2-kinase and alkaline phosphatase. The enzyme released Pi from the 2-position of fructose 2,6-bisphosphate and formed fructose 6-phosphate in stoichiometric amounts. The enzyme displays hyperbolic kinetics towards fructose 2,6-bisphosphate, with a Km value of 0.3 microM. It is strongly inhibited by fructose 6-phosphate. The inhibition is counteracted by L-glycerol 3-phosphate. Phosphorylation of the enzyme by cyclic-AMP-dependent protein kinase causes inactivation, which is reversible by the action of protein phosphatase 2A.

D-myo-inositol (1,4)-bisphosphate 1-phosphate. Partial purification from rat liver and characterization

A specific 1-phosphatase acting on myo-inositol (1,4)-biphosphate with a high affinity (Km = 0.9 microM) has been purified 49-fold from soluble proteins of rat liver by anion exchange chromatography followed by gel filtration. This enzyme has a molecular weight of 58,000 as estimated by gel filtration, a pH optimum of 7.5, and requires Mg++ for activity. The only product formed from myo-inositol (1,4)-bisphosphate is the 4-monophosphate. Of 7 other inositol phosphates examined only myo-inositol (1,3,4)-triphosphate was a substrate.

Purification and partial characterization of a calmodulin-stimulated nucleoside triphosphatase from pea nuclei

A nucleoside triphosphatase/deoxynucleoside triphosphatase associated with the chromatin fraction from a highly purified preparation of pea nuclei has been isolated and characterized. The purified enzyme has a molecular weight of 47,000 as checked by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and it has an isoelectric point of 6.6. In the presence of divalent cations (Mg2+ = Mn2+ greater than Ca2+), this enzyme hydrolyzes nucleoside triphosphates or deoxynucleoside triphosphates. Hydrolysis is optimal at pH 7.5 and is significantly inhibited by relatively low concentrations of quercetin, but is not sensitive to vanadate, nitrate, or oligomycin. The enzyme has a rather broad nucleotide substrate specificity and has a Km for MgATP2- of 0.6 mM. The enzyme activity is stimulated over 3-fold by Ca2+ and calmodulin, and the stimulation is blocked by the Ca2+ chelator EGTA and by the calmodulin antagonists compound 48/80 and chlorpromazine.

Purification and properties of inositol-1,4-bisphosphate 4-phosphohydrolase from rat brain

Inositol-1,4-bisphosphate 4-phosphohydrolase (inositol-1,4-bisphosphatase) was highly purified from a soluble fraction of rat brain. On SDS-polyacrylamide gel electrophoresis, the purified enzyme gave a single protein band and its molecular weight was estimated to be 42000. The isoelectric point of the enzyme was 4.3. The enzyme specifically hydrolyzed the 4-phosphomonoester linkage of inositol 1,4-bisphosphate. The Km value for inositol 1,4-bisphosphate was 30 microM, and it required Mg2+ for activity. Ca2+ was a competitive inhibitor with a Ki value of 60 microM as regards the Mg2+ binding. Li+, which is known to be a strong inhibitor of inositol 1-phosphatase (EC 3.1.3.25), inhibited the enzyme activity and caused 50% inhibition at a concentration of 1 mM (IC50 = 1 mM). Li+ was an uncompetitive inhibitor of substrate binding with a Ki value of 0.6 mM. These inhibitory parameters of Li+ were quite similar to those for inositol 1-phosphatase (IC50 = 1 mM, Ki = 0.3 mM). Thus, the effect of Li+ on decreasing the free inositol level with a subsequent decrease in agonist-sensitive phosphoinositides, is caused by its inhibition of multiple enzymes involved in conversion of inositol 1,4-bisphosphate to inositol.