Metal ion specificity at the catalytic site of yeast enolase

A new, more gentle enzyme purification for yeast enolase was developed. A series of kinetic experiments was performed with yeast enolase where the concentration of Mg(II) is kept constant and at the Km' level; the addition of Mn(II), Zn(II), or Cu(II) gives a hyperbolic decrease in the enzyme activity. The final velocity of these mixed-metal systems is the same as the velocity obtained only with Mn(II), Zn(II), or Cu(II), respectively. The concentration of the second metal that gives half-maximal effect in the presence of Mg(II) is approximately the same as the apparent Km (Km') value measured for that cation alone. Direct binding of Mn(II) to apoenolase in the absence and presence of Mg(II) shows that Mn(II) and Mg(II) compete for the same metal site on enolase. In the presence of D-2-phosphoglycerate (PGA) and Mg(II), only a single cation site per monomer is occupied by Mn(II). Water proton relaxation rate (PRR) studies of enzyme-ligand complexes containing Mn(II) and Mn(II) in the presence of Mg(II) are consistent with Mn(II) binding at site I under both conditions. PRR titrations of ligands such as the substrate PGA or the inhibitors orthophosphate or fluoride to the enolase-Mn(II)-Mg(II) complex are similar to those obtained for the enolase-Mn(II) complex, also indicating that Mn(II) is at site I in the presence of Mg(II). High-resolution 1H and 31P NMR was used to determine the paramagnetic effect of enolase-bound Mn(II) on the relaxation rates of the nuclei of the competitive inhibitor phosphoglycolate. The distances between the bound Mn(II) and the nuclei were calculated.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of pH on the Mn2+ activation of and binding to yeast enolase: a functional study

The influence of pH on the activation of yeast enolase by Mn2+ was measured by steady-state kinetics. The pH influence on the binding of Mn2+ to apoenolase and the enolase-substrate complex was measured by EPR spectroscopy. At pH values above 6.6, activation by Mn2+ is fit by Michaelis-Menten kinetics, but at higher concentrations of Mn2+, inhibition is observed. Under conditions analogous to the kinetic studies, the enzyme binds two Mn2+ per dimer with a Kd in the micromolar range. In the presence of the substrate 2-phosphoglycerate, three thermodynamically distinct cation binding sites per monomer are detected and the binding constants are determined by a fit to the data. As the pH decreases, the reaction velocity decreases and the cation inhibition becomes minimal. Under these conditions, only two Mn2+ binding sites per monomer are observed; the third site must be the inhibitory site. The velocity and kinetic constants are minimally affected by buffer except at pH 5.8 with PIPES. Under these conditions, the velocity is only about 40% that observed with other buffers and only a single binding site for Mn2+ per monomer is detected in the presence or absence of substrate. A direct role in the catalytic mechanism by the second cation is called to question. The binding constant for Mn2+ at site I is independent of pH over the range from 7.5 to 5.2, and the binding at site II increases only slightly over this same pH range. These results indicate that the cation sites at positions I and II contain ligands that are pH independent over this range.(ABSTRACT TRUNCATED AT 250 WORDS)

An active-site lysine in avian liver phosphoenolpyruvate carboxykinase

The participation of lysine in the catalysis by avian liver phosphoenolpyruvate carboxykinase was studied by chemical modification and by a characterization of the modified enzyme. The rate of inactivation by 2,4-pentanedione is pseudo-first-order and linearly dependent on reagent concentration with a second-order rate constant of 0.36 +/- 0.025 M-1 min-1. Inactivation by pyridoxal 5'-phosphate of the reversible reaction catalyzed by phosphoenolpyruvate carboxykinase follows bimolecular kinetics with a second-order rate constant of 7700 +/- 860 M-1 min-1. A second-order rate constant of inactivation for the irreversible reaction catalyzed by the enzyme is 1434 +/- 110 M-1 min-1. Treatment of the enzyme with pyridoxal 5'-phosphate gives incorporation of 1 mol of pyridoxal 5'-phosphate per mole of enzyme or one lysine residue modified concomitant with 100% loss in activity. A stoichiometry of 1:1 is observed when either the reversible or the irreversible reactions catalyzed by the enzyme are monitored. A study of kobs vs pH suggests this active-site lysine has a pKa of 8.1 and a pH-independent rate constant of inactivation of 47,700 M-1 min-1. The phosphate-containing substrates IDP, ITP, and phosphoenolpyruvate offer almost complete protection against inactivation by pyridoxal 5'-phosphate. Modified, inactive enzyme exhibits little change in Mn2+ binding as shown by EPR. Proton relaxation rate measurements suggest that pyridoxal 5'-phosphate modification alters binding of the phosphate-containing substrates. 31P NMR relaxation rate measurements show altered binding of the substrates in the ternary enzyme.Mn2+.substrate complex. Circular dichroism studies show little change in secondary structure of pyridoxal 5'-phosphate modified phosphoenolpyruvate carboxykinase. These results indicate that avian liver phosphoenolpyruvate carboxykinase has one reactive lysine at the active site and it is involved in the binding and activation of the phosphate-containing substrates.

[Changes in RCBF and edema after transient cerebral ischemia--ischemic threshold of postischemic hypoperfusion]

Brain damage after transient cerebral ischemia may be related to changes in postischemic cerebral blood flow and brain edema. In this study, the relationship between postischemic cerebral blood flow and edema was evaluated in the gerbil. Bilateral carotid occlusion (for 1, 1.5, 5, 15, or 30 min) was carried out in 110 female gerbils (50-70 g) under anesthesia with 2% halothane in 30% O2 and 70% NO2. Cerebral blood flow was measured by a [14C]-iodoantipyrine method modified slightly for use in small animals, and brain edema was evaluated by a specific gravity method. The threshold duration of ischemia which gives rise to subsequent hypoperfusion or edema was also established. In another 52 female gerbils under the same anesthesia, we investigated the effect of ischemia of variable duration on postischemic blood pressure and blood gas. Throughout all experiments, rectal temperature was maintained at 37-38 degrees C with a heating pad. Student's t test was used to calculate statistical significance. Neither blood pressure nor blood gas did vary significantly at any time following recirculation. Each brain region showed the same pattern of blood flow change, one almost independent of duration of occlusion. Namely, after the release of occlusion, transient recovery of blood flow was observed. But the flow then fell to 30-40% of the normal value at 1 h, after which it returned up to normal at 6 h. The severity of postischemic hypoperfusion was also independent of occlusion time. Interestingly, we did not observe postischemic hyperemia in any experimental groups except mild hyperemia in hippocampus after 5 and 15 min ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Description of a procedure which allows isolation of viral nonstructural proteins from BHK vertebrate cells infected with the West Nile flavivirus in a state which allows their direct chemical characterization

We have developed a procedure which allows isolation of virus-coded nonstructural (NS) proteins from BHK cells infected with the West Nile (WN) flavivirus in a state of purity which allows their chemical characterization. A crude membrane fraction proved to be a suitable starting material. Incubation of crude membranes in buffer containing 1.2 M guanidine hydrochloride (GH) allows isolation of weakly washed membranes. Incubation of weakly washed membranes in the presence of either 5 M GH or 8 M urea allows the isolation of stringently washed membranes, as well as a soluble protein-containing wash which is called the differential wash. Stringently washed membranes contain proteins with apparent molecular weights of 14, 19, 23, 29, and 50 kDa as predominant constituents. These proteins are of sufficient purity after SDS-PAGE to allow amino-terminal sequence determination. Together with the genome RNA sequence these analyses show that these molecules represent the virus-coded proteins NS 2b, NS 2a, pre M, NS 4b, and E, respectively. Similar analysis of the proteins present in the differential wash shows that the proteins NS 5, NS 3, and NS 1 are major constituents of this material. The carboxy-terminal sequences of NS 5 and NS 1 have also been determined.

Analogs of oxalacetate as potential substrates for phosphoenolpyruvate carboxykinase

Structural analogs of the substrate oxalacetate were examined as potential substrates and inhibitors for chicken liver mitochondrial phosphoenolpyruvate (P-enolpyruvate) carboxykinase. Steady-state kinetics were employed to characterize the inhibitory effects of these substrate analogs with the enzyme. Assays were carried out in both carboxylation and decarboxylation reaction directions. Pyruvate, beta-hydroxypyruvate, beta-mercaptopyruvate, beta-fluoropyruvate, DL-lactate, glycolate, glycoaldehyde, glyoxylate, glyphosate, and DL-aspartate showed no inhibitory effects by steady-state kinetics. Oxalate, acetopyruvate, and DL-, D-, and L-glycerate exhibited weak noncompetitive inhibition of the P-enolpyruvate carboxykinase-catalyzed reaction. DL-3-Nitro-2-hydroxypropionic acid, 3-nitro-2-oxopropionic acid, DL-malate, malonate, tartronate, and alpha-ketobutyrate all show weak inhibition with estimated inhibition constants greater than 20 nM. Several of these compounds were investigated by 31P NMR to determine if they function as phosphoryl acceptors for GTP. None of the compounds tested act as phosphoryl acceptors in the enzyme-catalyzed reaction. Chicken liver mitochondrial phosphoenolpyruvate carboxykinase shows a remarkably high degree of specificity at the binding site of oxalacetate.

Catatonia and burns

Two case reports describing symptoms of catatonia associated with thermal injury are reported. The incidence of catatonia in a burn unit was found to be about three times that in a general hospital.

A histidine residue at the active site of avian liver phosphoenolpyruvate carboxykinase

The histidine-selective reagents diethylpyrocarbonate (DEPC) and dimethylpyrocarbonate were used to study active site residues of phosphoenolpyruvate carboxykinase. Both reagents show pseudo first-order inhibition of enzyme activity at 22 +/- 1 degree C with calculated second-order rate constants of 2.8 and 4.6 M-1 s-1, respectively. The inhibition appears partially reversible. Substrates affect the rate of inhibition: KHCO3 enhances the rate, Mn2+ has little effect, and phosphoenolpyruvate decreases the rate. The best protection is obtained by IDP or IDP and Mn2+. The kinetic studies show that modification of histidine is specific and leads to loss of enzymatic activity. Two histidines per enzyme are modified by DEPC, as measured by an absorption change at 240 nm, in the absence of substrate, leading to loss in activity. One histidine per molecule is modified in the presence of KHCO3, giving inactivation. Cysteine and lysine residues are not affected. A study of the inhibition rate constant as a function of pH gives a pKa of 6.7. Enzyme modified by DEPC in the absence of substrate (1% remaining activity) shows no binding of ITP or of phosphoenolpyruvate to the enzyme.Mn2+ complex as studied by proton relaxation rates. When enzyme is modified in the presence of KHCO3 (44% remaining activity), ITP and KHCO3 bind to the enzyme.Mn2+ complex similarly to the binding to native enzyme. Phosphoenolpyruvate binding to modified enzyme.Mn results in an enhancement of proton relaxation rates rather than the decrease observed with native enzyme.Mn. The CD spectra of histidine-modified enzyme show a decrease in alpha-helical and random structure with an increase in anti-parallel beta-sheet structure compared to native enzyme. These results show that avian phosphoenolpyruvate carboxykinase has 2 histidine residues which are reactive with DEPC and dimethylpyrocarbonate, and one of the 15 histidine residues in the protein is at or near the phosphoenolpyruvate binding site and is involved in catalysis.

A reactive cysteine in avian liver phosphoenolpyruvate carboxykinase

The modification of avian phosphoenolpyruvate carboxykinase by a variety of sulfhydryl reagents leads to inhibition. The inhibition is related to the loss of 1 highly reactive cysteine residue of the 13 cysteines present in the enzyme. Inhibition by reagents which yield a mixed disulfide was rapidly reversed by thiols. Reagents specific for vicinal sulfhydryl configurations were not potent inhibitors. The cysteine-modified enzyme continues to bind Mn2+ with the same stoichiometry and dissociation constant as the native enzyme. All of the substrates also bind to thiol-modified inactive enzyme. The modification of the reactive cysteine with the spin-labeled iodoacetate derivative leads to inactive enzyme with spin label stoichiometrically incorporated. The EPR spectrum showed an immobilized spin label on the enzyme. EPR studies of the perturbation of the phosphoenolpyruvate carboxykinase-bound spin label by bound Mn2+ showed a dipolar interaction between the two spins, estimated to be 10 A apart. The perturbation of the 1/T1 and 1/T2 values of the 31P resonances of ITP by spin-labeled enzyme indicates that this portion of the nucleotide binds 8-10 A from the spin label. These results indicate that the reactive cysteine is close to but not at the active site of the enzyme. The thiol group must be free and in its reduced form for the enzyme to be active. Perhaps modification of this group prevents conformational change(s) upon ligand binding necessary for the catalytic process.

Analyses of the terminal sequences of West Nile virus structural proteins and of the in vitro translation of these proteins allow the proposal of a complete scheme of the proteolytic cleavages involved in their synthesis

The proteolytic processes involved in the synthesis of the structural proteins of the West Nile (WN) flavivirus were analyzed: The carboxy-terminal sequences of the structural proteins were determined and the proteins translated in vitro in the presence of membranes from a mRNA coding for the structural polyprotein were analyzed. The results obtained indicate that the following proteolytic activities are involved in the synthesis and assembly of WN virus structural proteins: The growing peptide chain which contains the sequences of the structural proteins in the order C-pre-M-E is cleaved at three places by cellular signalase(s). This cleavage generates the primary amino acid sequence of the mature structural proteins pre-M and E (and the amino-terminus of the ensuing nonstructural protein NS 1). The amino-terminal part of the polyprotein containing the amino acid residues 1 to 123 is released as a molecule which migrates slightly slower than the mature viral core protein and which presumably is associated to the RER membranes via its carboxy-terminal sequence. This protein is called the anchored C virus particles the anchored C protein is converted into mature C protein by removal of the carboxy-terminal hydrophobic segment containing the amino acid residues 106 to 123. Presumably a virus-coded protease which can cleave the polyprotein after two basic amino acid residues is responsible for this cleavage. The cell-associated WN virus particles are constructed from the proteins C, pre-M, and E which contain the amino residues 1-105, 124-290, and 291-787 of the polyprotein, respectively. Cleavage of the pre-M protein between amino acid residues 215 and 216, presumably by a cellular enzyme located in the Golgi vesicles, and loss of the amino-terminal fragment of this protein are associated with the release of virus from the cells.

Stereoselective ligand interactions of chicken liver phosphoenolpyruvate carboxykinase with fluorophosphoenolpyruvate

The stereospecific interactions of chicken liver phosphoenolpyruvate carboxykinase (P-enolpyruvate carboxykinase) with the two geometric isomers of 3-fluorophosphoenolpyruvate (F-P-enolpyruvate) were examined. Previous studies have shown that the Z isomer of F-P-enolpyruvate is a substrate for P-enolpyruvate carboxykinase but the E isomer is a competitive inhibitor [T. H. Duffy and T. Nowak (1984) Biochemistry 23, 661-670]. The reasons for this substrate selectivity were investigated. Studies of the 1H, 19F, and 31P relaxation rates of the ligands in the binary Mn-ligand complexes indicate the formation of direct coordination complexes. The temperature and frequency dependence of the proton relaxation rates (PRR) of the respective enzyme-Mn-ligand complexes demonstrates that the perturbation of the electronic environment at the Mn(II) site on the enzyme is different upon binding of the inhibitor (E-F-P-enolpyruvate) in contrast to the binding of substrates (P-enolpyruvate or Z-F-P-enolpyruvate). Structural studies demonstrate that Z-F-P-enolpyruvate forms a second sphere coordination complex with enzyme-bound Mn(II). E-F-P-enolpyruvate exchanges slowly from the ternary complex and binds less than or equal to 10 A from the bound Mn(II). CD studies in the far-uv region demonstrate that the alpha-helical content of P-enolpyruvate carboxykinase is increased at the expense of antiparallel and parallel beta-sheet structure upon binding of Mn(II) and substrate (P-enolpyruvate or Z-F-P-enolpyruvate) to the apoenzyme, but show no such structural change upon binding of Mn(II) and E-F-P-enolpyruvate. Analogous results are observed from CD studies at the aromatic amino acid region (250-350 nm). The stereoselective catalytic activities of P-enolpyruvate carboxykinase with F-P-enolpyruvate analogs can be explained by different interactions of these ligands within the catalytic site of the enzyme.

Arginine residues at the active site of avian liver phosphoenolpyruvate carboxykinase

The presence of arginine at the active site of avian liver phosphoenolpyruvate carboxykinase was studied by chemical modification followed by a characterization of the modified enzyme. The arginine-specific reagents phenylglyoxal, 2,3-butanedione, and 1,2-cyclohexanedione all irreversibly inhibit the enzyme with second-order rate constants of 3.42 M-1 min-1, 3.13 M-1 min-1 and 0.313 M-1 min-1, respectively. The substrates phosphoenolpyruvate, IDP, and the activator Mn2+ offer little to modest protection from inhibition. Either CO2 or CO2 in the presence of any of the other substrates elicited potent protection against modification. Protection by CO2 against modification by phenylglyoxal or 1,2-cyclohexanedione gave a biphasic pattern. Rapid loss in activity to 40-60% occurred, followed by a very slow loss. Kinetics of inhibition suggest that the modification of arginine is specific and leads to loss of enzymatic activity. Substrate protection studies indicate an arginine residue(s) at the CO2 site of phosphoenolpyruvate carboxykinase. Apparently no arginine residues are at the binding site of the phosphate-containing substrates. Partially inactive (40-60% activity) enzyme, formed in the presence of CO2, has a slight change of its kinetic constants, and no alteration of its binding parameters or secondary structure as demonstrated by kinetic, proton relaxation rate, and circular dichroism studies. Labeling of enzyme with [(7-)14C]phenylglyoxal in the presence of CO2 (40-60% activity) showed 2 mol of phenylglyoxal/enzyme or 1 arginine or cysteine residue modified. Labeling of phosphoenolpyruvate carboxykinase in the absence of CO2 yielded 6 mol of label/enzyme. Labeling results indicate that avian phosphoenolpyruvate carboxykinase has 2 or 3 reactive arginine residues out of a total of 52 and only 1 or 2 are located at the active site and are involved in CO2 binding and activation.

The preparation and characterization of Cr(III) and Co(III) complexes of GDP and GTP and their interactions with avian phosphoenolpyruvate carboxykinase

The exchange inert coordination complexes, Cr(H2O)4GDP, Cr(H2O)4GTP, Cr(NH3)4GDP, Cr(NH3)4GTP, Co(NH3)4GDP, and Co(NH3)4GTP have been synthesized and characterized. The lambda and delta coordination isomers of Cr(H2O)4GDP, Cr(NH3)4GDP, and the four Cr(H2O)4GTP isomers have been separated by reverse phase HPLC and characterized by their CD spectra. While the isomers of Co(NH3)4GTP have not been successfully separated, 31P NMR spectroscopy reveals the presence of the lambda and delta forms. The complexes, Cr(H2O)4GDP, Co(NH3)4GDP, Cr(H2O)4GTP, and Co(NH3)4GTP, are linear competitive inhibitors of avian phosphoenolpyruvate carboxykinase. The Ki values of 30 microM, 540 microM, 40 microM, and 12 microM, respectively, were determined for these complexes using Mn-IDP as the nucleotide substrate in the phosphoenolpyruvate carboxylation direction or Mn-ITP as nucleotide substrate for the oxalacetate decarboxylation reaction. The lambda and delta isomers of Cr(H2O)4 GDP show little specificity (a twofold maximum difference in Ki) for the enzyme. The isomeric forms of Cr(H2O)4 GTP demonstrate no observed stereoselectivity of interaction with the enzyme. All of the complexes tested, except for Cr(NH3)4GDP and Co(NH3)4GDP, which have larger Ki values, are good substrate analogs for P-enolpyruvate carboxykinase. When the substrate is Mn-GTP, fixed at 0.2 mM at pH 6.0, enzyme activity is stimulated two- to two and a half-fold by Cr(H2O)4GTP. A Dixon plot reveals that the stimulatory effect is saturated at 0.4 mM Cr(H2O)4GTP. The interaction of the enzyme with Cr(H2O)4GTP appears to produce a "memory" effect which is manifest with guanosine nucleotide substrates, but which is not observed with the alternative substrate Mn-ITP.

Bovine liver dihydropyrimidine amidohydrolase: pH dependencies of the steady-state kinetic and proton relaxation rate properties of the Mn(II)-containing enzyme

The essential Zn(II) in bovine liver dihydropyrimidine amidohydrolase (DHPase) was removed by incubation with 2,6-dipicolinic acid and replaced with Mn(II). Electron paramagnetic resonance studies of Mn(II) binding show that there are four binding sites per tetramer, and the dissociation constant at pH 7.5 is 13.5 microM. The substitution of Mn(II) for Zn(II) increases the specific activity of the enzyme approximately sixfold but has only a small effect (twofold increase) on the Km for 5-bromo-5,6-dihydrouracil (BrH2Ura). The pH dependence of the catalytic properties of Mn(II)-DHPase is the same as for the Zn(II) enzyme (Lee, M., Cowling, R., Sander, E., and Pettigrew, D. (1986) Arch. Biochem. Biophys. 248, 368-378). The pH dependence is well described in terms of the ionization of a single group with a pK of about 6 in the free enzyme. The ionization of this group is required for catalytic activity. The substitution of Mn(II) for Zn(II) does not affect the pH dependence of DHPase catalysis and therefore strongly suggests that the ionizable group is an amino acid residue at or near the active site, rather than a metal-bound water molecule. The pH dependence of the enhancement of the paramagnetic effect of the DHPase-Mn complex on the relaxation rate of the solvent water protons also is well described in terms of the ionization of a single group with a pK of about 6. Ionization of the group which is involved in catalysis also perturbs the environment of the bound Mn(II). The ionization of the active site group does not affect the number of exchangeable water molecules but does affect the symmetry of the environment of the bound Mn(II) and its electron relaxation.

Analysis of the influence of proteolytic cleavage on the structural organization of the surface of the West Nile flavivirus leads to the isolation of a protease-resistant E protein oligomer from the viral surface

In order to analyze the organization of the membrane proteins pre M, M, and E of the West Nile (WN) flavivirus we have studied the influence of proteolytic cleavage of intact virus on the structure of these proteins. The amino acid sequence of all proteins is known, all six disulfides present in the viral E protein have been identified, and it has been suggested that the E protein contains regions R1, L1, R2, L2, and R3, which together form the E protein ectodomain followed by a carboxyterminal membrane anchor region (Th. Nowak and G. Wengler (1987) Virology 156, 127-137). The results of our analyses can be summarized as follows: (1) The surface of the WN virus contains E protein oligomers; the E protein molecules present in these structures contain two segments which are exposed to proteolytic attack; the segments are located in parts L1 and R3 of the E protein. (2) Proteolytic cleavage of these oligomers in these regions neither destroys nor releases the oligomers from the viral surface. (3) The WN virus surface contains a layer of 7-nm ring-shaped subunits identifiable by electron microscopy which are neither destroyed nor released by proteolytic cleavage. (4) An E protein trimer can be isolated from the surface of protease-treated WN virus. This trimer is morphologically similar to the 7-nm ring-shaped element which can be identified on the surface of native and protease-treated WN virus by electron microscopy.

Analysis of disulfides present in the membrane proteins of the West Nile flavivirus

Recently the primary structure of the structural proteins of the flaviviruses West Nile (WN) virus (Castle et al., 1985; Wengler et al., 1985) and yellow fever (YF) virus (Rice et al., 1985) have been determined. As a first step in a further characterization of the organization of the structural proteins we have now studied the disulfide bridges present in the WN virus membrane proteins. All three membrane proteins, pre M, M, and E, were analyzed. The results obtained can be summarized as follows: The pre M proteins of both WN and YF virus each contain 6 cysteine residues and the position of all of these residues is strictly conserved between both viruses. The M proteins of both viruses do not contain cysteine residues. The E proteins of these viruses contain 12 cysteines and the position of all of these residues is strictly conserved between both viruses. All cysteine residues of the WN virus-derived membrane proteins are present as intramolecular disulfides. The six disulfide bridges generated from the 12 cysteine residues in the WN virus-derived E protein have been identified as follows: Cys 1-Cys 2; Cys 3-Cys 8; Cys 4-Cys 6; Cys 5-Cys 7; Cys 9-Cys 10; Cys 11-Cys 12. The analyses of the amino acid sequence conservation between the E proteins of YF and WN virus and the characterization of the disulfides have been used to develop a description of the E protein in which the molecule is assumed to be composed of the segments R1, L1, R2, L2, and R3 followed by a membrane anchor region at the carboxy-terminal region of the molecule. Computer analyses of the hydrophilicity and of the secondary structure indicate that the R1 region might contain a cluster of viral epitopes.

Purification and characterization of phosphoenolpyruvate carboxykinase from the parasitic helminth Ascaris suum

Phosphoenolpyruvate carboxykinase has been purified from homogenates of Ascaris suum muscle strips to apparent homogeneity as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purification is a three-step procedure which yields pure enzyme in milligram quantities with good yield. The subunit molecular weight of the Ascaris enzyme is between 75,000 and 80,000. The native molecular weight is 83,000 as determined by gel filtration. The kinetic constants for substrates of the carboxylation reaction were determined and compared to those measured for the avian liver enzyme. From kinetic studies it appears likely that two separate roles for divalent metal ions exist in the catalytic process. Studies conducted with Mn2+ or with micromolar concentrations of Mn2+, in the presence of millimolar concentrations of Mg2+ suggest that Mn2+ but not Mg2+ binds directly to and activates the enzyme while either Mn2+ or Mg2+ may bind to the nucleotide resulting in the metal-nucleotide complex. The metal-nucleotide is the active form of the substrate for the reaction. In the presence of Mg2+, an increase in the Mn2+ concentration results in a decrease in the Km for P-enolpyruvate suggesting a direct role for Mn2+ stimulation and regulation of activity. The concentrations of Mn2+ and Mg2+ in Ascaris muscle strips were determined by atomic absorption spectroscopy and support the proposed hypothesis of a specific Mn2+ activation of the enzyme. The nucleotides ATP and ITP act as competitive inhibitors against GTP with KI values of 0.50 and 0.75 mM, respectively. ITP is a competitive inhibitor against both IDP and P-enolpyruvate, suggesting overlapping binding sites for the two substrates on the enzyme.

Stereochemistry of phosphoenolpyruvate carboxylation catalyzed by phosphoenolpyruvate carboxykinase

The stereochemistry of the carboxylation of phosphoenolpyruvate to yield oxalacetate, catalyzed by chicken liver phosphoenolpyruvate carboxykinase and by Ascaris muscle phosphoenolpyruvate carboxykinase, was determined. The substrate (Z)-3-fluorophosphoenolpyruvate was used for the stereochemical analysis. The carboxylation reaction was coupled to malate dehydrogenase to yield 3-fluoromalate, and the stereochemistry of the products was identified by 19F NMR. In separate experiments, the enantiomeric tautomers of 3-fluorooxalacetate were shown to be utilized by malate dehydrogenase to yield (2R,3R)- and (2R,3S)-3-fluoromalate in nearly identical amounts. The products were identified by 19F NMR. When (Z)-3-fluorophosphoenolpyruvate was used as a substrate for phosphoenolpyruvate carboxykinase from avian liver and from Ascaris, and malate dehydrogenase was used to trap the product, only a single diastereomer was observed. This product was shown to be (2R,3R)-3-fluoromalate in each case. The assignments were based on coupling constants taken from Keck et al. [Keck, R., Hess, H., & Rétey, J. (1980) FEBS Lett. 114, 287]. These results indicate that the stereochemistry of carboxylation, catalyzed by chicken phosphoenolpyruvate carboxykinase and by Ascaris phosphoenolpyruvate carboxykinase, is identical and takes place from the si side of the enzyme-bound phosphoenolpyruvate. The carboxylation reaction was run both in H2O and in D2O. No deuterium incorporation into fluoromalate was shown to occur. The product 3-fluorooxalacetate is thus released from phosphoenolpyruvate carboxykinase as the keto form and is reduced more rapidly by reduced nicotinamide adenine dinucleotide with malate dehydrogenase than by the occurrence of tautomerization.

31P-NMR studies of the metabolisms of the parasitic helminths Ascaris suum and Fasciola hepatica

31P-NMR has been applied to the study of the metabolisms of the intact parasitic helminths Ascaris suum (the intestinal roundworm) and Fasciola hepatica (the liver fluke). After calibration of the chemical shift of Pi in muscle extracts the internal pH of adult Ascaris worms and the effect of the pH of the external medium on the organism's internal pH were measured. Assignments of nearly all of the observable 31P resonances could be made. A large resonance from glycerophosphorylcholine whose function is unclear was observed but no signals from energy storage compounds such as creatine phosphate were detected. The profiles of the phosphorus-containing metabolites in both organisms were monitored as a function of time. Changes in sugar phosphate distributions but not ATP/ADP were observed. Studies of the drug closantel on Fasciola hepatica were performed. Initial effects of the drug were a decrease in glucose 6-phosphate and an increase in Pi with no substantial change in ATP levels as observed by 31P-NMR. Studies involving treatment with closantel followed by rapid freezing, extraction, and analytical determination of glycolytic intermediates confirmed NMR observations. This NMR method can serve as a simple noninvasive procedure to study parasite metabolism and drug effects on metabolism.

Primary structure of the West Nile flavivirus genome region coding for all nonstructural proteins

The genome RNA of the flavivirus West Nile (WN) virus has been transcribed into cDNA, the cDNA has been cloned, and the nucleotide sequences coding for the structural proteins have been determined (Castle et al., 1985; Wengler et al., 1985). We have now determined the nucleotide sequence coding for all viral nonstructural proteins which comprises 7929 nucleotides. Together with our earlier sequence analyses these data show that a long open reading frame (ORF) containing 10,290 nucleotides is present on the genome of WN virus. The two largest nonstructural proteins which can be detected in flavivirus-infected cells are the proteins NV5 and NV4 which have an apparent molecular mass of 97,000 and 74,000 Da, respectively. Both proteins were isolated by preparative polyacrylamide gel electrophoresis, and partial amino acid sequences of peptides derived from these proteins were determined. These analyses allow us to localize the nucleotide regions which code for these proteins and show that the region coding for the NV5 protein is located at the 3'-terminus of the long ORF. Together with our earlier analyses these data show that the protein sequences of virus-specific proteins are present on the viral polyprotein translated from the long ORF in the order V2-NV2-V3-(nonstructural proteins of up to 75,000 Da)-NV4-(nonstructural proteins of up to 45,000 Da)-NV5. Our data indicate that virus-specific structural and nonstructural proteins which are synthesized from a single long ORF accumulate in large amounts in infected cells. A possible role of the presence of these molecules, which are associated to cellular membranes, in the accumulation of membrane vesicles which characteristically occurs in flavivirus-infected cells is discussed.