Thallium (I) activation of pyruvate kinase

Attempted Self-Harm with Elemental Thallium Purchased Online: Case Report with Analytical Confirmation

INTRODUCTION

Thallium is a highly toxic metal, with most publications demonstrating poisoning from thallium salts. We report on a patient with elevated serum and urine thallium concentrations from an intentional ingestion of elemental thallium purchased from the internet for self-harm.

CASE REPORT

The regional poison center was contacted about an 18-year-old man who ingested a fragment from a 100-gram bar reported to be elemental thallium. Serial serum and urine thallium concentrations were obtained. Prussian blue was started on hospital day (HD) 2. A metal fragment was seen on abdominal x-ray and removed via colonoscopy on HD3. The ingested fragment was analyzed via inductively coupled plasma mass spectrometry (ICP-MS) and found to be 87.0% elemental thallium. The initial serum thallium concentration obtained on HD1 was 423.5 mcg/L (reference range < 5.1 mcg/L), which subsequently decreased to 4.5 mcg/L, 29 days after the ingestion. An initial random urine thallium concentration obtained on HD 3 was 1850.5 mcg/g creatinine (reference range < 0.4 mcg/g creatinine). The patient remained hospitalized for 23 days and, when seen in follow-up, had not developed any signs or symptoms of thallium toxicity.

DISCUSSION

Elemental thallium ingestion is a rare toxicologic exposure, with limited published clinical and analytical experience to guide management. This case report describes a patient with ingestion of elemental thallium who developed elevated serum and urine thallium concentrations and was treated with Prussian blue. Despite having elevated serum and urine thallium concentrations consistent with previous fatal exposures, more evidence is needed to understand the differences between elemental thallium and thallium salts.

Genome-Wide Analysis and Functional Characterization of Pyruvate Kinase (PK) Gene Family Modulating Rice Yield and Quality

Pyruvate kinase (PK) is one of the three rate-limiting enzymes of glycolysis, and it plays a pivotal role in energy metabolism. In this study, we have identified 10 PK genes from the rice genome. Initially, these genes were divided into two categories: cytoplasmic pyruvate kinase (PKc) and plastid pyruvate kinase (PKp). Then, an expression analysis revealed that OsPK1, OsPK3, OsPK4, OsPK6, and OsPK9 were highly expressed in grains. Moreover, PKs can form heteropolymers. In addition, it was found that ABA significantly regulates the expression of PK genes (OsPK1, OsPK4, OsPK9, and OsPK10) in rice. Intriguingly, all the genes were found to be substantially involved in the regulation of rice grain quality and yield. For example, the disruption of OsPK3, OsPK5, OsPK7, OsPK8, and OsPK10 and OsPK4, OsPK5, OsPK6, and OsPK10 decreased the 1000-grain weight and the seed setting rate, respectively. Further, the disruption of OsPK4, OsPK6, OsPK8, and OsPK10 through the CRISPR/Cas9 system showed an increase in the content of total starch and a decrease in protein content compared to the WT. Similarly, manipulations of the OsPK4, OsPK8, and OsPK10 genes increased the amylose content. Meanwhile, the grains of all CRISPR mutants and RNAi lines, except ospk6, showed a significant increase in the chalkiness rate compared to the wild type. Overall, this study characterizes the functions of all the genes of the PK gene family and shows their untapped potential to improve rice yield and quality traits.

The K+-Dependent and -Independent Pyruvate Kinases Acquire the Active Conformation by Different Mechanisms

Eukarya pyruvate kinases possess glutamate at position 117 (numbering of rabbit muscle enzyme), whereas bacteria have either glutamate or lysine. Those with E117 are K⁺-dependent, whereas those with K117 are K⁺-independent. In a phylogenetic tree, 80% of the sequences with E117 are occupied by T113/K114/T120 and 77% of those with K117 possess L113/Q114/(L,I,V)120. This work aims to understand these residues’ contribution to the K⁺-independent pyruvate kinases using the K⁺-dependent rabbit muscle enzyme. Residues 117 and 120 are crucial in the differences between the K⁺-dependent and -independent mutants. K⁺-independent activity increased with L113 and Q114 to K117, but L120 induced structural differences that inactivated the enzyme. T120 appears to be key in folding the protein and closure of the lid of the active site to acquire its active conformation in the K⁺-dependent enzymes. E117K mutant was K⁺-independent and the enzyme acquired the active conformation by a different mechanism. In the K⁺-independent apoenzyme of Mycobacterium tuberculosis, K72 (K117) flips out of the active site; in the holoenzyme, K72 faces toward the active site bridging the substrates through water molecules. The results provide evidence that two different mechanisms have evolved for the catalysis of this reaction.

Potassium dependency of enzymes in plant primary metabolism

Potassium is a macroelement essential to many aspects of plant life, such as photosynthesis, phloem transport or cellular electrochemistry. Many enzymes in animals or microbes are known to be stimulated or activated by potassium (K⁺ ions). Several plant enzymes are also strictly K⁺-dependent, and this can be critical when plants are under K deficiency and thus intracellular K⁺ concentration is low. Although metabolic effects of low K conditions have been documented, there is presently no review focusing on roles of K⁺ for enzyme catalysis or activation in plants. In this mini-review, we compile the current knowledge on K⁺-requirement of plant enzymes and take advantage of structural data to present biochemical roles of K⁺. This information is instrumental to explain direct effects of low K⁺ content on metabolism and this is illustrated with recent metabolomics data.

Review of Potentially Toxic Rare Earth Elements, Thallium and Tellurium in Plant-based Foods

In the last decades, there is an increasing inclusion of various trace metals and metalloids such as thallium, tellurium and rare earth elements (REEs; lanthanides, scandium, and yttrium) in the composition and production of alloys, in agricultural and medicinal applications, as well as in the manufacturing of hi-tech products. All these activities have led to an accumulation of the aforementioned elements both in soil and water bodies and consequently in the food chain, through discharges from mining and mineral processing, liquid industrial waste or disposal of urban and industrial products. It has been demonstrated that chronic exposure to some of these elements, even at low doses, might lead to a wide range of adverse health effects, even from the early stages of life, such as neurotoxicity, neurodevelopmental toxicity and hepatic alterations. Particularly in children, there have been studies suggesting that some of these elements might negatively affect the children's spatial learning and memory ability indirectly. Such effects are triggered by processes like the production of reactive oxygen species (ROS), lipid peroxidation and modulation of antioxidant activities. Nevertheless, the limited data from toxicological studies and their so-far naturally low occurrence levels in the environment acted as a deterrent in measuring their concentrations during routine analyses of metals in foodstuff. Thus, it is important to collect information on their occurrence data both in adults and in children's daily diet. This review sumrises the current knowledge on the concentration of these elements, in plant-based food products to identify whether a potential health risk occurs. As side projects, this Fellowship provided hands-on training on the evaluation of new biocides application and participation in the given advice to the Danish Food and Veterinary Administration, Danish Environmental Protection Agency, the Danish Medical Agency and the European Chemicals Agency.

Pyruvate kinase variant of fission yeast tunes carbon metabolism, cell regulation, growth and stress resistance

Cells balance glycolysis with respiration to support their metabolic needs in different environmental or physiological contexts. With abundant glucose, many cells prefer to grow by aerobic glycolysis or fermentation. Using 161 natural isolates of fission yeast, we investigated the genetic basis and phenotypic effects of the fermentation-respiration balance. The laboratory and a few other strains depended more on respiration. This trait was associated with a single nucleotide polymorphism in a conserved region of Pyk1, the sole pyruvate kinase in fission yeast. This variant reduced Pyk1 activity and glycolytic flux. Replacing the "low-activity" pyk1 allele in the laboratory strain with the "high-activity" allele was sufficient to increase fermentation and decrease respiration. This metabolic rebalancing triggered systems-level adjustments in the transcriptome and proteome and in cellular traits, including increased growth and chronological lifespan but decreased resistance to oxidative stress. Thus, low Pyk1 activity does not lead to a growth advantage but to stress tolerance. The genetic tuning of glycolytic flux may reflect an adaptive trade-off in a species lacking pyruvate kinase isoforms.

An overview of structure, function, and regulation of pyruvate kinases

In the last step of glycolysis Pyruvate kinase catalyzes the irreversible conversion of ADP and phosphoenolpyruvate to ATP and pyruvic acid, both crucial for cellular metabolism. Thus pyruvate kinase plays a key role in controlling the metabolic flux and ATP production. The hallmark of the activity of different pyruvate kinases is their tight modulation by a variety of mechanisms including the use of a large number of physiological allosteric effectors in addition to their homotropic regulation by phosphoenolpyruvate. Binding of effectors signals precise and orchestrated movements in selected areas of the protein structure that alter the catalytic action of these evolutionarily conserved enzymes with remarkably conserved architecture and sequences. While the diverse nature of the allosteric effectors has been discussed in the literature, the structural basis of their regulatory effects is still not well understood because of the lack of data representing conformations in various activation states. Results of recent studies on pyruvate kinases of different families suggest that members of evolutionarily related families follow somewhat conserved allosteric strategies but evolutionarily distant members adopt different strategies. Here we review the structure and allosteric properties of pyruvate kinases of different families for which structural data are available.

Salinity and crop yield

Thirty crop species provide 90% of our food, most of which display severe yield losses under moderate salinity. Securing and augmenting agricultural yield in times of global warming and population increase is urgent and should, aside from ameliorating saline soils, include attempts to increase crop plant salt tolerance. This short review provides an overview of the processes that limit growth and yield in saline conditions. Yield is reduced if soil salinity surpasses crop-specific thresholds, with cotton, barley and sugar beet being highly tolerant, while sweet potato, wheat and maize display high sensitivity. Apart from Na⁺ , also Cl^- , Mg²⁺ , SO₄ ^2- or HCO₃ ^- contribute to salt toxicity. The inhibition of biochemical or physiological processes cause imbalance in metabolism and cell signalling and enhance the production of reactive oxygen species interfering with cell redox and energy state. Plant development and root patterning is disturbed, and this response depends on redox and reactive oxygen species signalling, calcium and plant hormones. The interlink of the physiological understanding of tolerance processes from molecular processes as well as the agronomical techniques for stabilizing growth and yield and their interlinks might help improving our crops for future demand and will provide improvement for cultivating crops in saline environment.

Thallium stimulates ethanol production in immortalized hippocampal neurons

Lactate and ethanol (EtOH) were determined in cell culture medium (CCM) of immortalized hippocampal neurons (HN9.10e cell line) before and after incubation with Thallium (Tl). This cell line is a reliable, in vitro model of one of the most vulnerable regions of central nervous system. Cells were incubated for 48 h with three different single Tl doses: 1, 10, 100 μg/L (corresponding to 4.9, 49 and 490 nM, respectively). After 48 h, neurons were "reperfused" with fresh CCM every 24/48 h until 7 days after the treatment and the removed CCM was collected and analysed. Confocal microscopy was employed to observe morphological changes. EtOH was determined by head space-solid phase microextraction -gas chromatography -mass spectrometry (HS-SPME-GCMS), lactate by RP-HPLC with UV detection. Tl exposure had significant effects on neuronal growth rate and morphology. The damage degree was dose-dependent. In not exposed cells, EtOH concentration was 0.18 ± 0.013 mM, which represents about 5% of lactate concentration (3.4 ± 0.10 mM). After Tl exposure lactate and EtOH increased. In CCM of 100 and 10 μg/L Tl-treated cells, lactate increased 24 h after reperfusion up to 2 and 3.3 times the control value, respectively. In CCM of 10 and 100 μg/L Tl-treated cells 24 h after reperfusion, EtOH increased up to 0.3 and 0.58 mmol/L. respectively. These results are consistent with significant alterations in energy metabolism, despite the low doses of Tl employed and the relatively short incubation time.

Evaluation of cytogenetic and DNA damage caused by thallium(I) acetate in human blood cells

Although thallium is detrimental to all living organisms, information regarding the mutagenic and genotoxic effects of this element and its compounds remains scarce. Therefore, we tested the genotoxic and cytotoxic effects of thallium(I) acetate on human peripheral blood cells in vitro using structural chromosomal aberrations (SCAs), sister chromatid exchanges (SCEs), and single-cell gel electrophoresis (at pH >13 or 12.1) analysis. Whole blood samples were incubated with 0.5, 1, 5, 10, 50, or 100 µg/mL thallium salt. Exposure to this metal compound resulted in a clear dose-dependent reduction in the mitotic and replicative indices. An increase in SCAs was evident in the treated group compared with the control group, and significant differences were observed in the percentage of cells with SCAs when metaphase cells were treated with 0.5-10 µg/mL of thallium(I). The SCE test did not reveal any significant differences. We observed that a 1-h treatment with thallium(I) at pH > 13 significantly increased the comet length for all the concentrations tested; however, at pH 12.1, only the two highest concentrations affected the comet length. These results suggested that thallium(I) acetate induces cytotoxic, cytostatic, and clastogenic effects, as well as DNA damage.

The importance of polarity in the evolution of the K+ binding site of pyruvate kinase

In a previous phylogenetic study of the family of pyruvate kinase, we found one cluster with Glu117 and another with Lys117. Those sequences with Glu117 have Thr113 and are K+-dependent, whereas those with Lys117 have Leu113 and are K+-independent. The carbonyl oxygen of Thr113 is one of the residues that coordinate K+ in the active site. Even though the side chain of Thr113 does not participate in binding K+, the strict co-evolution between position 117 and 113 suggests that T113 may be the result of the evolutionary pressure to maintain the selectivity of pyruvate kinase activity for K+. Thus, we explored if the replacement of Thr113 by Leu alters the characteristics of the K+ binding site. We found that the polarity of the residue 113 is central in the partition of K+ into its site and that the substitution of Thr for Leu changes the ion selectivity for the monovalent cation with minor changes in the binding of the substrates. Therefore, Thr113 is instrumental in the selectivity of pyruvate kinase for K+.

LacZ β-galactosidase: structure and function of an enzyme of historical and molecular biological importance

This review provides an overview of the structure, function, and catalytic mechanism of lacZ β-galactosidase. The protein played a central role in Jacob and Monod's development of the operon model for the regulation of gene expression. Determination of the crystal structure made it possible to understand why deletion of certain residues toward the amino-terminus not only caused the full enzyme tetramer to dissociate into dimers but also abolished activity. It was also possible to rationalize α-complementation, in which addition to the inactive dimers of peptides containing the "missing" N-terminal residues restored catalytic activity. The enzyme is well known to signal its presence by hydrolyzing X-gal to produce a blue product. That this reaction takes place in crystals of the protein confirms that the X-ray structure represents an active conformation. Individual tetramers of β-galactosidase have been measured to catalyze 38,500 ± 900 reactions per minute. Extensive kinetic, biochemical, mutagenic, and crystallographic analyses have made it possible to develop a presumed mechanism of action. Substrate initially binds near the top of the active site but then moves deeper for reaction. The first catalytic step (called galactosylation) is a nucleophilic displacement by Glu537 to form a covalent bond with galactose. This is initiated by proton donation by Glu461. The second displacement (degalactosylation) by water or an acceptor is initiated by proton abstraction by Glu461. Both of these displacements occur via planar oxocarbenium ion-like transition states. The acceptor reaction with glucose is important for the formation of allolactose, the natural inducer of the lac operon.

Genetic toxicology of thallium: a review

This review summarizes the current knowledge about the general toxicity of thallium (Tl) and its environmental sources, with special emphasis placed on its potential mutagenic, genotoxic, and cytotoxic effects on both eukaryotic and prokaryotic cells. Tl is a nonessential heavy metal that poses environmental and occupational threats as well as therapeutic hazards because of its use in medicine. It is found in two oxidation states, thallous (Tl(+)) and thallic (Tl(3+)), both of which are considered highly toxic to human beings and domestic and wild organisms. Many Tl compounds are colorless, odorless and tasteless, and these characteristics, combined with the high toxicity of TI compounds, have led to their use as poisons. Because of its similarity to potassium ions (K(+)), plants and mammals readily absorb Tl(+) through the skin and digestive and respiratory systems. In mammals, it can cross the placental, hematoencephalic, and gonadal barriers. Inside cells, Tl can accumulate and interfere with the metabolism of potassium and other metal cations, mimicking or inhibiting their action. The effects of Tl on genetic material have not yet been thoroughly explored, and few existing studies have focused exclusively on Tl(+). Both in vivo and in vitro studies indicate that Tl compounds can have a weak mutagenic effect, but no definitive effect on the induction of primary DNA damage or chromosomal damage has been shown. These studies have demonstrated that Tl compounds are highly toxic and lead to changes in cell-cycle progression.

Selective Thallium (I) Ion Sensor Based on Functionalised ZnO Nanorods

Well controlled in length and highly aligned ZnO nanorods were grown on the gold-coated glass substrate by hydrothermal growth method. ZnO nanorods were functionalised with selective thallium (I) ion ionophore dibenzyldiaza-18-crown-6 (DBzDA18C6). The thallium ion sensor showed wide linear potentiometric response to thallium (I) ion concentrations ( M to  M) with high sensitivity of 36.87 ± 1.49 mV/decade. Moreover, thallium (I) ion demonstrated fast response time of less than 5 s, high selectivity, reproducibility, storage stability, and negligible response to common interferents. The proposed thallium (I) ion-sensor electrode was also used as an indicator electrode in the potentiometric titration, and it has shown good stoichiometric response for the determination of thallium (I) ion.

The impact of ions on allosteric functions in human liver pyruvate kinase

Experimental designs used to monitor the magnitude of an allosteric response can greatly influence observed values. We report here the impact of buffer, monovalent cation, divalent cation, and anion on the magnitude of the allosteric regulation of the affinity of human liver pyruvate kinase (hL-PYK) for substrate, phosphoenolpyruvate (PEP). The magnitudes of the allosteric activation by fructose-1,6-bisphosphate (Fru-1,6-BP) and the allosteric inhibition by alanine are independent of most, but not all buffers tested. However, these magnitudes are dependent on whether Mg(2+) or Mn(2+) is included as the divalent cation. In the presence of Mn(2+), any change in K(app-PEP) caused by Fru-1,6-BP is minimal. hL-PYK activity does not appear to require monovalent cation. Monovalent cation binding in the active site impacts PEP affinity with minimum influence on the magnitude of allosteric coupling. However, Na(+) and Li(+) reduce the magnitude of the allosteric response to Fru-1,6-BP, likely due to mechanisms outside of the active site. Which anion is used to maintain a constant monovalent cation concentration also influences the magnitude of the allosteric response. The value of determining the impact of ions on allosteric function can be appreciated by considering that representative structures used in comparative studies have often been determined using protein crystals grown in diverse buffer and salt conditions.

Dichotomic Phylogenetic Tree of the Pyruvate Kinase Family

K+ dependence was assumed to be a feature of all pyruvate kinases until it was discovered that some enzymes express K+ -independent activity. Almost all the K+ -independent pyruvate kinases have Lys at position 117, instead of the Glu present in the K+ -dependent muscle enzyme. Mutagenesis studies show that the internal positive charge substitutes for the K+ requirement (Laughlin, L. T. & Reed, G. H. (1997) Arch. Biochem. Biophys. 348, 262-267). In this work a phylogenetic analysis of pyruvate kinase was performed to ascertain the abundance of K+ -independent activities and to explore whether the K+ activating effect is related to the evolutionary history of the enzyme. Of the 230 studied sequences, 46% have Lys at position 117, and the rest have Glu. Pyruvate kinases with Lys117 and Glu117 are separated in two clusters. All of the enzymes of the Glu117 cluster that have been characterized are K+ -dependent, whereas those of the Lys117 cluster are K+ -independent. Thus, there is a strict correlation between the dichotomy of the tree and the dependence of activity on K+. 77% of the pyruvate kinases that possess Lys117 have Lys113/Gln114; they also have Ile, Val, or Leu at position 120. These residues are replaced by Glu117 and Thr113/Lys114/Thr120 in 80% of K+ -dependent pyruvate kinases. Structural analysis indicates that these residues are in a hinge region involved in the acquisition of the catalytic conformation of the enzyme. The route of conversion from K+ -independent to K+ -dependent pyruvate kinases is described. A plausible explanation of how enzymes developed K+ dependence is put forth.

Lone pair effect in thallium(I) macrocyclic compounds

The role of the inert (lone) pair of electrons in thallium(I) salts is studied by comparison of the compounds [Tl@18-crown-6]+ X- (X = TlI4, ClO4) and [K@18-crown-6]+ ClO4-. In contrast to common introductory chemistry textbook opinions, the paradigm that s-p hybridization is a prerequisite for an inert electron pair to become stereochemically active in compounds of the heavier main group elements has to be revised. Instead, an inert pair of electrons is expected to become stereochemically involved whenever it is forced to participate in antibonding orbital interactions with its surroundings, and there is the possibility for a structural distortion that minimizes these repulsive forces. The structural distortion will occur to such an extent that repulsive orbital interactions and attractive electrostatic interactions counterbalance. Our results also provide an explanatory background for many of the rules of thumb that are found in the literature about why and when an inert electron pair is expected to become stereochemically active in a certain compound.

Pyruvate kinase revisited: the activating effect of K+

For more than 50 years, it has been known that K(+) is an essential activator of pyruvate kinase (Kachmar, J. F., and Boyer, P. D. (1953) J. Biol. Chem. 200, 669-683). However, the role of K(+) in the catalysis by pyruvate kinase has not been totally understood. Previous studies without K(+) showed that the affinity of ADP-Mg(2+) depends on the concentration of phosphoenolpyruvate, although the kinetics of the enzyme at saturating K(+) concentrations show independence in the binding of substrates (Reynard, A. M., Hass, L. F., Jacobsen, D. D. & Boyer, P. D. (1961) J. Biol. Chem. 236, 2277-2283). Here, we explored the kinetics of the enzyme with and without K(+). The results show that without K(+), the kinetic mechanism of pyruvate kinase changes from random to ordered with phosphoenol-pyruvate as first substrate. V(max) with K(+) was about 400 higher than without K(+). In the presence of K(+), the affinities for phosphoenol-pyruvate, ADP-Mg(2+), oxalate, and ADP-Cr(2+) were 2-6-fold higher than in the absence of K(+). This as well as fluorescence data also indicate that K(+) is involved in the acquisition of the active conformation of the enzyme, allowing either phosphoenolpyruvate or ADP to bind independently (random mechanism). In the absence of K(+), ADP cannot bind to the enzyme until phosphoenolpyruvate forms a competent active site (ordered mechanism). We propose that K(+) induces the closure of the active site and the arrangement of the residues involved in the binding of the nucleotide.

1H NMR studies of homo and mixed ligand complexes of Tl+ ion with several polyazamacrocycles

Proton NMR was used as a probe to study the interaction of the Tl(+) ion with 9-18-membered macromonocyclic tri-, tetra-, and hexaamines in dimethylformamide (DMF) solution. A study of proton chemical shift of ligands as a function of Tl(+) ion to ligand mole ratio revealed that the complexation reactions occur in a stepwise manner. Formation of a 1:1 complex is followed by the addition of a second complexant molecule to form a homo-sandwich complex for triazamacrocycle ligands and a mixed ligand complex in the case of hexamethylhexacyclen (HMHCY) and 1,4,7-triazacyclononane ([9]aneN(3)). The formation constants of resulting 1:1 and 1:2 (homo and mixed ligand sandwich) complexes in DMF solution were evaluated from computer fitting of the chemical shift-mole ratio data. The mixed ligand complexes may be more stable than the parent complex in which both ligands are the same. The influence of cavity size and substitution of methyl groups on nitrogen atoms of the macrocyclic ring the stability of the resulting complexes is discussed. The geometries of the tri- and tetraazamacrocycle ligands and their Tl(+) ion complexes were optimized by an ab initio method, and the calculated binding energies of resulting complexes were compared. Both the experimental and theoretical studies revealed that, in the presence of methyl groups, the stability of triazamacrocycle complexes with Tl(+) ion was decreased.

Thallium toxicity and the role of Prussian blue in therapy

Thallium salts have been used as medicinal agents, as key ingredients in a variety of manufacturing processes, and as a potent rodenticide. Additionally, environmental concerns are growing, as thallium is a waste product of coal combustion and the manufacturing of cement. Thallium salts are rapidly and nearly completely absorbed by virtually all routes, with gastrointestinal exposure being the most common route to produce toxicity. Thallium enters cells by a unique process governed by its similarity in charge and ionic radius to potassium. Although the exact mechanism of toxicity has not been established, thallium interferes with energy production at essential steps in glycolysis, the Krebs cycle, and oxidative phosphorylation. Additional effects include inhibition of sodium-potassium-adenosine triphosphatase and binding to sulfhydryl groups. The major manifestations of toxicity consist of a rapidly progressive, ascending, extremely painful sensory neuropathy and alopecia. Unlike exposure to most metal salts, gastrointestinal symptoms of thallium toxicity are relatively minor, and constipation is more characteristic than diarrhoea. Many other findings such as an autonomic neuropathy, cranial nerve abnormalities, altered mental status, motor weakness, cardiac, hepatic, and renal effects are described, but are less specific. Thallium also crosses the placenta freely and produces abnormalities in animals as well as fetal demise, overt toxicity and congenital abnormalities in humans. There are no controlled trials of treatments in thallium-poisoned patients. Thus, the literature is predominated by very small animal studies and case reports with very limited data. Strong evidence speaks against the use of traditional metal chelators such as dimercaprol (British Anti-Lewisite) and penicillamine, and the latter may cause redistribution of thallium into the central nervous system. Likewise, forced potassium diuresis appears harmful. The use of single- or multiple-dose activated charcoal is supported by in vitro binding experiments and some animal data, and charcoal haemoperfusion may be a useful adjunct. Multiple animal studies give evidence for enhanced elimination and improved survival with Prussian blue. Unfortunately, despite the fact that many humans have been treated with Prussian blue, the data presented are insufficient to comment definitively on its efficacy. However, Prussian blue's safety profile is superior to that of other proposed therapies and it should be considered the drug of choice in acute thallium poisoning. Public health efforts should focus on greater restrictions on access to, and use of, thallium salts.

Monitoring active site alterations upon mutation of yeast pyruvate kinase using 205Tl+ NMR

The interaction of the monovalent cation with wild type (WT) yeast pyruvate kinase (YPK) and with the T298S, T298C, and T298A mutants was investigated by 205Tl+ NMR to monitor possible structural alterations at the active site by Thr-298 mutation. TlNO3 activates WT YPK with a kcat value similar to that obtained with KCl and an apparent Ka of 0.96 +/- 0.07 mm in the presence of Mn2+ and fructose 1,6-bisphosphate. With the three mutants, Tl+ is a better activator than is K+ based on kcat values. Tl+ activation and inhibition of YPK is affected by mutation of the active site Thr-298. The effect of Mn2+ on the 1/T value of 205Tl+1 in the presence of the WT and mutant YPK complexes was determined at 173 MHz (300 MHz, 1H) and 346 MHz (600 MHz, 1H). For each complex studied, 1/pT2p >> 1/pT1p and 1/pT1p is frequency-dependent suggesting fast exchange conditions. The values of 1/pT1p differ for each mutant. A correlation time of 0.65 +/- 0.35 ns was estimated for the Mn2+-205Tl+ interaction. The Tl+-Mn2+ distances at the active site of YPK were calculated from the paramagnetic contribution of Mn2+ to 1/T1M of YPK-bound 205Tl+. The calculated Tl+-Mn2+ distance for the Thr-298 mutants is decreased by about 1 A from 6.0 +/- 0.2 A observed with WT. The results suggest conformational alterations at the active site of YPK where phosphoryl transfer occurs upon mutation of Thr-298. These conformational changes may, in part, explain the alteration in kcat and kcat/Km,PEP observed with the Thr-298 mutants.

Selectivity of pyruvate kinase for Na+ and K+ in water/dimethylsulfoxide mixtures

In aqueous media, muscle pyruvate kinase is highly selective for K+ over Na+. We now studied the selectivity of pyruvate kinase in water/dimethylsulfoxide mixtures by measuring the activation and inhibition constants of K+ and Na+, i.e. their binding to the monovalent and divalent cation binding sites of pyruvate kinase, respectively [Melchoir J.B. (1965) Biochemistry 4, 1518-1525]. In 40% dimethylsulfoxide the K0.5 app for K+ and Na+ were 190 and 64-fold lower than in water. Ki app for K+ and Na+ decreased 116 and 135-fold between 20 and 40% dimethylsulfoxide. The ratios of Ki app/K0.5 app for K+ and Na+ were 34-3.5 and 3.3-0.2, respectively. Therefore, dimethylsulfoxide favored the partition of K+ and Na+ into the monovalent and divalent cation binding sites of the enzyme. The kinetics of the enzyme at subsaturating concentrations of activators show that K+ and Mg2+ exhibit high selectivity for their respective cation binding sites, whereas when Na+ substitutes K+, Na+ and Mg2+ bind with high affinity to their incorrect sites. This is evident by the ratio of the affinities of Mg2+ and K+ for the monovalent cation binding site, which is close to 200. For Na+ and Mg2+ this ratio is approximately 20. Therefore, the data suggest that K+ induces conformational changes that prevent the binding of Mg2+ to the monovalent cation binding site. Circular dichroism spectra of the enzyme and the magnitude of the transfer and apparent binding energies of K+ and Na+ indicate that structural arrangements of the enzyme induced by dimethylsulfoxide determine the affinities of pyruvate kinase for K+ and Na+.

Activation of ribokinase by monovalent cations

Carbohydrate kinases frequently require a monovalent cation for their activity. The physical basis of this phenomenon is, however, usually unclear. We report here that Escherichia coli ribokinase is activated by potassium with an apparent K(d) of 5 mM; the enzyme should therefore be fully activated under physiological conditions. Cesium can be used as an alternative ion, with an apparent K(d) of 17 mM. An X-ray structure of ribokinase in the presence of cesium was solved and refined at 2.34 A resolution. The cesium ion was bound between two loops immediately adjacent to the anion hole of the active site. The buried location of the site suggests that conformational changes will accompany ion binding, thus providing a direct mechanism for activation. Comparison with structures of a related enzyme, the adenosine kinase of Toxoplasma gondii, support this proposal. This is apparently the first instance in which conformational activation of a carbohydrate kinase by a monovalent cation has been assigned a clear structural basis. The mechanism is probably general to ribokinases, to some adenosine kinases, and to other members of the larger family. A careful re-evaluation of the biochemical and structural data is suggested for other enzyme systems.

High pressure-induced changes of biological membrane. Study on the membrane-bound Na(+)/K(+)-ATPase as a model system

In order to study the pressure-induced changes of biological membrane, hydrostatic pressures of from 0.1 to 400 MPa were applied to membrane-bound Na(+)/K(+)-ATPase from pig kidney as a model system of protein and lipid membrane. The activity showed at least a three-step change induced by pressures of 0.1-100 MPa, 100-220 MPa, and 220 MPa or higher. At pressures of 100 MPa or lower a decrease in the fluidity of lipid bilayer and a reversible conformational change in transmembrane protein is induced, leading to the functional disorder of membrane-associated ATPase activity. A pressure of 100-220 MPa causes a reversible phase transition in parts of the lipid bilayer from the liquid crystalline to the gel phase and the dissociation of and/or conformational changes in the protein subunits. These changes could cause a separation of the interface between alpha and beta subunits and between protein and the lipid bilayer to create transmembrane tunnels at the interface. Tunnels would be filled with water from the aqueous environment and take up tritiated water. A pressure of 220 MPa or higher irreversibly destroys and fragments the gross membrane structure, due to protein unfolding and interface separation, which is amplified by the increased pressure. These findings provide an explanation for the high pressure-induced membrane-damage to subcellular organelles.

Dimethylsulfoxide promotes K+-independent activity of pyruvate kinase and the acquisition of the active catalytic conformation

Pyruvate kinase requires K+ for maximal activity; the enzyme exhibits 0.02% of maximal activity in its absence [Kayne, F. J. (1971) Arch. Biochem. Biophys. 143, 232-239]. However, pyruvate kinase entrapped in reverse micelles exhibits an important K+-independent activity [Ramírez-Silva, L., Tuena de Gómez-Puyou, M., & Gómez-Puyou, A. (1993) Biochemistry 32, 5332-5338]. It is possible that the amount of water, as well as interactions of the protein with the micelles, can account for this behavior. We therefore explored the solvent effects on the catalytic properties of muscle pyruvate kinase. The enzyme exhibited an activity of 19.4 micromol x min(-1) x mg(-1) in 40% dimethylsulfoxide, compared with 280 and 0.023 micromol x min(1) x mg(-1) observed with and without K+ in water, respectively. pH activity profiles and kinetic constants for the substrates of pyruvate kinase in dimethylsulfoxide without K+ were similar to those in 100% water with K+, and differed from those in water without K+. The spectral center of mass of the emission spectrum of pyruvate kinase in 100% water exhibited a blue shift of 3.5 nm in the presence of Mg(2+), phosphenolpyruvate, and K+, ligands that induce the active conformation of the enzyme. The spectral center of mass of the apoenzyme in 30-40% dimethylsulfoxide coincided with that of the enzyme-Mg(2+)-phosphenolpyruvate-K+ complex in 100% water. The water relaxation rate enhancement factor and binding of phosphenolpyruvate to the pyruvate kinase-Mn(2+)-(CH3)4N+ complex in 30-40% dimethylsulfoxide were similar to those of the pyruvate kinase-Mn(2+)-K+ complex in water. The aforementioned results indicate that when muscle pyruvate kinase is without K+, 30-40% dimethylsulfoxide induces its active conformation.

Monovalent cation activation in Escherichia coli inosine 5'-monophosphate dehydrogenase

Inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes the oxidation of inosine 5'-monophosphate (IMP) to xanthosine 5'-monophosphate with the concomitant reduction of NAD to NADH. Escherichia coli IMPDH is activated by K(+), Rb(+), NH(+)(4), and Cs(+). K(+) activation is inhibited by Li(+), Na(+), Ca(2+), and Mg(2+). This inhibition is competitive versus K(+) at high K(+) concentrations, noncompetitive versus IMP, and competitive versus NAD. Thus monovalent cation activation is linked to the NAD site. K(+) increases the rate constant for the pre-steady-state burst of NADH production, possibly by increasing the affinity of NAD. Three mutant IMPDHs have been identified which increase the value of K(m) for K(+): Asp13Ala, Asp50Ala, and Glu469Ala. In contrast to wild type, both Asp13Ala and Glu469Ala are activated by all cations tested. Thus these mutations eliminate cation selectivity. Both Asp13 and Glu469 appear to interact with the K(+) binding site identified in Chinese hamster IMPDH. Like wild-type IMPDH, K(+) activation of Asp50Ala is inhibited by Li(+), Na(+), Ca(2+), and Mg(2+). However, this inhibition is noncompetitive with respect to K(+) and competitive with respect to both IMP and NAD. Asp50 interacts with residues that form a rigid wall in the IMP site; disruption of this wall would be expected to decrease IMP binding, and the defect could propagate to the proposed K(+) site. Alternatively, this mutation could uncover a second monovalent cation binding site.

Subunit dissociation and inactivation of pyruvate kinase by hydrostatic pressure oxidation of sulfhydryl groups and ligand effects on enzyme stability

The effect of hydrostatic pressure on the stability of tetrameric rabbit muscle pyruvate kinase was investigated by enzyme activity measurements, size-exclusion chromatography, circular dichroism and fluorescence spectroscopies. Under nonreducing conditions, enzyme activity was irreversibly inhibited by increasing pressure and was completely abolished at 350 MPa. Inhibition was dependent on the concentration of pyruvate kinase, indicating that it was related to pressure-induced subunit dissociation. Size-exclusion chromatography of pressurized samples confirmed a decrease in the proportion of tetramers and an increase in monomers relative to native samples. Addition of dithiothreitol immediately following pressure release led to full recovery of both enzyme activity and of native tetramers. Furthermore, no irreversible inhibition of pyruvate kinase was observed if pressure treatment was carried out in the presence of dithiothreitol. These data suggest that pressure-dissociated monomers undergo conformational changes leading to oxidation of sulfhydryl groups, which prevents correct refolding of native tetramers on decompression. These conformational changes are relatively subtle, as indicated by the lack of significant changes in far-UV circular dichroism and intrinsic fluorescence emission spectra of previously pressurized samples. The effects of various physiological ligands on the pressure stability of pyruvate kinase were also investigated. A slight protection against inhibition was observed in the simultaneous presence of K+, Mg2+ and ADP. Both phosphoenolpyruvate and the allosteric inhibitor, phenylalanine, caused marked stabilization against pressure, suggesting significant energy coupling between binding of these ligands and stabilization of the tetramer.

Expression and characterization of recombinant pyruvate phosphate dikinase from Entamoeba histolytica

The parasite Entamoeba histolytica is an organism whose main energetic source comes from glycolysis. It has the singularity that several of its glycolytic enzymes use pyrophosphate as an alternative phosphate donor. Thus, pyruvate phosphate dikinase (PPDK), an inorganic pyrophosphate (PPi)-dependent enzyme, substitutes pyruvate kinase present in humans. We previously cloned and sequenced the gene that codifies for PPDK in E. histolytica. We now report its expression in a bacterial system and its purification to 98% homogeneity. We determined its K(m) for phosphoenolpyruvate, AMP and PPi (21, < 5 and 100 microM, respectively). Unlike PPDK from maize and bacteria and pyruvate kinase from other cells, EhPPDk is dependent on divalent cations but does not require monovalent cations for activity. The enzyme has an optimum pH of 6.0, it is labile to low temperatures and has a tetrameric structure. Since EhPPDK is a PPi-dependent enzyme, we also tested the effect of some pyrophosphate analogs as inhibitors of activity. Studies on the function and structure of this enzyme may be important for therapeutic research in several parasitic diseases, since it has no counterpart in humans.

The monovalent cation requirement of rabbit muscle pyruvate kinase is eliminated by substitution of lysine for glutamate 117

The crystal structure of rabbit muscle pyruvate kinase complexed with Mn2+, K+, and pyruvate revealed a binding site of K+ [T. M. Larsen, L. T. Laughlin, H. M. Holden, I. Rayment, and G. H. Reed (1994) Biochemistry 33, 6301-6309]. Sequence comparisons of rabbit muscle pyruvate kinase and pyruvate kinases from Corynebacterium glutamicum and Escherichia coli, which do not exhibit a requirement for activation by monovalent cations, indicate that the only substitutions in the K+ binding site are conservative. Glu 117 in the rabbit muscle enzyme, which is close to the K+ site, is, however, replaced by Lys in these two bacterial pyruvate kinases. The proximity of Glu 117 to K+ in the structure of the rabbit enzyme and conservation of the binding site in the bacterial enzymes which lack a dependence on monovalent cations suggested that a protonated epsilon-amino group of Lys 117 in these bacterial enzymes may provide an "internal monovalent cation." Site-specific mutant forms of the rabbit enzyme corresponding to E117K, E117A, E117D, and E117K/K114Q pyruvate kinase were examined to test this hypothesis. The E117K pyruvate kinase exhibits 12% of the activity of the fully activated wild-type enzyme but is > 200-fold more active than the wild-type enzyme in the absence of activating monovalent cations. Moreover, the activity of E117K pyruvate kinase exhibits no stimulation by monovalent cations in the assay mixtures. Both E117A and E117D pyruvate kinases retain activation by monovalent cations but have reduced activities relative to wild type. The results are consistent with the hypothesis that pyruvate kinases that do not require activation by monovalent cations supply an internal monovalent cation in the form of a protonated epsilon-amino group of Lys. The results also support the assignment of the monovalent cation in the active site of pyruvate kinase.

The contribution of water to the selectivity of pyruvate kinase for Na+ and K+

For many years it has been known that K+ is an essential activator of pyruvate kinase [Kachmar, J. F. & Boyer, P. D. (1953) J. Biol. Chem. 200, 669-683] and that Na+ induces relatively small enhancements of activity. The effect of these two alkali metal ions on the activity of pyruvate kinase entrapped in the low water environment of reverse 'micelles formed with cetyltrimethylammonium, hexanol, n-octane and various water concentrations was studied. In reverse micelles with 3.6% water, the activity with 7 mM Na+ is more than 82 times higher than in aqueous solution with an equivalent Na+ concentration. As the concentration of water in reverse micelles is raised, the activating effect of relatively low concentrations of Na+ (or K+) decreases simultaneously to a more than 100-fold increase in the concentration of Na+ or K+ required for attaining half-maximal activation. Similar results were obtained with NH4+, Rb+ and Cs+. Therefore, the amount of water in the system is critical for observing activation by alkali metal ions. In fact, the concentration of Na+ required for half-maximal activation in standard aqueous media is higher than the concentrations that can be experimentally assayed. As evidenced from fluorescence and kinetic data, it appears that the entrapment of pyruvate kinase in reverse micelles does not produce gross structural alterations. Therefore, it is suggested that in conventional aqueous systems, the basis of the high discrimination between Na+ and K+ by pyruvate kinase is the higher energy required for desolvating Na+. Nevertheless, at all the water concentrations studied, the activities reached with K+ were higher than with Na+ which suggests that the Na+ form of the enzyme has a lower catalytic capacity than the K+-enzyme complex.

The phospholipid flippase activity of gastric vesicles

We found that isolated gastric vesicles contain a novel Mg2+-ATP-dependent phospholipid translocation (flippase) activity. Fluorescence analogue of phosphatidylcholine, 2-(12-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)amino)dodecanoyl-1-hexadecanoyl-sn-glycero-3- phosphocholine, was ATP-dependently translocated from the outer (cytosolic) to inner (luminal) leaflet of the lipid membrane bilayer of hog gastric vesicles. The translocation was saturable and depended on time and the ATP concentration (Km = 3.1 microM). The basal Mg2+-ATPase activity of gastric vesicles in the absence of K+ showed high (Km = 1.6 microM) and low (Km = 80 microM) affinities for ATP, indicating that the present flippase activity is driven mostly by the high affinity Mg2+-ATPase activity. It required Mg2+ but not K+. Verapamil, which is an inhibitor of mouse mdr2 phosphatidylcholine flippase, did not inhibit the present flippase activity. Isolated sarcoplasmic reticulum vesicles that contain Ca2+-ATPase did not show any flippase activity. Fluorescence analogues of phosphatidylserine and phosphatidylethanolamine were similarly translocated by the gastric flippase. These phospholipid flippase activities were inhibited by 2-methyl-8-(phenylmethoxy)imidazo[1,2-a]pyridine-3-acetonitrile (SCH 28080) (IC50 = 0.14-0.25 microM), a specific K+-ATPase inhibitor of gastric H+,K+-ATPase rich in gastric vesicles. IC50 value for the SCH 28080-inhibitable Mg2+-ATPase activity was about 0.13 microM, indicating that the phospholipid translocation was driven mostly by the SCH 28080-sensitive Mg2+-ATPase activity. Possible physiological roles of flippases were discussed in relation with the gastric acid secretory and cytoprotective mechanisms.

Structure of rabbit muscle pyruvate kinase complexed with Mn2+, K+, and pyruvate

The molecular structure of rabbit muscle pyruvate kinase, crystallized as a complex with Mn2+, K+, and pyruvate, has been solved to 2.9-A resolution. Crystals employed in the investigation belonged to the space group P1 and had unit cell dimensions a = 83.6 A, b = 109.9 A, c = 146.8 A, alpha = 94.9 degrees, beta = 93.6 degrees, and gamma = 112.3 degrees. There were two tetramers in the asymmetric unit. The structure was solved by molecular replacement, using as the search model the coordinates of the tetramer of pyruvate kinase from cat muscle [Muirhead, H., Claydon, D. A., Barford, D., Lorimer, C. G., Fothergill-Gilmore, L. A., Schiltz, E., & Schmitt, W. (1986) EMBO J.5, 475-481]. The amino acid sequence derived from the cDNA coding for the enzyme from rabbit muscle was fit to the electron density. The rabbit and cat muscle enzymes have approximately 94% sequence identity, and the folding patterns are expected to be nearly identical. There are, however, three regions where the topological models of the cat and rabbit pyruvate kinases differ. Mn2+ coordinates to the protein through the carboxylate side chains of Glu 271 and Asp 295. These two residues are strictly conserved in all known pyruvate kinases. In addition, the density for Mn2+ is connected to that of pyruvate, consistent with chelation through a carboxylate oxygen and the carbonyl oxygen of the substrate. The epsilon-NH2 of Lys 269 and the OH of Thr 327 lie on either side of the methyl group of bound pyruvate. Spherical electron density, assigned to K+, is located within a well-defined pocket of four oxygen ligands contributed by the carbonyl oxygen of Thr 113, O gamma of Ser 76, O delta 1 of Asn 74, and O delta 2 of Asp 112. The interaction of Asp 112 with the side chains of Lys 269 and Arg 72 may mediate, indirectly, monovalent cation effects on activity.

Monovalent cations and inorganic phosphate alter branched-chain alpha-ketoacid dehydrogenase-kinase activity and inhibitor sensitivity

Potassium ion protects the branched-chain alpha-ketoacid dehydrogenase complex against inactivation by thermal denaturation and protease digestion. Rubidium was effective but sodium and lithium were not, suggesting that the ionic size of the cation is important for stabilization of the enzyme. Thiamine pyrophosphate stabilization of the complex [Danner, D. J., Lemmon, S. K., and Elsas, S. J. (1980) Arch. Biochem. Biophys. 202, 23-28] was found dependent on the presence of potassium ion. Studies with resolved components indicate that the thiamine pyrophosphate-dependent enzyme of the complex, i.e., the 2-oxoisovalerate dehydrogenase (lipoamide) (EC 1.2.4.4), is the component stabilized by potassium ion. Branched-chain alpha-ketoacid dehydrogenase-kinase activity measured by inactivation of the branched-chain alpha-ketoacid dehydrogenase complex was maximized at a potassium ion concentration of 100 mM. Stimulation of kinase activity was also found with rubidium ion but not with lithium and sodium ions. All salts tested increased the efficiency of inactivation by phosphorylation, i.e., decreased the degree of enzyme phosphorylation required to cause inactivation of the complex. The effectiveness and efficacy of alpha-chloroisocaproate as an inhibitor of branched-chain alpha-ketoacid dehydrogenase kinase were enhanced by the presence of monovalent cations, and further increased by inorganic phosphate. These findings suggest that monovalent cations and anions, particularly potassium and phosphate, cause structural changes in the dehydrogenase-kinase complex that alter its susceptibility to phosphorylation and responsiveness to kinase inhibitors.