PMIDA-Modified Fe3O4 Magnetic Nanoparticles: Synthesis and Application for Liver MRI

The surface modification of Fe₃O₄-based magnetic nanoparticles (MNPs) with N-(phosphonomethyl)iminodiacetic acid (PMIDA) was studied, and the possibility of their use as magnetic resonance imaging contrast agents was shown. The effect of the added PMIDA amount, the reaction temperature and time on the degree of immobilization of this reagent on MNPs, and the hydrodynamic characteristics of their aqueous colloidal solutions have been systematically investigated for the first time. It has been shown that the optimum condition for the modification of MNPs is the reaction at 40 °C with an equimolar amount of PMIDA for 3.5 h. The modified MNPs were characterized by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric, and CHN elemental analyses. The dependence of the hydrodynamic characteristics of the MNP colloidal solutions on the concentration and pH of the medium was studied by the dynamic light scattering method. On the basis of the obtained data, we can assume that the PMIDA molecules are fixed on the surface of the MNPs as a monomolecular layer. The modified MNPs had good colloidal stability and high magnetic properties. The calculated relaxivities r₂ and r₁ were 341 and 102 mmol^-1 s^-1, respectively. The possibility of using colloidal solutions of PMIDA-modified MNPs as a T₂ contrast agent for liver studies in vivo (at a dose of 0.6 mg kg^-1) was demonstrated for the first time.

Improving CRISPR-Cas specificity with chemical modifications in single-guide RNAs

CRISPR systems have emerged as transformative tools for altering genomes in living cells with unprecedented ease, inspiring keen interest in increasing their specificity for perfectly matched targets. We have developed a novel approach for improving specificity by incorporating chemical modifications in guide RNAs (gRNAs) at specific sites in their DNA recognition sequence ('guide sequence') and systematically evaluating their on-target and off-target activities in biochemical DNA cleavage assays and cell-based assays. Our results show that a chemical modification (2'-O-methyl-3'-phosphonoacetate, or 'MP') incorporated at select sites in the ribose-phosphate backbone of gRNAs can dramatically reduce off-target cleavage activities while maintaining high on-target performance, as demonstrated in clinically relevant genes. These findings reveal a unique method for enhancing specificity by chemically modifying the guide sequence in gRNAs. Our approach introduces a versatile tool for augmenting the performance of CRISPR systems for research, industrial and therapeutic applications.

Design and Synthesis of Phosphonoacetic Acid (PPA) Ester and Amide Bioisosters of Ribofuranosylnucleoside Diphosphates as Potential Ribonucleotide Reductase Inhibitors and Evaluation of Their Enzyme Inhibitory, Cytostatic and Antiviral Activity

Continuing our investigations on inhibitors of ribonucleotide reductase (RNR), the crucial enzyme that catalyses the reduction of ribonu-cleotides to deoxyribonucleotides, we have now prepared and evaluated 5′-phosphonoacetic acid, amide and ester analogues of adenosine, uridine and cytidine with the aim to verify both substrate specificity and contribution to biological activity of diphosphate mimic moieties. A molecular modelling study has been conducted on the RNR R1 subunit, in order to verify the possible interaction of the proposed bioisosteric moieties. The study compounds were finally tested on the recombinant murine RNR showing a degree of inhibition that ranged from 350 μM for the UDP analogue 5′-deoxy-5′- N-(phosphon-acetyl)uridine sodium salt (amide) to 600 μM for the CDP analogue 5′- O-[(diethyl-phosphon)acetyl]cytidine (ester). None of the tested compounds displayed noteworthy cytostatic activity at 100–500 μM concentrations, whereas ADP analogue 5′- N-[(diethyl-phosphon) acetyl]adenosine (amide) and 5′-deoxy-5′- N-(phos-phon-acetyl)adenosine sodium salt (amide) showed a moderate inhibitory activity (EC50: 48 μM) against HSV-2 and a modest inhibitory activity (EC50: 110 μM) against HIV-1, respectively.

Surface modification of CoCr alloy using varying concentrations of phosphoric and phosphonoacetic acids: albumin and fibrinogen adsorption, platelet adhesion, activation, and aggregation studies

CoCr alloy is commonly used in various cardiovascular medical devices for its excellent physical and mechanical properties. However, the formation of blood clots on the alloy surfaces is a serious concern. This research is focused on the surface modification of CoCr alloy using varying concentrations (1, 25, 50, 75, and 100 mM) of phosphoric acid (PA) and phosphonoacetic acid (PAA) to generate various surfaces with different wettability, chemistry, and roughness. Then, the adsorption of blood plasma proteins such as albumin and fibrinogen and the adhesion, activation, and aggregation of platelets with the various surfaces generated were investigated. Contact angle analysis showed PA and PAA coatings on CoCr provided a gradient of hydrophilic surfaces. FTIR showed PA and PAA were covalently bound to CoCr surface and formed different bonding configurations depending on the concentrations of coating solutions used. AFM showed the formation of homogeneous PA and PAA coatings on CoCr. The single and dual protein adsorption studies showed that the amount of albumin and fibrinogen adsorbed on the alloy surfaces strongly depend on the type of PA and PAA coatings prepared by different concentrations of coating solutions. All PA coated CoCr showed reduced platelet adhesion and activation when compared to control CoCr. Also, 75 and 100 mM PA-CoCr showed reduced platelet aggregation. For PAA coated CoCr, no significant difference in platelet adhesion and activation was observed between PAA coated CoCr and control CoCr. Thus, this study demonstrated that CoCr can be surface modified using PA for potentially reducing the formation of blood clots and improving the blood compatibility of the alloy.

Expression, purification, crystallization and preliminary X-ray diffraction analysis of the aspartate transcarbamoylase domain of human CAD

Aspartate transcarbamoylase (ATCase) catalyzes the synthesis of N-carbamoyl-L-aspartate from carbamoyl phosphate and aspartate in the second step of the de novo biosynthesis of pyrimidines. In prokaryotes, the first three activities of the pathway, namely carbamoyl phosphate synthetase (CPSase), ATCase and dihydroorotase (DHOase), are encoded as distinct proteins that function independently or in noncovalent association. In animals, CPSase, ATCase and DHOase are part of a 243 kDa multifunctional polypeptide named CAD. Up-regulation of CAD is essential for normal and tumour cell proliferation. Although the structures of numerous prokaryotic ATCases have been determined, there is no structural information about any eukaryotic ATCase. In fact, the only detailed structural information about CAD is that it self-assembles into hexamers and trimers through interactions of the ATCase domains. Here, the expression, purification and crystallization of the ATCase domain of human CAD is reported. The recombinant protein, which was expressed in bacteria and purified with good yield, formed homotrimers in solution. Crystallization experiments both in the absence and in the presence of the inhibitor PALA yielded small crystals that diffracted X-rays to 2.1 Å resolution using synchrotron radiation. The crystals appeared to belong to the hexagonal space group P6(3)22, and Matthews coefficient calculation indicated the presence of one ATCase subunit per asymmetric unit, with a solvent content of 48%. However, analysis of the intensity statistics suggests a special case of the P21 lattice with pseudo-symmetry and possibly twinning.

Oxygen isotope signature of UV degradation of glyphosate and phosphonoacetate: tracing sources and cycling of phosphonates

The degradation of phosphonates in the natural environment constitutes a major route by which orthophosphate (Pi) is regenerated from organic phosphorus and recently implicated in marine methane production, with ramifications to environmental pollution issues and global climate change concerns. This work explores the application of stable oxygen isotope analysis in elucidating the CP bond cleavage mechanism(s) of phosphonates by UV photo-oxidation and for tracing their sources in the environment. The two model phosphonates used, glyphosate and phosphonoacetic acid were effectively degraded after exposure to UV irradiation. The isotope results indicate the involvement of both ambient water and atmospheric oxygen in the CP bond cleavage and generally consistent with previously posited mechanisms of UV-photon excitation reactions. A model developed to calculate the oxygen isotopic composition of the original phosphonate P-moiety, shows both synthetic phosphonates having distinctly lower values compared to naturally derived organophosphorus compounds. Such mechanistic models, based on O-isotope probing, are useful for tracing the sources and reactions of phosphonates in the environment.

Synthesis and in vitro evaluation of aspartate transcarbamoylase inhibitors

The design, synthesis, and evaluation of a series of novel inhibitors of aspartate transcarbamoylase (ATCase) are reported. Several submicromolar phosphorus-containing inhibitors are described, but all-carboxylate compounds are inactive. Compounds were synthesized to probe the postulated cyclic transition-state of the enzyme-catalyzed reaction. In addition, the associated role of the protonation state at the phosphorus acid moiety was evaluated using phosphinic and carboxylic acids. Although none of the synthesized inhibitors is more potent than N-phosphonacetyl-l-aspartate (PALA), the compounds provide useful mechanistic information, as well as the basis for the design of future inhibitors and/or prodrugs.

N-phosphonacetyl-L-isoasparagine a potent and specific inhibitor of Escherichia coli aspartate transcarbamoylase

The synthesis of a new inhibitor, N-phosphonacetyl-L-isoasparagine (PALI), of Escherichia coli aspartate transcarbamoylase (ATCase) is reported, as well as structural studies of the enzyme.PALI complex. PALI was synthesized in 7 steps from beta-benzyl L-aspartate. The KD of PALI was 2 microM. Kinetics and small-angle X-ray scattering experiments showed that PALI can induce the cooperative transition of ATCase from the T to the R state. The X-ray structure of the enzyme.PALI complex showed 22 hydrogen-bonding interactions between the enzyme and PALI. The kinetic characterization and crystal structure of the ATCase.PALI complex also provides detailed information regarding the importance of the alpha-carboxylate for the binding of the substrate aspartate.

Picosecond dynamics of T and R forms of aspartate transcarbamylase: A neutron scattering study

E. coli aspartate transcarbamylase (ATCase) is a 310 kDa allosteric enzyme which catalyses the first committed step in pyrimidine biosynthesis. The binding of its substrates, carbamylphosphate and aspartate, induces significant conformational changes. This enzyme shows homotropic cooperative interactions between the catalytic sites for the binding of aspartate. This property is explained by a quaternary structure transition from T state (aspartate low affinity) to R state (aspartate high affinity) accompanied by a 5% increase of radius of gyration of ATCase. The same quaternary structure change is observed upon binding of the bisubstrate analogue PALA (N-(phosphonacetyl)-L-aspartate. Owing to the large incoherent neutron scattering cross-section of the hydrogen atom and the abundance of this element in proteins, inelastic neutron scattering gives a global view of protein dynamics as sensed via the individual motions of its hydrogen atoms. We present neutron scattering results of the local dynamics (few angstroms), at short time (few tens of picoseconds), of ATCase in T and R forms. Compared to the T form, we observe an increased mobility of the protein in the R form that we associate to an increase of accessible surface area to the solvent. Beyond this specific result, this highlights the key role of the accessible surface area (ASA) in dynamic contribution to inelastic neutron data in the picosecond time scale. In particular, we want to stress out (i) that a difference at the picosecond time scale does not allow to conclude to a difference in the dynamics at a longer time scale and to address whether the T state is looser than the R state (ii) how challenging is, any comparison in terms of general dynamics (tense or relaxed) between dynamic values deduced from experimental neutron data on proteins with different sequences and therefore ASA. This caveat holds particularly when comparing dynamics of a mesophile with the corresponding extremophile.

Adenosine phosphonoacetic acid is slowly metabolized by NDP kinase

NDP kinase catalyzes the last step in the phosphorylation of nucleotides. It is also involved in the activation by cellular kinases of nucleoside analogs used in antiviral therapies. Adenosine phosphonoacetic acid, a close analog of ADP already proposed as an inhibitor of ribonucleotide reductase, was found to be a poor substrate for human NDP kinase, as well as a weak inhibitor with an equilibrium dissociation constant of 0.6 mM to be compared to 0.025 mM for ADP. The X-ray structure of a complex of adenosine phosphonoacetic acid and the NDP kinase from Dictyostelium was determined to 2.0 A resolution showing that the analog adopts a binding mode similar to ADP, but that no magnesium ion is present at the active site. As ACP may also interfere with other cellular kinases, its potential as a drug targeting NDP kinase or ribonucleotide reductase is likely to be limited due to strong side effects. The design of new molecules with a narrower specificity and a stronger affinity will benefit from the detailed knowledge of the complex ACP-NDP kinase.

Functional annotation and kinetic characterization of PhnO from Salmonella enterica

Phosphorus is an essential nutrient for all living organisms. Under conditions of inorganic phosphate starvation, genes from the Pho regulon are induced, allowing microorganisms to use phosphonates as a source of phosphorus. The phnO gene was previously annotated as a transcriptional regulator of unknown function due to sequence homology with members of the GCN5-related N-acyltransferase family (GNAT). PhnO can now be functionally annotated as an aminoalkylphosphonic acid N-acetyltransferase which is able to acetylate a range of aminoalkylphosphonic acids. Studies revealed that PhnO proceeds via an ordered, sequential kinetic mechanism with AcCoA binding first followed by aminoalkylphosphonate. Attack by the amine on the thioester of AcCoA generates the tetrahedral intermediate that collapses to generate the products. The enzyme also requires a divalent metal ion for activity, which is the first example of this requirement for a GNAT family member.

Aspartate transcarbamylase (ATCase) of Escherichia coli: a new crystalline R-state bound to PALA, or to product analogues citrate and phosphate

Structures of the R-state of Escherichia coli ATCase maintained with carbamyl phosphate and succinate, phosphonoacetamide and malonate, or N-phosphonacetyl-l-aspartate (PALA) have previously been made in the space group P321, in which the two independent r (regulatory) and two independent c (catalytic) chains are repeated by crystallographic symmetry to yield the holoenzyme c(6)r(6), ((c(3))(2)(r(2))(3)). The exploration of a new crystalline R-state P2(1)2(1)2(1) was undertaken to examine the c(3).c(3) expansion of 11 A in the T-to-R transition, and to further test whether intermolecular contacts influence the binding of PALA. The results show that the expansion along the 3-fold axis is 10 A, and that the binding modes of the six crystallographic independent PALA molecules are virtually identical to one another, and to modes described previously. As further test, the PALA, a bisubstrate analogue, was displaced by citrate and phosphate, where citrate is an analogue of product carbamylaspartate. The results support the conclusions about the binding of the three previously studied analogues, and further support, within about 0.5 A, the structure proposed for the transition state [Gouaux, J. E., Krause, K. L., and Lipscomb, W. N. (1987) Biochem. Biophys. Res. Commun. 142, 893-897; Jin, L., Stec, B., Lipscomb, W. N., and Kantrowitz, E. R. (1999) Proteins: Struct., Funct., Genet. 37, 729-742].

Tetrahedral Intermediates in Thiamin Diphosphate-Dependent Decarboxylations Exist as a 1‘,4‘-Imino Tautomeric Form of the Coenzyme, Unlike the Michaelis Complex or the Free Coenzyme

Two circular dichroism signals observed on thiamin diphosphate (ThDP)-dependent enzymes, a positive band in the 300-305 nm range and a negative one in the 320-330 nm range, were investigated on yeast pyruvate decarboxylase (YPDC) and on the E1 subunit of the Escherichia coli pyruvate dehydrogenase complex (PDHc-E1). Addition of the tetrahedral ThDP-acetaldehyde adduct, 2-alpha-hydroxyethylThDP, to PDHc-E1 generates the positive band at 300 nm, consistent with the formation of the 1',4'-iminopyrimidine tautomer, as also demonstrated for phosphonolactylthiamin diphosphate, a stable analogue of the tetrahedral ThDP-pyruvate adduct 2-alpha-lactylThDP (Jordan, F. et al. (2003) J. Am. Chem. Soc. 125, 12732-12738). Therefore, we suggest that all tetrahedral ThDP-bound covalent complexes will also prefer this tautomer, and that the 4'-aminopyrimidine of ThDP participates in multiple steps of acid-base catalysis on ThDP enzymes. Studies with YPDC and PDHc-E1, and their active center variants, in conjunction with chemical models, enabled assignment of the negative band at 330 nm to a charge-transfer transition between the 4'-aminopyrimidine tautomer (presumed electron donor) and the thiazolium ring (presumed electron acceptor) of ThDP, with no significant contributions from any amino acid side chain of the proteins. However, in both YPDC and PDHc-E1, the presence of substrate or substrate surrogate was required to enable detection, suggesting that the band at 320-330 nm be used as a reporter for the Michaelis complex, involving the amino tautomer, on both enzymes. As the positive band near 300 nm reports on the 1',4'-imino tautomer of ThDP, methods are now available for kinetic monitoring of both tautomeric forms.

Biochemical properties of phosphonoacetate and thiophosphonoacetate oligodeoxyribonucleotides

Phosphorus-modified phosphonoacetate and thiophosphonoacetate oligodeoxyribonucleotides were chemically synthesized and their biochemical properties evaluated. Under physiological pH, these DNA analogs possess negative charge and form stable, complementary A-like DNA:RNA heteroduplexes when analyzed via circular dichroism spectroscopy. Phosphonoacetate and thiophosphonoacetate oligomers were found to stimulate RNase H activity and to be completely resistant to degradation by snake venom phosphodiesterase, DNase I and HeLa cell nuclear extract. Further research has demonstrated that neutral, esterified forms of these analogs can be taken up by cells. Phosphonoacetate and thiophosphonoacetate oligomers therefore represent a new class of oligodeoxyribonucleotide analogs having phosphorus- carbon bonds with considerable potential for use in biological research.

Aspartate transcarbamylase from the hyperthermophilic archaeon Pyrococcus abyssi: thermostability and 1.8A resolution crystal structure of the catalytic subunit complexed with the bisubstrate analogue N-phosphonacetyl-L-aspartate

The Pyrococcus abyssi aspartate transcarbamylase (ATCase) shows a high degree of structural conservation with respect to the well-studied mesophilic Escherichia coli ATCase, including the association of catalytic and regulatory subunits. The adaptation of its catalytic function to high temperature was investigated, using enzyme purified from recombinant E.coli cells. At 90 degrees C, the activity of the trimeric catalytic subunit was shown to be intrinsically thermostable. Significant extrinsic stabilization by phosphate, a product of the reaction, was observed when the temperature was raised to 98 degrees C. Comparison with the holoenzyme showed that association with regulatory subunits further increases thermostability. To provide further insight into the mechanisms of its adaptation to high temperature, the crystal structure of the catalytic subunit liganded with the analogue N-phosphonacetyl-L-aspartate (PALA) was solved to 1.8A resolution and compared to that of the PALA-liganded catalytic subunit from E.coli. Interactions with PALA are strictly conserved. This, together with the similar activation energies calculated for the two proteins, suggests that the reaction mechanism of the P.abyssi catalytic subunit is similar to that of the E.coli subunit. Several structural elements potentially contributing to thermostability were identified: (i) a marked decrease in the number of thermolabile residues; (ii) an increased number of charged residues and a concomitant increase of salt links at the interface between the monomers, as well as the formation of an ion-pair network at the protein surface; (iii) the shortening of three loops and the shortening of the N and C termini. Other known thermostabilizing devices such as increased packing density or reduction of cavity volumes do not appear to contribute to the high thermostability of the P.abyssi enzyme.

Phosphine and methane generation by the addition of organic compounds containing carbon-phosphorus bonds into incubated soil

Formation of phosphine and methane in anaerobic incubation systems was investigated under stirred and unstirred conditions. The PH3 and CH4 levels in the headspace, as well as the matrix-bound PH3 content in the stirred soil, significantly increased upon the addition of phosphonoacetic acid (P(O)(OH)2CH2COOH). Both the levels of matrix-bound PH3 and CH4 are positively correlated to the buffered dithionite fraction of reactive phosphorus in the soil samples, while a negative correlation was observed between matrix-bound PH3/CH4 levels and the reactive phosphorus fraction.

Alternate modes of binding in two crystal structures of alkaline phosphatase-inhibitor complexes

Two high resolution crystal structures of Escherichia coli alkaline phosphatase (AP) in the presence of phosphonate inhibitors are reported. The phosphonate compounds, phosphonoacetic acid (PAA) and mercaptomethylphosphonic acid (MMP), bind competitively to AP with dissociation constants of 5.5 and 0.6 mM, respectively. The structures of the complexes of AP with PAA and MMP were refined at high resolution to crystallographic R-values of 19.0 and 17.5%, respectively. Refinement of the AP-inhibitor complexes was carried out using X-PLOR. The final round of refinement was done using SHELXL-97. Crystallographic analyses of the inhibitor complexes reveal different binding modes for the two phosphonate compounds. The significant difference in binding constants can be attributed to these alternative binding modes observed in the high resolution X-ray structures. The phosphinyl group of PAA coordinates to the active site zinc ions in a manner similar to the competitive inhibitor and product inorganic phosphate. In contrast, MMP binds with its phosphonate moiety directed toward solvent. Both enzyme-inhibitor complexes exhibit close contacts, one of which has the chemical and geometrical potential to be considered an unconventional hydrogen bond of the type C-H...X.

Insights into the mechanisms of catalysis and heterotropic regulation of Escherichia coli aspartate transcarbamoylase based upon a structure of the enzyme complexed with the bisubstrate analogue N-phosphonacetyl-L-aspartate at 2.1 A

A high-resolution structure of Escherichia coli aspartate transcarbamoylase has been determined to 2.1 A; resolution in the presence of the bisubstrate analog N-phosphonacetyl-L-aspartate (PALA). The structure was refined to a free R-factor of 23.4% and a working R-factor of 20.3%. The PALA molecule is completely saturated with interactions to side chain and backbone groups in the active site, including two interactions that are contributed from the 80s loop of the adjacent catalytic chain. The charge neutralization of the bound PALA molecule (and presumably the substrates as well) induced by the electrostatic field of the highly positively charged active site is an important factor in the high binding affinity of PALA and must be important for catalysis. The higher-resolution structure reported here departs in a number of ways from the previously determined structure at lower resolution. These modifications include alterations in the backbone conformation of the C-terminal of the catalytic chains, the N- and C-termini of the regulatory chains, and two loops of the regulatory chain. The high-resolution of this structure has allowed a more detailed description of the binding of PALA to the active site of the enzyme and has allowed a detailed model of the tetrahedral intermediate to be constructed. This model becomes the basis of a description of the catalytic mechanism of the transcarbamoylase reaction. The R-structural state of the enzyme-PALA complex is an excellent representation of the form of the enzyme that occurs at the moment in the catalytic cycle when the tetrahedral intermediate is formed. Finally, improved electron density in the N-terminal region of the regulatory chain (residues 1 to 7) has allowed tracing of the entire regulatory chain. The N-terminal segments of the R1 and R6 chains are located in close proximity to each other and to the regulatory site. This portion of the molecule may be involved in the observed asymmetry between the regulatory binding sites as well as in the heterotropic response of the enzyme.

Assessment of the allosteric mechanism of aspartate transcarbamoylase based on the crystalline structure of the unregulated catalytic subunit

The lack of knowledge of the three-dimensional structure of the trimeric, catalytic (C) subunit of aspartate transcarbamoylase (ATCase) has impeded understanding of the allosteric regulation of this enzyme and left unresolved the mechanism by which the active, unregulated C trimers are inactivated on incorporation into the unliganded (taut or T state) holoenzyme. Surprisingly, the isolated C trimer, based on the 1.9-A crystal structure reported here, resembles more closely the trimers in the T state enzyme than in the holoenzyme:bisubstrate-analog complex, which has been considered as the active, relaxed (R) state enzyme. Unlike the C trimer in either the T state or bisubstrate-analog-bound holoenzyme, the isolated C trimer lacks 3-fold symmetry, and the active sites are partially disordered. The flexibility of the C trimer, contrasted to the highly constrained T state ATCase, suggests that regulation of the holoenzyme involves modulating the potential for conformational changes essential for catalysis. Large differences in structure between the active C trimer and the holoenzyme:bisubstrate-analog complex call into question the view that this complex represents the activated R state of ATCase.

Structural modeling and electrostatic properties of aspartate transcarbamylase from Saccharomyces cerevisiae

In Saccharomyces cerevisiae the first two reactions of the pyrimidine pathway are catalyzed by a multifunctional protein which possesses carbamylphosphate synthetase and aspartate transcarbamylase activities. Genetic and proteolysis studies suggested that the ATCase activity is carried out by an independently folded domain. In order to provide structural information for ongoing mutagenesis studies, a model of the three-dimensional structure of this domain was generated on the basis of the known X-ray structure of the related catalytic subunit from E. coli ATCase. First, a model of the catalytic monomer was built and refined by energy minimization. In this structure, the conserved residues between the two proteins were found to constitute the hydrophobic core whereas almost all the mutated residues are located at the surface. Then, a trimeric structure was generated in order to build the active site as it lies at the interface between adjacent chains in the E. coli catalytic trimer. After docking a bisubstrate analog into the active site, the whole structure was energy minimized to regularize the interactions at the contact areas between subunits. The resulting model is very similar to that obtained for the E. coli catalytic trimer by X-ray crystallography, with a remarkable conservation of the structure of the active site and its vicinity. Most of the interdomain and intersubunit interactions that are essential for the stability of the E. coli catalytic trimer are maintained in the yeast enzyme even though there is only 42% identity between the two sequences. Free energy calculations indicate that the trimeric assembly is more stable than the monomeric form. Moreover an insertion of four amino acids is localized in a loop which, in E. coli ATCase, is at the surface of the protein. This insertion exposes hydrophobic residues to the solvent. Interestingly, such an insertion is present in all the eukaryotic ATCase genes sequences so far, suggesting that this region is interacting with another domain of the multifunctional protein.

Polymeric calcium phosphate cements: setting reaction modifiers

In this study, the effects of several additives on the setting behavior and mechanical properties of polymeric calcium phosphate cements were investigated. The cements were derived from a polycarboxylic acid (PCA) and a calcium phosphate cement (CPC) powder that consisted of equimolar amounts of tetracalcium phosphate (TTCP) and dicalcium phosphate (DCPA). Retardation of the setting reaction in the PCA-CPC cements was observed by adding tribasic sodium phosphate and fluorides such as stannous fluoride, zirconium(IV) fluoride and titanium(IV) fluoride. It was found that increasing the concentration of these additives decreased the mechanical strength of the cements. However, improvements in both setting and mechanical properties for the PCA-CPC cements were observed by the combined use of 8% (w/w) stannous fluoride and 10% (w/w) tartaric acid. The mechanical properties of the PCA-CPC cement also were improved by adding calcium acetate, calcium methacrylate, zirconium(IV) sulfate and phosphonoacetic acid.

"Carba" peptide bond surrogates. Different approaches to Gly-psi(CH2-CH2)-D,L-Xaa pseudo-dipeptide units

Racemic "carba" pseudo-dipeptide units such as Gly-psi(CH2-CH2)-D,L-Xaa were obtained either through the Horner-Emmons condensation of N-tert.-butyloxycarbonyl-beta alaninal with the appropriate substituted triethyl phosphonacetate, or from commercially available 3-carbethoxy-2-piperidone.

Quantitation of the Na(+)-Pi cotransporter in renal cortical brush border membranes. [14C]phosphonoformic acid as a useful probe to determine the density and its change in response to parathyroid hormone

To determine the density of Na(+)-Pi symporters in brush border membranes (BBM) from rat renal cortex, [14C] phosphonoformic acid [( 14C] PFA), a competitive inhibitor of Na(+)-Pi cotransport, was employed as a probe. The [14C]PFA binding was measured in BBM vesicles (BBMV) under equilibrated conditions (extra-vesicular Na+, K+, and H+ = intravesicular Na+, K+, and H+) to avoid modulatory effects of these solutes. BBMV were preincubated in media without or with addition of molar excess of Pi (greater than 20 times) to determine the Pi-protectable PFA-binding sites, and then [14C] PFA binding was determined. Only the [14C]PFA binding in the presence of Na+ displaceable by an excess of Pi was saturated and was independent of intravesicular volume of BBMV. This value denoted as "Pi-protectable Na(+)-[14C]PFA binding," was analyzed by Scatchard plot showing BmaxPFA = 375 +/- 129 pmol of PFA/mg protein, KDPFA = 158 +/- 18 microM; the Hill coefficient was congruent to 1. For Na(+)-dependent binding of [3H]phlorizin, in the same BBMV, Bmax = 310 +/- 37 pmol/mg protein and KD V 2.2 +/- 0.5 microM. BBMV prepared from cortex of thyroparathyroidectomized rats infused with phosphaturic doses of parathyroid hormone (PTH) were compared with vehicle-infused controls. Administration of PTH resulted in decrease of BmaxPFA (-38%) and of Na(+)-gradient-dependent uptake of 32Pi (-35%), but KDPFA was not changed. Neither BmaxPhl and KDPhl for Na(+)-phlorizin binding, nor the Na(+)-gradient-dependent uptake of [3H]D-glucose differed between PTH-treated and control rats. We conclude: (a) measurement of Pi-protectable Na(+)-[14C]PFA binding determines numbers and affinity of Na(+)-Pi symporters in renal BBMV; (b) the affinity of PFA for Na(+)-Pi symporter is similar to apparent affinity for Pi (KmPi), as determined from measurements of Na(+)-gradient-dependent 32Pi uptake by BBMV; (c) both Na(+)-Pi symporter and [Na+]D-glucose symporters are present within renal BBM in a similar range of density; (d) PTH decreases the number of Na(+)-Pi cotransporters in BBMV commensurate with the parallel decrease of Na(+)-gradient-dependent Pi transport, whereas the affinity of Na(+)-Pi symporters for Pi is not changed. These observations support the hypothesis that PTH decreases capacity for Na(+)-dependent Pi reabsorption by internalization of Na(+)-Pi symporters in BBM of renal proximal tubules.