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PubMed2001

Fructose 2,6-bisphosphate activates pyrophosphate: fructose-6-phosphate 1-phosphotransferase and increases triose phosphate to hexose phosphate cycling in heterotrophic cells.

The aim of this work was to establish the influence of fructose 2,6-bisphosphate (Fru-2,6-P2) on non-photosynthetic carbohydrate metabolism in plants. Heterotrophic callus lines exhibiting elevated levels of Fru-2,6-P2 were generated from transgenic tobacco (Nicotiana tabacum L.) plants expressing a modified rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. Lines containing increased amounts of Fru-2,6-P2 had lower levels of hexose phosphates and higher levels of 3-phosphoglycerate than the untransformed control cultures. There was also a greater redistribution of label into the C6 position of sucrose and fructose, following incubation with [1-13C]glucose, in the lines possessing the highest amounts of Fru-2,6-P2, indicating a greater re-synthesis of hexose phosphates from triose phosphates in these lines. Despite these changes, there were no marked differences between lines in the metabolism of 14C-substrates, the rate of oxygen uptake, carbohydrate accumulation or nucleotide pool sizes. These data provide direct evidence that physiologically relevant changes in the level of Fru-2,6-P2 can affect pyrophosphate: fructose-6-phosphate 1-phosphotransferase (PFP) activity in vivo, and are consistent with PFP operating in a net glycolytic direction in the heterotrophic culture. However, the results also show that activating PFP has little direct effect on heterotrophic carbohydrate metabolism beyond increasing the rate of cycling between hexose phosphates and triose phosphates.

Animals↗
PubMed1978

The influence of protonation or alkylation of the phosphate group on the e.s.r. spectra and on the rate of phosphate elimination from 2-methoxyethyl phosphate 2-yl radicals.

The e.s.r. spectra of 1-yl, 2-yl and 3'-yl methoxyethyl phosphate radicals derived from CH3OCH2CH2-OPO3H2 by hydrogen abstraction have been measured in aqueous solutions and the hyperfine constants determined. The coupling constants vary strongly with protonation or alkylation of the phosphate group. The 2-yl radicals eliminate phosphate. The rate-constants for the elimination (ke) have been estimated by e.s.r. measurements and by product studies as a function of pH using 60Co gamma-radiolysis. The ke values vary from approximately 0.3 s(-1) for the CH3OCHCH2OPO3--radical and approximately 10(3) s-1 for CH3OCHCH2OPO3H-, to approximately 3 X 10(6) S-1 for CH3OCHCH2OPO3H2. Alkylation of the phosphate group increases the elimination rate-constant to a similar extent as protonation. The results support a recent mechanism which described the OH-radical-induced single-strand breaks of DNA in aqueous solution starting from the C-4' radical of the sugar moiety. It is further concluded the C-4' radical of DNA eliminates the 3'-phosphate group faster than the 5'-phosphate group.

Alkylation↗
PubMed1974

Seven-membered cyclic phosphate: synthesis and properties of S-cycloadenosine 2',5'-cyclic phosphate and cordycepin 2',5'-cyclic phosphate. Studies of nucleosides and nucleotides. LXIV. 1). Purine cyclonucleosides 25.

8,3'-Anhydro-8-mercapto-9-beta-D-xylofuranosyladenine (8,3'-s-cycloadenosine) was phosphorylated with cyanoethyl phosphate and DCC to 5'-phosphate. After 6-amino group was benzoylated, the monophosphate was treated with DCC to give a cyclic phosphate (II). The structure of compound II was elucidated as 8,3'-s-cycloadenosine 2',5'-cyclic phosphate by UV, NMR and CD spectra, as well as enzymatic hydrolyses. When compound II was desulfurized with Raney nickel, cordycepin 2',5'-cyclic phosphate (III) was obtained. Although compound III could be obtained from cordycepin 5'-phosphate with DCC, the yield was extremely low.

Benzyl Compounds↗
PubMed1984

Phosphate inhibition of spinach leaf sucrose phosphate synthase as affected by glucose-6-phosphate and phosphoglucoisomerase.

The inhibition patterns of inorganic phosphate (Pi) on sucrose phosphate synthase activity in the presence and absence of the allosteric activator glucose-6-P was studied, as well as the effects of phosphoglucoisomerase on fructose-6-P saturation kinetics with and without Pi. In the presence of 5 millimolar glucose-6-P, Pi was a partial competitive inhibitor with respect to both substrates, fructose-6-P and uridine diphosphate glucose. In the absence of glucose-6-P, the inhibition patterns were more complex, apparently because of the interaction of Pi at the activation site as well as the catalytic site. In addition, substrate activation by uridine diphosphate glucose was observed in the absence of effectors. The results suggested that Pi antagonizes glucose-6-P activation of sucrose phosphate synthase by competing with the activator for binding to the modifier site.The fructose-6-P saturation kinetics were hyperbolic in the absence of phosphoglucoisomerase activity, but became sigmoidal by the addition of excess phosphoglucoisomerase. The sigmoidicity persisted in the presence of Pi, but sucrose phosphate synthase activity was decreased. The apparent sigmoidal response may represent the physiological response of sucrose phosphate synthase to a change in hexose-P concentration because sucrose phosphate synthase operates in the cytosol in the presence of high activities of phosphoglucoisomerase. Thus, the enzymic production of an activator from a substrate represents a unique mechanism for generating sigmoidal enzyme kinetics.

Journal Article↗
PubMed1990

Binding of nucleoside triphosphates, inorganic phosphate, and other polyanionic ligands to the N-terminal region of rat brain hexokinase: relationship to regulation of hexokinase activity by antagonistic interactions between glucose 6-phosphate and inorganic phosphate.

Mg2(+)-chelates of several nucleoside triphosphates were shown to increase the inactivation of rat brain hexokinase (ATP:D-hexose-6-phosphotransferase, EC 2.7.1.1) by 0.6 M guanidine hydrochloride, with ATP-Mg2+ having the greatest effect; unchelated forms did not significantly affect inactivation. Since catalytic activity has been associated with the C-terminal half of the molecule, these results were interpreted as indicating a destabilization of this C-terminal region by binding of these chelates to the substrate nucleotide sites, with the particular effectiveness of ATP-Mg2+ reflecting the specificity for this species as a phosphoryl donor. These compounds were also shown to bind to the N-terminal half of the enzyme, as judged by their ability to protect against denaturation by guanidine hydrochloride and subsequent digestion with trypsin. Both free and Mg2(+)-chelated forms afforded protection, with the unchelated nucleotides being most effective; a preference for ATP was seen only with the chelated forms. Thus, it was concluded that the N-terminal half of hexokinase contains a relatively nonspecific nucleotide binding site, distinct from the substrate nucleotide site previously shown to reside in the C-terminal half. On the basis of this same ability to protect the N-terminal half against denaturation and proteolysis, several other polyanionic ligands were shown to bind to this region of the molecule. These included inorganic phosphate, its analogs, sulfate and arsenate, and its homologs, pyrophosphate and tripolyphosphate. All of these anionic ligands were also shown to antagonize inhibition by the glucose 6-phosphate (Glc-6-P) analog, 1,5-anhydroglucitol 6-phosphate. The allosteric site for binding of Glc-6-P has previously been shown to reside in the N-terminal half of the molecule, and it is suggested that the antagonism of inhibition by Glc-6-P (or its analog) by these anionic ligands results from interaction with an anion binding site for which the 6-phosphate group of inhibitory hexose 6-phosphates must compete. A model depicting possible relationships between ligand binding sites on brain hexokinase, and how their interactions might lead to observed regulatory properties, is developed based on these and previous studies of ligand binding as well as evidence that mammalian hexokinases (Mr 100,000) have evolved by duplication and fusion of a gene coding for an ancestral hexokinase with Mr 50,000 and which, like the mammalian enzyme, was sensitive to inhibition by Glc-6-P.

Animals↗
PubMed2006

Novel highly biodegradable biphasic tricalcium phosphates composed of alpha-tricalcium phosphate and beta-tricalcium phosphate.

Novel biodegradable biphasic tricalcium phosphates (BTCP) composed of alpha-tricalcium phosphate (alpha-TCP) and beta-tricalcium phosphate (beta-TCP) were successfully synthesized by heating amorphous calcium phosphate precursors with different structures at 800 degrees C for 3 h. The ratio of alpha-TCP and beta-TCP in the calcium phosphate particle can be controlled by aging time and pH value during synthesis of the amorphous precursor.

Biocompatible Materials↗
PubMed1969

The active chemical state of D-glyceraldehyde 3-phosphate in its reactions with D-glyceraldehyde 3-phosphate dehydrogenase, aldolase and triose phosphate isomerase.

Glyceraldehyde 3-phosphate exists as the geminal diol and the free aldehyde in the molar ratio 29:1 in aqueous solution. The rate constant of the conversion of diol into aldehyde is 8.7x10(-2)sec.(-1) in the pH range 7.3-8.6 at 20 degrees . The free aldehyde is the substrate for d-glyceraldehyde 3-phosphate dehydrogenase. Over a wide concentration range of enzyme the rate of conversion of diol into aldehyde is the rate-limiting process in the catalytic oxidation of d-glyceraldehyde 3-phosphate by NAD(+). Aldolase and triose phosphate isomerase both liberate d-glyceraldehyde 3-phosphate as the aldehyde. This suggests that the relatively slow diol-aldehyde interconversion does not restrict the rate of glycolysis.

Chemical Phenomena↗
PubMed1984

Hexose phosphate binding sites of fructose-6-phosphate,2-kinase:fructose-2,6-bisphosphatase. Interaction with N-bromoacetylethanolamine phosphate and 3-bromo-1,4-dihydroxy-2-butanone 1,4-bisphosphate.

N-Bromoacetylethanolamine phosphate and 3-bromo-1,4-dihydroxy-2-butanone 1,4-bisphosphate have been tested in order to study the hexose phosphate binding sites of a bifunctional enzyme, fructose-6-P,2-kinase:fructose-2,6-bisphosphatase. N-Bromoacetylethanolamine phosphate is a competitive inhibitor with respect to fructose-6-P (Ki = 0.24 mM) and a noncompetitive inhibitor with ATP (Ki = 0.8 mM). The reagent inactivates fructose-6-P,2-kinase but not fructose-2,6-bisphosphatase, and the inactivation is prevented by fructose-6-P. The inactivation reaction follows pseudo first-order kinetics to completion and with increasing concentrations of N-bromoacetylethanolamine phosphate a rate saturation effect is observed. The concentration of the reagent giving the half-maximum inactivation is 2.2 mM and the apparent first order rate constant is 0.0046 s-1. The enzyme alkylated by N-bromoacetylethanolamine-P has lost over 90% of the kinase activity, retains nearly full activity of fructose-2,6-bisphosphatase, and its inhibition by fructose-6-P is not altered. 3-Bromo-1,4-dihydroxy-2-butanone 1,4-bisphosphate is also a competitive inhibitor of fructose-6-P,2-kinase with respect to fructose-6-P in the forward reaction and fructose-2,6-P2 in the reverse direction. This reagent inhibits 93% of fructose-6-P,2-kinase but activates fructose-2,6-bisphosphatase 3.7-fold. 3-Bromo-1,4-dihydroxy-2-butanone 1,4-bisphosphate alters the fructose-2,6-P2 saturation kinetic curve from negative cooperativity to normal Michaelis-Menten kinetics with K0.5 of 0.8 microM. The reagent, however, has no effect on the fructose-6-P inhibition of the phosphatase. These results strongly suggest that hexose phosphate binding sites of fructose-6-P,2-kinase and fructose-2,6-bisphosphatase are distinct and located in different regions of this bifunctional enzyme.

Adenosine Triphosphate↗
PubMed1983

Inorganic phosphate determination in the presence of a labile organic phosphate: assay for carbamyl phosphate phosphatase activity.

An assay was developed for carbamyl phosphate phosphatase (CAP phosphatase) activity. This enzyme activity is difficult to assay because of the chemical instability of carbamyl phosphate (CAP). We chose to measure CAP phosphatase activity by monitoring release of inorganic phosphate (Pi). This entailed preparing CAP with low endogeneous Pi, constructing enzyme incubation and deproteination conditions for minimal chemical hydrolysis of CAP and employing a Pi assay designed for use with labile organic phosphates. With Ehrlich ascites tumor cell homogenate, CAP phosphatase assay was linear with time and increments of protein. In developing this procedure, the most critical aspect was the definition of reliable conditions for measuring Pi. The Pi assay used is a modification of the method of Baginski, Foa, and Zak [Clin. Chim. Acta 15, 155-158 (1967)] which is insensitive to hydrolysis of organophosphates after the last reagent is added. Color development in the Pi assay is significantly inhibited by cold and by greater than 0.3 mmol of trichloroacetic acid or greater than 1.5 mumol of CAP. However, under conditions utilized for measuring CAP phosphatase activity there is no interference. Absorbance in the Pi assay is stable and linear from 5 to 300 nmol of Pi. The assay is also suitable for measurement of ATPase activity and Pi determinations in general.

Acid Anhydride Hydrolases↗
PubMed1984

Experimental synovitis induced by aluminium phosphate in rabbits. Comparison of the changes produced in synovial tissue and in articular cartilage by aluminium phosphate, carrageenin, calcium hydrogen phosphate dihydrate, and natural diamond powder.

The goal of this experimental study was to examine the effect on articular tissue of tribasic aluminium phosphate (crystalline and amorphous forms) after intraarticular injection in rabbit and to compare it with that of various phlogistic compounds such as carrageenin, calcium hydrogen phosphate dihydrate and diamond powder, as a control. Synovium and cartilage were studied with light microscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive micro-analysis (EDM). Crystalline and amorphous aluminium phosphate could induce a synovitis with articular effusion in rabbits. With TEM, lysosomal inclusions of phagocytosed material were observed. Through SEM coupled with EDM, aluminium associated with phosphate was found in cellular elements.

Aluminum Compounds↗
PubMed1999

Reaction of calcium phosphate cements with different amounts of tetracalcium phosphate and dicalcium phosphate anhydrous.

Calcium phosphate cements (CPCs) with different amounts of tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA) (TTCP/DCPA molar ratio from 0.25 to 2.00) were prepared to further understand the setting reaction and the factors that could influence the properties of CPCs. Quantitative X-ray diffraction patterns, Fourier transform IR spectra, and diametral tensile strength of the set mass were measured along with pH measurements of the CPC suspension. Calcium-deficient hydroxyapatite (d-HAP) with a calcium to phosphate molar ratio of approximately 1.5 was formed initially in the CPC setting consisting of an equimolar mixture of TTCP and DCPA. This gradually transformed into stoichiometric HA (s-HA) with increasing incubation time. The s-HA was formed in the initial stage when the CPC contained an excess amount of TTCP. In contrast, maturation to s-HAP was slow when the CPC contained excess amounts of DCPA. The highest mechanical strength of set CPC was associated with an equimolar mixture of TTCP and DCPA, and the mechanical strength decreased as the TTCP/DCPA molar ratio deviated from 1.00. We concluded, therefore, that the setting reaction and the nature of the resulting set mass are dependent on the molar ratios of TTCP and DCPA.

Bone Cements↗
PubMed1989

Calcium phosphate cements: study of the beta-tricalcium phosphate--monocalcium phosphate system.

The possibility of making cements based on beta-tricalcium phosphate (beta-TCP), a promising bone graft material, was investigated. Upon admixture with water, beta-TCP/monocalcium phosphate monohydrate (MCPM) mixtures were found to set and harden like conventional hydraulic cements. Beta-TCP powders with larger particle size, obtained by sintering at higher temperatures, increased the ultimate strength of the cement. Results show that setting occurs after dissolution of MCPM, as a result of the precipitation of dicalcium phosphate dihydrate (DCPD) in the paste. The ultimate tensile strength of the hardened cement is proportional to the amount of DCPD formed. Upon ageing above 40 degrees C, DCPD transforms progressively into anhydrous dicalcium phosphate (DCP), thereby decreasing the strength. Ageing of the pastes in 100% r.h. results in a decay of the mechanical properties. This can be ascribed to an intergranular dissolution of the beta-TCP aggregates as a result of the pH lowering brought about by the MCPM to DCPD conversion.

Biocompatible Materials↗
PubMed1991

Calcium phosphate cements: effect of fluorides on the setting and hardening of beta-tricalcium phosphate-dicalcium phosphate-calcite cements.

Increasing amounts of fluoride ions have been found to increase the setting and hardening rates of beta-tricalcium phosphate-dicalcium phosphate dihydrate-calcite cements. Thus, fluoride-containing cements reached a diametral strength of about 1.5 MPa after 15 d, but the fluoride-free reference sample reached only 0.45 MPa. The acceleration of setting and hardening is correlated to an increased rate of hydroxyapatite formation in the cement at the expense of dicalcium phosphate dihydrate and calcite. Adding monocalcium phosphate monohydrate and fluoride to the blends resulted in a marked decrease of their setting time from about 1 h down to 8 min, without greatly affecting their final strength.

Apatites↗
PubMed1997

In vivo study of a calcium phosphate cement consisting of alpha-tricalcium phosphate/dicalcium phosphate dibasic/tetracalcium phosphate monoxide.

Prehardened calcium phosphate cement consisting of alpha-tricalcium phosphate (alpha-TCP), dicalcium phosphate dibasic (DCPD) and tetracalcium phosphate monoxide (TeCP) was implanted in rabbit mandibles and back muscles, and studied histologically and microradiographically. In the mandibles, new bone formation occurred around the implants and increased in quantity the longer the implant period lasted. Histology, microradiography and scanning electron microscopy (SEM) demonstrated direct contact of bone and cement. Bone response to this cement was essentially the same as to hydroxyapatite (HA) ceramics, known as a biocompatible bone substitute. Material resorption was recognized, which increased with the implant period and was greater in the surface bound by soft tissue than the surface bound by bone tissue. In the back muscles, however, no calcified tissue formation occurred. Resorption proved to be faster than in the case of the mandible implants. It was concluded that the cement, in prehardened form, has good biocompatibility and is a promising material as a bone substitute.

Animals↗