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At least 19 recordsLinked to original sources
PubMed1983

Regulation of Spinach Leaf Sucrose Phosphate Synthase by Glucose-6-Phosphate, Inorganic Phosphate, and pH.

Sucrose phosphate synthase was partially purified from spinach leaves and the effects and interactions among glucose-6-P, inorganic phosphate (Pi), and pH were investigated. Glucose-6-P activated sucrose phosphate synthase and the concentration required for 50% of maximal activation increased as the concentration of fructose-6-P was decreased. Inorganic phosphate inhibited sucrose phosphate synthase activity and antagonized the activation by glucose-6-P. Inorganic phosphate caused a progressive increase in the concentration of glucose-6-P required for 50% maximal activation from 0.85 mm (minus Pi) to 9.9 mm (20 mm Pi). In the absence of glucose-6-P, Pi caused partial inhibition of sucrose phosphate synthase activity (about 65%). The concentration of Pi required for 50% maximal inhibition decreased with a change in pH from 6.5 to 7.5. When the effect of pH on Pi ionization was taken into account, it was found that per cent inhibition increased hyperbolically with increasing dibasic phosphate concentration independent of the pH. Sucrose phosphate synthase had a relatively broad pH optimum centered at pH 7.5. Inhibition by Pi was absent at pH 5.5, but became more pronounced at alkaline pH, whereas activation by glucose-6-P was observed over the entire pH range tested. The results suggested that glucose-6-P and Pi bind to sites distinct from the catalytic site, e.g. allosteric sites, and that the interactions of these effectors with pH and concentrations of substrate may be involved in the regulation of sucrose synthesis in vivo.

Journal Article↗
PubMed1986

Changes in creatine phosphate, inorganic phosphate, and the purine pattern in dog hearts with time of coronary artery occlusion and effect thereon of mioflazine, a nucleoside transport inhibitor.

A detailed analysis was made of the changes in canine myocardium, with time of occlusion, in several important metabolites such as creatine phosphate and adenosine triphosphate (luminometry), inorganic phosphate (spectrophotometry), and most of the purines and nicotinamide adenine dinucleotide (high performance liquid chromatography). Within 1 min there was a significant reduction in creatine phosphate and a significant increase in inorganic phosphate, adenosine diphosphate, and adenosine monophosphate. A decrease in adenosine triphosphate became apparent after 4 min, concomitant with a progressive rise in the nucleosides, which reached almost 50% of the total purines after 64 min of occlusion. The formation of hypoxanthine was detectable in 50% only of all animals, suggesting a lack of active nucleoside phosphorylase in the others. Nicotinamide adenine dinucleotide, although decreasing slightly, was by far the most constant of all variables measured during at least 30 min of ischaemia. Therefore, this component is suggested to be a useful internal standard, thus minimising analytical and biological variations. Mioflazine, a potent nucleoside transport inhibitor (I50 3 X 10(-8) mol X litre-1), when given orally at 2.5 mg X kg-1, did not affect any of the changes with the exception of the nucleosides, where the drug completely inverts the adenosine to inosine ratio. The contribution of adenosine to the total nucleosides changed from 20% in the controls to 80% with treatment during at least 16 min of occlusion, there being no overlap between the groups. It is concluded therefore that adenosine is not deaminated in the cell where it is produced. It is not yet clear how this notable effect of mioflazine could be linked to its remarkable protective effect against ischaemia.

Animals↗
PubMed1981

Divalent metal ion, inorganic phosphate, and inorganic phosphate analogue binding to yeast inorganic pyrophosphatase.

Four different techniques, equilibrium dialysis, protection of enzymatic activity against chemical inactivation, 31P relaxation rats, and water proton relaxation rates, are used to study divalent metal ion, inorganic phosphate, and inorganic phosphate analogue binding to yeast inorganic pyrophosphatase, EC 3.6.1.1. A major new finding is that the binding of a third divalent metal ion per subunit, which has elsewhere been implicated as being necessary for enzymatic activity [Springs, B., Welsh, K. M., & Cooperman, B. S. (1981) Biochemistry (in press)], only becomes evident in the presence of added inorganic phosphate and that, reciprocally, inorganic phosphate binding to both its high- and low-affinity sites on the enzyme is markedly enhanced in the presence of divalent metal ions, with Mn2+ causing an especially large increase in affinity. The results obtained allow evaluation of all of the relevant equilibrium constants for the binding of Mn2+ and inorganic phosphate or of Co2+ and inorganic phosphate to the enzyme and show that the high-affinity site has greater specificity for inorganic phosphate than the low-affinity site. In addition, they provide. The results obtained allow evaluation of all of the relevant equilibrium constants for the binding of Mn2+ and inorganic phosphate or of Co2+ and inorganic phosphate to the enzyme and show that the high-affinity site has greater specificity for inorganic phosphate than the low-affinity site. In addition, they provide. The results obtained allow evaluation of all of the relevant equilibrium constants for the binding of Mn2+ and inorganic phosphate or of Co2+ and inorganic phosphate to the enzyme and show that the high-affinity site has greater specificity for inorganic phosphate than the low-affinity site. In addition, they provide evidence against divalent metal ion inner sphere binding to phosphate for enzyme subunits having one or two divalent metal ions bound per subunit and evidence for a conformational change restricting active-site accessibility to solvent on the binding of a third divalent metal ion per subunit.

Binding Sites↗
PubMed2001

Toxicological review of inorganic phosphates.

Inorganic phosphate salts are widely used as food ingredients and in a variety of commercial applications. The United States Food and Drug Administration (FDA) considers inorganic phosphates "Generally Recognized As Safe" (GRAS) (FDA, 1973a, 1979) [FDA: Food and Drug Administration 1973a. GRAS (Generally Recognized as Safe) food ingredients-phosphates. NTIS PB-221-224, FDA, Food and Drug Administration, 1979. Phosphates; Proposed Affirmation of and Deletion From GRAS Status as Direct and Human Food Ingredients. Federal Register 44 (244). 74845-74857, 18 December (1979)] and the European Union (EU) allows inorganic phosphates to be added directly to food (EU Directive 95/2/EC as amended by 98/72/EC). In this review, data on the acute, subchronic and chronic toxicity, genotoxicity, teratogenicity and reproductive toxicity from the published literature and from unpublished studies by the manufacturers are reviewed. Based on the toxicity data and similar chemistry, the inorganic phosphates can be separated into four major classes, consisting of monovalent salts, divalent salts, ammonium salts and aluminum salts. The proposed classification scheme supports the use of toxicity data from one compound to assess the toxicity of another compound in the same class. However, in the case of eye and skin irritation, the proposed classification scheme cannot be used because a wide range of responses exists within each class. Therefore, the eye and skin hazards associated with an individual inorganic phosphate should be assessed on a chemical-by-chemical basis. A large amount of toxicity data exists for all four classes of inorganic phosphates. The large and comprehensive database allows an accurate assessment of the toxicity of each class of inorganic phosphate. Overall, all four classes of inorganic phosphates exhibit low oral, inhalation and dermal toxicities. Based on these data, humans are unlikely to experience adverse effects when the daily phosphorus consumption remains below 70 mg/kg/day (JECFA, 1964, 1982a) [JECFA (Joint FAO/WHO Expert Committee on Food Additives 1964. Specifications for the Identity and Purity of Food Additives and their Toxicological Evaluation) Emulsifiers, Stabilizers, Bleaching, and Maturing Agents. Technical Report Series of the World Health Organization 281; ECFA (Joint FAO/WHO Expert Committee on Food Additives 1982a. Phosphoric Acid and Phosphate Salts. ICS/FA/82)].

Animals↗
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↗
PubMed1984

Allosteric interactions of glycogen phosphorylase b. A crystallographic study of glucose 6-phosphate and inorganic phosphate binding to di-imidate-cross-linked phosphorylase b.

The binding to glycogen phosphorylase b of glucose 6-phosphate and inorganic phosphate (respectively allosteric inhibitor and substrate/activator of the enzyme) were studied in the crystal at 0.3 nm (3A) resolution. Glucose 6-phosphate binds in the alpha-configuration at a site that is close to the AMP allosteric effector site at the subunit-subunit interface and promotes several conformational changes. The phosphate-binding site of the enzyme for glucose 6-phosphate involves contacts to two cationic residues, Arg-309 and Lys-247. This site is also occupied in the inorganic-phosphate-binding studies and is therefore identified as a high-affinity phosphate-binding site. It is distinct from the weaker phosphate-binding site of the enzyme for AMP, which is 0.27 nm (2.7A) away. The glucose moiety of glucose 6-phosphate and the adenosine moiety of AMP do not overlap. The results provide a structural explanation for the kinetic observations that glucose 6-phosphate inhibition of AMP activation of phosphorylase b is partially competitive and highly co-operative. The results suggest that the transmission of allosteric conformational changes involves an increase in affinity at phosphate-binding sites and relative movements of alpha-helices. In order to study glucose 6-phosphate and phosphate binding it was necessary to cross-link the crystals. The use of dimethyl malondi-imidate as a new cross-linking reagent in protein crystallography is discussed.

Adenosine Monophosphate↗
PubMed1984

Effect of parathyroid hormone on total phosphate and inorganic phosphate in blood, plasma, and urine of domestic fowl.

Previous work has shown that parathyroid hormone causes the kidneys of domestic fowl to secrete inorganic phosphate (Pi) into the urine. Secreted Pi is not derived directly from plasma Pi. The objective of the present study was to determine if direct or indirect correlations exist between the urinary Pi excretion rate and the concentration of phosphate in plasma or whole blood. Parathyroid hormone dissolved in gelatin carrier vehicle was injected into immature domestic fowl to cause net renal Pi secretion. Control birds were injected with carrier vehicle alone, resulting in net Pi reabsorption. Urine, plasma, and whole blood samples obtained during net Pi secretion were compared with samples obtained during net Pi reabsorption. Each sample was assayed for total and inorganic phosphate. Neither time course nor point by point comparisons of individual samples provided any evidence that secreted Pi is derived from organic phosphate in plasma or whole blood. These experiments suggest that phosphate pools within the kidneys may serve as a source for the Pi that is secreted in response to parathyroid hormone.

Animals↗
PubMedDate not supplied

Effect of hemodialysis on red cell organic and inorganic phosphates.

Red cell 2,3 diphosphoglyceric acid (DPG), total nucleotide phosphate, inorganic phosphate, and plasma inorganic phosphate were measured at the onset and termination of 22 hemodialyses performed for chronic renal failure in 19 patients. Plasma inorganic phosphate decreased from a mean of 2.05 mM/liter +/- 0.14 to 1.26 mM/liter +/- 0.09 (p less than 0.005) while intracellular inorganic phosphate fell from 1.76 mM/liter +/- 0.12 RBC water to 1.21+/-mM/liter (p less than 0.005). Mean DPG was 6.37 mM/liter RBC +/- 0.23 at the start of the dialysis and 6.08 mM/liter +/- 0.24 at the termination (p=n.s.). Mean nucleotide concentration was 5.90 mM phosphorous content/liter RBC+/-0.26 at the start and 5.95 mM/liter 0.21 at the end (p=n.s.). Although there were significant decreases in intracellular and plasma inorganic phosphate concentrations immediately following dialysis, neither the DPG nor nucleotide phosphate concentrations were significantly altered by this short-term procedure. Red cell water inorganic phosphorous rapidly attained identical concentration with plasma water inorganic phosphorous.

Adenosine Diphosphate↗
PubMed1995

Effects of creatine phosphate and inorganic phosphate on the sarcoplasmic reticulum of saponin-treated rat heart.

1. Ventricular trabeculae from rat heart were permeabilized by treatment with saponin. In the presence of 150 nM Ca2+, application of 20 mM caffeine released Ca2+ from the sarcoplasmic reticulum (SR), resulting in a transient contracture. Ca2+ released from the SR was detected using fura-2 fluorescence. The amplitudes of the caffeine-induced Ca2+ transients were used to assess SR Ca2+ content. 2. In the absence of creatine phosphate (CP), introduction of 5-30 mM inorganic phosphate (Pi) caused a net release of Ca2+ from the SR. Subsequent caffeine-induced Ca2+ and tension transients were smaller in the presence of Pi. Under these conditions, 30 mM Pi decreased the caffeine-induced Ca2+ transients by 45 +/- 3.1% (mean +/- S.D., n = 14). On removal of Pi, the [Ca2+] transiently decreased and the caffeine-induced Ca2+ transients returned to control levels over 4-6 min. 3. In the presence of CP (5-15 mM), the Ca2+ transients were unaffected by the introduction of Pi (5-30 mM) or slightly increased in amplitude. Pi (30 mM) significantly increased the caffeine-induced Ca2+ transients by 7 +/- 8.8% (mean +/- S.D., n = 19, P < 0.05) in the presence of 15 mM CP. The release of Ca2+ on addition of Pi and decrease in [Ca2+] on Pi withdrawal was less pronounced or absent completely in the presence of CP. The inhibitory effects of Pi on caffeine-induced Ca2+ release became apparent as the [CP] was decreased from 5 to 0 mM. 4. In the presence of the creatine phosphokinase inhibitor dinitro-fluorobenzene (DNFB) the effects of Pi (in the presence of CP) were qualitatively similar to the results obtained in the absence of CP, although the decrease in caffeine-induced Ca2+ release was less pronounced. 5. These results suggest that the rise in [Pi]i during ischaemia or anoxia will have little effect on the regulation of Ca2+ by the SR while the [CP]i remains above 5 mM. However, as the [CP] decreases below 5 mM, the accumulation of Pi within the cytosol will progressively reduce the SR Ca2+ content. CP may act in conjunction with endogenous creatine phosphokinase to modify the response of the SR to Pi, and possible mechanisms are considered.

Adenosine Triphosphate↗
PubMed1986

Bound adenosine 5'-triphosphate formation, bound adenosine 5'-diphosphate and inorganic phosphate retention, and inorganic phosphate oxygen exchange by chloroplast adenosinetriphosphatase in the presence of Ca2+ or Mg2+.

When the heat-activated chloroplast F1 ATPase hydrolyzes [3H, gamma-32P]ATP, followed by the removal of medium ATP, ADP, and Pi, the enzyme has labeled ATP, ADP, and Pi bound to it in about equal amounts. The total of the bound [3H]ADP and [3H]ATP approaches 1 mol/mol of enzyme. Over a 30-min period, most of the bound [32P]Pi falls off, and the bound [3H]ATP is converted to bound [3H]ADP. Enzyme with such remaining tightly bound ADP will form bound ATP from relatively high concentrations of medium Pi with either Mg2+ or Ca2+ present. The tightly bound ADP is thus at a site that retains a catalytic capacity for slow single-site ATP hydrolysis (or synthesis) and is likely the site that participates in cooperative rapid net ATP hydrolysis. During hydrolysis of 50 microM [3H]ATP in the presence of either Mg2+ or Ca2+, the enzyme has a steady-state level of about one bound [3H]ADP per mole of enzyme. Because bound [3H]ATP is also present, the [3H]ADP is regarded as being present on two cooperating catalytic sites. The formation and levels of bound ATP, ADP, and Pi show that reversal of bound ATP hydrolysis can occur with either Ca2+ or Mg2+ present. They do not reveal why no phosphate oxygen exchange accompanies cleavage of low ATP concentrations with Ca2+ in contrast to Mg2+ with the heat-activated enzyme. Phosphate oxygen exchange does occur with either Mg2+ or Ca2+ present when low ATP concentrations are hydrolyzed with the octyl glucoside activated ATPase. Ligand binding properties of Ca2+ at the catalytic site rather than lack of reversible cleavage of bound ATP may underlie lack of oxygen exchange under some conditions.

Adenosine Diphosphate↗
PubMed1995

Lack of Control in Inorganic Phosphate Uptake by Catharanthus roseus (L.) G. Don Cells (Cytoplasmic Inorganic Phosphate Homeostasis Depends on the Tonoplast Inorganic Phosphate Transport System?).

Inorganic phosphate (Pi) uptake by Catharanthus roseus (L.) G. Don cells was studied in relation to its apparent uncontrolled uptake using 31P-nuclear magnetic resonance spectroscopy. Kinetics of Pi uptake by the cells indicated that apparent Km and Vm were about 7 [mu]M and 20 [mu]mol g-1 fresh weight h-1, respectively. Pi uptake in Murashige-Skoog medium under different Pi concentrations and different initial cell densities followed basically the same kinetics. When supplied with abundant Pi, cells absorbed Pi at a constant rate (Vm) for the first hours and accumulated it in the vacuole. As the endogenous pool expanded, the rate of Pi uptake gradually decreased to nil. Maximum Pi accumulation was 100 to 120 [mu]mol g-1 fresh weight if cell swelling during Pi uptake (about 2-fold in cell volume) was not considered. Results indicated that (a) the rate of Pi uptake by Catharanthus cells was independent of initial cell density and was constant over a wide range of Pi concentrations (2 mM to about 10 [mu]M) unless the cells were preloaded with excess Pi, and (b) there was no apparent feedback control over the Pi uptake process in the plasma membrane to avoid Pi toxicity. The importance of the tonoplast Pi transport system in cytoplasmic Pi homeostasis is discussed.

Journal Article↗
PubMed2001

Determination of high-energy phosphate compounds and inorganic phosphate by reversed-phase high-performance liquid chromatography: evaluation of myocardial metabolic status in aerobically perfused and hypoxic mouse heart.

The present paper describes a simple HPLC method designed for measuring high-energy phosphate (HEP) compounds in a single run and inorganic phosphate (Pi) in an other short run under the same HPLC conditions. Inorganic phosphate was estimated by using thymidine phosphorylase (EC 2.4.2.4) which catalyzes a reaction involving inorganic phosphate to produce 2-deoxyribose 1-phosphate and thymine. The thymine/Pi stoichiometry was 1. The method provides a reproducible instrument for evaluating myocardial high-energy metabolism under physiological and pathological conditions.

Animals↗
PubMed1987

Effects of glucagon on renal and extrarenal handling of inorganic phosphate in mice: evidence for inorganic phosphate mobilizing activity.

Glucagon administration is known to increase urinary inorganic phosphate (Pi) excretion. We have now confirmed that this effect also occurs in mice. While, at lower glucagon doses, phosphaturia was accompanied by a decrease in plasma Pi, at the highest dose of glucagon plasma Pi was not altered in spite of a massive increase in urinary Pi. This suggested that glucagon may additionally have another effect on phosphate homeostasis, i.e. of mobilizing Pi from body stores. In order to distinguish between the renal and extrarenal effects of glucagon, the animals were fed a low-phosphate diet, a procedure known to blunt the effect of several phosphaturic agents. Under these conditions, any Pi mobilized from body stores should be reflected by an increment of plasma Pi. Glucagon phosphaturia was indeed blunted under this condition. Furthermore, plasma Pi increased spontaneously by 0.82 +/- 0.14 mmol/l (mean +/- SEM) in an experimental period of 8 h during which the mice were fasted. In mice injected with zinc-protamine-glucagon subcutaneously at 4 and 16 micrograms/g of body weight, plasma Pi increased by 1.45 +/- 0.17 and 2.38 +/- 0.14 mmol/l during 8 h, respectively. Thus, it appears that exogenous glucagon is a strong Pi-mobilizing hormone. Furthermore, during the recovery phase following insulin-induced hypoglycemia, in which glucagon is thought to play a primordial role, a similar Pi mobilization to that obtained after glucagon administration was observed. Thus, since glucagon is released during fasting to maintain the homeostasis of blood glucose, it is conceivable that the mobilization of Pi induced by fasting might also have been caused by endogenous glucagon and that this hormone might be involved in Pi homeostasis.

Animals↗