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PubMed1985

[Metabolism of phosphate-limited Streptomyces cultures. III. The ambivalent effect of phosphates in nourseothricin-producing cultures of Streptomyces noursei JA 3890b].

A common condition in the evolution of organisms and their metabolism seems to be a latent lack of available phosphate in the natural environment. Accordingly, the phosphate dependent metabolisms of the soil-living streptomycetes should be stamped by lack of phosphate, too. The biosynthesis of the streptothricin antibiotic nourseothricin by Streptomyces noursei 3890b is initiated by limitation of soluble phosphate in the fermentation medium. At the other side is shown that a certain rate of feeding of phosphate during the fermentation increases the nourseothricin biosynthesis. An ambivalente role of phosphate on the secondary metabolite biosynthesis is stated. The limitation of phosphate leads to a special physiological state of the producer, characterized by secondary product formation and dephosphorylating activities in cells. This state is temporally stabilized by the presence of a sufficient phosphate supply, realized by enzymatic hydrolysis of complex phosphate-containing substrates or by a direct feeding of inorganic phosphate to the fermentations. The occurrence of different physiological states in respect to the phosphate-dependent metabolism is described by S-shaped functions of the relationship between specific growth rate and the phosphate concentration in the medium. The special behaviour of Streptomyces noursei cells at phosphate limitation is discussed to be the result of the dephosphorylating activities in cells, hydrolyzing phosphoester-bonds of regulatory metabolites as well as energy-rich compounds.

Anti-Bacterial Agents↗
PubMed1982

CMP-dependent incorporation of [14C]Glycerol 3-phosphate into phosphatidylglycerol and phosphatidylglycerol phosphate by rabbit lung microsomes.

Rabbit lung microsomes were found to catalyze CMP-dependent incorporation of [14C]glycerol 3-phosphate into a total lipid extract. The radioactively labeled products in the lipid extract were identified as phosphatidylglycerol and phosphatidylglycerol phosphate. CMP-dependent incorporation of [14C]glycerol 3-phosphate by lung microsomes proceeded optimally at pH 7.4 and required Mn2+. The apparent Km value for CMP in this reaction was calculated to be 0.19 mM. No other cytidine nucleotide could substitute completely for CMP in supporting [14C]glycerol 3-phosphate incorporation into lipid. Cytosine-beta-D-arabinofuranoside-5'-monophosphate-dependent incorporation of [14C]glycerol 3-phosphate was observed at pH 8.5 but not at pH 6.8 CMP-dependent incorporation of [14C]glycerol 3-phosphate by microsomes was inhibited by inositol. The optimal in vitro rates of CMP-dependent and CDP diacylglycerol-dependent incorporation of [14C]glycerol 3-phosphate into lipid were similar (approximately 1 nmol . mg-1 protein . h-1) and were not additive. Both CMP -dependent and CDP diacylglycerol-dependent incorporation of [14C]glycerol 3-phosphate by lung microsomes appeared to involve CDPdiacylglycerol:glycerol-3-phosphate phosphatidyltransferase. However, the specific activity of this enzyme in a particular subcellular fraction did not relate directly in the extent of CMP-dependent [14C]glycerol 3-phosphate incorporation in that fraction. Preincubation of lung microsomes with 5 mM CMP plus 3 mM phosphatidylinositol increased CMP-dependent incorporation of [14C]glycerol 3-phosphate. When lung microsomes were depleted specifically of phosphatidylinositol by incubating with a phosphatidylinositol-specific phospholipase C, CMP-dependent incorporation was diminished. The Mn2+ requirement for CMP-dependent incorporation of [14C] glycerol 3-phosphate, its phosphatidylinositol requirement and its inhibition by Triton X-100 (0.2%) were not features shared by CDPdiacylglycerol-dependent incorporation of [14C]glycerol 3-phosphate but were characteristics of the reverse reaction catalyzed by CDPdiacylglycerol: inositol phosphatidyltransferase. Together with the previous finding of a developmental increase in the CMP content of fetal rabbit lung, these observations are consistent with a role for CMP in the regulation of the phosphatidylinositol and phosphatidylglycerol content of lung surfactant during lung maturation.

Animals↗
PubMed1978

The fate of 14C in glucose 6-phosphate synthesized from [1-14C]Ribose 5-phosphate by enzymes of rat liver.

1. Glucose 5-phosphate was synthesized from ribose 5-phosphate by an enzyme extract prepared from an acetone-dried powder of rat liver. Three rates of ribose 5-phosphate utilization were observed during incubation for 17 h. An analysis of intermediates and products formed throughout the incubation revealed that as much as 20% of the substrate carbon could not be accounted for. 2. With [1-14C]ribose 5-phosphate as substrate, the specific radioactivity of [14C]glucose 6-phosphate formed was determined at 1, 2, 5 and 30 min and 3, 8 and 17 h. It increased rapidly to 1.9-fold the initial specific radioactivity of [1-14C]ribose 5-phosphate at 3 h and then decreased to a value approximately equal to that of the substrate at 6 h, and finally at 17 h reached a value 0.8-fold that of the initial substrate [1-14C]ribose 5-phosphate. 3. The specific radioactivity of [14C]ribose 5-phosphate decreased to approx. 50% of its inital value during the first 3 h of the incubation and thereafter remained unchanged. 4. The distribution of 14C in the six carbon atoms of [14C]glucose 6-phosphate formed from [1-14C]ribose 5-phosphate at 1, 2, 5 and 30 min and 3, 8 and 17 h was determined. The early time intervals (1--30 min) were characterized by large amounts of 14C in C-2 and in C-6 and with C-1 and C-3 being unlabelled. In contrast, the later time intervals (3--17 h) were characterized by the appearance of 14C in C-1 and C-3 and decreasing amounts of 14C in C-2 and C-6. 5. It is concluded that neither the currently accepted reaction sequence for the non-oxidative pentose phosphate pathway nor the 'defined' pentose phosphate-cycle mechanism can be reconciled with the labelling patterns observed in glucose 6-phosphate formed during the inital 3 h of the incubation.

Animals↗
PubMed1991

Effects on calcium and phosphate metabolism and on parathyroid function of acute administration of tricalcium phosphate.

The effects of the ingestion of tricalcium phosphate on calcium and phosphate metabolism and on parathyroid function were evaluated in 10 young adults. Each subject was studied during a control period of two hours before and during an experimental period of four hours after ingestion of a single oral dose of tricalcium phosphate containing 1500 mg of calcium and 770 mg of phosphorus. Serum and urinary calcium and phosphate and the nephrogenous cAMP fraction were measured. Significant rises in serum (from 2.32 +/- 0.05 to 2.44 +/- 0.08 mmol/l) and urinary (from 1.08 +/- 0.65 to 3.43 +/- 1.38 mumols/l GF) calcium and in serum phosphate (from 1.05 +/- 0.18 to 1.28 +/- 0.14 mmol/l) occurred. Unexpectedly, the acute supply of calcium in the form of tricalcium phosphate did not provoke significant alteration of nephrogenous cAMP level. In order to assess the respective effects of calcium and of phosphate, similar tests with ingestion of similar amounts either of calcium (as a glucoheptogluconate salt) or of phosphate were subsequently performed in the same subjects. Significant increases in serum total calcium were observed after calcium glucoheptogluconate as after tricalcium phosphate. However, the effects on parathyroid function differed, since a significant (p less than 0.001) decrease in nephrogenous cAMP followed the ingestion of calcium glucoheptogluconate. Otherwise, a stimulating effect of phosphate on parathyroid function was observed. These findings suggest that the respective effects of calcium and of phosphate are counterbalanced when administered as tricalcium phosphate, resulting in the absence of parathyroid suppression.

Adult↗
PubMed1984

Analysis of the metabolic turnover of the individual phosphate groups of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. Validation of novel analytical techniques by using 32P-labelled lipids from erythrocytes.

We have developed methods that yield estimates of the 32P content of each of the individual phosphate groups of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate, thus extending the information available from studies of the labelling of these lipids in intact cells or membrane preparations. The analyses are undertaken with the deacylated lipids. Assay of the 5-phosphate of phosphatidylinositol 4,5-bisphosphate is achieved by the use, under conditions of first-order kinetics, of a 5-phosphate-specific phosphomonoesterase present in isolated erythrocyte membranes [Downes, Mussat & Michell (1982) Biochem. J. 203, 169-177]. Assay of the 4-phosphate of phosphatidylinositol 4-phosphate and of the total monoester phosphate content (4-phosphate plus 5-phosphate) of phosphatidylinositol 4,5-bisphosphate employs alkaline phosphatase from bovine intestine. The radioactivity of the 1-phosphate is that remaining as organic phosphate after exhaustive alkaline phosphatase treatment. The methodology has been validated by using lipids from human erythrocytes: these contain no 32P in their 1-phosphate. These methods should be of substantial value in studies of the many cells that show rapid hormonal perturbations of phosphatidylinositol 4,5-bisphosphate metabolism.

Alkaline Phosphatase↗
PubMed2002

A phosphate transporter from Medicago truncatula involved in the acquisition of phosphate released by arbuscular mycorrhizal fungi.

Many plants have the capacity to obtain phosphate via a symbiotic association with arbuscular mycorrhizal (AM) fungi. In AM associations, the fungi release phosphate from differentiated hyphae called arbuscules, that develop within the cortical cells, and the plant transports the phosphate across a symbiotic membrane, called the periarbuscular membrane, into the cortical cell. In Medicago truncatula, a model legume used widely for studies of root symbioses, it is apparent that the phosphate transporters known to operate at the root-soil interface do not participate in symbiotic phosphate transport. EST database searches with short sequence motifs shared by known phosphate transporters enabled the identification of a novel phosphate transporter from M. truncatula, MtPT4. MtPT4 is significantly different from the plant root phosphate transporters cloned to date. Complementation of yeast phosphate transport mutants indicated that MtPT4 functions as a phosphate transporter, and estimates of the K(m) suggest a relatively low affinity for phosphate. MtPT4 is expressed only in mycorrhizal roots, and the MtPT4 promoter directs expression exclusively in cells containing arbuscules. MtPT4 is located in the membrane fraction of mycorrhizal roots, and immunolocalization revealed that MtPT4 colocalizes with the arbuscules, consistent with a location on the periarbuscular membrane. The transport properties and spatial expression patterns of MtPT4 are consistent with a role in the acquisition of phosphate released by the fungus in the AM symbiosis.

Amino Acid Sequence↗
PubMed2002

A chloroplast phosphate transporter, PHT2;1, influences allocation of phosphate within the plant and phosphate-starvation responses.

The uptake and distribution of Pi in plants requires multiple Pi transport systems that must function in concert to maintain homeostasis throughout growth and development. The Pi transporter PHT2;1 of Arabidopsis shares similarity with members of the Pi transporter family, which includes Na(+)/Pi symporters of fungal and animal origin and H(+)/Pi symporters of bacterial origin. Sequence comparisons between proteins of this family revealed that plant members possess extended N termini, which share features with chloroplast transit peptides. Localization of a PHT2;1-green fluorescent protein fusion protein indicates that it is present in the chloroplast envelope. A Pi transport function for PHT2;1 was confirmed in yeast using a truncated version of the protein lacking its transit peptide, which allowed targeting to the plasma membrane. To assess the in vivo role of PHT2;1 in phosphorus metabolism, we identified a null mutant, pht2;1-1. Analysis of the mutant reveals that PHT2;1 activity affects Pi allocation within the plant and modulates Pi-starvation responses, including the expression of Pi-starvation response genes and the translocation of Pi within leaves.

Arabidopsis↗
PubMed1980

Evidence for the participation of independent translocation for phosphate and glucose 6-phosphate in the microsomal glucose-6-phosphatase system. Interactions of the system with orthophosphate, inorganic pyrophosphate, and carbamyl phosphate.

The interactions of Pi, PPi, and carbamyl-P with the hepatic glucose-6-phosphatase system were studied in intact and detergent-disrupted microsomes. Penetration of PPi and carbamyl-P into intact microsomes was evidenced by their reactions with the enzyme located exclusively on the luminal surface. Lack of effects of carbonyl cyanide m-chlorophenylhydrazone and valinomycin + KCl indicated that pH gradients and/or membrane potentials that could influence the kinetics of the system are not generated during metabolism of PPi and glucose-6-P by intact microsomes. With disrupted microsomes, only competitive interactions were seen among glucose-6-P, Pi, PPi, and carbamyl-P. With intact microsomes, Pi, PPi, and carbamyl-P were relatively weak, noncompetitive inhibitors of glucose-6-phosphatase, and PPi hydrolysis was inhibited competitively by Pi and carbamyl-P but noncompetitively by glucose-6-P. Analysis of the kinetic data in combination with findings from other studies that a variety of inhibitors of the glucose-6-P translocase (T1) does not affect PPi hydrolysis provide compelling evidence that permeability of microsomes to Pi, PPi, and carbamyl-P is mediated by a second translocase (T2). Some properties of the microsomal anion transporters are described. If the characteristics of the glucose-6-phosphatase system as presently defined in intact microsomes apply in vivo, glucose-6-P hydrolysis appears to be the predominant, if not the exclusive, physiologic function of the system. Both the "noncompetitive character" and the relative ineffectiveness of Pi as an inhibitor of glucose-6-phosphatase of intact microsomes result from the rate limitation imposed by T1 that prevents equilibration of glucose-6-P across the membrane. In microsomes from fed rats, where T1 is less rate restricting, about one-half as much Pi was required to give 50% inhibition compared with microsomes from fasted or diabetic rats. Thus, any treatment or agent that alters the kinetic relationship between transport and hydrolysis of glucose-6-P (e.g. endocrine or nutritional status) is an essential consideration in analyses of kinetic data for the glucose-6-phosphatase system.

Animals↗
PubMed1994

A structural analysis of phosphate and sulphate binding sites in proteins. Estimation of propensities for binding and conservation of phosphate binding sites.

The high resolution X-ray structures of 38 proteins that bind phosphate containing groups and 36 proteins binding sulphate ions were analysed to characterise the structural features of anion binding sites in proteins. 34 of the 66 phosphates found were in close proximity to the amino terminus of an alpha-helix. 27% of phosphate groups bind to only one amino acid, but there is a wide distribution, with 3% of phosphates binding to seven residues. Similarly, there is a large variability in the number of contacts each phosphate group makes to the protein. This ranges from none (3% of phosphates) to nine (3% of phosphates). The most common number of contacts is two (23% of phosphates). The most commonly found residue at helix-type binding sites is glycine, followed by Arg, Thr, Ser and Lys. At non-helix binding sites, the most commonly found residue is Arg followed by Tyr, His, Lys and Ser. There is no typical phosphate binding site. There are marked differences between propensities for phosphate binding at helix and non-helix type binding sites. Non-helix binding sites show more discrimination between the types of residues involved in binding when compared to the helix set. The propensities for binding of the amino acids reveal the expected trend of positively charged and polar residues being good at binding (although that for lysine is unexpectedly low) with the bulky non-polar residues being poor at binding. Bulky residues are less likely to bind with the amide nitrogen. Sulphate binding sites show similar trends. Analysis of multiple sequence alignments that include phosphate and sulphate binding proteins reveals the degree of conservation at the binding site residues compared to the average conservation of residues in the protein. Phosphate binding site residues are more conserved than sulphate binding sites.

Crystallography, X-Ray↗
PubMed1984

Phosphate uptake by superficial and deep nephron brush border membranes. Effect of the dietary phosphate and parathyroid hormone.

Dietary phosphate restriction and acute parathyroidectomy in rat are known to be associated with a selective increase in phosphate uptake by renal cortical brush border membranes (BBM). Conversely, phosphate loading and parathyroid hormone (PTH) administration result in a decrease of this uptake. In the present study, we investigated whether the response of the membrane to these various stimuli implies similar or different modifications of the kinetic properties of this membrane, whether these modifications affect one or both of the two systems of phosphate transport previously described, whether both superficial and deep nephron populations are involved, and whether the two stimuli: dietary phosphate, and parathyroid activity, are additive or not. Kinetic studies of phosphate (PO4) uptake by BBM vesicles were performed in seven groups of rats: control (N), acutely thyroparathyroidectomized (TPTX), PTH loaded (PTH), phosphate loaded (P+), phosphate depleted (P-), phosphate depleted with acute thyroparathyroidectomy (P-TPTX), and phosphate depleted-PTH loaded (P-PTH). In each of these experimental conditions, superficial and deep nephrons were investigated. Results indicated that 1. BBM from deep nephrons present a greater capacity for PO4 transport than those from superficial nephrons; 2. Whereas a dual system of PO4 uptake is observed in superficial BBM, deep BBM present only one single system; 3. Phosphate in the diet influences PO4 uptake by BBM to a greater extent in the deep than in the superficial nephrons; 4. PTH status on the contrary, equally influences both populations; 5. TPTX does not significantly enhance PO4 uptake in phosphate depleted rats; however, PTH loading curtails this uptake; 6.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1979

Effect of dietary phosphate intake on phosphate transport by isolated rat renal brush-border vesicles.

Renal brush-border membrane vesicles isolated from rats kept for 6-8 weeks on a low-phosphate diet (0.15% of dry matter) showed a markedly faster Na(+)-dependent phosphate uptake than did membrane vesicles isolated from animals kept on a high-phosphate diet (2% of dry matter). Phosphate-uptake rate by brush-border membrane vesicles isolated from animals on a low-phosphate diet remained significantly increased after acute parathyroidectomy. Dietary adaptation was also observed in animals that had been parathyroidectomized before exposure to the different diets. In animals on the low-phosphate diet parathyrin administration inhibited phosphate uptake by brush-border vesicles only if the animals were repleted with P(i) (5ml of 20mm-NaH(2)PO(4)) 1h before being killed. After acute phosphate loading and parathyrin administration the difference in the transport rate between the two dietary groups remained statistically significant. The results suggest that the adaptation of proximal-tubule phosphate transport to dietary intake of phosphate is reflected in the Na(+)/phosphate co-transport system located in the luminal membrane of the proximal-tubule cell. Since the dietary effects on phosphate transport by brush-border membranes are only partially reversed by acute changes in parathyrin concentration and are also observed in chronically parathyroidectomized animals, the adaptation of the Na(+)/phosphate co-transport system to dietary phosphate intake seems to involve an additional mechanism independent of parathyrin.

Alkaline Phosphatase↗
PubMed1983

Tubular capacity of phosphate transport in phosphate-deprived rats: effects of nicotinamide and PTH.

The present study tested the hypothesis that nicotinamide and/or parathyroid hormone (PTH) would reverse the resistance of phosphate-deprived rats to the phosphaturic effects of phosphate infusions by reducing the tubular capacity for phosphate transport. The response to progressively increasing rates of phosphate infusion (1, 2, and 3 mumol/min) was evaluated in thyroparathyroidectomized (TPTX) rats fed low phosphate diet (0.07%) for 4 days and treated with either vehicle or nicotinamide 2 h before the experiment. Following phosphate infusions, phosphate excretion was unchanged in vehicle-treated rats but increased progressively in nicotinamide-treated rats to 60% of the final rate of infusion. In a second series conducted in the presence of exogenous PTH, vehicle-treated rats responded to phosphate infusions by increasing phosphate excretion to 80-90% of the rate of infusion. In the presence of both nicotinamide and PTH, the rate of phosphate excretion matched the rate of infusion. Vehicle-treated rats displayed the highest reabsorptive rates without reaching a transport maximum, whereas a lower maximum reabsorptive rate was evident in nicotinamide-treated rats. However, in both groups given PTH, not only was the transport maximum lower but it declined with further increases in filtered load. We conclude that nicotinamide and PTH reverse the effects of dietary phosphate deprivation by decreasing the tubular capacity for phosphate transport.

Animals↗
PubMed1986

Effect of dexamethasone on segmental phosphate reabsorption in phosphate-deprived rats.

These experiments were designed to test the hypothesis that avid phosphate reabsorption by the pars recta accounts for the resistance to the phosphaturic effects of acute dexamethasone (DEX) and parathyroid hormone (PTH) infusions in rats fed a low-phosphate diet. Acute infusion of DEX [0.4 mg/(kg X h)] increased the fractional delivery of phosphate (FDPi) to the late proximal tubule from 7.1 +/- 2.1 to 14.4 +/- 3.5%, whereas FDPi to the early distal tubule and urine were not different. PTH alone [1 U/(kg X min)] increased FDPi to the late proximal tubule from 4.0 +/- 1.1 to 15.7 +/- 3.7%, whereas FDPi to the early distal tubule or urine was not different. The combination of DEX and PTH further increased FDPi to the late proximal tubule (32.7 +/- 6.4%) and resulted in an increase in fractional excretion of phosphate (FEPi), in spite of the fact that the FDPi to the early distal tubule was not significantly increased. The increased delivered load of phosphate to the pars recta following inhibition of phosphate transport in superficial proximal convoluted tubules resulted in a comparable increase in phosphate reabsorption in the pars recta, based on linear regression analysis, in rats fed low-phosphate diet but not in rats fed normal phosphate diet. These results demonstrate that acute infusion of DEX or PTH inhibits fractional phosphate reabsorption in the superficial proximal tubule but does not result in an increase in FEPi due at least in part to avid phosphate reabsorption in the superficial pars recta in rats fed low-phosphate diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorption↗
PubMed1985

Effects of phosphate and calcium infusion on renal phosphate transport in the dog.

The effect of phosphate infusion on renal tubular handling of calcium and phosphate was examined in dogs which had been thyroparathyroidectomized (TPTX) immediately prior to the studies. Phosphate infusions in TPTX animals caused a small decrease in total and ultrafilterable plasma calcium, and decreased phosphate reabsorptive capacity in the proximal tubule and loop segment. Infusion of CaCl2 during phosphate loading to offset the fall in plasma calcium prevented the reduction in proximal phosphate reabsorptive capacity. However, between the proximal and distal sampling site, the reduction in phosphate reabsorptive capacity could not be prevented by CaCl2 administration. These data are consistent with the presence of two phosphate transport systems; one in the early proximal tubule, modulated by changes in plasma calcium level, and a second in the loop segment, which is independent of calcium. While the data suggest that the depression of proximal phosphate reabsorption during phosphate infusion may be secondary to the fall in plasma calcium concentration, they do not exclude a direct effect of infused phosphate on proximal phosphate reabsorption that may be antagonized by an opposing direct effect of the calcium infusion.

Animals↗