Metabolism of inositol phosphates in the protozoan Paramecium. Characterization of a novel inositol-hexakisphosphate-dephosphorylating enzyme

Fluorometric detection of phytase enzyme activity and phosphate ion based on gelatin supported silver nanoclusters

Phytase is the commercial enzyme for bioconversion of phytate substrate to digestible phosphate ions. Recently silver nanoclusters (AgNCs) have received great attention as the optical transducer nanoparticles in biosensors structure. The novel detection platform was developed to detect the phytase enzyme activity and phosphate ions based on fluorescence quenching of AgNCs. The AgNCs were synthesized through gelatin supported reaction and characterized by TEM, FTIR and XRD analysis. The hydrolytic effect of phytase enzyme and subsequent phosphate release led to suppression of AgNCs fluorescence. The linear range was observed for enzyme in the range of 0.5-5 U/mL with the detection limit of 0.2 U/mL. Also, the same fluorescence quenching effect was observed in the presence of phosphate ion in the linear range of 1 to 16 µM with a detection limit of 0.5 µM. The proposed mechanism showed effectiveness of detection strategy for detection of phytase enzyme and phosphate ion.

Analytical Methods for Determination of Phytic Acid and Other Inositol Phosphates: A Review

From the early precipitation-based techniques, introduced more than a century ago, to the latest development of enzymatic bio- and nano-sensor applications, the analysis of phytic acid and/or other inositol phosphates has never been a straightforward analytical task. Due to the biomedical importance, such as antinutritional, antioxidant and anticancer effects, several types of methodologies were investigated over the years to develop a reliable determination of these intriguing analytes in many types of biological samples; from various foodstuffs to living cell organisms. The main aim of the present work was to critically overview the development of the most relevant analytical principles, separation and detection methods that have been applied in order to overcome the difficulties with specific chemical properties of inositol phosphates, their interferences, absence of characteristic signal (e.g., absorbance), and strong binding interactions with (multivalent) metals and other biological molecules present in the sample matrix. A systematical and chronological review of the applied methodology and the detection system is given, ranging from the very beginnings of the classical gravimetric and titrimetric analysis, through the potentiometric titrations, chromatographic and electrophoretic separation techniques, to the use of spectroscopic methods and of the recently reported fluorescence and voltammetric bio- and nano-sensors.

Phytase, a New Life for an “Old” Enzyme

Phytases are phosphohydrolytic enzymes that initiate stepwise removal of phosphate from phytate. Simple-stomached species such as swine, poultry, and fish require extrinsic phytase to digest phytate, the major form of phosphorus in plant-based feeds. Consequently, this enzyme is supplemented in these species’ diets to decrease their phosphorus excretion, and it has emerged as one of the most effective and lucrative feed additives. This chapter provides a comprehensive review of the evolving course of phytase science and technology. It gives realistic estimates of the versatile roles of phytase in animal feeding, environmental protection, rock phosphorus preservation, human nutrition and health, and industrial applications. It elaborates on new biotechnology and existing issues related to developing novel microbial phytases as well as phytase-transgenic plants and animals. And it targets critical and integrated analyses on the global impact, novel application, and future demand of phytase in promoting animal agriculture, human health, and societal sustainability.

Emerging roles of phosphoinositide-specific phospholipases C in the ciliates Tetrahymena and Paramecium

Phospholipases C (PLCs) that hydrolyze inositol phospholipids regulate vital cellular functions in both eukaryotic and prokaryotic organisms. The PLC superfamily consists of eukaryotic phosphoinositide-specific PLCs (PI-PLCs), bacterial PLCs and trypanosomal PLCs.1 PI-PLCs hydrolyze phosphatidylinositol-4,5-bisphosphate (PtdIns4,5P(2)) to produce inositol-1,4,5-trisphosphate (Ins1,4,5P(3)) and constitute a hallmark feature of eukaryotic cells. In metazoa, this reaction is coupled to receptor signaling via specific PI-PLC isoforms and results in acute increase of cytosolic Ca(2+) levels by Ins1,4,5P(3)-sensitive Ca(2+) channels (IP(3)-receptors, IP3Rs).2 A striking result of many studies so far has been the presence of a single PI-PLC gene in all unicellular eukaryotes investigated, as opposed to expansion of PI-PLC isoforms in metazoa;3 this has suggested that a single housekeeping PI-PLC represents an archetypal and simplified form of PI-PLC signaling.3 Several studies however have noted a unique expansion of PI-PLC/IP3R pathway components in ciliates.4,5 In a recent paper we showed the presence of multiple functional PI-PLC genes in Tetrahymena thermophila and biochemical characterization, pharmacological studies and study of their expression patterns suggested that they are likely to serve distinct non-redundant roles.4 In this report we discuss these studies and how they advance our understanding of PI-PLC functions in ciliates.

Uronema marinum: identification and biochemical characterization of phosphatidylcholine-hydrolyzing phospholipase C

Phosphatidylcholine (PC)-specific phospholipase D (PC-PLD) and phosphatidylcholine (PC)-specific phospholipase C (PC-PLC) activities have been detected in Uronema marinum. Partial purification of PC-PLC revealed that two distinct forms of PC-PLC (named as mPC-PLC and cPC-PLC) were existed in membrane and cytosol fractions. The two PC-PLC enzymes showed the preferential hydrolyzing activity for PC with specific activity of 50.4 for mPC-PLC and 28.3 pmol/min/mg for cPC-PLC, but did not hydrolyze phosphatidylinositol or phosphatidylethanolamine. However, the biochemical characteristics and physiological roles of both enzymes were somewhat different. mPC-PLC had a pH optimum in the acidic region at around, pH 6.0, and required approximately 0.4 mM Ca2+ and 2.5 mM Mg2+ for maximal activity. cPC-PLC had a pH optimum in the neutral region at around, pH 7.0, and required 1.6 mM Ca2+ and 2.5 mM Mg2+ for maximal activity. cPC-PLC, but not mPC-PLC, showed a dose-dependent inhibitory effect on the luminal-enhanced chemiluminescence (CL) responses and the viability of zymosan-stimulated phagocytes of olive flounder, indicating that cPC-PLC may contribute to the parasite evasion against the host immune response. Our results suggest that U. marinum contains PC-PLD as well as two enzymatically distinct PC-PLC enzymes, and that mPC-PLC may play a role in the intercellular multiplication of U. marinum and cPC-PLC acts as a virulence factor, serving to actively disrupt the host defense mechanisms.

Calcium in ciliated protozoa: sources, regulation, and calcium-regulated cell functions

In ciliates, a variety of processes are regulated by Ca2+, e.g., exocytosis, endocytosis, ciliary beat, cell contraction, and nuclear migration. Differential microdomain regulation may occur by activation of specific channels in different cell regions (e.g., voltage-dependent Ca2+ channels in cilia), by local, nonpropagated activation of subplasmalemmal Ca stores (alveolar sacs), by different sensitivity thresholds, and eventually by interplay with additional second messengers (cilia). During stimulus-secretion coupling, Ca2+ as the only known second messenger operates at approximately 5 microM, whereby mobilization from alveolar sacs is superimposed by "store-operated Ca2+ influx" (SOC), to drive exocytotic and endocytotic membrane fusion. (Content discharge requires binding of extracellular Ca2+ to some secretory proteins.) Ca2+ homeostasis is reestablished by binding to cytosolic Ca2+-binding proteins (e.g., calmodulin), by sequestration into mitochondria (perhaps by Ca2+ uniporter) and into endoplasmic reticulum and alveolar sacs (with a SERCA-type pump), and by extrusion via a plasmalemmal Ca2+ pump and a Na+/Ca2+ exchanger. Comparison of free vs total concentration, [Ca2+] vs [Ca], during activation, using time-resolved fluorochrome analysis and X-ray microanalysis, respectively, reveals that altogether activation requires a calcium flux that is orders of magnitude larger than that expected from the [Ca2+] actually required for local activation.

Formation of myo-inositol phosphates by Aspergillus niger 3-phytase

Kinetics of phytate hydrolysis by Aspergillus niger phytase and correlation between the amount of released phosphate and creation of lower myo-inositol phosphates were investigated. Phytase was able to hydrolyze myo-inositol hexakis-, pentakis-, tetrakis-, and trisphosphates. Finally, about 56% of total phosphate were released and myo-inositol bisphosphate was detected as the end-product.

Translocation between membranes and cytosol of p42IP4, a specific inositol 1,3,4,5-tetrakisphosphate/phosphatidylinositol 3,4, 5-trisphosphate-receptor protein from brain, is induced by inositol 1,3,4,5-tetrakisphosphate and regulated by a membrane-associated 5-phosphatase

The highly conserved 42-kDa protein, p42IP4 was identified recently from porcine brain. It has also been identified similarly in bovine, rat and human brain as a protein with two pleckstrin homology domains that binds Ins(1,3,4,5)P4 and PtdIns(3,4,5)P3 with high affinity and selectivity. The brain-specific p42IP4 occurs both as membrane-associated and cytosolic protein. Here, we investigate whether p42IP4 can be translocated from membranes by ligand interaction. p42IP4 is released from cerebellar membranes by incubation with Ins(1,3,4,5)P4. This dissociation is concentration-dependent (> 100 nM), occurs within a few minutes and and is ligand-specific. p42IP4 specifically associates with PtdIns(3, 4,5)P3-containing lipid vesicles and can dissociate from these vesicles by addition of Ins(1,3,4,5)P4. p42IP4 is only transiently translocated from the membranes as Ins(1,3,4,5)P4 can be degraded by a membrane-associated 5-phosphatase to Ins(1,3,4)P3. Then, p42IP4 re-binds to the membranes from which it can be re-released by re-addition of Ins(1,3,4,5)P4. Thus, Ins(1,3,4,5)P4 specifically induces the dissociation from membranes of a PtdIns(3,4,5)P3 binding protein that can reversibly re-associate with the membranes. Quantitative analysis of the inositol phosphates in rat brain tissue revealed a concentration of Ins(1,3,4,5)P4 comparable to that required for p42IP4 translocation. Thus, in vivo p42IP4 might interact with membranes in a ligand-controlled manner and be involved in physiological processes induced by the two second messengers Ins(1,3,4,5)P4 and PtdIns(3,4,5)P3.

Cloning and characterization of a gene encoding phosphatidyl inositol-specific phospholipase C from Trypanosoma cruzi

A gene encoding phosphatidyl inositol-4,5-bisphosphate phospholipase C (PLC) was cloned from the protozoan parasite Trypanosoma cruzi. A partial cDNA encoding putative PLC was obtained by a polymerase chain reaction (PCR) using degenerate oligonucleotide primers corresponding to conserved regions of PLCs. A 2178-bp protein coding region of the T. cruzi PLC gene, composed from cDNA and genomic clones, encodes a putative PLC with a calculated molecular mass of 82,032 Da and an isoelectric point of 5.93. The deduced amino acid sequence of T. cruzi PLC exhibited 23-42% overall identities with the PLCs from other organisms. Among them, PLC from Ictalurus punctatus revealed the highest identity to T. cruzi PLC. The percentage identities of the entire proteins and the catalytic X/Y domains suggested that T. cruzi PLC is more evolutionarily related to the PLCs of higher eukaryotes than to those of lower unicellular eukaryotes. The tetrad analysis of the segregants of the Saccharomyces cerevisiae PLC1/plc1::HIS3 diploid strain transformed with the T. cruzi PLC-expressing plasmid showed that expression of T. cruzi PLC suppressed the growth defect caused by the plc1 disruption in yeasts. Temperature-sensitive phenotype of the S. cerevisiae plc1-mutant haploid strain was also suppressed by the expression of T. cruzi PLC. The phosphatidyl inositol-4,5-biphosphate (PtdIns(4,5)P2) hydrolyzing activity of T. cruzi PLC was demonstrated in the lysate from the plc1-temperature sensitive yeast mutant strain transformed with the T. cruzi PLC-expressing plasmid. The yeast-expressed T. cruzi PLC showed an absolute Ca2+ dependence which was similar to mammalian PLC isoforms: the half-maximal activity at 0.5-1 x 10(-5) M Ca2+ and the maximal activity at 1-2 x 10(-4) M Ca2+.

Phytase: sources, preparation and exploitation

This review deals with phytase (myo-inositol hexakisphosphate phosphohydrolase) and covers microbiological sources, phytase occurrence in plants and animals, its purification, physico-chemical and molecular properties. Protein engineering of phytase and potential enzyme applications are discussed.

A novel, phospholipase C-independent pathway of inositol 1,4,5-trisphosphate formation in Dictyostelium and rat liver

In an earlier study a mutant Dictyostelium cell-line (plc-) was constructed in which all phospholipase C activity was disrupted and nonfunctional, yet these cells had nearly normal Ins(1,4,5)P3 levels (Drayer, A.L., Van Der Kaay, J., Mayr, G.W, Van Haastert, P.J.M. (1990) EMBO J. 13, 1601-1609). We have now investigated if these cells have a phospholipase C-independent de novo pathway of Ins(1,4,5)P3 synthesis. We found that homogenates of plc- cells produce Ins(1,4,5)P3 from endogenous precursors. The enzyme activities that performed these reactions were located in the particulate cell fraction, whereas the endogenous substrate was soluble and could be degraded by phytase. We tested various potential inositol polyphosphate precursors and found that the most efficient were Ins(1,3,4,5,6)P5, Ins(1,3,4,5)P4, and Ins(1,4,5,6)P4. The utilization of Ins(1,3,4,5,6)P5, which can be formed independently of phospholipase C by direct phosphorylation of inositol (Stephens, L.R. and Irvine, R.F. (1990) Nature 346, 580-582), provides Dictyostelium with an alternative and novel pathway of de novo Ins(1,4,5)P3 synthesis. We further discovered that Ins(1,3,4,5,6)P5 was converted to Ins(1,4,5)P3 via both Ins(1,3,4,5)P4 and Ins(1,4,5,6)P4. In the absence of calcium no Ins(1,4,5)P3 formation could be observed; half-maximal activity was observed at low micromolar calcium concentrations. These reaction steps could also be performed by a single enzyme purified from rat liver, namely, the multiple inositol polyphosphate phosphatase. These data indicate that organisms as diverse as rat and Dictyostelium possess enzyme activities capable of synthesizing the second messengers Ins(1,4,5)P3 and Ins(1,3,4,5)P4 via a novel phospholipase C-independent pathway.

Non-radioactive, isomer-specific inositol phosphate mass determinations: high-performance liquid chromatography-micro-metal-dye detection strongly improves speed and sensitivity of analyses from cells and micro-enzyme assays

A microbore high-performance liquid chromatographic (HPLC) method is presented allowing rapid and sensitive mass analysis of inositol phosphates from cells and tissues. An analysis starting from inorganic phosphate up to inositol hexakisphosphate displaying a similar isomer selectivity as compared to the standard metal-dye detection system takes about 15 min. The detection sensitivity was about 15 pmol for inositol trisphosphate, about 10 pmol for inositol tetrakisphosphate, about 5 pmol for inositol pentakisphosphate and less than 5 pmol for inositol hexakisphosphate. The method was validated regarding day-to-day variations and variations at the same day of retention times and peak areas of standard inositol phosphates. Standard deviations of retention times ranged from 0.25 to 0.62% (same day) and from 0.64 to 1.61% (day-to-day variations). Ranges of standard deviations of peak areas were between 2.24% and 3.91% (same day) and 6.13% and 13.8% (day-to-day variations). Linearity of the post-column complexometric metal-dye detection system was demonstrated in the range of a few picomoles and at least 800 pmol. The method was applied to the analysis of inositol phosphates in Jurkat T-lymphocytes and assays from minute amounts of enzymes interconverting inositol phosphates. While measurements of inositol phosphates from cell extracts are now possible using significantly reduced cell numbers, micro-enzyme assays are feasible in reasonable repeated analysis times and with sufficient isomer selectivity. In conclusion, a substantial improvement towards speed of analysis and detection sensitivity of inositol phosphate mass analysis was achieved by microbore metal-dye detection HPLC.

Inositol phosphate structural requisites for Ca2+ influx

To understand how inositol phosphates (InsP) cause Ca2+ influx, we injected 37 highly purified compounds containing a total of 49 InsP positional isomers into Xenopus oocytes. The eight InsP that stimulated Ca2+ influx were those that had the highest potency at releasing intracellular Ca2+, indicating that their common target was the inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] receptor. To cause Ca2+ influx, these InsP had to be injected in a much higher concentration than the minimal concentration required to release intracellular Ca2+. Such high InsP concentrations could inhibit ongoing oscillatory intracellular Ca2+ release. In addition, we found that InsPs could not elicit further intracellular Ca2+ release during the course of Ca2+ influx. Our data are consistent with the "capacitative Ca2+ entry" hypothesis, which states that InsP stimulate Ca2+ influx by depleting the InsP-sensitive intracellular Ca2+ store. In this context, we would suggest that to deplete the InsP-sensitive intracellular Ca2+ store, InsP may have to be present in a sufficiently high concentration to override the oscillatory Ca(2+)-refilling mechanisms of the stores.

Direct visualization of a vast cortical calcium compartment in Paramecium by secondary ion mass spectrometry (SIMS) microscopy: possible involvement in exocytosis

The plasma membrane of ciliates is underlaid by a vast continuous array of membrane vesicles known as cortical alveoli. Previous work had shown that a purified fraction of these vesicles actively pumps calcium, suggesting that alveoli may constitute a calcium-storage compartment. Here we provide direct confirmation of this hypothesis using in situ visualization of total cell calcium on sections of cryofixed and cryosubstituted cells analyzed by SIMS (secondary ion mass spectrometry) microscopy a method never previously applied to protists. A narrow, continuous, Ca-emitting zone located all along the cell periphery was observed on sections including the cortex. In contrast, Na and K were evenly distributed throughout the cell. Various controls confirmed that emission was from the alveoli, in particular, the emitting zone was still seen in mutants totally lacking trichocysts, the large exocytotic organelles docked at the cell surface, indicating that they make no major direct contribution to the emission. Calcium concentration within alveoli was quantified for the first time in SIMS microscopy using an external reference and was found to be in the range of 3 to 5 mM, a value similar to that for sarcoplasmic reticulum. After massive induction of trichocyst discharge, this concentration was found to decrease by about 50%, suggesting that the alveoli are the main source of the calcium involved in exocytosis.

Second messenger specificity of the inositol trisphosphate receptor: reappraisal based on novel inositol phosphates

To further understand how the second messenger D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] interacts with its intracellular receptor, we injected 47 highly purified inositol phosphate (InsP) positional isomers in Xenopus oocytes and compared their potency in releasing intracellular Ca2+. The potency of the Ca(2+)-releasing InsPs spanned four orders of magnitude. Seven compounds, including the novel inositol 1,2,4,5-tetrakisphosphate [D/L-Ins (1,2,4,5)P4] and D/L-Ins(1,4,6)P3, had a very high potency. All of these highly active InsPs shared the following structure: two D-trans-equatorial phosphates (eq-P) and one equatorial hydroxyl (eq-OH) attached to ring carbons D-4, D-5, and D-6 (or to the structurally equivalent D-1, D-6, and D-5 carbons). This permissive structure was not sufficient for Ca2+ release, because it was also found in two inactive compounds, Ins(1,6)P2 and Ins(1,3,6)P3. To be active, InsPs also required the structural equivalent of a D-3 eq-OH and/or a D-1 eq-P. Together, our data reveal how the structure of the InsP molecule affects its ability to release Ca2+.

31P-NMR analysis of Entamoeba histolytica. Occurrence of high amounts of two inositol phosphates

Perchloric-acid extracts of axenic Entamoeba histolytica were investigated by 31P-NMR spectroscopy. All major 31P resonances observed were assigned to specific compounds. The cells contained inorganic phosphate (1039 nmol/g wet cells), pyrophosphate (16 nmol/g wet cells), nucleoside diphosphates (91 nmol/g wet cells), nucleoside triphosphates (275 nmol/g wet cells), NAD(P) (60 nmol/g wet cells), phosphocholine (184 nmol/g wet cells), phosphoethanolamine (214 nmol/g wet cells), cytidine 5'-diphosphocholine (41 nmol/g wet cells) and cytidine 5'-diphosphoethanolamine (55 nmol/g wet cells). The latter four compounds may act as intermediates in the salvage pathway for the synthesis of phosphatidylethanolamine and phosphatidylcholine. E. histolytica trophozoites also contained two inositol phosphates in large quantities, InsP3 (0.26 mumol/g wet cells) and InsP7 (0.11 mumol/g wet cells). These components were identified by 31P-NMR, using homonuclear J-resolved and two-dimensional 1H-31P correlative, analyses as myo-inositol trisphosphate, Ins(2,4,6)P3, and pentakisphospho-myo-inositol diphosphate, Ins(1,2,3,4,6)P5(5)P2.

Effects of inositol starvation on the levels of inositol phosphates and inositol lipids in Neurospora crassa

An inositol-requiring strain of Neurospora crassa was labelled during growth in liquid medium with [3H]inositol, and the levels of inositol phosphates and phosphoinositides were determined under inositol-sufficient and inositol-starved conditions. Because the mutant has an absolute requirement for inositol, the total mass of inositol-containing compounds could be determined. Inositol-containing lipids were identified by deacylation and co-migration with standards on h.p.l.c.; PtdIns3P, PtdIns4P, and PtdIns(4,5)P2 were found in approximately equal amounts, in addition to large amounts of PtdIns. Inositol starvation decreased the level of PtdIns to 10% of the sufficient level, and decreased the levels of the other phosphoinositides to about 25%. A number of inositol phosphates were found, including several InsP3s, InsP4s and InsP5s and phytic acid. Ins(1,4,5)P3 was identified by co-migration with standards on h.p.l.c. and by digestion with inositol phosphomonoesterase. High concentrations of all inositol phosphates were found in the extracellular medium in inositol-starved cultures. Inositol starvation on both liquid and solid agar media decreased the intracellular levels of some inositol phosphates, but increased the levels of phytic acid and several other inositol phosphates which may be its precursors and/or breakdown products. These results may indicate that inositol starvation induces phytic acid synthesis as a protection against the free-radical production and lipid peroxidation characteristic of inositol-less death.