Microassay of phosphate provides a general method for measuring the activity of phosphatases using physiological, nonchromogenic substrates such as lysophosphatidic acid

Tumour suppressor PTEN activity is differentially inducible by myo-inositol phosphates

Tumour evolution and efficacy of treatments are controlled by the microenvironment, the composition of which is primarily dependent on the angiogenic reaction to hypoxic stress. Tumour angiogenesis normalization is a challenge for adjuvant therapy strategies to chemo-, radio- and immunotherapeutics. Myo-inositol trispyrophosphate (ITPP) appears to provide the means to alleviate hypoxia in the tumour site by a double molecular mechanism. First, it modifies the properties of red blood cells (RBC) to release oxygen (O₂ ) in the hypoxic sites more easily, leading to a rapid and stable increase in the partial pressure of oxygen (pO₂ ). And second, it activates the endothelial phosphatase and tensin homologue deleted on Chromosome 10 (PTEN). The hypothesis that stable normalization of the vascular system is due to the PTEN, a tumour suppressor and phosphatase which controls the proper angiogenic reaction was ascertained. Here, by direct biochemical measurements of PTEN competitive activity in relation to PIP2 production, we show that the kinetics are complex in terms of the activation/inhibition effects of ITPP with an inverted consequence towards the kinase PI3K. The use of the surface plasmon resonance (SPR) technique allowed us to demonstrate that PTEN binds inositol derivatives differently but weakly. This method permitted us to reveal that PTEN is highly sensitive to the local concentration conditions, especially that ITPP increases the PTEN activity towards PIP3, and importantly, that PTEN affinity for ITPP is considerably increased by the presence of PIP3, as occurs in vivo. Our approach demonstrates the validity of using ITPP to activate PTEN for stable vessel normalization strategies.

Altered plasma levels of lysophospholipids in response to adrenalectomy of rats

The aim of this study was to investigate effects of reduced stress hormone by adrenalectomy on rat plasma levels of lysophosphatidic acid (LPA) and other lysophospholipids. We measured activities of lysophospholipase D (lysoPLD) in plasma and lipid phosphate phosphatase (LPP) in blood by determining choline and inorganic phosphate, respectively. LPA, lysophosphatidylcholine (LPC), lysophosphatidylethanolamine (LPE), lysophosphatidylinositol (LPI), lysophosphatidylserine (LPS) and lysophosphatodylglycerol were quantified by LC-MS/MS. In adrenalectomized rats, plasma levels of LPA, LPE, LPS and LPI, but not LPC, were increased. The increased level of LPA were due to decreased LPC level, increases plasma activity of lysoPLD toward LPC and decreased LPP activity toward LPA. Daily injections of deoxycoricosterone into rats selectively reversed increased level of LPS. Our results suggest enzymatic mechanism for increased plasma level of LPA, and indicate that the circulating levels of lysophospholipids including LPA in rats are differently affected by artificial suppression of release of adrenergic hormones.

A review of phosphatidate phosphatase assays

Phosphatidate phosphatase (PAP) catalyzes the penultimate step in the synthesis of triacylglycerol and regulates the synthesis of membrane phospholipids. There is much interest in this enzyme because it controls the cellular levels of its substrate, phosphatidate (PA), and product, DAG; defects in the metabolism of these lipid intermediates are the basis for lipid-based diseases such as obesity, lipodystrophy, and inflammation. The measurement of PAP activity is required for studies aimed at understanding its mechanisms of action, how it is regulated, and for screening its activators and/or inhibitors. Enzyme activity is determined through the use of radioactive and nonradioactive assays that measure the product, DAG, or Pi However, sensitivity and ease of use are variable across these methods. This review summarizes approaches to synthesize radioactive PA, to analyze radioactive and nonradioactive products, DAG and Pi, and discusses the advantages and disadvantages of each PAP assay.

Semi-rationally engineered variants of S-adenosylmethionine synthetase from Escherichia coli with reduced product inhibition and improved catalytic activity

S-adenosylmethionine synthetase (MAT) catalyzes the synthesis of S-adenosylmethionine (SAM) from ATP and L-methionine. SAM is the major methyl donor for more than 100 transmethylation reactions. It is also a common cosubstrate involved in transsulfuration and aminopropylation. However, product inhibition largely restrains the activity of MAT and limits the enzymatic synthesis of SAM. In this research, the product inhibition of MAT from Escherichia coli was reduced via semi-rational modification. A triple variant (Variant III, I303 V/I65 V/L186 V) showed a 42-fold increase in K_i,ATP and a 2.08-fold increase in specific activity when compared to wild-type MAT. Its K_i,ATP was 0.42 mM and specific acitivity was 3.78 ±0.19 U/mg. Increased K_i,ATP means reduced product inhibition which enhances SAM accumulation. The SAM produced by Variant III could reach to 3.27 mM while SAM produced by wild-type MAT was 1.62 mM in the presence of 10 mM substrates. When the residue in 104^th of Variant III was further optimized by site-saturated mutagenesis, the specific activity of Variant IV (I303 V/I65 V/L186 V/N104 K) reached to 6.02 ±0.22 U/mg at 37 °C, though the SAM concentration decreased to 2.68 mM with 10 mM substrates. Analysis of protein 3D structure suggests that changes in hydrogen bonds or other ligand interactions around active site may account for the variety of product inhibition and enzyme activity. The Variant III and Variant IV with reduced inhibition and improved enzyme activity in the study would be more suitable candidates for SAM production in the future.

The Catalytic Efficiency of Lipin 1β Increases by Physically Interacting with the Proto-oncoprotein c-Fos

Phosphatidic acid (PA) is a central precursor for membrane phospholipid biosynthesis. The lipin family is a magnesium-dependent type I PA phosphatase involved in de novo synthesis of neutral lipids and phospholipids. The regulation of lipin activity may govern the pathways by which these lipids are synthesized and control the cellular levels of important signaling lipids. Moreover, the proto-oncoprotein c-Fos has an emerging role in glycerolipid synthesis regulation; by interacting with key synthesizing enzymes it is able to increase overall phospho- and glycolipid synthesis. We studied the lipin 1β enzyme activity in a cell-free system using PA/Triton X-100 mixed micelles as substrate, analyzing it in the presence/absence of c-Fos. We found that lipin 1β kcat value increases around 40% in the presence of c-Fos, with no change in the lipin 1β affinity for the PA/Triton X-100 mixed micelles. We also probed a physical interaction between both proteins. Although the c-Fos domain involved in lipin activation is its basic domain, the interaction domain is mapped to the N-terminal c-Fos. In conclusion, we provide evidence for a novel positive regulator of lipin 1β PA phosphatase activity that is not achieved via altering its subcellular localization or affinity for membranes but rather through directly increasing its catalytic efficiency.

Characterization of the yeast actin patch protein App1p phosphatidate phosphatase

Yeast App1p is a phosphatidate phosphatase (PAP) that associates with endocytic proteins at cortical actin patches. App1p, which catalyzes the conversion of phosphatidate (PA) to diacylglycerol, is unique among Mg(2+)-dependent PAP enzymes in that its reaction is not involved with de novo lipid synthesis. Instead, App1p PAP is thought to play a role in endocytosis because its substrate and product facilitate membrane fission/fusion events and regulate enzymes that govern vesicular movement. App1p PAP was purified from yeast and characterized with respect to its enzymological, kinetic, and regulatory properties. Maximum PAP activity was dependent on Triton X-100 (20 mm), PA (2 mm), Mg(2+) (0.5 mm), and 2-mercaptoethanol (10 mm) at pH 7.5 and 30 °C. Analysis of surface dilution kinetics with Triton X-100/PA-mixed micelles yielded constants for surface binding (Ks(A) = 11 mm), interfacial PA binding (Km(B) = 4.2 mol %), and catalytic efficiency (Vmax = 557 μmol/min/mg). The activation energy, turnover number, and equilibrium constant were 16.5 kcal/mol, 406 s(-1), and 16.2, respectively. PAP activity was stimulated by anionic lipids (cardiolipin, phosphatidylglycerol, phosphatidylserine, and CDP-diacylglycerol) and inhibited by zwitterionic (phosphatidylcholine and phosphatidylethanolamine) and cationic (sphinganine) lipids, nucleotides (ATP and CTP), N-ethylmaleimide, propranolol, phenylglyoxal, and divalent cations (Ca(2+), Mn(2+), and Zn(2+)). App1p also utilized diacylglycerol pyrophosphate and lyso-PA as substrates with specificity constants 4- and 7-fold lower, respectively, when compared with PA.

Maturation of endogenous glucose production in preterm and term calves

Glucose disposability is often impaired in neonatal calves and even more in preterm calves. The objective of this study was to investigate ontogenic maturation of endogenous glucose production (eGP) in calves and its effects on postnatal glucose homeostasis. Calves (n = 7 per group) were born preterm (PT; delivered by section 9 d before term) or at term (T; spontaneous vaginal delivery), or spontaneously born and fed colostrum for 4 d (TC). Blood samples were taken immediately after birth and before and 2h after feeding at 24h after birth (PT; T) or on d 4 of life (TC) to determine metabolic and endocrine changes. After birth (PT and T) or on d 3 of life (TC), fasted calves were gavaged with deuterium-labeled water to determine gluconeogenesis (GNG) and intravenously infused with [U(13)C]-glucose to measure eGP and glucose oxidation (GOx) in blood plasma. After slaughter at 26h after birth (PT, T) or on d 4 of life (TC), glycogen concentrations in liver and hepatic mRNA concentrations and enzyme activities of pyruvate carboxylase, phosphoenolpyruvate carboxykinase (PEPCK), and glucose-6-phosphatase were measured. Preterm calves had the lowest plasma concentrations of cortisol and 3,5,3'-triiodothyronine at birth. Plasma glucose concentrations from d 1 to 2 decreased more, but plasma concentrations of lactate and urea and glucagon:insulin ratio were higher in PT than in T and TC calves. The eGP, GNG, GOx, as well as hepatic glycogen concentrations and PEPCK activities, were lowest in PT calves. Results indicate impaired glucose homeostasis due to decreased eGP in PT calves and maturation of eGP with ontogenic development.

Intestinal glucose absorption but not endogenous glucose production differs between colostrum- and formula-fed neonatal calves

Glucose supply markedly changes during the transition to extrauterine life. In this study, we investigated diet effects on glucose metabolism in neonatal calves. Calves were fed colostrum (C; n = 7) or milk-based formula (F; n = 7) with similar nutrient content up to d 4 of life. Blood plasma samples were taken daily before feeding and 2 h after feeding on d 4 to measure glucose, lactate, nonesterified fatty acids, protein, urea, insulin, glucagon, and cortisol concentrations. On d 2, additional blood samples were taken to measure glucose first-pass uptake (FPU) and turnover by oral [U-(13)C]-glucose and i.v. [6,6-(2)H(2)]-glucose infusion. On d 3, endogenous glucose production and gluconeogenesis were determined by i.v. [U-(13)C]-glucose and oral deuterated water administration after overnight feed deprivation. Liver tissue was obtained 2 h after feeding on d 4 and glycogen concentration and activities and mRNA abundance of gluconeogenic enzymes were measured. Plasma glucose and protein concentrations and hepatic glycogen concentration were higher (P < 0.05), whereas plasma urea, glucagon, and cortisol (d 2) concentrations as well as hepatic pyruvate carboxylase mRNA level and activity were lower (P < 0.05) in group C than in group F. Orally administered [U-(13)C]-glucose in blood was higher (P < 0.05) but FPU tended to be lower (P < 0.1) in group C than in group F. The improved glucose status in group C resulted from enhanced oral glucose absorption. Metabolic and endocrine changes pointed to elevated amino acid degradation in group F, presumably to provide substrates to meet energy requirements and to compensate for impaired oral glucose uptake.

Foot-and-mouth disease virus 2C is a hexameric AAA+ protein with a coordinated ATP hydrolysis mechanism

Foot-and-mouth disease virus (FMDV), a positive sense, single-stranded RNA virus, causes a highly contagious disease in cloven-hoofed livestock. Like other picornaviruses, FMDV has a conserved 2C protein assigned to the superfamily 3 helicases a group of AAA+ ATPases that has a predicted N-terminal membrane-binding amphipathic helix attached to the main ATPase domain. In infected cells, 2C is involved in the formation of membrane vesicles, where it co-localizes with viral RNA replication complexes, but its precise role in virus replication has not been elucidated. We show here that deletion of the predicted N-terminal amphipathic helix enables overexpression in Escherichia coli of a highly soluble truncated protein, 2C(34–318), that has ATPase and RNA binding activity. ATPase activity was abrogated by point mutations in the Walker A (K116A) and B (D160A) motifs and Motif C (N207A) in the active site. Unliganded 2C(34–318) exhibits concentration-dependent self-association to yield oligomeric forms, the largest of which is tetrameric. Strikingly, in the presence of ATP and RNA, FMDV 2C(34–318) containing the N207A mutation, which binds but does not hydrolyze ATP, was found to oligomerize specifically into hexamers. Visualization of FMDV 2C-ATP-RNA complexes by negative stain electron microscopy revealed hexameric ring structures with 6-fold symmetry that are characteristic of AAA+ ATPases. ATPase assays performed by mixing purified active and inactive 2C(34–318) subunits revealed a coordinated mechanism of ATP hydrolysis. Our results provide new insights into the structure and mechanism of picornavirus 2C proteins that will facilitate new investigations of their roles in infection.

The Saccharomyces cerevisiae PHM8 gene encodes a soluble magnesium-dependent lysophosphatidic acid phosphatase

Phosphate is the essential macronutrient required for the growth of all organisms. In Saccharomyces cerevisiae, phosphatases are up-regulated, and the level of lysophosphatidic acid (LPA) is drastically decreased under phosphate-starved conditions. The reduction in the LPA level is attributed to PHM8, a gene of unknown function. phm8Delta yeast showed a decreased LPA-hydrolyzing activity under phosphate-limiting conditions. Overexpression of PHM8 in yeast resulted in an increase in the LPA phosphatase activity in vivo. In vitro assays of the purified recombinant Phm8p revealed magnesium-dependent LPA phosphatase activity, with maximal activity at pH 6.5. The purified Phm8p did not hydrolyze any lipid phosphates other than LPA. In silico analysis suggest that Phm8p is a soluble protein with no transmembrane domain. Site-directed mutational studies revealed that aspartate residues in a DXDXT motif are important for the catalysis. These findings indicated that LPA plays a direct role in phosphate starvation. This is the first report of the identification and characterization of magnesium-dependent soluble LPA phosphatase.

Colorimetric determination of pure Mg(2+)-dependent phosphatidate phosphatase activity

The malachite green-molybdate reagent was used for a colorimetric assay of pure Mg2(+)-dependent phosphatidate phosphatase activity. This enzyme plays a major role in fat metabolism. Enzyme activity was linear with time and protein concentration, and with the concentration of water-soluble dioctanoyl phosphatidate. The colorimetric assay was used to examine enzyme inhibition by phenylglyoxal, propranolol, and dimethyl sulfoxide. Pure enzyme and a water-soluble phosphatidate substrate were required for the assay, which should be applicable to a well-defined large-scale screen of Mg2(+)-dependent phosphatidate phosphatise inhibitors (or activators).

Development of a spectrophotometric assay for cyclase activity

We describe the development of a rapid colorimetric assay for soluble guanylate cyclase (sGC) activity adapted for a 96-well microplate. The assay greatly decreases the analysis time and cost over traditional methodologies based on radio- and immunoassays and high-performance liquid chromatography (HPLC) separations. The method does not demonstrate any significant interference with chemicals commonly used for sGC purification and reaction kinetics. The assay converts the inorganic pyrophosphate produced in the cyclase reaction to inorganic phosphate, which is then measured using a modified Fiske-Subbarow assay. We used the assay to compare the reaction kinetics of preparations of sGC from a commercial source with those from our lab with Mg(2+)-guanosine 5'-triphosphate (GTP) or Mn(2+)-GTP as a substrate. The commercial preparation was found to have a specific activity of around 1.5 micromol/min/mg, which is significantly lower than expected, as was the fold-activation upon addition of nitric oxide (NO). Our laboratory preparation had a higher specific activity that was consistent with results from HPLC assays. We determined that the human isoform of sGC is more active in the basal and NO forms with Mn(2)-GTP as a substrate than Mg(2+)-GTP, a feature more similar to rat lung sGC than the more commonly studied bovine lung.

Rapid Purification of Native Dynamin I and Colorimetric GTPase Assay

Dynamin I is a large GTPase enzyme required in membrane constriction and fission during multiple forms of endocytosis. The first method described here is for the rapid purification of native dynamin from peripheral membrane extracts of sheep brain using ammonium sulfate precipitation and affinity purification on recombinant SH3 domains. The method greatly enriches for dynamin I at high purity and allows for large-scale biochemical and functional studies. The second method is a nonradioactive, high-throughput colorimetric GTPase assay for dynamin activity. The approach is based on terminating incubations with EDTA and the use of malachite green for high-sensitivity detection of inorganic phosphate release. The two methods will facilitate high-throughput screens for potential dynamin inhibitors or activators.

Bismuth citrate in the quantification of inorganic phosphate and its utility in the determination of membrane-bound phosphatases

This paper describes a rapid and sensitive method to determine inorganic phosphate, even in the presence of labile organic phosphate compounds and large quantities of proteins. The method eliminates the use of sodium arsenite, a highly toxic compound, substituting bismuth citrate for it to stabilize the phosphomolybdic acid complex formed during the interaction of inorganic phosphate and molybdate reduced by ascorbic acid. This method has also been adapted to microplates and has been used to determine the activities of Na/K ATPase and alkaline phosphatase of intestinal basolateral and luminal plasma membranes.