Inorganic pyrophosphatase from bovine retinal rod outer segments

Identification of the inorganic pyrophosphate metabolizing, ATP substituting pathway in mammalian spermatozoa

Inorganic pyrophosphate (PPi) is generated by ATP hydrolysis in the cells and also present in extracellular matrix, cartilage and bodily fluids. Fueling an alternative pathway for energy production in cells, PPi is hydrolyzed by inorganic pyrophosphatase (PPA1) in a highly exergonic reaction that can under certain conditions substitute for ATP-derived energy. Recombinant PPA1 is used for energy-regeneration in the cell-free systems used to study the zymology of ATP-dependent ubiquitin-proteasome system, including the role of sperm-borne proteasomes in mammalian fertilization. Inspired by an observation of reduced in vitro fertilization (IVF) rates in the presence of external, recombinant PPA1, this study reveals, for the first time, the presence of PPi, PPA1 and PPi transporter, progressive ankylosis protein ANKH in mammalian spermatozoa. Addition of PPi during porcine IVF increased fertilization rates significantly and in a dose-dependent manner. Fluorometric assay detected high levels of PPi in porcine seminal plasma, oviductal fluid and spermatozoa. Immunofluorescence detected PPA1 in the postacrosomal sheath (PAS) and connecting piece of boar spermatozoa; ANKH was present in the sperm head PAS and equatorial segment. Both ANKH and PPA1 were also detected in human and mouse spermatozoa, and in porcine spermatids. Higher proteasomal-proteolytic activity, indispensable for fertilization, was measured in spermatozoa preserved with PPi. The identification of an alternative, PPi dependent pathway for ATP production in spermatozoa elevates our understanding of sperm physiology and sets the stage for the improvement of semen extenders, storage media and IVF media for animal biotechnology and human assisted reproductive therapies.

Mass spectrometric proteome analysis suggests anaerobic shift in metabolism of Dauer larvae of Caenorhabditis elegans

The Dauer larva is a non-feeding alternative larval stage of some nematodes specialized for long-term survival and dispersal. In this study we compared proteome maps obtained from Dauer larvae with those from the corresponding third larval stage (L3) of the feeding life cycle of C. elegans wild-type strain N2. We demonstrate at the protein level that altered metabolism may participate in longevity determination of Dauers. We detected huge amounts of alcohol dehydrogenase (CE12212) and aldehyde dehydrogenase (CE29809) in Dauer animals, indicating highly active fermentative pathways. Inorganic pyrophosphatase (CE05448) that enables to metabolize pyrophosphate as a high-energy source was over-expressed in Dauers. An interesting differentially expressed protein was phosphatidylethanolamine-binding protein (CE38516) that was found in high abundance in samples from Dauer larvae. Protein synthesis may be lowered in Dauer animals by the reduced expression of splicing factor rsp-3 (CE31089) and methionyl-tRNA synthase (CE34219). We observed significantly lower amounts of the pepsin-like aspartyl protease 1 (CE21681) in non-feeding Dauers, which is in agreement with reduced nutrient digestion. Finally, the hypothetical protein R08E5.2 (CE33294) was present in high abundance in L3 animals.

Human mitochondrial pyrophosphatase: cDNA cloning and analysis of the gene in patients with mtDNA depletion syndromes

Pyrophosphatases (PPases) catalyze the hydrolysis of inorganic pyrophosphate generated in several cellular enzymatic reactions. A novel human pyrophosphatase cDNA encoding a 334-amino-acid protein approximately 60% identical to the previously identified human cytosolic PPase was cloned and characterized. The novel enzyme, named PPase-2, was enzymatically active and catalyzed hydrolysis of pyrophosphate at a rate similar to that of the previously identified PPase-1. A functional mitochondrial import signal sequence was identified in the N-terminus of PPase-2, which targeted the enzyme to the mitochondrial matrix. The human pyrophosphatase 2 gene (PPase-2) was mapped to chromosome 4q25 and the 1.4-kb mRNA was ubiquitously expressed in human tissues, with highest levels in muscle, liver, and kidney. The yeast homologue of the mitochondrial PPase-2 is required for mitochondrial DNA maintenance and yeast cells lacking the enzyme exhibit mitochondrial DNA depletion. We sequenced the PPA2 gene in 13 patients with mitochondrial DNA depletion syndromes (MDS) of unknown cause to determine if mutations in the PPA2 gene of these patients were associated with this disease. No pathogenic mutations were identified in the PPA2 gene of these patients and we found no evidence that PPA2 gene mutations are a common cause of MDS in humans.

Expression of new human inorganic pyrophosphatase in thyroid diseases: its intimate association with hyperthyroidism

Inorganic pyrophosphatase (PPase) controls the level of inorganic pyrophosphate produced by biosynthesis of protein, RNA, and DNA. Thus, PPase is essential for life. PPase expression is unclear in the thyroid. We cloned a new human PPase, phospholysine phosphohistidine inorganic pyrophosphate phosphatase (LHPPase), and established a rabbit polyclonal anti-LHPPase antibody. This is the first study to determine the PPase expression by immunohistochemistry and Western blot. Intranuclear LHPPase expression of thyrocytes was enhanced most prominently in Graves' disease and autonomously functional thyroid nodule. To estimate a regulating factor of subcellular localization of LHPPase, we examined its expression of Graves' disease-derived thyrocytes in vitro with the disease-originated serum. Nuclear expression of LHPPase was lost in cultured thyrocytes even with the serum, while its cytoplasmic expression was retained. The data suggest that increased expression of LHPPase is associated with hyperthyroidism. Intranuclear expression of LHPPase may not be regulated by Graves' disease-derived serum factors.

Pyrophosphatase of the roundworm Ascaris suum plays an essential role in the worm's molting and development

Previous studies indicated that inorganic pyrophosphatase of Ascaris suum (AsPPase) plays an important role in larval survival in the host. Here we describe a precise role for AsPPase in larval molting and development and also describe the potential role of recombinant AsPPase (rAsPPase) in protective immunity to A. suum infection. Using reverse transcriptase PCR analysis, we found that disruption of AsPPase gene function by RNA interference resulted in suppression of AsPPase mRNA levels. RNA interference also caused inhibition of molting of third-stage larvae (31%) and suppression of native protein expression, as demonstrated by a 56% reduction in enzyme activity and quantified by immunoblot and immunofluorescence analyses, suggesting that AsPPase has a role in the molting process. The anatomic location of the AsPPase native enzyme in the hypodermis of larvae along with its elevated expression prior to and during the molting process supports such a role. Anti-rAsPPase immunoglobulin G (IgG) also resulted in 57% inhibition of molting of A. suum lung-stage third-stage larvae to fourth-stage larvae in vitro with developmental arrest. Antigenic epitopes of AsPPase overlapped the enzyme active sites. Mice immunized with rAsPPase exhibited high antigen-specific IgG antibody responses and were protected (>70%) against a challenge A. suum migratory-phase infection. Splenic T cells from rAsPPase-immunized mice produced low levels of T helper 1-type cytokines (gamma interferon and interleukin-2) in vitro but exhibited an elevated interleukin-10 response. A significantly high level of IgG1 subclass antibodies was found in immunized mice. Our results establish that AsPPase has a critical role in the molting and development of Ascaris roundworms and suggest the potential of AsPPase for use as a candidate vaccine against ascariasis.

Temperature and metal ions-dependent activity of the family I inorganic pyrophosphatase from the swine roundworm Ascaris suum

Temperature dependence, heat stability and metal ions-dependent activity were examined on the Family I inorganic pyrophosphatase (PPase) recently identified from Ascaris suum. Recombinant A. suum PPase (rAsPPase) showed an optimal activity at 55 degrees C. The rAsPPase was heat stable at 40 degrees C in the absence of added Mg(2+) and at 50 degrees C in its presence. The enzyme required divalent metal ions for its activity. The preferences for the metal ions (5 mM concentration) were in the order: Mg(2+)> Co(2+)> Cu(2+)> Fe(2+)> Zn(2+)> Mn(2+). On the contrary, enzyme activity was inhibited by Ca(2+). These findings suggest that catalytic features of AsPPase are consistent with the Family I PPases reported from a wide range of organisms.

A novel calcium-dependent soluble inorganic pyrophosphatase from the trypanosomatidLeishmania major

A single-copy gene IPP encoding a putative soluble inorganic pyrophosphatase (LmsPPase, EC 3.6.1.1) was identified in the genome of the parasite protozoan Leishmania major. The full-length coding sequence (ca. 0.8 kb) was obtained from genomic DNA by polymerase chain reaction (PCR) and cloned into an Escherichia coli expression vector, and was overexpressed for functional protein purification and characterization. The recombinant LmsPPase, purified to electrophoretic homogeneity by a two-step chromatography procedure, exhibited a predicted molecular mass of ca. 30 kDa. The enzyme has an absolute requirement for divalent cations, exhibits a pH optimum of 7.5-8.0 and does not hydrolyze polyphosphates or adenosine triphosphate (ATP). LmsPPase differs from previously studied soluble pyrophosphatases with respect to cation selectivity, Ca(2+) being far more effective than Mg(2+). Comparisons to known sPPases show a short N-terminal extension predicted to be a mitochondrial transit peptide, and changes in active-site residues and the neighboring region. Subcellular fractionation of L. major promastigotes suggests a mitochondrial localization. Molecular phylogenetic analysis indicates that LmsPPase is a highly divergent eukaryotic Family I sPPase, perhaps an ancestral class of eukaryotic sPPases functionally adapted to a calcium-rich, probably mitochondrial, environment.

Inorganic pyrophosphatase in the roundworm Ascaris and its role in the development and molting process of the larval stage parasites

Inorganic pyrophosphatase (PPase) is an important enzyme that catalyzes the hydrolysis of inorganic pyrophosphate (PPi) into ortho-phosphate (Pi). We report here the molecular cloning and characterization of a gene encoding the soluble PPase of the roundworm Ascaris suum. The predicted A. suum PPase consists of 360 amino acids with a molecular mass of 40.6 kDa and a pI of 7.1. Amino acid sequence alignment and phylogenetic analysis indicates that the gene encodes a functional Family I soluble PPase containing features identical to those of prokaryotic, plant and animal/fungal soluble PPases. The Escherichia coli-expressed recombinant enzyme has a specific activity of 937 micro mol Pi.min-1.mg-1 protein corresponding to a kcat value of 638 s-1 at 55 degrees C. Its activity was strongly dependent on Mg2+ and was inhibited by Ca2+. Native PPases were expressed in all developmental stages of A. suum. A homolog was also detected in the most closely related human and dog roundworms A. lumbricoides and Toxocara canis, respectively. The enzyme was intensely localized in the body wall, gut epithelium, ovary and uterus of adult female worms. We observed that native PPase activity together with development and molting in vitro of A. suum L3 to L4 were efficiently inhibited in a dose-dependent manner by imidodiphosphate and sodium fluoride, which are potent inhibitor of both soluble- and membrane-bound H+-PPases. The studies provide evidence that the PPases are novel enzymes in the roundworm Ascaris, and may have crucial role in the development and molting process.

Method for real-time detection of inorganic pyrophosphatase activity

A sensitive and simple method for real-time detection of inorganic pyrophosphatase (PPase) (EC 3.6.1.1) activity has been developed. The method is based on PPase-induced activation of the firefly luciferase activity in the presence of inorganic pyrophosphate (PPi). PPi inhibits the luciferase activity, but in the presence of PPase the luciferase activity is restored and the luminescence output increases. The assay yields linear responses between 8 and 500 mU. The detection limit was found to be 8 mU PPase. The method was used to detect the hydrolytic activity of PPases from Saccharomyces cerevisiae, Escherichia coli, and Bacillus stearothermophilus. As substrate for the luciferase, adenosine 5'-phosphosulfate can replace ATP, which is an advantage for detection of PPase activity in crude extracts containing ATP-hydrolyzing activities. The method can be used for kinetic and inhibition studies as well as for detection of PPase activity during different purification procedures.

Inorganic pyrophosphate generation and disposition in pathophysiology

Inorganic pyrophosphate (PP(i)) regulates certain intracellular functions and extracellular crystal deposition. PP(i) is produced, degraded, and transported by specialized mechanisms. Moreover, dysregulated cellular PP(i) production, degradation, and transport all have been associated with disease, and PP(i) appears to directly mediate specific disease manifestations. In addition, natural and synthetic analogs of PP(i) are in use or currently under evaluation as prophylactic agents or therapies for disease. This review summarizes recent developments in the understanding of how PP(i) is made and disposed of by cells and assesses the body of evidence for potentially significant physiological functions of intracellular PP(i) in higher organisms. Major topics addressed are recent lines of molecular evidence that directly link decreased and increased extracellular PP(i) levels with diseases in which connective tissue matrix calcification is disordered. To illustrate in depth the effects of disordered PP(i) metabolism, this review weighs the roles in matrix calcification of the transmembrane protein ANK, which regulates intracellular to extracellular movement of PP(i), and the PP(i)-generating phosphodiesterase nucleotide pyrophosphatase family isoenzyme plasma cell membrane glycoprotein-1 (PC-1).

Vertebrate photoreceptors

The basis of the duplex theory of vision is examined in view of the dazzling array of data on visual pigment sequences and the pigments they form, on the microspectrophotometry measurements of single photoreceptor cells, on the kinds of photoreceptor cascade enzymes, and on the electrophysiological properties of photoreceptors. The implications of the existence of five distinct visual pigment families are explored, especially with regard to what pigments are in what types of photoreceptors, if there are different phototransduction enzymes associated with different types of photoreceptors, and if there are electrophysiological differences between different types of cones.

High-throughput assay for inorganic pyrophosphatases using the cytosolic enzymes of Saccharomyces cerevisiae and human as an example

This paper describes the development of a new, malachite green based, enzymatic assay for the identification of specific inhibitors of inorganic pyrophosphatase (iPPase) from Saccharomyces cerevisiae for antifungal drug discovery. The human iPPase was used as counterscreen. The coding regions of both enzymes were amplified, cloned into a vector providing a His-tag at the C-terminus, expressed in Escherichia coli, and purified by metal chelate affinity chromatography. Since the complete human sequence had not been published previously, the human iPPase was cloned on the basis of expressed sequence tag data. The human sequence was confirmed and showed about 55% amino acid identity with the yeast enzyme and 95% identity with an already published bovine enzyme. Both recombinant iPPases were characterized with regard to their biochemical properties, showing that the His-tag did not influence the specific activity, pH optimum, inhibitor profile, or dimerization. The enzyme activity was determined by quantifying released phosphate by complex formation with malachite green. The resulting complex was quantified spectrophotometrically. The assay was adapted to a microtiter plate format. Thus, it is possible to screen a large compound pool for iPPase inhibitors in a short period of time.

Bacillus subtilis ORF yybQ encodes a manganese-dependent inorganic pyrophosphatase with distinctive properties: the first of a new class of soluble pyrophosphatase?

The N-terminal 15 amino acids of the major protein associated with inorganic pyrophosphatase activity in Bacillus subtilis WB600 are identical to those of B. subtilis ORF yybQ. This ORF was amplified from B. subtilis WB600 DNA by PCR and cloned into an overexpression vector in Escherichia coli. Induction of overexpression produced a soluble protein of 34,000 Da by SDS-PAGE and by matrix-assisted laser desorption and ionization mass spectrometry. The overexpressed protein had a high specific activity for the hydrolysis of magnesium pyrophosphate, and was specifically and reversibly activated by Mn2+ ions. These properties are identical to those of inorganic pyrophosphatase purified from B. subtilis WB600. No significant similarity was found between the derived sequence of the B. subtilis yybQ-encoded protein and published sequences of identified inorganic pyrophosphatases of Eukarya, Bacteria or Archaea domains. However, there is significant similarity to three putative proteins of unknown function from the archaea Methanococcus jannaschii and Archaeoglobus fulgidus, and from Streptococcus gordonii. The genomes of B. subtilis, M. jannaschii and A. fulgidus do not contain sequences similar to those of hitherto known soluble inorganic pyrophosphatases. The present findings, together with a survey of the properties of inorganic pyrophosphatases from 38 different sources, suggest that the B. subtilis yybQ-encoded protein is the first fully characterized member of a new class of inorganic pyrophosphatase.

3-phosphohistidine and 6-phospholysine are substrates of a 56-kDa inorganic pyrophosphatase from bovine liver

A 56-kDa inorganic pyrophosphatase isolated from bovine liver hydrolyzed PPi, imidodiphosphate, 3-phosphohistidine, and 6-phospholysine at rates of 0.11, 0.44, 1.09, and 1.22 mumol/min/mg protein, respectively, in a reaction mixture containing 1 mM MgCl2 at pH 8.2. The hydrolysis of imidodiphosphate was influenced by various treatments in a different manner from that of N-phosphorylated amino acids, indicating that the pyrophosphatase has two different catalytic sites for imidodiphosphate and N-phosphorylated amino acids, respectively. Evidence for separate catalytic sites consists of the following findings: the activity on hydrolysis of imidodiphosphate gave a bell-shaped pH curve with a peak at pH 6.5, while the activity on hydrolysis of N-phosphorylated amino acids maintained a high level in the pH range between 6.0 and 9.5. One hundred micromolar p-chloromercuriphenyl sulfonate inhibited the hydrolysis of imidodiphosphate by 35% and did not inhibit that of N-phosphorylated amino acids. Two millimolar magnesium chloride repressed the hydrolysis of imidodiphosphate and had no inhibitory effect on the hydrolysis of N-phosphorylated amino acids. Moreover, methylenediphosphonic acid, an analog of imidodiphosphate, stimulated the hydrolysis of imidodiphosphate in the presence of MgCl2, while it potentiated the substrate inhibition on hydrolysis of N-phosphorylated amino acids.

Purification and characterization of hepatic inorganic pyrophosphatase hydrolyzing imidodiphosphate

A 56-kDa inorganic pyrophosphatase consisting of 33-kDa subunits was purified from bovine liver almost to homogeneity. This hydrolase required divalent cations such as MgCl2, CoCl2, and MnCl2 to hydrolyze PPi and was insensitive to 2 mM sodium fluoride. The purified hydrolase released 2.1 mumol Pi from PPi per minute per milligram of protein in the presence of 1 mM MgCl2, and the Km for PPi was 0.14 mM. It also hydrolyzed imidodiphosphate to yield Pi and ammonia even in the absence of a divalent cation. The purified hydrolase liberated 2.2 mumol Pi from imidodiphosphate per minute per milligram of protein and the Km for imidodiphosphate was 0.12 mM. Addition of 80 microM MgCl2, CoCl2, or MnCl2 to the reaction mixture increased the hydrolysis rate of imidodiphosphate by 1.5-, 2.0-, and 3.4-fold, respectively. In the absence of divalent cations, the enzymatic hydrolysis of imidodiphosphate was inhibited competitively by PPi (Ki = 0.13 mM). Moreover, one-half of the maximal hydrolysis of imidodiphosphate was inhibited by 10 microM trans-1,2-diaminocyclohexane N,N,N',N'-tetraacetic acid or 45 microM p-chloromercuriphenyl sulfonate. When the hydrolase was treated with heat or SDS, two activities capable of hydrolyzing PPi and imidodiphosphate gave similar inactivation profiles, indicating that one hydrolase participated in the hydrolysis of both substrates.

Modulation of the calcium sensitivity of bovine retinal rod outer segment guanylyl cyclase by sodium ions and protein kinase A

Guanylyl cyclases (GC, EC 4.6.1.2) serve as receptors that produce cGMP in response to ligand binding. The production of cGMP is essential for the ability of retinal photoreceptor cells to restore the dark state after photoexcitation. GC activity is enhanced in rod outer segments (ROS) by a decrease in the cytosolic free Ca2+ concentration. We recently developed a new real-time assay to measure initial rates of ROS GC activity with much improved precision [Wolbring, G. & P. P. M. Schnetkamp (1995) Biochemistry 34, 4689-4695]. With this assay we examined the Ca2+ sensitivity of ROS GC, and we report here that protein kinase A-mediated phosphorylation and Na+ cause significant shifts in the IC50 for Ca2+ of the particulate guanylyl cyclase from bovine retinal rod outer segments. The IC50 for Ca2+ ranged between 30 and 270 nM Ca2+ dependent on the presence of Na+, choline, cAMP, cGMP, 8-bromo-cAMP, 8-bromo-cGMP, or the catalytic subunit of protein kinase A.

The mechanisms of vertebrate light adaptation: speeded recovery versus slowed activation

Light adaptation in vertebrate photoreceptors is commonly attributed to a feedback mechanism that reduces the amplitude of the receptor potential by speeding the inactivation of the transduction cascade and hastening the recovery process. Recent studies have challenged this model and suggest instead that desensitization originates mainly from changes in the activation phase rather than the recovery phase of the response. This has important implications for understanding the molecular mechanisms that underlie the control of sensitivity in this G-protein-coupled, signal-transduction pathway.

Control of photoreceptor proteins by Ca2+

A decrease of cytoplasmic Ca(2+)-concentration in vertebrate photoreceptor cells after illumination is necessary for light adaptation. Although the mechanisms of adaptation is not completely understood, several Ca(2+)-dependent cellular processes have been discovered. Some involve calcium-binding proteins like recoverin, guanylyl cyclase-activating protein and calmodulin, and their target proteins rhodopsin kinase, guanylyl cyclase, the cGMP-gated channel, and NO synthase. The activity of several enzymes or channels is directly controlled by Ca2+ and does not involve calcium-binding proteins. These proteins are pyrophosphatase, protein kinase C and the cGMP-gated channel.

Regulation of bovine rod outer segment membrane guanylate cyclase by ATP, phosphodiesterase and metal ions

In vertebrate retina, rod outer segment is the site of visual transduction. The inward cationic current in the dark-adapted outer segment is regulated by cyclic GMP. A light flash on the outer segment activates a cyclic GMP phosphodiesterase resulting in rapid hydrolysis of the cyclic nucleotide which in turn causes a decrease in the dark current. Restoration of the dark current requires inactivation of the phosphodiesterase and synthesis of cyclic GMP. The latter is accomplished by the enzyme guanylate cyclase which catalyzes the formation of cyclic GMP from GTP. Therefore, factors regulating the cyclase activity play a critical role in visual transduction. But regulation of the cyclase by some of these factors--phosphodiesterase, ATP, the soluble proteins and metal cofactors (Mg and Mn)--is controversial. The availability of different types of cyclase preparations, dark-adapted rod outer segments with fully inhibited phosphodiesterase activity, partially purified cyclase without PDE contamination, cloned rod outer segment cyclase free of other rod outer segment proteins, permitted us to address these controversial issues. The results show that ATP inhibits the basal cyclase activity but enhances the stimulation of the enzyme by soluble activator, that cyclase can be activated in the dark at low calcium concentrations under conditions where phosphodiesterase activity is fully suppressed, and that greater activity is observed with manganese as cofactor than magnesium. These results provide a better understanding of the controls on cyclase activity in rod outer segments and suggest how regulation of this cyclase by ATP differs from that of other known membrane guanylate cyclases.

A rich complexity emerges in phototransduction

Over the past two decades there has been an explosive growth in our understanding of phototransduction, leading to the development of a comprehensive scheme for the process. On the basis of this scheme the finer details of the process are being elucidated. Additional protein components and pathways have been identified, successful quantitative models of parts of the process have been developed, and a detailed understanding of the molecular basis of physiological function has begun to emerge. Here we summarize the most recent developments.