cDNA cloning of human and rat brain myo-inositol monophosphatase. Expression and characterization of the human recombinant enzyme

Myo-D-inositol Trisphosphate Signalling in Oomycetes

Oomycetes are pathogens of plants and animals, which cause billions of dollars of global losses to the agriculture, aquaculture and forestry sectors each year. These organisms superficially resemble fungi, with an archetype being Phytophthora infestans, the cause of late blight of tomatoes and potatoes. Comparison of the physiology of oomycetes with that of other organisms, such as plants and animals, may provide new routes to selectively combat these pathogens. In most eukaryotes, myo-inositol 1,4,5 trisphosphate is a key second messenger that links extracellular stimuli to increases in cytoplasmic Ca2+, to regulate cellular activities. In the work presented in this study, investigation of the molecular components of myo-inositol 1,4,5 trisphosphate signaling in oomycetes has unveiled similarities and differences with that in other eukaryotes. Most striking is that several oomycete species lack detectable phosphoinositide-selective phospholipase C homologues, the enzyme family that generates this second messenger, but still possess relatives of myo-inositol 1,4,5 trisphosphate-gated Ca2+-channels.

Osmoregulation by the myo-inositol biosynthesis pathway in turbot Scophthalmus maximus and its regulation by anabolite and c-Myc

The induction of the myo-inositol biosynthesis (MIB) pathway in euryhaline fishes is an important component of the cellular response to osmotic challenge. The MIPS and IMPA1 genes were sequenced in turbot and found to be highly conserved in phylogenetic evolution, especially within the fish species tested. Under salinity stress in turbot, both MIPS and IMPA1 showed adaptive expression, a turning point in the level of expression occurred at 12 h in all tissues tested. We performed an RNAi assay mediated by long fragment dsRNA prepared by transcription in vitro. The findings demonstrated that knockdown of the MIB pathway weakened the function of gill osmotic regulation, and may induce a genetic compensation response in the kidney and gill to maintain physiological function. Even though the gill and kidney conducted stress reactions or compensatory responses to salinity stress, this inadequately addressed the consequences of MIB knockdown. Therefore, the survival time of turbot under salinity stress after knockdown was obviously less than that under seawater, especially under low salt stress. Pearson's correlation analysis between gene expression and dietary myo-inositol concentration indicated that the MIB pathway had a remarkable negative feedback control, and the dynamic equilibrium mediated by negative feedback on the MIB pathway played a crucial role in osmoregulation in turbot. An RNAi assay with c-Myc in vivo and the use of a c-Myc inhibitor (10058-F4) in vitro demonstrated that c-Myc was likely to positively regulate the MIB pathway in turbot.

Co-crystallization of human inositol monophosphatase with the lithium mimetic L-690,330

Lithium, which is still the gold standard in the treatment of bipolar disorder, has been proposed to inhibit inositol monophosphatase (IMPase) and is hypothesized to exert its therapeutic effects by attenuating phosphatidylinositol (PI) cell signalling. Drug-discovery efforts have focused on small-molecule lithium mimetics that would specifically inhibit IMPase without exhibiting the undesired side effects of lithium. L-690,330 is a potent bisphosphonate substrate-based inhibitor developed by Merck Sharp & Dohme. To aid future structure-based inhibitor design, determination of the exact binding mechanism of L-690,330 to IMPase was of interest. Here, the high-resolution X-ray structure of human IMPase in complex with L690,330 and manganese ions determined at 1.39 Å resolution is reported.

High-Throughput Cell-Based Screening Using Scintillation Proximity Assay for the Discovery of Inositol Phosphatase Inhibitors

Inositol monophosphatase is a potential drug target for developing lithium-mimetic agents for the treatment of bipolar disorder. Enzyme-based assays have been traditionally used in compound screening to identify inositol monophosphatase inhibitors. A cell-based screening assay in which the compound needs to cross the cell membrane before reaching the target enzyme offers a new approach for discovering novel structure leads of the inositol monophosphatase inhibitor. The authors have recently reported a high-throughput measurement of G-protein-coupled receptor activation by determining inositol phosphates in cell extracts using scintillation proximity assay. This cell-based assay has been modified to allow the determination of inositol monophosphatase activity instead of G-protein-coupled receptors. The enzyme is also assayed in its native form and physiological environment. The authors have applied this cell-based assay to the high-throughput screening of a large compound collection and identified several novel inositol monophosphatase inhibitors. ( Journal of Biomolecular Screening 2004:132-140)

Structural elucidation of the NADP(H) phosphatase activity of staphylococcal dual-specific IMPase/NADP(H) phosphatase

NADP(H)/NAD(H) homeostasis has long been identified to play a pivotal role in the mitigation of reactive oxygen stress (ROS) in the intracellular milieu and is therefore critical for the progression and pathogenesis of many diseases. NAD(H) kinases and NADP(H) phosphatases are two key players in this pathway. Despite structural evidence demonstrating the existence and mode of action of NAD(H) kinases, the specific annotation and the mode of action of NADP(H) phosphatases remains obscure. Here, structural evidence supporting the alternative role of inositol monophosphatase (IMPase) as an NADP(H) phosphatase is reported. Crystal structures of staphylococcal dual-specific IMPase/NADP(H) phosphatase (SaIMPase-I) in complex with the substrates D-myo-inositol-1-phosphate and NADP(+) have been solved. The structure of the SaIMPase-I-Ca(2+)-NADP(+) ternary complex reveals the catalytic mode of action of NADP(H) phosphatase. Moreover, structures of SaIMPase-I-Ca(2+)-substrate complexes have reinforced the earlier proposal that the length of the active-site-distant helix α4 and its preceding loop are the predisposing factors for the promiscuous substrate specificity of SaIMPase-I. Altogether, the evidence presented suggests that IMPase-family enzymes with a shorter α4 helix could be potential candidates for previously unreported NADP(H) phosphatase activity.

Defective craniofacial development and brain function in a mouse model for depletion of intracellular inositol synthesis

myo-Inositol is an essential biomolecule that is synthesized by myo-inositol monophosphatase (IMPase) from inositol monophosphate species. The enzymatic activity of IMPase is inhibited by lithium, a drug used for the treatment of mood swings seen in bipolar disorder. Therefore, myo-inositol is thought to have an important role in the mechanism of bipolar disorder, although the details remain elusive. We screened an ethyl nitrosourea mutant mouse library for IMPase gene (Impa) mutations and identified an Impa1 T95K missense mutation. The mutant protein possessed undetectable enzymatic activity. Homozygotes died perinatally, and E18.5 embryos exhibited striking developmental defects, including hypoplasia of the mandible and asymmetric fusion of ribs to the sternum. Perinatal lethality and morphological defects in homozygotes were rescued by dietary myo-inositol. Rescued homozygotes raised on normal drinking water after weaning exhibited a hyper-locomotive trait and prolonged circadian periods, as reported in rodents treated with lithium. Our mice should be advantageous, compared with those generated by the conventional gene knock-out strategy, because they carry minimal genomic damage, e.g. a point mutation. In conclusion, our results reveal critical roles for intracellular myo-inositol synthesis in craniofacial development and the maintenance of proper brain function. Furthermore, this mouse model for cellular inositol depletion could be beneficial for understanding the molecular mechanisms underlying the clinical effect of lithium and myo-inositol-mediated skeletal development.

The Concise Guide to PHARMACOLOGY 2013/14: enzymes

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Enzymes are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, nuclear hormone receptors, catalytic receptors and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

A safe lithium mimetic for bipolar disorder

Lithium is the most effective mood stabilizer for the treatment of bipolar disorder, but it is toxic at only twice the therapeutic dosage and has many undesirable side effects. It is likely that a small molecule could be found with lithium-like efficacy but without toxicity through target-based drug discovery; however, lithium’s therapeutic target remains equivocal. Inositol monophosphatase is a possible target but no bioavailable inhibitors exist. Here we report that the antioxidant ebselen inhibits inositol monophosphatase and induces lithium-like effects on mouse behaviour, which are reversed with inositol, consistent with a mechanism involving inhibition of inositol recycling. Ebselen is part of the National Institutes of Health Clinical Collection, a chemical library of bioavailable drugs considered clinically safe but without proven use. Therefore, ebselen represents a lithium mimetic with the potential both to validate inositol monophosphatase inhibition as a treatment for bipolar disorder and to serve as a treatment itself.

Cloning, expression, purification, crystallization and X-ray analysis of inositol monophosphatase from Mus musculus and Homo sapiens

Inositol monophosphatase (IMPase) catalyses the hydrolysis of inositol monophosphate to inositol and is crucial in the phosphatidylinositol (PI) signalling pathway. Lithium, which is the drug of choice for bipolar disorder, inhibits IMPase at therapeutically relevant plasma concentrations. Both mouse IMPase 1 (MmIMPase 1) and human IMPase 1 (HsIMPase 1) were cloned into pRSET5a, expressed in Escherichia coli, purified and crystallized using the sitting-drop method. The structures were solved at resolutions of 2.4 and 1.7 Å, respectively. Comparison of MmIMPase 1 and HsIMPase 1 revealed a core r.m.s. deviation of 0.516 Å.

Evaluation of the tolerability and immunogenicity of ultraviolet C-irradiated autologous platelets in a dog model

BACKGROUND

The THERAFLEX ultraviolet (UV) platelets (PLTs) pathogen reduction system for PLT concentrates (PCs) operates using ultraviolet C (UVC) light at a wavelength of 254 nm. UVC treatment can potentially alter proteins, which may affect drug tolerance in humans and influence the immunogenicity of blood products. This preclinical study in beagle dogs was designed to evaluate the safety pharmacology of UVC-irradiated PCs after intravenous administration and to determine whether they are capable of eliciting humoral responses to PLTs and plasma proteins.

STUDY DESIGN AND METHODS

Six beagle dogs each were transfused once every other week for 10 weeks with UVC-irradiated or nonirradiated PCs. All PCs were autologous canine single-donor products prepared from whole blood. Safety pharmacology variables were regularly assessed. The impact of UVC irradiation on PLT and plasma proteomes was analyzed by one- and two-dimensional gel electrophoresis. Serum samples were tested for UVC-induced antibodies by Western blot and flow cytometry.

RESULTS

Dogs transfused with UVC-irradiated PCs showed no signs of local or systemic intolerance. Few but significant changes in PLT protein integrity were observed after UVC irradiation. Even after repeated administration of UVC-irradiated PCs, no antibodies against UVC-exposed plasma or PLT proteins were detected.

CONCLUSIONS

Repeated transfusions of autologous UVC-treated PCs were well tolerated in all dogs studied. UVC irradiation did not cause significant plasma or PLT protein modifications capable of inducing specific antibody responses in the dogs. High-resolution proteomics combined with antibody analysis introduces a comprehensive and sensitive method for screening of protein modifications and antibodies specific for pathogen reduction treatment.

Expression and functions of myo-inositol monophosphatase family genes in seed development of Arabidopsis

Myo-inositol monophosphatase (IMP) catalyzes the dephosphorylation of myo-inositol 3-phosphate in the last step of myo-inositol biosynthesis. IMP is also important in phosphate metabolism and is required for the biosynthesis of cell wall polysaccharides, phytic acid, and phosphatidylinositol. In Arabidopsis, IMP is encoded by VTC4. There are, however, two additional IMP candidate genes, IMPL1 and IMPL2, which have not yet been elucidated. In our genetic studies of Arabidopsis IMP genes, only the loss-of-function mutant impl2 showed embryonic lethality at the globular stage. All IMP genes were expressed in a similar manner both in the vegetative and reproductive organs. In developing seeds, expression of IMP genes was not coupled with the expression of the genes encoding myo-inositol phosphate synthases, which supply the substrate for IMPs in the de novo synthesis pathway. Instead, expression of IMP genes was correlated with expression of the gene for myo-inositol polyphosphate 1-phosphatase (SAL1), which is involved in the myo-inositol salvage pathway, suggesting a possible salvage pathway role in seed development. Moreover, the partial rescue of the impl2 phenotype by histidine application implies that IMPL2 is also involved in histidine biosynthesis during embryo development.

glpX gene of Mycobacterium tuberculosis: heterologous expression, purification, and enzymatic characterization of the encoded fructose 1,6-bisphosphatase II

The glpX gene (Rv1099c) of Mycobacterium tuberculosis (Mtb) encodes Fructose 1,6-bisphosphatase II (FBPase II; EC 3.1.3.11); a key gluconeogenic enzyme. Mtb possesses glpX homologue as the major known FBPase. This study explored the expression, purification and enzymatic characterization of functionally active FBPase II from Mtb. The glpX gene was cloned, expressed and purified using a two step purification strategy including affinity and size exclusion chromatography. The specific activity of Mtb FBPase II is 1.3 U/mg. The enzyme is oligomeric, followed Michaelis-Menten kinetics with an apparent km = 44 μM. Enzyme activity is dependent on bivalent metal ions and is inhibited by lithium and inorganic phosphate. The pH optimum and thermostability of the enzyme have been determined. The robust expression, purification and assay protocols ensure sufficient production of this protein for structural biology and screening of inhibitors against this enzyme.

VTC4 is a bifunctional enzyme that affects myoinositol and ascorbate biosynthesis in plants

Myoinositol synthesis and catabolism are crucial in many multiceullar eukaryotes for the production of phosphatidylinositol signaling molecules, glycerophosphoinositide membrane anchors, cell wall pectic noncellulosic polysaccharides, and several other molecules including ascorbate. Myoinositol monophosphatase (IMP) is a major enzyme required for the synthesis of myoinositol and the breakdown of myoinositol (1,4,5)trisphosphate, a potent second messenger involved in many biological activities. It has been shown that the VTC4 enzyme from kiwifruit (Actinidia deliciosa) has similarity to IMP and can hydrolyze l-galactose 1-phosphate (l-Gal 1-P), suggesting that this enzyme may be bifunctional and linked with two potential pathways of plant ascorbate synthesis. We describe here the kinetic comparison of the Arabidopsis (Arabidopsis thaliana) recombinant VTC4 with d-myoinositol 3-phosphate (d-Ins 3-P) and l-Gal 1-P. Purified VTC4 has only a small difference in the V(max)/K(m) for l-Gal 1-P as compared with d-Ins 3-P and can utilize other related substrates. Inhibition by either Ca(2+) or Li(+), known to disrupt cell signaling, was the same with both l-Gal 1-P and d-Ins 3-P. To determine whether the VTC4 gene impacts myoinositol synthesis in Arabidopsis, we isolated T-DNA knockout lines of VTC4 that exhibit small perturbations in abscisic acid, salt, and cold responses. Analysis of metabolite levels in vtc4 mutants showed that less myoinositol and ascorbate accumulate in these mutants. Therefore, VTC4 is a bifunctional enzyme that impacts both myoinositol and ascorbate synthesis pathways.

Structural and biochemical characterization of the type II fructose-1,6-bisphosphatase GlpX from Escherichia coli

Gluconeogenesis is an important metabolic pathway, which produces glucose from noncarbohydrate precursors such as organic acids, fatty acids, amino acids, or glycerol. Fructose-1,6-bisphosphatase, a key enzyme of gluconeogenesis, is found in all organisms, and five different classes of these enzymes have been identified. Here we demonstrate that Escherichia coli has two class II fructose-1,6-bisphosphatases, GlpX and YggF, which show different catalytic properties. We present the first crystal structure of a class II fructose-1,6-bisphosphatase (GlpX) determined in a free state and in the complex with a substrate (fructose 1,6-bisphosphate) or inhibitor (phosphate). The crystal structure of the ligand-free GlpX revealed a compact, globular shape with two alpha/beta-sandwich domains. The core fold of GlpX is structurally similar to that of Li+-sensitive phosphatases implying that they have a common evolutionary origin and catalytic mechanism. The structure of the GlpX complex with fructose 1,6-bisphosphate revealed that the active site is located between two domains and accommodates several conserved residues coordinating two metal ions and the substrate. The third metal ion is bound to phosphate 6 of the substrate. Inorganic phosphate strongly inhibited activity of both GlpX and YggF, and the crystal structure of the GlpX complex with phosphate demonstrated that the inhibitor molecule binds to the active site. Alanine replacement mutagenesis of GlpX identified 12 conserved residues important for activity and suggested that Thr(90) is the primary catalytic residue. Our data provide insight into the molecular mechanisms of the substrate specificity and catalysis of GlpX and other class II fructose-1,6-bisphosphatases.

Relevance between lipid metabolism-associated genes and rat liver regeneration

AIM

Lipids are important in constituting cell structure and participating in many biological processes, particularly in energy supplementation to cells. The aim of the present study is to elucidate the action of lipid metabolism-associated genes on rat liver regeneration (LR).

METHODS

Lipid metabolism-associated genes were obtained by collecting website data and retrieving related articles, and their expression changes in the regenerating rat liver were checked by the Rat Genome 230 2.0 array.

RESULTS

In total, 280 genes involved in lipid metabolism were proven to be LR-associated by comparing the gene expression discrepancy between the partial-hepatectomy and sham-operation groups. The initial and total expression numbers of these genes occurring in the initial phase, G(0)/G(1) transition, cell proliferation, cell differentiation, and structure-functional rebuilding of LR were 128, 33, 135, 6, and 267, 147, 1026, 306, respectively, illustrating that these genes were initially expressed mainly in the initiation stage and functioned in different phases. Upregulation (850 times) and downregulation (749 times), as well as 25 types of expression patterns, showed that the physiological and biochemical activities were diverse and complicated in LR.

CONCLUSION

According to the results of the chip detection, it was presumed that fatty acid synthesis at 24-66 h, leukotriene and androgen synthesis at 16-168 h, prostaglandin synthesis at 2-96 h, triglyceride synthesis at 18-24 h, glycosphingolipid synthesis at 0.5-66 h, metabolism of phosphatidyl inositol and sphingomyelin at 2-16 h, and cholesterol catabolism at 30-168 h were enhanced. Throughout almost the whole LR, the genes participating in estrogen, glucocorticoid, and progesterone synthesis, and triglyceride catabolism were upregulated, while phospholipid and glycosphingolipid catabolism were downregulated.

Barley (Hordeum vulgare L.) inositol monophosphatase: gene structure and enzyme characteristics

The cellular myo-inositol (Ins) pool is important to many metabolic and signaling pathways in plants. Ins monophosphatase (IMPase; EC 3.1.3.25) activity is essential for the de novo synthesis of myo-Inositol (Ins), and for recycling of Ins in Ins(1,4,5)P3. However, proteins encoded by at least one family of IMP genes also have L-galactose-1-P phosphatase activity important to ascorbic acid synthesis, indicating a bifunctionality that links these two branches of carbon metabolism. As part of research into the regulation of Ins synthesis and supply during seed development, the barley IMP-1 gene and gene products were studied. The 1.4 kb barley IMP-1 promoter contains one low temperature response element (RE), two heat shock REs, one gibberellin and two auxin REs, and five sugar REs. Barley IMP-1 is expressed in all tissues assayed, and expression levels were not greatly altered by abiotic stress treatments. Reduced use of Ins for Ins P6 synthesis in developing seed of barley low phytic acid (lpa) mutants results in Ins accumulation, and IMP-1 expression is reduced in proportion to the increase in Ins level. The barley recombinant enzyme had a lower Km, indicating higher affinity, for D/L-Ins(3)P1 (Km = 9.7 microM) as compared with reported Km (Ins P1) values for other eukaryotic IMPases (43-330 microM) or with a reported Km (L-Gal-1P) of 150 microM for a kiwifruit (Actinidia deliciosa) enzyme. These and other data indicate that the barley IMP-1 gene is regulated at least in part in response to Ins metabolic needs, and that the enzyme it encodes displays catalytic properties well suited for a role in Ins synthesis, in addition to other roles as an L-gal-1-P phosphatase important to ascorbate synthesis, or as an IMPase important to Ins(1,4,5)P3 signal recycling.

Functional identification of sll1383 from Synechocystis sp PCC 6803 as L-myo-inositol 1-phosphate phosphatase (EC 3.1.3.25): molecular cloning, expression and characterization

The genome sequence of the cyanobacterium Synechocystis sp. PCC6803 revealed four Open reading frame (ORF) encoding putative inositol monophosphatase or inositol monophosphatase-like proteins. One of the ORFs, sll1383, is approximately 870 base pair long and has been assigned as a probable myo-inositol 1 (or 4) monophosphatase (IMPase; EC 3.1.3.25). IMPase is the second enzyme in the inositol biosynthesis pathway and catalyses the conversion of L-myo-inositol 1-phosphate to free myo-inositol. The present work describes the functional assignment of ORF sll1383 as myo-inositol 1-phosphate phosphatase (IMPase) through molecular cloning, bacterial overexpression, purification and biochemical characterization of the gene product. Affinity (K (m)) of the recombinant protein for the substrate DL-myo-inositol 1-phosphate was found to be much higher (0.0034 +/- 0.0003 mM) compared to IMPase(s) from other sources but in comparison V (max) ( approximately 0.033 mumol Pi/min/mg protein) was low. Li(+) was found to be an inhibitor (IC(50) 6.0 mM) of this enzyme, other monovalent metal ions (e.g. Na(+), K(+) NH (4) (+) ) having no significant effect on the enzyme activity. Like other IMPase(s), the activity of this enzyme was found to be totally Mg(2+) dependent, which can be substituted partially by Mn(2+). However, unlike other IMPase(s), the enzyme is optimally active at approximately 42 degrees C. To the best of our knowledge, sll1383 encoded IMPase has the highest substrate affinity and specificity amongst the known examples from other prokaryotic sources. A possible application of this recombinant protein in the enzymatic coupled assay of L-myo-inositol 1-phosphate synthase (MIPS) is discussed.

Inositol-1 (or 4)-monophosphatase from Glycine max embryo axes is a phosphatase with broad substrate specificity that includes phytate dephosphorylation

A phosphate-hydrolyzing activity from Glycine max embryo axes was purified by a series of chromatographic steps and electroelution from activity gels, and demonstrated to be an inositol-1 (or 4)-monophosphatase by partial internal amino acid sequence. This enzyme hydrolyzed ATP, sodium pyrophosphate (NaPPi), inositol hexakisphosphate, and inositol 1-monophosphate, but not p-nitrophenyl phosphate, ADP, AMP or glucose 6-P. Using NaPPi as substrate, the highly purified protein hydrolyzed up to 0.4 mmol phosphate min(-1) mg(-1) protein and had a Km(avg) of 235 microM for NaPPi. Since NaPPi is relatively inexpensive and readily available, we used this as substrate for the subsequent characterization. We observed the following: (a) specific inhibition by Li and NaF but not by butanedione monoxime, or orthovanadate; (b) activation by Cu(2+) and Mg(2+); (c) optimum activity at pH 7.4; and (d) temperature stability after 1-h incubations at 37-80 degrees C, with maximum activity at 37 degrees C. The partially purified protein was detected by in-gel activity assays and the band was electroeluted to yield a highly purified protein. Analysis by SDS-PAGE and native IEF-PAGE yielded a single major polypeptide of 29 kDa and pI approximately 5.9, respectively. In addition, in-gel activity from embryo axes and whole hypocotyls at early germination times revealed one high and one intermediate molecular weight isoform, but only the intermediate one corresponded to IMPase. Throughout the post-imbibition period, the activity of the high molecular weight isoform disappeared and IMPase increased, indicating an increasing expression of the enzyme as germination and growth proceeded. These data indicate that the inositol-1 (or 4)-monophosphatase present in the embryo axis of G. max has a wide phosphate substrate specificity, and may play an important role in phosphate metabolism during the germination process.

Inositol monophosphatase regulates localization of synaptic components and behavior in the mature nervous system of C. elegans

Although recent studies have provided significant molecular insights into the establishment of neuronal polarity in vitro, evidence is lacking on the corresponding phenomena in vivo, including correct localization of synaptic components and the importance of this process for function of the nervous system as a whole. RIA interneurons act as a pivotal component of the neural circuit for thermotaxis behavior in the nematode Caenorhabditis elegans and provide a suitable model to investigate these issues, having a neurite clearly divided into pre- and post-synaptic regions. In a screen for thermotaxis mutants, we identified the gene ttx-7, which encodes myo-inositol monophosphatase (IMPase), an inositol-producing enzyme regarded as a bipolar disorder-relevant molecule for its lithium sensitivity. Here we show that mutations in ttx-7 cause defects in thermotaxis behavior and localization of synaptic proteins in RIA neurons in vivo. Both behavioral and localization defects in ttx-7 mutants were rescued by expression of IMPase in adults and by inositol application, and the same defects were mimicked by lithium treatment in wild-type animals. These results suggest that IMPase is required in central interneurons of the mature nervous system for correct localization of synaptic components and thus for normal behavior.

Spatial expression patterns and biochemical properties distinguish a second myo-inositol monophosphatase IMPA2 from IMPA1

Lithium is used in the clinical treatment of bipolar disorder, a disease where patients suffer mood swings between mania and depression. Although the mode of action of lithium remains elusive, a putative primary target is thought to be inositol monophosphatase (IMPase) activity. Two IMPase genes have been identified in mammals, the well characterized myo-inositol monophosphatase 1 (IMPA1) and myo-inositol monophosphatase 2 (IMPA2). Several lines of genetic evidence have implicated IMPA2 in the pathogenesis of not only bipolar disorder but also schizophrenia and febrile seizures. However, little is known about the protein, although it is predicted to have lithium-inhibitable IMPase activity based on its homology to IMPA1. Here we present the first biochemical study comparing the enzyme activity of IMPA2 to that of IMPA1. We demonstrate that in vivo, IMPA2 forms homodimers but no heterodimers with IMPA1. Recombinant IMPA2 exhibits IMPase activity, although maximal activity requires higher concentrations of magnesium and a higher pH. IMPA2 shows significantly lower activity toward myo-inositol monophosphate than IMPA1. We therefore screened for additional substrates that could be more efficiently dephosphorylated by IMPA2, but failed to find any. Importantly, when using myo-inositol monophosphate as a substrate, the IMPase activity of IMPA2 was inhibited at high lithium and restricted magnesium concentrations. This kinetics distinguishes it from IMPA1. We also observed a characteristic pattern of differential expression between IMPA1 and IMPA2 in a selection of tissues including the brain, small intestine, and kidney. These data suggest that IMPA2 has a separate function in vivo from that of IMPA1.

Identification of mitogen-activated protein kinase docking sites in enzymes that metabolize phosphatidylinositols and inositol phosphates

BACKGROUND

Reversible interactions between the components of cellular signaling pathways allow for the formation and dissociation of multimolecular complexes with spatial and temporal resolution and, thus, are an important means of integrating multiple signals into a coordinated cellular response. Several mechanisms that underlie these interactions have been identified, including the recognition of specific docking sites, termed a D-domain and FXFP motif, on proteins that bind mitogen-activated protein kinases (MAPKs). We recently found that phosphatidylinositol-specific phospholipase C-gamma1 (PLC-gamma1) directly binds to extracellular signal-regulated kinase 2 (ERK2), a MAPK, via a D-domain-dependent mechanism. In addition, we identified D-domain sequences in several other PLC isozymes. In the present studies we sought to determine whether MAPK docking sequences could be recognized in other enzymes that metabolize phosphatidylinositols (PIs), as well as in enzymes that metabolize inositol phosphates (IPs).

RESULTS

We found that several, but not all, of these enzymes contain identifiable D-domain sequences. Further, we found a high degree of conservation of these sequences and their location in human and mouse proteins; notable exceptions were PI 3-kinase C2-gamma, PI 4-kinase type IIbeta, and inositol polyphosphate 1-phosphatase.

CONCLUSION

The results indicate that there may be extensive crosstalk between MAPK signaling and signaling pathways that are regulated by cellular levels of PIs or IPs.

Rv2131c gene product: An unconventional enzyme that is both inositol monophosphatase and fructose-1,6-bisphosphatase

Inositol monophosphatase is an enzyme in the biosynthesis of myo-inostiol, a crucial substrate for the synthesis of phosphatidylinositol, which has been demonstrated to be an essential component of mycobacteria. In this study, the Rv2131c gene from Mycobacterium tuberculosis H37Rv was cloned into the pET28a vector and the recombinant plasmid was transformed into Escherichia coli BL21 (DE3) strain, allowing the expression of the enzyme in fusion with a histidine-rich peptide on the N-terminal. The fusion protein was purified from the soluble fraction of the lysed cells under native conditions by immobilized metal affinity chromatography (IMAC). The purified Rv2131c gene product showed inositol monophosphatase activity but with substrate specificity that was broader than those of several bacterial and eukaryotic inositol monophosphatases, and it also acted as fructose-1,6-bisphosphatase. The dimeric enzyme exhibited dual activities of IMPase and FBPase, with K(m) of 0.22+/-0.03mM for inositol-1-phosphate and K(m) of 0.45+/-0.05mM for fructose-1,6-bisphosphatase. To better understand the relationship between the function and structure of the Rv2131c enzyme, we constructed D40N, L71A, and D94N mutants and purified these corresponding proteins. Mutations of D40N and D94N caused the proteins to almost completely lose both the inositol monophosphatase and fructose-1,6-bisphosphatase activities. However, L71A mutant did not cause loss either of the activities, but the activity toward the inositol was 12-fold more resistant to inhibition by lithium (IC(50) approximately 60mM). Based on the substrate specificity and presence of conserved sequence motifs of the M. tuberculosis Rv2131c, we proposed that the enzyme belonged to class IV fructose-1,6-bisphosphatase (FBPase IV).

The preparation of new phosphorus-centered functional groups for modified oligonucleotides and other natural phosphates

Efforts to develop synthetic methodologies allowing the preparation of alpha,alpha- difluorophosphonothioates, alpha,alpha-difluorophosphonodithioates, alpha,alpha-difluorophosphono- trithioates, and alpha,alpha-difluorophosphinates are reviewed in the light of applications in the field of modified oligonucleotides and cyclitol phosphates. Two successful approaches have been developed, based either on the addition of phosphorus-centered radicals onto gem-difluoroalkenes or on a process involving the addition of lithiodifluorophosphono- thioates 91 onto a ketone and the subsequent deoxygenation reaction of the adduct. The radical route successfully developed a practical route to alpha,alpha-difluoro-H-phosphinates which proved to be useful intermediates to a variety of phosphate isosters. The ionic route led to the first preparation of phosphonodifluoromethyl analogues of nucleoside- 3'-phosphates.

MJ0917 in archaeon Methanococcus jannaschii is a novel NADP phosphatase/NAD kinase

NAD kinase phosphorylates NAD(+) to form NADP(+). Conversely, NADP phosphatase, which has not yet been identified, dephosphorylates NADP(+) to produce NAD(+). Among the NAD kinase homologs, the primary structure of MJ0917 of hyperthermophilic archaeal Methanococcus jannaschii is unique. MJ0917 possesses an NAD kinase homologous region in its C-terminal half and an inositol-1-phosphatase homologous region in its N-terminal half. In this study, MJ0917 was biochemically shown to possess both NAD kinase and phosphatase activities toward NADP(+), NADPH, and fructose 1,6-bisphosphate, but not toward inositol 1-phosphate. With regard to the phosphatase activity, kinetic values indicated that NADP(+) is the preferred substrate and that MJ0917 would function as a novel NADP phosphatase/NAD kinase showing conflicting dual activities, viz. synthesis and degradation of an essential NADP(+). Furthermore, in vitro analysis of MJ0917 showed that, although MJ0917 could supply NADP(+), it prevented excess accumulation of NADP(+); thus, it has the ability to maintain a high NAD(+)/NADP(+) ratio, whereas 5'-AMP would decrease this ratio. The evolutionary process during which MJ0917 arose is also discussed.

The first crystal structure of the novel class of fructose-1,6-bisphosphatase present in thermophilic archaea

As the first structure of the novel class of fructose-1,6-bisphosphatase (FBPase) present in thermophilic archaea, we solved the crystal structure of the ST0318 gene product (St-Fbp) of Sulfolobus tokodaii strain 7. The St-Fbp structure comprises a homooctamer of the 422 point-group. The protein folds as a four-layer alpha-beta-beta-alpha sandwich with a novel topology, which is completely different from the sugar phosphatase fold. The structure contains an unhydrolyzed FBP molecule in the open-keto form, as well as four hexacoordinated magnesium ions around the 1-phosphoryl group of FBP. The arrangement of the catalytic side chains and metal ligands is consistent with the three-metal ion assisted catalysis proposed for conventional FBPases. The structure provides an insight into the structural basis of the strict substrate specificity of St-Fbp.

Genomic organization and regulation of the LeIMP-1 and LeIMP-2 genes encoding myo-inositol monophosphatase in tomato

Myo-inositol (inositol) monophosphatase (IMP), an enzyme which catalyzes the synthesis of free inositol from various inositol monophosphates, is encoded by a small multigene family in many organisms. The tomato IMP gene family encodes three IMP isoforms with identical in vitro biochemical properties. To determine the role of each tomato LeIMP gene in plant growth, we isolated the genomic DNA copies of the LeIMP-1 and LeIMP-2 genes. The LeIMP-1 gene spans approximately 5.8 kb and consists of 12 exons, whereas the LeIMP-2 gene consists of an uninterrupted, single open reading frame (ORF). We have previously shown that steady-state levels of LeIMP-2 mRNA were very low in comparison to LeIMP-1 and LeIMP-3 mRNA levels. To determine whether LeIMP-2 gene expression was spatially restricted to a discreet domain within the plant we constructed transgenic plants containing an LeIMP-2 promoter::uidA gene fusion. Analysis of transgenic seedlings revealed that the LeIMP-2 promoter directed gene expression within epidermal and cortex cells of specific stem/leaf junctions in an abaxial-specific pattern and in the shoot apical meristem. Further, inositol, the product of IMP catalysis, and Li+, an inhibitor of IMP catalysis, decreased expression of the LeIMP-2 promoter as measured by a decrease in beta-glucuronidase activity after treatment.

Crystal structure of a dual activity IMPase/FBPase (AF2372) from Archaeoglobus fulgidus. The story of a mobile loop

Several hyperthermophilic organisms contain an unusual phosphatase that has dual activity toward inositol monophosphates and fructose 1,6-bisphosphate. The structure of the second member of this family, an FBPase/IMPase from Archaeoglobus fulgidus (AF2372), has been solved. This enzyme shares many kinetic and structural similarities with that of a previously solved enzyme from Methanococcus jannaschii (MJ0109). It also shows some kinetic differences in divalent metal ion binding as well as structural variations at the dimer interface that correlate with decreased thermal stability. The availability of different crystal forms allowed us to investigate the effect of the presence of ligands on the conformation of a mobile catalytic loop independently of the crystal packing. This conformational variability in AF2372 is compared with that observed in other members of this structural family that are sensitive or insensitive to submillimolar concentrations of Li(+). This analysis provides support for the previously proposed mechanism of catalysis involving three metal ions. A direct correlation of the loop conformation with strength of Li(+) inhibition provides a useful system of classification for this extended family of enzymes.

Purification and biochemical characterization of Mycobacterium tuberculosis SuhB, an inositol monophosphatase involved in inositol biosynthesis

Phosphatidylinositol is an essential component of mycobacteria, and phosphatidylinositol-based lipids such as phosphatidylinositolmannosides, lipomannan, and lipoarabinomannan are major immunomodulatory components of the Mycobacterium tuberculosis cell wall. Inositol monophosphatase (EC 3.1.3.25) is a crucial enzyme in the biosynthesis of free myo-inositol from inositol-1-phosphate, a key substrate for the phosphatidylinositol synthase in mycobacteria. Analysis of the M. tuberculosis genome suggested the presence of four M. tuberculosis gene products that exhibit an inositol monophosphatase signature. In the present report, we have focused on SuhB, which possesses the highest degree of homology with human inositol monophosphatase. SuhB gene was cloned into an E. coli expression vector to over-produce a His-tagged protein, which was purified and characterized. SuhB required divalent metal ions for functional inositol monophosphatase activity, with Mg(2+) being the strongest activator. Inositol monophosphatase activity catalyzed by SuhB was inhibited by the monovalent cation lithium (IC(50) = 0.9 mM). As anticipated, inositol-1-phosphate was the preferred substrate (K(m) = 0.177 +/- 0.025 mM; k(cat) = 3.6 +/- 0.2 s(-)(1)); however, SuhB was also able to hydrolyze a variety of polyol phosphates such as glucitol-6-phosphate, glycerol-2-phosphate, and 2'-AMP. To provide further insight into the structure-function relationship of SuhB, different mutant proteins were generated (E83D, D104N, D107N, W234L, and D235N). These mutations almost completely abrogated inositol monophosphatase activity, thus underlining the importance of these residues in inositol-1-phosphate dephosphorylation. We also identified L81 as a key residue involved in sensitivity to lithium. The L81A mutation rendered SuhB inositol monophosphatase activity 10-fold more resistant to inhibition by lithium (IC(50) = 10 mM). These studies provide the first steps in the delineation of the biosynthesis of the key metabolite inositol in M. tuberculosis.

Stereochemistry at phosphorus of the reaction catalyzed by myo-inositol monophosphatase

myo-Inositol monophosphatase (IMPase), the proposed target for lithium therapy for manic depression, is an important enzyme in the biosynthesis of second messengers. Earlier studies have shown that the IMPase-catalyzed hydrolysis of myo-inositol monophosphates to inorganic phosphate and myo-inositol proceeds by direct attack of water at phosphorus. However, research groups have independently proposed either an in-line displacement (with inversion of stereochemistry at phosphorus) or an adjacent attack with a pseudorotation (with retention of stereochemistry at phosphorus). Here, the elucidation of the stereochemical pathway is presented. The IMPase-catalyzed hydrolysis of D-1-S(p)-myo-inositol [(17)O]-thiophosphate in the presence of H(2)(18)O gave inorganic R(p)-[(16)O,(17)O,(18)O]-thiophosphate, with inversion of configuration at phosphorus. This is only consistent with an in-line displacement, and it rules out the controversial adjacent/pseudorotation mechanism. This result will assist in the design of alternative inhibitors of IMPase.

Characterization and regulation of inositol monophosphatase activity in Mycobacterium smegmatis

Mycobacterium tuberculosis and related members of the genus Mycobacterium contain a number of inositol-based lipids, such as phosphatidylinositol mannosides, lipomannan and lipoarabinomannan. The synthesis of phosphatidylinositol in M. smegmatis is essential for growth and myo-inositol is a key metabolite for mycobacteria. Little is known about the biosynthesis of inositol in mycobacteria and the only known de novo pathway for myo-inositol biosynthesis involves a two-step process. First, cyclization of glucose 6-phosphate to afford myo-inositol 1-phosphate via inositol-1-phosphate synthase and, secondly, dephosphorylation of myo-inositol 1-phosphate by inositol monophosphatase (IMP) to afford myo-inositol. The following report examines IMP activity in M. smegmatis extracts, with regard to pH dependence, bivalent cation requirement, univalent cation inhibition, regulation by growth and carbon source. We show that IMP activity, which is optimal at the end of the exponential growth phase in Sauton's medium, is Mg(2+)-dependent. Moreover, IMP activity is inhibited by Li(+) and Na(+), with Li(+) also being able to inhibit growth of M. smegmatis in vivo. This study represents a first step in the delineation of myo-inositol biosynthesis in mycobacteria.

Characterization of two genes, Impa1 and Impa2 encoding mouse myo-inositol monophosphatases

The enzyme myo-inositol monophosphatase (Impa) catalyzes the synthesis of free myo-inositol from various myo-inositol monophosphates in the phosphatidylinositol signaling system. Impa is a lithium-blockable enzyme that has been hypothesized to be the biological target for lithium-salts used as mood-stabilizing drugs in the treatment of manic-depressive (bipolar) illness. As an initial step to explore the functional consequences of reduced or absent Impa activity in an animal model we here report the isolation of two Impa-encoding mouse genes, Impa1 and Impa2. Impa1 spans approximately 17.5 kb and contains nine exons of 46--1354 bp encoding a protein of 277 amino acids. Impa2 spans at least 19.5 kb and contains eight exons of 46--444 bp size encoding a protein of 290 amino acids. The genomic structure including the positions of the exon-intron splice sites seems to be conserved among myo-inositol monophosphatase genes in mammalian species. One or more Impa-like genes do also exist in evolutionary more distant species like invertebrates, plants and bacteria. The proteins encoded by the non-vertebrate genes seem to be equally related to Impa1 and Impa2. We therefore suggest that the Impa1 and Impa2 genes duplicated from a common ancestral gene after the evolutionary divergence of vertebrates.

Lithium-related genetics of bipolar disorder

Lithium is a potent prophylactic medication and mood stabilizer in bipolar disorder. However, clinical outcome is variable, and its therapeutic effect manifests after a period of chronic treatment, implying a progressive and complex biological response process. Signal transduction systems known to be perturbed by lithium involve phosphoinositide (PI) turnover, activation of the Wnt pathway via inhibition of glycogen synthase kinase-3beta (GSK-3beta), and a growth factor-induced, Akt-mediated signalling that promotes cell survival. These pathways, acting in synergy, probably prompt the amplification of lithium signal causing such immense impact on the neuronal network. The sequencing of the human genome presents an unparallelled opportunity to uncover the full molecular repertoire involved in lithium action. Interrogation of high-resolution expression microarrays and protein profiles represents a strategy that should help accomplish this goal. A recent microarray analysis on lithium-treated versus untreated PC12 cells identified multiple differentially altered transcripts. Lithium-perturbed genes, particularly those that map to susceptibility regions, could be candidate risk-conferring factors for mood disorders. Transcript and protein profiling in patients could reveal a lithium fingerprint for responsiveness or nonresponsiveness, and a signature motif that may be diagnostic of a specific phenotype. Similarly, lithium-sensitive gene products could provide a new generation of pharmacological targets.

Expression of yeast INM1 encoding inositol monophosphatase is regulated by inositol, carbon source and growth stage and is decreased by lithium and valproate

Inositol monophosphatase plays a vital role in the de novo biosynthesis of inositol and in the phosphoinositide second messenger signalling pathway. We cloned the Saccharomyces cerevisiae open reading frame (ORF) YHR046c (termed INM1), which encodes inositol monophosphatase, characterized the protein Inm1p and analysed expression of the INM1 gene. INM1 was expressed in bacteria under the control of the lacZ promoter. The purified protein has inositol monophosphatase activity that is inhibited by the antibipolar drug lithium, but not valproate. In the inm1Delta:URA3 null mutant, inositol monophosphatase activity was reduced but not eliminated. The disruption had little effect on growth in the presence of lithium or valproate and no effect on growth in the absence of inositol. To characterize the regulation of INM1, we examined the effects of inositol, carbon source, growth phase, and the antibipolar drugs lithium and valproate on INM1 expression using an INM1-lacZ reporter gene. Unlike all other phospholipid biosynthetic enzyme-encoding genes studied, which contain the UASINO regulatory element, INM1 expression is increased in the presence of inositol. In addition, INM1 expression was repressed during growth in glycerol and derepressed as glucose-grown cells entered stationary. Both lithium and valproate, which cause a decrease in intracellular inositol, effect a decrease in INM1 expression. A model is presented to account for regulation of INM1 expression.

Overexpression, purification, and analysis of complementation behavior of E. coli SuhB protein: comparison with bacterial and archaeal inositol monophosphatases

The E. coli suhB gene product, which has been suggested to participate in posttranscriptional control of gene expression, also possesses inositol-1-phosphatase (I-1-Pase) activity. To test if SuhB I-1-Pase activity is sufficient for its function in cells, we have cloned the genes for three other I-1-Pases (from the archaea Methanococcus jannaschii and Archaeoglobus fulgidus, and from the bacterium Thermotoga maritima) into the E. coli expression vector pET23a(+) and examined if these extragenic I-1-Pases could complement the suhB mutation in E. coli strain CG1307 (which also has a mutation in dnaB and a cold-sensitive phenotype). None of these I-1-Pase genes restored growth at 30 degrees C although they generated active I-1-Pase enzymes (as measured by I-1-Pase specific activities of crude protein extracts from the transformed CG1307 cells). In contrast, the pET23a(+) recombinant plasmid with the wild-type E. coli suhB gene complemented the cold sensitivity of the chromosomal mutant suhB and restored the temperature-sensitive growth of the dnaB mutation in the double mutant strain CG1307. Further evidence that this relief of the suppressor behavior of the suhB mutation is not related to the I-1-Pase activity of the SuhB protein was provided by construction of the E. coli SuhB mutant D87N. This mutant protein is inactive as an I-1-Pase but fully functional in changing the temperature sensitivity of the E. coli double mutant strain. Therefore, I-1-P phosphatase activity is neither sufficient nor required for complementation of suhB mutant suppressor effects. The wild-type E. coli SuhB protein was also overexpressed to very high levels and purified to homogeneity in high yield (1 mg/10 mL of culture). The major differences of the E. coli I-1-Pase from all the other characterized I-1-Pases are that it exists as a monomer (rather than a dimer or tetramer) in solution and is more hydrophobic. These physical differences, rather than the I-1-Pase activity, may be involved in the biological role of wild-type SuhB in E. coli.

X-ray structure of yeast Hal2p, a major target of lithium and sodium toxicity, and identification of framework interactions determining cation sensitivity

The product of the yeast HAL2 gene (Hal2p) is an in vivo target of sodium and lithium toxicity and its overexpression improves salt tolerance in yeast and plants. Hal2p is a metabolic phosphatase which catalyses the hydrolysis of 3'-phosphoadenosine-5'-phosphate (PAP) to AMP. It is, the prototype of an evolutionarily conserved family of PAP phosphatases and the engineering of sodium insensitive enzymes of this group may contribute to the generation of salt-tolerant crops. We have solved the crystal structure of Hal2p in complex with magnesium, lithium and the two products of PAP hydrolysis, AMP and Pi, at 1.6 A resolution. A functional screening of random mutations of the HAL2 gene in growing yeast generated forms of the enzyme with reduced cation sensitivity. Analysis of these mutants defined a salt bridge (Glu238 ellipsis Arg152) and a hydrophobic bond (Va170 ellipsis Trp293) as important framework interactions determining cation sensitivity. Hal2p belongs to a larger superfamily of lithium-sensitive phosphatases which includes inositol monophosphatase. The hydrophobic interaction mutated in Hal2p is conserved in this superfamily and its disruption in human inositol monophosphatase also resulted in reduced cation sensitivity.

Characterization of a tetrameric inositol monophosphatase from the hyperthermophilic bacterium Thermotoga maritima

Inositol monophosphatase (I-1-Pase) catalyzes the dephosphorylation step in the de novo biosynthetic pathway of inositol and is crucial for all inositol-dependent processes. An extremely heat-stable tetrameric form of I-1-Pase from the hyperthermophilic bacterium Thermotoga maritima was overexpressed in Escherichia coli. In addition to its different quaternary structure (all other known I-1-Pases are dimers), this enzyme displayed a 20-fold higher rate of hydrolysis of D-inositol 1-phosphate than of the L isomer. The homogeneous recombinant T. maritima I-1-Pase (containing 256 amino acids with a subunit molecular mass of 28 kDa) possessed an unusually high V(max) (442 micromol min(-1) mg(-1)) that was much higher than the V(max) of the same enzyme from another hyperthermophile, Methanococcus jannaschii. Although T. maritima is a eubacterium, its I-1-Pase is more similar to archaeal I-1-Pases than to the other known bacterial or mammalian I-1-Pases with respect to substrate specificity, Li(+) inhibition, inhibition by high Mg(2+) concentrations, metal ion activation, heat stability, and activation energy. Possible reasons for the observed kinetic differences are discussed based on an active site sequence alignment of the human and T. maritima I-1-Pases.

Expression of human inositol monophosphatase suppresses galactose toxicity in Saccharomyces cerevisiae: possible implications in galactosemia

A suppressor of galactose toxicity in a gal7 yeast strain (lacking galactose 1-phosphate uridyl transferase) has been isolated from a HeLa cell cDNA library. Analysis of the plasmid clone indicated that the insert has an ORF identical to that of hIMPase (human myo-inositol monophosphatase). The ability of hIMPase to suppress galactose toxicity is sensitive to the presence of Li(+) in the medium. A gal7 yeast strain harboring a plasmid containing cloned hIMPase grows on galactose as a sole carbon source. hIMPase mediated galactose metabolism is dependent on the functionality of GAL1 as well as GAL10 encoded galactokinase and epimerase respectively. These results predicted that the UDP-glucose/galactose pyrophosphorylase mediated pathway may be responsible for the relief of galactose toxicity. Experiments conducted to test this prediction revealed that expression of UGP1 encoded UDP-glucose pyrophosphorylase can indeed overcome the relief of galactose toxicity. Moreover, expression of UGP1 allows a gal7 strain to grow on galactose as a sole carbon source. Unlike the hIMPase mediated relief of galactose toxicity, UGP1 mediated relief of galactose toxicity is lithium insensitive. Based on our results and on the basis of available information on galactose toxicity, we suggest an alternative explanation for the molecular mechanism of galactose toxicity.

Inhibitors of inositol monophosphatase

Inositol monophosphatase (IMPase) catalyses the hydrolysis of myo-inositol monophosphates to myo-inositol, which is required in the phosphatidyl inositol cell signalling pathway. Here the enzyme structure, mechanism and inhibition of IMPase are reviewed. Lithium, an effective therapy for manic depression, is an uncompetitive inhibitor. In the search for alternative inhibitors to lithium, substrate-based inhibitors, bisphosphonates, terpenoid and tropolone analogues are described.

Inhibition of myo-inositol monophosphatase isoforms by aromatic phosphonates

alpha-Hydroxyphosphonates are moderately potent (Ki = 6-600 microM) inhibitors of the enzyme myo-inositol monophosphatase (McLeod et al., Med. Chem. Res. 1992, 2, 96). Hydroxy-[4-(5,6,7,8-tetrahydronaphtyl-1-oxy)phenyl]methyl phosphonate (3) was resynthesized and its inhibitory potency towards the recombinant bovine brain enzyme confirmed (Ki = 20 microM). Similar aromatic difluoro-, keto-, and ketodifluorophosphonates (5, 7, 9) were inactive. Compound 3 was 15-fold less active on the human as compared to the bovine enzyme. Molecular modeling suggested that the hydrophobic part of the inhibitor interacts with amino acid side chains that are located at the interface between the enzyme subunits in an area (amino acids 175-185) with low similarity between the two isozymes. Phe-183 in the human enzyme was replaced with leucine, the corresponding residue in the bovine isoform. The three isozymes (human wild-type, bovine wild-type and human F183L) had similar kinetic properties, except that the bovine enzyme was less effectively inhibited by high concentrations of the activator Mg2+. The F183L mutant enzyme had a twofold increased affinity for compound 3 as compared to the human wild-type form. We conclude that residue 183 contributes to the binding of aromatic hydroxyphosphonates to IMPase, but it is not the only determining factor for inhibitor specificity with respect to different isozymes.

Cloning and expression of the inositol monophosphatase gene from Methanococcus jannaschii and characterization of the enzyme

Inositol monophosphatase (EC 3.1.3.25) plays a pivotal role in the biosynthesis of di-myo-inositol-1,1'-phosphate, an osmolyte found in hyperthermophilic archaeal. Given the sequence homology between the MJ109 gene product of Methanococcus jannaschii and human inositol monophosphatase, the MJ109 gene was cloned and expressed in Escherichia coli and examined for inositol monophosphatase activity. The purified MJ109 gene product showed inositol monophosphatase activity with kinetic parameters (K(m) = 0.091 +/- 0.016 mM; Vmax = 9.3 +/- 0.45 mumol of Pi min-1 mg of protein-1) comparable to those of mammalian and E. coli enzymes. Its substrate specificity, Mg2+ requirement, Li+ inhibition, subunit association (dimerization), and heat stability were studied and compared to those of other inositol monophosphatases. The lack of inhibition by low concentrations of Li+ and high concentrations of Mg2+ and the high rates of hydrolysis of glucose-1-phosphate and p-nitrophenylphosphate are the most pronounced differences between the archaeal inositol monophosphatase and those from other sources. The possible causes of these kinetic differences are discussed, based on the active site sequence alignment between M. jannaschii and human inositol monophosphatase and the crystal structure of the mammalian enzyme.