Disorder-to-order active site capping regulates the rate-limiting step of the inositol pathway.

Myo-inositol-1-phosphate synthase (MIPS) catalyzes the NAD+-dependent isomerization of glucose-6-phosphate (G6P) into inositol-1-phosphate (IMP), controlling the rate-limiting step of the inositol pathway. Previous structural studies focused on the detailed molecular mechanism, neglecting large-scale conformational changes that drive the function of this 240 kDa homotetrameric complex. In this study, we identified the active, endogenous MIPS in cell extracts from the thermophilic fungus Thermochaetoides thermophila. By resolving the native structure at 2.48 Å (FSC = 0.143), we revealed a fully populated active site. Utilizing 3D variability analysis, we uncovered conformational states of MIPS, enabling us to directly visualize an order-to-disorder transition at its catalytic center. An acyclic intermediate of G6P occupied the active site in two out of the three conformational states, indicating a catalytic mechanism where electrostatic stabilization of high-energy intermediates plays a crucial role. Examination of all isomerases with known structures revealed similar fluctuations in secondary structure within their active sites. Based on these findings, we established a conformational selection model that governs substrate binding and eventually inositol availability. In particular, the ground state of MIPS demonstrates structural configurations regardless of substrate binding, a pattern observed across various isomerases. These findings contribute to the understanding of MIPS structure-based function, serving as a template for future studies targeting regulation and potential therapeutic applications.

Phosphonate and α-Fluorophosphonate Analogues of d-Glucose 6-Phosphate as Active-Site Probes of 1l-myo-Inositol 1-Phosphate Synthase.

The biosynthesis of myo-inositol (mI) is central to the function of many organisms across all kingdoms of life. The first and rate-limiting step in this pathway is catalyzed by 1l-myo-inositol 1-phosphate synthase (mIPS), which converts d-glucose 6-phosphate (G6P) into 1l-myo-inositol 1-phosphate (mI1P). Extensive studies have shown that this reaction occurs through a stepwise NAD+-dependent redox aldol cyclization mechanism producing enantiomerically pure mI1P. Although the stereochemical nature of the mechanism has been elucidated, there is a lack of understanding of the importance of amino acid residues in the active site. Crystal structures of mIPS in the ternary complex with substrate analogues and NAD(H) show different ligand orientations. We therefore proposed to use isosteric and isoelectronic analogues of G6P to probe the active site. Here, we report the synthesis of the methylenephosphonate, difluoromethylenephosphonate, and (R)- and (S)-monofluoromethylenephosphonate analogues of G6P and their evaluation as inhibitors of mIPS activity. While the CH2 and CF2 analogues were produced with slight modification of a previously described route, the CHF analogues were synthesized through a new, shorter pathway. Kinetic behavior shows that all compounds are reversible competitive inhibitors with respect to G6P, with Ki values in the order CF2 (0.18 mM) < (S)-CHF (0.24 mM) < (R)-CHF (0.59 mM) < CH2 (1.2 mM). Docking studies of these phosphonates using published crystal structures show that substitution of the oxygen atom of the substrate changes the conformation of the resulting inhibitors, altering the position of carbon-6 and carbon-5, and this change is more pronounced with fluorine substitution.

Stereochemistry in the Reaction of the myo-Inositol Phosphate Synthase Ortholog Ari2 during Aristeromycin Biosynthesis.

The myo-inositol-1-phosphate synthase (MIPS) ortholog Ari2, which is encoded in the aristeromycin biosynthetic gene cluster, catalyzes the formation of five-membered cyclitol phosphate using d-fructose 6-phosphate (F6P) as a substrate. To understand the stereochemistry during the Ari2 reaction in vivo, we carried out feeding experiments with (6S)-d-[6-2H1]- and (6R)-d-[6-2H1]glucose in the aristeromycin-producing strain Streptomyces citricolor. We observed retention of the 2H atom of (6S)-d-[6-2H1]glucose and no incorporation of the 2H atom from (6R)-d-[6-2H1]glucose in aristeromycin. This indicates that Ari2 abstracts the pro-R proton at C6 of F6P after oxidation of C5-OH by nicotinamide adenine dinucleotide (NAD+) to generate the enolate intermediate, which then attacks the C2 ketone to form the C-C bond via aldol-type condensation. The reaction of Ari2 with (6S)-d-[6-2H1]- and (6R)-d-[6-2H1]F6P in vitro exhibited identical stereochemistry compared with that observed during the feeding experiments. Furthermore, analysis of the crystal structure of Ari2, including NAD+ as a ligand, revealed the active site of Ari2 to be similar to that of MIPS of Mycobacterium tuberculosis, supporting the similarity of the reaction mechanisms of Ari2 and MIPS.

A Cotton (Gossypium hirsutum) Myo-Inositol-1-Phosphate Synthase (GhMIPS1D) Gene Promotes Root Cell Elongation in Arabidopsis.

Myo-inositol-1-phosphate synthase (MIPS, EC 5.5.1.4) plays important roles in plant growth and development, stress responses, and cellular signal transduction. MIPS genes were found preferably expressed during fiber cell initiation and early fast elongation in upland cotton (Gossypium hirsutum), however, current understanding of the function and regulatory mechanism of MIPS genes to involve in cotton fiber cell growth is limited. Here, by genome-wide analysis, we identified four GhMIPS genes anchoring onto four chromosomes in G. hirsutum and analyzed their phylogenetic relationship, evolutionary dynamics, gene structure and motif distribution, which indicates that MIPS genes are highly conserved from prokaryotes to green plants, with further exon-intron structure analysis showing more diverse in Brassicales plants. Of the four GhMIPS members, based on the significant accumulated expression of GhMIPS1D at the early stage of fiber fast elongating development, thereby, the GhMIPS1D was selected to investigate the function of participating in plant development and cell growth, with ectopic expression in the loss-of-function Arabidopsis mips1 mutants. The results showed that GhMIPS1D is a functional gene to fully compensate the abnormal phenotypes of the deformed cotyledon, dwarfed plants, increased inflorescence branches, and reduced primary root lengths in Arabidopsis mips1 mutants. Furthermore, shortened root cells were recovered and normal root cells were significantly promoted by ectopic expression of GhMIPS1D in Arabidopsis mips1 mutant and wild-type plants respectively. These results serve as a foundation for understanding the MIPS family genes in cotton, and suggest that GhMIPS1D may function as a positive regulator for plant cell elongation.

Preparation of 1L-myo-Inositol 1-Phosphate as a Substrate of Phosphatidylinositol Phosphate Synthase.

Novel drugs possessing a mechanism of action specific to pathogenic mycobacteria, including Mycobacterium tuberculosis, are needed. In 2010, we discovered that the biosynthetic pathway of phosphatidylinositol, which is a membrane phospholipid, differs between humans and mycobacteria. The key enzyme responsible for this difference is phosphatidylinositol phosphate (PIP) synthase, which is present only in a few bacteria belonging to the phylum Actinobacteria. Discovering compounds that inhibit the activity of this enzyme will lead to the development of new drugs specific to pathogenic mycobacteria. Measuring PIP synthase activity requires the isotope-labeled substrate 1l-myo-inositol 1-phosphate (1l-Ino-1P). Because this substrate is not commercially available, we synthesized it from [14C] glucose 6-phosphate ([14C] Glc-6P), using a crude enzyme solution isolated from the methanoarchaeon 1l-Ino-1P synthase. The activity of 1l-Ino-1P synthase in the crude enzyme mixture was low, and quantitative analysis of the synthesized 1l-Ino-1P was inaccurate due to impurities present in the crude enzyme mixture. In the present study, we describe a method for synthesizing 1l-Ino-1P using a solution containing recombinant 1l-Ino-1P synthase derived from the hyperthermophilic archaeon Aeropyrum pernix. In addition, we elucidate the conditions leading to the almost complete conversion of Glc-6P into 1l-Ino-1P using this enzyme. Quantitation of the synthesized 1l -Ino-1P was performed by colorimetry and gas liquid chromatography. Further, we confirmed that isotope-labeled 1l-Ino-1P, which is difficult to quantitate by gas liquid chromatography, can be accurately quantified by colorimetry. We also confirmed that 1d-inositol 1-phosphate cannot be a substrate for PIP synthase.

Myo-inositol phosphate synthase expression in the European eel (Anguilla anguilla) and Nile tilapia (Oreochromis niloticus): effect of seawater acclimation.

A single MIPS gene (Isyna1/Ino1) exists in eel and tilapia genomes with a single myo-d-inositol 3-phosphate synthase (MIPS) transcript identified in all eel tissues, although two MIPS spliced variants [termed MIPS(s) and MIPS(l)] are found in all tilapia tissues. The larger tilapia transcript [MIPS(l)] results from the inclusion of the 87-nucleotide intron between exons 5 and 6 in the genomic sequence. In most tilapia tissues, the MIPS(s) transcript exhibits much higher abundance (generally >10-fold) with the exception of white skeletal muscle and oocytes, in which the MIPS(l) transcript predominates. SW acclimation resulted in large (6- to 32-fold) increases in mRNA expression for both MIPS(s) and MIPS(l) in all tilapia tissues tested, whereas in the eel, changes in expression were limited to a more modest 2.5-fold increase and only in the kidney. Western blots identified a number of species- and tissue-specific immunoreactive MIPS proteins ranging from 40 to 67 kDa molecular weight. SW acclimation failed to affect the abundance of any immunoreactive protein in any tissue tested from the eel. However, a major 67-kDa immunoreactive protein (presumed to be MIPS) found in tilapia tissues exhibited 11- and 54-fold increases in expression in gill and fin samples from SW-acclimated fish. Immunohistochemical investigations revealed specific immunoreactivity in the gill, fin, skin, and intestine taken from only SW-acclimated tilapia. Immunofluorescence indicated that MIPS was expressed within gill chondrocytes and epithelial cells of the primary filaments, basal epithelial cell layers of the skin and fin, the cytosol of columnar intestinal epithelial and mucous cells, as well as unknown entero-endocrine-like cells.

Ubiquitous distribution of phosphatidylinositol phosphate synthase and archaetidylinositol phosphate synthase in Bacteria and Archaea, which contain inositol phospholipid.

In Eukarya, phosphatidylinositol (PI) is biosynthesized from CDP-diacylglycerol (CDP-DAG) and inositol. In Archaea and Bacteria, on the other hand, we found a novel inositol phospholipid biosynthetic pathway. The precursors, inositol 1-phosphate, CDP-archaeol (CDP-ArOH), and CDP-DAG, form archaetidylinositol phosphate (AIP) and phosphatidylinositol phosphate (PIP) as intermediates. These intermediates are dephosphorylated to synthesize archaetidylinositol (AI) and PI. To date, the activities of the key enzymes (AIP synthase, PIP synthase) have been confirmed in only three genera (two archaeal genera, Methanothermobacter and Pyrococcus, and one bacterial genus, Mycobacterium). In the present study, we demonstrated that this novel biosynthetic pathway is universal in both Archaea and Bacteria, which contain inositol phospholipid, and elucidate the specificity of PIP synthase and AIP synthase for lipid substrates. PIP and AIP synthase activity were confirmed in all recombinant cells transformed with the respective gene constructs for four bacterial species (Streptomyces avermitilis, Propionibacterium acnes, Corynebacterium glutamicum, and Rhodococcus equi) and two archaeal species (Aeropyrum pernix and Sulfolobus solfataricus). Inositol was not incorporated. CDP-ArOH was used as the substrate for PIP synthase in Bacteria, and CDP-DAG was used as the substrate for AIP synthase in Archaea, despite their fundamentally different structures. PI synthase activity was observed in two eukaryotic species, Saccharomyces cerevisiae and Homo sapiens; however, inositol 1-phosphate was not incorporated. In Eukarya, the only pathway converts free inositol and CDP-DAG directly into PI. Phylogenic analysis of PIP synthase, AIP synthase, and PI synthase revealed that they are closely related enzymes.

Comparative modeling and virtual screening for the identification of novel inhibitors for myo-inositol-1-phosphate synthase.

Myo-inositol-1-phosphate (MIP) synthase is a key enzyme in the myo-inositol biosynthesis pathway. Disruption of the inositol signaling pathway is associated with bipolar disorders. Previous work suggested that MIP synthase could be an attractive target for the development of anti-bipolar drugs. Inhibition of this enzyme could possibly help in reducing the risk of a disease in patients. With this objective, three dimensional structure of the protein was modeled followed by the active site prediction. For the first time, computational studies were carried out to obtain structural insights into the interactive behavior of this enzyme with ligands. Virtual screening was carried out using FILTER, ROCS and EON modules of the OpenEye scientific software. Natural products from the ZINC database were used for the screening process. Resulting compounds were docked into active site of the target protein using FRED (Fast Rigid Exhaustive Docking) and GOLD (Genetic Optimization for Ligand Docking) docking programs. The analysis indicated extensive hydrogen bonding network and hydrophobic interactions which play a significant role in ligand binding. Four compounds are shortlisted and their binding assay analysis is underway.

Molecular cloning and characterization of a cDNA encoding kiwifruit L-myo-inositol-1-phosphate synthase, a key gene of inositol formation.

L-myo-inositol-1-phosphate synthase (MIPS; EC 5.5.1.4) is the key enzyme involved in de novo synthesis of myo-inositol, leading to numerous cellular functions. We isolated an open reading frame of Actinidia deliciosa MIPS (AdMIPS), which is 1,533 bp long and codes for 510 amino acids, with a predicted molecular weight of 56.3 kDa. Sequence analysis revealed its high similarity with MIPS proteins from other organisms. Gene expression and enzyme activity were highest in flower and young fruit. Transcription of AdMIPS was also detected in other tissues. Moderate drought drastically induced expression in the leaves whereas salinity stress induced transcription and enzyme activity in the leaves, phloem, and roots with different degrees. However, a longer period of saline exposure suppressed both expression and enzyme activity in all sampled tissues, indicating that AdMIPS is salt-sensitive.

Phosphonate and α-fluorophosphonate analogs of d-glucose 6-phosphate as active-site probes of 1l-myo-inositol 1-phosphate synthase.

Phosphate ester analogs in which the bridging oxygen is replaced with a methylene or fluoromethylene group are well known non-hydrolyzable mimics of use as inhibitors and substrate analogs for reactions involving phosphate esters. Properties of the replaced oxygen are often best mimicked by a mono-fluoromethylene group, but such groups are challenging to synthesize and can exist as two stereoisomers. Here, we describe the protocol for our method of synthesizing the α-fluoromethylene analogs of d-glucose 6-phosphate (G6P), as well as the methylene and difluoromethylene analogs, and their application in the study of 1l-myo-inositol-1-phosphate synthase (mIPS). mIPS catalyzes the synthesis of 1l-myo-inositol 1-phosphate (mI1P) from G6P, in an NAD-dependent aldol cyclization. Its key role in myo-inositol metabolism makes it a putative target for the treatment of several health disorders. The design of these inhibitors allowed for the possibility of substrate-like behavior, reversible inhibition, or mechanism-based inactivation. In this chapter, the synthesis of these compounds, expression and purification of recombinant hexahistidine-tagged mIPS, the mIPS kinetic assay and methods for determining the behavior of the phosphate analogs in the presence of mIPS, and a docking approach to rationalizing the observed behavior are described.

Probing myo-inositol 1-phosphate synthase with multisubstrate adducts.

The synthesis of a series of carbohydrate-nucleotide hybrids, designed to be multisubstrate adducts mimicking myo-inositol 1-phosphate synthase first oxidative transition state, is reported. Their ability to inhibit the synthase has been assessed and results have been rationalised computationally to estimate their likely binding mode.

Crystal structure of a trapped catalytic intermediate suggests that forced atomic proximity drives the catalysis of mIPS.

1-L-myo-inositol-phosphate synthase (mIPS) catalyzes the first step of the unique, de novo pathway of inositol biosynthesis. However, details about the complex mIPS catalytic mechanism, which requires oxidation, enolization, intramolecular aldol cyclization, and reduction, are not fully known. To gain further insight into this mechanism, we determined the crystal structure of the wild-type mIPS from Archaeoglobus fulgidus at 1.7 Å, as well as the crystal structures of three active-site mutants. Additionally, we obtained the structure of mIPS with a trapped 5-keto-glucose-6-phosphate intermediate at 2 Å resolution by a novel (to our knowledge) process of activating the crystal at high temperature. A comparison of all of the crystal structures of mIPS described in this work suggests a novel type of catalytic mechanism that relies on the forced atomic proximity of functional groups. The lysine cluster is contained in a small volume in the active site, where random motions of these side chains are responsible for the progress of the complex multistep reaction as well as for the low rate of catalysis. The mechanism requires that functional groups of Lys-274, Lys-278, Lys-306, and Lys-367 assume differential roles in the protonation/deprotonation steps that must occur during the mIPS reaction. This mechanism is supported by the complete loss of activity of the enzyme caused by the Leu-257 mutation to Ala that releases the lysine containment.

Identification and organization of chloroplastic and cytosolic L-myo-inositol 1-phosphate synthase coding gene(s) in Oryza sativa: comparison with the wild halophytic rice, Porteresia coarctata.

The gene coding for rice chloroplastic L-myo-inositol-1-phosphate synthase (MIPS; EC 5.5.1.4) has been identified by matrix-assisted laser desorption time-of-flight mass spectrometry analysis of the purified and immunologically cross-reactive approximately 60 kDa chloroplastic protein following two-dimensional polyacrylamide gel electrophoresis, which exhibited sequence identity with the cytosolic MIPS coded by OsINO1-1 gene. A possible chloroplastic transit peptide sequence was identified upstream of the OsINO1-1 gene upon analysis of rice genome. RT-PCR and confocal microscope studies confirmed transcription, effective translation and its functioning as a chloroplast transit peptide. Bioinformatic analysis mapped the chloroplastic MIPS (OsINO1-1) gene on chromosome 3, and a second MIPS gene (OsINO1-2) on chromosome 10 which lacks conventional chloroplast transit peptide sequence as in OsINO1-1. Two new PcINO1 genes, with characteristic promoter activity and upstream cis-elements were identified and cloned, but whether these proteins can be translocated to the chloroplast or not is yet to be ascertained. Electrophoretic mobility shift assay carried out with nuclear extract of Porteresia coarctata leaves grown under both control and stressed condition shows binding of nuclear proteins with the upstream elements. Nucleotide divergence among the different Oryza and Porteresia INO1 genes were calculated and compared.

The structure and mechanism of myo-inositol-1-phosphate synthase.

The first and rate-limiting step in the biosynthesis of myo-inositol is the conversion of D-glucose 6-phosphate to 1L-myo-inositol 1-phosphate catalyzed by 1L-myo-inositol 1-phosphate synthase (MIP synthase). MIP synthase has been identified in a wide variety of organisms from bacteria to humans and is relatively well-conserved throughout evolution. It is probably homotetrameric in most if not all cases and always requires NAD+ as a cofactor, with NADH being reconverted to NAD+ in the catalytic cycle. This review focuses on the structure and mechanism of MIP synthase, with a particular emphasis on the mechanistic insights that have come from several recent structures of the enzyme. These include the structure of the enzyme from Saccharomyces cerevisiae, Archeoglobus fulgidus and Mycobacterium tuberculosis.

An evolutionary analysis identifies a conserved pentapeptide stretch containing the two essential lysine residues for rice L-myo-inositol 1-phosphate synthase catalytic activity.

A molecular evolutionary analysis of a well conserved protein helps to determine the essential amino acids in the core catalytic region. Based on the chemical properties of amino acid residues, phylogenetic analysis of a total of 172 homologous sequences of a highly conserved enzyme, L-myo-inositol 1-phosphate synthase or MIPS from evolutionarily diverse organisms was performed. This study revealed the presence of six phylogenetically conserved blocks, out of which four embrace the catalytic core of the functional protein. Further, specific amino acid modifications targeting the lysine residues, known to be important for MIPS catalysis, were performed at the catalytic site of a MIPS from monocotyledonous model plant, Oryza sativa (OsMIPS1). Following this study, OsMIPS mutants with deletion or replacement of lysine residues in the conserved blocks were made. Based on the enzyme kinetics performed on the deletion/replacement mutants, phylogenetic and structural comparison with the already established crystal structures from non-plant sources, an evolutionarily conserved peptide stretch was identified at the active pocket which contains the two most important lysine residues essential for catalytic activity.

Crystal structure of inositol 1-phosphate synthase from Mycobacterium tuberculosis, a key enzyme in phosphatidylinositol synthesis.

Phosphatidylinositol (PI) is essential for Mycobacterium tuberculosis viability and the enzymes involved in the PI biosynthetic pathway are potential antimycobacterial agents for which little structural information is available. The rate-limiting step in the pathway is the production of (L)-myo-inositol 1-phosphate from (D)-glucose 6-phosphate, a complex reaction catalyzed by the enzyme inositol 1-phosphate synthase. We have determined the crystal structure of this enzyme from Mycobacterium tuberculosis (tbINO) at 1.95 A resolution, bound to the cofactor NAD+. The active site is located within a deep cleft at the junction between two domains. The unexpected presence of a zinc ion here suggests a mechanistic difference from the eukaryotic inositol synthases, which are stimulated by monovalent cations, that may be exploitable in developing selective inhibitors of tbINO.

1L-myo-inositol-1-phosphate synthase.

1L-myo-Inositol-1-phosphate synthase catalyzes the conversion of D-glucose 6-phosphate to 1L-myo-inositol-1-phosphate, the first committed step in the production of all inositol-containing compounds, including phospholipids, either directly or by salvage. The enzyme exists in a cytoplasmic form in a wide range of plants, animals, and fungi. It has also been detected in several bacteria and a chloroplast form is observed in alga and higher plants. The enzyme has been purified from a wide range of organisms and its active form is a multimer of identical subunits ranging in molecular weight from 58,000 to 67,000. The activity of the synthase is stimulated by NH4Cl and inhibited by glucitol 6-phosphate and 2-deoxyglucose 6-phosphate. Structural genes (INO1) encoding the 1L-myo-inositol-1-phosphate synthase subunit have been isolated from several eukaryotic microorganisms and higher plants. In baker's yeast, Saccharomyces cerevisiae, the transcriptional regulation of the INO1 gene has been studied in detail and its expression is sensitive to the availability of phospholipid precursors as well as growth phase. The regulation of the structural gene encoding 1L-myo-inositol-1-phosphate synthase has also been analyzed at the transcriptional level in the aquatic angiosperm, Spirodela polyrrhiza and the halophyte, Mesembryanthemum crystallinum.

Identification of the INO1 gene of Mycobacterium tuberculosis H37Rv reveals a novel class of inositol-1-phosphate synthase enzyme.

1L-myo-inositol (inositol) is vital for the biogenesis of mycothiol, phosphatidylinositol and glycosylphosphatidylinositol anchors linked to complex carbohydrates in Mycobacterium tuberculosis. All these cellular components are thought to play important roles in host-pathogen interactions and in the survival of the pathogen within the host. However, the inositol biosynthetic pathway in M. tuberculosis is not known. To delineate the pathways for inositol formation, we employed a unique combination of tertiary structure prediction and yeast-based functional assays. Here, we describe the identification of the gene for mycobacterial INO1 that encodes inositol-1-phosphate synthase distinct in many respects from the eukaryotic analogues.

Recombinant expression of a functional myo-inositol-1-phosphate synthase (MIPS) in Mycobacterium smegmatis.

Myo-inositol-1-phosphate synthase (MIPS, E.C. 5.5.1.4) catalyzes the first step in inositol production-the conversion of glucose-6-phosphate (Glc-6P) to myo-inositol-1-phosphate. While the three dimensional structure of MIPS from Mycobacterium tuberculosis has been solved, biochemical studies examining the in vitro activity have not been reported to date. Herein we report the in vitro activity of mycobacterial MIPS expressed in E. coli and Mycobacterium smegmatis. Recombinant expression in E. coli yields a soluble protein capable of binding the NAD(+) cofactor; however, it has no significant activity with the Glc-6P substrate. In contrast, recombinant expression in M. smegmatis mc(2)4517 yields a functionally active protein. Examination of structural data suggests that MtMIPS expressed in E. coli adopts a fold that is missing a key helix containing two critical (conserved) Lys side chains, which likely explains the inability of the E. coli expressed protein to bind and turnover the Glc-6P substrate. Recombinant expression in M. smegmatis may yield a protein that adopts a fold in which this key helix is formed enabling proper positioning of important side chains, thereby allowing for Glc-6P substrate binding and turnover. Detailed mechanistic studies may be feasible following optimization of the recombinant MIPS expression protocol in M. smegmatis.

Direct Ionic Regulation of the Activity of Myo-Inositol Biosynthesis Enzymes in Mozambique Tilapia.

Myo-inositol (Ins) is a major compatible osmolyte in many cells, including those of Mozambique tilapia (Oreochromis mossambicus). Ins biosynthesis is highly up-regulated in tilapia and other euryhaline fish exposed to hyperosmotic stress. In this study, enzymatic regulation of two enzymes of Ins biosynthesis, Ins phosphate synthase (MIPS) and inositol monophosphatase (IMPase), by direct ionic effects is analyzed. Specific MIPS and IMPase isoforms from Mozambique tilapia (MIPS-160 and IMPase 1) were selected based on experimental, phylogenetic, and structural evidence supporting their role for Ins biosynthesis during hyperosmotic stress. Recombinant tilapia IMPase 1 and MIPS-160 activity was assayed in vitro at ionic conditions that mimic changes in the intracellular milieu during hyperosmotic stress. The in vitro activities of MIPS-160 and IMPase 1 are highest at alkaline pH of 8.8. IMPase 1 catalytic efficiency is strongly increased during hyperosmolality (particularly for the substrate D-Ins-3-phosphate, Ins-3P), mainly as a result of [Na+] elevation. Furthermore, the substrate-specificity of IMPase 1 towards D-Ins-1-phosphate (Ins-1P) is lower than towards Ins-3P. Because MIPS catalysis results in Ins-3P this results represents additional evidence for IMPase 1 being the isoform that mediates Ins biosynthesis in tilapia. Our data collectively demonstrate that the Ins biosynthesis enzymes are activated under ionic conditions that cells are exposed to during hypertonicity, resulting in Ins accumulation, which, in turn, results in restoration of intracellular ion homeostasis. We propose that the unique and direct ionic regulation of the activities of Ins biosynthesis enzymes represents an efficient biochemical feedback loop for regulation of intracellular physiological ion homeostasis during hyperosmotic stress.