Inositol Phospholipid Metabolism Research Articles

Insights into the regulation mechanisms of algal extracellular polymeric substances secretion upon the exposures to anatase and rutile TiO2 nanoparticles

As an important part of extracellular secondary metabolites, extracellular polymeric substances (EPS) can play a significant role in protecting cells from the threat of exogenous substances, including nanoparticles (NPs). However, the regulation mechanisms of EPS secretion under NPs exposure remain largely unknown. This study investigated the signaling pathways and molecular responses related to EPS secretion of algae (Chlorella pyrenoidosa) upon the exposures to anatase and rutile TiO2 NPs (nTiO2-A and nTiO2-R, respectively) at two similar toxic (20% and 50% of algal growth inhibition) concentrations. The results showed that EPS responded to nTiO2 stress via excess secretion and compositional variation, and nTiO2-A induced more EPS secretion than nTiO2-R at similar toxicity concentrations. The up-regulation of the Ca2+ signaling pathway might play a greater role in promoting EPS secretion under nTiO2-R exposure compared with nTiO2-A exposure, while the significantly increased intracellular ROS could mainly account for the increased EPS secretion under nTiO2-A exposure. The up-regulated genes related to biological synthesis and protein metabolism and the enhanced biosynthetic metabolism might be the direct causes of the increased EPS secretion. The increased ROS could have a greater effect on the amino acid metabolism and related genes upon the exposure to nTiO2-A than nTiO2-R to induce more EPS secretion. More serious membrane damage caused by nTiO2-R than nTiO2-A would affect the intracellular inositol phospholipid metabolism more severely, while the inositol phospholipid pathway and Ca2+ signaling pathway might agree and communicate with each other inherently to regulate EPS secretion upon nTiO2-R exposure. The findings address the regulation mechanisms of algal EPS secretion under nTiO2 exposure and provide new insights into algal bio-responses to nTiO2 exposure.

Pathways for external alkalinization in intact and in microwounded Chara cells are differentially sensitive to wortmannin

ABSTRACTProton flows across the plant cell membranes play a major role in electrogenesis and regulation of photosynthesis and ion balance. The profiles of external pH along the illuminated internodal cells of characean algae consist of alternating high- and low-pH zones that are spatially coordinated with the distribution of photosynthetic activity of chloroplasts underlying these zones. The results based on confocal laser scanning fluorescence microscopy, pH microsensors, and pulse-amplitude-modulated chlorophyll microfluorometry revealed that the coordination of H+ transport and photosynthesis is disrupted by the 2 different environmental cues (low light and wounding) and by a chemical, wortmannin interfering with the inositol phospholipid metabolism. On the one hand, the transition from moderate to low irradiance diminished the peaks in the profiles of photosystem II (PSII) quantum efficiency but did not remove the pH bands. On the other hand, the microwounding of the internode with a glass micropipette, impacting primarily the cell wall, resulted in a rapid local alkalinization of the external medium (by 2–2.5 pH units) near the cell surface, thus mimicking the appearance of natural pH bands. Despite their seeming similarity, the alkaline bands of intact cells were eliminated by wortmannin, whereas the wound-induced alkalinization was insensitive to this drug. Furthermore, the attenuation of natural pH bands in wortmannin-treated cells was accompanied by the enhancement in spatial heterogeneity of PSII efficiency and electron transport rates, which indicates the complexity of chloroplast–plasma membrane interactions. The results suggest that the light- and wound-induced alkaline areas on the cell surface are associated with different ion-transport systems.

High Cell Density Upregulates Calcium Oscillation by Increasing Calcium Store Content via Basal Mitogen-Activated Protein Kinase Activity.

Calcium releases of non-excitable cells are generally a combination of oscillatory and non-oscillatory patterns, and factors affecting the calcium dynamics are still to be determined. Here we report the influence of cell density on calcium increase patterns of clonal cell lines. The majority of HeLa cells seeded at 1.5 x 104/cm2 showed calcium oscillations in response to histamine and ATP, whereas cells seeded at 0.5 x 104/cm2 largely showed transient and sustained calcium increases. Cell density also affected the response of HEK293 cells to ATP in a similar manner. High cell density increased the basal activity of the mitogen-activated protein (MAP) kinase and calcium store content, and both calcium oscillation and calcium store content were down-regulated by a MAP kinase inhibitor, U0126. Thus, MAP kinase-mediated regulation of calcium store likely underlie the effect of cell density on calcium oscillation. Calcium increase patterns of HeLa cells were conserved at any histamine concentrations tested, whereas the overexpression of histamine H1 receptor, which robustly increased histamine-induced inositol phospholipid hydrolysis, converted calcium oscillations to sustained calcium increases only at high histamine concentrations. Thus, the consequence of modulating inositol phospholipid metabolism was distinct from that of changing cell density, suggesting the effect of cell density is not attributed to inositol phospholipid metabolism. Collectively, our results propose that calcium increase patterns of non-excitable cells reflect calcium store, which is regulated by the basal MAP kinase activity under the influence of cell density.

Signaling in lymphocyte activation.

The fate of T and B lymphocytes, the key cells that direct the adaptive immune response, is regulated by a diverse network of signal transduction pathways. The T- and B-cell antigen receptors are coupled to intracellular tyrosine kinases and adaptor molecules to control the metabolism of inositol phospholipids and calcium release. The production of inositol polyphosphates and lipid second messengers directs the activity of downstream guanine-nucleotide-binding proteins and protein and lipid kinases/phosphatases that control lymphocyte transcriptional and metabolic programs. Lymphocyte activation is modulated by costimulatory molecules and cytokines that elicit intracellular signaling that is integrated with the antigen-receptor-controlled pathways.

Sensory receptors—design principles revisited

Sensory receptors—design principles revisited

Expression pattern and sub‐cellular distribution of phosphoinositide specific phospholipase C enzymes after treatment with U‐73122 in rat astrocytoma cells

Phosphoinositide specific phospholipase C (PI-PLC) enzymes interfere with the metabolism of inositol phospholipids (PI), molecules involved in signal transduction, a complex process depending on various components. Many evidences support the hypothesis that, in the glia, isoforms of PI-PLC family display different expression and/or sub cellular distribution under non-physiological conditions such as the rat astrocytes activation during neurodegeneration, the tumoural progression of some neoplasms and the inflammatory cascade activation after lipopolysaccharide administration, even if their role remains not completely elucidated. Treatment of a cultured established glioma cell line (C6 rat astrocytoma cell line) induces a modification in the pattern of expression and of sub cellular distribution of PI-PLCs compared to untreated cells. Special attention require PI-PLC beta3 and PI-PLC gamma2 isoforms, whose expression and sub cellular localization significantly differ after U-73122 treatment. The meaning of these modifications is unclear, also because the use of this N-aminosteroid compound remains controversial, inasmuch it has further actions which might contribute to the global effect recorded on the treated cells.

Plant phospholipid signaling: “in a nutshell”

Since the discovery of the phosphoinositide/phospholipase C (PI/PLC) system in animal systems, we know that phospholipids are much more then just structural components of biological membranes. In the beginning, this idea was fairly straightforward. Receptor stimulation activates PLC, which hydrolyses phosphatidylinositol4,5-bisphosphate [PtdIns(4,5)P2] into two second messengers: inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol (DG). While InsP3 difuses into the cytosol and triggers the release of calcium from an internal store via ligand-gated calcium channels, DG remains in the membrane where it recruits and activates members of the PKC family. The increase in calcium, together with the change in phosphorylation status, (in)activates a variety of protein targets, leading to a massive reprogramming, allowing the cell to appropriately respond to the extracellular stimulus. Later, it became obvious that not just PLC, but a variety of other phospholipid-metabolizing enzymes were activated, including phospholipase A, phospholipase D, and PI 3-kinase. More recently, it has become apparent that PtdIns4P and PtdIns(4,5)P2 are not just signal precursors but can also function as signaling molecules themselves. While plants contain most of the components described above, and evidence for their role in cell signaling is progressively increasing, major differences between plants and the mammalian paradigms exist. Below, these are described "in a nutshell."

New insights into the regulation and function of serine/threonine kinases in T lymphocytes

The development of T lymphocytes in the thymus and the function of mature T cells in adaptive immune responses are choreographed by antigen receptors, costimulatory molecules, adhesion molecules, cytokines, and chemokines. These extrinsic stimuli are coupled to a diverse network of signal transduction pathways that control the transcriptional and metabolic programs that determine T-cell function. At the core of T-lymphocyte signal transduction is the regulated metabolism of inositol phospholipids and the production of two key lipid second messengers: polyunsaturated diacylglycerols (DAGs) and phosphatidylinositol (3-5) triphosphate [PI-(3-5)-P(3)]. The object of the present review is to discuss facts, controversies, and unresolved issues about DAG and PI-(3,4,5)-P(3) production in T lymphocytes and to discuss some of the serine/threonine kinases that control unique aspects of T-lymphocyte biology and coordinate T-cell participation in adaptive immune responses.

A plasma-membrane linker for the phosphoinositide-specific phospholipase C in tobacco plants

We previously screened genes that were transcriptionally activated during the early stage of wound response in tobacco plants (Nicotiana tabacum), and isolated a particular clone, which encoded a membrane-located protein, designated as NtC7. Upon overexpression in tobacco plants, NtC7 conferred a marked tolerance to osmotic stress, suggesting it to be involved in maintenance of osmotic adjustments. In this study, we searched for proteins which interact with NtC7 by the yeast two-hybrid screening, and isolated a clone encoding phosphoinositide-specific phospholipase C, designated as NtPI-PLC. Physical interaction between NtC7 and C2 domain of NtPI-PLC was confirmed by the pull-down assay. Expression of fused protein to green-fluorescence protein in onion epidermal cell layers indicated both proteins to predominantly localize to the plasma membrane. Their interaction in planta was shown by the bimolecular fluorescence complementation, which exhibited a clear fluorescence of reconstituted yellow fluorescence protein. Transcripts of NtC7 and NtPI-PLC were markedly increased 30 to 60 min after wounding. PI-PLC is one of key enzymes in metabolism of inositol phospholipids, which function in signal transduction and also in response to stresses including osmotic changes. It was shown to localize to plasma-membrane and, to a lesser extent, to cytosol. However, molecular mechanism of membrane localization has remained to be determined, because of the apparent lack of domains for membrane association. The present results suggest that one of such mechanisms is tethering NtPI-PLC to the plasma membrane through interaction with NtC7, which possesses a transmembrane domain at the C-terminus.Addendum to: Tamura T, Hara K, Yamaguchi Y, Koizumi N, Sano H. Osmotic stress tolerance of transgenic tobacco expressing a gene encoding a membrane -located receptor-like protein from tobacco plants. Plant Physiol 2003; 131:454-62; PMID: 12586870; DOI: 10.1104/pp.102.011007.

Polyamines, PI(4,5)P2, and actin polymerization

Spermine (SPM) and spermidine (SPD) activate isolated phosphatidylinositol-4-phosphate 5-kinases (PI(4)P5K), enzymes that convert phosphatidylinositol-4-phosphate to phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). PI(4,5)P2 formation is known to be involved in cellular actin reorganization and motility, functions that are also influenced by polyamines. It has not been proven that endogenous polyamines can control inositol phospholipid metabolism. We evoked large decreases in SPD and putrescine (PUT) contents in HL60 cells, using the ornithine decarboxylase inhibitor, alpha-difluoromethylornithine (DFMO), which resulted in decreases in PI(4,5)P2 content per cell and inositol phosphate formation to 76.9 +/- 3.5% and 81.5 +/- 4.0% of control, respectively. Accurately reversing DFMO-evoked decreases in SPD content by incubating cells with exogenous SPD for 20 min rescued these decreases. DFMO treatment and SPD rescues also changed the ratio of total cellular PI(4,5)P2 to PIP suggesting involvement of a SPD-sensitive PI(4)P5K. PUT and SPM were not involved in DFMO-evoked changes in cellular PI(4,5)P2 contents. In DFMO-treated HL60 cells, the percent of total actin content that was filamentous was decreased to 59.1 +/- 5.8% of that measured in paired control HL60 cells, a finding that was rescued following reversal of DFMO-evoked decreases in SPD and PI(4,5)P2 contents. In slowly proliferating DMSO-differentiated HL60 cells, inositol phospholipid metabolism was uncoupled from SPD control. We conclude: in rapidly proliferating HL60 cells, but not in slowly proliferating differentiated HL60 cells, there are endogenous SPD-sensitive PI(4,5)P2 pools, probably formed via SPD-sensitive PI(4)P5K, that likely control actin polymerization.

Neomycin inhibits platelet functions and inositol phospholipid metabolism upon stimulation with thrombin, but not with ionomycin or 12-O-tetradecanoyl-phorbol 13-acetate

Gel-filtered human platelets that had been pre-labelled with [32P]Pi were stimulated with thrombin, ionomycin or the phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA). The effect of the hexacationic aminoglycoside antibiotic, neomycin, on platelet physiological responses, such as aggregation and secretion, as well as changes in phosphoinositide metabolism was studied. Neomycin strongly inhibited thrombin-induced aggregation and secretion whereas the antibiotic had no effect on ionomycin- or TPA-induced platelet functions. The thrombin-induced enhancement of inositol phospholipid metabolism was strongly inhibited by the presence of neomycin whereas the TPA- or ionomycin-induced increase in inositol [32P]polyphospholipids remained unaffected. The inhibitory effect of some other aminoglycoside antibiotics was compared to that of neomycin and the data demonstrate that the inhibition of platelet secretion and phosphatidic acid production was dependent on the cationic charge of the antibiotic. It is suggested that neomycin inhibits signal transduction in platelets at a level prior to the inositol-phospholipid-specific phosphodiesterase.

Intrathecal drug therapy for chronic pain: from basic science to clinical practice.

Intrathecal drug therapy for chronic pain: from basic science to clinical practice.

Pleiotropic alterations in lipid metabolism in yeast sac1 mutants: relationship to "bypass Sec14p" and inositol auxotrophy.

SacIp dysfunction results in bypass of the requirement for phosphatidylinositol transfer protein (Sec14p) function in yeast Golgi processes. This effect is accompanied by alterations in inositol phospholipid metabolism and inositol auxotrophy. Elucidation of how sac1 mutants effect "bypass Sec14p" will provide insights into Sec14p function in vivo. We now report that, in addition to a dramatic accumulation of phosphatidylinositol-4-phosphate, sac1 mutants also exhibit a specific acceleration of phosphatidylcholine biosynthesis via the CDP-choline pathway. This phosphatidylcholine metabolic phenotype is sensitive to the two physiological challenges that abolish bypass Sec14p in sac1 strains; i.e. phospholipase D inactivation and expression of bacterial diacylglycerol (DAG) kinase. Moreover, we demonstrate that accumulation of phosphatidylinositol-4-phosphate in sac1 mutants is insufficient to effect bypass Sec14p. These data support a model in which phospholipase D activity contributes to generation of DAG that, in turn, effects bypass Sec14p. A significant fate for this DAG is consumption by the CDP-choline pathway. Finally, we determine that CDP-choline pathway activity contributes to the inositol auxotrophy of sac1 strains in a novel manner that does not involve obvious defects in transcriptional expression of the INO1 gene.

Sac1p plays a crucial role in microsomal ATP transport, which is distinct from its function in Golgi phospholipid metabolism.

Analysis of microsomal ATP transport in yeast resulted in the identification of Sac1p as an important factor in efficient ATP uptake into the endoplasmic reticulum (ER) lumen. Yet it remained unclear whether Sac1p is the authentic transporter in this reaction. Sac1p shows no homology to other known solute transporters but displays similarity to the N-terminal non-catalytic domain of a subset of inositol 5'-phosphatases. Furthermore, Sac1p was demonstrated to be involved in inositol phospholipid metabolism, an activity whose absence contributes to the bypass Sec14p phenotype in sac1 mutants. We now show that purified recombinant Sac1p can complement ATP transport defects when reconstituted together with sac1Delta microsomal extracts, but is unable to catalyze ATP transport itself. In addition, we demonstrate that sac1Delta strains are defective in ER protein translocation and folding, which is a direct consequence of impaired ATP transport function and not related to the role of Sac1p in Golgi inositol phospholipid metabolism. These data suggest that Sac1p is an important regulator of microsomal ATP transport providing a possible link between inositol phospholipid signaling and ATP-dependent processes in the yeast ER.

Effect of C 2 ceramide on the inositol phospholipid metabolism (uptake of 32P, 3H-serine and 3H-palmitic acid) and apoptosis-related morphological changes in Tetrahymena

Sphingomyelin metabolites have significant role in the regulation of many life processes of mammalian cells. In the present experiments the influence of phospholipid turnover and apoptosis related morphologic signs by one of this metabolite, C 2 ceramide was studied, and compared to the control, untreated cells, in the unicellular Tetrahymena. The incorporation of phospholipid head group components (serine, phosphorus) show a clear time-dependence; while the incorporation of fatty acid component (palmitic acid) is very fast: no significant alterations were found between 5- and 60-min incubations. C 2 ceramide treatment didn't alter 3H-palmitic acid incorporation into phospholipids, however 3H-serine incorporation was mainly inhibited. The amount of total incorporated 32P was also decreased, on the other hand the lover concentration C 2 ceramide (10 μM) elevated the synthesis of inositol phospholipids. The higher concentration of C 2 ceramide (50 μM) had inhibitory effect on the synthesis of each phospholipids examined. This means that in the presence of the C 2 ceramide the synthesis, recovery and turnover of phospholipids, participating in signal transduction, are altered. However these observations were based the uptake of labeled phospholipid precursors, which gives information on the dynamics of the process, without using lipid mass measurements. C 2 ceramide also caused the rounding off the cells, DNA degradation and nuclear condensation. These latter observations point to morphological signs of apoptosis. The results call attention to the role of sphingomyelin metabolites on signalization of unicellulars, to the cross-talk between the inositol phospholipids and sphingomyelin metabolites, and the role of these molecules in the apoptotic processes at a low evolutionary level.

The effects of lithium therapy on thyroid and thyrotropin-releasing hormone.

Lithium is used in the prophylaxis of bipolar depressive disorder in augmentation treatment of depression and in the therapy of some cases of unipolar depression. Lithium affects cell function via its inhibitory action on adenosine triphosphatase (ATPase) activity, cyclic adenosine monophosphate (cAMP), and intracellular enzymes. The inhibitory effect of lithium on inositol phospholipid metabolism affects signal transduction and may account for part of the action of the cation in manic depression. Lithium also alters the in vitro response of cultured cells to thyrotropin-releasing hormone (TRH) and can stimulate DNA synthesis. Lithium is concentrated by the thyroid and inhibits thyroidal iodine uptake. It also inhibits iodotyrosine coupling, alters thyroglobulin structure, and inhibits thyroid hormone secretion. The latter effect is critical to the development of hypothyroidism and goiter. Effects on brain deiodinase enzymes and alterations in thyroid hormone receptor concentration in the hypothalamus are under investigation in relation to the therapeutic effect of lithium. The ion affects many aspects of cellular and humoral immunity in vitro and in vivo. This accounts for a rise in antithyroid antibody titer in patients having these antibodies before lithium administration whereas there is no induction of thyroid antibody synthesis de novo. Goiter, due to increased thyrotropin (TSH) after inhibition of thyroid hormone release, occurs at various reported incidence rates from 0%-60% and is smooth and nontender. Subclinical and clinical hypothyroidism due to lithium is usually associated with circulating anti-thyroid peroxidase (TPO) antibodies but may occur in their absence. Iodine exposure, dietary goitrogens, and immunogenetic background may all contribute to the occurrence of goiter and hypothyroidism during long-term lithium therapy. It is currently unclear whether the reported association of lithium therapy and hyperthyroidism are causal, although there is suggestive epidemiological evidence. Finally, lithium therapy is associated with exaggerated response of both TSH and prolactin to TRH in 50%-100% of patients, although basal levels are not usually high. It is probable that the hypothalamic pituitary axis adjusts to a new setting in patients receiving lithium.

Effects of Brefeldin A on Phosphatidylcholine Phospholipase D and Inositolphospholipid Metabolism in HL‐60 Cells

The involvement of the small GTP-binding protein ADP-ribosylation factor (ARF) in guanosine 5'-[gamma-thio]triphosphate (GTP[S])-dependent activation of phospholipase D (PLD) in HL-60 cells has been well established in vitro. In this study, we tested the effect of brefeldin A, which prevents ARF activation by inhibiting guanine-nucleotide-exchange activity, on PLD stimulation by receptor agonists (formyl-Met-Leu-Phe and ATP) and by the phorbol ester phorbol 12-myristate 13-acetate (PMA) in differentiated HL-60 cells. However, brefeldin A did not affect the activation of PLD at a time (1 h) when it eliminated the activity of the trans-Golgi enzyme galactosyltransferase. It also did not inhibit PLD activity in Golgi-enriched membranes treated with GTP[S] with or without ARF in vitro. However, longer times of brefeldin A treatment (>6 h), progressively and completely inhibited the activation of PLD by formyl-Met-Leu-Phe and partly inhibited (approximately 50%) the activation by PMA. In contrast, long-term brefeldin A treatment did not inhibit the effect of GTP[S] on PLD in permeabilized HL-60 cells. Long-term brefeldin A treatment completely inhibited inositol phosphate production in response to formyl-Met-Leu-Phe and ATP, indicating that it affected inositolphospholipid-specific phospholipase C activity. These data indicate that the rapid inhibitory effect of brefeldin A on Golgi function is not associated with inhibition of receptor-mediated or PMA-mediated PLD activation in HL-60 cells. However, longer-term effects, presumably arising from the disruption of the Golgi, lead to a total inhibition of agonist activation of PLD and inositolphospholipid-specific phospholipase C. In summary, these results do not support a role for brefeldin-A-sensitive ARF in agonist regulation of PLD in HL-60 cells.

Effect of 3-Amino-1-Propanol on the Phosphatidylinositol (PI) and Glycosyl Phosphatidylinositol (GPI) Systems of Tetrahymena

3-Amino-1-propanol (AP), a substance replacing ethanolamine in phosphatidylethanolamine (PE) significantly reduced 32P incorporation to phosphatidylinositol (PI) and glycosyl -phosphatidylinositol (GPI) in the unicellular organism Tetrahymena pyriformis. At 10 mM, AP completely inhibited the incorporation of 32P into PI. 3H-arachidonate incorporation into PI was also inhibited, while that into diacylglycerol (DAG) was high. The experiments indicate the presence and metabolism of inositol phospholipids and GPI in T. pyriformis.

Formation of nascent secretory vesicles from the trans-Golgi network of endocrine cells is inhibited by tyrosine kinase and phosphatase inhibitors.

Recent evidence suggests that secretory vesicle formation from the TGN is regulated by cytosolic signaling pathways involving small GTP-binding proteins, heterotrimeric G proteins, inositol phospholipid metabolism, and protein serine/threonine phosphorylation. At the cell surface, protein phosphorylation and dephosphorylation on tyrosine residues can rapidly modulate cytosolic signaling pathways in response to extracellular stimuli and have been implicated in the internalization and sorting of signaling receptors. to determine if phosphotyrosine metabolism might also regulate secretory vesicle budding from the TGN, we treated permeabilized rat pituitary GH3 cells with inhibitors of either tyrosine phosphatases or tyrosine kinases. We demonstrate that the tyrosine phosphatase inhibitors pervanadate and zinc potently inhibited budding of nascent secretory vesicles. Tyrphostin A25 (TA25) and other tyrosine kinase inhibitors also prevented secretory vesicle release, suggesting that vesicle formation requires both phosphatase and kinase activities. A stimulatory peptide derived from the NH2 terminus of the small GTP-binding protein ADP ribosylation factor 1 (ARF1) antagonized the inhibitory effect of TA25, indicating that both agents influence the same pathway leading to secretory vesicle formation. Antiphosphotyrosine immunoblotting revealed that protein tyrosine phosphorylation was enhanced after treatment with tyrosine phosphatase or kinase inhibitors. Subcellular fractionation identified several tyrosine phosphorylated polypeptides of approximately 175, approximately 130, and 90-110 kD that were enriched in TGN-containing Golgi fractions and tightly membrane associated. The phosphorylation of these polypeptides correlated with inhibition of vesicle budding. Our results suggest that in endocrine cells, protein tyrosine phosphrylation and dephosphorylation are required for secretory vesicle release from the TGN.

Identity of zinc ion-dependent acid phosphatase from bovine brain and myo-inositol 1-phosphatase

A 62 kDa Zn 2+-dependent acid phosphatase has been purified from bovine brain. The protein was carboxymethylated and then cleaved by endoproteinase Glu-C, trypsin and CNBr. Several fragments were subjected to structural analysis either by using mass spectrometry or automated peptide sequencing. The four sequenced peptides were compared with the known protein sequences contained in the EMBL Data Bank. All four peptide sequences were identical to the corresponding amino-acid sequences present in myo-inositol 1-phosphatase from bovine brain. Furthermore we found that the amino-acid composition of Zn 2+-dependent acid phosphatase purified in our laboratory is very similar to that of myo-inositol 1-phosphatase, and that several peptide fragments have molecular weights (measured by mass spectrometry techniques) identical to those expected for cleavage-fragments originated from the authentic myo-inositol 1-phosphatase. This is one of the key enzymes in the receptor-stimulated inositol phospholipid metabolism and it has been considered as the probable target of Li + ion during LiCl therapy in manic-depressive patients. The comparison of the Zn 2+-dependent acid phosphatase and the Mg 2+-dependent myo-inositol-l-phosphatase activities, measured at different purification steps, shows that the ratio between the two activities was remarkably constant during enzyme purification. We also demonstrated that in the presence of Mg 2+ this enzyme efficiently catalyses the hydrolysis of myo-inositol 1-phosphate, and that the Li + ion inhibits this activity. Furthermore, the thermal treatment of the enzyme causes a time-dependent parallel decrease of both Zn-dependent p-nitrophenyl phosphatase (assayed at pH 5.5) and Mg 2+-dependent myo-inositol-l-phosphatase (assayed at pH 8.0) activities, suggesting the hypothesis that the same protein possesses both these activities.