Analysis of the metabolic turnover of the individual phosphate groups of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. Validation of novel analytical techniques by using 32P-labelled lipids from erythrocytes

Design, synthesis and evaluation of a tripodal receptor for phosphatidylinositol phosphates

Phosphatidylinositol phosphates (PIPs) are membrane phospholipids that play crucial roles in a wide range of cellular processes. Their function is dictated by the number and positions of the phosphate groups in the inositol ring (with seven different PIPs being active in the cell). Therefore, there is significant interest in developing small-molecule receptors that can bind selectively to these species and in doing so affect their cellular function or be the basis for molecular probes. However, to date there are very few examples of such molecular receptors. Towards this aim, herein we report a novel tripodal molecule that acts as receptor for mono- and bis-phosphorylated PIPs in a cell free environment. To assess their affinity to PIPs we have developed a new cell free assay based on the ability of the receptor to prevent alkaline phosphatase from hydrolysing these substrates. The new receptor displays selectivity towards two out of the seven PIPs, namely PI(3)P and PI(3,4)P₂. To rationalise these results, a DFT computational study was performed which corroborated the experimental results and provided insight into the host-guest binding mode.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Pathogen destruction versus intracellular survival: the role of lipids as phagosomal fate determinants

Phagocytosis is a key component of the innate immune response and of the clearance of apoptotic bodies. Phagosome formation and subsequent maturation require extensive cytoskeletal rearrangement and precisely choreographed vesicular fusion and fission events. The objectives of this review are to highlight the functional importance of lipids in the phagocytic process, to discuss how pathogenic microorganisms can in some cases manipulate host lipid metabolism to either co-opt or disrupt phagosome maturation and promote their own survival, and to describe how defective phagosomal lipid metabolism can result in disease.

Visualization and manipulation of phosphoinositide dynamics in live cells using engineered protein domains

There is hardly a membrane-associated molecular event that is not regulated by phosphoinositides, a minor but critically important class of phospholipids of cellular membranes. The rapid formation, elimination, and conversion of these lipids in specific membrane compartments are ensured by a wealthy number of inositol lipid kinases and phosphatases with unique localization and regulatory properties. The existence of multiple inositol lipid pools have been indicated by metabolic labeling studies, but the level of functional compartmentalization revealed by the identification of numerous protein effectors acted upon by phosphoinositides could not have been foreseen. The changing perception of inositides from just serving as lipid precursors of second messengers to becoming highly dynamic local membrane-bound regulators poses new challenges concerning the detection of their rapid localized changes. Moreover, it is increasingly evident that manipulation of lipids in highly defined compartments would be a highly superior approach to soaking the cells with a particular phosphoinositide when studying the local regulation of the lipid on any effectors. In this review, we will summarize our efforts to improve our tools in studying phosphoinositide dynamics and discuss our views on the values of these methods compared to other options currently used or being explored.

Metabolism and possible compartmentalization of inositol lipids in isolated rat-liver nuclei

(1) The removal of the nuclear envelope from isolated rat-liver nuclei by washing with Triton X-100 (TX-100) was assessed by electron microscopy. All the envelope was removed by 0.04% (w/v) TX-100. (2) After this removal, phosphorylation of inositol lipids and diacylglycerol (DAG) from [gamma-32P]ATP still occurs, despite the near complete absence of detectable (by mass assay) DAG and PtdIns. This suggests that the majority of these two lipids in nuclei are present in the nuclear membrane, but the small amounts remaining after extraction, defined as intranuclear, are available for phosphorylation by lipid kinases (36% for DAG and 24% for PtdIns respectively, when expressed as a percentage of incorporation of intact nuclei). (3) PtdIns(4,5)P2 did not follow the same pattern as PtdIns and DAG; after removal of the nuclear membrane, 40% of the mass of this lipid was left in the nucleus. Moreover, a similar amount of PtdIns(4,5)P2 was also resistant to extraction with even higher concentrations of detergent, suggesting that PtdIns(4,5)P2 has a discrete intranuclear location, probably bound to nuclear proteins. (4) Addition of exogenous substrates, PtdIns, PtdIns(4)P and DAG, to membrane-depleted nuclei resulted in reconstitution of the majority of lipid phosphorylations from [gamma-32P]ATP (70%, 90% and 94% of intact nuclei respectively), suggesting a predominantly intranuclear location for the respective kinases. (5) Nuclei also showed phosphomonoesterase and phosphatidic acid hydrolase activity; dephosphorylation of pre-radiolabelled PtdIns(4)P, PtdIns(4,5)P2 and phosphatidic acid was observed when [gamma-32P]ATP was removed. However, some of the radioactivity was apparently resistant to these enzymes, suggesting the existence of multiple pools of these lipids. (6) Addition of excess non-radiolabelled ATP to nuclei pre-labelled with [gamma-32P]ATP resulted in an initial increase in the label in PtdIns(4,5)P2, implying a precursor-product relationship between the radiolabelled pools of PtdIns(4)P and PtdIns(4,5)P2. This was confirmed by analysis of the incorporation of 32P into the 4'-phosphate group of PtdIns(4)P and the individual 4'- and 5'-phosphate groups of PtdIns(4,5)P2. The data from these experiments also indicated that PtdIns(4,5)P2 can be produced from a pre-existing pool of PtdIns(4)P, as well as de novo from PtdIns. (7) Taken together our data suggest that isolated rat-liver nuclei have an intranuclear inositol lipid metabolism mechanism utilizing enzymes and substrates equivalent to those found in cytosol and plasma membrane, and that there may be some, but not complete, compartmentalization of the components of the nuclear inositol cycle.

Metabolic evidence for PtdIns(4,5)P2-directed phospholipase C in permeabilized plant protoplasts

Comparison of the sequences of the genes encoding phospholipase C (PLC) which have been cloned to date in plants with their mammalian counterparts suggests that plant PLC is similar to PLCdelta of mammalian cells. The physiological role and mechanism of activation of PLCdelta is unclear. It has recently been shown that Ins(1,4,5)P3 may not solely be the product of PtdIns(4,5)P2-directed PLC activity. Enzyme activities capable of producing Ins(1,4,5)P3 from endogenous inositol phosphates are present in Dictyostelium and also in rat liver. Significantly it has not been directly determined whether Ins(1,4,5)P3 present in higher plants is the product of a PtdIns(4, 5)P2-directed PLC activity. Therefore we have developed an experimental strategy for the identification of d-Ins(1,4,5)P3 in higher plants. By the use of a short-term non-equilibrium labelling strategy in permeabilized plant protoplasts, coupled to the use of a 'metabolic trap' to prevent degradation of [32P]Ins(1,4,5)P3, we were able to determine the distribution of 32P in individual phosphate esters of Ins(1,4,5)P3. The [32]Ins(1,4,5)P3 identified showed the same distribution of label in individual phosphate esters as that of [32P]PtdIns(4,5)P2 isolated from the same tissue. We thus provide in vivo evidence for the action of a PtdIns(4,5)P2-directed PLC activity in plant cells which is responsible for the production of Ins(1,4,5)P3 observed here. This observation does not, however, exclude the possibility that in other cells or under different conditions Ins(1,4,5)P3 can be generated by alternative routes.

Phosphatidylinositol 3,5-bisphosphate defines a novel PI 3-kinase pathway in resting mouse fibroblasts

PtdIns(3,5)P2 is identified as the product of an agonist-independent, wortmannin-sensitive pathway in resting mouse cells. Results are presented here to indicate that PtdIns(3,5)P2 is formed by phosphorylation of PtdIns3P at the D-5 position, and they suggest that relatively constant cellular levels of PtdIns3P and PtdIns(3, 5)P2 are maintained by the concerted action of PtdIns3P 5-kinase and PtdIns(3,5)P2 5-phosphatase. These studies imply a novel mechanism for the action of PtdIns-specific phosphoinositide 3-hydroxykinases in mammalian cells.

Inositol phosphates in the duckweed Spirodela polyrhiza L

We have undertaken an analysis of the inositol phosphates of Spirodela polyrhiza at a developmental stage when massive accumulation of InsP6 indicates that a large net synthesis is occurring. We have identified Ins3P, Ins(1,4)P2, Ins(3,4)P2 and possibly Ins(4,6)P2, Ins(3,4,6)P3, Ins(3,4,5,6)P4, Ins (1,3,4,5,6)P5, D- and/or L-Ins(1,2,4,5,6)P5 and InsP6 and revealed the likely presence of a second InsP3 with chromatographic properties similar to Ins(1,4,5)P3. The higher inositol phosphates identified show no obvious direct link to pathways of metabolism of second messengers purported to operate in higher plants, nor do they resemble the immediate products of plant phytase action on InsP6.

Metabolic evidence for the order of addition of individual phosphate esters in the myo-inositol moiety of inositol hexakisphosphate in the duckweed Spirodela polyrhiza L

The aquatic monocotyledonous plant Spirodela polyrhiza was labelled with [33P]Pi for short periods under non-equilibrium conditions. An InsP6 fraction was obtained and dissected by using enantiospecific (enzymic) and non-enantiospecific (chemical) means to determine the relative labelling of individual phosphate substituents on the inositol ring of InsP6. Phosphates in positions D-1, -2, -3, -4, -5 and -6 contained approx. 21%, 32-39%, 9-10%, 14-16%, 19-23% and 16-18% of the label respectively. We conclude from the foregoing, together with identities [described in the preceding paper, Brearley and Hanke (1996) Biochem. J. 314, 215-225] of inositol phosphates found in this plant at a developmental stage associated with massive accumulation of InsP6, that synthesis of InsP6 from myo-inositol proceeds according to the sequence Ins3P-->Ins(3,4)P2-->Ins(3,4,6)P3-->Ins(3,4,5,6)P4-->Ins(1,3,4,5,6 ) P5-->InsP6 in Spirodela polyrhiza. These results represent the first description of the synthetic sequence to InsP6 in the plant kingdom and the only comprehensive description of endogenous inositol phosphates in any plant tissue. The sequence described differs from that reported in the slime mould Dictyostelium discoideum.

Quantitative chromatographic analysis of inositol phospholipids and related compounds

The metabolism of phospholipids and the mobilization of second messengers such as inositol-1,4,5-trisphosphate, 1,2-diacylglycerol (DAG) and arachidonic acid (AA) from phospholipids is commonly studied by radiolabelling phospholipids with [3H]myo-inositol or [32P]ATP and measuring the incorporation of radioactivity in different phospholipids or their hydrolysis products. However, for the radiolabelling method to accurately reflect changes in the compound's mass, it is essential that the tissue is labelled to isotopic equilibrium which is difficult to achieve. To circumvent the disadvantages of the radiolabelling method, several analytical procedures have been developed for the mass analysis of phospholipids and inositolphosphates (IPs). Quantitation of the mass or the radiolabelling of phospholipids is a complex multi-step procedure that involves quantitative isolation of phospholipids, fractionation of individual phospholipids and either determination of radioactivity in each component or the measurement of their mass. Phospholipids, DAG and AA are extracted from tissue sample with organic solvents such as chloroform-methanol (2:1) containing HCl or formic acid. The extract is separated by TLC, cartridge-column chromatography or HPLC on a reversed-phase column. Phospholipids are quantitated by measuring inorganic phosphate, absorption at 200 nm or mass spectrometry. Inositol phosphates are extracted with perchloric acid or trichloroacetic acid and separated by ion-exchange cartridge-column or HPLC with an ion-exchange column. IPs are quantitated by measuring inorganic phosphate or by using enzymatic reaction, metal-dye coupling, NMR or mass spectrometry.

Inositol trisphosphate metabolism in Saccharomyces cerevisiae: identification, purification and properties of inositol 1,4,5-trisphosphate 6-kinase

Ins(1,4,5)P3 metabolism was examined in Saccharomyces cerevisiae extracts. S. cerevisiae contains readily detectable Ins(1,4,5)P3 kinase activity that is predominantly soluble, but phosphomonoesterase activity acting on Ins(1,4,5)P3 was not detected in either soluble or particulate preparations from this organism. We have purified the kinase activity approximately 685-fold in a rapid four-step process, and obtained a stable preparation. The enzyme has an apparent native molecular mass of approximately 40 kDa, and displays Michaelis-Menten kinetics with respect to its two substrates, ATP and Ins(1,4,5)P3. The Km for ATP was 2.1 mM, and that for Ins(1,4,5)P3 was 7.1 microM. The enzyme appeared to be the first step in the conversion of Ins(1,4,5)P3 into an InsP5, and the partially purified preparation contained another activity that converted the InsP4 product into an InsP5. The InsP4 product of the partially purified kinase was not metabolized by human erythrocyte ghosts and co-chromatographed with an Ins(3,4,5,6)P4 [L-Ins(1,4,5,6)P4] standard, identifying it as D-Ins(1,4,5,6)P4. The yeast enzyme is thus an Ins(1,4,5)P3 6-kinase. This activity may be an important step in the production of inositol polyphosphates such as InsP5 and InsP6 in S. cerevisiae.

Phosphatidylinositol 3,4,5-trisphosphate is formed from phosphatidylinositol 4,5-bisphosphate in thrombin-stimulated platelets

Platelets accumulate PtdIns(3,4,5)P3 and PtdIns(3,4)P2 in response to thrombin and thrombin-receptor-directed peptide in a GTP-dependent manner. These phosphoinositides are considered to be mediators of signaling events in a variety of cells. We have examined the metabolic route by which PtdIns(3,4,5)P3 and PtdIns(3,4)P2 are synthesized by briefly (10 min) incubating platelets with high activities of [32P]Pi, followed by 20 or 60 s exposure to thrombin, and analysing the relative radioactivities of the individual phosphate groups in the resulting labelled PtdIns(3,4,5)P3 and PtdIns(3,4)P2. The phosphate group possessing the highest specific activity under such non-equilibrium labelling conditions indicates the last one added in a metabolic sequence. The thrombin-stimulated rate of labelling of PtdIns(3,4)P2 was significantly slower than that of PtdIns(3,4,5)P3. Increased labelled PtdIns3P was not detected within 60 s. The measured relative radioactivities decreased in the order 3 > 5 > 4 >> 1 for PtdIns(3,4,5)P3 and 3 > 4 >> 1 for PtdIns(3,4)P2. On the basis of the results of both rate-of-labelling and specific radioactivity analyses we conclude that PtdIns(3,4,5)Pa is formed by 3-OH phosphorylation of PtdIns(4,5)P2, whereas PtdIns(3,4)P2, may be formed by 3-OH phosphorylation of PtdIns4P and/or dephosphorylation of PtdIns(3,4,5)P3. These findings point to the activation of phosphoinositide 3-kinase as a critical receptor-regulated step in thrombin-stimulated platelets.

Pathway of synthesis of 3,4- and 4,5-phosphorylated phosphatidylinositols in the duckweed Spirodela polyrhiza L

[3H]Inositol and [32P]Pi labelling of the aquatic plant Spirodela polyrhiza L. revealed the presence of PtdIns(3,4)P2, in addition to PtdIns3P, PtdIns4P and PtdIns(4,5)P2 previously identified [Brearley and Hanke (1992) Biochem. J. 283, 255-260]. PtdIns(3,4,5)P3 was not detected. Throughout a 40 min [32P]Pi-labelling period the specific radioactivity of the gamma-phosphate of ATP and of the ATP pool as a whole increased. Chemical and enzymic dissection of phosphoinositides obtained from plants labelled for 35 min with [32P]Pi showed that over 99.7% of the label in PtdIns3P and PtdIns4P was accounted for by the monoester phosphates. The 3- and 4-monoester phosphates of PtdIns(3,4)P2 accounted for 23.1% and 76.6% respectively of the label, whereas the 4- and 5-monoester phosphates of PtdIns(4,5)P2 accounted for 21.1% and 78.6% respectively. These results are consistent with the synthesis of PtdIns(4,5)P2 via PtdIns4P. The labelling of the individual phosphates of PtdIns(3,4)P2 is, however, inconsistent with synthesis from PtdIns(4,5)P2 via PtdIns(3,4,5)P3, but instead suggests that PtdIns(3,4)P2 is synthesized by 4-phosphorylation of PtdIns3P. These results afford the first evidence that in plants in vivo, synthesis of PtdIns(4,5)P2 follows the pathway described in animal cells and also that plants possess PtdIns3P 4-kinase activity similar to that reported from animal cells.

In vivo formation of polyphosphoinositide isomers and association with progression of murine polycystic kidney disease

Polyphosphoinositide isomers have been demonstrated to be important mediators of cell proliferation in vitro. The present study demonstrates, for the first time, the in vivo formation of the novel isomer, phosphatidylinositol(3)phosphate, in the kidney and liver of intact animals following intraperitoneal administration of [3H]myo-inositol. The formation of renal [3H]phosphatidylinositol(3)phosphate relative to total [3H]phosphatidylinositol-phosphate was positively correlated with cyst proliferation and renal enlargement in a murine model of polycystic kidney disease. Furthermore, despite no difference in the formation of renal [3H]phosphatidylinositol(4)phosphate, a markedly lower accumulation (by 48%) of [3H]phosphatidylinositol(4,5)bisphosphate was observed in the diseased animals as compared to controls. These results indicate that further studies on the in vivo formation of specific polyphosphoinositide isomers in disease states characterized by abnormal growth and oncogene expression are warranted.

Mass assay for inositol 1-phosphate in rat brain by high-performance liquid chromatography and pulsed amperometric detection

A high-performance liquid chromatographic method for direct mass measurement of inositol 1-phosphate (I(1)P) in rat brain is described. Separation of I(1)P from its isomers and from endogenous components is achieved by polymeric anion-exchange chromatography with a sodium hydroxide/sodium acetate mobile phase. Detection is performed at high pH by pulsed amperometric detection at a gold electrode. Sample preparation involves liquid-liquid extraction and ion-exchange solid-phase extraction, prior to HPLC. The method is sufficiently sensitive and selective to enable facile determination of basal levels of I(1)P in small amounts of brain tissue. The applicability of the method is demonstrated by the in vivo monitoring of I(1)P levels in rat brain after administration of the inositol monophosphatase inhibitor lithium and the cholinergic agonist pilocarpine. The method is a significant improvement over existing published mass assays for I(1)P by virtue of its simplicity, speed, sensitivity, and ruggedness.

Phosphatidylinositol metabolism in cells transformed by polyomavirus middle T antigen

Associated with the middle T antigen of polyomavirus is a novel phosphatidylinositol (PtdIns) kinase activity which phosphorylates PtdIns at the D-3 position of the inositol ring. We have undertaken an analysis of myo-[3H]inositol-containing compounds in a panel of NIH 3T3 cell lines stably transfected with transforming and nontransforming middle T antigen mutants. All cell lines from which PtdIns 3-kinase activity coprecipitated with middle T antigen exhibited modestly elevated levels of PtdIns(3)P and compounds with predicted PtdIns(3,4)P2 and PtdIns(3,4,5)P3 structures. Complex formation between middle T antigen and PtdIns 3-kinase correlated not with an increase in total inositol phosphate levels but rather with elevated levels of InsP2 and InsP4. A specific increase in the level of an InsP2 species which comigrated in high-pressure liquid chromatography analysis with Ins(3,4)P2 was observed. These results suggest that association of the polyomavirus middle T antigen with PtdIns 3-kinase activates a distinct inositol metabolic pathway.

Product-precursor relationships amongst inositol polyphosphates. Incorporation of [32P]Pi into myo-inositol 1,3,4,6-tetrakisphosphate, myo-inositol 1,3,4,5-tetrakisphosphate, myo-inositol 3,4,5,6-tetrakisphosphate and myo-inositol 1,3,4,5,6-pentakisphosphate in intact avian erythrocytes

Avian erythrocytes were incubated with myo-[3H]inositol for 6-7 h and with [32P]Pi for the final 50-90 min of this period. An acid extract was prepared from the prelabelled erythrocytes, and the specific radioactivities of the gamma-phosphate of ATP and of both the myo-inositol moieties (3H, d.p.m./nmol) and the individual phosphate groups (32P, d.p.m./nmol) of [3H]Ins[32P](1,3,4,6)P4,[3H]Ins[32P](1,3,4,5)P4, [3H]Ins[32P](3,4,5,6)P4 and [3H]Ins[32P](1,3,4,5,6)P5 were determined. The results provide direct confirmation that one of the cellular InsP4 isomers is Ins(1,3,4,5)P4 which is synthesized by sequential phosphorylation of the 1,4,5 and 3 substitution sites of the myo-Ins moiety, precisely as previously deduced [Batty, Nahorski & Irvine (1985) Biochem. J. 232, 211-215; Irvine, Letcher, Heslop & Berridge (1986) Nature (London) 320, 631-634]. This is compatible with the proposed synthetic route from PtdIns via PtdIns4P, PtdIns(4,5)P2 and Ins(1,4,5)P3. The data also suggest that, in avian erythrocytes, the principle precursor of Ins(1,3,4,5,6)P5 is Ins(3,4,5,6)P4. Furthermore, if the gamma- (and/or beta-) phosphate of ATP is the precursor of the phosphate moieties of Ins(3,4,5,6)P4, then this isomer must be derived from the phosphorylation of Ins(3,4,6)P3. If the gamma- (and/or beta-) phosphate of ATP similarly acts as the ultimate precursor to all of the phosphates of Ins(1,3,4,6)P4, then, in intact avian erythrocytes, the main precursor of Ins(1,3,4,6)P4 is Ins(1,4,6)P3. This contrasts with the expectation, based on results with cell-free systems, that Ins(1,3,4,6)P4 is synthesized by the direct phosphorylation of Ins(1,3,4)P3.

PDGF-dependent tyrosine phosphorylation stimulates production of novel polyphosphoinositides in intact cells

A phosphatidylinositol (PI) kinase activity associated with certain protein tyrosine kinases important in cell proliferation phosphorylates the 3' hydroxyl position of PI to produce phosphatidylinositol-3-phosphate (PI-3-P). Here we report that, in addition to PI-3' kinase activity, anti-phosphotyrosine (alpha-P-tyr) immunoprecipitates from platelet-derived growth factor (PDGF)-stimulated smooth muscle cells (SMC) contain lipid kinase activities that utilize the substrates phosphatidylinositol-4-phosphate (PI-4-P) and phosphatidylinositol-4,5-bisphosphate (PI-4,5-P2). These activities are absent in alpha-P-tyr immunoprecipitates from quiescent SMC. The product of PI-4-P phosphorylation appears to be phosphatidylinositol-3,4-bisphosphate (PI-3,4-P2), a lipid not previously reported. The product of PI-4,5-P2 phosphorylation is phosphatidylinositol-trisphosphate (PIP3). PI-3-P was detected in quiescent SMC and increased only slightly in response to PDGF. PIP3 and the putative PI-3,4-P2 appeared only after the addition of mitogen. Both the temporal production of these novel phospholipids after PDGF stimulation and the observation of the enzymatic activities that produce them in alpha-P-tyr immunoprecipitates suggest that these phospholipids are excellent candidates for mediators of the PDGF mitogenic response.

Inositol lipid metabolism in rat hippocampal formation slices: basal metabolism and effects of cholinergic agonists

Incubation of rat hippocampal formation slices under steady-state conditions with [3H]inositol leads to only three phospholipids becoming labelled: phosphatidylinositol, phosphatidylinositol 4-phosphate, and phosphatidylinositol 4,5-bisphosphate. All three lipids incorporate [32P]Pi into their phosphodiester phosphate group with the polyphosphoinositides also incorporating this tracer into their monoester phosphate groups. As the concentrations of these lipids remain constant during these labelling processes we conclude that the phosphodiester phosphate, the inositol moiety, and the monoester phosphate groups undergo metabolic turnover in hippocampal formation slices incubated in vitro. The rate of incorporation of [3H]inositol into all three inositol phospholipids was stimulated by the addition of methacholine to the medium. Moreover, following steady-state labelling of the inositol lipids with [3H]inositol, methacholine in the presence of 10 mM LiCl caused a transient fall of 13% in the radiochemical concentration of phosphatidylinositol 4,5-bisphosphate after only 30 s stimulation and a fall of 15% in the radiochemical concentration of phosphatidylinositol after 30 min. Concomitantly, there was an approximately stoichiometric rise in the radiochemical concentration of inositol phosphates. Thus, we suggest that methacholine stimulates an inositol phospholipid phosphoinositidase C in rat hippocampal formation slices.

Synthesis of myo-inositol 1,3,4,5,6-pentakisphosphate from inositol phosphates generated by receptor activation

myo-[3H]Inositol 1,3,4,5,6-pentakisphosphate can be made from myo-[3H]inositol 1,4,5-trisphosphate in a rat brain homogenate or soluble fraction. Although D-myo-inositol 3,4,5,6-tetrakisphosphate can be phosphorylated by a soluble rat brain enzyme to give myo-inositol 1,3,4,5,6-pentakisphosphate, it is not an intermediate in the pathway from myo-inositol 1,4,5-trisphosphate. The intermediates in the above pathway are myo-inositol 1,3,4,5-tetrakisphosphate, myo-inositol 1,3,4-trisphosphate and myo-inositol 1,3,4,6-tetrakisphosphate [Shears, Parry, Tang, Irvine, Michell & Kirk (1987) Biochem. J. 246, 139-147; Balla, Guillemette, Baukal & Catt (1987) J. Biol. Chem. 262, 9952-9955], and it is catalysed by soluble kinase activities of similar anion-exchange mobility and Mr value. Compounds with chromatographic and chemical properties consistent with the structures myo-inositol 1,3,4,5-tetrakisphosphate, myo-inositol 1,3,4,6-tetrakisphosphate and myo-inositol 3,4,5,6-tetrakisphosphate are present in avian erythrocytes, human 1321 N1 astrocytoma cells and primary-cultured murine bone-marrow-derived macrophages. The amounts of these inositol tetrakisphosphates rise upon muscarinic cholinergic stimulation of the astrocytoma cells or stimulation of macrophages with platelet-activating factor.

An inositol tetrakisphosphate-containing phospholipid in activated neutrophils

Inositol (1,4,5)triphosphate (InsP3) and tetrakisphosphate (InsP4) have been observed in a variety of cell types and have been proposed to play roles in the receptor-mediated rise in intracellular Ca2+ (refs 2, 3). Recently, they have been shown to act synergistically in the activation of a Ca2+-dependent K+ channel in lacrimal acinar cells. InsP3 is the product of phospholipase C (PLC) action on phosphatidylinositol 4,5-bisphosphate (PtdInsP2) whereas InsP4 is believed to arise from phosphorylation of InsP3 by a cytosolic kinase. Although sought as a source for InsP4, PtdInsP3 has not been identified in any specific cell type. There were early reports of InsP4-containing phospholipids in crude extract from bovine brain, but this finding was later withdrawn. Recently, however, a membrane-bound enzyme (Type 1 PI kinase) which adds phosphate onto the 3 position of inositol phospholipids has been identified and the phosphatidylinositol-3-phosphate (PtdIns(3)P) product characterized. This suggests that several forms of phosphoinositides may exist and could be precursors for some of the variety of soluble inositol phosphate products which have been reported in recent years. Here we report the appearance of another novel phosphoinositide containing four phosphates, phosphatidylinositol trisphosphate (PtdInsP3) which we find only in activated but not in unstimulated neutrophils from human donors.

Metabolism of inositol bis-, tris-, tetrakis- and pentakis-phosphates in GH3 cells

Previous studies demonstrated a multiplicity of isomers of inositol phosphates in GH3 rat pituitary tumour cells. In order to determine their origin, we have investigated the metabolism of radiolabelled inositol phosphates (IPn) in GH3 cell homogenates by using h.p.l.c. I(1,4,5)P3 is either phosphorylated to I(1,3,4,5)P4 (in the presence of ATP) or dephosphorylated to I(1,4)P2 (in the absence of ATP). I(1,4)P2 is dephosphorylated to I(4)P (greater than 95%). I(1,3,4,5)P4 hydrolysis yields two products. By using dual-labelled [32P, 3H]I(1,3,4,5)P4 with 32P in either the 3 or the 4/5 position, we have identified the probable configuration of these isomers. The predominant (greater than 97%) IP3 formed is I(1,3,4)P3, with a minor I(1,4,5)P3 peak. Subsequent I(1,3,4)P3 hydrolysis yields two IP2 isomers [the major (approximately equal to 85%) is I(3,4)P2; the minor (approximately equal to 15%) is I(1,3)P2] and two IP isomers (the major (approximately equal to 90%) is I(3)P [L-I(1)P], the minor I(4)P). IP5 is very slowly dephosphorylated to and IP4 of undetermined isomeric configuration. We have also examined GH3 cell lipids for the presence of phosphoinositides either more highly phosphorylated than PIP2 (as potential sources of the IP4/IP5 and IP6 in these cells) or phosphorylated in positions other than 1, 4 and 5, and have been unable to find evidence of either. These data suggest two main routes of metabolism for I(1,4,5)P3 in GH3 cells: either (1) phosphorylation to I(1,3,4,5)P4, and the subsequent consecutive dephosphorylation to I(1,3,4)P3, I(3,4)P2 and finally L-I(1)P [D-I(3)P]; or (2) dephosphorylation to I(1,4)P2 and, subsequently, I(4)P.

The dephosphorylation of inositol 1,4-bisphosphate to inositol in liver and brain involves two distinct Li+-sensitive enzymes and proceeds via inositol 4-phosphate

1. Hydrolysis of both enantiomers of inositol 1-phosphate and both enantiomers of inositol 4-phosphate to inositol is inhibited by LiCl in liver and brain. 2. The phosphatase activity is predominantly soluble. 3. Inositol 1,4-bisphosphate is also hydrolysed by the soluble fraction of liver and brain. 4. Bisphosphatase activity is inhibited by LiCl, but is less sensitive than monophosphatase activity. 5. The product of bisphosphatase in liver and brain is inositol 4-phosphate.

Phospholipid turnover during cell-cycle traverse in synchronous Chinese-hamster ovary cells. Mitogenesis without phosphoinositide breakdown

The turnover of phospholipids was investigated in quiescent serum-starved Chinese-hamster ovary (CHO-K1) cells stimulated to progress through the cell cycle by the addition of dialysed bovine serum. A variety of radiolabelling techniques were employed to study the rapid effects of serum on phospholipids and later events during G1 and S phases of the cell cycle. Pulse-labelling studies using [32P]Pi revealed that there was a stimulation of the synthesis rate of all phospholipids investigated during the initial few hours after serum addition. The greatest stimulation (20-fold) was observed in phosphatidylcholine, and the smallest in the polyphosphoinositides (PPIs). Mock stimulation with serum-free medium caused a similar increase in PPI turnover, but little or no effect on turnover of other phospholipids. This effect could be accounted for by a stimulation of the turnover of cellular ATP pools increasing [32P]ATP specific radioactivity. Late G1 and S phases were associated with a decrease in the rate of synthesis of all phospholipids. Phosphatidic acid was the only phospholipid whose labelling fell below that in mock-stimulated cells during the period of the cell cycle. Stimulation of serum-starved cells that had been prelabelled with myo-[2-3H]inositol caused no change in the amounts of inositol trisphosphate, but both serum-stimulated and mock-stimulated cells exhibited similar small decreases in both inositol bisphosphate and inositol monophosphate, of approx. 30% after 30 s. When cells were serum-stimulated in the presence of 10 mM-Li+, there was no increase in the size of the total inositol phosphate pool. We conclude that mitogenic stimulation and cell-cycle traverse cause profound and complex effects on phospholipid turnover in CHO-K1 cells, but there is no evidence for a role of inositol lipid turnover in the proliferative response to serum in this cell line.

Synthesis of polyphosphoinositides in nuclei of Friend cells. Evidence for polyphosphoinositide metabolism inside the nucleus which changes with cell differentiation

Previous work demonstrated the existence of phosphatidylinositol kinase and phosphatidylinositol phosphate kinase in rat liver nuclei, with the suggestion that these activities are in the nuclear membrane [Smith & Wells (1983) J. Biol. Chem. 258, 9368-9373]. Here we show that highly purified nuclei from Friend cells, washed free of nuclear membrane by Triton, can incorporate radiolabel from [gamma-32P]ATP into phosphatidic acid, phosphatidylinositol phosphate and phosphatidylinositol 4,5-bisphosphate. The degree of radiolabelling of phosphatidylinositol bisphosphate is highly dependent on the state of differentiation of the cells, being barely detectable in growing cells and much greater after dimethyl sulphoxide-induced differentiation; this difference is mostly due to different amounts of phosphatidylinositol phosphate in the isolated nuclei. We suggest that polyphosphoinositides are made inside the nucleus and that they have a role in chromatin function; either the phospholipids themselves play a role, or there is a possibility of intranuclear signalling by inositide-derived molecules.

The dephosphorylation pathway of D-myo-inositol 1,3,4,5-tetrakisphosphate in rat brain

Dephosphorylation of inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] was measured in both the soluble and the particulate fractions of rat brain homogenates. Analysis of the hydrolysis of [4,5-32P]Ins(1,3,4,5)P4 showed that for both fractions the 5-phosphate of Ins(1,3,4,5)P4 was removed and inositol 1,3,4-trisphosphate [Ins(1,3,4)P3] was specifically produced. In the soluble fraction, Ins(1,3,4)P3 was further hydrolysed at the 1-phosphate position to inositol 3,4-bisphosphate[Ins(3,4)P2]. DEAE-cellulose chromatography of the soluble fraction separated the phosphatase activities into three peaks. The first hydrolysed both Ins(1,3,4,5)P4 and inositol 1,4,5-trisphosphate, the second inositol 1-phosphate and the third Ins(1,3,4)P3 and inositol 1,4-bisphosphate, [Ins(1,4)P2]. Further purification of the third peak on either Sephacryl S-200 or Blue Sepharose could not dissociate these two activities [i.e. with Ins(1,4)P2 and Ins(1,3,4)P3 as substrates]. The dephosphorylation of Ins(1,3,4)P3 could be inhibited by the addition of Li+.

Adenosine triphosphate stimulates inositol phospholipid metabolism and prostacyclin formation in adrenal medullary endothelial cells by means of P2-purinergic receptors

In the adrenal medulla, chromaffin cells secrete high concentrations of catecholamines, ATP, peptides and other factors that must pass through an endothelial cell barrier to enter the bloodstream. We have measured the effect of several of these chromaffin cell secretory products on cultured bovine adrenal medullary endothelial cells and have found that only ATP stimulates prostacyclin formation. The stimulation of prostacyclin formation by ATP coincides with the metabolism of inositol phospholipids and the accumulation of the putative second messenger inositol trisphosphate. The time course, concentration dependence, and P2-purinergic receptor specificity were similar for ATP-stimulated prostacyclin formation and ATP-stimulated inositol phospholipid metabolism. Thus, the increase in prostacyclin formation may be secondary to mobilization of intracellular Ca2+ by inositol trisphosphate, leading to activation of phospholipase A2, liberation of arachidonic acid, and the conversion of arachidonic acid to prostacyclin. We propose that the function of ATP, which is often colocalized with cell-specific hormones in secretory cells, may be to regulate blood flow in the adrenal medulla and other endocrine tissues by interacting with adjacent endothelial cells.

The metabolism of inositol 4-monophosphate in rat mammalian tissues

Rat brain soluble fraction contains an enzymatic activity that dephosphorylates inositol 1,4-bisphosphate (Ins(1,4)P2). We have used anion exchange h.p.l.c. in order to identify the inositol monophosphate product of Ins(1,4)P2 hydrolysis (i.e. Ins(1)P1, Ins(4)P1 or both). When [3H]Ins(1,4)P2 was used as substrate, we obtained an inositol monophosphate isomer that was separated from the co-injected standard [3H]Ins(1)P1. This suggested an Ins(1,4)P21-phosphatase pathway leading to the production of the inositol 4-monophosphate isomer. The dephosphorylation of [32P]Ins(4)P1 was measured in rat brain, liver and heart soluble fraction and was Li+-sensitive. Chromatography of the soluble fraction of a rat brain homogenate on DEAE-cellulose resolved a monophosphate phosphatase activity that hydrolyzed both [3H]Ins(1)P1 and [4-32P]Ins(4)P1 isomers.

Multiple metabolic pools of phosphoinositides and phosphatidate in human erythrocytes incubated in a medium that permits rapid transmembrane exchange of phosphate

1. A Hepes-based medium has been devised which allows rapid Pi exchange across the plasma membrane of the human erythrocyte. This allows the metabolically labile phosphate pools of human erythrocytes to come to equilibrium with [32P]Pi in the medium after only 5 h in vitro. 2. After 5-7 h incubation with [32P]Pi in this medium, only three phospholipids, phosphatidic acid (PtdOH), phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol 4,5-bisphosphate (PtdIns4,5P2) are radioactively labelled. The concentrations of PtdIns4P and PtdIns4,5P2 remain constant throughout the incubation, so this labelling process is a reflection of the steady-state turnover of their monoester phosphate groups. 3. During such incubations, the specific radioactivities of the monoesterified phosphates of PtdIns4, PtdIns4,5P2 and PtdOH come to a steady value after 5 h that is only 25-30% of the specific radioactivity of the gamma-phosphate of ATP at that time. We suggest that this is a consequence of metabolic heterogeneity. This heterogeneity is not a result of the heterogeneous age distribution of the erythrocytes in human blood. Thus it appears that there is metabolic compartmentation of these lipids within cells, such that within a time-scale of a few hours only 25-30% of these three lipids are actively metabolized. 4. The phosphoinositidase C of intact human erythrocytes, when activated by Ca2+-ionophore treatment, only hydrolyses 50% of the total PtdIns4,5P2 and 50% of 32P-labelled PtdIns4,5P2 present in the cells: this enzyme does not discriminate between the metabolically active and inactive compartments of lipids in the erythrocyte membrane. Hence at least four metabolic pools of PtdIns4P and PtdIns4,5P2 are distinguishable in the human erythrocyte plasma membrane. 5. The mechanisms by which multiple non-mixing metabolic pools of PtdOH, PtdIns4P and PtdIns4,5P2 are sustained over many hours in the plasma membranes of intact erythrocytes are unknown, although some possible explanations are considered.

Enzymatic fluorometric assay for myo-inositol trisphosphate

The determination of myo-inositol trisphosphate by an enzymatic fluorometric assay is presented. The method involves the acid extraction of water-soluble inositol polyphosphates followed by separation by anion-exchange chromatography. Samples are subsequently neutralized by passage over a Dowex Cl- resin and elution with lithium chloride. Samples are then desalted with ethanol. Following dephosphorylation with alkaline phosphatase, free myo-inositol is measured enzymatically via the NAD-dependent oxidation to scyllo-inosose with myo-inositol dehydrogenase. The efficiency of recovery, assay specificity, and an application to the measurement of inositol polyphosphates in hormone-stimulated tissue are discussed.

Enzymic dephosphorylation of D-myo-inositol 1,4-bisphosphate in rat brain

Inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and inositol 1,4-bisphosphate [Ins(1,4)P2] phosphatase activities were measured in both 180,000 g (60 min) particulate and supernatant fractions of rat brain homogenates. Although Ins(1,4,5)P3 was mostly hydrolysed by a particulate phosphatase [Erneux, Delvaux, Moreau & Dumont (1986) Biochem. Biophys. Res. Commun. 134, 351-358], Ins(1,4)P2 phosphatase was predominantly soluble. The latter enzyme was Mg2+-dependent and sensitive to thiol-blocking agents (e.g. p-hydroxymercuribenzoate). In contrast with Ins(1,4,5)P3 phosphatase activity measured in the soluble fraction, Ins(1,4)P2 phosphatase was insensitive to 0.001-1 mM-2,3-bisphosphoglycerate. Lithium salts, widely used in psychiatric treatment, inhibited both Ins(1,4)P2 and Ins(1)P1 phosphatase activities of the crude soluble fraction. In particular, 50% inhibition of phosphatase activity, with 2 microM-Ins(1,4)P2 as substrate, was achieved at 3-5 mM-LiCl. At these concentrations, LiCl did not change Ins(1,4,5)P3 phosphatase activity measured in the same fraction with 1-4 microM-Ins(1,4,5)P3 as substrate. Chromatography of the soluble fraction of a rat brain homogenate on DEAE-cellulose resolved three phosphatase activities. These forms, peaks I, II and III, dephosphorylated Ins(1,4,5)P3, Ins(1)P1 and Ins(1,4)P2 respectively. If LiCl (10 mM) was included in the assay mixture, it inhibited both peak-II Ins(1)P1 phosphatase and peak-III Ins(1,4)P2 phosphatase, suggesting the existence of at least two Li+-sensitive phosphatases.

Effects of bombesin and insulin on inositol (1,4,5)trisphosphate and inositol (1,3,4)trisphosphate formation in Swiss 3T3 cells

The effects of bombesin and insulin, separately and in combination, have been studied in Swiss mouse 3T3 cells. Bombesin caused a rapid transfer of 3H from the lipid inositol pool of prelabeled cells into inositol phosphates. Label in inositol tetrakisphosphate (InsP4) and in Ins1,4,5P3 and Ins1,3,4P3 rose within 10 sec of stimulation and that in Ins1,4P2, another InsP2 and InsP1, more slowly. Insulin, which had little effect on its own, increased the turnover of inositol lipids due to acute bombesin stimulation and also enhanced the DNA synthesis evoked by prolonged bombesin treatment. The results suggest that bombesin acting as a growth factor, uses inositol lipids as part of its transduction mechanism and that insulin acts synergistically to enhance both inositol phosphate formation and DNA synthesis.

Bradykinin-induced changes in inositol trisphosphate mass in MDCK cells

Bradykinin produces increases in cytosolic calcium in MDCK cells. We have extracted and separated Inositol 1,4,5 trisphosphate by HPLC and after-acid hydrolysis and conversion to the hexatrifluoro-acetyl derivative quantitated by negative ion chemical ionization mass spectrometry the mass of inositol trisphosphate in MDCK cells. Bradykinin causes an increase in the mass of Inositol trisphosphate from basal levels of 152 pmoles/mg cell protein to 537 pmoles/mg cell protein by 10 secs of stimulation. We conclude that bradykinin stimulates PLC hydrolysis of PIP2 with rapid release of IP3 in sufficient amount to account for the increase in cytosolic Ca++.

The labelling of polyphosphoinositides with [32P]Pi and the accumulation of inositol phosphates in vasopressin-stimulated hepatocytes

When hepatocytes were incubated with [32P]Pi, the kinetics for the labelling of the monoester phosphate groups of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate were similar to each other and slightly slower than that for the labelling of the gamma-phosphate of ATP. Analysis of the water-soluble 3H-labelled materials derived from [3H]inositol-labelled hepatocytes revealed that, in addition to inositol and its mono-, bis- and tris-phosphates (Ins, InsP, InsP2 and InsP3), these cells contained two unidentified radioactive compounds which co-eluted with InsP on anion-exchange chromatography. When [3H]inositol-labelled hepatocytes were stimulated with 0.23 microM-vasopressin in the presence of 10 mM-Li+, there was an accumulation of radioactivity in InsP, InsP2 and InsP3 but not in Ins or the two unidentified compounds. Further analysis of these inositol phosphates by h.p.l.c. revealed that vasopressin also stimulates the accumulation of inositol tetrakisphosphate (InsP4) in these cells. Vasopressin-stimulated InsP and InsP2 accumulations were maximal in the presence of 1-10 mM-Li+ but InsP3 accumulation continued to increase up to 50 mM-Li+. Accumulated inositol phosphates were retained within the cell. Li+ from 1 to 50 mM did not influence the extent of vasopressin-stimulated inositol lipid degradation in hepatocytes. In the absence of Li+, radioactivity in vasopressin-stimulated hepatocytes accumulated almost entirely in free inositol. The vasopressin-stimulated accumulation of inositol phosphates in the presence of 10 mM-Li+ was abolished by a V1-vasopressin antagonist. Inositol phosphate accumulation was not influenced by ionophore A23187, dimethyl sulphoxide or indomethacin.

Turnover of phosphomonoester groups and compartmentation of polyphosphoinositides in human erythrocytes

The turnover of phosphomonoester groups of phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] was investigated in human erythrocytes by short-term labelling with [32P]Pi. The procedure applied ensured a quantitative extraction of erythrocyte polyphosphoinositides as well as their reliable separation for the determinations of pool sizes and specific radioactivities. The pool sizes of phosphatidylinositol (PtdIns), PtdIns4P and PtdIns(4,5)P2 are 25, 11 and 44 nmol/ml of cells respectively. Under steady-state conditions, the phosphorylation fluxes from [gamma-32P]ATP into PtdIns4P and PtdIns(4,5)P2 are in the ranges 14-22 and 46-94 nmol X h-1 X ml of cells-1 respectively. Only 25-60% of total PtdIns4P and 6-10% of total PtdIns(4,5)P2 take part in the rapid tracer exchange, i.e. are compartmentalized. In isolated erythrocyte ghosts, the turnover of PtdIns4P approximately corresponds to that in intact erythrocytes, although any compartmentation can be excluded in this preparation. Under the conditions of incubation employed, the turnover of PtdIns(4,5)P2 is more than one order of magnitude smaller in isolated ghosts than that obtained for intact erythrocytes.

Extracellular ATP activates polyphosphoinositide breakdown and Ca2+ mobilization in Ehrlich ascites tumor cells

The effects of extracellular ATP on phosphoinositide metabolism and intracellular Ca2+ homeostasis were studied in Ehrlich ascites tumor cells. Cytosolic [Ca2+] was measured using either quin 2 or the recently described indicator fura 2. Addition of 0.5-25 microM extracellular ATP to intact cells results in a rapid mobilization of Ca2+ from a nonmitochondrial, intracellular Ca2+ store. Likewise, direct addition of 0.2-2 microM myo-1,4,5-inositol trisphosphate (IP3) to digitonin-permeabilized Ehrlich cells induces a rapid and reversible release of Ca2+ from a nonmitochondrial pool. Under the same conditions which facilitate intracellular Ca2+ mobilization, extracellular ATP also triggers a rapid breakdown of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) and accumulation of IP3. A maximal 18% decrease of the polyphosphoinositide is observed 40-60 s after the addition of 25 microM ATP; within 5 min PtdIns(4,5)P2 returns to or exceeds the original, prestimulus level. These conditions also trigger a rapid accumulation of phosphatidic acid (1.7-fold increase within 5 min). Paralleling these ATP-induced changes in phospholipid levels is a substantial accumulation of the mono-, bis-, and trisphosphate derivatives of inositol; most significantly, a 2-fold increase in the IP3 level is observed within 30 s after ATP addition. These results suggest that in these tumor cells, extracellular ATP elicits changes in phosphoinositide metabolism similar to those produced by a wide variety of Ca2+-mobilizing hormones and growth factors.

Characterization of D-myo-inositol 1,4,5-trisphosphate phosphatase in rat brain

Rat brain homogenates contain significant amounts of inositol 1,4,5-trisphosphate phosphatase in both 180,000xg (60 min) particulate and supernatant fractions. As other membrane-bound enzymes (e.g. guanylate cyclase), particulate inositol 1,4,5-trisphosphate phosphatase activity is highly sensitive to low concentrations of Triton X-100 (0.03%). Higher concentrations of detergent (1%) partially solubilized the enzyme. Thiol blocking agents (e.g. p-hydroxymercuribenzoate) inactivate inositol 1,4,5-trisphosphate phosphatase activity (an effect reversed with 2-mercaptoethanol). It is thus suggested that enzymatic activity requires the presence of -SH groups.

Dependence on Ca2+ of the activities of phosphatidylinositol 4,5-bisphosphate phosphodiesterase and inositol 1,4,5-trisphosphate phosphatase in smooth muscles of the porcine coronary artery

The activities of phosphatidylinositol 4,5-bisphosphate (PIP2) phosphodiesterase (PDE) and inositol 1,4,5,-trisphosphate (IP3) phosphatase in the particulate and cytosol fractions prepared from porcine coronary artery smooth muscles were examined using 32P-labelled PIP2 and IP3 as substrates, respectively. The activity of PIP2 PDE, as assessed from the production of IP3, in the cytosol fraction was about 10-fold higher than that in the particulate fraction. In the absence of MgCl2, the activity of PIP2 PDE in both fractions showed no causal relation to the free Ca2+ concentration in the physiological range of 10(-7)-10(-5) M, but was enhanced remarkably by 10(-4) M free Ca2+. The addition of 1 mM-MgCl2 to the assay medium markedly inhibited the activity of PIP2 PDE in both fractions in the presence of free Ca2+ (10(-8)-10(-5) M). In the absence of MgCl2, 10(-5)M-acetylcholine (ACh) produced IP3, and this action was blocked by 3 X 10(-6) M-atropine. The ACh-induced activation of PIP2 PDE ceased in the presence of 1 mM-MgCl2; however, the reactivation occurring on the addition of 10 microM-guanosine 5'-[gamma-thio]triphosphate did not depend on the free Ca2+ concentrations (10(-7)-10(-5)M). The activities of IP3 phosphatase, determined from decrease in the amount of IP3 in the particulate and cytosol fractions, had much the same potency in both fractions. The activity of IP3 phosphatase in the cytosol fraction was enhanced by MgCl2 in a concentration-dependent manner, the maximal value occurring at 1 mM-MgCl2, and was also enhanced in the presence of physiological concentrations of free Ca2+ (10(-7)-10(-6) M). These findings suggest that the activation of PIP2 PDE which occurs with application of ACh in the presence of guanine nucleotides and 1 mM-MgCl2 is independent of the free Ca2+ concentration, and that the hydrolysis of IP3 by phosphatase increases, depending on the concentration of free Ca2+.