Ca2+-induced polyphosphoinositide breakdown due to phosphomonoesterase activity in chicken erythrocytes

Restricted diffusion of integral membrane proteins and polyphosphoinositides leads to their depletion in microvesicles released from human erythrocytes

The protein and phospholipid composition of microvesicles released from normal human erythrocytes after ATP depletion, on aging or by treatment with merocyanine 540, dimyristoyl phosphatidylcholine or Ca2+/ionophore A23187 has been compared with the composition of the original cell membrane. It has been shown that these microvesicles are depleted of band 3, glycophorin and phosphatidylinositol 4,5-bisphosphate relative to phospholipid by 40% or more. These data are interpreted to mean that less than half of these membrane components are free to diffuse laterally in the lipid bilayer. Acetylcholinesterase was found to be enriched 2-3-fold in microvesicles, possibly because the removal of non-diffusing proteins from the vesiculating region of the lipid bilayer allows more space for freely diffusing proteins like acetylcholinesterase to enter the microvesicle membrane.

Inhibition of adenylate cyclase activity by polyamines in human erythrocyte plasma membranes

Polyamines (spermidine, spermine and putrescine) inhibited the adenylate cyclase activity in a concentration dependent manner in human erythrocyte plasma membranes. Spermidine (Spd) exhibited more inhibitory effect than spermine (Spm) and putrescine (Put). On the contrary, the addition of amino acids (arginine, glutamine and lysine) did not influence the basal enzyme activity. Other cations (polylysine, polyarginine and polyglutamine) also did not affect the enzyme activity. Addition of all the three polyamines (Spd, Spm and Put) in the reaction mixture exhibited moderate inhibitory effect on the adenylate cyclase activity whether it was basal or activated with sodium fluoride or with forskolin. Since the three polyamines exhibited maximum inhibitory effect at 10 microM concentration which is within physiological limit for mammalian tissues, we suggest that there may be a regulatory function of these molecules on adenylate cyclase activity in human erythrocytes.

Purification and kinetic properties of a soluble phosphatidylinositol-4-phosphate kinase of the bovine adrenal medulla with emphasis on its inhibition by calcium ions

A phosphatidylinositol-4-phosphate kinase (PIP kinase, EC 2.7.1.68) was purified about 1000-fold from the cytosolic fraction of bovine adrenal medulla by ammonium sulfate precipitation, anion exchange, phosphocellulose and gel permeation chromatography. The PIP kinase preparation was free from other polyphosphoinositide metabolizing activities, and some of its catalytic properties were studied using both membrane-bound and liposomal/micellar phosphatidylinositol 4-phosphate (PIP) as the substrate. Using a native substrate (chromaffin granule ghosts) enriched in PIP, the Km value for MgATP was about 130 microM at 5 mM Mg2+, and optimum activity was obtained at pH 7.2. ADP inhibited the PIP kinase activity in a mixed non-competitive manner. The enzyme was stimulated by Mg2+ and spermidine, whereas free calcium effectively inhibited the PIP kinase activity (I50 at about 0.1 microM Ca2+). This inhibition was independent of calmodulin and CaATP (80 microM) was not a substrate for the PIP kinase. The presence of near intracellular concentrations of potassium decreased the sensitivity to calcium ions (I50 of about 50 microM), but the inhibitory effect was still evident in the physiologically interesting concentration range. The concentration for half-maximal inhibition was three orders of magnitude higher (I50 of about 200 microM) using a liposomal/micellar suspension of purified PIP as the substrate, which demonstrates the importance of using a native substrate in studying the regulatory properties of this enzyme. It is concluded that the high sensitivity of the PIP kinase activity to calcium ions is likely to be physiologically significant, as recently proposed for the phosphatidylinositol (PI) kinase (Husebye, E.S. and Flatmark, T. (1988) Biochim. Biophys. Acta 968, 261-265), and may represent a negative feedback control of the cytosolic calcium concentration. The cationic amphiphile trifluoperazine was a potent inhibitor of PIP kinase activity (I50 at about 15 microM), and may represent a useful tool to study PI kinase selectively in membranes or cells containing both kinases.

Effect of exogenously added acylphosphatases on inositol lipid metabolism in human platelets

In this paper we demonstrate that human platelets contain an acylphosphatase isoenzyme. We then investigated the effect of exogenously added human muscle and erythrocyte acylphosphatases on inositol lipid content in human platelets permeabilized with saponin. Alterations in the level of the polyphosphoinositides were observed: in particular, the levels of phosphatidylinositol 4,5-bisphosphate, and of phosphatidylinositol 4-monophosphate were decreased, whereas the level of phosphatidylinositol was increased. These results suggest that acylphosphatases promote polyphosphoinositide dephosphorylation, possibly through intracellular Ca2+ mobilization.

Phosphoinositide hydrolysis by guanosine 5'-[gamma-thio]triphosphate-activated phospholipase C of turkey erythrocyte membranes

Phosphatidylinositol (PtdIns), phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] of turkey erythrocytes were labelled by using either [32P]Pi or [3H]inositol. Although there was little basal release of inositol phosphates from membranes purified from labelled cells, in the presence of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) the rate of accumulation of inositol bis-, tris- and tetrakis-phosphate (InsP2, InsP3 and InsP4) was increased 20-50-fold. The enhanced rate of accumulation of 3H-labelled inositol phosphates was linear for up to 20 min; owing to decreases in 32P specific radioactivity of phosphoinositides during incubation of membranes with unlabelled ATP, the accumulation of 32P-labelled inositol phosphates was linear for only 5 min. In the absence of ATP and a nucleotide-regenerating system, no InsP4 was formed, and the overall inositol phosphate response to GTP[S] was decreased. Analyses of phosphoinositides during incubation with ATP indicated that interconversions of PtdIns to PtdIns4P and PtdIns4P to PtdIns(4,5)P2 occurred to maintain PtdIns(4,5)P2 concentrations; GTP[S]-induced inositol phosphate formation was accompanied by a corresponding decrease in 32P- and 3H-labelled PtdIns, PtdIns4P and PtdIns(4,5)P2. In the absence of ATP, only GTP[S]-induced decreases in PtdIns(4,5)P2 occurred. Since inositol monophosphate was not formed under any condition, PtdIns is not a substrate for the phospholipase C. The production of InsP2 was decreased markedly, but not blocked, under conditions where Ins(1,4,5)P3 5-phosphomonoesterase activity in the preparation was inhibited. Thus the predominant substrate of the GTP[S]-activated phospholipase C of turkey erythrocyte membranes is PtdIns(4,5)P2. Ins(1,4,5)P3 was the major product of this reaction; only a small amount of Ins(1:2-cyclic, 4,5)P3 was released. The effects of ATP on inositol phosphate formation apparently involve the contributions of two phenomena. First, the P2-receptor agonist 2-methylthioadenosine triphosphate (2MeSATP) greatly increased inositol phosphate formation and decreased [3H]PtdIns4P and [3H]PtdIns(4,5)P2 in the presence of a low (0.1 microM) concentration of GTP[S]. ATP over the concentration range 0-100 microM produced effects in the presence of 0.1 microM-GTP[S] essentially identical with those observed with 2MeSATP, suggesting that the effects of low concentrations of ATP are also explained by a stimulation of P2-receptors. Higher concentrations of ATP also increase inositol phosphate formation, apparently by supporting the synthesis of substrate phospholipids.(ABSTRACT TRUNCATED AT 400 WORDS)

Polyphosphoinositides are present in plasma membranes isolated from fusogenic carrot cells

Fusogenic carrot cells grown in suspension culture were labeled 12 hours with myo-[2-(3)H]inositol. Plasma membranes were isolated from the prelabeled fusogenic carrot cells by both aqueous polymer two-phase partitioning and Renografin density gradients. With both methods, the plasma membrane-enriched fractions, as identified by marker enzymes, were enriched in [(3)H]inositol-labeled phosphatidylinositol monophosphate (PIP) and phosphatidylinositol bisphosphate (PIP(2)). An additional [(3)H]inositol-labeled lipid, lysophosphatidylinositol monophosphate, which migrated between PIP and PIP(2) on thin layer plates, was found primarily in the plasma membrane-rich fraction of the fusogenic cells. This was in contrast to lysophosphatidylinositol which is found primarily in the lower phase, microsomal/mitochrondrial-rich fraction.

Does the inositol tris/tetrakisphosphate pathway exist in rat heart?

Appearance of two isomers of inositol trisphosphate (InsP3) was observed when [3H]inositol prelabelled rat heart ventricles were stimulated for 10 and 30 s with noradrenaline. In contrast, inositol tetrakisphosphate (InsP4) could not be detected. However the existence of the inositol tris/tetrakisphosphate pathway was demonstrated by studying [3H]inositol 1,4,5-trisphosphate (Ins-1,4,5-P3) metabolism in a soluble fraction of rat heart. There, [3H]Ins-1,4,5-P3 was phosphorylated to form [3H]Ins-1,3,4,5-P4. Raising [Ca2+] from 1 nM to 1 microM increased InsP3 kinase activity by 2-fold (EC50 for Ca2+ approx. 56 nM). This effect appeared to be due to an increase of the apparent Vmax of the enzyme while the apparent Km was unchanged.

The formation of lysophosphatidylinositol phosphate in human platelet microsomes

The formation of lysophosphatidylinositol phosphate (lysoPIP) from lysophosphatidylinositol (lysoPI) via kinase activity was studied in microsomal preparations from human platelets. For this purpose, [3H]lysoPI or [3H]phosphatidylinositol ([3H]PI) was prepared and incubated in the presence or absence of ATP, MgCl2 and Triton X-100, and the appearances of radioactivity in [3H]lysoPIP and [3H]phosphatidylinositol phosphate ([3H]PIP), respectively, were monitored using thin layer chromatography. Both lysoPI and PI phosphorylations were completely dependent upon the presence of ATP and MgCl2 in the incubation medium; Triton X-100 addition stimulated both reactions, with the stimulation of PI conversion being considerably greater than that for lysoPI can be converted to lysoPIP by phosphorylation in human platelet microsomes. The potential significance of this enzymatic reaction in stimulated cells is discussed in relation to the generation of inositol-1,4,5-trisphosphate, an important intracellular second messenger.

Externally applied adenosine-5'-triphosphate causes inositol triphosphate accumulation in cultured chick myotubes

In striated muscle, adenosine-5'-triphosphate (ATP) potentiates the responses to acetylcholine. The underlying biochemical events are unknown. Here we report that ATP, externally applied to chick myotubes, induces a rapid, dose-dependent accumulation of intracellular inositol triphosphate which is correlated with a decrease in phosphatidyl 4,5-bisphosphate. Adenosine-5'-diphosphate, adenosine-5'-monophosphate and adenosine are less potent while beta, gamma-imido ATP is equipotent motoneurons and/or skeletal muscle controls the activation of a polyphosphoinositide phosphodiesterase via a cell membrane P2-purinoceptor, thus modulating skeletal muscle responses to transmitter release.

Changes in membrane polypeptides, polyphosphoinositides and phosphatidate in dense fractions of sickle cells

When sickle erythrocytes were fractionated on discontinuous isotonic stractan gradients the denser fractions, which were rich in irreversibly sickled cells contained less polyphosphoinositides and more phosphatidate than either lighter sickle cell fractions or normal cells. These changes could be due to activation of a polyphosphoinositide phosphodiesterase in the denser cells. Membrane polypeptide analysis of the denser fractions also showed a marked depletion of band 4.1 and a protein of molecular mass about 110 kDa but an increased amount of a 180 kDa polypeptide which might be a breakdown product of ankyrin. These biochemical alterations could be consequences of Ca2+ accumulation in the denser sickle cells and may contribute to the structural alterations which give rise to irreversibly sickled cells.