Biosynthesis of Diphosphoinositide in Brain

Discoveries of the phosphatidate phosphatase genes in yeast published in the Journal of Biological Chemistry

This JBC Review on the discoveries of yeast phosphatidate (PA) phosphatase genes is dedicated to Dr. Herbert Tabor, Editor-in-Chief of the Journal of Biological Chemistry (JBC) for 40 years, on the occasion of his 100th birthday. Here, I reflect on the discoveries of the APP1, DPP1, LPP1, and PAH1 genes encoding all the PA phosphatase enzymes in yeast. PA phosphatase catalyzes PA dephosphorylation to generate diacylglycerol; both substrate and product are key intermediates in the synthesis of membrane phospholipids and triacylglycerol. App1 and Pah1 are peripheral membrane proteins catalyzing an Mg²⁺-dependent reaction governed by the DXDX(T/V) phosphatase motif. Dpp1 and Lpp1 are integral membrane proteins that catalyze an Mg²⁺-independent reaction governed by the KX ₆RP-PSGH-SRX ₅HX ₃D phosphatase motif. Pah1 is PA-specific and is the only PA phosphatase responsible for lipid synthesis at the nuclear/endoplasmic reticulum membrane. App1, Dpp1, and Lpp1, respectively, are localized to cortical actin patches and the vacuole and Golgi membranes; they utilize several lipid phosphate substrates, including PA, lyso-PA, and diacylglycerol pyrophosphate. App1 is postulated to be involved in endocytosis, whereas Dpp1 and Lpp1 may be involved in lipid signaling. Pah1 is the yeast lipin homolog of mice and humans. A host of cellular defects and lipid-based diseases associated with loss or overexpression of PA phosphatase in yeast, mice, and humans, highlights its importance to cell physiology.

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.

A retrospective: use of Escherichia coli as a vehicle to study phospholipid synthesis and function

Although the study of individual phospholipids and their synthesis began in the 1920s first in plants and then mammals, it was not until the early 1960s that Eugene Kennedy using Escherichia coli initiated studies of bacterial phospholipid metabolism. With the base of information already available from studies of mammalian tissue, the basic blueprint of phospholipid biosynthesis in E. coli was worked out by the late 1960s. In 1970s and 1980s most of the enzymes responsible for phospholipid biosynthesis were purified and many of the genes encoding these enzymes were identified. By the late 1990s conditional and null mutants were available along with clones of the genes for every step of phospholipid biosynthesis. Most of these genes had been sequenced before the complete E. coli genome sequence was available. Strains of E. coli were developed in which phospholipid composition could be changed in a systematic manner while maintaining cell viability. Null mutants, strains in which phospholipid metabolism was artificially regulated, and strains synthesizing foreign lipids not found in E. coli have been used to this day to define specific roles for individual phospholipid. This review will trace the findings that have led to the development of E. coli as an excellent model system to study mechanisms underlying the synthesis and function of phospholipids that are widely applicable to other prokaryotic and eukaryotic systems. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Synthesis and function of membrane phosphoinositides in budding yeast, Saccharomyces cerevisiae

It is now well appreciated that derivatives of phosphatidylinositol (PtdIns) are key regulators of many cellular processes in eukaryotes. Of particular interest are phosphoinositides (mono- and polyphosphorylated adducts to the inositol ring in PtdIns), which are located at the cytoplasmic face of cellular membranes. Phosphoinositides serve both a structural and a signaling role via their recruitment of proteins that contain phosphoinositide-binding domains. Phosphoinositides also have a role as precursors of several types of second messengers for certain intracellular signaling pathways. Realization of the importance of phosphoinositides has brought increased attention to characterization of the enzymes that regulate their synthesis, interconversion, and turnover. Here we review the current state of our knowledge about the properties and regulation of the ATP-dependent lipid kinases responsible for synthesis of phosphoinositides and also the additional temporal and spatial controls exerted by the phosphatases and a phospholipase that act on phosphoinositides in yeast.

The Saccharomyces cerevisiae LSB6 gene encodes phosphatidylinositol 4-kinase activity

The LSB6 gene product was identified from the Saccharomyces Genome Data Base (locus YJL100W) as a putative member of a novel type II phosphatidylinositol (PI) 4-kinase family. Cell extracts lacking the LSB6 gene had a reduced level of PI 4-kinase activity. In addition, multicopy plasmids containing the LSB6 gene directed the overexpression of PI 4-kinase activity in cell extracts of wild-type cells, in an lsb6Delta mutant, in a pik1(ts) stt4(ts) double mutant, and in an pik1(ts) stt4(ts) lsb6Delta triple mutant. The heterologous expression of the S. cerevisiae LSB6 gene in Escherichia coli resulted in the expression of a protein that possessed PI 4-kinase activity. Although the lsb6Delta mutant did not exhibit a growth phenotype and failed to exhibit a defect in phosphoinositide synthesis in vivo, the overexpression of the LSB6 gene could partially suppress the lethal phenotype of an stt4Delta mutant defective in the type III STT4-encoded PI 4-kinase indicating that Lsb6p functions as a PI 4-kinase in vivo. Lsb6p was localized to the membrane fraction of the cell, and when overexpressed, GFP-tagged Lsb6p was observed on both the plasma membrane and the vacuole membrane. The enzymological properties (pH optimum, dependence on magnesium or manganese as a cofactor, the dependence of activity on Triton X-100, the dependence on the PI surface concentration, and temperature sensitivity) of the LSB6-encoded enzyme were very similar to the membrane-associated 55-kDa PI 4-kinase previously purified from S. cerevisiae.

Phosphatidylinositol phosphate 5-kinase: purification and inhibition studies

A membrane-associated phosphatidylinositol phosphate 5-kinase has been purified approximately 110,000-fold from sheep brains. The purification procedure involves: sodium chloride (1M) extraction of the membrane, 20-40% ammonium sulfate fractionation, phosphocellulose (P-11) chromatography, a second phosphocellulose chromatography, hydroxyapatite chromatography, heparin Sepharose chromatography, HPLC SP(SO3- polymer)-cation exchange chromatography, and HPLC gel filtration. The purified enzyme exhibited a final specific activity of 1750 nmole/min/mg of protein. The molecular mass of the enzyme was estimated to be approximately 60 kDa by SDS-PAGE and 130 kDa by HPLC gel filtration. Kinetic measurements showed that the apparent Km value of phosphatidylinositol phosphate 5-kinase for the utilization of ATP is 43 microM. The 2'(3')-0-(2,4,6-trinitrophenyl) derivative of ATP was found to be an inhibitor of the enzyme. The mode of inhibition is competitive, with a Ki value of 55 microM.

Formation of phosphatidylinositol-4-phosphate in human peripheral blood eosinophils

To evaluate the presence of phosphatidylinositol-4-phosphate in peripheral blood eosinophils, venous blood was drawn from normal healthy volunteers. The eosinophils were isolated on a Percoll gradient and were incubated with [gamma32P]ATP in the presence of Mg2+. After stopping the reaction, lipid extraction was performed with acidic medium and phospholipids were separated by thin-layer chromatography on 1% (w/v) oxalic acid- and potassium oxalate-impregnated silica gel plates. Considerable amounts of radioactivity were found to be incorporated into phosphatidylinositol-4-phosphate on both plates. This reaction requires ATP and Mg2+, and maximal response is seen at 10 microM ATP and 20 mM Mg2+. The reaction is dependent upon the time and temperature of the assay system. No significant superoxide anion generation from the eosinophils incubated with ATP at the concentrations used in the study was observed. These results suggest the possible presence of phosphatidylinositol kinase which catalyses the formation of phosphatidylinositol-4-phosphate from endogenous phosphatidylinositol in human peripheral blood eosinophils.

Affinity labelling of the active site of brain phosphatidylinositol 4-kinase with 5'-fluorosulphonylbenzoyl-adenosine

5'-p-Fluorosulphonylbenzoyl-adenosine (FSO2BzAdo), an affinity labelling analogue of ATP, was used to label the active site of sheep brain phosphatidylinositol 4-kinase (PtdIns 4-kinase). The incubation of PtdIns 4-kinase with concentrations of FSO2BzAdo as low as 50 microM resulted in considerate inactivation of the enzyme. (e.g. 55% less after 60 min with 50 microM FSO2BzAdo). The kinetics of inactivation of PtdIns 4-kinase by FSO2BzAdo suggest a two-step mechanism, in which a rapid reversible binding of FSO2BzAdo to the enzyme is followed by a covalent sulphonation step. The first-order rate constant (k2) for the inactivation of PtdIns 4-kinase was calculated to be 0.063 min-1, and the steady-state constant of inactivation (Ki) to be 200 microM. Preincubation of the enzyme with either ATP plus Mg2+, or PtdIns alone, prior to addition of FSO2BzAdo reduced the degree of inactivation of the enzyme; suggesting that FSO2BzAdo binds within the active site PtdIns 4-kinase. Moreover, since ATP plus Mg2+ provided the greatest protection against inactivation, it is concluded that the main site of labelling of PtdIns 4-kinase by FSO2BzAdo is within the ATP-binding site of the enzyme. Results obtained from chemical modification experiments, which employed pyridoxal 5'-phosphate and tetranitromethane, are consistent with a catalytically-essential lysine being present within the ATP-binding site of PtdIns 4-kinase. Therefore, it is hypothesised that the inactivation of PtdIns 4-kinase by FSO2BzAdo may be due to the labelling of this lysine residue.

Purification and chemical modification of a phosphatidylinositol kinase from sheep brain

A membrane-bound phosphatidylinositol (PtdIns) kinase has been purified approximately 9500-fold to apparent homogeneity from sheep brains. The purification procedure involves: solubilisation of the membrane fraction with Triton X-100, ammonium sulphate fractionation and a number of ion-exchange and gel-filtration chromatography steps. The purified enzyme exhibited a final specific activity of 1149 nmol.min-1.mg-1. The molecular mass of the enzyme was estimated to be 55 kDa by SDS/PAGE and 150 +/- 10 kDa by HPLC gel filtration in the presence of Triton X-100. Kinetic measurements have shown that the apparent Km value of PtdIns kinase for the utilisation of PtdIns is 22 microM and for ATP 67 microM. Mg2+ was the most effective divalent cation activator of PtdIns kinase, with maximal enzymatic activity reached at a concentration of 10 mM Mg2+. In addition to adenosine and ADP, the 2'(3')-O-(2,4,6-trinitrophenyl) derivative of ATP was found to be a strong competitive inhibitor of the enzyme, with a Ki of 32 microM. Enzymatic activity was found to be stimulated by Triton X-100 but inhibited by deoxycholate.

Synthesis of polyphosphoinositides in transverse tubule and sarcoplasmic reticulum membranes of frog skeletal muscle

Synthesis of polyphosphoinositides has been studied in transverse (T-) tubule and sarcoplasmic reticulum (SR) membrane fractions of frog skeletal muscle, following 32P-labeling with [gamma-32P]ATP. Purified SR and T-tubule fractions respectively synthesize 9.4 +/- 0.8 and 71.9 +/- 9.8 pmol PtdInsP/mg per min, indicating nearly 8-fold higher activity of PtdIns kinase in the T-tubules than in the SR. The activity of this enzyme in both membrane systems is maximum at pH 7 and pCa 6. PtdInsP2 is synthesized from the endogenous PtdInsP, only in T-tubule membranes by the action of PtdInsP kinase. This lipid is the most intensely 32P-labeled phosphoinositide (181.7 +/- 9.2 pmol/mg per min) in these membranes. PtdIns kinase in the T-tubule and SR membranes, and PtdInsP kinase in the former are modulated by the free [Mg2+]. Loss of radiolabel from transiently maximal 32P-incorporation in polyphosphoinositides in T-tubule membranes, concomitant with a decrease in the ATP concentration in the incubation buffer, shows the occurrence of phosphoinositidases in these membranes. Under the conditions used, no such activities were evident in SR membranes. Compound 48/80, a mixture of condensation products of N-methyl-p-methoxyphenethylamine with formaldehyde, known to block phosphoinositidase C and phospholipase A2, causes a dose-dependent increase in the 32P-label of PtdInsP, in T-tubule membranes. The synthesis of lyso PtdInsP2, a deacylated form of PtdInsP2 which occurs in nearly equal quantities in both T-tubule and SR membranes, may result from a mechanism independent of phospholipase A2.

Bimodal lipid substrate dependence of phosphatidylinositol kinase

Phosphatidylinositol (PI) kinase activity was solubilized from rat liver microsomes and partially purified by chromatography on hydroxyapatite and Reactive Green 19-Superose. Examination of the ATP dependence using a mixed micellar assay gave a Km of 120 microM. The dependence of reaction rate on PI was more complicated. PI kinase bound a large amount of Triton X-100, and as expected for a micelle-associated enzyme utilizing a micelle-associated lipid substrate, the reaction rate was dependent on the micellar mole fraction, PI/(PI + Triton X-100), with a Km of 0.02 (unitless). Activity showed an additional dependence on bulk PI concentration at high micelle dilution. These results demonstrated two kinetically distinguishable steps leading to formation of a productive PI/enzyme(/ATP) complex. The rate of the first step, which probably represents exchange of PI from the bulk micellar pool into enzyme-containing micelles, depends on bulk PI concentration. The rate of the second step, association of PI with enzyme within a single micelle, depends on the micellar mole fraction of PI. Depression of the apparent Vmax at low ionic strength suggested that electrostatic repulsion between negatively charged PI/Triton X-100 mixed micelles inhibits PI exchange, consistent with a model in which intermicellar PI exchange depends on micellar collisions.

Phosphatidylinositol kinase, phosphatidylinositol-4-phosphate kinase and diacylglycerol kinase activities in rat brain subcellular fractions

Subcellular fractions isolated and purified from rat brain cerebral cortices were assayed for phosphatidylinositol (PI-), phosphatidylinositol-4-phosphate (PIP-), and diacylglycerol (DG-) kinase activities in the presence of endogenous or exogenously added lipid substrates and [gamma-32P]ATP. Measurable amounts of all three kinase activities were observed in each subcellular fraction, including the cytosol. However, their subcellular profiles were uniquely distinct. In the absence of exogenous lipid substrates, PI-kinase specific activity was greatest in the microsomal and non-synaptic plasma membrane fractions (150-200 pmol/min per mg protein), whereas PIP-kinase was predominantly active in the synaptosomal fraction (136 pmol/min per mg protein). Based on percentage of total protein, total recovered PI-kinase activity was most abundant in the cytosolic, synaptosomal, microsomal and mitochondrial fractions (4-11 nmol/min). With the exception of the microsomal fraction, a similar profile was observed for PIP-kinase activity when assayed in the presence of exogenous PIP (4 nmol/20 mg protein in a final assay volume of 0.1 ml). Exogenous PIP (4 nmol/20 mg protein) inhibited PI-kinase activity in most fractions by 40-70%, while enhancing PIP-kinase activity. PI- and PIP-kinase activities were observed in the cytosolic fraction when assayed in the presence of exogenously added PI or PIP, respectively, but not in heat-inactivated membranes containing these substrates. When subcellular fractions were assayed for DG-kinase activity using heat-inactivated DG-enriched membranes as substrate, DG-kinase specific activity was predominantly present in in the cytosol. However, incubation of subcellular fractions in the presence of deoxycholate resulted in a striking enhancement of DG-kinase activities in all membrane fractions. These findings demonstrate a bimodal distribution between particulate and soluble fractions of all three lipid kinases, with each exhibiting its own unique subcellular topography. The preferential expression of PIP-kinase specific activity in the synaptic membranes is suggestive of the involvement of PIP2 in synaptic function, while the expression of PI-kinase specific activity in the microsomal fraction suggests additional, yet unknown, functions for PIP in these membranes.

Regulation of phosphatidylinositol kinase activity in Saccharomyces cerevisiae

The effects of growth phase and carbon source on membrane-associated phosphatidylinositol kinase in cell extracts of Saccharomyces cerevisiae were examined. Phosphatidylinositol kinase activity increased 2- and 2.5-fold in glucose- and glycerol-grown cells, respectively, in the stationary phase as compared with the exponential phase of growth. The increase in phosphatidylinositol kinase activity in the stationary phase of growth correlated with an increase in the relative amounts of phosphatidylinositol 4-phosphate, the product of the reaction. The increase in phosphatidylinositol kinase activity was not due to the presence of water-soluble effector molecules in cell extracts as indicated by mixing experiments. Phosphatidylinositol kinase activity decreased in cell extracts of exponential-phase cells preincubated under phosphorylation conditions which favor cyclic AMP-dependent protein kinase activity. Phosphatidylinositol kinase activity was not affected in cell extracts of stationary-phase cells preincubated under phosphorylation conditions.

The inhibitory effect of cyclic AMP on phosphatidylinositol kinase is not mediated by the cAMP dependent protein kinase

Addition of cAMP to cells has been shown to inhibit phosphatidylinositol (PI) metabolism. cAMP has been reported to inhibit an enzyme in this pathway, PI kinase and it has been suggested that this inhibition is due to phosphorylation of PI kinase by the cAMP dependent protein kinase (PKA). In the present study we directly investigated if the inhibitory effect of cAMP was mediated by PKA. In membranes derived from murine hepatocytes we found that cAMP inhibited PI kinase but other adenine derivatives were more potent inhibitors. Moreover, it was found that the effects of the derivatives were unlikely to be due secondarily to the production of cAMP via their interaction with adenosine receptors. Through studies employing an inhibitor of PKA, mutant cells lacking PKA, and addition of purified catalytic subunit of PKA, we found that the inhibitory effect of cAMP was not mediated by PKA. In addition, the inhibitory effect of cAMP and adenosine was retained upon partial purification of PI kinase. Pulse chase experiments affirmed that the inhibitory effect was not due to breakdown of PI but rather to inhibition of its synthesis. We conclude that the inhibitory effect of cAMP and related compounds on PI kinase is not mediated by PKA dependent phosphorylation but rather appears to be a direct effect of these agents.

Studies and perspectives of protein kinase C

Protein kinase C, an enzyme that is activated by the receptor-mediated hydrolysis of inositol phospholipids, relays information in the form of a variety of extracellular signals across the membrane to regulate many Ca2+-dependent processes. At an early phase of cellular responses, the enzyme appears to have a dual effect, providing positive forward as well as negative feedback controls over various steps of its own and other signaling pathways, such as the receptors that are coupled to inositol phospholipid hydrolysis and those of some growth factors. In biological systems, a positive signal is frequently followed by immediate negative feedback regulation. Such a novel role of this protein kinase system seems to give a logical basis for clarifying the biochemical mechanism of signal transduction, and to add a new dimension essential to our understanding of cell-to-cell communication.

Variant of A431 cells isolated by ricin A-conjugated monoclonal antibody directed to EGF receptor: phosphorylation of EGF receptor and phosphatidylinositol

A monoclonal antibody specific for human EGF receptors was cross-linked to subunit A of toxic ricin. Using this conjugate, we isolated a variant of A431 cells, designated C1-B7, with approximately 40 times less EGF binding capacity. Unlike the parental cells, the C1-B7 variant was resistant to EGF-induced suppression of cell growth. The EGF receptors retained in this variant were of high-affinity type and susceptible to EGF-induced autophosphorylation. Membrane prepared from C1-B7 cells was highly phosphorylated in the presence of 2 microM [gamma-32P]-ATP, primarily on the lipid components shown as phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. This same level of lipid phosphorylation was observed on A431 membrane only in the presence of higher ATP concentrations. After addition of EGF to A431 membrane, phosphatidylinositol phosphorylation was significantly decreased with a concomitant increase in EGF-dependent protein phosphorylation. Thus, the EGF-dependent receptor-mediated protein phosphorylation precedes phosphatidylinositol phosphorylation. These observations support the idea that the growth inhibitory effect of EGF on A431 cells is caused by high ATP consumption due to the EGF-induced protein phosphorylation and reduction of phosphatidylinositol turnover.

Calcium- and calmodulin-dependent phosphorylation of diphosphoinositide in acetylcholine receptor-rich membranes from electroplax of Narke japonica

The phosphorylation of phosphoinositides in the acetylcholine receptor (AChR)-rich membranes from the electroplax of the electric fish Narke japonica has been examined. When the AChR-rich membranes were incubated with [gamma-32P]ATP, 32P was incorporated into only two inositol phospholipids, i.e., tri- and diphosphoinositide (TPI and DPI). Even after the alkali treatment of the membrane, AChR-rich membranes still showed a considerable DPI kinase activity upon addition of exogenous DPI. It is likely that the 32P-incorporation into these lipids was realized by the membrane-bound DPI kinase and phosphatidyl inositol (PI) kinase. Such a membrane-bound DPI kinase was activated by Ca2+ (greater than 10(-6) M), whereas the PI kinase appeared to be inhibited by Ca2+. The effect of Ca2+ on the DPI phosphorylation was further enhanced by the addition of ubiquitous Ca2+-dependent regulator protein calmodulin. Calmodulin antagonists such as chlorpromazine (CPZ), trifluoperazine (TFP), and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) inhibited the phosphorylation of DPI in the AChR-rich membranes. It is suggested that the small pool of TPI in the plasma membrane is replenished by such Ca2+- and calmodulin-dependent DPI kinase responding to the change in the intracellular Ca2+ level.

Inhibition of phosphatidylinositol-4-phosphate kinase by its product phosphatidylinositol-4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PIP2) is enzymatically produced when high speed supernatant fraction from bovine retina is incubated with [gamma-32P]ATP and phosphatidylinositol 4-phosphate (PIP) as substrates. Exogenously added PIP2 inhibits PIP kinase activity 50% at equimolar concentrations of product and substrate. Ca2+-dependent phosphodiesteratic activity, resulting in the loss of PIP2 and PIP and concommitant increase in myo-inositol 1,4,5-trisphosphate and myo-inositol 1,4-bisphosphate, was observed when soluble retinal fractions were incubated with heat-inactivated 32P-prelabeled guinea pig nerve ending membranes as substrate. It is suggested that polyphosphoinositides are under stringent and complex control and that upon receptor activation-mediated stimulation of phosphodiesteratic degradation release of the feedback inhibition shown here may occur and result in the synthesis and replenishment of PIP2.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

Mitogens increase phosphorylation of phosphoinositides in thymocytes

In many cell systems the interaction of ligands with their receptors causes rapid breakdown and resynthesis of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). Recent work has focused on the role of the degradation products of PtdIns(4,5)P2 as intermediates in the activation of cell function and growth: inositol trisphosphate (InsP3) can release Ca2+ from intracellular stores and diacylglycerol is thought to activate protein kinase C. This enzyme is also activated by phorbol esters (for example, 12-O-tetradecanoyl phorbol 13-acetate, TPA) and this is assumed to account for the pleiotropic effects of TPA on cell function and growth. Mouse thymocytes are not mitogenically stimulated by TPA alone, but it is a potent co-mitogen in combination with either concanavalin A (Con A) or A23187 (A. N. Corps and J.C.M., unpublished observations). Here we show that mitogenic concentrations of TPA, A23187 and Con A each cause an increase in the net phosphorylation of phosphatidylinositol (PtdIns) to PtdIns(4,5)P2 in mouse thymocytes. This is consistent with simulation by the mitogens of the same phosphoinositide phosphorylations in intact cells as recently demonstrated for the isolated products of the src and ros viral oncogenes in a cell-free system.

Turnover of inositol phospholipids and signal transduction

Various extracellular informational signals such as those from a group of hormones and some neurotransmitters appear to be passed from the cell surface into the cell interior by two routes, protein kinase C activation and Ca2+ mobilization. Both routes usually become available as the result of an interaction of a single ligand and a receptor and act synergistically to evoke subsequent cellular responses such as release reactions. The signal-dependent breakdown of inositol phospholipids, particularly phosphatidylinositol bisphosphate, now appears to be a key event for initiating these processes.

Membrane-associated phosphatidylinositol kinase from Saccharomyces cerevisiae

Membrane-associated phosphatidylinositol kinase (ATP:phosphatidylinositol 4-phosphotransferase, EC 2.7.1.67) was partially purified 93-fold from Saccharomyces cerevisiae. Activity was dependent on magnesium ions (10 mM) and the optimum pH was 8.5. The apparent Km values for ATP and phosphatidylinositol were 0.21 mM and 71 microM, respectively. Activity was stimulated by sodium cholate and inhibited by sodium, potassium, lithium, and fluoride ions.

Phospholipid synthesis in the squid giant axon: enzymes of phosphatidylinositol metabolism

We examined the properties of several enzymes of phospholipid metabolism in axoplasm extruded from squid giant axons. The following synthetic enzymes, CDP-diglyceride: inositol transferase (EC 2.7.8.11), ATP:diglyceride phosphotransferase, diglyceride kinase (EC 2.7.2.-), and phosphatidylinositol kinase (EC 2.7.1.67), were all present in axoplasm. Phospholipid exchange proteins, which catalyzed the transfer of phosphatidylinositol and phosphatidylcholine between membrane preparations and unilamellar lipid vesicles, were also found. However, we did not find conditions under which the synthesis of CDP-diglyceride, phosphatidylserine, and phosphatidylinositol-4,5-diphosphate could be measured. Subcellular fractionation by differential centrifugation showed that the axoplasmic inositol transferase and phosphatidylinositol kinase activities were largely "microsomal," while the diglyceride kinase and exchange protein activities were primarily "cytosolic."

Properties of phosphatidylinositol kinase activities in rabbit erythrocyte membranes

Properties of phosphatidylinositol kinase activities in rabbit erythrocyte membranes were studied by measuring 32P incorporation into di- and triphosphoinositide from Mg-[gamma-32P]ATP. The Km's for 32P incorporation into di- and triphosphoinositide were 110 and 48 microM ATP, respectively. The optimal temperature for 32P incorporation into diphosphoinositide was at 32 degrees C, whereas the optimum for triphosphoinositide labeling occurred at 43 degrees C. Differences in the effects of pH on the rate of 32P incorporation into di- and triphosphoinositide were also found. At 37 degrees C but not at 25 degrees C 32P-labeled diphosphoinositide was phosphorylated to triphosphoinositide in the presence of Mg-ATP. Triton X-100 partially inhibited 32P incorporation into diphosphoinositide but completely inhibited the synthesis of triphosphoinositide. At physiological concentrations, 0.4 mM MgCl2 half-maximally activated di- and triphosphoinositide synthesis. Higher concentrations of MgCl2 (5 to 50 mM) decreased 32P incorporation into diphosphoinositide and greatly enhanced 32P incorporation into triphosphoinositide. NaCl or KCl (less than or equal to 100 mM) did not have any effects on polyphosphoinositide synthesis, whereas 150 to 300 mM NaCl or KCl decreased synthesis of diphosphoinositide and increased synthesis of triphosphoinositide. Further studies showed that 50 mM MgCl2 and 200 mM NaCl or KCl stimulate kinase-mediated phosphorylation of diphosphoinositide to triphosphoinositide. Triton X-100 inhibited the ability of 50 mM MgCl2 and neomycin to stimulate phosphorylation of diphosphoinositide to triphosphoinositide. The pathways for synthesis of di- and triphosphoinositides in erythrocyte membranes are discussed.

Metabolism of phosphoinositides in the rat erythrocyte membrane. A reappraisal of the effect of magnesium on the 32P incorporation into polyphosphoinositides

The metabolism of phosphoinositides was investigated in the red blood cell membrane of the rat by measuring 32P-incorporation into phospholipids after incubation of membranes with [gamma-32P]ATP in a medium containing magnesium. A new chromatographic procedure has been developed which facilitates the separation of triphosphoinositide, diphosphoinositide and phosphatidylinositol from the phospholipids present in lipid extracts of incubated 'ghost' under our experimental conditions only two phospholipids, diphosphoinositide and triphosphoinositide, were 32P-labelled. Furthermore, the results indicate that either di-or triphosphoinositide could be labelled preferentially, depending upon the magnesium concentration of the incubation medium. This clarifies some apparent discrepancies reported in the literature between the 32 P labelling of polyphosphoinositides observed in intact erythrocytes and that observed with 'ghost' membranes. In addition, the enzymatic pathways involved in the phosphoinositide metabolism are discussed.

Detergent solubilization and hydrophobic chromatography of rat brain phosphatidylinositol kinase

Rat brain microsomal phosphatidylinositol kinase activity was maximally activated in the presence of either 3 mM sodium deoxycholate, 2% Triton-X-100, or 30-40 mM octylglucoside. Among these detergents, 1% Triton-X-100 was most effective in solubilizing the enzyme, and after treatment with this agent, 100% of the activity was recovered in the high speed supernatant. Octylglucoside solubilized 40% of the enzyme at concentrations below its critical micelle concentration of 25 mM and up to 80% at higher levels. Solubilized phosphatidylinositol kinase failed to absorb to adenosine nucleotide affinity resins. However, when the Triton-X-100 extract was chromatographed on an uncharged hydrophobic resin, consisting of dodecyl chains attached to Sepharose 4B by ether bonds, nearly all the enzyme activity was retained, and from 44-85% could be eluted with 8 mM sodium deoxycholate. Solubilization followed by hydrophobic chromatography resulted in several-fold purification of phosphatidylinositol kinase and may have disrupted interactions of the enzyme with other hydrophobic proteins sufficiently to allow its substantial purification by conventional or affinity chromatography techniques.