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

Phosphoinositide lipids as signaling molecules: common themes for signal transduction, cytoskeletal regulation, and membrane trafficking

Signaling roles for phosphoinositides that involve their regulated hydrolysis to generate second messengers have been well characterized. Recent work has revealed additional signaling roles for phosphoinositides that do not involve their hydrolysis. PtdIns 3-P, PtdIns 3,4,5-P3, and PtdIns 4,5-P2 function as site-specific signals on membranes that recruit and/or activate proteins for the assembly of spatially localized functional complexes. A large number of phosphoinositide-binding proteins have been identified as the potential effectors for phosphoinositide signals. Common themes of localized signal generation and the spatially localized recruitment of effector proteins appear to underlie mechanisms employed in signal transduction, cytoskeletal, and membrane trafficking events.

Effects of ethanol on phosphorylation of lipids in rat synaptic plasma membranes

Synaptic plasma membranes (SPM) isolated from rat cerebral cortex contain lipid kinases for conversion of phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), and diacylglycerol (DG) to PIP, phosphatidylinositol 4,5-bisphosphate (PIP2), and phosphatidic acid (PA), respectively. These anionic phospholipids are important in signal transduction mechanisms and are required for synaptic function. The effect of ethanol and other aliphatic alcohols on phosphorylation of these lipids in SPM has not been established. Incubation of SPM with [gamma-32P]ATP resulted in labeling of PIP, lyso-PIP, PIP2, and PA. Ethanol (50-200 mM) added to the incubation system showed a dose-dependent decrease in labeling of PIP2, but not PIP or PA. To a lesser extent, labeling of PIP2 was also inhibited by 1-propanol, but neither isopropanol nor 1-butanol could alter the PIP2 labeling pattern. Under similar incubation conditions, labeling of PIP and PA in SPM was not altered by ethanol, 1-propanol, iso-propanol, but 1-butanol stimulated PIP labeling with a peak at 25 mM. Addition of exogenous PIP to the incubation mixture led to an increase in labeling of PIP2, suggesting that the endogenous PIP pool in SPM is limiting for the synthesis of PIP2 in SPM. Interestingly, when SPM were incubated with exogenous PIP, addition of ethanol (50-100 mM) to this incubation mixture resulted in an increase in PIP2 labeling. Taken together, these results suggest a specific effect of ethanol on PIP kinase in SPM, and this effect seems to be dependent on the location and/or amount of PIP in the membrane.

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.

Phosphorylation of lipids in rat primary glial cells and immortalized astrocytes (DITNC)

Phosphatidylinositol 4,5-bisphosphate, a substrate in the signal transduction pathway involving phospholipase C, is synthesized in cell membranes by phosphorylation of phosphatidylinositol (PI) and phosphatidylinositol 4-phosphate (PI-4-P). Incubation with [gamma-32P]adenosine triphosphate of membranes isolated from primary glial cells in culture and from an immortalized astrocyte cell line (DITNC) and subsequent separation of the lipids by high-performance thin-layer chromatography revealed a number of labeled lipid bands. Further analysis of the deacylated products by high-performance liquid chromatography indicated the presence of two PI-4-P bands and a band corresponding to lysoPIP (PIP, PI phosphate). The two PI-4-P bands were more prominent in the samples from astrocyte cell membranes than from synaptic plasma membranes and plasma membranes. Analysis of the fatty acid composition of PI by gas-liquid chromatography revealed that both 20:3n-9 and 20:4n-6 were present in PI of cultured cell membranes, whereas the brain membranes contained exclusively 20:4n-6. The two PI-4-P bands in these membranes can be attributed to the presence of different molecular species of PI. Assignment of the fraction corresponding to lysoPIP was consistent with the observation that an increase in label in this lipid band occurred upon incubation of DITNC cell supernatant with lysoPI (1-stearoyl PI). This suggests that endogenous lysoPI present in the cell membranes can be phosphorylated to form lysoPIP.

Cellular distribution of polyphosphoinositides in rat hepatocytes

The distribution of total phospholipids, phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) was studied in isolated rat hepatocytes: (i) by mass assay and isotopic labelling in the fractions of plasma membranes, microsomes, mitochondria and nuclei prepared from isolated hepatocytes and (ii) by immunolocalization of PIP2 with a specific antibody (kt3g) in whole hepatocytes and isolated nuclei. Mass measurement and isotopic labelling showed that PIP was distributed in all four fractions. PIP2 was present in the plasma membrane and the nuclei. In whole cells, PIP2 was also detected in the plasma membrane by immunolocalization with the anti-PIP2 antibody kt3g. In unpolarized single hepatocytes, PIP2 distributed evenly throughout the plasma membrane. However, in polarized cell couplets, PIP2 was the most often undetectable in the lateral domain between the cells, and distributed preferentially in the sinusoidal domain of the plasma membrane. These results suggest that hepatocytes segregate PIP2 in particular domains of their plasma membrane. In purified fractions of nuclei, immunolocalization experiments showed that PIP2 was present uniquely in the nuclear envelope.

[Calcium and liver]

Cells expand energy to lower the concentration of free calcium in the cytosol ([Ca2+]i) to a very low level. Extracellular Ca2+ entering via channels situated in the plasma membrane is expelled into the extracellular medium by a Ca(2+)-Mg(2+)-ATPase or by Na(+)-Ca2+ exchangers. The Ca2+ that enters the cell is sequestered, once inside the cytosol, by a Ca(2+)-Mg(2+)-ATPase, which concentrates Ca2+ in specialized domains of the endoplasmic reticulum. The nucleus and the mitochondria also concentrate Ca2+, but less efficiently. The stimulation of numerous receptors by hormones, growth factors and neurotransmitters coupled to GTP-binding proteins provokes a rapid increase in [Ca2+]i by mobilizing Ca2+ from intra- and extracellular compartments. Membrane coupling is ensured by the activation of a phospholipase C-beta, which hydrolyses a doubly phosphorylated phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). The inositol (1,4,5)-trisphosphate (InsP3) consequently formed binds to a receptor consisting in 4 homologous of 250 kDa each. The InsP3 receptor has been localized to a specialized region, rich in Ca2+, of the endoplasmic reticulum. The receptor has been purified and its sequence obtained. Reincorporated into planar bilayers, it displays the properties of a channel. In the cell, opening of the InsP3 receptor-channel provokes the release of the Ca2+ accumulated within the endoplasmic reticulum. Analyzing the kinetics of channel opening by the methods of rapid mixing, rapid filtration or flash photolysis of caged InsP3 has revealed that InsP3 opens the channel within a very short time, probably less than 30 msec. The InsP3 receptor-channel is autoregenerative. With the sustained stimulation of a Ca2+ influx the release of Ca2+ leads to an augmentation of [Ca2+]i, which is responsible for triggering cellular responses. The complexity of Ca2+ signals produced by stimulated cells has been revealed by studies in which highly effective techniques have been used to detect Ca2+ ions in the cytosol, such as bioluminescent proteins, fluorescent indicators or ionic currents sensitive to Ca2+. It appears that variations in [Ca2+]i induced by stimulation consist of oscillations of which the frequency, but not the amplitude, depends on the concentration of the hormone. Moreover, by summing the images picked up with a video recorder, it has been possible to demonstrate the changes in [Ca2+]i at the subcellular level and the waves of Ca2+ in stimulated cells.

Evidence for a new inositol phospholipid in rat brain mitochondria

Phosphorylation of phosphatidylinositol (PI), phosphatidylinositol monophosphate (PIP) and diacylglycerol (DAG) was studied in rat brain cortex myelin, synaptosomal and mitochondrial fractions, with ATP as phosphate donor and endogenous phospholipids as substrate. All fractions had PI, PIP and DAG phosphorylating activity with their own characteristic subcellular distribution. However, in the mitochondrial fraction an unidentified lipid was phosphorylated, which had a slower Rf value than PIP2 on TLC. After hydrolysis of the polar head group of the lipid and separation on anion exchange columns, it appeared to be a phosphoinositide. The elution profile showed that it was not phosphatidylinositol trisphosphate, or a lyso-compound. The available evidence suggests that the unknown inositol phospholipid in rat brain mitochondria is a phosphatidylinositol 4,5-bisphosphate isomer, although the possibility of it being a glycosyl-phosphoinositide cannot be excluded.

Brain phosphatidic acid and polyphosphoinositide formation in a broken cell preparation: regional distribution and the effect of age

The effect of age on phosphate incorporation into phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidic acid (PA) was studied. Lysed crude synaptosomal fractions of different brain regions of 3-month-old and 32-month-old Brown Norway rats were used. The brain regions tested were the hippocampus, frontal cortex, occipital/parietal cortex, entorhinal/pyriformal cortex, striatum/septum, thalamus and hypothalamus. The individual specific phosphorylating activities were unevenly distributed within the brain of Brown Norway rats. Strikingly, the distribution of phosphate incorporation into PIP2 was opposite from that of phosphate incorporation into PA. Phosphate incorporation into PA decreased (-15%) with age in almost all brain regions tested, whereas phosphate incorporation into PIP2 decreased with age only in the frontal cortex (-20%) and in the hypothalamus (-8%). The effects of age may reflect a deterioration of phosphoinositide metabolism, with its function in signal transduction coupled to receptors via G-proteins, in the brain regions involved. In addition, there was an age related decrease in protein content and total phospholipid phosphorus content of lysed crude synaptosomal preparations of all brain regions. The high correlation between the changes in these parameters may be indicative of a decrease in the number or size of synaptosomes with age in the brain regions involved.

Polyphosphoinositide synthesis and protein phosphorylation in the plasma membrane from full-grown Bufo arenarum oocytes

1. Polyphosphoinositide content and phosphorylation of lipids and proteins were analyzed in oocytes of the toad Bufo arenarum Hensel. 2. Plasma membrane-enriched fractions obtained from full-grown, prophase-arrested oocytes incorporated 32P into both phospholipids and proteins after incubation with [gamma-32P]ATP in an Mg(2+)-containing medium. Phosphatidylinositol 4-phosphate (PIP), phosphatidate (PA) and phosphatidylinositol-4,5-bisphosphate (PIP2) were the only labelled lipids. The 32P incorporation depended on incubation time, the amount of protein, and the ATP concentration. 3. Autoradiography of polyacrylamide gel electropherograms and scintillation counting showed that the radioactivity was mainly associated with a group of membrane proteins having an M(r) of 87,000. 4. This paper provides evidence for the capacity of prophase-arrested oocytes from Bufo arenarum to synthesize polyphosphoinositides and to phosphorylate distinct membrane proteins.

Brief chronic effects of lithium administration on rat brain phosphoinositides and phospholipids

Lithium is known to exert its biochemical action on cells and tissues by inhibiting the enzymic conversion of inositol monophosphates to inositol. However, it is not clear whether this inhibitory action may lead to changes in the de novo biosynthesis of phosphatidylinositol and its phosphorylated derivatives. This biosynthetic scheme may have an important bearing with regard to the receptor-mediated signal transduction mechanism involving hydrolysis of polyphosphoinositides and release of inositol trisphosphate as second messenger for mobilization of intracellular calcium. In this study, the effects of brief chronic lithium administration on metabolism of brain phosphoinositides and other phospholipids were examined using the radiotracer technique with 32Pi as precursor. Sprague Dawley rats that were treated with lithium (3-4 meq/kg body wt) twice daily for 2-6 days consistently indicated an increase in the labeling of phosphatidylinositol 4,5-bisphosphates and a decrease in labeling of phosphatidylinositols and phosphatidylethanolamines. These phospholipid changes were found in both cortex and hippocampus and appeared to occur primarily in the synaptosomal fraction. Although the extent of the phospholipid changes could vary depending on both duration and dose levels of the lithium administered, these results demonstrated subtle effects of lithium on depressing the biosynthesis of phosphatidylinositol as well as phosphatidylethanolamine but perhaps a compensative increase in the synthesis of the phosphatidylinositol 4,5-bisphosphates.

Synthesis, transfer, and phosphorylation of phosphoinositides in cardiac membranes

Compartmentation of phosphoinositide synthesis and transfer of endogenous phosphatidylinositol (PI) were characterized in membrane fractions prepared from rabbit myocardium. De novo synthesis of PI was highly enriched in free sarcoplasmic reticulum (551 pmol.mg-1. min-1) compared with that in sarcolemma (26.8 pmol.mg-1. min-1) and junctional sarcoplasmic reticulum (178 pmol.mg-1. min-1). In contrast, PI phosphorylation was highly enriched in sarcolemma (2.69 nmol.mg-1.min-1) compared with that in free sarcoplasmic reticulum (0.22 nmol.mg-1.min-1) and junctional sarcoplasmic reticulum (0.38 nmol.mg-1.min-1). Phosphorylation of endogenous phosphatidylinositol 4-phosphate to phosphatidylinositol 4,5-bisphosphate was also enriched in sarcolemma (38.5 pmol.mg-1.min-1) compared with that in free sarcoplasmic reticulum (less than 5.0 pmol.mg-1.min-1) and junctional sarcoplasmic reticulum (6.5 pmol.mg-1.min-1). Transfer of endogenous PI was characterized as a mechanism to overcome compartmentation of PI synthesis in cardiac membranes. A 29-kDa PI transfer protein was purified 1,500-fold from cytosol of rabbit myocardium. Both cytosol and purified PI transfer protein catalyzed the transfer of endogenous PI from microsomal sites of synthesis to sarcolemma. In conclusion, synthesis of PI is highly enriched in free sarcoplasmic reticulum, whereas phosphorylation of phosphoinositides is highly enriched in sarcolemma. A 29-kDa PI transfer protein in myocardial cytosol can mediate in vitro transfer of de novo-synthesized PI to the sarcolemma.

Partial purification and characterization of phosphatidylinositol kinases from human platelets

Most of human platelet phosphatidylinositol (PI) kinase activity (approx. 80%) was associated with the membrane fraction and its majority was released by the extraction with Triton X-100 after KCl treatment. Two major activity peaks (mPIK-I and mPIK-III) were obtained by Mono Q column chromatography. They were distinct from each other with regard to Mr (76,000 and 80,000 as determined by gel-filtration chromatography), apparent Km values for ATP, effect of arachidonic acid and phosphatidylserine and detergent requirement. Triton X-100 inhibited the activity of mPIK-I but rather weakly enhanced the mPIK-III activity, and sodium cholate remarkably inhibited both mPIK-I and mPIK-III activities. Their products were identified to be phosphatidylinositol 4-phosphate. On the other hand, about 20% of PI kinase activity was recovered from the cytosolic fraction and two activity peaks (cPIK-I and cPIK-II) were resolved on Mono Q column chromatography. There were no significant differences in biochemical properties between cPIK-I and cPIK-II. Both of them had Mr approx. 550,000 as determined by gel-filtration chromatography and were activated by sodium cholate to a greater extent than by Triton X-100. The results suggest that the major PI kinases (mPIK-I and mPIK-III) are PI 4-kinase and mPIK-I is distinct from PI 4-kinases in other sources especially with regard to the effect of Triton X-100.

Metabolism of phosphatidylinositol in plasma membranes and synaptosomes of rat cerebral cortex: a comparison between endogenous vs exogenous substrate pools

The metabolism of phosphatidylinositols (PI) labeled with [14C]arachidonic acid within plasma membranes or synaptosomes was compared to the metabolism of PI prelabeled with [14C]arachidonic acid and added exogenously to the same membranes. Incubation of membranes containing the endogenously-labeled PI pool in the presence of Ca2+ resulted in the release of labeled arachidonic acid, as well as a small amount of labeled diacylglycerol. Labeled arachidonic acid was effectively reutilized and returned to the membrane phospholipids in the presence of adenosine triphosphate (ATP), CoA, and lysoPI. Although Ca2+ promoted the release of labeled diacylglycerol from prelabeled plasma membranes, this amount was only 17% of the maximal release, i.e., release in the presence of deoxycholate and Ca2+. This latter condition is known to fully activate the PI-phospholipase C, and incubation of prelabeled plasma membranes resulted in a six-fold increase in labeled diacylglycerols. On the other hand, when exogenously labeled PI were incubated with plasma membranes in the presence of Ca2+, the labeled diacylglycerols released were 59% of that compared to the fully activated condition. The phospholipase C action was calcium-dependent, regardless of whether exogenous or endogenous substrates were used in the incubation. In contrast to plasma membranes, intact synaptosomes had limited ability to metabolize exogenous PI even in the presence of Ca2+, although the activity of phospholipase C was similar to that in the plasma membranes when assayed in the presence of deoxycholate and Ca2+. These results suggest that discrete pools of PI are present in plasma membranes, and that the pool associated with the acyltransferase is apparently not readily accessible to hydrolysis by phospholipase C.

Phosphoinositide metabolism in cultured glioma and neuroblastoma cells: subcellular distribution of enzymes indicate incomplete turnover at the plasma membrane

The hypothesis that the small portion of cellular phosphoinositide participating in signal transduction might be preferentially recycled within the plasma membrane was tested in rat glioma (C6) and murine neuroblastoma (N1E-115) cells. Percoll density gradient centrifugation was used to isolate a purified plasma membrane fraction and the subcellular distribution of all enzymes mediating phosphoinositide turnover was assessed. A small but significant proportion of PtdInsP2-specific phosphodiesterase was located in the plasma membrane but only two of the five enzymes required to replace PtdInsP2 (diacylglycerol kinase and PtdInsP kinase) also were present. CTP:phosphatidate cytidylyltransferase and CMP-phosphatidate:inositol phosphatidyltransferase were located exclusively in a microsomal fraction containing enriched levels of endoplasmic reticulum markers. Thus, diacylglycerol from agonist-stimulated cleavage of PtdInsP2, or phosphatidic acid formed from it, must be transferred to the endoplasmic reticulum for conversion to PtdIns. Plasma membrane also lacked PtdIns kinase. If the soluble PtdIns kinase has access to membrane-bound substrate, PtdIns may be phosphorylated to PtdInsP before or during transport to the plasma membrane. Phosphorylation by the predominantly plasma membrane PtdInsP kinase to form PtdInsP2 completes the cycle. PtdInsP phosphatase was present in all membrane fractions suggesting that PtdInsP can be returned to the PtdIns pool in plasma membrane and elsewhere. PtdInsP2 phosphatase was almost exclusively in the cytosol suggesting that reversible interchange between PtdInsP and PtdInsP2 in the plasma membrane may be modulated by the ability of this phosphatase to act on PtdInsP2 in the membrane. Thus, PtdIns resynthesis in the plasma membrane of these cells does not occur and is not required for phosphoinositide-mediated signal transduction.

Differences in the metabolism of inositol and phosphoinositides by cultured cells of neuronal and glial origin

Phosphoinositide and inositol metabolism was compared in glioma (C6), neuroblastoma (N1E-115) and neuroblastoma X glioma hybrid (NG 108-15) cells. All cell lines had similar proportions of phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), and phosphatidylinositol 4,5-bisphosphate (PIP2). Neuroblastoma and hybrid cells had almost identical phospholipid and phosphoinositide compositions and similar activities for the enzymes metabolizing polyphosphoinositides (PI kinase, PIP phosphatase, PIP kinase, PIP2 phosphatase, PIP2 phosphodiesterase). Glioma cells differed by having greater proportions of ethanolamine plasmalogen and sphingomyelin, lower PIP kinase, 3-5-fold higher PIP phosphatase activity and 10-15-fold greater PIP2 phosphodiesterase activity. Higher PIP phosphatase and PIP2 diesterase activities appear to be characteristic of cells of glial origin, since similar activities were found in primary cultures of astroglia. Glioma cells also metabolize inositol differently. In pulse and pulse-chase experiments, glioma cells transported inositol into a much larger water-soluble intracellular pool and maintained a concentration gradient 30-times greater than neuroblastoma cells. Label in intracellular inositol was less than in phosphoinositides in neuroblastoma and exchanged rapidly with extracellular inositol. In glioma, labeling of intracellular inositol greatly exceeded that of phosphoinositides. As a consequence, radioactivity in prelabeled phosphoinositides could not be effectively chased from glioma cells by excess unlabeled inositol. Such differences between cells of neuronal and glial origin suggest different and possibly supportive roles for these two cell types in maintaining functions regulated through phosphoinositide-linked signalling systems in the central nervous system.

Increased phosphatidylinositol kinase activity in psoriatic epidermis

Phosphatidylinositol (PI) kinase is activated by growth factors, such as epidermal growth factor (EGF), and is thought to be involved in cellular proliferation. Psoriasis is a hyperproliferative epidermal disease in which EGF receptor expression is altered and phospholipase C activity is increased. Considering the potential importance of growth factor stimulated phosphoinositide metabolism in the genesis of abnormal growth, we measured PI kinase activity in epidermal keratome biopsies from normal skin and the lesional and nonlesional skin of psoriatic patients. The PI kinase activity in 10 psoriatic involved plaques was increased 6.7-fold (Vmax = 67.1 +/- 23.9 pmol formed/min/mg protein +/- SE) when compared with 11 normal epidermal biopsies (Vmax = 10.0 +/- 1.3 pmol/min/mg protein, p less than 0.025). Similar results were noted when enzyme activity was standardized using DNA content. The apparent Km of PI kinase for ATP in involved psoriatic biopsies (0.45 +/- 0.14 mM) was also significantly (p less than 0.025) increased compared with normals (0.11 +/- 0.02 mM). The PI kinase activity in 11 biopsies of nonlesional psoriatic epidermis was not statistically different from normal epidermis. Both psoriatic and normal PI kinases required Mg++ and were inhibited by Ca++. The polyamine, spermine, a known activator of PI kinase in other tissues, stimulated normal but not psoriatic epidermal PI kinase. Both normal and psoriatic PI kinase activities had an apparent mol wt of 85,000. Increased synthesis of phosphoinositides by PI kinase in psoriatic tissue may provide more substrate for phospholipase C; a key enzyme in growth factor-mediated signal transduction.

Evidence for the presence of diacylglycerol kinase in rat brain myelin

The previous demonstration that incubation of brain slices with [32P]phosphate brings about rapid labeling of phosphatidic acid in myelin suggests that the enzyme involved should be present in this specialized membrane. DAG kinase (ATP:1,2-diacyglycerol 3-phosphotransferase, E.C. 2.7.1.107) is present in rat brain homogenate at a specific activity of 2.5 nmol phosphatidic acid formed/min/mg protein, while highly purified myelin had a much lower specific activity (0.29 nmol/min/mg protein). Nevertheless, the enzyme appears to be intrinsic to this membrane since it can not be removed by washing with a variety of detergents or chelating agents, and it could not be accounted for as contamination by another subcellular fraction. Production of endogenous, membrane-associated, diacylglycerol (DAG) by PLC (phospholipase C) treatment brought about translocation from soluble to particulate fractions, including myelin. Another level of control of activity involves inactivation by phosphorylation; a 10 min incubation of brain homogenate with ATP resulted in a large decrease in DAG kinase activity in soluble, particulate and myelin fractions.

Regulation of polyphosphoinositide synthesis in cardiac membranes

The relative distribution of phosphatidylinositol (PI) and phosphatidylinositol-4-phosphate (PIP) kinase activities in enriched cardiac sarcolemma (SL), sarcoplasmic reticulum (SR), and mitochondrial fractions was investigated. PI and PIP kinase activities were assayed by measuring 32P incorporation into PIP and phosphatidylinositol 4,5-bisphosphate (PIP2) from endogenous and exogenous PI in the presence of [gamma-32P]ATP. PI and PIP kinase activities were present in SL, SR, and mitochondrial fractions prepared from atria and ventricles although the highest activities were found in SL. A similar membrane distribution was found for PI kinase activity measured in the presence of detergent and exogenous PI. PI and PIP kinase activities were detectable in the cytosol providing exogenous PI and PIP and Triton X-100 were present. Further studies focused on characterizing the properties and regulation of PI and PIP kinase activities in ventricular SL. Alamethacin, a membrane permeabilizing antibiotic, increased 32P incorporation into PIP and PIP2 4-fold. PI and PIP kinase activities were Mg2+ dependent and plateaued within 15-20 min at 25 degrees C. Exogenous PIP and PIP2 (0.1 mM) had no effect on PIP and PIP2 labeling in SL in the absence of Triton X-100 but inhibited PI kinase activity in the presence of exogenous PI and Triton X-100. Apparent Km's of ATP for PI and PIP kinase were 133 and 57 microM, respectively. Neomycin increased PIP kinase activity 2- to 3-fold with minor effects on PI kinase activity. Calmidazolium and trifluoperazine activated PI kinase activity 5- to 20-fold and completely inhibited PIP kinase activity. Quercetin inhibited PIP kinase 66% without affecting PI kinase activity. NaF and guanosine 5'-O-(3-thiotriphosphate) had no effect on PI and PIP kinase activities, indicating that these enzymes were not modulated by G proteins. The probability that PIP and PIP2 synthesis in cardiac sarcolemma is regulated by product inhibition and phospholipase C was discussed.

Time course for labeling of brain membrane phosphoinositides and other phospholipids after intracerebral injection of [32P]-ATP. Evaluation by an improved HPTLC procedure

An improved two-dimensional HPTLC procedure was developed for separating phospholipids including individual phosphoinositides, phosphatidic acids and plasmalogens. This procedure was used to examine the time course for uptake of label by phospholipids in brain subcellular membranes after intracerebral injection of [gamma-32P]-ATP. There were considerable differences in the phospholipid labeling pattern among different subcellular fractions. In particular, a high proportion of labeled phosphatidylinositol 4,5-bisphosphates and phosphatidic acids was found in the myelin fraction during the initial 4 hr after injection. In other subcellular fractions, labeling of phosphoinositides was maximum at 2 hr, but with prolonged time, poly-phosphoinositides started to show a decline in radioactivity whereas labeling of other phospholipids continued to show a steady increase instead. Results indicate at least two different modes for the uptake of label by brain membrane phospholipids after intracerebral injection of [32P]-ATP.