Phosphatidylcholine biosynthesis in isolated hamster heart

Statistical analysis of the processes controlling choline and ethanolamine glycerophospholipid molecular species composition

The regulation and maintenance of the cellular lipidome through biosynthetic, remodeling, and catabolic mechanisms are critical for biological homeostasis during development, health and disease. These complex mechanisms control the architectures of lipid molecular species, which have diverse yet highly regulated fatty acid chains at both the sn1 and sn2 positions. Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) serve as the predominant biophysical scaffolds in membranes, acting as reservoirs for potent lipid signals and regulating numerous enzymatic processes. Here we report the first rigorous computational dissection of the mechanisms influencing PC and PE molecular architectures from high-throughput shotgun lipidomic data. Using novel statistical approaches, we have analyzed multidimensional mass spectrometry-based shotgun lipidomic data from developmental mouse heart and mature mouse heart, lung, brain, and liver tissues. We show that in PC and PE, sn1 and sn2 positions are largely independent, though for low abundance species regulatory processes may interact with both the sn1 and sn2 chain simultaneously, leading to cooperative effects. Chains with similar biochemical properties appear to be remodeled similarly. We also see that sn2 positions are more regulated than sn1, and that PC exhibits stronger cooperative effects than PE. A key aspect of our work is a novel statistically rigorous approach to determine cooperativity based on a modified Fisher's exact test using Markov Chain Monte Carlo sampling. This computational approach provides a novel tool for developing mechanistic insight into lipidomic regulation.

Trimetazidine effect on phospholipid synthesis in ventricular myocytes: consequences in alpha-adrenergic signaling

The anti-anginal drug trimetazidine (TMZ) has been shown to increase the synthesis of phospholipids in ventricular myocytes, including phosphatidyl-inositol (PI). This study focused on the consequences of increasing PI metabolism on alpha-adrenergic signaling pathway in cultured rat cardiomyocytes. In the cells treated with TMZ, the synthesis of PI from inositol was largely increased as compared with the control (+55% in 60 min). The stimulation of alpha-adrenergic receptors by phenylephrine (PE) induced a dose-dependent production of inositide phosphates (IPs) by phospholipase C (PLC) activation. However, the amount of available IPs was significantly lower in TMZ-treated cells, in a dose-dependent manner. This effect was observed in the presence and absence of the IP1-phosphatase inhibitor LiCl. The in vitro determination of PLC activity revealed that this effect could not be attributed to the direct inhibition of the enzyme by TMZ. The TMZ-induced reduction of IPs in the PE-stimulated cardiomyocytes should be attributed to the increase of inositol recycling and incorporation in membrane structures, elicited by increased phospholipid synthesis. The consequences of this reduction in IPs availability were investigated on the cardiomyocyte hypertrophy induced by alpha-adrenergic chronic stimulation. Acute stimulation with PE increased protein synthesis (+50%), but this increase was largely prevented by TMZ. In conclusion, TMZ reduces cell available IPs, by accelerating their recycling in membranes as PI. This effect results in a cytoprotection in the pathological process of hypertrophy elicited by chronic alpha-adrenergic stimulation.

Influence of trimetazidine on the synthesis of complex lipids in the heart and other target organs

Trimetazidine exerts antianginal properties at the cellular level, without haemodynamic effect in clinical and experimental conditions. This cytoprotection was attributed to a decreased utilization of fatty acids for energy production, balanced by an increased incorporation in structural lipids. This study evaluated the influence of Trimetazidine on complex lipid synthesis from [2-(3)H] glycerol, in ventricular myocytes, isolated rat hearts and in vivo in the myocardium and several other tissues. In cardiomyocytes, Trimetazidine increased the synthesis of phosphatidyl-choline (+ 80%), phosphatidyl-ethanolamine (+ 210%), phosphatidyl-inositol (+ 250%) and cardiolipid (+ 100%). The common precursor diacylglycerol was also increased (+ 40%) whereas triacylglycerol was decreased (-70%). Similar results were obtained in isolated hearts with 10 microm Trimetazidine (phosphatidyl-choline + 60%, phosphatidyl-ethanolamine + 60%, phosphatidyl-inositol + 100% and cardiolipid + 50%), the last two phospholipids containing 85% of the radioactivity. At 1 microm, Trimetazidine still stimulated the phospholipid synthesis although the difference was found significant only in phosphatidyl-inositol and cardiolipid. In vivo studies (10 mg/kg per day for 7 days and 5 mg/kg, i.p. before the experiment) revealed significant changes in the intracellular lipid biosynthesis, with increased labelling of phospholipids and reduced incorporation of glycerol in nonphosphorous lipids. Trimetazidine increased the glycerol uptake from plasma to the other tissues (liver, cochlea, retina), resulting in an altered lipid synthesis. The anti-anginal properties of Trimetazidine involve a reorganisation of the glycerol-based lipid synthesis balance in cardiomyocytes, associated with an increased uptake of plasma glycerol that may contribute to explain the pharmacological properties reported in other organs.

Lysophosphatidylethanolamine acyltransferase activity is elevated during cardiac cell differentiation

We examined if elevation in lysophosphatidylethanolamine acyltransferase activity was associated with elevation in phosphatidylethanolamine content during differentiation of P19 teratocarcinoma cells into cardiac myocytes. P19 cells were induced to undergo differentiation into cardiac myocytes by the addition of 1% dimethylsulfoxide to the medium. Immunofluorescence microscopy revealed the presence of striated myosin at 8 days post-dimethylsulfoxide addition confirming differentiation into cardiac cells. The content of phosphatidylethanolamine was increased 2.1-fold (P<0.05) in differentiated cells compared to undifferentiated cells, whereas the content of phosphatidylcholine was reduced 29% (P<0.05). There were no alterations in the pool sizes of other phospholipids, including cardiolipin. The relative abundance of fatty acids in phospholipids of P19 cells was 18:1 > 18:0 > 16:1 = 18:2 > 16:0 = 14:0 > 20:4 and differentiation did not affect the relative amounts of these fatty acids within individual phospholipids. When cells were incubated with [1,3-(3)H]glycerol, radioactivity incorporated into phosphatidylethanolamine was elevated 5.8-fold, whereas radioactivity incorporated into phosphatidylcholine was unaltered. Ethanolaminephosphotransferase, cholinephosphotransferase and membrane CTP:phosphocholine cytidylyltransferase activities were elevated in differentiated cells compared to undifferentiated cells, whereas membrane and cytosolic phospholipase A2 activities were unaltered. Lysophosphatidylethanolamine acyltransferase activities were elevated 2.4-fold (P<0.05). Lysophosphatidylcholine acyltransferase, monolysocardiolipin acyltransferase, acyl-Coenzyme A synthetase and acyl-Coenzyme A hydrolase activities were unaltered in differentiated cells compared to undifferentiated cells. We postulate that during cardiac cell differentiation, the observed elevation in lysophosphatidylethanolamine acyltransferase activity accompanies the elevation in phosphatidylethanolamine mass, possibly to maintain the fatty acyl composition of this phospholipid within the membrane.

Is the cytoprotective effect of trimetazidine associated with lipid metabolism?

Trimetazidine is an anti-ischemic compound devoid of hemodynamic effect, which was recently suspected to induce cardioprotection at the cellular level by a mechanism involving lipid metabolism. The effect on trimetazidine was evaluated in vivo by determination of rat cardiac fatty acid composition, and in vitro by investigation of the phospholipid metabolism in cultured rat cardiomyocytes. In rats, a 4-week trimetazidine treatment induced a significant decrease in the phospholipid content in linoleic acid, balanced by a small increase in oleic and stearic acids. These changes were not correlated with similar alterations in plasma fatty acid composition. In isolated cells, the time-dependent incorporation of labeled precursors of membrane phospholipid ([3H]inositol, [14C]ethanolamine, [14C]choline, [3H]glycerol, [14C]arachidonic acid, and [14C]linoleic acid 10 micromol/L) was compared in trimetazidine-treated cells and control cells. In trimetazidine-treated cells, arachidonic acid incorporation was increased in the phospholipid, but not in other lipid fractions. This enhanced fatty acid utilization elicited a net increase in the total arachidonic acid uptake. The incorporation of [14C] inositol in phosphatidylinositol was strongly stimulated by trimetazidine, although the uptake of inositol was not altered. The difference was significant within 30 minutes, and reached +70%(in trimetazidine-treated cells) after 150 minutes. A similar result was obtained with ethanolamine as phosphatidylethanolamine precursor, where turnover increased by 50% in trimetazidine-treated cells. Conversely, the incorporation of choline in phosphatidylcholine was not significantly affected by the presence of trimetazidine. In conclusion, trimetazidine appears to interfere with the metabolism of phospholipids in cardiac myocytes in a manner that could indicate an increased phosphatidylinositol turnover and a redirection of cytidine triphosphate (CTP) utilization toward phosphatidylethanolamine instead of phosphatidylcholine turnover. This overall phospholipid turnover increase may contribute to a reorganization of the fatty acid utilization balance in the heart, which could lead to a lowered availability of fatty acids for energy production.

The effects of lidocaine and hypoxia on phospholipid biosynthesis in the isolated hamster heart

In this study, the effects of lidocaine and hypoxia on the biosynthesis of phospholipids in the hamster heart were examined. Hamster hearts were perfused with [1,3-3H]glycerol under normal and hypoxic conditions, and in the absence or presence of 0.5 mg/mL lidocaine. After perfusion, the radioactivity incorporated into the various phospholipid fractions was determined. With the exception of phosphatidylcholine, the synthesis of phospholipids was generally stimulated by lidocaine perfusion. In contrast, hypoxia caused a general decrease in phospholipid biosynthesis which was partially restored by lidocaine. ATP and CTP levels were severely reduced under hypoxic conditions, but their levels were not restored by lidocaine treatment. The activities of enzymes for phospholipid synthesis were determined under the various perfusion conditions. The activity of phosphatidic acid phosphatase was elevated by lidocaine and decreased by hypoxic treatment. The activity of CTP:phosphatidic acid cytidylyltransferase was increased under hypoxia, with or without lidocaine. Despite the reduction in phosphatidylcholine biosynthesis, no change in the activity of cytidine diphosphocholine (CDPcholine):diacylglycerol cholinephosphotransferase was detected following lidocaine or hypoxic perfusion. However, enzyme activity was inhibited by the presence of lidocaine in the assay mixture. Our results indicate that the reduction in phospholipid biosynthesis under hypoxic conditions was caused mainly by diminishing high-energy nucleotide levels. The enhancement of phospholipid biosynthesis by lidocaine appeared to be mediated in part by modulation of enzyme activities.

Cholestane-3 beta, 5 alpha, 6 beta-triol stimulates phospholipid synthesis and CTP-phosphocholine cytidyltransferase in cultured LLC-PK cells

The present study was conducted to examine the effect, if any, of triol on the rate of total or individual phospholipid synthesis by LLC-PK cells in culture. LLC-PK cells were incubated in medium with or without 10 micrograms/ml of 5 alpha-cholestane-3 beta, 5 alpha,6 beta-triol (triol) for 24 h. Triol-treated and control cells were then incubated with medium containing either [14C]glycerol or [32P]phosphate for 1, 6 or 12 hr. In triol-treated cells, the amount of labeled glycerol and [32P]phosphate incorporated into glycerophospholipids and phospholipids (PL), respectively, were higher in triol-treated cells than in control cells, indicating a higher rate of PL synthesis in triol-treated cells. The results also showed that the increase in PL synthesis was higher in magnitude for some PL than others, thus disturbing the ratios among the PL fractions in the cell membrane. CTP-phosphocholine cytidyltransferase activity was greatly enhanced in the cytosolic as well as the particulate fractions of the triol-treated cells, which explains the increase of PC synthesis under triol effect. The rate of [3H]acetate incorporation into the total and free fatty acid fractions was significantly increased in triol-treated cells. The activation of the cytidyl transferase enzyme was related to the enhanced de novo synthesis and cellular uptake of fatty acids in triol-treated cells, which make fatty acids more available in these cells and can upregulate the enzyme. The increased synthesis of phospholipids in the triol cells and the increased level of phospholipid in these cells (as micrograms lipid phosphorus/mg cell protein) observed in our previous study indicate changes in the phospholipid head group composition of the triol cells. These changes can affect several membrane properties and membrane bound enzymes.

Metabolism and actions of CDP-choline as an endogenous compound and administered exogenously as citicoline

CDP-choline, supplied exogenously as citicoline, has beneficial physiological actions on cellular function that have been extensively studied and characterized in numerous model systems. As the product of the rate-limiting step in the synthesis of phosphatidylcholine from choline, CDP-choline and its hydrolysis products (cytidine and choline) play important roles in generation of phospholipids involved in membrane formation and repair. They also contribute to such critical metabolic functions as formation of nucleic acids, proteins, and acetylcholine. Orally-administered citicoline is hydrolyzed in the intestine, absorbed rapidly as choline and cytidine, resynthesized in liver and other tissues, and subsequently mobilized in CDP-choline synthetic pathways. Citicoline is efficiently utilized in brain cells for membrane lipid synthesis where it not only increases phospholipid synthesis but also inhibits phospholipid degradation. Exogenously administered citicoline prevents, reduces, or reverses effects of ischemia and/or hypoxia in most animal and cellular models studied, and acts in head trauma models to decrease and limit nerve cell membrane damage, restore intracellular regulatory enzyme sensitivity and function, and limit edema. Thus, considerable accumulated evidence supports use of citicoline to enhance membrane maintenance, membrane repair, and neuronal function in conditions such as ischemic and traumatic injuries. Beneficial effects of exogenous citicoline also have been postulated and/or reported in experimental models for dyskinesia, Parkinson's disease, cardiovascular disease, aging, Alzheimer's disease, learning and memory, and cholinergic stimulation.

The effect of lidocaine on de novo phospholipid biosynthesis in the isolated hamster heart

Lidocaine is used clinically as an antiarrhythmic agent, but its effect on cardiac phospholipid metabolism has not been defined. In this study, hamster hearts were perfused with [1,3-3H]glycerol in the presence of 0.5 mg/mL lidocaine. The incorporation of radioactivities into lysophosphatidic acid, phosphatidic acid, phosphatidylethanolamine, cytidine diphosphate diacylglycerol, phosphatidylinositol, phosphatidylserine, diacylglycerol and triacylglycerol were enhanced by lidocaine treatment, whereas the labelling of phosphatidylcholine was reduced. Analyses of enzyme activities in the heart after perfusion with lidocaine revealed that the activities of phosphatidate phosphatase and acyl-coenzyme A (CoA):1,2-diacylglycerol acyltransferase were enhanced. The presence of lidocaine in the assay did not directly stimulate these enzymes. However, the activity of acyl-CoA:glycerol-3- phosphate acyltransferase was stimulated by lidocaine whereas the activity of cytidine diphosphocholine:1,2-diacylglycerol cholinephosphotransferase was inhibited by lidocaine. We conclude that lidocaine affects the regulation of phospholipid biosynthesis in the heart by both direct and indirect modulation of phospholipid biosynthetic enzymes.

Biphasic modulation of choline uptake and phosphatidylcholine biosynthesis by vasopressin in rat cardiac myocytes

The effect of vasopressin on choline uptake and phosphatidylcholine biosynthesis in isolated rat heart myocytes was investigated. Myocytes were incubated with labelled choline in the presence of 0.05-1.0 microM vasopressin. Uptake of choline was enhanced (25%) by a low concentration (0.2 microM) of vasopressin, but was attenuated (19%) by a higher vasopressin concentration (1.0 microM). The biosynthesis of phosphatidylcholine was also affected by vasopressin in a biphasic manner. At low concentrations of vasopressin, a general increase in cytosine triphosphate:phosphocholine cytidylyltransferase activity was observed that caused an enhanced conversion of phosphocholine to phosphatidylcholine via the cytidine diphosphocholine pathway. At high vasopressin concentrations, a decrease in the activity of cytidylyltransferase was detected, which was caused by the translocation of the enzyme from the microsomal fraction to the cytosolic fraction. The decrease in enzyme activity coincides with a reduction in the conversion of labelled phosphocholine to phosphatidylcholine. In view of the fact that phospholipid biosynthesis in rat hepatocytes is inhibited by vasopressin at all concentrations, the biphasic modulation of phosphatidylcholine biosynthesis in rat heart myocytes illustrates the diverse effects of this hormone in different mammalian tissues.

Effects of selenium supplement on the de novo biosynthesis of glycerolipids in the isolated rat heart

The effect of selenium supplement on glycerolipid biosynthesis in the isolated rat heart was investigated. Selenium was administered to the rat by intraperitoneal injection of 4.33 mumol/kg per day for 3 consecutive days. Animals administered with an equal volume of saline were used as controls. Hearts from both animal groups were perfused in Krebs-Henseleit buffer containing labelled glycerol. Subsequent to perfusion, the radioactivity associated with each glycerolipid group was determined. Selenium supplement caused elevations in the labelling of phosphatidic acid and phosphatidylcholine but not in other phospholipids, diacylglycerol or triacylglycerol. The mechanisms for the enhancement of labelling into phosphatidic acid and phosphatidylcholine were examined. The activity of the enzymes responsible for the synthesis of phosphatidic acid in the rat heart was not changed by selenium supplement. However, a 51% increase in the acyl-CoA level was detected which might account for the elevated labelling of phosphatidic acid in the selenium supplemented animal. The 2-fold increase in the activity of CDPcholine:diacylglycerol cholinephosphotransferase might also account for the increase in the labelling of phosphatidylcholine in the heart of the selenium-supplemented rat. It is clear from this study that selenium plays a regulatory role in the control of cellular lipid metabolism.

Effect of exogenous CDP-choline on choline metabolism in isolated adult rat ventricular myocytes under normoxic and hypoxic conditions

The purpose of this study was to examine the effect of exogenous CDP-choline on choline metabolism and phosphatidylcholine biosynthesis in adult rat ventricular myocytes. Choline uptake and metabolism were examined, using [methyl3H] choline. CDP-choline in the medium produced a concentration dependent reduction in the amount of radio-label in phosphocholine and phospholipid but it did not alter choline uptake into the myocytes. CDP-choline also did not antagonize the effect of hypoxia on phosphatidylcholine synthesis; rather it accentuated the hypoxia-induced reductions in cellular phosphocholine and phosphatidylcholine biosynthesis. These results indicate that the exogenous administration of CDP-choline alters choline metabolism in the heart by reducing the formation of phosphocholine and phosphatidylcholine without altering choline uptake and suggest an effect of a CDP-choline metabolite on choline metabolism which is not effective in opposing the effect of hypoxia on phosphatidylcholine biosynthesis.

A genetic abnormality of cardiac myocytes from the blind mutant (RC) chick heart: abnormalities of cardiac structure and choline transport

A new genetic cardiomyopathy was identified in a blind mutant avian strain. Cardiac myocytes were cultured from 7-day-old chick embryos from Rhode Island Red chickens and from another strain of this species that has been identified to have several abnormalities, the most striking of which is blindness. Cardiac myocytes were maintained in tissue culture. Morphologically, in culture, the cardiac myocyte from the blind mutant strain assumed a spherical shape and showed abnormalities of sarcolemmal membrane compared to control myocytes from heterozygous animals. Choline uptake and metabolism were examined, using [methyl 3H] choline, because it is a sarcolemmal transport process and choline is metabolised to phosphatidylcholine, an important phospholipid for cellular structure and function. Choline uptake through the active transport process was markedly and significantly reduced in the mutant cells compared to control cells, while choline metabolism to phosphatidylcholine was not significantly altered. These results demonstrate a new abnormality of cardiac myocytes, a cardiomyopathy that can be studied in cell culture and one with abnormalities of cellular choline transport.

Phosphatidylcholine metabolism in ischemic and hypoxic hearts

The rates of phosphatidylcholine biosynthesis in the isolated hamster hearts under ischemic and hypoxic conditions were examined. Global ischemia was produced by perfusion of the heart with a reduced flow, whereas hypoxia was produced by perfusion with a N2-saturated buffer. A 51% reduction in the biosynthesis of phosphatidylcholine was observed in the ischemic heart. The reduction was caused by a severe decrease in ATP level which resulted in a diminished conversion of choline into phosphocholine. A 22% reduction in the biosynthetic rate of phosphatidylcholine was also detected in the hypoxic heart. The reduction was caused by a diminished level of CTP which resulted in a decreased conversion of phosphocholine to CDP-choline. No compensatory mechanism was triggered during ischemia, but the CTP: phosphocholine cytidylyltransferase activity was enhanced in the hypoxic heart. Our results demonstrate the possible rate-limiting role of choline kinase and reconfirm the regulatory role of the cytidylyltransferase in the biosynthesis of phosphatidylcholine.

The effect of amino acids on choline uptake and phosphatidylcholine biosynthesis in mammalian hearts

The uptake of choline in mammalian hearts in the presence of amino acids was examined. Isolated hamster, guinea pig, rat and rabbit hearts were perfused with labeled choline in the presence and absence of amino acids. Neutral amino acids enhanced choline uptake in the hamster heart, but not in the guinea pig, rat and rabbit hearts. Phosphatidylcholine biosynthesis in these hearts was not affected by the presence of amino acids. Choline uptake in the hamster myocytes was also enhanced by neutral amino acid. The enhancement of choline uptake suggests a direct interaction between the amino acid and the transport of choline into the myocardiac cells. The different responses in choline uptake to neutral amino acids indicate that the regulation of choline uptake in the hearts may be different between mammalian species.

Phospholipid base exchange enzyme activity in sarcolemmal membranes from the heart of cardiomyopathic hamsters

The activity of phospholipid base exchange enzymes has been evaluated in cardiac sarcolemmal membranes from Syrian Golden hamsters and from a hamster strain (UM-X7.1) characterized by a genetic form of hypertrophic cardiomyopathy. No choline base exchange activity and only a little serine base exchange activity were detected, whereas the ethanolamine base exchange enzyme was found highly active in membranes from both strains. For this reason, the present study is focussed on the ethanolamine base exchange enzyme. The apparent Km for ethanolamine of ethanolamine base exchange enzyme from Syrian Golden membranes and from UM-X7.1 strain membranes are 18 and 32 microM, respectively. The specific activity of the sarcolemmal ethanolamine base exchange enzyme is lower in the UM-X7.1 strain than in Syrian Golden hamsters. The calcium-dependence of the enzyme appears different when the membranes from the two strains are compared. Indeed, after removal of the membrane-bound divalent cations, comparable activities are found in both membrane preparations, whereas, upon addition of Ca2+ to the incubation mixtures, the activity of the enzyme is enhanced in the membranes from Syrian Golden strain more than in those from UM-X7.1 strain. The cholesterol content of sarcolemmal membranes is higher in the cardiomyopathic strain than in the Syrian Golden hamsters. A possible relation between changes of the membrane lipid composition and of the ethanolamine base exchange activity is discussed.

Solubilization and modulation of acyl-CoA:1-acyl-glycerophosphocholine acyltransferase activity in rat liver microsomes

The acylation of 1-acyl-glycerophosphocholine is an important mechanism for the maintenance of the asymmetrical distribution of acyl groups in phosphatidylcholine. The majority of acyl-CoA:1-acyl-glycerophosphocholine acyltransferase is located in the microsomal fraction. In this study, the rat liver microsomes were incubated with various detergents, and the solubilized enzyme was separated from the remainder by centrifugation. Sodium cholate, sodium deoxycholate and octylglucopyranoside caused the solubilization of 14-25% of the enzyme activity. The acyl specificity of the solubilized enzyme was similar to the insoluble enzyme, indicating that there was no selective solubilization of any acyl specific acyltransferase. The solubilized enzyme did not display any lipid requirement, and its activity was inhibited by phosphatidylcholine, phosphatidylethanolamine and 1,2-diacylglycerol. Kinetic studies with varying concentrations of acyl-CoAs revealed that the inhibition by 1,2-diacylglycerol was essentially uncompetitive. The modulation of acyltransferase activity by 1,2-diacylglycerol may be an important mechanism for controlling the acylation of lysophosphatidylcholine.

Phospholipase D activity in subcellular membranes of rat ventricular myocardium

The phospholipase D (PL D), which catalyzes the formation of phosphatidic acid (PA), was studied in rat myocardium using 14C-labelled phosphatidylcholine (PC) as an exogenous substrate. Subcellular distribution experiments indicated the presence of PL D in particulate fractions only. Different procedures for the isolation of purified cardiac subcellular organelles showed the presence of PL D in sarcolemma (SL), sarcoplasmic reticulum (SR) and mitochondria with 14-, 11- and 5-fold enrichment when compared to the homogenate value, respectively. The activity of SL PL D was observed over a narrow acid pH range with an optimum at 6.5, and it showed a high specificity for PC while phosphatidylethanolamine and phosphatidylinositol showed a low rate of hydrolysis. Under optimal conditions, PA formation was linear for a 90-min period of incubation and the reaction rate was constant for 10 to 100 micrograms SL protein in the assay medium. The SR PL D displayed properties similar to those seen with the SL PL D. In membrane fractions PL D was also found to catalyze a transphosphatidylation reaction for the synthesis of phosphatidylglycerol. Assessment of the intramembranal levels of radioactive 1,2-diacylglycerol (DAG) in the absence or presence of KF suggested the presence of an active PA phosphohydrolase activity. This study indicates that a PC-specific PL D activity is localized in different membrane systems of the myocardium and may be associated with PA phosphohydrolase to act in a coordinated manner. The functional significance of PL D-dependent formation of PA in cardiac membranes is discussed.

Lysophosphatidylcholine as an intermediate in phosphatidylcholine metabolism and glycerophosphocholine synthesis in cultured cells: an evaluation of the roles of 1-acyl- and 2-acyl-lysophosphatidylcholine

Previous studies in our laboratory have shown that the principal pathway of phosphatidylcholine (PtdCho) degradation in cultured mouse N1E-115 neuroblastoma, C6 rat glioma, primary rat brain glia and human fibroblasts is PtdCho----lysophosphatidylcholine (lysoPtdCho)----glycerophosphocholine (GroPCho)----glycerophosphate plus choline (Morash, S.C. et al. (1988) Biochim. Biophys. Acta 961, 194-202). GroPCho is the first quantitatively major degradation product in this pathway, and could be formed by phospholipases A1 or A2, followed by lysophospholipase, or by a co-ordinated attack releasing both fatty acids by phospholipase B. The quality and quantities of lysoPtdCho present in cells reflect the nature of the initial hydrolysis step (A1 or A2), specificities of the lysophospholipases, and activities of acyltransferases that form PtdCho from lysoPtdCho. The present study was undertaken to elucidate the relative importance of these pathways by examining the fate of exogenous 1-acyl and 2-acyl-lysoPtdCho incubated with N1E-115 and C6 cells in culture. By fatty acid composition, endogenous lysoPtdCho was found to be mainly 1-acyl in both cell types based on a predominance of saturated acyl species; this suggested either preferential further deacylation or reacylation of 2-acyl-lysoPtdCho, or that 2-acyl-lysoPtdCho was not formed. Exogenous 1- and 2-acyl-lysoPtdCho specifically radiolabelled with choline and/or fatty acid were incubated either singly or as equimolar mixtures with cells. Cell association was rapid and not reversible by washing and both species were taken up at similar rates. The 2-acyl species was acylated to PtdCho faster than the 1-acyl species in both cell lines. Acylation of both lyso species was higher in C6 compared to N1E-115 cells. Hydrolysis of lysoPtdCho to GroPCho was higher in N1E-115 cells and with 1-acyl-lysoPtdCho. Transacylation between two molecules of lysoPtdCho was a minor pathway. These results document the variety and relative importance of reactions of lysoPtdCho metabolism; under similar conditions, 1- and 2-acyl-lysoPtdCho are handled differently. Both species turn over actively, but only the 1-acyl species accumulates while 2-acyl-lysoPtdCho is likely to be reacylated to form PtdCho.

Effects of chlorpromazine and trifluoperazine on choline metabolism and phosphatidylcholine biosynthesis in cultured chick heart cells under normoxic and anoxic conditions

The effects of chlorpromazine and trifluoperazine on phosphatidylcholine biosynthesis in the heart were investigated in isolated cardiac cells under normoxic and anoxic conditions. The cells were obtained from 7-day-old chick embryos and were maintained in culture. After 96 hr, cells were maintained either in an incubator with oxygen at room air concentration (normoxia) or in an incubator containing 95% nitrogen and 5% CO2 (anoxia). Pulse chase experiments with [methyl-3H]choline were conducted using a 2-hr incubation with choline. Chlorpromazine and trifluoperazine at 10(-5) M produced a significant (P less than 0.05) increase in the incorporation of choline into both phosphocholine and phospholipid. High concentrations of chlorpromazine or trifluoperazine i.e. 10(-4) M, damaged myocardial cells as reflected in a significant (P less than 0.05) reduction in cellular protein and a further reduction in labelled choline in phosphocholine or phospholipid after adjusting for the lower protein concentrations. Anoxia altered choline metabolism but 6 hr of anoxia was the minimum time needed for the effect to be observable. Anoxia, for 24 hr, produced a significant (P less than 0.05) reduction in labelled choline in phosphocholine without a significant change in incorporation of label in phospholipid or cellular protein. Both chlorpromazine and trifluoperazine at 10(-5) M prevented anoxic-induced changes in phosphocholine metabolism. Thus, chlorpromazine and trifluoperazine affect phospholipid biosynthesis in cardiac cells and prevent anoxia-induced changes in phosphatidylcholine biosynthesis.

Ethanolamine and choline transport in cultured bovine aortic endothelial cells

The transport of the polar head groups, ethanolamine and choline, was examined in cultured bovine aortic endothelial cells. Both ethanolamine and choline are taken up by high- and low-affinity systems. The K'm and V'max for the Na+-dependent, high-affinity ethanolamine and choline transport system are 3.0 and 3.0 microM and 5.4 and 7.3 pmol/mg protein/min, respectively. Ethanolamine and choline competitively influence one another's transport as the presence of 50 microM ethanolamine increases the K'm but not the V'max of choline uptake. Likewise, 50 microM choline increases the K'm but not the V'max of ethanolamine transport. The concentration of ethanolamine that inhibits maximal velocity of 5 microM choline by 50% is 9.7 microM, while 12 microM choline inhibits 5 microM ethanolamine maximal velocity by 50%. Uptake of both head groups is only partially Na+-dependent and is inhibited similarly by 2-methylethanolamine and 2,2-dimethylethanolamine at all concentrations examined. Hemicholinium-3, a classic inhibitor of high-affinity, Na+-dependent choline transport, reduces both ethanolamine and choline accumulation in a concentration-dependent fashion, but has a greater effect on choline transport at higher concentrations. The major portion of these data is consistent with our hypothesis that the uptake of physiological concentrations of ethanolamine and choline may occur through the same transport system. However, the results of the effect of hemicholinium-3 and the extent of Na+-dependency of choline and ethanolamine uptake could be interpreted as meaning that separate transport systems for choline and ethanolamine exist which cross react or that a single transport system exists which has separate active sites for the two compounds.

The effect of calcium ions and the calcium ionophore A23187 on choline uptake and phosphatidylcholine biosynthesis in chick embryo hearts

The effect of increases in extracellular calcium [Ca]o and the calcium ionophore A23187 on choline uptake and phosphatidylcholine biosynthesis was assessed in isolated cardiac myocytes. The cells were obtained from 7-day old chick embryos and were maintained in culture. Choline uptake was examined using [methyl 3H] choline. A23187 was found to increase choline uptake through the saturable choline uptake process. Pulse chase experiments using [methyl 3H] choline showed that after a 2 h incubation with choline, about 85% of the label was recovered in phosphocholine with most of the rest in phospholipid and a small amount in CDP-choline and glycerol phosphocholine. Increases in [Ca]o up to 10 mM did not affect the amount of label in phosphocholine or phospholipid, the rate of disappearance of label from phosphocholine, or the rate of appearance of labelled choline in phospholipid. In contrast, A23187, at concentrations up to 10(-4) M, was associated with a significant (p less than 0.05) increase in choline in the phosphocholine and phospholipid pool compared to control cells. The time course of the disappearance of choline from the phosphocholine pool and appearance in phospholipid pool was not significantly different between control cells and those treated with A23187. A23187 increased choline uptake via the specific uptake process. The effect on choline uptake may be attributed to the action of A23187 to facilitate the release of calcium from specific intracellular calcium storage sites rather than a nonspecific increase in [Ca]i that may have resulted from the increase in [Ca]o.

Phosphatidylcholine metabolism in cultured cells: catabolism via glycerophosphocholine

The catabolism of phosphatidylcholine (PtdCho) has been studied in cultured murine neuroblastoma (N1E-115), C6 glioma, rat brain primary glia, and human fibroblast cells. Cells were pulse labelled for 96 h with [methyl-3H]choline followed by a chase for up to 24 h in medium containing 4 mM choline. Measurement of the radioactivity and mass of choline-containing compounds in these cells indicated that the major degradative pathway is PtdCho----lysophosphatidylcholine (lysoPtdCho)----glycerophosphocholine (GroPCho)----choline. At all times during the chase, PtdCho, sphingomyelin and lysoPtdCho comprised 72-92% of the cell-associated radioactivity; the remaining 10-30% was water-soluble and was chiefly GroPCho (30-80%) in all cell lines. In fibroblasts, however, phosphocholine (PCho) was also a major labelled water-soluble component (33-54%). The specific activity of GroPCho closely parallelled that of PtdCho in fibroblasts, but decreased faster than PtdCho in C6 and N1E-115 cells. We postulate that this may be due to distinct pools of PtdCho in the cell with differing rates of turnover. The changes in specific activity of PCho suggest that the major portion is formed by synthesis rather than as a degradative product. However, the inability to reduce the specific activity of this fraction to that of the intracellular choline suggests that a portion may be derived from either PtdCho or GroPCho.

Effect of diethyl ether on phosphatidylcholine biosynthesis in hamster organs

The effect of diethyl ether anesthesia on phosphatidylcholine biosynthesis in hamster organs was investigated. Ether administration did not affect the incorporation of radioactive choline into phosphatidylcholine in the liver, heart, lung, brain and spleen. A significant (29%) decrease in the labeling of phosphatidylcholine was detected in the kidney of ether-treated hamsters. Reduction in phosphatidylcholine labeling was not due to a diminished radioactive choline uptake but a decrease in the conversion of phosphocholine to CDP-choline. The accumulation of labeled phosphocholine was caused by the translocation of CTP:phosphocholine cytidylyltransferase from microsomal (more-active) form to cytosolic (less-active) form. Ether administration appears to modulate the cytidylyltransferase in hamster kidney differently than that in other hamster organs.

On the plasmalogenation of myocardial choline glycerophospholipid during maturation of various vertebrates

1. The plasmalogen profiles of a series of hearts from fish to mammals were obtained by various TLC analyses. 2. All specimens (ventricular) contained ethanolamine plasmalogen and some choline plasmalogen, as well. 3. The distribution of these two plasmalogen species was relatable, in part, to (a) phylogeny and (b) ontogeny. 4. There were exceptions. 5. The appearance of choline plasmalogen was preceded by its alkylacyl precursor, suggesting plasmalogenation by a base-specific delta 1-alkyl desaturase. 6. From the data, we have raised some questions as to the metabolic role played by the plasmalogens and precursors as occupants of myocardial mitchondrial membranes.

Phosphocholine phosphatase and alkaline phosphatase are different enzymes in hamster heart

The CDP-choline pathway is the major pathway for the synthesis of phosphatidylcholine in the hamster heart. The formation of phosphocholine from choline was regarded as the first committed reaction in this pathway. We demonstrated earlier that the phosphocholine pool in the heart was substantially less than that found in other tissues, and we observed that a substantial amount of the phosphocholine was hydrolyzed back to choline by a phosphatase. This phosphatase was located in the microsomal fraction of the heart, and unlike alkaline phosphatase, it was not inhibited by amino acids. The pH optima and heat sensitivity of phosphocholine phosphatase were also found to differ from alkaline phosphatase. Phosphocholine did not inhibit the hydrolysis of p-nitrophenylphosphate, but a "mixed type" inhibition of the hydrolysis of phosphocholine was observed in the presence of p-nitrophenylphosphate. Our data support the hypothesis that these two activities originate from separate and distinct enzymes, and we postulate that the cardiac phosphocholine phosphatase may play a role in the regulation of the phosphocholine pool size in the hamster heart.

Phospholipid methylation in canine myocardium: kinetic characteristics and the effect of endotoxin administration

The kinetic characteristics and the effect of endotoxin administration on the enzymatic methylation of phospholipids in dog heart microsomes were studied using S-adenosyl-L-[methyl-3H]methionine as a methyl donor. Kinetic studies in control dogs reveal that the stepwise methylation of phosphatidylethanolamine to phosphatidylcholine was catalyzed by three different enzymes. Methyltransferase I catalyzed the methylation of phosphatidylethanolamine to phosphatidyl-N-monomethylethanolamine, had a very low Km (approximately 1.5 microM) for S-adenosylmethionine, and a pH optimum of 6.5, and it was stimulated by Mg2+ and Ca2+. Methyltransferase II catalyzed the methylation of phosphatidyl-N-monomethylethanolamine to phosphatidyl-N,N-dimethylethanolamine, had a low Km (8-12 microM) for S-adenosylmethionine, and a pH optimum of 8.5, and it was stimulated by low concentrations (less than 1 mM) of Ca2+ but was unaffected by Mg2+. Methyltransferase III catalyzed the formation of phosphatidylcholine from phosphatidyl-N,N-dimethylethanolamine, had a high Km (approximately 33 microM) for S-adenosylmethionine, and a pH optimum of 9.5, and it was unaffected by Mg2+ or Ca2+. Experiments with trypsin digestion indicate that methyltransferases I and III were partially embedded while methyltransferase II was completely exposed to the surface of the membrane. Endotoxin administration (2 and 4 hr) decreased the Km and Vmax by 30 to 36% and 24 to 37.7%, respectively, for S-adenosylmethionine. Since the enzymatic methylation of phospholipids has been implicated to play an important role in the regulation of membrane structure and function, the endotoxin-induced decreases in the Km and Vmax of phospholipid-methylating enzymes in dog heart microsomes may contribute to the development of myocardial dysfunction in endotoxin shock.

Phosphatidylethanolamine methyltransferase: evidence for influence of diet fat on selectivity of substrate for methylation in rat brain synaptic plasma membranes

Male weanling rats were fed diets containing 20% (w/w) fat differing in fatty acid composition for 24 days. Synaptic plasma membranes were isolated from the brain and the fatty acid composition of phosphatidylethanolamine and phosphatidylcholine was determined. In vitro assays of phosphatidylethanolamine methyl-transferase activity were performed on fresh membrane samples to assess effect of dietary fat on the rate of phosphatidylethanolamine methylation for phosphatidylcholine synthesis via the phosphatidylethanolamine methyltransferase pathway. Dietary level of n-6 and ratio of n-6 to n-3 fatty acids influenced membrane phospholipid fatty acid composition and activity of the lipid-dependent phosphatidylethanolamine methyltransferase pathway. Rats fed a diet rich in n-6 fatty acids produced a high ratio of n-6/n-3 fatty acids in synaptosomal membrane phosphatidylethanolamine, and elevated rates of methylation of phosphatidylethanolamine to phosphatidylcholine by phosphatidylethanolamine methyltransferases, suggesting that the pathway exhibits substrate selectivity for individual species of phosphatidylethanolamine containing long-chain homologues of dietary n-6 and n-3 fatty acids (20:4(n-6), 22:4(n-6), 22:5(n-6) and 22:6(n-3). It may be concluded that diet alters the membrane content of n-6, n-3 and monounsaturated fatty acids, and that change in phosphatidylethanolamine species available for methylation to phosphatidylcholine alters the rate of product synthesis in vivo by the phosphatidylethanolamine methyltransferase pathway.

Membrane phospholipid metabolism during myocardial ischaemia: past, present and future

Alterations in myocardial membrane phospholipids may play an important role in the pathogenesis of ischaemic myocardial cell injury. Studies in canine myocardium, perfused rat heart, and cultured myocardial cells have demonstrated that the accumulation of free arachidonic acid correlates with the development of irreversible cell injury. Accumulation of other phospholipid hydrolysis products, including amphiphilic compounds such as lysophosphatidylcholine, has also been reported. The biochemical mechanisms which are responsible for phospholipid hydrolysis and arachidonic acid accumulation during ischaemia are unknown. This manuscript provides a synopsis of previous work in this field and suggests new directions for the field of myocardial phospholipid metabolism.

Enhancement of choline uptake by glycine in hamster heart

Choline uptake by the isolated hamster heart has been shown to be inhibited by exogenous ethanolamine. In this study, the effect of glycine on choline uptake was investigated. At 0.01-1.0 mM glycine in the perfusate, an enhancement of choline uptake (30%) by the isolated heart was observed. Despite the higher choline uptake, the presence of glycine did not affect the rate of phosphatidylcholine biosynthesis. At higher glycine concentration (50 mM), the enhancement of choline uptake was abolished. Exogenous choline had no effect on the uptake of glycine. We postulate that choline and glycine are transported by separate mechanisms, and that glycine may play a regulatory role in the control of choline uptake by the hamster heart.

Impaired biosynthesis of highly unsaturated phosphatidylcholines: a hypothesis on the molecular etiology of some muscular dystrophies

A brief review of the literature concerning the synthesis of phosphatidylcholine and phosphatidylethanolamine in muscle suggests that the cytidine pathways are replaced by the recently proposed acyl-specific de novo and salvage glycerolphosphodiester pathways (Infante, 1984) in fully differentiated muscle. An analysis of published data suggests an impaired synthesis of 4,7,10,13,16,19-docosahexaenoic phosphatidylcholine, at the level of de novo sn-3-glycerolphosphorylcholine synthesis, as the primary defect in Duchenne and (dy) murine muscular dystrophies. This phosphatidylcholine species is postulated to be required for optimum sarcoplasmic Ca2+ transport activity. It is proposed that this impairment initiates the secondary series of events which lead to the observed pathology of these diseases. Based on some predictions of the hypothesis, potential diagnosis and treatments are suggested.

The base-exchange enzyme activities of sarcolemma and sarcoplasmic reticulum from rat heart

The Ca2+ dependent incorporation of [14C]ethanolamine, L-[14C]serine and [14C]choline into phosphatidylethanolamine, phosphatidylserine and phosphatidylcholine, respectively, were investigated in membrane preparations from rat heart. The ethanolamine and serine base-exchange enzyme-catalyzed reactions were associated with the sarcolemma and sarcoplasmic reticulum. There was a 17.2-fold and 6.8-fold enrichment, respectively, of the serine and the ethanolamine base-exchange enzyme activities in the sarcolemma compared to the starting whole homogenate. The sarcoplasmic reticulum was enriched in the ethanolamine and serine base-exchange enzyme activities. The choline base-exchange enzyme activity of all membranes fractions was negligible compared to the ethanolamine or serine base-exchange enzyme activities. The apparent Km for the ethanolamine and serine base-exchange enzyme in sarcolemma was 14 microM and 25 microM, respectively. The pH optimum for these base-exchange activities was 7.5-8.0. There was a dependence upon Ca2+ for these reactions with a 1 or 4 mM concentration required for maximal activity. The properties of the sarcoplasmic reticulum base-exchange enzymes were similar to the sarcolemmal base-exchange enzymes.

Increased arachidonate incorporation in perfused heart phospholipids from vitamin E-deficient rats

When perfused with exogenous arachidonic acid (AA), the rat heart incorporates this eicosanoid precursor into the phospholipids. The incorporation of AA into phosphatidylcholine was 6-fold higher than the incorporation of AA into phosphatidylethanolamine. When vitamin E-deficient rat hearts were perfused with AA, there was a marked increase in incorporation of AA into both phosphatidylcholine and phosphatidylethanolamine compared to rats fed with a vitamin E supplemented diet. The result of this study suggests that vitamin E has a regulatory role in phospholipid biosynthesis in the mammalian heart.

Catabolism of exogenous and endogenous sphingomyelin and phosphatidylcholine by homogenates and subcellular fractions of cultured neuroblastoma cells. Effects of anesthetics

Cultured murine neuroblastoma cells contain a neutral, Mg2+-stimulated sphingomyelinase and an alkaline phosphatidylcholine-hydrolyzing activity that are enriched in the plasma membrane fraction. The reaction products of sphingomyelin catabolism are phosphocholine and ceramide and those of phosphatidylcholine, glycerophosphocholine and fatty acid. These reactions were studied with endogenous as well as exogenous liposomal substrates. With both exogenous and endogenous substrates, the sphingomyelinase activity was stimulated two- to threefold by Mg2+ and a further three- to fourfold by volatile anesthetic agents. Stimulation was concentration-dependent and corresponded to anesthetic potency: methoxyflurane greater than halothane greater than enflurane. Greater than 80% of the plasma membrane sphingomyelin was hydrolyzed within 2 h in the presence of Mg2+ and anesthetic. In contrast, the activity with exogenous and endogenous phosphatidylcholine was unaffected by Mg2+ or Ca2+ and was markedly inhibited (50-80%) by anesthetic agents. The degree of inhibition was concentration-dependent and corresponded to anesthetic potency. The quantitative importance of choline-containing lipids in cell membranes, the relatively exclusive localization of the neutral Mg2+-stimulated sphingomyelinase in cells of neural origin, the totally different type of hydrolytic attack on phosphatidylcholine, and the reciprocal effects of anesthetics on the hydrolysis of these two lipids strongly suggest important roles for these activities in cell membranes in general and in the neuron in particular.

Regulation of phospholipid synthesis by intracellular phospholipases in fetal rabbit type II pneumocytes

Exposure of fetal type II pneumocytes to phospholipase A2 inhibitors led to significantly reduced choline uptake and decreased synthesis of total and disaturated phosphatidylcholines from both [methyl-14C]choline and [9,10(n)-3H]palmitate precursors. The percentage of the total synthesized phosphatidylcholine recovered as disaturated phosphatidylcholine was increased when compared to that in control cultures, suggesting that unsaturated phosphatidylcholine synthesis was reduced to a greater extent than that of the disaturated species. Synthesis of sphingomyelin and phosphatidylethanolamine from labeled palmitate was also reduced, whereas that of phosphatidylinositol and phosphatidylglycerol was significantly increased. Addition of phospholipase C resulted in increased synthesis of phosphatidylcholine from both labeled precursors; no significant changes were found in synthesis of most of the other 3H-labeled lipids. Added phospholipase A2 did not lead to any changes in either choline or palmitate incorporation. However, when melittin (a phospholipase A2 activator) was added to the cultures, greater incorporation of both palmitate and choline was observed, along with a significant increase in the percentage of total cellular radioactivity in 14C-labeled lipids, indicating also stimulation of phosphatidylcholine synthesis. A marked increase in CTP: phosphorylcholine cytidylyltransferase activity was found after treatment of the cultures with phospholipase C. Exposure to quinacrine also increased the activity of this enzyme. Addition of phospholipase C and melittin to prelabeled pneumocyte cultures accelerated degradation of cell phospholipids and the release of free fatty acids as the main degradation products. These findings suggest that intracellular phospholipases are regulators of synthesis of surfactant phospholipids in fetal type II pneumocytes, and that activation or inhibition of these phospholipases could represent a mechanism through which hormones and pharmacological agents modify surfactant and other phospholipid synthesis.

Membrane-bound CTP:phosphocholine cytidylyltransferase regulates the rate of phosphatidylcholine synthesis in HeLa cells treated with unsaturated fatty acids

The influence of fatty acids on phosphatidylcholine biosynthesis in HeLa cell cultures was investigated. Oleate and other unsaturated fatty acids stimulated the incorporation of phospho[Me-3H]choline into phosphatidylcholine from 5 to 20-fold, while saturated fatty acids were without effect. Stimulation of the reaction catalyzed by CTP:phosphocholine cytidylyltransferase (CTP:cholinephosphate cytidylyltransferase, EC 2.7.7.15) by 1 mM oleate was evident within 15 min and could be reversed within 40 min after removal of the oleate-supplemented cell medium. Cytidylyltransferase activity was 11-fold higher in homogenates from cells exposed to oleate. Treatment of the HeLa cells with oleate produced almost complete translocation of the cytidylyltransferase from the cytosol to the microsomal fraction. Additional support for conversion of the cytidylyltransferase to a membrane-bound form in oleate-treated cells was obtained in studies with digitonin. Exposure of control cells to digitonin for 2 min caused the release of 60% of the total cytidylyltransferase into the medium, while oleate-treated cells leaked only 5% of the enzyme in the presence of digitonin. Finally, oleate (50 microM) was shown to promote complete aggregation of cytosolic cytidylyltransferase with microsomes from HeLa cells and 22-fold stimulation of the enzyme activity. It appears that the rate of phosphatidylcholine biosynthesis is governed by the amount of cytidylyltransferase bound to endoplasmic reticulum in HeLa cells.

Acyl specificity of hamster heart CDP-choline 1,2-diacylglycerol phosphocholine transferase in phosphatidylcholine biosynthesis

The acyl specificity of 1,2-diacylglycerol: CDP-choline phosphocholine transferase (EC 2.7.8.2) for the formation of phosphatidylcholine with the appropriate acyl groups in hamster heart was investigated. Enzyme activity was determined in the microsomal fraction with 1,2-diacylglycerols of known acyl content. Maximum enzyme activity was obtained with diacylglycerol containing a monoenoic acyl group at the C-2 position of the glycerol moiety, regardless of the acyl group at the C-1 position. The specificity of the enzymes was also investigated by perfusing the isolated hamster heart with labelled glycerol. Comparison of the molecular species of the labelled diacylglycerols and phosphatidylcholine subsequent to perfusion revealed that the specificity of phosphocholine transferase was not limited to the monoenoic species of diacylglycerol. The difference in specificity observed between the in vitro assay and the perfusion study may partly be attributed to the presence of detergent in the enzyme assay mixture (to facilitate solubility of diacylglycerol). It is concluded that in the hamster heart, phosphocholine transferase has only limited ability to select the appropriate acyl groups for phosphatidylcholine biosynthesis. It appears that the majority of the newly formed phosphatidylcholine in the heart via the CDP-choline pathway is subsequently resynthesized by deacylation-reacylation process.

Ethanolamine inhibits choline uptake in the isolated hamster heart

The effect of exogenous ethanolamine on phosphatidylcholine biosynthesis in the isolated hamster heart was investigated. Hamster hearts were perfused with [Me-3H]choline in the presence of 0.05-0.5 mM ethanolamine. Incorporation of label into phosphatidylcholine was decreased 26-63% at 0.1-0.5 mM ethanolamine. Similar decreases in the labelling of the metabolites of the CDP-choline pathway were observed at these ethanolamine concentrations. The observed decrease in phosphatidylcholine labelling at 0.1-0.5 mM ethanolamine was attributed to an inhibition of labelled choline uptake by ethanolamine. The inhibitory role of ethanolamine to choline uptake was examined by comparison to hemicholinium-3. Both compounds inhibited choline uptake in a competitive manner. Intracellular choline, phosphocholine and CDP-choline concentrations were not altered under all experimental conditions. It can be concluded that exogenous ethanolamine has no immediate effect on the rate of phosphatidylcholine biosynthesis in the isolated hamster heart. The reduced labelling of phosphatidylcholine in the presence of ethanolamine is a direct result of the reduction of labelled choline taken up by the heart.

Effects of cyclic AMP analogues and phosphodiesterase inhibitors on phospholipid biosynthesis in fetal type II pneumocytes

Purified type II pneumocytes grown in monolayer cultures after isolation from fetal rabbit lung organotypic cultures were employed to investigate effects of cAMP analogues and phosphodiesterase inhibitors on [methyl-14C]choline and [9-10(n)3H]palmitate incorporation into cell lipids. After 24 h exposure to 0.5 mM N6,O2-dibutyryl-cAMP or 8-bromo-cAMP, a significant increase was found in the rate of incorporation of choline into phospholipids. Addition of 1 mM 1-methyl-3-isobutylxanthine or aminophylline also increased incorporation of choline into phospholipids but did not significantly change the incorporation of choline into sphingomyelin. These effects were not due to increased uptake of choline or changes in the pool size of the precursor. Cyclic AMP analogues also stimulated the rate of incorporation of palmitate into most lipid fractions but did not alter the relative percentages of incorporation of either precursor into any of the phospholipids. Phosphodiesterase inhibitors did not significantly change the rate of incorporation of palmitate into neutral lipids and most phospholipids, except for a decrease into sphingomyelin, phosphatidylinositol and phosphatidylethanolamine. However, they increased the percentage of incorporation of palmitate into phosphatidylcholine and decreased the percentage of incorporation into most other phospholipids. These data clearly indicate that cAMP can stimulate the synthesis of phospholipids within the type II pneumocytes. This effect is probably a general stimulation effect for the cAMP analogues but methylxanthines may selectively increase the synthesis of surfactant lipids such as phosphatidylcholine while decreasing that of other membrane-associated phospholipids.

Differential effects of anesthetics on phosphatidylethanolamine biosynthesis in hamster tissue

The anesthetic effects of diethyl ether and sodium pentobarbital on phosphatidylethanolamine biosynthesis in hamster organs were investigated. No significant difference was observed in the livers and hearts between control and anesthesized animals. However, a 30% reduction in phosphatidylethanolamine labelling was detected in the kidney of the diethyl ether-anesthesized hamsters. Such reduction appears to be caused by an inhibition of phosphoethanolamine cytidylyltransferase, a rate-limiting enzyme in the major pathway for the biosynthesis of phosphatidylethanolamine. The uptake of ethanolamine by the organs was not affected during anesthesia.