Effect of epidermal growth factor on enzymes of phospholipid biosynthesis in lung and liver of fetal rat in vivo and in vitro

Epidermal growth factor (EGF), a mitogenic polypeptide that binds to cell surface receptors, is an important regulator of cell differentiation and fetal lung surfactant synthesis, and may be used as a potential novel therapeutic agent in prematurity. Nevertheless, the distinct role in lung development and its mechanisms of action are not well understood. We investigated in vivo the systemic effect of intrafetally administered EGF (200 ng/g fetal body weight) and maternally administered dexamethasone (DEXA; 0.2 and 2.0mg/kg maternal body weight) on the activity of important enzymes of the phospholipid synthesis in the fetal rat lung and liver: choline kinase (EC 2.7.1.32), cholinephosphate cytidyltransferase (EC 2.7.7.15), choline phosphotransferase (EC 2.7.8.2), lysolecithin acyltransferase (EC 2.3.1.23) and glycerolphosphate phosphatidyltransferase (EC 2.7.8.5). Additionally, in vivo and in vitro effects of DEXA on EGF receptor synthesis, and the effects of EGF on protein content and morphogenesis of the fetal rat lung organoid culture, were evaluated. Whereas DEXA induced the activity of all investigated enzymes of phospholipid synthesis and increased EGF receptor synthesis, EGF has no effects on the enzymes, either in vivo or in vitro. EGF enhanced protein synthesis and morphogenesis in vitro. With respect to our data and the literature, we hypothesize that DEXA and EGF may act on different cellular sides. Whereas glucocorticoids induce surfactant phospholipid synthesis, EGF should be more involved in cell proliferation and morphogenesis.

The effect of growth temperature on the phospholipid and fatty acyl compositions of non-proteolytic Clostridium botulinum

A non-proteolytic strain of Clostridium botulinum (NCIB 4270) was found to have a complex lipid composition, comprising five major phosphorus-containing lipids: phosphatidylethanolamine (PE), phosphatidylglycerol (PG), diphosphatidylglycerol (DPG), phosphatidylserine (PS) and a glycophospholipid of unknown structure (GPL), in order of abundance. Changing the growth temperature did not alter the lipid composition either qualitatively or quantitatively. The main fatty acyl components of the lipids are 14:0, 16:0 and 16:1. When the growth temperature was lowered from 37 to 8 degrees C, there was an increase in 14:0 from 16.4 to 37.5%, an increase in 16:1 from 10.5 to 22.5%, and a decrease in the proportion of 16:0 from 40.3 to 19.1%. There was also a decrease in the proportion of cyclopropane fatty acids (15:0cyc and 17:0cyc) from 7.3 to 0.5%, and in the equivalent chain length of the total fatty acids from 15.9 to 15.3 as the temperature was lowered. The same temperature-dependent changes occurred in the five major lipid classes examined. Despite reports of the presence of plasmalogenic forms of phospholipids (i.e. those lipids which have the acyl chain in the sn-1 position replaced by an alk-1-enyl group) in some Clostridium spp., none were detected in C. botulinum NCIB 4270 using either commercially available spray reagents or by gas-liquid chromatographic analysis of the products or acid methanolysis of total lipid extracts. It is concluded that non-proteolytic C. botulinum lacks plasmalogens, typical of other clostridia, in its membranes and instead modulates its fatty acid composition in response to temperature changes in a manner that is typical of other (non-clostridial) bacteria.

Alteration of fatty acid composition in a pgsA3 mutant of Escherichia coli

We examined the fatty acid composition in an Escherichia coli pgsA3 mutant lacking the potential to synthesize phosphatidylglycerolphosphate, a precursor of phosphatidylglycerol. The contents of C18:1cis-9 (oleic acid) and C18:1cis-11 (cis-vaccenic acid) in the total phospholipids extracted from the pgsA3 mutant growing at 37 degrees C were higher and that of C14:0 was lower than the wild type cells, resulting in a higher level of unsaturation of fatty acids (ratio of unsaturated fatty acids to saturated ones) in the mutant. The higher level of the unsaturated fatty acids in the pgsA3 mutant was more obvious in cardiolipin than in phosphatidylethanolamine. On the other hand, at 28 degrees C, at which the pgsA3 mutant shows limited cell growth, the content of unsaturated fatty acids in cardiolipin decreased in the pgsA3 mutant compared with the wild type. We consider that the pgsA3 mutant maintains cellular homeostasis by altering the level of unsaturated fatty acids in cardiolipin, and the mechanism is influenced by temperature.

Regulation of cardiolipin biosynthesis in H9c2 cardiac myoblasts by cytidine 5'-triphosphate

The regulation of cardiolipin biosynthesis by CTP in H9c2 cardiac myoblasts was investigated. H9c2 cells were incubated in the presence of cyclopentenylcytosine which is converted to cyclopentenylcytosine-triphosphate, a potent and specific inhibitor of CTP synthetase. Incubation of cells for 12 h with cyclopentenylcytosine reduced the cellular pool size of CTP to less than 10% of control cells but did not influence the pool size of other nucleotides. The de novo biosynthesis of phosphatidylcholine from [methyl-3H]choline, phosphatidylethanolamine from [1-3H]ethanolamine, and biosynthesis of all glycerol containing phospholipids from [U-14C]glycerol or [1,3-3H]glycerol were reduced approximately 50% after preincubation of the cells with cyclopentenylcytosine. In contrast, radioactive glycerol accumulated in phosphatidic acid, diacylglycerol, and triacylglycerol in cyclopentenylcytosine-treated cells compared with controls suggesting a re-routing of phospholipid biosynthesis away from CTP utilizing reactions toward neutral lipid synthesis. The de novo biosynthesis of all phospholipids was restored to control levels by addition of cytidine to the medium which elevated CTP levels. Cyclopentenylcytosine did not affect the in vitro enzyme activities involved in cardiolipin biosynthesis in these cells. In addition, the resynthesis of cardiolipin and most phospholipids from [1-14C]linoleic acid was not affected by cyclopentenylcytosine. Our findings indicate that the cellular CTP level may regulate cardiolipin biosynthesis in H9c2 cardiac myoblasts and support the notion that the cellular CTP level may be a universal signal/switch for all phospholipid biosynthesis in eukaryotic cells.

Growth phenotypes of Escherichia coli carrying a mutation of acidic phospholipid synthesis

We identified novel phenotypes of a pgsA3 mutant lacking the potential to synthesize phosphatidylglycerophosphate, a precursor of phosphatidylglycerol. The first phenotype is limited cell growth at a high temperature, under the condition of low salt. The phenotype was co-transduced with a phenotype lacking the potential to synthesize phosphatidylglycerol in a P1 transduction experiment, and was restored by transformation with a plasmid containing a wild type pgsA gene. The second phenotype of the pgsA3 mutant was resistant to growth in the presence of a low concentration of kanamycin (4 micrograms/ml). P1 transduction and transformation with the plasmid containing the wild-type pgsA gene revealed that the pgsA3 mutation was also responsible for the second phenotype.

Isolation and expression of the Rhodobacter sphaeroides gene (pgsA) encoding phosphatidylglycerophosphate synthase

The Rhodobacter sphaeroides pgsA gene (pgsARs), encoding phosphatidylglycerophosphate synthase (PgsARs), was cloned, sequenced, and expressed in both R. sphaeroides and Escherichia coli. As in E. coli, pgsARs is located immediately downstream of the uvrC gene. Comparison of the deduced amino acid sequences revealed 41% identity and 69% similarity to the pgsA gene of E. coli, with similar homology to the products of the putative pgsA genes of several other bacteria. Comparison of the amino acid sequences of a number of enzymes involved in CDP-diacylglycerol-dependent phosphatidyltransfer identified a highly conserved region also found in PgsARs. The pgsARs gene carried on multicopy plasmids was expressed in R. sphaeroides under the direction of its own promoter, the R. sphaeroides rrnB promoter, and the E. coli lac promoter, and this resulted in significant overproduction of PgsARs activity. Expression of PgsARs activity in E. coli occurred only with the E. coli lac promoter. PgsARs could functionally replace the E. coli enzyme in both a point mutant and a null mutant of E. coli pgsA. Overexpression of PgsARs in either E. coli or R. sphaeroides did not have dramatic effects on the phospholipid composition of the cells, suggesting regulation of the activity of this enzyme in both organisms.

A regulatory mechanism for the balanced synthesis of membrane phospholipid species in Escherichia coli

The mechanism that assures the balanced synthesis of zwitterionic (phosphatidylethanolamine) and acidic phospholipids (phosphatidylglycerol and cardiolipin) in Escherichia coli has been examined by genetically manipulating the two enzymes at the biosynthetic branch point, i.e., phosphatidylglycerophosphate synthase, encoded by pgsA, and phosphatidylserine synthase, encoded by pssA. A mutant in which the most part of the pssA gene was replaced with a drug resistance gene lacked phosphatidylserine synthase and phosphatidylethanolamine and required divalent metal ions for growth, as did a previously reported insertion-inactivated pssA mutant. When this mutant harbored a plasmid containing a Bacillus subtilis gene that encodes membrane-bound phosphatidylserine synthase, the phosphatidylethanolamine content was dependent on its activity, in contrast to that with the soluble E. coli counterpart. A defective mutation, pgsA3, caused reductions not only in acidic-phospholipid synthesis but also in phosphatidylethanolamine synthesis, despite the normal level of phosphatidylserine synthase activity. These results, together with previous observations, indicate that phosphatidylserine synthesis requires the membrane-associated form of phosphatidylserine synthase, which is related to the membrane-levels of acidic phospholipids, thus yielding balanced compositions of zwitterionic and acidic phospholipids.

Eukaryotic phospholipid biosynthesis

The current status of the biochemistry of phospholipid biosynthesis is presented. The review focuses on the identification and characterization of molecular tools such as purified enzymes and cloned genes and cDNAs for those enzymes. The enzymes discussed are those involved in the biosynthesis of the major phospholipid classes, namely, phosphatidate, phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, sphingomyelin, phosphatidylinositol and its phosphorylated derivatives, and cardiolipin. The review centers on the pathways in mammals and yeast. Novel genetic approaches used to delineate pathways and clone cDNAs are discussed. The regulatory roles played by some of the enzymes involved in controlling the biosynthetic pathways are presented.

Thapsigargin selectively stimulates synthesis of phosphatidylglycerol in N1E-115 neuroblastoma cells and phosphatidylinositol in C6 glioma cells

Phospholipid metabolism was studied in N1E-115 neuroblastoma and C6 glioma cells exposed to thapsigargin, a selective inhibitor of endoplasmic reticulum Ca(2+)-ATPase that raises the cytosolic free Ca2+ concentration [Ca2+]i. Thapsigargin caused only a transient increase of [Ca2+]i (< 1 min) in N1E-115 cells similar in magnitude and duration to agonist-induced calcium release mediated by inositol trisphosphate. Sustained elevation of [Ca2+]i due to influx of extracellular calcium, as occurs in most other cell lines including C6 cells, did not occur in N1E-115 cells. Increased uptake of inorganic phosphate (Pi) associated calcium influx was observed in C6 but not in N1E-115 cells. Thapsigargin affected phospholipid synthesis in both cell lines, most likely by inhibiting phosphatidic acid phosphohydrolase as indicated by diversion of [3H]oleic acid incorporation from triacylglycerol to phospholipid synthesis and stimulation of [32P]Pi incorporation into anionic phospholipids at the expense of phosphatidylcholine synthesis. The response to increased phosphatidate/phosphatidyl-CMP availability was cell specific. Thapsigargin (> 100 nM) selectively stimulated phosphatidylglycerol synthesis 20-30-fold in N1E-115 neuroblastoma cells while phosphatidylinositol synthesis was increased < 2-fold. In contrast, phosphatidylglycerol was not affected in C6 glioma cells and phosphatidylinositol synthesis was stimulated 8-fold by thapsigargin (> 1 microM). Agonist-stimulated calcium release did not increase phosphatidylglycerol synthesis in N1E-115 cells. Thapsigargin-stimulated phosphatidylglycerol synthesis and agonist-stimulated phosphatidylinositol synthesis could occur at the same time. Similar results were obtained with TMB-8, an inhibitor of intracellular Ca2+ release that decreases diacylglycerol utilization by blocking choline uptake and phosphatidylcholine synthesis without affecting resting [Ca2+]i. Thus [Ca2+]i does not directly mediate the effects of thapsigargin, TMB-8 or agonist stimulation on anionic phospholipid metabolism. These additional effects may limit the use of thapsigargin to assess Ca(2+)-dependence of phospholipid metabolism associated with Ca(2+)-mediated signal transduction.

Biochemical studies of three Saccharomyces cerevisiae acyl-CoA synthetases, Faa1p, Faa2p, and Faa3p

The efficiency and specificity of protein N-myristoylation appear to be influenced by the availability of myristoyl-CoA and other potential acyl-CoA substrates of myristoyl-CoA:protein N-myristoyltransferase. Recent studies have revealed that Saccharomyces cerevisiae contains at least three acyl-CoA synthetase genes (FAA for fatty acid activation). We have expressed Faa1p, Faa2p, and Faa3p in a strain of Escherichia coli that lacks its own endogenous acyl-CoA synthetase (FadD). Each S. cerevisiae acyl-CoA synthetase contained a carboxyl-terminal His tag so that it could be purified to homogeneity in a single step using nickel chelate affinity chromatography. In vitro assays of C3:0-C24:0 fatty acids indicate that Faa1p prefers C12:0-C16:0, with myristic and pentadecanoic acid (C15:0) having the highest activities. Faa2p can accommodate a wider range of acyl chain lengths: C9:0-C13:0 are preferred and have equivalent activities, although C7:0-C17:0 fatty acids are tolerated as substrates with no greater than a 2-fold variation in specific activity. The myristoyl-CoA synthetase activities of Faa1p and Faa2p are 2 orders of magnitude greater than that of Faa3p in vitro. Faa3p has a preference for C16 and C18 fatty acids with a cis-double bond at C-9-C-10. The temperature optimum for Faa1p is 30 degrees C, while Faa2p and Faa3p have the greatest activities at 25 degrees C. These in vitro observations were confirmed using two in vivo assays: (i) measurement of the ability of each S. cerevisiae acyl-CoA synthetase to direct the incorporation of exogenously derived tritiated myristate, palmitate, or oleate into cellular phospholipids produced in a fadD- strain of E. coli during exponential growth at 24 or 37 degrees C and (ii) measurement of the incorporation of [3H]myristate into a yeast N-myristoylprotein coexpressed with Nmt1p and Faa1p, Faa2p, or Faa3p in the fadD- strain.

beta-Adrenergic priming of rats in vivo modulates the effect of beta-agonist in vitro on surfactant phospholipid metabolism of isolated lungs

To evaluate the effects of multiple beta-adrenergic stimulations on pulmonary surfactant phospholipids, perfused lungs from beta-adrenergic primed and non-primed rats were challenged with the beta-agonist terbutaline in vitro. Cell-free lung lavage, lavagable alveolar cells and lung tissue were analysed for phospholipid content and incorporation of precursors. In lung lavage, terbutaline in vitro doubled the incorporation of 14C-choline and 3H-palmitate into total phosphatidylcholine (PC) and of 3H-palmitate into phosphatidylglycerol (PG). beta-adrenergic priming in vivo prior to terbutaline in vitro lowered the increase of precursor incorporation. For lavagable cells, terbutaline in vitro increased the incorporation of 3H-palmitate into PC. Priming in vivo reduced this effect and diminished the specific 3H-choline incorporation into lavagable cell PC below control level. For lung tissue, priming increased the amounts of PC and disaturated PC (DSPC) whereas terbutaline in vitro decreased DSPC in both primed and non-primed lungs. Terbutaline in vitro slightly increased the incorporation of 14C-choline and 3H-palmitate into PC and DSPC in non-primed but not in primed lungs. beta-adrenergic blockade by ICI 118.551 prevented all effects but generally increased 3H-palmitate incorporation into the phospholipids and, in lavagable cells, the amount of PC. We conclude that long-term beta-adrenergic treatment may alter the metabolism of pulmonary surfactant phospholipids by increasing tissue PC and DSPC and by decreasing the secretion of newly-synthesized PC.

Effect of pentoxifylline on the inhibition of surfactant synthesis induced by TNF-alpha in human type II pneumocytes

Tumor necrosis factor alpha (TNF-alpha) seems to play an important role in the pathogenesis of the adult respiratory distress syndrome (ARDS). This study was designed to determine the effect of TNF-alpha and pentoxifylline (PTXF) on surfactant synthesis by isolated human type II pneumocytes. In order to isolate the pneumocytes, lungs obtained from both previously healthy multiple organ donors (n = 11) and patients who underwent surgical excision for lung cancer (n = 8) were used. Surfactant synthesis was measured by the incorporation of labeled glucose into the two most important phospholipid components of surfactant: phosphatidylcholine (PC) and phosphatidylglycerol (PGL). The pneumocytes of the donor group showed a greater degree of PC synthesis than those from the cancer group (3.44 +/- 0.19 versus 2.15 +/- 01.5 pmol/micrograms protein, p < 0.001). The synthesis of PC by pneumocytes in both the donor (1.13 +/- 0.19 versus 3.44 +/- 0.19 pmol/micrograms protein, p < 0.01) and cancer (0.99 +/- 0.11 versus 2.15 +/- 0.15 pmol/micrograms protein, p < 0.01) groups was decreased by TNF-alpha (100 ng/ml). This effect was blocked by PTXF (100 micrograms/ml), a substance that also increased PC production in the control-group pneumocytes from cancer patients, the final PC levels being similar to those of the donors in the absence of TNF-alpha. These results suggest that one of the mechanisms of TNF-alpha participation in the pathophysiology of ARDS is inhibition of surfactant synthesis, and support the hypothesis of in vivo production of TNF-alpha in lung-cancer patients, with subsequent chronic exposure of the lung epithelial cells to this cytokine.(ABSTRACT TRUNCATED AT 250 WORDS)

A somatic cell mutant defective in phosphatidylglycerophosphate synthase, with impaired phosphatidylglycerol and cardiolipin biosynthesis

Phosphatidylglycerophosphate (PGP) synthase catalyzes a reaction involved in the synthesis of phosphatidylglycerol (PG), which serves as a metabolic precursor for cardiolipin (CL), found primarily in the mitochondrial membranes of eukaryotic cells. We isolated a Chinese hamster ovary cell mutant (designated PGS-S) with a specific lesion in PGP synthase by using an in situ enzymatic assay for the enzyme. This mutant was obtained by introducing a second mutation into mutant PGS-P that had been generated by first-step mutagenesis. The PGP synthase activities in cell extracts of mutant PGS-S grown at 33 and 40 degrees C were 14 and 1% of those in the wild type cells, respectively; in addition, PGP synthase in cell extracts of mutant PGS-S exhibited higher sensitivity to heat than that of the wild type. Mutant PGS-S also showed a temperature-dependent defect in the synthesis of PG and CL in vivo, together with temperature sensitivity for cell growth. A temperature-resistant revertant of mutant PGS-S simultaneously restored PGP synthase activity and the ability to synthesize PG and CL in vivo to nearly the same levels as those of mutant PGS-P. These results constitute genetic evidence that PGP synthase is responsible for PG synthesis and is essential for cell growth.

Requirement of phosphatidylglycerol for flagellation of Escherichia coli

We report that phosphatidylglycerol is required for flagellation of Escherichia coli. Cells carrying the pgsA3 mutation did not form swarm rings in semisolid agar. P1 transduction experiments revealed that the potential for phosphatidylglycerol synthesis and for the formation of swarm rings was co-transducible. The pgsA3 mutant transformed with the wild type pgsA+ gene cloned into the R-plasmid vector had the potential for both phosphatidylglycerol synthesis and cell motility. Electromicroscopic and SDS-PAGE analyses showed that the pgsA3 mutation causes the lack of flagellation.

Biosynthesis and characterization of phosphatidylglycerophosphoglycerol, a possible intermediate in lipoteichoic acid biosynthesis in Streptococcus sanguis

A membrane enzyme preparation from Streptococcus sanguis was shown to convert sn-[14C]glycerol 3-phosphate and CDP-diacylglycerol (or deoxyCDP-diacylglycerol) into a series of progressively higher-molecular-weight [14C]oligophosphoglycerophospholipids in vitro. The first oligophosphoglycerophospholipid to accumulate (termed lipid-1) was purified to homogeneity; chemical analysis, gas-liquid chromatography and chemical degradation studies indicated the most likely structure to be phosphatidylglycerophosphoglycerol (PGpG). PGpG is formed directly from two molecules of phosphatidylglycerol (PG), one molecule of PG serving as a sn-glycerol 1-phosphate (pG) donor and the second serving as the pG acceptor, with co-production of diacylglycerol. These oligophosphoglycerophospholipids may be intermediates in the biosynthesis of lipoteichoic acids.

The phosphonic acid analog of phosphatidylglycerol phosphate: influence on Escherichia coli growth and physiology

At 20 microM, rac-3,4-dihydroxybutyl-1-phosphonate (DBP) has only a slight bacteriostatic effect on Escherichia coli. However, cells lose viability when the medium also contains either 20 mM magnesium or calcium ions. Magnesium ions stimulate the incorporation of DBP into (1,2-diacyl)-sn-glycerol-D-4'-phosphoryloxy-3'-hydroxybutyl-1'-pho sphonate, the phosphonate analog of phosphatidylglycerol phosphate. Much higher DBP concentrations are needed to block the growth of a pgsA3 mutant than to block the growth of an isogenic wild-type strain. The DBP-treated pgsA mutant also has a much higher survival rate when stored in the cold than does the DBP-treated wild-type strain. Furthermore, the pgsA3 mutant grows normally in the presence of DBP and magnesium ions. Treatment with DBP and magnesium ions does not appear to disrupt the cell's inner or outer membranes. However, it does block macromolecular and phosphoglyceride synthesis. A combination of 20 microM rac-DBP and 0.5 mM spermidine or 0.125 mM spermine is bacteriostatic. These studies indicate that the PGP analog contributes to DBP's bacteriostatic effect when the growth medium contains low concentrations of magnesium or calcium ions and is responsible for its bactericidal effect when the medium contains high concentrations of these ions.

Calcium-independent effects of TMB-8. Modification of phospholipid metabolism in neuroblastoma cells by inhibition of choline uptake

TMB-8 [8-(NN-diethylamino)-octyl-3,4,5-trimethoxybenzoate] blocks agonist-stimulated release of Ca2+ from intracellular sites in many cell lines and is often used to distinguish between dependence on extracellular and intracellular Ca2+. In N1E-115 neuroblastoma cells, TMB-8 did not alter the resting cytosolic Ca2+ concentration in unstimulated cells, yet phospholipid metabolism was greatly affected. At concentrations of TMB-8 (25-150 microM) that inhibit Ca2+ release, phosphatidylcholine formation was inhibited, whereas synthesis of phosphatidylinositol, phosphatidylglycerol and phosphatidylserine was stimulated. Unlike other cationic amphipathic compounds, TMB-8 did not inhibit phosphatidate phosphatase or enzymes in the pathway from choline to phosphatidylcholine. Choline transport was the major site of action. TMB-8 was a competitive inhibitor (Ki = 10 microM) of low-affinity (Kt = 20 microM) choline transport. When added at the same time as labelled precursor, TMB-8 also decreased cellular uptake of phosphate and inositol, but not that of ethanolamine or serine. In prelabelled cells, continued uptake and incorporation of phosphate and inositol were not affected. Under these conditions phosphatidylinositol synthesis was increased 2-fold and, like the effect on phosphatidylcholine, reached a plateau at 100 microM-TMB-8. Phosphatidylglycerol synthesis increased linearly with TMB-8 concentration to 40-fold stimulation at 150 microM, suggesting a selective effect on synthesis of phosphatidylglycerol from CDP-diacylglycerol. Phosphatidylserine synthesis was also increased up to 3-fold. These Ca(2+)-independent effects limit the use of TMB-8 in studies of cell signalling that involve stimulated phosphatidylinositol and phosphatidylcholine metabolism.

Surfactant phospholipids: synthesis and storage

Pulmonary surfactant, a complex consisting of 90% lipids and 10% specific proteins, lines the alveoli of the lung and prevents alveolar collapse and transudation by lowering the surface tension at the air-liquid interface. Dipalmitoylphosphatidylcholine constitutes approximately 50% of the surfactant lipids and is primarily responsible for the surface tension-lowering property of the surfactant mixture. This phospholipid, together with the other surfactant phospholipids, is produced at the endoplasmic reticulum of the alveolar type II epithelial cells. The characteristic lamellar bodies in these cells serve as storage depot for the surfactant before this is secreted onto the alveolar surface. This article reviews the pathways via which the surfactant lipids are synthesized, our current knowledge of the regulation of these pathways, and what is known about intracellular traffic of phospholipids from their site of synthesis to the lamellar bodies.

Biosynthesis of the unique trans-delta 3-hexadecenoic acid component of chloroplast phosphatidylglycerol: evidence concerning its site and mechanism of formation

As in most higher plants, chloroplast membranes of the green alga Dunaliella salina contain phosphatidylglycerol (PG) that is rich in trans-delta 3-hexadecenoic acid (16:1t), a fatty acid found nowhere else in the cell. After labeling D. salina with exogenous [3H]myristic acid [( 3H]14:0), the cis-unsaturated fatty acids of monogalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol as well as PG had higher specific radioactivities in chloroplast envelopes than in thylakoids. In contrast, 16:1t was very slow to become radioactive, and its specific radioactivity was several times higher in isolated thylakoids than in envelopes after brief (3-20 min) labeling with [3H]14:0. Analysis of individual PG molecular species revealed that the fatty acid paired with 16:1t was also labeled slowly. Thus linoleate (18:2) released from a 16:1t-containing PG had a 350-fold (at 3 min) to 20-fold (at 60 min) lower specific radioactivity than did 18:2 from a palmitate (16:0)-containing PG. The findings suggest that the substrates for trans-desaturation are 16:0-containing PG molecular species which are readily labeled from [3H]14:0 in the envelope but are diluted by the large pool of thylakoid PG before penetrating to the desaturation site. By examining the labeling patterns of individual PG molecular species classes, it was concluded that D. salina 16:1t is formed from 16:0 linked to 18:2/16:0 PG and 18:3/16:0 PG by a trans-desaturase located within the inner recesses of the thylakoid compartment.

Phosphatidylglycerol acetal of plasmenylethanolamine as an intermediate in ether lipid formation in Clostridium butyricum

The formation and turnover of the recently discovered phosphatidylglycerol acetal of plasmenylethanolamine was investigated in Clostridium butyricum. Incorporation of phosphate into the phospholipids was studied by using [32P]orthophosphate in pulse and pulse-chase experiments in growing cells. Among the ethanolamine-containing lipids, the diacyl form of phosphatidylethanolamine was labeled most rapidly, followed by the phosphatidylglycerol acetal of plasmenylethanolamine and plasmenylethanolamine. The glycerol acetal of plasmenylethanolamine was labeled most slowly. There was rapid turnover of approximately one half of the newly labeled phosphatidylethanolamine pool. Since the kinetics of labeling of the small pool of phosphatidylglycerol acetal of plasmenylethanolamine and the larger pool of plasmenylethanolamine were similar during the early time courses of pulse and pulse-chase experiments, the results argue against the derivation of phosphatidylglycerol acetal of plasmenylethanolamine from plasmenylethanolamine. The results are consistent with the derivation of glycerol acetal of plasmenylethanolamine from either phosphatidylglycerol acetal of plasmenylethanolamine or plasmenylethanolamine.

Biosynthesis of phosphatidylglycerol and phosphatidylglycerolphosphate in submitochondrial membranes isolated from guinea pig liver is absolutely dependent on CDP-diglycerides imported from microsomal membranes

The biosynthesis of radioactively labelled phosphatidylglycerol via phosphatidylglycerophosphate in outer and inner mitochondrial membranes isolated from guinea pig liver was found to depend absolutely on CDP-diglycerides, which could not be biosynthesized in these membranes. The requirement for CDP-diglycerides in the biosynthesis of labelled phosphatidylglycerol could be fulfilled by the transfer of biosynthesized [3H]CDP-diglycerides from the microsomal membranes to the outer and inner mitochondrial membranes.

Comparison of the biosynthesis and composition of polyglycerophosphatides and phosphatidylinositols in mitochondria and microsomes isolated from neonatal and adult rat heart and liver

The level of biosynthesis and the composition of polyglycerophosphatides (phosphatidylglycerol, phosphatidyglycerolphosphate, and diphosphatidylglycerol or cardiolipin) and phosphatidylinositols were examined in mitochondria and microsomes, respectively, isolated from neonatal and adult rat heart and liver. Biosynthesis of [3H]polyglycerophosphatides [( 3H]phosphatidylglycerol and [3H]phosphatidylglycerolphosphate) was 4.5 times higher in neonatal than in adult heart mitochondria, whereas in the respective liver mitochondria this synthesis was only 15% higher in neonatal mitochondria. The biosynthesis of [3H]phosphatidylinositol was twice as high in neonatal as in adult heart microsomes, but very similar in the respective liver microsomes. The major biosynthesized polyglycerophosphatide was [3H]phosphatidylglycerol. The accumulation of [3H]phosphatidylglycerolphosphate depended on the origin of the mitochondria. Under our experimental conditions [3H]phosphatidylinositol was the only synthesized phosphoinositide in all microsomes. The biosynthesis of cardiolipin depended on the origin of the mitochondria and was highest in adult rat liver mitochondria and lowest in adult heart mitochondria. In all cases the biosynthesized [14C,3H] cardiolipin from [14C]phosphatidylglycerol and [3H]CDP-diglycerides had a ratio of 14C/3H around unity. The biosynthesis of [3H]CDP-diglycerides, the key precursor for the biosynthesis of phosphatidylglycerol, phosphatidylinositol, and cardiolipin, was 30% higher in neonatal than in adult heart microsomes and very similar in the respective liver microsomes. The subcellular localization of the enzymes required for the biosynthesis of the lipids and liponucleotides examined was found to be the same in membranes isolated from neonatal and adult rat heart and liver.

Formation of acylphosphatidylglycerol by a lysosomal phosphatidylcholine:bis(monoacylglycero)phosphate acyl transferase

A delipidated soluble fraction prepared from a mitochondrial-lysosomal fraction of rabbit alveolar macrophages that catalyzes transacylation of lysophosphatidylglycerol to form bis(monoacylglycero)phosphate was also found to transfer oleic acid from [14C]dioleoyl phosphatidylcholine to form acylphosphatidylglycerol. The reaction was dependent on the presence of bis(monoacylglycero)phosphate and was maximal at a concentration of 44 microM when the ratio of fatty acid transferred to fatty acid released was 0.28. Addition of phosphatidylglycerol had only a small effect. Homogenates of rat liver also catalyzed the reaction and after subcellular fractionation the activity was localized to lysosomes. The lysosomal activity was solubilized by delipidation with butanol to give a preparation with a specific activity 2462 times that of the homogenate. Optimal activity of soluble preparations from both macrophages and liver was at pH 4.5, with little activity above 6.0. Release of free fatty acid was also stimulated under conditions of optimal acyl transfer. Both acyl transfer and release of fatty acid were inhibited by Ca2+, detergents, chlorpromazine, lysophosphatidylcholine, and oleic acid. When there was disproportional inhibition, acyl transfer was always more affected. These results suggest that sequential acylation of lysophosphatidylglycerol to form bis(monoacylglycero)phosphate and then acylphosphatidylglycerol constitute a mechanism in the lysosome for the transport and partition of fatty acids released by the lysosomal phospholipases.

Alteration of the phospholipid composition of Escherichia coli through genetic manipulation

In order to study the function of individual phospholipids, we have constructed a strain of Escherichia coli in which the ratio of phosphatidylethanolamine to phosphatidylglycerol plus cardiolipin can be regulated. In this strain (HDL1001) the normal expression of the phosphatidylglycerophosphate synthase does not occur due to the presence of the pgsA30 allele (Heacock, P. N., and Dowhan, W. (1987) J. Biol. Chem. 262, 13044-13049). A second chromosomal copy of the pgsA gene is fused to the lacOP region in single copy within the lac operon. Strain HDL1001 is absolutely dependent for growth on an inducer of the lac operon. In addition, the level of the pgsA gene product, the content of the two major acidic phospholipids, and the growth rate are dependent on the level of inducer in the growth medium. Cells remain viable in the absence of inducer as evidenced by a rapid return to normal growth after the readdition of inducer. The growth rate and phospholipid composition are affected only after the level of phosphatidylglycerophosphate synthase drops below about 15% of normal levels; both phosphatidic acid and (d)CDP-diacylglycerol also begin to increase to significant levels. At the point of cell arrest the level of the major acidic phospholipids is reduced by about 90% of wild type levels.