Phosphatidylserine Synthesis Research Articles

Mechanism of the ATP-dependent phosphatidylserine synthesis in liver subcellular fractions

It has been shown that the ATP-dependent incorporation of [ 14C]serine into phosphatidylserine in rat liver mitochondrial and microsomal fractions is prevented by EGTA. On the other hand, at low (μM) Ca 2+ concentrations, serine incorporation is strongly stimulated by ATP and Mg 2+. This stimulatory effect is reduced by calcium ionophore A23187. It is therefore suggested that the ATP-dependent process is that of serine base-exchange reaction, stimulated by endogenous Ca 2+ accumulated inside the microsomal vesicles by Ca 2+,Mg 2+-ATPase. The mitochondrial activity can be accounted for by contamination by the endoplasmic reticulum. Phosphatidylserine; Base-exchange reaction; ATPase, Ca 2+,Mg 2+-; (Endoplasmic reticulum)

Phospholipid metabolism and T cell activation: Receptor triggering is associated with the inhibition of phosphatidylserine synthesis

Activation of Jurkat T lymphocytes to produce IL2 is accompanied by a strong inhibition of phosphatidylserine (PS) synthesis. This inhibition was obtained either with the mitogenic lectin PHA, anti-CD3 monoclonal antibodies (mAb), anti-CD2 mAb or anti-Ti mAb. Bypassing membrane receptor signalling, by using a Ca 2+ ionophore or a protein phosphatase inihibitor, sodium ortho-vanadate, also results in a marked inhibition of PS synthesis. Activators of phospholidid —Ca 2+ dependent protein kinase C (PKC) did not significantly modify PS synthesis, suggesting that the observed changes only involve the transduction of the first activation signal. PS being a necessary cofactor for PKC, our results strongly suggest that the inhibition of PS synthesis induced by receptor triggering exerts a feed back control on PKC therefore leading to a transient activation of the enzyme upon full lymphocyte activation.

Increased Incorporation of L-(U-14C)serine into Phosphatidylserine in Polymorphonuclear Cells from Cluster Headache Patients

It has recently been demonstrated by our group that polymorphonuclear cells (PMNs) from cluster headache patients incorporate more arachidonic acid (AA) into phosphatidylserine (PS) than PMNs from controls. In the present report, the incorporation of L-(U-14C)serine into PS in PMNs from 14 healthy volunteers and 12 cluster headache patients was studied. PMNs from controls incorporated 1194 +/- 578 (mean +/- SD) cpm of L-(U-14C)serine into PS, 268 +/- 292 cpm into phosphatidylethanolamine, and 57 +/- 71 cpm into sphingomyeline. The corresponding figures in cluster headache patients were 2365 +/- 841 cpm, 291 +/- 207 cpm, and 88 +/- 66 cpm, respectively. Incorporation of L-(U-14C)serine into PS was significantly increased (p less than 0.0004) in PMNs from cluster headache patients, whereas no significant difference was seen in other lipids. The results confirm that patients with cluster headache have an increased incorporation of precursors into PS in isolated PMNs, and they indicate that this is due to an increased de novo synthesis of PS.

IL-1 signaling for IL-2 production in T cells involves a rise in phosphatidylserine synthesis

IL-1 signaling for IL-2 production in T cells involves a rise in phosphatidylserine synthesis

Regulation of phospholipid biosynthesis in Saccharomyces cerevisiae by inositol. Inositol is an inhibitor of phosphatidylserine synthase activity.

The addition of inositol to the growth medium of Saccharomyces cerevisiae resulted in rapid changes in the rates of phospholipid biosynthesis. The partitioning of the phospholipid intermediate CDP-diacylglycerol was shifted to phosphatidylinositol at the expense of phosphatidylserine and its derivatives phosphatidylethanolamine and phosphatidylcholine. Serine at 133-fold greater concentrations than that of inositol shifted the partitioning of CDP-diacylglycerol to phosphatidylserine at the expense of phosphatidylinositol but to a much lesser degree. Kinetic experiments with pure phosphatidylserine synthase and phosphatidylinositol synthase indicated that the partitioning of CDP-diacylglycerol between phosphatidylserine and phosphatidylinositol was not governed by the affinities both enzymes have for their common substrate CDP-diacylglycerol. Instead, the main regulation of phosphatidylinositol and phosphatidylserine synthesis was through the exogenous supply of inositol. The Km of inositol (0.21 mM) for phosphatidylinositol synthase was 9-fold higher than cytosolic concentration of inositol (24 microM). The Km of serine (0.83 mM) for phosphatidylserine synthase was 3-fold below the cytosolic concentration of serine (2.6 mM). Therefore, inositol supplementation resulted in a dramatic increase in the rate of phosphatidylinositol synthesis, whereas serine supplementation resulted in little affect on the rate of phosphatidylserine synthesis. Inositol also contributed to the regulation of phosphatidylinositol and phosphatidylserine synthesis by having a direct affect on phosphatidylserine synthase activity. Kinetic experiments with pure phosphatidylserine synthase showed that inositol was a noncompetitive inhibitor of the enzyme with a Ki of 65 microM.

Disruption of the CHO1 gene encoding phosphatidylserine synthase in Saccharomyces cerevisiae.

A Saccharomyces cerevisiae mutant that lacked phosphatidylserine synthase [EC 2.7.8.8] (CDP-1,2-diacyl-sn-glycerol: L-serine O-phosphatidyltransferase) completely was constructed by disrupting its structural gene, CHO1. Over two-thirds of its coding region, from the starting to the 200th codon, was replaced with a LEU2 DNA fragment. This new cho1 mutant showed no detectable synthesis of phosphatidylserine but grew slowly in a medium that contained either ethanolamine or choline. These results indicate that phosphatidylserine synthase and most probably phosphatidylserine are dispensable in S. cerevisiae but necessary for its optimal growth. Additional supplementation with myo-inositol raised the cellular content of phosphatidylinositol and improved the growth of the mutant, suggesting the importance of the negative charges of the membrane surface. The CHO1-disrupted mutant, when grown on choline, accumulated phosphatidylethanolamine to a significant level even after extensive dilution of the initial culture. It segregated prototrophic revertants that could synthesize phosphatidylethanolamine without recovery of phosphatidylserine synthesis. These results imply the presence of a route(s) for the formation of ethanolamine or its phosphorylated derivative in S. cerevisiae.

IL-1 signaling for IL-2 production in T cells involves a rise in phosphatidylserine synthesis.

Activation of Jurkat T cells with anti-TCR, anti-CD3, anti-CD2, or PHA is accompanied by a strong inhibition of phosphatidylserine (PS) synthesis. The inhibition of the synthesis of this phospholipid could be partially reversed by IL-1. In Jurkat cells, IL-1 did not activate phosphodiesterases as demonstrated by the lack of change of inositol triphosphate and diacylglycerol levels as well as the lack of change in cytosolic Ca2+ concentration. Furthermore, IL-1 did not modify the intracellular level of cGMP and cAMP, suggesting that the observed rise of PS synthesis could play the role of mediator IL-1 action. As PS is a necessary cofactor for the activation of protein kinase C, our results suggest strongly that IL-1 modulate protein kinase C activity in the activated lymphocyte through its action on PS synthesis.

Compartmentalization and differential labeling of phospholipids of rat liver subcellular membranes

The in vivo, interorganelle movement of phospholipids synthesized by different biosynthesis routes has been investigated in rat liver. Rats were injected with [ methyl- 3H]choline, [1- 3H]ethanolamine or [3- 3H]serine into the portal vein. Subcellular membranes (endoplasmic reticulum, Golgi apparatus, plasma membrane and mitochondria) were isolated, and the specific radioactivites of the phospholipids in each membrane were determined. There was a very rapid distribution of phospholipids from their sites of synthesis to the other organelles. In the plasma membrane, for example, the specific radioactivity of phosphatidylcholine derived from choline, ethanolamine or serine was as high as, or higher than, in the endoplasmic reticulum at all times examined. In addition, the specific radioactivity of phosphatidylserine (derived from serine) in the plasma membrane was approximately double that in the endoplasmic reticulum, even though the latter is the major site of phosphatidylserine synthesis. There was no evidence for the sequential flow of phospholipid from the endoplasmic reticulum, via the Golgi apparatus, to the plasma membrane. The experiments also demonstrated that the various subcellular membranes were labeled to different extents with phospholipids synthesized from different biosynthetic routes. It is unlikely that there is sufficient phospholipid biosynthetic enzyme activity in subcellular organelles other than the endoplasmic reticulum (Vance, J.E. and Vance, D.E. (1988) J. Biol. Chem. 263, 5898–5909) and the mitochondria for phosphatidylserine decarboxylase, to account for the efficient labeling of phospholipids of the plasma membrane, mitochondria and Golgi apparatus. The data suggest that although phospholipids can move very rapidly from one organelle to another, and within the plane of the lipid bilayer, there is neither a rapid mixing of newly synthesized phopholipids with the endogenous phospholipid pool, nor a rapid mixing of phospholipids derived from different biosynthetic origins.

Differential effects of chloroquine on the phospholipid metabolism of plasmodium-infected erythrocytes

The effect of the antimalarial drug chloroquine (CQ) on the phospholipid metabolism in Plasmodium knowlesi-infected simian erythrocytes has been studied by incubating cells with different labeled precursors and various concentrations of CQ. The drug induced considerable modifications of this metabolism but at the same time decreased nucleic acid and protein synthesis as well as the output of 14CO 2 from radioactive glucose. Phosphatidylcholine biosynthesis was severely reduced. However, under these conditions, CQ had the early effect of markedly increasing phosphatidylinositol labeling from radioactive inositol, fatty acids, 1-( 14C)palmitoyl-lysophosphatidylcholine, but not from glycerol. Synthesis of phosphatidylserine from ( 14C)sefine and of phosphatidylethanolamine from labeled glycerol, ethanolamine, and serine was increased, especially at high CQ concentrations when the whole metabolism of the parasite was severely reduced. These effects reflect a deep differential effect of CQ on the intense phospholipid metabolism of the Plasmodium-infected erythrocytes, which might involve a redirecting of phospholipid metabolism similar to that induced by other cationic amphiphilic drugs, and a compensatory synthesis resulting from the severe blockage of phosphatidylcholine synthesis.

Insulin and oxytocin effects on phosphoinositide metabolism in adipocytes.

The effects of hormones on phosphoinositide metabolism were examined in rat adipocytes prelabeled with 32Pi or [3H]inositol. Oxytocin and vasopressin produced large decreases in labeled polyphosphoinositides and increases in phosphatidic acid and inositol phosphates, whereas insulin was without effect, although it stimulated lipogenesis from glucose. Likewise, insulin did not elevate 1,2-diacylglycerol measured chemically by high pressure liquid or thin-layer chromatography in fat cells or pads. It also did not increase the radioactivity in 1,2-diacylglycerol in ghosts prepared from fat cells previously labeled with [3H]arachidonic acid, although oxytocin and vasopressin increased this. It is therefore concluded that insulin does not stimulate the breakdown of polyphosphoinositides to yield 1,2-diacylglycerol and inositol phosphates in adipocytes and that the insulin-like actions of oxytocin must be due to other changes. Insulin induced small, but significant and equal increases (40% at 30 min) in the incorporation of [3H] inositol into phosphatidylinositol, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate in adipocytes. The effects were not dependent upon glucose and were not evident before 15 min. Oxytocin also produced large increases in the labeling of the three phosphoinositides. Insulin stimulated the incorporation of [3H]glycerol into the three phosphoinositides and also phosphatidic acid, phosphatidylserine, and phosphatidylethanolamine by 50-100% in cells incubated without glucose. No changes in the labeling of glycerol 3-phosphate, lysophosphatidic acid, phosphatidylcholine, and triacylglycerol were detected, and there was a small increase (30%) in 1,2-diacylglycerol labeling. It is concluded that insulin increases the synthesis of phosphatidylinositol, phosphatidylinositol 4-phosphate, phosphatidylinositol 4,5-bisphosphate, phosphatidylethanolamine, and phosphatidylserine in fat cells partly by stimulating a reaction(s) located between glycerol 3-phosphate and phosphatidic acid in the biosynthetic pathway.

A new pathway for phosphatidylserine synthesis in rat liver microsomes

It has been shown that the phosphate moiety of glycerophosphate is incorporated into phosphatidylserine of rat liver microsomes, but not of mitochondria. The reaction is dependent on CMP. This observation suggests that the new pathway of acyl-specific synthesis of phosphatidylserine proposed by J.P. Infante [(1984) FEBS Lett. 170, 1–14] can proceed in rat liver microsomes.

Phorbol ester inhibits phosphatidylserine synthesis in human promyelocytic leukaemia HL60 cells. Possible involvement of free radicals and correlation with phosphorylation of nuclear protein 1b

Treatment of human promyelocytic leukaemia HL60 cells in conditioned medium with 12-O-tetradecanoylphorbol 13-acetate (TPA) for 4 h resulted in 25-30% inhibition of labelling of phosphatidylserine (PS) with [U-14C]serine. PS labelling was 40% lower, and no inhibitory TPA effect was observed when the experiments were performed in fresh medium. Cycloheximide or puromycin also inhibited PS labelling by 38-44%; their inhibitory effects were non-additive with that of TPA and occurred only in conditioned medium. Catalase (CAT) and superoxide dismutase (SOD), both free-radical scavengers, and H7, a protein kinase C inhibitor, reversed to various extents the inhibitory effect of TPA on PS synthesis. On the other hand, chlorobenzoic acid, a free-radical-generating agent, also inhibited PS synthesis by 22% after 4 h treatment when conditioned medium was used. When ethanolamine was added to cells in conditioned medium to quench PS formation through the exchange of free serine with the ethanolamine moiety of phosphatidylethanolamine (PE), PS labelling was decreased by 33% and the inhibitory TPA effect was significantly decreased. On the other hand, ethanolamine had marginal quenching effect on PS labelling when added to cells in fresh medium. TPA increased the phosphorylation of various proteins in the cells, including protein lb (Mr 80,000; pI 5.5) shown to be localized mainly in the nuclear fraction. Chlorobenzoic acid selectively stimulated the phosphorylation of protein lb, whereas CAT and SOD specifically attenuated the TPA-stimulated phosphorylation of this protein. All these agents affected phosphorylation of protein lb only if conditioned medium was used. The findings suggested that net synthesis of PS through the base-exchange mechanism was stimulated in HL60 cells by cell products present in the conditioned medium. TPA inhibited this stimulated PS synthesis by a mechanism which appeared to involve active oxygen species and protein synthesis and might be related to the phosphorylation of protein lb.

Phosphatidylethanolamine biosynthesis in rat mammary carcinoma cells that require and do not require ethanolamine for proliferation.

Epithelial cells and some of their transformed derivatives require ethanolamine to grow normally in defined culture medium. When these cells are cultured without ethanolamine, the amount of cellular phosphatidylethanolamine is considerably reduced. Using a set of rat mammary carcinoma cell lines whose growth is responsive (64-24 cells) and not responsive (22-1 cells) to ethanolamine, the biochemical mechanism of ethanolamine responsiveness was investigated. The biosynthesis and metabolism of phospholipid, particularly of those involving phosphatidylethanolamine, were thus compared between the two types of cells. The incorporation of [3H]serine into phosphatidylserine and phosphatidylethanolamine in 64-24 cells was 60 and 37%, respectively, of those in 22-1 cells. However, the activity of phosphatidylserine decarboxylase was virtually the same in these cell lines. When these cells were cultured in the presence of [32P]phosphatidylcholine and [32P]phosphatidylethanolamine, the rate of accumulation of 32P-labeled phosphatidylserine from the radioactive phosphatidylethanolamine was considerably reduced in 64-24 cells compared to that in 22-1 cells, although the rate of synthesis of phosphatidylserine and phosphatidylethanolamine from the radioactive phosphatidylcholine was similar between the two cell lines. The rate of labeling phosphatidylcholine from the radioactive phosphatidylethanolamine was also reduced in 64-24 cells, although the difference was not as great as that of phosphatidylserine. Incorporation of 32P into phosphatidylethanolamine was correlated with the concentration of ethanolamine in the culture medium in 64-24 cells, whereas in 22-1 cells the incorporation was not influenced by ethanolamine. Enzyme activities of the CDP-ethanolamine pathway were not significantly different between the two cell lines. The rate of degradation of phosphatidylethanolamine was also similar in these cell lines. These results show that ethanolamine responsiveness of 64-24 cells, and probably other epithelial cells, is due to a limited ability to synthesize phosphatidylserine resulting from a limited base-exchange activity utilizing phosphatidylethanolamine.

The membrane-associated enzyme phosphatidylserine synthase is regulated at the level of mRNA abundance.

To precisely define the functional sequence of the CHO1 gene from Saccharomyces cerevisiae, encoding the regulated membrane-associated enzyme phosphatidylserine synthase (PSS), we subcloned the original 4.5-kilobase (kb) CHO1 clone. In this report a 2.8-kb subclone was shown to complement the ethanolamine-choline auxotrophy and to repair the defect in the synthesis of phosphatidylserine, both of which are characteristic of cho1 mutants. When this subclone was used as a hybridization probe of Northern and slot blots of RNA from wild-type cells, the abundance of a 1.2-kb RNA changed in response to alterations in the levels of the soluble phospholipid precursors inositol and choline. The addition of inositol led to a 40% repression of the 1.2-kb RNA level, while the addition of choline and inositol led to an 85% repression. Choline alone had little repressive effect. The level of 1.2-kb RNA closely paralleled the level of PSS activity found in the same cells as determined by enzyme assays. Disruption of the CHO1 gene resulted in the simultaneous disappearance of 1.2-kb RNA and PSS activity. Cells bearing the ino2 or ino4 regulatory mutations, which exhibit constitutively repressed levels of a number of phospholipid biosynthetic enzymes, had constitutively repressed levels of 1.2-kb RNA and PSS activity. Another regulatory mutation, opi1, which causes the constitutive derepression of PSS and other phospholipid biosynthetic enzymes, caused the constitutive derepression of the 1.2-kb RNA. When cho1 mutant cells were transformed with the 2.8-kb subclone on a single-copy plasmid, the 1.2-kb RNA and PSS activity levels were regulated in a wild-type fashion. The presence of the 2.8-kb subclone on a multicopy plasmid, however, led to the constitutive overproduction of 1.2-kb RNA and PSS in cho1 cells.

The membrane-associated enzyme phosphatidylserine synthase is regulated at the level of mRNA abundance.

To precisely define the functional sequence of the CHO1 gene from Saccharomyces cerevisiae, encoding the regulated membrane-associated enzyme phosphatidylserine synthase (PSS), we subcloned the original 4.5-kilobase (kb) CHO1 clone. In this report a 2.8-kb subclone was shown to complement the ethanolamine-choline auxotrophy and to repair the defect in the synthesis of phosphatidylserine, both of which are characteristic of cho1 mutants. When this subclone was used as a hybridization probe of Northern and slot blots of RNA from wild-type cells, the abundance of a 1.2-kb RNA changed in response to alterations in the levels of the soluble phospholipid precursors inositol and choline. The addition of inositol led to a 40% repression of the 1.2-kb RNA level, while the addition of choline and inositol led to an 85% repression. Choline alone had little repressive effect. The level of 1.2-kb RNA closely paralleled the level of PSS activity found in the same cells as determined by enzyme assays. Disruption of the CHO1 gene resulted in the simultaneous disappearance of 1.2-kb RNA and PSS activity. Cells bearing the ino2 or ino4 regulatory mutations, which exhibit constitutively repressed levels of a number of phospholipid biosynthetic enzymes, had constitutively repressed levels of 1.2-kb RNA and PSS activity. Another regulatory mutation, opi1, which causes the constitutive derepression of PSS and other phospholipid biosynthetic enzymes, caused the constitutive derepression of the 1.2-kb RNA. When cho1 mutant cells were transformed with the 2.8-kb subclone on a single-copy plasmid, the 1.2-kb RNA and PSS activity levels were regulated in a wild-type fashion. The presence of the 2.8-kb subclone on a multicopy plasmid, however, led to the constitutive overproduction of 1.2-kb RNA and PSS in cho1 cells.

Effect of parathyroid hormone on phospholipid metabolism in osteoblast-like rat osteogenic sarcoma cells.

Previous results have shown that 1,25-dihydroxycholecalciferol [1,25(OH)2D3] enhances the synthesis of phosphatidylserine (PS) and suppresses the synthesis of phosphatidylethanolamine (PE) in osteoblast-like rat osteogenic sarcoma UMR 106 cells [Matsumoto, Kawanobe, Morita & Ogata (1985) J. Biol. Chem. 260, 13704-13709]. In the present study, the effect of parathyroid hormone (PTH) on phospholipid metabolism is examined by using these cells. Treatment of UMR 106 cells with human PTH-(1-34)-peptide suppresses the synthesis of phosphatidylethanolamine in a dose- and time-dependent manner without affecting the synthesis of PS. The maximal effect on PE synthesis is obtained with 2.4 nM-human PTH-(1-34)-peptide when the cells are treated for 48 h or longer. In addition, when human PTH-(1-34)-peptide is added together with the maximal dose of 1,25(OH)2D3, there is a further decline in PE synthesis, whereas the stimulation of PS synthesis by 1,25(OH)2D3 is not altered. Because methylation of PE is suggested to affect hormone receptor-adenylate cyclase coupling, the observed change in PE metabolism by PTH and 1,25(OH)2D3 may be, at least in part, involved in the development of desensitization phenomenon to PTH in these cells.

Metabolism of the phospholipid precursor inositol and its relationship to growth and viability in the natural auxotroph Schizosaccharomyces pombe.

Phospholipid metabolism in the fission yeast Schizosaccharomyces pombe was examined. Three enzymes of phospholipid biosynthesis, cytidine diphosphate diacylglycerol synthase (CDP-DG), phosphatidylinositol (PI) synthase, and phosphatidylserine (PS) synthase, were characterized in extracts of S. pombe cells. Contrary to an earlier report, we were able to demonstrate that CDP-DG served as a precursor for PI and PS biosynthesis in S. pombe. S. pombe is naturally auxotrophic for the phospholipid precursor inositol. We found that S. pombe was much more resistant to loss of viability during inositol starvation than artificially generated inositol auxotrophs of Saccharomyces cerevisiae. The phospholipid composition of S. pombe cells grown in inositol-rich medium (50 microM) was similar to that of S. cerevisiae cells grown under similar conditions. However, growth of S. pombe at low inositol concentrations (below 30 microM) affected the ratio of the anionic phospholipids PI and PS, while the relative proportions of other glycerophospholipids remained unchanged. During inositol starvation, the rate of PI synthesis decreased rapidly, and there was a concomitant increase in the rate of PS synthesis. Phosphatidic acid and CDP-DG, which are precursors to these phospholipids, also increased when PI synthesis was blocked by lack of exogenous inositol. The major product of turnover of inositol-containing phospholipids in S. pombe was found to be free inositol, which accumulated in the medium and could be reused by the cell.

Phosphatidylserine biosynthesis in cultured Chinese hamster ovary cells. I. Inhibition of de novo phosphatidylserine biosynthesis by exogenous phosphatidylserine and its efficient incorporation.

The effect of phosphatidylserine exogenously added to the medium on de novo biosynthesis of phosphatidylserine was investigated in cultured Chinese hamster ovary cells. When cells were cultured for several generations in medium supplemented with phosphatidylserine and 32Pi, the incorporation of 32Pi into cellular phosphatidylserine was remarkably inhibited, the degree of inhibition being dependent upon the concentration of added phosphatidylserine. 32Pi uptake into cellular phosphatidylethanolamine was also partly reduced by the addition of exogenous phosphatidylserine, consistent with the idea that phosphatidylethanolamine is biosynthesized via decarboxylation of phosphatidylserine. However, incorporation of 32Pi into phosphatidylcholine, sphingomyelin, and phosphatidylinositol was not significantly affected. In contrast, the addition of either phosphatidylcholine, sphingomyelin, phosphatidylethanolamine, or phosphatidylinositol to the medium did not inhibit endogenous biosynthesis of the corresponding phospholipid. Radiochemical and chemical analyses of the cellular phospholipid composition revealed that phosphatidylserine in cells grown with 80 microM phosphatidylserine was almost entirely derived from the added phospholipid. Phosphatidylserine uptake was also directly determined by using [3H]serine-labeled phospholipid. Pulse and pulse-chase experiments with L-[U-14C] serine showed that when cells were cultured with 80 microM phosphatidylserine, the rate of synthesis of phosphatidylserine was reduced 3-5-fold whereas the turnover of newly synthesized phosphatidylserine was normal. Enzyme assaying of extracts prepared from cells grown with and without phosphatidylserine indicated that the inhibition of de novo phosphatidylserine biosynthesis by the added phosphatidylserine appeared not to be caused by a reduction in the level of the enzyme involved in the base-exchange reaction between phospholipids and serine. These results demonstrate that exogenous phosphatidylserine can be efficiently incorporated into Chinese hamster ovary cells and utilized for membrane biogenesis, endogenous phosphatidylserine biosynthesis thereby being suppressed.

Rapid accumulation of diacyl lipid in rat liver microsomes by selective acylation of 2-acyl-sn-glycero-3-phosphorylserine

2-Acyl- sn-glycero-3-phosphoryl[U- 14C]serine (2-acylGPS) was prepared by digesting 1,2-dioleoyl-sn-glycero-3-phosphoryl[U- 14C]serine, mixed with pig brain phosphatidylserine (PS), with Rhizopus arrhizus lipase. The labeled lysolipid was shown to be actively acylated by rat liver microsomes in the presence of acyl-CoA thioesters, with the formation of diacylPS. In experiments with unlabeled lysolipid and [ 14C]acyl-CoA thioesters, digestion of the products with pancreatic phospholipase A 2 showed that the acyl groups had been incorporated almost exclusively at position-1. The acylation reactions were dependent on pH of the incubation medium, time of incubation, concentration of microsomal protein and of lysolipid and thioester substrates. With [ 14C]lysolipid there was a clear preference for long-chain saturated over unsaturated acyl group donors and highest acylation rates were recorded with stearoyl-CoA. Since stearate is the major fatty acid at position-1 of tissue PS, these findings suggest that deacylation-reacylation reactions may be of importance in regulating the fatty acid profile of this lipid. The observed rates of acylation of 2-acylGPS in vitro were high in relation to the endogenous level of PS in the microsomal membranes, and short-term incubations led to net synthesis of PS with a many-fold increase in its concentration. The question is raised whether this is a latent activity expressed under optimal assay conditions in vitro, conditions that may not obtain in a cell, or whether the high acylating potential is indicative of an important function for the addition, or turnover, of acyl groups at position-1 of the serine lipid.

Phosphatidylserine synthesis in adult dirofilaria immitis females

Phosphatidylserine synthesis in adult Dirofilria immitis females. International Journal for Parasitology 16: 9–11. Phosphatidylserine synthesis in extracts of Dirofilaria immitis females was found to occur exclusively by way of a calcium-stimulated enzyme-catalyzed exchange of l-serine for the base components of preformed phospholipids. Under specified conditions the rate of serine incorporation into phospholipid(s) was around 44 pmol min −1 mg −1 protein.