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J E Vance

Publications and source records attributed to J E Vance.

At least 55 records · Page 3Linked to original sources
PubMed1993

Assembly of rat hepatic very low density lipoproteins in the endoplasmic reticulum.

The intracellular site of assembly of hepatic very low density lipoproteins has been investigated. Two endoplasmic reticulum fractions and Golgi vesicles (relatively free from endosomal contamination) were isolated from rat liver and the luminal contents were released. The apoB-containing entities were separated from the lumen of the endoplasmic reticulum and Golgi vesicles by an immunoaffinity isolation procedure. The amount of each lipid moiety (triacylglycerols, cholesterol plus cholesteryl esters and phospholipids) associated with a unit mass of apoB was shown to be very similar in the luminal contents isolated from each of the three fractions. Moreover, the apoB-containing particles that were isolated from the endoplasmic reticulum and Golgi were mainly present in a fraction of density < 1.02 g/ml. The average diameter of these lipoprotein particles was shown by negative staining electron microscopy to be in the size range of very low density lipoproteins and low density lipoproteins. The size and composition of the intracellular lipoproteins were very similar to those of both very low density lipoproteins isolated from the culture medium from rat hepatocytes and plasma very low density lipoproteins. The conclusion from this study is that the full complement of lipids associate with apoB at an early stage during the secretory pathway, in the endoplasmic reticulum.

Animals↗
PubMed1992

Circulating activated T lymphocytes in autoeczematization.

BACKGROUND AND DESIGN: Autoeczematization, the symmetric diffuse spread of a previously localized dermatitis, has an unclear etiology although some investigators have postulated that activated T lymphocytes play a role. Two estimates of activation of peripheral T lymphocytes are the cell surface expression of the HLA-DR antigen and the interleukin 2 receptor (IL-2R). We measured the percentage of circulating activated T lymphocytes in nine patients with autoeczematization compared with normal controls (n = 10), patients with stasis dermatitis (n = 6), and patients with severe (n = 10) or mild (n = 8) psoriasis. The percentage of activated T lymphocytes was determined by fluorescent, activated cell sorter analysis of peripheral leukocytes doubly stained with antibodies to T lymphocytes and HLA-DR antigen or IL-2R. RESULTS: Statistically significant elevations of HLA-DR- and IL-2R--positive T lymphocytes were seen in autoeczematization patients (P less than .004 and P less than .04, respectively) and those with severe psoriasis (P less than .004 for HLA-DR antigen and IL-2R). Percentages of HLA-DR- and IL-2R--positive T lymphocytes in patients with mild psoriasis were not significantly elevated. Two patients with autoeczematization had a reduction of their previously elevated levels of HLA-DR- and IL-2R--positive T lymphocytes after treatment. CONCLUSIONS: These data suggest a possible role for circulating activated T lymphocytes in the pathogenesis of autoeczematization and possibly in severe psoriasis.

Antigens, Surface↗
PubMed1991

Brefeldin A does not inhibit the movement of phosphatidylethanolamine from its sites for synthesis to the cell surface.

Brefeldin A, a fungal metabolite which interrupts trafficking of proteins via the Golgi by causing disassembly of the Golgi stacks, has been used to investigate the mechanism of movement of phosphatidylethanolamine (PtdEtn) from its sites of synthesis to the cell surface. PtdEtn is made in hepatocytes by two major pathways, (a) from CDP-ethanolamine on the endoplasmic reticulum and (b) by decarboxylation of phosphatidylserine in mitochondria. Monolayer cultures of rat hepatocytes were incubated with radiolabeled precursors of PtdEtn ([3H]ethanolamine or [3H]serine) in the presence or absence of brefeldin A. The movement of newly made PtdEtn to the plasma membrane was studied by treatment of intact cells with trinitrobenzene sulfonate which reacted only with PtdEtn on the outside surface of the cells to produce N-trinitrophenyl-PtdEtn; PtdEtn in intracellular membranes remained underivatized by this reagent. Using this method, the incorporation of radioactivity into cell surface and intracellular PtdEtn could be differentiated. The studies showed that PtdEtn made by the two different biosynthetic routes was rapidly transported to the outside leaflet of the plasma membrane. However, the kinetics and the extent of labeling of the cell surface PtdEtn, relative to that of the intracellular PtdEtn pool, were different from each labeled precursor. The incorporation of [3H]ethanolamine into PtdEtn on the cell surface gradually increased to a constant level of 1.8% of the label of intracellular PtdEtn after 3 h. In contrast, after 0.5-1 h, cell surface PtdEtn labeled from [3H]serine comprised 4% of the intracellular PtdEtn pool; the extent of labeling gradually declined to a constant level of approximately 2.4% by 3 h. Brefeldin A did not interrupt the movement of PtdEtn, derived from either biosynthetic origin, to the cell surface even though protein secretion was greatly reduced. Thus, apparently PtdEtn and proteins are independently transported to the cell surface of hepatocytes.

Animals↗
PubMed1991

Calreticulin, and not calsequestrin, is the major calcium binding protein of smooth muscle sarcoplasmic reticulum and liver endoplasmic reticulum.

The distribution of calsequestrin and calreticulin in smooth muscle and non-muscle tissues was investigated. Immunoblots of endoplasmic reticulum proteins probed with anti-calreticulin and anti-calsequestrin antibodies revealed that only calreticulin is present in the rat liver endoplasmic reticulum. Membrane fractions isolated from uterine smooth muscle, which are enriched in sarcoplasmic reticulum, contain a protein band which is immunoreactive with anti-calreticulin but not with anti-calsequestrin antibodies. The presence of calreticulin in these membrane fractions was further confirmed by 45Ca2+ overlay and "Stains-All" techniques. Calreticulin was also localized to smooth muscle sarcoplasmic reticulum by the indirect immunofluorescence staining of smooth muscle cells with anti-calreticulin antibodies. Furthermore, both liver and uterine smooth muscle were found to contain high levels of mRNA encoding calreticulin, whereas no mRNA encoding calsequestrin was detected. We have employed an ammonium sulfate precipitation followed by Mono Q fast protein liquid chromatography, as a method by which calsequestrin and calreticulin can be isolated from whole tissue homogenates, and by which they can be clearly resolved from one another, even where present in the same tissue. Calreticulin was isolated from rabbit and bovine liver, rabbit brain, rabbit and porcine uterus, and bovine pancreas and was identified by its amino-terminal amino acid sequence. Calsequestrin cannot be detected in preparations from whole liver tissue, and only very small amounts of calsequestrin are detectable in ammonium sulfate extracts of uterine smooth muscle. We conclude that calreticulin, and not calsequestrin, is a major Ca2+ binding protein in liver endoplasmic reticulum and in uterine smooth muscle sarcoplasmic reticulum. Calsequestrin and calreticulin may perform parallel functions in the lumen of the sarcoplasmic and endoplasmic reticulum.

Amino Acid Sequence↗
PubMed1991

Newly made phosphatidylserine and phosphatidylethanolamine are preferentially translocated between rat liver mitochondria and endoplasmic reticulum.

The translocation of: (i) phosphatidylserine (PtdSer) from its site of synthesis on microsomal membranes to its site decarboxylation in mitochondrial membranes and (ii) phosphatidylethanolamine (PtdEtn) from the mitochondria to its site of methylation to phosphatidylcholine on microsomal membranes has been reconstituted in cell-free systems consisting of rat liver mitochondria and microsomes. Two types of systems have been reconstituted. In one, the translocation of newly made PtdSer or PtdEtn was examined by incubation of microsomes and mitochondria with [3-3H]serine. In the other, membranes were prelabeled with radioactive PtdSer or PtdEtn, and the transfer of these two lipids between mitochondria and microsomes was monitored. For the transfer of both PtdSer from microsomes to mitochondria and PtdEtn from mitochondria to microsomes, newly made phospholipids were translocated much more readily than pre-existing phospholipids. The data suggest that with respect to their translocation between these two organelles, the pools of newly synthesized PtdSer and PtdEtn were distinct from the pools of "older" phospholipids pre-existing in the membranes. Transfer of neither phospholipid in vitro depended on the presence of cytosolic proteins (i.e. soluble phospholipid transfer proteins) or on the hydrolysis of ATP, although there was some stimulation of PtdSer transfer by ATP and several other nucleoside mono-, di-, and triphosphates. The data are consistent with a collision-based mechanism in which the endoplasmic reticulum and mitochondria come into contact with one another, thereby effecting the transfer of phospholipids. The proposal that there is contact between the endoplasmic reticulum and mitochondria is supported by the recent isolation of a membrane fraction having many, but not all, of the properties of the endoplasmic reticulum, but which was isolated in association with mitochondria (Vance, J. E. (1990) J. Biol. Chem. 265, 7248-7256).

Animals↗
PubMed1991

Biosynthesis of membrane lipids in rat axons.

Compartmented cultures of sympathetic neurons from newborn rats were employed to test the hypothesis that the lipids required for maintenance and growth of axonal membranes must be synthesized in the cell body and transported to the axons. In compartmented cultures the distal axons grow into a compartment separate from that containing the cell bodies and proximal axons, in an environment free from other contaminating cells such as glial cells and fibroblasts. There is virtually no bulk flow of culture medium or small molecules between the cell body and axonal compartments. When [methyl-3H]choline was added to the cell body-containing compartment the biosynthesis of [3H]-labeled phosphatidylcholine and sphingomyelin occurred in that compartment, with a gradual transfer of lipids (less than 5% after 16 h) into the axonal compartment. Surprisingly, addition of [methyl-3H]choline to the compartment containing only the distal axons resulted in the rapid incorporation of label into phosphatidylcholine and sphingomyelin in that compartment. Little retrograde transport of labeled phosphatidylcholine and sphingomyelin (less than 15%) into the cell body compartment occurred. Moreover, there was minimal transport of the aqueous precursors of these phospholipids (e.g., choline, phosphocholine and CDP-choline) between cell compartments. Similarly, when [3H]ethanolamine was used as a phospholipid precursor, the biosynthesis of phosphatidylethanolamine occurred in the pure axons, and approximately 10% of the phosphatidylethanolamine was converted into phosphatidylcholine. Experiments with [35S]methionine demonstrated that proteins were made in the cell bodies, but not in the axons. We conclude that axons of rat sympathetic neurons have the capacity to synthesize membrane phospholipids. Thus, a significant fraction of the phospholipids supplied to the membrane during axonal growth may be synthesized locally within the growing axon.

Animals↗
PubMed1991

Role of intrarenal angiotensin II and alpha-adrenoceptors in renal vasoconstriction with acute hypoxemia and hypercapnic acidosis in conscious dogs.

To evaluate our previous observation of renal vasoconstriction during combined acute hypoxemia and hypercapnic acidosis preceded by acute hypoxemia, we studied 13 conscious mongrel uninephrectomized dogs with chronic renal catheters and controlled sodium intake (80 meq/day for 4 days). Five dogs were studied during combined acute hypoxemia (PaO2, 37 +/- 1 mm Hg) and hypercapnic acidosis (PaCO2, 59 +/- 1 mm Hg; pH 7.20 +/- 0.01). Each dog was studied during infusion of 1) the intrarenal vehicle (n = 5), 2) the intrarenal alpha 1-antagonist prazosin (0.2 micrograms.kg-1.min-1, n = 5), 3) intrarenal [Sar1,Ala8]angiotensin II (70 ng.kg-1.min-1, n = 5), and 4) intrarenal prazosin and [Sar1,Ala8]angiotensin II (n = 4). Immediate induction of combined hypoxemia and hypercapnic acidosis after control measurements during intrarenal vehicle infusion resulted in a decrease in effective renal plasma flow and glomerular filtration rate, increase in renal vascular resistance, and decrease in filtered sodium load in the first 20 minutes of the blood gas derangement. Intrarenal administration of [Sar1,Ala8]angiotensin II failed to reverse the effects of the combined blood gas derangement on renal function. In contrast, intrarenal prazosin administration either alone or in combination with [Sar1,Ala8]angiotensin II abrogated the increase in renal vascular resistance, decrease in glomerular filtration rate, and fall in filtered sodium load. These studies identify a major role for alpha 1-adrenoceptors in the renal vasoconstriction during combined hypoxemia and hypercapnic acidosis.

Acidosis↗
PubMed1991

Secretion of VLDL, but not HDL, by rat hepatocytes is inhibited by the ethanolamine analogue N-monomethylethanolamine.

The role of phospholipids in the assembly and secretion of very low density lipoproteins (VLDL) has been investigated by incubation of monolayer cultures of rat hepatocytes with monomethylethanolamine, an analogue of ethanolamine and choline. The cellular concentration of phosphatidylmonomethylethanolamine was increased 17-fold in response to treatment of hepatocytes with monomethylethanolamine. The secretion of phosphatidylcholine, triacylglycerol, and the apolipoproteins BH, BL, and E into VLDL was inhibited by approximately 50% in hepatocytes incubated with monomethylethanolamine, compared to untreated cells. Cell viability was unaffected by treatment with the ethanolamine analogue, as was cellular protein synthesis. The mechanism by which monomethylethanolamine reduced VLDL secretion was examined. Since monomethylethanolamine is a structural analogue of ethanolamine and choline, an obvious hypothesis for explanation of the effect on VLDL secretion was that phosphatidylcholine biosynthesis, which is required for VLDL secretion (Z. Yao and D. E. Vance. 1988. J. Biol. Chem. 263: 2998-3004) was inhibited. However, the biosynthesis of phosphatidylcholine from [3H]choline or from [3H]glycerol was not significantly reduced in the analogue-treated, compared with the untreated, hepatocytes. Nor was the incorporation of [3H]glycerol into cellular triacylglycerol altered in the monomethylethanolamine-treated cells. Furthermore, addition of monomethylethanolamine to hepatocytes did not reduce the rate of biosynthesis of phosphatidylethanolamine either from CDP-ethanolamine or from phosphatidylserine, nor was phosphatidylserine biosynthesis from [3-3H]serine affected. The 50% inhibition of VLDL secretion elicited by monomethylethanolamine was apparently specific for VLDL because there was no difference in secretion of HDL (lipid or apoprotein moieties) or albumin by cells incubated with or without the ethanolamine analogue. The experiments showed that inhibition of VLDL secretion by monomethylethanolamine was not the result of decreased biosynthesis of phospholipids, triacylglycerols, or cholesteryl esters. More subtle effects of the ethanolamine/choline analogue, for example interference by the increased amount of phosphatidylmonomethylethanolamine, in the process of assembly of lipids with apoB remain a possibility.

Animals↗
PubMed1990

Stimulation of apolipoprotein secretion in very-low-density and high-density lipoproteins from cultured rat hepatocytes by dexamethasone.

The effects of dexamethasone (a synthetic glucocorticoid) and insulin on the secretion of very-low-density lipoprotein (VLDL) and high-density lipoprotein (HDL) were investigated. Rat hepatocytes in monolayer culture were preincubated for 15 h in the presence or absence of combinations of 100 nM-dexamethasone and 2 nM-, 10 nM- or 50 nM-insulin. Dexamethasone increased [3H]oleate incorporation into secreted triacylglycerol by 2.7-fold and the mass of triacylglycerol secreted by 1.5-fold. Insulin alone decreased these parameters and antagonized the effect of dexamethasone. Dexamethasone increased the secretion of [3H]leucine in apolipoprotein (apo) E, and in the large (BH) and small (BI) forms of apo B in VLDL by about 7.1-, 3.6- and 4.0-fold respectively. Insulin alone decreased the secretion of these 3H-labelled apolipoproteins in VLDL. However, 2 nM-insulin with dexamethasone increased the secretion of 3H-labelled apo BH and apo BL by a further 0.8- and 3.2-fold respectively; 50 nM-insulin decreased the secretions of apo E, apo BH and apo BL in VLDL. Similar effects for dexamethasone or insulin alone were also obtained for the masses of apo E and apo BL + H secreted in VLDL. Albumin secretion was not significantly altered by either dexamethasone or insulin alone, but in combination they stimulated by 2.1-2.6-fold. Insulin or dexamethasone alone had little effect on the secretion of apolipoproteins in the HDL fraction. However, dexamethasone plus 2 nM-insulin increased the incorporation of [3H]leucine into apo AI, apo AH plus apo C, apo AIV and apo E of HDL by about 1.8-, 1.6-, 1.7- and 2.0-fold respectively. The apo E in the bottom fraction represented about 69% of the total 3H-labelled apo E secreted. The responses in the total secretion of apo E from the hepatocytes resembled those seen in HDL. The interactions of insulin and dexamethasone are discussed in relation to the general regulation of lipoprotein metabolism, the development of hyperlipidaemias and the predisposition to premature atherosclerosis.

Albumins↗
PubMed1990

Lipoproteins secreted by cultured rat hepatocytes contain the antioxidant 1-alk-1-enyl-2-acylglycerophosphoethanolamine.

Monolayer cultures of rat hepatocytes have been examined for their ability to secrete ethanolamine plasmalogen as a component of nascent lipoproteins. In culture medium from these cells, ethanolamine plasmalogen comprises approx. 20-30% of total ethanolamine glycerophospholipids when measured either as phospholipid mass or by the incorporation of [1-3H]ethanolamine. An approximately equal distribution of the plasmalogen was found throughout all lipoprotein density fractions. The content of plasmalogen in whole rat serum, was 36% of total ethanolamine glycerophospholipids. In contrast, in rat liver and cultured hepatocytes the amount of ethanolamine plasmalogen was 5-fold lower than in serum or culture medium (approx. 5% of total ethanolamine phospholipids). Normal human plasma also contains ethanolamine plasmalogen in relatively large amounts (approx. 50% of total ethanolamine phospholipids). Thus, a major function of plasmalogen biosynthetic enzymes in liver may be the provision of ethanolamine plasmalogen for secretion into lipoproteins. Previous studies (e.g., Zoeller, R.A. et al. (1988) J. Biol. Chem. 263, 11590-11596) have suggested that ethanolamine plasmalogen may function as an antioxidant for the protection of lipid and protein membrane components against oxidation. Oxidized, but not native, low-density lipoprotein is rapidly taken up by macrophages with the formation of foam cells characteristic of atherosclerotic lesions (Steinbrecher, U.P. et al. (1984) Proc. Natl. Acad. Sci. USA 81, 3883-3887). Thus, the presence of plasmalogen as part of newly secreted lipoprotein particles may prevent their oxidation and subsequent uptake by macrophages.

Animals↗
PubMed1990

The assembly of lipids into lipoproteins during secretion.

The process of assembly and secretion of lipoproteins is discussed with particular reference to the role of lipids. The majority of circulating lipoproteins is produced by the liver (80%) with the remainder being supplied by the intestine. The liver secretes both very low density lipoproteins and high density lipoproteins, but the assembly and secretion of these two types of particles may follow different routes. The major lipid components of lipoproteins are triacylglycerols, cholesterol, cholesterol esters and phospholipids. The biosynthesis of these lipids occurs on membranes of the endoplasmic reticulum, with many of the enzymes also being present in the Golgi; the roles of these two subcellular organelles in the assembly of lipoproteins are discussed. There appears to be a compartmentalization of lipids in cells, such that defined pools, often those newly-synthesized, are preferred, or even required, for lipoprotein assembly. The process of hepatic very low density lipoprotein secretion appears to be regulated by the supply of lipids. Indeed, the synthesis of new lipid may be a major driving force in lipoprotein assembly and secretion.

Animals↗
PubMed1990

Phospholipid synthesis in a membrane fraction associated with mitochondria.

A crude rat liver mitochondrial fraction that was capable of the rapid, linked synthesis of phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) labeled from [3-3H] serine has been fractionated. PtdSer synthase, PtdEtn methyltransferase, and CDP-choline:diacylglycerol cholinephosphotransferase activities were present in the crude mitochondrial preparation but were absent from highly purified mitochondria and could be attributed to the presence of a membrane fraction, X. Thus, previous claims of the mitochondrial location of some of these enzymes might be explained by the presence of fraction X in the mitochondrial preparation. Fraction X had many similarities to microsomes except that it sedimented with mitochondria (at 10,000 x g). However, the specific activities of PtdSer synthase and glucose-6-phosphate phosphatase in fraction X were almost twice that of microsomes, and the specific activities of CTP:phosphocholine cytidylyltransferase and NADPH:cytochrome c reductase in fraction X were much lower than in microsomes. The marker enzymes for mitochondria, Golgi apparatus, plasma membrane, lysosomes, and peroxisomes all had low activities in fraction X. Polyacrylamide gel electrophoresis revealed distinct differences, as well as similarities, among the proteins of fraction X, microsomes, and rough and smooth endoplasmic reticulum. The combined mitochondria-fraction X membranes can synthesize PtdSer, PtdEtn, and PtdCho from serine. Thus, fraction X in combination with mitochondria might be responsible for the observed compartmentalization of a serine-labeled pool of phospholipids previously identified (Vance, J. E., and Vance, D. E. (1986) J. Biol. Chem. 261, 4486-4491) and might be involved in the transfer of lipids between the endoplasmic reticulum and mitochondria.

Animals↗
PubMed1990

Hypersecretion of VLDL, but not HDL, by hepatocytes from the JCR:LA-corpulent rat.

The JCR:LA-corpulent male rat, when homozygous for the cp gene (cp/cp) is hyperlipidemic and prone to atherosclerosis. Both male and female cp/cp rats have markedly elevated serum levels of triacylglycerols and phospholipids [Dolphin, P.J. et al. 1987. Biochim. Biophys. Acta. 919: 140-148]. In the present study, monolayer cultures of hepatocytes were prepared from male and female, corpulent and lean, rats. There was a marked hypersecretion of all very low density lipoprotein (VLDL) lipid and apoprotein components from corpulent-derived cells. The increased secretion most likely accounts for the increased levels of VLDL lipids and apoproteins previously observed in serum. In contrast, there was no difference between the corpulent and lean hepatocytes in their secretion of high density lipoprotein (HDL) lipids and apoproteins. The difference in triacylglycerol secretion between the lean and corpulent cells was sustained even when the cells were cultured for 24, 48, and 72 h prior to the experiment, by which time the hormonal differences between the corpulent and lean animals would have been largely eliminated. The magnitude of the difference in triacyglycerol secretion did not diminish with increasing time in culture. The biochemical basis responsible for the hypersecretion of VLDL has not yet been established. However, preliminary results suggest that there is an inherent difference in glycerolipid metabolism in the two types of hepatocytes.

Animals↗
PubMed1989

The use of newly synthesized phospholipids for assembly into secreted hepatic lipoproteins.

We have recently shown (Vance, J.E. (1988) Biochim. Biophys. Acta 963, 70-81) that the percent distribution of molecular species of phosphatidylcholine (PC) derived from [methyl-3H]choline and [3-3H]serine, and phosphatidylethanolamine (PE) derived from [3-3H]serine were different in secreted lipoproteins and in the cultured hepatocytes from which the lipoproteins were produced. The species 1-stearoyl-2-arachidonoyl PC and PE were selectively not secreted. How this selection occurs is not known. One possible explanation is that secreted phospholipids are representative of the newly synthesized pool, whereas the molecular species composition of bulk cellular phospholipids has been altered by selective deacylation or by deacylation-reacylation. This hypothesis has been tested. The percent distribution of radioactivity from [1-3H]ethanolamine, [3-3H]serine and [methyl-3H]choline in nascent cellular and secreted PE and PC molecular species was examined by high-performance liquid chromatography. From [3H]serine labeling, the percent distribution of [3H]PE species in the medium after 4 h resembled closely that in cells 0.5 h, but not 4 h, after labeling. Thus, nascent phosphatidylserine-derived PE was immediately earmarked for secretion before remodeling occurred. Similarly, newly made rather than 'old' PE and PC from alternative biosynthetic sources may be preferred for assembly into lipoproteins. In addition, PE methyltransferase apparently preferred newly made, rather than remodelled, serine-derived PE for methylation to PC. In no instance (i.e., neither for any phospholipid nor any precursor) was there evidence that 'old' rather than 'new' phospholipid was specifically selected for secretion.

Animals↗
PubMed1988

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 phospholipids with the endogenous phospholipid pool, nor a rapid mixing of phospholipids derived from different biosynthetic origins.

Animals↗
PubMed1988

Compartmentalization of phospholipids for lipoprotein assembly on the basis of molecular species and biosynthetic origin.

Specific pools of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are compartmentalized on the basis of their biosynthetic origin for assembly into lipoproteins by rat hepatocytes in culture (Vance, J.E. and Vance, D.E. (1986) J. Biol. Chem. 261, 4486-4491). The present experiments confirm that PC and PE derived from serine, rather than from ethanolamine, are preferred for assembly into lipoproteins. One possible explanation for this compartmentalization is that defined molecular species of phospholipids are selected for secretion. Thus, the molecular species distribution of PC and PE of cultured rat hepatocytes incubated with one of [3H]choline, [3H]ethanolamine or [3H]serine was compared in the cells and in the lipoproteins secreted into the culture medium using high-performance liquid chromatography. The percent distribution of molecular species of PC and PE labeled from ethanolamine was the same in the cells and the medium. On the contrary, the percent distribution of molecular species of PC derived from [3H]choline, and PC and PE derived from [3H]serine, was different in the cells and secreted lipoproteins; the species 1-stearoyl-2-arachidonyl-PC and -PE were selectively not secreted. However, selection of defined molecular species of cellular PC labeled from serine would only account for the specific radioactivity of secreted PC labeled from serine being 46% higher than in the hepatocytes at 4 h, whereas the specific radioactivity of medium PC was actually 300% higher than in the cells. A comparison of the labeling pattern of molecular species of PC and PE using [3H]serine and [3H]ethanolamine as precursors suggests that PE methyltransferase may prefer certain species of PE as substrate if the PE was derived from either serine or ethanolamine.

Animals↗