Phospholipid metabolism of stimulated lymphocytes. Preferential incorporation of polyunsaturated fatty acids into plasma membrane phospholipid upon stimulation with concanavalin A

Rabbit thymocytes were isolated and incubated for various lengths of time with concanavalin A. The cultures were pulsed for the last 12.5 min of incubation with equimolar mixtures of radioactively labelled fatty acids, either [3H]arachidonate and [14C]oleate or [3H]arachidonate and [14C]palmitate, and the uptake of each fatty acid into phospholipid of plasma membrane was determined. Upon binding of the mitogen, the fatty acids were incorporated at an increased rate with a new steady state being reached between 12.5 and 42.5 min after stimulation. Initially after 12.5 min, when the two fatty acids were added together, no preferential incorporation of the polyunsaturated fatty acid arachidonate was seen compared to the saturated or monounsaturated ones, palmitate or oleate. However shortly thereafter arachidonate, when compared to palmitate or oleate, started to be preferentially incorporated into plasma membrane phospholipid so that by 4 h after activation, only arachidonate was incorporated at an increased rate: the uptake of palmitate and oleate had reverted to that of unstimulated cells. In contrast, when palmitate or oleate were added alone, after 4 h of activation incorporation was increased similar to that of arachidonate, suggesting that all long chain fatty acids compete for the same activated enzyme(s). A detailed analysis of incorporation into phospholipid species showed that all fatty acids were taken up with the highest rate into phosphatidylcholine. After activation, fatty acid incorporation was increased by approx. 50% for phosphatidylcholine: the highest stimulation rates were observed with phosphatidylinositol (3-7-fold) and phosphatidylethanolamine (2-3-fold). The data suggest that shortly after stimulation with mitogens, the membrane phospholipids start to change by replacing saturated and monounsaturated fatty acids by polyunsaturated ones, thus creating a new membrane.     

Phosphatidyglycerol in rat lung. II. Comparison of occurrence, composition, and metabolism in surfactant and residual lung fractions.

A comparison of the occurrence, fatty acid composition, and metabolism of phosphatidyglycerol and phosphatidylcholine in the surfactant and residual fraction of rat lung has been carried out. The surfactant and residual fractions were separated by discontinuous sucrose density gradient centrifugation. The surfactant fraction was found to contain 69 percent phosphatidylcholine and 7 percent phosphatidylglycerol. The residual fraction contained 46 percent phosphatidylcholine and 3 percent phosphatidylglycerol. Phosphatidylcholine and phosphatidylglycerol were found to contain 85 and 79 percent palmitate in the surfactant fraction and 67 and 68 percent in the residual fraction, respectively. Isolated rat lungs were perfused with medium containing [U-14C]glucose, [9,10-3H]palmitate, and [1-14C]acetate and the incorporation into palmitate isolated from the alpha and beta position of phosphatidylcholine and phosphatidylglycerol was determined. Each radioactive substrate was found to be incorporated into palmitate of phosphatidylcholine equally at the alpha and beta position of the surfactant fraction. In the residual fraction the specific activity of the beta position palmitate was found to be twice that of the alpha position. The incorporation of [9,10-3H]palmitate and [1-14C]acetate into palmitate at the alpha and beta positions of phosphatidylglycerol was similar in both the surfactant and residual fractions. In each case palmitate at the alpha position had approximately twice the specific activity of that at the beta position. The incorporation of [U-14C]glucose into phosphatidylglycerol of the surfactant fraction was, however, greater in palmitate at the beta position than at the alpha. The results show that phosphatidylglycerol is associated with the lung surfactant fraction and suggest that palmitate esterified to the alpha and beta positions of phosphatidylglycerol and phosphatidylcholine occurs at different rates and is dependent upon the precursor source of palmitate.     

Synthesis of saturated phosphatidylcholine and phosphatidylglycerol by freshly isolated rat alveolar type II cells

Saturated phosphatidylcholine and phosphatidylglycerol are important components of pulmonary surface active material, but the relative contributions of different pathways for the synthesis of these two classes of phospholipids by alveolar type II cells are not established. We purified freshly isolated rat type II cells by centrifugal elutriation and incubated them with [1-14C]palmitate as the sole exogenous fatty acid in one series of experiments or with [9,10-3H]palmitate, mixed fatty acids (16:0, 18:1 and 18:2), and [U-14C]glucose in another series of experiments. Type II cells readily incorporated [1-14C]palmitate into saturated phosphatidic acid (55-59% of total phosphatidic acid), saturated diacylglycerol (82-87% of total diacylglycerol), saturated phosphatidylcholine (69-76% of total phosphatidylcholine), and saturated phosphatidylglycerol (55-59% of total phosphatidylglycerol). Saturated phosphatidic acid, diacylglycerol and phosphatidylglycerol were nearly equally labeled in the sn-1 and sn-2 positions, whereas saturated phosphatidylcholine was preferentially labeled in the sn-2 position. With [9,10-3H]palmitate and [U-14C]glucose, the labeling patterns of phosphatidic acid, diacylglycerol and phosphatidylglycerol were similar to each other but different from that of phosphatidylcholine. The glucose label was found predominantly in the unsaturated phosphatidylcholines at early times (3-10 min) and in the saturated phosphatidylcholines at later times (30-90 min). Similarly, the 3H/14C ratio was very high in saturated phosphatidylcholine and always above that in saturated diacylglycerol. We conclude that freshly isolated type II cells synthesize saturated phosphatidic acid, diacylglycerol, phosphatidylcholine and phosphatidylglycerol and that under our in vitro conditions the deacylation-reacylation pathway is important for the synthesis of saturated phosphatidylcholine but is less important for the synthesis of saturated phosphatidylglycerol. By the assumptions stated in the text during the pulse chase experiment de novo synthesis of saturated phosphatidylcholine from saturated diacylglycerol accounted for 25% of the total synthesis of saturated phosphatidylcholine.     

Phospholipid metabolism of stimulated lymphocytes: Preferential incorporation of polyunsaturated fatty acids into plasma membrane phospholipid upon stimulation with concanavalin A

Rabbit thymocytes were isolated and incubated for various lengths of time with concanavalin A. The cultures were pulsed for the last 12.5 min of incubation with equimolar mixtures of radioactively labelled fatty acids, either [³H]arachidonate and [¹⁴C]oleate or [³H]arachidonate and [¹⁴C]palmitate, and the uptake of each fatty acid into phospholipid of plasma membrane was determined. Upon binding of the mitogen, the fatty acids were incorporated at an increased rate with a new steady state being reached between 12.5 and 42.5 min after stimulation. Initially after 12.5 min, when the two fatty acids were added together, no preferential incorporation of the polyunsaturated fatty acid arachidonate was seen compared to the saturated or monounsaturated ones, palmitate or oleate. However shortly thereafter arachidonate, when compared to palmitate or oleate, started to be preferentially incorporated into plasma membrane phospholipid so that by 4 h after activation, only arachidonate was incorporated at an increased rate: the uptake of palmitate and oleate had reverted to that of unstimulated cells. In contrast, when palmitate or oleate were added alone, after 4 h of activation incorporation was increased similar to that of arachidonate, suggesting that all long chain fatty acids compete for the same activated enzyme(s). A detailed analysis of incorporation into phospholipid species showed that all fatty acids were taken up with the highest rate into phosphatidylcholine. After activation, fatty acid incorporation was increased by approx. 50% for phosphatidylcholine: the highest stimulation rates were observed with phosphatidylinositol (3–7-fold) and phosphatidylethanolamine (2–3-fold). The data suggest that shortly after stimulation with mitogens, the membrane phospholipids start to change by replacing saturated and monounsaturated fatty acids by polyunsaturated ones, thus creating a new membrane.     

Proteolipid formation in Mycoplasma capricolum. Influence of cholesterol on unsaturated fatty acid acylation of membrane proteins

Mycoplasma capricolum, a procaryotic sterol and fatty acid auxotroph was grown on media supplemented with [3H]palmitate or [3H]oleate. The isolated bacterial membranes were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Of the more than 50 membrane polypeptides revealed by Coomassie blue staining, approximately 25 were labeled with [3H]palmitate and only about 6 were labeled with [3H]oleate. Exhaustive delipidation of the membranes with chloroform:methanol did not alter the labeling pattern. Treatment of delipidated membranes by mild alkaline hydrolysis released up to 71% of the [3H]palmitate and 93% of the [3H]oleate. The data suggest that numerous membrane proteins of M. capricolum are covalently modified by acylation with saturated and unsaturated fatty acids. Cerulenin, a specific inhibitor of fatty acid synthesis had no effect on the labeling of mycoplasma membrane proteins by either [3H]palmitate or [3H]oleate. A small amount of membrane-associated cholesterol previously shown to stimulate sequentially the synthesis of unsaturated phospholipid, RNA, and protein (Dahl, J. S., and Dahl, C. E. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 692-696) specifically enhances the acylation of certain proteolipids by oleate but not by palmitate.     

Phosphatidylglycerol synthesis in spinach chloroplasts: characterization of the newly synthesized molecule

Intact chloroplasts from spinach (Spinacia oleracea L., hybrid 424) readily incorporate [¹⁴C]glycerol-3-phosphate and [¹⁴C]acetate into diacylglycerol, monoacylglycerol, diacylglycrol, free fatty acids (only when acetate is the precursor), phosphatidic acid, phosphatidylcholine, and most notably phosphatidylglycerol. The fraction of phosphatidylglycerol synthesized is greatly increased by the presence of manganese chloride in the reaction mixture. Glycerol-3-phosphate-labeled phosphatidylglycerol is equally labeled in the two glycerol moieties of the molecule. Acetate-labeled phosphatidylglycerol is equally labeled in both acyl groups. Position one contains primarily oleate, linoleate and small amounts of palmitate. Position two contains primarily palmitate. No radioactive trans-Δ³-hexadecenoate was detected. The labeling patterns indicate that the radioactive phosphatidylglycerol is the product of de novo chloroplast lipid biosynthesis and furthermore, phosphatidylglycerol may be a substrate for fatty acid desaturation.     

Lipids and lipid metabolism in the brown alga, Fucus serratus

The lipids of the brown alga Fucus serratus were isolated, identified and quantified. The major acyl lipids were the three glycosylglycerides, diacylgalactosylglycerol, diacyldigalactosylglycerol and diacylsulphoquinovosylglycerol. These represent over 70% of the total acyl lipids. The fatty acid compositions of the major lipids were examined and most showed rather distinctive fatty acid contents. For example, diacylgalactosylglycerol was enriched in n-3 polyunsaturated fatty acids while phosphatidylcholine and phosphatidylethanolamine had very high levels of arachidonate. Phosphatidylglycerol contained the unusual trans-Δ3-hexadecenoic acid. The labelling of lipids and fatty acids from [¹⁴C]acetate was examined and the distribution of label between individual components as a function of the incubation period and in algae collected at different times of the year is reported. Algae collected in the winter incorporated much more radioactivity into non-esterified fatty acids when compared to algae collected in the summer. All algae could label myristate, palmitate, stearate and oleate at high rates. Longer incubation times allowed the labelling of polyunsaturated fatty acids such as linoleic acid.     

Incorporation of acyl moieties of phospholipids into murein lipoprotein in intact cells of Escherichia coli by phospholipid vesicle fusion.

The biosynthesis of the acyl moieties in murein lipoprotein was studied by fusion of [3H]palmitate-labeled phospholipid vesicles with intact cells of an fadD mutant of Escherichia coli. A linear increase in the incorporation of [3H]palmitate radioactivity into both the ester- and amide-linked fatty acids in lipoprotein was observed during a 3-h chase after the fusion. Addition of chloramphenicol completely prevented the incorporation of [3H]palmitate from phospholipids to lipoprotein. These results strongly support our hypothesis that the acyl moieties in phospholipids are the precursors for the fatty acids in murein lipoprotein of E. coli. Among the major glycerophosphatides in E. coli, no specificity was observed regarding the efficacy of the donor.     

The kinetics of incorporation in vivo of [14C]acetate and [14C]carbon dioxide into the fatty acids of glycerolipids in developing leaves

1. The patterns of incorporation of (14)C into glycerolipid fatty acids of developing maize leaf lamina from supplied [1-(14)C]acetate and from (14)CO(2) during steady-state photosynthesis were similar. Oleate of phosphatidylcholine and palmitate of phosphatidylglycerol attained linear rates of labelling more rapidly than did other fatty acids, particularly the linoleate and linolenate of monogalactosyl diacylglycerol. 2. After the transfer of lamina from labelled to unlabelled acetate, there was a decrease in labelled oleate and linoleate of phosphatidylcholine and a concomitant increase in the amount of radioactivity in the linoleate and linolenate of monogalactosyl diacylglycerol. 3. The rapidly labelled phospholipids, phosphatidylcholine and phosphatidylglycerol, were shown by differential and sucrose-density-gradient centrifugation to be associated with different organelles, the former being mainly in a low-density membrane fraction, probably microsomal, and the latter mainly in chloroplasts. 4. During a 48h period after supplying spinach leaves with [(14)C]acetate, radioactivity was lost from the oleate of phosphatidylcholine present in fractions sedimented at 12000g and 105000g, and accumulated in the linolenate of monogalactosyl diacylglycerol of the chloroplast. 5. It is proposed that the phosphatidylcholine of some non-plastid membranes is intimately involved in the process of oleate desaturation and that this lipid serves as a donor of unsaturated C(18) fatty acids to other lipids, principally monogalactosyl diacylglycerol, of the chloroplasts.     

Lipoproteins of Haemophilus influenzae type b.

Haemophilus influenzae type b Minn A produced 12 lipoproteins with apparent molecular weights of between 14,000 and 67,000. The lipoproteins were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography of delipidated extracts of cells grown in [3H]palmitate. When the delipidated cell extracts were subjected to acid methanolysis, tritium was quantitatively recovered as palmitate and methyl palmitate, indicating that the [3H]palmitate had not been degraded and reincorporated into nonlipid material during cell growth. One of the lipoproteins comigrated with outer membrane protein (OMP) P6. OMP P6 was purified from [3H]palmitate-labeled cells. The purified protein preparation contained both amide- and ester-linked fatty acids. We conclude that (i) H. influenzae type b produces several lipoproteins, and (ii) one of these lipoproteins is OMP P6, a protein under consideration as a vaccine component.     

Activation of human monocytic cells by Borrelia burgdorferi and Treponema pallidum is facilitated by CD14 and correlates with surface exposure of spirochetal lipoproteins.

Here we examined the involvement of CD14 in monocyte activation by motile Borrelia burgdorferi and Treponema pallidum. B. burgdorferi induced secretion of IL-8 by vitamin D3-matured THP-1 cells, which was inhibited by a CD14-specific mAb known to block cellular activation by LPS and the prototypic spirochetal lipoprotein, outer surface protein A. Enhanced responsiveness to B. burgdorferi also was observed when THP-1 cells were transfected with CD14. Because borreliae within the mammalian host and in vitro-cultivated organisms express different lipoproteins, experiments also were performed with "host-adapted" spirochetes grown within dialysis membrane chambers implanted into the peritoneal cavities of rabbits. Stimulation of THP-1 cells by host-adapted organisms was CD14 dependent and, interestingly, was actually greater than that observed with in vitro-cultivated organisms grown at either 34 degrees C or following temperature shift from 23 degrees C to 37 degrees C. Consistent with previous findings that transfection of Chinese hamster ovary cells with CD14 confers responsiveness to LPS but not to outer surface protein A, B. burgdorferi failed to stimulate CD14-transfected Chinese hamster ovary cells. T. pallidum also activated THP-1 cells in a CD14-dependent manner, although its stimulatory capacity was markedly less than that of B. burgdorferi. Moreover, cell activation by motile T. pallidum was considerably less than that induced by treponemal sonicates. Taken together, these findings support the notion that lipoproteins are the principle component of intact spirochetes responsible for monocyte activation, and they indicate that surface exposure of lipoproteins is an important determinant of a spirochetal pathogen's proinflammatory capacity.     

Role of carnitine and carnitine palmitoyltransferase as integral components of the pathway for membrane phospholipid fatty acid turnover in intact human erythrocytes.

The deacylation and reacylation process of phospholipids is the major pathway of turnover and repair in erythrocyte membranes. In this paper, we have investigated the role of carnitine palmitoyltransferase in erythrocyte membrane phospholipid fatty acid turnover. The role of acyl-L-carnitine as a reservoir of activated acyl groups, the buffer function of carnitine, and the importance of the acyl-CoA/free CoA ratio in the reacylation process of erythrocyte membrane phospholipids have also been addressed. In intact erythrocytes, the incorporation of [1-14C]palmitic acid into acyl-L-carnitine, phosphatidylcholine, and phosphatidylethanolamine was linear with time for at least 3 h. The greatest proportion of the radioactivity was found in acyl-L-carnitine. Competition experiments using [1-14C]palmitic and [9,10-3H]oleic acid demonstrated that [9,10-3H]oleic acid was incorporated preferentially into the phospholipids and less into acyl-L-carnitine. When an erythrocyte suspension was incubated with [1-14C]palmitoyl-L-carnitine, radiolabeled palmitate was recovered in the phospholipid fraction, and the carnitine palmitoyltransferase inhibitor, 2-tetradecylglycidic acid, completely abolished the incorporation. ATP depletion decreased incorporation of [1-14C]palmitic and/or [9,10-3H]oleic acid into acyl-L-carnitine, but the incorporation into phosphatidylcholine and phosphatidylethanolamine was unaffected. In contrast, ATP depletion enhanced the incorporation into phosphatidylcholine and phosphatidylethanolamine of the radiolabeled fatty acid from [1-14C]palmitoyl-L-carnitine. These data are suggestive of the existence of an acyl-L-carnitine pool, in equilibrium with the acyl-CoA pool, which serves as a reservoir of activated acyl groups. The carnitine palmitoyltransferase inhibition by 2-tetradecylglycidic acid or palmitoyl-D-carnitine caused a significant reduction of radiolabeled fatty acid incorporation into membrane phospholipids, only when intact erythrocytes were incubated with [9,10-3H]oleic acid. These latter data may be explained by the differences in rates and substrates specificities between acyl-CoA synthetase and the reacylating enzymes for palmitate and oleate, which support the importance of carnitine palmitoyltransferase in modulating the optimal acyl-CoA/free CoA ratio for the physiological expression of the membrane phospholipids fatty acid turnover.     

Fatty acid metabolism in sn-glycerol-3-phosphate acyltransferase (plsB) mutants.

Fatty acid metabolism was examined in Escherichia coli plsB mutants that were conditionally defective in sn-glycerol-3-phosphate acyltransferase activity. The fatty acids synthesized when acyl transfer to glycerol-3-phosphate was inhibited were preferentially transferred to phosphatidylglycerol. A comparison of the ratio of phospholipid species labeled with 32Pi and [3H]acetate in the presence and absence of glycerol-3-phosphate indicated that [3H]acetate incorporation into phosphatidylglycerol was due to fatty acid turnover. A significant contraction of the acetyl coenzyme A pool after glycerol-3-phosphate starvation of the plsB mutant precluded the quantitative assessment of the rate of phosphatidylglycerol fatty acid labeling. Fatty acid chain length in membrane phospholipids increased as the concentration of the glycerol-3-phosphate growth supplement decreased, and after the abrupt cessation of phospholipid biosynthesis abnormally long chain fatty acids were excreted into the growth medium. These data suggest that the acyl moieties of phosphatidylglycerol are metabolically active, and that competition between fatty acid elongation and acyl transfer is an important determinant of the acyl chain length in membrane phospholipids.     

Fatty acids reverse the cyclic AMP inhibition of triacylglycerol and phosphatidylcholine synthesis in rat hepatocytes.

The influence of cyclic AMP analogues and fatty acids on glycerolipid biosynthesis in monolayer cultures of rat hepatocytes was investigated. Chlorophenylthio-cyclic AMP and adenosine 3':5'-cyclic phosphorothioate inhibited the rate of triacylglycerol synthesis from [1(3)-3H]glycerol, and phosphatidylcholine synthesis from [Me-3H]-choline. Supplementation of the hepatocytes with palmitate (1 mM) reversed chlorophenylthio-cyclic AMP inhibition of triacylglycerol synthesis. Similarly, cyclic AMP analogue-inhibition of phosphatidylcholine synthesis was abolished when the cells were simultaneously incubated with oleate (3 mM). Reactivation of phosphatidylcholine synthesis in chlorophenylthio-cyclic AMP-supplemented cells with oleate was accompanied by conversion of CTP: phosphocholine cytidylyltransferase into the membrane-bound form, since these cells released the enzyme more slowly after treatment with digitonin. The opposing actions of cyclic AMP and fatty acids are discussed in relation to the regulation of glycerolipid biosynthesis during starvation, diabetes and stress.     

Acylated proteins in Borrelia hermsii, Borrelia parkeri, Borrelia anserina, and Borrelia coriaceae.

Borrelia hermsii, Borrelia parkeri, Borrelia anserina, and Borrelia coriaceae produced several lipoproteins identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography of bacteria grown in [3H]palmitate. Five major acylated proteins were demonstrated by sequential alkaline and acid hydrolysis. High-pressure liquid chromatography of isolated proteins confirmed that covalently bound radioactivity was represented by fatty acids.     

Mechanism for binding of fatty acids to hepatocyte plasma membranes

The purpose of this study was to examine the interaction between fatty acids and plasma membranes from liver cells. We were unable to reproduce the reported effect of heating on the capacity of these membranes to bind [3H]oleate (Stremmel et al. 1985 Proc. Natl. Acad. Sci. USA. 82: 4-8). In fact, the distribution of [3H]oleate between plasma membranes and unilamellar vesicles of lipids extracted from these membranes was in favor of the lipids, indicating the absence of a detectable amount of binding to a putative fatty acid binding protein in plasma membranes. Radius of curvature of vesicles (125 A vs 475 A) had no effect on the partitioning of fatty acid. In addition, the distribution of [3H]oleate between plasma membranes and other phases had the properties of a partition coefficient over a 200-fold range of [3H]oleate. There was no evidence in this experiment for a binding isotherm, i.e., binding of [3H]oleate at a specific site, superimposed on the nonspecific partitioning of [3H]oleate into the lipids of the plasma membrane. There was no competition between [14C]oleate and [3H]palmitate for entry into plasma membranes. Finally, rates of uptake of [14C]oleate and [3H]palmitate by perfused rat liver were not affected by the presence of the other fatty acid in perfusates. These data indicate that the avidity of hepatocyte plasma membranes for [3H]oleate is a simple consequence of the physical chemical properties of oleate, lipids, and water. The data exclude the idea that the uptake of fatty acids into cells is the result of binding proteins and/or catalyzed reactions at the water-membrane interface of the cell or within the plane of the plasma membrane.     

Identification of the thiol ester linked lipids in apolipoprotein B

Human plasma low-density lipoproteins of 1.032-1.043 g/mL density were totally delipidized. The reduced and carboxymethylated apolipoprotein B was incubated with 50 mM [14C]methylamine at pH 8.5 at 30 degrees C. Covalent incorporation of [14C]methylamine was observed with concomitant generation of new sulfhydryl groups, which could be blocked with [3H]- or [14C]iodoacetic acid. One type of the [14C]methylamine-modified products was separated from the protein and was found to be lipid in nature. Its Rf on thin-layer chromatography (TLC) was similar to that of the synthetic N-methyl fatty acyl amides. After purification with TLC and transesterification in 3 N methanolic HCl, methyl esters of C16 and C18 fatty acids at 1:1 ratio were identified by gas-liquid chromatography. The transesterification method was verified with the known N-methyl fatty acyl amides. These results suggest the presence of labile thiol ester linked palmitate and stearate in apolipoprotein B. Under mild alkaline conditions, the thiol ester bonds are broken by methylamine and form N-methyl fatty acyl amides and release new-SH groups. Intramolecular thiol ester bonds linked between cysteine side chains and acidic amino acid residues were also found present, which will be reported separately.     

Differences in partitioning of meal fatty acids into blood lipid fractions: a comparison of linoleate, oleate, and palmitate

There has been much interest in the health effects of dietary fat, but few studies have comprehensively compared the acute metabolic fate of specific fatty acids in vivo. We hypothesized that different classes of fatty acids would be variably partitioned in metabolic pathways and that this would become evident over 24 h. We traced the fate of fatty acids using equal amounts of [U-(13)C]linoleate, [U-(13)C]oleate, and [U-(13)C]palmitate given in a test breakfast meal in 12 healthy subjects. There was a tendency for differences in the concentrations of the tracers in plasma chylomicron-triacylglycerol (TG) (oleate > palmitate > linoleate). This pattern remained in plasma nonesterified fatty acid (NEFA) and very low-density lipoprotein (VLDL)-TG (P 相似文献   

The role of the acyl-CoA pool in the synthesis of polyunsaturated 18-carbon fatty acids and triacylglycerol production in the microsomes of developing safflower seeds

Microsomes isolated from the developing cotyledons of the seeds of the safflower varieties, very-high-linoleate, Gila and high-oleate, were capable of exchanging the acyl groups in acyl-CoA with the fatty acids in position 2 of phosphatidylcholine. The specificity of the 'acyl-exchange' towards the acyl moiety in acyl-CoA was selective in the order: oleate greater than linoleate greater than linolenate. Stearoyl-CoA was completely selected against when presented in a mixed substrate with unsaturated 18-carbon acyl-CoAs. Microsomes, of the very-high-linoleate safflower variety, rapidly desaturated in situ-labelled [14C]oleoylphosphatidylcholine in the presence of NADH. Little oleate desaturation, however, was observed in the microsomes of the high-oleate variety. Microsomes of the Gila and high-oleate varieties of safflower rapidly synthesised phosphatidic acid by the acylation of glycerol 3-phosphate with acyl-CoA. The phosphatidic acid was metabolised to diacylglycerol, which was further acylated to triacylglycerol. A strong selectivity for linoleoyl-CoA was found for the acylation of glycerol 3-phosphate in both the Gila and high-oleate microsomes. On the basis of these results, we propose that the pattern of 18-carbon unsaturated fatty acids in the triacylglycerols of all 'oil'-producing seeds is a direct reflection of the fatty acids in the acyl-CoA pool. This, in turn, is governed by: A, the rate and specificity of the acyl exchange between acyl-CoA and phosphatidylcholine; B, the rate of oleate (and linoleate) desaturation in phosphatidylcholine; and C, the rate and specificity of the glycerophosphate acyltransferase.