Quantitative evaluation of two pathways for phosphatidylcholine biosynthesis in rat brain in vivo

[Me-³H] choline and [³²P] orthophosphate were injected intraventricularly into adult female rats. After variable intervals from injection (1–10 min) the animals were sacrificed by means of a microwave apparatus, and phosphorylcholine and choline phosphoglycerides extracted from brain and counted for radioactivity content after separation. The kinetic constants (K) for phosphorylcholine incorporation into lipids were determined both for [³²P] and [³H] labeling. From the data obtained by these procedures it is concluded that base-exchange reactions for choline incorporation into lipids are operating in rat brain in vivo and that they represent a rapidly equilibrating system.     

The utilization of ethanolamine and serine for ethanolamine phosphoglyceride synthesis by human Y79 retinoblastoma cells

Phospholipid synthesis was investigated in human Y79 retinoblastoma cells, a cultured cell line of retinal origin that retains many neural characteristics. Ethanolamine is taken up by Y79 cells through a high-affinity transport system and is utilized to synthesize ethanolamine and choline phosphoglycerides. High-affinity ethanolamine uptake has a K'm of 40.6 microM and a V'max of 1.06 nmol/min/mg protein, and the process is Na+ dependent. Choline is the only compound tested that reduced ethanolamine uptake, and very high choline concentrations were required to produce this effect. The cells incorporate ethanolamine into phosphatidylethanolamine and ethanolamine plasmalogen at equivalent rates, and the rates of catabolism of these phospholipids are similar. Only a small quantity of ethanolamine is incorporated into phosphatidylcholine, but the amount is not reduced by the addition of choline. Serine is incorporated into phosphatidylserine, which then is converted to phosphatidylethanolamine. Ethanolamine reduces but does not abolish this conversion. Unlike ethanolamine, only a small amount of serine is incorporated into ethanolamine plasmalogen. It is possible that the ethanolamine high-affinity uptake system is necessary to provide a neural cell with enough free ethanolamine for ethanolamine plasmalogen synthesis.     

In vivo studies on pathways for the biosynthesis of lecithin in the rat

The in vivo biosynthesis of lecithin in rats has been studied with the precursors choline-1,2-(14)C, ethanolamine-1,2-(14)C and methionine-CH(3)-(14)C or -CH(3)-(3)H. Lecithin synthesis from choline is rapid in all organs. No sex difference was observed in this pathway. The biosynthesis of lecithin by methylation of phosphatidyl ethanolamine is of quantitative significance in the liver, but not in extrahepatic tissues. More lecithin is synthesized by this pathway in female rats. In liver the lecithin synthesized via both pathways enters a common pool which is in rapid equilibrium with lecithin of blood plasma. A sex difference in the utilization of radioactive ethanolamine for the formation of phosphatidyl ethanolamine was observed (greater utilization in the female). Incorporation of ethanolamine into phospholipids of extrahepatic tissues was slow in both sexes. With labeled methionine as precursor the liver cytidine diphosphate (CDP) choline had a specific activity identical with that of liver lecithin after 20 min, while the specific activity of phosphoryl choline remained low. With labeled choline as precursor the phosphoryl choline reached a specific activity 50 times that of lecithin after 20 min, while the specific activity of CDP choline was only four times that of lecithin. These findings indicate that the reaction: CDP choline + diglyceride right harpoon over left harpoon phosphatidyl choline + CMP is freely reversible in vivo.     

Quantitative assessment of crosstalk between the two isoprenoid biosynthesis pathways in plants by NMR spectroscopy

Plants have been shown to use the mevalonate pathway for the biosynthesis of sterols and triterpenes in the cytoplasm and the recently discovered deoxyxylulose phosphate pathway for the biosynthesis of a variety of hemiterpenes, monoterpenes, diterpenes, as well as for the biosynthesis of carotenoids and the phytol side chain of chlorophyll in plastids. Despite the compartmental separation, at least one terpene precursor can be exchanged between the two pathways. In order to assess quantitatively the crosstalk between the two isoprenoid pathways, [2-¹³C₁]mevalonolactone or [U-¹³C₆]glucose were supplied to cell cultures of Catharanthus roseus grown under illumination or in darkness. Sitosterol, lutein and phytol were isolated and analysed by NMR spectroscopy. The incorporations of exogenous [2-¹³C₁]mevalonolactone were 48% and 7% into the DMAPP and IPP precursors of sitosterol and lutein, respectively. With [U-¹³C₆]glucose as precursor, at least 95% of sitosterol precursors were obtained via the mevalonate pathway, whereas phytol appeared to be biosynthesised via the deoxyxylulose phosphate pathway (approximately 60%) as well via the mevalonate pathway (approximately 40%). The apparent ratios for the contribution of the two pathways depend on the nature of the precursor supplied as well as the nature of the target compound. Thus, crosstalk between the two terpenoid pathways cannot be explained in detail by a simple two compartment model and requires an additional in depth study of complex regulatory mechanisms.     

Regulation of ethanolamine and choline phosphatide biosynthesis in isolated rat intestinal villus cells

The effects of ethanolamine, choline, and different fatty acids on phospholipid synthesis via the CDP-ester pathways were studied in isolated rat intestinal villus cells. The incorporation of [14C]glucose into phosphatidylethanolamine was stimulated severalfold by the addition of ethanolamine and long-chained unsaturated fatty acids, while the addition of lauric acid inhibited the incorporation of radioactivity into phosphatidylethanolamine. At concentrations of ethanolamine higher than 0.2 mM, phosphoethanolamine accumulated, but the concentrations of CDP-ethanolamine and the incorporation of radioactivity into phospatidylethanolamine did not increase further. The incorporation of [14C]glucose into phosphatidylcholine responded in a way similar to that of phosphatidylethanolamine, except that a 10-fold higher concentration of choline was required for maximal stimulation. CCC inhibited the incorporation of choline into phosphatidylcholine. In contrast with hepatocytes, villus cells did not form phosphatidylcholine via phospholipid N-methylation. The data indicate that, in intestinal villus cells, the cytidylyltransferase reactions are rate limiting in the synthesis of phosphatidylethanolamine and probably also of phosphatidylcholine. The availability of diacylglycerol and its fatty acid composition may also significantly affect the rate of phospholipid synthesis.     

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.     

Quantitative evaluation of siRNA delivery in vivo

Effective small interfering RNA (siRNA)-mediated therapeutics require the siRNA to be delivered into the cellular RNA-induced silencing complex (RISC). Quantitative information of this essential delivery step is currently inferred from the efficacy of gene silencing and siRNA uptake in the tissue. Here we report an approach to directly quantify siRNA in the RISC in rodents and monkey. This is achieved by specific immunoprecipitation of the RISC from tissue lysates and quantification of small RNAs in the immunoprecipitates by stem-loop PCR. The method, expected to be independent of delivery vehicle and target, is label-free, and the throughput is acceptable for preclinical animal studies. We characterized a lipid-formulated siRNA by integrating these approaches and obtained a quantitative perspective on siRNA tissue accumulation, RISC loading, and gene silencing. The described methodologies have utility for the study of silencing mechanism, the development of siRNA therapeutics, and clinical trial design.     

Co-ordinate regulation of ethanolamine kinase and phosphoethanolamine cytidylyltransferase in the biosynthesis of phosphatidylethanolamine in rat liver.

Essential-fatty acid deficiency produces a 52% increase in the rate of phosphatidyl-ethanolamine synthesis in rat liver as calculated from results obtained in vivo [Trewhella & Collins (1973) Biochem. Biophys. Acta 296, 34--50]. This flux change was used to test the possible regulatory roles of ethanolamine kinase and of phosphoethanolamine cytidylyltransferase, which are rate-limiting enzymes of the cytidine pathway for the synthesis of phosphatidylethanolamine [Infante (1977) Biochem. J. 167, 847--849]. The results show that essential-fatty acid deficiency produces 50% and 53% increases respectively in the specific activity of these enzymes, accounting for the increased rate of phosphatidylethanolamine synthesis produced by this dietary insufficiency. This evidence leads to the conclusion that ethanolamine kinase and phosphoethanolamine cytidylyl-transferase have co-ordinated regulatory roles in the flux control of the cytidine pathway, and its sphinganine 1-phosphate lyase branch reaction, for the synthesis of phosphatidylethanolamine.     

Serine and ethanolamine incorporation into different plasmalogen pools: Subcellular analyses of phosphoglyceride synthesis in cultured glioma cells

In cultured glioma cells, plasma membrane (PM) is enriched in phosphatidylserine (PtdSer) and plasmalogens (1-O-alk-1-enyl-2-acyl-sn-glycero-3-phosphoethanolamine). Serine can be a precursor of headgroups of both ptdSer and ethanolamine phosphoglycerides (PE) including plasmalogens and non-plasmalogen PE (NP-PE). Synthesis of phospholipids was investigated at the subcellular level using established fractionation procedures and incorporation of [³H(G)]L-serine and [1,2-¹⁴C]ethanolamine. Specific radioactivity of PtdSer from [³H]serine was 2-fold greater in PM than in microsomes, reaching maximum by 2–4 h. Labeled plasmalogen from [³H]serine appeared in PM by 4 h and increased to 48 h, whereas almost no plasmalogen accumulated in microsomes within 12 h. In contrast, labeled plasmalogen from [1,2-¹⁴C]ethanolamine appeared in both PM and microsomes at early incubation times and became enriched in PM beyond 12 h. Thus, in glioma cells: (1) greater and faster accumulation of labeled PtdSer in PM may reflect direct synthesis from serine within PM; (2) PM is a major source of PtdSer for decarboxylation and PE synthesis; (3) NP-PE in both PM and microsome provides headgroup for synthesis of plasmalogen; and, (4) plasmalogen synthesis may involve different intracellular pools depending on headgroup origin.Abbreviations NP-PE nonplasmenylethanolamine phosphoglycerides including both diacyl and alkylacyl species - PE total ethanolamine phosphoglycerides: plasmalogen-plasmenylethanolamine or alkenylacyl ethanolamine phosphoglyceride (1-O-alk-1-enyl-2-acyl-sn-glycero-3-phosphoethanolamine) - PL phospholipid - PM plasma membrane - PtdCho phosphatidylcholine - PtdSer phosphatidylserine     

Influence of chain length and unsaturation on the effects of fatty acids on phosphoglyceride biosynthesis in isolated rat and pig hepatocytes.

Hepatocytes isolated from rat or pig by collagenase perfusion were incubated with [3H]glcyerol and different albumin-bount fatty acids. Among C22 fatty acids docosahexaenoic acid stimulated phosphatidylethanolamine synthesis in rat hepatocytes most effectively. Addition of docosahexaenoic acid plus either palmitic or stearic acid resulted almost in the same stimulation whereas combinations of this acid with lauric or myristic acid had no effect. Lauric acid and myristic acid alone inhibited phosphatidylethanolamine synthesis. The chain length specificity for monoenoic fatty acids was similar, the hexadecenoic and octadecenoic acids (both cis and trans) being most stimulatory. The addition of 0.2 mM ethanolamine markedly stimulated phosphatidylethanolamine synthesis, but most effects of fatty acids were similar in its presence or absence.     

Evidence for two enzymatic pathways for omega-oxidation of docosanoic acid in rat liver microsomes

We studied the omega-oxidation of docosanoic acid (C22:0) in rat liver microsomes. C22:0 and 22-hydroxy-docosanoic acid (omega-hydroxy-C22:0) were used as substrates, and the reaction products were analyzed by electrospray ionization mass spectrometry. In the presence of NADPH, omega-oxidation of C22:0 produced not only the hydroxylated product, omega-hydroxy-C22:0, but also the dicarboxylic acid of C22:0, docosanedioic acid (C22:0-DCA). When rat liver microsomes were incubated with omega-hydroxy-C22:0 in the presence of either NAD+ or NADPH, C22:0-DCA was formed readily. Formation of C22:0-DCA from either C22:0 or omega-hydroxy-C22:0 with NADPH as cofactor was inhibited strongly by miconazole and disulfiram, whereas no inhibition was found with NAD+ as cofactor. Furthermore, omega-oxidation of C22:0 was reduced significantly when molecular oxygen was depleted. The high sensitivity toward the more specific cytochrome P450 inhibitors ketoconazole and 17-octadecynoic acid suggests that hydroxylation of C22:0 and omega-hydroxy-C22:0 may be catalyzed by one or more cytochrome P450 hydroxylases belonging to the CYP4A and/or CYP4F subfamily. This study demonstrates that C22:0 is a substrate for the omega-oxidation system in rat liver microsomes and that the product of the first hydroxylation step, omega-hydroxy-C22:0, may undergo further oxidation via two distinct pathways driven by NAD+ or NADPH.