Phosphatidylcholine synthesis

Acetylcholine Precursors. Early efforts to treat dementia using cholinomimetics focused on choline [62-49-7] (12) supplement therapy (Fig. 3). This therapy, analogous to L-dopa [59-92-7] therapy for Parkinson s disease, is based on the hypothesis that increasing the levels of choline in the brain bolsters acetylcholine (ACh) synthesis and thereby reverses deficits in cholinergic function. In addition, because choline is a precursor of phosphatidylcholine as well as ACh, its supplementation may be neuroprotective in conditions of choline deficit (104). 

Phosphatidylethanolamine synthesis begins with phosphorylation of ethanol-amine to form phosphoethanolamine (Figure 25.19). The next reaction involves transfer of a cytidylyl group from CTP to form CDP-ethanolamine and pyrophosphate. As always, PP, hydrolysis drives this reaction forward. A specific phosphoethanolamine transferase then links phosphoethanolamine to the diacylglycerol backbone. Biosynthesis of phosphatidylcholine is entirely analogous because animals synthesize it directly. All of the choline utilized in this pathway must be acquired from the diet. Yeast, certain bacteria, and animal livers, however, can convert phosphatidylethanolamine to phosphatidylcholine by methylation reactions involving S-adenosylmethionine (see Chapter 26). 

Korgel, B. A. and Monbouquette, H. G. (1996). Synthesis of Size-Monodisperse CdS Nanocrystals Using Phosphatidylcholine Vesicles as True Reaction Compartments. /. Phys. Chem., 100, 346-351. 

The regulation of triacylglycerol, phosphatidylcholine, and phosphatidylethanolamine biosynthesis is driven by the availability of free fatty acids. Those that escape oxidation are preferentiaUy converted to phos-phohpids, and when this requirement is satisfied they are used for triacylglycerol synthesis. 

The mechanisms involved in the establishment of lipid asymmetry are not well understood. The enzymes involved in the synthesis of phospholipids are located on the cytoplasmic side of microsomal membrane vesicles. Translocases (flippases) exist that transfer certain phospholipids (eg, phosphatidylcholine) from the inner to the outer leaflet. Specific proteins that preferentially bind individual phospholipids also appear to be... 

Materials. Egg phosphatidylcholine (PC), bovine brain phosphatidylserine (PS) were obtained from Avanti Polar Lipids Inc. (Birmingham, AL) and cholesterol was from Sigma (St. Louis, MO). Ganglioside GMj, bovine, was obtained from Calbiochem (San Diego, CA). Diethylenetriamine pentaacetic acid distearylamide complex (DPTA-SA) was synthesized according to ref. 17 and nlIn-DTPA-SA was prepared as described (7). This lipophilic radiolabel is not transferred to the serum components from liposomes (unpublished data), nor is it rapidly metabolized in vivo (7). The synthesis of N-(glutaryl)phosphatidylethanolamine(NGPE) has been described (18). Dipalmitoyl deoxyfluorouridine(dpFUdR) was synthesized as described (24). 

Therefore, it is currently believed that anandamide is formed from membrane phospholipids (Fig. 4) through a pathway that involves (1) a trans-acylation of the amino group of phosphatidylethanolamine with arachidonate from the sn-1 position of phosphatidylcholine and (2) a D-type phosphodiesterase activity on the resulting A-arachidonylphosphati-dylethanolamide (NAPE). Synthesis of anandamide is presumably regulated at the levels of both enzymes, the A-acyltranferase and the phospholipase D, by stimuli that raise intracellular calcium or by receptors linked with cAMP and PKA. It has been shown that anandamide is formed when neurons are depolarized and, therefore, the intracellular calcium ion levels are elevated (Cadas, 1996). 

The effect of Li+ upon the synthesis and release of acetylcholine in the brain is equivocal Li+ is reported to both inhibit and stimulate the synthesis of acetylcholine (reviewed by Wood et al. [162]). Li+ appears to have no effect on acetyl cholinesterase, the enzyme which catalyzes the hydrolysis of acetylcholine [163]. It has also been observed that the number of acetylcholine receptors in skeletal muscle is decreased by Li+ [164]. In the erythrocytes of patients on Li+, the concentration of choline is at least 10-fold higher than normal and the transport of choline is reduced [165] the effect of Li+ on choline transport in other cells is not known. A Li+-induced inhibition of either choline transport and/or the synthesis of acetylcholine could be responsible for the observed accumulation of choline in erythrocytes. This choline is probably derived from membrane phosphatidylcholine which is reportedly decreased in patients on Li+ [166],... 

As mentioned above, PAF and PAF-like molecules are rapidly synthesized by keratinocytes following UV exposure. We suggest that two mechanisms are involved. UV-induced free radical formation leads to membrane oxidation and the formation of oxidized phosphatidylcholine. The PAF-like molecules bind to PAF receptors in either a paracrine or autocrine fashion. This induces the release of arachidonic acid from the membrane, activates PI.A2 and promotes the synthesis of bona fide PAF.55 The newly synthesized PAF then binds to PAF receptors, which upregulates the production of more PAF and downstream biological modifiers such as eicosanoids and cytokines. Ultimately this activates the cascade of events that leads to immune suppression. 

Figure 1. Control of mitochondrial biogenesis by the nuclear genome. Most mitochondrial proteins, including cytochrome c, are nuclear gene products which are subsequently imported into mitochondria. Similarly, most enzymes involved in synthesis of mitochondrial phosphoplipids are encoded in the nuclear genome. Being located in the endoplasmatic reticulum, they synthesize phosphatidylcholine (PtdCho), phosphatidylserine (PtdSer), phosphatidylglycerol (PG) and phosphatidylinositol (Ptdins). The phospholipids are transferred to the outer membrane. The imported lipids then move into the inner membrane at contact sites. Mitochondria then diversify phospholipids. They decarboxylate phosphatidylserine to phosphatidylethanolamine (PtdEtN), but the main reaction is the conversion of imported phosphatidylglycerol to cardiolipin (CL). Cardiolipins localize mainly in the outer leaflet of the inner membrane.

Figure 1. Synthetic pathway for PS and PE in mammalian cells. The major steps occuring in the synthesis and interconversion of PS and PE are shown. The PS synthases condense serine with a phosphatidyl moiety derived from PC and PE. The nascent PS can be converted to PE by decarboxylation. PE can also be formed by transfer of a phosphoethanolamine moiety from CDP-ethanolamine to diacylglycerol via the Kennedy pathway. The abbreviations used are PC, phosphatidylcholine PS, phosphatidylserine PE, phosphatidylethanolamine DG, diacylglycerol PSD, phosphatidylserine decarboxylase PSS, PS synthase.

Figure 1. Pathways for the synthesis of phosphatidylcholine, phosphatidylethanolamine and sphingomyelin. Abbreviations CK, choline kinase CPT, cholinephosphotransferase CT, CTP phosphooholine cytidylyltransferase DAG, diacylglycerol PC, phosphatidylcholine PE, phosphatidylethanolamine PEMT, phosphatidylethanolamine-N-methyltransferase SM, sphingomyelin SMase, sphingomyelinase SMsyn, sphingomyelin synthase.

Relation of phosphatidylcholine synthesis with the sphingomyelin pathway... 

Figure 2. Effect of Ca-ceramide on phosphatidylcholine and phosphatidylethanolamine synthesis in rat-2 fibroblasts. Cells were treated for 2 h in the absence (open bars) or presence (hatched bars) of 25pM C6-ceramide and the following parameters were determined i) the incorporation of [ H]choline and [ H]ethanolamine into phosphatidylcholine (PC) and phosphatidylethanolamine (PE), respectively (panel A) and in CDP-choline and CDP-ethanolamine, respectively (panel B) and ii) the in vitro activity of choline- and ethanolaminephosphotransferase (CPT and EPT) (panel C).

Wright, M.M., Henneberry, A.L., Lagace, T.A., Ridgway, N.D., and McMaster, C.R., 2001, Uncouphng Famesol Induced Apoptosis from its Inhibition of Phosphatidylcholine Synthesis. . Biol Chem. 276 25254-25261. 

Zerouga M, Stillwell W, Jenski LJ. Synthesis of a novel phosphatidylcholine conjugated to docosahexaenoic acid and methotrexate that inhibits cell proliferation. Anticancer Drugs 2002 13 301. 

Glycerophospholipids are used for membrane synthesis and for producing a hydrophilic surface layer on lipoproteins such as VLDL. In cell membranes, they also serve as a reservoir of second messengers such as diacylglycerol, inositol 1,4,5-triphosphate, and arachidonic acid. Their structure is similar to triglycerides, except that the last fatty acid is replaced by phosphate and a water-soluble group such as choline (phosphatidylcholine, lecithin) or inositol (phosphatidyl-inositol). 

Figure 11.21 Outline of synthesis of phosphatidylinositol, phosphatidylserine, phosphatidylethanolamine and phosphatidylcholine. Note in the synthesis of phosphatidylinositol, the free base, inositol, is used directly. Inositol is produced in the phosphatase reactions that hydrolyse and inactivate the messenger molecule, inositol trisphosphate (IP3). This pathway recycles inositol, so that it is unlikely to be limiting for the formation of phosphatidylinositol bisphosphate (PIP )- This is important since inhibition of recycling is used to treat bipolar disease (mania) (Chapter 12, Figure 12.9). Full details of the pathway are presented in Appendix 11.5. Inositol, along with choline, is classified as a possible vitamin (Table 15.3).

Figure 11.24 A summary of the reactions involved in synthesis of sphingomyelin. Reaction between serine and palmitoyl-CoA produces 3-oxosphinganine, which is converted to sphingamine. Attachment of a Long-chain fatty acid to the amino group of sphinganine produces dihydroxyceramide. Ceramide reacts with phosphatidylcholine the phosphocholine component forms an ester bond with the hydroxyl group at position one of ceramide.

Ma LY, LaCagnin LB, Bowman L, et al. 1989. Carbon tetrachloride inhibits synthesis of pulmonary surfactant disaturated phosphatidylcholines and ATP production in alveolar type II cells. Acta Biochem Biophys 1003 136-144. 

Figure 11.7 Salvage pathway for phosphatidylcholine synthesis. (Modified from Schmidli etal., 1991.)...

Schmidli, P. K., Schurtenberger, R, and Luisi, P. L. (1991). Liposome-mediated enzymatic synthesis of phosphatidylcholine as an approach to self-replicating liposomes. J. Am. Chem. Soc., 113, 8127-30. 

Diethanolamine has been shown to inhibit choline uptake into cultured Syrian hamster embryo (SHE) and Chinese hamster ovary cells and to inhibit the synthesis of phosphatidylcholine in in-vitro systems in a concentration-dependent, competitive and reversible manner (Lehman-McKeeman Gamsky, 1999, 2000). Diethanolamine treatment caused a marked reduction in hepatic choline metabolite concentrations in mice following two weeks of dermal dosing. The most pronounced reduction was in the hepatic concentration of phosphocholine, the intracellular storage form of choline (Stott et al, 2000). Moreover, the pattern by which choline metabolites were altered was similar to the pattern of change that has been observed following dietary choline deprivation in rodents (Pomfret et al, 1990). Excess choline also prevented diethanolamine-induced inhibition of phosphatidylcholine synthesis and incorporation of diethanolamine into SHE cell phospholipids (Lehman-McKeeman Gamsky, 2000). 

Lehman-McKeeman, L.D. Gamsky, E.A. (1999) Diethanolamine inhibits choline uptake and phosphatidylcholine synthesis in Chinese hamster ovary cells. Biochem. biophys. Res. Comm., 262, 600-604... 

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