Phosphocholine derivatives

This alcohol is the main constituent of the mammalian-derived phospha-tidycholines. The only exception has been the report of a diol present in very small amounts (as a phosphocholine derivative) in lung tissue. 

The most convincing example that inhibition of PC synthesis can induce apoptosis comes from a cell line with a temperature-sensitive defect in one of the enzymes in the CDP-choline pathway (Cui et al, 1996). The MT58 cell line is a Chinese hamster ovary (CHO) derived cell line with a mutation in CTP phosphocholine cytidylyltransferase (CT), which renders the... 

Transfer of a phosphocholine residue to the free OH group gives rise to phosphatidylcholine (lecithin enzyme l-alkyl-2-acetyl-glycerolcholine phosphotransferase 2.7.8.16). The phosphocholine residue is derived from the precursor CDP-choline (see p. 110). Phos-phatidylethanolamine is similarly formed from CDP-ethanolamine and DAG. By contrast, phosphatidylserine is derived from phosphatidylethanolamine by an exchange of the amino alcohol. Further reactions serve to interconvert the phospholipids—e.g., phosphatidylserine can be converted into phosphatidylethanolamine by decarboxylation, and the latter can then be converted into phosphatidylcholine by methylation with S-adenosyl methionine (not shown see also p. 409). The biosynthesis of phosphatidylino-sitol starts from phosphatidate rather than DAG. 

This enzyme [EC 2.3.1.23], also called lysolecithin acyl-transferase and lysophosphatidylcholine acyltransferase, catalyzes the reaction of an acyl-CoA derivative with 1-acyl-5 n-glycero-3-phosphocholine to yield coenzyme A and l,2-diacyl-5 n-glycero-3-phosphocholine. The enzyme preferentially acts on unsaturated acyl-CoA derivatives, but l-acyl-5 n-glycero-3-phosphoinositol can also act as the acceptor. 

An aliphatic ketone (9-heptadecanone) and two keto derivatives of stearic acid (as potassium salts) containing a ketone functionality either at position 5 or 12 were incorporated into bilayers of the phospholipid l,2-dihexadecyl-sn-glycero-3-phosphocholine. Infrared spectra of these mixtures were measured as a function of temperature and amount of added cholesterol. It was found that the presence of cholesterol in these bilayers induces changes in the location of the guest ketone and that these changes are dependent on both temperature and cholesterol concentration. It is also demonstrated that, in the gel phase, the presence of cholesterol induces larger intersheadgroup separations and, therefore, water penetrates deeper into the lipid bilayer. 

In a typical spectrum, the peak at 524 represents the mass ion MH+ and mlz 482 represents the deacetylated derivative. Also an additional fragment of importance in use in structure studies is the mlz 184, which represents the 0-phosphocholine residue. These are the major fragments obtained from 1-0-hexadccyl-2-acetyl-.y -glycero-3-phosphocholine. Similar information can be derived from a similar derivative in which the acetyl group is replaced by an oleoyl residue. In this case the mass ion MH+ would be 746 and mlz 184 would represent the 0-phosphocholine group. If a linoleoyl (at sn-2), hexa-decyl (at rn-l)-substituted derivative were present, then a mass ion of 744 would be detected. With regard to the polar head groups, an ethanolamine phospholipid would yield on FAB an mlz 142, indicative of a protonated O-phosphoethanolamine. 

The water-soluble fraction (from the original extraction of the reaction mixture described above) is evaporated to dryness, dissolved in 2 ml water, and applied to a Dowex AG 50 W x 8 column (H form, 200-400 mesh-column size, 0.6 x 5 cm). Water is used as the eluant, and the P-containing components are collected. The latter could be evaporated to dryness and silylated using the reagent pyridine/Tri-Sil TBT and BSTFA (1 2 2, v/v) (Pierce Chemical Co., Rockford, 1L). Subsequently an aliquot of this TMS derivative can be analyzed directly by the gas-liquid chromatography technique of Karlsson (1970). For further information on the detection and assay of choline and phosphocholine, suggested reading is an article by Kennedy (1991). 

Using a physical-chemical protocol, Cymerman-Craig et al. (1966) employed phospholipase C attack on the naturally occurring alkenylacylglycero-phosphocholine to yield alkenylacylglycerol. The latter derivative was subjected to base-catalyzed methanolysis to produce alkenylglycerol. The latter... 

HPLC column was directed into a triple quadropole mass spectrometer equipped with an atmospheric pressure articulated ion spray (electrospray) source. Using MS-MS conditions, dissociation of the parent ions yielded daughter ions comparable to the IM+H]+ of PAF and other molecules, such as O-phosphocholine. Quantitative analyses were obtained by selective ion monitoring. Amounts as low as 0.3 ng of PAF and similar compounds were detected. Interestingly with the lyso derivatives, the limited of detection was 3 ng. The primary alkyl chain lengths present in the neutrophil-derived PAF were 16 0 and 18 0. No mention was made of the presence of any 1-0-long chain acyl analogs. 

Although the occurrence of analogs of PAF in mammalian cells was always considered a possibility, Mueller et al. (1984) first reported the presence of 1-0 long-chain acyl-2-acetyl-glycero-3-phosphocholine in rabbit neutrophils. Later, Tokumura et al. (1989) isolated a vasopressor active phosphatidylcholine fraction from bovine brain. Treatment of the fraction with phospholipase C yielded a diglyceride component which was converted to the t-butyldimethylsilyl derivative. Analysis of the latter by gas-liquid chroma-... 

The rate-controlling enzymes in the salvage pathways are the phosphocholine and phosphoethanolamine citidyltransferases. Thus, rat liver cell choline, phosphocholine, and CDP-choline concentrations are 0.23, 1.3, and 0.03 mM, respectively. For the ethanolamine derivatives, these are 1.09, 3.83, and 0.24 mM, respectively. The bottlenecks are clearly in the formation of the CDP derivatives. Phosphocholine citidyltransferase of cytosol is phosphorylated and inactive. Phosphorylation prevents its interaction with the ER. It is activated when it is dephosphorylated and translocated to the ER. cAMP-dependent phosphorylation then promotes its removal from the ER and its inactivation. Ethanolamine... 

PC can come from the diet or be synthesized in the body. Dietary PC is cleaved by a pancreatic enzyme, phospholipase B, that catalyzes hydrolysis of the fatty add from carbon-2 (C-2) of the glycerol backbone. This hydrolysis produces a free fatty add and l-acyh3-phosphocholine glycerol. The latter compound is called lysolecithin, because its parent compound PC is known as lecithin. Lysoledthin and the free fatty acid are absorbed by the enterocyte, converted back to PC, and then packaged into chylomiorons that are released into the lymphatic system. A small fraction of the choline derived from dietary PC enters the bloodstream as free choline. 

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