Cholinephosphotransferase

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.

Figure 3. Inhibition of phosphatidylcholine biosynthesis by apoptosis-inducing compounds. The target enzyme for the inhibition of PC biosynthesis is shown for several compounds that have further in common that they all induce apoptosis. Abbreviations are as follows CK, choline kinase CPT, cholinephosphotransferase CT, CTP phosphocholine cytidylyltransferase PC, phosphatidylcholine.

Voziyan, P.A., Goldner, C.M., and Melnykovych, G., 1993, Famesol inhibits phosphatidylcholine biosynthesis in cultured cells by decreasing cholinephosphotransferase activity. Biochem. J. 295 757-762... 

This enzyme [EC 2.7.S.2] (also known as CDP-choline l,2-diacylglycerol cholinephosphotransferase, diacylglycerol cholinephosphotransferase, phosphoryl-choline glyceride transferase, alkylacylglycerol cholinephosphotransferase, and l-alkyl-2-acetylglycerol cholinephosphotransferase) catalyzes the reaction of CDP-choline with 1,2-diacylglycerol to produce CMP and a phosphatidylcholine. l-Alkyl-2-acylglycerol derivatives can also serve as substrates. 

It is postulated that inhibition of PtdCho synthesis and the release of choline are key steps associated with excitotoxicity and are common to NMDA and AMPA receptor stimulation. The mechanism of inhibition of PtdCho is not fully understood. Metabolic labeling experiments in cortical cultures demonstrate that NMDA receptor over activation does not modify the activity of phosphochohne or phospho-ethanolamine cytidylyltransferases but strongly inhibits choline and ethanolamine phosphotransferase activities. This effect is observed well before any significant membrane damage and cell death. Moreover, cholinephosphotransferase activity is lower in microsomes from NMDA-treated cells. These results show that membrane... 

Finally PC is made by releasing the CMP group in the process of fusing phosphocholine to diacylglycerol (DAG) by the enzyme CDP-choline 1,2-diacylglycerol cholinephosphotransferase (CPT McMaster and Bell, 1997). One cloned isoform of CPT seems specific for CDP-choline, whereas another CPT also can synthesize PE from CDP-ethanolamine and DAG (Henneberry and McMaster, 1999 Henneberry et al., 2000). 

Henneberry, A.L., and McMaster, C.R., 1999, Cloning and expression of a human choline/ethanol-aminephosphotransferase synthesis of phosphatidylcholine and phosphatidylethanolamine. BiochemJ. 339 291-298 Henneberry, A.L., Wistow, G., and McMaster, C.R., 2000, Cloning, genomic organization, and characterization of a human cholinephosphotransferase. J. Biol. Chem. 275 29808-29815... 

The CDP-choline pathway is the major pathway for the synthesis of PC in the lung, and cholinephosphotransferase (CPT) is a terminal enzyme in this pathway. Regulation of PC metabolism is one of the vital aspects of the cell cycle with implications in the control of cell proliferation as well as in apoptosis (Cui et al., 1996 Baburina and Jackowski, 1998). 

Sikpi, M. and Das, S. (1987). Development of cholinephosphotransferase in guinea pig lung mitochondria and microsomes. Biochim Biophys Acta 899, 35-43. 

Sinha Roy, S., Mukherjee, S., Kabir, S., Rajaratnam, V., Smith, M., and Das, S.K. (2005). Inhibition of cholinephosphotransferase activity in lung injury induced by 2-chloroethyl ethyl sulfide, a mustards analog. J Biochem Mol Toxicol 19, 289-97. 

Stith, I.E. and Das, S.K. (1981). Pulmonary surfactant lipids studies on cholinephosphotransferase in developing guinea pig lung. Indian Biologist 13, 120-8. 

This enzyme was also discovered by Kennedy and co-workers and is considered to be located on the ER but is also found on the Golgi, MAM, and nuclear membranes (C.R. McMaster, 1997). Although the enzyme has been known for more than four decades, and despite intense efforts in many laboratories, the cholinephosphotransferase... 

Cholinephosphotransferase acts at a branchpoint in the metabolism of DG (Fig. 1) that can be converted into PE, TG, and PA. Most studies indicate that cells and tissues contain an excess of cholinephosphotransferase activity. Hence, the amount of active enzyme does not normally limit PC biosynthesis. However, it is clear that the in vivo activity of cholinephosphotransferase is regulated by substrate supply. The supply of one of the substrates (CDP-choline) is regulated by the activity of CT (Section 3.4), and the supply of DG seems to be controlled by the availability of fatty acids. Excess DG that is not utilized for PC or PE biosynthesis is stored in the liver as TG. 

The CT reaction usually limits the rate of PC biosynthesis. The first evidence in support of this conclusion was drawn from the relative pool sizes of the aqueous precursors (in rat liver, choline = 0.23 mM, phosphocholine =1.3 mM, CDP-choline = 0.03 mM). Calculation of these values assumes that 1 g wet tissue is equivalent to 1 ml and that there is no compartmentation of the pools. The second assumption may not be valid as there is evidence for compartmentation of PC precursors (M.W. Spence, 1989). The concentration of phosphocholine is 40-fold higher than that of CDP-choline, consistent with a bottleneck in the pathway at the reaction catalyzed by CT. Pulse-chase experiments illustrate this bottleneck more vividly. After a 0.5 h pulse of hepatocytes with [methyl- H]choline, more than 95% of radioactivity in the precursors of PC was in phosphocholine, with the remainder in choline and CDP-choline. When the radioactivity was chased with unlabeled choline, labeled phosphocholine was quantitatively converted to PC (Fig. 5). The radioactivity in CDP-choline remained low during the chase and CDP-choline was rapidly converted to PC. There was minimal radioactivity in choline which suggests that choline is immediately phosphorylated after it enters the cell. It is important to note that if a cell or tissue is in a steady state, pool sizes and reaction rates do not change. Thus, although the rate of PC synthesis is determined by the CT reaction, the rates of the reactions catalyzed by choline kinase and cholinephosphotransferase are the same as that of the reaction catalyzed by CT, otherwise, the pool sizes of precursors would change. For example, if the choline kinase reaction were faster than the CT reaction, the amount of phosphocholine would increase. Thus, CT sets the pace of the pathway. 

Fig. 5. Biosynthesis of membrane phospholipids from alkyldihydroxyacetone-P (alkyl-DHAP), the first detectable intermediate formed in the biosynthetic pathway for ether-linked glycerolipids. Enzymes catalyzing the reactions are (1) NADPH alkyl-DHAP oxidoreductase, (II) acyl-CoA 1 -alkyl-2-lyso-sn-glycero-3-phosphate acyltransferase, (III) l-alkyl-2-acyl-in-glycero-3-phosphate phosphohydrolase, (IV) ATP 1-alkyl- /i-glycerol phosphotransferase, (V) CDP-choline l-alkyl-2-acyl-sn-glycerol cholinephosphotransferase (dithiothreitol-sensitive), (VI) CDP-ethanolamine l-alkyl-2-acyI-sn-glycerol ethanolaminephosphotransferase, and (VII) acyl-CoA 1 -alkyl-2-acy 1-OT-glycerol acyltransferase.

Fig. 7. Biosynthesis of choline plasmalogens (plasmenylcholines) via modification of the sn-3 polar head group of ethanolamine plasmalogens (plasmenylethanolamines). These reactions are proposed to be catalyzed directly by (1) a base exchange enzyme or (II) At-methyltransferase. A combination of other enzymatic reactions could also result in replacement of the ethanolamine moiety of plasmenylethanolamine to produce plasmenylcholines the enzymes responsible include (IB) phospholipase C, (IV) the reverse reaction of ethanolamine phosphotransferase, (V) phospholipase D, (VI) phosphohydtolase, and (VII) cholinephosphotransferase. AdoMet, 5-adenosyl-L-methionine AdoHcy, 5-adenosyl-L-homocysteine Etn, ethanolamine GPE, sn-glycero-...

Fig. 8. Biosynthesis of plasmenylcholine via the modification of the sn-2 acyl and sn-3 head-group moieties of plasmenylethanolamine. The reactions are catalyzed by the following enzymes (I) PLAj, (II) CoA-independent transacylase, (HI) lysophospholipase C, (IV) lysophospholipase D, (V) phosphotransferase, (VI) acyl-CoA acyl-transferase, (VII) phosphohydrolase, and/or (VIII) cholinephosphotransferase. Etn, ethanolamine Cho, choline GPE, s/i-glycero-3-phosphoethanolamine GPC, 5/i-glycero-3-phosphocholine.

Fig. 11. Biosynthesis of platelet-activating factor (PAF) via the de novo pathway. The three-step reaction sequence, beginning with l-alkyl-2-lyso-s -glycero-3-phosphate as the precursor, is catalyzed by (I) acetyl-CoAialkyllysoglycerophosphate acetyltransferase, (II) alkylacetylglycero-phosphate phosphohydrolase, and (HI) dithiothreitol-insensitive CDP-choline alkylacetylglycerol cholinephosphotransferase.

Vial, H. J., Thuet, J. J. and Philippot, J. R. (1984) Cholinephosphotransferase and ethanolaminephosphotransferase activities in Plasmodium knowlesi-infected erythrocytes. Their use as parasite-specific markers. Biochim. Biophys. Acta 795 372-383. 

---