Cytidylyltransferases

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.

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... 

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.

Baburina, 1., and Jackowski, S., 1998, Apoptosis triggered by l-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine is prevented by increased expression ofCTP phosphochohne cytidylyltransferase. J. Biol. Chem. 273 2169-2173... 

Clement, J.M., and Kent, C., 1999, CTP phosphochohne cytidylyltransferase insights into regulatory mechanisms and novel functions. Biochem. Biophys. Res. Commun. 257 643-650... 

Kent C., 1997, CLP phosphochohne cytidylyltransferase. Biochim. Biophys. Acta 1348 79-90... 

Wieder, T., PerUtz, C., Wieprecht, M., Huang, R.T., Geilenm C.C. and Orfanosm, CR., 1995, Two new sphingomyelin analogs inhibit phosphatidylcholine biosynthesis by decreasing membrane-bound CTP phosphocholine cytidylyltransferase levels in HaCaT cells. Biochem. J. 311 873-879... 

Phosphocholine reacts with a nucleoside triphosphate (cytidine triphosphate) in a reaction, catalysed by cholinephosphate cytidylyltransferase, which produces cytidine diphosphochoUne ... 

This enzyme [EC 2.7.7.14], also referred to as ethanol-amine-phosphate cytidylyltransferase and phosphoryl-ethanolamine transferase, catalyzes the reaction of CTP with ethanolamine phosphate to produce CDP-ethanol-amine and pyrophosphate. 

ETHANOLAMINE N-METHYLTRANSEERASE Ethanolamine phosphate cytidylyltransferase, CTP ETHANOLAMINEPHOSPHATE CYTIDYLYLTRANSFERASE... 

PHOSPHATIDATE PHOSPHATASE PHOSPHATIDATE CYTIDYLYLTRANSFERASE PHOSPHATIDATE PHOSPHATASE Phosphatidolipase,... 

This assumption comes from the high amount of Neu2en5Ac found in serum and urine of this patient the compound is considered to be derived from CMP-Neu5Ac in a nonenzymic, elimination reaction found to occur under physiological conditions (see later)170 238 239. Thus, lack of acylneuraminate cytidylyltransferase (EC 2.7.7.43) activity as the reason for a diminution of the concentration of CMP-Neu5Ac and, therefore, lessened feedback-inhibition of UDP-GlcNAc 2-epi-merase, can largely be excluded. 

Additional regulation of phosphatidylcholine and phosphatidylethanolamine biosynthesis occurs at the second step in the biosynthetic sequence (see fig. 19.4) where either CDP-choline or CDP-ethanolamine are made. For phosphatidylcholine biosynthesis, the activity of CTP phos-phocholine cytidylyltransferase (which makes CDP-choline) is governed by an unusual mechanism. The enzyme... 

CTP-phosphocholine cytidylyltransferase (CT) is found in two forms, an active form bound to cellular membranes and an inactive form when in a soluble form. The active form of CT is favored by low concentrations of phosphatidylcholine, high concentrations of diacylglycerol and fatty acids and the unphosphorylated state of the enzyme. 

Phosphatidylcholine biosynthesis appears to be regulated principally at the step catalyzed by CTP phos-phocholine cytidylyltransferase. Does the type of regulation observed make biochemical sense Draw a chemical reaction mechanism for this enzyme. 

CMP-KDO synthetase (cytidine-5 -triphosphate cytidine-5 -mon-ophosphate-3-deoxy-D-manno-octulosonate cytidylyltransferase), the next enzyme in the pathway, catalyzes the formation of the nucleotide sugar, CMP-KDO from CTP and KDO. This enzyme was first studied by Ghalambor and Heath (IT). We have purified this enzyme to homogeneity (27). T i apparent K values for CTP nd KDO in the presence of 10 mM Mg were determined to be 2 x 10 M and 2.9 x 10 M, respectively. The enzy tic reaction was dependent upon the addition of CTP, KDO and Mg but did not require a reducing agent. The formation of CMP-KDO was not inhibited by the addition of CDP, CMP, KDO-8-phosphate or N-acetylneuraminic acid to the complete reaction mixture. In agreement with Ghalambor and Health (17), neither KDO-8-phosphate nor N-acetylneuraminic acid could substitute for KDO in the reaction mixture. Pyrophosphate, one of the end products, is a weak inhibitor of the reaction with an apparent Ijq value of 5.0 mM. The addition of CMP,CD or any of the other mono- or di-nucleotides did not inhibit the reaction. 

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... 

Lyso-PtdCho is not simply a lipid metabolite producing neurotrophic and neurotoxic effects. It participates in many signal transduction processes. Lyso-PtdCho activates protein kinases such as protein kinase C, protein kinase A, and c-jun terminal kinase. Lyso-PtdCho stimulates phospholipase D and inhibits CTP-phosphocholine cytidylyltransferase (Gomez-Munoz et al., 1999 Boggs et al., 1995). Lyso-PtdCho acts as an agonist for certain G-protein coupled receptors and can be converted to another bioactive lipid, lyso-phosphatidic acid. 

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