In order to investigate the mechanisms involved in some brain disorders at the membrane level, we studied the kinetics and biochemical properties of brain CTP : choline-phosphate cytidylyltransferase (EC 2.7.7.15), the rate-limiting enzyme of the two-step biosynthesis of phosphatidylcholine. This enzyme catalyzes the biosynthesis of CDPcholine from choline phosphate and CTP. We found that its subcellular localization (mainly in microsomal and cytosolic fractions) was different from that of phosphatidylethanolamine N-methyltransferase (EC 2.1.1.17), the enzyme of the alternative pathway for phosphatidylcholine synthesis.CTP:choline-phosphate cytidylyltransferase showed a K,,, of 10 mM for CTP and 0.3 mM for choline phosphate and exhibited a random mechanism. CDPcholine, the reaction product, was a competitive inhibitor of choline phosphate and CTP utilization and had a Ki of 0.090 mM. Both particulate and soluble enzymes required Mg2+ and exhibited an optimal pH at about 7. Cytosolic activity was enhanced by addition of unsaturated fatty acids or phospholipids extracted from brain membranes. Such an enhancement was increased with the centrifugation time used for preparing the soluble enzyme.Phosphatidylcholine, a major phospholipid of mammalian cells, is mainly produced from choline phosphate through Kennedy's pathway [l]. This lipid has primarily a structural function in biologicdl membranes. It may also provide fatty acids, like arachidonic acid for prostaglandin, leukotriene and thromboxane synthesis [2]. Phospholipid synthesis in the brain can modify the affinity of many receptors for their ligands, both endogenous, like catecholamines [3], and exogenous, like benzodiazepines [4].CTP : choline-phosphate cytidylyltransferase catalyzes the synthesis of CDPcholine, the rate-limiting step of phosphatidylcholine synthesis in various tissues [5]. The enzyme from several sources exhibits similar properties [6]. In both lung and liver [7], this enzyme is present under two forms. a) A low-molecular-mass form (L form), which is cytosolic and which presents very low activity. The cytosolic L form exhibits a greater affinity for membranes after in vitro addition of unsaturated fatty acids [8] or in vivo after cholesterol-or cholate-rich diet [9]. b) A high-molecular-mass form (H form), located in cytosol and membranes. The H form, which is the active one, is a mixed aggregate of L form and lipids [lo]. The addition of phosphatidylglycerol or some other lipids [ l l , 121 Choline-phosphate cytidylyltransferase from lung has been extensively studied [lo -131, but few reports concerning the brain enzyme are available yet [14-161. Nevertheless, studying the brain enzyme is of double interest: (a) cholinephosphate cytidylyltransferase may be modified by lipid environment changes, which occur in some cerebral diseases such as ischemia [17], where the membrane balance is altered and the lipid breakdown increases; (b) CDPcholine, the product of the enzyme reaction, has been shown to exert protective effects in brain ischemia ...

Rats were chronically intoxicated with triethyltin in the drinking water (0.002%) for a period of 15 days. Starting with day 5 of the intoxication period a decrease in the body weight was observed and, in parallel, the development of a cerebral oedema could be followed by measuring white matter density. At the same time, an increase of phosphatidylethanolamine-N-methyltransferase and cholinephosphate cytidylyltransferase activities was noted. This increase might be a compensatory mechanism for counteracting the membrane damages induced by triethyltin.