ENHANCEMENT BY CYTIDINE OF MEMBRANE PHOSPHOLIPID-SYNTHESIS

Cytidine, as cytidine 5'-diphosphate choline, is a major precursor in the synthesis of phosphatidylcholine in cell membranes. In the present study, we examined the relationships between extracellular levels of cytidine, the conversion of [C-14]choline to [C-14]phosphatidylcholine, and the net syntheses of phosphatidylcholine and phosphatidylethanolamine by PC12 cells. The rate at which cytidine (as [H-3]cytidine) was incorporated into the PC12 cells followed normal Michaelis-Menten kinetics (k(m) = 5-mu-M; V(max) = 12 x 10(-3) mmol/mg of protein/min) when the cytidine concentrations in the medium were below 50-mu-M; at higher concentrations, intracellular [H-3]cytidine nucleotide levels increased linearly. Once inside the cell, cytidine was converted mainly into cytidine triphosphate. In pulse-chase experiments, addition of cytidine to the medium caused a time- and dose-dependent increase (by up to 30%) in the incorporation of [C-14]choline into membrane [C-14]-Phosphatidylcholine. When the PC12 cells were supplemented with both cytidine and choline for 14 h, small but significant elevations (p < 0.05) were observed in their absolute contents of membrane phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine, all increasing by 10-15% relative to their levels in cells incubated with choline alone. Exogenous cytidine, acting via cytidine triphosphate. can thus affect the synthesis and levels of cell membrane phospholipids.