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 [ 14 C]choline to [ 14 C]phosphatidylcholine, and the net syntheses of phosphatidylcholine and phosphatidyleth‐anolamine by PC12 cells. The rate at which cytidine (as [ 3 H]cytidine) was incorporated into the PC12 cells followed normal Michaelis‐Menten kinetics ( K m = 5 μ M; V max = 12 × 10 −3 mmol/mg of protein/min) when the cytidine concentrations in the medium were below 50 μ M; at higher concentrations, intracellular [ 3 H]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 [ 14 C]choline into membrane [ 14 C]‐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, phospha‐tidylethanolamine, 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.