Regulation of Phospholipid Metabolism in Differentiating Cells from Rat Brain Cerebral Hemispheres in Culture PATTERNS OF ACETYLCHOLINE, PHOSPHOCHOLINE, AND CHOLINE PHOSPHOGLYCERIDES LABELING FROM [METHYL-“C]CHOLINE*

mM. At low choline concentrations (high affinity uptake) most of the [“Clcholine label was present in the phosphocholine fraction prior to the appearance of label in phospholipids. At high choline concentrations (low affinity uptake) a large proportion of the radioactivity was converted into acetylcholine. The dissimilarities between the formation of phosphocholine and acetylcholine as a function of choline concentration might be explained by the existence of two mutually independent enzymatic activities with different K, affinities for choline. Kinetic data augmented by double label studies, suggested that formation of choline phosphoglyceride proceeds entirely via a phosphocholine intermediate. Nearly all radioactivity in the lipid fraction is incorporated into choline phosphoglycerides. A higher turnover rate of choline incorporation into choline phosphoglycerides, accompanied by an increase in the levels of glycerophosphocholine, was observed in older cultures as compared to younger cultures. The metabolic implications of these findings in cultured brain cells in comparison with other in oitro systems are discussed.