Phosphatidylcholine biosynthesis in Tetrahymena pyriformis

1. 1. Phosphatidylcholine biosynthesis in Tetrahymena pyriformis was investigated. Experiments in vivo using [Me-14C]methionine showed incorporation of radioactivity into the choline portion of phosphatidylcholine. No methylation of 2-aminoethylphosphonic acid was detected. Incorporation of the intact choline molecule into lecithin was demonstrated when the cells were grown on [Me-14C]choline. 2. 2. The transfer of the methyl group of [ Me- 14 C ] S - adenosyl - l - methionine to form lecithin was demonstrated in cell-free systems. The activity was localized in the microsomal cell fraction. The enzyme system accepted exogenous phosphatidylmonomethylethanolamine as substrate but not phosphatidylethanolamine or phosphatidyldimethylethanolamine. 3. 3. CDP-choline: diglyceride phosphocholinetransferase activity was localized in the mitochondria. The enzyme was twice as active with the optimal concentration of Mn2+ as Mg2+; the enzymatic activity was inhibited by low concentrations of Ca2+ or Hg2+.