Biosynthesis of phosphatidylcholine by enzyme preparations from spinach leaves

The enzymic incorporation of choline-1,2-(14)C from CDP-choline-1,2-(14)C into phosphatidylcholine by spinach leaf preparations was characterized. The enzyme catalyzing the incorporation, choline phosphotransferase, had a pH optimum of about 8.0 and required either Mn(2+) or Mg(2+) as cofactor. The saturation concentration of Mn(2+) was 0.3 mm and that for Mg(2+) was 13 mm. The K(m) for CDP-choline was 10 micro m. The choline phosphotransferase was inhibited by sulfhydryl reagents. The enzyme was inactivated at 30 degrees C, but this inactivation could be prevented by dithiothreitol and Mn(2+). Preincubation of the enzyme with Mn(2+) prevented inhibition by sulfhydryl reagents. The incorporation of diglyceride-U-(14)C into phosphatidylcholine was also studied. The enzyme did not show any diglyceride specificity when exogenous diglyceride was added, indicating that fatty acid distribution in phosphatidylcholine of spinach is not controlled by choline phosphotransferase.