Biosynthesis of phosphatidylethanolamine by enzyme preparations from plant tissues.

The enzymic utilization of cytidine diphosphoethanolamine in the synthesis of phosphatidylethanolamine is localized in the microsomal fraction of spinach (Spinacia oleracea) leaves. The metal ion requirement can be satisfied by Mn(2+) (saturation approximately 0.6 mm) or Mg(2+) (saturation approximately 25 mm). The enzyme has a pH optimum of 8.0 in the presence of Mn(2+) and 7.5 in the presence of Mg(2+). A Michaelis constant of 20 mum was determined for cytidinediphos-phoethanolamine. Enzyme activity was stimulated by thiol compounds and inhibited by thiol reagents. No inhibition was obtained with cytidine monophosphate and Tween 80.The in vitro biosynthesis of phosphatidylethanolamine was inhibited by cytidine diphosphocholine and the biosynthesis of phosphatidylcholine was inhibited by cytidine diphosphoethanolamine. Activities of the two synthetic systems were indistinguishable on the basis of susceptibility to lyophilization and inhibition by thiol reagents.