THE Jona~ar, OP BIOLOGICAL CHEYIBTRY

CDP-diglyceride: L-serine phosphatidyltransferase (phosphatidylserine synthetase) of Escherichia coli is tightly associated with ribosomes in crude cell-free extracts. The synthetase has now been separated from ribosomes by extraction with solutions containing 5 M NaCl and has been purified loo-fold. The partially purified enzyme is devoid of contaminating hydrolytic activities and nearly free of RNA. The enzyme catalyzes exchange reactions of CMP with CDP-diglyceride, and of serine with phosphatidylserine. Furthermore, the enzyme catalyzes the formation of CDP-diglyceride from phosphatidylserine and CMP, although the equilibrium strongly favors synthesis of phosphatidylserine. A phosphatidyl-enzyme intermediate may thus be involved in the action of this enzyme. Under the conditions employed in the assay system, however, this intermediate seems to be unstable, since the synthetase also hydrolyzes phosphatidylserine and CDP-diglyceride at a slow rate.