Conversion of Phosphatidylethanolamine to Phosphatidylcholine in Rat Liver

The enzymatic activities for the synthesis of phosphatidylcholine from phosphatidylethanolamine and Sadenosyl-L-methionine have been solubilized from rat liver microsomes by ultrasonic disruption in the presence of 0.2% Triton X-100. Significant co-purification of the activities was achieved by chromatography of the Triton extract on octyl-Sepharose CL-4B and, subsequently, on Sepharose 6B. Throughout purification, the ratio of the activities for the N-methylation of phosphatidylethanolamine, phosphatidyl-N-monomethylethanolamine, and phosphatidyl-N,N-dimethylethanolamine remained unchanged. Additional properties of the purified enzyme(s) were examined. The three activities had a similar pH profile with an optimum at pH 9.5 and were inhibited in an identical fashion by Triton X-100. It was not possible to estimate kinetic parameters with phosphatidylethanolamine as a substrate because of the rapid methylation of the product (phosphatidyl-Nmonomethylethanolamine) to phosphatidyl-N,N-dimethylethanolamine and phosphatidylcholine. However, the initial reaction rate for the formation of phosphatidyl-N-monomethylethanolamine from phosphatidylethanolamine and S-adenosyl+methionine was estimated to be 0.08 nmol*min-‘*mg-‘. The apparent kinetic constants for the methylation of phosphatidyl-Nmonomethylethanolamine and phosphatidyl-N,N-dimethylethanolamine were determined. S-Adenosyl-Lhomocysteine was a competitive inhibitor with respect to S-adenosyl-L-methionine for the methylation of phosphatidyl-N-monomethylethanolamine and phosphatidyl-N,N-dimethylethanolamine. The results suggest that the conversion of phosphatidylethanolamine to phosphatidyl-N-monomethylethanolamine is the rate-limiting step.