The phospholipid-N-methyltransferase of rat brain microsomes

The phospholipid-N-methyltransferase activity of rat brain microsomes had an optimum pH of 11.0 in the absence or presence of phosphatidylethanolamine (PE) but pH 10.0 in the presence of phosphatidylmonomethylethanolamine (PMME) or phosphatidyldimethylethanolamine (PDME). An apparent Km for S-adenosyl methonine from 0.10 to 0.12 mM was observed with exogenous methylated phospholipids PMME or PDME. Methylated neutral lipid was the major lipid produced in the absence of the exogenous acceptors. Two exogenous phospholipids, PMME and PDME, significantly stimulated microsomal phospholipid-N-methyltransferase activity and the predicted methylated phospholipids were the major products. PE additions did not cause any stimulation of methylated lipid formation. Preincubation of particles at temperatures from 40 to 100 degrees C resulted in a loss in the microsomal phospholipid-N-methyltransferase activity that was stimulated by PMME and PDME.