Detection of Cytosolic Phosphatidylethanolamine N -Methyltransferase in Rat Brain

Phosphatidylethanolamine N-methyltransferase (PEMT) is known to be a membrane-associated protein. However, cytosolic PEMT was detected when sufficient amounts of exogenous phospholipids were added in the incubation media. The methylation of phospholipids was measured by the incorporation of the $[^3H]-methyl$ group from S-adenosylmethionine and the methylated phospholipids were analyzed by thinlayer chromatography. The essence of the assay condition for the cytosolic enzyme was the inclusion of 200 ${\mu}g$ of each substrate, phosphatidylethanolamine (PE), phosphatidyl N-monomethylethanolamine (PME) and phosphatidyl N,N-dimethylethanolamine (PDE), in the reaction mixture of 100 ${\mu}l$. The subcellular fractionation of brain PEMT activities revealed that approximately 38.1 % for PME, 39.5% for PDE, and 22.4% for PC formation was present in the cytosolic fraction. The general properties of cytosolic PEMT were characterized and compared with those of neuronal nuclei PEMT.