Identification and Properties of Methyltransferases That Synthesize Phosphatidylcholine in Rat Brain Synaptosomes

Abstract

Abstract: Rat brain was found to enzymatically methylate phospholipids to form phosphatidylcholine with S ‐adenosyl‐ l ‐methionine serving as the methyl donor. Methyltransferase activity was localized in the microsomes and synaptosomes. In synaptosomes, at least two enzymes were found to be involved in the formation of phosphatidylcholine. The first methyltransferase which catalyzes the methylation of phosphatidylethanolamine to form phosphatidyl‐ N ‐monomethylethanolamine was found to have a pH optimum of 7.5, a low K m for 5‐adenosyl‐ l ‐methionine and a partial requirement for Mg 2 . Methyltransferase I is tightly bound to membranes. The second methyltransferase (II) catalyzes the successive methylations of phosphatidyl‐ N ‐monomethylethanolamine to phosphatidyl‐ N , N ‐dimethylethanolamine and then to phosphatidylcholine. In contrast to methyltransferase I, methyltransferase II has a pH optimum of 10.5, a high apparent K m for S ‐adenosyl‐ l ‐methionine and no requirement for Mg 2 . Methyltransferase II is easily solubilized by sonication. The highest specific activity for both enzymes was found in the synaptosomal plasma membrane.