Synthesis of phosphatidylcholine from phosphatidylethanolamine by at least two methyltransferases in rat pituitary extracts

Rat pituitary extracts contain at least two methyltransferases that methylate phosphatidylethanolamine to phosphatidylcholine using S-adenosylmethionine as the methyl donor. The first enzyme methylates phosphatidylethanolamine to phosphatidyl-N-monomethylethanolamine and has a high Km (40-42 microM) for S-adenosylmethionine, whereas the second enzyme(s) catalyzes two successive methylations of phosphatidyl-N-monomethylethanolamine to phosphatidyl-N,N-dimethylethanolamine and then to phosphatidylcholine and has a low Km (6.7 microM) for S-adenyl-L-methionine. The first enzyme is loosely bound to the membrane fraction; therefore it appears in both particulate (20,000 X g) and supernatant (20,000 X g) fractions, whereas the second enzyme(s) is tightly bound to the membrane and thus appears only in the particulate fraction. Both methyltransferases have two pH optima of 6.5 and 9.5 (9.5 activity greater than 6.5 activity) and they do not require Mg2+.